WO2001084074A1 - High precision laser monitor for manufacture of optical coatings and method for its use - Google Patents

High precision laser monitor for manufacture of optical coatings and method for its use Download PDF

Info

Publication number
WO2001084074A1
WO2001084074A1 PCT/US2001/001155 US0101155W WO0184074A1 WO 2001084074 A1 WO2001084074 A1 WO 2001084074A1 US 0101155 W US0101155 W US 0101155W WO 0184074 A1 WO0184074 A1 WO 0184074A1
Authority
WO
WIPO (PCT)
Prior art keywords
electromagnetic energy
signal
optical
response
producing
Prior art date
Application number
PCT/US2001/001155
Other languages
French (fr)
Inventor
Stephen E. Moody
David L. Cunningham
Original Assignee
Orca Photonic Systems, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Orca Photonic Systems, Inc. filed Critical Orca Photonic Systems, Inc.
Priority to AU2001232792A priority Critical patent/AU2001232792A1/en
Publication of WO2001084074A1 publication Critical patent/WO2001084074A1/en

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
    • G01B11/0625—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating with measurement of absorption or reflection

Definitions

  • Tre present invention relates to a method and apparatus for monitoring a manufacturing process, and more particularly, no a method ana apparatus for high precision monitoring of the manufacture of optical coatings.
  • Thin film coatings deposited on glass or similar substrates are use ⁇ extensively in optical technology to tailor the behavior of optical components. Such coatings are used to increase or decrease reflectivity, to modify the polarization of light, or to filter or otherwise control the wavelength spectrum cf light Demg processed.
  • Coatings of dielectric materials offer the highest and most flexible performance to the optical components manufacturer. These coatings frequently consist of multiple discrete layers of differing materials, wmcn are applied successively to the substrate. Such coatings are generally manufactured by vacuum deposition or sputtering of material onto the substrate. In order to achieve the desired component performance, the optical thickness of the individual layers of the coatings must be precisely controlled.
  • optical coatings are designed as a stack of layers alternating between high and low index-of-refraction materials. These layers are usually 1/4 wavelength m optical thickness, or some multiple of 1/4 wavelength, although it is not necessary that they be. As the optical thickness of a layer that is currently being deposite ⁇ approaches 1/4 wavelengtn, the transmission (or reflectance) through the part reaches a turning point, either a maximum or a minimum. Depositior should be stopped accurately at the turning point, ana a new layer or another material begun.
  • Transmission through a partially complete coating may become quite small. For a narrow bandpass filter, transmission may be less than 1 percent, with accurate location of the turning points still required.
  • the science of the optical monitoring approach requires a controlled wavelength spectrum of the light being measured, since the interaction between the coating's optical thickness and the light depends on wavelength. The allowable width of the optical spectrum used for the measurement depends strongly on the type and specification of the component being manufactured.
  • the invention is an apparatus for monitoring the optical thickness of a layer of an optical coating that is being deposited on a medium.
  • the apparatus includes a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy, and a divider to divide the electromagnetic energy into a reference portion and a second portion.
  • the apparatus also includes a reference detector to receive the reference portion of the electromagnetic energy ana to produce a reference signal response thereto, a transmitter to transmit the second portion of the electromagnetic energy to the optical coating, a receiver to receive the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating and to produce a received energy signal m response thereto, and a detector to process the received energy signal and to produce a detector signal response thereto.
  • the apparatus further includes a first gam-programmable preamplifier to receive the reference signal and a first ga control signal and produce a first output signal m response tnereto, a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal in response thereto, and a comparator to receive the first and second output signals and produce a comparison signal representing their difference.
  • the apparatus includes an electronic circuit to receive the comparison signal and to produce the first and second gam control signals and a transmission monitoring signal m response thereto, the electronic circuit operating to drive the comparison signal toward zero by controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes in the first and second gam control signals needed to drive the comparison signal toward zero.
  • the invention is an apparatus for controlling the deposited optical thicknesses of layers of an optical coating that is being deposited on a medium to a predetermined thickness in accordance with a predetermined coating design.
  • the apparatus includes a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a tnermal source of electromagnetic energy, a ⁇ ivider to ⁇ ivide the electromagnetic energy into a reference portic and a second portion, and a reference detector to receive t r e reference portion of the electromagnetic energy and to produce a reference signal m response thereto.
  • the apparatus also includes a transmitter to transmit the second portion of the electromagnetic energy to the optical coating, a receiver to receive the second portion of the electromagnetic energy after the second portior of the electromagnetic energy has been affected by the optical coating and to produce a received energy signal in response thereto, and a detector to process the received energy signal and to produce a detector signal m response thereto.
  • the apparatus further includes a first gam-programmable preamplifier to receive the reference signal and a first gam control signal and produce a first output signal in response thereto, a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal m response thereto, and a comparator to receive the first and second output signals ana produce a comparison signal representing their difference.
  • the apparatus includes an electronic circuit, coating deposition control circuit, and an optical coating depositor.
  • the electronic circuit receives the comparison signal and produces the first and second gam control signals and a transmission monitoring signal m response thereto.
  • the electronic circuit operates to drive the comparison signal toward zero by controlling the first and second ga control signals, and generates the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero.
  • the coating deposition control circuit receives the transmission monitoring signal, a deposition status information signal, and process design information and produces a process control signal m response thereto.
  • the optical coating depositor ⁇ eposits the optical coating on the medium, the optical coating depositor being responsive to the process control signal, and produces the deposition status information signal, whereby the first and second ga control signals are indicative of the thickness of the optical coating on the medium, so that the optical coating depositor terminates deposition of the optical coating when the optical coating has the predetermined thickness.
  • the invention is a metnod for analyzing the thickness of an optical coating on a me ⁇ mm .
  • the method includes the steps of a > producing electromagnetic energy having a spectral brightness substantially greatei than a thermal source of electromagnetic energy and b) dividing the electromagnetic energy into a reference portion and a second portion.
  • the method also includes c) receiving the reference portion of the electromagnetic energy and producing a reference signal in response thereto, d) transmitting the second portion of the electromagnetic energy to the optical coating, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating and producing a received energy signal m response thereto, and f) processing the received energy signal and producing a detector signal m response thereto .
  • the method further includes the steps of g) receiving the reference signal and a first gam control signal and producing a first output signal m response thereto, h) receiving the received energy signal and a second gam control signal and producing a second output signal in response thereto, and I) receiving the reference and detector signals and producing a comparison signal representing their difference.
  • the method still further includes the steps of j ) receiving the comparison signal and producing the first and second gam control signals ana a transmission monitoring signal response thereto, K) anvmg the comparison signal toward zero by controlling the first and second gam control signals, and 1) generating the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero.
  • the invention is a method for depositing an optical coating on a medium, the optical coating having a predetermined thickness.
  • the method includes the steps of a) providing a source of electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy and b) dividing the electromagnetic energy into a reference portion and a second portion.
  • the method also includes the steps of c) receiving the reference portion of the electromagnetic energy and producing a reference signal m response thereto, d) transmitting the electromagnetic energy to the optical coating, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating, and f) processing the electromagnetic energy received by the receiver and producing a detector signal m response thereto.
  • the method further includes the steps of g) receiving the reference and detector signals and a gam control signal, and producing a comparison signal in response to the reference and detector signals, h) transmitting the frequency control signal to the source of the electromagnetic energy to determine the instantaneous substantially single frequency of electromagnetic energy produced by the source of the electromagnetic energy, I) transmitting the amplitude modulation signal to the source of the modulation to determine the instantaneous modulation of tne electromagnetic energy produced by the source of the electromagnetic energy, the amplitude modulation signal also oemg transmitted to the amplifier, ; transmitting the comparison signal to the amplifier, and k) receiving tne gam control signal and producing the comparison signal response thereto, whereby the source produces the electromagnetic energy at the predetermined frequency, and the gam control signal is transmitted, the ga control signal being indicative of the tnickness of the optical coating on the medium.
  • the method also includes the step of 1) depositing the octxcal coat g on the medium, the deposition being responsive to the gam control signal, whereby the first and second gam control signals are indicative of the thickness of the optical coating on the medium, so that the deposition of the optical coating terminates when the optical coatmg has the predetermined thickness.
  • the invention is a method for controlling the deposited optical thickness of each layer of an optical coatmg that is being deposited on a medium to a predetermined thickness m accordance with a predetermined coatmg design.
  • the method includes the steps of a) producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy, and b) dividing the electromagnetic energy into a reference portion and a second portion.
