Abstract:
An optical transceiver comprising: an optical transmitter having plurality of light sources controllable to generate optical signals in different optical channels, an output aperture, an optical multiplexer that multiplexes optical signals generated by the light sources and transmits them to exit the transceiver from the output aperture; an optical receiver having a plurality of optical sensors, an input aperture for receiving optical signals in a plurality of optical channels, a demultiplexer that demultiplexes signals received at the input aperture, and directs signals received in different channels to different optical sensors of the plurality of optical sensors; and a QSFP compliant housing that houses the transmitter and receiver.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application 61/316,155 filed on Mar. 22, 2010, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments of the invention relate to providing small optical receivers, transmitters and transceivers that support high data transmission rates for telecommunications and data communications. 
       BACKGROUND 
       [0003]    The amount of information transferred over the various local and global communications networks is growing at a staggering rate. A recent white paper published in 2008 by Cisco Systems Inc. entitled “Approaching the Zettabyte Era” predicts that global IP traffic will increase from about 10 exabytes per month in 2008 to over 40 exabytes per month in 2012. The rapid increase in global communication traffic has generated a need for faster and smaller communications components. 
         [0004]    In November 2006 a group of leading communications companies promulgated a specification for a physically small optical transceiver capable of supporting data transfer rates of up to 40 Gbits per second. The latest update of the specification was issued in March 2009. 
         [0005]    The specification defines an optical transceiver, comprising four independent optical transmit channels and four independent optical receive channels. Each transmit channel is required to be capable of transmitting data at up to 10 gigabits per second (Gbps). The specified transceiver is configured to multiplex the data from the four independent transmit channels and transmit the multiplexed data over a single mode fiber (SMF), hereinafter a “transmit fiber”, for a total aggregated transceiver transmission data rate of 40 Gbps. Each receive channel is required to be capable of receiving data at up to 10 gigabits per second (Gbps) for a total aggregated transceiver receive data rate of 40 Gbps. The transceiver is configured to receive data over a single SMF “receiver fiber” and demultiplex the received data to the four receive channels. The specified transceiver is designed to replace four standard SFP transceivers and occupy a space of only about 30% more than one of the standard SFP transceivers. It is referred to as a “Quad Small Form-factor Pluggable” optical module, and is commonly referred to by its acronym “QSFP”. 
       SUMMARY 
       [0006]    An embodiment of the invention relates to providing a small optical receiving module comprising a Planar Lightwave Circuit (PLC) for receiving optical signals transmitted in a plurality of different optical channels over a single optical fiber, demultiplexing the signals, and generating electrical signals responsive to the demultiplexed signals. 
         [0007]    In accordance with an embodiment of the invention, the PLC comprises an optical filter, optionally a thin film filter (TFF), optically coupled to an output port of a Mach Zehnder Interferometer (MZI) for each optical channel for demultiplexing optical signals received by the receiving module to the optical channel. An optical sensor, optionally a photodiode (PD), is coupled to the channel&#39;s TFF and generates electrical signals responsive to the optical signals demultiplexed to the channel. The receiving optical module is also referred to as a Receiving Optical Sub-Assembly (ROSA) 
         [0008]    An embodiment of the invention relates to providing a small optical transmitting module comprising a Planar Lightwave Circuit (PLC) for generating optical signals in a plurality of different optical channels and multiplexing the signals for transmission over a single optical fiber. In accordance with an embodiment of the invention, for each optical channel the module comprises a light source coupled to an input port of an MZI for generating optical signals in the channel and multiplexing the generated signals. Optionally the light source comprises a laser diode. Optionally the laser diode is a coarse wavelength division multiplexing (CWDM) distributed feedback (DFB) laser diode. The transmitting module is also referred to as a transmitting optical sub-assembly (TOSA). 
         [0009]    An aspect of some embodiments of the invention, relate to providing a QSFP transceiver comprising a ROSA, i.e. a receiving optical sub-assembly, and a TOSA, i.e. a transmitting optical subassembly, in accordance with embodiments of the invention. 
