Frequency Noise Suppression of a Single Mode Laser with an Unbalanced Fiber Interferometer for Subnanometer Interferometry

Abstract

We present a method of noise suppression of laser diodes by an unbalanced Michelson fiber interferometer. The unstabilized laser source is represented by compact planar waveguide external cavity laser module, ORIONTM (Redfern Integrated Optics, Inc.), working at 1540.57 nm with a 1.5-kHz linewidth. We built up the unbalanced Michelson interferometer with a 2.09 km-long arm based on the standard telecommunication single-mode fiber (SMF-28) spool to suppress the frequency noise by the servo-loop control by 20 dB to 40 dB within the Fourier frequency range, remaining the tuning range of the laser frequency.

Noise Suppression on the Tunable Laser for Precise Cavity Length Displacement Measurement

“…The power spectral density of the in-loop beat frequency signal measured in dBm units by the RFSA has been normalized to the amplitude of the carrier and recalculated to the dBc units. Finally based on the assumption that for small variations of the frequency , the intensity variations of the interference signal depends linearly on the time delay of the interferometer and on the maximal intensity variations [ 27 ]: the power spectral density data was recalculated to the power spectral density of the frequency noise [ 22 ].…”

“…The detection bandwidth and the sensitivity to phase fluctuations are determined by the delay introduced by the beam travelling through the fiber spool in the long arm [ 22 ]. The RF signal from the photodetector is coupled into the RF spectral analyzer (RFSA) Agilent N9000A and analyzed.…”

“…Laser phase noise suppression was already achieved by the methods based on unbalanced fiber interferometer [ 20 , 21 ]. We have already presented the set-up for the laser noise suppression of planar waveguide external cavity laser module [ 22 ] whereas in this work we present the phase noise suppression of a DFB laser diode with wide tuning range to monitor the displacement of the Fabry-Perot cavity with sub-nm uncertainty. We have also presented that the continuous high accuracy displacement measurement with one single mode DFB laser is important in monitoring of the low expansion materials [ 4 ] and unlike the displacement measurements for calculable capacitor [ 23 ] we are able to track the cavity displacement over the whole tuning range of the laser.…”