Abstract:
The invention broadly comprises an optical add-drop multiplexer having at least two multi-port circulators (V, W) connected in parallel and interconnected by in-fibre Bragg gratings ( 40, 70 ) in a manner such that one port (V 1 ) of one of the circulators (V) is arranged to receive ( 20 ) a multi-channel input signal ( 10 ) and another port (V 8 ) of the same circulator (V) is arranged to output ( 80 ) a multi-channel output signal.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a multichannel optical add-drop multiplexer employing multi-port circulators and optical fibre Bragg gratings.  
         BACKGROUND OF THE INVENTION  
         [0002]    Multichannel add-drop multiplexers are devices that can drop more than one wavelength channel from an optical network and then add to the network a number of channels, usually the same number as have been dropped. Throughout the specification, the terms wavelength channel, wavelength and channel are used synonymously.  
           [0003]    A number of optical add-drop multiplexer designs have been proposed and demonstrated. Such designs include an arrayed waveguide grating multiplexer, a Mach-Zender interferometer employed in conjunction with fibre Bragg gratings, and an optical circulator employed in conjunction with a fibre Bragg grating. The latter design is the most practical because of the low insertion loss, low cross talk, and polarisation insensitivity.  
           [0004]    Recently the inventors have proposed and demonstrated a single channel optical circulator and fibre Bragg grating based optical add-drop multiplexer using a single circulator with either one or two fibre Bragg gratings. It has been demonstrated that this optical add-drop multiplexer can tolerate up to 30 dB power difference for in-band channels, and 20 dB power difference for out-of-band channels. Such a design is particularly useful for local area network applications where different channels at different points may have largely different powers.  
           [0005]    In principle, a multichannel optical add-drop multiplexer can be built simply by cascading multiple single-channel optical add-drop multiplexers. However, this approach is not desirable for two reasons. The first is that by increasing the number of circulators used and, hence, the number of ports through which light will be directed, an unacceptable increase will occur in the insertion loss. The second reason is that if only a certain number of channels are required at one location within the optical network then it is advantageous not to guide all of the different wavelengths through all the circulators. Therefore, it is desirable that the channels that are not required at a certain location be dropped and then added in a manner which avoids them having to pass through too many ports of the circulators.  
           [0006]    The inventors have developed an alternative to the cascaded arrangement and one which avoids at least some of the above stated difficulties.  
         SUMMARY OF THE INVENTION  
         [0007]    Broadly defined, the present invention provides an optical add-drop multiplexer having at least two multi-port circulators connected in parallel and interconnected by in-fibre Bragg gratings in a manner such that one port of one of the circulators is arranged to receive a multi-channel input signal and another port of the same circulator is arranged to output a multi-channel output signal.  
           [0008]    It will be understood that, as a circulator has at least three ports, the term “multi-port circulator” refers to a circulator with four or more ports and is used in this manner throughout this document.  
           [0009]    The invention may be defined further as providing an optical add-drop multiplexer comprising at least two multi-port circulators, one port of a first of the circulators being arranged to receive a multi-channel input signal and another port of the first circulator being arranged to output a multi-channel output signal. At least one in-fibre Bragg grating is located between two further ports of at least the first circulator, and additional ports of the first circulator are connected by further in-fibre Bragg gratings to ports of the or at least one of the further circulator(s) in a manner to place the circulators in parallel. Also, additional in-fibre Bragg gratings are provided to interconnect ports of the circulators in a manner to permit dropping and adding of at least two channels of the multi-channel input signal from and to ports of one or other of the circulators to which no in-fibre Bragg gratings are connected.  
           [0010]    The or each “further” circulator as above specified may be constituted by at least two cascaded circulators.  
           [0011]    An advantage of the add-drop multiplexer of the present invention is the significant decrease in insertion losses to the channels that are not required to be dropped at a certain location within a network. These channels, after entering the first multi-port circulator, are transmitted through Bragg gratings and are delivered from another port in the first multi-port circulator. Thus, these channels are transmitted through a minimum number of ports.  
