Abstract:
A chirped RZ-AMI optical transmitter includes a first logic gate for receiving a first signal obtained by pre-coding binary data and a second signal having a waveform obtained by inverting and delaying the first signal, and outputting a third signal obtained by logically operating on the first and second signals. A second logic gate receives a fourth signal having a waveform obtained by inverting the first signal, and a fifth signal having a waveform obtained by delaying the first signal. The second logic gate outputs a sixth signal obtained by logically operating on the fourth and fifth signals. A Mach-Zehnder Modulator (MZM) modulates an input light according to the third and sixth signals and outputs an RZ-AMI optical signal.

Description:
CLAIM FOR PRIORITY  
       [0001]     This application claims priority to an application entitled “Chirped RZ-AMI Optical Transmitter,” filed in the Korean Intellectual Property Office on Oct. 20, 2004 and assigned Serial No.2004-83926, 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 an optical transmitter using a Mach-Zehnder Modulator (MZM).  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  is a block diagram showing the basic construction of a typical Return-to-Zero Alternate-Mark-Inversion (RZ-AMI) optical transmitter using an MZM and a Delay Interferometer (DI), and  FIG. 2  is a diagram showing processing signals of the RZ-AMI optical transmitter shown in  FIG. 1 . The RZ-AMI optical transmitter  100  includes a pre-coder  110 , a modulator driver  120 , a Continuous Wave (CW) laser  130 , an MZM  140  and a DI  150 .  
         [0006]     The pre-coder  110  pre-codes and outputs binary data S 11  which are input Non-Return-to-Zero (NRZ) signals. The modulator driver  120  receives the input from the pre-coder  110 , amplifies it and outputs the amplified signal as a pre-coded signal S 12 . The pre-coder  110  may include a 1-bit delay element and an exclusive-OR element. The MZM  140  intensity &amp; phase-modulates and outputs, according to the amplified signal, a light input from the CW laser  130 . The bias position of the MZM  140  is located at a null point corresponding to a minimum value in a transfer characteristic function of the MZM  140 . The DI  150  splits the modulated S 13  input from the MZM  140  into a first and a second optical signal, delays the first optical signal by 0.5 bit, i.e., one half of a bit period, and outputs an optical signal S 15  obtained by combining the first delayed optical signal and the second optical signal so that they destructively interfere. Then, an RZ-AMI optical signal is obtained by phase-modulating the destructively-interfered optical signal S 15  each bit by means of a phase modulator so that the optical signal S 15  has an inversed phase. The RZ-AMI modulation scheme known in the art has characteristics in which an optical signal includes intensity information and a phase of the optical signal is inverted alternately with each bit. In particular, in indicating the intensity of an RZ-AMI optical signal, as in the case of an RZ signal, a shift in energy of the RZ-AMI optical signal from a level  0  to a level  1 , with a subsequent return to the level  0 , indicates a single bit. Accordingly, since the RZ-AMI optical signal has the same change of intensity as that in the RZ signal, the RZ-AMI optical signal has an advantage in an RZ modulation scheme. For example, the RZ-AMI optical signal is tolerant to a non-linearity of an optical fiber in a transmission system having a data speed more than 20 Gb/s. Further, since the phase of the optical signal is inverted alternately each bit, a frequency component of a carrier is suppressed. Therefore, the RZ-AMI optical signal is tolerant to not only the Brillouin non-linearity effect but also the non-linearity effect such as the Intra-channel Four-Wave-Mixing (IFWM) and the Intra-channel Cross-Phase-Modulation (IXPM).  
         [0007]     However, the RZ-AMI optical transmitter  100  as described above is expensive due to the expensive parts, particularly the MZM  140 , the DI  150  and the phase modulator. Therefore, a system with the RZ-AMI optical transmitter  100  tends to require a non-competitive price.  
