Field Programmable Gate Array‐Based Control Systems for Highly Stable Laser Applications

We present a hybrid laser frequency stabilization method combining modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) for the cesium D2 transition. In a typical pump-probe setup, the error signal is a combination of the DC-coupled MTS error signal and the AC-coupled FMS error signal. This combines the long-term stability of the former with the high signal-to-noise ratio of the latter. In addition, we enhance the long-term frequency stability with laser intensity stabilization. By measuring the frequency difference between two independent hybrid spectroscopies, we investigate the short-and long-term stability. We find a long-term stability of 7.8 kHz characterized by a standard deviation of the beating frequency drift over the course of 10 h and a short-term stability of 1.9 kHz characterized by an Allan deviation of that at 2 s of integration time.

Synchronous achievement of laser frequency stabilization and tunability via modulation transfer spectroscopy on the rubidium D1 line

We realize a high-performance compact optically pumped cesium beam atomic clock utilizing the Faraday laser simultaneously as the pumping and detection lasers. The Faraday laser, which is frequency stabilized by the modulation transfer spectroscopy (MTS) technique, has a narrow linewidth and superior frequency stability. Measured by the optical heterodyne method between two identical systems, the linewidth of the Faraday laser is 2.5 kHz after MTS locking, and the fractional frequency stability of the Faraday laser is optimized to $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}/\sqrt{\ensuremath{\tau}}$. Based on this high-performance Faraday laser, the cesium beam clock realizes a signal-to-noise ratio of 39 600 in 1-Hz bandwidth when the cesium oven temperature is $130{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Compared with hydrogen maser, the fractional frequency stability of the Faraday-laser-pumped cesium beam clock can reach $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}/\sqrt{\ensuremath{\tau}}$ and decreases to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ at 10 000 s when the cesium oven temperature is set at $110{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. This Faraday-laser-pumped cesium beam clock demonstrates its excellent performance and its great potential in the fields of timekeeping, navigation, and communication. Moreover, as a high-performance optical frequency standard, the Faraday laser can contribute to the development of other applications in quantum metrology, precision measurement, and atomic physics.

We present a novel method of stabilizing a distributed-feedback laser. It’s the fi rst time to our knowledge that the time-measurement technique is used in laser frequency stabilization. We obtain the laser frequency deviation information from the Fabry-Perot interferometer based on the pulse delay time. In contrast to traditional approaches, the laser can be stabilized in the quasi-continuous spectru m that the interferometer covering. Our method can obtain the error signal from a high signal to noise ratio (SNR) of the voltage signal and not limited by the frequency references. It also avoids many traditional problems, such as power insensitive, modulation, lo w-level signal, and finite freq uency references. A relative frequency fluctuation less than 0.1 MHz is achieved and the root of an Allan variance is about 10 -11 for an average time of 10 s. Keywords: Spectroscopy, distributed-feedba ck lasers, laser stabilization 1. INTRODUCTION Frequency stabilization of lasers is crucial for many applications, including high-resolution molecular and atomic spectroscopy, coherent optical communication, precision measurement and Bose-Einstein condensation experiments.In general, laser is locked to a frequency reference by analyzing the error signal. There are many methods to obtain the error signal, including saturated absorption spectroscopy [1-4], modulation transfer spectroscopy [5,6], Sagnac interferometry [7,8], sub-Doppler spectrum [9], and electromagnetically induced transparency [10] etc.. Typically, the error signal from voltage amplitude will decrease to a low level when the laser is locked. So it is greatly influenced by environm ent noises, like electromagnetic interference and thermal noise. And the other limitation of above schemes is that they can only stabilize the laser in some specific frequencies, such as the transmission peak of an interferometer, an atomic spectral line or a molecular absorption line, where the choices of frequency refe rences is limited to a finite number.

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The precise stabilization of laser frequencies is crucial for various applications in atomic physics, particularly in atomic clock experiments and ion cooling. The Pound-Drever-Hall (PDH) technique is a technique for locking laser frequencies to high-finesse optical cavities, ensuring exceptional frequency stability. This study focuses on designing an optical cavity as well as implementing the PDH technique to stabilize a 369-nm laser, essential for ytterbium ion (Yb+) cooling in atomic clock experiments. The results show that the 369-nm laser can maintain stability within a range of 5 MHz within 12 minutes, leading to effective stabilization for practical applications.

A preliminary prototype of spaceborne laser frequency stabilization system for inter-satellite laser ranging missions has been constructed and demonstrated on ground. The system is based on hydroxide-catalysis bonding and fiber optics. The laser frequency control is driven by an in-house-designed digital controller featuring self-analysis and auto-locking functions. Laser frequency noise less than 30 Hz/Hz1/2 from 0.7 Hz to 10 Hz has been measured by beat note analysis. The laser stabilization system has been tested on a laser interferometer test bed. Preliminary experimental results are reported in this paper.