           [0012]    A further advantage is the reduction of cross-talk between different channels and, in particular, channels which are not required for dropping or adding at a location within the network where other channels are being dropped or added. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    A preferred embodiment of the optical add-drop multiplexer will now be described, by way of example only, with reference to the accompanying circuit diagrams in which:  
         [0014]    [0014]FIG. 1 shows a circuit diagram employing two eight port circulators arranged in parallel to enable the dropping and adding of three channels; and  
         [0015]    [0015]FIG. 2 shows a circuit diagram employing three eight port circulators which enables the adding and dropping of five channels. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    In the circuit shown in FIG. 1 light in the wavelength range λ 1  . . . λ n    10  is launched  20  into port V 1  of the multi-port circulator V. The light is then guided within the circulator to port V 2 . At port V 2  all the light exits the circulator through an optical fibre  30 . The optical fibre  30  has three Bragg gratings A, B and C along its length. These three Bragg gratings, A, B and C, reflect most of the light guided through the fibre  30  at the wavelengths λ a , λ b , and λ c , while all other wavelengths, namely the wavelengths in the range λ T =[(λ 1 , . . . , λ n ) (λ a , λ b , λ c )], are transmitted through the Bragg gratings A, B and C. The transmitted wavelengths λ T  are guided by the fibre  30  and launched back into the circulator V at port V 7  where it is further guided through the circulator V to port V 8 . At port V 8  all light in the wavelength region λ T  exits from the first circulator V. Light in the range of wavelengths λ T  may be required for dropping and adding at a later point within the network.  
         [0017]    At port V 2  where light at the wavelengths λ a , λ b , and λ c  is now present as a consequence of it being reflected by the Bragg gratings A, B and C respectively. From port V 2  light at the wavelengths λ a , λ b , and λ c , is guided through the circulator V to port V 3 . At port V 3  all the light exits through an optical fibre  40  containing a Bragg grating A′ which reflects at the wavelength λ a  and transmits light at all other wavelengths. Therefore, at port V 3  only light at the wavelength λ a  is present while light at the wavelengths λ b  and λ c  is transmitted through the fibre  40  to port W 2  of the circulator W. Light at the wavelength λ a  is then guided through the circulator V from port V 3  to port V 4  where it is dropped out of the network for use at a desired location.  
         [0018]    Reference is now made to the circulator W. At port W 1  of the circulator W light at the wavelength λ a  is added to the network after having been dropped at port V 4  of the circulator V. The light at the wavelength λ a  is then guided through the circulator W to port W 2  where light at the wavelengths λ b , and λ c  is also present since it was transmitted through the Bragg grating A′ and guided by the fibre  40  from port V 3  of the circulator V. Therefore, at port W 2  of the circulator W light at the three wavelengths λ a , λ b , and λ c  is present. At port W 2  all of the three wavelengths λ a , λ b , and λ c  are then guided through the circulator W to port Wee where they exit through an optical fibre  60  containing a Bragg grating B′ which reflects at the wavelength λ b  and transmits at all other wavelengths. Thus, only λ b  remains at port W 3  while λ a  and λ c  are transmitted through the optical fibre  60  to port W 6  of the circulator W. From port W 3  light at the wavelength λ b  is guided through the circulator W to port W 4  where it is dropped out of the network for use at a desired location. Light at the wavelength λ b  is then added at port W 5  of the circulator W. From port W 5  light at the wavelength λ b  is then guided to port W 6  of the circulator W where light at the wavelengths λ a  and λ c  is present after having been transmitted through the Bragg grating B and guided through the optical fibre  60  from port W 3  of the circulator W. Thus light at all three wavelengths λ a , λ b , and λ c  is present at port W 6  of the circulator W. From port W 6  the light is guided to port W 7  where it exits through an optical fibre  70  containing a Bragg grating C′ which reflects light at the wavelength λ c  and transmits light at all other wavelengths. Thus, from port W 7  light at the wavelength λ c  is guided to port W 8  of the circulator W where it is dropped from the network.  
