Patent Publication Number: US-2011069966-A1

Title: Optical transceiver with electrical dispersion equalization

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrical dispersion equalization applicable to an optical transceiver that receives and transmits optical signals, and to an optical transceiver implementing the same. 
     2. Related Background Art 
     An optical communication system, whose distance between the optical ports is less than a few kilo-meters and a transmission speed is less than a several hundreds of mega bit per second (Mbps) generally uses a multi-mode fiber for the transmission medium, a light-emitting diode (LED) for an optical signal source and a photodiode (PD) for a light-receiving device. 
     While, an optical communication system with the transmission speed of 2.5 Gbps, 10 Gbps or higher uses the single-mode fiber as a transmission medium and a laser diode (LD) as an optical signal source, which follows the international standard of, for instance, Synchronous Digital Hierarchy (SDH) and Synchronous Optical NETwork (SONET). 
     Recently, an optical communication system that combines a multimode fiber with an LD is directed to a higher transmission rate using a cost effective multi-mode fiber. However, because a core of the multimode fiber may propagate many modes each showing specific transmission speed, the optical signal to be transmitted in the multimode fiber is easily deformed, which makes it hard to transmit an optical signal in a higher transmission rate. 
     One solution has been suggested in the United State Patent Application published as US-2009/0041468A to compensate the signal deformation in the multimode fiber due to different transmission speed specific to the mode, in which the dispersion caused in the multimode fiber may be electrically compensated for an electrical signal converted from the received optical signal. This technique to compensate the dispersion electrically has been called as the Electronic Dispersion Compensator (EDC). The standard, IEEE 802.3aq, has ruled to use the EDC to realize the transmission rate of 10 Gbps for already installed multimode fiber. 
       FIG. 7  is a functional block diagram of an optical transceiver  1 , while,  FIG. 8  is a block diagram of an EDC implemented within the optical transceiver  1 . The optical transceiver  1  shown in  FIG. 7  comprises a transmitter unit  2  and a receiver unit  3 . The transmitter unit  2  recovers an input electrical signal by a clock and data recovery (CDR)  4 , drives a semiconductor laser diode (LD) based on the recovered electrical signal by an LD driver  5 . The LD emits an optical signal modulated with the recovered electrical signal to an optical fiber, which is not illustrated in  FIG. 7 . 
     The receiver unit  3  includes a photodiode (PD)  7 , a trans-impedance amplifier (TIA)  8 , an EDC  9  and another CDR  10 . The PD receives an optical signal from an optical fiber and generates a photocurrent. The TIA  8 , which operates as a pre-amplifier, converts the photocurrent into a voltage signal. The EDC  9  electrically compensates the voltage signal, and the CDR  10  extracts a data and a clock contained in the compensated voltage signal. 
     The EDC  9 , as shown in  FIG. 8 , is one type of digital filters, in particular, the EDC  9  shown in  FIG. 8  is called as the transversal filter. The EDC  9  includes a delay unit  11 , a multiplier  12  and an adder  13 . The delay unit  11  includes a plurality of delay elements each delaying an input thereof by a period T and outputting thus delayed signal to the multiplier  12 . The multiplier  12  also includes a plurality of multiplying units each multiplying the output of the delay unit with a tap coefficient. The adder  13  generates a sum of respective outputs of the multiplying unit  12 . The EDC  9  further includes a smaller  14 , a slicer  15 , and an error detector  17 . The sampler  14  samples the output of the adder  13  by a period of T, and holds the sampled signal with at least a period T. The slicer  15  compares the output of the sampler  14  with a preset reference to generate a binary signal. The input of the slicer  15 , which is an analogue signal, and the output thereof, which is a binary signal, are compared and a difference therebetween is fed to the tap controller  16 . The tap controller  16  adjusts the tap coefficients provided to respective multiplier units such that the output of the error detector  17  minimizes. 
       FIG. 8  is the functional block diagram of the EDC with the feedforward arrangement, but, another type of the EDC has been known as the feedback configuration. 
     The IEEE 802.3aq only defines the transmission rate of 10 Gbps, but other standards, such as those called as the fiber channel, has ruled the multi-transmission rates. Specifically, the FC-PI-4 (Fiber Channel Physical Interface) has ruled three transmission rates of 8.5 Gbps, 4.25 Gbps, and 2.125 Gbps, but has ruled that the EDC is necessary to be implemented only for the transmission rate 8.5 Gbps. 
