RELATED APPLICATION

[0001] This application is a continuation of U.S. Application No. 16/419,481, filed May 22, 2019. The entire teachings of the above application(s) are incorporated herein by reference.

BACKGROUND

[0002] The disclosure relates to a photonic integrate circuit (PIC) system and, more particularly, to an improved integrated optical polarizer to generate highly linear, polarized, single mode light to obtain the performance required by the application system.

[0003] An optic system comprising one or more PIC devices (e.g., chips) may be constructed for applications that require a stable output state of polarization. In systems for such applications, a light signal consisting of different polarizations may be processed with different efficiencies, and performance degradation may result when orthogonal polarizations are present. It is thus desirable for such systems to operate with a single polarization, which requires polarizing optical elements in the optical system. Integrated polarizers may be incorporated into a PIC to generate and maintain the light in a signal linear polarization state. Optic birefringence may be deliberately introduced into the components and waveguides, so the waveguides and components conduct and process light of a single linear polarization.

[0004] Components that provide a measure of polarization sensitivity may be used in conjunction with optical fibers for such applications. A problem arises, however, concerning the polarization state of the propagating waves. When linearly polarized light is coupled into a single-mode fiber, the light may become slightly randomized in polarization, and there may be conversion from linear to elliptical polarization. When light emerges from the fiber, it is highly likely that both orthogonal polarizations, known as the transverse magnetic (TM) and transverse electric (TE) modes in planar waveguides, will be excited and will be coupled to components of polarization sensitivity.

[0005] Applications that may employ an integrated polarizer may include, for example, optic interferometric modulators, optic interferometric sensors, wavelength-division multiplexing, electro-optic switching arrays, coherent optical communication modules, among others. One example of optic interferometric sensors is a hybrid fiber optic gyroscope (FOG), comprising a PIC and optic fiber coil.

[0006] A polarizer is a critical component in a FOG. A polarizer filters out spurious phase shifts due to parasitic interference and constrains the gyroscope bias. In a FOG that employs a Sagnac interferometer, it is a common practice to use sensing coils of polarization maintaining (PM) fiber. Due to stress to the birefringent PM fiber, the two

counterpropagating waves at a point in the Sagnac interferometer may each exhibit a different state of polarization, and therefore each counterpropagating wave may be subject to a different effective refractive index. The different effective refractive indices may further produce a degree of non-reciprocity and yield a spurious phase difference at the output.

[0007] It has been shown theoretically that if a polarizer is placed at the base waveguide of the Y-junction of a Sagnac interferometer, to launch linearly polarized light in one of the polarization modes, the Sagnac interferometer circuit is substantially reciprocal. Light is thus filtered at the input, and the two counterpropagating waves that are coming back through this same polarizer are perfectly in phase at the output.

[0008] In practice, however, the rejection of the spurious phase error is limited by the polarization extinction ratio (PER) of the polarizer. There remains a residual phase difference between both counterpropagating waves if the polarizer is imperfect. It was further revealed that the bias error is limited by the amplitude PER, e, of the polarizer instead by its intensity PER, e ² , which is typically the basis for measuring and evaluating a polarizer PER. This implies that a polarizer with very high intensity PER is required to suppress the phase error caused by the polarization randomization in the Sagnac interferometer. As one example, to control a maximum phase measurement error in a FOG of 10 ^-7 radians, a polarizer with 93 dB intensity PER would be required.

[0009] Optical waveguide polarizers function by allowing only one polarization state, either TE or TM mode, to propagate while the other polarization state is eliminated. An effective polarizer is characterized by low insertion loss and high PER. Integrated polarizers are based on the different propagation properties of the polarization modes that permit discrimination of either the TE or TM field by means of a strong attenuation on the mode in cross polarization.

[0010] Many types of integrated polarizers have been realized over the years, including metal-clad waveguides, birefringence waveguides in crystalline substrate such as LiNb0 ₃ , waveguides that activate plasmonic modes by using metals, transition-metal oxides, graphene or transparent conducting oxides, dielectric polarizers based on resonant tunneling effect, shallowly-etched silicon-on-insulator ridge optical waveguides, symmetric etching of silicon nanowires, and waveguides with subwavelength gratings. These different types of polarizers require either additional layer of material for the enhancement of mode discrimination, or additional steps of process for the required structures, or fabrication with very tight accuracy, or special crystalline substrates.

