Patent Description:
Recent advancements in integrated photonics provided a means for interfacing with fiber optics, demonstrating high degrees of robustness, resulting in reduced size, weight, and power consumption when compared to free-spaced optics. Thus, the integrated photonics platform offers a high-efficiency coupler with the ability to be applied to many different integrated photonics platforms. US Patent No. <CIT> relates to a resonator fiber optic gyroscope including one or more light sources to produce a first light and a second light and an optical fiber resonator. Al's 'Improving coupling efficiency of fiber-waveguide coupling with a double tip coupler' relates to a double-tip coupler that comprises two inversely and laterally tapered waveguides for light coupling between a fiber and a sub-micron silicon nitride waveguide. US Patent No. <CIT> relates to a spot-size converter for coupling light between first and second waveguides respectively. US Patent no. <CIT> relates to a composite optical waveguide is constructed using an array of waveguide cores, in which one core is tapered to a larger dimension, so that all the cores are used as a composite input port, and the one larger core is used as an output port.

The Embodiments of the present invention provide methods and systems for an RFOG according to the independent claims and will be understood by reading and studying the following specification.

In examples, a system and method are provided according to the appended claims.

Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which:.

In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text.

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.

Embodiments provide a high-efficiency fiber-to-waveguide coupler implemented in a resonator fiber-optic gyroscope (RFOG) is described herein. The fiber-to-waveguide coupler includes a multi-layer waveguide structure that matches an integrated photonics mode to the mode supported by the standard optical fiber. The on-chip design of a multi-layer waveguide structure enables ultra-high efficiency fiber-to chip transfer without many of the coupling methodologies which increase the size, cost, and points of error within a traditional RFOG.

The multi-layer photonics interface with on-chip design allows sufficiently high efficiency pass through rates. For an initial hand-off between the fiber mode to the waveguide mode, the design is theoretically simulated to be greater than about <NUM>% and the following adiabatic transition into a single waveguide layer is theoretically simulated to be greater than about <NUM>%.

The on-chip design of the multi-layer waveguide structure enables ultra-high efficiency fiber-to-chip transfer without ball lens-based coupling methodology. The on-chip waveguides direct optical signals, eliminating the need for discrete micro-optic beam splitters to steer optical signals. Furthermore, the waveguide preserves optical signal polarization, eliminating the need for discrete micro-optic polarizers, half waveplates, and Faraday rotators, and the polarization ratio can be improved further by incorporating on-chip polarizing elements into the device design. Because the chip manufacturing is based on standard, micro- and nanofabrication processes, the design can be produced quickly and relatively inexpensively.

<FIG> illustrates a block diagram of one example embodiment of an RFOG with an integrated photonics interface <NUM>. The embodiment of <FIG> is but one RFOG architecture in which the present invention can be implemented.

In the illustrated embodiment, the RFOG with an integrated photonics interface <NUM> includes an optical resonator coil <NUM>, an integrated photonics interface <NUM>, a coherent light source system <NUM>, and a processing system <NUM>.

The optical resonator coil <NUM> has a first port and a second port. The first port and the second port of the optical resonator coil <NUM> are coupled to the integrated photonics interface <NUM>. The integrated photonics interface <NUM> includes integrated waveguides which direct optical signals through the integrated photonics interface <NUM>. One of the integrated waveguides closes the loop of the optical resonator coil <NUM>. Other integrated waveguides provide connections for the first port.

In one embodiment, the optical resonator coil <NUM> is comprised of turns or windings of optical fiber. The first photodetector 106a, the second photodetector 106b, the third photodetector 106c, the fourth photodetector 106d, the fifth photodetector 106e, and the coherent light source system <NUM> are coupled to the integrated photonics interface <NUM>.

Photodetectors convert incident optical signals into electrical signals. The amplitude of the electrical signal generated by the photodetector is linearly related to the intensity of the incident optical signal.

