Passive homodyne demodulator and sensor system

• US 4,609,290 A
• Filed: 06/06/1983
• Issued: 09/02/1986
• Est. Priority Date: 06/06/1983
• Status: Expired due to Term

• Current Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Original Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Inventors: Cahill, Richard F.
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/501,628
• Time in Patent Office: 1,184 Days
• Field of Search: 356/345, 356/351, 250/227, 367/149
• US Class Current: 356/477
• CPC Class Codes: G01D 5/344 using polarisation G01D5/35303 takes precedence
• 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 passive device which allows processing of signals impressed upon a light beam and arranged so that sensitivity is maximized and signal dropout, due to environmental effects such as temperature, do not occur. The passive homodyne demodulator includes a beam combiner, for combining the impressed light beam with a reference beam, beamsplitters, polarizing beamsplitters, and birefringent elements used as polarization shifters, so that ouputs are produced which are the sine and cosine of the phase change between the sensor and reference beams due to changes in the effective optical path length difference of the reference and sensor fibers due to the effect sensed. These signals are then electronically converted into an electrical output which linearly represents the phase of the signal originally impressed on the sensor fiber.

Claims

1. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, said means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam including;

means to produce said reference light beam with a polarization defined as vertical and said sensor input light beam with a polarization defined as horizontal, said means to mix said reference light beam and said sensor output light beam including;

optical coupler means having;

a reference input connected to receive said reference light beam;

a sensor input connected to receive said sensor output light beam;

a first output out through which passes a first mixed beam having a vertical reference light beam component and a horizontal sensor output light beam component; and

a second output out through which passes a second mixed beam having a vertical reference light beam component and a horizontal sensor output light beam component, and wherein said means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

also include;

means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams, said first, second, third, and fourth demodulated light beams, having amplitudes of K+sin φ

, K-sin φ

, K+cos φ

, and K-cos φ

respectively where K is a constant having a magnitude of at least 1;

first polarized beamsplitter means which separate said mix of said reference light beam and said sensor output light beam into first and second predetermined component beams thereof;

second polarized beamsplitter means which separate said first predetermined component beams into third and fourth predetermined component beams;

means to optically orient said second polarized beamsplitter means at 45°

with respect to said first polarized beamsplitter means;

third polarized beamsplitter means which separate said second predetermined component beams into fifth and sixth predetermined component beams, said third predetermined component beams;

means to optically orient said third polarized beamsplitter means at 45°

with respect to said first polarized beamsplitter means;

means to combine said third predetermined component beams into said first demodulated light beam;

means to combine said fourth predetermined component beams into said second demodulated light beam;

means to combine said fifth predetermined component beams into said third demodulated light beam; and

means to combine said sixth predetermined component beams into said fourth demodulated light beam.

2. The sensor system as defined in claim 1 wherein said means to mix said reference light beam and said sensor output light beam include:

an optical coupler having;

a reference input connected to receive said reference light beam;

a sensor input connected to receive said sensor output light beam;

a first output out through which passes a first mixed reference and sensor output light beam; and

a second output out through which passes a second mixed reference and sensor output light beam, said first and second mixed reference and sensor output light beams being optically coupled to said means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

.

3. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, wherein said means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

also include;

means to demodulate said mix of reference light beam and said sensor output light beam into at least a second demodulated light beam having an amplitude which is a function of cos φ

;

first polarized beamsplitter means which separate said mix of said reference light beam and said sensor output light beam into first and second predetermined component beams thereof;

second polarized beamsplitter means which separate said first predetermined component beams into third and fourth predetermined component beams;

third polarized beamsplitter means which separate said second predetermined component beams into fifth and sixth predetermined component beams;

means to combine said third predetermined component beams into said first demodulated light beam; and

means to combine said fifth predetermined component beams into said second demodulated light beam.

4. The sensor system as defined in claim 3 wherein said second polarized beamsplitter means are optically oriented at 45°

with respect to said first polarized beamsplitter means and said third polarized beamsplitter means are optically orientated at 45°

with respect to said first polarized beamsplitter means.

5. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, said means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

also including;

means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams, said first, second, third, and fourth demodulated light beams, having amplitudes of K+sin φ

, K-sin φ

, K+cos φ

, and K-cos φ

respectively where K is a constant having a magnitude of at least 1 including;

first polarized beamsplitter means which separate said mix of said reference light beam and said sensor output light beam into first and second predetermined component beams thereof;

second polarized beamsplitter means which separate said first predetermined component beams into third and fourth predetermined component beams, said second polarized beamsplitter means being optically oriented at 45°

with respect to said first polarized beamsplitter means;

third polarized beamsplitter means which separate said second predetermined component beams into fifth and sixth predetermined component beams, said third polarized beamsplitter means being optically oriented at 45°

with respect to said first polarized beamsplitter means;

means to combine said third predetermined component beams into said first demodulated light beam;

means to combine said fourth predetermined component beams into said second demodulated light beam;

means to combine said fifth predetermined component beams into said third demodulated light beam; and

means to combine said sixth predetermined component beams into said fourth demodulated light beam.

6. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

including;

first, second, third, and fourth detectors for converting light signals into electrical signals;

a second beamsplitter positioned to split said reference light beam into first and second reference light beams;

a third beamsplitter positioned to split said sensor output light beam into first and second sensor output light beams;

a fourth beamsplitter positioned to split said first reference light beam and said first sensor output light beam into third and fourth reference light beam and third and fourth sensor output light beams, directing said third reference light beam and said third sensor output light beam onto said first detector and said fourth reference light beam and said fourth sensor output light beam onto said second detector;

a phase shifter positioned to shift the phase of said second reference light beam 90°

;

a fifth beamsplitter positioned to split said phase shifted second reference light beam and said second sensor output light beam into fifth and sixth reference light beam and fifth and sixth sensor output light beams, directing said fifth reference light beam and said fifth sensor output light beam onto said third detector and said sixth reference light beam and said sixth sensor output light beam onto said fourth detector.

7. The sensor system as defined in claim 6 wherein said second, third, fourth, and fifth beamsplitters are fiber optic couplers.

8. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, wherein said means to mix said reference light beam and said sensor output light beam include;

means to linearize the polarizations of said sensor output light beam and said reference light beam;

a second beamsplitter block having first, second, third, and fourth sides, and a vertical axis, said vertical axis being parallel to said second and fourth sides;

means to couple said sensor output light beam into said second beamsplitter block first side at 45°

to said vertical axis thereof; and

means to couple said reference light beam into said second beamsplitter block second side at 45°

to said vertical axis thereof and an average 90°

out of phase from said sensor output light beam, said second beamsplitter producing a first mixed beam at said third side and a second mixed beam at said fourth side, and wherein said means to demodulate said mix of said reference light beam and said sensor output light beam include;

a third beamsplitter positioned receive said first mixed beam from said second beamsplitter and to split said first mixed beam into said first demodulated light beam and a second demodulated light beam having an amplitude of K-sin φ

where K is a constant having a magnitude of at least 1; and

a fourth beamsplitter positioned receive said second mixed beam from said second beamsplitter and to split said second mixed beam into a third demodulated light beam having an amplitude of K+cos φ and

a fourth demodulated light beam having an amplitude of K=cos φ

.

9. The sensor system as defined in claim 8 wherein said means to demodulate said mix of said reference light beam and said sensor output light beam include:

first, second, third, and fourth detectors for converting light signals into electrical signals, said third and fourth beamsplitters being block beamsplitters having;

first, second, and third sides, said first side of said third and fourth beamsplitters being mounted to said third and fourth sides of said second beamsplitter respectively, said first detector being mounted to said second side of said third beamsplitter, said second detector being mounted to said third side of said third beamsplitter, said third detector being mounted to said second side of said fourth beamsplitter, said fourth detector being mounted to said third side of said fourth beamsplitter.

10. The sensor system as defined in claim 8 wherein said means to demodulate said mix of said reference light beam and said sensor output light beam include:

first, second, third, and fourth detectors for converting light signals into electrical signals, said third and fourth beamsplitters being block beamsplitters having;

first, second, and third sides, said first side of said third and fourth beamsplitters being mounted to said third and fourth sides of said second beamsplitter respectively;

first optical fiber means to couple said first detector to said second side of said third beamsplitter;

second optical fiber means to couple said second detector to said third side of said third beamsplitter;

third optical fiber means to couple said third detector to said second side of said fourth beamsplitter; and

fourth optical fiber means to couple said fourth detector to said third side of said fourth beamsplitter.

