[3] Wu, Z. L., Tang, J.F.; Shi, B. X. Measurement of weak absorption in optical coatings by transvers、 phototherowal deflection technique. Accepted by Optica Acta Sinica (Chinese).

[4] Temple, P. A. Experimental and theoretical considerations in thin-film laser calorimetry. Opt. Eng. 23(2), 325;1984.

[5] Walther, H.G.; Welsch E. Calculation and measurement of the absorption in multilayer films by means of photoacoustics. Thin Solid Films 142,27;1986.

[6] Bennet, H. E. Simple expressions for predicting the effect of volume and interface absorption and scattering in highrefrectance or antireflectance multilayer coatings. J. Opt. Soc. Am. 70(3), 268,1980.

[7] Roche, P. Antiscattering tranparent monolayers, theory and experiment. J. Opt. Soc. Am. A. 1(10), 1032;1984.

Figure 1. Sample configuration showing a thin wedge of material which has been deposited on the entrance surface of the substrate. The bare region to the left is used to measure A and the step in the foreground is used to determine the film thickness profile.

S

Table 1. Deposition methods and related charactrizations of the samples investigated(:6328A, substrate: Kg).

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.

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 -5 -6 be 10 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

-4

-4

-4

-4

mw

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