In one embodiment, the processing system <NUM> is a state machine. In another embodiment, the processing system comprises a processor circuitry coupled to memory circuitry. The processing circuitry may be implemented with at least one of a microprocessor, a microcontroller, an application specific integrated circuit, and/or a gate array. The memory circuitry may be implemented by at least one of random access memory, read only memory, Flash memory, magnetic memory such as a hard drive, and/or optical memory such as an optical drive and optical disc. The processing circuitry may execute software and/or firmware stored in the memory circuitry, e.g. to determine angular rate of rotation.

The processing system <NUM> determines the angular rate of rotation of the optical resonator coil <NUM> by processing signals from the coherent light source system <NUM>. The memory may include program instructions which are executed by the processor to determine the angular rate of rotation <NUM> of the optical resonator coil <NUM> about an input axis <NUM>. The angular rate of rotation <NUM> is determined by the difference between the resonant frequencies of the optical resonator in the clockwise and counterclockwise directions. This is measured by locking the frequencies, fcw and fccw, of the clockwise (CW) slave optical signal (or first optical signal) 111a and the counter-clockwise (CCW) slave optical signals 103b (or second optical signal) to the CW and CCW resonances, respectively, of the optical resonator. Thus, the processing system <NUM> uses a frequency difference, Δf, between the frequency, fcw, of the CW slave optical signal and the frequency, fccw, of the CCW slave optical signal to derive the rotation rate output of the optical resonator, e.g. the RFOG with an optical processing system with optical power control. The CW optical power is controlled via photodetector 106d, CW optical Power Servo System 110i and CW VOA 110j, where photodetectors 106d is connected to the CW optical power control port of the Multi-layer Photonics Interface <NUM>. Likewise, The CCW optical power is controlled via photodetector 106e, CCW optical Power Servo System <NUM> and CW VOA <NUM>, where photodetectors 106e is connected to the CCW optical power control port of the Multi-layer Photonics Interface <NUM>.

In the illustrated embodiment, the first optical signal 111a is combined with a master optical signal (or second optical signal) 111b in an optical combiner (combiner) 110i prior to being coupled to the integrated photonics system <NUM>, and then to the optical resonator coil <NUM>. The CCW slave optical signal 103b is also coupled to the integrated photonics system <NUM> and the optical resonator coil <NUM>. Signals representative of fccw and fcw are respectively provided by a CCW resonance tracking servo system <NUM> and a CW resonance tracking servo system <NUM> in the coherent light source system <NUM>. The angular rate of rotation <NUM> about the input axis <NUM> is an angular rate of rotation, Ω, <NUM> where Ω = (λ* Δf * P) / (<NUM> * A), λ is substantially equal to the average wavelength of the clockwise optical signal 103a and the counter-clockwise light signal 103b. Further, A is the area enclosed by the optical resonator coil <NUM> and P is the perimeter of the optical resonator path of the optical resonator coil <NUM> and the multi-layer photonics interface <NUM>, optically connecting light between two fiber ends of coil <NUM>.

The coherent light source system <NUM> generates a clockwise (CW) optical signal 103a and counter-clockwise (CCW) optical signal 103b which are coupled to the optical resonator coil <NUM> by the integrated photonics system <NUM>. In one embodiment, the CW optical signal 103a and the CCW optical signal 103b are linearly polarized. In another embodiment, the linearly polarized CW optical signal 103a and the linearly polarized CCW optical signal 103b are each substantially linearly polarized in one direction, e.g. horizontal or vertical, or P or S polarized. The direction of polarization is ideally the same for the linearly polarized CW optical signal 103a and the linearly polarized CCW optical signal 103b. Polarized CW optical signal 103a' and polarized CCW optical signal 103b' propagate respectively clockwise and counter-clockwise around the optical resonator coil <NUM>.

The integrated photonics system <NUM> transfers optical feedback signals which are converted to electrical feedback signals by the first photodetector 106a, the second photodetector 106b, the third photodetector 106c, the fourth photodetector 106d, and the fifth photodetector 106e. The first photodetector 106a, the second photodetector 106b, the third photodetector 106c, the fourth photodetector 106d, and the fifth photodetector 106e respectively generate a first feedback signal 105a, a second feedback signal 105b, a third feedback signal 105c, a fourth feedback signal 105d, and a fifth feedback signal 105e which are coupled to the coherent light source system <NUM>.

