WO1995006853A1

WO1995006853A1 - INTEGRATED 3x3 COUPLER FOR A FIBER OPTIC GYRO

Info

Publication number: WO1995006853A1
Application number: PCT/US1994/009585
Authority: WO
Inventors: Franklin Bert Bossler
Original assignee: Alliedsignal Inc.
Priority date: 1993-08-30
Filing date: 1994-08-25
Publication date: 1995-03-09

Abstract

An opto-electronic integrated circuit (OEIC) combines the functions of four of the six separate components of a 3x3-coupler-type fiber optic gyro, reducing the entire optical circuit to just three components: source, OEIC, and coil. The OEIC also eliminates the six fusion splices formerly used to connect the components, by using self-aligning grooves to attach the fibers to the waveguides on the OEIC. Low cost is enhanced by using silicon for the substrate of the OEIC, although it is also possible to use other materials for use at other wavelengths.

Description

INTEGRATED 3X3 COUPLER FOR A FIBER OPTIC GYRO

Background of the Invention

Field of the Invention

The present invention relates to fiber optic gyros and more particularly to utilizing an opto-electronic integrated circuit to combine functions within a fiber optic gyro.

Description of the Prior Art

A unique configuration of a Fiber Optic Gyro utilizing a 3x3 coupler was described in the prior art: "Low-cost Fiber Optic Gyroscope", by E. Hartl et al, at the SPIE International Symposium OE/Fibers '91,3-6 September 1991, Boston, Massachusetts. This configuration is shown in Figure 1 and further described in other studies: "Fiber Optic Rate Gyro Study, Phase 1", GEC Marconi Electronic Systems Corporation Document R 256A525, Appendix I, 20 February 1992, and "Fiber Optic Rate Gyro (FORG) Study Final Report" Litton Guidance and Control Systems, 14

February 1992, both in response to Mercer University RFQ 00314.

These prior art techniques require an optical circuit consisting of six discrete parts: a pigtailed source module, a 3x3 fused fiber coupler, the fiber¬ optic sensing coil, and three pigtailed detectors with associated preamplifier circuits. Such components are expensive and bulky because they require sealed packages. Furthermore, these components must be assembled using fusion or other splices at six places. Such splices are also labor intensive, expensive and bulky.

Summary of the Invention

The present invention utilizes an opto-electronic integrated circuit to combine the functions of the 3x3 coupler and the three detector-preamplifier circuits, as shown in Figure 2. This reduces the entire gyro optical circuit to three parts; pigtailed source module 12, integrated opto-electronic circuit 14 and coil 16. This simplification should greatly reduce the cost of the components as well as the hands-on labor required to assemble them.

In order to obtain these advantages, the present invention utilizes a crystalline silicon or other semiconductor substrate common in the electronics art. The required optical waveguides are fabricated on one surface, before or after the required analog electronics circuits are fabricated in the usual fashion on or near the same or opposite surface. The use of silicon is preferred for its low cost, its well- known processes, and so that the optical wavelength may be in the vicinity of 820 nm, to take advantage of the almost double (1.89x) sensitivity for the gyro compared to the 1550 nm wavelength used in the prior studies. Alternatively, InP or GaAlAs or other material could be used if operation at 1.3 or 1.55 u wavelengths are required.

Brief Description of the Drawings

Figure 1 illustrates an optical schematic of the prior art.

Figure 2 illustrates an optical schematic of a fiber optic gyroscope utilizing the present invention.

Figure 3 illustrates a possible arrangement of the present invention.

Figure 4 illustrates a layout of a 2 mm. x 8 mm. opto-electronic integrated circuit of the present invention.

