Patent Description:
Aspects of the present disclosure relate to electronic components. More specifically, certain implementations of the present disclosure relate to methods and systems for eliminating polarization dependence for <NUM> degree incidence MUX/DEMUX designs.

Conventional approaches for multiplexing and demultiplexing may be costly, cumbersome, and/or inefficient—e.g., they may be complex and/or time consuming, and/or may have limited responsivity due to losses.

Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present disclosure as set forth in the remainder of the present application with reference to the drawings.

<CIT> provides systems, methods, and apparatus for a photonic chip. The photonic chip includes one or more electronic components in addition to one or more optical components. An optical coupler can be utilized for coupling external optical fibers or sources with the optical components. The optical coupler can include a beam splitter for splitting an incident light having both trans-electric (TE) and trans-magnetic (TM) polarizations into two beams having only TE and TM polarizations. The light beam with TM polarization is incident on a grating coupler on the chip having a horn section, which includes gratings. The light beam is reflected onto the grating coupler such that the direction of TM polarization is within the first plane of incidence, and the first beam of light is incident on the first plurality of gratings at an angle with respect to a normal to the plane of the first grating coupler.

<CIT> is directed to a tunable filter that includes a polarizing beam splitter and a half-wave plate that produce two equally polarized beams from an arbitrarily polarized input light. The polarized beams are directed toward a tunable component that consists of an optical substrate coated with a filter and a reflective element on its back and front surface, respectively. The pass and stop beams emerging from the tunable component are retroreflected by roof structures and passed again through the filter. As a result of this combination of components, the filter's performance is not dependent on polarization and the filter's output channels are not shifted or deviated by tuning.

System and methods are provided for eliminating polarization dependence for <NUM> degree incidence MUX/DEMUX designs, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the present disclosure, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.

As utilized herein, circuitry or a device is "operable" to perform a function whenever the circuitry or device comprises the necessary hardware and code (if any is necessary) to perform the function, regardless of whether performance of the function is disabled or not enabled (e.g., by a user-configurable setting, factory trim, etc.).

<FIG> is a block diagram of a photonically-enabled integrated circuit with polarization independent MUX/DEMUX, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there are shown optoelectronic devices on a photonically-enabled integrated circuit <NUM> comprising optical modulators 105A-105D, photodiodes 111A-111D, monitor photodiodes 113A-113D, and optical devices comprising couplers 103A-103C and grating couplers 117A-<NUM>. There are also shown electrical devices and circuits comprising amplifiers 107A-107D, analog and digital control circuits <NUM>, and control sections 112A-112D. The amplifiers 107A-107D may comprise transimpedance and limiting amplifiers (TIA/LAs), for example. Coupling optics <NUM> may comprise beam splitters, thin film filters, mirrors, prisms, etc..

In an example scenario, the photonically-enabled integrated circuit <NUM> comprises a CMOS photonics die with a laser assembly <NUM> coupled to the top surface of the IC <NUM>. The laser assembly <NUM> may comprise one or more semiconductor lasers with isolators, lenses, and/or rotators for directing one or more continuous-wave (CW) optical signals to the couplers 104A-104D. The CW optical signals may be at different wavelengths for CWDM operation, such as CWDM4, for example. The photonically enabled integrated circuit <NUM> may comprise a single chip, or may be integrated on a plurality of die, such as with one or more electronics die and one or more photonics die.

The grating couplers 104A-104D comprise grating structures with grating spacing and width configured to couple optical signals of a specific wavelength and polarization into the IC <NUM>. A lens array may be incorporated between the grating couplers 104A-104D and the laser assembly <NUM> for focusing of the optical signal to the grating couplers for increased coupling efficiency.

Optical signals are communicated between optical and optoelectronic devices via optical waveguides <NUM> fabricated in the photonically-enabled integrated circuit <NUM>. Single-mode or multi-mode waveguides may be used in photonic integrated circuits. Single-mode operation enables direct connection to optical signal processing and networking elements. The term "single-mode" may be used for waveguides that support a single mode for each of the two polarizations, transverse-electric (TE) and transverse-magnetic (TM), or for waveguides that are truly single mode and only support one mode. Such one mode may have, for example, a polarization that is TE, which comprises an electric field parallel to the substrate supporting the waveguides. Two typical waveguide cross-sections that are utilized comprise strip waveguides and rib waveguides. Strip waveguides typically comprise a rectangular cross-section, whereas rib waveguides comprise a rib section on top of a waveguide slab. Of course, other waveguide cross section types are also contemplated and within the scope of the disclosure.

