Integrated resonant micro-optical gyroscope and method of fabrication

An integrated optic gyroscope is disclosed which is based on a photonic integrated circuit (PIC) having a bidirectional laser source, a pair of optical waveguide phase modulators and a pair of waveguide photodetectors. The PIC can be connected to a passive ring resonator formed either as a coil of optical fiber or as a coiled optical waveguide. The lasing output from each end of the bidirectional laser source is phase modulated and directed around the passive ring resonator in two counterpropagating directions, with a portion of the lasing output then being detected to determine a rotation rate for the integrated optical gyroscope. The coiled optical waveguide can be formed on a silicon, glass or quartz substrate with a silicon nitride core and a silica cladding, while the PIC includes a plurality of III–V compound semiconductor layers including one or more quantum well layers which are disordered in the phase modulators and to form passive optical waveguides.

FIELD OF THE INVENTION

The present invention relates in general to optical rotation rate sensors (also termed optical gyroscopes or optical gyros), and in particular to an optical gyroscope comprising a photonic integrated circuit.

BACKGROUND OF THE INVENTION

Accurate sensing of angular velocity is an essential element in the guidance and control of vehicles of many types including land vehicles, manned or unmanned aircraft, guided missiles and smart munitions. Many types of partially or completely integrated optical gyroscopes have been disclosed in attempts to provide a rotation rate sensor having certain advantages over conventional rotating mass gyroscopes in terms of improved reliability and robustness, reduced size and power, and lower cost (see e.g. U.S. Pat. Nos. 4,326,803; 5,327,215; 5,408,492; 6,259,089; and 6,587,205 which are incorporated herein by reference).

Optical gyroscopes (i.e. gyros) are based on the well-known Sagnac effect which defines a linear relationship between the rotation rate of light propagating in a circuital path and a phase shift Δφ of the light propagating around the path in opposite directions given by:

Δ⁢⁢ϕ=8⁢π⁢⁢A⁢⁢Ωλ⁢⁢c
where A is the area of the circuital path, Ω is the angular rotation rate, λ is the wavelength of the light, and c is the speed of light. The phase shift Δφ of the light for a given rotation rate Ω can be increased N-fold when the circuital path with the area A is looped around N times. A further increase in sensitivity of the optical gyroscope can be achieved when the circuital path forms a resonator. In this case, the phase shift Δφ will be increased by a finesse, F, of the resonator. Further details of the operation of various types of optical gyroscopes can be found in a book chapter by S. Merlo et al entitled “Fiber Gyroscope Principles” (inHandbook of Fibre Optic Sensing Technology, chapter 16, John Wiley & Sons, Ltd., 2000).

The integrated optical gyroscope of the present invention with a bidirectional laser source, phase modulators and waveguide detectors formed as a photonic integrated circuit (PIC) on a compound semiconductor substrate and with a passive ring resonator formed separately and attached thereto represents an advance in the art which is expected to provide many of the advantages listed above while being more easily fabricable using current technology than a completely-integrated device with the passive ring resonator formed on the same substrate as the PIC.

These and other advantages of the present invention will become evident to those skilled in the art.

SUMMARY OF THE INVENTION

The present invention relates to an integrated optic gyroscope that comprises a bidirectional laser source formed on a compound semiconductor substrate to provide a lasing output from each end thereof, a pair of optical waveguide phase modulators also formed on the substrate to provide a phase modulation for each lasing output from the bidirectional laser source, and a plurality of passive optical waveguides formed on the substrate to direct each lasing output to an edge thereof after passing through one of the optical waveguide phase modulators. The integrated optical gyroscope also comprises a passive ring resonator that is adapted to receive each lasing output from the edge of the compound semiconductor substrate, to propagate each lasing output around the resonator in a different direction, and to direct a portion of each lasing output out of the resonator after propagating around the resonator. A pair of waveguide photodetectors, which are also formed on the compound semiconductor substrate and optically coupled to the edge thereof, can then receive the portion of each lasing output from the passive ring resonator and generate therefrom output electrical signals that can be used to determine a rotation of the passive ring resonator.

The passive ring resonator can comprise either an optical fiber, or a coiled optical waveguide formed on another substrate (e.g. comprising silicon, glass, or quartz). In embodiments of the present invention where the passive ring resonator is provided as an optical fiber, the integrated optic gyroscope can further include a fiber optic splitter to couple each lasing output into the passive ring resonator, and to couple the portion of each lasing output out of the passive ring resonator after propagating around the resonator.

In other embodiments of the present invention where the integrated optic gyroscope comprises a coiled optical waveguide formed on another substrate (e.g. comprising silicon, glass or quartz), an adiabatic mode-matching region can be provided on this other substrate proximate to an edge thereof to optically couple the passive ring resonator to the passive optical waveguides on the compound semiconductor substrate. The coiled optical waveguide can comprise a waveguide core of silicon nitride surrounded by a waveguide cladding of silica, and generally further includes a waveguide crossing. The coiled optical waveguide is preferably a single-mode waveguide, with the coiled optical waveguide transmitting the lasing output in a transverse-electric (TE) polarization state while suppressing the transmission of the lasing output in a transverse-magnetic (TM) polarization state. The passive ring resonator can further include a 2×2 evanescent waveguide coupler and a pair of 1×2 lateral mode interference splitters to couple each lasing output into the passive ring resonator, and to couple the portion of each lasing output out of the resonator after propagating around the resonator.