[0011] It is worthwhile to mention that high-PER, low-loss polarizers have been reported in ultrathin Si ₃ N ₄ waveguide. Series of connected s-shape bending waveguides that have ultra-loose mode confinement result in high propagation loss in TM mode while keep TE- mode loss low (see, e.g., FIG.6E). The highest PER is 75dB measured at 1600nm on a planer polarizer comprising a half period of s-bend waveguide of 3.5pm core wide and of 40nm core height (FIG.6A). The waveguide typically has birefringence within the range of 0.011 ~

0.014 at 1550nm wavelength (see, e.g., J. F. Bauters, M. J. R. Heck, D. Dai, J. S. Barton, D.

J. Blumenthal, and J. E. Bowers,“Ultralow-Loss Planar Si3N4 Waveguide Polarizers,” IEEE Photonics Journal, 5(1), Article S/N: 6600207 (2013)).

SUMMARY

[0012] The described embodiments are directed to a compact integrated polarizer with ultrahigh polarization extinction ratio that suitable to construct high-end tactical grade, or navigation grade, fiber optical gyroscope. The integrated polarizer of the described embodiments may exhibit a high intensity polarization extinction ratio (PER), while being relatively compact in size.

[0013] The integrated polarizer of the described embodiments has low insertion loss, high polarization extinction ratio and compact device size, and may be fabricated using the same materials as other components in a PIC system. An effective method and device are provided which can be mass produced using mature microfabrication technology and with low manufacture cost.

[0014] In one aspect, the invention may be an integrated optical polarizer comprising a planar substrate, an optical waveguide integrated on the substrate, and a polarizer section of the optical waveguide. The optical waveguide may have an input that defines an input direction of propagating light and an output that defines an output direction of propagating light that is opposite of the input direction of propagating light. The polarizer section of the optical waveguide may conduct a first polarization mode and attenuate a second polarization mode. The polarizer section of the optical waveguide may (i) be birefringent, and (ii) have a confinement characteristic of at most six percent.

[0015] The polarizer section and other components of photonic integrated circuit may comprise the same materials. The optical waveguide and the polarizer may be monolithically fabricated on the substrate. The integrated optical polarizer may further comprise a first Y- splitter having a first base waveguide, and a second Y-splitter having a second base waveguide. The integrated optical polarizer, the first Y-splitter, and the second Y-splitter may be (i) monolithically fabricated on a common substrate, and (ii) configured such that the first base waveguide is optically coupled to a first end of the integrated waveguide optical polarizer, and the second base waveguide is optically coupled to a second end of the integrated waveguide optical polarizer, consistent with a fiber optic gyroscope architecture.

[0016] The integrated optical polarizer may further comprise a pair of trenches formed along the both sides of the integrated waveguide optical polarizer. The pair of trenches may be configured to deflect light radiated from the integrated waveguides away from the integrated circuit plane to prevent stray light from coupling into the integrated circuit. The integrated optical polarizer may further 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.

[0017] The substrate may be a silicon wafer, and the optical waveguide may comprise a silicon nitride waveguide core and a silicon oxide waveguide cladding. The silicon nitride waveguide core may have a thickness of less than 50nm. The optical waveguide may comprise a waveguide core having a rectangular cross-section with an aspect-ratio greater than 50. The integrated polarizer waveguide may be configured to support a TE polarization mode, and not configured to support a TM polarization mode.

[0018] In another aspect, the invention may be an integrated optical polarizer comprising a planar substrate, an optical waveguide integrated on the substrate, and a polarizer section of the optical waveguide that conducts a first polarization mode and attenuates a second polarization mode. The polarizer section of the optical waveguide may (i) be birefringent, (ii) have a confinement characteristic of at most six percent, and (iii) 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. [0019] The polarizer section and other components of photonic integrated circuit may comprise the same materials. The optical waveguide and the polarizer may be monolithically fabricated on the substrate. The integrated optical polarizer may further comprise a first Y- splitter having a first base waveguide, and a second Y-splitter having a second base waveguide. The integrated optical polarizer, the first Y-splitter, and the second Y-splitter are (i) monolithically fabricated on a common substrate, and (ii) configured such that the first base waveguide is optically coupled to a first end of the integrated waveguide optical polarizer, and the second base waveguide is optically coupled to a second end of the integrated waveguide optical polarizer, consistent with a fiber optic gyroscope architecture.