The optical modulators 105A-105D comprise Mach-Zehnder or ring modulators, for example, and enable the modulation of the continuous-wave (CW) laser input signals. The optical modulators 105A-105D may comprise high-speed and low-speed phase modulation sections and are controlled by the control sections 112A-112D. The high-speed phase modulation section of the optical modulators 105A-105D may modulate a CW light source signal with a data signal. The low-speed phase modulation section of the optical modulators 105A-105D may compensate for slowly varying phase factors such as those induced by mismatch between the waveguides, waveguide temperature, or waveguide stress and is referred to as the passive phase, or the passive biasing of the MZI.

In an example scenario, the high-speed optical phase modulators may operate based on the free carrier dispersion effect and may demonstrate a high overlap between the free carrier modulation region and the optical mode. High-speed phase modulation of an optical mode propagating in a waveguide is the building block of several types of signal encoding used for high data rate optical communications. Speed in the several Gb/s may be required to sustain the high data rates used in modern optical links and can be achieved in integrated Si photonics by modulating the depletion region of a PN junction placed across the waveguide carrying the optical beam. In order to increase the modulation efficiency and minimize the loss, the overlap between the optical mode and the depletion region of the PN junction must be carefully optimized.

One output of each of the optical modulators 105A-105D may be optically coupled via the waveguides <NUM> to the grating couplers 117E-<NUM>. The other outputs of the optical modulators 105A-105D may be optically coupled to monitor photodiodes 113A-113D to provide a feedback path. The IC <NUM> may utilize waveguide based optical modulation and receiving functions. Accordingly, the receiver may employ an integrated waveguide photo-detector (PD), which may be implemented with epitaxial germanium/SiGe films deposited directly on silicon, for example.

The grating couplers 104A-104D and 117A-<NUM> may comprise optical gratings that enable coupling of light into and out of the photonically-enabled integrated circuit <NUM>. The grating couplers 117A-117D may be utilized to couple light received from optical fibers into the photonically-enabled integrated circuit <NUM>, and the grating couplers 117E-<NUM> may be utilized to couple light from the photonically-enabled integrated circuit <NUM> into optical fibers. The grating couplers 104A-104D and 117A-<NUM> may comprise single polarization grating couplers (SPGC) and/or polarization splitting grating couplers (PSGC). In instances where a PSGC is utilized, two input, or output, waveguides may be utilized, as shown for grating couplers 117A-117D, although these may instead be SPGCs.

The optical fibers may be epoxied, for example, to the CMOS chip, using a fiber coupler that selectively deflects optical signals of different wavelengths to and from different grating couplers on the chip <NUM>, with each coupler, such as each of the grating couplers 117A-<NUM> being configured to couple optical signals of different wavelengths.

The photodiodes 111A-111D may convert optical signals received from the grating couplers 117A-117D into electrical signals that are communicated to the amplifiers 107A-107D for processing. In another embodiment of the disclosure, the photodiodes 111A-111D may comprise high-speed heterojunction phototransistors, for example, and may comprise germanium (Ge) in the collector and base regions for absorption in the <NUM>-<NUM> optical wavelength range, and may be integrated on a CMOS silicon-on-insulator (SOI) wafer.

The analog and digital control circuits <NUM> may control gain levels or other parameters in the operation of the amplifiers 107A-107D, which may then communicate electrical signals off the photonically-enabled integrated circuit <NUM>. The control sections 112A-112D comprise electronic circuitry that enables modulation of the CW laser signal received from the splitters 103A-103C. The optical modulators 105A-105D may require high-speed electrical signals to modulate the refractive index in respective branches of a Mach-Zehnder interferometer (MZI), for example.

In operation, the photonically-enabled integrated circuit <NUM> may be operable to transmit and/or receive and process optical signals. Optical signals may be received from optical fibers by the grating couplers 117A-117D and converted to electrical signals by the photodetectors 111A-111D. The electrical signals may be amplified by transimpedance amplifiers in the amplifiers 107A-107D, for example, and subsequently communicated to other electronic circuitry, not shown, in the photonically-enabled integrated circuit <NUM>.