  • the method also includes the steps of c) receiving the reference portion of the electromagnetic energy and producing a reference signal m response thereto, d) transmitting the second portion of the electromagnetic energy to the optical coatmg layer, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating layer and producing a received energy signal m response thereto, and f) processing the received energy signal ana producing a detector signal m response thereto.
  • the method further includes the steps of g rece ⁇ / ⁇ ng tne reference signal and a first gam control and producing a first output signal m response thereto, hi receiving the detector signal and a second gam control signal and producing a second output signal response thereto.
  • the method also includes the steps of I) receiving the first ana second output signals and producing a comparison signal representing their d_fference, j ) receiving the comparison signal a ⁇ d producing the first and second ga control signals and a transmission moritorm ⁇ signal in response thereto, the electronic circuit operating to drive the comparison signal towar ⁇ zero oy controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes the first and second gam control signals needed to drive the comparison signal toward zero, and k) receiving the transmission monitoring signal, a deposition status information signal, and process design information and producing a process control signal m response thereto.
  • the method further includes the step of 1) depositing the optical coatmg layer on the medium response to the process control signal, and producing the deposition status information signal, whereby the first and second ga control signals are indicative of the thickness of the optical coatmg layer on the medium, so that deposition of the optical coatmg layer ceases when the optical coatmg layer has the predetermined thickness.
  • the invention is an apparatus for depositing an optical coatmg on a medium, the optical coat g having a predetermined thickness.
  • the apparatus includes means for producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy and means for dividing the electromagnetic energy into a reference portion and a second portion.
  • the apparatus also includes means for receiving the reference portion of the electromagnetic energy and for ⁇ producing a reference signal m response thereto, means for transmitting tne secon ⁇ portion of the electromagnetic energy to the optical coating, means for receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg
  • the apparatus includes means for receivm ⁇ the reference s_gna ⁇ and a first ga control signal and for producing a first output ⁇ signal m response thereto, means for receiving the received energy signal and a second gam control signal and for producing a second output signal m response thereto and means for receiving the reference and detector signals and for producing a comparison signal representing their difference.
  • the apparatus also includes means for receiving the comparison signal and for producing the first and second gam control signals and a transmission monitoring signal in response thereto, means for driving the comparison signal toward zero by controlling the first and second gam control signals, and means
  • FIG. 1 is a block diagram of the preferred embodiment of the invention.
  • the apparatus 10 monitors the optical thickness of an optical coatmg being applied to a part 12 under test.
  • the apparatus 10 includes a laser 14, an acousto-optic modulator 16, reference photodetector 18, and an optical receiver 2C.
  • Tne apparatus 10 also includes a gam-programmable nulling preamplifier 22, a phase-sensitive detector 24, and a computer 26.
  • a tunable single-frequency laser such as a model 6328P external-cavity tunable ⁇ iode laser produced by New Focus
  • the laser 14 can be any source of electromagnetic energy having spectral brightness substantially greater thar a thermal source of electromagnetic ener ⁇ , (for example, an amplified spontaneous emission source, such as a model IQ-2300 from EXFO, Inc., with suitable filtering).
  • the acousto-optic modulator 16 (for example, an IntraActicn model FCM401E5CA fiber-pigtailed acousto-optic modulator) modulates the beam of light 30 in accordance with a signal that it receives at a signal port 32.
  • the acousto-optic modulator 16 produces a modulated beam of light 34 which is transmitted to an optical power divider 35.
  • the modulated beam of light 34 might also be obtained directly from the laser 14 by modulating the laser' s ⁇ rive current to vary the laser' s output power or by some other form of direct modulation.
  • the modulator 16 could be electro-optic, mechanical, or liquid crystal.
  • the optical power divi ⁇ er 35 which can be an MP Opticon model WA15080102CP0FC fiber power splitter, then transmits a reference portion of the modulated beam of light 34 to the reference photodetector 18 over a conduit 37 and a second portion of the modulated beam of light 34 over a conduit 41 (sucn as an optical fiber) to an optical transmitter 36.
  • the optical transmitter 36 transmits a beam of light 38 (which can represent approximately 92 percent of the output power of the optical power divider 35) through the part 12, producing a beam of light 39.
  • the beam of light 39 is received by an optical receiver 40 which produces a received energy signal m response to the beam of light 39.
  • the received energy signal is transmitted to the optical rece. ⁇ er 20 over a _me 42.
  • Tne optical receiver 20 is preferably a photodiode ana the line 42 is preferably an optical conduit sucn as an optical fioer.
  • the reference photodetector 18 and the optical receiver 20 produce reference and detector signals respectively m resoonse to the reference portion of the modulated beam of light 34 and m response to the received energy signal produced by the optical receiver 40.
  • the optical transmitter 36 can reflect the beam of light 38 from the part 12, producing the beam of light 39.
  • the gain-programmable nulling preamplifier 22 receives the reference and detector signals respectively produced by the reference photodetector 18 and the optical receiver 20 over the respective lines 44 and 46.
  • a first oscillator 47 produces an AM modulating signal.
  • the AM modulating signal is transmitted over the line 48 to the signal port 32 on the acousto-optic modulator 16, where it impresses AM modulation on beam 34.
  • the AM modulating signal is also transmitted over the line 50 to the phase-sensitive detector 24, which may be a Stanford Research Systems model 510 lock-m amplifier.
  • the gam-programmable nulling preamplifier 22 also produces a response signal which is transmitted to the phase-sensitive detector 24 over the line 52.
  • the phase-sensitive detector 24 In response to the AM modulating signal and the response signal, the phase-sensitive detector 24 produces a comparison signal which is transmitted to the computer 26 over the line 54. Deposition status information and process design information is transmitted to the computer 26, as will be described supsequently .
  • the computer 26 processes the output signal and produces a gam control signal m response.
  • the gam control signal is transmitted to the gam-programmable nulling preamplifier 22 over the line 56.
  • the computer 26 also processes the output signal, the deposition status information and tne process design information, and produces both a transmission monitor si ⁇ nai and a process control signal m response.
  • a second oscillator 57 also produces an FM modulating signal which is sent to the tunable smgle-fre ⁇ ⁇ ency laser 14 over the line 58 to control the frequency of the light produced by the tunable single-frequency laser 14.
  • FIG. 2 is a block diagram of a portion of the preferred embodiment of the invention, showing further details of the gam-programmable nulling preamplifier 22.
  • the ⁇ am-programmable nulling preamplifier 22 includes signal and reference preamplifiers 80 and 82, respectively, a ⁇ _ffere ⁇ t ⁇ al amplifier 84, ana a computer terface 86.
  • Tne signai and reference preamplifiers 80 and 82 are low noise gam-programmable transimpedance preamplifiers.
  • the transimpedance gam of the preamplifier 80 can be adjusted accord with the adjustment of a resistance within a range of 0 to 10.24 Gohms, and the ga of the preamplifier 82 can be adjusted in accord with the adjustment of a resistance within a range of 0 to 40.96 Mohms.
  • Figure 2 also shows the reference photodetector 18, the optical receiver 20, the phase-sensitive detector 24 ana the computer 26.
  • the reference photodetector 18 receives part of the modulated beam of light 34 from the conduit 37, and, on the conduit 42, optical receiver 20 receives the signal produced by the optical receiver 40 m response to the beam of light 39.
  • the reference photodetector 18 transmits the signal it produces over line 44 to the reference preamplifier 82.
  • the optical receiver 20 transmits the signal it produces to the signal preamplifier 80 over the line 46.
  • the gam- programmable nulling preamplifier 22 and the differential amplifier 84 collectively constitute a comparator.
  • the differential amplifier 84 receives an output signal produced by the siqnal preamplifier 80 on its positive terminal and receives an output signal produced by the reference preamplifier 82 on its negative terminal.
  • the differential amplifier 84 produces an output signal, which is transmitted to the phase-sensitive detector 24 over the line 52.
  • the signal produced by the phase-sensitive ⁇ etector 2 ⁇ is transmitted to the computer 26 over the line 54, and the ga control signal produced by the computer 26 is transmitted to the computer interface 86 over the line 56.
  • the computer interface 86 produces first and second gam control signals which are transmitted to the preamplifiers 80 and 82 over the respective lines 90 and 92.
  • the tunable single-frequency laser 14 preferably operates m the wavelength range of 1525 to 1630 nanometers and the FM dither signal on the line 58 causes the wavelength of the light produced by the tunable single-frequency laser 14 to vary approximately 0.05 nm peak-to-peak, although the amount of dither depends upon the part 12 or other operational aspects of the apparatus 10.
  • the specific wavelength of the light pro ⁇ uced by the tunable single-frequency laser 14 is chosen according to the design of the coatmg being deposited on the part 12.
  • Conventional photodiode bias circuits 94 and 96 are used with the reference photodetector 18 and the optical receiver 20, respectively.
  • the optical frequency modulation reduces errors induced by parasitic optical interference within the measurement system.
  • the apparent magnitude of the signal that is transmitted through (or reflected from) a layer will vary slightly with the frequency of the light source because of parasitic interference within the apparatus and the device being tested. If this effect is not compensated, slow drifts m the frequency of the light source will produce apparent drifts m the value of the observed transmission monitor signal.
  • the variation of apparent transmission with the instantaneous frequency of the light source is a periodic, or multiperiodic, function.
  • the characteristic period of the function is related to the geometric characteristics of the various sources of interference, especially the part under test.
  • the optimum value of the FM-modulation amplitude is the one that causes the frequency excursion of the light source to just cover one period of this parasitic interferometer transmission function.
  • the practical performance of the coatmg monitor system can be significantly enhanced by optimizing the FM- modulation amplitude to correspond to one period of the dominant periodic function. This adjustment can be accomplished either with a manual optimization process, or automatically with a low- frequency servo loop that quantifies the short-term noise of the measured signal (which results substantially from the interaction of light source frequency with the parasitic interferometer behavior), and adjusts the FM-modulation extent to minimize that noise.

