Patent Description:
Aspects of the present disclosure relate to optoelectronic components. More specifically, certain implementations of the present disclosure relate to methods and systems for near normal 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.

In <CIT>, an optical multiplexer/demultiplexer includes: a plurality of light emission/reception elements; a plurality of light branching elements allowing a part of incident light to pass through and reflecting the remaining part; a plurality of connection elements arranged on the optical path connecting the corresponding light emission/reception elements and the light branching elements; and waveguide elements arranged on the optical path of a reflected light from a light branching element to another light branching element where the reflected light comes. All the connection elements are formed into a single connection element block. This suppresses increase of the assembling time and the production cost. Moreover, by using a spherical mirror requiring a smaller number of design parameters as a connection element, the yield can be increased even if the connection elements are formed in to a single block of array. Moreover, by using a concave mirror as a waveguide element and arranging all the waveguide elements into a single waveguide element block, it is possible to obtain a large alignment allowance of the waveguide element block, which facilitates assembling.

In <CIT>, an optical wavelength division multiplexer and demultiplexer is provided wherein the optical couplings and alignment of the device are achieved by forming and adhesively joining a prefabricated optical block to a molded coupling module to avoid post-fabrication alignment and adjustment of the optical pathway otherwise required. One or more adhesive overflow channels are formed in said upper surface of the molded coupling module to receive excess adhesive used to join the optical block to the molded coupling module. Dams are formed adjacent each of said adhesive overflow channels to contain adhesive used to join the optical block to the molded coupling module, and to facilitate the use of low viscosity adhesive which in turn achieves a tighter, more parallel bond between the adhesively bonded parts.

System and methods are provided for near normal 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 near normal incidence multiplexers/demultiplexers. 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.

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 coupler 103A. 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 dies, 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 IC130. A lens array may be incorporated between the grating couplers 104A-104D and the laser assembly <NUM> for focusing of the optical signals 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 signal. 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 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 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 coupled to the CMOS chip, using coupling optics <NUM> that selectively deflect 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 coupling optics <NUM> are described further with respect to <FIG>.

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. 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. An optical transceiver chip 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.

One example 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. The input to such an IC 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 photodetectors and converted into a suitable electrical signal by going through a Transimpedance 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, and mirrors deflecting optical signals down to near-normal incidence on the chip while allowing other wavelength signals to pass through. These are shown further with respect to <FIG>.

<FIG> is a schematic illustrating thin film filters with a launching filter. Referring to <FIG>, there is shown a transceiver <NUM> with optical signals coupled via fibers <NUM> and a coupler <NUM>. The coupler <NUM> comprises mirrors <NUM>, glass <NUM>, thin film filters (TFFs) <NUM>, and a lens array <NUM>. The coupler <NUM> may be configured to direct optical signals into photonics die <NUM> at near-normal incidence.

The fibers <NUM> may comprise one or more optical fibers for coupling optical signals to and from the coupler <NUM> and photonics die <NUM> and may be within the fiber coupler <NUM>, which may comprise lenses, filters, or polarization controlling components. For example, the fiber coupler <NUM> may comprise a lens for focusing the optical signals from the fibers <NUM> onto the mirror <NUM>, or focus signals from the mirror <NUM> to the fibers <NUM>. In another example, the fiber coupler <NUM> may comprise polarization splitting components to spatially separate signals of different polarizations before reflecting off the mirror <NUM>. The fibers may comprise single mode or multi-mode fiber. In an example scenario, one fiber is used to couple signals into the coupler <NUM> and subsequently to the photonics die <NUM>, while a second fiber receives optical signals from the photonics die via the coupler <NUM>.

The glass <NUM> may comprise a machined and/or polished highly transparent structure on which optical components such as mirrors and filters may be formed. For example, one or more layers of a highly reflective metal, such as gold, for example, may be deposited on highly polished surfaces of the glass <NUM>, thereby forming mirror <NUM>. Similarly, filter structures may be formed by depositing stacks of dielectric layers on the glass <NUM>, thereby forming the TFFs <NUM>, for example. Accordingly, the glass <NUM> may guide optical signals from the fiber <NUM> end to the TFF <NUM> end, and vice versa.