Integrated photonics platforms allow the full functionality of an optical transceiver to be integrated on a single chip or a plurality of chips in a flip-chip bonded structure. An optical transceiver contains optoelectronic circuits that create and process the optical/electrical signals on the transmitter (Tx) and the receiver (Rx) sides, as well as optical interfaces that couple the optical signals to and from a fiber. The signal processing functionality may include modulating the optical carrier, detecting the optical signal, splitting or combining data streams, and multiplexing or demultiplexing data on carriers with different wavelengths.

An important commercial application of silicon photonics is high speed optical transceivers, i.e., ICs that have optoelectronic transmission (Tx) and receiving (Rx) functionality integrated in the same chip or a plurality of bonded chips in a small package. The input to such an IC or ICs is either a high speed electrical data-stream that is encoded onto the Tx outputs of the chip by modulating the light from a laser or an optical data-stream that is received by integrated photo-detectors and converted into a suitable electrical signal by going through a Trans-impedance Amplifier (TIA)/Limiting Amplifier (LA) chain. Such silicon photonics transceiver links have been successfully implemented at baud-rates in the tens of GHz.

One method for increasing data rates in optical transceivers is to multiplex a plurality of optical signals at different wavelengths for concurrent transmission through the optical fiber, which may then be demultiplexed at the receiving end. To this end, multiplexers and demultiplexers (MUX/DEMUX) may be utilized to combine/separate the different optical wavelengths. This may be accomplished with thin film filters (TFFs) tuned to different wavelengths, deflecting optical signals down to near-normal incidence on the chip into corresponding grating couplers while allowing other wavelength signals to pass through. These structures are shown in <FIG> as coupling optics <NUM> and in further detail with respect to <FIG>.

<FIG> is a schematic illustrating a MUX/DEMUX with thin film filters, in accordance with an embodiment of the disclosure. Referring to <FIG>, there is shown a MUX/DEMUX <NUM> comprising fibers <NUM>, a fiber coupler <NUM>, Rx grating couplers 205A, Tx grating couplers 205B, a lens array <NUM>, and TFF structure <NUM>. The Rx grating couplers 205A may be similar to the grating couplers 117A-117D and the Tx grating couplers 205B may be similar to the grating couplers 117E-<NUM>, for example, shown with respect to <FIG>.

The fiber coupler <NUM> may comprise a ferrule for receiving ends of the optical fibers <NUM> and coupling optical signals into the TFF structure <NUM> comprising TFFs 209A-209D. Each of the TFFs 209A-209B comprises reflectors for configurable wavelengths, meaning all but a desired wavelength pass through each of the TFFs 209A-209D while the desired wavelength is reflected down toward the lens array <NUM>. In an example scenario, each of the TFFs 209A-209D is configured for a different wavelength, as indicated by wavelengths λ1-λ4 below the grating couplers 205A/205B in the side view of <FIG>, and may reflect signals whether they are received from the fibers <NUM> or from the photonics die <NUM> for coupling into the fibers <NUM>. The TFFs 209A-209D are capable of operating in both a CWDM4 transmitting mode, where optical signals from the photonics die <NUM> are coupled into one of the fibers <NUM>, and also in a CWDM4 receiving mode where optical signals from the fibers <NUM> are reflected down to the photonics die <NUM>, although other numbers of Tx and Rx channels are possible. The TFFs 209A-209D may be configured for reduced polarization dependence loss (PDL) at <NUM> degrees and it may be assumed that the performance at each polarization individually is repeatable and acceptable, but the separation between transfer functions should be considered in structure design.

The lens array <NUM> may comprise silicon lens structures for focusing optical signals received from the TFFs 209A-209D onto grating couplers 205A situated beneath the array. In addition, the lens array <NUM> may focus optical signals received from the grating couplers 205A/205B into the TFFS 209A-209D for transmission through one of the fibers <NUM>.

In operation, in a demultiplexing example, CWDM optical signals may be received from one of the fibers <NUM>, where the other fiber is for receiving signals from the photonics die <NUM>, the received signals being coupled into the TFFs <NUM>. Each TFF 209A-209D may reflect an optical signal of one wavelength down to a respective Rx grating coupler 205A, while allowing the other three wavelength optical signals to pass. Each optical signal reflected downward is then focused by the lens array <NUM> onto the corresponding grating coupler 205A, which then couples the optical signal into a waveguide running parallel to the top surface of the photonics die <NUM>.

