Development of a metrology instrument for mapping the crystallographic axis in large optics
A metrology instrument has been developed to scan crystals and map the peak tuning angles for frequency conversion from the infrared to the ultra violet over large apertures. The need for such a device emerged from the National Ignition Facility (NIF) program where frequency conversion crystals have been found to have significant crystallographic axis wander at the large NIF aperture size of 4 1 cm square. With only limited access to a large aperture laser system capable of testing these crystals, scientists have been unable to determine which crystal life-cycle components most affect these angular anomalies. A system that can scan crystals with a small diameter probe laser beam and deliver microradian accuracy and repeatability from probe point to probe point is needed. The Crystal Alignment Verification Equipment (CAVE) is the instrument designed to meet these needs and fit into the budget and time constraints of the ongoing NIF development. In order to measure NIF crystals, the CAVE has a workspace of 50 x 50 cm and an angular measurement accuracy of 10 {micro}radians. Other precision requirements are probe beam energy measurement to 2% of peak, thermal control to 20 0. 1°C around the crystal, crystal mounting surface flatness of 1 {micro}m over 40 cm square, and clean operations to Class 100 standards. Crystals are measured in a vertical position in a kinematic mount capable of tuning the crystal to 1 {micro}radian. The mirrors steering the probe beam can be aligned to the same precision. Making tip/tilt mounts with microradian level adjustment is relatively commonplace. The real precision engineering challenge of the CAVE system is maintaining the angular alignment accuracy of the probe laser relative to the crystal for each spatial position to be measured. The design team determined that a precision XY stage with the required workspace and angular accuracy would be prohibitive to develop under the given tight time constraints. Instead the CAVE uses commercially available slides and makes up for their inaccuracies with metrology. The key to the CAVE device is referencing all angular measurements relative to a master reference surface. The angles of the probe laser and the crystal mount are measured relative to a 61 cm optical flat for every spatial location. Autocollimation and imaging are used to achieve the microradian angular precision. Slide errors are removed by alignment of the crystal and the beam for each new probe point through active feedback from the autocollimation system. The frst prototype CAVE system is being constructed and tested at Lawrence Livermore National Laboratory. The mechanical and optical errors are being measured and analyzed to verify the critical performance specification of 10 {micro} radians. Others facing the same challenge of designing point-to-point probe system for measuring optical axes can use the knowledge gained from this design.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE Office of Defense Programs (DP)
- DOE Contract Number:
- W-7405-Eng-48
- OSTI ID:
- 5779
- Report Number(s):
- UCRL-JC-132252; 39DP02000; ON: DE00005779
- Resource Relation:
- Conference: American Society for Precision Engineering, 13th Annual Meeting, St. Louis, MO, October 25-30, 1998
- Country of Publication:
- United States
- Language:
- English
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