Photonic integrated circuit and interference fiber optic gyroscope

Abstract

To provide a multifunctional photonic integrated circuit (PIC) suitable for the manufacture of a fiber optic gyroscope (FOG).SOLUTION: A multifunctional photonic integrated circuit (PIC) 102 is configured and arranged to indicate a scale factor of substantially high stability and accuracy. The PIC 102 may comprise, for example, a high optical birefringence and low propagation loss waveguide, low wavelength-dependent split-ratio Y-junctions 114, 116, a high extinction ratio linear polarizer 118, and high efficiency fiber-to-waveguide mode-size converters. Considerations for ensuring high-level FOG performance are addressed by, for example, optimization of a waveguide structure, functional requirements for individual components, and combined effects of circuit layout. A high-end, tactical grade FOG may be built using the disclosed PIC, after connecting to a polarization maintaining optical fiber coil, a light source, and a photodetector.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Patent Application No. 62 / 744,505, filed October 11, 2018, the entire teachings of which are incorporated herein by reference. [Background technology]

[0002] Fiber optic gyroscopes (FOGs), accelerometers and FOG-based inertial navigation systems (INSs) constitute key parts of an integrated sensor system essential for the performance of high-precision autonomous vehicles (such as automobiles). The INSs are used to sense the vehicle's position and orientation. For vehicle position estimation, vehicles may use a combination of a Global Positioning System (GPS) and an INS. The accuracy of the GPS system has improved significantly since 2000. The GPS system identifies signals that facilitate an overall mean user range error (URE) of less than 7.8 m 95% of the time. GPS itself cannot provide sufficient accuracy for position estimates for autonomous vehicles, although the actual performance often exceeds the specifications. On the other hand, GPS provides good long-term stability in position measurements. In contrast, an INS can be highly accurate in the short term, but suffers from long-term drift in position measurements.

[0003] GPS and INS are sometimes used in combination to overcome the disadvantages of each individual location sensing system. This combined operation becomes even more important in autonomous vehicle applications, since GPS accuracy degrades near buildings, bridges, and trees. In such environments, the GPS receiver may not be able to acquire signals from a sufficient number of satellites, resulting in degraded performance. This is of great concern in urban areas where skyscrapers form "urban canyons" where GPS availability is very limited. INS can be used to improve GPS accuracy and fill performance gaps such as those caused by urban canyons. An INS coupled to a GPS can perform dead reckoning on the current position and calculate orientation and speed continuously without the need for an external reference.

[0004] For vehicle attitude estimation, INS sensors can be fused with GPS signals to instantly detect the roll, pitch and heading of a moving vehicle. Accurate determination of rotation angles is a critical task for autonomous vehicle control systems to ensure safety, which requires highly accurate and stable scale factors of the gyroscopes associated with the autonomous vehicle. Summary of the Invention

[0005] Some embodiments described herein are directed to an Interferometric Fiber Optic Gyroscope (IFOG). Example embodiments of an IFOG, as described herein, can be configured to exhibit a scale factor having stability above a predetermined stability threshold and accuracy above a predetermined accuracy threshold.

[0006] Other embodiments described herein are directed to IFOG components such as photonic integrated circuits (PICs) based on highly polarization-maintaining (PM) waveguides with a Si3N4 core and cladding material including SiO2. Passive functional FOG components may be integrated into the PIC. Such components may include low-loss couplers used in Sagnac interferometers and source / detector transceivers, high polarization extinction ratio (PER) polarizers, high-efficiency fiber-coupled mode size converters, and one or more features for stray light elimination.

[0007] In one aspect, the invention can be a photonic integrated circuit (PIC) comprising a polarizer and an interferometric Y-junction. The polarizer can comprise a polarizer waveguide and have a first polarizer port and a second polarizer port. The polarizer can be configured to have a polarization extinction ratio (PER) of at least 75 dB and a birefringence of at least 0.012 within the operating wavelength range of the PIC. The interferometric Y-junction can have a base waveguide, a first branch waveguide, and a second branch waveguide that are polarization-maintaining (PM), have a predetermined length difference relative to each other, and can be configured to maintain a single transverse mode of light.

[0008] In one embodiment, the H-parameter of each of the polarizer waveguide, the base waveguide, the first branch waveguide and the second branch waveguide is less than −40 dB / m.

[0009] In other embodiments, the difference in waveguide length between the first branch waveguide and the second branch waveguide may be greater than the depolarization lengths of the first branch waveguide and the second branch waveguide. The polarizer waveguide, the first branch waveguide, and the second branch waveguide may each comprise a Si3N4 core surrounded by a SiO2 cladding material. The core may have a width-to-height aspect ratio of at least 50. The waveguide may comprise a plurality of waveguide bends optically connected in series. The plurality of waveguide bends may comprise a first 180 degree bend, a second 180 degree bend, and at least one additional bend of at least 90 degrees.

[0010] An embodiment may further include at least one trench formed along a side of the waveguide, the trench being configured to deflect light emitted from the waveguide away from the PIC to prevent stray light from coupling into the PIC.

