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Journal of Physics Conference Series

Laser linewidth measurement using beat-note technique

P Parkprom, N Chattrapiban, Pimonpan Sompet, Nithiwadee Thaicharoen

Published Dec 1, 2023• 1 citation
Publisher

Abstract

Abstract Lasers play a crucial role in quantum and atomic physics experiments. To prepare atoms in a desired quantum state, these lasers must possess specific properties, including tunability and a narrow spectral linewidth. In this study, we construct and employ an external cavity diode laser to prepare cold rubidium atoms. We control the laser’s stability using an electronic system and evaluate its performance by measuring the spectral linewidth through the beat signal technique. The measurement indicates a linewidth narrower than 1 MHz, which makes the laser suitable for subsequent experiments. Finally, we adjust the laser frequency to match the rubidium transition frequency between states 5S 1/2, F = 2 ↔ 5P 3/2, F′ = 2, and successfully utilizing the laser to prepare cold rubidium atoms in a magneto-optical trap.

DOI
10.1088/1742-6596/2653/1/012081

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Abstract Lasers play a crucial role in quantum and atomic physics experiments. To prepare atoms in a desired quantum state, these lasers must possess specific properties, including tunability and a narrow spectral linewidth. In this study, we construct and employ an external cavity diode laser to prepare cold rubidium atoms. We control the laser’s stability using an electronic system and evaluate its performance by measuring the spectral linewidth through the beat signal technique. The measurement indicates a linewidth narrower than 1 MHz, which makes the laser suitable for subsequent experiments. Finally, we adjust the laser frequency to match the rubidium transition frequency between states 5S 1/2, F = 2 ↔ 5P 3/2, F′ = 2, and successfully utilizing the laser to prepare cold rubidium atoms in a magneto-optical trap.
This thesis describes the design, construction and characterization of an external cavity diode laser (ECDL) within the context of AMO research – specifically, ultracold rubidium experiments. The main benefit over other forms of laser light is the ECDL’s low cost and narrow linewidth. Having a more narrow linewidth allows us to affect specific electron states, such as for laser cooling, more precisely than broader alternatives. We find that building such a laser in house is feasible with scan range up to 4 GHz. We also note that attention to external noise, through mechanical vibrations but more importantly through temperature drifts, is necessary to produce an ECDL with a stable optical frequency.
We report laser frequency stabilization with modulation transfer spectroscopy (MTS) on 85 Rb atoms. With both PZT (piezo-electric transducer) slow-loop feedback and current fastloop feedback to the laser head, we get a linewidth narrowing less than 5 kHz simultaneously. Laser injection to a laser diode and frequency beating with another polarization spectroscopy based stabilization setup are also employed to check the narrow linewidth property. With the help of the technique, a linewidth around kHz-level laser is obtained and pave the way for the locking of the lattice laser of ytterbium clock with transfer cavity technique. The setup can be used as a frequency reference for precise frequency control of atomic clock system.
The 5S1/2 --> 7S1/2 two-photon transition of atomic rubidium, which is 100 times weaker than the 5S-5D transition, is observed with an extended-cavity diode laser and a vapor cell. Signals with a signal-to-noise ratio of 280 are obtained with a laser power of 10 mW, and the observed linewidth is 3 MHz. The laser wavelength is 760 nm and is locked on the transitions to a stability of 2 x 10(-11). For the first time to our knowledge, the isotope shift of this transition is measured to be 130(4) MHz in atomic frequency.
The diode laser system for laser cooling on the 1S 0 → 3P1 intercombination line of strontium discussed in this thesis allows us to cool and trap an atomic strontium sample to 15 muK. Samples in this temperature range are useful for the development of the next generation of atomic frequency standards, cold collision studies, and as a step towards quantum degeneracy. This laser system consists of a Littrow configuration external-cavity diode laser, which is frequency locked to a high finesse cavity. The cavity is subsequently locked to an absorption feature to produce a few mW of tunable, stabilized, single mode power at the target wavelength. We present the design and characterization of this system, including a linewidth measurement of 70 kHz, as well as preliminary results from cooling and trapping.
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.
The advancement of cold atom gravimeters (CAG) is based on the precise control and stabilization of lasers used for the atom interferometry.The 5S → 5P transition in rubidium (Rb) at 780.241 nm is crucial for such quantum technology applications.In this study, the hyperfine structure of 5 2 S1/2 → 5 2 P3/2, D2 transition line of 87 Rb atoms obtained through saturation absorption spectroscopy, are used as a reference for the frequency stabilization of an external cavity diode laser (ECDL) operating at 780 nm.The ECDL is locked to the hyperfine saturated absorption peak of 87 Rb (F = 2 → 3').This laser beam is combined with another 780 nm ECDL laser to obtain the beat frequency spectrum, generating a beat note signal of ~ 6.8 GHz, which is needed for phase stabilization between lasers.These are critical for the Raman beam generation in the CAG, as the 87 Rb atoms ground hyperfine states (F = 1 and F = 2) have separation of ~ 6.834 GHz, and two Raman beams with an 6.8 GHz offset can drive transitions between these hyperfine states of 87 Rb to realize the CAG interferometry scheme.