In the illustrated embodiment, the coherent light source system <NUM> comprises a PDH servo system 110a, a CW slave optical source 110c, a master optical source 110b, a CCW slave optical source 110d, a CW resonance tracking servo <NUM>, a CW optical phase lock loop 110e, a CCW optical phase lock loop 110f, a CCW resonance tracking servo system <NUM>, an optical combiner (combiner) 110i, a CW optical power servo system <NUM>, a CCW optical power servo system <NUM>, a CW variable optical attenuator (VOA) 110j, and a CCW VOA <NUM>. The PDH servo system 110a is configured to receive a first feedback signal 105a, and is coupled to the master optical source 110b and the first photodetector 106a.

The CW resonance tracking servo <NUM> is configured to receive the third feedback signal 105c, and is coupled to the CW optical phase lock loop 110e and the third photodetector 106c. The CW optical phase lock loop 110e is coupled to the CW slave optical source 110c and the master optical source 110b. The CCW resonance tracking servo system <NUM> is configured to receive the second feedback signal 105b, and is coupled to the CCW optical phase lock loop 110f and the second photodetector 106b. The CCW optical phase lock loop 110f is coupled to the CCW slave optical source 110d and the master optical source 110b.

The CW optical power servo system <NUM> is configured to receive the fourth feedback signal 105d, and is coupled to the CW VOA 110j and the fourth photodetector 106d. The CCW optical power servo system <NUM> is configured to receive the fifth feedback signal 105e, and is coupled to the CCW VOA <NUM>. The fourth photodetector 106d and the fifth photodetector 106e receive respectively a portion of the CW optical signal 103a' and a portion of the CCW optical signal 103b' circulating in the optical resonator, and convert such optical signals respectively to electrical signals, respectively the fourth feedback signal 105d and the fifth feedback signal 105e. The amplitude of the fourth feedback signal 105d and the fifth feedback signal 105e is indicative of the power levels of respectively the CW optical signal 103a' and the CCW optical signal 103b'. The fourth feedback signal 105d and the fifth feedback signal 105e are respectively received by the CW optical power servo system <NUM> and the CCW optical power servo system <NUM>. The CW optical power servo system <NUM> and the CCW optical power servo system <NUM> are electrical circuits configured to generate electrical signals to control the attenuation of respectively the CW VOA 110j and the CCW VOA <NUM> to maintain a constant power level of the CW optical signal 103a' and the CCW optical signal 103b' propagating in the optical resonator <NUM>. Although, a variable optical attenuator is illustrated for pedagogical purposes herein, other devices such as a variable gain optical amplifier can be used in lieu of a variable optical attenuator.

The optical combiner 110i combines a first optical signal 111a emitted from the CW slave optical source 110c and a second optical signal 111b emitted from the master optical source 110b, and forms the CW optical signal 103a. The CCW slave optical source 110d generates the CCW optical signal 103b.

The first feedback signal 105a and the PDH servo system 110a lock the carrier frequency of the second optical signal 111b emitted by the master optical source 110b to a longitudinal resonant frequency or to a frequency that is offset from a longitudinal resonant frequency of the optical resonator by a fixed frequency (offset frequency). The longitudinal resonant frequency equals q multiplied by a free spectral range (FSR) of the optical resonator, where q is an integer. In some embodiments, the carrier frequency of the second optical signal 111b is locked to q times FSR, or in some embodiments it is locked to (q + ½) * FSR. The difference between the longitudinal resonance frequency (or resonance frequency) and the corresponding carrier frequency is the frequency offset.

The third feedback signal 105c and the CW resonance tracking servo system <NUM> and the CW optical phase lock loop 110e lock the carrier frequency of the first optical signal 111a emitted by the CW slave optical source 110c to a resonant frequency or to a frequency that is offset from a CW resonant frequency of the optical resonator by a fixed offset frequency. The CW resonant frequency equals p multiplied by a free spectral range (FSR) of the optical resonator, where p is an integer. In some embodiments, the carrier frequency of the first optical signal 111a is locked to p times FSR, or in some embodiments it is locked to (p + ½) * FSR.

