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Canadian Journal of Physics

A home-built lock-in amplifier for laser frequency stabilization

K Sowka, M. Weel, S. Cauchi, L Cockins, Anantharaman Kumarakrishnan

Published Sep 1, 2005• 14 citations

Abstract

We describe an inexpensive lock-in amplifier that can be built using discrete off-the-shelf RF components and home-built analog circuits. This lock-in has been used in a feedback loop to lock the frequency of a laser to an atomic transition. The frequency stability was tested by trapping rubidium atoms. The feedback loop involves obtaining a modulated saturated absorption signal from a vapor cell using an acousto-optic modulator to modulate the laser frequency. The absorption signal is sent to the lock-in to generate an error signal, proportional to the first derivative of the absorption, which is fed back to the laser to complete the feedback loop. We also demonstrate a simplified optical setup for viewing the saturated absorption spectrum. In this case, the signal consists of narrow saturated absorption spectra riding on top of a Doppler-broadened pedestal. We show that it is possible to greatly reduce the effect of this background and lock the laser to the atomic resonance by modifying the home-built lock-in to generate an error signal proportional to the third derivative of the absorption. The results of this work can be adapted for advanced undergraduate laboratory work.PACS Nos.: 01.50Pa, 07.50.e, 42.62Fi, 42.60Fc

DOI: 10.1139/p05-049

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We describe an inexpensive lock-in amplifier that can be built using discrete off-the-shelf RF components and home-built analog circuits. This lock-in has been used in a feedback loop to lock the frequency of a laser to an atomic transition. The frequency stability was tested by trapping rubidium atoms. The feedback loop involves obtaining a modulated saturated absorption signal from a vapor cell using an acousto-optic modulator to modulate the laser frequency. The absorption signal is sent to the lock-in to generate an error signal, proportional to the first derivative of the absorption, which is fed back to the laser to complete the feedback loop. We also demonstrate a simplified optical setup for viewing the saturated absorption spectrum. In this case, the signal consists of narrow saturated absorption spectra riding on top of a Doppler-broadened pedestal. We show that it is possible to greatly reduce the effect of this background and lock the laser to the atomic resonance by modifying the home-built lock-in to generate an error signal proportional to the third derivative of the absorption. The results of this work can be adapted for advanced undergraduate laboratory work.PACS Nos.: 01.50Pa, 07.50.e, 42.62Fi, 42.60Fc

A sub-MHz frequency stability of the laser is crucial in the quantum-atom experiment where precise atomic transition is required for control and manipulation of atom like in the atom quantum computing. In this work, we have built the frequency stabilization system for the lasers used in our cold Rubidium atom experiment in the magneto-optical trap (MOT) and the optical tweezers. We utilized a Rubidium vapour glass cell as a frequency reference, which provides atomically absolute reference point with low complexity setup. By mean of the absorption of the laser beam by the Rubidium vapour, the Doppler-free saturated absorption (DFSA) experiment was setup to produce the D2 line spectra. The acousto-optic modulator (AOM) was used to sinusoidally dither the laser frequency and consequently the resonance peaks in the spectra. Detailing in this work, the AOM sinusoidal dithering and the dithered DFSA signal had orthogonality and were processed together to produce the error signal used to stabilize the laser frequency to the resonant peak. This frequency stabilization system achieved the frequency standard deviation below 0.75 MHz over 200 minutes period. The system also allowed for stabilized frequency tunability over a range of 55-60 MHz about the target resonance peak. This performance is significant for stabilizing the lasers in the MOT experiment and related quantum-atom research.

Laser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth of an atomic transition sets a lower bound on the Doppler cooling temperature. The 2s-2p transition in lithium-6 has a Doppler temperature of ~140 μK In contrast, the 2s-3p transition has a narrower linewidth and thus provides a lower temperature limit of ~20 μK. We present a method for stabilizing a laser to an atomic line in a vapor cell using modulation transfer spectroscopy and a home-built lock-in amplifier. Our results demonstrate successful locking of a 323 nm laser to the 2s-3p transition. The stabilized laser provides a second stage of magneto-optical trapping that results in a factor of ∼5 increase in the phase space density before evaporating to degeneracy in an optical dipole trap.

In this study, a miniature laser frequency stabilization module for cold atom sensing applications was designed and realized.The module can lock the laser frequency to the saturated absorption spectrum of the D2 line of rubidium (Rb) atoms using the frequency modulation spectroscopy method.The module core is a tiny Rb cell, co-packaged with the saturation absorption optical setup, temperature control circuit, and detection circuit through a joint optical-mechanical-electrical design for a total volume of only 40 mm × 15 mm × 30 mm.The frequency fluctuation of the 780 nm laser after frequency stabilization by the module was within 1 MHz over 1000 s, which is adequate for magneto-optical trap experiments.The results verify the feasibility of the module as a frequency reference and provide a light source for portable cold atom sensing devices.

We report the frequency stabilization of a diode laser by applying the dichroic-atomic-vapor laser lock (DAVLL) method to a thin Rb vapor cell. A 1-mm-long Rb vapor cell is used to obtain a sub-Doppler spectrum that is required for frequency locking, which is easier than in the case of using a saturated absorption spectrometer. A 794-nm diode laser is tuned to the Rb D 1 line, and sub-Doppler signals are observed by introducing a pumping beam and a probe beam in a perpendicular direction. We obtain error signals by subtracting the signals of σ+ polarized beams from that of σ- polarized beams in the presence of a magnetic field and stabilized the 794 nm diode laser frequency without modulating laser parameters. The stability of the 794-nm diode laser is evaluated using other independently stabilized diode laser.

An all-optical locking technique without extra electrical feedback control system for a semiconductor laser has been used in stabilizing the laser frequency to a hyperfine crossover transition of 87Rb 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 with 1 MHz level accuracy. The optical feedback signal is generated from the narrow-band Faraday anomalous dispersion optical filter (FADOF) with nonlinear saturation effect. The peak transmission of the narrow-band FADOF corresponding to 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 crossover transition is 18.6 %. The bandwidth is as wide as 38.9 MHz as the laser frequency changes. After locking, the laser frequency fluctuation is reduced to 1.7 MHz. The all-optical laser locking technique can be improved to much higher accuracy with increased external cavity length. The laser we have realized can provide light exactly resonant with atomic transitions used for other atom-light interaction experiments.