Abstract

A high precision laser monitor (10) for manufacture of optical coatings and method for its use. A laser (14) produces a beam of light which is divided into a reference portion and a second portion. First and second gain-programmable preamplifiers (22) receive the reference signal and the detector signal, respectively, and produce respective first and second output signals. A comparator compares the first and second output signals and produce a comparison signal. The computer (26) operates to drive the comparison signal toward zero by controlling first and second gain-amplifiers control signals and produces a transmission monitoring signal by analyzing the changes in the first and second gain control signals needed to drive the comparison signal toward zero. The computer (26) uses the transmission monitoring signal to control the coating depositor to achieve the desired thickness for each layer of the coating.

Description

Description
L:GU PRECISION ASER ONITOR FOR MANUFACTURE OF OPTICAL COATINGS
AND METHOD FOR ITS USE
Technical Field
Tre present invention relates to a method and apparatus for monitoring a manufacturing process, and more particularly, no a method ana apparatus for high precision monitoring of the manufacture of optical coatings.
Bac-cαround of the Invention
Thin film coatings deposited on glass or similar substrates are useα extensively in optical technology to tailor the behavior of optical components. Such coatings are used to increase or decrease reflectivity, to modify the polarization of light, or to filter or otherwise control the wavelength spectrum cf light Demg processed.
Coatings of dielectric materials (as opposed to metallic coatings) offer the highest and most flexible performance to the optical components manufacturer. These coatings frequently consist of multiple discrete layers of differing materials, wmcn are applied successively to the substrate. Such coatings are generally manufactured by vacuum deposition or sputtering of material onto the substrate. In order to achieve the desired component performance, the optical thickness of the individual layers of the coatings must be precisely controlled.
It has long been understood that the optical thickness of coating layers can be monitored during deposition by observing tne optical transmission through the part, and, hence, the coating being manufactured. U.S. Patent 4,024,291 describes an embodiment of this concept which is specifically for the control of vapor deposition. U.S. Patent 4,336,119 essentially describes the application of the same measurement approach to coating deposition by reactive sputtering.
In all of the known optical monitoring approaches, light is emitted from a light source, transmitteα through or reflected from the part being monitored, and subsequently measured at an optical detector. Most optical coatings are designed as a stack of layers alternating between high and low index-of-refraction materials. These layers are usually 1/4 wavelength m optical thickness, or some multiple of 1/4 wavelength, although it is not necessary that they be. As the optical thickness of a layer that is currently being depositeα approaches 1/4 wavelengtn, the transmission (or reflectance) through the part reaches a turning point, either a maximum or a minimum. Depositior should be stopped accurately at the turning point, ana a new layer or another material begun.
Transmission through a partially complete coating may become quite small. For a narrow bandpass filter, transmission may be less than 1 percent, with accurate location of the turning points still required. The science of the optical monitoring approach requires a controlled wavelength spectrum of the light being measured, since the interaction between the coating's optical thickness and the light depends on wavelength. The allowable width of the optical spectrum used for the measurement depends strongly on the type and specification of the component being manufactured.
In the past, this function has been obtained by use of an incandescent light source, coupled with an optical monocnromator that filters out a portion of the broad spectrum emitted by the incandescent lamp. As the monochromator transmission passband is made narrower, the total optical power from the light source that is transmitted through the system is reduced proportionately. This effect, along with low transmission of a partial coating and the need for very high precision measurements, places important practical limits on the minimum bandwidth that can be obtained from a given monitoring system. As the received optical power is reduced, measurement noise and error increase, reducing the useful precision of the monitor. Manufacture of some components requires a combination of very narrow bandwidth and very high precision that cannot oe easily obtained from "white light" optical monitors because of these effects .
It is possible to overcome the limitation on monitor bandwidth by using a laser light source m place of tne filtered incandescent source. This disclosure describes the implementation of a complete, high precision coating monitor system using a laser light source to obtain very narrow measurement bandwidth. The inventive dual-beam laser spectrometer apparatus described m detail subsequently provides variable nulling which can reduce the adverse effects of cnanges m the intensity of the laser light source and allows high-precision measurements of optical transmission through thin film coatings by means of the use of a precision tracking gain-programmable amplifier. Also, the appropriate use of optical frequency modulation reduces errors induced by parasitic optical interference within the measurement system.
Summary of the Invention
According to one aspect, the invention is an apparatus for monitoring the optical thickness of a layer of an optical coating that is being deposited on a medium. The apparatus includes a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy, and a divider to divide the electromagnetic energy into a reference portion and a second portion. The apparatus also includes a reference detector to receive the reference portion of the electromagnetic energy ana to produce a reference signal response thereto, a transmitter to transmit the second portion of the electromagnetic energy to the optical coating, a receiver to receive the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating and to produce a received energy signal m response thereto, and a detector to process the received energy signal and to produce a detector signal response thereto.
The apparatus further includes a first gam-programmable preamplifier to receive the reference signal and a first ga control signal and produce a first output signal m response tnereto, a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal in response thereto, and a comparator to receive the first and second output signals and produce a comparison signal representing their difference. In addition, the apparatus includes an electronic circuit to receive the comparison signal and to produce the first and second gam control signals and a transmission monitoring signal m response thereto, the electronic circuit operating to drive the comparison signal toward zero by controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes in the first and second gam control signals needed to drive the comparison signal toward zero.
According to another aspect, the invention is an apparatus for controlling the deposited optical thicknesses of layers of an optical coating that is being deposited on a medium to a predetermined thickness in accordance with a predetermined coating design. The apparatus includes a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a tnermal source of electromagnetic energy, a αivider to αivide the electromagnetic energy into a reference portic and a second portion, and a reference detector to receive tre reference portion of the electromagnetic energy and to produce a reference signal m response thereto.
The apparatus also includes a transmitter to transmit the second portion of the electromagnetic energy to the optical coating, a receiver to receive the second portion of the electromagnetic energy after the second portior of the electromagnetic energy has been affected by the optical coating and to produce a received energy signal in response thereto, and a detector to process the received energy signal and to produce a detector signal m response thereto. The apparatus further includes a first gam-programmable preamplifier to receive the reference signal and a first gam control signal and produce a first output signal in response thereto, a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal m response thereto, and a comparator to receive the first and second output signals ana produce a comparison signal representing their difference.