The second feedback signal 105b, the CCW resonance tracking servo system <NUM>, and CCW optical phase lock loop 110f lock the carrier frequency of the CCW optical signal 103b emitted by the CCW slave optical source 110d to a resonant frequency or to a frequency that is offset from a CCW resonant frequency of the optical resonator by a fixed offset frequency. The CCW resonant frequency equals m multiplied by a free spectral range (FSR) of the optical resonator, where m is an integer. In some embodiments, the carrier frequency of the CCW optical signal 103b is locked to m times FSR, or in some embodiments it is locked to (m + ½) * FSR. In one embodiment, q, p, and m are different integer numbers.

When the CW and CCW carrier frequencies are both locked to offset frequencies, the frequency offsets from optical resonator resonant frequencies are substantially equal; hence, the difference between the carrier frequencies of the first optical signal 111a and the CCW optical signal 103b are equal to Δf as described above, and rotation rate can be derived from Δf as described above.

In one embodiment, the frequency offsets of the first optical signal 111a and the CCW optical signal 103b are substantially zero frequency. In another embodiment, each frequency offset is substantially one half of a free spectral range of the optical resonator. In all embodiments, the first optical signal 111a, the second optical signal 111b, and CCW optical signal 103b are frequency modulated, e.g. by frequency modulating respectively the CW slave optical source 110c, the master optical source 110b, and the CCW slave optical source 110d, to determine optical resonator resonant frequencies. In one embodiment, modulation frequencies for the CW slave optical source 110c, the master optical source 110b, and the CCW slave optical source 110d are all different, so that the CCW resonance tracking servo system <NUM>, the CW resonance tracking servo system <NUM>, and the PDH servo system 110a may distinguish detected optical signals from each of the CW slave optical source 110c, the master optical source 110b, and the CCW slave optical source 110d. The frequency modulation causes each of the first optical signal 111a, the second optical signal 111b, and the CCW optical signal 103b to have a spectrum of frequencies centered about a corresponding carrier frequency.

In embodiments where the carrier frequencies of the first optical signal 111a and the CCW optical signal 103b, respectively emitted by the CW slave optical source 110c and the CCW slave optical source 110d, are locked to a frequency offset of substantially one half free spectral range from a resonant frequency of the optical resonator in the CW and CCW directions respectively, the odd sidebands of the CW slave optical source 110c and the CCW slave optical source 110d are locked onto resonant frequencies in the CW and CCW directions respectively. This condition is substantially the same for measuring rotation rate as the case of locking CW and CCW slave optical source carrier frequencies to CW and CCW resonant frequencies of the optical resonator.

In one embodiment, the carrier frequencies of first optical signal 111a, the second optical signal 111b, and the CCW optical signal 103b are controlled as follows. The carrier frequency of the second optical signal 111b is locked to a resonance or an offset frequency corresponding to the CW direction of the optical resonator as shown in <FIG>. The photodetector PD1 106a detects an incident optical signal, and generates the corresponding first feedback 105a which is provided to the PDH servo system 110a. The PDH servo system 110a is responsive to a frequency component in first feedback signal 105a that is related to the frequency of modulation applied to the carrier frequency of the master optical source 110b. Portions of the second optical signal 111b, emitted by the master optical source 110b, are respectively coupled to the CW optical phase lock loop 110e and the CCW optical phase lock loop 110f. A portion of the first optical signal 111a, emitted by the CW slave optical source 110c, and a portion of the CCW optical signal 103b, emitted by the CCW slave optical source 110d, are respectively coupled to the CW optical phase lock loop 110e and the CCW optical phase lock loop 110f. The carrier frequency of the first optical signal 111a is locked to the carrier frequency of the second optical signal 111b within a tunable difference frequency of f1 by the CW optical phase lock loop 110e. The carrier frequency of the CCW optical signal is locked to the master carrier frequency within a tunable difference frequency of f2 by the CCW optical phase lock loop 110f. A tunable difference frequency is a frequency within the tuning range of the corresponding servo system. The CW resonance tracking servo system <NUM> and the CCW resonance tracking servo system <NUM> control tunable difference frequencies f1 and f2 so that the carrier frequency of the first optical signal 111a is locked to the CW resonance of the optical resonator (or at a frequency offset of substantially one half free spectral range from it) and the carrier frequency of the CCW optical signal 103b is locked to the CCW resonance of the optical resonator (or to a frequency offset of substantially one half free spectral range from it). The tunable difference frequencies f1 and f2 are controlled such that the desirable offset of the CW and CCW carrier frequencies from resonance (e.g., substantially zero or substantially one half free spectral range) are maintained during rotation rate changes, and changing environmental conditions, e.g. temperature and vibrations, that can cause the optical resonator resonances to shift over time.

