Abstract:
An apparatus includes a bank of optical detectors, an input optical filter and a selector. The optical detectors are configured to output respective detection indications in response to detecting a presence of an optical signal. The input optical filter is configured to receive an input optical signal having an input wavelength, and to route the input optical signal to one of the optical detectors in the bank depending on the input wavelength. The selector is configured to select an output wavelength based on the detection indications of the optical detectors, and to cause generation and transmission of an output optical signal at the selected output wavelength.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to optical communication, and particularly to methods and systems for wavelength negotiation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Some optical systems, such as Wavelength Division Multiplexing (WDM) systems, transmit and receive modulated optical signals over optical fibers. Various techniques for allocating wavelength to optical carriers are known in the art. 
         [0003]    For example, U.S. Patent Application Publication 2005/0244161, whose disclosure is incorporated herein by reference, describes an optical transmission system in which a transmission section outputs plural monochromatic-wavelength lights individually, a first allocation section allocates a wavelength of a monochromatic-wavelength light based on a power of the monochromatic-wavelength light individually outputted from the transmission section from among the plural monochromatic-wavelength lights, a notification section issues a notification of wavelength information of the monochromatic-wavelength lights allocated by the first allocation section to the transmission section, and a first control section controls wavelengths of the monochromatic-wavelength lights to be outputted from the transmission section based on the wavelength information of the notification issued from the notification section. 
         [0004]    As another example, U.S. Pat. No. 7,720,384, whose disclosure is incorporated herein by reference, describes a transponder in a WDM apparatus, which detects a wavelength of received light having one of plural wavelengths multiplexed or divided by a WDM unit, determines a transmission wavelength which is a wavelength of a light to be transmitted to the WDM unit on the basis of the wavelength of the received light, and sets to convert the wavelength of the light to be transmitted to the WDM unit into the transmission wavelength, and transmit the converted light. 
       SUMMARY OF THE INVENTION 
       [0005]    An embodiment of the present invention that is described herein provides an apparatus including a bank of optical detectors, an input optical filter and a selector. The optical detectors are configured to output respective detection indications in response to detecting a presence of an optical signal. The input optical filter is configured to receive an input optical signal having an input wavelength, and to route the input optical signal to one of the optical detectors in the bank depending on the input wavelength. The selector is configured to select an output wavelength based on the detection indications of the optical detectors, and to cause generation and transmission of an output optical signal at the selected output wavelength. 
         [0006]    In some embodiments, the apparatus further includes multiple lasers that are set to multiple respective output wavelengths, and the selector is configured to activate, based on the detection indications of the optical detectors, only a selected laser from among the multiple lasers that has the selected output wavelength. 
         [0007]    In an embodiment, the selector is configured to activate the selected laser by connecting only the selected laser to a power supply. In another embodiment, the apparatus includes an output optical filter, which is configured to route outputs of the multiple lasers to a modulator that modulates data onto the selected output wavelength. 
         [0008]    In a disclosed embodiment, the selector is configured to select the output wavelength to be equal to the input wavelength. In an example embodiment, each of the optical detectors is non-tunable and simultaneously covers all possible values of the output wavelength. In an embodiment, the bank of optical detectors and the input optical filter are fabricated in a silicon layer that is disposed on a single insulator substrate. In another embodiment, the optical detectors are mounted on a single glass substrate, and the input optical filter is fabricated in the glass substrate. 
         [0009]    There is additionally provided, in accordance with an embodiment of the present invention, an apparatus including a single substrate and optoelectronic circuitry, which is fabricated on the single substrate and which is configured to receive an input optical signal having an input wavelength, to identify the input wavelength, to select an output wavelength depending on the identified input wavelength, and to generate and transmit an output optical signal having the output wavelength. 
         [0010]    In some embodiments, the single substrate includes an insulator, and the optoelectronic circuitry includes a Silicon Photonics (SiP) device fabricated in a Silicon-on-Insulator (SOI) configuration. In alternative embodiments, the single substrate includes a glass substrate, and the optoelectronic circuitry is disposed on the glass substrate. In an embodiment, the optoelectronic circuitry is fabricated in two or more semiconductor layers separated by one or more interposer layers. 
         [0011]    There is also provided, in accordance with an embodiment of the present invention, a method including receiving an input optical signal having an input wavelength. The input optical signal is routed using an input optical filter, depending on the input wavelength, to one of multiple optical detectors that are configured to output respective detection indications in response to detecting a presence of an optical signal. An output wavelength is selected based on the detection indications of the optical detectors. An output optical signal is generated and transmitted at the selected output wavelength. 