11. The sensor system as defined in claim 8 wherein said third and fourth beamsplitters are block beamsplitters having first, second, and third sides, said means to demodulate said mix of said reference light beam and said sensor output light beam include:

first optical fiber means to couple said second beamsplitter to said first side of said third beamsplitter;

second optical fiber means to couple said second beamsplitter to said first side of said fourth beamsplitter;

first, second, third, and fourth detectors for converting light signals into electrical signals, said first detector being mounted to said second side of said third beamsplitter, said second detector being mounted to said third side of said third beamsplitter, said third detector being mounted to said second side of said fourth beamsplitter, said fourth detector being mounted to said third side of said fourth beamsplitter.

12. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, wherein said means to mix said reference light beam and said sensor output light beam include;

means to linearize the polarizations of said sensor output light beam and said reference light beam;

a quarter wave plate having a first and an opposite second side;

means to couple said reference light beam to said quarter wave plate first side; and

a transparent plate having a first side and an opposite second side, said opposite second side of said transparent plate having a partially reflective coating thereon, said opposite second sides of said quarter wave plate and said transparent plate being positioned together whereby said quarter wave plate and said transparent plate produce a first mixed beam out of said first side of said quarterwave plate and a second mixed beam out of said first side of said transparent plate, and wherein said means to demodulate said mix of said reference light beam and said sensor output light beam include;

a second beamsplitter positioned receive said first mixed beam from said quarter wave plate and to split said first mixed beam into said first demodulated light beam and a second demodulated light beam having an amplitude of K-sin φ

where K is a constant having a magnitude of at least 1; and

a third beamsplitter positioned receive said second mixed beam from said transparent plate and to split said second mixed beam into a third demodulated light beam having an amplitude of K+cos φ and

a fourth demodulated light beam having an amplitude of K-cos φ

.

13. The sensor system as defined in claim 12 wherein said second and third beamsplitters are block beamsplitters having first, second, and third sides, said means to demodulate said mix of said reference light beam and said sensor output light beam further include:

first optical fiber means to couple said first side of said quarter wave plate to said first side of said second beamsplitter;

second optical fiber means to couple said first side of said transparent plate to said first side of said third beamsplitter;

first, second, third, and fourth detectors for converting light signals into electrical signals, said first detector being mounted to said second side of said second beamsplitter, said second detector being mounted to said third side of said second beamsplitter, said third detector being mounted to said second side of said third beamsplitter, said fourth detector being mounted to said third side of said third beamsplitter.

14. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam;

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

including;

means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams, said first, second, third, and fourth demodulated light beams, having amplitudes of K+sin φ

, K-sin φ

, K+cos φ

, and K-cos φ

respectively where K is a constant having a magnitude of at least 1; and

a first detector positioned to receive said at least first demodulated light beam to convert said amplitude into a first electrical signal which is K-sin φ

, and said means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams including;

a second detector positioned to receive said second demodulated light beam to convert said amplitude into a second electrical signal which is K-sin φ

;

a third detector positioned to receive said third demodulated light beam to convert said amplitude into a third electrical signal which is K+cos φ

; and

a fourth detector positioned to receive said fourth demodulated light beam to convert said amplitude into a fourth electrical signal which is K-cos φ

; and

processing means to transform said first, second, third, and fourth electrical signals into a fifth electrical signal representative of φ

, said processing means including;

first means to combine said first and second electrical signals into an output electrical signal representative of sin φ

;

second means to combine said third and fourth electrical signals into an output electrical signal representative of cos φ

;

first differentiator means connected to differentiate said electrical signal representative of sin φ

to produce an electrical signal, φ

cos φ

;

second differentiator means connected to differentiate said electrical signal representative of cos φ

to produce an electrical signal, -φ

sin φ

;

first multiplier means to multiply said output signals of said first means to combine and said second differentiator means together into an output electrical signal representative of -φ

sin² φ

;

second multiplier means to multiply said output signals of said second means to combine and said first differentiator means together into an output electrical signal representative of -φ

cos² φ

;

third combiner means connected to said output electrical signals representative of -φ

sin² φ and

φ

cos² φ

to produce therefrom an output electrical signal, φ

; and

integrator means connected to said output electrical signal, φ

, to produce therefrom φ

.

15. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam;

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

including;

means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams, said first, second, third, and fourth demodulated light beams, having amplitudes of K+sin φ

, K-sin φ

, K+cos φ

, and K-cos φ

respectively where K is a constant having a magnitude of at least 1; and

a first detector positioned to receive said at least first demodulated light beam to convert said amplitude into a first electrical signal which is K-sin φ

, and said means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams including;

a second detector positioned to receive said second demodulated light beam to convert said amplitude into a second electrical signal which is K-sin φ

;

a third detector positioned to receive said third demodulated light beam to convert said amplitude into a third electrical signal which is K+cos φ

; and

a fourth detector positioned to receive said fourth demodulated light beam to convert said amplitude into a fourth electrical signal which is K-cos φ

; and

processing means to transform said first, second, third, and fourth electrical signals into a fifth electrical signal representative of φ

, said processing means including;

first means to combine said first and second electrical signals into an output electrical signal representative of sin φ

;

second means to combine said third and fourth electrical signals into an output electrical signal representative of cos φ

;

first low pass filter means connected to said electrical signal representative of sin φ

to produce a first low pass filter electrical signal output;

second low pass filter means connected to said electrical signal representative of cos φ

to produce a second low pass filter electrical signal output;

first multiplier means to multiply said output signals of said first means to combine and said second low pass filter means together into a first multiplier means output electrical signal;

second multiplier means to multiply said output signals of said second means to combine and said first low pass filter means together into a second multiplier means output electrical signal;

third combiner means connected to said output electrical signals of said first and second multipliers to produce therefrom a third combiner means output electrical signal; and

first band pass filter means connected to said third combiner means output electrical signal to produce therefrom φ

when φ

is much less than 1.

16. A sensor system using light as the sensing medium including:

a light source which produces a light beam of predetermined coherence;

first beamsplitter means which split said light beam of predetermined coherence into a reference light beam and a sensor input light beam;

means to impress the effect to be sensed on said sensor input light beam in the form of relative phase changes, φ

, thereof with respect to said reference light beam to generate a sensor output light beam;

means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam;

means to mix said reference light beam and said sensor output light beam; and

means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

, said means to produce a relative linear polarization difference of 90°

between said reference light beam and said sensor input light beam including;

means to produce said reference light beam with a polarization defined as vertical and said sensor input light beam with a polarization defined as horizontal, said means to mix said reference light beam and said sensor output light beam including;

optical coupler means having;

a reference input connected to receive said reference light beam;

a sensor input connected to receive said sensor output light beam;

a first output through which passes a first mixed beam having a vertical reference light beam component and a horizontal sensor output light beam component; and

a second output out through which passes a second mixed beam having a vertical reference light beam component and a horizontal sensor output light beam component, and wherein said means to demodulate said mix of said reference light beam and said sensor output light beam into at least a first demodulated light beam having an amplitude which is a function of sin φ

also include;

means to demodulate said mix of reference light beam and said sensor output light beam into second, third, and fourth demodulated light beams, said first, second, third, and fourth demodulated light beams, having amplitudes of K+sin φ

, K-sin φ

, K+cos φ

, and K-cos φ

respectively where K is a constant having a magnitude of at least 1;

first polarized beamsplitter means which separate said said mix of said reference light beam and said sensor output light beam into first and second predetermined component beams traveling in different directions;

reflecting means positioned to intercept said second predetermined component beams and to direct them parallel to said first predetermined component beams;

second polarized beamsplitter means which separate said first and second predetermined component beams into third, fourth, fifth, and sixth predetermined component beams;

means to optically orientate said second polarized beamsplitter means at 45°

with respect to said first polarized beamsplitter means;

means to combine said third predetermined component beams into said first demodulated light beam;

means to combine said fourth predetermined component beams into said second demodulated light beam;

means to combine said fifth predetermined component beams into said third demodulated light beam; and

means to combine said sixth predetermined component beams into said fourth demodulated light beam.