John M. Halpin

High-Throughput Laser Peening of Metals Using a High-Average-Power Nd: Glass Laser System

Laser shot peening, a surface treatment for metals, is known to induce residual compressive stresses to depths over 1 mm providing improved component resistance to various forms of failure. Recent information also suggests that thermal relaxation of the laser induced stress is significantly less than that experienced by other forms of surface stressing that involve significantly higher levels of cold work. We have developed a unique solid state laser technology employing Nd:glass amplifier slabs and SBS phase conjugation that enables this process to move into high throughput production processing.

Design of a production process to enhance optical performance of 3ω optics

Using the Phoenix pre-production conditioning facility we have shown that raster scanning of 3ω optics using a XeF excimer laser and mitigation of the resultant damage sites with a CO2 laser can enhance their optical damage resistance. Several large-scale (43 cm x 43 cm) optics have been processed in this facility. A production facility capable of processing several large optics a week has been designed based on our experience in the pre-production facility. The facility will be equipped with UV conditioning lasers -- 351-nm XeF excimer lasers operating at 100 Hz and 23 ns. The facility will also include a CO2 laser for damage mitigation, an optics stage for raster scanning large-scale optics, a damage mapping system (DMS) that images large-scale optics and can detect damage sites or precursors as small as ≈15 μm, and two microscopes to image damage sites with ≈5 μm resolution. The optics will be handled in a class 100 clean room, within the facility that will be maintained at class 1000.

Enhanced performance of large 3ω optics using UV and IR lasers

We have developed techniques using small-beam raster scanning to laser-condition fused silica optics to increase their damage threshold. Further, we showed that CO2 lasers could be used to mitigate and stabilize damage sites while still on the order of a few tens of microns in size, thereby greatly increasing the lifetime of an optic. We recently activated the Phoenix pre-production facility to condition and mitigate optics as large as 43 cm x 43 cm. Several full-scale optics have been processed in Phoenix. The optics were first photographed using a damage mapping system to identify scratches, digs, or other potential sites for initiation of laser damage. We then condition the optic, raster scanning with the excimer laser. The first scan is performed at a low fluence. A damage map is then acquired and any new damage sites or any sites that have grown in size are mitigated using the CO2 laser. The process is repeated at successively higher fluences until a factor of 1.7 above the nominal operating fluence is reached. After conditioning, optics were tested in a large beam 3ω laser and showed no damage at fluences of 8 J/cm2 average.

Design of an illumination technique to improve the identification of surface flaws on optics

An edge illumination technique has been designed using a monochromatic light source that improves the identification of surface flaws on optics. The system uses a high-resolution CCD camera to capture images of the optics. Conventional edge illumination methods using white light sources have been plagued by light leaking around the optics causing high background levels. The background combined with lower resolution cameras has made it difficult to determine size and intensity characteristics of the flaws. Thus photographs taken of the optics are difficult to analyze quantitatively and do not allow for the detection of small, faintly illuminated sites. Infrared diodes have been utilized to illuminate large-scale (43 cm x 43 cm) fused silica optics, and a two-dimensional array CCD camera has been used to collect the image data. Flaw sizes as small as ~10 μm have been detected. A set of frames has been built to support the infrared sources where one diode array per side is magnetically attached to the frame. The diodes inject light into the optic causing the sites to illuminate, which can be detected by the camera. A customized mounting design has been implemented to secure the frames to the stage, or base, for image acquisition. The design uses a dual bracket assembly to support the frames. With this design for optical illumination, quantitative data has been obtained of the surface flaws. A comparison of the peak intensity, total integrated intensity and size of the flaws measured in these images and the size of the flaws as measured using a microscope will be presented.

Diode-pumped Nd:YAG laser with 38 W average power and user-selectable, flat-in-time subnanosecond pulses

A diode-pumped injection-seeded Nd:YAG laser system with an average output power of 38 W is described. The laser operates at 300 Hz with pulse energies up to 130 mJ. The temporal pulse shape is nominally flat in time and the pulse width is user selectable from 350 to 600 ps. In addition, the spatial profile of the beam is near top hat with contrast <10%.

Image analysis algorithms for the advanced radiographic capability (ARC) grating tilt sensor at the National Ignition Facility

The Advance Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a laser system designed to produce a sequence of short pulses used to backlight imploding fuel capsules. Laser pulses from a short-pulse oscillator are dispersed in wavelength into long, low-power pulses, injected in the NIF main laser for amplification, and then compressed into high-power pulses before being directed into the NIF target chamber. In the target chamber, the laser pulses hit targets which produce x-rays used to backlight imploding fuel capsules. Compression of the ARC laser pulses is accomplished with a set of precision-surveyed optical gratings mounted inside of vacuum vessels. The tilt of each grating is monitored by a measurement system consisting of a laser diode, camera and crosshair, all mounted in a pedestal outside of the vacuum vessel, and a mirror mounted on the back of a grating inside the vacuum vessel. The crosshair is mounted in front of the camera, and a diffraction pattern is formed when illuminated with the laser diode beam reflected from the mirror. This diffraction pattern contains information related to relative movements between the grating and the pedestal. Image analysis algorithms have been developed to determine the relative movements between the gratings and pedestal. In the paper we elaborate on features in the diffraction pattern, and describe the image analysis algorithms used to monitor grating tilt changes. Experimental results are provided which indicate the high degree of sensitivity provided by the tilt sensor and image analysis algorithms.

Optical design of a high power fiber optic coupler

Specifications, design, and operation of an optical system that couples a high-power copper vapor laser beam into a large core, multimode fiber are described. The approach used and observations reported are applicable to fiberoptic delivery applications.

Pulse Contrast Measurement on the NIF Advanced Radiographic Capability (ARC) Laser System

Accurate characterization of pulse contrast for high peak power lasers is critical to the success of experiments exploring inertial confinement fusion. The Advanced Radiographic Capability (ARC) laser at the National Ignition Facility (NIF) is a petawatt class laser system that produces pulses in the picosecond regime for the creation of diagnostic x-rays. ARC leverages four of the NIF’s beamlines for final amplification while implementing a separate front-end and pre-amplification stage, known as the High-Contrast ARC Front End (HCAFE). To characterize pulse contrast at the output of HCAFE, a means of measurement at long times (>500 ps) has been developed using a photodiode that has achieved a dynamic range of over 100 dB and 125 dB after deconvolution. Within hundreds of picoseconds of the main pulse, a commercial third-order cross-correlator (Amplitude Technologies Sequoia) is used to characterize the pulse contrast. Together, these diagnostics provide the necessary data for ensuring pulse contrast requirements can be met on ARC. Efforts were made to mitigate existing pre-pulses and to increase the stability of the system as a long-term operational companion to the NIF. We describe the development and testing of the photodiode diagnostic and the analysis of the data resulting from contrast measurements. Details are also provided regarding the identification and mitigation of pre-pulses within the HCAFE system.