DE19741122A1 - Interferometric near-field device, e.g. near-field microscope for measuring and structuring material - Google Patents

Abstract

The device includes a fiber amplifier (F) connected to a fiber laser (C) at one end and tapered to a fine point (K) at the other end. The point lies opposite the material (I) being measured, and lies at a distance determined by the diameter (L) of the point. The material to be measured is an integral component of the device.

Description

The invention relates to an arrangement according to the preamble of Claim 1.

So that optical arrangements that at a radiation exit opening exploit existing near field, e.g. B. near-field optical microscope Being able to measure or structure samples, the distance to the sample must be checked when scanning.

It is known to use optical near-field arrangements for measuring or for writing / reading. Such an arrangement, which uses the intensity effects in a fiber laser to determine the distance, is described by D. Betzig and others in Appl. Phys. Lett. 63 (26), 1993, page 3550 | 1 | described. The use of a near field arrangement for storing or reading out data is described by E. Betzig inter alia in Appl. Phys. Lett. 61 (2), 1992, page 142. In addition, 32 patents are listed with the corresponding patent numbers (US Patent Database), which reflect the device and application aspect:
5214282: Method and apparatus for processing a minute portion of a specimen
5539197: Scanning near-field optical microscope having a medium denser than air in the gap between the probe chip and the sample being measured
5333495: Method and apparatus for processing a minute portion of a specimen
5479024: Method and apparatus for performing near-field optical microscopy
5581083: Method for fabricating a sensor on a probe tip used for atomic force microscopy and the like
5517280: Photolithography system
5294790: Probe unit for near-field optical scanning microscope
5473157: Variable temperature near-field optical microscope
5449901: Fine surface observing apparatus
5354985: Near field scanning optical and force microscope including cantilever and optical waveguide
5018865: Photon scanning tunneling microscopy
4829177: Point projection photoelectron microscope with hollow needIe
5583286: Integrated sensor for scanning probe microscope
5581193: Multiple source and detection frequencies in detecting threshold phenomena associated with and / or atomic or molecular spectra
5559328: Small cavity analytical instruments
5548113: Co-axial detection and illumination with shear force dithering in a near-field scanning optical microscope
5546223: Method for external excitation of subwavelength light sources that is integrated into feedback methodologies
5538898: Method suitable for identifying a code sequence of a biomolecule
5504366: System for analyzing surfaces of samples
5471064: Precision machining method, precision machining apparatus and data storage, apparatus using the same
5469734: Scanning apparatus linearization and calibration system
5461600: Hig-density optical data storage unit and method for writing and reading information
5397896: Multiple source and detection frequencies in detecting threshold phenomena associated with and / or atomic or molecular spectra
5382789: Near field scanning optical microscope
5304795: High resolution observation apparatus with photon scanning microscope
5288999: Manufacturing method including near-field optical microscopic examination of a semiconductor wafer
5288998: Manufacturing method including photoresist processing using a near-field optical probe
5288997: Manufacturing method, including near-field optical microscopic examination of a magnetic bit pattern
5288996: Near-field optical microscopic examination of genetic material
5281814: System for imaging and detecting threshold phenomena associated with and / or atomic or molecular spectra of a substance by reflection of an AC electrical signal
5199090: Flying magnetooptical read / write head employing an optical integrated circuit waveguide
5105305: Near-field scanning optical microscope using a fluorescent probe.

Interferometric methods for determining the distance between Radiation outlet and sample are from S. Pilevar u. a. in Ultramicroscopy 61, 1995, page 223 | 2 | or by U. Schwarz u. a. in opt. Comm. 134, 1997, page 301 | 3 | described.

The known methods for determining the distance are based on a direct interaction between fiber tip and sample. Checking the distance Fiber tip testing can be done in different ways.

The control mechanism most commonly used is based on the measurement of the damping of an oscillating tip by shear forces (E. Betzig et al. In Appl. Phys. Lett. 60, 1992, page 2484). This attenuation is measured optically in that a laser beam is directed laterally onto the tip and the modulation of the laser intensity caused by the movement of the tip is measured. There are a variety of variations on this process. The following serious disadvantages cannot be eliminated. The movement of the tip must be tracked at a very short distance from the sample (<100 nm); sample roughness means that the laser intensity varies without there being any damping of the sample movement. The laser intensity irradiated from the side is approx. 10 ⁵ times more intense than the light that passes through the fiber tip and, if the sample is examined spectroscopically, can lead to falsification of the measurement result via scattered radiation. In addition, the control system must be set up each time the tip is replaced.