As shown in <FIG>, the first optical signal 111a emitted from the CW slave optical source 110c and the second optical signal 111b emitted from the master optical source 110b are combined in optical combiner 110i prior to being coupled to the integrated photonics system <NUM>. Optical combiner 110i may be a bulk optic beam splitter, or a fiber optical direction coupler, or another waveguide directional optical coupler.

In one embodiment, the master optical source 110b, the CW slave optical source 110c and the CCW slave optical source 110d each respectively comprise a LASER. Each optical source may include two or more optical outputs provided, e.g., to the optical resonator and optical phase lock loop(s) and implemented with an optical splitter. One or more optical sources may include a phase modulator to frequency modulate respective LASER(s). One or more optical sources may include intensity (or amplitude) modulators to compensate for phase modulation to amplitude modulation noise in phase modulator(s), to equalize the amplitude levels of the baseband component emitted by the slave optical sources, and to stabilize the second optical signal 111b emitted by the master optical source 110b. Each optical source may include optical isolators to prevent leakage of undesired signal into such sources. In another embodiment, the PDH servo system 110a, the CW resonance tracking servo system <NUM>, and the CCW resonance tracking servo system <NUM> are respectively implemented with electronic circuitry.

The coherent light source system <NUM>, or components therein, may include components not shown in <FIG>. Such components may be used to amplitude and phase modulate optical signals, and to amplify or split optical signals. In one embodiment, such components include variable optical attenuator(s), intensity modulator(s), phase modulator(s), optical amplifier(s), optical isolator(s), and optical passive device(s).

<FIG> illustrates a detailed view of a fiber-to-waveguide optical coupler <NUM> further exemplified in application number <CIT>. In some embodiments, the integrated photonics interface shares similar structural anatomy to the fiber-to-waveguide optical coupler. The optical coupler <NUM> comprises a waveguide structure <NUM> surrounded by and embedded in a cladding <NUM>. The waveguide structure <NUM> includes a first waveguide layer <NUM>, and a second waveguide layer <NUM> separated from the first waveguide layer <NUM> by a predetermined distance. In some embodiments, more than two waveguide layers may be used, with each subsequent waveguide using either the structure of the first waveguide layer <NUM> or the structure of the second waveguide layer <NUM> as described below.

The first waveguide layer <NUM> has a proximal end <NUM> and a distal end <NUM>. The first waveguide layer <NUM> includes a first pair of waveguides <NUM>, <NUM> that extend from the proximal end <NUM> along a first portion of the first waveguide layer <NUM>. The first pair of waveguides <NUM>, <NUM> each widen along a second portion of the first waveguide layer <NUM> such that the first pair of waveguides <NUM>, <NUM>, merge into a single waveguide <NUM> toward the distal end <NUM>.

The second waveguide layer <NUM> has a proximal end <NUM> and a distal end <NUM>. The second waveguide layer <NUM> includes a second pair of waveguides <NUM>, <NUM> that extend from the proximal end <NUM> along a first portion of the second waveguide layer <NUM>. The second pair of waveguides <NUM>, <NUM> each narrow along a second portion of the second waveguide layer <NUM> to respective distal tips <NUM>, <NUM> at the distal end <NUM>.

