Abstract:
An optical transmission apparatus using duobinary modulation adapted to an optical communication system using dense wavelength division multiplexing (DWDM) is disclosed. The duo-binary optical transmission apparatus includes an electro-absorption modulated laser (EML) for performing a light intensity modulation operation based on an electrical data signal and producing a modulated optical signal, the EML having a light source for outputting carriers and a modulator for performing the light intensity modulation operation, a precoder for inputting an inverted signal of the electrical data signal and encoding it to an encoded signal, and a modulator for performing a phase modulation of the optical signal modulated by the EML and outputting a duo-binary optical signal, according to the encoded signal.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “OPTICAL TRANSMISSION APPARATUS USING DUOBINARY MODULATION,” filed in the Korean Intellectual Property Office on Jul. 20, 2004 assigned Serial No. 2004-56305, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical transmission apparatus, and more particularly to an optical transmission apparatus using duobinary modulation adapted to an optical communication system using dense wavelength division multiplexing (DWDM).  
         [0004]     2. Description of the Related Art  
         [0005]     Duobinary modulation converts a binary signal into a ternary signal or multi-level signal, in which the phase of the binary signal is inverted at a digit ‘0’. Since a duo-binary signal requires a relatively narrow line width as compared to an OOK (On-Off Keying) signal with respect to optical spectrum, it beneficially reduces channel intervals in optical communication systems adopting dense wavelength division multiplexing (DWDM). Also, since the duobinary signal has a relatively large tolerance with respect to dispersion of an optical fiber, it can be transmitted 2 to 3 times farther than a transmission distance of the OOK signal without dispersion compensation. Also, no carrier tone component is included in an optical spectrum so that the duobinary signal is rarely affected by stimulated Brillouin scattering (SBS). Also, even though the duobinary signal is processed by an RZ (Return to Zero) modulation method, no DC frequency component is included therein. This means that it can be easily transformed by the VSB (Vestigial Sideband) modulation method at a receiving terminal such that the tolerance with respect to dispersion of an optical fiber can be increased.  
         [0006]      FIG. 1  is a block diagram illustrating a conventional optical transmission apparatus  100  using duobinary modulation utilizing an electrical low bandpass filter.  
         [0007]     As shown in  FIG. 1 , the conventional optical transmission apparatus  100  includes a precoder  101 , two driving amplifiers  102  and  103 , two low bandpass filters  104  and  105 , a laser source  106 , and a Mach-Zehnder (M-Z) interferometer type optical intensity modulator  107 .  
         [0008]     In operation, input binary data is encoded in the precoder  101 . Generally, a precoder may include a one bit delay and exclusive OR (XOR) gate as a logic element. The encoded binary data (for example, Q data and /Q data corresponding to inverted data of Q) are input to the two low bandpass filters  104  and  105  via the driving amplifiers  102  and  103 , respectively. Ideally, the low bandpass filters  104  and  105  should be cosine square filters, but may also be a Bessel-Thomson filter. If a bandwidth of the low bandpass filters  102  and  103  is a line width of 3 dB corresponding to ¼ of a transmission speed for a binary data signal (for example, a filter of 2.5 GHz for data of 10 Gb/s), binary signals passing through the low bandpass filters  104  and  105  are transformed into band-limited ternary signals. The band-limited ternary signals are applied to the M-Z interferometer type optical intensity modulator  107  to modulate carriers output from the laser source  106 , therefore optical duobinary signals are generated. Here, bias of the M-Z interferometer type optical intensity modulator  107  is performed at a null point corresponding to a minimum value in a transfer characteristic function.  
         [0009]      FIG. 2  includes eye diagrams according to transmission distances of the optical transmission apparatus  100  of  FIG. 1 , which are measured at the 2 31 -1 PRBS (Pseudo Random Binary Sequence). Here, the first eye diagram of  FIG. 2 , BB (back to back), shows a characteristic before signal transmission. As shown in  FIG. 2 , an optical duobinary signal transmitted through a single mode optical fiber shows an enhanced transmission characteristic to a distance of 100 km, but its transmission distance is restricted to approximately 200 km, since crosstalk gradually increases at a distance over 100 km such that signal quality is decreased. Therefore, substantially, the optical duobinary signal cannot be used in a region between 200 km and 250 km, which is called a ‘metro region.’ Also, since the optical duobinary signal in  FIG. 1  is greatly dependent on characteristics of the electrical low bandpass filter, performance of the electrical low bandpass filter may be largely changed according to a pattern distance of the inputted binary signal. Especially, system penalties occur more frequently in a 2 31 -1 PRBS pattern with various patterns than in a 2 7 -1 PRBS pattern. Also, the conventional apparatus has disadvantages in that time jitters of signals occur more in the optical duobinary signals than in general NRZ signals due to non-ideal characteristics of the electrical low bandpass filter.  