The lens array <NUM> may comprise a micro-machined silicon structure, for example, with lens structures formed therein that are operable to focus optical signals received from the TFFs <NUM> to specific spots on the photonics die <NUM>, such as grating couplers. While convex lensing structures are shown, other shapes may be utilized depending on desired focal length, the dielectric constant of the lens material used, and space requirements, for example.

The TFFs <NUM> may comprise stacks of alternating dielectric constant materials resulting in structures that are reflective at most wavelengths but allow light of specific wavelengths to pass through. Each of the TFFs <NUM> may be tuned to different wavelengths, which may be useful for CWDM applications. While four TFFs <NUM> are shown in <FIG>, any number of TFFs may be used depending on the number of desired different wavelengths.

The mirrors <NUM> may comprise a highly reflective material, such as a metal, formed on the glass <NUM>, for directing the optical signals to the TFFs <NUM> from the fibers <NUM>. The fibers <NUM> may comprise collimators at their outputs for providing collimated beams to the coupler <NUM>.

In operation, optical signals may be coupled into the coupler <NUM> via the fibers <NUM> and reflected by the mirrors <NUM> and TFFs <NUM>, resulting in a multi-reflection configuration. The TFFs <NUM> are each configured to reflect all signals except for those in a specific wavelength range. In this manner, specific wavelength optical signals may be coupled to specific locations on the photonics die <NUM>, preferably to grating couplers tuned to the specific wavelength. The mirrors <NUM> enable an essentially vertical impingement on the TFFs <NUM>, so that further reflecting structures are not needed after the TFFs <NUM> for desired near-normal incidence on the photonics die <NUM>, thereby maximizing the coupling efficiency of optical signals in to the photonics die <NUM>.

<FIG> is a schematic illustrating thin film filters integrated with collimating lenses. Referring to <FIG>, there is shown a transceiver <NUM> with optical signals coupled fiber coupler <NUM> and die coupler <NUM>. The fiber coupler <NUM> comprises a lens <NUM>, a V-groove <NUM>, and a reflector <NUM>. In an example scenario, the fiber coupler <NUM> comprises two-fiber V-grooves for aligning two fibers <NUM> (with only one shown in the cross-section of <FIG>). The fiber coupler <NUM> may also comprise polarization controlling components. For example, the fiber coupler <NUM> may comprise a lens/filter <NUM> for focusing the optical signals from the fibers <NUM> onto the reflector <NUM>, or focus signals from the reflector <NUM> to the fibers <NUM>. In another example, the lens/filter <NUM> in the fiber coupler <NUM> may comprise polarization splitting components to spatially separate signals of different polarizations before reflecting off the reflector <NUM>.

In another example, the lens <NUM> may comprise a graded index (GRIN) lens, for example, for focusing optical signals from the fiber <NUM> onto the reflector <NUM> so that reflected signals impinge on the desired TFF 309A-309D, and also for focusing signals received from the photonics die <NUM> via the die coupler <NUM> into the fibers <NUM>. The reflector <NUM> may comprise highly reflective material, such as a metal, to direct optical signals from the fiber <NUM> onto the first of the TFFs 309A-309D, resulting in a multi-reflection configuration.

The die coupler <NUM> comprises glass <NUM> with mirror <NUM>, thin film filters (TFFs) 309A-309D, and a lens array <NUM> formed thereon. The mirror <NUM> may be formed by depositing a highly reflective metal on the top surface of the glass <NUM> and may extend the length of the grating couplers 313A-313D in the photonics die <NUM> to allow for multiple reflections.

The TFFs 309A-309D are each configured to reflect all optical signals except for those in a specific wavelength range. In this manner, specific wavelength optical signals may be coupled to corresponding grating couplers <NUM> on photonics die <NUM>, tuned to a specific wavelength for that grating coupler. The mirror <NUM> and TFFs 309A-309D enable an essentially vertical impingement on the TFFs 309A-309D, so that further reflecting structures are not needed after the TFFs 309A-309D for desired near-normal incidence on the grating couplers 313A-313D on the photonics die <NUM>.

