For a variety of applications, it is important to measure the sensitivity of the pointing of a beam emerging from a collimator, as a function of temperature changes. A straightforward method for carrying out this measurement is based on using interferometry for monitoring the changes in beam pointing, which presents its own problems. The added temperature dependence and complexity issues relating to using an interferometer are addressed by not using an interferometer in the first place. Instead, the collimator is made part of an arrangement that uses a minimum number of low-cost, off-the-shelf materials and by using a quad diode to measure changes in beam pointing.
In order to minimize the influence of the test arrangement on the outcome of the measurement, several steps are taken. The collimator assembly is placed on top of a vertical, 1-m-long, fused silica tube. The quad diode is bonded to a fused silica bar, which, in turn, is bonded to the lower end of the fused silica tube. The lower end of the tube rests on a self-aligning support piece, while the upper end of the tube is kept against two rounded setscrew tips, using a soft rubber string. This ensures that very little stress is applied to the tube as the support structure changes dimensions due to thermal expansion. Light is delivered to the collimator through a bare fiber in order to minimize variable bending torque caused by a randomly relaxing, rigid fiber jacket.
In order to separate the effect of temperature on the collimator assembly from the effect temperature has on the rest of the setup, multiple measurements are taken with the collimator assembly rotated from measurement to measurement. Laboratory testing, with 1-m spacing between the collimator and the quad diode, has shown that the sensitivity of the arrangement is better than 100 nm rms, over time spans of at least one hour, if the beam path is protected from atmospheric turbulence by a tube. The equivalent sensitivity to detecting changes in pointing angle is 100 nanoradians.
This work was done by Alex Abramovici, Timothy E. Cox, Randall C. Hein, and Daniel R. MacDonald of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47529
This Brief includes a Technical Support Package (TSP).
Method for Measuring Collimator-Pointing Sensitivity to Temperature Changes
(reference NPO-47529) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically focused on the method for measuring collimator-pointing sensitivity to temperature changes, referenced as NPO-47529. It was prepared under the Commercial Technology Program of NASA to disseminate results from aerospace-related developments that have broader technological, scientific, or commercial applications.
The research was conducted at the Jet Propulsion Laboratory (JPL) at the California Institute of Technology, under a contract with NASA. The primary objective of the study is to understand and mitigate the temperature sensitivity of collimators used in laser systems, which is crucial for maintaining precision in various applications, including space exploration.
Key aspects of the testing methodology include the use of a long baseline alongside a short one to differentiate between collimator effects and quad diode effects. The test arrangement is designed to minimize temperature sensitivity through specific features such as a 1-meter spacer, axial symmetry, and the use of low expansion coefficient materials. Fused silica is employed for the quad diode mount, spacer, and collimator mount due to its homogeneity and low coefficient of thermal expansion (CTE). The bonding of components is done using 2216 epoxy, which has been shown to limit movement in response to temperature changes to a maximum of 50 nm/°K.
The document also discusses the assembly method of the collimator, which involves a spacer and a retaining ring. It notes that the thermal behavior may vary between units due to the lack of precise centering, a common issue with friction joints. The vertical orientation of the fused silica tube is highlighted as a means to stabilize the collimator assembly, with additional support from precision aluminum shims to ensure even placement.
Overall, the findings emphasize the importance of careful material selection and assembly techniques in reducing temperature-induced errors in laser systems. The document serves as a valuable resource for researchers and engineers working in the field of aerospace technology, providing insights into the challenges and solutions related to temperature sensitivity in optical systems. For further inquiries or assistance, contact information for JPL's Innovative Technology Assets Management is provided.
