Fiber-optic measurement device, gyrometer, and navigation and inertial-stabilization system

1. A fiber-optic measurement device ( 10 ) of the type in which a parameter to be measured generates a phase difference Δφ p between two counter-propagating waves ( 24 , 25 ), including:

a light source ( 11 ),

a fiber-optic SAGNAC ring interferometer ( 20 ), preferably single-mode, including a coil ( 21 ) and a splitting element ( 23 ), in which said two counter-propagating waves ( 24 , 25 ) propagate, said ring interferometer ( 20 ) having a proper frequency f p ,

an electromagnetic radiation detector ( 14 ), receiving the luminous power ( 37 ) exiting from said ring interferometer ( 20 ) and delivering a modulated electrical signal ( 38 ) representative of the luminous power ( 37 ), which is function of the total phase difference Δφ t between said two counter-propagating waves ( 24 , 25 ) at the output of said ring interferometer ( 20 ),

a modulation chain ( 30 ) adapted to modulate said luminous power ( 37 ) exiting from said ring interferometer ( 20 ), said modulation chain ( 30 ) including:

at least one digital/analog converter ( 31 ) adapted to process a digital control signal to deliver an analog control signal,

an amplifier ( 32 ) adapted to process said analog control signal to deliver a modulation control voltage V m (t),

at least one phase modulator ( 33 ) placed in said interferometer ( 20 ), which, when subjected at the input to said modulation control voltage V m (t), is adapted to generate at the output a phase-shift modulation φ m (t), said phase-shift modulation φ m (t) introducing between said two counter-propagating waves ( 24 , 25 ) a phase-difference modulation Δφ m (t) such that: Δφ m (t)=φ m (t)−φ m (t−Δτ g ), Δτ g =1/(2f p ) being the transit-time difference between said two counter-propagating waves ( 24 , 25 ) determined between said phase modulator ( 33 ) and said splitting element ( 23 ), and

signal processing means ( 110 ) including:

an analog/digital converter ( 111 ) digitizing said modulated electrical signal ( 38 ) received from the detector ( 14 ) and representative of said power received to deliver a digital electrical signal, and

a digital processing unit ( 112 ) adapted to process said digital electrical signal to deliver a signal function of said phase difference Δφ p and of said parameter to be measured,

feedback means ( 120 ) adapted to process said signal function of said phase difference A to generate a feedback signal,

biasing means ( 130 ) adapted to generate a biasing signal,

means ( 140 ) for controlling said modulation chain ( 30 ), adapted to process said feedback signal and said biasing signal to deliver said digital control signal at the input of said modulation chain ( 30 ), such that said modulation control voltage V m (t) at the input of said phase modulator ( 33 ) is the sum of a feedback modulation voltage V cr (t) and a biasing modulation voltage V b (t), said phase modulator ( 33 ) being adapted, when it is subjected to said feedback modulation voltage V cr (t), to generate a feedback phase-shift modulation φ cr (t) ( 121 ), said feedback phase-shift modulation φ cr (t) ( 121 ) introducing a feedback phase-difference modulation Δφ cr between said two counter-propagating waves ( 24 , 25 ) allowing to keep at zero the sum of said phase difference Δφ p and said feedback phase-difference modulation Δφ cr , and

means ( 150 ) for controlling the gain of said modulation chain ( 30 ) allowing to keep adjusted the transfer function of said modulation chain ( 30 ),

said fiber-optic measurement device ( 10 ) being characterized in that said biasing means ( 130 ) are adapted to generate said biasing signal such that said phase modulator ( 33 ) generates a biasing phase-shift modulation φ b (t), when it is subjected to said biasing modulation voltage V b (t), said biasing phase-shift modulation φ b (t) being the sum of:

a first biasing phase-shift modulation component φ b1 (t) ( 34 A), introducing a first biasing phase-difference modulation component Δφ b1 (t) ( 34 ) between said two counter-propagating waves ( 24 , 25 ), said first biasing phase-difference modulation component Δφ b1 (t) ( 34 ) being a square-wave periodic modulation, of levels +π and −π, at a first biasing modulation frequency f b1 such that f b1 =(2k 1 +1) f p , k 1 being a natural number and f p said proper frequency, and

a second biasing phase-shift modulation component φ b2 (t) ( 35 A), introducing a second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) between said two counter-propagating waves ( 24 , 25 ), said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) being a periodic modulation, of extreme amplitudes +π/a and −π/a, a being a non-zero real number such that |a|>1, at a second biasing modulation frequency f b2 such that f b2 =(2k 2 +1) f p , k 2 being a non-zero natural number such that k 2 >k 1 , and f p said proper frequency.

2. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein said first biasing phase-difference modulation component Δφ b1 (t) ( 34 ) has a duty factor of 50%.

3. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) has a duty factor of 50%.

4. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein k 1 =0.

5. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein k 2 >2.

6. The fiber-optic measurement device ( 10 ) according to claim 5 , wherein k 2 >4.

7. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) is a square-wave modulation.

8. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) is a sine-wave modulation.

9. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein:

said second biasing modulation frequency f b2 =(2k 2 +1) f p is such that f b2 =(2k 21 +1) f b1 , k 21 being a non-zero natural integer, and f b1 =(2k 1 +1) f p being the first biasing modulation frequency, and

said first biasing phase-difference modulation component Δφ b1 (t) ( 34 ) and said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) are in phase quadrature.

10. The fiber-optic measurement device ( 10 ) according to claim 1 , wherein said feedback phase-shift modulation φ cr (t) ( 121 ) is a stair-step modulation.

11. The fiber-optic measurement device ( 10 ) according to claim 10 , wherein said feedback phase-shift modulation φ cr (t) ( 121 ) and said first biasing phase-difference modulation component φ b1 (t) ( 34 A) are synchronous, each stair-step of said feedback phase-shift modulation φ cr (t) ( 121 ) having a duration Δτ g and said first biasing phase-shift modulation component φ b1 (t) ( 34 A) being a first biasing modulation frequency f b1 such that f b1 =f p , f p being the proper frequency.

12. The fiber-optic measurement device according to claim 1 , wherein said feedback phase-shift modulation φ cr (t) falls down by 2π when it exceeds 2π.

13. The fiber-optic measurement device according to claim 1 , wherein:

said digital processing unit ( 112 ) demodulates said digital electrical signal in phase with said second biasing phase-difference modulation component Δφ b2 (t) ( 35 ) independently of the first biasing phase-difference modulation component Δφ b1 (t) ( 34 ), and

said means ( 150 ) for controlling the gain of said modulation chain ( 30 ) demodulate said digital electrical signal so as to provide a signal function of the transfer function of said modulation chain ( 30 ).

14. A gyrometer, characterized in that it is compliant with the measurement device ( 10 ) of claim 1 , the parameter to be measured being a component of the rotational speed of the ring interferometer ( 20 ).