         [0008]     Since a chirped RZ signal has been known to be tolerant to the non-linearity effect, it is observed by the present inventors that the RZ-AMI optical signal may also have the same advantages. Accordingly, a chirped RZ-AMI modulation scheme may be a very superior modulation scheme having advantages of a chirped RZ signal and an RZ-AMI optical signal. What is needed is a chirped RZ-AMI optical transmitter that is low-priced and tolerant to the non-linearity effect.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been made to solve the above-mentioned problems occurring in the prior art, and, in one aspect, the present invention provides a chirped RZ-AMI (Return-to-Zero Altemate-Mark-Inversion) optical transmitter having two logic gates and a Mach-Zehnder Modulator (MZM). The first logic gate receives a first signal obtained by pre-coding binary data, and a second signal having a waveform obtained by inverting and delaying the first signal. An output signal is obtained by logically operating on the first and second signals. A second logic gate receives a fourth signal having a waveform obtained by inverting the first signal, and a fifth signal having a waveform obtained by delaying the first signal. A sixth signal is obtained by logically operating on the fourth and fifth signals. The MZM modulates, according to the third and sixth signals, inputted light and outputs an RZ-AMI optical signal.  
         [0010]     In accordance with another aspect of the present invention, there is provided a chirped RZ-AMI (Return-to-Zero Alternative-Mark-Inversion) optical transmitter having a first logic gate for receiving a first signal obtained by pre-coding binary data and a second signal having a waveform obtained by inverting and delaying the first signal. A third signal for output is obtained by logically operating on the first and the second signals. A second logic gate receives a fourth signal having a waveform identical to a waveform of the second signal and a fifth signal having a waveform identical to a waveform of the first signal. A sixth signal for output is obtained by logically operating on the fourth and the fifth signals. A Mach-Zehnder Modulator (MZM) modulates, according to the third signal and sixth signals, inputted light and outputs an RZ-AMI optical signal. 
     
    
     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 the same or similar features are annotated with like reference numbers throughout the several views:  
         [0012]      FIG. 1  is a block diagram showing the basic construction of a typical RZ-AMI optical transmitter using an MZM and a DI;  
         [0013]      FIG. 2  is a timing diagram showing processing signals of the RZ-AMI optical transmitter of  FIG. 1 ;  
         [0014]      FIG. 3  is a block diagram showing a chirped RZ-AMI optical transmitter according to a first preferred embodiment of the present invention;  
         [0015]      FIG. 4  is a timing diagram of signal processing for the optical transmitter in  FIG. 3  according to a first combination of a first and a second logic gate;  
         [0016]      FIG. 5  is a timing diagram of signal processing for the optical transmitter in  FIG. 3  according to a second combination of a first and a second logic gate;  
         [0017]      FIG. 6  is a polar map showing a positive chirp;  
         [0018]      FIG. 7  is a timing diagram of signal processing for the optical transmitter shown in  FIG. 3  according to a third combination of a first and a second logic gate;  
         [0019]      FIG. 8  is a timing diagram of signal processing for the optical transmitter shown in  FIG. 3  according to a fourth combination of a first and a second logic gate;  
         [0020]      FIG. 9  is a polar map showing a negative chirp; and  
         [0021]      FIG. 10  is a block diagram showing a chirped RZ-AMI optical transmitter according to a second preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     For the purposes of clarity and simplicity, detailed description of known functions and configuration incorporated herein is omitted for clarity of presentation.  
         [0023]      FIG. 3  is a block diagram showing a chirped RZ-AMI optical transmitter according to a first preferred embodiment of the present invention.  
         [0024]     The chirped RZ-AMI optical transmitter  200  includes a pre-coder  210 , a first and a second branching means  212 ,  214 , a first and a second delay  220 ,  230 , a first and a second logic gate  250 ,  255 , a first and a second modulator driver  260 ,  270 , a CW laser  280 , and an MZM  290 .  
         [0025]     The pre-coder  210  pre-codes input NRZ binary data S 21 , divides the pre-coded signal (or 2 level signal) into two branch signals, inverts one of the branch signals, and outputs the remaining branch signal (non-inverted signal) S 22  and the inverted signal {overscore (S)} 22 . 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 the 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.  