The waveguide structure <NUM> is configured to couple an optical fiber <NUM> to an integrated photonics platform, such that an integrated photonics mode is matched to a fiber mode supported by an optical fiber <NUM>.

The first and second waveguide layers <NUM>, <NUM> can be composed of various higher index optically transmissive materials, such as silicon, silicon nitride (SiNx), silicon oxynitride (SiON), silicon carbide (SiC), diamond, silicon germanium (SiGe), germanium, gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), lithium niobite (LiNbO<NUM>), titanium dioxide (TiO<NUM>), or combinations thereof.

The cladding <NUM> can be composed of various lower index materials, such as silicon dioxide (SiO<NUM>), silicon oxynitride (SiON), zinc oxide (ZnO) (used with Si waveguide or other similarly high index waveguide), aluminum oxide (Al<NUM>O<NUM>), calcium fluoride (CaF<NUM>), or combinations thereof.

The first pair of waveguides <NUM>, <NUM> can each have a thickness of about <NUM> to about <NUM>, and the second pair of waveguides <NUM>, <NUM> can each have a thickness of about <NUM> to about <NUM>.

<FIG> illustrates a detailed view of a fiber-to-waveguide optical coupler 200a. In some embodiments, the integrated photonics interface shares similar structural anatomy to the fiber-to-waveguide optical coupler. The optical coupler 200a comprises a waveguide structure 202a surrounded by and embedded in a cladding 204a. The waveguide structure 202a includes a first waveguide layer 210a, and a second waveguide layer 220a separated from the first waveguide layer 210a by a predetermined distance.

The first waveguide layer 210a has a proximal end 212a and a distal end 214a. The first waveguide layer 210a includes a first pair of waveguides 216a, 218a that extend from the proximal end 212a along a first portion of the first waveguide layer 210a. In some embodiments, the first pair of waveguides 216a, 218a each narrow along a second portion of the first waveguide layer 210a to respective distal tips 217a, 219a at the distal end 214a.

The second waveguide layer 220a has a proximal end 222a and a distal end 224a. The second waveguide layer 220a includes a second pair of waveguides 226a, 228a that extend from the proximal end 222a along a first portion of the second waveguide layer 220a. In some embodiments, the second pair of waveguides 226a, 228a each narrow along a second portion of the second waveguide layer 220a to respective distal tips 227a, 229a at the distal end 224a.

The waveguide structure 220a is configured to couple an optical fiber 240a to an integrated photonics platform, such that an integrated photonics mode is matched to a fiber mode supported by an optical fiber 240a.

In some embodiments, the first pair of waveguides 216a, 218a and the second pair of waveguides 226a, 228a do not narrow along the second portion of the respective waveguide layers 210a, 220a. In some embodiments, the first pair of waveguides 216a, 218a comprise a first pair of waveguides for a second fiber-to-waveguide optical coupler. Similarly, the second pair of waveguides 226a, 228a comprise a second pair of waveguides for a second optical coupler. At the distal end 214a, the first pair of waveguides 216a couple to a second optical fiber opposite the optical fiber 240a. Similarly, at the distal end 224a, the second pair of waveguides 226a couple to the second optical fiber opposite the optical fiber 240a. Thus, the optical fiber 240a and the second optical fiber optically couple along the first pair of waveguides 216a, 218a and the second pair of waveguides 226a, 228a. The two fiber-to-waveguide optical couplers share the first waveguide layer 210a and the second waveguide layer 220a.

The first and second waveguide layers 210a, 220a can be composed of various higher index optically transmissive materials, such as silicon, silicon nitride (SiNx), silicon oxynitride (SiON), silicon carbide (SiC), diamond, silicon germanium (SiGe), germanium, gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), lithium niobite (LiNbO<NUM>), titanium dioxide (TiO<NUM>), or combinations thereof.

