Abstract:
A duo-binary optical transmitter tolerant to a chromatic dispersion includes a pre-coder for generating a first 2-level signal from input binary data and generating a second signal having a waveform obtained by inverting the first signal, a Mach-Zehnder Modulator (MZM) for generating a Differential Phase Shift Keying (DPSK) modulated optical signal by modulating an input light according to the first signal and the second signal, and a Delay Interferometer (DI) for splitting the DPSK modulated optical signal into a first split signal and a second split signal, delaying the second split signal, and generating a duo-binary optical signal by interfering the first split signal with the second delayed split signal, wherein a time required for delaying the second split signal is set to 0.5˜0.8 bit.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “Duo-binary Optical Transmitter tolerant to Chromatic Dispersion,” filed in the Korean Intellectual Property Office on Oct. 4, 2004 and assigned Serial No. 2004-78766, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical transmitter, and more particularly to a duo-binary optical transmitter for outputting duo-binary optical signals by means of a Mach-Zehnder Modulator (MZM) and a Delay Interferometer (DI).  
         [0004]     2. Description of the Related Art  
         [0005]     Modulation schemes of an optical communication system have been developed to increase the transmission speed and transmission efficiency. An existing On-Off Keying (OOK) modulation scheme has disadvantages in that it has a wide spectrum of an optical signal, thus easily influenced by a chromatic dispersion of an optical fiber. In contrast, a duo-binary modulation scheme has advantages in that it has a narrow spectrum of an optical signal and thereby less influenced by the chromatic dispersion of an optical fiber.  
         [0006]      FIG. 1  is a block diagram showing a conventional duo-binary optical transmitter. As shown, the optical transmitter includes a Pulse Pattern Generator (PPG)  110 , a pre-coder  120 , a first and a second Low Pass Filter (LPF)  130  and  140 , a first and a second amplifier (AMP)  150  and  160 , a Light Source (LS)  170 , and an MZM  180 .  
         [0007]     In operation, the PPG  110  outputs binary data  112  and the pre-coder  120  outputs a first signal  122 , which is obtained by pre-coding the binary data  112  into a 2-level signal, and a second signal  124  having a waveform inverse corresponding to the first signal  122 . The first LPF  130  outputs a third signal  132  obtained by converting the first signal  122  into a 3-level signal, and the second LPF  140  outputs a fourth signal  142  obtained by converting the second signal  124  into a 3-level signal. The first AMP  150  amplifies and outputs the third signal  132 , and the second AMP  160  amplifies and outputs the fourth signal  142 . The LS  170  is a Distributed FeedBack (DFB) laser and outputs a light  172  having a predetermined wavelength. The MZM  180  modulates the light input from the LS  170  according to the amplified third signal  152  and fourth signal  162 , then output a 2-level duo-binary optical signal  182 .  
         [0008]     Since the conventional optical transmitter uses the 3-level third signal and fourth signal, the 2-level duo-binary optical signal output from the MZM may be deteriorated due to a non-linearity of the first and the second AMP. In order to solve this problem, a method has been proposed, in which a 2-level duo-binary optical signal is generated by applying a 2-level signal to the MZM. According to this method, an optical transmitter generates a Differential Phase Shift Keying (DPSK) modulated optical signal by applying 2-level signals output from a pre-coder to an MZM, and then generates a 2-level duo-binary optical signal by inputting the generated optical signal into a 1-bit DI. However, this method is much less tolerant to the chromatic dispersion of an optical fiber.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a duo-binary optical transmitter tolerant to chromatic dispersion of an optical fiber.  