For multiplexing, optical signals of different wavelengths λ1-λ4 may be coupled from the Tx grating couplers 205B in the photonics die <NUM> and focused by the lens array <NUM> onto a respective TFF 209A-209D. Each TFF then reflects the signal into the Tx fiber of the fibers <NUM>. The TFFs 209A-209D allows each wavelength optical signal to pass through, other than the one reflected from the die <NUM>, thereby generating a CWDM4 optical signal in the fiber.

<FIG> illustrates optical coupling via different thin film filters for different polarization optical signals, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a CWDM MUX/DEMUX <NUM> comprising optical fibers <NUM>, a fiber coupler <NUM>, a lens array <NUM>, s-polarization TFFs 309A, p-polarization TFFs 309B, and photonic die <NUM>. Typically, p-polarized light may be understood to have an electric field direction parallel to the plane of incidence on a device, and s-polarized light has the electric field oriented perpendicular to that plane.

The fibers <NUM>, fiber coupler <NUM>, lens array <NUM>, and photonic die <NUM> may be substantially similar to similarly named structures described above. The s-TFFs 309A comprise thin film filters that are tuned for s-polarization optical signals received either from the fibers <NUM> or from the photonic die <NUM>, and the p-TFFs 309B comprise thin film filters that are tuned for p-polarization optical signals received either from the fibers <NUM> or from the photonic die <NUM>.

The deviation in reflection between the s and p polarization can be utilized to an advantage in a CWDM system, with the first <NUM>-filter stack, the s-TFFs 309A, configured for s-polarization optical signals so that each filter is engineered to push the p-polarization out of band, meaning they are not reflected downward. The p-polarizations may then be collected by a second <NUM>-filter stack, the p-TFFs 309B. Transmission of optical signals out of the photonic die <NUM> may be via the s-TFFs 309A, for example, or both sets of TFFs. In this manner, demultiplexing and multiplexing may be enabled based on polarization dependence of TFFs.

<FIG> illustrates spectral response for s-polarization and p-polarization thin film filters, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a CWDM MUX/DEMUX <NUM>, similar to the CWDM MUX/DEMUX <NUM> described with respect to <FIG>. By rotating a TFF, the transmission properties for s-polarization are different with respect to p-polarization, as shown by the plot below with ∼ <NUM> difference of the p-polarization transmission spectra for the s-TFFs. Optical signals with p-polarization will pass through the s-TFFs at wavelengths all the way down to ∼ <NUM>, whereas s-polarized optical signals only down to ∼ <NUM>. In this embodiment, p-polarization signals from the fibers pass through the s-TFFs while s-polarization signals are reflected downward into the photonic chip. The p-polarization signals are then reflected down to the photonic chip via the p-TFFs.

Illustrates spectral response for p-polarization thin film filters, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a CWDM MUX/DEMUX <NUM>, similar to the CWDM MUX/DEMUX described with previously. The plot below the MUX/DEMUX <NUM> illustrates the transmission through the last p-TFF, showing that the all p-polarization signals less than -<NUM> wavelength range will be reflected downward into the photonic chip via the p-TFFs, but since most all of these wavelengths will have been reflected downward by previous TFFs, only the last remaining signal is actually reflected.

<FIG> illustrates spectral response for stacked thin film filters, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there are shown spectral responses for stacked s-TFFs on top and stacked p-TFFs on bottom, where the solid lines in the upper plot represent the transition between transmission/reflection for the s-polarized signal by the s-TFFs, indicating that all the p-polarized signals transmit through the S-TFFs. In the lower plot, the dashed lines represent transmission curves for p-TFFs. The stacked TFFs thus enable four CWDM bands for each polarization. In this manner, CWDM is enabled with configurable polarization and wavelength.

<FIG> illustrates a beam splitter for spatially separating different polarizations, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a beam splitter <NUM> comprising a mirror <NUM> on one surface of a rhomboid prism <NUM> and a polarization splitting thin film stack <NUM> on a trapezoidal prism <NUM>. The rhomboid prism <NUM> and the trapezoidal prism <NUM> may comprise transparent material on which reflective materials may be formed. Similarly, the thin film stack <NUM> may be formed on a sloped surface of the trapezoidal prism <NUM>, which may be configured to reflect optical signals of one polarization downward while allowing signals of another polarization to pass through.

