Chinese Journal of Lasers, Volume. 52, Issue 7, 0701003(2025)
Automatic Frequency Stabilization System Based on Sideband PDH Technique with Transfer Cavity
Transfer cavity locking is a high-performance, low-cost frequency stabilization method that is widely used in optical clocks and quantum computing with neutral atoms or ions. Typically, the transfer cavity-locked laser is not tunable, making it unsuitable for cold atom experiments. To address this problem, this study presents an automatic frequency-locking system for a cooling laser of mercury atoms. The wavelength of the cooling laser is 254 nm, which is quadrupled from a 1015-nm slave laser locked to the transfer cavity. To lock the deep-ultraviolet (DUV) laser to the atomic transition, we build an automatic frequency-locking system based on sideband PDH (Pound-Drever-Hall) technology. An analog-to-digital converter (ADC) is employed to acquire the saturated absorption spectra (SAS) and lock-in amplified signals, and an algorithm is developed to identify different atomic transitions. The processed signal is then fed back to the sideband frequency, realizing automatic frequency sweeping, peak searching, and frequency locking.
To derive the sideband PDH technique, sidebands are generated using two cascaded fiber electro-optical modulators (FEOMs) to modulate a 1062-nm master laser. Because the cavity is locked to one sideband, adjusting the sideband frequency can change the cavity length, which in turn changes the frequency of the 1015-nm slave laser. In addition, to achieve automatic frequency locking, we develop a program based on LabVIEW that integrates both algorithm and hardware communication functionalities. Before operation, we need only to set up an appropriate signal threshold, and the system automatically identifies the atomic transition based on this value. The program controls the ADC to collect and analyze the SAS and lock-in amplifier signals. The regulation signal is then fed into the signal generator to change the sideband frequency for scanning and locking. Consequently, when the 1015-nm slave laser is locked to the transfer cavity using the sideband PDH technique, the sideband frequency is regulated by a program that references the SAS signal and lock-in amplifier signal, and the laser frequency can be locked to the atomic transition.
To demonstrate the excellent locking performance of the transfer cavity system, we first measure the frequency noise power spectrum and deep-ultraviolet laser linewidth during locking. The results show that transfer locking significantly reduces the low-frequency noise of the laser (Fig. 6). The linewidth of the two DUV lasers is 450 kHz after being locked to the slave laser by an optical phase-lock loop (Fig. 7), which can be used for laser cooling of mercury atoms. Using a wavelength meter, we also measure the frequency fluctuations of the slave laser after locking. The results show that the frequency of the slave laser has no significant drift within the accuracy of the wavelength meter (Fig. 9). Subsequently, the 254-nm DUV laser is locked to different atomic transitions. For 202Hg, only a single transition occurs in the broad Doppler profile. The system can normally lock the slave laser to this transition, and the SAS signal after locking exhibits excellent stability (Fig. 8). In addition, the transitions of 199Hg and 204Hg are very close to each other, and the system can identify them and lock the 254-nm DUV laser to them separately by inputting different signal thresholds (Fig. 10). Furthermore, the system offers high flexibility; after locking, detuning can be changed or switched to another atomic transition simply by changing the sideband frequency. Finally, we modulate the master laser in this system, effectively overcoming the issue of insufficient broadband FEOM at a certain wavelength. However, the slow frequency drift of the master laser can be directly compensated at the sideband frequency without introducing an additional FEOM.
We developed an automatic frequency stabilization system based on sideband PDH technology with a transfer cavity. By referencing the SAS signal and lock-in amplifier signal, we employ the sideband PDH technique to shift the frequency, allowing the 1015-nm slave laser to automatically lock to different transitions of the mercury atom. This method not only allows the slave laser to be locked to the atomic transition, but also achieves a narrow linewidth of the deep-ultraviolet laser. This system has a user-friendly interface that provides very simple operation for users. In short, this method combines the advantages of the SAS and transfer cavity, significantly improving frequency stability while ensuring that the laser is locked to the atomic transition. The method enables flexible detuning of the slave laser, making it widely applicable to various cold atom experiments.
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Haiyang Song, Li Ma, Jiahui Xie, Shiyuan Zhu, Zhen Xu. Automatic Frequency Stabilization System Based on Sideband PDH Technique with Transfer Cavity[J]. Chinese Journal of Lasers, 2025, 52(7): 0701003
Category: laser devices and laser physics
Received: Oct. 28, 2024
Accepted: Dec. 3, 2024
Published Online: Apr. 14, 2025
The Author Email: Xu Zhen (xuzhen@siom.ac.cn)
CSTR:32183.14.CJL241296