[0020] The integrated optical polarizer may further comprise a pair of trenches formed along the both sides of the integrated waveguide optical polarizer. The pair of trenches may be configured to deflect light radiated from the integrated waveguides away from the integrated circuit plane, to prevent stray light from coupling into the integrated circuit. The integrated optical polarizer may further comprise 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.

[0021] The substrate may be a silicon wafer, and the optical waveguide may comprise a silicon nitride waveguide core and a silicon oxide waveguide cladding. The silicon nitride waveguide core may have a thickness of less than 50nm. The optical waveguide may comprise a waveguide core having a rectangular cross-section with an aspect-ratio greater than 50. The integrated polarizer waveguide may be configured to support a TE polarization mode, and not configured to support a TM polarization mode.

[0022] In another aspect, the invention may be an integrated waveguide optical polarizer comprising a planar substrate, and an integrated waveguide formed over the substrate. The optical waveguide may have an input that defines an input direction of propagating light and an output that defines an output direction of propagating light that is opposite of the input direction of propagating light. The optical waveguide may have a polarizer that conducts one polarization mode and attenuates the other polarization mode. The integrated waveguide may be birefringent and have a higher effective modal refractive index than the refractive index of the cladding material, such that the difference between the two refractive indices is less than 0.004. The polarizer may comprise a plurality of arc waveguides, each having a length of p/2 radians. The arc waveguides may be arranged such that (i) each arc waveguide is

characterized by a bending radius configured to facilitate radiation loss of a polarization mode having a larger mode-field dimension, (ii) the arc waveguides are optically connected in series to form an arc waveguide train, (iii) a number the arcs in the arc waveguide train facilitates a specific polarization extinction ratio, and (iv) at least one individual group of two or three adjacent waveguides in the arc waveguide train are arranged such that centers of the arc waveguides in each of the individual groups are located on a same side of the arc waveguide train, and the centers of the arc waveguides in adjacent individual groups are located on the different sides of the arc waveguide train.

[0023] The polarizer section may comprise the same materials as the rest of photonic integrated circuit. The structure of the integrated polarizer and the rest of photonic integrated circuit may be monolithically fabricated on the substrate.

[0024] The integrated waveguide optical polarizer may further comprise a first Y-splitter having a first base waveguide, and a second Y-splitter having a second base waveguide. The integrated optical polarizer, the first Y-splitter, and the second Y-splitter may be (i) monolithically fabricated on a common substrate, and (ii) configured such that the first base waveguide is optically coupled to a first end of the integrated waveguide optical polarizer, and the second base waveguide is optically coupled to a second end of the integrated waveguide optical polarizer, consistent with a fiber optic gyroscope architecture.

[0025] The integrated waveguide optical polarizer may further comprise a pair of trenches formed along the both sides of the integrated waveguide optical polarizer. The pair of trenches may be configured to deflect light radiated from the integrated waveguides away from the integrated circuit plane to prevent stray light from coupling into the integrated circuit.

[0026] The arc waveguide train may comprise more than five of the arc waveguides of p/2 radian length. The substrate may be a silicon wafer, the waveguide core may be silicon nitride, and the waveguide cladding may be silicon oxide. The waveguide may be formed by an ultrathin core such that the silicon nitride thickness is less than 50nm. The optical waveguide may comprise a waveguide core having a rectangular cross-section with an aspect-ratio greater than 50. The integrated polarizer waveguide may be configured to support a TE polarization mode, and may not be configured to support a TM polarization mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing will be 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. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.

[0028] FIG. 1 A shows an example embodiment of a photonic integrated circuit (PIC) built on a substrate, according to the invention.

[0029] FIG. IB shows another example embodiment of a PIC built on a substrate, according to the invention.

[0030] FIGs. 2A and 2B depict an example embodiment of a waveguide structure for polarizer construction, according to the invention.

[0031] FIGs. 3 A and 3B depict an example of the dependence of waveguide transmission on the wavelength of the TE and TM modes of the polarizer.

[0032] FIGs. 4A and 4B show the behaviors of example embodiments of integrated polarizers, according to the invention.

[0033] FIG. 5 shows that the propagation loss of an example waveguide for various differences between effective index of waveguide and index of cladding material, according to the invention.

[0034] FIGs. 6A through 6E show examples of prior implementations of integrated polarizers.

[0035] FIGs. 7A through 7E illustrate examples of integrated polarizers according to embodiments of the invention.

[0036] FIGs. 8A through 8G illustrate alternative examples of integrated polarizers according to embodiments of the invention.

