Abstract:
An optical transmitter including a multi-lambda source to output injection light consisting of a plurality of injection wavelengths in channels, a circulator having a first port, a second port, and a third port, the circulator receiving the injection light at the first port, and outputting the received injection light to the second port, and further receiving signal light at the second port, and outputting the received signal light to the third port, an arrayed waveguide grating having a multiplexing port connected to the second port of the circulator, and a plurality of demultiplexing ports, spectrum-slicing injection light received from the circulator at the multiplexing port into a plurality of injection channels, and outputting the injection channels to the demultiplexing ports and further receiving and multiplexing a plurality of signal channels at the demultiplexing ports, into a signal light, and outputting the signal light to the multiplexing port, and a plurality of reflective semiconductor optical amplifiers connected to the demultiplexing ports of the arrayed waveguide grating, respectively, each of the reflective semiconductor optical amplifiers receiving an associated one of the injection channels, and amplifying the associated injection channel to generate an associated one of the signal channels.

Description:
CLAIM OF PRIORITY 
     This application claims priority to that patent application entitled “OPTICAL TRANSMITTER AND PASSVIE OPTICAL NETWORK USING THE SAME,” filed in the Korean Intellectual Property Office on Jun. 11, 2004 and assigned Serial No. 2004-42951, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength division multiplexing (WDM) optical network, and, more particularly, to an optical transmitter used in a WDM optical network, and a passive optical network using the optical transmitter. 
     2. Description of the Related Art 
     In WDM passive optical networks (PONs), particular wavelengths are assigned to respective subscribers each having an optical network unit (ONU). Accordingly, such a WDM PON ensures communication security, while being capable of easily accommodating a separate communication service required by a subscriber. Furthermore, PONs allow for the expansion of a subscriber communication capacity and it is also possible to simply increase the number of subscribers by assigning new wavelengths to new subscribers. 
     Generally, WDM PONs use a double star type topology. In the double star type topology, a remote node is installed in an area where a plurality of subscribers are distributed near one another. The remote node is connected to a central office via a single feeder fiber. The ONU of each subscriber is connected to the remote node by an independent distribution fiber. A multiplexed signal of downstream optical signals from the central office is transmitted to the remote node via the feeder fiber, and then demultipexed by an arrayed waveguide grating (AWG), for example. Thereafter, the downstream optical signals are transmitted to the individual ONUs via the respective distribution optical fibers. Upstream signal channels, i.e., wavelengths, outputted from respective ONUs are transmitted to the remote node, and then multiplexed by the AWG of the remote node. The resultant multiplexed signal of the upstream signal channels, i.e., wavelengths, is transmitted to the central office. 
     Recently, spectrum-sliced light sources have been actively researched for a wavelength division multiplexing light source. Such a spectrum-sliced light source slices incoherent light having a sufficiently wide wavelength band flat profile, using an optical filter or AWG, to provide a large number of wavelength-divided channels. In this case, it is thus unnecessary to use individual light sources, each respectively having particular oscillation wavelength, and a corresponding wavelength stabilizing device. For such a spectrum-sliced light source, a light emitting diode (LED), a superluminescent diode (SLD), a Fabry-Perot (FP) laser, a fiber amplifier light source, a picosecond pulse light source, etc. have been proposed. For example, injection light of a broad band generated from an incoherent light source such as an LED or fiber amplifier light source may be spectrum-divided using an optical filter or AWG and the resultant spectrum-divided injection channels, i.e., wavelengths, are provided to a reflective semiconductor optical amplifier, which is not provided with any isolator. Thus, the amplified light in the individual channels may be used for transmission of optical signals. 
       FIG. 1  is a block diagram illustrating an optical transmitter used in a typical PON.  FIG. 2  is a diagram depicting waveforms of injection light A, and the signal light B shown in  FIG. 1 . As shown in  FIG. 1 , the optical transmitter  100  includes a broadband light source (BSL)  110 , a circulator (CIR)  120 , an AWG  130 , N reflective semiconductor optical amplifiers (RSOAs)  140 - 1  to  140 -N. 
