WO2001086332A1

WO2001086332A1 - Optical boxcar integrator/delay generator

Info

Publication number: WO2001086332A1
Application number: PCT/US2001/004751
Authority: WO
Inventors: Alan R. Sugg; Marshall J. Cohen
Original assignee: Sensors Unlimited Inc, A Division Of Finisar
Priority date: 2000-05-09
Filing date: 2001-02-14
Publication date: 2001-11-15
Also published as: AU2001259021A1

Abstract

An optical boxcar integrator/delay generator measures the pulse shape characteristics of very short optical pulses using an optical waveguide delay line array (10). A substrate (12) having a refractive index n1 is photolithographically etched to create B waveguide branches or cores (14a-14f) having an index of refraction n2 that is higher than n1. A cladding material having an index of refraction n3 covers the substrate and the B waveguide branches. Each of the B branches has a different length L and a common optical signal input point (18) and B different optical signal output points (22a-22f). The optical input signal (20) is typically in the 10 Gb/s range, or greater. The B optical output signals are optically coupled to individual photodiodes in an array which in turn are electrically connected to individual integrating amplifiers to produce multiple sample outputs that can be analyzed for pulse shape information.

Description

TITLE: OPTICAL BOXCAR INTEGRATOR/DELAY GENERATOR

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the priority of provisional US patent Application No. 60/203,530 filed May 9, 2000 by Alan R. Sugg and Marshall J. Cohen and entitled "Optical Boxcar Integrator / Delay Generator" and the entire contents thereof are hereby incorporated by reference in total into this application.

BACKGROUND OF THE INVENTION

1. Field ofthe Invention

This invention relates to an optical boxcar integrator / delay generator used to determine the shape of optical pulses having a high Gb/s rate.

2. Description of Related Art

It is important to quickly and accurately determine optical pulse dispersion in optical communications networks so that correction for aberrations can be made in real time before damage is done to the original data. In order to measure optical dispersion, it is necessary to determine the shape and time of an optical pulse. To do so requires measuring the intensity of the pulse at timed intervals spaced more narrowly than the width in time ofthe pulse. According to prior art techniques this was commonly accomplished with a boxcar integrator coupled with a time delay generator. The boxcar integrator integrates, i.e., averages, a signal over a narrow time interval period. The time delay generated determines the starting time ofthe integration. This technique requires a repeated signal. Multiple measurements are made with each measurement delayed in time from the previous measurement.

The time delays involved in optical pulses are often too short for the electronic technique described above. In a lOGb/s optical communications system, for example, the pulse widths are of the order of 50 ps wide. To adequately profile the pulse width would require a sampling precision of 10 ps or less. It is generally not possible to achieve satisfactory results with conventional electronic delay generators. Typical ofthe prior art are the following disclosures found in US and foreign patent documents.

Japanese Patent 52-76827 describes, for example, an information processing system in which light pulse intelligence is broken up into approximately equal parts which, in turn, are conveyed by a plurality of light fibers of different lengths. Similarly, US Patent 3,777,149 describes a signal detection/delay equalizer for use in a fiber optic communication system. The disclosure describes the take-off of different wave energy components at different angles from the transmission line.

US Patent 3,777,150 includes a similar teaching and is directly related to US Patent 3,777,149 described above. US Patent 3,988,614 describes an equalization scheme in which optical fiber signal dispersion is compensated for by dispersing the light signal into a spectrum, detecting the different portions ofthe spectrum, and producing signals in response thereto which are subjected to time delay compensation.

US Patent 3,838,278 describes a "Optical Switching Network Utilizing Organ Arrays of Optical Fibers." The use of organ arrays for the purpose of splitting up pulses is also discussed and described in other parts ofthe patent literature including: US Patents 3,925,727; 3,958,229 and 3,991,318.