The bidirectional laser source in the integrated optic gyroscope can comprise a distributed feedback (DFB) laser. The bidirectional laser source and the waveguide photodetectors can all be formed from a plurality of compound semiconductor layers epitaxially deposited on the compound semiconductor substrate, with the plurality of compound semiconductor layers including one or more quantum wells. Each passive optical waveguide and the phase modulators can also be formed from the same compound semiconductor layers with each quantum well therein being disordered or etched away. An ion-implanted region can also be formed between the bidirectional laser source and each phase modulator to provide electrical isolation between these elements, with the ion-implanted region extending partway through the compound semiconductor layers.

The compound semiconductor substrate and the silicon, glass or quartz substrate can each include one or more alignment waveguides formed thereon to facilitate alignment of each passive optical waveguide on the compound semiconductor substrate to the passive ring resonator on the silicon, glass or quartz substrate in preparation for attaching the substrates together edge-to-edge. The two substrates can then be attached together using a UV-cured epoxy adhesive. The compound semiconductor substrate can further include an alignment laser or an alignment photodetector or both optically coupled to at least one of the plurality of alignment waveguides on the compound semiconductor substrate.

The present invention further relates to an integrated optic gyroscope comprising a passive ring resonator formed on a first substrate, and a photonic integrated circuit (PIC) formed on a second substrate, with the two substrates being attached together. The passive ring resonator further comprises a coiled optical waveguide having a plurality of loops, a pair of input optical waveguides optically coupled to the coiled optical waveguide to receive lasing light from an edge of the first substrate and to convey the lasing light into the coiled optical waveguide in each of two counterpropagating directions; and a pair of output optical waveguides coupled to the coiled optical waveguide to receive a portion of the lasing light out from the coiled optical waveguide and to convey the portion of the lasing light to the edge of the first substrate after propagating around the coiled optical waveguide. The PIC formed on the second substrate comprises a bidirectional distributed feedback (DFB) laser to generate the lasing light and to emit the lasing light from each end thereof, a pair of optical waveguide phase modulators optically coupled to each end of the DFB laser to phase modulate the lasing light, a passive optical waveguide to convey the lasing light from each phase modulator to an edge of the second substrate wherefrom the lasing light is coupled into the input optical waveguides on the first substrate, and a waveguide photodetector to receive the portion of the lasing light from each output optical waveguide on the first substrate and generate therefrom an electrical output signal indicative of a rotation rate of the passive ring cavity.

The first substrate can comprise silicon, glass or quartz; and the second substrate can comprise a III–V compound semiconductor (e.g. GaAs or InP). The passive ring resonator, and each input and output optical waveguide can comprise a waveguide core of silicon nitride surrounded by a waveguide cladding of silica (i.e. silicon dioxide or a silicate glass). The passive ring resonator is preferably made birefringent to transmit the lasing light from the DFB laser in a transverse electric (TE) mode while attenuating any transmission of the lasing light in a transverse magnetic (TM) mode. The coiled optical waveguide can also include one or more waveguide crossings.

The input optical waveguides and the output optical waveguides can be optically coupled to the coiled optical waveguide through a 2×2 evanescent waveguide coupler. A pair of 1×2 lateral mode interference splitters can also be used to optically couple the input and output optical waveguides to the coiled optical waveguide. An adiabatic mode-matching region can be formed proximate to the edge of the first substrate to facilitate coupling of the input and output optical waveguides on the first substrate to the passive optical waveguides on the second substrate.

The PIC can comprise a plurality of III–V compound semiconductor layers which include a pair of low-refractive-index cladding layers sandwiched about a high-refractive-index core layer. The high-refractive-index core layer can further include one or more quantum wells therein, with the quantum wells being intact in the DFB laser and in the waveguide photodetectors, and being disordered or etched away within the phase modulators and passive optical waveguides. A grating can also be formed in one of the low-refractive-index cladding layers of the DFB laser, with an etch-stop layer being optionally formed below the grating. An electrical isolation region can also be provided between each phase modulator and the DFB laser.

The first and second substrates can be attached together at the edges thereof (e.g. with a UV-cured epoxy adhesive). To facilitate alignment of the input and output optical waveguides on the first substrate to the passive optical waveguides on the second substrate in preparation for attaching the substrates together, a plurality of alignment waveguides can be provided on each of the substrates. Additionally, an alignment laser can be formed on the second substrate and optically coupled to one or more of the alignment waveguides on the first substrate. The second substrate can also include an alignment photodetector optically coupled to a curved alignment waveguide on the second substrate to detect lasing light from the alignment laser.

The present invention also relates to a method for forming an integrated optic gyroscope. This method comprises steps for epitaxially growing on a compound semiconductor substrate a plurality of compound semiconductor layers including at least one quantum well layer; forming a plurality of active optical elements from the compound semiconductor layers including a bidirectional laser source, a pair of optical waveguide phase modulators optically coupled to the bidirectional laser source, and a pair of waveguide photodetectors; disordering or etching away a portion of the compound semiconductor layers and forming therefrom a plurality of passive optical waveguides, with the passive optical waveguides connecting the pairs of phase modulators and waveguide photodetectors to an edge of the compound semiconductor substrate; and connecting a passive ring resonator to the edge of the compound semiconductor substrate, with the passive ring resonator being optically coupled to the passive optical waveguides to receive a phase-modulated lasing output from each phase modulator, and to direct a portion of the phase-modulated lasing output to each waveguide photodetector after propagating around the passive ring resonator. The passive ring resonator, which can comprise an optical fiber or a coiled optical waveguide formed on another substrate, can be attached to the compound semiconductor substrate with an adhesive (e.g. a UV-cured epoxy).

Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, there is shown a schematic plan view of a first example of an integrated optic gyroscope10formed according to the present invention. The integrated optic gyroscope10comprises a pair of substrates12and14which are processed separately to form elements of the apparatus10thereon, and which are then attached together with an adhesive16(e.g. a epoxy adhesive that is cured with ultraviolet light, also termed a UV-cured epoxy). One of the substrates12comprises silicon (e.g. a monocrystalline silicon substrate), glass or quartz (i.e. crystalline quartz or fused silica); and the other substrate14comprises a compound semiconductor substrate which can be formed, for example, from a III–V compound semiconductor material such as gallium arsenide (GaAs) or indium phosphide (InP).

By utilizing two substrates12and14, the various optical elements on each substrate12or14can be optimized in terms of performance and manufacturing cost. The silicon, glass or quartz substrate12allows the formation of passive optical components including a relatively long passive ring resonator18with a low waveguide transmission loss, and at a lower cost than would generally be possible using a compound semiconductor substrate. Additionally, the compound semiconductor substrate14allows the formation of certain active optical elements including a bidirectional laser source20which cannot easily be formed on the other substrate12due to the lack of a direct energy bandgap in the silicon, glass or quartz. Other active optical elements including a pair of optical waveguide phase modulators22and a pair of waveguide photodetectors24and a plurality of passive optical waveguides26can also be provided on the compound semiconductor substrate14to form a photonic integrated circuit (PIC)28which can used be in combination with the passive ring resonator18formed on the silicon, glass or quartz substrate12, or alternately in combination with a passive ring resonator18′ formed from an optical fiber30as shown inFIG. 5.

To form the PIC28inFIG. 1, a plurality of III–V compound semiconductor layers can be epitaxially grown on the compound semiconductor substrate14by an epitaxial growth method such as molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), both of which are well-known in the art and therefore need not be described here in detail. These epitaxial layers, which are shown in the schematic cross-section view inFIG. 2Ataken along the section line1—1inFIG. 1, include a pair of low-refractive-index cladding layers including a lower cladding layer32and an upper cladding layer32′ which are sandwiched about a high-refractive-index core layer34. The core layer34also preferably includes one or more quantum wells36therein. Those skilled in the art will understand that the term “quantum well” refers to a thin epitaxial layer about 30 nanometers thick or less wherein a quantum confinement of carriers (i.e. electrons and holes) occurs. Also, the terms “low-refractive-index” and “high-refractive-index” are used herein to indicate the existence of a difference Δn in the refractive index between the cladding layers32and32′ and the core layer34which is used for optical waveguiding in a direction normal to the plane of the substrate14.

One of the cladding layers32or32′ can be doped n-type, and the other cladding layer can be p-type doped, with the dopant concentration in each cladding layer32and32′ generally being in the range of 1–5×1017cm−3. The core layer34can be undoped (i.e. intrinsic) to form a p-i-n structure in the epitaxial layers. The compound semiconductor substrate14can be doped the same type as the lower cladding layer32, with the dopant concentration in the substrate14being, for example, on the order of 1018cm−3.

Those skilled in the art will understand that additional epitaxial layers can be provided which are not shown inFIG. 2Aincluding a buffer layer having the same semiconductor alloy composition as the substrate14and located between the substrate14and the lower cladding layer32; and a heavily doped (e.g. to 1018–1019cm−3) cap layer above the upper cladding layer32′. Additionally, one or more etch-stop layers can be optionally provided to facilitate the formation of a rib structure44used for lateral definition of the laser source20, phase modulators22, waveguide photodetectors24and passive optical waveguides26so that these elements all operate in a single mode.

As an example, to form the PIC28at an operating wavelength near 980 nanometers, the compound semiconductor substrate14can comprise gallium arsenide (GaAs), the cladding layers32and32′ can comprise aluminum gallium arsenide (AlGaAs), the waveguide core layer34can comprise GaAs, and each quantum well36can comprise indium gallium arsenide (InGaAs) which can be strained due to a slight lattice mismatch to the surrounding GaAs and AlGaAs and to the GaAs substrate14. Each AlGaAs cladding layer32and32′ can be, for example, 1.5 μm thick. The core layer34can be, for example, 0.2 μm thick and can be optionally graded in semiconductor alloy composition (e.g. from Al0.3Ga0.7As to GaAs) to form a graded-index separate confinement heterostructure (GRINSCH) structure. Each InGaAs quantum well36can be, for example, 8 nanometers (nm) thick. When multiple InGaAs quantum wells36are utilized, each adjacent pair of the InGaAs quantum wells36can be separated by a GaAs barrier layer which can be, for example, 10 nm thick. Using MOCVD, each GaAs or InGaAs layer above can be epitaxially grown at a temperature of about 620° C., and each AlGaAs layer can be grown at a temperature of about 720° C.

