Abstract:
According to at least one embodiment, a system comprises a Sagnac interferometric loop and a semiconductor optical amplifier (SOA) located at an asymmetric position on that loop, wherein the Sagnac interferometric loop and the SOA are operable to perform signal conversion on an input signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Application 60/499,536, filed Sep. 2, 2003, the disclosure of which is hereby incorporated herein by reference. 

   GOVERNMENT LICENSE RIGHTS 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of ARO MURI DAAD 19-00-1-0165 awarded by the Army Research Office. 

   TECHNICAL FIELD 
   This invention relates to wavelength conversion in a fiber optics-based communication system. 
   BACKGROUND 
   In communication systems, signals are often transmitted over very long distances. Such long distances may cause signals to become degraded, for example, by attenuation and interference. Accordingly, some systems use signal converters to receive a degraded signal and restore its shape and amplitude. 
   Fiber optics-based communication systems in the past have used electrical signal converters that receive the light signal from the optical transmission medium, change that signal to an electric signal, restore the shape and amplitude, and change the signal back to light for transmission over another optical medium. Advances in fiber optics technology have allowed for the development of optical wavelength conversion, which performs the conversion without changing the signal to an electric signal. However, traditional optical wavelength converters have suffered from the effects of the pattern dependent effect and poor extinction ratios. 
   BRIEF SUMMARY 
   The present invention is directed to a system and method in which various embodiments demonstrate wavelength conversion using a Sagnac interferometer with an SOA. Some example embodiments provide for the alleviation of pattern-dependent effect and the improvement of extinction ratios using an optimal interference phase bias. According to at least one embodiment, a system comprises a Sagnac interferometric loop and a semiconductor optical amplifier (SOA) located at an asymmetric position on that loop, wherein the Sagnac interferometric loop and the SOA are operable to perform signal conversion on an input signal. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is an illustration of the structure of a Sagnac interferometric loop with an SOA at an asymmetric position, adapted according to various embodiments; 
       FIG. 2(   a ) is an illustration of an example input NRZ signal, which may be used with some embodiments of the present invention; 
       FIG. 2(   b ) is an illustration of a signal produced from XGM effect conversion; 
       FIG. 2(   c ) is an illustration of a signal produced from conversion using a Sagnac interferometer, adapted according to various embodiments; 
       FIG. 2(   d ) is an illustration of a signal produced from conversion using a Sagnac interferometer, adapted according to various embodiments; 
       FIG. 3  is an illustration of an example signal converter employing a Sagnac interferometer, adapted according to various embodiments; 
       FIG. 4(   a ) is an eye diagram of an input signal which may be used with one or more embodiments; 
       FIGS. 4(   b ) and ( c ) are eye diagrams of a wavelength converted signals according to various embodiments; 
       FIG. 4(   d ) shows the eye diagram of a converted data signal using XGM effect; 
       FIG. 5(   a ) is an illustration of a data pattern of an input signal; 
       FIG. 5(   b ) is an illustration of the data pattern of the converted signal using XGM effect; 
       FIG. 5(   c ) is an illustration of a data pattern of a converted signal using the Sagnac interferometric loop, as illustrated in  FIG. 3 ; 
       FIG. 6  is an illustration of the results of Bit Error Rate (BER) measurement of converted signals; and 
       FIG. 7  is an illustration of the optical power penalty of converted signals at different wavelengths. 
   

   DETAILED DESCRIPTION 
   When a Semiconductor Optical Amplifier (SOA) is placed at an off-center position inside a Sagnac interferometric loop, the present invention demonstrates multiple functionalities using short optical Return-to-Zero (RZ) signals, such as demultiplexing for optical time division multiplexing (OTDM) systems, optical switching and packet routing, optical sampling, optical regeneration, and the like. For Non-Return-to-Zero (NRZ) signals, waveform restoration using the present invention is possible. In one embodiment, the present invention provides all-optical wavelength conversion for NRZ signals with a fiber-based Sagnac interferometric loop with an SOA at an asymmetric position. Analysis shows that the phase bias of the interference is critical for the wavelength conversion operation. With the Sagnac interferometric structure, the present invention can provide reduced pattern dependent effect and higher extinction ratios when compared to Cross Gain Modulation (XGM)-based wavelength converters. As explained further below, in one embodiment, the 10 Gb/s Bit Error Rate (BER) measurement shows a reduced power penalty of the converted signal using a Sagnac interferometric loop. 
   While other systems have adopted Mach-Zehnder and Michelson interferometers together with SOAs to demonstrate wavelength conversion operation of NRZ signals, the exemplary embodiments described herein employ Sagnac interferometers as NRZ wavelength converters.  FIG. 1  is a high-level diagram of the structure of Sagnac interferometric loop  100  having SOA  101  at an asymmetric position. Input signal  10  is separated by 50:50 coupler  102  into clockwise (CW) component  11  and counter clockwise (CCW) component  12 . CW and CCW components  11 ,  12  pass through SOA  100  at different times due to the asymmetric position of SOA  101  in loop  100 . Polarization controller  103  is used to control the loop birefringence and the consequent interference when the two components (CW  11  and CCW  12 ) come back to 50:50 coupler  102 . 
   For purposes of description, consider a wave plate with retardation φ and orientation θ (to account for the loop birefringence). CW component  11  and CWW component  12  experience a phase shift and gain of φ CW , G CW  and φ CCW  and G CCW , respectively. Input light  10  is linearly polarized with polarization direction α. By incorporating different phase shifts and gains for the CW and CCW light components into the analysis of Sagnac loop  100 , the transmission to the input optical signal can be expressed as: 
   