US Patent 4,515,428 describes an "Integrated Optical Star Coupler" and includes a discussion of various prior art fabrication techniques therefor including photolithography. Japanese Patent 3-167,429 describes another integrated optical splitter of possible relevance. Likewise German Patent 28,42,824 includes a general discussion of an optical fiber pulse dispersion measurement device using an optical fiber delay structure.

Lastly, the following patents include disclosures of possibly relevant, but less important, structures and systems with regard to optical signal processing: US Patents 5,684,586; 5,982,530 and 6,122,419.

Insofar as understood, none ofthe foregoing prior art teaches or suggests a satisfactory structure for determining the shape and time of an optical pulse where the spacing between the pulses is relatively small compared to the width ofthe pulses themselves. SUMMARY OF THE INVENTION

Briefly described, the invention comprises an optical boxcar integrator/delay generator apparatus for measuring the pulse shape characteristics of very short optical pulses using an optical waveguide delay line array. A substrate having a refractive index nl is photolitho- graphically etched to create B waveguide branches or cores having an index of refraction n2 that is higher than nl . A cladding material having an index of refraction n3 covers the substrate and the B waveguide branches. Each of the B branches has a different length L and a common optical signal input point and B different optical signal output points. The optical input signal is typically in the 10 Gb/s range or above and is delayed by a different amount of time T from input to output given that the input signal travels over B different paths of different length. The B output signals are optically coupled to individual photodiodes in an array which in turn are electrically connected to individual integrating amplifiers to produce multiple sample outputs that can be analyzed for pulse shape information.

According to one embodiment, the substrate material is SiO₂ having an nl of approximately 1.5 n narrow channels of waveguide cores having a width of approximately 3-12 μm, depending upon the index of the waveguide material, are defined photolitho- graphically in silicon, having an index of refraction of 3.4, or SiN_x having an index of refraction of 1.9. The core ofthe waveguide is subsequently either passivated with SiO₂ or left exposed to air in order to define the other clad region n3. The result is a silicon or SiN_x channel having an index n2 of approximately 3.4 or 1.9 respectively.

Light entering the channels is confined to the microscopic waveguides B by the differences in the indices of refraction. The propagation speed ofthe pulse down a channel is the speed of light in vacuum divided by the index of refraction of the channel n2. The signals captured by the integrating photodiode receiver ray can be more accurately analyzed to determine the shape ofthe original optical pulse. The simultaneous measurement of several outputs is the equivalent of measuring a single output multiple times.

The invention may be more fully understood by reference to the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I a illustrates a top plan view ofthe waveguide delay line array according to the third embodiment ofthe invention.

FIG. lb is an edge view ofthe waveguide delay line array shown in FIG la above. FIG. 2 is a block diagram schematic of the waveguide delay array of FIGs. la and lb shown in the context of an optical boxcar integrator.

• FIG. 3 a illustrates the effect of variable time delay on a narrow pulse shape. FIG. 3b illustrates the effect of variable time delay on broadened pulse shapes. DETAILED DESCRIPTION OF THE INVENTION During the course of this description like elements will have the same numbers according to the different figures that illustrate the invention.

A critical aspect ofthe preferred embodiment ofthe invention is the implementation of time delays using an optical waveguide delay line array (OWDLA) 10 as illustrated in FIGs la and lb. Light 20 enters the OWDLA 10 at a common point 18 and is split approximately equally into B multiple branches 14a, 14b, 14c, 14d, 14e, 14f. The number of branches B is determined by the specific application. In the illustration of FIG la, six branches B are shown. Each ofthe

B branches 14a-14f has a different length L of LI through L6 respectively. As shown in FIG la the length LI of waveguide 14a is shorter than the length of L2 of waveguide 14b which in turn is shorter than the length of L3 waveguide 14c, etc. FIG lb is an edge view ofthe array illustrated in FIG 1 a. Light emerges as optical signals

22a-22f from each ofthe B waveguides 14a-14f after light 20 has entered the common point 18.