Another distance control method is based on the use of Tunnel microscopy (U. Dürig et al. In J. Appl. Phys. 59, 1986, page 3318). For The method is very suitable for homogeneous samples. The main disadvantage is that the samples must be conductive. Other methods are based on the Use of atomic force microscopes (AFM) described by N.F. van Hulst u. a. in scan sample microsc. 36, 1992, page 36. An interesting one Control mechanism for the tip-sample distance was developed by K. Karrai et al. a. in Appl. Phys. Lett. 66, 1995, page 1842, the Fiber tip attached to an arm of a quartz tuning fork. By Interaction of the tip with the sample results in a damping of the Measuring system, which is expressed in a change in the resonant circuit quality. The high preparation effort has a disadvantageous effect. The high circle quality causes the scanning (scanning) speed to be slow. Furthermore was tries to dampen the impedance change of a shear force Exploiting piezo vibrators for distance control (J. W. Hsu et al. In Rev. Sci. Instr. 66, 1995, page 3177). Typical control frequencies (scan rates) of the the methods mentioned are less than or equal to 10 kHz.

With the methods mentioned, a safety margin must be observed cannot be guaranteed between the radiation exit opening and the sample. The Betzig | 1 | suggests using a fiber laser in which one of the Fiber ends is extended to a tip and at which the sample as End reflector used for the laser (3-mirror laser), allowed Changes in reflection on the sample surface in changes in intensity  to implement. The distance control takes place according to the tried and tested Standard methods.

The in | 2 | described method is limited to the metrological aspect of the signal processing, whereby that of the sample in the radiation entrance opening reflects back light with a reference beam is compared (Mach-Zehnder interferometer). The in | 3 | described The arrangement is the same as that originally used in | 1 | described measuring method, with Except for the lasers used.

The change in laser intensity, which is caused by the change in reflection is caused on the sample surface, due to the change in the quality of the coupled to the laser Resonators. 3-mirror arrangements, of which in | 1 | and | 3 | is reported are already from Eichler u. a. in journal f. Applied Physics 28 (3), 1969, page 125 | 4 | and Herziger u. a. in Phys. Lett 24A (12) 1967, page 684 been.

The in | 1 | - | 3 | described methods are based on the separation of the Methods for recording the measurement signal in sample spectroscopy and the regulation for compliance with the sample-fiber tip distance in the event that the sample should be measured in a spatially resolved manner. The former is purely visual, the latter takes advantage of the techniques described above, which are essential is mechanical in nature. Furthermore, they are in | 1 | described procedures and the corresponding follow-up procedures in the pulse mode of Solid-state lasers occurring relaxation oscillation in the Limited scanning speed. The main disadvantage of all of the above Methods that are specific to the method used in sample spectroscopy low light intensities have to do with the measuring principle justified, which is based solely on the detection of changes in intensity, so that the signal-to-noise ratio cannot meet the requirements. The technological effort, the z. B. in | 2 | is driven to this  Remedying the disadvantage is disproportionate to the achieved Results.

The object of the invention is an arrangement for Material characterization as well as for reading or writing in To create information in the nanometer range that the distance control Unified sample and the working process on the sample which is compact, is easy to use and inexpensive.

This object is achieved by an arrangement with the features of the claim 1 solved.

Diode-pumped fiber lasers in combination with fiber amplifiers offer one promising starting point for the realization more compact and easy-to-use interferometric near-field arrangements based on the Principle of a laser interferometer based. Near field arrangements of this type benefit from the development of fiber lasers, whose emission spectrum of From ultraviolet to infrared, and from the perfection of technology the manufacture of fiber tips. In addition to the emission wavelength of The radiation form - constant light, pulse - can also be selected with fiber laser.

The interferometric near-field arrangement mentioned advantageously combines Way a fiber laser with an undamped measuring resonator.

In an advantageous embodiment, the measuring resonator contains a radiation-amplifying medium, which is also designed as a fiber. The Fiber lasers and the fiber amplifier are rarer with the same ions Earth doped in a known manner. The level of the doping concentration is decisive for the laser or amplifier effect, can in known to be chosen differently. The fiber that as Amplifier is provided (fiber amplifier), is unilaterally known Methods with a tip. The diameter of the  Radiation exit opening of the tip can be in a known manner Requirements are adjusted. The material to be examined (sample) as third mirror, completes the laser interferometer in a known manner, compare | 4 |. A major advantage of the interferometric mentioned Near field arrangement is that the measurement signal is that of a change in distance assigned laser frequency change is used. The calibration of the Frequency change with a known change in distance allows the Absolute specification of a sample height profile. The output power of the laser is, as with the described arrangements | 1 |, | 3 |, as a further measured variable available as they are used in local refractive, reflective or absorptive Changes on the sample also varied. The superiority of mentioned arrangement with respect to the signal-to-noise ratio is to to use a comparison, the transition from AM broadcast reception to FM reception analog.

