John Peterson

Beyond ray tomography: Wavepaths and Fresnel volumes

Two techniques that account for the band-limited nature of seismic data are incorporated into tomographic traveltime inversion schemes. The first technique, the wavepath algorithm, is based upon the wave equation, the Born approximation, and an adjoint method for computing Frechet derivatives. Computation of a single wavepath requires the forward propagation of the seismic wavefield, as well as the reverse propagation of a residual wavefield. The second technique, the Fresnel volume approach, is based upon the paraxial ray approximation. The Fresnel volume algorithm requires little more computation than does conventional ray tracing and an order of magnitude less computer time than our calculation of wavepaths. When the Fresnel volume sensitivity functions are normalized by the area of the Fresnel ellipse perpendicular to the ray, the sensitivity estimates are very similar to the wavepaths. In particular, there is heightened sensitivity to velocity structure near the source and receiver locations. The normalization by the Fresnel ellipse area is necessary to ensure ray theoretical results in the limit of infinite frequency. Tomographic inversion based upon wavepaths or Fresnel volumes is more appropriate when considering the arrival time of the peak of the initial pulse rather than the first-arrival time. Furthermore, using the traveltime of the peak instead of the first-arrival time reduces the bias of tomograms to high velocity anomalies. The raypath, wavepath, and Fresnel volume techniques were applied to a set of cross-borehole traveltime observations gathered at the Grimsel Rock Laboratory. All methods imaged a low velocity fracture zone in the granitic site, in agreement with independent well information. Estimates of model parameter resolution are similar for the wavepath and Fresnel volume schemes. The source-receiver regions are the most well resolved areas. However, the model parameter resolution computed using a conventional ray-based formalism is more evenly distributed over the cross-borehole area.

Combined advanced finishing and UV-laser conditioning for producing UV-damage-resistant fused-silica optics

Laser-induced damage initiation on fused silica optics can limit the lifetime of the components when used in high power UV laser environments. For example in inertial confinement fusion research applications, the optics can be exposed to temporal laser pulses of about 3 nsec with average fluences of 8 J/cm2 and peak fluences between 12 and 15 J/cm2. During the past year, we have focused on optimizing the damage performance at a wavelength of 355-nm (3(omega) ), 3-nsec pulse length, for optics in this category by examining a variety of finishing technologies with a challenge to improve the laser damage initiation density by at least two orders of magnitude. In this paper, we describe recent advances in improving the 3(omega) damage initiation performance of laboratory-scale zirconium oxide and cerium oxide conventionally finished fused silica optics via application of processes incorporating magnetorheological finishing (MRF), wet chemical etching, and UV laser conditioning. Details of the advanced finishing procedures are described and comparisons are made between the procedures based upon large area 3(omega) damage performance, polishing layer contamination, and optical subsurface damage.

CO2-laser polishing for reductoin of 351-nm surface damage initiation in fused silica

We have applied a carbon dioxide (CO2) raster scanning laser polishing technique on two types of fused silica flat optics to determine the efficacy of CO2-laser polishing as a method to increase the 351-nm laser damage resistance of optic surfaces. R-on-1 damage test results show that the fluence for any given 355-nm damage probability is 10-15 J/cm2 higher (at 3 ns pulse length, scaled) for the CO2-laser polished samples. Poor quality and good quality surfaces respond to the treatment such that their surface damage resistance is brought to approximately the same level. Surface stress and the resultant effect on wavefront quality remain key technology issues that would need to be addressed for a robust deployment.

UV-laser conditioning for reduction of 351-nm damage initiation in fused silica

This paper describes the effect of 355-nm laser conditioning on the concentration of UV-laser-induced surface damage sites on large-aperture fused silica optics. We will show the effect of various 355-nm laser conditioning methodologies on the reduction of surface-damage initiation in fused silica samples that have varying qualities of polishing. With the best, generally available fused silica optic, we have demonstrated that 355-nm laser conditioning can achieve up to 10x reduction in surface damage initiation concentration in the fluence range of 10-14 J/cm2 (355- nm at 3 ns).

Mitigation of laser damage on National Ignition Facility optics in volume production

The National Ignition Facility has recently achieved the milestone of delivering over 1.8 MJ and 500 TW of 351 nm laser energy and power on target, which required average fluences up to 9 J/cm2 (3 ns equivalent) in the final optics system. Commercial fused silica laser-grade UV optics typically have a maximum operating threshold of 5 J/cm2. We have developed an optics recycling process which enables NIF to operate above the laser damage initiation and growth thresholds. We previously reported a method to mitigate laser damage with laser ablation of the damage site to leave benign cone shaped pits. We have since developed a production facility with four mitigation systems capable of performing the mitigation protocols on full-sized (430 mm) optics in volume production. We have successfully repaired over 700 NIF optics (unique serial numbers), some of which have been recycled as many as 11 times. We describe the mitigation systems, the optics recycle loop process, and optics recycle production data.

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%.

Effects of Nonlinear Absorption in BK7 and Color Glasses at 355 nm

We have demonstrated a simple experimental technique that can be used to measure the nonlinear absorption coefficients in glasses. We determine BK7, UG1, and UG11 glasses to have linear absorption coefficients of 0.0217 ± 10% cm-1, 1.7 ± 10% cm-1, and 0.82 ± 10% cm-1, respectively, two-photon absorption cross-sections of 0.025 ± 20% cm/GW, 0.035 ± 20% cm/GW, and 0.047 ± 20% cm/GW, respectively, excited-state absorption cross-sections of 8.0 x 10-18 ± 20% cm2, 2.8 x 10-16 ± 20% cm2, and 5 x 10-17 ± 20% cm2, respectively, and solarization coefficients of 8.5 x 10-20 ± 20% cm2, 2.5 x 10-18 ± 20% cm2, and 1.3 x 10-19 ± 20% cm2, respectively. For our application, nonlinear effects in 10-cm of BK7 are small (≤ 2%) for 355-nm fluences < 0.2 J/cm2 for flat-top pulses. However, nonlinear effects are noticeable for 355-nm fluences at 0.8 J/cm2. In particular, we determine a 20% increase in the instantaneous absorption from linear, a solarization rate of 4% per 100 shots, and a 10% temporal droop introduced in the pulse, for 355-nm flat-top pulses at a fluence of 0.8 J/cm2. For 0.5-cm of UG1 absorbing glass the non-linear absorption has a similar effect as that from 10-cm of BK7 on the pulse shape; however, the effects in UG11 are much smaller.

Combined advanced finishing and UV-laser conditioning for producing UV-damage-resistant fused silica optics

During the past year, we have focused on optimizing the laser damage performance for fused silica optics at 355-nm, 3-nsec pulse length, via application of magnetorheological finishing (MRF), wet chemical etching, and UV laser conditioning.