         [0019]    Light at the wavelength λ c  is added to the network at port V 5  of the circulator V. From port V 5  the light at wavelength λ c  is guided to port V 6  where light at all three wavelengths is present, since light at the wavelengths λ a  and λ b  has been transmitted through the Bragg grating C and the optical fibre  70  from port W 7  of the circulator W. From port V 6  of the circulator V light at all three wavelengths λ a , λ b , and λ c  is guided to port V 7  of the circulator V. At port V 7  all the light exits through the optical fibre  30  which contains three Bragg grating reflecting at the wavelengths λ a , λ b , and λ c  so that all the light is reflected back to port V 7  from which it is then guided to port V 8  of the circulator V where it exits  80  the circuit to continue through the network where it may be dropped and added at later stages as required.  
         [0020]    The circuit diagram shown in FIG. 2 employs an arrangement of circulators and optical fibre Bragg gratings which enables the dropping and adding of 5 different channels at the wavelengths λ a , λ b , λ c , λ d  and λ e .  
         [0021]    In the circuit shown in FIG. 2 light in the wavelength range λ V1  . . . λ n    100  is launched  200  into port X 1  of the multi-port circulator X. The light is then guided within the circulator to port X 2 . At port X 2  all the light exits the circulator through an optical fibre  300 . The optical fibre  300  has three Bragg gratings A, B, C, D and E along its length. These three Bragg gratings, A, B, C, D and E reflect most of the light guided through the fibre  300  at the wavelengths λ a , λ b , λ c , λ d  and λ e  while all other wavelengths, namely the wavelengths in the range λ t =[(λ 1 , . . . , λ n )−(λ a , λ b , λ c , λ d , λ e )] are transmitted through the Bragg gratings A, B, C, D and E. The transmitted wavelengths λ t  are guided by the fibre  300  and launched back into the circulator X at port X 7  where it is further guided through the circulator X to port X 8 . At port X 8  all light in the wavelength region λ t  is output from the circulator X. Light in the range of wavelengths λ t  may be required for dropping and adding at a later point within the network.  
         [0022]    At port X 2  where light at the wavelengths λ a , λ b , λ c , λ d  and λ e  is now present since it was reflected by the Bragg gratings A, B, C, D and E respectively. From port X 2  light at the wavelengths λ a , λ b , λ c , λ d  and λ e  is guided through the circulator X to port X 3 . At port X 3  all the light exits through an optical fibre  400  containing a Bragg grating A′ which reflects at the wavelength λ a  and transmits light at all other wavelengths. Therefore, at port X 3  only light at the wavelength λ a  is present while light at the wavelengths λ b , λ c , λ d  and λ e  is transmitted through the fibre  400  to port Y 2  of the circulator Y. Light at the wavelength λ a  is then guided through the circulator X from port X 3  to port X 4  where it is dropped out of the network for use at a desired location.  
