This work represents a radical simplification in the design of the optical head needed for high-precision laser ranging applications. The optical head is now a single fiber-optic collimator with dimensions of order of 1×1×2 cm, which can be easily integrated into the system being measured with minimal footprint. Previous heads were significantly larger, with multiple optical elements requiring careful alignment. The new design has only one optical fiber per head, rather than four, making it much easier to multiplex between tens or hundreds of heads. It is capable of subnanometer precision, consistent with the demanding requirements of new missions.
By combining a large number of multiplexed, low-cost, ultra-compact optical heads, it will be possible to form dense optical trusses, with minimal footprint, for the stabilization of large precision structures. The compact heads could be integrated with a piezoelectric actuator inside a tube to provide an "active strut" — a lightweight, structural member with infinite stiffness and zero thermal expansion — which can form the basis for extremely stable, lightweight structures. The compact heads could also be used as a tool in the laboratory for monitoring vibration and drifts in the dimensions of an experiment. Laser metrology may thus attain the same level of utility as accelerometers and thermistors. Simple magnetic or snap-on mounts could be used to position and point the heads and target retro-reflectors.
This advance in optical-head design is enabled by the application of pseudo- random noise (PRN) codes to optical metrology systems. This code is used to discriminate between multiple optical returns that have different propagation delays. In this way, the phase of the signal from the reference surface on the optical head can be measured independently of the phase of the signal from the more distant target retro-reflector.
A prototype system has been assembled in the laboratory. The data were recorded on a digital oscilloscope, and the signal processing was applied off-line in software. Future work will include the implementation of the software algorithms in a real-time FPGA-based signal processing system, and the demonstration of different multiplexing schemes.
This work was done by Serge Dubovitsky, Daniel Shaddock, Brent Ware, and Oliver Lay of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-45102, volume and number of this NASA Tech Briefs issue, and the page number.