Abstract:
A WDM optical source, in adopting a spectrum-sliced method, advantageously needs neither an optical source with specific generation wavelength nor a wavelength-stabilizing circuit for stabilizing wavelength. Affording high power and a very narrow line width, the WDM optical source can provide a broadcasting service without signal distortion by a chromatic dispersion effect and also avoids the expense of an amplifier and/or external modulator, thereby relieving subscribers of the economical burden. The practical use of WDM-PON and of WDM-PON using an inventive WDM optical source can therefore be brought to fruition and broadcasting service can be provided economically.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority from an application entitled “Wavelength-Division-Multiplexed Optical Source and Passive Optical Network System Employing the Same,” filed in the Korean Intellectual Property Office on Jan. 15, 2003 and assigned Serial No. 2003-2622, 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 a wavelength-division-multiplexed optical source and a passive optical network system employing the same, and more particularly, to a wavelength-division-multiplexed optical source for providing data services and broadcasting services, and to a passive optical network system employing the same.  
           [0004]    2. Description of the Related Art  
           [0005]    A Wavelength-Division-Multiplexed Passive Optical Network (WDM-PON) provides broadband communication services at very high speed by using intrinsic wavelengths assigned to each subscriber. Therefore, the WDM-PON can keep a communication secret with certainty and can easily accommodate an increase in communication capacity as well as a special communication service requested by a subscriber. In particular, the WDM-PON can be reconfigured for new subscriber terminals just by adding intrinsic wavelengths to be assigned to each additional terminal. Advantageously, the WDM-PON can thus easily be made to accommodate extra subscriber terminals.  
           [0006]    However, a central office (CO) and subscriber terminals of which the WDM-PON is comprised must have at least one optical source with an assigned oscillation wavelength and at least one wavelength-stabilizing circuit for stabilizing the wavelength of the optical source, which imposes a high cost burden on the subscribers to the WDM-PON. The WDM-PON has not yet been put to practical use for this reason in spite of its many advantages. There accordingly exists a need for an economical optical source in order to put the WDM-PON to practical use.  
           [0007]    Implementation of a broadcasting service through a WDM-PON, instead of through another hybrid coaxial network (HFC network) as is currently conventional, would also advantageously reduce cost. Therefore, a number of studies for utilizing a WDM-PON for a broadcasting service are vigorously being pursued, and a variety of methods for providing a broadcasting service have been proposed. Examples of methods for providing a broadcasting service include through a distributed feedback laser (DFB laser), through a distributed feedback laser array (DFB laser array), and through a spectrum-sliced light source. The characteristics of each method are as follows.  
           [0008]    The broadcasting service provision method using a DFB laser directly modulates a distributed feedback laser in accordance with broadcasting service signals, amplifies the modulated signals through an optical amplifier, and outputs the amplified signals through a power splitting optical link to provide the broadcasting service to each subscriber terminal. The power splitting optical link is provided with a special link so as to be differentiated from the optical link of WDM for data service.  
           [0009]    This method complicates the manufacturing procedure and requires the use of high-priced elements which are necessary to provide accurate wavelength selectivity and wavelength stability of a WDM optical source. The method further requires a special power splitting optical link so as to be differentiated from the optical link of WDM for data service. Subscribers are therefore burdened by additional construction cost and continuous investment from the viewpoint of maintenance and operation.  
           [0010]    The broadcasting service provision method using a DFB laser array, carrying some of the same disadvantages as the broadcasting service provision using a DFB laser, electrically multiplexes data service signals and broadcasting service signals of differing frequency bands, modulates directly each distributed feedback laser in accordance with the multiplexed signals, and then outputs the signals through optical link of WDM to provide the broadcasting service to each subscriber terminal. Similar to the case of the broadcasting service provision method using a DFB laser, this method complicates manufacturing and requires the use of high-priced elements which are needed to provide accurate wavelength selectivity and wavelength stability of the WDM optical source. Also characteristic of this method is degradation of data service signals and broadcasting service signals due to their simultaneous provision through one channel.  