       SUMMARY OF THE INVENTION  
       [0010]     One aspect of the present invention relates to an optical transmission apparatus using duobinary modulation capable of increasing a transmission distance of duobinary optical signals, such that the duobinary optical signals can be used in a region 200 km to 250 km apart therefrom.  
         [0011]     Another aspect of the present invention relates to an optical transmission apparatus using duobinary modulation capable of minimizing pattern dependence of duobinary signals, which is caused when an electrical low bandpass filter is not used.  
         [0012]     One embodiment of the present invention is directed to a duo-binary optical transmission apparatus including an electro-absorption modulated laser (EML) for performing a light intensity modulation operation based on an electrical data signal and producing a modulated optical signal. The EML has a light source for outputting carriers and a modulator for performing the light intensity modulation operation. The apparatus also includes a precoder for inputting an inverted signal of the electrical data signal and encoding it to an encoded signal, and a modulator for performing a phase modulation of the optical signal modulated by the EML and outputting a duo-binary optical signal, according to the encoded signal.  
         [0013]     Another embodiment of the present invention is directed to a duo-binary optical transmission apparatus including an electro-absorption modulated laser (EML) for performing a light intensity modulation operation based on an electrical data signal and produce a modulated optical signal. The EML has a light source for outputting carriers and a modulator for performing the light intensity modulation operation. The apparatus also includes a first driving amplifier for amplifying the electrical data signal and driving the EML base on the amplified electrical data signal, a precoder for inputting an inverted signal of the electrical data signal and encoding it to an encoded signal, a second driving amplifier for amplifying the encoded signal and providing the amplified encoded signal to the modulator, a time delay for adjusting a delay time of the amplified encoded signal of the first driving amplifier of modulator and fitting it to the modulated optical signal of the EML, and a modulator for performing a phase modulation of the optical signal modulated by the EML and outputting a duo-binary optical signal, according to an output signal of the time delay. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above and other aspects, embodiments and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  is a block diagram illustrating a conventional optical transmission apparatus using duobinary modulation utilizing an electrical low bandpass filter;  
         [0016]      FIG. 2  shows eye diagrams according to transmission distances of the conventional optical transmission apparatus of  FIG. 1 ;  
         [0017]      FIG. 3  is a block diagram illustrating an optical transmission apparatus using duobinary modulation according to one embodiment of the present invention;  
         [0018]      FIG. 4  is graphs illustrating chirp characteristics according to a bias voltage of an EML (Electro-absorption Modulated Laser) according to the present invention;  
         [0019]      FIG. 5  is graphs illustrating transmission characteristics according to a bias voltage of an EML according to the present invention;  
         [0020]      FIG. 6   a  is a block diagram illustrating a precoder used in an optical transmission apparatus using duobinary modulation according to one embodiment of the present invention;  
         [0021]      FIGS. 6   b  is a block diagram illustrating a precoder used in an optical transmission apparatus using duobinary modulation according to another embodiment of the present invention;  
         [0022]      FIG. 7   a  is an eye-diagram after an NRZ data signal is modulated based on a light intensity modulation operation of an EML;  
         [0023]      FIG. 7   b  is an eye-diagram after light intensity modulated signals modulated by an EML are modulated by a phase modulation operation in a Mach-Zehnder optical modulator;  
         [0024]      FIG. 8  is an eye-diagram according to a transmission distance of the duobinary optical transmission apparatus of  FIG. 3 ; and  
         [0025]      FIG. 9  is graphs illustrating characteristics of receive sensitivity according to a transmission distance of the duobinary optical transmission apparatus, comparing the present invention with the prior art.  
     
    
     DETAILED DESCRIPTION  
       [0026]     Now, embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention. Also, the terms used in the following description are terms defined taking into consideration the functions obtained in accordance with the embodiments of the present invention.  
         [0027]      FIG. 3  is a block diagram illustrating an optical transmission apparatus  200  using duobinary modulation according to one embodiment of the present invention.  