     The broadband light source  110  outputs injection light A having a flat profile in a sufficiently broad wavelength band into N light beams of wavelengths λ 1  to λ N . ( FIG. 2 ). 
     The circulator  120  has a first port  120 - 1  connected to the broadband light source  110 , a second port  120 - 2  connected to a multiplexing port MP of the AWG  130 , and a third port  120 - 3  connected to a transmission link. The circulator  120  receives the injection light A at the first port  120 - 1 , and outputs the injection light A to the second port  120 - 2 . The circulator  120  also receives signal light B at the second port  120 - 2 , and outputs the signal light B to the third port  120 - 3 . 
     The AWG  130  has N demulitplexing ports DP 1  to DP N , in addition to the multiplexing port MP. The demulitplexing ports DP 1  to DP N  are connected to the RSOAs  140 - 1  to  140 -N, respectively. For example, the N-th demultiplexing port DP N  is connected to the N-th RSOA  140 -N. The AWG  130  spectrum-slices the injection light A inputted to the multiplexing port MP, and outputs the resultant light beams to the demultiplexing ports DP 1  to DP N , respectively. The AWG  130  further multiplexes the signal channels, i.e., wavelengths, inputted to the respective demulitplexing ports DP 1  to DP N , and outputs a resultant multiplexed signal to the multiplexing port MP. The AWG  130  has wavelength transmission characteristics having periodically repeated free special ranges (FSRs). The AWG  130  has N wavelengths in an arbitrary FSR thereof. That is, the FSR has transmission wavelengths respectively corresponding to the N wavelengths. 
     The first through N-th RSOAs  140 - 1  to  140 -N receive the first through N-th injection signals on the N channels and output first through N-th signal channels. For example, the N-th RSOA  140 -N receives the N-th injection channels, amplifies the injection signal, and outputs the N-th injection signal, which has an increased peak power level. In this case, the N-th signal channel has an N-th wavelength. 
     In the above-mentioned optical transmitter, however, the injection light outputted from the broadband light source exhibits loss caused by mismatching of spectrums of the AWG and spectrums of the insertion light as well as insertion loss while passing through the AWG because it has a wide and flat profile. Such loss may be in the order of 3 dB. Hence, there is a need in the industry for a means to optically multiplex/demultiplex optical signals without incurring such loss. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems incurred in the related art, and it is an object of the invention to provide an optical transmitter capable of achieving maximal energy efficiency, and a PON using the optical transmitter. 
     In accordance with one aspect, the present invention provides an optical transmitter comprising a multi-lambda source to output injection light consisting of a plurality of injection channels, a circulator having a first port, a second port, and a third port, the circulator receiving the injection light at the first port, outputting the received injection light to the second port, and further receiving a signal light at the second port, and outputting the received signal light to the third port, an arrayed waveguide grating (AWG) having a multiplexing port connected to the second port of the circulator, and a plurality of demultiplexing ports, for spectrum-slicing the received injection light provided by the circulator at the multiplexing port into a plurality of injection signals, and providing the injection channels to the demultiplexing ports, and further receiving a plurality of signal channels at the demultiplexing ports and multiplexing the received\signal channels into a signal light, and outputting the signal light to the multiplexing port, and a plurality of reflective semiconductor optical amplifiers connected to the demultiplexing ports of the arrayed waveguide grating, respectively, each of the reflective semiconductor optical amplifiers further receiving an associated one of the injection channels, and amplifying the associated injection channel to generate an associated one of the signal channels. 
