Laser Frequency Stabilization Research Articles

Polarization spectroscopy of an excited state transition in Rubidium

We investigate polarization spectroscopy of an excited state transition in room-temperature rubidium vapor. By applying a circularly polarized coupling beam, resonant with the 52S1/2 → 52P3/2 transition, we induce anisotropy in the atomic medium that is then probed by scanning a probe beam across the 52P3/2 → 62S1/2 transition. By performing polarimetry on the probe beam, a dispersive spectral feature is observed. We characterize the excited-state polarization spectrum as a function of coupling intensity for both isotopes and find that at high intensities, Autler-Townes splitting results in a sub-feature, which theoretical modelling shows is enhanced by Doppler averaging. This spectroscopic technique produces a narrow dispersive signal which is ideal for laser frequency stabilization to excited-state transitions.

ALADIN laser frequency stability and its impact on the Aeolus wind error

Abstract. The acquisition of atmospheric wind profiles on a global scale was realized by the launch of the Aeolus satellite, carrying the unique Atmospheric LAser Doppler INstrument (ALADIN), the first Doppler wind lidar in space. One major component of ALADIN is its high-power, ultraviolet (UV) laser transmitter, which is based on an injection-seeded, frequency-tripled Nd:YAG laser and fulfills a set of demanding requirements in terms of pulse energy, pulse length, repetition rate, and spatial and spectral beam properties. In particular, the frequency stability of the laser emission is an essential parameter which determines the performance of the lidar instrument as the Doppler frequency shifts to be detected are on the order of 108 smaller than the frequency of the emitted UV light. This article reports the assessment of the ALADIN laser frequency stability and its influence on the quality of the Aeolus wind data. Excellent frequency stability with pulse-to-pulse variations of about 10 MHz (root mean square) is evident for over more than 2 years of operations in space despite the permanent occurrence of short periods with significantly enhanced frequency noise (> 30 MHz). The latter were found to coincide with specific rotation speeds of the satellite's reaction wheels, suggesting that the root cause are micro-vibrations that deteriorate the laser stability on timescales of a few tens of seconds. Analysis of the Aeolus wind error with respect to European Centre for Medium-Range Weather Forecasts (ECMWF) model winds shows that the temporally degraded frequency stability of the ALADIN laser transmitter has only a minor influence on the wind data quality on a global scale, which is primarily due to the small percentage of wind measurements for which the frequency fluctuations are considerably enhanced. Hence, although the Mie wind bias is increased by 0.3 m s−1 at times when the frequency stability is worse than 20 MHz, the small contribution of 4 % from all Mie wind results renders this effect insignificant (< 0.1 m s−1) when all winds are considered. The impact on the Rayleigh wind bias is negligible even at high frequency noise. Similar results are demonstrated for the apparent speed of the ground returns that are measured with the Mie and Rayleigh channel of the ALADIN receiver. Here, the application of a frequency stability threshold that filters out wind observations with variations larger than 20 or 10 MHz improves the accuracy of the Mie and Rayleigh ground velocities by only 0.05 and 0.10 m s−1, respectively, however at the expense of useful ground data.

THE LASER TECHNOLOGY FOR EARTHQUAKE'S FORECAST AND FOR DIFFERENT APPROACHES OF SEISMIC HAZARD ASSESSMENT

Historically, the first laser-deformograph was developed by group of Geophysical Observatory of the Tavria National University named after I. Vernadsky (formerly Simferopol State University named after M.V. Frunze) and started to work in 1981. This laser complex allowed to carry out the measurements of the Earth’s long time deformation. The measuring volume of the observatory was located in an adit (depth of about 20 meters), which connects the right rangefinder post with the main battery structure and has a series of sealed baffles (doors, hatches) that isolate it from external influences. In the capacity of the main tools for studying oscillatory processes in the environment, the Geophysical Observatory used two-beam laser interferometers of the Michelson type with spaced beams, which have very high metrological characteristics and use the wavelength of a frequency-stabilized laser as a reference. Engineering support of the interferometric complexes’ functioning in the Geophysical Observatory was carried out by: F.N. Pankov, A.V. Buklersky, V.I. Tokarev [5].

Broadening the dynamic range of the Pound–Drever–Hall frequency stabilization technique

The Pound–Drever–Hall technique is widely used for laser frequency or optical cavity length stabilization. We propose a method to broaden its dynamic range. The conventional error signal divided by the transmission power serves as the new error signal, which increases the dynamic range in the linear regime. The use of an extended Kalman filter improves the resilience to noise when the transmitted power is low. This expands the dynamic range into the nonlinear regime and achieves a deterministic auto-lock even in the presence of a large disturbance. Such a nonlinear extension is, however, limited to an auto-lock for a previously locked state because the extended Kalman filter requires accurate prior knowledge of the initial state. For demonstration purposes, we simulate the implementation of this method for locking a ten-meter scale cavity with a single-stage suspension.

Single step zero-thermal-expansion temperature measurement of optical reference cavities.