In operation, an input optical signal is received in the rhomboid prism <NUM> where the s-polarization signals may be reflected laterally by the thin film stack <NUM> and the p-polarization signals may pass through the thin film stack <NUM> and continue downward to grating couplers in the photonic die. The mirror <NUM> formed on the rhomboid prism <NUM> adjacent to the thin film stack reflects the reflected s-polarization signals down to the photonic die. In this manner, the different polarizations are translated spatially from each other, where the displacement beam splitting may be greater than <NUM> if desired.

<FIG> illustrates an optical transceiver with a spatial separation beam splitter, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a transceiver <NUM> comprising Tx fiber 801A, Rx fiber 801B, a fiber coupler <NUM>, a photonic die <NUM>, and beam splitter <NUM>. The photonic die <NUM> may be similar to the photonic die described earlier with respect to <FIG>, and comprises the Tx grating couplers 805A and Rx grating couplers 805B and 805C. In an example scenario, the Tx couplers 805A and Rx grating couplers 805B may comprise single polarization grating couplers and the Rx grating couplers 805C may comprise polarization splitting grating couplers.

The prism 807C may comprise a transparent structure with thin film filters formed on sloped surfaces for reflecting desired signals down to the Rx grating couplers 805B and 805C as well as from the Tx grating couplers 805A to the Tx fiber 801A via the splitter prism 807A. The prism 807A may also have thin films formed on an angled surface thereby forming TFF 809A for splitting signals of different polarizations upon hitting the sloped surface, while mirror prism 807B comprises layers formed on an angled surface to provide a mirror <NUM> for reflecting signals from the TFF 809A to the Rx grating couplers 805C.

The transceiver <NUM> incorporates the beam splitter <NUM> comprising the TFF 809A in the splitter prism 807A and the mirror <NUM> in the mirror prism 807B to spatially separate signals of different polarizations, such that the different Rx grating couplers 805B and 805C may be utilized for different polarizations and wavelengths from a single received CWDM signal. In addition, the transceiver comprises a <NUM>th TFF for the <NUM>th p-polarization, as illustrated in <FIG>.

Each TFF 809B-809F may be designed to reflect the s-polarization of one CWDM band and p-polarization of the previous CWDM band while allowing all others to pass through. This uses the same approach as the previous implementation of <FIG>, where the band-edges of the p- and s-polarization transmissions are deliberately separated. In this example, the delta between them is set to <NUM> (CWDM channel spacing). The delay between the two polarizations can be readily compensated on silicon, such as with a few hundred microns of extra waveguide length on one side, for example.

In operation, the transceiver <NUM> is operable to receive and transmit CWDM4 signals through the use of spatially separated polarization splitters and wavelength sensitive thin film filters. Four optical signals at different CWDM wavelengths may be generated in the photonics die <NUM>, such as described previously, and coupled out of the die via the Tx grating couplers 805A. The TFFs 809B-809F reflect each of the signals out of the TFF prism 807C into the splitter prism 807A and into the Tx fiber 801A, thereby generating a CWDM4 signal transmitted into the fiber 801A.

Similarly, a CWDM signal may be received via the Rx fiber 801B and coupled to the beam splitter <NUM> where one polarization passes through the TFF 809A to the TFF prism 807C, where each of the TFFs 809B-809F reflects a particular wavelength and polarization signal down to the Rx grating couplers 805B, which couple the corresponding wavelength signal into the photonic die <NUM> for processing. The other polarization signals at the TFF 809A are reflected laterally to the mirror <NUM>, which reflects the signals into the TFF prism 807C, where the TFFs 809B-809F each reflect a specific wavelength and polarization signal down to the Rx grating couplers 805C, which couple the signals into the photonic die <NUM> for processing.

<FIG> illustrates the spectral bands for a CWDM stack of thin film filters, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there are shown stacked TFF spectral bands, showing band <NUM> through band <NUM>, for a CWDM4 application, ranging from <NUM> - <NUM>. Each filter may be tuned to reflect the s-polarization of one CWDM band and the p-polarization of the previous CWDM band. Therefore, the first TFF may reflect the p-polarization for band <NUM> and the s-polarization for band <NUM>, the second TFF for the p-polarization of band <NUM> and the s-polarization for band <NUM>, and so on. In this manner, five TFFs may be utilized for <NUM> CWDM signals.