     Because the EDC is necessary to set the unit delay T as a reciprocal of the transmission rate, namely, T equal to 100 ps for the transmission rate of 10 Gbps and so on. When the EDC is operated for multiple transmission rates but the single and permanent unit delay T, the EDC tends to cause an incorrect operation or to diverge the tap coefficients. Moreover, the optical transceiver operable in multi transmission modes is necessary to implement with devices responsible to a high transmission rates, that is, the devices are operable in a higher frequency. When such devices are used in relatively lower frequency, the noises with higher frequency components are amplified and generated by the devices, which degrades the optical sensitivity of the transceiver. For instance, the PD and the TIA operable for the transmission rate of 8.25 Gbps are used for the transmission rate of 4.25 Gbps; the optical sensitivity degrades by 3 dB because the frequency bandwidth of the devices is twice. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention relates to an optical transceiver that comprises: a PD, a pre-amplifier, and an EDC. The optical transceiver of the invention may be operable for multi transmission rates. The PD receives an optical signal from a transmission optical fiber and generates a photocurrent corresponding to the optical signal. The pre-amplifier, which may be a type of trans-impedance amplifier, may convert the photocurrent into a voltage signal. The EDC receives the voltage signal provided from the pre-amplifier and generates a compensated signal. A feature of the present invention is that the EDC automatically adjusts the tap coefficients thereof for the first transmission rate, sets only one of the tap coefficients to be TRUE, while, the other of the tap coefficients to be FALSE for the second transmission rate slower than the first transmission rate, and fixes the tap coefficients to preset values for the third transmission rate slower than the second transmission rate. 
     The EDC of the present invention may operate as a transversal filter to compensate the input deformed signal for the first transmission rate, as a low-pass-filter to reduce noises with high frequency components generated in the PD and the pre-amplifier for the third transmission rate, and pass through the voltage signal provided from the pre-amplifier. Accordingly, the optical transceiver may be operable in multi transmission rate even the transceiver implements with the EDC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a functional block diagram of an optical transceiver according to an embodiment of the present invention; 
         FIG. 2  is a functional block diagram of an EDC according to an embodiment of the present invention; 
         FIGS. 3A and 3B  are the functional block diagram of an optical transceiver according to another embodiment of the invention; 
         FIGS. 4A to 4C  show eye diagrams of a signal input to the EDC for respective transmission rates; 
         FIGS. 5A to 5C  show frequency responses of the EDC for respective transmission rates; 
         FIGS. 6A to 6C  show eye diagrams of a signal output from the EDC for respective transmission rates; 
         FIG. 7  is a functional block diagram of a conventional optical transceiver; and 
         FIG. 8  is a functional block diagram of an EDC implemented within the conventional optical transceiver shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, preferred embodiments according to the present invention will be described as referring to accompanying drawings.  FIG. 1  shows an optical transceiver according to an embodiment of the invention. The optical transceiver  21  shown in  FIG. 1  comprises a transmitter section  22  and a receiver section  23  similar to the arrangement of a conventional optical transceiver shown in  FIG. 7 . The transmitter section  22  includes a clock and data recovery (CDR)  24 , an LD-driver  25 , and a semiconductor laser diode (hereinafter denoted as LD)  26 . The CDR  24  reshapes and recovers the electrical data input to the optical transceiver  21 . The LD-driver  25  drives the LD by the signal provided from the CDR  24 . The LD  26  emits signal light which is modulated by the electrical signal recovered by the CDR  24 . 
     The receiver section  23  includes a semiconductor photodiode (PD)  27 , a pre-amplifier  28 , an electrical dispersion correction (EDC)  29 , and another CDR  30 . The PD  27  generates a photocurrent by receiving signal light provided from the optical fiber. The pre-amplifier  28 , which may be an arrangement of a trans-impedance amplifier (TIA), converts the photocurrent into a voltage signal. The EDC  29  electrically compensates the dispersion superposed in the voltage signal provided from the TIA  28 . The CDR  30  reshapes and recovers the signal compensated by the EDC  29  and outputs thus reshaped electrical signal. A feature of the optical transceiver  21  according to the present embodiment is that the EDC  29  may vary or adjust the tap co-efficient thereof in the multiplier unit depending on the transmission rate, for instance 5 Gbps, 10 Gbps or 20 Gbps, the information of which is externally provided. 
