W. H. Lowdermilk

Materials for optical coatings in the ultraviolet.

In this work, we used 248-nm 20-nsec laser pulses to measure laser-damage thresholds for halfwave-thick single layers of fifteen potential UV coating materials, for highly reflective coatings made of thirteen combinations of these materials, and for antireflective coatings made using five combinations of the materials. Refractive index, absorption, position of the UV absorption edge, stress, and environmental stability were measured for the halfwave-thick single layers. The first three of these parameters are closely related and were generally correlated with damage thresholds of the single-layer coatings. Thresholds of HR coatings were correlated with absorption in the high-index materials used in the coatings and with refractive index of the low-index materials. Thresholds of AR films were not well correlated with properties of single-layer coatings.

Carbon dioxide laser polishing of fused silica surfaces for increased laser-damage resistance at 1064 nm

Mechanically polished fused silica surfaces were heated with continuous-wave CO(2) laser radiation. Laser-damage thresholds of the surfaces were measured with 1064-nm 9-nsec pulses focused to small spots and with large-spot, 1064-nm, 1-nsec irradiation. A sharp transition from laser-damage-prone to highly laser-damage-resistant took place over a small range in CO(2) laser power. The transition to high damage resistance occurred at a silica surface temperature where material softening began to take place as evidenced by the onset of residual strain in the CO(2) laser-processed part. The small-spot damage measurements show that some CO(2) laser-treated surfaces have a local damage threshold as high as the bulk damage threshold of SiO(2). On some CO(2) laser-treated surfaces, large-spot damage thresholds were increased by a factor of 3-4 over thresholds of the original mechanically polished surface. These treated parts show no obvious change in surface appearance as seen in bright-field, Nomarski, or total internal reflection microscopy. They also show little change in transmissive figure. Further, antireflection films deposited on CO(2) laser-treated surfaces have thresholds greater than the thresholds of antireflection films on mechanically polished surfaces.

Scandium oxide coatings for high-power UV laser applications.

Scandium oxide has proved to be a damage-resistant high-index material in laser coatings for use at 248 nm. The results of damage threshold measurements on laser reflectors and antireflection coatings of various designs, material combinations, and deposition temperatures are presented. The most significant effects are observed for overcoat layers on high reflectors and undercoat layers on antireflection coatings. Thresholds >6 J/cm(2) for 20 nsec pulses were observed in both cases.

Gradient-index AR film deposited by the sol–gel process

Gradient-index alkali borosilicate antireflection films for use in laser systems were deposited by the sol-gel process. Laser damage thresholds of these films, measured with 1.06-microm 1-nsec pulses, were four times greater than thresholds of widely used, multilayer, antireflection coatings.

High performance avalanche transistor switchout for external pulse selection at 1.06 μm

The design and performance of an avalanche transistor switchout are described. The device selects a single pulse from a train of cw or Q-switched mode-locked pulses and introduces less than +/-1% amplitude variation in the selected pulse. The prepulse rejection ratio exceeds 10(7), and a lifetime of greater than 10(7) shots has been achieved.

Antireflective surfaces for high-energy laser optics formed by neutral-solution processing

Antireflective surfaces were produced on optical borosilicate glass by controlled corrosion in nearly neutral solutions. The surfaces have reflectivity below 0.1% and median threshold for laser-induced damage by 1-nsec 1.06-microm pulses of 12 J/cm(2); twice the median damage threshold of thin film antireflection coatings.

Influence of deposition parameters on laser-damage threshold of silica-tantala AR coatings

A series of 4-layer silica-tantala antireflection coatings was deposited under 18 different combinations of substrate temperature (175, 250, and 325 degrees C), oxygen pressure (0.5,1.0, and 2.0 x 10(-4) Torr), and rate of deposition (1.5 and 5 A/sec). Measurements of laser-damage threshold for 1064-nm, 1-nsec pulses, average absorption, net stress, and reflectivity were then made on these coatings. Coatings deposited at the lowest temperature had the highest damage thresholds. Damage thresholds were found not to be directly related to average absorption or net stress. Coatings deposited on fused silica substrates which had been polished by a bowl-feed process had generally higher damage thresholds than coatings deposited on conventionally polished fused silica or on BK-7 glass polished by either conventional or bowl-feed processes. Baking coatings in air for 4 h at 400 degrees C generally reduced average absorption and net stress, changed the net stress from compression to tension and, in some cases, increased the damage threshold.

A Review of 1064-nm Damage Tests of Electron-Beam Deposited Ta 2 O 5 /SiO 2 Antireflection Coatings

Damage tests of Ta 2 O 5 /SiO 2 antireflection films deposited under a variety of conditions showed that thresholds of films deposited at 175 C were greater than thresholds of films deposited at either 250 C or 325 C. Deposition at high rate and low oxygen pressure produced highly absorptive films with low thresholds. Thresholds did not correlate with film reflectivity or net stress in the films, and correlated with film absorption only when the film absorption was greater than 10 4 ppm. Baking the films for four hours at 400 C reduced film absorption, altered net film stress, and produced an increase in the average damage threshold.

Laser-damage thresholds of thin-film optical coatings at 248 nm

We have measured the laser-induced damage thresholds for 248 nm wavelength light of over 100 optical coatings from commercial vendors and research institutions. All samples were irradiated once per damage site with temporally multi-lobed, 20-ns pulses generated by a KrF laser. The survey included high, partial, and dichroic reflectors, anti-reflective coatings, and single layer films. The samples were supplied by ten vendors. The majority of samples tested were high reflectors and antireflective coatings. The highest damage thresholds were 8.5 to 9.4 J/cm/sup 2/, respectively. Although these represent extremes of what has been tested so far, several vendors have produced coatings of both types with thresholds which consistently exceed 6 J/cm/sup 2/. Repeated irradiations of some sites were made on a few samples. These yielded no degradation in threshold, but in fact some improvement in damage resistance. These same samples also exhibited no change in threshold after being retested seven months later.

Graded-index Antireflective Coatings for High Power Lasers Deposited by the Sol-Gel Process

Gradient-index alkali borosilicate antireflection films for use in laser systems were deposited by the sol-gel process. Laser damage thresholds of these films, measured with 1.07 ..mu..m, 1-ns pulses, were four times greater than thresholds of widely-used, multilayer, antireflection coatings.