GB2237382A

GB2237382A - Photothermal deflection spectroscopy

Info

Publication number: GB2237382A
Application number: GB9020322A
Authority: GB
Inventors: Enrico Masetti; Marco Montecchi
Original assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Comitato Nazionale per la Ricerca e per lo Sviluppo dell Energia
Current assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA
Priority date: 1989-09-26
Filing date: 1990-09-18
Publication date: 1991-05-01
Anticipated expiration: 2010-09-18
Also published as: IT1231979B; FR2652414A1; IT8948405A0; DE4030315A1; FR2652414B1; GB2237382B; GB9020322D0

Abstract

An optoelectronic device for measuring the optical absorption of a beam of radiation (107) in thin layer materials (104) by photothermal deflection spectroscopy, comprises a laser source (100), an optical beam splitter (103) and a double position sensor (106). One of the beams (102) produced by the beam splitter passes through the region adjacent the zone heated by the beam (107). The refractive index of the medium in which the layer (104) is immersed is affected locally by the heating and causes beam (102) to be deflected across the sensor (106). Deflection of the other beam (101), which is not affected by the heating caused by beam (107), is used to compensate for vibration and for instability of the laser (100). <IMAGE>

Description

4.

k 17:2 C-11 -::> OPTOELECTRONIC DEVICES The present invention relates to a device for measuring the optical absorption in thin layer materials by using the photothermal deflection spectroscopy. More particularly this invention relates to an optoelectronic device for reducing the effects associated to the instability in the direction of the analyzing beam due to inherent phenomenons of the laser and to mechanical vibrations in a measuring instrument called "Photothermal Deflection Spectroscopy" (P.D.S.).

The measurement of the optical absorption of thin covering for optical systems has become even more noticeable for the last years due to the increasing spread of the optical devices and in particular of the laser sources in the industrial processes.

A measuring system which has been successful, even though it is not free of drawbacks, is the above mentioned P.D.S. system which allows the energy absorbed by a probe and transformed into heat to be measured. Even though such system allows very important results to be obtained, it encounters a serious limitation in trying to deal with measurements on an industrial scale and not only in research laboratories because of the remarkable incidence of the noise on the reliability of the measurements.

g Thus the measurement is strongly influenced by the noise because of the high sensitivity of the sensor, and then it is necessary to use a phase lock analyzer operating at a preset modulation frequency in order to derive the "real" signal, as better seen afterwards.

The noise has several causes:

a) instability of the sighting of the analyzing laser beam; b) instability of the power of the laser beam; c) turbulence of liquids and gases crossed by the probe; d) mechanical vibrations of several components; e) electronic circuit noise.

In practice the following arrangements are provided to mitigate such causes. As for:

a) and b): using lasers of high quality; c): intubating, where possible, the analyzing laser beam in hollow cylinders (better vacuum cylinders) or cylinders which can be filled with transparent materials such as plexiglas, glass or other suitable material; d): providing anti-torsion mounting and antivibration optical benches; e): using instruments of high quality.

Many of such arrangements, in particular those of item d), require a considerable financial burden and moreover increase remarkably weight and size of P.D.S. apparatus.

It should be noted that a good anti-vibration bench should have a complex pneumatic suspension system and a high inertial mass, i.e. a heavy weight. Obviously this is a limit to the spreading of the technique.

It has now been found possible to reduce the effects caused by the mechanical vibrations d) and the instability of the analyzing laser beam a) at very low cost by providing an optical system for doubling the analyzing laser beam, two position sensors and a suitable electronic circuit are required.

By means of the present invention there is provided an optoelectronic device which is part of an absorption measuring system using P.D.S. and allows both the noise generated by the mechanical vibrations and the noise due to the direction instability of the analyzing laser beams to be strongly reduced, and then the sensitivity of the apparatus to be increased.

According to the present invention there is provided an optoelectronic device for measuring the optical absorption in thin layer materials using the photothermal deflection spectroscopy, which comprises a laser source, an optical doubler between the laser 1 - 4 source and a probe, and a double position sensor.

The optical doubler may comprise a beam divider separating an input beam in two beams of similar intensity having directions orthogonal to each other, and a prism having a total internal reflexion, the prism being placed downstream of the divider. A cylindrical covering,lens may be placed between the optical doubler and the probe. The two beams from the divider may be a measurement beam crossing the probe at the area illuminated by a pump beam, and a control beam parallel to the measurement beam and independent thereof. It is possible that the reciprocal positions and orientations of the measurement beam and the control beam are provided by positioning the total reflexion prism. The cylindrical converging lens can focus both the measurement beam and the control beam in the area adjacent to the probe to be measured.

