A frequency-modulated laser interferometer for nanometer-scale position sensing at cryogenic temperatures

摘要

The continually increasing sensitivity required for advancement of far-infrared astronomy dictates that the next generationof space-based observatories must employ cryogenically cooled telescopes and instruments. Cryogenic operation ofinterferometers such as those proposed for future space missions poses particular challenges, including the need for robustlow power dissipation cryogenic position metrology. Instrumentation must be cooled to <4 K to avoid a noise contributionfrom self-emission and often contain moving components whose position must be measured precisely at cryogenictemperatures.In 2018, we reported on the development of a three-phase fiber-fed laser homodyne interferometer for optical positionmetrology that achieved a displacement uncertainty of 2.3 nm RMS at 4 K. In that design, one arm of the interferometerhad an additional 2 m of optical fiber to carry the probe signal to the 4 K work space. Subsequently, a 2 m, armored,differential fiber pair was developed to balance the lengths of the probe and reference interferometric beams that weresubject to thermal gradients. Although this led to an improved dynamic performance in the measurement of an oscillatingtarget, low velocity performance was limited by 1/f noise in the photodetector circuit.Building on that work, we present the design and review the performance of a new frequency-modulated laserinterferometer system we have developed that improves upon the three-phase system by eliminating the need for adifferential fiber pair in cryogenic applications and achieves 29 nm RMS uncertainty for mechanical displacementvelocities from 0 to ～4 mm/s.