To form the PIC28for operation at a wavelength near 1.55 μm, the compound semiconductor substrate14can comprise indium phosphide (InP), with the various layers32,32′,34and each quantum well36being formed from different compositions of indium gallium arsenide phosphide (InGaAsP) and with the exact InGaAsP semiconductor alloy compositions for the layers32,32′ and34being selected so that the refractive index for the layers32and32′ is less than that for the core layer34, and with the InGaAsP composition of each quantum well36being selected to provide an energy bandgap near 1.55 μm. After a distributed Bragg reflector (DBR) grating38has been formed as described hereinafter, an iron-doped InP layer can be regrown over the grating38.

After epitaxial growth of the various layers used to form the photonic integrated circuit28, an etch mask can be formed over the compound semiconductor substrate14in preparation for etching partway down through the upper cladding layer32′ to form the DBR grating38at the location of the bidirectional laser source20. The etching can be performed, for example, using chlorine reactive ion beam etching. The etch mask can be formed photolithographically or by direct e-beam writing as known to the art, with the DBR grating38comprising a plurality of teeth each with a width substantially equal to one-quarter-wavelength of the lasing to be generated with the source20, and with adjacent teeth generally being uniformly spaced by one-quarter-wavelength of the lasing. The teeth can be, for example, 80 nm deep. The bidirectional laser source20can be, for example, 400 μm long and operates single frequency with a linewidth that is preferably on the order of 3 MHz or less, and most preferably 1 MHz or less.

To facilitate formation of the DBR grating38an etch-stop layer (not shown) can be epitaxially grown in the upper cladding layer32′ with a semiconductor alloy composition that is different from the remainder of the layer32′. As an example, when the upper cladding layer32′ comprises AlGaAs, the etch-stop layer can comprise indium gallium phosphide (InGaP), with the InGaP etch-stop layer being, for example, 20 nm thick. The etch-stop layer allows the upper cladding layer32′ to be etched down to a precise depth with the etch-stop layer also providing a passivated surface upon which epitaxial regrowth can take place (e.g. to form a regrown portion40of the upper cladding32′ with a different AlGaAs semiconductor alloy composition). Alternatively, a timed etch can be used to form the DBR grating38, in which case, a passivation layer (e.g. comprising GaAs) can be provided in the upper cladding32′ so that etching of the DBR grating38can terminate at the passivation layer whereupon the epitaxial regrowth can take place with the different AlGaAs semiconductor alloy composition. The epitaxial regrowth can be performed in the same manner as the growth of the cladding layers32and32′ (e.g. at about 720° C. using MOCVD).

Once the DBR grating38has been formed, selected regions of each quantum well36can be locally disordered to reduce the propagation loss of light in the phase modulators22and passive waveguides26to about 10 dB-cm−1or less. This disordering produces a disordered quantum well36′ having a slightly higher energy bandgap which results in a blue-shift of up to 50 nm or more in an absorption edge therein; and this blue-shift in the absorption edge substantially reduces a transmission loss for light within the waveguide core34for the phase modulators22and passive optical waveguides26. In other embodiments of the present invention, another etching step using chlorine reactive ion beam etching can be performed with the etching being timed to etch down through each quantum well36in the phase modulators22and passive waveguides26thereby removing each quantum well36from these elements of the PIC28. The regrown portion40can then be used to cover the remainder of the waveguide core34in the phase modulators36and passive optical waveguides26.

The passive optical waveguides26are used to direct a lasing output, which is generated by the source20and passes through one of the phase modulators22, to an edge of the substrate14so that the lasing output can be directed into the passive ring resonator18. Other passive optical waveguides26receive a portion of the lasing output back from the passive ring resonator18after traversing around the resonator18and direct this portion of the lasing output to one of the waveguide photodetectors24.

The disordering of each quantum well36can be produced, for example, by impurity-free vacancy diffusion which utilizes a silicon dioxide layer deposited over regions of the substrate14wherein each quantum well36is to be disordered. A silicon nitride layer can be deposited over the remainder of the substrate14to protect an upper surface of the compound semiconductor layers during a subsequent rapid thermal annealing step (e.g. at 875–1000° C. for 1–4 minutes) which is used to produce the disordered quantum wells36′. The silicon dioxide and silicon nitride layers can each be up to about 0.5 μm thick and can be deposited by low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD).

During the thermal annealing step, gallium from the compound semiconductor layers diffuses upward into the silicon dioxide layer leaving behind vacancies that then diffuse downward to the quantum wells36where the vacancies produce an intermixing of gallium, indium and aluminum from the quantum wells36and adjoining layers (e.g. the cladding layers32and32′ and any barrier layers). This intermixing changes the semiconductor alloy composition in each disordered quantum well36′ thereby increasing the energy bandgap therein and producing the blue-shift in the absorption edge. In the phase modulators22and passive optical waveguides26wherein the disordered quantum well36is produced, the blue-shift in the absorption edge can be over 50 nm; whereas in areas where the quantum well36is not disordered, the absorption edge is generally shifted to a much smaller extent (≦10 nm).

