Abstract:
An optical transceiver for the coherent communication system is disclosed. The optical transceiver follows the standard of the CFP transceiver and installs a wavelength tunable laser diode (LD) as a light source for the optical transmission and a local light for the optical reception; an optical modulator of the Mach-Zehnder type made of dielectric material; and an optical receiver to recover the DP-QPSK optical signal The housing of the optical transceiver provides a front auxiliary area and a rear auxiliary area to install a slender optical modulator and to bend an inner fiber with a large radius.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present application relates to an optical transceiver, in particular, the present application relates to an optical transceiver capable of performing the coherent optical communication. 
         [0003]    2. Background Arts 
         [0004]    As the mass of information to be transmitted on the optical communication system, endeavors have been continuously devoted to enhance the transmission capacity not only to increase the transmission speed but to raise the modulation degree, such as the polarization modulation, the phase modulation, a combination of these two modulations, and so on. These modulations have been known as the coherent modulation technique in the wireless communication. Recently, various documents have reported to introduce the coherent modulation in the optical communication system. 
         [0005]    The coherent modulation counts the phase of the light as one information unit. Accordingly, comparing the phase of the transmitted light with the reference light, the in-phase component and the quadrature component may be utilized as the information unit. The former (In-phase component) is a component whose phase matches with that of the reference light, while, the latter is a component whose phase is difference by 90° against that of the reference light. In the coherent modulation, the reference light is called as the local light whose frequency is precisely matched with that of the signal light, while, the phase is optional against the signal light, The optical communication system may utilize, in addition to those two components in the phase, two polarizations may be utilized as the information unit. Such a modulation is called as Dual-Polarization Dual-Phase-Shift-Keying (DP-QPSK). 
         [0006]    The DP-QPSK modulation requests the local light to be extremely stable in the phase thereof, namely, extremely narrow linewidth of the laser emission, and to be compact as possible. In order to get the narrow line width, the optical modulation is carried out by an optical modulator independent of the LD, in particular, the coherent modulation generally introduces, what is called, the Mach-Zehender (MZ) modulator. 
         [0007]    When the MZ modulator is primarily made of dielectric material, typically lithium niobate (LN), and a substantial longitudinal length is inevitable to secure an effective electrical-to-optical interaction because of relatively smaller electro-optic effect of the material. 
         [0008]    Most optical transceivers distributing in the field are defined in the outer dimensions and electrical interfaces thereof in respective standards. Because the optical transceivers are operated under controls by the host system, diverse outer dimensions and electrical interfaces delay the diffusion of such optical transceivers. Accordingly, a coherent optical transceiver is necessary to install a lot of optical components, compared with conventional optical transceivers adopting only the amplitude modulation, within a housing having a limited inner space. 
       SUMMARY OF THE INVENTION 
       [0009]    An aspect of the present invention relates to an optical transceiver that is applicable to a coherent communication system. The optical transceiver of the present application comprises an optical receptacle, a wavelength tunable laser diode (LD), a polarization maintaining coupler (PMC), an optical modulator, an optical receiver, and a housing. The optical receptacle receives an external optical connector. The LD generates a laser light. The PMC splits the laser light output from the LD into two beams. The optical modulator modulates one of the beams split by the PMC to generate a modulated optical beam. The optical receiver recovers data contained in an optical signal externally provided through the external optical connector by multiplying another of the beams that is output from the PMC with the external optical signal. The housing encloses the optical receptacle, the LD, the PMVC, the optical modulator, and the optical receiver therein. The housing provides a front panel to mount the optical receptacle. A feature of the optical transceiver of the present application is that the housing provides a front auxiliary area protruding from the front panel where a portion of the optical modulator and a portion of the LD are installed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0011]      FIG. 1  shows a perspective drawing of an optical transceiver according to the present embodiment; 
           [0012]      FIG. 2  shows an inside of the optical transceiver; 
           [0013]      FIG. 3  magnifies a primary portion inside of the optical transceiver; 
           [0014]      FIG. 4  views the inside of the optical transceiver from the bottom by removing the bottom housing; 
           [0015]      FIG. 5  schematically illustrates a functional block diagram of the optical transceiver primary in the optical system thereof; 
           [0016]      FIG. 6  is a perspective view of the iTLA; 
           [0017]      FIG. 7  schematically shows the inside of the LD module; 
           [0018]      FIG. 8A  schematically illustrates the inside of the ICR, and  FIG. 8B  shows a functional block diagram of the ICR; 
           [0019]      FIG. 9  is an exploded view of the housing, the top and bottom housings, and the front panel, where  FIG. 9  illustrates the housing bottom up; 
           [0020]      FIG. 10  illustrates the arrangement of the components and the wiring of the inner fibers within the space; 
           [0021]      FIG. 11  magnifies the rear portion of the top housing; and 
           [0022]      FIG. 12  is a plan view of the rear portion illustrated in  FIG. 11 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0023]    Next, some preferable embodiments according to the present application will be described. In the description of drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicated explanations. 