         [0010]    There is therefore provided in accordance with an embodiment of the invention an optical transceiver comprising: an optical transmitter having, a plurality of light sources controllable to generate optical signals in different optical channels, an output aperture, and an optical multiplexer that multiplexes optical signals generated by the light sources and transmits them to exit the transceiver from the output aperture; an optical receiver having, a plurality of optical sensors, an input aperture for receiving optical signals in a plurality of optical channels, a demultiplexer that demultiplexes signals received at the input aperture and directs signals received in different channels to different optical sensors of the plurality of optical sensors; and a QSFP compliant housing that houses the transmitter and receiver. Optionally, each optical sensor is shielded by a thin film filter that transmits light in one of the plurality of optical channels and blocks light in the other of the plurality of optical channels. Additionally or alternatively, the receiver comprises a planar light circuit (PLC). 
         [0011]    In an embodiment, the receiver comprises a planar light circuit (PLC). Optionally, the transmitter comprises a PLC. Optionally, the transmitter PLC and the receiver PLC are positioned adjacent each other. Optionally, the transceiver comprises a conducting panel located between the transmitter and receiver PLCs. Optionally, the transmitter and receiver PLCs are substantially coplanar. Optionally, the transmitter and receiver PLCs are mounted to a same planar support base. 
         [0012]    In an embodiment of the invention, the transmitter and receiver PLCs are integrally formed on a same substrate. Optionally, the substrate comprises a glass. 
         [0013]    In an embodiment of the invention, the multiplexer comprises a Mach Zehnder Interferometer (MZI). Optionally, the multiplexer comprises a cascade of at least three MZIs. 
         [0014]    In an embodiment of the invention, the demultiplexer comprises a Mach Zehnder Interferometer (MZI). Optionally, the demultiplexer comprises a cascade of at least three MZIs. 
         [0015]    There is further provided in accordance with an embodiment of the invention, a receiving optical module for demultiplexing optical signals, the module comprising: a planar optical substrate having an input aperture for receiving optical signals in each of a plurality of different optical channels; at least one Mach-Zehnder interferometer formed in the optical substrate and comprising an entry waveguide that receives optical signals that enter the module via the input aperture, and first and second exit waveguides via which received optical signals in different optical channels of the plurality of optical channels exit the interferometer; and a different optical sensor for each exit waveguide that receives optical signals that exit the interferometer via the exit waveguide. Optionally, each optical sensor is shielded by a thin film filter that transmits light in one of the plurality of optical channels and blocks light in the other of the plurality of optical channels. 
         [0016]    There is further provided in accordance with an embodiment of the invention, a transmitting optical module for multiplexing and transmitting optical signals, the module comprising: a planar optical substrate having an output aperture for transmitting optical signals; at least one Mach-Zehnder interferometer formed in the optical substrate comprising an exit waveguide optically coupled to the output aperture and first and second entry waveguides; and different light sources optically coupled to the substrate that generate optical signals in different optical channels that are received by the interferometer in different entry waveguides, which optical signals exit the interferometer via the exit waveguide. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0017]    Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
           [0018]      FIG. 1  schematically shows a ROSA, in accordance with an embodiment of the invention; 
           [0019]      FIG. 2A  shows a graph of transmittance for an array of cascaded MZIs comprised in a ROSA, in accordance with an embodiment of the invention; 
           [0020]      FIGS. 2B  shows a graph of transmittance for a TFF comprised in a ROSA, in accordance with an embodiment of the invention; 
           [0021]      FIGS. 2C  shows a graph of transmittance of a combination of MZIs and TFFs comprised in a ROSA, in accordance with an embodiment of the invention; 
           [0022]      FIG. 3  schematically shows a TOSA, in accordance with an embodiment of the invention; 
           [0023]      FIG. 4  schematically shows a QSFP transceiver comprising the ROSA and TOSA shown in  FIG. 1  and  FIG. 2  respectively in accordance with an embodiment of the invention; and 
           [0024]      FIG. 5  schematically shows another QSFP, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  schematically shows a ROSA  20  in accordance with an embodiment of the invention. ROSA  20  is configured to receive optical signals in a plurality of optionally four different optical channels defined respectively by four different wavelength bands, demultiplex the optical signals and generate electrical signals for each channel responsive to the demultiplexed optical signals. The optical channels and the wavelength bands that define the channels are identified and referred to by wavelength symbols λ 1 , λ 2 , λ 3 , or λ 4 . Optical signals in channels λ 1 , λ 2 , λ 3 , or λ 4  are represented by shaded block arrows labeled respectively by Sλ 1 , Sλ 2 , Sλ 3 , or Sλ 4 . 
         [0026]    A process by which ROSA demultiplexes optical signals and generates electrical signals for demultiplexed optical signals for each channel, in accordance with an embodiment of the invention, is discussed below following description of the architecture of ROSA  20 . In  FIG. 1  ROSA is schematically shown processing signals in the four channels. 