         [0012]    There is further provided, in accordance with an embodiment of the present invention, a method including providing a single substrate, and fabricating on the single substrate optoelectronic circuitry, which receives an input optical signal having an input wavelength, identifies the input wavelength, selects an output wavelength depending on the identified input wavelength, and generates and transmits an output optical signal having the output wavelength. 
         [0013]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram that schematically illustrates a system for wavelength auto-negotiation, in accordance with an embodiment of the present invention; and 
           [0015]      FIG. 2  is a flow chart that schematically illustrates a method for wavelength auto-negotiation, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0016]    Embodiments of the present invention that are described herein provide methods and systems for automatic wavelength negotiation, for use in optical communication equipment. In the disclosed embodiments, an auto-negotiation system receives an input optical signal having an input wavelength, and transmits an output optical signal having an output wavelength. The input wavelength is unknown a-priori. The system automatically identifies the input wavelength, and sets the output wavelength depending on the identified input wavelength. 
         [0017]    In some embodiments, the system comprises a bank of optical detectors, and an optical filter that routes the input signal to one of the optical detectors based on the input wavelength. As a result, one of the optical detectors (whose identity depends on the input wavelength) will indicate positive detection, while the other optical detectors will indicate negative detection. 
         [0018]    The detection indications of the optical detectors are used to control a selector circuit, which selects one of multiple lasers set to different output wavelengths. The selector circuit activates the appropriate laser depending on the detection indications, which in turn depend on the input wavelength. The laser output is modulated with the data for transmission, so as to produce the output optical signal at the selected output wavelength. 
         [0019]    Several implementation variations of the wavelength negotiation system and its components, for use in various wavelength ranges, are described herein. Typically, the entire system is fabricated on a single substrate, e.g., in an integrated Silicon Photonics (SiP) device. 
         [0020]    The disclosed configurations do not use any tunable optical elements that scan the range of possible wavelengths, but rather use a bank of detectors that operate in parallel. As a result, wavelength detection and setting is extremely fast, typically below 1 mS. This high-speed performance is especially important for optical network switching and routing applications. 
       System Description 
       [0021]      FIG. 1  is a block diagram that schematically illustrates a system  20  for wavelength auto-negotiation, in accordance with an embodiment of the present invention. A system such as system  20  can be used, for example, in an optical network switch or router, or in any other suitable type of optical communication equipment. In a typical application, system  20  is used in a network element of a Wavelength Division Multiplexing (WDM) network. In the present context, the term “WDM” refers to any and all types and variants of WDM, such as, for example, Dense WDM (DWDM), Coarse WDM (CWDM) or any other suitable variant. 
         [0022]    System  20  receives an input optical signal (referred to as “input signal” for brevity) over an input port  28 , and transmits an output optical signal (referred to as “input signal” for brevity) over an output port  32 . The system automatically identifies the wavelength of the input signal (referred to herein as “input wavelength”) and sets the wavelength of the output signal (referred to herein as “output wavelength”) depending on the identified input wavelength. 
         [0023]    In a typical application, although not necessarily, system  20  sets the output wavelength to be equal to the input wavelength. Typically, the input signal and the output signal are exchanged with the same remote node. Alternatively, however, the source of the input signal and the destination of the output signal may be different nodes. 
         [0024]    Typically, system  20  is fabricated on a single substrate  24 . In an example embodiment, the entire system  20  is implemented in a Silicon Photonics (SiP) device, using a Silicon On Insulator (SOI) configuration. In another embodiment, the entire system  20  is fabricated on a single glass substrate. The choice of substrate and fabrication process may depend, for example, on the optical band of the input and output signals. These implementation aspects are addressed in greater detail below. 
         [0025]    Input port  28  and output port  32  may comprise, for example, optical fibers or optical waveguides. The input and output ports may be coupled to the same fiber or waveguide or to different fibers or waveguides. In one example embodiment, the input and output ports are coupled to different optical fibers in the same optical cable. 
         [0026]    In the embodiment of  FIG. 1 , the input signal is provided to a main reception path, which comprises a high-speed photo-detector (PD)  36  and a high-speed Trans-Impedance Amplifier (TIA)  40 . PD  36  demodulates the optical signal and produces an electrical signal. TIA  40  amplifies the electrical signal and thus outputs a high-speed stream of data. The received data is provided as output of system  20 . 
         [0027]    In the embodiment of  FIG. 1 , the input wavelength is assumed to be selected from a set of N possible wavelengths denoted λ 1  . . . λN. The actual input wavelength of the input signal is unknown a-priori, and is identified by system  20 . In an example embodiment, N=18, the wavelength spacing is 20 nm, and the possible input wavelengths are given in the following table: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 example wavelength set 
               