[0011] An embodiment may further comprise a first connector configured to be connected to a light source, a second connector configured to be connected to a light detector, a third connector configured to be connected to a first port of the fiber coil, a fourth connector configured to be connected to a second port of the fiber coil, and a source-Y junction having a base waveguide, a first branch waveguide and a second branch waveguide. The first branch waveguide of the source-Y junction may be optically coupled to the first connector. The second branch waveguide of the source-Y junction may be optically coupled to the second connector. The base waveguide of the source-Y junction may be optically coupled to the first polarizer port. The first branch waveguide of the interferometric Y junction may be optically coupled to the third connector. The second branch waveguide of the interferometric Y junction may be optically coupled to the fourth connector. The base waveguide of the interferometric Y junction may be optically coupled to the second polarizer port.

[0012] The base and branch waveguides may have a birefringence of at least 0.012. The base and branch waveguides may have a propagation loss of less than 0.5 dB / cm. The base and branch waveguides may comprise thin Si3N4 waveguides having a core thickness of 10 nm to 100 nm and a core width of 1 micron to 10 microns. The polarizer waveguide may comprise three 180 degree curves joined in series with each other to form an m-shape.

[0013] In another aspect, the invention may be a photonic integrated circuit (PIC) comprising a first connector configured to be optically coupled to a light source, a second connector configured to be optically coupled to a light detector, a third connector configured to be optically coupled to a first port of a fiber coil, a fourth connector configured to be optically coupled to a second port of the fiber coil, a first coupler having a first branch port, a second branch port and a common base port, a second coupler having a first branch port, a second branch port and a common base port, and a polarizer having a first polarizer port and a second polarizer port and comprising a polarizer waveguide. The first branch port of the first coupler may be optically coupled to the first connector via the first waveguide, the second branch port of the first coupler may be optically coupled to the second connector via the second waveguide, and the common base port of the first coupler may be optically coupled to the first polarizer port via the third waveguide. The first branch port of the second coupler may be optically coupled to the third connector via a fourth waveguide, the second branch port of the second coupler may be optically coupled to the fourth connector via a fifth waveguide, and the common base port of the second coupler may be optically coupled to the second polarizer port via a sixth waveguide. The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide may each have a birefringence of at least 0.012, each have a propagation loss of less than 0.5 dB / cm, and each be configured to support a single transverse mode of light.

[0014] The sixth waveguide may be longer than a depolarization length associated with the sixth waveguide. A difference between the fourth and fifth waveguide lengths may be longer than a depolarization length associated with the fourth and fifth waveguides. The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide and the sixth waveguide may each have a width-to-height aspect ratio of at least 50.

[0015] The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide may each comprise a Si3N4 core surrounded by a SiO2 cladding material. The waveguide may comprise a plurality of waveguide bends optically connected in series. The plurality of waveguide bends may comprise a first 180 degree bend, a second 180 degree bend, and at least one additional bend of at least 90 degrees.

[0016] The polarizer waveguide may comprise three 180 degree curves joined together in series to form an "m" shape.

[0017] In another aspect, the invention can be an interferometric fiber optic gyroscope (IFOG) that can include a photonic integrated circuit (PIC), a sensing fiber coil, and a phase modulator. The PIC can include a polarizer having a polarization extinction ratio (PER) of at least 75 dB and a birefringence of at least 0.012. The PIC can further include a waveguide that is polarization-maintaining and configured to maintain a single transverse mode of light. The sensing fiber coil can have a first end and a second end. The phase modulator can be disposed at the first end of the sensing fiber coil.

[0018] In another aspect, the invention may be an interferometric fiber optic gyroscope (IFOG) comprising a photonic integrated circuit (PIC), a sensing fiber coil having a first end and a second end, and a phase modulator disposed at the first end of the sensing fiber coil. The PIC may further comprise a first connector configured to be optically coupled to a light source, a second connector configured to be optically coupled to a photodetector, a third connector configured to be optically coupled to a first port of the fiber coil, a fourth connector configured to be optically coupled to a second port of the fiber coil, a first coupler having a first branch port, a second branch port, and a common base port, a second coupler having a first branch port, a second branch port, and a common base port, and a polarizer having a first polarizer port and a second polarizer port and comprising a polarizer waveguide. The first branch port of the first coupler may be coupled to the first connector via the first waveguide. The second branch port of the first coupler may be coupled to the second connector via the second waveguide. The common base port of the first coupler may be coupled to the first polarizer port via the third waveguide. The first branch port of the second coupler may be coupled to the third connector via the fourth waveguide. The second branch port of the second coupler may be coupled to the fourth connector via the fifth waveguide. The common base port of the second coupler may be coupled to the second polarizer port via the sixth waveguide. The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide and the sixth waveguide may each have a birefringence of at least 0.012, each have a propagation loss of less than 0.5 dB / cm and each be configured to support a single transverse mode of light.

[0019] The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide may each comprise a Si3N4 core surrounded by a SiO2 cladding material. The polarizer waveguide, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide may each have a width-to-height aspect ratio of at least 50.