         [0023]    Reference is now made to circulator Y. At port Y 1  of the circulator Y light at the wavelength λ a  is added to the network after having been dropped at port X 4  of the circulator X. The light at the wavelength λ a  is then guided through the circulator Y to port Y 2  where light at the wavelengths λ b , λ c , λ d  and λ e  is also present since it was transmitted through the Bragg grating A′ and guided by the fibre  400  from port X 3  of the circulator X. Therefore, at port Y 2  of the circulator Y light at the five wavelengths λ a , λ b , λ c , λ d  and λ e  is present. At port Y 2  all of the five wavelengths λ a , λ b , λ c , λ d  and λ e  are then guided through the circulator Y to port Y 3  where they exit through an optical fibre  600  containing a Bragg grating B′ which reflects at the wavelength λ b  and transmits at all other wavelengths. Thus, only λ b  remains at port Y 3  while λ a , λ c , λ d  and λ e  are transmitted through the optical fibre  600  to port Y 6  of the circulator Y. From port Y 3  light at the wavelength λ b  is guided through the circulator Y to port Y 4  where it is dropped out of the network for use at a desired location. Light at the wavelength λ b  is then added at port Y 5  of the circulator Y. From port Y 5  light at the wavelength λ b  is then guided to port Y 6  of the circulator Y where light at the wavelengths λ a , λ c , λ d  and λ e  is present after having been transmitted through the Bragg grating B and guided through the optical fibre  600  from port Y 3  of the circulator Y. Thus light at all five wavelengths λ a , λ b , λ c , λ d  and λ e  is present at port Y 6  of the circulator Y. From port Y 6  the light is guided to port Y 7  where it exits through an optical fibre  700  containing a Bragg grating C′ which reflects light at the wavelength λ e  and transmits light at all other wavelengths. Thus, from port Y 7  light at the wavelength λ c  is guided to port Y 8  of the circulator Y where it is dropped from the network.  
         [0024]    Reference is now made to the circulator Z. At port Z 1  of the circulator Z light at the wavelength λ c  is added to the network after having been dropped at port Y 8  of the circulator Y. The light at the wavelength λ c  is then guided through the circulator Z to port Z 2  where light at the wavelengths λ a , λ b , λ d  and λ e  is also present since it was transmitted through the Bragg grating C′ and guided by the fibre  700  from port Y 7  of the circulator Y. Therefore, at port Z 2  of the circulator Z light at the five wavelengths λ a , λ b , λ c , λ d  and λ e  is present. At port Z 2  all of the five wavelengths λ a , λ b , λ c , λ d  and λ e  are then guided through the circulator Z to port Z 3  where they exit through an optical fibre  800  containing a Bragg grating D′ which reflects at the wavelength λ d  and transmits at all other wavelengths. Thus, only λ d  remains at port Z 3  while λ a , λ b , λ c  and λ e  are transmitted through the optical fibre  700  to port Z 6  of the circulator Z. From port Z 3  light at the wavelength λ d  is guided through the circulator Z to port Z 4  where it is dropped out of the network for use at a desired location. Light at the wavelength λ d  is then added at port Z 5  of the circulator Z. From port Z 5  light at the wavelength λ d  is then guided to port Z 6  of the circulator Z where light at the wavelengths λ a , λ b , λ c  and λ e  is present after having been transmitted through the Bragg grating d and guided through the optical fibre  800  from port Z 3  of the circulator Z. Thus light at all five wavelengths λ a , λ b , λ c , λ d  and λ e  is present at port Z 6  of the circulator Z. From port Z 6  the light is guided to port Z 7  where it exits through an optical fibre  900  containing a Bragg grating E′ which reflects light at the wavelength λ e  and transmits light at all other wavelengths. Thus, from port Z 7  light at the wavelength λ e  is guided to port Z 8  of the circulator Z where it is dropped from the network.  
         [0025]    Light at the wavelength λ e  is added to the network at port X 5  of the circulator X. From port X 5  the light at wavelength λ e  is guided to port X 6  where light at all five wavelengths λ a , λ b , λ c , λ d  and λ e  is present, since light at the wavelengths λ a , λ b , λ c  and λ d  has been transmitted through the Bragg grating E′ and the optical fibre  900  from port Z 7  of the circulator Z. From port X 6  of the circulator X light at all five wavelengths λ a  λ b , λ c , λ d  and λ e  is guided to port X 7  of the circulator X. At port X 7  all the light exits through the optical fibre  300  which contains three Bragg grating reflecting at the wavelengths λ a , λ b , λ c , λ d  and λ e  so that all the light is reflected back to port X 7  from which it is then guided to port X 8  of the circulator X where it exits  950  the circuit to continue through the network where it may be dropped and added at later stages as requires.  
         [0026]    Variations and modifications may be made in the embodiments of the invention as above described without departing from th scope of the invention as defined in the following statements of claim.