The lenses 311A-311D may comprise a micro-machined silicon structure, for example, that is operable to focus optical signals received from the TFFs 309A-309D to specific grating couplers 313A-313D, and also to focus optical signals from the photonics die <NUM> onto the TFFs 309A-309D. The TFFs 309A-309D and lenses 311A-311D may be formed and/or machined on the same structure. While convex lensing structures are shown, other shapes may be utilized depending on desired focal length, the dielectric constant of the lens material used, and space requirements, for example.

The TFFs 309A-309D may comprise stacks of alternating dielectric constant materials resulting in structures that are reflective at most wavelengths but allow light of specific wavelengths to pass through. Each of the TFFs 309A-309D may be tuned to different wavelengths, which may be useful for CWDM applications. While four TFFs 309A-309D are shown in <FIG>, any number of TFFs may be used depending on the number of desired different wavelengths.

In an example scenario, the TFFs 309A-309D and lens array <NUM> subassembly may first be precision attached to the photonics die <NUM>, followed by the fiber coupler <NUM> being actively aligned to the TFF 309A-309D and lenses 311A-311D subassembly.

In operation, a CWDM4 optical signal may be coupled into the fiber coupler <NUM> via the fibers <NUM> and directed by the reflector <NUM> down to the first TFF 309A, where the optical signal with the wavelength that corresponds to TFF 309A will pass through to the lens 311A, while all other optical signals reflect up to the mirror <NUM>. The reflected signals are again reflected down by the mirror <NUM>, but to the second TFF 309B where a second wavelength optical signal passes through while the remaining signals are reflected to the mirror <NUM>. This continues until the last remaining optical signal passes through the last TFF 309D and lens 311D to the last grating coupler 313D. In this manner, a CWDM signal may be demultiplexed and processed individually by the photonic and electronic circuitry in the photonics die, as described previously with respect to <FIG>, for example.

In this manner, specific wavelength optical signals may be coupled to specific locations on the photonics die <NUM>, to grating couplers tuned to the specific wavelength. The mirror <NUM> enables an essentially vertical impingement on the TFFs 309A-309D, so that further reflecting structures are not needed after the TFF 309D for desired near-normal incidence on the photonics die <NUM>, thereby maximizing the coupling efficiency of optical signals in to the photonics die <NUM>.

<FIG> illustrates an integrated MUX/DEMUX with lenses integrated on silicon, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a MUX/DEMUX <NUM> formed in a silicon substrate <NUM> with lenses/TFFs 407A-407D formed on one surface and a back-side mirror 403A formed on the opposite side. The lenses/TFFs 407A-407D may comprise lenses incorporated with TFFs, in that stacks of dielectric layers for TFFS may be deposited on lenses formed in the silicon substrate <NUM>, for example. In another example embodiment, the lenses may be deposited on the Si substrate <NUM> and then dielectric stacks may then be formed on the lenses. The dielectric stacks may be configured such that only optical signals of a desired wavelength pass through to be focused by each lens.

The fiber <NUM> may comprise a single mode or multi-mode fiber that is placed in a V-groove <NUM> formed in the Si substrate <NUM> for proper alignment. The lid <NUM> may comprise physical support and hermetic sealing, for example, for the optical end of fiber <NUM>. The V-groove <NUM> may be formed in the silicon with an angled surface for receiving optical signals from the fiber <NUM>, where the angled surface may comprise an anti-reflection (AR) coating <NUM> to reduce or eliminate unwanted reflections back into the fiber <NUM>. In an example scenario, the V-groove <NUM> and mirror angled surface may be formed in the same etch step, using potassium hydroxide (KOH), for example, which would give a degree of self-alignment. Using such an etch, which follows crystal planes, may enable exceptionally tight angle control. In this manner, the various planes of the mirrors and V-groove are defined by crystallographic axes of the silicon. An opposite surface to the AR coating <NUM> may be coated with a highly reflective metal to form mirror 403B.

The arrangement of the AR coating, mirrors 403A and 403B, and lenses/TFFs 407A-407D enable a multi-reflection configuration, where at each lens/TFF 407A-407D most wavelengths are reflected while only a desired wavelength is transmitted through the lens/TFF 407A-407D to a coupler on a photonics die. Utilizing a silicon substrate for the MUX/DEMUX <NUM> may enable tight thickness and coplanarity control for the multi-reflection configuration. In an example scenario, the Si substrate <NUM> may be ~ <NUM> thick and the angle of reflection at the backside mirror 403A may be ~ <NUM> degrees, in that an incoming optical signal is at <NUM> degrees from normal incidence.