       FIG. 2  shows an example of the EDC  29  with a function of the selectable tap co-efficient. The EDC  29  shown in  FIG. 2 , similar to the conventional EDC shown in  FIG. 8 , includes the input delay unit  31  with the transversal type, the multiplier unit  32 , and the sum unit  33 . The delay unit  31  includes a plurality of delay elements  31   a  each delaying the output of the upward delay element by a period T. The first delay element delays the input of the delay unit  31 . The multiplier unit  32  multiplies respective outputs of the delay elements with the tap co-efficient specific to respective delay units. The sum unit  33  adds respective outputs of the multiplier unit  32  and outputs thus added result. The delay unit  31 , the multiplier unit  32 , and the sum unit  33  carry out function of the self-convolution. 
     The EDC  29  further includes sampler  34  which has a function of, what is called, the sample-and-hold for sampling the output provided from the sum unit  33  for a period T and holding the sampled output which is provided to the slicer  35 . The slicer  35  has a function of the comparator where the input thereof, which is equivalent to the held output of the sampler  34 , is compared with a preset reference and outputs a binary signal. The error detector  37  compares this binary output with the input of the slicer  35 ; namely, the analogue output of the slicer  34  and sends the compared results to the tap controller  36 . The tap controller  36  adjusts the tap co-efficient provided to the multiplier  32  based on the output of the error detector  37  so as to minimize the output of the error detector  37 . 
     A feature of the EDC  29  according to the present embodiment is that the tap controller  36  receives the RATE signal and adjusts the tap co-efficient depending on the RATE signal. Specifically, when the optical transceiver  21  is operated in a high speed mode, the EDC  29  adjusts the tap co-efficient by the feedback loop of the sum unit  33 , the sampler  34 , the slicer  35 , the error detector  37  and the tap controller  36 . While, the transceiver  21  operates in a relatively slow speed mode, the tap controller  36  switches the input thereof from the output of the error detector  37  to the preset  38  which provides the fixed tap coefficient. When the multiplier  32  is provided with the set of the fixed tap co-efficient, the EDC  29  operates as a transversal filter to pass only relatively lower frequency components. The EDC  29  operating as a transversal filter, noises of the PD  27  and the TIA  28 , whose operable frequency bandwidths are set broader because they are necessary to respond higher frequencies and inevitably cause noises in high frequencies, may be reduced and the sensitivity of the receiver unit  23  may be enhanced. 
     Assuming cases where the transmission rates of the transceiver are 5 Gbps, 10 Gbps, and 20 Gbps, the delay time T of the delay unit  31  is 50 ps, and the frequency bandwidth of the PD  27  and that of the TIA  28  are 7.5 GHz applicable to the rate of 10 Gbps; the EDC  29  may compensate the deformation in the waveform due to not only the dispersion in the transmission line but the limited frequency bandwidth of the PD  27  and the TIA  28  by adjusting the tap coefficient. When the transmission speed is 10 Gbps where the PD  27  and the TIA  28  become adequately operable, the EDC  29  may set the tap coefficient such that only one of the tap coefficients is set to be TRUE “1”, while, the other tap coefficients are set to be FALSE “0”, which equivalently passes the EDC  29 . Moreover, when the rate signal RATE indicates the transmission speed of 5 Gbps, the EDC  29  sets the tap coefficients such that the EDC  29  operates as an LPF (low-pass-filter) to eliminate the high frequency noise due to the PD  27  and the TIA  29 . 
     A feature of the present invention shown in  FIGS. 1 and 2 , and described above is applicable to another type of a transceiver whose block diagram is shown in  FIGS. 3A and 3B . The optical transceiver  21   a  shown in  FIG. 3A  has the transmitter unit  22   a  including only the LD-driver  25  (LDD) and the LD  26 , while, the receiver unit  23   a  has the same arrangement with those of the transceiver  21  shown in  FIG. 1 . The optical transceiver  21   a  in  FIG. 3A  eliminates the CDR in the transmitter unit  22   a  and the deformation appearing in the transmitter unit  22   a  may be compensated in the receiver unit  23   a  involved in the other optical transceiver optically coupled with the present transceiver  21   a.    