Settling processes (relaxation oscillations) play in the above Arrangement does not matter because the laser works in a stationary state. The Radiation losses at the tapered fiber end are in the Arrangement advantageously through the reinforcement in the fiber amplifier compensated. Another advantage of the arrangement for spectroscopic applications exist in the few nanometers amount tunable wavelength of the laser.

According to the invention, height differences ΔL _v on a sample surface in the range 10 ^-2 nm <ΔL _v <100 nm can be detected with the arrangement mentioned. The particular advantage of the interferometric near-field arrangement described here lies in the detection of small changes in refractive index or degree of absorption, which takes place via intensity feedback. With a spatial resolution of <100 nm, differences in reflectance <10 ^-6 , refractive index differences Δn of Δn <10 ^-5 and absorption degree variations of approx. 10 ^-3 can be detected.

According to the invention, it should be provided that the laser length is at least λ / 2 can be moved.

For the data storage area of application, the described near field arrangement can be used to process data - very high packing density (writing, reading). Target values are e.g. B. storage densities greater than 200 bit / µm ² at a write speed of about 10 Mbit / s or a read speed of 1 to 10 Mbit / s.

The invention is in the field of laser measurement technology, in particular in Areas in which the spatial resolution is in the nanometer range. The technical problem with which the invention is concerned is a To create an arrangement that allows the distance over a contactless Keep sample constant in the nanometer range and local changes absorptive, refractive and reflexive nature, which a spatial extension in the Possess nanometer range. According to the invention time-resolved measurements possible. The arrangement is said to be compact, be user-friendly and insensitive to environmental influences. The Control speed (scan rate) should be greater than in known ones Methods and an increase in sensitivity when detecting reflective, Enable refractive and absorptive changes (sample spectroscopy).

For this purpose, the solution according to the invention provides that a diode pumped Fiber laser and a fiber amplifier pumped with the same diode in one 3 mirror arrangement are summarized. The fiber laser is as 2-mo denlaser trained as he z. B. by I. M. Jauncay u. a. Electr. Lett. 24 (1), 1988, page 24 | 5 | is described. The fiber amplifier is reflection-free spliced the fiber laser. The third mirror is realized through the rehearsal, which is thereby an integral part and object of investigation at the same time. The The reaction of the sample to the laser properties can be formalized coupled resonators | 4 | describe.  

The fiber laser and fiber amplifier use to generate radiation or amplification one doped standard monomode fiber. The fiber core is a transversely monomodal optical fiber that is cladding is surrounded.

The diode laser radiation, the beam parameter product of which the Laser fiber should be adjusted, is used to radiation-enhancing material that is in both the core of the laser as well in which the amplifier fiber is embedded, to pump. Optical pumping takes place according to the tandem principle, d. H. the in radiation-amplifying material of the laser does not absorb part of the Pump radiation is used to pump the radiation-amplifying material in the Amplifier used. The fiber that forms the fiber amplifier is one-sided tapered according to known methods and has a light exit opening the tip, whose diameter according to the manufacturing technology in 100 nm range (or below). The coupled resonator (external Resonator) is by a mirror, the laser resonator and external Have resonator in common (center mirror), and that of the top opposite sample formed. The operating principle of the invention Arrangement can be described as follows. The determination of the height profile when scanning the sample, laser interferometry is used. It is for that necessary, the intensity emitted by the laser to a frequency analysis undergo. According to the invention in the near field arrangement Exploitation of a laser frequency used when changing the length of the external resonator occurs. The difference frequency of two laser modes, from which one should be unaffected is used as a measurement signal. The different lengths of the external resonator caused by the Height profile of the sample are determined (nm range) correspond directly to the measurable difference frequencies. The aforementioned rule principle for determination the height profile of the sample takes advantage of a resonant frequency of the laser (mode) is shifted when a resonance frequency (mode) of the external resonator is sufficiently close. Since the  Length variation of the external resonator according to the height variation on the sample in the nm range is always close to one Laser resonance worked. In the case of the coincidence of those to be influenced Fashion with a mode of the external resonator, one measures the undisturbed Difference frequency of the two laser modes. A small change in length of the external resonator causes this difference frequency to change. This is the basis for measuring the height profile in units of Difference frequency. A defined change in length of the external Resonators allows calibration so that an absolute specification of the Differences in height is possible.