[0020] The patent or application contains at least one drawing executed in color. Copies of such patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0021] The foregoing will become apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views, and in which: The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the embodiments, and in which: [Brief description of the drawings]

[0022] [Figure 1] 1 illustrates an exemplary embodiment of an interferometric fiber optic gyroscope (IFOG) 100 constructed and arranged in accordance with an embodiment of the present invention. [Figure 2A] 1 shows the TE mode performance compared to the TM mode performance of a polarizer according to an embodiment of the present invention. [Figure 2B] 1 shows the TE mode performance compared to the TM mode performance of a polarizer according to an embodiment of the present invention. [Figure 3A] The mode size of the PIC waveguide and the mode size of the fiber splice point compared to the mode size of the PM fiber are shown. [Figure 3B] The mode size of the PIC waveguide and the mode size of the fiber splice point compared to the mode size of the PM fiber are shown. [Figure 3C] The mode size of the PIC waveguide and the mode size of the fiber splice point compared to the mode size of the PM fiber are shown. [Figure 4] 1 shows a setup for measuring PIC waveguide birefringence and the PER of a polarizer, according to an embodiment of the present invention. [Diagram 5] 1 shows white light interferometry data showing cross-coupling locations at a PIC at a PIC-fiber splice and a fiber-fiber splice, according to an embodiment of the present invention. [Figure 6] The relationship between the PER of the polarizer and the interference intensity between the TE and TM modes is shown for different power ratios between the two modes. [Figure 7A] 1 shows the waveguide birefringence as a function of waveguide core thickness and width. [Figure 7B] The threshold line at which the waveguide begins to support the first higher-order TE mode is shown superimposed on the contour plot of the waveguide birefringence. [Figure 8A] 1 shows the waveguide propagation loss as a function of the waveguide core thickness and width. [Figure 8B] The dependence of the propagation loss on the core width for different core heights is shown. [Figure 9] The mode field versus increasing waveguide core height is shown in Fig. 9(a)-9(c). The mode field versus increasing waveguide core width is shown in Fig. 9(d)-9(f). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] A description of an exemplary embodiment follows.

[0024] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

[0025] An embodiment of the invention may include an interferometric fiber optic gyroscope (IFOG) configured to exhibit a scale factor having stability above a predetermined stability threshold and precision above a predetermined precision threshold. Exemplary embodiments may include IFOG components such as a photonic integrated circuit (PIC) based on a highly polarization-maintaining (PM) single-mode waveguide having a Si3N4 core and a cladding material including SiO2. Passive functional IFOG components may be integrated into the PIC such as low-loss couplers used in the Sagnac interferometer and source / detector transceivers, high polarization extinction ratio (PER) polarizers, high efficiency fiber-coupled mode size converters, and one or more features for stray light elimination.

[0026] The use of ultra-thin (e.g., 100 nm or less) Si3N4 waveguides in PICs offers several advantages. For example, Si3N4 waveguides transmit light in the 830 nm wavelength range, and IFOGs have larger scale factors, such as shorter operating wavelengths, given the specific fiber length and coil size. Also, birefringence as high as 0.02 can be achieved in Si3N4 waveguides, and high birefringence waveguides can provide substantial phase error suppression. Si3N4 waveguides configured with ultra-high aspect ratios can exhibit very low propagation loss. The amorphous nature of Si3N4 facilitates the inclusion of light source couplers in PICs and facilitates circuit layout for geometries conducive to preventing stray light from recombining. Furthermore, Si3N4 is a well-established material in the microelectronics industry, where it is used as an electrical and thermal insulator in electrical circuits. The manufacturing lines associated with Si3N4 processing are very mature and have a known performance history. Si3N4 also features a set of optical properties that make it an ideal choice for many applications requiring the integration of photonic devices onto an integration platform.

[0027] FIG. 1 illustrates an exemplary embodiment of an IFOG 100 constructed and arranged in accordance with the present invention. The IFOG 100 includes a PIC 102, a fiber array 104, a sensing fiber coil 106, a PZT modulator 108, a light source 110, and a photodetector 112. The PIC 102 includes a source Y-junction 114, an interference Y-junction 116, and a polarizer 118. Each Y-junction includes a base waveguide that splits into a pair of branch waveguides. The base and branch waveguides are high PM single mode waveguides. The Y-junctions are also referred to herein as couplers. The waveguides have the properties of being "highly polarization maintaining" by being (i) highly birefringent, and (ii) having low scattering. Specific value ranges for birefringence and scattering are described herein that correspond to specific thresholds for PM.

[0028] As shown in FIG. 1, when the two fiber ends of the sensing fiber coil 106 are connected to the ends of the two branch waveguides of the interferometric Y-junction 116 in the PIC 102 via the fiber array 104, a Sagnac interferometer is formed. A polarizer 118 connects the base waveguide of the source Y-junction 114 to the base waveguide of the interferometric Y-junction 116. In this example embodiment, the polarizer 118 has an "m" shape, but other shapes may alternatively be used. The polarizer 118 polarizes the input light from the light source 110 coming through the source Y-junction 114. The polarizer 118 also filters out cross-coupled erroneous signals in the light returning from the sensing fiber coil 106 by extinguishing the TM modes due to the high PER of the polarizer.

[0029] A phase modulator 108, realized by a fiber-wound oscillating lead zirconate titanate (PZT) disk, is located near one end of the sensing fiber coil 106 and included therein to form a coil / modulation assembly 120. A sinusoidal modulation signal can be applied to the phase modulator 108 in a controlled manner with a modulation / demodulation process suitable for stable Sagnac phase-shift readout.

