Demodulators for optical fiber interferometers with [3.times.3] outputs

• US 5,313,266 A
• Filed: 08/17/1992
• Issued: 05/17/1994
• Est. Priority Date: 08/17/1992
• Status: Expired due to Fees

• Current Assignee: Chief of Naval Research
• Original Assignee: Chief of Naval Research
• Inventors: Keolian, Robert M., Garrett, Steven L., Cameron, Charles B.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US07/931,228
• Time in Patent Office: 638 Days
• Field of Search: 356/345, 356/350, 250/227.19, 250/227.27, 385/12, 385/14
• US Class Current: 356/477
• CPC Class Codes: G01D 5/35329 using interferometer with two arms in transmission, e.g. Mach-Zender interferometer
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125

Abstract

A highly sensitive optical fiber interferometer sensor comprising a laser light source, a [2×2] optical fiber coupler to split the beam in two, a differential transducer which converts a signal of interest into optical phase shift in the laser light transmitted through the two optical fibers in the interferometer and a [3×3] optical fiber complex which recombines the two beams, producing interference which can be electronically detected. The use of the [3×3] coupler permits Passive Homodyne demodulation of the phase-modulated signals provided by the interferometer without feedback control or modulation of the laser itself and without requiring the use of electronics within the interferometer.

Claims

1. A highly sensitive interferometer sensor comprising:

a radiation source means for generating a coherent beam of radiation,

a first coupler means for splitting said beam into two beams,

a differential transducer means for converting a signal of interest into phase shift in said two beams,

a second coupler means for recombining said two beams to produce three phase-modulated signals x, y and z,

a radiation converter means for converting the three phase modulated signals into a set of three electrical signals, a, b, and c,

a symmetric demodulator circuit for converting the set of three electrical signals a, b and c into a single electrical signal ξ

which is proportional to the signal of interest.

2. The sensor of claim 1 wherein said symmetric demodulator circuit includes

modulation stripper means for selecting a component of the electrical signal proportional to the beam intensity which is being modulated by a phase shift the output signals of said modulation stripper being u, v and w, and

circuit means for forming a phase shift signal ξ

which is proportional to ##EQU152## from u, v and w.

3. The sensor of claim 1 wherein said symmetric demodulation circuit includes

a summing and scaling circuit means for summing the three electrical signals a, b, and c from said radiation convertor and scaling the sum by one-third to form a signal d,

three difference amplifier means for taking the difference between each of said signals a, b and c and said signal d to form signals u, v, and w, and

circuit means for forming a phase shift signal ξ

which is proportional to ##EQU153## from u, v and w.

4. The sensor of claim 1 wherein said symmetric demodulation circuit includes

circuit means for forming the signals

5. The sensor of claim 1 wherein said symmetric demodulator circuit includes:

a modulation stripper means for scaling and combining the three phase-modulated electrical signals a, b, and c from said radiation convertor means to form three new signals e,f and g,

a derivative cross multiplier means for generating three electrical signals k,l and m which are proportional to the time derivatives of the electrical signals e,f and g from said modulation stripper means and for generating a signal proportional to the sum of the products of the electrical signal group from said modulation stripper means with differences between the electrical signal of time derivatives,

a modulation depth compensator means for removing the modulation power dependence from the output of the signal from said derivative cross multiplier means, and

an integrating circuit means for generating an electrical signal ξ

which is proportional to the signal of interest from the signal from said modulation depth compensator means.

6. The sensor of claim 1 wherein said symmetric demodulation circuit includes

a summing and scaling circuit means for summing the three first phase-modulated electrical signals and scaling the sum by one-third,

first three difference amplifier means for taking the difference between each of the three first electrical signals and the output of said summing and scaling circuit to produce three second electrical signals,

differentiating circuit means for taking three time derivatives of said three second electrical signals u,v, and w to produce three third electrical signals u,v,w,

second three difference amplifier means for taking the difference signals between all three combinations of pairs of the three second electrical signals to produce three fourth electrical signals (w-v),(u-w),(v-u),

multiplying circuit means for multiplying each of said three fourth electrical signals by one of the three third electrical signals to produce three fifth electrical signals u(w-v),v(u-w),w(v-u),

squaring circuit means for taking the squares of the three second electrical signals to produce three sixth electrical signals u²,v²,w²,

first summing circuit means for summing said three sixth electrical signals to produce a seventh electrical signal u² +v² +w²,

second summing circuit means for summing said three fifth electrical signals to produce an eighth electrical signal u(w-v)+v(u-w)+w(v-u) ,

division circuit means for dividing said eighth electrical signal by said seventh electrical signal to produce a ninth electrical signal ##EQU155## integrating circuit means for producing a tenth electrical signal proportional to phase shift signal by integrating said ninth electrical signal ##EQU156##

7. of claim 1 wherein said symmetric demodulator circuit includes circuit means for forming the signals