DETAILED DESCRIPTION

[0037] A description of example embodiments follows.

[0038] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. [0039] FIG. 1A shows an example embodiment of a photonic integrated circuit (PIC) 100 built on a substrate 102. The PIC comprises an integrated polarizer 104 (the area enclosed by the dotted line) and two Y-junctions 106a, 106b, with their base waveguides 108a, 108b, respectively, connecting to the both ends of the polarizer 104, respectively. The polarizer comprises six arc waveguides, each of p/2 radian length and of the same radius, connected in series, and the directions of its input and output ports are in opposite directions. As used herein, the direction of an input or output port refers to the direction of propagation of light associated with that port. Arrow 110a represents the direction of light associated with a first port 112a of the polarizer 104, and arrow 110b represents the direction of light associated with a second port 112b of the polarizer 104. As shown in FIG. 1 A, arrows 110a and 110b are in opposite directions

[0040] FIG. IB shows another example embodiment of a PIC 120 built on a substrate 122. The PIC comprises an integrated polarizer 124 and two Y-junctionsl26a, 126b, with their base waveguides 128a, 128b, respectively, connecting to the both ends of the polarizer 124, respectively. The polarizer 124 has six arc waveguides, each of p/2 radian length and of the same radius, connected in series, and the directions of its input port 130a and output port 130b are in the same direction.

[0041] FIGs. 2A and 2B depict an example embodiment of a waveguide structure for construction of the polarizers described herein, in a cross-section view. The waveguide comprises a substrate 202, a waveguide core 204 surrounded by a waveguide cladding material 206. The substrate 202, which in the example embodiment may be a silicon wafer, is a carrier for the waveguide structure. The waveguide core 204 has a higher refractive index than that of the waveguide cladding material 206, which in the example embodiment may be silicon oxide. The waveguide core 204 may be configured to have a high aspect ratio, such that the waveguide structure supports only a single mode, the fundamental transverse mode. The waveguide may be characterized by a high birefringence, and the waveguide may have a mode confinement that is loose. Due to the waveguiding conditions described above, only a small percentage of the mode field is distributed within the waveguide core, and large percentage of the mode field is stretched out into the cladding material as an evanescent wave.

[0042] FIG. 2A presents a contour map 208a of the TE mode superposed on the waveguide cross-section profile. FIG. 2B is a contour map 208b of the TM mode superposed on the same waveguide profile. The steps have a same value between the adjacent contour lines in the two figures. The above-mentioned waveguiding conditions also result in a larger mode-field dimensions of the TM mode than that of the TE mode, as indicated by a comparison of FIG. 2A and FIG. 2B. Because of the TM mode is more loosely confined than the TE mode, the TM mode experiences a higher bending loss than that of the TE mode. The layout of the waveguide is configured as bended in shape as shown by the waveguide layouts in the area enclosed by the dotted line (reference number 104) in FIG. 1.

[0043] It is well known that the evanescent wave (i.e., the percentage of the mode field extended in the waveguide cladding area 206 beyond the core 204) increases when the wavelength of the guided light becomes longer, which is equivalent to the decrease of normalized frequency, or v-number, of the waveguide. If the waveguide is arranged to bend at a fixed radius, the waveguide is characterized by a high propagation loss (i.e., becomes “leaky”), when the wavelength is longer than a certain“critical wavelength.” Since the TM mode has a looser mode confinement than that of the TE mode, the critical wavelength for TM mode is shorter than that for TE mode in the waveguide. There will thus be a wavelength window in which the TE mode has low propagation loss while the TM mode is highly attenuated. This configuration constitutes a polarizer of low TE mode loss but high TM mode discrimination.

[0044] FIGs. 3 A and 3B demonstrate an example of the dependence of waveguide transmission on the wavelength of the TE and TM modes of the polarizer described in the FIG. 1 A. In the FIG. 3A and 3B, the y-axis is in linear scale and in logarithmic scale, respectively. In a wavelength window between 810nm to 910nm, the PER of the polarizer is between 50dB to 170dB, while the TE mode propagation loss is less than 0.3dB.

[0045] The PER of a polarizer can be further improved while a low-loss TE mode transmission can be maintained. FIGs. 4A and 4B show the behaviors of example embodiments of integrated polarizers, comprised of three half-circle bending waveguides, each having different bending radius as shown in bending radius key 402. When the bending radius decreases from 1350pm to 600 p , the PER of the polarizers increases from 20dB to 133dB, while the loss of the TE mode of all the polarizers is less than 0.5dB.

