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Unit for stabilizing optical frequency combs
Hucl, Václav ; Čížek, Martin ; Šmíd, Radek ; Lazar, Josef ; Číp, Ondřej
Optical frequency synthesizers (femtosecond combs) are used mainly in the field of precise measurement of optical frequencies, laser spectroscopy and interferometer distance surveying. The repetition and offset frequencies belong to main controlled parameters of such synthesizer. These two parameters need to be locked to ultra-stable frequency standards i.e. H-masers, Cs- or Rb- clocks. We present the complete digital signal processing chain for phase-locked loop (PLL) control of the repetition and offset frequency. The setup is able to overcome some dropouts causes the femtosecond laser instabilities. The presented work gives the possibility of long-time operation of femtosecond combs which is necessary when the optical frequency stability measurement of ultra-stable lasers is performed.
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Electronics for reducing the sideband noise of a laser diode by utilizing an unbalanced fiber interferometer
Čížek, Martin ; Šmíd, Radek ; Číp, Ondřej
Measuring length changes of optical resonators requires using of lasers with a narrow spectral width. Moreover, for tracking a whole range of possible lengths a widely-tunable laser is required. Laser sources based on DFB laser diodes have a wide optical frequency tuning range, however their spectral linewidth is up to single MHz. The usual way of narrowing the linewidth is based on stabilizing the wavelength of the laser by locking it on an etalon resonator cavity using for example a PDH setup. This of course leads to losing the tunability. We present a different approach where an unbalanced fiber Michelson interferometer with a measuring arm formed of a long optical fiber spool is used as a an optical frequency discriminator. The output electrical signal from the photodetector proportional to the optical frequency changes is used in a fast servo loop. The controller slightly detunes the laser diode by adjusting its pump current and locks optical frequency changes to a zero value. This leads to reducing the sideband noise of the laser by up to 60 dB. The whole method is independent on the actual wavelength of the laser and works in the whole tuning range of the DFB diode.
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Narrowing the linewidth of laser diode via fiber interferometer
Šmíd, Radek ; Argence, B. ; Číp, Ondřej ; Santarelli, G.
In this work we present a method of stabilization of laser diodes by unbalanced Michelson fiber interferometer. The unstabilized laser source is represented by compact planar waveguide external cavity laser module ORION (Redfern Integrated Optics, Inc.) working at 1040.57 nm with < 3 kHz linewidth. Wavelength of the laser matches P(13) 13C2H2 absorption line with tunability range +/- 10 pm. We built Michelson interferometer with 1 km arm based on SMF-28 fiber spool to suppress the frequency noise by fast PI control loop up to 33 kHz by modulation voltage. We were able to decrease the noise level by -60 dBc/Hz up to 33 kHz modulation limit range.
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Fabry-Perot displacement interferometry in air
Šmíd, Radek ; Durand, M. ; Wang, Y. ; Lawall, J.
In earlier work, we have demonstrated the use of Fabry-Perot interferometry to measure displacements of up to 50 mm with a fractional accuracy of 4 x 10-10 in vacuum. The initial work was done at a wavelength of 633 nm, and the more recent work was done at 1560 nm. We now seek to extend the approach to displacement measurements in air. A well-known approach to compensate for the refractive index of air without relying on independent measurements of the temperature and pressure is to use two wavelengths. In this work, we supplement our implementation at 1560 nm with an additional laser at 1064 nm in order to control for the dispersion of air.
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Evaluation of lenght of the Fabry-Perot cavity with ultra-low expansion spacer with optical frequency comb
Šmíd, Radek ; Čížek, Martin ; Číp, Ondřej
The ultra-low expansion materials play a crucial role in laser stabilization and they are essential for laser stalibization. Blocks made from ultra-low expansion material are used as a base for precise interferometric measurements and in AFM microscopes. In our experiment we have monitored the free spectral range (FSR) changes and absolute length of a Fabry-Perot cavity (FPC) with a spacer made from an ultra-low expansion ceramics (Zerodur) with the expansion coefficient (CTE) of 2.8 • 10-8K-1. Experiment was carried out in the vacuum chamber at the pressure as low as 1 O'5 Pa. A fiber based femtosecond mode-locked laser with 100 MHz repetition frequency and central wavelength of 1550 nm was locked to a GPS disciplined crystal oscillator with a short term stability better than 10"12 and long term stability controlled by GPS clocks. An auxiliary tunable DFB laser diode was locked to a certain optical mode of the FPC. A beat-note signal from optical mixing between the laser and a stabilized femtosecond frequency comb is detected and processed. The absolute optical frequency of this mode is defined by saturation to acetylene cell. The FPC spacer of Zerodur material of 175 mm length was analyzed during the temperature cycling process from 22 - 35°C. Measured CTE of the spacer material was (0.069 ± 0.005) • 10-6K-1. Material exhibited hysteresis in the spacer length up to 8 nm during cycling process.
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