         [0026]     The first branching means  212  divides the branch signal input from the pre-coder  210  into two branch signals. This may be accomplished through parallel connection of conductive wires.  
         [0027]     The first delay  220  delays one of the branch signals having passed through the first branching means  212  by 0.5 bit, and outputs the delayed signal.  
         [0028]     The second branching means  214  divides the inverted signal input from the pre-coder  210  into two branch signals. This may, as in the case of the first branching means  212 , be accomplished through parallel connection of conductive wires.  
         [0029]     The second delay  230  delays one of the branch signals having passed through the second branching means  214  by 0.5 bit, and outputs the delayed signal.  
         [0030]     The first logic gate  250  receives the branch signal (first signal) S 22  having passed through the first branching means  212  and the delayed signal (second signal) S 24  input from the second delay  230 . The first logic gate  250  logically operates on the received first and second signal to output the third signal S 25 .  
         [0031]     The second logic gate  255  receives the branch signal (fourth signal) {overscore (S)} 22  having passed through the second branching means  214  and the delayed signal (fifth signal) S 23  input from the first delay  220 . The second logic gate  255  logically operates on the received fourth and fifth signal to output the sixth signal S 26 .  
         [0032]     The first modulator driver  260  amplifies the third signal S 25  input from the first logic gate  250 .  
         [0033]     The second modulator driver  270  amplifies the sixth signal S 26  input from the second logic gate  255 .  
         [0034]     The CW laser  280  outputs a light having a predetermined wavelength and the MZM  290  outputs a chirped RZ-AMI optical signal S 27  obtained by modulating, according to the amplified third and the sixth signals, the light input from the CW laser  280 . The MZM  290  includes a dual-arm. The third signal is applied to one arm of the dual-arm, and the sixth signal is applied to the other arm of the dual-arm. The MZM  290  may have a z-cut structure having a dual-arm. The bias position of the MZM  290  is located at a null point corresponding to a minimum value of a modulator transfer characteristic.  
         [0035]     The electric field S 27 (E) of the chirped RZ-AMI optical signal is determined by a difference between waveforms of the third signal and the sixth signal output from the first and the second logic gates  250 ,  255 . The phase S 27 (P) of the chirped RZ-AMI optical signal is determined by a sum of waveforms of the third signal and the sixth signal output from the first and the second logic gates  250 ,  255 . A chirp characteristic of the chirped RZ-AMI optical signal changes according to combinations of the first and the second logic gates  250 ,  255 , providing the chirped RZ-AMI optical signal with a positive chirp or a negative chirp. Hereinafter, the chirped RZ-AMI optical signal will be said to have positive chirp when it has a phase that increases as its intensity increases and decreases when the intensity decreases, i.e., a phase that varies directly with intensity.  FIG. 6  shows a polar map representing positive chirp. By contrast, the chirped RZ-AMI optical signal has negative chirp when its phase varies inversely with intensity, so that phase increases when intensity decreases and such that phase decreases when intensity increases.  FIG. 9  shows a polar map representing negative chirp.  
         [0036]     When the delay times of the first and the second delay  220 ,  230  are set to correspond to about one half of a transmission speed of the chirped RZ-AMI optical signal, for example, about 12.5 ps in a case of a transmission speed of 40 Gb/s, an RZ of about 50% occurs. The duty cycle of the chirped RZ-AMI optical signal (RZ signal) may be adjusted by adjusting the delay times of the first and the second delay  220 ,  230 . As is typical of the AMI signal, the phase of the chirped RZ-AMI optical signal inverts with each bit.  
         [0037]     The optical transmitter  200  may be realized by means of various combinations of the first and the second logic gates  250 ,  255 .  
         [0038]      FIG. 4  is a diagram showing processing signals of the optical transmitter  200  according to a first combination of the first and the second logic gates  250 ,  255 . The first combination is the case in which NAND logic gates as employed as the first and the second logic gates  250 ,  255 .  