         [0027]    ROSA  20  comprises an optical substrate  22  optionally formed in the shape of a rectangular plate having top a surface  24  and an “entry surface”  28  optionally perpendicular to surface  24 . A second “reflecting surface”  26 , opposite entry surface  28 , is optionally oriented at 45° relative to the top face surface and is formed so that it reflects light in the optical channels for which ROSA  20  is designed to demultiplex optical signals. An input aperture  20  for receiving optical signals is located on entry surface  28 . Edges of substrate  22  that are normally not seen in the perspective of  FIG. 1  are shown with dashed lines. 
         [0028]    Substrate  22  comprises a first, “input” Mach-Zehnder Interferometer (MZI)  30  cascaded with second and third, “output” MZIs  40  and  50 . Input MZI  30  comprises an entry waveguide  31 , exit waveguides  32 , and  33  and a delay waveguide  34 . Input waveguide  31  intersects first surface  28  and the intersection forms input aperture  29 . Exit waveguides  32  and  33  of input MZI  30  are connected respectively with input waveguides  41  and  51  of output MZIs  40  and  50  respectively. Output MZI  40  has exit waveguides  42  and  43  that intersect reflecting surface  26  at reflecting apertures  61 , and  62  respectively. Output MZI  50  has exit waveguides  52  and  53  that intersect reflecting surface  29  at reflecting apertures  63  and  64 . 
         [0029]    The component waveguides of MZIs  30 ,  40  and  50  are formed in substrate  22  using any of various methods and materials known in the art. Optionally, the waveguides are formed by an ion exchange process such as described in PCT Publication WO 2006/054302. In some embodiments of the invention substrate  22  and the waveguides are constructed in a photolithographic process, such as a CMOS process, using techniques and materials known in the art of fabricating semiconductor dies. Optionally, waveguides that are joined together, for example, exit waveguides  32  and input waveguide  41  are formed simultaneously as a single waveguide unitary waveguide. 
         [0030]    Light propagating in an exit waveguide  42 ,  43 ,  52 , or  53  that is incident on the reflecting aperture associated with the waveguide is reflected out of the waveguide towards top surface  24 . Light sensors  71 ,  72 ,  73  and  74 , optionally photodiodes (PDs), having light sensitive regions  76  are mounted on top surface  24  with their respective light sensitive regions aligned to receive light from exit waveguides  42 ,  43 ,  52 , and  53  reflected by reflecting apertures  61 ,  62 ,  63  and  64  respectively. Optionally, each photodiode  71 ,  72 ,  73  and  74  is mounted to top surface  24  with an optical filter  81 ,  82 ,  83  and  84  located between its sensitive region  76  and the top surface. Optical filters  81 ,  82 ,  83  and  84  are relatively narrow band pass filters, such as thin film filters (TFFs), each of which transmits light in a different one of the four optical channels for which ROSA  20  demultiplexes light. 
         [0031]    Operation of ROSA  20  is described with respect to an optical signal Sλ 1 , Sλ 2 , Sλ 3 , and Sλ 4  in each of wavelength bands λ 1 , λ 2 , λ 3 , or λ 4 . Signals Sλ 1 , Sλ 2 , Sλ 3 , and Sλ 4  are shown entering ROSA  20  and entry waveguide  31  of input MZI  30  through input aperture  29 . Delay waveguide  34  of MZI  30  is configured to introduce a phase difference in optical signals that are coupled into and propagate in the delay waveguide so that optical signals Sλ 1  and Sλ 2  in optical channels λ 1 , λ 2  that enter the MZI exit the MZI, as shown in the figure, via exit waveguide  33  and enter MZI  40 . Optical signals Sλ 3  and Sλ 4  in optical channels λ 3  and λ 4  on the other hand exit input MZI via exit waveguide  34  and enter MZI  50 . Delay waveguide  44  in MZI  40  that receives optical signals Sλ 1  and Sλ 2  is configured to separate the optical signals so that optical signals Sλ 1  and Sλ 2  leave the MZI via exit waveguides  43  and  43  respectively. The signals are respectively incident on reflecting apertures  61  and  62  and are reflected out of the waveguides by the reflecting apertures towards light sensitive regions  76  of PDs  71  and  72  respectively. After reflected signals Sλ 1  and Sλ 2  are filtered by passage through TFFs  81  and  82  respectively, PDs  71  and  72  generate electrical signals represented by arrows  91  and  92  responsive to the demultiplexed optical signals Sλ 1  and Sλ 2  that they receive. 