An alternate process has been developed to allow selective disordering of each quantum well36while utilizing a full-surface overlayer of silicon dioxide. This alternate process utilizes a polymer film instead of the silicon nitride layer for protecting regions of the substrate14wherein each quantum well36is to be preserved from disordering. The polymer film, which can be about up to a few tens of nanometers thick, prevents gallium loss from the epitaxial layers thereby preventing vacancy formation and the resultant intermixing and disordering of the quantum wells36. The polymer film can be deposited in a reactive ion etching system using a plasma containing methyl fluoride (CHF3) and oxygen (O2) A full-surface layer of silicon dioxide (e.g. about 0.5 μm thick) can be initially be blanket deposited over the substrate14. A photolithographically-patterned etch mask can then be formed over portions of this silicon dioxide layer which are to be removed to expose the underlying compound semiconductor layers at locations wherein each quantum well36is to be protected from disordering. The substrate14can then be placed into the reactive ion etching system and the polymer film can be blanket deposited over the entire substrate14from the CHF3/O2plasma. Another thin (e.g. 100 nm thick) layer of silicon dioxide can then be deposited over the entire substrate14to prevent surface decomposition when the annealing step described above is carried out to selectively disorder each quantum well36in the phase modulators22and passive optical waveguides26.

An ion implanted region42can be formed proximate to one or both ends of each phase modulator22as needed to electrically isolate the phase modulator22from the adjacent laser source20and from the passive optical waveguides26. The ion implanted region42can be formed by implanting hydrogen ions (i.e. protons) or oxygen ions at an energy sufficient to penetrate through at least a majority of the thickness of the upper cladding32′, and if needed through the waveguide core34and into the lower cladding32.

The phase modulators22, which operate under reverse biasing and which can be modulated at a frequency of up to about 1 GigaHertz (GHz), can be, for example, 150–600 μm long. An applied reverse-bias voltage to each phase modulator22produces an electric field across the waveguide core34thereby changing the refractive index therein slightly, and this generates a phase shift for the lasing output passing through the phase modulator22. By modulating the applied reverse-bias voltage, the lasing output can be phase modulated (e.g. to produce a sawtooth serrodyne modulation). Since each quantum well36has been disordered or etched away within the phase modulators22, any absorption of the lasing output is substantially reduced. Each phase modulator22preferably has a voltage-phase figure-of-merit of about 25 degrees-volt−1-mm−1or better, and provides more than a 2π phase shift of the lasing output.

After disordering or etching away each quantum well36, a rib structure44can be etched down into or through the regrown portion40of the upper cladding32′ as shown inFIG. 2B, which is a schematic cross-section view along the section line2—2inFIG. 2A. This etching step can also be performed using chlorine reactive beam etching. The rib structure44provides lateral mode confinement to form a single-mode optical waveguide for the various elements of the PIC28including the bidirectional laser source20, phase modulators22, waveguide photodetectors24and passive optical waveguides26. After etching the rib structure44, which can be 1–3 μm wide, a low-index spacer layer46of an electrically-insulating material (e.g. silicon dioxide, silicon nitride, or a polymer such as BCB, PMMA or photoresist) can be deposited over the substrate14and etched away over top the active optical elements including the bidirectional laser source20, each phase modulator22and each waveguide photodetector24. A plurality of patterned upper electrodes48can then be formed over each active optical element to provide an electrical connection thereto (e.g. by evaporation or sputtering). A full-surface lower electrode50can be provided on an underside of the substrate14to form a common electrical connection to these active optical elements.

The waveguide photodetectors24can be, for example, 30–50 μm long or more, and can be optically connected to the edge of the substrate14through a passive optical waveguide26as shown inFIG. 1. The waveguide photodetectors24can be operated by applying a reverse-bias voltage of a few volts across the electrodes48and50to absorb any of the lasing output incident therein after traversing around the passive ring resonator18and generate therefrom an electrical output signal. The presence of one or more quantum wells36in each waveguide photodetector24results in a relatively strong absorption of the lasing output therein and provides a relatively high detection efficiency.

In other embodiments of the present invention, a selective oxidation of one or more high-aluminum-content AlxGa1-xAs layers epitaxially grown within the upper cladding32′, or the lower cladding32, or both with an aluminum content, x, in the range of 0.8≦x≦1.0 can be used to provide a lateral mode confinement and a lateral current confinement to define the single-mode optical waveguide for the bidirectional laser source20, phase modulators22, waveguide photodetectors24and passive optical waveguides26. In this case, the rib structure44can be made several microns wider than the single-mode optical waveguide to be formed, and can be etched down to expose the high-aluminum-content AlxGa1-xAs layer(s) on both sides of the rib structure44. Exposure of the high-aluminum-content AlxGa1-xAs layer(s) to an elevated temperature in the range of 350 to 500° C. and to a high humidity environment can then be used to selectively oxidize the high-aluminum-content AlxGa1-xAs layer(s) from each side inward over time and to leave an unoxidized portion of the high-aluminum-content AlxGa1-xAs layer(s) that can be about 1–3 μm wide. This converts the high-aluminum-content AlxGa1-xAs layer(s) to an aluminum oxide (e.g. Al2O3) while not substantially altering the remainder of the cladding layers32and32′ or the regrown layer40which have a lower aluminum content (i.e. AlxGa1-xAs with 0.3≦x≦0.6). The aluminum oxide provides a reduced index of refraction compared to the unoxidized AlxGa1-xAs and this lateral index step across the width of the various elements of the PIC28defines a single-mode optical waveguide therein.