[0030] The combination of polarizer 118, interferometric Y-junction 116, and coil / modulation assembly 120 constitutes the so-called "minimal configuration" of the IFOG optical system. Light along the clockwise (CW) path and light along the counterclockwise (CCW) path undergo the same one reflection and one transmission at interferometric Y-junction 116, and the Sagnac interferometer is configured to operate with the optical reciprocity required for a very stable IFOG. Light that is input to the Sagnac interferometer and output from the Sagnac interferometer passes through the same polarizer 118 and single mode waveguide. Thus, this exemplary embodiment constitutes a single mode single polarization filter. Any signal that is in a different state than the input (e.g., a different state in either spatial or polarization mode) is undesirable and is substantially rejected by the single mode single polarization filter.

[0031] Mechanisms that generate erroneous non-reciprocal phase shifts can perturb the Sagnac phase shift. The described embodiments act to mitigate or eliminate such mechanisms in PIC designs. Such erroneous phase shift signals can be introduced, for example, by the slight non-reciprocity of the Y-junction in the PIC. The Y-junction effectively operates as a pair of waveguides arranged at an angle to each other and can be considered as a two-mode waveguide near the junction. When the input light from the base waveguide of the Y-junction reaches the coupling region of the Y-junction, the evanescent waves overlap in the two-mode waveguide, the fundamental symmetric mode is converted to a second antisymmetric mode, and the two lobes of the mode field are split and guided almost perfectly into the two branch waveguides. For the two return signals entering the coupling region, the fundamental mode is formed and guided by the base waveguide, while the second antisymmetric mode, which is higher than the cutoff of the waveguide, is radiated to the substrate. Since there is residual loss between the symmetric and antisymmetric modes, light transmitted through the junction in different directions experiences different losses (depending on the particular direction), so spurious phase shifts at the base port of the Y-junction occur due to slight non-reciprocity at the Y-junction. A single-mode single polarizer filter placed at the base waveguide of the interferometric Y-junction operates to equalize the phase of the light waves transmitted in opposite directions. The advantage of the minimal configuration built with the PIC architecture is that the polarizer 118 can be substantially aligned with the interferometric Y-junction 116 with a single-mode waveguide filter in between. If a high PER polarizer and a single-mode waveguide are implemented after the base waveguide of the interferometric Y-junction 116, no further phase error generation occurs at the junction.

[0032] Erroneous phase signals can also arise due to light coupled from the operating polarization mode into its orthogonal mode. The PM fiber and the PIC waveguide each support two polarization modes, and the Sagnac interferometer provides two parallel light paths, and any light coupling into the unwanted polarization mode can generate nonreciprocal parasitic paths. Although the majority of cross-coupling points are in the fiber coil, cross-coupling can also occur at the fiber-PIC waveguide junction and / or within the path in the PIC (e.g., at an interferometric Y-junction). To prevent such nonreciprocal parasitic paths, the polarizer in the single-mode single polarizer filter is constructed to have a substantially high PER.

[0033] On the other hand, the high PER filter cannot eliminate spurious phase errors generated by cross-coupling from the interferometric Y-junction 116 due to two points located at approximately the same distance within the depolarization length of the fiber. In the exemplary embodiment of the IFOG PIC, the high birefringence of the waveguides provides excellent polarization maintenance, so the polarization cross-coupling at the interferometric Y-junction is negligible. Most polarization coupling occurs at the connection point of the coil fiber to the PIC. To avoid phase errors generated at the connection point, one of the branch waveguides of the interferometric Y-junction is configured to be longer than the other branch to provide a fixed phase shifter between the connection points. The length of the phase shifter is configured to be longer than the depolarization length, which is about 1.8 mm in the exemplary embodiment.

[0034] Since the two reflections at the connection interface and the interference Y-junction constitute a Michelson interferometer, the reflections at the two connection points can also generate an erroneous phase signal. Similarly, if the path difference of the two branch waveguides is longer than the decoherence length, the erroneous phase can be avoided. Since the decoherence length of the waveguide in the exemplary embodiment is about 28 μm, the path difference of the two branch waveguides is configured to be at least 28 μm.

[0035] Other erroneous phase signals are characteristic of multifunction optical integrated circuits (MIOCs) fabricated on LiNbO3 substrates. LiNbO3 MIOCs are widely used in commercial FOG products. However, increasing the integration by fitting two Y-junctions into a single circuit has not been successful because the in-plane birefringence of the crystalline LiNbO3 substrate limits the freedom of optical circuit layout, so the two Y-junctions must be arranged in a straight line. Unguided radiation (generated along the base waveguide of the Y-junction due to asymmetric modes from the interfering Y-junction) is received by each of the branch waveguides of the source Y-junction. Incoming parasitic signals from spurious recombination are added to the main signal and result in phase errors. To prevent the source Y-junction from receiving radiation from the antisymmetric modes in the FOG PIC, the waveguides connect the base waveguides configured to form a 180-degree path loop, as shown in Figure 1. In fact, the polarizer 118 connecting the source Y-junction 114 to the interference Y-junction 116 is configured in an "m" shape that allows the two Y-junctions to be oriented in the same direction.