             
          
           
               
                   
                 Wavelength 
                 Nominal center 
               
               
                   
                 index 
                 wavelength [nm] 
               
               
                   
                   
               
               
                   
                 λ1 
                 1271 
               
               
                   
                 λ2 
                 1291 
               
               
                   
                 λ3 
                 1311 
               
               
                   
                 λ4 
                 1331 
               
               
                   
                 λ5 
                 1351 
               
               
                   
                 λ6 
                 1371 
               
               
                   
                 λ7 
                 1391 
               
               
                   
                 λ8 
                 1411 
               
               
                   
                 λ9 
                 1431 
               
               
                   
                 λ10  
                 1451 
               
               
                   
                 λ11  
                 1471 
               
               
                   
                 λ12  
                 1491 
               
               
                   
                 λ13  
                 1511 
               
               
                   
                 λ14  
                 1531 
               
               
                   
                 λ15  
                 1551 
               
               
                   
                 λ16  
                 1571 
               
               
                   
                 λ17  
                 1591 
               
               
                   
                 λ18  
                 1611 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    Alternatively, any other suitable set of wavelengths can be used. 
         [0029]    In order to identify the input wavelength, an optical coupler  44  samples a portion of the optical signal for processing. In the present example, coupler  44  comprises a 3 dB coupler that samples 50% of the signal power. Alternatively, however, any other suitable coupling ratio can be used. 
         [0030]    The sampled input signal is filtered by a receive (RX) WDM filter  48 . (As noted above, the term WDM refers to WDM variants such as DWDM and CWDM, as well.) Filter  48  comprises an input, and N outputs corresponding to the N possible input wavelengths λ 1  . . . λN. The filter routes the optical signal from the input to the appropriate output depending on the signal wavelength. In other words, if the input wavelength is λk, then filter  48  routes the signal to the k th  output. 
         [0031]    WDM filter  48  may be implemented in any suitable way and using any suitable optical technology. In an example embodiment, the filter comprises a prism or optical grating, which deflects the optical signal at an angle that depends on the wavelength. An optical grating of this sort may be etched into substrate  24  using photolithography, or otherwise fabricated in the substrate. 
         [0032]    System  20  comprises a bank of N optical detectors that are coupled to the N respective outputs of filter  48 . Each optical detector outputs a binary detection indication, which indicates the presence or absence of optical signal at the detector. In the present example, each optical detector comprises a PD  52  followed by a TIA  56 . Each TIA  56  outputs a Direct Current (DC) voltage level that indicates whether or not light was detected by the respective PD  52 . 
         [0033]    It should be noted that each optical detector in the bank (PD  5 +TIA  56  in the present example) is broadband and not tunable, i.e., it detects the presence or absence of light regardless of the specific wavelength of the detected light. Moreover, each optical detector typically indicates only whether or not an optical signal is present, and does not provide any indication of the signal wavelength. In other words, the detection indications of the optical detectors are binary or Boolean. 
         [0034]    It should also be noted that, unlike PD  36  and TIA  40 , PDs  52  and TIAs  56  do not perform demodulation but only energy detection. As such, PDs  52  and TIAs  56  may comprise low-speed (and therefore low-cost) components. In alternative embodiments, the bank of optical detectors may be implemented using any other suitable elements. 
         [0035]    The bank of N optical detectors produces a set of N binary detection indications (outputs of TIAs  56 ). When the input wavelength is λk, then the k th  detection indication will be positive (i.e., optical signal is detected) and the other detection indications will be negative (i.e., optical signal is not detected). 
         [0036]    The N binary detection indications are provided to a selector  60 . Selector  60  comprises an input that is connected to a power supply  64 , and N outputs that are connected respectively to N lasers  68  set to wavelengths λ 1  . . . λN. Selector  60  is configured to connect its input (the power supply) to one of its outputs (i.e., to one of lasers  68 ) depending on the selection inputs (i.e., depending on the binary detection indications). In this manner, selector  60  selects and activates one of lasers  68  depending on the detection indications, i.e., based on the identified input wavelength. The other lasers remain inactive since they are disconnected from the power supply. 
