Method and apparatus for maintaining incident optical signal having low degree of polarization in specific state of polarization

Abstract

An optical circuit includes a light source having a low degree of polarization, an input section connected to the light source, and an output section connected to the input section. The input section has an optical waveguide incapable of maintaining a state of polarization. The output section has at least one optical waveguide having no branches. The optical waveguide having no branches has, in at least part thereof, an optical waveguide capable of maintaining a state of polarization.

What is claimed is:  
     
         1 . A method of maintaining an incident optical signal having a low degree of polarization in a specific state of polarization at an output terminal of an optical waveguide, characterized by comprising the steps of: 
 inputting the optical signal having a low degree of polarization to an input section having as an input terminal an optical waveguide incapable of maintaining a state of polarization; and    outputting an optical signal having the specific state of polarization to an output terminal of a polarized wave output section through said polarized wave output section having, in at least part thereof, an optical waveguide capable of maintaining a state of polarization.    
     
     
         2 . A method according to  claim 1 , wherein said optical waveguide of said polarized wave output section includes no branch.  
     
     
         3 . A method according to  claim 1 , wherein the optical signal is an optical signal output from an edge-emitting diode.  
     
     
         4 . A method according to  claim 1 , wherein the optical signal has a degree of polarization of not more than 30%.  
     
     
         5 . A method according to  claim 1 , wherein said optical waveguide capable of maintaining a state of polarization is one fiber selected from the group consisting of a PANDA fiber (Polarization maintaining and absorption reducing fiber), an optical fiber having increased structural axial asymmetry, and an elliptical jacket fiber.  
     
     
         6 . A method according to  claim 1 , wherein said input section is directly connected to said polarized wave output section.  
     
     
         7 . A method according to  claim 1 , wherein said input section is connected to said polarized wave output section through an optical component incapable of controlling a state of polarization.  
     
     
         8 . A method according to  claim 1 , further comprising the step of controlling state-of-polarization control means at an input of said optical waveguide capable of maintaining a state of polarization to change another specific state of polarization from one specific state of polarization.  
     
     
         9 . A method according to  claim 8 , wherein said state-of-polarization control means controls a state of polarization on a longitude of a Poincaré sphere.  
     
     
         10 . An optical circuit characterized by comprising: 
 a light source having a low degree of polarization;    an input section connected to said light source; and    a polarized wave output section connected to said input section, said input section having an optical waveguide incapable of maintaining a state of polarization, said polarized wave output section having at least one optical waveguide with no branches, and said optical waveguide with no branches having, in at least part thereof, an optical waveguide capable of maintaining a state of polarization.    
     
     
         11 . A circuit according to  claim 10 , further comprising state-of-polarization control means arranged at an input of said optical waveguide capable of maintaining a state of polarization.  
     
     
         12 . A circuit according to  claim 10 , wherein said light source comprises an edge-emitting diode.  
     
     
         13 . A circuit according to  claim 10 , wherein said light source has a degree of polarization of not more than 30%.  
     
     
         14 . A circuit according to  claim 10 , wherein said optical waveguide capable of maintaining a state of polarization is a PANDA fiber, and said optical waveguide incapable of maintaining a state of polarization is a single-mode optical fiber.  
     
     
         15 . A circuit according to  claim 10 , wherein said input section further comprises an optical multiplexer/demultiplexer, and a first optical waveguide connecting said light source and said optical multiplexer/demultiplexer, said polarized wave output section comprises an output terminal connecting a test sample, the other output terminal to an optical system, an optical detector, and a second optical waveguide connecting said optical detector and said optical multiplexer/demultiplexer, 
 said polarized wave output section further comprises a third optical waveguide connecting said optical system and said optical multiplexer/demultiplexer, and a fourth optical waveguide connecting said optical multiplexer/demultiplexer and a test sample,    said light source comprises a light source having a low degree of polarization, said first and second optical waveguides comprise optical waveguides incapable of maintaining a state of polarization,    said third optical waveguide comprises an optical waveguide incapable of maintaining a state of polarization and an optical waveguide capable of maintaining a state of polarization, and    said fourth optical waveguide comprises an optical waveguide incapable of maintaining a state of polarization and an optical waveguide capable of maintaining a state of polarization, whereby constituting a Michelson optical circuit.    
     
     
         16 . A circuit according to  claim 15 , further comprising arithmetic means for calculating a return loss of the test sample on the basis of an output from said optical detector, and means for displaying the return loss calculated by said arithmetic means.  
     
     
         17 . A circuit according to  claim 10 , wherein said polarized wave output section comprises a connection terminal to an optical system, a terminal connecting the test sample, a multiplexer/demultiplexer, a third optical waveguide connecting said optical system and said optical multiplexer/demultiplexer, a fourth optical waveguide connecting said terminal connecting a test sample and said optical multiplexer/demultiplexer, an optical detector, and a second optical waveguide connecting second optical detector and said optical multiplexer/demultiplexer, said input section comprises a first optical waveguide connecting said light source and said optical multiplexer/demultiplexer, said light source comprises a light source having a low degree of polarization, and said first optical waveguide comprises a first portion incapable of maintaining a state of polarization, 
 said first portion is connected to said optical multiplexer/demultiplexer by a second portion capable of maintaining a state of polarization, and said second, third, and fourth optical waveguides comprise optical waveguides capable of maintaining a state of polarization to constitute a Michelson optical circuit.    
     
     
         18 . A circuit according to  claim 15 , wherein said optical waveguides capable of maintaining a state of polarization are formed at portions of said third and fourth optical waveguides, which are susceptible to disturbances.