           [0011]    The broadcasting service provision method using a spectrum-sliced light source modulates directly or indirectly an optical source outputting optical signals of wide bandwidth in accordance with broadcasting service signals, spectrally slices the modulated signals, and outputs plenty of wavelength-sliced channels generated as the result through optical link of the WDM to provide the broadcasting service to each subscriber terminal. This method therefore doesn&#39;t need an optical source with specific generation wavelength and a wavelength-stabilizing circuit for stabilizing the wavelength. Examples of an optical source for the spectrum-sliced method are a light emitting diode (LED), a super luminescent diode (SLD) and a fiber amplifier light source.  
           [0012]    Disadvantageously, transmission performance may be degraded for the broadcasting service provision method using a spectrum-sliced light source, because this method causes some distortion of the broadcasting service signals by chromatic dispersion effect. The receive sensitivity may also be degraded, because signal-to-signal beat noise generated in an optical receiver exists in the bandwidth of the broadcasting service signals. Although the LED and the SLD have extremely wide optical bandwidth and may cut the construction cost, narrow modulation bandwidth causes the transmissible capacity of the broadcasting service signals to be small, and the low output of optical sources require the addition of an optical amplifier for compensating the loss generated by the spectrum slicing. Another optical source and yet another optical amplifier must be additionally included so as to provide more capacity for broadcasting service signals. Also, although the fiber amplifier light source may provide high power for spectrum-sliced channels, use of the light source entails the high price of an external modulator.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention has been made to solve the above-mentioned problems and provides an economical wavelength-division-multiplexed (WDM) optical source for putting a wavelength-division-multiplexed passive optical network (WDM-PON) to practical use.  
           [0014]    In another aspect, the present invention provides an economical WDM optical source for economically providing broadcasting services in the WDM-PON.  
           [0015]    In a further aspect, the present invention provides a central office system of the WDM-PON for providing economical broadcasting services.  
           [0016]    As an alternative aspect, the present invention provides a local office system of the WDM-PON for providing economical broadcasting services.  
           [0017]    In yet another aspect, the present invention provides subscriber terminals of the WDM-PON to afford economical broadcasting services.  
           [0018]    An inventive wavelength-division-multiplexed optical source comprises: a pump laser; a first optical amplifier, operated by rear-pumping of the pump laser, for generating amplified spontaneous emission noise (ASE noise); a first multiplexer/demultiplexer having a first input/output terminal on one side and a plurality of second input/output terminals on the other side, for demultiplexing signals inputted into the first input/output terminal and outputting the demultiplexed signals to the second input/output terminals, and for multiplexing signals inputted into the second input/output terminals and outputting the multiplexed signals to the first input/output terminal; a plurality of mirrors, connected to the second input/output terminals in one-to-one correspondence, for inputting again the demultiplexed signals outputted through the second input/output terminals; a circulator for transmitting signals inputted from the first optical amplifier to the first input/output terminal, and for outputting multiplexed signals inputted from the first input/output terminal; a second optical amplifier, operated by rear-pumping of the pump laser, for amplifying multiplexed signals outputted from the circulator; an optical splitter for splitting the multiplexed signals amplified by the second optical amplifier and for outputting split signals to the first optical amplifier and for external transmission, respectively; and an external modulator for modulating the signals outputted for external transmission according to preset broadcasting signals and for outputting the modulated signals to a transmission link.  
           [0019]    In accordance with another aspect of the present invention, there is provided a passive optical network system including a central office, a local office, and a plurality of subscriber terminals, the central office being connected with the local office through an optical fiber and providing optical communication service to the subscriber terminals through the local office, the central office comprising: a first wavelength-division-multiplexed (WDM) optical source for providing a downstream broadcasting service to the subscriber terminals; a second WDM optical source for providing a downstream data service to the subscriber terminals; a plurality of optical receivers for receiving upstream data service signals transmitted from each subscriber terminal and converting the received signals to electric signals; a plurality of first wavelength division multiplexers for multiplexing/demultiplexing upstream/downstream data service signals to provide upstream/downstream data services to the subscriber terminals; a second multiplexer/demultiplexer for multiplexing a plurality of downstream data service signals outputted from the first wavelength division multiplexers, and for demultiplexing upstream data service signals to be transmitted to the first wavelength division multiplexers; and a second wavelength division multiplexer for multiplexing the multiplexed signals inputted from said second multiplexer/demultiplexer and the multiplexed signals inputted from the first WDM optical source, for demultiplexing upstream data service signals inputted from the local office and for outputting the demultiplexed signals to said second multiplexer/demultiplexer.  