According to the invention, the second independent measured variable is next to the Difference frequency change the laser intensity change available that it allows locally reflective, refractive and absorptive change on (in) the Sample.

In the following, the explanations for individual figures Formation of the interferometric near-field arrangement according to the invention be clearly illustrated.

It shows schematically in detail:

FIG. 1 is an interferometric near field according to the invention;

FIG. 2 shows an alternative diagram of the interferometric near-field arrangement according to the invention shown in FIG. 1;

3 shows a section through a standard single-mode fiber ;

FIG. 4a shows a possible technical embodiment;

Figure 4b is a possible technical specifications for applications in which not only high spatial resolution (nm range) additionally high time resolution (femtoseconds) are required.

FIG. 5 shows the result of a measurement of the pump radiation absorption as a function of fiber length, illustrated by the example of a Nd ³⁺ -doped single-mode silica fiber (doping amount ~1000 ppm Nd ₂ O _3);

Figure 6a shows the change in length of the Verstärkerresonators (height profile of the sample) corresponding change in the difference frequency of the two modes of the fiber laser (parameters:. L _v = 13 cm, m ₂ = 3 x 10 ^5; n _L = 2 × 10 ¹⁴ s ^-1, R ₂ = T ₂ = 0.5, R ₃ = 0.01);

Figure 6b shows the change in length of the Verstärkerresonators (height profile of the sample) corresponding change in the difference frequency of the two modes of the fiber laser (parameters:. L _v = 13 cm, m ₂ = 3 x 10 ^5; n _L = 2 × 10 ¹⁴ s ^-1 R ₂ = T ₂ = 0.5, R ₃ = 0.04);

Figure 7a shows the frequency for understanding the reaction of the Verstärkerresonators to the laser resonant frequency useful scheme for lasers and amplifiers in the event of coincidence position, a laser mode coincides with a mode of the Verstärkerresonators match.

FIG. 7b shows the resonance frequency scheme (like FIG. 7a) in the event of a change in the length of the amplifier resonator; FIG. 7a, which leads to a shift in the laser mode;

Fig. 8 shows the results of a measurement of the small signal gain, the example of Nd ³⁺ -doped single-mode silica fiber of FIG. 5;

Figure 9 shows the percent change of the laser power .DELTA.P / P and the percentage change of effective reflectivity |. .DELTA.R _eff | / R _eff as a function of change in the reflectivity of the mirror .DELTA.R ₃ M ₃ (sample).

The interferometric near-field arrangement shown in FIG. 1 essentially consists of two single-mode quartz glass fibers, one of which is designed as a fiber laser C, the other as a fiber amplifier F. The length of the fiber laser is in a known manner | 5 | chosen so that only two longitudinal modes for the selected excitation (pumping) range can oscillate. The fiber laser resonator is closed off by mirroring M ₁ and an intrafiber reflection grating M ₂ , the fiber amplifier resonator, ie the resonator coupled to the fiber laser by M ₂ , M ₃ . (An elegant, technologically complex implementation of a 2-mode laser is described by SV Chernikov inter alia in Opt. Lett. 18 (23), 1993, page 2023.)

FIG. 2 shows an equivalent scheme of the operation of the arrangement. The fibers are standard single-mode fibers. The section through such a fiber with the associated refractive index-n profile is shown schematically in FIG. 3. Characteristic fiber data for a laser wavelength ∼1 µm are: core diameter D ∼5 µm; Diameter (cladding) N ∼90 µm, thickness (coating) O ∼30 µm. The numerical aperture of such fibers is generally ∼0.2.

Rare earth metals come into question as the doping of the core D. The doping level can be adapted to the requirements of the arrangement to be implemented and can be implemented using known manufacturing methods. In addition to quartz glass, phosphate or fluoride glass can be used as the starting material for fiber production. The choice of the dopants determines the emission wavelength of the laser in a known manner. The doping of the fiber amplifier material must correspond to the fiber laser with respect to the dopant; the doping level can be freely selected according to the requirements. Standard semiconductor lasers A, which radiate in basic mode, are to be used as pump lasers. The pump radiation H serves to excite the radiation-amplifying material D. In order to achieve the required pump power density in the fiber core, it is advisable to use the well-known master laser power amplifier (MOPA) combination. The length of the fiber laser C is selected according to the invention so that emission takes place in two longitudinal modes and that a substantial part of the pump radiation (<80%) enters the fiber amplifier F and is converted there into excitation of the radiation-amplifying material. A prerequisite for the functionality of the arrangement is the known fact that the emission wavelength of the MOPA and the absorption band of the dopant have to match in the range from 1 to 2 nm. Fiber laser and fiber amplifiers (laser resonator C and external resonator G) are connected to each other in accordance with standard methods via a reflection-free splice E (reflectivity «10 ^-4 ). The fiber tip K with the light exit opening L is attached to one end of the amplifier fiber using standard methods. It faces Sample I. The distance LI, designated GF, is known to be comparable to the size of the light exit opening L. The radiation losses which are known to occur in fibers which are tapered to a point are taken into account by an attenuation P. The sample, which has a reflectivity, characterized by M ₃ , represents the final mirror of the external resonator. The laser radiation B generated in the fiber laser C and amplified in the fiber amplifier F serves for the contactless control of the distance GF from the sample (range 10 to 100 nm) and for measuring absorptive, refractive and reflective variations on the sample or for writing information into the sample.