         [0028]     With reference to  FIG. 3 , the optical transmission apparatus  200  includes an EML (Electro-absorption Modulated Laser)  210 , a precoder  220 , a time delay  230 , and an optical modulator  240 . The apparatus  200  can further include a first driving amplifier  250  for driving the EML  210 , a second driving amplifier  260  for driving the optical modulator  240 , and a polarizer  270 .  
         [0029]     The EML  210  modulates an input electrical data signal, i.e., an NRZ (non return to zero) data signal, to an optical signal in a modulated format. The EML  210  is an element that can be manufactured by combining a laser diode (LD) with an electro-absorption (EA) modulator, which can be singly integrated on a substrate such that it can be mass-produced. Also, it should be understood by one of ordinary skill in the art that the EML  210  is relatively small-sized and cost-effective. If the level of input NRZ data signal is small, it is amplified by the first driving amplifier  250  such that the amplified and input NRZ data signal can drive the EML  210 .  
         [0030]      FIGS. 4 and 5  are graphs illustrating characteristics of the electro-absorption (EA) modulator within the EML (for example, samples A 1  and A 2 ). An extinction ratio and driving voltage of sample A 1  are 4.65 dB, and 0V to −1.15V, respectively. Also, an extinction ratio and driving voltage of sample A 2  are 4.6 dB, and 0V to −1.07V, respectively.  
         [0031]      FIG. 4  illustrates chirp characteristics according to a bias voltage of an EML according to one aspect of the present invention, in which the chirp characteristics are gradually changed from a positive region to a negative region as an inverse voltage supplied to the electro-absorption (EA) modulator is increased.  
         [0032]      FIG. 5  illustrates transmission characteristics according to a bias voltage of an EML according to another aspect of the present invention, in which a loss of the electro-absorption (EA) modulator is increased and thusly an output signal is decreased, as an inverse voltage supplied to the electro-absorption (EA) modulator is increased.  
         [0033]     With reference to  FIG. 3 , the precoder  220  encodes an inverted signal of the inputted electrical data signal, i.e., a /NRZ (Non Return to Zero) data signal. The precoder  220  may be implemented as shown in the embodiments of  FIGS. 6   a  and  6   b.    
         [0034]      FIGS. 6   a  and  6   b  are block diagrams illustrating precoders.  FIG. 6   a  is a block diagram illustrating one embodiment of the present invention including a one-bit time delay  61  and a logic exclusive OR gate (hereinafter referred to as XOR gate)  62 .  FIG. 6   b  is a block diagram illustrating another embodiment including a logic AND gate (hereinafter referred to as AND gate)  63  and a T flip-flop (called T-FF for short)  64 . Even though the precoders shown in  FIGS. 6   a  and  6   b  are implemented to encode the /NRZ data signal, they may be implemented to further include an inverter before each of their input terminals so that they can encode NRZ data signal.  
         [0035]     The second driving amplifier  260  amplifiers output signal of the precoder  220  to drive the optical modulator  240 .  
         [0036]     The time delay  230  adjusts delay time of output signal amplified by the second driving amplifier  260  and fits it to the optical signal modulated by the EML  210 . Even though the time delay  230  is located between the second driving amplifier  260  and the optical modulator  240  in this embodiment of the present invention, the embodiment can be modified, for example, such that it may be located at an input lead of the precoder  220  or an input lead of the second driving amplifier  260 .  
         [0037]     The optical modulator  240  performs phase modulation of the optical signal modulated by the EML  210  based on an output signal of the time delay  230 , and outputs duobinary optical signal. The optical modulator  240  may be implemented with various types of Mach-Zehnder (M-Z) interferometer optical intensity modulators (hereinafter referred to M-Z optical modulator) such as an X-cut LiNbO 3  modulator, a Z-cut LiNbO 3  modulator, a polymer modulator, or an optical fiber type modulator. Generally, an M-Z optical modulator modulates the intensity of input light according to driving signal applied to its electrode, and, at the same time, performs phase modulation of the input light. To obtain the proper characteristics, the M-Z optical modulator should be employed.  
         [0038]     The polarizer  270  adjusts polarization direction of the optical signals from the EML  210 . This is done so that the polarization direction of the optical signals from the EML  210  is consistent with a specific polarization direction of the optical modulator  240 , for example, a polarization direction with highest modulation efficiency.  
         [0039]     Now, the operation of the optical transmission apparatus  200  will be described in detail below.  