[0046] The polarization mode to be eliminated should be loosely (weakly) confined by the waveguide, which causes that mode to experience high losses through radiation. The degree of the confinement of the mode is dependent on An = RI _{effectivejnodal} - M cladding material , which is the difference of the effective modal refractive index of the waveguide

(RI _{effective modal} ) from the refractive index of the cladding material (RI _C iaddm matcnai)· A waveguide with a smaller difference (D//) between these refractive indices experiences a looser (or equivalently, weaker) modal confinement, since a larger percentage of the mode- field portion is extended into the surrounding cladding area, which results in a higher radiation loss associated with the waveguide. FIG. 5 shows that the propagation loss of the waveguide that has index differences of 0.0016, 0.0041, 0.0076 and 0.0122 is 456dB/cm, 72.2dB/cm, 3.95dB/cm and 0.06dB/cm, respectively.

[0047] FIGs. 6A through 6E show examples of prior implementations of integrated polarizers, which typically comprise one or more single s-bend (i.e., p/2 radian length, or equivalently, 90 degree length) waveguides. FIGs. 6A and 6D are examples of two p/2 radian length waveguide bends, FIGs. 6B and 6C are examples of four p/2 radian length waveguide bends, and FIG. 6E shows a waveguide with multiple shallowly waving p/2 s- bends.

[0048] FIGs. 7A through 7E illustrate five examples of integrated polarizers according to embodiments of the invention. These embodiments arrange the two waveguide connectors on the different side of the integrated circuit such that it enables the input and output signals of the polarizer to propagate in the same direction. For example, FIG. 7A shows input light 702 and output light 704. As illustrated, the waveguide arrangement facilitates the input light 702 and the output light 704 oriented in the same direction.

[0049] The integrated polarizers depicted in FIGs. 7A through 7E comprise a series connection of from six to fourteen p/2 radian length arc bends. The waveguide shown in FIG. 7A comprises six p/2 radian arc bends. The waveguide shown in FIG. 7B comprises eight p/2 radian arc bends. The waveguide shown in FIG. 7C comprises ten p/2 radian arc bends. The waveguide shown in FIG. 7D comprises twelve p/2 radian arc bends, and the waveguide shown in FIG. 7E comprises fourteen p/2 radian arc bends.

[0050] The circles shown at the ends of the waveguides in FIGs. 7A through 7E represent optical connectors 706. Further optical connections, for example to integrated waveguides or optical fibers, can be realized at these connectors. A pair of trenches can be formed along the both sides of the integrated waveguide optical polarizer, so that the trenches deflect the radiated light from the integrated waveguides out of the integrated circuit plane, and prevent stray light from coupling into the integrated circuit.

[0051] The integrated polarizers depicted in FIGs. 8A through 8G are examples of polarizers according to embodiments of the invention. These embodiments arrange the connectors on the same side of the integrated circuit such that it enables the input and output signals of the polarizer to propagate in opposite directions. For example, FIG. 8A shows input light 802 and output light 804. As illustrated, the waveguide arrangement facilitates the input light 802 and the output light 804 oriented in the opposite directions.

[0052] The integrated polarizers depicted in FIGs. 8A through 8G comprise a series connection of from two to fourteen p/2 radian length arc bends. The waveguide shown in FIG. 8A comprises two p/2 radian arc bends. The waveguide shown in FIG. 8B comprises four p/2 radian arc bends. The waveguide shown in FIG. 8C comprises six p/2 radian arc bends. The waveguide shown in FIG. 8D comprises eight p/2 radian arc bends. The waveguide shown in FIG. 8E comprises ten p/2 radian arc bends. The waveguide shown in FIG. 8F comprises twelve p/2 radian arc bends, and the waveguide shown in FIG. 8G comprises fourteen p/2 radian arc bends.

[0053] The circles shown at the ends of the waveguides in FIGs. 8A through 8G represent optical connectors 806. Further optical connections, for example to integrated waveguides or optical fibers, can be realized at these connectors. A pair of trenches can be formed along the both sides of the integrated waveguide optical polarizer, so that the trenches deflect the radiated light from the integrated waveguides out of the integrated circuit plane, and prevent stray light from coupling into the integrated circuit.

[0054] While example embodiments have been particularly shown and described, it will 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 encompassed by the appended claims.