DE69001873T2 - UNIPOLARIZATION CONTAINING INTEGRATED-OPTICAL COMPONENTS FOR OPTICAL GYROSCOPES. - Google Patents

Description

Technical area

This invention relates to integrated optics circuits (IOCs) and, in particular, to IOCs for use in optical gyroscopes.

State of the art

As is well known, the working principle of the fiber optic gyroscope (FOG) is the Sagnac effect. When light traverses an optical fiber loop rotating about an axis perpendicular to its plane, the optical travel time of the light signal varies depending on the rotation speed of the loop. For two optical signals traversing the loop in opposite directions, the Sagnac phase difference between them (S; in radians) is proportional to the rotation speed and is given by: (Equation 1)

where: L is the length of the fiber loop, d is the loop diameter, λ is the optical signal wavelength, c is the speed of light, and X is the loop rotation speed in rad/s.

This difference, which serves as a measure of velocity, can be increased by using a fiber optic coil to increase the loop length (L). To mitigate the need to measure a DC value, a sine phase modulator at one end of the loop can be used to modulate the beams. The modulation acts on the counter-circulating beams at different times due to the optical propagation delay in the coil, which results in a dither of the phase difference. This improves detection sensitivity because it allows the use of sensitive AC processing.

When opposing rays of unit intensity are added interferometrically, the total intensity is:

1 = 1/2*(1 + cos P) (Equation 2)

where P is the total phase difference. The Bessel expansion of the intensity expression yields a component:

F = k*sin(S) (Equation 3)

with the modulation frequency f. This can be taken as an analog output signal of the phase dither Sagnac interferometer. The coefficient k is:

k = 2+J₁*[2A*sin(π*f*T)] (Equation 4)

where the term 2A*sin(π*f*T) is the dither amplitude resulting from a modulation with the modulation frequency (f) and the amplitude (A) at a coil delay time T. The amplitude is maximized when f = 1/2T (the coil natural frequency).

The analog output signal F of frequency f is proportional to the rotation at sufficiently low speeds. It can be measured directly as an indication of the rotation, or the output signal can be continuously zeroed by a servo loop that adds an optical phase bias in opposition to the Sagnac phase difference. This can be achieved by adding a repeated linear ramp phase modulator (serrodyne modulation) to one end of the fiber coil. If the peak ramp amplitude is 2 π rad, the serrodyne modulation produces an effectively constant phase difference bias between the oppositely directed beams. The phase bias amplitude is proportional to the ramp repetition frequency, which provides an easily measurable representation of the loop rotation speed.

In FOG operation, the counter-circulating beams are assumed to travel identical paths in the absence of rotation and no applied phase bias, i.e., "reciprocity." However, different spatial modes and orthogonal polarization modes of the fiber coil are not always degenerate. Power coupled between modes can perturb the optical phase at the detector, causing drift in the FOG. Furthermore, it can be difficult to optimize the phase modulation of the desired and undesired polarization components of the beam simultaneously, and gyro output error can occur.

To eliminate crosstalk between spatial modes and to control polarization, known Sagnac gyros include both a single spatial mode filter and a single polarizer filter in the beam propagation path. The filters are located in the optical path common to the source beam and the interfering beams; between the optical detector and the loop beam splitter. This "reciprocal configuration" restricts the optical signals to a single spatial mode and a single polarization. The polarizing filter may be a bulk optical polarizer such as a Glan-Thompson prism, which gives extinction ratios of 60 dB, or any of a variety of fiber or integrated optical (10) polarizers. The single-mode filter may comprise a short length of single-mode fiber or single-mode waveguide.

However, if the extinction ratio of the polarization filter is not sufficiently high, polarized light in the filter can Light, which may be considerably depolarized after passing through the loop, may arrive at the detector with considerable power in the unwanted polarization. In some cases, the filter extinction ratio must be as high as 100 to 140 dB to reduce this power to acceptable levels.

Publications that demonstrate the state of the art just described are listed in Selected Papers on Fiber Optic Gyroscopes, SPIE Milestone Series, Volume MS8, pp. 1-637 (1989), which contains a large number of original articles on the design, development and manufacture of fiber optic gyroscopes.

US-A-4 547 262 describes a proton exchange process for forming an optical waveguide and components in an optical substrate made of lithium tantalate. The process described is, however, limited to the formation of "passive" optical components.