[0036] In an example embodiment, one or more trenches may be etched along with the waveguide of a PIC (not shown) to reduce or eliminate incoming stray light by deflecting the stray light out of the plane, or by absorbing the stray light (e.g., by configuring the trenches to be filled with an absorbing material), or both.

[0037] Ultra-thin Si3N4 waveguides can be used to produce polarizers with high PER and low loss. Very loosely confined waveguides with various combinations of curves and s-shapes can result in high propagation loss in TM mode while keeping TE mode loss low. An exemplary embodiment of a plane polarizer with a half period of s-bend waveguide with a core width of 3.5 μm and a core height of 40 nm shows a PER of 75 dB measured at 1600 nm. This exemplary waveguide has a birefringence of 0.011 to 0.014 at 1550 nm wavelength.

[0038] The polarizer 118 of the exemplary embodiment comprises an ultra-thin Si3N4 waveguide that is loosely confined and has a group birefringence of 0.015 at 830 nm wavelength. 1.5 periods of s-bends that make up an m-shaped bend are included between the two Y-junctions 114, 116. As shown in FIG. 2A, the exemplary embodiment of the m-bend polarizer 118 may exhibit a loss of 0.15 dB in the TE mode and 71.9 dB in the TM mode at 830 nm wavelength after a propagation distance of 8.1 mm along the m-bend waveguide.

[0039] 2B shows the wavelength-dependent loss of the TE and TM modes of the "m-shaped" bent polarizer 118 of the example embodiment. The average loss of the TM mode in the wavelength window between 800 nm and 850 nm is 67.5 dB, while the average loss of the TE mode in the wavelength window is only 0.2 dB. As shown in FIG. 1, the parallel arrangement of the two Y-junctions at the interval of the m-shaped bent polarizer length effectively prevents the remaining part of the PIC from recombining with the non-guided light of the antisymmetric mode emitted from the interfering Y-junction.

[0040] Each individual output port of the PIC is placed in a fiber array 104 that connects to a corresponding external component fiber. In the exemplary embodiment, the fibers are panda-type PM fibers with a mode field diameter of 4.5±0.5 μm. The fibers are aligned to a v-groove substrate with the same pitch as the waveguide array on the PIC. Each fiber is aligned with its slow axis coinciding with the PIC TE mode direction. The PIC waveguide is loosely confined, but still has a smaller mode field size than that of the fiber. The exemplary embodiment utilizes an in-plane inverted taper to adiabatically expand the mode field to an optimized size that facilitates substantial matching to the fiber mode field. The mode field at the fiber connection point is forced into a more circular distribution and is of a larger mode size (as shown in FIG. 3A) than the base PIC waveguide (as shown in FIG. 3B). The degree of modal overlap between the waveguide mode field and the fiber mode field at the junction is η=0.92 (see FIG. 3C). On the other hand, when the fiber is directly coupled to a base PIC waveguide with an elliptical mode distribution (see FIG. 3B), the coupling coefficient is η=0.81.

[0041] Because such a simple structure keeps fabrication costs low, the exemplary embodiment implements an in-plane tapered transition, and the coupling efficiency can be as high as 0.48 dB at an operating wavelength of 850±50 nm.

[0042] Figure 4 shows the setup for the measurement of the PIC waveguide birefringence and the PER of the polarizer 118. The waveguide birefringence is measured using a white light interferometer (WLI) or optical coherence domain polarimetry (OCDP).

[0043] WLI utilizes incoherent light as the light source of a Michelson interferometer, whose two arms correspond to two polarization paths respectively of the PM optical system under investigation. A moving mirror changes the relative path difference between the two polarization modes. Since the light source has a very short coherence length, an interference peak appears at the position where cross-polarization coupling occurs when the moving mirror scans evenly along the optical path of the fixed arm. The intensity and position of the cross-coupling point are measured, which reveals the cross-polarization coupling, component loss and waveguide birefringence information of the optical system. The same light source, which is a pigtailed superluminescent diode (SLD), is used for both WLI measurement and IFOG light source 110, so the measurement is accurate and practical. The group birefringence and PER measured in this way fully reflect the role of the components in the corresponding IFOG.

[0044] As shown in FIG. 4, the pigtailed SLD 402 is spliced ​​to the source port fiber 404 of the fiber array 104. The output is a fiber 406 connected to one of the fiber coil ports which becomes the input of the WLI 408. The measured WLI data shows five significant cross-coupling locations as shown in FIG. 5. The cross-coupling location 502 close to zero is the location of the 45 degree polarization analyzer 410 of the WLI. The second cross-coupling location 504 is generated by cross-polarization coupling at the fiber connection to one of the coil ports. The third cross-coupling location 506 is induced by cross-coupling at the connection point of the source fiber. The fourth cross-coupling location 508 comes from the splice between the fiber connecting to the PIC and the fiber pigtailed to the SLD source. The fifth cross-coupling location 510 is the transmission of the SLD source through the optical system. The distance d between the second peak 504 and the third peak 506 is the optical path difference between the TE and TM modes of the PIC waveguide. The waveguide birefringence can be determined by the ratio of the distance d between the two peaks to the length of the waveguide. In the exemplary embodiment, the waveguide group birefringence is 0.015, which indicates a short 1.8 mm decoherence length of the waveguide polarization and a discoherence length i of 28 μm.

