Patent Publication Number: US-2005141898-A1

Title: System for all-optical clock recovery

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2003-96221, filed on Dec. 24, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to an all-optical clock recovery device, and more particularly, to a system for all-optical clock recovery that is essential in optical signal processing such as 3R regeneration and demultiplexing.  
      2. Description of the Related Art  
      A clock signal can be obtained electrically and optically. One such optical method is an all-optical method using injection locking.  
       FIGS. 1 and 2  are block diagrams schematically showing embodiments of a conventional all-optical clock recovery device using injection locking.  
      Referring to  FIGS. 1 and 2 , the conventional all-optical clock recovery device includes an optical signal controller  10 , a laser unit  20 , and an optical clock output unit  30 .  
      The optical signal controller  10  includes a variable optical amplifier  12  that amplifies optical signals λ σ  to a predetermined magnitude, a first optical bandpass filter (OBPF)  14  that selects an optical signal λ S  from the amplified optical signals λ σ , and a polarization controller (PC)  16  that controls the polarization state of the optical signal λ S . Here, the PC  16  may or may not be required depending on the type of laser used.  
      The laser unit  20  includes a laser  25  and a transmission unit ( 27   a  or  27   b ) that transmits an optical clock λ C  from the laser  25  according to the input and output ports of the laser  25 . The transmission unit ( 27   a  or  27   b ) is disposed on a front end of the laser  25 . In a case where the input and output ports of the laser  25  are the same, an optical circulator (OC)  27   a  is used as the transmission unit, and in a case where the input and output ports are different, an optical isolator (OI)  27   b  is used as the transmission unit. The transmission unit ( 27   a  or  27   b ) transmits the optical clock λ C  from the laser to the optical clock output unit  30  and prevents them from returning to the optical signal controller  10 .  
      The optical clock output unit  30  includes an optical amplifier  32  that amplifies the optical clock λ C  from the laser unit  20 , and a second OBPF  35  that outputs only the optical clock λ C  among the optical signal λ S  and the optical clock λ C .  
      Since an optical signal, not an optical pulse, is injected into the laser, the optical clock output from the above system may have a pattern effect and a relatively large timing jitter.  
     SUMMARY OF THE INVENTION  
      According to an aspect of the present invention, there is provided a system for all-optical clock recovery including: an optical signal controller that controls the magnitude and the polarization state of an optical signal, a laser unit that receives the optical signal output from the optical signal controller and outputs an optical clock, and an optical regeneration loop that controls the phase, polarization, and strength of the optical clock output from the laser unit so as to provide the laser unit with the optical clock, the phase, polarization, and strength of which are controlled, and the optical signal output from the optical signal controller.  
      The laser unit may include a laser and a unit for transmitting the optical clock output from the laser to the optical regeneration loop.  
      The laser may be a passively mode-locked laser diode, a mode-locked fiber ring laser, or a self-pulsating laser diode.  
      If the input and output ports of the laser are the same, the unit for transmitting the optical clock may be an optical circulator. If the input and output ports of the laser are different, the unit for transmitting the optical clock may be an optical isolator.  
      The optical regeneration loop may include a variable optical amplifier that controls the magnitude of the optical clock output from the laser unit, an optical bandpass filter that passes only the optical clock among the optical signal and the optical clock, a variable optical delay line that controls the phase of the optical clock, a first optical splitter that feeds back some of the optical clock to the laser unit and outputs the remaining optical clock, and a second optical splitter that combines the optical clock, the phase of which is controlled by the variable optical delay line, with the optical signal provided from the optical signal controller and provides the combined signal to the laser unit disposed on a rear end of the variable optical delay line.  
      Also, the optical regeneration loop may further include a polarization controller, disposed between the first and second optical splitters, that controls the polarization state of the feedback optical clock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIGS. 1 and 2  are schematic block diagrams of a conventional system for an all-optical clock recovery; and  
       FIGS. 3 and 4  are block diagrams of embodiments of a system used for all-optical clock recovery and having an optical regeneration loop, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Throughout the drawings, like reference numerals refer to like elements.  
      Referring to  FIGS. 3 and 4 , an all-optical clock recovery device according to embodiments of the present invention includes an optical signal controller  110 , a laser unit ( 120  or  130 ), to which an optical signal controlled by the optical signal controller  110  is input, and an optical regeneration loop  150  that controls the phase, polarization, and strength of an optical clock output by the laser unit ( 120  or  130 ) and provides the laser unit ( 120  or  130 ) with the optical clock and the optical signal.  
      Here, the optical signal controller  110  includes a first variable optical amplifier (VOA)  111  that controls the strength of optical signals, a first optical bandpass filter (OBPF)  113  that transmits an optical signal with a desired wavelength, the strength of which is controlled, and a first polarization controller (PC)  115  that controls the polarization state of the filtered optical signal. Here, the first PC  115  may or may not be required depending on the type of laser used.  