In operation, a CWDM4 optical signal may be coupled into the MUX/DEMUX <NUM> via the fiber <NUM> and angled slightly downward by the change of index of refraction of the Si substrate <NUM>, before being reflected downward by the mirror 403B to the backside mirror 403A. The angle of the mirror 403B may be configured such that the reflection of the optical beam is such that it is centered on the lens/TFF 407A when reflected back upward by the backside mirror 403A. In an example scenario, the angle of the mirror 403B may be defined by crystalline planes of the silicon, formed by anisotropic etching, for example, thereby enabling very tight distribution of this angle. At the lens/TFF 407A, a first wavelength optical signal, λ<NUM> in this example, passes through while the remaining wavelength optical signals are reflected back downward to the backside mirror 403A. The optical signal at wavelength λ<NUM> is coupled to a photonics die coupled to the MUX/DEMUX <NUM>.

After being reflected by the lens/TFF 407A, the optical signal thus comprises signals of wavelengths λ<NUM>- λ<NUM>, as indicated in <FIG> at the second reflection at the backside mirror 403A. This signal then reaches the lens/TFF 407B where the λ<NUM> wavelength optical signal passes through while the λ<NUM> and λ<NUM> wavelength signals are reflected back to the backside mirror 403A. This reflection and transmission continues until each optical signal is communicated through one of the lenses/TFFs 407A-407D, demonstrating a CWDM4 demultiplexer. Although a four wavelength DEMUX is shown in <FIG>, any number of wavelengths may be demultiplexed based on the number of lenses/TFFs incorporated in the structure.

In addition, the MUX/DEMUX <NUM> may multiplex signals by essentially operating in the reverse direction to that described above. In this case, optical signals of four different wavelengths may be received from a photonic chip, pass through the corresponding lens/TFF 407A-407D and be reflected by the backside mirror 403A. As the next lens/TFF is tuned to a different wavelength, each signal will thus be reflected by the other lenses/TFFs 407A-407D to the backside mirror 403A consecutively until reaching the mirror 403B and AR coating <NUM>, before being coupled into the fiber <NUM>, thereby generating a CWDM4 signal.

<FIG> illustrates an integrated MUX/DEMUX with thin film filters integrated on silicon and coupled to a lens array and photonics die, in accordance with an example embodiment of the disclosure. Referring to <FIG>, there is shown a MUX/DEMUX <NUM> formed in a silicon substrate <NUM> with TFFs 509A-509D formed on one surface and a back-side mirror 503A formed on the opposite side. The TFFs 509A-509D may comprise dielectric stacks that may be configured such that only optical signals of a desired wavelength pass through while reflecting other optical wavelengths.

The lenses 507A-507D may comprise lenses formed in or on a silicon substrate and may be operable to focus received signals onto corresponding grating couplers in the photonics die <NUM>. In addition, the lenses 507A-507D may be operable to focus optical signals received from grating couplers in the photonics die <NUM> to the TFFs 509A-509D.

The fiber <NUM> may comprise a single mode or multi-mode fiber that is placed in a V-groove <NUM> formed in the Si substrate <NUM> for proper alignment. The lid <NUM> may comprise physical support and hermetic sealing, for example, for the optical end of fiber <NUM>. The V-groove <NUM> may be formed in the silicon with an angled surface for receiving optical signals from the fiber <NUM>, where the angled surface may comprise an anti-reflection (AR) coating <NUM> to reduce or eliminate unwanted reflections back into the fiber <NUM>. In an example scenario, the V-groove <NUM> and mirror angled surface may be formed in the same etch step, using potassium hydroxide (KOH), for example, which would give a degree of self-alignment. Using such an etch, which follows crystal planes, may enable exceptionally tight angle control. In this manner, the various planes of the mirrors and V-groove are defined by crystallographic axes of the silicon. An opposite surface to the AR coating <NUM> may be coated with a highly reflective metal to form mirror 503B.

