The objective of the above-mentioned invention is to present a new type of microphone-called optical microphone. The operation of the microphone is baaed on the de¬ flection of a He-Ne laser beam on to a filmy reflector m the shape of an extended drop. This reflector is attached by one of its extremities to the camera. The other extremi¬ ty, which remains unattached, is situated at approximately 0.1 mm from a duct that leads to the chamber from which ori¬ ginates the pressure wave. This chamber can be either the mouthpiece of a common microphone (audio) , or the chamber which contains the sample m a conventional photoacoustic camera. As is well known, the photoacoustic camera is the principal part of the photoacoustic spectrometer and gauges of radiation which have essentially a photoacoustic effect 1,2. The fundamental component of a photoacoustic camera is the transducer which in the final analysis converts the oπ- gmal signals due to the absorption of electromagnetic ra¬ diation through the sample into eletric signals. The trans¬ ducer more commonly used in photoacoustic cameras are capa- citive and electret microphones.

Recently optic methods for the detection of photo- acoustic signals have been proposed as an alternative to the method employing conventional microphones. The present invention consists of a ^N new method for the detection of photoacoustic signals.

Among the principal applications of the microphone whose patent priority is being reαueate , one of them refers to systems for the detection of photoacoustic spectrometers and gauges of electromagnetic radiation. These two types of equipment are already available commercially.

The optical microphone substitutes with advantage the capacitive and electret microphones in experimental si¬ tuations where the photoacoustic camera has to be placed in toxic, corrosive or humid environment in fields of ionizing radiation. In such cases the conventional microphones function inadequately and may suffer irreversible damage.

TECHNICAL STAGE - Various researches have proposed optical methods for the detection of photoacoustic signals. CHOI and DIEBOLD proposed a method in which a He-Ne laser bean strikes a reflector diaphragm in a Helmholtz resonant photoacoustic camera. The pressure waves which originate from the periodic absorption of the modulated light through the sample, modulate the movement on the surface of the dia¬ phragm so that the intensity of the reflected beam varies in the same modulation frequency as that of the excitation beam. This variation is measured through an iris-photodiode mon¬ tage.

CHUANG and ZARE, with an equipment similar to that of Choi and Diebold, used a position detector for measuring the reflection of the laser beam on the diaphragm.

More recently, PARK and DIEBOLD discribed an inter- ferometric microphone in which a Fabri-Perot interferometer was used .

ET

The present invention is similar to those mentioned in references 3, 4, and 5 only in what concerns the utiliza ^¬ tion of laser beam, which is reflected and then detect¬ ed by a photodiode. The differences are as follows: D A conventional photoacoustic camera for solids is used, instead of a Helmholtz resonant photoacoustic one.

2) Instead of a diaphragm, a thin film in the shape of an extended drop is used to reflect the laser beam.

3) The camera is not sealed, since the duct which carries the pressure waves is open at the extremity where the film is unattached, at an approximate distance of 0.1 mm. DETAILED TECHNICAL DESCRIPTION - The camera was made of aluminium, in the shape of a prism. The film of the op¬ tical microphone was made of a reflective mylar film 25 mm thick. It has the shape of an extended drop 13.0 mm long, 2.5mm at the wider extremity, nd 1.5mm at the narrower 15one, which latter is glued to a support so that other extre¬ mity is at an approximate distance of 0.1. mm, over the duct of the photoacoustic camera. In the drawings which accompany this report, fig. 1 shows from four angles details of the photoacoustic camera and the film of the optical microphone. In A we have a side view with 1 showing the film. In B, a front view with 2 showing the quartz window, 3 the duct, and 4 the camera which contains the sample. In C, a top view in which 5 shows the window support, 3 the duct, and 1 the film. In D, a perspe¬ ctive view in which 5 shows the quarts window support, 1 the film, and 6, the laser beam.

Fig. 2 shows a schematic diagram of the experimental arrangement used to make the measurements and tests with the optical microphone. In this figure, 7 represents the He-Ne laser, 8 the modulator, 9 the light source, 10 the photo- acoustic camera, 1 the reflective film, 11 the position de¬ tector, 12 the chamber, sealed to isolate all turbulence from the air outside, 13 the wires which conduct the elec¬ tric signals coming from the position detector, 14 the lock- in amplifier, and 15 the whire which conducts the electric signal of reference coming from the modulator. The light source used for the tests was a 200 halogenic lamp of tungsten whose luminous beam was modulated by a mechanical chopper (PAR, model 192) . The proof beam is a He-Ne laser of 2mW of luminous power. The laser beam, after having been reflected from the film, strikes the position detec¬ tor which is composed of 2 silicon photodiodes placed 0.1 mm apart.

The distance between the reflective film and the position detector was approximately 4 cm. The luminous intensity converted into electric sig¬ nals by the photodiodes was amplified by a PAR differential amplifier (model 116} , and processed by a PAR lock-in am¬ plifier, model 124-A.

The proof laser, the photoacoustic camera and the position detector were placed in a closed receptacle so as to minimize the air turbulence.

One of the most interesting characteristic of the optical microphone is that it makes possible the adjusting

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of its resonance frequency. This can be done by varying, for instance, the length of the reflective film. This cha¬ racteristic is especially important when the microphone is used in a photoacoustic camera. In tests made with a film 13 cm long, a maximum Bignal of 12 mV was obtained at a mo¬ dulation frequency of Hz. When shorter (about 7 cm) film was used, the maximum signal obtained was at 58 Hz.

In the tests that have been made, the optical micro¬ phone showed linear response to the excitation beam power. This measuring was done with the excitation power varying

-4 -z from 1.6 x 10 U to 5.4 x 10 W. The responsiveness was of 0.204 V/W at 17 Hz. Also for this modulation frequency, the minimum detectable power - the signal/noise relation

-6 being equal to 1 - was of 2.5 x 10 W. To use the microphone in conjunction with audio it suffices to open the quartz window which isolates the sam¬ ple chamber in the photoacousti camera.

The results obtained can be greatly improved with the development of better tecniques - that of cutting and that of positioning the reflective film. By modifying the duct end so that it would have the same shape as that of the film, which then could be placed inside it, its sensi- tivmess could be greatly improved. This can be done with the use of a laser cutting-machine. Films of the same shape, but narrower, would also improve the results. The major cause of the noises was the fact that it has not been possible to use an antivibration table on which to place the optical instruments. If these resources are available

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the results will be greatly improved.

It is important to emphasise that the optical micro¬ phone described above may be used with a Helmholtz resonant photo-acoustic camera. In this case the optical microphone will produce a signal of high amplitude when its resonance frequency coincides with the resonance frequency of the Helmholtz camera.

CONCLUSIONS - The optical microphone has the follow¬ ing main characteristics: 1. It presents linear response in conjunction with the power of incident radiation.

2. It possesses a resonance frequency which is the function of the reflective film dimensions.

3. It makes possible the utilization of photo- acoustic cameras in humid, toxic, and corrosive environ¬ ments, as well as in fields of ionizing radiation.

4. It can be used as a conventional microphone in audio equipment.

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