Fiber optic frequency shifter

A fiber optic frequency shifter comprising:

• a length of optical fiber having a core and a cladding, a portion of the cladding having been removed from said length of optical fiber to form an interaction region;
• a block of birefringent material adjacent said interaction region so that a light wave propagating in said length of optical fiber has an evanescent field penetrating into said block;
• and
• means for controlling the birefringence of said block to form a rotating birefringence in said length of optical fiber to cause the length of optical fiber to act as a rotating half-wave plate upon light propagating therein.

A fiber optic frequency shifter for shifting an optical input signal having a frequency f.sub.o to produce an optical output signal having frequency f.sub.o .+-.2f, where f is the amount of frequency shift, comprising:

• a length of optical fiber;
• means for forming a rotating half-wave plate in said length of optical fiber;
• means for controlling the rotation rate of said half-wave plate to control the difference between the frequency input to said half-wave plate and the frequency of the signal output from said half-wave plate;
• and
• feedback means for processing the output signal of said half-wave plate to maintain the intensity of components in the output signal having the frequency f.sub.o below a predetermined threshold value.

A method for shifting an input optical signal of a frequency f.sub.o to an output optical frequency f.sub.o .+-.2f, comprising the steps of:

• removing a portion of the cladding from a length of optical fiber to form an interaction region;
• placing a block of birefringent material adjacent the interaction region so that a light wave propagating in the length of optical fiber has an evanescent field extending into said block;
• and
• controlling the birefringence of the block to a rotating birefringence in the length of optical fiber to cause the length of optical fiber to act as a rotating half-wave plate upon light propagating therein.

A method of shifting an input optical frequency of a frequency f.sub.o to an output optical frequency f.sub.o .+-.2f, comprising the steps of:

• selecting a length of optical fiber;
• forming a rotating half-wave plate in the length of optical fiber;
• controlling the rotation rate of the half-wave plate to control the amount of the frequency shift;
• and
• processing the output signal of the half-wave plate to maintain the intensity of components in the optical output signal having the frequency f.sub.o below a predetermined threshold value.