         [0010]     In one embodiment, there is provided a duo-binary optical transmitter tolerant to a chromatic dispersion, the duo-binary optical transmitter including: a pre-coder for generating a first 2-level signal from input binary data and for generating a second signal having a waveform obtained by inverting the first signal; a Mach-Zehnder Modulator (MZM) for generating a Differential Phase Shift Keying (DPSK) modulated optical signal by modulating an input light according to the first signal and the second signal; and a Delay Interferometer (DI) for splitting the DPSK modulated optical signal into a first split signal and a second split signal, delaying the second split signal, and generating a duo-binary optical signal by interfering the first split signal with the second delayed split signal, wherein the time required for delaying the second split signal is set to 0.5˜0.8 bit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a block diagram showing a conventional duo-binary optical transmitter;  
         [0013]      FIG. 2  is a block diagram showing a duo-binary optical transmitter according to an embodiment of the present invention;  
         [0014]      FIG. 3  is an eye diagram showing a back-to-back monitoring result for a duo-binary optical signal obtained by setting the delay time of the optical transmitter shown in  FIG. 2  to 0.8 bit;  
         [0015]      FIG. 4  is an eye diagram showing a monitoring result for a duo-binary optical signal after the duo-binary optical signal obtained by setting the delay time of the optical transmitter shown in  FIG. 2  to 0.8 bit has been transmitted 160 km;  
         [0016]      FIG. 5  is an eye diagram showing a back-to-back monitoring result for a duo-binary optical signal obtained by setting the delay time of the optical transmitter shown in  FIG. 2  to 1 bit;  
         [0017]      FIG. 6  is an eye diagram showing a monitoring result for a duo-binary optical signal after the duo-binary optical signal obtained by setting the delay time of the optical transmitter shown in  FIG. 2  to 1 bit has been transmitted 160 km;  
         [0018]      FIG. 7  is a graph illustrating the change of a receiver sensitivity according to the delay time of the optical transmitter shown in  FIG. 2 ;  
         [0019]      FIG. 8  is a graph showing a comparison of the optical transmitter shown in  FIG. 2  having a delay time of 0.7 T and the optical transmitter having a delay time of 1.0 T; and  
         [0020]      FIG. 9  is a graph showing an optimal delay time range of the optical transmitter shown in  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0021]     Hereinafter, embodiments according to the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configuration incorporated herein will be omitted as it may make the subject matter of the present invention unclear.  
         [0022]      FIG. 2  is a block diagram showing a duo-binary optical transmitter according to an embodiment of the present invention. As shown, the inventive optical transmitter includes an LS  240 , a pre-coder  210 , a first and a second AMP  220  and  230 , an MZM  250  and a DI  300 .  
         [0023]     The pre-coder  210  pre-codes input Non-Return-to-Zero (NRZ) binary data, divides the pre-coded signal (or 2-level signal) into a first branch signal and a second branch signal, inverts the second branch signal, and outputs the first branch signal (non-inverted signal)  212  and the second branch signal (inverted signal)  214 . Further, the pre-coder  210  may include a 1-bit delay element, an exclusive-OR element, a branching means (e.g., parallel connection of conductive wires) for dividing outputs of the delay element and the exclusive-OR element into two branch signals, and an inverter for inverting one of the two branch signals.  
         [0024]     The first AMP  220  is a modulator driver which amplifies and outputs the first signal  212  input from the pre-coder  210 . The second AMP  230  is a modulator driver which amplifies and outputs the second signal  214  input from the pre-coder  210 .  
         [0025]     The LS  240  outputs a light  242  having a predetermined wavelength and may use a Continuous Wave (CW) laser, a DFB laser, etc.  
         [0026]     The MZM  250  outputs a NRZ-DPSK modulated optical signal  252  obtained by modulating the light  242  input from the LS  240  according to the first amplified signal  222  and the second amplified signal  232 . The MZM  250  includes a dual-arm, the first amplified signal  222  is applied to one of the dual-arm, and the second amplified signal  232  is applied to the other of the dual-arm. Furthermore, the MZM  250  may use a LiNbO 3  modulator including a dual-arm.  
         [0027]     The DI  300  includes a splitter  260 , a delay  270  and a coupler  280 . The DI  300  splits the NRZ-DPSK modulated optical signal  252  into a first and a second split signal  262  and  264 . Further, the DI  300  delays the second split signal  264  and outputs a duo-binary optical signal  282  obtained by interfering the first split signal  262  with the second delayed split signal  264 .  
         [0028]     The splitter  260  splits the NRZ-DPSK modulated optical signal  252  input from the MZM  250  into the first and the second split signal  262  and  264 .  
         [0029]     The delay  270  delays and outputs the second split signal  264  input from the splitter  260 . Herein, it is preferred that the delay  270  has a delay time set to 0.5 to 0.8 bit.  
         [0030]     The coupler  280  outputs the duo-binary optical signal  282  obtained by interfering the first split signal  262  input from the splitter  260  with a second delayed split signal  272  input from the delay  270 .  