[0045] We measured the PER from the source port to the coil port using two different methods, the rotation analysis method and the WLI method. Although we measured the PER of the circuit with half of the PIC and therefore two additional Y-junctions, the straight waveguide and two Y-junctions maintain low loss propagation in both TE and TM modes, and the polarization cross-coupling is less than 60 dB, so most of the polarization effects come from the m-shaped polarizer. In the WLI measurement, the light from the SLD source launches both TE and TM modes of light into the PIC circuit, where the TE and TM modes experience different attenuation after circuit transmission. The normalized WLI signal, which is the interference intensity between the fast and slow guided waves when a moving mirror is present at the position where the two guided waves are superimposed, is

number

[0046] The same port-to-port PER of the PIC may alternatively be measured using a rotating polarimetry method. Linearly polarized light from the source port was launched with the polarization plane at 45 degrees to the main polarization axis. The output light from the coil port, collimated using a low distortion large aperture lens, is passed through a polarization analyzer, a Glan-Thompson polarizer with a PER of 50 dB, and received by an optical power meter. The PER is measured by determining the difference between the maximum and minimum power as the analyzer is rotated through 360 degrees. The measured port-to-port PER in the PIC of the example embodiment is -45.3 dB, which is consistent with the WLI measurement described herein. The difference in PER of 3.1 dB by the two methods may be explained by the less than perfect alignment of the two fiber-to-PIC connections.

[0047] The propagation loss of the PIC waveguide of the exemplary embodiment can be determined by measuring its transmission spectrum. The input light can be provided by a wavelength-swept laser that had a full width at half maximum (FWHM) of 0.02 nm. The laser can be coupled to a PM fiber and linearly polarized with an in-line fiber polarizer. The light output from the fiber tip can be arranged to launch only the TM mode of the waveguide. The fiber tip can be positioned approximately 0.2 mm away from the waveguide facet to avoid local Fabry-Perot effects. The transmitted light can be close-coupled to a second PM fiber whose output light is received by a power meter. The laser wavelength is scanned from 830 nm to 830.6 nm with a step size of 0.02 nm. The average power at each wavelength step is recorded and the Fourier transform of the transmission spectrum is compared to the theoretical spectrum. The propagation loss of the PIC waveguide of the exemplary embodiment is estimated to be 0.24 dB / cm. There is no strong variation in the propagation loss in the wavelength range for IFOG operation.

[0048] A waveguide Y-junction with a broadband 3 dB splitting ratio (i.e., equal 50-50 splitting of power from the base of the Y-junction to its branches) is a key component of the Sagnac interferometer. Maintaining a 50-50 splitting ratio in the operating wavelength range is important for noise reduction and bias and scale factor stability. Phase errors induced by backscattered signals generated in the fiber coil can be eliminated with the 50-50 coupler ratio. A Y-junction that does not exhibit a 50-50 splitting ratio in the FOG minimum configuration (i) may generate more antisymmetric modes radiated from the interfering Y-junction toward the base waveguide, (ii) may couple more stray light back into the circuit, and (iii) may generate more phase errors. A Sagnac interferometer with a Y-junction that does not exhibit a 50-50 splitting ratio may also have a smaller scale factor compared to the use of a Y-junction with a 50-50 splitting ratio. If the splitting ratio varies with wavelength, an additional scale factor error can be introduced since the centroid wavelength of the SLD varies with temperature.

[0049] The described PIC embodiment uses a simple symmetric Y-junction due to its compactness, good tolerance to manufacturing errors, and the success of this type of coupler for FOG applications on LiNbO3 substrates. The example embodiment of the PIC Y-junction exhibits a splitting ratio of 0.5±0.0068 over the 816 nm to 846 nm wavelength range. The very close to 50-50 splitting ratio over the entire gyro operating wavelength range demonstrates the beneficial effect of using an integrated Y-junction in an IFOG PIC. This precise splitting ratio contributes to good temperature stability of the gyroscope bias and scale factor in these PIC IFOG prototypes.

[0050] The theoretical coupling efficiency of a PM fiber to a PIC waveguide with an inverted taper to expand the mode field size is 0.48 dB. The described embodiment consistently achieves a coupling efficiency of 0.9 dB per connection in a pigtailed PIC using the fiber array alignment method described herein. Since the propagation loss is known to be 0.24 dB / cm, the pigtail loss can be easily estimated as if the two connections have the same coupling efficiency.

[0051] The measured mode field of the PIC waveguide at the fiber connection end is 1 / e 2 It has a size of 4.3 μm (vertical) × 4.3 μm (horizontal) for a peak power of 1000 GHz. For the mode field measurements, linearly polarized light was coupled into the source port of the PIC to excite only the TE mode of the waveguide. The output pattern from one of the coil ports was projected onto the sensing plane of a CCD camera using a large aperture, low distortion lens. The actual size of the mode field was calibrated using a PM Panda fiber, whose outer diameter was measured using a calibrated microscope. For the calibration, a cleaved fiber tip was placed in the same location as the PIC output, and the fiber end image was projected onto the surface of the CCD camera using the same optics.

