Figure 1: Set up for the production of layers with arbitrary index profiles. The subsystem consists of the microprocessor board (μ), the D/A converter board (D/A), the control panel (B) and the rate

measurement system (M).

Figure 2: Index of refraction as a function of layer thickness for a gradual interface between SiO2 and HfO2. The horizontal scale indicates the layer thickness in units of geometrical thickness.

SiO2 produced by codeposition. Circles indicate index of refraction data calculated with the aid of an AES-profile of the sample.

Figure 4:

TiO2

Transmission spectra of high reflecting coatings of and SiO2. The dashed curve corresponds to the gradual system. EXP. indicates the experimental curve.

111

139

Figure 5:

Damage site on a halfwave layer system of HfO2/SiO2. The first layer is totally ablated.

abrupt

Figure

6:

POSITION (nm)

Calculated temperature distribution in the first layer pair of a qw stack of HfO2/SiO2 for 1.064 μm. The calculation was carried out with a laser pulse duration of 15 ns, an input energy of 30 mJ and a spot diameter of 300 μm. The horizontal scale indicates the position in units of geometrical thickness.

Figure 7:

POSITION

Simulated packing density in a layer system consisting of a dense layer (Pgr, particle diameter 1.5, mobility value 1.3) and layer with lower packing density (Pr particle diameter 1.2, mobility value 1.1). A density of 100 % corresponds to the closest packed structure. P, is the sum of Pr and Pr.

Figure 8: Simulated packing density of two layers with properties equal to figure 7. The interface between the layers is codeposited. It consists of 6000 particles which are concentrated according to a linear alteration of the deposition rate in the interface region.

Manuscript Received

1-17-89

Measurements of Pulse Damage Thresholds
of AR Coated CdTe at 10.6μm

J. G. Grimm, R. S. Eng, C. Freed, N. W. Harris

Massachusetts Institute of Technology
Lincoln Laboratory

P.O. Box 73

Lexington, MA 02173-0073

R. G. O'Donnell

Ford Aerospace Corp., Lexington, MA 02173

Laser induced damage thresholds (LIDT) measurement results on AR-coated single and polycrystalline CdTe samples using 35μs flat top pulses from a CO2 laser MOPA system are reported. Single-shot LIDT's are in excess of 50 J/cm2. The LIDT's for cumulative pulses in the 50k shots regime and pulse repetition frequency in the 1-5Hz range have been measured. The LIDT has been found to be dependent approximately on the square root of the pulse width. The problem of protecting ARcoatings from aqueous solution has also been investigated.

Key Words: anti-reflection coatings; cadmium telluride; CO2 laser MOPA; laser induced damage threshold; pulse repetition frequency effect.

1. Introduction

The laser induced damage thresholds of a number of semiconductor materials transparent in the IR spectral region have been measured and reported in the open literature. Despite this there are still many unanswered questions on the dependence of laser induced damage threshold (LIDT) of anti-reflection coatings on pulse length, pulse repetition frequency (prf), and cumulative number of shots. We would like to report our method of generating nearly flat-top medium energy CO2 laser pulses of large width and our measurements of the damage thresholds of a number of AR-coated CdTe samples using these wide laser pulses. Single-shot laser induced damage thresholds of AR coated samples exceeding 50J/cm2 have been observed. We have investigated prf and cumulative laser pulse effects on LIDT. Some of the samples have been tested successfully for many thousand laser shots without any observable surface damage.

2. Experimental Setup

There are three major components for the damage test measurement setup, namely, the laser beam from a laser master oscillator-power amplifier (MOPA) system, the damage test sample holder, and the damage monitoring instruments. The CO2 laser beam used in the damage testing station basically comes out of a small laser MOPA system. The output pulse width can be adjusted from 5μs to 80us prior to any measurements. For the 35μs wide pulse, in particular, the available output energy is about 230mJ. Mode matching this output pulse to a spot size (1/e2 diameter) of about 0.79mm produces a peak energy density of over 90J/cm2 at the center of the damage test housing for LIDT testing of CdTe crystals (or other infrared optical components). The pulse energy is approximately proportional to the pulse width. Based on our previous work, this level of energy should be sufficient for LIDT measurements of AR-coated crystals available at this time.

The main components of the CO2 laser MOPA system are shown in a block diagram in figure 1. A lowpressure hybrid TE CO2 laser, consisting of a cw dc-discharge gain cell in series with a custom made commercial pulsed-discharge gain cell (Pulse Systems, Inc. Model Dual LP-30 laser gain cell) sharing the same stable resonator configuration laser cavity,2 produces a smooth single TEMq00 mode, 80mJ pulsed This work was sponsored by the Department of the Navy for SDIO under contract F19628-85-C-0002. "The views expressed are those of the author and do not reflect the official policy or position of the U.S. Government."