     
       
         
           
             
               
                 
                   
                     
                       T 
                       = 
                         
                       ⁢ 
                       
                         
                           1 
                           4 
                         
                         ⁢ 
                         
                           { 
                           
                             
                               G 
                               CW 
                             
                             + 
                             
                               G 
                               CCW 
                             
                             - 
                             
                               2 
                               ⁢ 
                               
                                 
                                   
                                     G 
                                     CW 
                                   
                                   ⁢ 
                                   
                                     G 
                                     CCW 
                                   
                                 
                               
                               ⁢ 
                               
                                 { 
                                 
                                   [ 
                                   
                                     cos 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         
                                           φ 
                                           CCW 
                                         
                                         - 
                                         
                                           φ 
                                           CW 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                           
                         ⁢ 
                         
                           ( 
                           
                             
                               
                                 cos 
                                 2 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                                 ) 
                               
                             
                             + 
                             
                               cosϕ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   sin 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     2 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     θ 
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                         ] 
                       
                       - 
                     
                   
                 
                 
                   
                     
                       
                           
                         ⁢ 
                         
                           [ 
                           
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     φ 
                                     CCW 
                                   
                                   - 
                                   
                                     φ 
                                     CW 
                                   
                                 
                                 ) 
                               
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                                 ) 
                               
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   α 
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                         } 
                       
                       } 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
     
   
   The transmission can also be expressed as: 
   
     
       
         
           
             
               
                 T 
                 = 
                 
                   
                     1 
                     4 
                   
                   ⁢ 
                   
                     { 
                     
                       
                         G 
                         CW 
                       
                       + 
                       
                         G 
                         CCW 
                       
                       - 
                       
                         2 
                         ⁢ 
                         A 
                         ⁢ 
                         
                           
                             
                               G 
                               CW 
                             
                             ⁢ 
                             
                               G 
                               CCW 
                             
                           
                         
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 φ 
                                 CCW 
                               
                               - 
                               
                                 φ 
                                 CW 
                               
                               + 
                               
                                 φ 
                                 B 
                               
                             
                             ) 
                           
                         
                       
                     
                     } 
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
           
             
               
                 
                   where 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                 
                 = 
                 
                   
                     
                       
                         ( 
                         
                           
                             
                               cos 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                               ) 
                             
                           
                           + 
                           
                             cos 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 sin 
                                 2 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ϕ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 α 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ) 
               
             
           
           
             
               
                 
                   and 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     φ 
                     B 
                   
                 
                 = 
                 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ϕ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 α 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               cos 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 2 
                                 ⁢ 
                                 θ 
                               
                               ) 
                             
                           
                           + 
                           
                             cos 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 sin 
                                 2 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ) 
               
             
           
         
       
     
   
   One of ordinary skill in the art will understand that the Transmission in Equations 1 and 2 above may also be expressed, respectively, as: 
   
     
       
         
           
             
               
                 
                   
                     
                       T 
                       = 
                         
                       ⁢ 
                       
                         
                           1 
                           4 
                         
                         ⁢ 
                         
                           { 
                           
                             
                               G 
                               CW 
                               2 
                             
                             + 
                             
                               G 
                               CCW 
                               2 
                             
                             - 
                             
                               2 
                               ⁢ 
                               
                                 G 
                                 CW 
                               
                               ⁢ 
                               
                                 G 
                                 CCW 
                               
                               ⁢ 
                               
                                 { 
                                 
                                   [ 
                                   
                                     
                                       cos 
                                       ⁡ 
                                       
                                         ( 
                                         
                                           
                                             ϕ 
                                             CCW 
                                           
                                           - 
                                           
                                             ϕ 
                                             CW 
                                           
                                         
                                         ) 
                                       