         [0040]     With reference to  FIG. 3 , an NRZ (Non Return to Zero) data signal is amplified by a first driving amplifier  250  such that the EML  210  performs a light intensity modulation operation based the amplified signal. A DC bias voltage and a driving voltage are adjusted such that the EML  210  is operated in a region with positive chirp characteristics and a relatively low extinction ratio.  FIG. 7   a  is an eye-diagram after an NRZ data signal is modulated based on a light intensity modulation operation of an EML, which is implemented under conditions that a driving voltage for driving the EML V DRIVING  is 1.1V pp , and a bias voltage V BIAS  is −0.55V. In this case, the extinction ratio is approximately 5 dB.  
         [0041]     A /NRZ data signal is encoded by the precoder  220  and then the encoded /NRZ data signal is amplified by the second driving amplifier  260 . The amplified /NRZ data signal from the second driving amplifier  260  is delayed by the time delay  230  to apply it to the M-Z optical modulator in the optical modulator  240 . The operating conditions of the M-Z optical modulator in the optical modulator  240  are a bias position and a magnitude of applying signal, so that the bias position is located at a null point corresponding to a minimum of modulator transfer characteristics and the magnitude of the applied signal is twice the half-wave voltage, Vπ, of the optical modulator. Also, a delay time of the time delay  230  is adjusted to perform a phase modulation at the center of a digit ‘0’ of light intensity modulated signal. Under such operation conditions, the M-Z optical modulator in the optical modulator  240  operates as a phase modulator, and performs phase modulation of the optical intensity modulation signal in the EML  210 .  
         [0042]      FIG. 7   b  is an eye-diagram after light intensity modulated signals modulated by an EML are modulated by a phase modulation operation in a Mach-Zehnder optical modulator. When a signal with a relatively small extinction ratio is phase-modulated at the center of a digit ‘0’, since an offset interference occurs at the center of a digit ‘0’, an eye-diagram with respect to the signal can be obtained like that of a general duobinary optical signal. Since phase-modulation is performed by the M-Z optical modulator, a phase transition occurs at the center of a digit ‘0’, which causes offset interference. If a general phase modulator is used, since a phase transition does not occur at the center of a digit ‘0’, offset interference is not generated. Especially, since the input data signal does not pass through the electrical low band pass filter, time jitters also can be reduced to those of the NRZ data signal and dependence with respect to a pattern can be reduced by a relatively large ratio. One advantages of this embodiment of the present invention can be seen by comparing the BB (Back-to-Back) eye-diagram of  FIG. 8  with the eye-diagrams of the prior art of  FIG. 2 .  
         [0043]      FIG. 8  is an eye-diagram according to a transmission distance of the duobinary optical transmission apparatus of  FIG. 3 , in which the eye-diagram is obtained at a 2 31 -1 PRBS distance. As shown in the drawing, pulses are spread out to a distance of 50 km due to a positive chirp, but the pulses show effects to be recompressed between 50 km and 200 km due to phase modulation generated at the center of zero level (a digit 0). After that, even though the pulse signal gradually increases crosstalk over a distance of 200 km, the pulses can be transmitted to a distance of 285 km.  
         [0044]      FIG. 9  illustrates characteristics of receive sensitivity according to a transmission distance of the duobinary optical transmission apparatus, compared to the prior art discussed above. Reference numerals  91  and  92  are graphs showing characteristics of the EMLs of samples A 1  and A 2  as in  FIGS. 4 and 5 , respectively, and reference numeral  93  shows a graph using a conventional optical transmission apparatus using duobinary modulation. Regarding back-to-back (BB) characteristics, the optical transmission apparatus using duobinary modulation is superior to the conventional optical transmission apparatus by approximately 1 dB, and also can transmit a data signal to a distance of 50 km to 75 km farther than that of the prior art. The optical fiber for transmission is operated in a standard single mode with a distribution of 17 ps/nm/km at 1550 nm, which are measured in a 2 31 -1 PRBS distance.  
         [0045]     As mentioned above, the optical transmission apparatus  200  modulates intensity of electrical data signals in a region having a positive chirp and a relatively low extinction ratio using an EML, which can be integrated in a single chip, and phase-modulates using an optical modulator such that an input data signal can be transmitted to a region of 250 km. Therefore, when utilizing aspects of the present invention, a duobinary modulation technology can cover a “metro region” around 250 km.  
         [0046]     Also, since embodiments of the present invention do not use an electrical low bandpass filter, pattern dependence and time jitters can be largely reduced such that signal quality is maintained in good conditions.  
         [0047]     Although embodiments of the present invention have been disclosed 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.