In addition, the apparatus includes an electronic circuit, coating deposition control circuit, and an optical coating depositor. The electronic circuit receives the comparison signal and produces the first and second gam control signals and a transmission monitoring signal m response thereto. The electronic circuit operates to drive the comparison signal toward zero by controlling the first and second ga control signals, and generates the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero. The coating deposition control circuit receives the transmission monitoring signal, a deposition status information signal, and process design information and produces a process control signal m response thereto. The optical coating depositor αeposits the optical coating on the medium, the optical coating depositor being responsive to the process control signal, and produces the deposition status information signal, whereby the first and second ga control signals are indicative of the thickness of the optical coating on the medium, so that the optical coating depositor terminates deposition of the optical coating when the optical coating has the predetermined thickness.
According to a further aspect, the invention is a metnod for analyzing the thickness of an optical coating on a meαmm . The method includes the steps of a > producing electromagnetic energy having a spectral brightness substantially greatei than a thermal source of electromagnetic energy and b) dividing the electromagnetic energy into a reference portion and a second portion. The method also includes c) receiving the reference portion of the electromagnetic energy and producing a reference signal in response thereto, d) transmitting the second portion of the electromagnetic energy to the optical coating, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating and producing a received energy signal m response thereto, and f) processing the received energy signal and producing a detector signal m response thereto .
The method further includes the steps of g) receiving the reference signal and a first gam control signal and producing a first output signal m response thereto, h) receiving the received energy signal and a second gam control signal and producing a second output signal in response thereto, and I) receiving the reference and detector signals and producing a comparison signal representing their difference. The method still further includes the steps of j ) receiving the comparison signal and producing the first and second gam control signals ana a transmission monitoring signal response thereto, K) anvmg the comparison signal toward zero by controlling the first and second gam control signals, and 1) generating the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero. According to a still further aspect, the invention is a method for depositing an optical coating on a medium, the optical coating having a predetermined thickness. The method includes the steps of a) providing a source of electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy and b) dividing the electromagnetic energy into a reference portion and a second portion. The method also includes the steps of c) receiving the reference portion of the electromagnetic energy and producing a reference signal m response thereto, d) transmitting the electromagnetic energy to the optical coating, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating, and f) processing the electromagnetic energy received by the receiver and producing a detector signal m response thereto.
The method further includes the steps of g) receiving the reference and detector signals and a gam control signal, and producing a comparison signal in response to the reference and detector signals, h) transmitting the frequency control signal to the source of the electromagnetic energy to determine the instantaneous substantially single frequency of electromagnetic energy produced by the source of the electromagnetic energy, I) transmitting the amplitude modulation signal to the source of the modulation to determine the instantaneous modulation of tne electromagnetic energy produced by the source of the electromagnetic energy, the amplitude modulation signal also oemg transmitted to the amplifier, ; transmitting the comparison signal to the amplifier, and k) receiving tne gam control signal and producing the comparison signal response thereto, whereby the source produces the electromagnetic energy at the predetermined frequency, and the gam control signal is transmitted, the ga control signal being indicative of the tnickness of the optical coating on the medium.
The method also includes the step of 1) depositing the octxcal coat g on the medium, the deposition being responsive to the gam control signal, whereby the first and second gam control signals are indicative of the thickness of the optical coating on the medium, so that the deposition of the optical coating terminates when the optical coatmg has the predetermined thickness.
According to yet another aspect, the invention is a method for controlling the deposited optical thickness of each layer of an optical coatmg that is being deposited on a medium to a predetermined thickness m accordance with a predetermined coatmg design. The method includes the steps of a) producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy, and b) dividing the electromagnetic energy into a reference portion and a second portion. The method also includes the steps of c) receiving the reference portion of the electromagnetic energy and producing a reference signal m response thereto, d) transmitting the second portion of the electromagnetic energy to the optical coatmg layer, e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating layer and producing a received energy signal m response thereto, and f) processing the received energy signal ana producing a detector signal m response thereto.
The method further includes the steps of g receι/±ng tne reference signal and a first gam control
Figure imgf000011_0001
and producing a first output signal m response thereto, hi receiving the detector signal and a second gam control signal and producing a second output signal response thereto. The method also includes the steps of I) receiving the first ana second output signals and producing a comparison signal representing their d_fference, j ) receiving the comparison signal a^d producing the first and second ga control signals and a transmission moritormα signal in response thereto, the electronic circuit operating to drive the comparison signal towarα zero oy controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes the first and second gam control signals needed to drive the comparison signal toward zero, and k) receiving the transmission monitoring signal, a deposition status information signal, and process design information and producing a process control signal m response thereto.
The method further includes the step of 1) depositing the optical coatmg layer on the medium response to the process control signal, and producing the deposition status information signal, whereby the first and second ga control signals are indicative of the thickness of the optical coatmg layer on the medium, so that deposition of the optical coatmg layer ceases when the optical coatmg layer has the predetermined thickness. According to a still further aspect, the invention is an apparatus for depositing an optical coatmg on a medium, the optical coat g having a predetermined thickness. The apparatus includes means for producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy and means for dividing the electromagnetic energy into a reference portion and a second portion. The apparatus also includes means for receiving the reference portion of the electromagnetic energy and for ^ producing a reference signal m response thereto, means for transmitting tne seconα portion of the electromagnetic energy to the optical coating, means for receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg
1" and for producing a receiveα energy signal m response thereto, means for processing the received energy signal and for producing a detector signal m response thereto. In addition, the apparatus includes means for receivmα the reference s_gnaι and a first ga control signal and for producing a first output ι signal m response thereto, means for receiving the received energy signal and a second gam control signal and for producing a second output signal m response thereto and means for receiving the reference and detector signals and for producing a comparison signal representing their difference.
20 The apparatus also includes means for receiving the comparison signal and for producing the first and second gam control signals and a transmission monitoring signal in response thereto, means for driving the comparison signal toward zero by controlling the first and second gam control signals, and means
25 for generating the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero.
Detailed Description of the Preferred Embodiment of the 30 Invention
