Abstract:
An optical add/drop multiplexer for adding or dropping a channel to an optical signal. The optical add/drop multiplexer includes a wavelength-division multiplexer to receive and transmit an optical signal, and a plurality of demultiplexing ports, each demultiplexing port is a path for a demultiplexed channel of the optical signal; and a plurality of add/drop multiplexers, wherein respective add/drop multiplexers are connected to respective demultiplexing ports, each of the add/drop multiplexers having a reflector for transmitting or reflecting an input channel, wherein each add/drop multiplexers is configured to add and/or drop a channel to/from from the wavelength-division multiplexer using the reflector.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority under 35 U.S.C. § 119 to an application entitled “Optical Add/Drop Multiplexer,” filed in the Korean Intellectual Property Office on May 2, 2003 and assigned Ser. No. 2003-28231, the contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a wavelength division multiplexing (WDM) system, and in particular, to an optical add/drop multiplexer for adding or removing a predetermined channel to or from a multiplexed optical signal.  
           [0004]    2. Description of the Related Art  
           [0005]    Practical uses of WDM technology have been developed, where a plurality of channels with different wavelengths are transmitted using a single-core optical fiber. Accordingly, the transmission of an optical signal at a very high rate is facilitated. The development of optical device technology enables the ability to establish/switch an optical signal path, or to add/drop an optical signal. Thus, a WDM optical communication network can be configured.  
           [0006]    In general, an optical add/drop multiplexer includes a pair of wavelength-division multiplexers (WDMs) and a plurality of optical switches. An arrayed-waveguide grating (AWG) is widely used as a WDM for a number of reasons, such as simple channel extension, easy control, and excellent integration capability. A 2×2 space switch or a wavelength-dependent fiber brag grating (FBG) is usually used as an optical switch.  
           [0007]    [0007]FIG. 1 illustrates the structure of a conventional optical add/drop multiplexer. Referring to FIG. 1, the optical add/drop multiplexer includes first and second circulators (C 1  and C 2 )  120  and  140 , each circulator is connected to an optical fiber  110  for transmission of a multiplexed optical signal. In addition, the optical add/drop multiplexer has a plurality of ports, first to nth FBGs  131  to  133 , and first and second WDMs (WDM 1  and WDM 2 )  150  and  160 . Here, n is a natural number equal to or greater than  3 . For notational simplicity, if reference numeral “###” denotes a circulator  120  or  140 , its m-th port will be provided with reference numeral “###m”, where m is a natural number. A multiplexed optical signal input/output to/from the optical add/drop multiplexer includes a plurality of channels at different wavelengths. It is assumed here that an m th  channel has wavelength m.  
           [0008]    The first circulator  120  has first to third ports  1201  to  1203  for outputting an input optical signal to a lower port. The first circulator  120  outputs an optical signal received from the first port  1201  to the second port  1202 . And, it outputs an optical signal received from the second port  1202  to the third port  1203 .  
           [0009]    The first to n th  FBGs  131  to  133  are connected between the second ports  1202  and  1402  of the first and second circulators  120  and  140 . These FBGs pass an optical signal in an off state and reflect only a predetermined channel from the optical signal in an on state. For example, the second FBG  132  is set to reflect only a second channel λ2, and the nth FBG  133  is set to reflect only an n th  channel λn.  
           [0010]    The first WDM  150  has a first multiplexing port (MP 1 )  151  and 11 th  to 1n th  demultiplexing ports (DP 11  to DP 1 n)  152  to  154 . The first MP  151  is connected to the third port  1203  of the first circulator  120 . The first WDM  150  outputs a channel received from the first MP  151  to a DP corresponding to the wavelength of the received channel. For example, the first WDM  150  outputs the second channel λ2 received through the first MP  151  to the 12 th  DP  153 , and the n th  channel λn to the 1nth DP  154 .  
           [0011]    The second circulator  140  outputs an optical signal received from the first port  1401  to the second port  1402 , and an optical signal received from the second port  1402  to the third port  1403 .  
           [0012]    The second WDM  160  has a second MP  161  (MP 2 ) and 21 th  to 2n th  DPs (DP 21  to DP 2 n)  162  to  164 . The second MP  161  is connected to the first port  1401  of the second circulator  140 . The second WDM  160  outputs channels received from the DPs  162  to  164  to the second MP  161 .  
           [0013]    The optical add/drop multiplexer drops the first channel λ1 from an input optical signal in a first case, and adds the second channel λ2 to the optical signal in a second case. These two cases will be described below.  
