Transducer system

1. A transducer for converting an acoustic signal into an electric signal, comprising:

a) a Fabry-Perot (FP) interferometer illuminated by a laser radiation with wavelength λ from a laser, a FP transmittance function being: T=1/(1+F sin 2 (2π·d·n/λ)), where F is a coefficient of finesse, d is a distance between two mirrors, and n is a refractive index of a medium in a space between the mirrors, wherein the medium is a gas or a liquid, wherein:

i. the distance d is constant and is off a resonance distance for a wavelength λ 0 ; the mirrors themselves and the distance between the mirrors being non-movable and non-deformable;

ii. the distance d corresponds to an operation point of the transducer, the operation point being achieved where a derivative of T is at a maximum or at a minimum value and a second derivative of T is equal to zero; and

iii. the space receives an acoustic signal, changing the refractive index n of the medium in the space and thus influencing the laser radiation propagation; and

b) a first laser radiation detector detecting an intensity of an outputted laser radiation influenced by the acoustic signal and generating an electric signal, wherein:

i. the electric signal is split into an AC (alternating current) and a DC (direct current) components by a separator;

ii. the AC component of the electric signal is outputted, and the acoustic signal is recovered;

iii. the DC component enters a feedback loop connected to the laser; and

iv. the DC component controls a drive current of the laser to irradiate wavelength λ being different from wavelength λ 0 , wherein λ corresponds to the operation point of the transducer where the second derivative of T is equal to zero.

2. The transducer of claim 1 , further comprising a photodiode detecting a signal characteristic of the laser, the transducer further including a differential amplifier, with the photodiode connected to supply its signal to the differential amplifier.

3. The transducer of claim 2 , with the differential amplifier chosen from a group consisting of a reference detector and a separate amplifier or preamplifier.

4. The transducer of claim 1 , further comprising an electric power source providing a variable control current for the laser, the first detector being connected to the electric power source for control action.

5. The transducer of claim 1 , wherein the DC component slowly altering or adapting the wavelength of the laser for at least partially compensating for environmental influences on the transmission function T of the interferometer.

6. The transducer of claim 1 , with a differential amplifier having a first input coupled to an output of the first detector and a second input coupled to an output of a second electromagnetic radiation detector integrated in the laser.

7. The transducer of claim 1 , wherein a bias tee separates the electrical signal into the AC and the DC component.

8. The transducer of claim 1 , wherein a current source for the laser performs frequency modulation of the laser radiation.

9. The transducer of claim 1 , wherein the feedback loop is connected to the laser to determine a drive current and uses the drive current as an error signal.

10. A method of converting an acoustic signal into an electric signal, comprising the steps of:

a) generating a laser radiation with wavelength λ from a laser;

b) coupling the laser radiation into a space between two mirrors of a Fabry-Perot (FP) interferometer, a FP transmittance function being: T=1/(1+F sin 2(2π·d·n/λ)), where F is a coefficient of finesse, d is a distance between the mirrors, and n is a refractive index of a medium in a space between the mirrors, wherein the medium is a gas or a liquid;

c) arranging the mirrors and the distance between mirrors to be non-movable and non-deformable;

d) placing mirrors at the distance d, the distance d being off a resonance distance for a wavelength λ 0 ; the distance d corresponding to an operation point of the transducer, wherein the operation point is achieved where a derivative of T is at a maximum or at a minimum value and a second derivative of T is equal to zero;

e) coupling an acoustic signal into the space between the mirrors to influence the laser radiation propagation;

f) detecting an influenced laser radiation with a first electromagnetic radiation detector to generate an electric signal;

g) splitting the electric signal into an AC (alternating current) and a DC (direct current) component;

h) recovering the acoustic signal from the AC component;

i) controlling an electric drive current powering the laser to irradiate wavelength λ being different from wavelength λ 0 , wherein λ corresponds to the operation point of the transducer where the second derivative of T is equal to zero.

11. The method of claim 10 , further comprising: generating a reference signal by converting the laser radiation directly behind the laser into a voltage and forming a difference signal between the electric signal indicative of the acoustic signal and the reference signal.

12. The method of claim 10 , further comprising: modulating a frequency of the laser to compensate for undesired optical feedback.

13. The method of claim 12 , wherein a modulation frequency is above 500 MHz.

14. The method of claim 10 , further comprising using the electric drive current powering the laser as an error signal of the feedback loop.

15. The method of claim 10 , further comprising stabilizing the laser in its radiation frequency using a reference interferometer in front of a reference detector.

16. The method of claim 10 , further comprising a differential signal evaluation, in which a positive as well as a negative edge or a shoulder of a transmission curve are evaluated and a differential signal is generated.

17. The method of claim 10 , further comprising: modulating the wavelength of the laser radiation with a frequency that is significantly different from a highest acoustic wave frequency to be measured.

18. The method of claim 10 , wherein a modulating frequency is above 20 kHz.