To date, the prior art lacks a proton exchange process that allows the formation of "active" optical components, where "active" refers to components that operate according to the well-known electro-optic effect. Examples of such "active" devices include optical modulators that change the local refractive index of the waveguide in response to an applied electric field, thereby changing the phase of the optical signal passing near it.

Description of the invention

The aim of the present invention is to provide a fiber optic gyro (FOG) system having a high polarization extinction ratio.

According to one aspect of the invention, there is provided a fiber optic gyro (FOG) system, with:

source means for providing a source light beam; interferometer loop means comprising optical fiber waveguides for counter-circulating two light beams in phased shift around a rotating loop, the phased shift providing a differential phase whose magnitude is proportional to the rotational speed of the loop;

an integrated optical circuit (IOC) device having a waveguide array disposed on a major surface of a substrate of refractive material, the waveguide array comprising a beam splitter/combiner device and two loop guide sections, the beam splitter/combiner device being arranged to split the source light beam into the two light beams for counter-circulating through the loop guide sections and the interferometric loop and to combine the phase-shifted light beams returned from the loop into an interference signal representing the magnitude of the differential phase;

a detector device which, in response to the interference signal, provides a signal representation of the magnitude of the differential phase; and

means for coupling the source light beam to the IOC means and for coupling the interference signal from the IOC to the detector means;

characterized,

that the IOC device further comprises a modulator device arranged on the main surface for electro-optically modulating the phase of each of the two light beams; and

that the waveguide arrangement in the main surface is formed by a two-stage proton exchange (TSPE) process which includes the following steps:

Immersing the substrate in a benzoic acid bath at a temperature of 150 ºC to 250 ºC for a period of two to sixty minutes;

Removing the substrate from the bath following the immersion step; and

Annealing the substrate for a period of one to five hours at a temperature of 300 ºC to 400 ºC.

According to a second aspect of the invention there is provided an integrated optical circuit (IOC) for use in a fiber optic gyro (FOG) system having a light source for providing a source light beam; an interferometric loop for counter-circulating two light beams around the loop with a differential phase proportional to a rotational speed of the loop; detector circuitry for detecting the magnitude of the differential phase by sensing the magnitude of an interference signal resulting from the combination of the counter-circulating light beams; and signal coupling circuitry for presenting the source light beam from the light source to the IOC and applying the interference signal from the IOC to the detector circuitry; the integrated optical circuit (IOC) comprising a refractive material substrate having a major surface (54); and

a waveguide arrangement means connected to the signal coupling circuitry and comprising a beam splitter/combiner means and two loop conductor sections, the beam splitter/combiner means being arranged to split the source light beam into two light beams for counter-circulating through the loop conductor sections and the interferometric loop and to combine the counter-circulating light beams from the loop into an interference signal representing the magnitude of the differential phase, characterized in that

that the IOC further comprises a modulator device arranged on the main surface adjacent to the loop conductor sections for electro-optically modulating the phase of each of the two light beams; and

that the waveguide arrangement in the main surface (54) is formed by a two-stage proton exchange (TSPE) process which includes the steps:

Immersing the substrate in a benzoic acid bath at a temperature of 150 ºC to 250 ºC for a period of two to sixty minutes;

Removing the substrate from the bath following the immersion step; and

Annealing the substrate for a period of one to five hours at a temperature of 300 ºC to 400 ºC.

Further, according to the present invention, the substrate material may be LiNbO₃ or LiTaO₃. Further, according to the present invention, the substrate material has an X-cut orientation.

Further according to the present invention, the IOC includes a beam splitter for splitting the incident light beam into two counter-circulating beams presented to the sensing loop and integrated optical (10) elements for phase modulating the counter-circulating beams. Further according to the invention, the beam splitter includes a symmetrical Y-junction. Further according to the present invention, the beam splitter includes a directional coupler.

The FOG system of the present invention uses proton exchanged (PE) waveguide structures in either LiNbO3 or LiTaO3 to produce an IOC that has a high degree of polarization control. The IOC contains a beam splitter such as a symmetric Y-junction or directional coupler, phase modulators, and single-mode input/output waveguides. The waveguides provide polarization extinction ratios of greater than 55 dB.

These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as shown in the attached drawing.

Short description of the drawing

Fig. 1 is a perspective view of a FOG-IOC embodiment according to the present invention; and

Fig. 2 is a schematic representation of a FOG system in which the IOC embodiment of Fig. 1 can be used.