         [0024]      FIG. 1  shows a perspective drawing of an optical transceiver  1  according to the present embodiment. The optical transceiver  1 , which follows the multi-source agreement (MSA) of what is called, Centium Form factor Pluggable (CFP), includes a top housing  2 , a bottom housing  3 , two fastening screws  4 , and a front panel  5 . The description below assumes that “front” or “forward” corresponds to a side where the front panel is provided, and “rear” corresponds to a side opposite to the front, and a direction from the front to the rear is longitudinal direction. 
         [0025]    However, these descriptions are only for explanation sakes and do not narrower the scope of the present invention. 
         [0026]    The top and bottom housings,  2  and  3 , which are made of metal die casting, has a longitudinal length of 144 mm from the front panel  5  to the rear end, and a width of 82 mm in the front panel  5 . The fastening screws  4  are provided in respective sides of the front panel  5  to latch the optical transceiver  1  with the host system. 
         [0027]      FIG. 2  shows an inside of the optical transceiver  1  viewed from the top;  FIG. 3  magnifies a primary portion of the optical transceiver  1 ; and  FIG. 4  views the inside of the optical transceiver  1  from the bottom by removing the bottom housing  3 . As shown in  FIGS. 2 to 4 , the fastening screws  4  are set in the pockets  3   a  appearing in  FIG. 9  formed by the top and bottom housings,  2  and  3 , in respective sides of the optical transceiver  1 . The rear ends  4   a  of the fastening screws  4  extrude from the electrical plug  6  provided in the rear end of the optical transceiver  1 . Mating the electrical plug  6  with an electrical connector provided in the host system, the fastening screws  4  may be fastened with the electrical connector. 
         [0028]    Specifically, the end  4   a  of the fastening screw  4  extrudes in respective outer sides of the electrical plug  6  which provides terminals for radio frequency (RE) signals and those for power supplies, a total number of which exceeds  100  counts with a pitch of 0.8 mm. Mating the end  4   a  of the fastening screws  4  with female holes provided in respective sides of the optical connector, the optical transceiver  1  may be securely and precisely set in the host system and communicate with the host system. 
         [0029]    Inner space formed by the top and bottom housings,  2  and  3 , of the optical transceiver  1  installs two drivers  11 , an optical modulator  12 , an intelligent tunable laser assembly (iTLA)  13 , a polarization maintaining coupler (PMC)  14 , an integrated coherent receiver (ICR)  15 , a digital signal processor (DSP)  16 , semi-rigid cables  17 , and an optical receptacle  18 . Some of those elements are mounted on a circuit board. In the present optical transceiver  1 , the electrical plug  6  is independent of the circuit board. 
         [0030]    The inner space of the optical transceiver  1  installs the optical modulator  12 , which has an extended and slim rectangular housing, in a side along the driver  11 . Four semi-rigid cables  17  electrically connect the driver  11  with the optical modulator  12 . The semi-rigid cable  17  is a co-axial cable sheathed with, for instance, copper so as to be flexibly and freely bent and to keep a bent shape. Accordingly, the semi-rigid cable  17  enhances the flexibility of the disposition of respective components within the inner space. 
         [0031]    Also, the optical transceiver  1  provides inner fibers, F 1  to F 5 . Five inner fibers, F 1  to F 5 , are enclosed within the inner space in the present embodiment. These inner fibers, F 1  to F 5 , optically couple the optical modulator  12 , the iTLA  13 , the PMC  14 , the ICR  15 , and the optical receptacle  18 . Specifically, the inner fiber F 1 , which is forwardly brought out from the optical modulator  12  then turned rearward, couples the optical modulator  12  with the optical receptacle  18 . Details of the arrangements of the inner fibers, F 1  to F 5 , will be described later. Four optical signals modulated by the optical modulator  12  are multiplexed and output through the optical connector C 1  set in the optical receptacle  18 , Also, an external optical signal is input to the other optical connector C 2 . 