The B branches 14a-14f are implemented as microscopic optical waveguides. Each waveguide has an index of refraction n2 and is surrounded by materials with lower indices, i.e., nl and n3 as shown. The index of refraction n2 of each waveguide ofthe B branches 14a-14f are preferably identical, but could be made different if it was defined to dramatically change the delay characteristics. According to the implementation shown in FIGS la and lb, the substrate material 12 is SiO₂ in which the index of refraction nl is approximately 1.5. B narrow channels of waveguide cores 14a-14f, having a width of approximately 3-12 μm, depending upon the index of the waveguide materials, are defined photolithographically in silicon, which has an index of refraction nl of 3.4, or SiN_x which has an index of refraction nl of 1.9. The core ofthe waveguides 14a-14f is subsequently either passivated with SiO₂ or left exposed to air in order to define the other clad region 16 having index of refractions n3. The result is a silicon or SiN_x channel 14a-14f with an index n2 of approximately 3.4 or 1.9 respectively. Light 20 enters the channels through a common point 18 and is confined to the six microscopic waveguides 14a-14f by the differences in the indices of refraction nl and n3. Light exits each ofthe waveguides 14a-14f as delayed light output pulses 22a-22f respectively. The propagation ofthe speed of a light pulse down a channel 14a-14f is the speed of light in vacuum divided by the index of refraction ofthe channel n2. In the example given here, the propagation speed is:

3xl0¹⁰ cm/s ÷ 1.5 = 2xl0¹⁰ cm/s The time delay from one channel 14a-14f to the next is the difference in channel length L (L1-L6 shown in the Figures 1 a and lb). For an incremental time delay of 1 ps, the difference in channel length L1-L6 must be: ΔT = 10-^I s=(ΔL)*n2Λ ΔL /2xl0¹⁰ cm/s

ΔL =0.02 cm = 200μm This is a reasonable geometry for standard photolithographic techniques. Typical optical outputs are illustrated in Figures 3a and 3b, which includes both narrow and broadened shapes respectively. The schematic block diagram of FIG. 2 describes the integrated optical boxcar integrator/delay generator incorporating the OWDLA 10 illustrated in FIG 1. As shown FIG. 2, light 20 enters the OWDLA 10 at a common point 18. It is split equally into six different pulses that propagate down waveguides 14a-14f each having a different length LI through L6 respectively. While six different waveguides 14a-14f are illustrated it will be appreciated by those of ordinary skill in the art that more or less than six waveguides 14a- 14f may be required depending upon the application. The light pulses emerge as delay outputs 22a-22f respectively from the waveguides 14a-14f. Each output light pulse 22a-22f is delayed in time from the pulse from each adjacent waveguide 14a-14f. The emerging pulses of light 22a-22f are detected using a matching high-speed photodiode array 24a-24f. Each element ofthe receiver array 34 consists of a high-speed photodiode 24a-24f each coupled respectively to a high speed integrating amplifier 26a-26f. A satisfactory high speed integrating amplifier 26a-26f is available commercially from Applied Microcircuits Corporation as part number AMCC79025; also referred to as the S572025 - Quad Transimpedance Amplifier; the device of elements 24a-24f is fully manufacturable according to the teachings of this disclosure. The S7025 is a 2.5 Gbps transimpedance amplifier (TIA) designed for high speed fiber optic communications. The Current Mode Logic technology incorporated provides for superior drive performance required to operate at GHz data rates. The advanced low input noise construction allows input signals as low as 6 microamps to be reliably detected. Peak input currents as high as 500 microamps are easily amplified with low duty cycle distortion. The S7025 signal outputs are limited to 900 millivolts differential. This provides the widest possible dynamic output range without exceeding the input voltage of an attached SERDES. The S7025 is provided in a die package. The die availability provides for easy integration directly into a PIN diode array package. As special die design is available for multi-angle bonding for greater flexibility in tight packages.