The cladding 204a can be composed of various lower index materials, such as silicon dioxide (SiO<NUM>), silicon oxynitride (SiON), zinc oxide (ZnO) (used with Si waveguide or other similarly high index waveguide), aluminum oxide (Al<NUM>O<NUM>), calcium fluoride (CaF<NUM>), or combinations thereof.

The first pair of waveguides 216a, 218a can each have a thickness of about <NUM> to about <NUM>, and the second pair of waveguides 226a, 228a can each have a thickness of about <NUM> to about <NUM>.

<FIG> is an example graphical representation of an integrated photonics interface <NUM>. In the example, the integrated photonics interface <NUM> comprises four waveguides: a first waveguide <NUM>, a second waveguide <NUM>, a third waveguide <NUM>, and a fourth waveguide <NUM> integrated into one or more layers of the integrated photonics interface <NUM>. The integrated photonics interface <NUM> can be a multi-layer photonics interface where each layer comprises one or more waveguides. The orientation and scale of the waveguides on the integrated photonics interface are scaled to exemplify the features of the device, and should not be considered the actual orientation of the waveguides within the integrated photonics interface <NUM>. In some examples, the waveguides comprise a high index optically transmissive material.

In the example, the first waveguide <NUM> comprises a first port <NUM> and a second port <NUM>. The waveguides of the integrated photonics interface <NUM> are configured to preserve the optical signal polarization and to guide the optical signal with little signal loss. In the example, the first port <NUM> and the second port <NUM> of the first waveguide <NUM> optically couple to an optical resonator coil, a fiber resonator coil, or another type of resonator. The optical couplings at the first port <NUM> and the second port <NUM> of the first waveguide <NUM> are configured preserve the signal polarization and preserve greater than <NUM>% of the signal. In some examples, connections at the first port <NUM> and the second port <NUM> comprise an integrated photonics interface as depicted in <FIG>.

In the example, the second waveguide <NUM> comprises a first port <NUM> and a second port <NUM>. In the example, the first port <NUM> and the second port <NUM> of the second waveguide <NUM> are a first beam transmission port and a second beam transmission port respectively. In some examples, the first port <NUM> and the second port <NUM> of the second waveguide <NUM> each optically couple to one or more photodetectors within an RFOG. In the example given in <FIG>, the first port <NUM> couples to the second photodetector 106b and the second port <NUM> couples to the third photodetector 106c. The second waveguide <NUM> is configured to be optically coupled <NUM> to the first waveguide <NUM>. The optical coupling <NUM> is configured to allow a portion of the light transmitted through the first waveguide <NUM> through to the second waveguide <NUM>. In some examples, connections at the first port <NUM> and the second port <NUM> comprise an integrated photonics interface as depicted in <FIG>.

In the example, the third waveguide <NUM> comprises a first port <NUM> and a second port <NUM>. In the example, the first port <NUM> and the second port <NUM> of the third waveguide <NUM> are a first beam reflection port and a Multi-frequency laser source port respectively and optically couple to the Multi-frequency laser source. In some examples, the first port <NUM> of the third waveguide <NUM> each optically couples to a photodetector within an RFOG. The third waveguide <NUM> is configured to be optically coupled <NUM> to the first waveguide <NUM>. The optical coupling <NUM> is configured to allow a portion of the light transmitted through the third waveguide <NUM> through to the first waveguide <NUM> and thus the attached resonator coil. In some examples, connections at the first port <NUM> and the second port <NUM> comprise an integrated photonics interface as depicted in <FIG>.

In the example, the fourth waveguide <NUM> comprises a first port <NUM> and a second port <NUM>. In the example, the first port <NUM> and the second port <NUM> of the fourth waveguide <NUM> are a Multi-frequency laser source port and a second beam reflection port respectively and optically couple to the Multi-frequency laser source. In some examples, the second port <NUM> of the fourth waveguide <NUM> each optically couples to a photodetector within an RFOG. The fourth waveguide <NUM> is configured to be optically coupled <NUM> to the first waveguide <NUM>. The optical coupling <NUM> is configured to allow a portion of the light transmitted through the third waveguide <NUM> through to the first waveguide <NUM> and thus the attached resonator coil. In some examples, connections at the first port <NUM> and the second port <NUM> comprise an integrated photonics interface as depicted in <FIG>.