A possible technical configuration is shown schematically in FIG. 4a. In the selected example, the fiber of predetermined length is wound around a cylinder Q made of dielectric material and fixed. By applying a voltage to such a cylinder provided with contacts according to known methods, the laser resonator length C can be changed by increasing the cylinder diameter. It is known that the laser power can be stabilized by means of electronic control on the piezo cylinder and the laser modes can be kept at the center of gravity of the amplifier profile. The measurement of the laser intensity can be done with an electronic detection system R _P consisting of receiver S _A , S _E ; Measuring device U, V, R _P : S _A , V; S _E , U take place. Changes in intensity that occur due to absorption in the sample can be detected with a radiation receiver S _A R _{P, A} : S _A , V. By means of a divider mirror T, the laser intensity P on the coupling-in side can be converted into a detection system R _{P, E} : S _E , U can be reflected. This detection system R _PE should consist of a receiver S _E for radiation intensity. Part of the electrical receiver signal should be used in absorption measurements to form the differential intensity DP = P _A - P _E and detected with an oscillograph, cf. G. Herziger et al. f. Appl. Physik 18, 2, 1964, page 67, the other part should be available for electronic frequency analysis U (spectrum analyzer). An advantageous feature of the invention is the tandem pump arrangement for lasers and amplifiers with a powerful diode laser (MOPA). If the material is to be influenced or measured with a high repetition frequency (<100 MHz) and / or extremely short pulses (femto- / pico-seconds intensity half-width), the technical configuration of the arrangement according to the invention shown in FIG. 4b should be selected. In this case, the radiation from a short-pulse laser with coupled transport fiber W is coupled into the amplifier fiber, according to known technology, via a directional coupler X, both realized according to known methods. The wavelength of the radiation from the short-pulse laser does not have to be in the spectral amplification range of the fiber amplifier, it should be adapted to the structuring or measurement process.

With the inventive combination of interferometric Near field arrangement and short pulse laser can be a defined distance from Material can be set at any point on the material surface, so that the area to be irradiated is always the same size and the Structuring or measurement of the material can be done with pulses, their temporal latitude and repetition frequency can be selected by the coupled one Short pulse lasers are determined.

In FIG. 5, a measurement is to pump radiation absorption as inter alia P. glass fiber and Integr Optics 16 (1), 1997, page 103 |. 6 | was described for an Nd ³⁺ -doped single-mode quartz glass fiber (doping level approx. 1000 ppm Nd ₂ O ₃ ). For a laser in which two longitudinal modes oscillate, a length of £ 10 cm should be provided; compare | 5 |. The fiber length required for two-mode operation is determined according to the in | 6 | described cutting process determined. According to FIG. 5, less than 10% of pump radiation is absorbed for the stated fiber length £ 10 cm. For a given fiber length, the percentage of pump radiation absorbed can be changed by varying the doping level according to known methods. Both the laser frequency and the laser intensity are advantageously available as measured variables in the near-field arrangement according to the invention. The measuring principle according to the invention of the laser frequency change, which is based on the frequency reaction of the amplifier resonator on the laser resonator, can be described as follows.

If no external resonator is coupled to the laser, the undisturbed natural frequencies of the laser are obtained

ν _L = m ₁ .c.π / 2π.nL _L (1)

m ₁ - integer, n - refractive index.

By coupling the external resonator, the natural laser frequency is shifted so that it applies to the disturbed natural frequency

4π (ν _L + Δν _L ) L _L / c + δ = 2 πm ₁ (2)

ie the total phase shift must again be an integer multiple of π. The phase shift δ at the center mirror is M ₂

the index L was omitted at R _2.3 . With ν = ν _L + ν _L and development of the trigonometric functions up to the second order follows:

The standardized representation of the frequency shift of the laser resonance frequency ν _{1, L} as a function of the change in amplifier resonator length is shown in FIG. 6a.