Figure 1 is a block diagram of the preferred embodiment of the invention. The apparatus 10 monitors the optical thickness of an optical coatmg being applied to a part 12 under test. The apparatus 10 includes a laser 14, an acousto-optic modulator 16, reference photodetector 18, and an optical receiver 2C. Tne apparatus 10 also includes a gam-programmable nulling preamplifier 22, a phase-sensitive detector 24, and a computer 26. The laser 14, which is preferably a tunable single-frequency laser (such as a model 6328P external-cavity tunable αiode laser produced by New Focus), transmits a beam of light 30 to the acousto-optic modulator 16. The laser 14 can be any source of electromagnetic energy having spectral brightness substantially greater thar a thermal source of electromagnetic enerσ , (for example, an amplified spontaneous emission source, such as a model IQ-2300 from EXFO, Inc., with suitable filtering). The acousto-optic modulator 16 (for example, an IntraActicn model FCM401E5CA fiber-pigtailed acousto-optic modulator) modulates the beam of light 30 in accordance with a signal that it receives at a signal port 32. The acousto-optic modulator 16 produces a modulated beam of light 34 which is transmitted to an optical power divider 35. Tnose skilled m the arts would know that the modulated beam of light 34 might also be obtained directly from the laser 14 by modulating the laser' s αrive current to vary the laser' s output power or by some other form of direct modulation. In principle, those skilled in the arts would know that the modulator 16 could be electro-optic, mechanical, or liquid crystal. The optical power diviαer 35, which can be an MP Opticon model WA15080102CP0FC fiber power splitter, then transmits a reference portion of the modulated beam of light 34 to the reference photodetector 18 over a conduit 37 and a second portion of the modulated beam of light 34 over a conduit 41 (sucn as an optical fiber) to an optical transmitter 36. The optical transmitter 36 transmits a beam of light 38 (which can represent approximately 92 percent of the output power of the optical power divider 35) through the part 12, producing a beam of light 39. The beam of light 39 is received by an optical receiver 40 which produces a received energy signal m response to the beam of light 39. The received energy signal is transmitted to the optical rece.^er 20 over a _me 42. Tne optical receiver 20 is preferably a photodiode ana the line 42 is preferably an optical conduit sucn as an optical fioer. The reference photodetector 18 and the optical receiver 20 produce reference and detector signals respectively m resoonse to the reference portion of the modulated beam of light 34 and m response to the received energy signal produced by the optical receiver 40. Optionally, the optical transmitter 36 can reflect the beam of light 38 from the part 12, producing the beam of light 39. The beam of light 39 can then be received b^ tr= optical receiver 40 to produce the detector signal.
The gain-programmable nulling preamplifier 22 receives the reference and detector signals respectively produced by the reference photodetector 18 and the optical receiver 20 over the respective lines 44 and 46. A first oscillator 47 produces an AM modulating signal. The AM modulating signal is transmitted over the line 48 to the signal port 32 on the acousto-optic modulator 16, where it impresses AM modulation on beam 34. The AM modulating signal is also transmitted over the line 50 to the phase-sensitive detector 24, which may be a Stanford Research Systems model 510 lock-m amplifier. The gam-programmable nulling preamplifier 22 also produces a response signal which is transmitted to the phase-sensitive detector 24 over the line 52. In response to the AM modulating signal and the response signal, the phase-sensitive detector 24 produces a comparison signal which is transmitted to the computer 26 over the line 54. Deposition status information and process design information is transmitted to the computer 26, as will be described supsequently . The computer 26 processes the output signal and produces a gam control signal m response. The gam control signal is transmitted to the gam-programmable nulling preamplifier 22 over the line 56. The computer 26 also processes the output signal, the deposition status information and tne process design information, and produces both a transmission monitor siσnai and a process control signal m response.
A second oscillator 57 also produces an FM modulating signal which is sent to the tunable smgle-freα^ency laser 14 over the line 58 to control the frequency of the light produced by the tunable single-frequency laser 14.
Figure 2 is a block diagram of a portion of the preferred embodiment of the invention, showing further details of the gam-programmable nulling preamplifier 22. The σam-programmable nulling preamplifier 22 includes signal and reference preamplifiers 80 and 82, respectively, a α_ffere^tιal amplifier 84, ana a computer terface 86. Tne signai and reference preamplifiers 80 and 82 are low noise gam-programmable transimpedance preamplifiers. The transimpedance gam of the preamplifier 80 can be adjusted accord with the adjustment of a resistance within a range of 0 to 10.24 Gohms, and the ga of the preamplifier 82 can be adjusted in accord with the adjustment of a resistance within a range of 0 to 40.96 Mohms. Figure 2 also shows the reference photodetector 18, the optical receiver 20, the phase-sensitive detector 24 ana the computer 26. The reference photodetector 18 receives part of the modulated beam of light 34 from the conduit 37, and, on the conduit 42, optical receiver 20 receives the signal produced by the optical receiver 40 m response to the beam of light 39. The reference photodetector 18 transmits the signal it produces over line 44 to the reference preamplifier 82. The optical receiver 20 transmits the signal it produces to the signal preamplifier 80 over the line 46. In the present embodiment, the gam- programmable nulling preamplifier 22 and the differential amplifier 84 collectively constitute a comparator. The differential amplifier 84 receives an output signal produced by the siqnal preamplifier 80 on its positive terminal and receives an output signal produced by the reference preamplifier 82 on its negative terminal. The differential amplifier 84 produces an output signal, which is transmitted to the phase-sensitive detector 24 over the line 52.
The signal produced by the phase-sensitive αetector 2^ is transmitted to the computer 26 over the line 54, and the ga control signal produced by the computer 26 is transmitted to the computer interface 86 over the line 56. In response, the computer interface 86 produces first and second gam control signals which are transmitted to the preamplifiers 80 and 82 over the respective lines 90 and 92.
The tunable single-frequency laser 14 preferably operates m the wavelength range of 1525 to 1630 nanometers and the FM dither signal on the line 58 causes the wavelength of the light produced by the tunable single-frequency laser 14 to vary approximately 0.05 nm peak-to-peak, although the amount of dither depends upon the part 12 or other operational aspects of the apparatus 10. The specific wavelength of the light proαuced by the tunable single-frequency laser 14 is chosen according to the design of the coatmg being deposited on the part 12. Conventional photodiode bias circuits 94 and 96 are used with the reference photodetector 18 and the optical receiver 20, respectively. The dual-beam laser spectrometer apparatus described, and its method of operation, both provide variable nulling which can reduce the adverse effects of changes in the intensity of tne tunable single-frequency laser 14 and allows high-precision measurements of optical transmission through thin film coatings by means of the use of a precision tracking gam-programmaole amplifier .
Also, the optical frequency modulation reduces errors induced by parasitic optical interference within the measurement system. The apparent magnitude of the signal that is transmitted through (or reflected from) a layer will vary slightly with the frequency of the light source because of parasitic interference within the apparatus and the device being tested. If this effect is not compensated, slow drifts m the frequency of the light source will produce apparent drifts m the value of the observed transmission monitor signal. The variation of apparent transmission with the instantaneous frequency of the light source is a periodic, or multiperiodic, function. The characteristic period of the function is related to the geometric characteristics of the various sources of interference, especially the part under test.
The optimum value of the FM-modulation amplitude is the one that causes the frequency excursion of the light source to just cover one period of this parasitic interferometer transmission function. The practical performance of the coatmg monitor system can be significantly enhanced by optimizing the FM- modulation amplitude to correspond to one period of the dominant periodic function. This adjustment can be accomplished either with a manual optimization process, or automatically with a low- frequency servo loop that quantifies the short-term noise of the measured signal (which results substantially from the interaction of light source frequency with the parasitic interferometer behavior), and adjusts the FM-modulation extent to minimize that noise.
While the foregoing is a detailed description of the preferred embodiment of the invention, there are many alternative embodiments of the invention that would occur to those skilled in the art and which are within the scope of the present invention. Accordingly, the present invention is to be determined by the following claims.