There may be a double sensor divided in four sectors forming two couples which are evenly illuminated by the measurement beam and the control beam under non-absorption conditions. It is possible that the four photocurrents which are generated by the four sectors of the sensor are conveyed to an electronic circuit, the output of which is a linear combination thereof with alternate signs so as to eliminate from the measurement the effects due to the - 5 instability of the laser beam and the mechanical vibrations.

All of the optical doubling and focusing means of the analyzing beam may be assembled so as to be integral with one another. It is possible that the doubling_assembly making the laser beams parallel to each other is capable of being applied to analyzing apparatus both of "transversal" and "collinear" type.

A real advantage achieved by the present invention is that the P.D.S. apparatus does not need to be placed on anti-vibration tables, thus providing a wider spread in the industrial field. Another advantage is the very low cost.

The invention will be now described with reference to and as illustrated in, but in no manner limited to, the accompanying drawings, in which:

Fig. 1 shows schematically the operating principle of a transversal P.D.S. apparatus; Fig. 2 shows the operating principle of a collinear P.D.S. apparatus; Fig. 3 shows the diagram of a two-sector position sensor; Fig. 4 shows the operation of the position sensor with a laser beam; Fig. 5 shows the block diagram of the electronic circuit associated to the position sensor; 1 Fig. 6 shows the application of the invention to a transversal P.D.S. system (the application to a collinear P.D.S. system is similar); Fig. 7 shows the block diagram of the electronic circuit associated to the position sensor in the foursector arrangement.

The heat transmission from the probe to the environmental medium (generally a transparent fluid) causes in the latter a gradient of the refractive index able to deviate a laser beam crossing the medium. The amount of such a deviation is proportional to the absorbed electromagnetic radiation. The proportionality constant is provided by measuring under the same conditions the deviation relative to a probe having a known absorption. Absorptions up to 10-6 can be presently measured by P.D.S. techniques. In Figs. 1 and 2 the diagrams of a "transversal" and a 1.1collinear" P.D.S. apparatus for measuring probes under the form of thin layers are shown, respectively. The electromagnetic radiation of pump P from either a lamp followed by a monochromator or a laser beam hits probe C in the analysis field. The analyzing beam from a laser L (tipically a He-Ne laser of a few milliwatts power) is focused by a converging lens LC so that near the probe the transversal dimension of the beam is lower than that of the beam of pump P. In the arrangement of Fig. 1 the focusing further allows the distance between the analyzing laser beam and the probe to be reduced with consequent increase of the sensitivity (the amplitude of the thermal wave generated by the heating of the probe is being reduced in the exponential form while moving away from the surface). To increase the deviation the medium in which the probe is immersed should have a high gradient of the refractive index as a function of temperature. Generally the position sensor SP is formed of a pair of adjacent, opposed photodiodes. In Fig. 3 a two-sector detector is shown; it is centrally placed with respect to the laser spot (without absorption) and is oriented so that the axis of the photodiodes is almost parallel to the plane in which the deviated beams from the analyzing beam due to the absorption are lying (Fig. 4).

The two photocurrents are transformed into voltages by a suitable electric circuit (Fig. 5) supplying an output proportional to their difference. For small values of angle "all of Fig. 4 the output signal of the circuit of Fig. 5 is modulated at the same frequency as that of modulation of the electromagnetic pump radiation, and the value thereof is proportional to the absorption of the probe.

In Fig. 6 the device according to the invention is schematically shown. The analyzing beam 100 generated by a laser source is doubled in a measurement beam 102 and a control beam 101 by means of an optical system. consisting of an optical dividing head 1.03, which divides the incoming beam in two beams of the same intensity having directions orthogonal to each other, and a prism providing a total internal reflexion and allowing, once suitably positioned, the. measurement beam to be disposed parallel to the control beam at the desired mutual distance.

Both beams 101 and 102 are focused in a plane perpendicular to the surface of the probe 104 by a converging cylindrical lens 105 and transverse in parallel to each other the distance be:tween the lens and the double position sensor 106 which in the illustrated embodiment has four quadrants. However,, only measurement beam 102 passes through the area of the probe illuminated by the pump beam 107. The measurement beam and the control beam hit the sensor so as to evenly illuminate (without absorption) each a couple of sectors.