      Laser units  120  and  130  include lasers  125  and  135 , respectively. Here, the lasers  125  and  135  can have a common input and output port, as shown in  FIG. 3 , or different input and output ports, as shown in  FIG. 4 . The lasers  125  and  135  may be, for example, a passively mode-locked laser diode, a mode-locked fiber ring laser, or a self-pulsating laser diode.  
      In a case where the input and output ports of the laser  125  are the same, the laser unit  120  includes an optical circulator (OC)  127  that designates the proceeding direction of the optical clock, as shown in  FIG. 3 . The OC  127  is installed on a front end of the laser  125  to pass the input optical signal and/or the optical clock in one direction only. In a case where the input and output ports of the laser  135  are different, the laser unit  130  includes an optical isolator (OI)  137  that passes the input optical signal and/or the optical clock in one direction, as shown in  FIG. 4 . If the optical clock proceeds towards both directions of an optical regeneration loop, the phase (or timing) and polarization state of the optical clocks traveling in different directions may be different from each other in the lasers  125  and  135 , which would negatively affect clock recovery. In other words, the OC  127  and the OI  137  enable the optical signal and the optical clock output from the laser to proceed in a clockwise direction of the optical regeneration loop  150  and not return to the optical signal controller  110 .  
      The optical regeneration loop  150  includes a second VOA  152  that controls the strength of the optical clock output from the laser unit ( 120  or  130 ), a second OBPF  154  that passes only the optical clock among the optical signal and the optical clock, and a first optical splitter (OS)  156  that feeds back some of the optical clock filtered by the second OBPF  154  to the laser unit ( 120  or  130 ) and outputs the rest of the optical clock. Additionally, the optical regeneration loop  150  includes a second PC  158  that controls the polarization state of the optical clock that is returned to the laser unit ( 120  or  130 ) by the first OS  156 ; a variable optical delay line (VODL)  160  that controls the phase of the optical clock, the polarization state of which is controlled by the second PC  158 ; and a second OS  162  that combines the optical clock, the phase of which is controlled by the VODL  160 , with the optical signal provided from the optical signal controller  110  and provides the laser unit ( 120  and  130 ) with the combined signal. The second PC  160  may or may not be required depending on the type of laser used.  
      Operations of the all-optical clock recovery device having the above structure will be described below.  
      The magnitude of the optical signals λ σ  is controlled and the optical signal with a desired wavelength is transmitted within the optical signal controller  110 , for example, by the first VOA  111  and the first OBPF  113 . Here, λ S  denotes the selected wavelength. The filtered optical signal is input into the first PC  115 , which controls the polarization state of the filtered optical signal, and is passed to the laser ( 125  or  135 ) of the respective laser unit ( 120  or  130 ) after first being passed through the second OS  162 .  
      The laser ( 125  or  135 ) outputs the optical clock due to the input optical signal. Here, in a case where the input and output ports of the laser  125  are the same, the OC  127  provides the laser  125  with the input optical signal and transmits the optical clock output from the laser  125  to the optical regeneration loop  150 . In a case where the input and output ports of the laser  135  are different, the OI  137  transmits the optical signal output from the optical signal controller  110  to the laser  135  and prevents the optical clock and the optical signal output from the laser  135  from being transmitted in a reverse direction so that the optical clock and the optical signal proceed in only one direction of the optical regeneration loop  150 .  
      The strength (magnitude) of the optical clock output from the laser unit ( 120  or  130 ) is controlled by the second VOA  152 , and only the optical clock is passed by the second OBPF  154  with the optical signal filtered off. The optical clock is split by the first OS  156  into one part that will be output and the other that will be looped back. Accordingly, the OS  156  either outputs the optical clock kc or passes the optical clock λ C  to the second PC  158  which controls the polarization state of the optical clock λ C . The VODL  160  controls the optical clock % c such that its phase coincides with that of the optical signal input by the optical signal controller  110 . In addition, the optical clock, the phase of which is controlled, is combined with the optical signal from the optical signal controller  110  by the second OS  162 , and the combined signal is provided to the laser unit ( 120  or  130 ). Since the optical clock, as well as the optical signal, are input to the laser unit ( 120  or  130 ), a pattern effect and timing jitter of the optical clock output from the first OS  156  can be reduced.  
      As described above, according to the present invention, an optical clock with reduced pattern effect and timing jitter can be recovered using an optical regeneration loop without the need for electrical-to-optical conversion or optical-to-electrical conversion. In addition, the optical clock recovered from the optical signal can be used to process the optical signal.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.