         [0031]      FIG. 3  is an eye diagram showing a back-to-back monitoring result for the duo-binary optical signal obtained by setting the delay time of the optical transmitter to 0.8 bit. As shown in  FIG. 3 , when the delay time of the delay  270  is set to 0.8 bit, one can see that the duo-binary optical signal has a wide window (or eye)  310  as a result of the back-to-back monitoring and has a periodic ripple  330  in a zero rail  320 .  
         [0032]     The improvement of dispersion characteristics of the duo-binary optical signal due to the periodic ripple  330  in the zero rail  320  may be evidence as follows.  
         [0033]      FIG. 4  is an eye diagram showing a monitoring result for the duo-binary optical signal after the duo-binary optical signal obtained by setting the delay time of the optical transmitter to 0.8 bit has been transmitted for 160 km. As shown in  FIG. 4 , one can see that the duo-binary optical signal has a window  340  smaller than that of the case in the back-to-back monitoring of  FIG. 3 , but the window  340  is formed wider.  
         [0034]      FIG. 5  is an eye diagram showing a back-to-back monitoring result for the duo-binary optical signal obtained by setting the delay time of the optical transmitter to 1 bit. As shown in  FIG. 5 , when the delay time of the delay  270  is set to 1 bit, one can see that the duo-binary optical signal has a wide window  410  as a result of the back-to-back monitoring and does not have a periodic ripple in a zero rail  420 .  
         [0035]      FIG. 6  is an eye diagram showing a monitoring result for the duo-binary optical signal after the duo-binary optical signal obtained by setting the delay time of the optical transmitter to 1 bit has been transmitted 160 km. As shown in  FIG. 6 , one can see that the duo-binary optical signal has a smaller window  430  and a greatly distorted waveform due to the chromatic dispersion of an optical fiber, as compared with the case in which the optical transmitter has the delay time set to 0.8 bit.  
         [0036]      FIG. 7  is a graph illustrating the change of a receiver sensitivity according to the delay time of the optical transmitter. In  FIG. 7 , a transmission distance represents a distance in which the duo-binary optical signal output from the optical transmitter is propagated in a standard single mode fiber. Here, a ‘T’ represents a delay time constant corresponding to 1 bit and a ‘t’ represents a delay time variable. Equivalent lines having a value within a range of −21˜−16 dBm represent corresponding receiver sensitivities, respectively. When the transmission distance is 0 km, the most favorable receiver sensitivity is shown at a delay time of 1.0 T. When the transmission distance increases, the most favorable receiver sensitivity is shown at a delay time of 0.5˜0.8 T.  
         [0037]      FIG. 8  is a graph showing a comparison of the optical transmitter having a delay time of 0.7 T versus a delay time of 1.0 T. As shown in  FIG. 8 , when the delay time of the delay  270  is set to 0.7 T, one can see that it is possible to more than doubling the transmission distance without compensating for the chromatic dispersion of an optical fiber, as compared with the case in which the delay time of the delay  270  is set to 1.0 T. That is, when the delay time is 1.0 T, it is possible to obtain a receiver sensitivity below −19 dBm up to about 100 km. However, when the delay time is 0.7 T, it is possible to obtain a receiver sensitivity below −19 dBm up to about 200 km.  
         [0038]      FIG. 9  is a graph showing an optimal delay time range of the optical transmitter. In a typical optical communication system, a span representing a distance between optical repeaters is generally set to 80 km in order to compensate for dispersion. As shown in  FIG. 9 , when the delay time of the optical transmitter is set to 0.5˜0.8 T, one can see that it is possible to transmit an optical signal up to two spans without the need of a dispersion compensation. That is, when the optical transmitter is applied to an existing optical communication system having a span of 80 km, it is possible to reduce the number of optical repeaters by one half.  
         [0039]     As described above, a duo-binary optical transmitter according to the present invention does not use an LPF and sets a delay time of a DI to 0.5˜0.8 bit, so that the duo-binary optical transmitter is tolerant to the chromatic dispersion of an optical fiber. Therefore, it is possible to reduce a manufacturing cost of an optical communication system employing the optical transmitter.  
         [0040]     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, including the full scope of equivalents thereof.