Acousto-optic fiber-optic frequency shifter using periodic contact with a surface acoustic wave

Claim 1

An apparatus for shifting the optical frequency of an optical signal from a first optical frequency to a second optical frequency, comprising: an optical fiber having first and second propagation paths for propagation of said optical signal therein, said first propagation path having a first optical propagation constant k 1 at said first optical frequency, said second propagation path having a second optical propagation constant k 2 at said second optical frequency; means for generating an acoustic wave having an acoustic frequency equal to the difference between said first optical frequency and said second optical frequency; acoustic propagation means for propagating said acoustic wave in proximity to said optical fiber, said acoustic propagation means having an acoustic propagation constant k a ; and means for positioning said optical fiber in acoustic contact with said acoustic propagation means at periodic locations along said fiber so that said acoustic wave stresses said optical fiber at said periodic locations to cause optical energy to be coupled from said first optical propagation path to said second optical propagation path and to be shifted in frequency by an amount equal to the acoustic frequency, said periodic locations spaced apart from each other by a distance Λ g and having an effective propagation constant k g equal to 2π/Λ g , said effective propagation constant k g having a value such that the difference between said acoustic propagation constant k a and said effective propagation constant k g is substantially equal to the difference between said first optical propagation constant k 1 an said second optical propagation constant k 2 .

Claim 2

An apparatus for shifting the optical frequency of an optical signal from a first optical frequency to a second optical frequency, comprising: an optical fiber having first and second propagation paths for propagation of said optical signal therein, said first propagation path having a first optical propagation constant k 1 at said first optical frequency, said second propagation path having a second optical propagation constant k 2 at said second optical frequency; means for generating an acoustic wave having an acoustic frequency equal to the difference between said first optical frequency and said second optical frequency; acoustic propagation means for propagating said acoustic wave in proximity to said optical fiber, said acoustic propagation means having an acoustic propagation constant k a ; and means for positioning said optical fiber in acoustic contact with said acoustic propagation means at periodic locations along said fiber so that said acoustic wave stresses said optical fiber at said periodic locations to cause optical energy to be coupled from said first optical propagation path to said second optical propagation path and to be shifted in frequency by an amount equal to the acoustic frequency, said periodic locations spaced apart from each other by a distance Λ g and having an effective propagation constant k g equal to 2π/Λ g , said effective propagation constant k g having a value such that the difference between said acoustic propagation constant k a and said effective propagation constant k g is substantially equal to the difference between said first optical propagation constant k 1 an said second optical propagation constant k 2 .

Claim 5

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths; means for generating acoustic energy; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, said applying means comprising a substrate having a surface for propagating said acoustic energy, said surface formed to provide an elongated narrow waveguide which interfaces with said fiber at said plural locations to provide acoustic contact therebetween, the width of said waveguide being not substantially larger than that of the optical fiber to tightly confine the acoustic energy to the interface between the waveguide and the fiber.

Claim 6

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths; means for generating acoustic energy; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, said applying means comprising a substrate having a surface for propagating said acoustic energy, said surface formed to provide an elongated narrow waveguide which interfaces with said fiber at said plural locations to provide acoustic contact therebetween, the width of said waveguide being not substantially larger than that of the optical fiber to tightly confine the acoustic energy to the interface between the waveguide and the fiber.

Claim 7

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths, said first optical propagation path having a first optical propagation constant k 1 , said second optical propagation path having a second optical propagation constant k 2 ; means for generating acoustic energy, said acoustic energy having an acoustic propagation constant k a ; and means for applying said acoustic energy to propagate along said fiber said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, said plural locations being spaced apart from each other by a distance Λ g selected so that said plural locations have an effective static propagation constant k g defined by; k.sub.g =2π/Λ.sub.g and wherein the difference between said first and second propagation constants, k 1 and k 2 , is substantially equal to the difference between said acoustic propagation constant k a and said effective static propagation constant k g .

Claim 8

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths, said first optical propagation path having a first optical propagation constant k 1 , said second optical propagation path having a second optical propagation constant k 2 ; means for generating acoustic energy, said acoustic energy having an acoustic propagation constant k a ; and means for applying said acoustic energy to propagate along said fiber said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, said plural locations being spaced apart from each other by a distance Λ g selected so that said plural locations have an effective static propagation constant k g defined by; k.sub.g =2π/Λ.sub.g and wherein the difference between said first and second propagation constants, k 1 and k 2 , is substantially equal to the difference between said acoustic propagation constant k a and said effective static propagation constant k g .

Claim 9

An apparatus for shifting the frequency of an optical signal between first optical frequency and a second optical frequency, comprising: a optical fiber having a longitudinal axis, and first and second optical propagation paths; means for generating acoustic energy; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propogation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, the distance between corresponding portions of said plural locations being defined as a spatial period Λ g , wherein said acoustic energy has a wavelength of Λ a along said fiber, and wherein said optical signal traveling in said fiber has a beat length L B , said distance selected so that said spatial period Λ g is defined by the following relationship: ##EQU9##

Claim 10

An apparatus for shifting the frequency of an optical signal between first optical frequency and a second optical frequency, comprising: a optical fiber having a longitudinal axis, and first and second optical propagation paths; means for generating acoustic energy; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propogation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency, the distance between corresponding portions of said plural locations being defined as a spatial period Λ g , wherein said acoustic energy has a wavelength of Λ a along said fiber, and wherein said optical signal traveling in said fiber has a beat length L B , said distance selected so that said spatial period Λ g is defined by the following relationship: ##EQU9##

Claim 11

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths; a device for generating acoustic energy, having a wavelength less than the beat length of an optical signal propagating along the two propagation paths of the optical fiber; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency.

