Abstract
The absolute distance between the mirrors of a Fabry-Perot cavity with a spacer from an ultra low expansion material was measured by an ultra wide tunable laser diode. The DFB laser diode working at 1542 nm with 1.5 MHz linewidth and 2 nm tuning range has been suppressed with an unbalanced heterodyne fiber interferometer. The frequency noise of laser has been suppressed by 40 dB across the Fourier frequency range 30–300 Hz and by 20 dB up to 4 kHz and the linewidth of the laser below 300 kHz. The relative resolution of the measurement was that corresponds to 0.3 nm (sub-nm) for 0.178 m long cavity with ability of displacement measurement of 0.5 mm.
Highlights
Measurement of the length of etalons and measurement of the length of passive Fabry-Perot cavities [1,2,3] or their displacement [4,5] is limited by vibration of mirrors, thermal fluctuations, speed of lock-loops and by the noise and the linewidth of the laser source
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 distributed feedback (DFB) laser diode with wide tuning range to monitor the displacement of the
The longer arm consists of a fiber spool represented by the single mode fiber of roughly 10 m, and an acousto-optic modulator (AOM) working at a fixed frequency of 80 MHz and Faraday mirror 2 (FM2)
Summary
Measurement of the length of etalons and measurement of the length of passive Fabry-Perot cavities [1,2,3] or their displacement [4,5] is limited by vibration of mirrors, thermal fluctuations, speed of lock-loops and by the noise and the linewidth of the laser source. FeedBack lasers (DFB) [14] can achieve roughly 1 MHz linewidth corresponding to 10−8 relative uncertainty of measurement and 1 nm uncertainty for 100 mm long cavity but offer wide tuning range of nms or hundreds of GHz. Better linewidth can be achieved for external cavity laser (ECL). The state-of-the-art ECLs include planar waveguides with fiber Bragg grating and reach linewidths under 3 kHz [17] but the tuning range is only 20 pm or 5 GHz. Further improvement of relative uncertainty of the displacement measurement can be achieved by laser frequency noise suppression. Frequency noise suppression can be achieved by locking and stabilization of the laser to an absorption cell [9,18] or to an ultrastable cavity from very low expansion material such as ULE [19] These methods have limited tuning range of treated. This paper includes the measurement set-up for cavity length displacement and new results including the measurement of beat frequency to a stable optical frequency comb
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