The high humidity environment can be produced, for example, by flowing nitrogen gas through water heated to about 80–95° C. to entrain water vapor, and then directing the resultant moisture-laden gas into the presence of the heated substrate14. The exact time required to oxidize each high-aluminum-content AlxGa1-xAs layer, which will depend on the temperature to which the substrate14is heated, the thickness of each high-aluminum-content AlxGa1-xAs layer, and the lateral extent to which each high-aluminum-content AlxGa1-xAs layer is to be oxidized, can be learned by practice of the present invention. After the oxidation step, the low-index spacer layer46can be deposited over the compound semiconductor substrate14, and the electrodes48and50can be formed on each side of the substrate14as described previously to complete the PIC28which can be about 3×8 millimeters in size.

As shown inFIG. 1, the photonic integrated circuit28can also include a plurality of alignment waveguides52which can be used to optically align the compound semiconductor substrate14to the silicon, glass or quartz substrate12immediately prior to attaching these two substrates together edge-to-edge. The alignment waveguides52can be formed identically to the passive optical waveguides26with one or more disordered quantum wells36′ therein. Although not shown inFIG. 1, each passive optical waveguide26and each alignment waveguide can be tilted slightly (up to about 10 degrees) with respect to the edge of the compound semiconductor substrate14to reduce a back reflection of the lasing light at the edge of the substrate14.

Light from an external source (e.g. a laser, light-emitting diode, lamp or fiber optic light source) can be directed through one or more of the alignment waveguides52on each side of the compound semiconductor substrate14and therefrom through a plurality of identically-spaced alignment waveguides52′ formed with a silicon nitride core62and silica cladding64on the substrate12. By detecting the amount of light transmitted through the alignment waveguides52and52′ with one or more external photodetectors (not shown) the waveguides52and52′ can be moved relative to each other until they are precisely aligned. This ensures that the passive optical waveguides26on the compound semiconductor substrate14will also be precisely aligned to corresponding single-mode optical waveguides formed on the silicon, glass or quartz substrate12. The two substrates12and14can then be permanently attached together using a UV-cured epoxy16. The alignment of the two substrates12and14can be performed using a commercial automated alignment system which can also include the external light source and detector and one or more computer-controlled multi-axis stages for holding the substrates12and14and aligning one of the substrates12or14relative to the other substrate.

In lieu of the alignment waveguides52and52′ used with an external light source and photodetector as described above or in combination therewith, an internal alignment laser56and a pair of alignment photodetectors58can be provided on the compound semiconductor substrate12. The alignment laser56and the alignment photodetectors58can be formed identically to the laser source20and waveguide photodetector24described previously, and at the same time. The alignment laser56can be activated with an electrical current to provide a lasing output into a curved alignment waveguide60which couples the lasing to an edge of the compound semiconductor substrate14and therefrom through another curved alignment waveguide60′ formed on the substrate12and then back to the alignment photodetector58. Each alignment photodetector58can be used to measure the amount of the lasing output as the two substrates12and14are moved relative to each other until the detected lasing output is maximized at which point the two substrates12and14can be permanently attached together as described previously.

In the first example of the present invention inFIG. 1, the passive ring resonator18can be formed on the silicon, glass or quartz substrate12with a waveguide core62comprising silicon nitride (or alternately doped silica, a polymer or a high-index glassy material such as Ta2O5, Hf2O5or TiO2) surrounded by a waveguide cladding64comprising silica (seeFIG. 3A). The term “silica” as used herein can refer to silicon dioxide (SiO2) or to any silicate glass including TEOS (deposited from the decomposition of tetraethylortho silicate by chemical vapor deposition at about 750° C.) and borophosphorous silicate glass (also termed BPSG). The term “silicon nitride” as used herein is intended to refer to stoichiometric silicon nitride (Si3N4), to non-stoichiometric silicon nitride (i.e. SixNy), and also to silicon oxynitride (SixNyOz). The use of silicon nitride with a refractive index of about 2 in combination with silica with a refractive index of about 1.45 provides a relatively large refractive index difference Δn which allows the formation of single-mode optical waveguides with a relatively small mode size as indicated by the dashed ellipse inFIG. 2A, and also with relatively low radiation and bending losses.

InFIG. 1, the single-mode optical waveguides are used to form a number of passive optical devices on the silicon, glass or quartz substrate12forming the passive ring resonator18including the alignment waveguides52′ and60′, a pair of input optical waveguides54for receiving the lasing output from each phase modulator22, a pair of output optical waveguides54′ optically coupled to the waveguide photodetectors24, a 2×2 evanescent waveguide coupler66, and a pair of optical splitters68located between the 2×2 evanescent waveguide coupler66and the input and output optical waveguides54and54′. The input and output optical waveguides54and54′ receive the lasing output (also termed lasing light) at the edge of the substrate12and couple the lasing output through the optical splitters68and coupler66to and from the coiled optical waveguide72. The input and output optical waveguides54and54′ preferably have a very low backscatter at the edge of the substrate12; and this can be achieved either by antireflection coating the ends of the waveguides54and54′ or alternately by tilting the waveguides54and54′ by a few degrees (e.g. up to about 10 degrees) with respect to the edge of the substrate12. Each of the above optical devices on the substrate12can have a waveguide structure as will be described in detail hereinafter.

Furthermore, an adiabatic mode-matching region70can be optionally provided on the substrate12at a termination of each input and output optical waveguide54and54′, and also at the termination of each optional alignment waveguide52′ and60′. The adiabatic mode-matching regions70facilitate mode matching with the PIC28which generally has a slightly larger mode profile for the lasing output than exists on the silicon, glass or quartz substrate12.