     In accordance with another aspect, the present invention provides a passive optical network comprising a central office including a first multi-lambda source to output upstream injection light consisting of a plurality of upstream injection channels, a first arrayed waveguide grating to receive the upstream injection light, to spectrum-slice the received upstream injection light into upstream injection channels, and to receive and multiplex a plurality of received downstream signal channels into downstream signal light, and a first group of reflective semiconductor optical amplifiers, each to receive an associated one of the upstream injection channels, and to amplify the associated upstream injection channel to generate an associated one of the downstream signal channels and a remote node connected to the central office via a feeder fiber, the remote node including a second arrayed waveguide grating to receive and demultiplex the downstream signal light, into the downstream signal channels and an optical network unit connected to the remote node via a plurality of distribution fibers to receive the respective downstream signal channels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a block diagram illustrating an optical transmitter used in a typical PON; 
         FIG. 2  is a diagram depicting waveforms of injection light A and signal light B of the optical transmitter shown in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating an optical transmitter according to an exemplary embodiment of the present invention; 
         FIG. 4  is a diagram depicting waveforms of injection light C and signal light D of the optical transmitter shown in  FIG. 3 ; 
         FIG. 5  is a graph showing a variation in transmission quality depending on a variation in half-width; and 
         FIG. 6  is a block diagram illustrating a PON according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the annexed drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein are omitted when it may make the subject matter of the present invention unclear. 
       FIG. 3  is a block diagram illustrating an optical transmitter according to an exemplary embodiment of the present invention.  FIG. 4  is a diagram depicting waveforms of injection light C and signal light D shown in  FIG. 3 . As shown in  FIG. 3 , the optical transmitter  200  includes a multi-lambda source (MLS)  210 , a circulator (CIR)  220 , an arrayed waveguide grating (AWG)  230 , and 2N reflective semiconductor optical amplifiers (RSOAs)  240 - 1  to  240 - 2 N. 
     The multi-lambda source  210  outputs injection light C consisting of 2N injection channels. In this case, the injection channels are represented by different wavelengths λ 1  to λ 2N . For example, the N-th injection channel has a wavelength λ N . The multi-lambda source  210  may be a multi-lambda laser, an incoherent multi-lambda source using an erbium-doped fiber amplifier (EDFA) and an AWG, or a Fabry-Perot laser diode. 
     The circulator  220  has a first port  220 - 1  connected to the multi-lambda source  210 , a second port  220 - 2  connected to a multiplexing port MP of the AWG  230 , and a third port  220 - 3  connected to a transmission link. The circulator  220  receives the injection light C at the first port  220 - 1 , and outputs the injection light C to the second port  220 - 2 . The circulator  220  also receives signal light D at the second port  220 - 2  to the third port  220 - 3 . 
     The AWG  230  has 2N demulitplexing ports DP 1  to DP 2N , in addition to the multiplexing port MP. The demulitplexing ports DP 1  to DP 2N  are connected to the RSOAs  240 - 1  to  240 - 2 N, respectively. For example, the N-th demultiplexing port DP N  is connected to the N-th RSOA  240 -N. The AWG  230  spectrum-slices the injection light C inputted to the multiplexing port MP, and outputs the resultant light beams to the demultiplexing ports DP 1  to DP 2N , respectively. The AWG  230  further multiplexes the 2N signal channels inputted to the demulitplexing ports DP 1  to DP 2N , and outputs the resultant multiplexed signal to the multiplexing port MP. The AWG  230  has wavelength transmission characteristics having periodically repeated free special ranges (FSRs). The AWG  230  has 2N wavelengths in an arbitrary FSR thereof. That is, the FSR has transmission wavelengths respectively corresponding to the 2N wavelengths. Also, the transmission spectrums of the AWG  230  match the spectrums of the injection light C, respectively. Accordingly, there is no loss caused by mismatching of spectrums. That is, each transmission line width of the AWG  230  is equal to or larger than the line width of each injection channel. 
     The first through 2N-th RSOAs  240 - 1  to  240 -2N receive the first through 2N-th injection channels, respectively, and output first through 2N-th signal channels, respectively. For example, the N-th RSOA  240 -N receives the N-th injection channel, amplifies the N-th injection channel, and outputs the N-th signal channel, which has an increased peak power level. In this case, the N-th signal channel has an N-th wavelength. In order to provide a lower half-width limit to the N-th signal channel, it is desirable for the N-th RSOA  240 -N to operate in a saturated state. 