Claims

1. An apparatus for monitoring the optical thickness of a .aver of an optical coatmg that is being deposited on a meαium, comprising: a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energ ; a divider to divide the electromagnetic energy into a reference portion and a second portion; a reference detector to receive the reference portion of tre electromagnetic energy and to produce a reference signal m response thereto; a transmitter to transmit the second portion of the electromagnetic energy to the optical coating; a receiver to receive the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coat g and to produce a received energy signal m response thereto; a detector to process the received energy signal and to produce a detector signal in response thereto; a first gam-programmable preamplifier to receive the reference signal and a first gam control signal and produce a first output signal m response thereto; a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal in response thereto; a comparator to receive the first and second output signals and produce a comparison signal representing their difference; and an electronic circuit to receive the comparison signal and to produce the first and second ga control signals and a transmission monitoring signal m response thereto, the electronic circuit operating to drive the comparison signal toward zero by controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero.
2. The apparatus of claim 1, wherein the source of electromagnetic energy is a tunable laser producing an instantaneous substantially single frequency of electromagnetic energy .
3. The apparatus of claim 2, wherein the frequency of the electromagnetic energy produceα by the source of electromagnetic energy is FM-modulated by an externally supplied modulation signal .
4. The apparatus of claim 1, wherein the source of electromagnetic energy is a laser producing a substantially fixed frequency of electromagnetic energy.
5. The apparatus of claim 4, wherein the frequency of the electromagnetic energy of the source of electromagnetic energy is FM-modulated by an externally supplied modulation signal.
6. The apparatus of claim 1, wherein the source of electromagnetic energy is a laser producing a plurality of frequencies that fall within a restricted bandwidth.
7. The apparatus of claim 1, wherein the source of electromagnetic energy is an amplified spontaneous emission light source combined with a bandwidth-limiting optical filter.
8. The apparatus of claim 1, wherein the source of electromagnetic energy includes an acousto-optic modulator.
9. The apparatus of claim 1, wherein the source of 5 electromagnetic energy is directly modulated.
10. The apparatus of claim 1, wherein the receiver receives the electromagnetic energy transmitted through the optical coatmg.
0 11. The apparatus of claim 1, wherein the receiver receives the electromagnetic energy reflected from the optical coatmg.
12. The apparatus of claim 1, wherein the comparator includes a phase-sensitive detector that has a predetermined phase 5 relationship to the modulation of the electromagnetic energy produced by the source of electromagnetic energy.
13. The apparatus of claim 1, wherein the first and second gain-programmable preamplifiers are respectively first and C second digital gam-programmable preamplifiers.
14. The apparatus of claim 13, wherein the gams of the first and second digital gain-programmable preamplifiers are programmable with very high precision thereby allowing a high 5 precision determination of coatmg optical thickness.
15. The apparatus of claim 1, wherein the electronic circuit is a programmed computer.
0 16. The apparatus of claim 15, wherein the frequency of the source of electromagnetic energy is modulated and the magnitude of the modulation is slowly varied to minimize measurement errors, and the modulation of the frequency of the electromagnetic energy produced by the source of electromagnetic energy is increased or decreased m order to minimize the short term variation the measured transmission value.
1". Tne apparatus of claim 16, wherein the modulation of the frequency of the source of electromagnetic energy is automatically increased or decreased by a servo loop, which slowly varies the magnitude of the modulation over a small range, measures the effect of the variation on the short-term measurement noise observed on the ga control signal, and changes the modulation of the frequency of the electromagnetic energy produced by the source of electromagnetic energy so as to produce reduced short-term measurement noise.
18. An apparatus for controlling the deposited optical thicknesses of layers of an optical coatmg that is being deposited on a medium to a predetermined thickness m accordance with a predetermined coat g design, the apparatus comprising: a source of electromagnetic energy to produce electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy; a divider to divide the electromagnetic energy into a reference portion and a second portion; a reference detector to receive the reference portion of the electromagnetic energy and to produce a reference signal in response thereto; a transmitter to transmit the second portion of the electromagnetic energy to the optical coating; a receiver to receive the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg and to produce a received energy signal m response thereto; a detector to process the received energy signal and to produce a detector signal m response thereto; a first gam-programmable preamplifier to receive the reference signal and a first gam control signal and produce a first output signal m response thereto; a second gam-programmable preamplifier to receive the detector signal and a second gam control signal and produce a second output signal response thereto; a comparator to receive the first and second output signals and produce a comparison signal representing their difference; an electronic circuit to receive the comparison signal and to produce the first and second gam control signals and a transmission monitoring signal m response thereto, the electronic circuit operating to drive the comparison signal toward zero b^ controlling the first and second gam control signals, and generating the transmission monitoring signal by analyzing the changes m the first and second gam control signals needed to drive the comparison signal toward zero; a coatmg deposition control circuit adapted to receive the transmission monitoring signal, a deposition status information signal, and process design information and produce a process control signal m response thereto; ana an optical coatmg depositor for depositing the optical coatmg on the medium, the optical coatmg depositor being responsive to the process control signal, and producing the deposition status information signal, whereby the first and second gam control signals are indicative of the thickness of the optical coatmg on the medium, so that the optical coatmg depositor terminates deposition of the optical coat g when the optical coatmg has the predetermined thickness.
19. The apparatus of claim 18, wherein the apparatus determines the thickness of the optical coat g while the optical coatmg depositor is depositing the optical coatmg upon the medium.
20. Tne apparatus of claim 18, wherein the source of electromagnetic energy is a tunable laser producing an instantaneous substantially single frequency of electromagnetic energy .
21. The apparatus of claim 20, wherein the frequency of the electromagnetic energy produced by the source of electromagnetic energy is FM-modulated by an externally supplied modulation signal .
22. The apparatus of claim 18, wherein the source of electromagnetic energy is a laser producing a substantially fixed frequency of electromagnetic energy.
23. The apparatus of claim 22, wherein the frequency of the electromagnetic energy produced by the source of electromagnetic energy is FM-modulated by an externally supplieα modulation signal .
24. The apparatus of claim 18, wherein the source of electromagnetic energy is a laser producing a plurality of frequencies that fall within a restricted bandwidth.
25. The apparatus of claim 18, wherein the source of electromagnetic energy is an amplified spontaneous emission light source combined with a bandwidth-limiting optical filter.
26. The apparatus of claim 18, wherein the source of electromagnetic energy includes an acousto-optic modulator.
21 . The apparatus of claim 18, wherein the source of electromagnetic energy includes means for αirectly modulating the source of electromagnetic energy.
5 28. The apparatus of claim 18, wherein the receiver receives the electromagnetic energy transmitted through the optical medium.
29. The apparatus of claim 18, wherein the receiver receives 10 the electromagnetic energy reflected from the optical medium.
30. The apparatus of claim 18, wherein the comparator includes a phase-sensitive detector that has a predetermined phase relationsmp to the modulation of the electromagnetic energy ι5 produced by the source of electromagnetic energy.
31. The apparatus of claim 18, wherein the first and second gam-programmable preamplifiers are respectively first and second digital gam-programmable preamplifiers.
20
32. The apparatus of claim 31, wherein the gams of the first and second digital gam-programmable preamplifiers are programmable with very high precision thereby allowing a high precision determination of coating optical thickness.
25
33. The apparatus of claim 18, wherein the electronic circuit is a programmed computer.
34. The apparatus of claim 33, wherein the frequency of the 30 source of electromagnetic energy is modulated and the magnitude of the modulation is slowly varied to minimize measurement errors, and the modulation of the frequency of the electromagnetic energy produced by the source of electromagnetic energy is increased or decreased order to minimize the snort term variation m the measured transmission value.