           [0020]    In accordance with another aspect of the present invention, there is provided a passive optical network system including a central office, a local office, and a plurality of subscriber terminals, the local office being connected to the central office and the subscriber terminals through optical fibers and providing optical communication service to the subscriber terminals, the local office comprising: a multiplexer/demultiplexer for demultiplexing optical signals for downstream data service and optical signals for downstream broadcasting service multiplexed and transmitted from the central office  100 , and for multiplexing upstream optical signals transmitted from the subscriber terminals.  
           [0021]    In accordance with another aspect of the present invention, there is provided a passive optical network system including a central office, a local office, and a plurality of subscriber terminals connected to the central office through the local office by optical fibers and being provided optical communication service provided from the central office, a subscriber terminal of said plurality comprising: a wavelength division multiplexer for demultiplexing optical signals transmitted downstream from the local office and dividing optical signals for downstream data service and optical signals for downstream broadcasting service and outputting the divided optical signals, and for multiplexing optical signals for upstream transmission from said subscriber terminal to the local office; a downstream data receiver for receiving optical signals for downstream data service demultiplexed by the wavelength division multiplexer and converting the received optical signals to electric signals; a downstream broadcasting receiver for receiving optical signals for downstream broadcasting service demultiplexed by the wavelength division multiplexer and converting the received optical signals to electric signals; and an upstream optical source for generating optical signals for upstream transmission to the local office through the wavelength division multiplexer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    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:  
         [0023]    [0023]FIG. 1 is a schematic view of a wavelength-division-multiplexed (WDM) optical source in accordance with an embodiment of the present invention;  
         [0024]    [0024]FIG. 2 is a schematic view of a WDM optical source in accordance with another embodiment of the present invention;  
         [0025]    [0025]FIG. 3 is a waveform view illustrating a spectrum form of a spectrum-sliced channel;  
         [0026]    [0026]FIG. 4 is a schematic view of a passive optical network system in accordance with an embodiment of the present invention;  
         [0027]    [0027]FIG. 5 illustrates a spectrum of signals multiplexed by a WDM in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 6A illustrates a spectrum of downstream signals demultiplexed by a waveguide grating router in a local office of a passive optical network system in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 6B illustrates a spectrum of upstream signals multiplexed by a waveguide grating router in a local office of a passive optical network system in accordance with an embodiment of the present invention;  
         [0030]    [0030]FIG. 7A describes band-pass characteristic of a first wavelength division multiplexer (MD_MUX# 1 ) in a central office of a passive optical network system in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 7B describes band-pass characteristic of a second wavelength division multiplexer (MD_MUX# 2 ) in a central office of a passive optical network system in accordance with an embodiment of the present invention; and  
         [0032]    [0032]FIG. 7C describes band-pass characteristic of a third wavelength division multiplexer (MD_MUX# 3 ) in a subscriber terminal of a passive optical network system in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    Hereinafter, a wavelength-division-multiplexed optical source and a passive optical network system employing the same according to preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, detailed description of known functions and configurations incorporated herein will be omitted when its inclusion might obscure the subject matter of the present invention unnecessarily.  
         [0034]    [0034]FIG. 1 is a schematic view of a wavelength-division-multiplexed (WDM) optical source in accordance with an embodiment of the present invention. Referring to FIG. 1, the WDM optical source according to an embodiment of the present invention comprises first and second optical amplifiers  30 ,  70 , a circulator  40 , a multiplexer/demultiplexer  50 , a plurality of mirrors  55 , a band-pass filter  60 , first and a second optical splitters  20 ,  80 , and an external modulator  90 .  