Figure 5 illustrates a detailed region of Figure

Detailed Description of the Invention

One embodiment of the present invention is sketched in Figure 3. The components in this structure are described below as: waveguide 21 and circuit layers 22, implanted on the surface of silicon substrate 20, 3x3 coupler 24, detector 26 (three places) , preamplifiers 28 (three places) , and grooves 29 (three places) for "blind" fiber alignment. Techniques which could be used to create waveguides 21 are known in the art. Other alternatives are being actively developed in all common materials and wavelengths. For the present invention, single- mode guides are required. Single-polarization guides are desirable, but not imperative. Losses below 0.1 db/cm are desired, but 0.5 db/cm could be accepted. The guides must have some bends. Techniques for design of short-radius bends are also known in the art.

3x3 coupler 24 has not been as extensively studied in the literature as the 2x2 coupler. A number of possible structures can be considered, any of which could apply to the present invention. An evanescent- field coupler in which the coupling is achieved by closely-spaced parallel waveguides, as sketched in Figure 3, has been made by A.T.& T. in an OEIC for Jet Propulsion Laboratory, at 1550 nm. An alternative structure that may be more appropriate for this application would be a "double-pitchfork" arrangement, in which the guiding of the waves in the coupling region is similar to that in the fused 3x3 coupler. That is, in the coupling region the lateral extent of the wave is much larger than the individual guiding cores, so that the single wave couples equally to all three output guides. Since it is difficult to make the three guides on a surface small enough to act as a single core, as is done in the fusion splice by drawing down the fused fiber bundle, one need only taper all three guides together into one guide in the center region. Techniques for overcoming the problems of fabricating a low-angle tapered intersection have been studied in connection with asymmetrical 2x2 couplers. Coupling from a surface waveguide to detector 26, either on the surface of the underlying substrate, or on an overlaid detector region, is the subject of intense effort today. The basic requirement is that the optical wave in the waveguide be redirected to impinge on the detector surface. One successful approach is to make an angled facet on the end of the waveguide which reflects the wave downward or upward by total internal reflection (TIR) . Unfortunately, this is not an easy structure to fabricate in a glass waveguide on silicon. It may be possible with careful polishing on the edge of a chip, or with a directional etch - D.E. Bossi et al, "Regrowth-Free Waveguide- Integrated Photodetector with Efficient Total-Internal Reflection Coupling", Photonics Technology Letters v.5, pgs. 166-169.

Another alternative is the use of a grating to deflect the wave into the detector surface - T. Sahara et al, "Integrated Optics. . .Periodic Structures, J. Quant. Elect. V.QE22, pgs. 845-867, 1986. A suitable grating period would be two or three times the wavelength of the radiation in the waveguide, (approximately the free-space wavelength Lambda sub zero, divided by the index of refraction of the waveguide itself, denoted n.) . Such a grating would scatter two waves at angles of plus and minus 40 to 60 degrees from the guide direction. The downward wave should be absorbed in the (silicon) substrate, while the upward wave would be reflected by TIR back down through the waveguide and into the substrate also. The grating can be formed in the waveguide itself by a periodic change in the width, depth, thickness, or cladding. The length of the grating must be in the order of 100 lambda, but need be only two to five times as wide as the waveguide. The area of the detector should be no problem.

Integration of the MOS-FET or MOD-FET preamplifier should be straightforward since only low-frequency analog signals are required. Electrical connections to the preamplifiers can be made by solder button flip- chip or other low-cost means which do not conflict with the clamping of the optical fibers discussed below. Techniques for automatic ("Blind") alignment of singlemode fibers to integrated opto-electronic circuits using grooves in the silicon have been demonstrated in the following references. - C.A. Armiento et al, "Passive Coupling of InGaAsP/InP Laser Array and Single-Mode Fibers Using Silicon Waferboard", Elect. Lett. 27, pgs. 1109-1110, 1991; and M.S. Cohen et al, "Passive Laser-Fiber Alignment by Index Method", Photonics Technology Letters, v.3 pgs. 985-987, 1991. These techniques have generally used two separate pieces: one with the grooves and a vertical cleaved or polished face perpendicular to the grooves and the surface, and the other with the waveguides on the surface and a perpendicular vertical surface on which the ends of the waveguides are accessible. In order to achieve the desired economy of fabrication, it is desirable that the V-grooves and the optical guide be on the same surface of one silicon chip.