       FIG. 3B  shows a functional block diagram of the other type of the optical transceiver  21   b  according to an embodiment of the invention. The optical transceiver  21   b  shown in  FIG. 3B  comprises a transmitter unit  22   b  including the LD-driver  25  and the LD  26 , and a receiver unit  23   b  that includes the PD  27  and the TIA  28 . This optical transceiver  21   b  shown in  FIG. 3B  eliminates the EDC in the receiver unit  223   b  in addition to an EDC involved in the transmitter unit. However, the host system coupled with the optical transceiver  21   b  has the function of the CDR and implements with the EDC  29 . Thus, the optical transceiver  21   b  includes only the PD  27  and the TIA  27  in the receiver unit  23   b.    
     Next, some results according to the present invention will be described, in which the results assumes the following: three transmission rates, 5 Gbps, 10 Gbps, and 20 Gbps, are selected, the total bandwidth of the optical fiber, the PD and the TIA is 6 GHz, the EDC  29  has the configuration of only the feedforward arrangement without any feedback arrangement with seven (C 0 ˜C 6 ) controllable taps and the unit delay T of 50 ps. 
       FIGS. 4A to 4C  show eye diagrams of the signal input to the EDC  29  for the transmission rates of 5, 10, and 20 Gbps, respectively. The eye diagrams for 5 and 10 Gbps show an enough eye but the eye opening penalty was found to be 2.2 dB for the transmission rate of 20 Gbps due to the limited band width of the optical fiber, the PD  27  and the TIA  28 . 
     When such deformed signal for the transmission rate of  20  Gbps is input to the EDC  29 , the EDC  29  automatically adjusts the tap coefficients, CO to C 6 , thereof to be −0.014, 0.08, −0.487, 2.98, −0.729, 0.216 and −0.06, respectively. For the transmission rate of 10 Gbps, the EDC  29  sets TRUE only for the tap coefficient C 3 , while, sets FALSE for the other tap coefficients. Under such tap coefficients, the EDC  29  passes through the input signal only with a delay of 100 ps. Finally, for the transmission rate of 5 Gbps, the fixed tap coefficients of, C2=0.25, C3=0.5, C4=0.25, and the other coefficients, C 0 , C 1 , C 5  and C 6 , are set to be zero (0), under which the EDC  29  operates as an LPF because the components multiplied by 0.25 superposed with the primary component multiplied with 0.5 by the unit delay of 50 ps. 
       FIGS. 5A to 5C  correspond to the response of the EDC  29  for respective transmission rates. As shown in  FIG. 5A , the frequency response of the EDC  29  for the rate 5 Gbps shows the LPF performance with the cut off frequency of 3.6 GHz. For the transmission rate of 10 Gbps, the EDC  29  shows no frequency response, as shown in  FIG. 5B . Further, the EDC  29  enhances the high frequency performance by showing a peak around 10 GHz to compensate the limited bandwidth of the other devices as shown in  FIG. 5C . 
       FIGS. 6A to 6C  show the eye diagram output from the EDC  29  operated under the condition described above. For the transmission rate of 5 Gbps, the output shows an enough eye without deforming the input waveform shown in  FIG. 4A . Because the EDC  29  operates as an LPF with the cut-off frequency of 3.6 GHz, the sensitivity of the receiver unit may be improved by about 2.2 dB. For the transmission rate of 10 Gbps, the output shows an enough eye, as shown in  FIG. 6B . Finally, for the transmission rate of 20 Gbps shown in  FIG. 6C , the output of the EDC  29  shows enough eye without penalty although some overshoots and undershoots are found. 
     Thus, the EDC according to the present invention may vary the tap control mode depending on the transmission rate; accordingly, the EDC may compensate the deformation appeared in the input signal for relatively higher transmission rate, while, the EDC may reduce the noise caused by the excess frequency bandwidth of the PD and the TIA for relatively lower transmission rate. The optical transceiver implementing with the EDC of the invention may enhance the optical sensitivity for the lower transmission rate, and may securely recover the transmitted optical signal for the higher transmission rate. 
     Although the present invention has been fully described in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.