35. The apparatus of claim 34, wherein the modulation of tne frequency of the source of electromagnetic energy is automatically increased or decreased by a servo loop, which slowly varies the magnitude of the modulation over a small range, measures the effect of the variation on the short-term measurement noise observed on the gam control signal, and changes the modulation of the frequency of the electromagnetic energy produced by the source of electromagnetic energy so as to produce reduced short-term measurement noise.
36. A method for analyzing the tnickness of an optical coatmg on a medium, comprising the steps of: a) producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy; b) dividing the electromagnetic energy into a reference portion and a second portion; c) receiving the reference portion of the electromagnetic energy and producing a reference signal m response thereto; d) transmitting the second portion of the electromagnetic energy to the optical coatmg; e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg and producing a received energy signal in response thereto; f) processing the received energy signal and producing a detector signal m response thereto; g) receiving the reference signal and a first ga control signal and producing a first output signal in response thereto; h) receiving the received energy signal and a second gain control signal and producing a second output signal m response thereto; i) receiving the reference and detector signals and producing a comparison signal representing their difference; j ) receiving the comparison signal and producing the first and second gam control signals and a transmission monitoring signal response thereto; k; driving the comparison signal toward zero by controlling the first and second gam control signals; and 1) generating the transmission monitoring signal by analyzing the changes m the first and second ga control signals needed to drive the comparison signal toward zero.
37. Tne method of claim 36, further including the step of: m) determining the thickness of the optical coating while the optical coat g is being deposited upon the medium.
38. The method of claim 36, wherein step a) includes producing an instantaneous substantially single frequency of electromagnetic energy.
39. The method of claim 36, wherein step a) includes modulating the frequency of the electromagnetic energy.
40. A method for depositing an optical coatmg on a medium, the optical coat g having a predetermined thickness, comprising the steps of: a) providing a source of electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy; b) dividing the electromagnetic energy into a reference portion and a second portion; c) receiving the reference portion of the electromagnetic energy and producing a reference signal response thereto; d) transmitting the electromagnetic energy to the optical coating; e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coating; f) processing the electromagnetic energy received by the receiver and producing a detector signal response thereto; g) receiving the reference and detector signals and a gam control signal, and producing a comparison signal m response to the reference and detector signals; h) transmitting the frequency control signal to the source of the electromagnetic energy to determine the instantaneous substantially single frequency of electromagnetic energy produced by the source of the electromagnetic energy; l) transmitting the amplitude modulation signal to the source of the modulation to determine the instantaneous modulation of the electromagnetic energy produced by the source of the electromagnetic energy, the amplitude modulation signal also being transmitted to the amplifier; j) transmitting the comparison signal to the amplifier; and k) receiving the gam control signal and producing the comparison signal m response thereto, whereby the source produces the electromagnetic energy at the predetermined frequency, and the ga control signal is transmitted, the gam control signal being indicative of the thickness of the optical coatmg on the medium; and
1) depositing the optical coatmg on the medium, the deposition being responsive to the gam control signal, whereby the first and second gam control signals are indicative of the thickness of the optical coat g on the medium, so that the deposition of the optical coatmg terminates when the optical coatmg has the predetermined thickness.
41. The method of claim 40, further including the step of: m) determining the thickness of the optical coating while the optical coatmg is being deposited upon the medium.
42. The method of claim 40, wherein the source of the instantaneous substantially single frequency of electromagnetic energy is a tunable laser.
43. The method of claim 40, wherein the frequency control signal controls the source by modulating the frequency of the electromagnetic energy.
44. A method for controlling the deposited optical thickness of each layer of an optical coatmg that is being deposited on a medium to a predetermined thickness in accordance with a predetermined coating design, the method comprising the steps of: a) producing electromagnetic energy having a spectral brightness substantially greater than a thermal source of electromagnetic energy; b) dividing the electromagnetic energy into a reference portion and a second portion; c) receiving the reference portion of the electromagnetic energy and producing a reference signal in response thereto; d) transmitting the second portion of the electromagnetic energy to the optical coating layer; e) receiving the second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg layer and producing a received energy signal in response thereto; f) processing the received energy signal and producing a detector signal in response thereto; g) receiving the reference signal and a first gam control signal and producing a first output signal response thereto; h) receiving the detector signal and a second gam control signal and producing a second output signal m response thereto; l) receiving the first and second output signals and producing a comparison signal representing their difference; j) receiving the comparison signal and producing the first and second gam control signals and a transmission monitoring signal m response thereto, the electronic circuit operating to drive the comparison signal toward zero by controlling the first and second ga control signals, and generating the transmission monitoring signal by analyzing the changes m tne first and second ga control siσnals needed to drive the comparison signal toward zero; k) receiving the transmission monitoring signal, a deposition status information signal, and process design information and producing a process control signal in response thereto; and
1) depositing the optical coatmg layer on the medium m response to the process control signal, and producing the deposition status information signal, whereby the first and second gam control signals are indicative of the thickness of the optical coat g layer on tne medium, so that deposition of the optical coatmg layer ceases when the optical coating layer has the predetermined thickness.
45. The method of claim 44, further including the step of: m) determining the thickness of the optical coat g while the optical coatmg is being deposited upon the medium.
46. The method of claim 44, wherein the electromagnetic energy is produced by a tunable laser.
47. The method of claim 44, wherein the frequency of the electromagnetic energy is modulated.
48. An apparatus for controlling a depositor that deposits an optical coating on a medium, the optical coatmg having a predetermined thickness, comprising: means for producing electromagnetic energy having a spectral bπgntness substantially greater than a thermal source of electromagnetic energy; means for dividing the electromagnetic energy into a reference portion and a second portion; means for receiving the reference portion of the electromagnetic energy and for producing a reference signal m response thereto; means for transmitting the seconα portion of the electromagnetic energy to the optical coating; means for receiving tne second portion of the electromagnetic energy after the second portion of the electromagnetic energy has been affected by the optical coatmg and for producing a received energy signal in response thereto; means for processing the received energy signal and for producing a detector signal in response thereto; means for receiving the reference signal and a first gam control signal and for producing a first output signal m response thereto; means for receiving the received energy signal and a second gam control signal and for producing a second output signal m response thereto; means for receiving the reference and detector signals and for producing a comparison signal representing their difference; means for receiving the comparison signal and for producing the first and second gam control signals and a transmission monitoring signal response thereto; means for driving the comparison signal toward zero by controlling the first and second ga control signals; and means for generating the transmission monitoring signal by analyzing the changes m tne first and second gam control signals needed to drive the comparison signal toward zero; and means for receiving the transmission monitoring signal, a deposition status information signal, and process design information and for producing a process control signal in response thereto, whereby the first and second gam control signals are indicative of the thickness of the optical coatmg on the medium, so that the optical coating depositor terminates deposition of the optical coating when the optical coating has the predetermined thickness.
49. The apparatus of claim 44, wherein the means for producing electromagnetic energy is a tunable laser.
PCT/US2001/001155 2000-04-28 2001-01-11 High precision laser monitor for manufacture of optical coatings and method for its use WO2001084074A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001232792A AU2001232792A1 (en) 2000-04-28 2001-01-11 High precision laser monitor for manufacture of optical coatings and method for its use