     As compared to the typical optical transmitter  100  of  FIG. 1 , the optical transmitter  200  having the above-described configuration can output 2N channels under the condition in which the same injection light power is used. The typical optical transmitter  100  of  FIG. 1  can output only N channels. That is, each channel outputted from the optical transmitter  200  of the present invention has a reduced half-width, but exhibits an increased peak power level at the same power, as compared to that of the typical optical transmitter  100 . Such effects are based on the fact that, although it has conventionally been regarded that the wider the half-width of each signal channel, the better the transmission quality, a similar transmission quality can be obtained in accordance with use of RSOAs, in spite of a reduction in half-width, as long as the same peak power level is given. 
       FIG. 5  is a graph explaining a variation in transmission quality depending on a variation in half-width. In  FIG. 5 , first curve  510  depicts a variation in the bit error of the first channel depending on a variation in reception power when the first channel has a half-width of 0.4 nm and a power of −18 dBm, and second curve  520  depicts a variation in the bit error of the first channel depending on a variation in reception power when the first channel has a half-width of 0.64 nm and a power of −16 dBm. Referring to  FIG. 5 , it can be seen that, although a variation in half-width occurs, similar bit error rates can be obtained at the same peak power level. These results may be analyzed as being based on the fact that, for example, in a WDM PON using RSOAs, the side mode suppression ratio (SMSR) to determine reception power depends on the peak power level of injection light inputted to the RSOAs. This analysis is based on an equation to derive an optical-signal-to-noise ratio (OSNR), that may be expressed as:
   OSNR= 58+Input Power−Noise Figure. 
       FIG. 6  is a block diagram illustrating a PON according to an exemplary embodiment of the present invention. As shown in  FIG. 6 , the PON  300  includes a central office  310 , a remote node (RN)  410  connected to the central office  310  via a feeder fiber (FF)  400 , and an ONU  440  connected to the remote node  410  via N distribution fibers (DFs)  430 - 1  to  430 -N. 
     The central office  310  includes N bi-directional transceivers (Bidi-TRxs)  320 - 1  to  320 -N, an AWG  360 , first and second multi-lambda sources (MLSs)  370  and  380 , and a coupler  390 . 
     The N bi-directional transceivers  320 - 1  to  320 -N are connected to the N demultiplexing ports DP 1  to DP N  of the AWG  360 , respectively. Each of the bi-directional transceivers  320 - 1  to  320 -N includes a receiver (Rx), a transmitter (Tx), and a filter (FT). For example, the N-th bi-directional transceiver  320 -N is connected to the N-th demultiplexing port DP N  of the AWG  360 , and includes the N-th receiver (Rx N )  330 -N, the N-th transmitter (TX N )  340 -N, and the N-th filter (FT N )  350 -N. 
     Each of the transmitters  340 - 1  to  340 -N amplifies the associated upstream injection channel, and outputs the associated downstream signal channel with an increased peak power level. For example, the N-th transmitters  340 -N amplifies the N-th upstream injection channel having the wavelength λ N , and outputs the N-th downstream signal channel having the wavelength λ N  with an increased peak power level. Each of the transmitters  340 - 1  to  340 -N includes an RSOA. It is desirable for the RSOA of each transmitter to operate in a saturated state, in order to provide a lower half-width limit to the associated signal channel. 
     The N receivers  330 - 1  to  330 -N receive N upstream signal channels having wavelengths λ N+1  to λ 2N , respectively. For example, the N-th receiver  330 -N receives the N-th upstream signal channel having the wavelength λ 2N . 
     Each of the filters  350 - 1  to  350 -N has a first port  350 - 1 . 1  connected to the associated receiver, a second port  350 - 1 . 2  connected to the associated transmitter, and a third port  350 - 1 . 3  connected to the associated demultiplexing port of the AWG  360 . For example, the N-th filter  350 -N has a first port  350 -N. 1  connected to the N-th receiver  330 -N, a second port  350 -N. 2  connected to the N-th transmitter  340 -N, and a third port  350 -N. 3  connected to the demultiplexing port DP N  of the AWG  360 . Each of the filters  350 - 1  to  350 -N receives the associated upstream injection channel at the third port, outputs the received upstream injection channel to the second port, receives the associated upstream signal channel at the third port, outputs the received upstream signal channel to the first port, receives the associated downstream signal channel at the second port, and outputs the received downstream signal channel to the third port. For example, the N-th filter  350 -N receives the N-th upstream injection channel at the third port  350 -N. 3 , outputs the received N-th upstream injection channel to the second port  350 -N. 2 , receives the N-th upstream signal channel at the third port  350 -N. 3 , outputs the received N-th upstream signal channel to the first port  350 -N. 1 , receives the N-th downstream signal channel at the second port  350 -N. 2 , and outputs the received N-th downstream signal channel to the third port  350 -N. 3 . 