         [0039]     In particular, the first logic gate  250  receives the first signal S 22  having passed through the first branching means  212  and the second signal S 24  input from the second delay  230 . The first logic gate  250 , implemented as a NAND gate, outputs the third signal S 25  as a 0 bit when the first signal is a 1 bit and the second signal is a 1 bit. The first logic gate  250  outputs the third signal S 25  as a 1 bit in the other three case, i.e., when the first signal is a 1 bit and the second signal is a 0 bit, the first signal is a 0 bit and the second signal is a 1 bit, and the first signal is a 0 bit and the second signal is a 0 bit.  
         [0040]     The second logic gate  255  receives the fourth signal {overscore (S)} 22  having passed through the second branching means  214  and the fifth signal S 23  input from the second delay  220 . The second logic gate  255  outputs the sixth signal S 26  as a 0 bit when the fourth signal is 1 bit and the fifth signal is 1 bit. Likewise, in the other three cases, the second logic gate  255  outputs the sixth signal as 1 bit, i.e., when the fourth signal is a 1 bit and the fifth signal is a 0 bit, the fourth signal is a 0 bit and the fifth signal is a 1 bit, and the fourth signal is a 0 bit and the fifth signal is a 0 bit.  
         [0041]     The NAND operation performed by each of the first and the second logic gates  250 ,  255  can be realized through a serial connection of a NOT logic gate and an AND logic gate. Since the intensity and the phase S 27 (P) of the chirped RZ-AMI optical signal S 27  output from the MZM  290  vary together, as evident from the bottom two waveforms in  FIG. 6 , it follows that the chirped RZ-AMI optical signal of the first combination of logic gates  250 ,  255  has positive chirp.  
         [0042]      FIG. 5  is a diagram showing processing signals of the optical transmitter  200  according to a second combination of the first and the second logic gate  250 ,  255 , in which the second logic gates are implemented as OR logic gates.  
         [0043]     As in the first combination, the first logic gate  250  receives the first signal S 22  having passed through the first branching means  212  and the second signal S 24  input from the second delay  230 . The first logic gate  250 , however, outputs the third signal S 25  as a 0 bit when at least one of the first and second signals is a 1 bit, and outputs the third signal as a 1 bit when the first signal is a 0 bit and the second signal is a 0 bit.  
         [0044]     Again, as in the first combination, the second logic gate 255 receives the fourth signal {overscore (S)} 22  having passed through the second branching means 214 and the fifth signal S 23  input from the second delay  220 . The second logic gate  255 , however, outputs the sixth signal S 26  as a 0 bit when at least one of the fourth and fifth signals is a 1 bit, and outputs the sixth signal of 1 bit when the fourth signal is a 0 bit and the fifth signal is a 0 bit. Since the intensity and the phase S 27 (P) of the chirped RZ-AMI optical signal S 27  output from the MZM  290  vary directly, as shown in  FIG. 6 , one can see that the chirped RZ-AMI optical signal of the second combination has, like that of the first combination, positive chirp.  
         [0045]      FIG. 7  illustrates signal processing of the optical transmitter  200  according to a third combination of the first and the second logic gates  250 ,  255 , which uses AND logic gates as the first and the second logic gates. The first logic gate  250  accordingly outputs the third signal S 25  as a 1 bit when the first signal is 1 bit and the second signal is 1 bit. The third signal is outputted as a 0 bit when one of the first and second signals is a 1 bit and the other is a 0 bit, , or when the first and second signals are both 0 bits. The second logic gate  255  operates analogously to output the sixth signal S 26  as a 1 bit when both the fourth and fifth signals are 1 bits, and to otherwise output the sixth signal as a 0 bit. Since the intensity and the phase S 27 (P) of the chirped RZ-AMI optical signal S 27  output from the MZM  290  of the third combination vary inversely, the chirped RZ-AMI optical signal has negative chirp as shown in  FIG. 9 .  