For a 13 cm long amplifier resonator, m ₂ = 2L _v / λ∼3 × 10 ⁵ (λ _las = 1.05 × 10 ^-4 cm). In this example, the frequency shift Δν _L is ∼ ± 6.5 × 10 ⁶ s ^-1 for a change in length of the amplifier resonator length of ± 76 nm.

An approximately linear frequency shift of the laser resonance frequency ν _{1, L} is obtained around the coincidence point = 0 when the amplifier resonator length changes. The maximum deviation from linearity is ∼17%.

In Fig. 6b the reflectance of the sample was increased to 4%. For a change in length of the amplifier resonator of ± 48 nm, a frequency shift of the laser mode of ± 1.7 × 10 ⁷ s ^{-1 is obtained} . The maximum deviation from linearity is <5%.

FIGS. 7a, b illustrate schematically how the measurement signal, the size of which is to be determined according to formula (4) or FIG. 6a, 6b, by difference frequency generation of the two laser modes is established. In Fig. 7a the frequencies of the two laser modes with ν _{1, L} ; ν _{2, L.} The difference frequency that can be measured with an electronic spectrum analyzer (R _p : U) is denoted by ν _{2, L} - ν _{1, L} = Δν _{Diff, L.} For the example shown, Δν _{Diff, L} = 2.1 × 10 ⁹ s ^-1 . The frequency bandwidth of the laser modes is designated δν _L. The index V on the relevant variables was used for the amplifier resonator. In the coincidence position, the laser frequency ν _{1, L} coincides with a mode frequency of the amplifier resonator, ν _{1, V} , ν _{1, L} - ν _{1, V} = 0. The condition is that the frequency of the second laser mode is as far from that of the next one Amplifier resonator mode is removed for their difference

Δν _{2, L-4, V} »δν _{L, V}

applies or numerically for the selected example: 3.2 × 10 ⁸ s ^-1 »2 × 10 ⁶ s ^-1 . Since ΔL _v = 0 in the coincidence position, it has the value 0 in FIGS. 6a, 6b . If sample I is moved past the tip, ΔL _v changes. The mode frequencies (ν _{1, V} to ν _{4, V} ) of the amplifier resonator thus change in accordance with equation (1), cf. FIG. 7b. (In Eq. (1) replace index L with V). According to Eq. (4) a shift in the laser frequency (ν _{1, L} ) of mode 1, L results. The spectral position ν _{2, L of} the unaffected mode 2, L does not change as long as the condition Δν _{2, L-4, V} »δν _{L, V is} fulfilled. Depending on the current length of the amplifier resonator L _v + ΔL _v , there is a current difference frequency Δν _{Diff, L} which differs from the undisturbed difference frequency Δν ^U _{Diff, L} (coincidence setting, = 0). The change in the differential frequency can be detected electronically (R _p : S _E , U). Using known electronic methods, the measurement signal can either be transformed into a low-frequency range or converted into a DC signal. The resolution limit of electronic spectral analyzers is approx. 100 s ^-1 , which corresponds to a height difference on the sample (change in length of the resonator) of ∼10 ^-2 nm. To measure such small height differences, a measuring time of 10 to 100 ms is required. The height profile of the sample is shown in the result of the measurement as a location-dependent difference frequency. If a constant distance between fiber tip and sample is required, the measured difference frequency can be used as a manipulated variable for a device for sample displacement (compensation of ΔL _v ). This enables a contactless scanning of sample I. Advantageously, according to the invention, the frequency change can be directly calibrated in units of length. To do this, it is only necessary to move the sample by means of a piezo actuator by a defined length ΔL _v out of the coincidence position = 0 and to measure the associated difference frequency. The scanning speed is given by the settling time of the laser, which can be derived from the frequency bandwidth of the modes. It is approximately 0.1 to 1 µs.

The condition Δν _{2, L-4, V} »δν _{L, V} can only be met, especially for the coupled resonator G, if this resonator is attenuated by introducing a radiation-amplifying material D. The frequency bandwidth is given by

δν _V = δν o | V / √G₀

(H. Boersch et al. Phys. Lett 4, 1963, page 191).

δν _V is the frequency bandwidth of the undamped resonator, δ o | V is the bandwidth without amplification, G ₀ is the amplification coefficient. . 8 with Fig G is ₀ cm for L _v = 13: G ₀ = 1.56 x 10 ^5. With a finesse F of the passive resonator F = 2 is δ o | V = Δν _{Diff, V} / 2 = 3.975 x 10 ^8/2 (. Cf. Fig. 7a), that is δν _V ~ 1 × 10 ⁶ s ^-1. The bandwidth of the laser is shown in | 5 | indicated with 1-2 × 10 ⁶ s ^-1 , so that the above condition for the selected example is very well met.