[0052] To achieve high performance of the IFOG, optical reciprocity must be maintained in the fiber Sagnac interferometer. The most sensible polarization configuration is to control the entire interferometer operating in one linear polarization mode. For this purpose, each component and connection that makes up the interferometer must be polarization maintaining. Any cross-polarization coupling and other types of non-reciprocity effects add erroneous signals to the Sagnac signal. For example, parasitic interference can be generated due to cross-polarization coupling in the Sagnac interferometer, due to recombination of non-guided stray light from adjacent components or connections, or due to back reflection or back scattering in non-smooth waveguides and connections. The high integration of the IFOG PIC brings new challenges to suppress non-reciprocity effects due to the high density of optical components placed in a small area. Each component and connection in the circuit can act as a drawback, coupling light of the operating polarization mode into the orthogonal polarization mode or generating non-guided stray light that is easily received by subsequent circuits and adjacent components. In the case of cross-polarization coupling, if the cross-coupled light is in the main polarization mode and merges with the main light within a waveguide length shorter than the depolarization length, an erroneous ratio signal will be added to the true signal due to interference. To prevent the formation of such parasitic interference, the waveguide length between the two cross-coupling points must be longer than the decoherence length. In the case of stray light recombination, the difference between the recombination path and the main path must be longer than the decoherence length. Fortunately, integrated waveguides can be constructed to have very high birefringence, thus providing very short decoherence and decoherence lengths for PIC waveguides. To provide single-mode PM waveguides with high birefringence and low propagation loss at 830 nm wavelength, Si3N4 / SiO2 waveguides with large width-to-height ratios (at least 50) provide an ideal platform.

[0053] To optimize the waveguide core dimensions, the effective refractive indices of the TE and TM modes are calculated as a function of the width and height of the Si3N4, respectively, using a commercial simulation tool. The waveguide birefringence is then calculated as the refractive index difference of the two polarization modes. On the Si3N4 height-width plane, the waveguide birefringence shows a smooth ridge profile in the Si3N4 height range between 0.03 μm and 0.04 μm (see FIG. 7A), while the refractive indices of the two modes increase monotonically with the width and height of the waveguide. At the opposite side of the "valley", in the region of larger waveguide height (see FIGS. 7A and 7B), the birefringence increases relatively steeply with the waveguide height. The selection of the width of the Si3N4 core is limited by the threshold at which the waveguide starts to support the first higher-order TE mode. The waveguide width must be located in the single-mode region, since the propagation loss increases rapidly in the higher-order modes, and the polarization-maintaining property is no longer maintained for the waveguide to support the higher-order modes. The solid magenta line is the simulated threshold line, and therefore the optimum waveguide core dimensions lie within the region enclosed by the 0.012 birefringence contour line and the higher mode threshold lines, or in the region in the lower right corner of Figure 7B.

[0054] Propagation loss is another waveguide property of interest for waveguide optimization. In general, optical attenuation is mainly attributed to surface scattering and bulk absorption, which are two mechanisms for a non-leaky straight waveguide. Since LPCVD Si3N4 core and SiO2 cladding materials have very low material loss and a wide transmission wavelength window (e.g., 0.4-6.7 μm), the dominant mechanism for waveguide loss is radiation loss resulting from scattering due to surface roughness of the waveguide walls between the Si3N4 core and the SiO2 cladding. Elimination of scattering loss due to sidewall roughness has been developed to allow three-dimensional (3D) waveguide structures to be considered. The notable result is that the top and bottom interfaces result in scattering losses about 3-4 orders of magnitude higher than those due to sidewalls for Si3N4 core heights between about 20 nm and 100 nm.

[0055] The following equation can be used as a coefficient for the radiation loss produced by surface roughness scattering in a symmetric single mode waveguide:

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[0056] An important measure for a highly accurate and sensitive FOG is to minimize cross-polarization coupling in the Sagnac interferometer. This is even more important in a PIC FOG, since the integrated waveguides will experience higher material stresses and more light scattering, and the distance between the functional components will be closer. Making the FOG PIC waveguides "highly" polarization-maintaining is important to mitigate cross-polarization coupling. To make a highly polarization-maintaining waveguide, the waveguide needs to be designed with very high birefringence and very low scattering, which makes the waveguide less likely to have the operating polarization coupled into the orthogonal polarization state due to stress in the waveguide material (high birefringence) and due to light scattering at the waveguide interface (low scattering). A parameter for the evaluation of the ability to maintain the polarization state is the H parameter, i.e., the retention parameter, which is:

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[0057] Figures 9(a), 9(b) and 9(c) show the simulated mode fields at a fixed waveguide width and increasing waveguide height from Figure 9(a) to Figure 9(c). The mode fields are dramatically confined to the SiO2 cladding at a core height of 10 nm in both the lateral and vertical directions. This is because the normalized electric field intensity at the core / cladding interface

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[0058] We can make a choice for the optimal core dimensions for the waveguide by considering the birefringence contour plot shown in FIG. 7B and the loss contour plot in FIG. 8A. We want the waveguide to have both high birefringence and low propagation loss. First, consider the waveguide birefringence map in FIG. 7B. In the single mode region, there are two candidate locations, one at the top of the birefringence ridge in the center of the birefringence map, and the other at the ridge of the steep birefringence gradient in the lower right corner. The birefringence at the two locations is 0.015 and 0.019, respectively. 0.019 is a 27% increase from 0.015. Meanwhile, for the propagation loss in FIG. 8A, the loss at the two corresponding locations is 0.22 dB / cm and 0.52 dB / cm. The loss more than doubles (136%) from 0.22 dB / cm to 0.52 dB / cm loss. We choose the waveguide dimensions to be located on top of the birefringent ridges with the further consideration that in the smooth ridge region the birefringence is more stable and tolerant to fabrication errors and a thinner Si3N4 core can achieve a mode field dimension with a better match to the fiber mode field of the sensing coil.