     The N downstream signal channels outputted from the N bi-directional transceivers  320 - 1  to  320 -N have different wavelengths λ 1  to λ N , respectively. The upstream injection channels inputted to the N bi-directional transceivers  320 - 1  to  320 -N have different wavelengths λ 1  to λ N , respectively. The upstream signal channels inputted to the N bi-directional transceivers  320 - 1  to  320 -N have different wavelengths λ N+1  to λ 2N , respectively. The range of the 2N wavelengths may be 25 to 200 GHz. 
     The AWG  360  includes N demultiplexing ports DP 1  to DP N , and a multiplexing port MP. The multiplexing port MP of the AWG  360  is connected to a first port  390 . 1  of the coupler  390 . The AWG  360  spectrum-slices the upstream injection light inputted to the multiplexing port MP, and outputs the resultant upstream injection channels having wavelengths λ 1  to λ N  to the demultiplexing ports DP 1  to DP N , respectively. For example, the AWG  360  outputs the spectrum-sliced N-th upstream injection channel having the wavelength λ N  to the N-th demultiplexing port DP N . The AWG  360  also demultiplexes upstream signal light inputted to the multiplexing port MP, and outputs the resultant upstream signal channels having wavelengths λ N+1  to λ 2N  to the demultiplexing ports DP 1  to DP N , respectively. For example, the AWG  360  outputs the demultiplexed N-th upstream signal channel having the wavelength λ 2N  to the N-th demultiplexing port DP N . Also, the AWG  360  multiplexes N downstream signal channels having wavelengths λ 1  to λ N  respectively inputted to the demultiplexing ports DP 1  to DP N , and outputs the resultant downstream signal light to the multiplexing port MP. The transmission spectrums of the AWG  360  match the spectrums of the upstream injection light, so that there is no loss caused by mismatching of the spectrums. That is, each transmission line width of the AWG  360  is equal to or larger than the line width of each upstream injection channel. 
     The first multi-lambda source  370  outputs upstream injection light consisting of N upstream injection channels having different wavelengths λ 1  to λ N , respectively, and the second multi-lambda source  380  outputs downstream injection light consisting of N downstream injection channels having different wavelengths λ N+1  to λ 2N , respectively. 
     The coupler  390  has four ports  390 . 1  to  390 . 4 . The first port  390 . 1  of the coupler  390  is connected to the multiplexing port MP of the AWG  360 , the second port  390 . 2  is connected to the second multi-lambda source  380 , the third port  390 . 3  is connected to the first multi-lambda source  370 , and the fourth port  390 . 4  is connected to the feeder fiber  400 . The coupler  390  receives the upstream injection light at the third port  390 . 3 , outputs the received upstream injection light to the first port  390 . 1 , receives the downstream injection light at the second port  390 . 2 , outputs the received downstream injection light to the fourth port  390 . 4 , receives the downstream signal light at the first port  390 . 1 , outputs the received downstream signal light to the fourth port  390 . 4 , receives the upstream signal light at the fourth port  390 . 4 , and outputs the received upstream signal light to the first port  390 . 1 . 