         [0032]    The optical receptacle  18  protrudes from the front panel  5 . The optical receptacle  18  is also coupled with the ICR  15  through another inner fiber F 2  which extends rearward from the optical receptacle  18  and makes a round within the inner space. The external signal input to the optical connector C 2  enters the ICR  15  and carried on the inner fiber F 2 . The iTLA  13  pierces through the opening  5   a  provided in the front panel  5 . The PMC  14  is set in a rear of the optical receptacle  18  and in side by side against the driver  11 . The DSP  16  is placed in the rear of the ICR  15 . The front panel  5  in the opening  5   a  thereof exposes not only the iTLA  13  but the optical receptacle  18  as receiving the optical connectors, C 1  and C 2 . 
         [0033]      FIG. 5  schematically illustrates a functional block diagram of the optical transceiver  1  primarily in the optical system thereof. Lines except inside thereof correspond to the optical paths, while, solid lines denote electrical paths. The iTLA  13  generates an optical signal with a wavelength within a band of 1.55 μm specifically, 1.53 to 1.57 μm. 
         [0034]      FIG. 6  is a perspective view of the iTLA  13 . The iTLA  13  includes an LD module  13 A that generates the optical signal, a base  13 B, a circuit board  13 C, and a connector  130 . A flexible printed circuit (FPC) board coupled with the connector  130  electrically connects the iTLA with circuits mounted on the mother board in the optical transceiver  1 .  FIG. 7  schematically shows the inside of the LD module  13 A, which installs an LD  13   a,  a wavelength detector  13   b  including an etalon filter  13   c,  monitor photodiodes (mPD),  13   d  and  13   e,  and so on. The LD  13   a  may generate laser emission with a line width thereof substantially equal to or narrower than 100 kHz. The wavelength band around 1.55 μm corresponds to the oscillation frequency of about 1.95 THz. Accordingly, the line width of around 100 kHz becomes equivalent to the stability of about 5×10 8 . 
         [0035]    Referring again to  FIG. 5 , the local beam output from the iTLA  13  is split by the PMC  14  as maintaining the polarization thereof. The polarization of the local beam is in parallel to the active layer of the LD  13   a , that is, because the LD  13   a  enclosed within the housing of the LD module  13 A, the polarization of the local beam output from the LD module  13 A is kept in substantially in parallel to the bottom of the housing. One of the local beams split by the PMC  14  enters the optical modulator  12 , but the other reaches the ICR  15 . The optical modulator  12 , which has a type of the LN modulator comprised of lithium niobate, modulates thus provided one of local beams based on the modulation signals provided from the driver  11  through the semi-rigid cables  17 . The modulation signals may have a frequency exceeding 10 GHz, sometimes reaching 40 GHz. The modulation signals thus provided correspond to Ix, Iy, Qx, and Qy, where I and Q mean the in-phase and quadrature, respectively; while, x and y correspond to the polarizations. Thus, the optical modulator  12  may perform the DP-QPSK modulation. 
         [0036]    The ICR  15 , which receives the other of the local beams splits by the PMC  14 , extracts the phase information of the input optical signal provided from the optical connector C 2  by performing the multiplication of two optical beams.  FIG. 8A  schematically illustrates the inside of the ICR  15 , and  FIG. 8B  is a functional block diagram of the ICR  15 . As shown in  FIG. 8A , the ICR  15  includes a variable optical attenuator (VOA)  15   a;  two PD units  15   b  each corresponding to respective polarizations and including two lanes for the in-phase signal (I) and the quadrature phase signal (Q), respectively; two 90° hybrids  15   c  to perform the multiplication of two beams; two skew adjustors  15   d;  and some optical components such as a polarization beam splitter (PBS), a BS, and lenses. The ICR  15  further provides a λ/2 plate  15 A to rotate the polarization of the signal beam in the optical path from the signal to the local, while, the local beam provided from the polarization maintaining fiber (PMF) is kept in the polarization thereof until respective hybrids  15   c.    
