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Manuscript Received 1-17-89

Laser Damage of YAG Surfaces

Ted McMinn

McDonnell Douglas Astronautics Co.
Missle and Defense Electronics Div.
St. Louis, Mo. 63166

and

Steve Seitel, Mark Babb
Montana Laser Optics, Inc.
Bozeman, Mt. 59772

A variety of uncoated and coated YAG substrates have been laser damaged at normal and non-normal incidence; as well as total internal reflection geometries. Testing was done at 1064 nm, with

~11 nsec pulses. Both polished and fractured uncoated surfaces
were tested. Typical polished surfaces damaged in the ~10-12
J/cm2 range, while the cleanest fractured surfaces survived
fluences > 90 J/cm2. An assortment of different normal and non-
normal incidence AR coatings was also tested. The correlation
between the damage threshold and the following items is examined:
high index materials in the coating design (mainly TiO2), electric
field profile within the coating, and ion assisted deposition. The AR
coating thresholds ranged from 1-19 J/cm2, with the better
coatings being consistently over 12 J/cm2. Prism samples were
fabricated to test YAG surfaces in total internal reflection.

Key words: coatings; fracture; lon assited deposition; Nd:YAG; total internal reflection; uncoated, YAG

1.0 Introduction

YAG is one of the most important crystalline host materials used for solid state lasers. As laser diode technology has matured over the past decade, diode pumped YAG lasers have become capable of efficient, high peak power operation. In all high power laser systems laser damage is a key concern. High efficiency extraction of stored energy into the laser mode results in optical energy fluences several times the saturation fluence. Improving the damage threshold therefore can directly impact the potential efficiency of the laser system.

In addition to the standard types of normal incidence AR coatings needed on the YAG crystal, certain face pumped, total internal reflection (TIR), slab laser geometries also require special coatings. These special coatings include dual polarization AR coatings for non-normal incidence YAG surfaces, as well as AR and isolation coatings for the TIR surfaces. In this report, we give the results of laser damage testing performed on a variety of uncoated and coated YAG surfaces.

2.0 Samples

Most of the substrate material was purchased from Allied Signal Corp., with a few of the samples coming from Laser Crystal Corp. The material was all undoped YAG, with the exception of a few of the uncoated and fractured samples which were Nd:YAG (~1 atomic %). Grinding and polishing was performed either at McDonnell Douglas Corp. (MDC), or by Virgo Inc. The samples were mostly disks, 25.4 mm in diameter and 2-3 mm thick. Both sides of the disks were polished to a "laser-grade" polish. The scratch/dig specification was 10/5 or better.

3.0 Measurements

All laser damage threshold measurements were made by Montana Laser Optics, Inc. (MLO). The threshold measurements are considered accurate to ±20%, and reproducible to ± 10%. Table 1 lists the test parameters which apply to any of the results in this report, except where specifically stated otherwise.

For this study, the term "laser damage" refers to any permanent laser-induced change to the surface. The term "threshold" refers to the least fluence which caused damage. Damage to the sample was initially detected by increased scatter by the test site from a coaxial HeNe probe beam. Damage at a site is then verified by Nomarski/darkfield microscopy at 150-300X. All the surfaces tested, except the TIR surfaces, were the entrance face of the sample with respect to the incident laser beam, see figure 1. The TIR test geometry is shown in figure 8

Some of the damage tests were conducted at 59.1° incidence. All damage fluence levels refer to the beam analysis plane which is normal to the incident laser beam. see figure 1. These fluence levels should be multiplied by the cosine of 59.1°, when comparing to normal incidence measurements.

The spread in the laser damage threshold values is defined in the following equation:

Spread

=

(a-b)/b

a= highest fluence which did not cause
damage

b= lowest fluence which did cause damage

(the damage threshold as defined by MLO)

The value of the spread is related to the spatial uniformity of a surface's damage threshold on a size scale set by the irradiated spot size.

4.0 Results

4.1 Uncoated

4.1.1 Polished Surfaces

Several uncoated YAG surfaces were laser damaged to establish a baseline from which to compare coated surface results. Table 3 contains the results from 16 uncoated YAG surfaces. The average damage threshold from table 3 is 11.1 J/cm2, with a standard deviation of 1.7 J/cm2. In addition to the table 3 results, one uncoated YAG surface was tested at both normal and 59° incidence. The 59° threshold results were: 26 J/cm2 for s-polarization, and 19 J/cm2 for p-polarization. The normal incidence threshold was 12 J/cm2. To compare the normal and 59° incidence results, one must multiply the 59° results by the cosine of 59°, or ~0.52. This multiplication results in 13.5 J/cm2 for s-polarization, and 9.9 J/cm2 for p-polarization. Both of the "converted" values are within two standard deviations of the average normal incidence threshold, and as such represent reasonable consistency between normal and non-normal