To form the various passive optical devices on the substrate12, an initial silica layer, which can be about 2-μm-thick, can be deposited or grown over the substrate12. In the case of a silicon substrate12, this can be done a conventional thermal steam oxidation process whereby an exposed surface of the silicon substrate12is oxidized and converted to silica in a steam ambient at atmospheric pressure or above, and at a high temperature of up to about 1200° C. Once the initial layer of silica is formed on the substrate12, an additional 1.5–3 μm thickness of silica can then be deposited over the substrate12(e.g. TEOS deposited by a plasma-enhanced CVD process). These two layers of silica form a lower portion of the waveguide cladding64.

A layer of silicon nitride about 0.12–0.15 μm thick can then be blanket deposited over the substrate12by LPCVD. The silicon nitride layer can be patterned by reactive ion etching to form the waveguide core62for each passive optical device being formed on the substrate12, with the waveguide core62being, for example, 0.8 μm wide for use at 980 nm or 1.2 μm wide for use at 1.55 μm. In other embodiments of the present invention, doped silica, a polymer or a high-index glassy material such as Ta2O5, Hf2O5or TiO2can be used for the waveguide core62in place of the silicon nitride.

Each optical waveguide formed on the silicon, glass or quartz substrate12operates in a single mode due to a form birefringence produced by a high aspect ratio of width to thickness of the waveguide core62. This ensures that the waveguide core62in a coiled optical waveguide72of the passive ring resonator18and other passive optical devices formed on the substrate12transmits the lasing output from the laser source20which is generated in a fundamental mode having a transverse-electric (TE) polarization state, and that the waveguide core62suppresses (i.e. attenuates) the transmission of the lasing output in any other mode including a fundamental mode having a transverse-magnetic (TM) polarization state. The high aspect ratio of the waveguide core62further acts to prevent any conversion from the TE polarization state to the TM polarization state and any uncontrolled movement between these two polarization states by providing a significantly different modal effective refractive index and optical phase velocity for these two polarization states.

Once the waveguide core62has been formed, an additional 3.5–4 μm of silica can be blanket deposited over the substrate12(e.g. as TEOS) to complete formation of the various passive optical devices on the substrate12. One or more high-temperature annealing steps at 1050–1200° C. for up to a few hours after deposition of the silica and silicon nitride layers can be used to reduce an absorption loss in each of the passive optical devices due to the presence of an H—O bond in the silica and due to an H—N bond in the silicon nitride.

To form each optional adiabatic mode-matching region70as shown inFIGS. 1 and 3B, a channel74can be etched to a uniform depth of up to about 5 μm into the silicon, glass or quartz substrate12prior to formation of the silica cladding64. After the formation of an initial layer of silica by deposition or the thermal steam oxidation process described previously, the initial layer of silica can be planarized to present a flat surface for subsequent depositions of silica and silicon nitride. The use of a planarization step (e.g. by chemical-mechanical polishing) allows the waveguide core62to be located at a uniform height over the entire substrate12. InFIG. 3B, the channel74formed in the substrate12provides added room for an expanded mode profile (shown by the dashed ellipse inFIG. 3B) for the lasing output in the adiabatic mode-matching region70to prevent absorption of the lasing output by the substrate12(e.g. when the substrate12comprises silicon). This expanded mode profile is produced by tapering or narrowing each side of the waveguide core62inward in the adiabatic mode-matching region70. This inward tapering or narrowing of the waveguide core62produces a gradual low-loss expansion of the mode profile with distance towards the edge of the substrate12.

InFIG. 1, the coiled optical waveguide72has a plurality of turns (e.g. 10–100 turns) to provide an overall effective optical path length which can be in the range of 0.5–5 meters with an average diameter for the coil turns of, for example, 15 millimeters, and with a finesse, F, of, for example, F=2 or higher. A waveguide bending loss for this average diameter of the passive ring resonator18has been calculated to be about 0.24 dB/turn. A waveguide crossing76is also required for each turn of the coiled optical waveguide72in order connect an innermost turn of the coiled optical waveguide72to the 2×2 evanescent waveguide coupler66; and each waveguide crossing76can present an additional crossing loss of about 0.25 dB. It is expected that this crossing loss can be further reduced by providing a tapered waveguide crossing76wherein each intersecting optical waveguide forming the crossing76is laterally tapered either inward or outward at the crossing76by up to a few tenths of a micron or more.

In the 2×2 evanescent waveguide coupler66inFIG. 1, equal amounts of the lasing output are coupled into the coiled optical waveguide72of the passive ring resonator18in each direction, and equal amounts of the lasing output are coupled out of the coiled optical waveguide72of the resonator18after propagating around the resonator18in each direction. The coupling provided by the 2×2 evanescent waveguide coupler66can be, for example, 1% in each direction.

Each optical splitter68can be a 2×2 evanescent waveguide splitter formed from two optical waveguides brought close enough together to provide for an evanescent coupling of the lasing output from one optical waveguide to the other optical waveguide similar to the 2×2 evanescent waveguide coupler66. Such a 2×2 evanescent optical splitter68preferably provides a −3 dB coupling for light propagating through the splitter68in each direction. An optical waveguide on one side of the 2×2 evanescent waveguide splitter68is not used and this optical waveguide can be terminated with a curved waveguide section as shown inFIG. 1.