         [0037]    By proper configuration of selector  60 , system  20  may apply any desired mapping between input wavelength and output wavelength. In an example embodiment, system sets the output wavelength to be equal to the input wavelength. In order to apply this mapping, selector  60  is configured to connect the power supply to the λk laser if the k th  detection indication is positive (i.e., if the input wavelength was identified as λk). Alternatively, however, selector  60  may be configured to apply any other suitable mapping, in which the output wavelength is not necessarily equal to the input wavelength. 
         [0038]    The N outputs of lasers  68  are connected to respective inputs of a transmit (TX) WDM filter  72 . Filter  72  is typically similar to RX WDM filter  48  (e.g., prism-based or optical grating) and may be fabricated in a similar way. Filter  72  routes the outputs of lasers  68  to its output. The output of filter  72  thus comprises an un-modulated optical carrier signal at the selected output wavelength. 
         [0039]    System  20  comprises a high-speed (electro-optical or non-electro-optical) modulator  76 , which is driven by a high-speed modulator driver  80 . Data for transmission is provided to driver  80 . Thus, modulator  76  modulates the optical carrier signal with the data for transmission. The resulting output optical signal, at the output wavelength, is sent over output port  32 . 
         [0040]    The system configuration shown in  FIG. 1  is an example configuration, which is chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable system configuration can be used. 
         [0041]      FIG. 2  is a flow chart that schematically illustrates a method for wavelength auto-negotiation, in accordance with an embodiment of the present invention. the method begins with system  20  receiving an input optical signal over input port  28 , at a reception step  90 . The input signal has a wavelength λk that is unknown a-priori. RX WDM filter  48  routes a sample of the input signal to the k th  optical detector, at an input routing step  94 . 
         [0042]    The output of the k th  optical detector controls selector  60  to connect power supply  36  to the λk laser, at a selection step  98 . TX WDM filter  72  routes the output of the λk laser to modulator  76 . Modulator  76  modulates and transmits the optical output signal, at a transmission step  102 . 
       Example Implementation Variations 
       [0043]    In some embodiments, system  20  is implemented as a single SiP device using a silicon process. In a SiP implementation, the various optical components (e.g., PDs  36  and  52  and WDM filters  48  and  72 ) are fabricated in a silicon layer that is disposed on an insulator (e.g., silica) substrate. This sort of implementation is suitable, for example, for input and output wavelengths on the order of 1100-1600 nm. The SiP device may be fabricated, for example, in a two-dimensional (2-D) configuration having a single silicon layer (or other semiconductor layer) disposed on the insulator substrate, with the various optical and electronic circuit elements fabricated or disposed on the silicon layer. 
         [0044]    Alternatively, the SiP device may be fabricated in a 2.5-D configuration. In such a configuration, some circuit elements are fabricated or disposed on the silicon (or other semiconductor) layer. An interposer layer is then disposed on the silicon layer. Additional circuit elements may be mounted on the interposer, and connected to the circuit using through vias that connect the upper and lower surfaces of the interposer. 
         [0045]    Further alternatively, the SiP device may be fabricated in a 3-D configuration. In a 3-D configuration, multiple silicon layers (or other semiconductor layer) are disposed on the insulator substrate, and separated from one another by one or more interposer layers. The various optical and electronic circuit elements are fabricated in the various silicon layers of this multi-layer structure. Typically, the optical components (e.g., PDs  52  and filters  48  and  72 ) are fabricated in the bottom silicon layer. 
         [0046]    In some embodiments, lasers  68  may comprise Vertical Cavity Surface Emitting Lasers (VCSELs) that are mounted on a silicon layer, e.g., using flip-chip or ball-grid interconnections. Alternatively, the lasers can be implemented by gluing of InP on the silicon, such that optical lasing occurs inside the silicon layer. Generally, the disclosed techniques can be used with any suitable type of laser, e.g., Directly Modulated Laser (DML) or Electro-absorption Modulated Laser (EML) implementations. 
         [0047]    Alternatively to SiP, system  20  may be fabricated on a glass substrate. This sort of implementation is suitable, for example, for input and output wavelengths on the order of 800-1100 nm. In these embodiments, lasers  68  typically comprise VCSELS, filters  48  and  72  are typically etched directly in the glass substrate, and the PDs are implemented using external components. 
         [0048]    Although the embodiments described herein mainly address High-Speed Electro-Optical (E-O) interconnect applications, the methods and systems described herein can also be used in other applications, such as in E-O telecommunication applications. 
         [0049]    It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.