In addition to the change in differential frequency, the change in intensity of the laser radiation, which can be measured with the power meter R _{p, is} available as a second independent measured variable. The laser power can be calculated as follows, see AE Siegman, Lasers, Univ. Sci. Books, Mill Valey, Calif. Cape. 12, page 487.

To do this, you must know:
the losses (radiation loss in the fiber tip, fiber losses, the area of the core D, the saturation intensity of the fiber (material constant), the small signal amplification and the effective reflectivity of the coupled resonator.

F - area of the core, T ₂ - transmittance of the coupling-out mirror, I _s - saturation intensity, R ₁ - reflectance of the mirror M ₁ , R _eff - effective reflectance of the mirror combination M ₂ , M ₃ . The effective reflectivity R _eff is a function of the coordination of the two resonators to one another, cf. | 4 |; G ₀ - small signal gain factor.

In Fig. 8 the measured small-signal gain is InG ₀ / L _L = g ₀ shown for in Fig. 5 described example. The determination of G ₀ is in | 6 | spelled out. For the given example, F = 2.8 × 10 ^-7 cm ² , I _s = 4.5 × 10 ⁴ W / cm ² , G ₀ ∼7 × 10 ⁷ for L _L + L _V = 18 cm. After | 4 | the influence of the coupled resonator can be taken into account by an effective reflectivity R _eff

R _eff = (1- (1-R ₂ ) (1-R ₃ )) / (1 + R ₃ R ₂ -2√R₂R₃cosx) (6)

x = 4πν ^x L _v / m ₂ c.

FIG. 9 shows the percentage change in laser power ΔP / P and the percentage change in the effective reflectivity | ΔR _eff | / R _eff as a function of the change in reflectance ΔR _{3 of} the mirror M ₃ . In order to demonstrate a change in ΔR ₃ ∼10 ^-2 , a percentage change in power ΔP / P of ∼5% must be detectable, ie the power measurement with R _p : S _E , V may only have an error <1%. The absolute power for the parameters mentioned is approx. 1 mW. The spatially resolved (100 nm range) determinable with the measuring arrangement R _p : S _A , S _E , V can be changed according to | 6 | with the characteristic data of the arrangement according to the invention are ≧ 10 ^-3 .

The minimum reflection (ΔR) or refractive index changes (Δn), which can be detected in a spatially resolved manner with the arrangement according to the invention (measuring arrangement R _p : S _E , U), are in the range ΔR ≧ 10 ^-6 ; Δn ≧ 10 ^-5 .

It is conceivable that samples to be examined have areas of different reflectivities in addition to a height profile. Under certain circumstances, this can interfere with the recording of the height profile of the sample or the maintenance of a constant distance from the fiber tip if only the difference in frequency is measured. As the comparison of FIGS. 6a, 6b shows, the measurement of the differential frequency alone is not clear. A change in reflectance can simulate a change in height. An equilateral measurement of the laser power eliminates the ambiguity, since according to FIG. 9 a change in reflectance is directly associated with a change in laser power.

The problem of noise, i. H. the increased spontaneous emission not essential, since only in a very limited Frequency range (frequency bandwidth of the modes) is worked.

For structuring the material through radiation-induced It is imperative to keep a constant distance to changes Comply with the material surface, otherwise the irradiated areas have differently changed and / or different sizes. The Regulation at constant distance is done by setting constant Differential frequency and laser power.

The power of the pump light source A is increased by increasing the diode current with a clock frequency adapted to the test conditions known methods increased (dc operation with superimposed pulses), which is a Increasing the power of the fiber laser can result in the material a radiation-dependent change can be made. Such changes with an adjusted write speed (0 to 100 MHz) can be advantageously coupled with the Short pulse laser W can be executed.  

The described arrangement for measuring or generating Structures in the nm range can be seen as an implementation variant that the Possibilities for (laser) interferometric measurement of distances in Nanometer range or the determination and generation of reflective, refractive and absorptive changes in a sample with the highest Location and time resolution by means of coupled resonators, the one being Fiber laser, the other designed as a fiber amplifier with a tapered end is exploited.