         [0035]    The first optical amplifier  30 , preferably configured as an erbium-doped fiber amplifier (EDFA) or a semiconductor optical amplifier, is operated with rear pumping by a pump laser diode  10  and generates amplified spontaneous emission (ASE) noise. The first optical amplifier  30  amplifies multiplexed signals inputted from the second optical splitter  80  and outputs the amplified signals to the circulator  40 .  
         [0036]    The circulator  40  transmits signals inputted from the first optical amplifier  30  to an input/output terminal  1 -N located on a second side of the multiplexer/demultiplexer  50 , and outputs multiplexed signals inputted from the input/output terminals  1 -N of the multiplexer/demultiplexer  50  to the band-pass filter  60 .  
         [0037]    The multiplexer/demultiplexer  50  has one input/output terminal at a first side and a plurality of input/output terminals  1 -N at the second side. The multiplexer/demultiplexer  50  accordingly demultiplexes signals inputted from the input/output terminal of the first side to output the demultiplexed signals to the input/output terminals  1 -N of the second side, and then multiplexes signals inputted from the input/output terminals  1 -N of the second side to output the multiplexed signals to the input/output terminal of the first side. It is preferred that the multiplexer/demultiplexer  50  be configured with a 1×N waveguide grating router (WGR).  
         [0038]    A plurality of mirrors  55  are connected in one-to-one correspondence to the plurality of input/output terminals  1 -N located at the second side of the multiplexer/demultiplexer  50  and are disposed such that the mirrors  55  input again, i.e., reflect back, each demultiplexed signal outputted from the input/output terminals  1 -N of the second side as input into the input/output terminals  1 -N of the second side.  
         [0039]    The band-pass filter  60  outputs the multiplexed signals inputted from the circulator  40  to the second optical amplifier  70 , while limiting the multiplexed signal to a preset wavelength band-pass for the WDM optical source.  
         [0040]    The second optical amplifier  70  is operated with rear pumping by a pump laser diode  10 , and amplifies multiplexed signals which are outputted from the circulator  40  and then transmitted through the band-pass filter  60 . It is preferred that the second optical amplifier  70  be configured as an erbium-doped fiber amplifier (EDFA) or a semiconductor optical amplifier.  
         [0041]    It is also preferred that each of the first and the second optical splitters  20 ,  80  be configured with 1×N splitter. The first optical splitter  20  splits signals of the pump laser diode  10  to feed the first and the second optical amplifiers  30 ,  70 . The second optical splitter  80  splits the multiplexed signals amplified by the second optical amplifier  70  for subsequent output to the first optical amplifier  30  and to outside the closed configuration.  
         [0042]    The external modulator  90  modulates the multiplexed signals that are split and outputted outside from the second optical splitter  80 , in accordance with preset broadcasting service signals, and then outputs the modulated signals to a transmission link. It is preferred that the external modulator  90  be configured with a LiNbO 3  modulator, an electro-absorption modulator or a semiconductor optical amplifier.  
         [0043]    [0043]FIG. 2 portrays another embodiment that differs from the embodiment of FIG. 1 in that the external modulator is configured with a semiconductor optical amplifier  95 . The semiconductor optical amplifier  95  can perform its modulation function at high speed due to its wide modulation bandwidth as well as perform optical amplification. The WDM optical source, modulating the WDM optical signals in accordance with broadcasting service signals and simultaneously amplifying the power, can therefore transmit more broadcasting service signals over longer distances.  
         [0044]    With reference to FIGS. 1 and 2, ASE noise generated from the first optical amplifier  30  with a wide spectrum band enters the multiplexer/demultiplexer  50  configured with an 1×N waveguide grating router (WGR) by means of the circulator  40  and is spectrum-split by the multiplexer/demultiplexer into N channels. The spectrally split channels are reflected back by the N mirrors  55  connected to the second end of the multiplexer/demultiplexer  50  and are then multiplexed in the multiplexer/demultiplexer  50 . The multiplexed signals are outputted to the circulator  40  and then transmitted by the circulator to the band-pass filter  60  which has the same band-pass as the free spectrum range (FSR) of the WGR for spectrum analysis. The filtered signals are amplified by the second optical amplifier  70  and are split by the second optical splitter  80  into signals destined for the first optical amplifier  30  and the external modulator  90 , respectively. The filtered signals inputted to external modulator  90  are transmitted to a transmission link after being modulated in accordance with broadcasting service signals.  