           [0014]    A controller (not shown) sets the first and second FBGs  131  and  132  to the on state and the other FBG  133  to the off state. In the first case, the first circulator  120  outputs an optical signal received through the first port  1201  to the second port  1202 , and the first FBG  131  reflects only the first channel λ 1  from the optical signal. The first circulator  120  outputs the first channel λ 1  received from the second port  1202  to the third port  1203 . The first WDM  150  outputs the first channel λ 1  received through the first MP  151  to the 11 th  DP  152 , thereby dropping the first channel λ 1 .  
           [0015]    In the second case, an optical signal, having passed through the first to n th  FBGs  131  to  133 , is input to the second port  1402  of the second circulator  140 . The second circulator  140  outputs the optical signal to the third port  1403 . The second WDM  160  outputs the second channel λ 2  received through the 22 th  DP  163  to the second MP  161 . The second circulator  140  outputs the second channel λ 1  received through the first port  1401  to the second port  1402 . The second FBG  132  reflects the second channel λ2. The second circulator  140  outputs the second channel λ 2  received through the second port  1402  to the third port  1403 , thereby adding the second channel λ 2  to the optical signal.  
           [0016]    As described above, a conventional optical add/drop multiplexer has the plurality of FBGs  131  to  133  connected in serial. Therefore, the number of FBGs through which a channel is dropped or added must pass differs depending on the wavelength of the channel. For example, if the first channel λ 1  is dropped, it is reflected from the first FBG  131 . On the other hand, if the third channel λ 3  is dropped, it must pass the first and second FBGs  131  and  132  twice. When a channel is added or dropped optical loss occurs during a pass through an FBG Therefore, its power differs according to its wavelength. Moreover, the FBGs  131  to  133  are usually controlled by ambient temperature and tension management, which takes a relatively long time. As a result, high-speed switching is difficult.  
         SUMMARY OF THE INVENTION  
         [0017]    Therefore, the present invention has been made to reduce or overcome the above mentioned problems involved with the related art. One object of the present invention is to provide an optical add/drop multiplexer which operates independently of the wavelength of an added or dropped channel and enables high-speed switching.  
           [0018]    It is another object of the present invention to provide a low-price optical add/drop multiplexer with a simplified structure.  
           [0019]    In accordance with the principles of the present invention, an optical add/drop multiplexer is provided, for adding or dropping a channel of an optical signal. The optical add/drop multiplexer includes a wavelength-division multiplexer to receive and transmit an optical signal, and a plurality of demultiplexing ports, each demultiplexing port is a path for a demultiplexed channel of the optical signal; and a plurality of add/drop multiplexers, wherein respective add/drop multiplexers are connected to respective demultiplexing ports, each of the add/drop multiplexers having a reflector for transmitting or reflecting an input channel, wherein each add/drop multiplexers is configured to add and/or drop a channel to/from from the wavelength-division multiplexer using the reflector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 illustrates a conventional optical add/drop multiplexer;  
         [0022]    [0022]FIG. 2 is a block diagram of an optical add/drop multiplexer according to the present invention;  
         [0023]    [0023]FIG. 3 illustrates an embodiment of the optical add/drop multiplexer according to the present invention; and  
         [0024]    [0024]FIGS. 4A and 4B illustrate the operation of an n th  add/drop multiplexer (ADM) illustrated in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail.  
         [0026]    An optical add/drop multiplexer illustrated in FIGS.  2  to  4 B includes circulating parts (CPs) or circulators, each CP or circulator having a plurality of ports. For notational simplicity, if reference numeral “###” denotes a CP or a circulator, its m-th port will be provided with reference numeral “###m”, where m is a natural number. A multiplexed optical signal input/output to/from the optical add/drop multiplexer includes a plurality of channels at different wavelengths. It is assumed here that an m th  channel has wavelength m.  
         [0027]    [0027]FIG. 2 is a block diagram of the optical add/drop multiplexer according to the present invention. Referring to FIG. 2, the optical add/drop multiplexer is comprised of a WDM  220  and first to n th  ADMs (ADM 1  to ADMn)  230  to  250  connected to the WDM  220 . Here, n is a natural number equal to or greater than 3.  
         [0028]    The WDM  220  includes an input port (IN)  221  and an output port (OUT)  222  which are connected to an optical fiber  210  for transmission of a multiplexed optical signal. First to nth DPs (DP 1  to DPn)  223  to  225  serve as paths for demultiplexed channels. The WDM  220  wavelength-division demultiplexes a multiplexed optical signal received through the input port  221  and outputs each demultiplexed channel to a DP corresponding to the wavelength of the demultiplexed channel. For example, the WDM  220  outputs the second channel λ 2  to the second DP  224  and the n th  channel λn to the n th  DP  225 . Conversely, the WDM  220  wavelength-division multiplexes a plurality of wavelengths λ 1  to λn received through the first to n th  DPs  223  to  225  and outputs the multiplexed optical signal through the output port  222 .  