     The remote node  410  includes an AWG  420 . The AWG  420  has a multiplexing port MP, and N demultiplexing ports DP 1  to DP N . The multiplexing port MP of the AWG  420  is connected to the feeder fiber  400 , and the demultiplexing ports DP 1  to DP N  of the AWG  420  are connected to the distribution fibers  430 - 1  to  430 -N, respectively. For example, the N-th demultiplexing port DP N  of the AWG  420  is connected to the N-th distribution fiber  430 -N. The AWG  420  spectrum-slices the downstream injection light inputted to the multiplexing port MP, and outputs the resultant downstream injection channels having wavelengths λ N+1  to λ 2N  to the demultiplexing ports DP 1  to DP N , respectively. For example, the AWG  420  outputs the spectrum-sliced N-th downstream injection channel having the wavelength λ 2N  to the N-th demultiplexing port DP N . The AWG  420  also demultiplexes downstream signal light inputted to the multiplexing port MP, and outputs the resultant downstream signal channels having wavelengths λ 1  to λ N  to the demultiplexing ports DP 1  to DP N , respectively. For example, the AWG  420  outputs the demultiplexed N-th upstream signal channel having the wavelength λ N  to the N-th demultiplexing port DP N . Also, the AWG  420  multiplexes N upstream signal channels having wavelengths λ N+1  to λ 2N  respectively inputted to the demultiplexing ports DP 1  to DP N , and outputs the resultant upstream signal light to the multiplexing port MP. The transmission spectrums of the AWG  420  match the spectrums of the downstream injection light, so that there is no loss caused by mismatching of the spectrums. That is, each transmission line width of the AWG  420  is equal to or larger than the line width of each downstream injection channel. 
     The ONU  440  includes N bi-directional transceivers  450 - 1  to  450 -N. The bi-directional transceivers  450 - 1  to  450 -N are connected to the N distribution fibers  430 - 1  to  430 -N, respectively. Each of the bi-directional transceivers  450 - 1  to  450 -N includes a receiver (RX), a transmitter (TX), and a filter (FT). For example, the N-th bi-directional transceiver  450 -N is connected to the N-th distribution fiber  430 -N, and includes the N-th receiver (RX N )  470 -N, the N-th transmitter (TX N )  480 -N, and the N-th filter (FT N )  460 -N. 
     Each of the transmitters  480 - 1  to  480 -N amplifies the associated upstream injection channel, and outputs the associated downstream signal channel with an increased peak power level. For example, the N-th transmitters  480 -N amplifies the N-th upstream injection channel having the wavelength λ 2N , and outputs the N-th downstream signal channel having the wavelength λ 2N  with an increased peak power level. Each of the transmitters  480 - 1  to  480 -N includes an RSOA. It is desirable for the RSOA of each transmitter to operate in a saturated state, in order to provide a lower half-width limit to the associated signal channel. 
     The N receivers  470 - 1  to  470 -N receive N upstream signal channels having wavelengths λ N+1  to λ 2N , respectively. For example, the N-th receiver  470 -N receives the N-th upstream signal channel having the wavelength λ 2N . 
     Each of the filters  460 - 1  to  460 -N has a first port connected to the associated distribution fiber, a second port connected to the associated receiver, and a third port connected to the associated transmitter. For example, the N-th filter  460 -N has a first port  460 -N 1  connected to the N-th distribution fiber  430 -N, a second port  460 -N 2  connected to the N-th receiver  470 -N, and a third port  460 -N 3  connected to the N-th transmitter  480 -N. Each of the filters  460 - 1  to  460 -N receives the associated downstream injection channel at the first port, outputs the received downstream injection channel to the third port, receives the associated upstream signal channel at the third port, outputs the received upstream signal channel to the first port, receives the associated downstream signal channel at the first port, and outputs the received downstream signal channel to the second port. For example, the N-th filter  460 -N receives the N-th downstream injection channel at the first port  460 -N 1 , outputs the received N-th downstream injection channel to the third port  460 -N 3 , receives the N-th upstream signal channel at the third port  460 -N 3 , outputs the received N-th upstream signal channel to the first port  460 -N 1 , receives the N-th downstream signal channel at the first port  460 -N 1 , and outputs the received N-th downstream signal channel to the second port  460 -N 2 . 
     As apparent from the above description, the bi-directional transceiver according to the present invention and the PON using the bi-directional transceiver can achieve maximal energy efficiency because they use multi-lambda sources. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the appended claims.