[0059] Although example embodiments have been specifically shown and described, it should be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments as encompassed by the appended claims. [Explanation of symbols]

[0060] 100: IFOG 102 :PIC 104: Fiber array 106: Sensing fiber coil 108: PZT modulator 110: Light source 112: Photodetector 114: Source Y junction 116: Interference Y-junction 118: Polarizer 120: Coil / Modulator Assembly 402: SLD (Superluminescent Diode) 404: Source port fiber 406: Fiber 408: WLI (White Light Interferometer) 410 :Analyzer 502: Central peak 504: Front Pigtail 506: Rear Pigtail 508: Splice 510: Source structure

Claims

1. 1. A photonic integrated circuit (PIC), comprising: a polarizer comprising a polarizer waveguide and having a first polarizer port and a second polarizer port, the polarizer being configured to have a polarization extinction ratio (PER) of at least 75 dB and a birefringence of at least 0.012 within an operating wavelength range of the PIC; an interferometric Y-junction comprising: a base waveguide, a first branch waveguide, and a second branch waveguide, the first branch waveguide being longer than the second branch waveguide, the first branch waveguide having a length that exceeds the depolarization length; and a first connector configured to be connected to a light source; a second connector configured to be connected to the photodetector; a third connector configured to be connected to the first port of the fiber coil; and a fourth connector configured to be connected to a second port of the fiber coil; and a source Y-junction having a base waveguide, a first branch waveguide and a second branch waveguide; Equipped with the first branch waveguide of the source Y junction is optically coupled to the first connector, the second branch waveguide of the source Y junction is optically coupled to the second connector, and the base waveguide of the source Y junction is optically coupled to the first polarizer port; the first branch waveguide of the interferometric Y junction is optically coupled to the third connector, the second branch waveguide of the interferometric Y junction is optically coupled to the fourth connector, and the base waveguide of the interferometric Y junction is optically coupled to the second polarizer port; The base waveguide and the branch waveguide are made of thin silicon having a core thickness of 10 nm to 100 nm and a core width of 1 micron to 10 microns. ３ N ４ A photonic integrated circuit comprising a waveguide.

2. 2. The photonic integrated circuit of claim 1, wherein the H-parameter of each of the polarizer waveguide, the base waveguide, the first branch waveguide, and the second branch waveguide is less than -40 dB / m.

3. The photonic integrated circuit of claim 1 , wherein a difference in waveguide length between the first branch waveguide and the second branch waveguide is greater than a depolarization length of the first branch waveguide and the second branch waveguide.

4. The polarizer waveguide, the first branch waveguide, and the second branch waveguide are each made of SiO ２ Si surrounded by cladding material ３ N ４ The photonic integrated circuit of claim 1 comprising a core.

5. The Si ３ N ４ The photonic integrated circuit of claim 4 , wherein the core has a width-to-height aspect ratio of at least 50.

6. 10. The photonic integrated circuit of claim 1, wherein the waveguide comprises a plurality of waveguide bends optically connected in series, the plurality of waveguide bends comprising a first 180 degree bend, a second 180 degree bend, and at least one additional bend of at least 90 degrees.

7. 10. The photonic integrated circuit of claim 1, further comprising at least one trench formed along a side of the waveguide, the trench configured to deflect light emitted from the waveguide away from the PIC to prevent stray light from coupling into the PIC.

8. The photonic integrated circuit of claim 1 , wherein the base waveguide and the branch waveguide have a birefringence of at least 0.

012.

9. The photonic integrated circuit of claim 1 , wherein the base waveguide and the branch waveguide have a propagation loss of less than 0.5 dB / cm.

10. The photonic integrated circuit of claim 1 , wherein the polarizer waveguide comprises three 180 degree curves joined together in series to form an m-shape.

11. 2. The photonic integrated circuit of claim 1, wherein the first waveguide, the second waveguide, and the base waveguide of the source Y-junction each have a width-to-height aspect ratio of at least 50.

12. The first waveguide, the second waveguide and the base waveguide of the source Y junction are each made of SiO ２ Si surrounded by cladding material ３ N ４ The photonic integrated circuit of claim 1 comprising a core.

13. 1. An interferometric fiber optic gyroscope (IFOG), comprising: A photonic integrated circuit (PIC) according to claim 1; a sensing fiber coil having a first port connected to the third connector and a second port connected to the fourth connector; a phase modulator connected in-line with the sensing fiber coil and disposed at the first port; An interferometric fiber optic gyroscope comprising:

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