         [0045]    Filtered signals inputted to the first optical amplifier  30  by means of the second optical splitter  80  are amplified in the first optical amplifier  30 , are inputted to the multiplexer/demultiplexer  50  by means of the circulator  40  to be demultiplexed, and are outputted after reflection as multiplexed signals. The multiplexed signals are amplified by the second optical amplifier  70  after being band pass filtered, and are inputted to the second optical splitter  80 . The second optical splitter  80  splits the filtered signals for output to the first optical amplifier  30  and the external modulator  90 /semiconductor optical amplifier  95 , respectively, and the signals inputted to the external modulator  90 /semiconductor optical amplifier  95  are modulated in accordance with broadcasting service signals.  
         [0046]    The WDM optical source, as shown in FIGS. 1 and 2, repeats the serial operation endlessly, thus generating the multiplexed signals with very narrow line width and high power and inputting the multiplexed signals to the external modulator  90 /semiconductor optical amplifier  95 . By avoiding a chromatic dispersion effect as well as signal-to-signal beat noise, the optical source can transmit more broadcasting service signals over longer distances.  
         [0047]    The increase in signal power is achieved, moreover, efficiently due to filtering by the band-pass filter  60 .  
         [0048]    If the bandwidth of the ASE noise signals outputted from the first optical amplifier  30  is wider than the free spectrum range (FSR) of the WGR configuring the multiplexer/demultiplexer  50 , the signals inputted to the multiplexer/demultiplexer  50  are spectrum-split into a variety of wavelengths spread as the period of the FSR of the WGR, as shown in FIG. 3. Transmission performance may be degraded owing to the spectrum spread in the wide wavelength band and a consequent increase in chromatic dispersion effect and signal-to-signal beat noise, if such signals are inputted to the external modulator  90  and are transmitted after being modulated in accordance with broadcasting service signals.  
         [0049]    It is therefore preferred that the band-pass filter  60  confine the spectrum band of the signals having been spectrum-split in the multiplexer/demultiplexer  50  to a band not exceeding a free spectrum range (FSR) of the WGR, so that the spectrum exists in only one wavelength. This allows the transmission of more broadcasting service signals and to a farther distance.  
         [0050]    [0050]FIG. 4 is a schematic view of a passive optical network system in accordance with an embodiment of the present invention. The passive optical network system comprises a central office  100 , a local office  200  and a plurality of subscriber terminals  300 , each apparatus being connected with one another through an optical fiber. The central office  100  provides optical communication service to the subscriber terminals  300  through the local office  200 . The local office  200  is connected to the central office  100  and the subscriber terminals  300  through an optical fiber so as to provide the subscriber terminals  300  optical communication service from the central office  100 . The multiplexer/demultiplexer  150  in the central office  100  and the multiplexer/demultiplexer  210  in the local office  200  are identical with the multiplexer/demultiplexer  50  (referring to the FIG. 1).  
         [0051]    The central office  100  includes two kinds of optical sources for simultaneously providing data service and broadcasting service downstream to the subscriber terminals  300 . For example, the central office  100  may include a multi-channel downstream broadcasting optical source  130  and a plurality of downstream data optical sources  110 . The central office  100  may also include a plurality of upstream optical receivers  120  for receiving the upstream data service signals transmitted from each subscriber terminal  300  to convert the received signals to electric signals. The configuration of the downstream broadcasting optical source  130  is that depicted and described in conjunction with FIG. 1 and therefore is not repeated here. The downstream broadcasting optical source  130  and the downstream data optical sources  110  preferably include band-pass filters having different band-pass from each other in order to generate optical signals that differ as to wavelength band. For example, if the downstream broadcasting optical source  130  comprises a first band-pass filter having a preset band-pass, it is preferred that each downstream data optical source  110  comprise a second band-pass filter having a band-pass different from the band-pass of the first band-pass filter. It is also preferred that the both band-pass filters be configured to have the same band-pass as a free spectrum range (FSR) of a multiplexer/demultiplexer  50  (referring to FIG. 1) included in the downstream broadcasting optical source  130 , and to have its center wavelength separated by more than a FSR of a multiplexer/demultiplexer  50  (referring to FIG. 1) from the center wavelength of the second band-pass filter. Avoiding an overlap in the FSRs allows optical receivers in subscriber terminals to distinguish data service channels from broadcasting service channels.  
