George Dubé

Laser Induced Surface Damage

A summary of recent investigations of surface damage of transparent dielectrics is presented. Damage threshold measurements made on Owens-Illinois ED-2 laser glass at normal incidence and at Brewsters angle are reported. For 30-nsec pulses at normal incidence, exit surface damage thresholds are typically 100 J/cm(2) for ED-2 glass. The observed ratio between entrance and exit damage thresholds for the two geometries can be explained by considering the electric field strengths at the surfaces and including interference between incident and reflected light waves. A similar analysis is applied to surface damage that occurs during total internal reflection. Finally the morphology of surface damage of ED-2 laser glass is described, and a model based upon reflections from the laser induced plasma is proposed to explain the observations.

Refractive beam steering

A number of concepts using the principle of the refraction of light have been developed to steer light beams. Refractive beam steering concepts involve the use of optical wedges in order to deviate a light beam. This principle is ideally suited for steering laser light since dispersion is minimal due to the monochromatic nature of the laser. The methods used to form the optical wedge and the means developed to adjust it are what distinguish the various concepts and have resulted in many patents over the years for their innovators. A new concept called a Lubricated Adjustable Optical Wedge (LAOW) has been recently developed that does not require complicated mechanical systems to form the wedge and provide the adjustment necessary to deviate the light beam. An optical wedge is formed using plano-convex and plano-concave lenses that are contacted together using a thin film of transparent index matching lubricant between the spherical surfaces. The forces of capillary action and surface tension provide the sole means of keeping the lens elements together. This technique has demonstrated a repeatability ≤±0.12 arc seconds in beam deviation angle.

Prism Coupling Measurement of Part-Per-Million Extinction in an Optical Thin Film During and After Deposition

A modified prism coupling technique was used inside a vacuum chamber to measure the very low extinction in an optical thin film as it was being deposited. More conventional modifications were also investigated.

Measuring part per million thin film absorption during deposition

Waveguiding was used to measure the extinction coefficient of a thin film while it was being deposited in a vacuum chamber. Experimental results are presented and compared to calculations and measurements by other techniques.

Abirefringent athermal windows for high energy lasers

Experimental results are presented on using one or more additional windows to reduce the distortion from existing windows transmitting high average power laser beams. A concept is presented for a compound window that will neither distort nor depolarize a high power beam.

Cleaning optical surfaces from the inside out

The advantages of backside irradiation and controlling the polarization and angle of incidence of laser beams used to conventionally clean surfaces have been described previously. This paper considers beams internally incident upon the surface of a transparent substrate at angles of incidence beyond the critical angle for total internal reflection. Attenuated total internal reflection provides an efficient means for both finding and removing absorbing contaminants. Beams may enter the substrate through an edge or a coupling prism. Erbium laser pulses were used to remove water from several dielectric and semiconductor materials. In some cases the water was explosively removed, with no sign of damage to the substrate. A 2.94 μm laser should be especially effective whenever water is present, either naturally from adsorption or capillary condensation or when added for steam laser cleaning. Unabsorbed light can be efficiently routed to clean adjacent areas through multiple total internal reflections. Thus some of the scanning is done at the speed of light. Theory indicates that ATR laser cleaning is effective for very small particles that cannot be removed by shock laser cleaning. This paper will describe attenuated total internal reflection laser cleaning and compare it to conventional laser cleaning techniques.

Lubricated adjustable optical wedge beam steerers

Lubricated (matched lens) adjustable optical wedges steer laser beams in two dimensions from a single center-of-scan that stays on the original beam axis. These patented refractive beam steerers offer a wide range of apertures, deviations and precision. They do not suffer from the blind spots inherent with conventional Risley Wedge Beam Steerers. Unlike mirrors, they are nearly immune to vibrations and they do not require rerouting or “dog-legging” the beam. We have tested dozens of transparent lubricants and found several that can handle high laser powers. Lubricated adjustable optical wedges may soon replace conventional beam alignment or steering hardware in a variety of applications. This paper will review lubricated adjustable optical wedges and other refractive beam steerers.Lubricated (matched lens) adjustable optical wedges steer laser beams in two dimensions from a single center-of-scan that stays on the original beam axis. These patented refractive beam steerers offer a wide range of apertures, deviations and precision. They do not suffer from the blind spots inherent with conventional Risley Wedge Beam Steerers. Unlike mirrors, they are nearly immune to vibrations and they do not require rerouting or “dog-legging” the beam. We have tested dozens of transparent lubricants and found several that can handle high laser powers. Lubricated adjustable optical wedges may soon replace conventional beam alignment or steering hardware in a variety of applications. This paper will review lubricated adjustable optical wedges and other refractive beam steerers.

A TWO-AXIS ADJUSTABLE-WEDGE REFRACTIVE BEAM ALIGNER

We describe a practical device for deviating a transmitted light beam up to several degrees. It keeps the pivot point on the original axis and can offer more sensitive angular control than conventional mirror mounts.

Coaxial digital pulse compression for high‐power Nd : glass lasers

A new method is suggested for using a single exploding mirror to efficiently combine any number of temporally separated coaxial laser pulses so that all pulses simultaneously irradiate a small target. Besides increasing the output and efficiency of the large Nd : glass laser systems being used in fusion research, this method would also isolate the laser from target reflections and protect the target from premature irradiation by amplified spontaneous emission.