         [0037]    Specifically, referring to  FIG. 8B , the ICR  15  receives the local beam from the iTLA  13  through the PMF and the signal beam from the optical connector C 2  through the signal mode fiber (SMF), Each beam is split into two beams by the BS and the PBS, respectively. One of the 90° hybrids  15   c  multiplies one of signal beams split by the PBS with one of the local beams also split but by the BS to generate the in-phase and the quadrature phase signals for the X-polarization, Ix and Qx. The other 90° hybrids  15   c  multiples one of the signal beams but passing through the λ/2 plate  15 A with one of local beams to generate the in-phase and the quadrature phase signals for the Y-polarization, Iy and Qy. Because the optical components set in the paths for the local beam and the signal beam except for the λ/2 maintain the polarization of the local beam, respective hybrids may exactly generate the signals for two polarizations, Four generated signals, Ix to Qy, are provided to the DSP  16  to recover information contained in the input optical signal, The DSP  16  provides the information thus recovered to the host system. 
         [0038]    The optical modulator  12 , the iTLA  13 , and/or the ICR  15  are necessary to be provided with a lot of DC biases for the stable operations thereof. For instance, the optical modulator  12  needs, in addition to the driving signals, biases to compensate the phases of the optical beams, to balance respective power of the optical outputs, and/or to monitor respective optical outputs. The ITU  13  requires, in addition to the bias current to generate an optical beam, to control the wavelength of the optical beam in the target one, to monitor the power of the output beam, and so on. Also the ICR  15  is necessary to be provided with biases for PDs and pre-amplifiers installed therein, commands to adjust the gains of the pre-amplifiers, and so on. The optical transceiver  1  provides such many biases by respective FPCs from the mother board. An optical transceiver  1  for the coherent communication system is inevitably requested to enclose those electrical and optical components within a housing whose outer dimensions are precisely determined in MSAs. Next, details of the housing of the optical transceiver  1  of the present embodiment will be described. 
         [0039]      FIG. 9  is an exploded view of the housing, namely, the top and bottom housings,  2  and  3 , and the front panel  5 , where  FIG. 9  illustrates the housing in bottom up. The top housing  2  provides in respective sides the cavities  2 A from the front to the rear to set the fastening screws  4  therein. The fastening screws  4  pierce the front panel  5 , the cavities  2 A, and protrude from the rear end. The top housing  2  also provides an extension  2 B extending forward from the opening  5   a  of the front panel  5 , The extension  2 B secures a front auxiliary area  51  covered with a ceiling  3 A of the bottom housing  3 . Although the extension  2 B protrudes from the front panel  5 , the extension  2 B does not interfere with the installation of the external fiber extracted from the optical connectors, C 1  and C 2 . 
         [0040]      FIG. 10  illustrates the arrangement of the components and the wiring of the inner fibers within the housing  2 . The front auxiliary area  51  installs the front portion of the optical modulator  12 . Accordingly, even when the optical modulator  12  in the dimensions thereof, in particular, the longitudinal length thereof, is longer than the longitudinal length of the optical transceiver  1  whose outer dimensions follows the CFP standard, the optical transceiver  1  may build an optical modulator of the MZ type primarily made of dielectric material such as lithium niobate (LiNbO 3 ). Because of smaller electrical-optical interactive co-efficient of dielectric materials, an optical modulator made of such material requires a length to show a substantial modulation degree. Without the front auxiliary area S 1 , no optical modulator of the MZ type made of dielectric material is available to be installed within the optical transceiver following the CFP standard. Moreover, the front auxiliary area S 1 , or the front extension  28 , does not interfere with the function for the optical transceiver  1  to be plugged within the host system and communicate therewith. That is, the CFP standard is silent for the arrangement of the front panel, only sets the limitation that the optical connector provided in a CFP transceiver is to have the type of the LC connector. Accordingly, the optical transceiver  1  of the present embodiment is an exclusive solution to install an optical modulator with the MZ type primarily made of dielectric materials, 
         [0041]    The top housing  2  provides in a rear end thereof a rear wall  2 D and an eaves  2 C extending outwardly from the rear wall  2 D. The rear wall  2 D faces the rear end  3 B of the bottom housing  3  as shown in  FIG. 9 . That is, the rear wall  20 , and the top and bottom housings,  2  and  3 , form the inner space to install the components therein. Referring to  FIG. 11 , which magnifies the rear portion of the top housing  2 , the rear wall  20  sets the electrical plug  6  thereon. As described later, the electrical plug  6  does not interfere with the wiring of the inner fiber F 3  extracted from the rear wall  20  and returning back into the inner space of the optical transceiver  1 . 