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         
                                           cos 
                                           2 
                                         
                                         ⁡ 
                                         
                                           ( 
                                           
                                             2 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             θ 
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         
                           
                             = 
                               
                             ⁢ 
                             
                               cos 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ϕ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   sin 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     2 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     θ 
                                   
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                         ] 
                       
                       - 
                     
                   
                 
                 
                   
                     
                       
                           
                         ⁢ 
                         
                           [ 
                           
                             sin 
                             ⁢ 
                             
                               ( 
                               
                                 
                                   ϕ 
                                   CCW 
                                 
                                 - 
                                 
                                   ϕ 
                                   CW 
                                 
                               
                               ) 
                             
                             ⁢ 
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ϕ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                                 ) 
                               
                             
                             ⁢ 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   α 
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                         } 
                       
                       } 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
           
             
               and 
             
             
               
                   
               
             
           
           
             
               
                 
                   
                     
                       T 
                       = 
                         
                       ⁢ 
                       
                         
                           1 
                           4 
                         
                         ⁢ 
                         
                           { 
                           
                             
                               G 
                               CW 
                               1 
                             
                             + 
                             
                               G 
                               CCW 
                               2 
                             
                             - 
                             
                               2 
                               ⁢ 
                               
                                 AG 
                                 CW 
                               
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         
                           G 
                           CCW 
                         
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 ϕ 
                                 CCW 
                               
                               - 
                               
                                 ϕ 
                                 CW 
                               
                               + 
                               
                                 ϕ 
                                 β 
                               
                             
                             ) 
                           
                         
                       
                       } 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
                 ) 
               
             
           
         
       
     
   