         [0029]    The first to n th  ADMs  230  to  250  are connected to the first to n th  DPs  230  to  250  in a one to one correspondence. Each ADM includes a CP and a reflector (R). The first to n th  ADMs  230  to  250  are similar in configuration. Thus, the first ADM  230  will be described as a representative.  
         [0030]    A first CP (CP 1 )  232  in the first ADM  230  has first to fifth ports  2321  to  2325  and outputs a channel received through a port to an adjacent lower port. For example, the first CP  232  outputs a channel received through the first port  2321  to the second port  2322 , and the channel received through the second port  2322  to the third port  2323 . The first port  2321  serves as a path for a channel that is added, and the fifth port  2325  serves as a path for a channel that is dropped. The third port  2323  is connected to the first DP  223 . The second port  2322  is connected to the fourth port  2324 .  
         [0031]    A first reflector (R 1 )  234  in the first ADM  230  is connected to the second and fourth ports  2322  and  2324  of the first CP  232 . It passes an input channel in an off state and reflects the channel in an on state. A bi-lateral reflector, which changes a transmittance and a transmitted wavelength according to a control signal and which is wavelength-independent, can be used as the first reflector  234 .  
         [0032]    The operation of dropping the first channel λ 1  from an input optical signal and adding it to the optical signal in the optical add/drop multiplexer will be described.  
         [0033]    A controller (not shown) sets the first reflector  234  to an on state and the other reflectors  244  to  254  to an off state in the first ADM  230 . For dropping the first channel λ 1 , the WDM  220  wavelength-division demultiplexes an optical signal received through its input port  221  and outputs the demultiplexed first channel λ 1  to the first DP  223  connected to the first ADM  230 . The first CP  232  of the first ADM  230  outputs the first channel λ 1  received through the third port  2323  to the fourth port  2324 , and the first reflector  234  reflects the first channel λ 1 . The first CP  232  drops the first channel λ 1  by outputting the first channel λ 1  received through the fourth port  2324  to the fifth port  2325 .  
         [0034]    For adding the first channel λ 1 , the first ADM  230  outputs the first channel λ 1  received through the first port  2321  to the second port  2322 , and the first reflector  234  reflects the input first channel λ 1 . The first CP  232  outputs the first channel λ 1  received through the second port  2322  to the third port  2323  connected to the first DP  223  of the WDM  220 . The WDM  220  wavelength-division multiplexes the channels λ 1  to λn received through the DPs  223  to  225  and outputs the multiplexed optical signal through the output port  222 .  
         [0035]    [0035]FIG. 3 illustrates an embodiment of the structure of the optical add/drop multiplexer according to the present invention. The optical add/drop multiplexer includes first and second WDMs (WDM 1  and WDM 2 )  320  and  360 , and first to n th  ADMs (ADM 1  to ADMn)  330  to  350  connected between the first and second WDMs  320  and  360 .  
         [0036]    The first WDM  320  is comprised of a first MP (MP 1 )  321  connected to an optical fiber  310  for transmission of a multiplexed optical signal and 11 th  to 1n th  DPs (DP 11  to DP 1 n)  322  to  324 . The first WDM  320  wavelength-division demultiplexes a multiplexed optical signal received through the first MP  321  and outputs each demultiplexed channel to a DP corresponding to the wavelength of the demultiplexed channel. For example, the first WDM  320  outputs the second channel λ 2  to the 12 th  DP  323  and the n th  channel λn to the 1n th  DP  324 . AWGs can be used as the first and second WDMs  320  and  360  (because of simple channel extension, easy control, and excellent integration capability).  
         [0037]    The first to n th  ADMs  330  to  350  are connected to the 11 th  to 1n th  DPs  322  to  324  in a one to one correspondence. Each ADM includes a pair of circulators and a reflector. The first to n th  ADMs  330  to  350  are similar in configuration. Thus, the first ADM  330  will be described as a representative.  
         [0038]    An 11 th  circulator (C 11 )  332  in the first ADM  330  has first to third ports  3321  to  3323  and outputs a channel received through a port to its adjacent lower port. The first port  3321  of the 11 th  circulator  332  is connected to the 11 th  DP  322  of the first WDM  320 . The 11 th  circulator  332  outputs the first channel λ 1  received through the first port  3321  to the second port  3322 , and the first channel λ 1  received through the second port  3322  to the third port  3323 , thereby dropping the first channel λ 1 .  