         [0052]    The central office  100  includes a plurality of a first wavelength division multiplexers (WD_MUX# 1 )  140 , multiplexer/demultiplexer  150 , and a second wavelength division multiplexer (WD_MUX# 2 )  160 .  
         [0053]    The first wavelength division multiplexer (WD_MUX# 1 )  140  communicates upstream data service signals on its multiplexing side and downstream data service signals on its demultiplexing side. Accordingly, (WD_MUX# 1 )  140  possibly can be configured to include a third band-pass filter having the same band-pass as a preset wavelength band of the downstream data optical source  110  and a fourth band-pass filter having the same band-pass as a preset wavelength band of an upstream optical source  310  in the subscriber terminal  300 . FIG. 7A describes band-pass characteristic of the first wavelength division multiplexer (MD_MUX# 1 )  140 .  
         [0054]    The multiplexer/demultiplexer  150  multiplexes a plurality of downstream data service signals outputted from the first wavelength division multiplexer (WD_MUX# 1 )  140  and demultiplexes upstream data service signals transmitted through the second wavelength division multiplexer (WD_MUX# 2 )  160 . Multiplexer/demultiplexer  150  is preferably composed of 1×N waveguide grating router (WGR).  
         [0055]    The second wavelength division multiplexer (WD_MUX# 2 )  160  multiplexes the multiplexed signals inputted from the multiplexer/demultiplexer  150  and the multiplexed signals inputted from the downstream broadcasting optical source  130 , and demultiplexes upstream data service signals inputted from the local office  200  to output the demultiplexed signals to the multiplexer/demultiplexer  150 . That is, the second wavelength division multiplexer (WD_MUX# 2 )  160  has an operation characteristic which passes the wavelength-division-multiplexed optical signals for upstream/downstream data service and the signals of the downstream broadcasting optical source. It is therefore possible for the second wavelength division multiplexer (WD_MUX# 2 )  160  to include a fifth band-pass filter having the same band-pass as a wavelength band of wavelength-division-multiplexed optical signals for upstream/downstream data service by the operation characteristic and a sixth band-pass filter having the same band-pass as a preset wavelength band of the downstream optical source  130 . FIG. 7B describes band-pass characteristic of the second wavelength division multiplexer (MD_MUX# 2 )  160 .  
         [0056]    The central office  100  preferably further comprises an optical amplifier (for example, an erbium-doped fiber amplifier) on an optical fiber connected to the local office  200  to amplify the downstream signals outputted from, and the upstream signals inputted to, the second wavelength division multiplexer (WD_MUX# 2 )  160 .  
         [0057]    The local office  200  comprises a multiplexer/demultiplexer  210  which demultiplexes multiplexed optical signals for downstream data service and multiplexed optical signals for downstream broadcasting service transmitted from the central office  100  and multiplexes upstream optical signals transmitted from the subscriber terminals  300 . It is preferred that the multiplexer/demultiplexer  210  be implemented as a 1×N waveguide grating router (WGR).  
         [0058]    The subscriber terminal  300  comprises a third wavelength division multiplexer (WD_MUX# 3 )  340 , an upstream optical source  310 , a downstream data receiver  320  and a downstream broadcasting receiver  330 .  
         [0059]    The third wavelength division multiplexer (WD_MUX# 3 )  340  demultiplexes optical signals transmitted downstream from the local office  200  and divides them for downstream data service and for downstream broadcasting service. The third wavelength division multiplexer (WD_MUX# 3 )  340  also multiplexes optical signals for upstream transmission from the subscriber terminal  300  to the local office  200  and outputs the multiplexed optical signals.  