One option is to use a saw-cut using a diamond saw or other mechanical method to form the vertical face on the wafer. Butt-coupling of the fiber to the waveguide layer would then be possible. A 1 mm diameter dentists burr (designed to be used at 300,000 rpm in an air- driven spindle) would only need to make a scarf about 0.5 mm wide at the surface. The burr would have to be at least 35 urn (0.0013") thick, but could be much thicker if necessary. Alternative mechanical methods such as ultrasonic grinding may also be employed to produce the vertical face.

Another possibility is first to form the vertical face by means of a reactive ion etch, with appropriate etch chemistry and enough acceleration potential to create a near-vertical hole about 40 microns deep. Then when the groove is subsequently etched, the etch process will produce an undercut under the surface waveguide layer, permitting a "face-contact" splice.

The fibers can be held in place by both an index- matching epoxy and a mechanical hold-down plate. The plate should preferably be transparent to the UV curing radiation, so that if necessary, particularly in development, performance can be verified with the clamp in place, prior to the curing cycle. The cover plate may also incorporate electrical connections to the preamps by means of the solder-bumps. Alignment would be facilitated if the plate incorporates matching grooves, which would permit blind assembly, by using the fibers themselves as the locating fiducial references. A close-fitting metal cover is very desirable, both to shield the detectors from stray light and to shield the preamps from electromagnetic interference.

Much effort is being put into the integration of lasers with opto-electronic circuits, especially at the popular communications wavelengths. If these efforts are successful in permitting low-cost fabrication, it may well be possible to include the source on the OEIC for the gyro, thus eliminating the cost of the pigtailed source from the gyro, and one of the fiber- grooves.

Much effort is also being expended in integrating mirrors, especially total-internal-reflectors, into waveguide circuits, to permit the size of the devices to be greatly reduced by allowing much larger bend angles. To date, these have been successful in InP and GaAlAs materials where the waveguides are formed in epitaxial material and the mirrors can be formed by Reactive-Ion-Etching (RIE) at high potentials. If RIE or mechanical methods are found suitable to create low- loss total internal reflector mirrors in glass waveguides on Si, a much smaller OEIC could be produced as suggested in Figures 4 and 5. (Unfortunately, blind alignment of surface waveguides to optical fiber in grooves etched by RIE would require precise control of the depth of the grooves. Depth control is not possible in todays art in InP or GaAlAs, whereas etched depth is relatively easy to control in silicon.) Figure 4 illustrates a layout of a 2 mm. x 8 mm. OEIC 82 using TIR mirrors 83 (6 places) throughout. Light from the source module (not shown) enters through the source module fiber 84 into the diamond saw-cut 85 and thence into the coupling region 88. Within the coupling region 88 there are three input waveguides and three output waveguides and the optical power is shared from each input waveguide to all output waveguides and from each output waveguide to all input waveguides. In the coupling region 88, the incident light is split into the three separate waveguides, one going to one end of the sensing coil (Coil A 89); a second going to the other end of the sensing coil (Coil B 90) ; and the remainder going to the straight-through detector 99 and its preamp 100. After traversing the sensing coil the light which was transmitted through Coil A 89 returns through Coil B 90 and reenters the coupling region 88. Similarly, the light which was transmitted through Coil B returns through Coil A and also reenters the coupling region 88. After traversing the coupling region again, this time in the direction back toward the source, a portion of each of the two returning waves is incident on each of the two detectors 87. The vector sum of the two returning light waves on each detector is an interference pattern. In the absence of a rotation of the sensing coil, the interference should deliver identical power to each of the detectors 87, and hence the output voltages of the preamps 86 should be equal, i.e., their difference should be zero. Rotation of the sensing coil will cause a relative phase shift between the two returning waves, such that the power in one detector 87 will increase, while the power in the other will decrease. The output voltage of the two preamps 86 will therefore be different, and this difference is a measure of the magnitude and direction of the rotation of the sensing coil. Changes in the intensity of the source module will, of course, cause proportional changes in the detected power in all detectors 87 and 99. By suitable analog circuitry or by digital computation techniques well known in the art, the output of preamp 100 can be used to compensate the difference of the outputs of the preamps 86 so that the output scale factor (expressed as volts per degree per second) is independent of the source intensity.