         [0032]    Similarly optical signals Sλ 3  and Sλ 4  that exit MZI  30  are input to MZI  50 , in which phase shift generated in optical signals propagating in delay waveguide  54  separates the signals so that they leave MZI  50  along waveguides  52  and  53  respectively. The exiting optical signals Sλ 3  and Sλ 4  are respectively incident on reflecting apertures  63  and  64 , which reflect the signals respectively towards PDs  73  and  74 . After signals Sλ 3  and Sλ 4  pass through and are filtered by TFFs  83  and  84 , the signals are received respectively by PDs  73  and  74  which generate output electrical signals  93  and  94  responsive thereto. 
         [0033]    Cascaded MZIs  30 ,  40  and  50  and band pass filters  81  . . .  84  operate in accordance with an embodiment of the invention to demultiplex optical signals in optical channels λ 1 , λ 2 , λ 3 , or λ 4  with reduced cross talk between signals in the optical channels. 
         [0034]    By way of example, assume that wavelength bands λ 1 , λ 2 , λ 3 , or λ 4  are CWDM wavelength bands (1270 nm-1610 nm) with channel spacing of 20 nm and central wavelengths 1270 nm, 1290 nm, 1310 nm and 1330 nm. If cascaded MZIs  30 ,  40  and  50  are properly tuned with appropriate phase shifts introduced by delay waveguides  34 ,  44  and  54 , they will have transmittance as a function of wavelength similar to that shown in a graph  100  in  FIG. 2A . Wavelength X is shown along an abscissa of the graph and transmittance “T” is shown in arbitrary units along an ordinate of the graph. Graph  100  shows that the cascaded MZIs have a relatively broad transmittance peak for each wavelength channel and that the peak decreases relatively slowly from a maximum transmittance at the central wavelength of the channel. As a result, optical signals in one optical channel adulterated with optical frequencies from an adjacent channel can often generate cross talk with the adjacent channel and influence electrical signals output by a PD  71 ,  72 ,  73  or  74  of the adjacent channel. 
         [0035]    Addition of narrow band pass filters such as TFFs  81 ,  82 ,  83  and  84  in accordance with an embodiment of the invention as shown for ROSA  20  in  FIG. 1  operate to reduce such cross talk.  FIG. 2B  shows a graph  102  of a typical transmittance curve as a function of frequency that can be realized for a TFF having a narrow band pass centered on an arbitrary frequency λ O .  FIG. 2C  shows a graph  104  of transmittance as a function of wavelength for cascaded MZIs  20 ,  30  and  40  with the addition of narrow band pass TFFs  81 ,  82 ,  83  and  84  centered on wavelengths 1270 nm, 1290 nm, 1310 nm, and 1330 nm. In the figure dashed curves  105  and  106  represent component transmittances of the cascaded MZIs and the TFFs prior to being multiplied, which are shown in graphs  102  and  104  ( FIGS. 2A and 2B ) respectively. The bold solid curve  108  represents the combined transmittance of the cascaded MZIs and TFFs. The addition of the TFFs sharply separates the transmittance curve peaks for the optical channels processed by ROSA  20  and operates to reduce cross talk between the channels. 
         [0036]      FIG. 3  schematically shows a TOSA  120 , generating and multiplexing optical signals in a plurality of optionally four different optical channels in accordance with an embodiment of the invention. 
         [0037]    TOSA  120  is similar to ROSA  20  and comprises an optical substrate  22  having a reflecting surface  26  and comprising cascaded MZIs  130 ,  140  and  150 . Optionally, cascaded MZIs  130 ,  140  and  150  are identical to cascaded MZIs  30 ,  40  and  50  comprised in ROSA  20  shown in  FIG. 1 . However, TOSA  120  comprises laser diodes, optionally distributed feedback (DFB) laser diodes (LDs),  181 ,  182 ,  183  and  184 , for generating optical signals in place of PDs  71 ,  72 ,  73  and  74  and their associated TFFs comprised in ROSA  20 . In addition, cascaded MZIs  130 ,  140  and  150  in TOSA  120  operate in “reverse” to multiplex signals rather than demultiplex signals. Waveguides  31 ,  41 , and  51 , referred to as entry waveguides in the discussion of ROSA  20  function as exit waveguides in TOSA  120 . And waveguides  32 ,  33 ,  42 ,  43 ,  52 , and  53  referred to as exit waveguides in the discussion of ROSA  20 , function as entry waveguides in TOSA  120 . Waveguide  31  intersects surface  28  to form an output aperture  129  on the surface for the TOSA. 