The modulation transfer spectroscopy in an ytterbium hollow cathode lamp at 399 nm is measured. The error signal for frequency locking is optimized by measuring the dependences of its slope, linewidth and magnitude on various parameters. Under the optimum condition, the laser frequency at 399 nm can be stabilized. The long-term stability of laser frequency is measured by monitoring the fluorescence signal of the ytterbium atomic beam induced by the locked laser. The laser frequency is shown to be tightly locked, and the stabilized laser is successfully applied to the cooling of ytterbium atoms.

Optical cooling is a key technique for preparing ultracold atoms in quantum technologies and precision experiments. We employ shortcut-to-adiabaticity (STA) techniques to accelerate and stabilize laser-based atomic cooling protocols. This approach improves the performance of conventional adiabatic momentum transfer schemes by addressing key limitations such as Doppler shifts, laser intensity fluctuations, and spontaneous emission. We first examine two- and three-level atomic systems subjected to counter-propagating laser pulses that induce momentum reduction through photon recoil. STA methods are then employed to construct pulse sequences that are robust against detuning errors and amplitude noise, outperforming standard π-pulse schemes in resilience. Meanwhile, we analyze the dissipative dynamics during the momentum transfer and demonstrate the superiority of the STA protocol in enhancing momentum transfer efficiency via accelerated control. The results demonstrate that STA can significantly improve both the efficiency and robustness of cooling. These findings have implications for applications in atomic physics, quantum information processing, and precision metrology.

No abstract available.

The Zeeman He-Ne laser frequency stabilization system using a surface stabilized ferroelectric liquid crystal cell for polarization switching in the frequency stabilization loop is presented. Our experiments showed that two circularly polarized laser beams with on opposite sense of circularity can be switched up to 3.7 kHz rate. No additional optical components (the quarterwave and halfwave plates) were needed in switching setup. During three hours of operation the long time stability of the laser frequency of 3 multiplied by 10<SUP>-8</SUP> was achieved. The laser stabilization system is suitable for the two frequency interferometer setups.

In a spin-exchange relaxation-free (SERF) atomic magnetometer, the pump and probe laser frequencies are tuned to the D1 transition line of alkali metal atoms to enhance measurement sensitivity. The D1 line, a non-cycling transition, exhibits a low absorption coefficient. This study theoretically analyzes the influence of different parameters on the absorption coefficient of the D1 line and proposes a polarization-dependent approach to enhance it. A frequency stabilization method based on modulation transfer spectroscopy (MTS) with an enhanced absorption coefficient is introduced. Compared with the conventional method, optimizing the probe beam's polarization increases the absorption coefficient, raising the amplitude of the MTS signal. The results show that using a stronger MTS signal for frequency stabilization reduces the laser's frequency noise. Using heterodyne measurement, we characterize the linewidth-narrowing effect of MTS locking and evaluate the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 2.2 MHz to 885 kHz after MTS stabilization. The Allan deviation measurements of the beat signal between two locked lasers are 2.6 × 10-11 /τ. This work provides a stable optical frequency reference for SERF atomic magnetometers and offers potential applications in extracting spatial information from optically pumped atoms.

10. Electric Arcs

Active frequency stabilization of a laser to an atomic or molecular resonance underpins many modern-day AMO physics experiments. With a flat background and high signal-to-noise ratio, modulation transfer spectroscopy (MTS) offers an accurate and stable method for laser locking. However, despite its benefits, the four-wave mixing process that is inherent to the MTS technique entails that the strongest modulation transfer signals are only observed for closed transitions, excluding MTS from numerous applications. Here we report for the first time, to the best of our knowledge, the observation of a magnetically tunable MTS error signal. Using a simple two-magnet arrangement, we show that the error signal for the Rb87F=2→F'=3 cooling transition can be Zeeman-shifted over a range of >15 GHzto any arbitrary point on the rubidium D2 spectrum. Modulation transfer signals for locking to the Rb87F=1→F'=2 repumping transition, as well as 1GHz red-detuned to the cooling transition, are presented to demonstrate the versatility of this technique, which can readily be extended to the locking of Raman and lattice lasers.

Two kinds of optical heterodyne saturation spectroscopies, namely, frequency modulation spectroscopy (FMS) and modulation transfer spectroscopy (MTS), are demonstrated for locking a fiber laser to the transition lines of metastable (4)He atoms around 1083 nm. The servo-loop error signals of FMS and MTS for stabilizing laser frequency are optimized by studying the dependence of the peak-to-peak amplitude and slope on the optical power of pump and probe beams. A comparison of the stabilization performances of FMS/MTS and polarization spectroscopy (PS) is presented, which shows that MTS exhibits relatively superior performance with the least laser frequency fluctuation due to its flat-background dispersive signal, originated from the four-wave mixing process. The Allan deviation of the stabilized laser frequency is 5.4 × 10(-12)@100 s with MTS for data acquired in 1000 s, which is sufficiently applicable for fields like laser cooling, optical pumping, and optical magnetometry.