Best way to carry out the invention

Referring first to Fig. 2, in the best mode embodiment of a FOG system 10 according to the present invention, the FOG system includes an IOC 12 that integrates several critical FOG optics functions on a single small substrate chip. The provision of the IOC optics shown in Fig. 2 results in a "reciprocal configuration" FOG system, as described in detail below. This ensures that only "non-reciprocal" effects such as rotation and time-varying propagation parameters affect the differential phase between the counter-circulating light beams.

In addition to the IOC 12, the system 10 includes a light source 14, light detector circuitry 16, an output tap 18 (which may be a directional coupler or a beam splitter), a fiber optic sensing coil 20, and control circuitry 22. In the case of high power FOG applications, the light source may comprise a superluminescent (semiconductor diode) (SLD). The SLD yields a broad spectrum, short coherence length (less than 100 microns) light beam. In FOG applications with lower power, a less expensive laser diode can be used. The source delivers the coherent light beam to the tap 18 via an output fiber 24.

The tap transmits a major portion of the incident source beam, approximately 50%, to the IOC 12 via a single mode fiber optic conductor 26. The detector circuitry connected to the fiber optic conductor 26 may include a well-known PIN diode transimpedance amplifier detector system.

The source beam incident on the IOC is filtered by a single polarization filter 30 and a single mode filter 32 located on the "common path" in the form of the waveguide section 33 in the IOC. This is the waveguide section that guides both the incident source beam to the sensing loop and the interference signal from the loop to the detector circuitry. The polarization filter attenuates the unwanted polarization mode. Ideally, the extinction ratio (the ratio of the output power in the desired polarization mode divided by the output power in the unwanted polarization mode) is as large as possible, e.g. in the range of 100 to 120 dB.

The polarization filters used in known FOGs typically include: bulk optical polarizers (such as a Glan-Thompson prism), optical waveguide polarizers, or thin film polarizers on titanium diffused LiNbO3 IOCs. The bulk polarizers have extinction ratios of up to 60 dB, but present difficulties in connecting to optical fibers and waveguides used elsewhere in the FOG. The other type of known polarizers typically have extinction ratios of less than 60 dB. In the proton exchanged (PE) FOG IOC of the present invention, the polarizer contains the entire PE waveguide matrix disposed on the IOC, which allows extinction ratios of greater than than 60 dB and can be easily connected to attached fiber optic cables.

The polarized output beam from filter 30 is presented to a single-mode filter 32, which may comprise a segment of the single-mode waveguide system on the IOC. The mode filtering provided by filter 32 may be supplemented by that of single-mode guide 26.

The single polarization, single spatial mode optical signal from filter 32 is applied to a beam splitter/combiner 34, which may be a Y-junction or a 3 dB directional coupler, which equally splits the filtered source light into two beams which are directed to waveguide sections 35 and 36, respectively, of the sensing loop. The guide section 35 couples the first beam through a dither modulator 37, and section 36 directs the second beam through a serrodyne modulator 38 into opposite ends 40 and 42, respectively, of the fiber sensing coil 20. The beams propagate in opposite directions through the coil as first and second counter-circulating beams and are returned from the loop to the splitter/combiner 34 through the opposite phase modulator. After recombination, the Sagnac difference phase created by rotation of the loop causes the counter-circulating beams to produce an interference signal. The interference signal is then fed back via the common path guide 33, through the mode filter and polarizer to the detector circuitry 16 via the output tap 18.

Each beam is modulated twice by each modulator as it moves through the loop; on entry and again on exit. The dither modulator 37 phase modulates each beam to produce the differential phase dither. The dither improves the measurement sensitivity by allowing an AC detection of the differential phase value. The dither is at a maximum when the modulation frequency is equal to the natural frequency of the fiber sensing coil.

The serrodyne modulator 38 provides a linear ramp phase modulation of each beam. If the peak ramp amplitude is 2 π radians and the flyback following each ramp segment is essentially instantaneous, the serrodyne modulation acting on the oppositely directed beams at slightly different times due to the optical delay in the coil produces an effectively constant differential phase bias. The bias can be controlled by a servo loop within the control circuitry 22 to be constantly opposite to the Sagnac phase difference, thereby making the phase difference zero, as is known in the art. The serrodyne ramp frequency then represents a gyro output signal that is proportional to the loop rotation speed.