         [0060]    The third wavelength division multiplexer (WD_MUX# 3 )  340  can be configured with a seventh band-pass filter for passing the wavelength band of the upstream optical source  310 , an eighth band-pass filter for passing optical signals for downstream data service, and a ninth band-pass filter for passing the optical signals for downstream broadcasting service, according to the operation characteristic. That is, because the third wavelength division multiplexer (WD_MUX# 3 )  340  has as an operation characteristic the function of passing the signals of the upstream optical source  310 , the signals of the downstream data optical source  110 , and the signals the downstream broadcasting optical source  130 . FIG. 7C is a spectrum illustrating band-pass characteristic of the third wavelength division multiplexer (MD_MUX# 3 )  340 .  
         [0061]    The upstream optical source  310  generates optical signals for upstream transmission to the local office  200  through the third wavelength division multiplexer (WD_MUX# 3 )  340 . The band-pass of the optical signals generated from the upstream optical source  310  is preferably confined to a different band-pass from that of the optical signals for downstream data service as well as a different band-pass from that of the optical signals for downstream broadcasting service. The downstream data receiver  320  receives optical signals for downstream data service demultiplexed by the third wavelength division multiplexer (WD_MUX# 3 )  340  and converts the received optical signals to electric signals.  
         [0062]    The downstream broadcasting receiver  330  receives optical signals for downstream broadcasting service demultiplexed by the third wavelength division multiplexer (WD_MUX# 3 )  340  and converts the received optical signals to electric signals.  
         [0063]    Operationally, optical signals generated from the downstream data optical source  110  and the downstream broadcasting optical source  130  in the central office  100 , are multiplexed by the second wavelength division multiplexer (WD_MUX# 2 )  160  and are transmitted to the local office  200 . Then, the multiplexer/demultiplexer  210  in the local office  200  demultiplexes the multiplexed signals, and divides the data service and broadcasting service optical signals for respective output to each pertinent channel. This is possible because the multiplexer/demultiplexer  210  is composed of 1×N waveguide grating router (WGR) the band-pass characteristic of which has a cyclic characteristic according to the free spectrum range (FSR).  
         [0064]    [0064]FIG. 5 is an exemplary conceptual diagram illustrating the spectra of multiplexed WDM optical signals for data service and for broadcasting service and a spectrum of multiplexed upstream optical signals, the spectra being mutually distinct by virtue of their respective disposition within the free spectrum range (FSR) of the WGR.  
         [0065]    [0065]FIG. 6A is an exemplary conceptual diagram illustrating a spectrum of channel signals for data service and for broadcasting service which are outputted to each subscriber terminal after being demultiplexed by the multiplexer/demultiplexer  210  in the local office  200 . FIG. 6B is an exemplary conceptual diagram illustrating a spectrum of upstream signals outputted from the multiplexer/demultiplexer  210  in the local office  200 .  
         [0066]    Optical signals for data service and the optical signals for broadcasting service are demultiplexed by the multiplexer/demultiplexer  210  for output to respective third wavelength division multiplexers (WD_MUX# 3 )  340 . The latter WDMs further demultiplex the received signals into optical signals that optical receivers  320 ,  330  convert into electrical signals.  
         [0067]    Meanwhile, upstream optical signals which are outputted from the upstream optical source  310  are transmitted to the local office  200  through the third wavelength division multiplexer (WD_MUX# 3 )  340  and are then multiplexed by the multiplexer/demultiplexer  210 . The latter multiplexed signals are transmitted to the central office  100 , where they are demultiplexed in the multiplexer/demultiplexer  150  after passing through the second wavelength division multiplexer (WD_MUX# 2 )  160 . The signals are then passed through the first wavelength division multiplexer (WD_MUX# 1 )  140  and are transmitted to the upstream optical receiver  120  to be convert to electric signals.  
         [0068]    As described above, the WDM optical source according to the present invention adopts a spectrum-slicing method that advantageously relieves the need for a WDM optical source with a specific generation wavelength or a wavelength-stabilizing circuit for stabilizing wavelength. The WDM optical source according to the present invention also provides WDM signals with high power and very narrow line width, and therefore a broadcasting service without signal distortion by a chromatic dispersion effect. Nor is there a need for an additional amplifier and/or external modulator, which are expensive and whose implementation would economically burden subscribers.  
         [0069]    While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.