<FIG> illustrates a beam splitter with a polarization rotator, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a beam splitter <NUM> with polarization rotation, the beam splitter comprising a mirror <NUM> formed on an angled surface of rhomboid prism <NUM>, a polarization splitting thin film stack <NUM> on trapezoidal prism <NUM>, a polarization rotator <NUM>, and a plate <NUM>. The mirror <NUM>, prisms <NUM> and <NUM>, and thin film stack <NUM> may be similar to elements described with respect to <FIG>.

In addition, a half-wave plate oriented at <NUM> degrees to the input signal formed or mounted below the mirror rotates the polarization so that only a single polarization, p-polarization in this example, is incident on the photonic die. The polarization rotator <NUM> may comprise a ½ wavelength polarization rotator plate where an incoming signal at one surface may be rotated by <NUM> degrees upon exiting from the opposite surface. For example, an s-polarized signal would be p-polarized upon traveling through the rotator <NUM>. Finally, a glass plate <NUM> may be placed adjacent to the half-wave plate polarization rotator <NUM> to planarize the bottom of the beam splitter <NUM>.

In operation, an input optical signal comprising s- and p-polarized signals may be received in the rhomboid prism <NUM> such that it hits the thin film stack <NUM>, which reflects the s-polarization signals laterally and transmits the p-polarization signals downward to grating couplers in the photonic die through the polarization rotator <NUM>. A mirror formed on a rhomboid prism adjacent to the thin film stack reflects the reflected s-polarization signals down to the photonic die, resulting in laterally displaced signals.

<FIG> illustrates an optical transceiver with a spatial separation beam splitter and polarization rotator, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown transceiver <NUM> comprising Tx fiber 1101A, Rx fiber 1101B, photonics die <NUM>, beam splitter <NUM>, and a fiber coupler <NUM>.

The beam splitter <NUM> may comprise TFF 1109A on an angled surface of splitter prism 1107A, mirror <NUM> on an angled surface of mirror prism 1107B, and a rotator <NUM> adjacent to the mirror prism 1107B. The rotator <NUM> may comprise a half-wave plate, for example, for rotating the polarization of the s-polarization to that of the p-polarization at the TFF 1109A. A glass plate <NUM> may be placed adjacent to the half-wave plate polarization rotator <NUM> to planarize the surface of the beam splitter <NUM> coupled to the TFF prism 1107C.

The photonic die <NUM> may comprise Tx grating couplers 1105A and Rx grating couplers 1105B and 1105C. In an example scenario, the grating couplers 1105A-1105C comprise single polarization grating couplers, as opposed to a combination of single polarization and polarization splitting grating couplers, which may provide lower coupling efficiency.

The input signal comprising signals of different wavelength and polarization from the Rx fiber 1101B, may be split into p-polarization signals transmitting through the polarization splitting TFF 1109A, through the plate <NUM>, and to the TFF prism 1107C. S-polarized signals may be reflected towards the mirror <NUM>, which then reflects them to the rotator <NUM>, which rotates the signals to p-polarization signals that are coupled to the TFF prism 1107C. Therefore, only p-polarization signals are transmitted out of the beam splitter <NUM>, which due to the orientation of the TFF 1109A, are s-polarized at the surface of the filters. Due to the rotation of the optical signals by the rotator <NUM> resulting in all signals having the same polarization coming out of the beam splitter <NUM>, single polarization grating couplers may be utilized throughout the transceiver <NUM>, as opposed to utilizing polarization splitting grating couplers with lower coupling efficiency.

The transceiver <NUM> incorporates the spatially separating beam splitter <NUM>, such that different Rx grating couplers 1105A may be utilized for different wavelengths. The delay between the different p-polarization signals at the surface of the photonic die <NUM> may be readily compensated on silicon, such as with a few hundred microns of extra waveguide length on one side, for example.

Since PSGCs can be replaced by SPGCs throughout the transceiver <NUM>, a significant improvement in Rx insertion loss may be demonstrated. This benefit is obtained irrespective of the exact implementation of the transceiver <NUM>.

<FIG> illustrates an optical transceiver with a single polarization MUX/DEMUX with beam displacer, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown transceiver <NUM> comprising Tx fiber 1201A, Rx fiber 1201B, a fiber coupler <NUM>, displacer <NUM>, TFF prism <NUM>, and photonics die <NUM>.