Special features ofthe S7205 include: SiGe BICMOS Technology

10 Gbps Throughout at 2.5 Gbps per Channel Data Rates Loss of Signal Detect Single 3.3 Volt Supply Low Total Jitter Generation Voltage Limited Output Source Terminated Differential Output 1.98 mm x 3.33 mm Die Package Meets IEEE -802.3z Specifications.

Some ofthe applications ofthe S7025 include:

• Gigabit Ethernet Optical Links

• Fiber Channel Optical Links • Parallel Fiber Interconnects

• Board-to-Board

• Shelf-to-Shelf

• Chassis-to-Chassis The receiver array 34 comprising elements 24a-24f, 26a-26f is triggered by a pulse 32 generated by a clock and recovery circuit 28. Upon receipt ofthe trigger 32, the receiver array 34 integrates and stores the output ofthe OWDLA 10 over a short period of time, typically less than 10 ps. The multiple sample signals 30a-3 Of captured by the integrating receiver array 24a-24f, 26a-26f can be analyzed to determine the optical pulse shape ofthe incoming optical signal 20. Simultaneous measurement of the several outputs is equivalent to measuring a single output several times.

The present invention has several significant advantages over the prior art. First, the optical boxcar integrator/generator enables signal measurements at time intervals much shorter than is possible than conventional prior art techniques. Second, an important application ofthe foregoing would be as an optical dispersion monitor used in conjunction with an adaptive dispersion compensation module. Undesirable broadening ofthe optical pulse shape in a fiber optic system commonly results from dispersion in the index of refraction of the optical glass fibers. The present invention has the ability to quickly measure the pulse shape, establish a degree of dispersion and allow an appropriate compensation to be inserted. The faster the compensation can take place, the better the degree of error correction the system is capable of handling.

While the invention has been described with reference to the preferred embodiment thereof, it will be appreciated by those of ordinary skill in the art that modifications can be made to the steps and structure of the invention without departing from the spirit and scope of the invention as a whole. For example, while 6 channels are shown and described, it is possible that fewer channels, but more likely, more channels, could be specified and fabricated.

Claims

WHAT IS CLAIMED IS:

Claim 1. An optical delay waveguide apparatus comprising: a substrate; and, a plurality of waveguide branches B located in said substrate, each branch B having a common signal input point and B signal output points, and wherein each branch has a length L.

Claim 2. The apparatus of claim 1 further comprising: pulse shape analyzing means for receiving signals from said B signal output points and deriving pulse shape information therefrom.

Claim 3. The apparatus of claim 2 wherein said input signal is an optical signal above the 1 Gb/s range.

Claim 4. The apparatus of claim 3 wherein said input signal is in the 10 Gb/s range, or greater.

Claim 5. The apparatus of claim 4 wherein each branch B has a different length L and wherein said optical input signal takes a different length of time to travel from said signal input point to said B signal output points.

Claim 6. The apparatus of claim 5 wherein said substrate has an index of refraction nl and said B waveguide branches have indexes of refraction n2 and wherein n2 is higher than nl .

Claim 7. The apparatus of claim 6 further comprising: a clad region covering said substrate and said B waveguide branches, wherein said clad region has an index of refraction n3 that is less than nl .

Claim 8. The apparatus of claim 7 wherein said B waveguide branches comprise a plurality of B photolithographically defined cores each having a width in the range of 3-12 microns.

Claim 9. The apparatus of claim 8 wherein said pulse shape analyzing means further comprises: photodiodes organized in an array and wherein each photodiode receives a different one of said B optical output signals; integrating amplifier means electrically connected to each of said photodiodes to integrate and amplify the signals therefrom thereby producing a plurality of sample signals for further pulse shape analyzing; and, clock and recovery means for generating a trigger signal to control said photodiodes and said integrating amplifier means.

Claim 10. The apparatus of claim 9 wherein said substrate comprises a material taken form the group comprising: silicon or SiNx with an index of refraction n2 of 3.4 or 1.9 respectively.