In the present invention, all elements of the FOG system except the light source 14, detector 16, tap 18 and sensing coil 20 are integrated on a single IOC substrate. In future embodiments, tap 18 may also be added to the IOC. This offers a number of advantages. First, the IOC can be mass-produced using technology similar to that used in semiconductor fabrication. This makes the FOG IOC less expensive than the discrete devices it replaces. Second, at phase modulator (37, 38) frequencies potentially extending into the gigahertz range, the wide bandwidth capabilities of 10 devices are advantageous. Most importantly, the high birefringence of proton-exchanged waveguides and their inherent ability to guide only one polarization preserves the input polarizations in the IOC. This is true even for components such as beam splitters, which are poorly designed in this respect. work when manufactured in fiber optic form.

Fig. 1 is a perspective view of an IOC 50 in accordance with the present invention. To refer to the schematic view of the IOC of Fig. 2, the IOC is shown with the same fiber optic connectors (26 at the input end and 40, 42 at the sense coil end) as in Fig. 2. The IOC includes a substrate 52 of crystalline material having a major surface 54 for receiving the integrated optical elements. The substrate has fiber optic mating ends 56, 58.

The substrate material is preferably an X-cut crystal; either LiNbO₃ or LiTaO₃. A Z-cut and a Y-cut crystal (in that order) may also be used. The substrate of Fig. 1 is an X-cut crystal with an extraordinary refractive index along the Z axis.

The 10-elements arranged on the surface 54 comprise the waveguide matrix which has the common path guide section 33, the Y-branch splitter 34 and the loop guide sections 35, 36. The waveguide matrix is fabricated by proton exchange as described in detail below. As a result, the matrix itself is a single-mode, single-polarization system. There is no need for separate, i.e. discrete, spatial mode and polarization filters 30, 32 as in Fig. 2.

Further 10 elements comprise the electrodes 37a, 37b for the dither modulator 37 and 38a, 38b for the serrodyne modulator 38. The paired electrodes are arranged symmetrically around the associated loop conductor sections 35, 36.

The FOG-IOC can be prepared using the two-step proton exchange (TSPE) process described and claimed in US-A-4 984 861 to Suchoski et al, entitled: Low-Loss Proton Exchanged Waveguides for Active Integrated Optic Devices, issued January 15, 1991.

Fabrication begins with the deposition of a masking layer of aluminum (Al) or chromium (Cr) on the substrate surface 54. The masking layer is patterned and etched to form the Y-junction splitter 34, the common path conductor section 33, and the loop conductor sections 35, 36. The etched mask confines the proton exchange to the patterned area. The actual channel pattern widths depend on the signal wavelength chosen, but range from 3 to 10 microns.

The crystal substrate is then immersed in a concentrated benzoic acid bath for two to sixty minutes. The acid bath is at a temperature of 150ºC to 250ºC. Following the bath, the crystal is annealed at an elevated temperature in the range of 300ºC to 400ºC for a period of one to five hours. The exact set of processing conditions depends on the substrate material chosen (whether LiNbO₃ or LiTaO₃), the wavelength, the crystal cut, and the modal dispersion requirements.

Metallization for the paired modulator electrodes 37a, 37b and 38a, 38b is deposited and patterned using photolithographic techniques and electrode materials known in the art. Electrical connections (not shown) can be made to the external electronic systems by wire bonding.

The TSPE process locally increases the extraordinary refractive index (inside the waveguide channels) and locally decreases the ordinary refractive index. As a result, in the X-cut FOG-IOC shown in Fig. 1, it is only possible to guide a guided optical mode polarized along the Z-axis (the extraordinary axis). is to support.

Reflections at the two IOC interfaces with the sense coil contribute to FOG offset errors. These reflections occur due to a refractive index mismatch between the fiber guides (32, 34) and the IOC matching waveguide sections. They produce large offset errors when the two IOC/fiber interferences are equidistant (within the light source coherence length) from the Y-branch splitter. The reflections can be minimized by angling the fiber matching ends 56, 58 of the IOC by an angle α, where α > 8º. As shown in Fig. 1, in the preferred embodiment all faces of the IOC are angled with a typical inclination of α = 10º.

As described above, additional FOG system elements such as tap 18 (Fig. 2) can be provided on the IOC. In addition, the present proton exchange IOC can be used in ring resonator FOG systems as well as the interferometric type described here.

While the invention has been described with reference to a best mode for carrying out the invention, it will be apparent to those skilled in the art that the foregoing and various other changes, omissions and additions in form and details thereof may be made without departing from the scope of this invention.