Claims (26)

1. Arrangement for measuring and structuring material (I) with the highest spatial and time resolution in the nanometer range with laser radiation, called interferometric near-field arrangement, which is constructed from a fiber laser (C) and a fiber amplifier (F), using a constant light pump source (A) the radiation-amplifying material (D) of both the laser and the amplifier is excited and the reaction of the fiber amplifier on the fiber laser is exploited, characterized in that the fiber amplifier (F) is spliced onto the fiber laser (C) at one end (E) and at the other Is tapered to a fine tip (K) that the tip (K) faces the material (I) and is at a distance from it which is determined by the diameter (L) of the tip and that the material to be examined (I) is an integral part of the arrangement. 2. Arrangement for measuring and structuring material (I) according to claim 1, characterized in that the fiber laser (C) and the fiber amplifier (F) is designed as a 3-mirror (M ₁ , M ₂ , M ₃ ) tandem arrangement, the Mechanism of action is based on the interferometric coupling of two resonators. 3. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that in the fiber amplifier (F) the radiation of a short pulse laser (W) additionally is coupled. 4. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that  the material (I) as an integral part of the arrangement Frequency repercussions on the fiber laser (F), which acts as a measured variable is used. 5. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the material (I) as an integral part of the arrangement, a Intensity repercussions on the fiber laser (F), which acts as a measured variable is used. 6. Arrangement for measuring and structuring material (I) according to Claims 1, 3 and 4, characterized in that the measured variables belonging to the frequency and intensity feedback can be used interactively or individually. 7. Arrangement for measuring and structuring material (I) according to Claims 3 and 4, characterized in that the measurement signal associated with the frequency feedback turns out to be Difference frequency represents two laser modes. 8. Arrangement for measuring and structuring material (I) according to Claim 6 characterized in that the difference frequency change can be calibrated in units of length. 9. Arrangement for measuring and structuring material (I) according to Claims 3 and 4, characterized in that  the measurement signal associated with the intensity feedback appears as Represents change in intensity of the laser radiation. 10. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the material (I) can be moved past the fiber tip (K) (Scan). 11. Arrangement for measuring and structuring material (I) according to Claims 6 and 9, characterized in that a difference in frequency by scanning the material height profile is caused. 12. Arrangement for measuring and structuring material (I) according to Claims 6 and 9, characterized in that a difference frequency change by scanning a plane Material surface occurs that reflects locally differently. 13. Arrangement for measuring and structuring material (I) according to Claim 6 and the following claims 7 to 10 characterized in that a difference frequency change and a laser intensity change at Scan a non-flat, locally different reflecting Material surface occur. 14. Arrangement for measuring and structuring material (I) according to Claims 6, 7 and 11, characterized in that  in the case of a flat, locally different reflecting Material surface has a clear characterization of the surface Because of the simultaneous availability of two measured variables (Difference frequency and laser intensity change) can take place. 15. Arrangement for measuring and structuring material (I) according to Claim 6 characterized in that time-dependent refractive index changes in the material a change in the Measured variable, difference frequency, if previously Characterization according to height and reflectance at this point was made. 16. Arrangement for measuring and structuring material (I) according to Claim 7 characterized in that absorption differences when scanning a flat material surface a change in the measured variable results in laser intensity. 17. Arrangement for measuring and structuring material (I) according to Claims 7 and 14, characterized in that time-dependent absorption differences at previously characterized locations a time dependency of the measured variable requires laser intensity. 18. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the fiber laser (F) only emits in two longitudinal modes. 19. Arrangement for measuring and structuring material (I) according to Claim 1  characterized in that the coordination of the modes of the fiber laser (F) with respect to the center of the Amplifier curve is done by changing the fiber length. 20. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the radiation losses occurring at the fiber tip (K) through the Gain in the fiber amplifier (attenuation) can be compensated. 21. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the wavelength of the laser radiation by suitable choice of the dopants radiation-amplifying material (D) and the pump light source (A) can be made changeable. 22. Arrangement for measuring and structuring material (I) according to Claim 1 characterized in that the pump light source in addition to the dc excitation current with superimposed Current pulses are applied, which lead to an increase in Lead fiber laser power. 23. Arrangement for measuring and structuring material (I) according to Claims 1 and 21, characterized in that a radiation-dependent change in the material with constant Distance fiber tip (K) - material (I) can be made. 24. Arrangement for measuring and structuring material (I) according to Claim 1  characterized in that the radiation from a short pulse laser (W) is coupled into the amplifier fiber becomes. 25. Arrangement for measuring and structuring material (I) according to Claims 1 and 24, characterized in that the wavelength, pulse width and repetition frequency of the short pulse laser (W) problem-specific can be selected. 26. Arrangement for measuring and structuring material (I) according to Claims 1, 24 and 25, characterized in that highest spatial resolution in the range <100 nm through precise adjustment the distance fiber tip - material combined with the highest time resolution <10 fs by coupling short-pulse laser radiation into the amplifier fiber is achieved.