         [0039]    A first reflector  334  (R 1 ) of the first ADM  330  is connected to the second port  3322  of the 11 th  circulator  332  and the second port  3362  of a 12 th  circulator (C 12 )  336 . It passes an input channel in an off state and reflects the channel in an on state. A bi-lateral reflector, which changes a transmittance according to a control signal and is wavelength-independent, can be used as the first reflector  334 .  
         [0040]    The 12 th  circulator  336  in the first ADM  330  has first to third ports  3361  to  3363  and outputs a channel received through a port to its adjacent lower port. The 12 th  circulator  336  outputs the first channel λ 1  received through the first port  3361  to the second port  3362 , and the first channel λ 1  received through the second port  3362  to the third port  3363 , thereby adding the first channel λ 1 .  
         [0041]    The second WDM  360  is comprised of a second MP (MP 2 )  361  connected to the optical fiber  310  and 21 th  to 2n th  DPs (DP 21  to DP 2 n)  362  to  366 . The second WDM  360  wavelength-division multiplexes a plurality of channels received through the 21 th  to 2n th  DPs  362  to  366  and outputs the multiplexed optical signal to the second MP  361 .  
         [0042]    The operation of dropping the first channel λ 1  from an input optical signal and adding the second channel λ 1  to the optical signal in the thus-constituted optical add/drop multiplexer will be described.  
         [0043]    A controller (not shown) sets the first reflector  334  to an on state and the other reflectors  344  to  354  to an off state in the first ADM  330 . For dropping the first channel λ 1 , the first WDM  320  wavelength-division demultiplexes an input optical signal and outputs the demultiplexed first channel λ 1  to the 11 th  DP  322  connected to the 11 th  circulator  332  of the first ADM  330 . The 11 th  circulator  332  outputs the first channel λ 1  received through the first port  3321  to the second port  3322 , and the first reflector  334  reflects the first channel λ 1 . The 11 th  circulator  332  drops the first channel λ 1  by outputting the first channel λ 1  received through the second port  3322  to the third port  3323 .  
         [0044]    For adding the first channel λ 1  in the first ADM  330 , the 12 th  circulator  336  outputs the first channel λ 1  received through the first port  3361  to the second port  3362 , and the first reflector  334  reflects the input first channel λ 1 . The 12 th  circulator  336  outputs the first channel λ 1  received through the second port  3362  to the third port  3363  connected to the 21 th  DP  362  of the second WDM  360 . The second WDM  360  wavelength-division multiplexes the channels λ 1  to λn received through the 21 th  to 2n th  DPs  362  to  366  and outputs the multiplexed optical signal through the second MP  361 .  
         [0045]    [0045]FIGS. 4A and 4B illustrate the operation of the n th  ADM illustrated in FIG. 3.  
         [0046]    [0046]FIG. 4A illustrates adding a dropped n th  channel λn in the n th  ADM  350 . For dropping the n th  channel λn, an n1 th  circulator (Cn 1 )  352  outputs the n th  channel λn received through a first port  3521  to a second port  3522 . An n th  reflector (Rn)  354  is set to an on state and reflects the input n th  channel λn. The n1 th  circulator  352  drops the n th  channel λn by outputting the n th  channel λn received through the second port  3522  to the third port  3523 . For adding the n th  channel λn, an n2 th  circulator (Cn 2 )  356  outputs the n th  channel λn received through a first port  3561  to a second port  3562 . The n th  reflector  354  reflects the n th  channel λn. The n2 th  circulator  362  adds the n th  channel λn by outputting the n th  channel λn received through the second port  3562  to the third port  3563 .  
         [0047]    [0047]FIG. 4B illustrates transmitting the n th  channel λn in the n th  ADM  350 . Referring to FIG. 4B, the n1 th  circulator  352  outputs the n th  channel λn received through the first port  3521  to the second port  3522 . The n th  reflector  354  is set to an off state and transmits the input n th  channel λn. The n2 th  circulator  356  outputs the n th  channel λn received through the second port  3562  to the third port  3563 .  
         [0048]    Advantageously, the optical add/drop multiplexer of the present invention adds and/or drops a channel independently of the wavelength, by using circulators as passive devices and wavelength-independent reflectors. In this manner, high-speed switching is possible and can be easily controlled.  
         [0049]    The use of the circulators and the wavelength-independent reflectors also minimizes the number of auxiliary devices, such as temperature controllers. As a result, the optical add/drop multiplexer has a simplified structure and a lower cost of fabrication is achieved.  
         [0050]    While the invention has been shown and described with reference to a certain preferred embodiment 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.