Let it be assumed that the couple 1,2 is illuminated by the measurement beam and the couple 3,4 by the control beam. The four photocurrents thus generated are conveyed to an electronic circuit the 1 -2 j j! 1 1 output of which is a linear combination thereof with alternate signs; V = alil + a2i2 + a3i3 + a4i4 In Fig. 7 the block diagram of an embodiment of the above mentioned electronic circuit is shown. If am and ac are the deviations of the measurement beam and the control beam, respectively, it will result in case of small deviations (am and ac << R/L): V(am, ac) = ImDl2/2(SlTl(l+ am/aOm) - S2T2(lam/aOm) + IcD34/2(S3T3(l-ac/a6c) - S4T4(l + ac/aOc) dV/dam = ImDl2/2((SlTl + S2T2)aOm) dV/dac = -IcD34/2((S3T3 + S4T4)/aOc) where Im and Ic are the intensities of the measurement beam and the control beam, respectively. D12 and D34 are the gains of the differential means; Sj (j-1... 4) are the photovoltaic conversion factors, and Tj (jI.. . 4) are the transductance factors. By varying D12 and D34 or Tj it is possible to make dV/dam = -dV/dac, in which case the same deviation in the measurement beam and in the control beam produces signals of the sam module but of opposite signs, and then V(am, ac) remains unchanged. The heating of the probe causes the deflection of the measurement beam but not that of the control beam, and the various noise sources (except for the electronic source) affect both beams.

1 - 10 If the influence has.the same intensity, frequency and phase, the value of V(am(t), ac (t)) does not vary. This is essentially the case of the instability in the sighting of the analyzing laser beam and of the mechanical vibrationsf if the measurement beam and the control beam hit the centre of the respective couples of photodiodes and if the optical doubling and focusing means of the probe have been mounted integral with one another.

From the foregoing it is evident that the device according to the invention is adapted to make the effects on the measurement due to the noise negligible, in particular those relative to the instability of the direction of the laser beam and to the mechanical vibration, with respect to the effects due to other noise sources. As already mentioned in the introduction, this object is achieved without using a laser having a high sighting of the beam and an expensive, cumbersome, optical anti-vibration bench.

R 1

Claims

- 11 CLAIMS

1. An optoelectronic device for measuring the optical absorption in thin layer materials using the photothermal deflection spectroscopy, which comprises a laser source, an optical doubler between the laser source and a probe, and a double position sensor.

2. A device as claimed in claim 1, in which the optical doubler comprises a beam divider separating an input beam in two beams of similar intensity having directions orthogonal to each other, and a prism having a total internal reflexion, the prism being placed downstream of the divider.

3. A device as claimed in claims 1 or 2, in which a cylindrical converging leis is placed between the optical doubler and the probe.

4. A device as claimed in claims 2 or 3, in which the two beams from the divider are a measurement beam crossing the probe at the area illuminated by a pump beam, and a control beam parallel to the measurement beam and independent thereof.

5. A device as claimed in claim 4, in which the reciprocal positions and orientations of the measurement beam and the control beam are provided by positioning the total reflexion prism.

6. A device as claimed in claims 4 or 5, in which a cylindrical converging lens focuses both the - 12 measurement beam and the control beam in the area adjacent to the probe to be measured.

7. A device as claimed in any of claims 4 to 6, in which there is a double sensor divided in four sectors forming two couples which are evenly illuminated by the measurement beam and the control beam under nonabsorption conditions.

8. A device as claimed in claim 7, in which four photocurrents which are generated by the four sectors of the sensor are conveyed to an electronic circuit, the output of which is a linear combination thereof with alternate signs so as to eliminate from the measurement the effects due to the instability of the laser beam and the mechanical vibrations.

9. A device as claimed in any of claims 1 to 8, in which all of optical doubling and focusing means of an analyzing beam generated by the laser source are assembled so as to be integral with one another.

10. A device as-claimed in claim 9, in which in the doubling assembly making the laser beams parallel to each other is capable of being applied to analyzing apparatus both of "transversal" and "collinear" type.

11. A device for measuring the optical absorption in thin layer materials using the photothermal deflection spectroscopy, substantially as hereinbefore particularly described and illustrated in any of t 1 v 1 v Figures of the accompanying drawings.

Published 1991 atIbe Patent Office. State House. 66/71 High Holborn. IA)ndonWCIR41?. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point, Cwmfelinfach. Cross Keys, Newport NP1 7HZ. Printed by Multiplex techniques lid. St Mary Cr-ay, Kent.