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US56083200A 2000-04-28 2000-04-28
US09/560,832 2000-04-28

Publications (1)

Publication Number Publication Date
WO2001084074A1 true WO2001084074A1 (en) 2001-11-08

Family

ID=24239548

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US2001/001155 WO2001084074A1 (en) 2000-04-28 2001-01-11 High precision laser monitor for manufacture of optical coatings and method for its use

Country Status (2)

Country Link
AU (1) AU2001232792A1 (en)
WO (1) WO2001084074A1 (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2388848A (en) * 2002-05-24 2003-11-26 Alps Electric Co Ltd Control of optical thin film deposition
US8315834B2 (en) 2003-12-17 2012-11-20 Siemens Energy, Inc. System and method for measuring coating thickness
CN112254658A (en) * 2020-09-28 2021-01-22 广东工业大学 Method for tracing magnitude of film thickness

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3802774A (en) * 1971-05-17 1974-04-09 Siemens Ag Method and apparatus for determining the thickness or width of work pieces
US4184767A (en) * 1975-07-21 1980-01-22 The United States Of America As Represented By The Secretary Of The Navy Frequency agile optical radar
US5299045A (en) * 1991-01-12 1994-03-29 Canon Kabushiki Kaisha Light detecting apparatus having a diffraction grating
US5394238A (en) * 1992-11-09 1995-02-28 Honeywell Inc. Look-ahead windshear detector by filtered Rayleigh and/or aerosol scattered light
US5859531A (en) * 1996-07-25 1999-01-12 Maurice; Lisa Brackenbury Displacement apparatus using a magnetic optic sensor and position dependent magnetic field

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3802774A (en) * 1971-05-17 1974-04-09 Siemens Ag Method and apparatus for determining the thickness or width of work pieces
US4184767A (en) * 1975-07-21 1980-01-22 The United States Of America As Represented By The Secretary Of The Navy Frequency agile optical radar
US5299045A (en) * 1991-01-12 1994-03-29 Canon Kabushiki Kaisha Light detecting apparatus having a diffraction grating
US5394238A (en) * 1992-11-09 1995-02-28 Honeywell Inc. Look-ahead windshear detector by filtered Rayleigh and/or aerosol scattered light
US5859531A (en) * 1996-07-25 1999-01-12 Maurice; Lisa Brackenbury Displacement apparatus using a magnetic optic sensor and position dependent magnetic field

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2388848A (en) * 2002-05-24 2003-11-26 Alps Electric Co Ltd Control of optical thin film deposition
GB2388848B (en) * 2002-05-24 2005-09-28 Alps Electric Co Ltd Apparatus and method for depositing optical thin film
US8315834B2 (en) 2003-12-17 2012-11-20 Siemens Energy, Inc. System and method for measuring coating thickness
CN112254658A (en) * 2020-09-28 2021-01-22 广东工业大学 Method for tracing magnitude of film thickness

Also Published As

Publication number Publication date
AU2001232792A1 (en) 2001-11-12

Similar Documents

Publication Publication Date Title
US8543010B2 (en) Bias control in an optical modulator and transmitter
US20050078317A1 (en) Synchronizing the filter wavelength of an optical filter with the wavelength of a swept local oscillator signal
US6587214B1 (en) Optical power and wavelength monitor
US4533247A (en) Optical transmission system
US6646753B2 (en) In-situ thickness and refractive index monitoring and control system for thin film deposition
US20060120733A1 (en) System and method for optical heterodyne detection of an optical signal including optical pre-selection that is adjusted to accurately track a local oscillator signal
US6094446A (en) Wavelength stabilizing apparatus of laser light source
JP2002504755A (en) Method and apparatus for monitoring and controlling laser emission wavelength
US20030206336A1 (en) Variable wavelength light source apparatus and optical amplifier using same
US20080002746A1 (en) Optical transmitters
JPH1068894A (en) Etalon device
US6798499B2 (en) Method of forming optical thin films on substrate at high accuracy and apparatus therefor
US7062166B2 (en) First and second derivative processing of wavelength multiplexed optical signals
US20020191279A1 (en) Automatic gain control and dynamic equalization of erbium doped optical amplifiers in wavelength multiplexing networks
US6992774B2 (en) Wavelength determining apparatus and method
US5956356A (en) Monitoring wavelength of laser devices
WO2001084074A1 (en) High precision laser monitor for manufacture of optical coatings and method for its use
US20030042407A1 (en) Variable optical attenuator using wavelength locked loop tuning
Tammela et al. Time-resolved frequency chirp measurement using a silicon-wafer etalon
US20090141748A1 (en) Tunable laser light source and controlling method of the same
US6646790B2 (en) Optical amplifier gain control monitoring
US6295151B1 (en) Optical transmission and receiving equipment
CN116676582A (en) Direct light control system and method of coating equipment
CA2351998A1 (en) Wavelength monitor apparatus and wavelength stabilizing light source
US7016382B2 (en) Method and apparatus for stabilizing laser wavelength

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): AE AG AL AM AT AU AZ BA BB BG BR BY BZ CA CH CN CR CU CZ DE DK DM DZ EE ES FI GB GD GE GH GM HR HU ID IL IN IS JP KE KG KP KR KZ LC LK LR LS LT LU LV MA MD MG MK MN MW MX MZ NO NZ PL PT RO RU SD SE SG SI SK SL TJ TM TR TT TZ UA UG US UZ VN YU ZA ZW

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): GH GM KE LS MW MZ SD SL SZ TZ UG ZW AM AZ BY KG KZ MD RU TJ TM AT BE CH CY DE DK ES FI FR GB GR IE IT LU MC NL PT SE TR BF BJ CF CG CI CM GA GN GW ML MR NE SN TD TG

121 Ep: the epo has been informed by wipo that ep was designated in this application
DFPE Request for preliminary examination filed prior to expiration of 19th month from priority date (pct application filed before 20040101)
REG Reference to national code

Ref country code: DE

Ref legal event code: 8642

122 Ep: pct application non-entry in european phase
NENP Non-entry into the national phase

Ref country code: JP