Location of all three detectors and preamps on the same substrate will also tend to make the scale factor independent of supply voltages, temperature, and other systematic and process variables.

Diamond saw-cuts 85, 91 and 92 are 0.25 x 0.50 mm. and 50 urn. deep. TIR mirrors 83 are 30 urn. square and are through the waveguide only. Figure 5 illustrates a detail of Region A of Figure 4. Three parallel waveguides 94, 95, 96 representing coupling region 88 of Figure 4 enter a separation region 97 and reflect off TIR mirrors 98 (two places) as shown. A minor variation on Figure 4 would be to have the two grooves for the coil fibers arranged to exit at the top and bottom of the figure, (eliminating two of the TIR mirrors) instead of being in parallel as shown. This would be appropriate if the OEIC is located on or near the coil itself, where the two ends of the coil emerge from the winding process in opposite directions. Careful fixturing could permit the coil fiber ends to be captured in the grooves of the OEIC without requiring the fibers to leave the vicinity of the coil. This would leave only the source fiber to be dressed away from the coil itself, reducing the optical assembly to only two components. Then, if the source itself can be integrated on the OEIC, (and if the thermal effects of the source power dissipation are not too detrimental to gyro performance) , the whole optical circuit could be one monolithic assembly, with no external fiber at all.

This invention is applicable to low-cost rate-gyro applications typical of those previously using mechanical rate-gyros built by Bendix-Cheshire, in production rates of hundreds to thousands per week.

It is not intend ied that this invention be limited to the hardware arrangement, or operational procedures shown disclosed. This invention includes all of the alterations and variations thereto as encompassed within the scope of the claims as follows.

Claims

What is claimed is:

1. An optical circuit for a fiber optic gyro comprising: pigtailed source module means; opto-electronic integrated circuit means connected to said pigtailed source module means; and, coil means connected to said opto-electronic integrated circuit means.

2. An optical circuit for a fiber optic gyro as claimed in claim 1 wherein said opto¬ electronic integrated circuit means comprises: a substrate; source module fiber means incorporated within said substrate for receiving incident light from said pigtailed source module means; coupling region means on said substrate connected to said source module fiber means for splitting said incident light into coil A light, coil B light and straight through light; straight through detector means on said substrate for receiving said straight through light; straight through preamp means on said substrate connected to said straight through detector means; detector means on said substrate for receiving returning light from said coil means, flowing back toward said pigtailed source module means; and, preamp means on said substrate connected to said detector means.

3. An optical circuit for a fiber optic gyro as claimed in claim 2 wherein said coupling region means comprises: three input waveguides; three output waveguides; means for sharing optical power from each of said three input waveguides to all of said three output waveguides; and, means for sharing optical power from each of said three output waveguides to all of said three input waveguides.

4. An optical circuit for a fiber optic gyro as claimed in claim 3 wherein said coupling region means further comprises: a plurality of total internal reflection (TIR) mirrors.

5. A method of making an optical circuit for a fiber optic gyro comprising the steps of: providing a pigtailed source module; connecting an opto-electronic integrated circuit to said pigtailed source module; and, connecting coil means to said opto-electronic integrated circuit.

6. A method of making an optical circuit for a fiber optic gyro as claimed in claim 5 wherein connecting an opto-electronic integrated circuit to said pigtailed source module and connecting coil means to said opto-electronic integrated circuit comprises the steps of: etching precision grooves in periphery of said opto-electronic integrated circuit; and, mechanically cutting vertical faces at end of said etched precision grooves; simply laying optical fibers into said precision grooves; and, clamping said optical fibers in place to provide blind alignment of said optical fibers instead of requiring precision active alignment.