Claim 12

An apparatus for shifting the frequency of an optical signal between a first optical frequency and a second optical frequency, comprising: an optical fiber having a longitudinal axis, and first and second optical propagation paths; a device for generating acoustic energy, having a wavelength less than the beat length of an optical signal propagating along the two propagation paths of the optical fiber; and means for applying said acoustic energy to propagate along said fiber, said applying means in acoustic contact with said fiber to cause at least a portion of the applied acoustic energy to be transferred to said fiber to stress said fiber, said applying means including means for concentrating the transferred acoustic energy at plural locations, spaced along said fiber, so that said acoustic energy stresses said fiber preferentially at each of said plural locations, said propagation of said acoustic energy causing light at said first frequency to be coupled at each of said plural locations from said first optical path to said second optical path and shifted to said second frequency.

Claim 24

An apparatus for shifting the optical frequency of an optical signal from a first optical frequency to a second optical frequency, comprising: an optical waveguide having first and second propagation paths for propagation of optical energy therein, said first propagation path having a first optical propagation constant k 1 at said first optical frequency, said second propagation path having a second optical propagation constant k 2 at said second optical frequency; an acoustic generator coupled to apply acoustic energy to said waveguide such that an acoustic wave propagates in accordance with an acoustic propagation constant k a , said acoustic energy interacting with said optical energy at plural locations along said waveguide such that optical energy is coupled from said first optical propagation path to said second optical propagation path substantially exclusively at said locations, said locations being spaced apart from each other by a distance Λg and having an effective propagation constant k g equal to 2π/Λ g , said effective propagation constant k g having a value such that the difference between said acoustic propagation constant k a and said effective propagation constant k g is substantially equal to the difference between said first optical propagation constant k 1 and said second optical propagation constant k 2 .

Claim 25

An apparatus for shifting the optical frequency of an optical signal from a first optical frequency to a second optical frequency, comprising: an optical waveguide having first and second propagation paths for propagation of optical energy therein, said first propagation path having a first optical propagation constant k 1 at said first optical frequency, said second propagation path having a second optical propagation constant k 2 at said second optical frequency; an acoustic generator coupled to apply acoustic energy to said waveguide such that an acoustic wave propagates in accordance with an acoustic propagation constant k a , said acoustic energy interacting with said optical energy at plural locations along said waveguide such that optical energy is coupled from said first optical propagation path to said second optical propagation path substantially exclusively at said locations, said locations being spaced apart from each other by a distance Λg and having an effective propagation constant k g equal to 2π/Λ g , said effective propagation constant k g having a value such that the difference between said acoustic propagation constant k a and said effective propagation constant k g is substantially equal to the difference between said first optical propagation constant k 1 and said second optical propagation constant k 2 .

Claim 26

A method of manufacturing a frequency shifter comprising an optical fiber having two propagation modes, and a substrate having a surface for propagating acoustic energy, said method comprising: mounting said fiber on said surface such that at least a component of said acoustic energy propagates along the length of said fiber; holding said fiber in acoustic contact with said surface such that said acoustic energy is preferentially applied to acoustically contact said fiber at spaced locations along said fiber, and is preferentially inhibited from acoustically contacting said fiber between said locations; and mounting an acoustic transducer to provide said acoustic energy at an acoustic wavelength which is significantly less than the beat length of said fiber for said modes.

Claim 27

A method of manufacturing a frequency shifter comprising an optical fiber having two propagation modes, and a substrate having a surface for propagating acoustic energy, said method comprising: mounting said fiber on said surface such that at least a component of said acoustic energy propagates along the length of said fiber; holding said fiber in acoustic contact with said surface such that said acoustic energy is preferentially applied to acoustically contact said fiber at spaced locations along said fiber, and is preferentially inhibited from acoustically contacting said fiber between said locations; and mounting an acoustic transducer to provide said acoustic energy at an acoustic wavelength which is significantly less than the beat length of said fiber for said modes.

Claim 40

A method of frequency shifting, comprising: generating acoustic energy: applying said acoustic energy to an optical fiber having first and second propagation modes such that at least a component of the acoustic energy propagates down the fiber; utilizing said acoustic energy to couple light between said modes over an interaction length; selecting an acoustic wavelength for said acoustic energy which is less than the beat length for said two modes to provide a power transfer characteristic for said interaction length which varies periodically such that said interaction length comprises forward coupling regions in which light is transferred from said first mode to said second mode and back coupling regions in which light is transferred from said second mode to said first mode; and inhibiting coupling in said back coupling regions to cause said coupling in said forward coupling regions to cumulatively couple light from said first mode to said second mode.

Claim 41

A method of frequency shifting, comprising: generating acoustic energy: applying said acoustic energy to an optical fiber having first and second propagation modes such that at least a component of the acoustic energy propagates down the fiber; utilizing said acoustic energy to couple light between said modes over an interaction length; selecting an acoustic wavelength for said acoustic energy which is less than the beat length for said two modes to provide a power transfer characteristic for said interaction length which varies periodically such that said interaction length comprises forward coupling regions in which light is transferred from said first mode to said second mode and back coupling regions in which light is transferred from said second mode to said first mode; and inhibiting coupling in said back coupling regions to cause said coupling in said forward coupling regions to cumulatively couple light from said first mode to said second mode.