Claims (14)

1. Fiber optic gyro (FOG) system (10), comprising: a source device (14) for providing a source light beam; an interferometer loop device (20) with optical fiber waveguides for counter-rotating two light beams in phased shift around a rotating loop (20), the phased shift resulting in a differential phase whose magnitude is proportional to the rotational speed of the loop (20); an integrated optical circuit (IOC) device (12) having a waveguide array (33-36) disposed on a major surface (54) of a substrate (52) of refracting material, the waveguide array (33-36) comprising a beam splitter/combiner device (34) and two loop conductor sections (35, 36), the beam splitter/combiner device (34) being arranged to splitter the source light beam into the two light beams for counter-circulating through the loop conductor sections (35, 36) and the interferometric loop (20) and to combine the phase-shifted light beams returned from the loop (20) into an interference signal representing the magnitude of the differential phase; a detector device (16) which, in response to the interference signal, provides a signal representation of the magnitude of the differential phase; and means (18) for coupling the source light beam to the IOC means (12) and for coupling the interference signal from the IOC (12) to the detector means (16); characterized, that the IOC device (12) further comprises a modulator device (37, 38) arranged on the main surface (54) for electro-optically modulating the phase each of the two light rays; and that the waveguide arrangement (33-36) in the main surface (54) is formed by a two-stage proton exchange (TSPE) process, which includes the following steps: Immersing the substrate (52) in a benzoic acid bath at a temperature of 150 ºC to 250 ºC for a period of two to sixty minutes; Removing the substrate (52) from the bath following the immersion step; and Annealing the substrate (52) for a period of one to five hours at a temperature of 300 ºC to 400 ºC. 2. The system of claim 1, wherein the IOC substrate material comprises LiNbO₃. 3. The system of claim 1, wherein the IOC substrate material comprises LiTaO₃. 4. The system of claim 2 or 3, wherein the substrate material further comprises X-cut crystal material. 5. The system of claim 2 or 3, wherein the substrate material further comprises Z-cut crystal material. 6. The system of claim 2 or 3, wherein the substrate material further comprises Y-cut crystal material. 7. A system according to any one of claims 1 to 6, wherein the steel splitter/combiner device comprises a symmetrical Y-junction. 8. An integrated optical circuit (IOC) for use in a fiber optic gyro (FOG) system having a light source (14) for providing a source light beam; an interferometric loop (20) for counter-circulating two light beams around the loop (20) with a differential phase proportional to a rotational speed of the loop (20); detector circuitry (16) for detecting the magnitude of the differential phase by sensing the magnitude of an interference signal resulting from the combination of the counter-circulating light beams; and signal coupling circuitry (18) for presenting the source light beam from the light source (14) to the IOC (12) and applying the interference signal from the IOC (12) to the detector circuitry (16); wherein the integrated optical circuit (IOC) comprises a refractive material substrate (12) having a major surface (54); and a waveguide arrangement device (33-36) connected to the signal coupling circuitry (18) and having a beam splitter/combiner device (34) and two loop conductor sections (35, 36), the beam splitter/combiner device (34) being arranged for splitting the source light beam into two light beams for counter-circulating through the loop conductor sections (35, 36) and the interferometric loop (20) and for combining the counter-circulating light beams from the loop (20) into an interference signal representing the magnitude of the differential phase, characterized in that that the IOC (12) further comprises modulator means (37, 38) arranged on the main surface (54) adjacent the loop conductor sections (35, 36) for electro-optically modulating the phase of each of the two light beams; and that the waveguide arrangement (33, 36) in the main surface (54) is formed by a two-stage proton exchange (TSPE) process, which includes the steps: Immersing the substrate (52) in a benzoic acid bath at a temperature of 150 ºC to 250 ºC for a period of two to sixty minutes; Removing the substrate (52) from the bath following the immersion step; and Annealing the substrate (52) for a period of one to five hours at a temperature of 300 ºC to 400 ºC. 9. The system of claim 8, wherein the IOC substrate material comprises LiNbO₃. 10. The system of claim 8, wherein the IOC substrate comprises LiTaO₃. 11. The system of claim 9 or 10, wherein the substrate material further comprises X-cut crystal material. 12. The system of claim 9 or 10, wherein the substrate material further comprises Z-cut crystal material. 13. The system of claim 9 or 10, wherein the substrate material further comprises Y-cut crystal material. 14. A system according to any one of claims 8 to 13, wherein the beam splitter/combiner means comprises a symmetrical Y-junction.