         [0046]      FIG. 8  represents optical transmitter  200  signal processing according to a fourth combination of the first and the second logic gate  250 ,  255 , the latter being implemented as NOR logic gates. The first logic gate  250  therefore outputs the third signal S 25  as a 1 bit when the first and second signals are 0 bits, and otherwise as a 1 bit. The second logic gate  255  analogously realizes NOR logic by the same input signals S 23 , {overscore (S)} 22  described above for the first three combinations. Since the intensity and the phase S 27 (P) of the chirped RZ-AMI optical signal S 27  of the fourth combination output from the MZM 290 vary inversely, it follows that the chirped RZ-AMI optical signal has negative chirp as shown in  FIG. 9 .  
         [0047]     The following table 1 represents formats and chirp signs for the first to the fourth combination.  
                                                         TABLE 1                                   First logic gate   Second logic gate   format   Sign of chirp                                    First   NAND   NAND   AMI   Positive       combination       Second   OR   OR   AMI   Positive       combination       Third   AND   AND   AMI   Negative       combination       Fourth   NOR   NOR   AMI   Negative       combination                  
 
         [0048]      FIG. 10  is a block diagram showing a chirped RZ-AMI optical transmitter according to a second preferred embodiment of the present invention. The optical transmitter  300  has nearly the same construction as that of the optical transmitter  200  shown in  FIG. 3 . However, they differ in that the optical transmitter  300  uses only one delay, that delay being disposed at a different position. Accordingly, the following discussion focuses, for brevity, on these differences.  
         [0049]     The optical transmitter  300  includes a pre-coder  310 , first and a second branching means  312 ,  314 , a delay  320 , first and a second logic gate  330 ,  340 , a first and a second modulator driver  350 ,  360 , a CW laser  370 , and an MZM  380 .  
         [0050]     The pre-coder  310  pre-codes input NRZ binary data, divides the pre-coded signal (or 2 level signal) into two signals, inverts one of the branch signals, and outputs the remaining branch signal (non-inverted signal) and the inverted signal.  
         [0051]     The first branching means  312  divides the branch signal input from the pre-coder  310  into two branch signals.  
         [0052]     The delay  320  delays the branch signal input from the pre-coder  310  by 0.5 bit, and outputs the delayed signal.  
         [0053]     The second branching means  314  divides the delayed signal input from the delay  320  into two branch signals.  
         [0054]     The first logic gate  330  receives one (first signal) of the branch signals having passed through the first branching means  312  and one (second signal) of the branch signals having passed through the second branching means  314 , and logically operates on the received first and second signals to output a third signal.  
         [0055]     The second logic gate  340  receives the remaining branch signal (fourth signal) having passed through the first branching means  314  and the remaining branch signal (fifth signal) having passed through the second branching means  312 , and logically operates on the received fourth and fifth signals to output a sixth signal.  
         [0056]     The first modulator driver  350  amplifies the third signal input from the first logic gate  330 .  
         [0057]     The second modulator driver  360  amplifies the sixth signal input from the second logic gate  340 .  
         [0058]     The CW laser  370  outputs a light having a predetermined wavelength and the MZM  380  outputs a chirped RZ-AMI optical signal obtained by modulating the light input from the CW laser  370  according to the amplified third and sixth signals.  
         [0059]     The optical transmitter  300  may be realized by means of various combinations of the first and the second logic gates  330 ,  360 .  
         [0060]     The following table 2 represents formats and chirp signs for a fifth to an eighth combination. Description for each combination is omitted.  
                                                         TABLE 2                                   First logic gate   Second logic gate   format   Sign of chirp                                    Fifth   NAND   OR   AMI   Positive       combination       Sixth   OR   NAND   AMI   Positive       combination       Seventh   AND   NOR   AMI   Negative       combination       Eighth   NOR   AND   AMI   Negative       combination                  
 
         [0061]     According to the present invention as described above, a chirped RZ-AMI optical transmitter with a low price can be realized by using only one MZM without an expensive DI. Moreover, since the chirped RZ-AMI optical transmitter outputs a chirped RZ-AMI optical signal, the chirped RZ-AMI optical transmitter is tolerant to the non-linearity effect.  
         [0062]     Although preferred embodiments of the present invention have 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.