   As shown in Equation 4, the phase bias of the interference can be tuned by adjusting the polarization of the input light and the loop birefringence. With a polarization controller, such as polarization controller  103 , embodiments of the present invention can provide full tunability of the phase bias. One of skill in the art will understand that there are various methods to calculate the gain and phase change of light passing through the SOA. Due to the asymmetric position of SOA  101  in Sagnac loop  100 , CW signal  11  reaches SOA  101  earlier than CCW signal  12  by a time delay of τ=2Δ χSOA /c fiber  where Δ χSOA  is the offset of SOA  101  from the center position of loop  100 , and c fiber  is the speed of light in the optical fiber. 
     FIG. 2(   a ) illustrates exemplary input NRZ signal  200 , which has a super Gaussian shape with peak power of 8 mW. In one embodiment of the invention, the following SOA parameters may be used: linewidth enhancement factor=4.0, saturation power=1.0 pJ, small signal gain=20 dB, SOA carrier lifetime=70 ps and the SOA offset from the loop center=20 ps. When a single SOA without being put into the Sagnac loop is used for wavelength conversion, converted signal  201  due to XGM effect is shown in  FIG. 2(   b ). Signal  201  shows large fluctuations of the amplitude level of bit “ 1 ” which shows pattern-dependent effect caused by the slow SOA recovery rate. The problem can be solved by decreasing the SOA recovery time, which may pose difficulty on SOA fabrications. With a Sagnac interferometric structure adapted according to embodiments of the present invention, converted signals  202  and  203  shown in  FIGS. 2(   c ) and ( d ) may be produced. In  FIG. 2(   c ), the phase bias for the interference is set to be zero, which has been the working condition for the applications of optical switching and regeneration.  FIG. 2(   d ) shows converted signal  203  for an optimal phase bias of −1.12, and shows reduced pattern dependent effect and improved extinction ratio compared with  FIG. 2(   b ). 
     FIG. 3  illustrates an exemplary embodiment of the present invention, which can be used for experimental purposes. Because the embodiment is experimental, the term “control” may be used to describe the input NRZ signal before it is converted. Such description merely means that the signal is yet to be converted and does not imply that the signal performs a controlling function with regard to any of the optical components of signal converter  300 . 
   Signal converter  300  includes Sagnac interferometer  301 . SOA  302 , such as an Alcatel  1901  with a working current of 195 mA, for example, is connected to the two output ports of 50:50 optical coupler  303 . Optical tunable delay line (OTDL)  304  can be tuned to finely adjust the offset of SOA  302  from the loop center (graphically illustrated by line  305 ). Polarization controllers PC 1   306  and PC 2   307  are used to adjust the polarization of the light propagating in the CW and CCW directions. A NRZ data signal may be generated by modulating continuous wave light from source  308  with a 10 Gb/s pseudo random bit sequence with length of 2 23 −1 from source  309 . The modulated light output from modulator  320  is amplified using a Erbium-doped fiber amplifier  311  and provided to the control port of loop  301 . Continuous wave light from tunable laser source  312 , such as an HP 8168C, for example, goes into input port  30  of Sagnac loop  301 . Tunable bandpass filter  310  used in this embodiment has a 3 dB bandwidth of about 1 nm. The output of loop  301  is connected to bit error rate tester  313  in this experimental embodiment. Although various parameters and component models are specifically named with regard to  FIG. 3 , it should be understood that one skilled in the art may determine that other parameters and/or component models may be used to produce comparable signal conversion when used with a Sagnac loop interferometer with an asymmetrical SOA, and those alternate embodiments are within the scope of the present invention. 
   In the exemplary embodiment depicted in  FIG. 3 , the mean optical power of the control NRZ data signal  31  (λ 1 =1545 nm) from laser source  308  is 5.3 dBm, and the optical power of the input light  30  (λ 2 =1535 nm) from laser source  312  is −2.7 dBm. The wavelength of light  30  from laser source  312  determines the wavelength of the output data  32 . In this embodiment, polarization controllers PC 1   306  and PC 2   307  are finely adjusted to optimize the wavelength converted signal.  FIG. 4(   a ) is an eye diagram of input NRZ signal  31  in this embodiment.  FIGS. 4(   b ) and ( c ) are eye diagrams of the wavelength converted signal  32  at 1535 nm when the SOA offset from the Sagnac loop center is 23 ps and 60 ps, respectively. As a comparison, wavelength conversion based on direct XGM effect using an SOA is conducted with optimized optical powers of the control data and the continuous wave light. The eye diagram of such converted data signal using XGM effect is shown in  FIG. 4(   d ). 
     FIG. 5(   a ) is an illustration of the data pattern of input NRZ signal  500  used in this embodiment.  FIG. 5(   b ) is an illustration of the data pattern of converted signal  501  using XGM effect, and  FIG. 5(   c ) is an illustration of the data pattern of converted signal  502  using Sagnac interferometric loop  301  with offset SOA  302  (23 ps). Comparing  FIGS. 5(   b ) and ( c ), it is clear that the pattern-dependent effect is alleviated with the Sagnac interferometric loop. When the offset of SOA  302  from loop center  305  is too large, the neighboring bits may interact with each other, causing the converted signal to deteriorate, as shown in  FIG. 4(   c ). Note that the CW and CCW components reach SOA  302  with timing difference of 120 ps with an SOA offset of 60 ps. 
   The results of Bit Error Rate (BER) measurement are shown in  FIG. 6 . The power penalties at 10 −9  are 4.3 dB and 1.4 dB for converted signal  501  (using XGM effect) and for converted signal  502  (using Sagnac interferometric loop  301  of  FIG. 3 ). The large power penalty of converted signal  501  from XGM effect is mainly due to the pattern-dependent effect. The optical power penalty of the converted signals  501  and  502  at different wavelength is shown in  FIG. 7 . Compared with XGM wavelength converters, Sagnac interferometric loop  301  with SOA  302  at an asymmetric position, as shown in  FIG. 3 , may reduce the power penalty by more than 2 dB. 
   Although the system depicted in  FIG. 3  is a test setup, implementations of the invention in practical applications are possible. For instance, in one scenario, the control and output ports of the loop may be connected to a fiber transmission line in a telecommunications system. A continuous wave light (similar to light  30  from laser source  312  in  FIG. 3 ) may then be input into the loop at the input port. As explained earlier, the wavelength of the continuous-wave light determines the wavelength of the output signal. Accordingly, the incoming signal on the fiber transmission line is converted by the interferometric loop and output onto the fiber transmission line to be transmitted to its destination. This conversion may show improved performance with regard to the pattern dependent effect and extinction ratios than conversion performed through XGM effect on the same signal. 
   It is an advantage of some embodiments that they may be adapted for use in optical networks to provide 10 Gb/s and above signal conversion, thereby facilitating the development of present and future communication networks. In fact, various embodiments may be fully integratable with present and future optical networks. 
   In summary, various embodiments using a fiber-based Sagnac interferometric loop with an SOA at 10 Gb/s are illustrated. Theoretical analysis shows that the phase bias of the interference has important influence on the wavelength conversion operation. The converted signals from the Sagnac interferometric loop of  FIGS. 1 and 3  may have improved performance compared with wavelength conversion using XGM effect. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.