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Applied Physics Letters

Laser frequency offset locking using electromagnetically induced transparency

S. C. Bell, D. Heywood, J. D. White, J. D. Close, Robert E Scholten

Published Apr 23, 2007• 63 citations
Publisher

Abstract

The authors have used an electromagnetically induced transparency resonance in rubidium as a dispersive reference to lock the relative frequency of two lasers to the atomic ground-state hyperfine splitting. The beat frequency between the two lasers directly generates a microwave signal at 3.036GHz (Rb85) or 6.835GHz (Rb87). High bandwidth (600kHz) feedback was achieved with only low-frequency (10MHz) electronics using the frequency modulation sideband method. The spectral width of the microwave beat frequency was reduced to less than 1kHz. The technique offers a convenient and low-cost method suitable for many topical two-frequency experiments, including coherent population trapping, slow light, lasing without inversion, and Raman sideband cooling.

DOI
10.1063/1.2734471

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The authors have used an electromagnetically induced transparency resonance in rubidium as a dispersive reference to lock the relative frequency of two lasers to the atomic ground-state hyperfine splitting. The beat frequency between the two lasers directly generates a microwave signal at 3.036GHz (Rb85) or 6.835GHz (Rb87). High bandwidth (600kHz) feedback was achieved with only low-frequency (10MHz) electronics using the frequency modulation sideband method. The spectral width of the microwave beat frequency was reduced to less than 1kHz. The technique offers a convenient and low-cost method suitable for many topical two-frequency experiments, including coherent population trapping, slow light, lasing without inversion, and Raman sideband cooling.
We present two techniques to lock a laser frequency on an atomic transition line by using an electromagnetically induced transparency (EIT) signals, which give optical and electric feedback. We employed these methods to the D2 line of Rb87 atoms. Spectral characteristics of subnatural linewidth EIT allow us to improve frequency stability. By optical feedback of EIT signals, we were able to obtain locking bandwidths of 12 MHz, while frequency stability was 5×10−13 at best after 100 s of integration using a frequency modulation technique.
Dual-frequency (DF) lasers with specific frequency intervals have crucial applications in geometric measurement, optical microwave generation, quantum precision measurement, etc. Here, we report a 780 nm DF rubidium Faraday laser, with two modes’ frequencies corresponding to the 85 Rb and 87 Rb D 2 transitions. The microwave signal generated from the two laser modes’ beat has a linewidth of 671 Hz and is tunable from 4.7–4.95 GHz to 5.94–6.7 GHz by adjusting the diode current. Moreover, we use modulation transfer spectroscopy to lock one of the laser modes to the 87 Rb hyperfine transition, and the frequency stability improves nearly an order of magnitude. The noise characteristics are also measured, where the phase noise is suppressed over 25 dB at 100 Hz offset compared to the microwave signal generated by two single-mode lasers’ heterodyne beat. This 780 nm DF rubidium Faraday laser has two coherent modes with frequencies related to Rb transitions, which provides a promising and convenient method for compact atomic physics applications, such as coherent population trapping clocks, atom gravimeters, and so on.
With the optical phase lock loop (OPLL) we made, we can achieve phase locking at frequency differences ranging from 0.5GHz to 7.5 GHz. This OPLL is fully applicable in atomic physics experiments, mostly in coherent lasers frequency locking. Two kinds of modulation modes were brought to ensure the frequency range and precision: the fast feedback current as the injection current and the slow feedback current to adjust the piezo-electric transducer. This device has been put into an optical lattice platform to lock a laser used for cooling and trapping atoms. The beat signal has a -3dB band width of 1Hz at 6.834GHz, corresponding to the hyperfine splitting of the ground state 87Rb atom.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
Coherent population trapping (CPT) resonances usually exhibit contrasts below 10% when interrogated with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to nearly 100% generating an additional light field through a nonlinear four-wave mixing interaction in the atomic vapor.1 A similar method can also be used to create a beat signal at the CPT resonance frequency that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance.2 This could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a miniature microfabricated saturated absorption spectrometer3 that produces a signal for locking a laser frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1 mm3 and the volume of the entire spectrometer is around 0.1 cm3.
Frequency locking to reference atomic lines using Rydberg electromagnetically induced transparency (EIT) has been recently introduced as an inexpensive and reliable technique for laser frequency stabilization. In this work, we carry out a systematic study of this technique using heterodyne beat spectroscopy. Two different commercial semi-conductor lasers are locked to the same reference frequency using EIT locking, and their relative frequency stability is analyzed and continuously monitored in real time. A substantial improvement in the laser frequency stability is achieved through searching for the optimal proportional–integral settings and EIT probe laser powers. The results show that the cutoff frequency of the beat signal can be lowered to less than 500 kHz. We also compare the frequencies of free running lasers with that of a locked laser and characterize their frequency drifts. This study is important in assessing the use of Rydberg EIT locking in atomic electrometers.