Alternately, each optical splitter68can comprise a 1×2 lateral mode interference (LMI) splitter68which is shown in the schematic plan view ofFIG. 4. The 1×2 LMI splitter68comprises an oversized waveguide interference section which can be, for example, 3–4 μm wide and 25–75 μm long. A portion of the lasing output from the passive ring resonator18enters the 1×2 LMI splitter68from a bottom side thereof inFIG. 4thereby exciting numerous lateral TE modes of the LMI splitter68and producing self imaging as is well known by those skilled in the art. This self imaging then excites a fundamental TE mode in each of the waveguides54exiting from a top side of the 1×2 LMI splitter68, thereby splitting the lasing output equally into two parts to produce a −3 dB coupling in this direction. The lasing output from the phase modulator22can also enter the 1×2 LMI splitter68at the top side thereof and be coupled into the passive ring resonator18with a −3 dB loss.

A plurality of passive ring resonators as described above can be formed on a 6-inch-diameter silicon, glass or quartz wafer and can be cut apart after fabrication to provide each individual passive ring resonator on a substrate12which is about 20 mm square. After the silicon, glass or quartz substrate12is attached to the compound semiconductor substrate14as described previously this assembly can be further attached to a heat sink (e.g. comprising copper) and packaged together with electronic circuitry for operation of the device10in a conventional dual-in-line package (DIP) about 24×50 mm in size.

The electronic circuitry, which can comprise one or more integrated circuit chips, can drive the bidirectional laser source20, provide a bias modulation and a serrodyne modulation having an analog or digital sawtooth waveform with a modulation amplitude corresponding to greater than a 2π phase shift to each phase modulator22, provide a phase sensitive detection of the output signal from each photodetector24and use the detected output signal in a feedback servo loop to drive a voltage controlled oscillator to generate the serrodyne modulation to each phase modulator22. In this way, both a clockwise and a counterclockwise path in the passive ring resonator18can be maintained at resonance so that the rotation rate can be read out from a difference in the serrodyne modulation frequencies applied to each phase modulator22.

A second example of the integrated optic gyroscope10of the present invention is schematically shown in plan view inFIG. 5. In the apparatus10ofFIG. 5, the PIC28can be formed as described previously with reference toFIGS. 1,2A and2B except that the alignment waveguides52and60, the alignment laser56and the alignment photodetector58can be omitted. The PIC28in the example ofFIG. 5is used in combination with a passive ring resonator18′ which comprises a single-mode optical fiber30. The use of the single-mode optical fiber30allows the formation of the passive ring resonator18′ with an arbitrary size and length of the single-mode optical fiber30which can be advantageous to provide a higher sensitivity for the detection of a much smaller rotation rate than is possible with the example ofFIG. 1. However, this increase in sensitivity is at the expense of a device10which is generally expected to be less rugged and compact compared with the totally integrated device10ofFIG. 1.

InFIG. 5, a pair of 1×2 fiber optic splitters80are used to couple each lasing output from the edge of the substrate14into a 2×2 fiber optic splitter82and therefrom into the single-mode optical fiber30. The 2×2 fiber optic splitter82couples about 1–2% of the lasing output into the passive ring resonator18′ formed from the single-mode optical fiber30. After propagating around the passive ring resonator18′ in each direction, a portion of the lasing output is coupled back through the fiber optic splitters80and82and to the waveguide photodetectors24on the compound semiconductor substrate14. The 1×2 fiber optic splitters80can be connected to the compound semiconductor substrate14using an optical adhesive (e.g. a UV-cured epoxy adhesive).

To improve mode matching between the single-mode optical fiber30and the passive optical waveguides26on the compound semiconductor substrate14, an adiabatic waveguide coupler (not shown) can be provided on the substrate14. The adiabatic waveguide coupler can be formed by laterally tapering the waveguide core34in a manner similar to that described with reference toFIG. 3B. This provides a lateral and vertical mode expansion of the lasing output in the compound semiconductor substrate14to more closely match a mode profile for the single-mode optical fiber30.

The apparatus10ofFIG. 5can be operated in the same manner described previously for the first example integrated optic gyroscope10of the present invention. Those skilled in the art will understand that other arrangements for the integrated optic gyroscope10are possible according to the teachings of the present invention. For example, the 2×2 evanescent waveguide coupler66and the optical couplers68formed on the silicon, glass or quartz substrate12can be substituted for the fiber optic splitters80and82inFIG. 5. This can be done, for example, by forming the substrate12as inFIG. 1except with the passive ring resonator18being omitted and with the 2×2 evanescent waveguide coupler66being connected through a pair of optical waveguides54which are terminated with adiabatic mode-matching regions70located at the edge of the substrate12opposite the compound semiconductor substrate14. The optical fiber30ofFIG. 5can then be connected to this edge of the silicon, glass or quartz substrate12to form a device10which is more integrated than that ofFIG. 5while providing a sensitivity higher than that ofFIG. 1. As another example, the optical couplers68can be formed on the compound semiconductor substrate14and coupled to an external fiber optic splitter82which is further optically connected to the optical fiber30. This latter arrangement helps to prevent any back reflection or scattering from the edge of the substrate14and the ends of the fiber optic splitter82connected thereto from being coupled into the passive ring resonator18′.

The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.