         [0042]    The rear wall  2 D also provides a groove  2   b  on a top thereof into which a gasket is set to shield the inner space, and two slits,  2   c  and  2   d,  in a center and a side thereof, respectively. The side slit  2   d  is formed in a position just behind the optical modulator  12 . Referring to  FIG. 10 , the inner fiber F 3  passes these slits,  2   c  and  2   d.  Specifically, the inner fiber F 3  pulled out from the optical modulator  12  passes the rear wall  2 D through the side slit  2   d,  rounded in the rear auxiliary area S 2  returns back to the inner space passing through the center slit  2   c,  and reaches the PMC  14  from the rear after running along the optical modulator  12  frontward, turned backward in the front auxiliary area Si, passing the inner connector  19 , and turned again frontward. Another inner fiber F 4  extracted from the iTLA  13  rearward reaches the PMC  14  from the front by rounding twice the optical modulator  12 . 
         [0043]    The inner fiber F 5 , which extends from the PMC  14  rearward, crosses laterally in the rear end of the inner space, runs frontward between the optical modulator  12  and one of the side walls, turns rearward in the front auxiliary area S 2 , and finally reaches the plug P provided in the front wall of the ICR  15 . The inner fiber F 2 , extracted rearward from the optical connector C 2 , rounds the inner space and reaches the other connector C 3  also provided in the front wall of the ICR  15 . 
         [0044]    The last inner fiber F 1 , which is extracted rearward from the other optical port  18 A of the optical receptacle  18 , reaches the optical modulator  12  from the front by being rounded in the rear of the inner space, running in the center thereof, and rounded again rearward in the front auxiliary area S 2 . That is, the inner fiber F 1  reaches the optical modulator  12  from the port  18 A as shaping an S-character. Two inner fibers, F 3  and F 4 , which are coupled with the PMC  14 , provide respective inner connectors  19 . Moreover, the inner fiber F 5 , which is also coupled with the PMC  14 , has the plug P in the end to the ICR  14  to maintain the polarization direction thereof. Thus, the PMC  14  may be easily replaced by detaching respective connectors. 
         [0045]    The optical transceiver  1  of the embodiment further provides a cover  20  to cover the rear auxiliary area S 2  into which the inner fiber F 3  is set, The inner fiber F 3 , which passes the rear wall  2 D through the side slit  2   d  behind the optical modulator  12 , rounds along the periphery of the rear auxiliary area S 2  and returns the inner space as passing through the center slit  2   c.  The cover  20  covers the inner fiver F 3  in the rear auxiliary area S 2 . The cover  20  is assembled with the top housing  2  by engaging three latches,  20 A to  20 C, with three holes,  20   e  to  20   g,  provided in the extension  2 C of the top housing  2 , as shown in  FIGS. 11 and 12 , where  FIG. 11  is a perspective view of the rear of the top housing and  FIG. 12  is a plan view thereof. 
         [0046]    The extension  2 C of the top housing  2  provides a hollow corresponding to the shape of the rear auxiliary area S 2 . The hollow has a diameter greater than  15  mm, which is a smallest diameter allowable for an ordinary single mode fiber. Setting the inner fiber F 3  along the periphery of the area S 2 , the round diameter of the inner fiber F 3  automatically becomes greater than 15 mm. The bent loss of the inner fiber F 3  may be thus suppressed. 
         [0047]    The optical transceiver  1  of the present embodiment thus described provides the front auxiliary area S 1  protruding from the front panel  5 . The front auxiliary area S 2  installs the front portion of the optical modulator  12  and that of the iTLA  13 . In particular, because the optical modulator having an enough longitudinal dimension to secure the electrical to optical interaction of the dielectric material may be partially set within the front auxiliary area S 1 , the optical transceiver  1  may be applicable for the coherent communication system. Also, the inner fibers, F 1  to F 5 , are rounded in the front auxiliary area S 1 , the installation of the inner fibers, F 1  to F 5 , may be effectively carried out without causing unnecessary bending stress in the inner fibers, F 1  to F 5 . 
         [0048]    Also, the optical transceiver  1  of the present embodiment provides the rear auxiliary area S 2  in the outside of the rear wall  2 D. The rear auxiliary area S 2  may provide a space to set and round the inner fiber F 3  there by a bending diameter greater than 15 mm. The inner fiber F 3  passes the side slit  2   d  behind the optical modulator  12 , rounds along the periphery of the auxiliary area S 2 , and passes the rear wall  2 D again through the center slit  2   c.  Thus, the auxiliary area S 2  may secure the bending diameter greater than 15 mm. The inner fiber F 3  in the rear auxiliary area S 2  may be securely protected by the cover  20 . 
         [0049]    In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.