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Laser Frequency Stabilization for Narrow Linewidth Cooling of Lithium-6

Adam Reed

Published Mar 1, 2011• 0 citations
OpenAlex

Abstract

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.

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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.
We have used the narrow 2 S 1/2 [arrow right] 3 P 3/2 transition in the ultraviolet (UV) to laser cool and magneto-optically trap (MOT) 6 Li atoms. Laser cooling of lithium atoms is usually performed on the 2 S 1/2 [arrow right] 2 P 3/2 (D2) transition, where temperatures of twice the Doppler limit, or ∼300 μ K for lithium, are achieved. The linewidth of the UV transition is seven times narrower than the D2 line, resulting in a lower Doppler limit. We show that a MOT operating on the UV transition reaches temperatures as low as 59 μ K. We load 6 million atoms from this UV MOT into a 1070 nm optical dipole trap (ODT). We show that the light shift of the UV transition in the ODT is small and blue-shifted, facilitating efficient loading. Evaporative cooling of a two spin-state mixture of 6 Li in the ODT produces a quantum degenerate Fermi gas with 3 million atoms in only 11 seconds.
We have used the narrow $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ transition in the ultraviolet (uv) to laser cool and magneto-optically trap (MOT) 6{}^{6}6Li atoms. Laser cooling of lithium is usually performed on the $2{S}_{1/2}\ensuremath{\rightarrow}2{P}_{3/2}$ (D2) transition, and temperatures of $\ensuremath{\sim}$300 $\ensuremath{\mu}$K are typically achieved. The linewidth of the uv transition is seven times narrower than the D2 line, resulting in lower laser cooling temperatures. We demonstrate that a MOT operating on the uv transition reaches temperatures as low as 59 $\ensuremath{\mu}$K. Furthermore, we find that the light shift of the uv transition in an optical dipole trap at 1070 nm is small and blueshifted, facilitating efficient loading from the uv MOT. Evaporative cooling of a two spin-state mixture of 6{}^{6}6Li in the optical trap produces a quantum degenerate Fermi gas with $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in a total cycle time of only 11 s.
Laser cooling of lithium atoms is usually performed on the 2S1/2 to 2P3/2 transition at 671 nm. The Doppler temperature of this transition, which is the limit temperature of laser cooling, is 140 µK. The 2S1/2 to 3P3/2 transition at 323 nm has a Doppler temperature 7 times smaller than that of the 671 nm transition. With the 323 nm transition, it is possible to reach a temperature low enough for an all-optical production of a Bose-Einstein condensate. The 323 nm light is produced by second-harmonic generation (SHG). We present our design of the laser system that generates light at 646 nm and the frequency-doubling cavity for SHG. The laser frequency is stabilized by a Fabry-Perot cavity and an Iodine transition.
The modulation transfer spectroscopy (MTS) technique used to stabilize two different transition lines of rubidium, required for laser cooling of atoms, is investigated. The laser source is based on a 1560 nm semiconductor laser diode that is amplified by an erbium doped fiber amplifier (EDFA). The laser output is frequency doubled with a fiber coupled periodically poled Lithium Niobate (PPLN) crystal to 780 nm. To address the weak spectral features of repumping laser, a theoretical study is conducted followed by experimental verification. The 87Rb D2 line F = 1 → F' = 0 transition temperature-enhanced MTS signal is observed, which provides a choice to stabilize the repumping laser frquency on the F = 1 → F ' = 2 transition using frequency shifting with acousto-optic modulators (AOMs). Compared with the orignal MTS signal, this method is shown to be more stable and accurate for laser frequency locking. Laer frequency locking is realized by controlling the driving current of the seed laser using digital PID feedback. After locking, the laser linewidth is 35.36 kHz.
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
We use integrated circuit-based frequency-to-voltage conversion of a frequency comb beat signal as the means for laser frequency stabilization that is suitable for narrow-line laser cooling. The method is compared to an atomic beam lock where the laser frequency instability for the new scheme shows an improvement of 2 orders of magnitude at sub-1 s and grants a lock-capture range that is approximately 30 times greater. We employ the locking method on a 1111.6 nm laser that is frequency doubled and used in a dual-wavelength magneto-optical trap for Yb171 atoms, producing atomic cloud temperatures of ∼20 μK.