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. 2016 Sep 6;16(9):1428.
doi: 10.3390/s16091428.

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

Radek Šmíd  1 , Martin Čížek  2 , Břetislav Mikel  3 , Jan Hrabina  4 , Josef Lazar  5 , Ondřej Číp  6
Affiliations

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

Radek Šmíd et al. Sensors (Basel). .

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 10 - 9 that corresponds to 0.3 nm (sub-nm) for 0.178 m long cavity with ability of displacement measurement of 0.5 mm.

Keywords: Fabry-Perot cavity; displacement measurement; heterodyne interferometry; noise suppression; unbalance Michelson interferometer.

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Conflict of interest statement

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Schematics of the set-up of noise suppression of Distributed FeedBack (DFB) laser diode. 90/10 and 50/50 are fiber splitters, FM1 and FM2 are Faraday mirrors, AOM—acusto-optic modulator, PD—photodetector, DDS—direct digital synthesizer, BPF—band pass filter, RFSA—Radiofrequency Spectrum Analyzer, LPF—low pass filter.
Figure 2
Figure 2
Scheme of the set-up for measurement of displacement of the Fabry-Perot cavity with Zerodur spacer under the temperature stable condition. EOM-electro-optic modulator, AOM2-frequency shifting acousto-optic modulator, VCO-voltage-control oscillator, OI-optic isolator, FPD-fast photodetector, PD-R-refection signal photodetector, PD-T-transmission signal photodetector, PBS-polarization beam splitter, PD-photodetector, LPF-low pass filter.
Figure 3
Figure 3
In-loop power spectral density of the frequency noise calculated from the RF spectrum measured on RF spectral analyzer Agilent N9000A (see RFSA in Figure 1).
Figure 4
Figure 4
Temporal evolution of the beat frequency between the stabilized optical frequency comb and free-running DFB laser (red) and between the stabilized optical frequency comb and the noise suppressed DFB laser (blue).
See this image and copyright information in PMC

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