The arrangement of the AR coating, mirrors 503A and 53B, and TFFs 509A-509D enable a multi-reflection configuration, where at each TFF 509A-509D most wavelengths are reflected while only a desired wavelength is transmitted through the TFF 509A-509D to a corresponding lens 507A-507D before being coupled to the photonics die <NUM>. Utilizing silicon substrates for the MUX/DEMUX <NUM> may enable tight thickness and coplanarity control for the multi-reflection configuration. In an example scenario, the Si substrate <NUM> may be ~ <NUM> thick and the angle of reflection at the backside mirror 403A may be ~ <NUM> degrees, in that an incoming optical signal is at <NUM> degrees from normal incidence.

In operation, a CWDM4 optical signal may be coupled into the MUX/DEMUX <NUM> via the fiber <NUM> and angled slightly downward by the change of index of refraction of the Si substrate <NUM>, before being reflected downward by the mirror 503B to the backside mirror 503A. The angle of the mirror 503B may be configured such that the reflection of the optical beam is such that it is centered on the TFF 509A when reflected back upward by the backside mirror 503A. In an example scenario, the angle of the mirror 503B may be defined by crystalline planes of the silicon, formed by anisotropic etching, for example, thereby enabling very tight distribution of this angle. At the TFF 509A, a first wavelength optical signal, λ<NUM> in this example, passes through while the remaining wavelength optical signals are reflected back downward to the backside mirror 503A. The optical signal at wavelength λ<NUM> may be focused by the lens 507A to be coupled to the photonics die <NUM>.

After being reflected by the TFF 509A, the optical signal thus comprises signals of wavelengths λ<NUM>- λ<NUM>, as indicated in <FIG> at the second reflection at the backside mirror 503A. This signal then reaches the TFF 509B where the λ<NUM> wavelength optical signal passes through while the λ<NUM> and λ<NUM> wavelength signals are reflected back to the backside mirror 503A. This reflection and transmission continues until each optical signal is communicated through one of the TFFs 509A-509D, demonstrating a CWDM4 demultiplexer. Although a four wavelength DEMUX is shown in <FIG>, any number of wavelengths may be demultiplexed based on the number of lenses and TFFs incorporated in the structure.

In addition, the MUX/DEMUX <NUM> may multiplex signals by essentially operating in the reverse direction to that described above. In this case, optical signals of four different wavelengths may be received from the photonics die <NUM>, pass through the corresponding lens 507A-507D and TFF 509A-509D and be reflected by the backside mirror 503A. As the next TFF is tuned to a different wavelength, each signal will thus be reflected by the other TFFs 509A-509C to the backside mirror 503A consecutively until reaching the mirror 503B and AR coating <NUM>, before being coupled into the fiber <NUM>, thereby generating a CWDM4 signal.

In an example embodiment of the disclosure, a method and system is described for near normal incidence MUX/DEMUX designs, according to claims <NUM>, <NUM> and <NUM>.

Claim 1:
A method for communication, the method comprising:
in an optical demultiplexer coupled to a photonics die, the optical demultiplexer comprising a plurality of thin film filters (407A-D) formed on a first surface of a substrate (<NUM>), an input fiber (<NUM>) arranged in a V-groove (<NUM>) of the first surface of the substrate (<NUM>), a first mirror (403B) on a sloping surface formed in the first surface of the substrate (<NUM>), and a second mirror (403A) formed on a second surface of the substrate (<NUM>) opposite to the first surface:
receiving via the input fiber an input optical signal comprising a plurality of wavelength optical signals;
reflecting the input optical signal from the first mirror (403B) to the second mirror (403A);
reflecting the input optical signal from the second mirror (403A) to a first of the plurality of thin film filters (407A-D);
communicating an optical signal at a first wavelength of the plurality of wavelength optical signals to the photonics die while reflecting others of the plurality of wavelength optical signals to the second mirror (403A);
reflecting the others of the plurality of wavelength optical signals from the second mirror (403A) to a second of the plurality of thin film filters (407A-D); and
communicating an optical signal at a second wavelength of the plurality of wavelength optical signals to the photonics die, wherein the substrate (<NUM>) comprises silicon.