The photonics die <NUM> may comprise Tx grating couplers 1205A and Rx grating couplers 1205B and 1205C. The TFF prism <NUM> may comprise TFFs 1209A-1209D for reflecting optical signals of a specific wavelength down to the Rx grating couplers 1205B and <NUM>-5C, as well as to reflect optical signals received from the Tx grating couplers 1205A to the beam splitter <NUM>.

The beam splitter <NUM> may comprise a beam displacer <NUM>, a rotator <NUM>, and a plate <NUM>, all in an optically transparent material. In the example shown in <FIG>, polarization separation is accomplished by using the beam displacer <NUM>, which may comprise a plate comprising a strongly birefringent crystal, such as yttrium orthovanadate (YVO4), with an optical axis at <NUM> degrees as shown. This crystal refracts the s-polarization laterally with respect to the p-polarization signal, which passes straight through to the half-wave plate rotator <NUM>, resulting in s-polarization signals out of the beam splitter <NUM>.

The rotator <NUM> may comprise a half-wave plate, for example, for rotating the polarization of the p-polarization signal s-polarization. Due to the rotation of the s-polarization signal to p-polarization via the rotator <NUM>, single polarization grating couplers may be utilized as opposed to polarization splitting grating couplers.

The transceiver <NUM> incorporates the spatially separating beam splitter <NUM>, such that different Rx grating couplers 1205B and 1205C may be utilized for different wavelengths. The delay between the laterally displaced signals at the surface of the photonic die <NUM> may be readily compensated on silicon, such as with a few hundred microns of extra waveguide length on one side, for example.

In an example embodiment of the disclosure, a method and system is described for eliminating polarization dependence for <NUM> degree incidence MUX/DEMUX designs. The system may comprise an optical transceiver, where the optical transceiver comprises an input optical fiber, a beam splitter, and a plurality of thin film filters coupled to a photonics die, and where the thin film filters are arranged above corresponding grating couplers in the photonics die.

The transceiver may be operable to receive an input optical signal comprising a plurality of different wavelength signals via the input optical fiber, split the input optical signal into signals of a first polarization and signals of a second polarization using the beam splitter by separating the signals of the second polarization laterally from the signals of the first polarization, communicate the signals of the first polarization and the second polarization to the plurality of thin film filters, and reflect signals of each of the plurality of different wavelength signals to corresponding grating couplers in the photonics die using the thin film filters. Optical signals may be communicated at a plurality of wavelengths out of the photonics die to the thin film filters.

Each of the optical signals from the photonics die may be reflected to the beam splitter using the thin film filters. The reflected optical signals from the photonics die may be communicated to an output fiber of the optical transceiver. Each of the thin film filters may be configured to reflect optical signals of the first polarization at a first wavelength and signals of the second polarization at a second wavelength. The beam splitter may comprise a thin film stack on an angled surface of a first prism, with the thin film stack being configured to reflect signals of the second polarization while allowing signals of the first polarization to pass through.

The separated signals of the second polarization may be communicated to the plurality of thin film filters using a mirror in the beam splitter formed on an angled surface of a second prism adjacent to the first prism. A polarization of the reflected signals of the second polarization may be rotated using a polarization rotator on the second prism before being communicated to the plurality of thin film filters. The signals of the second polarization may be separated laterally from the signals of the first polarization using a birefringent material. The birefringent material may allow signals of the first polarization to pass directly through to the plurality of thin film filters.

Claim 1:
A method for communication, the method comprising:
in an optical transceiver (<NUM>), the optical transceiver (<NUM>) comprising an input optical fiber (801B), a beam splitter (<NUM>), and a plurality of thin film filters (809B-F) coupled to a photonics die, the thin film filters (809B-F) being arranged above corresponding grating couplers in the photonics die:
receiving an input optical signal comprising a plurality of different wavelength signals via the input optical fiber (801B);
splitting the input optical signal into signals of a first polarization and signals of a second polarization using the beam splitter (<NUM>) by separating the signals
of the second polarization laterally from the signals of the first polarization;
communicating the signals of the first polarization and the second polarization to the plurality of thin film filters (809B-F).
separating, using the plurality of thin film filters (809B-F):
the communicated signals of the first polarization into a first plurality of signals that each correspond to one of the different wavelength signals,
the communicated signals of the second polarization into a second plurality of signals that each correspond to one of the different wavelength signals; and
communicating each of the signals of the first and second pluralities of signals to a corresponding grating coupler in the photonics die.