The waveguides <NUM>, <NUM>, <NUM>, <NUM> of the integrated photonics interface <NUM> can be composed of various higher index optically transmissive materials, such as silicon, silicon nitride (SiNx), silicon oxynitride (SiON), silicon carbide (SiC), diamond, silicon germanium (SiGe), germanium, gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), lithium niobite (LiNbO<NUM>), titanium dioxide (TiO<NUM>), or combinations thereof.

The waveguides <NUM>, <NUM>, <NUM>, <NUM> of the integrated photonics interface <NUM> can each have a thickness of about <NUM> to about <NUM>, and the second pair of waveguides <NUM>, <NUM> can each have a thickness of about <NUM> to about <NUM>.

<FIG> illustrates an exemplary method <NUM> of operating an RFOG with an integrated photonics interface. To the extent the method <NUM> shown in <FIG> is described as being implemented in the system shown in <FIG> and <FIG>, it is to be understood that other embodiments can be implemented in other ways. The blocks of the flow diagram have been arranged in a generally sequential manner for ease of explanation; however, it is to be understood that this arrangement is merely exemplary, and it should be recognized that the processing associated with the methods (and the blocks shown in the figures) can occur in a different order (for example, where at least some of the processing associated with the blocks is performed in parallel and/or in an event-driven manner).

In block <NUM>, receive at an integrated photonics interface a first optical signal and a second optical signal, e.g. from the coherent light source system <NUM>. The first optical signal and the second optical signal are received at separate waveguides within the integrated photonics interface. In some examples, the waveguides are on the same layer of the integrated photonics interface, in other examples, the waveguides are on separate layers.

In block <NUM>, inject a portion of the first optical signal into a first waveguide within the integrated photonics interface, the first waveguide optically coupled to an optical resonator, so that the first optical signal propagates in a first direction through the optical resonator. In block <NUM>, inject a portion of the second optical signal into the first waveguide within the integrated photonics interface so that the second optical signal propagates in a second direction through the optical resonator which is opposite to the first direction.

Claim 1:
A system for a resonant fiber optic gyroscope, the system comprising:
at least two fiber-to-waveguide optical couplers (<NUM>), each of the at least two fiber-to-waveguide optical couplers having a proximal end (<NUM>, <NUM>) and a distal end (<NUM>, <NUM>), each of the at least two fiber-to-waveguide optical couplers (<NUM>) having a waveguide structure (<NUM>) including,
a first waveguide layer (<NUM>) extending from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>), the first waveguide layer (<NUM>) including a first waveguide branch (<NUM>) that extends from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>) and a second waveguide branch (<NUM>) that extends from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>), and
a second waveguide layer (<NUM>) separated from the first waveguide layer (<NUM>), the second waveguide layer (<NUM>) extending from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>), the second waveguide layer (<NUM>) including a including a third waveguide branch (<NUM>) that extends from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>) and a fourth waveguide branch (<NUM>) that extends from the proximal end (<NUM>) to the distal end (<NUM>) of the fiber-to-waveguide optical coupler (<NUM>);
an optical resonator coil (<NUM>) comprised of turns or windings of optical fiber, further comprising a first port and a second port, wherein the first port is optically coupled to the proximal end (<NUM>, <NUM>) of a first fiber-to-waveguide optical coupler of the at least two fiber-to-waveguide optical couplers and the second port is optically coupled to the proximal end (<NUM>, <NUM>) of a second fiber-to-waveguide optical coupler of the at least two fiber-to-waveguide optical couplers (<NUM>), and the distal end of the first fiber-to-waveguide optical coupler being optically coupled to the distal end of the second fiber-to-waveguide optical coupler to form at least one waveguide; and
wherein the or each waveguide structure (<NUM>) is configured to match an integrated photonics mode to a fiber mode supported by an optical fiber.