         [0038]    In operation of TOSA  120 , each laser diode  181 ,  182 ,  183 , and  184  is controlled to respectively generate optical signals Sλ 1 , Sλ 2 , Sλ 3 , and Sλ 4  as required in each of optical channels λ 1 , λ 2 , λ 3 , or λ 4 . The optical signals, as shown in  FIG. 3 , are transmitted into substrate  22  so that they are reflected respectively by reflection apertures  61 ,  62 ,  63 , and  64  into waveguides  42 ,  43 ,  52 , and  53  respectively. Optical signals Sλ 1  and Sλ 2 , generated by LDs  181  and  182  are reflected respectively by reflecting apertures  61  and  62  into waveguides  42  and  43  of MZI  140 , which multiplexes the signal and transfers them to MZI  120  via waveguide  41 . Similarly, optical signals Sλ 3  and Sλ 4 , generated by LDs  183  and  184  are reflected by reflecting apertures  63  and  64  into waveguides  52  and  53  of MZI  150 . MZI  150  multiplexes the optical signals it receives and transfers them to MZI  130  via waveguide  51 . MZI  130  multiplexes optical signals Sλ 1 , Sλ 4 , Sλ 3 , and Sλ 4  it receives and transmits them via waveguide  31  to output aperture  129 . 
         [0039]    ROSA and TOSA  20  and  120  can be configured and produced sufficiently small so that they can be used in a QSFP transceiver, in accordance with an embodiment of the invention.  FIG. 4  schematically shows a QSFP  200  comprising ROSA  20  and TOSA  120 , in accordance with an embodiment of the invention. 
         [0040]    ROSA and TOSA  20  and  120  are optionally mounted to a support base  202  having a conducting panel  204  located between them to reduce mutual electromagnetic interference. A microcontroller  206 , is mounted to support base  202 , and is connected to LDs  181 ,  182 ,  183  and  184  of TOSA  120  to control generation of optical signals by the LDs in each of optical channels λ 1 , λ 2 , λ 3 , or λ 4  for multiplexing and transmission through output aperture  129 . Optionally microcontroller  206  is coupled to each of the LDs by a different laser diode driver (LDD, not shown) that controls current to the LD, and thereby light pulses generated by the LD. The microcontroller is also connected to PDs  71 ,  72 ,  73  and  74  comprised in ROSA  20  to receive electrical signals generated by the PDs responsive to optical signals received by QSFP transceiver  200  via input aperture  29  that the ROSA demultiplexes. Optionally, microcontroller  206  is coupled to each of the PDs by a different transimpedance amplifier/limiting amplifier (TIA/LIA, not shown) that receives current signals from the PD and converts it to a shaped voltage signal. A QSFP compliant connector  208  is electrically connected to microcontroller  206  for connecting QSFP  200  to a communication terminal. QSFP  200  is small enough to be housed in a QSFP compliant housing having length, width and height less than or equal to 70 mm, 18.35 mm and 8.5 mm. 
         [0041]      FIG. 5  schematically shows another QSFP  220  comprising a ROSA  222  and a TOSA  224 , in accordance with an embodiment of the invention. ROSA  222  is similar to and optionally comprises the same components as ROSA  20 . TOSA  224  is similar to and optionally comprises the same components as TOSA  120 . However, unlike QSFP  200 , in QSFP  220  the ROSA and TOSA (ROSA  222  and TOSA  224 ) are formed on a same glass substrate  226 . A groove  230  formed in an underside surface  228  of substrate  226  has a reflecting surface  232  that functions in place of reflecting surfaces  26  in ROSA  20  and TOSA  120  ( FIG. 1  and  FIG. 3  respectively). Reflecting surface  232  reflects optical signals generated by LDs in TOSA  224  into the TOSA&#39;s cascaded MZIs for multiplexing and transmission from QSFP transceiver  220  via output aperture  129 . The reflecting surface reflects optical signals that enter QSFP  220  via input aperture  29  and exit cascaded MZIs in ROSA  222  to PDs in the ROSA for generation of electrical signals responsive to the optical signals. 
         [0042]    In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
         [0043]    Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.