EB--electron beam; R--risitance; T--substrate temperature;

P--deposition pressure.

Table 2. Calculated bulk and interface absorption of singlelayer films from measured data at λ =6328A.

о

Table 3. Calculated, Z, and A/A (q,p) of the in

vestigated TiO2/Si02 coatings in comparison with

Manuscript Received
1-15-89

Measurement of Weak Absorption in Optical Coatings
by Means of Photothermal Deflection Technique

Wu Zhouling Tang Jinfa Shi Baixun
Zhejiang University, Hangzhou, PRC

An experimental setup has been built up to measure weak absorption in optical coatings based on transverse and collinear photothermal optical beam deflection technique (TPOBD and CPOBD). To caliberate the setup, a C film is used as the caliberating sample and an approperiate approximation is made based on the fact that optical coatings are usually thermally thin when the modulation frequency is relatively low. The sensitivity of weak absorption measurement of this experimental setup is assessed to -6 be 10-5 for TPOBD and 10 for CPOBD, with the He-Ne pump power being about 100 and the He-Ne probe beam about 2w. Some experimental results are given in this paper. The

mw

absorption losses of SiO2 ZrO2 and MgF2 single-layers are found to be 1.73x104, 5.29 x10-4 and 5.94x10 by TPOBD and 1.82x10-4, 5.70x104 and 6.27x104 by CPOBD. These

by TPOBD and 1.82x10

results are in good agreement with those measured by laser-calorimeter-method.

Key Words: absorption; optical coating; photothermal deflection.

1. Introduction

There is more or less absorption in all dielectric materials. Laser-induced damage of optical coatings is ofen attributed to the weak absorption existing in the films. It is essential to precisely measure weak absorption in dielectric thin films to improve the deposition process and raise the laser damage resistance. Generally, the absorption of high quality opti

cal thin films is of the 10-4-10-5 order of magnitude. photothermal deflection technique [1,2]

is a suitable one for measuring such weak absorption.

In this paper we report our recent measurements of weak absorption in optical thin films by an experimental setup based on photothermal deflection technique. The experimental results, which showed a good agreement with those measured by laser-calorimeter-method [3], approved the technique quite versatile and very simple.

2. Apparatus and Principles

TPOBD and CPOBD are two kinds of photothermal deflection technique. Their geometry is shown in figure 1. The physical basis of it can briefly described as follows: when a solid sample is illuminated by an intensity-modulated laser beam, the absorption of the energy will cause thermal waves in the sample and media adjacent to the sample, and hence coressponding index-of-refraction gradient in these media. By probing this index-of-refraction gradient with a second laser beam, one can relate its deflection to the optical absorption of the sample.

Based on TPOBD and CPOBD, we have built up an apparatus for the measurement of optical absorption of optical thin films. Details of the detection system are given in figure 2.

To caliberate the apparatus, we employ thermally-thin-sample approxiamation [2] and C film caliberating sample [4,5]. The absorptance of the measured sample is then given by

A = (S/S)A

where S---OBD amplitude signal of the sample;

S-OBD amplitude signal of the caliberating C film;

C

A--absorptance of the caliberating C film measured with high precision and accuracy by photometrical method [6].

3. Experimental Results and Discussion

Some single layer- and multilayer coatings were measured by both TPOBD and CPOBD. The results and their comparison with those by laser-calorimeter-method are summaried in table 1. Obviously, the results obtained by the three methods agree reasonably well.

Sources of error in our experiments include thermally-thin-approximation, measurement error of caliberating C film and precision of the apparatus. Although accurate analysis of these errors is not ready now, a primary evaluation of them is summarized in table 2, where the precision of the apparatus is derived from ten measurements of ZrO2 single layers.

The sensitivity of an experimental system is determined by its total noise level. In our

experiment, measurements of ZrO2 single layer (A--104) showed S <=1uv while SCPOBD=0.4V, <=1UV while STPOBD

uv

noise

and measurements of Au single layer (A--30%) showed S =70mV. From this noise 6 results, the sensitivity of the experimental system is assessed to be A--10 for CPOBD and A for TPOBD. This is satisfying for most optical coatings.

--10-5

The authors wish to acknoledge Prof. J.L.Zhou and Mr. K.Hu for their fruitful discussions.

4. References

[1] Jackson, W.B.; Amer, N.M.; Boccara, A.C.; Fournier, D. Photothermal deflection spectroscopy anddetection. Appl.Opt. 20(8):1333;1981.

[2] Murphy, J.C.; Aamodt, L.C. Photothermal spectroscopy using optical beam probing: mirage effct J. Appl. Phys. 51(9):4580; 1980.

[3] Jin S.Z.; Tang J.F. Measurement of weak absorption in optical thin films. Appl. Opt. 26(7): 2407;1987.

[4] Wu Z.L.; Tang J.F.; Shi B.X. measurement of weak absorption in optical coatings by transverse photothermal deflection technique. Accepted by Optica Acta Sinica.

[5] Wu Z.L.; Tang J.F.; Shi B.X. Measurement of weak absorption in optical coatings by collinear photothermal deflection technique. Accepted by Acta Optica Sinica.

[6] Deutsch, T.F. J. Opt. Soc. Am. 4(3):663; 1975.