A metrology system to measure the on-orbit movement of a ten meter mast has been built for the Nuclear Spectroscopic Telescope Array (NuSTAR) x-ray observatory. In this paper, the metrology system is described, and the performance is measured. The laser beam stability is discussed in detail. Pre-launch alignment and calibration are also described. The invisible infrared laser beams must be aligned to their corresponding detectors without deploying the telescope in Earth's gravity. Finally, a possible method for in-flight calibration of the metrology system is described.
Laser-based metrology has been identified as an enabling technology in the deployment of large, spaceborne
observatories, where nanometer-level knowledge of fiducial displacement drives overall system performance. In
particular, Nd:YAG NPRO (non-planar ring oscillator) based lasers have received considerable attention in this
application because of their inherent high coherence at wavelengths of interest (1064 and 1319nm). However, the use of
NPRO based lasers in decade long space missions is limited by typical 800nm-band pump laser diode wearout and
random failure rates. Therefore, reliably achieving multi-hundred milliwatt NPRO power over prolonged mission
lifetimes requires innovative pump architectures. In this paper we present a pump architecture capable of supporting
continuous NPRO operation over 5.5yrs at 300mW with reliability exceeding 99.7%. The proposed architecture relies
on a low-loss, high port count, all-fiber optical coupler to combine the outputs of multiple single-mode pump laser
diodes. This coupler is capable of meeting the exacting environmental requirements placed by a space mission, such as
SIM Lite.
We have designed and built a hollow-core fiber frequency reference cell, filled it with CO2, and used it to demonstrate
frequency stabilization of a 2.05 μm Tm:Ho:YLF laser using frequency modulation (FM) spectroscopy technique. The
frequency reference cell is housed in a compact and robust hermetic package that contains a several meter long hollow-core
photonic crystal fiber optically coupled to index-guiding fibers with a fusion splice on one end and a mechanical
splice on the other end. The package has connectorized fiber pigtails and a valve used to evacuate, refill it, or adjust the
gas pressure. We have demonstrated laser frequency standard deviation decreasing from >450MHz (free-running) to
<2.4MHz (stabilized).
The 2.05 μm laser wavelength is of particular interest for spectroscopic instruments due to the presence of many CO2
and H20 absorption lines in its vicinity. To our knowledge, this is the first reported demonstration of laser frequency
stabilization at this wavelength using a hollow-core fiber reference cell. This approach enables all-fiber implementation
of the optical portion of laser frequency stabilization system, thus making it dramatically more lightweight, compact, and
robust than the traditional free-space version that utilizes glass or metal gas cells. It can also provide much longer
interaction length of light with gas and does not require any alignment. The demonstrated frequency reference cell is
particularly attractive for use in aircraft and space coherent lidar instruments for measuring atmospheric CO2 profile.
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