State-of-the-art laser frequency stability has been pushed to the 10-17 level. The laser reference cavity is typically nested in a multi-layer thermal enclosure to increase vacuum thermal time constant and thermally controlled at the zero-thermal-expansion temperature to reduce the external temperature fluctuation effect. It is rather time consuming to accurately determine the zero-thermal-expansion temperature for a large thermal time constant system. Here we develop a fast method for measuring the zero-thermal-expansion temperature of the cavity by relying on just one single temperature scan. We first develop a theoretical model to predict the performance of the laser locked to the reference cavity, and then construct an evaluation system for verification of the model. The zero-thermal-expansion temperature of a 30-cm cavity is measured to be 4.3±0.5°C. The fast and high precision method for determining the zero-thermal-expansion temperature will be valuable in improving long-term frequency stabilities of cavity stabilized lasers.

An automatic frequency stabilized laser with hertz-level linewidth

The ultra-stable laser system has been widely used in a variety of fields. Recently, automatic laser frequency stabilization is crucial to some unmanned applications. This paper reports the development of the automatic laser frequency stabilization for the ultra-stable laser system with hertz-level linewidth. The hybrid automatic frequency stabilization circuits are combined with analog high-speed PID circuits and digital automatic controller. An external cavity diode laser is demonstrated to be frequency locked to a reference cavity with a control bandwidth over 2.1 MHz which can be improved. Beating with a second ultra-stable lasers, a median linewidth of 3.86 Hz is obtained via 203 repeated measurements. To characterize the reliability of automatic frequency stabilization, the laser is intendedly interrupted by 4432 times, while the laser frequency is relocked automatically. The mean time for the laser relocking is 4.86 s, and 90 % of the experiments can be relocked within 6 s. Moreover, one week of laser frequency stabilization with automatic relocking is demonstrated. The design of the auto-locking laser system can be applied for field and space applications.

Compact and high reliable frequency-stabilized laser system at 589 nm based on the distributed-feedback laser diodes

Compact and high reliable frequency-stabilized laser system at 589 nm based on the distributed-feedback laser diodes

Absolute frequency readout derived from ULE cavity for next generation geodesy missions.

The next generation of Gravity Recovery and Climate Experiment (GRACE)-like dual-satellite geodesy missions proposals will rely on inter-spacecraft laser interferometry as the primary instrument to recover geodesy signals. Laser frequency stability is one of the main limits of this measurement and is important at two distinct timescales: short timescales over 10-1000 seconds to measure the local gravity below the satellites, and at the month to year timescales, where the subsequent gravity measurements are compared to indicate loss or gain of mass (or water and ice) over that period. This paper demonstrates a simple phase modulation scheme to directly measure laser frequency change over long timescales by comparing an on-board Ultra-Stable Oscillator (USO) clocked frequency reference to the Free Spectral Range (FSR) of the on-board optical cavity. By recording the fractional frequency variations the scale correction factor may be computed for a laser locked to a known longitudinal mode of the optical cavity. The experimental results demonstrate a fractional absolute laser frequency stability at the 10 ppb level (10-8) at time scales greater than 10 000 seconds, likely sufficient for next generation mission requirements.

Large-tuning-range frequency stabilization of an ultraviolet laser by an open-loop piezoelectric ceramic controlled Fabry-Pérot cavity.

We demonstrate a laser frequency stabilization method with large tuning range to stabilize a UV laser by installing piezoelectric ceramic actuators into a Fabry-Pérot cavity with an ultra-low expansion spacer. To suppress piezoelectric drift, a two-layer symmetrical structure is adopted for the piezoelectric actuator, and a 14.7 GHz tuning range is achieved. The short-term drift of the piezoelectric ceramics caused by temperature and creep is eliminated, and the long-term drift is 0.268 MHz/h when the Fabry-Pérot cavity is sealed in a chamber without a vacuum environment. The long-term frequency drift is mainly caused by stress release and is eliminated by compensating the cavity voltage with an open loop. Without the need for an external reference or a vacuum environment, the laser frequency stabilization system is greatly simplified, and it can be extended to wavelengths ranging from ultraviolet to infrared. Owing to its simplicity, stability, and large tuning range, it is applicable in cold atom and trapped ion experiments.

Terahertz frequency counter based on a semiconductor-superlattice harmonic mixer with four-octave measurable bandwidth and 16-digit precision

We have developed a broadband and high-precision terahertz (THz) frequency counter based on a semiconductor-superlattice harmonic mixer (SLHM). Comparison of two THz frequencies determined using two independent counters and direct measurement of frequency-stabilized THz-quantum cascade lasers by a single counter showed a measurement uncertainty of less than 1 × 10−16 over a four-octave range from 120 GHz to 2.8 THz. Further extension of this measurable range was indicated by the research regarding the higher-harmonics generation of a local oscillator for the SLHM. This compact and easy-to-handle THz counter operating at room temperature is available for high-resolution spectroscopy of ultracold molecules proposed for detecting temporal changes in physics constants as well as many THz applications requiring a wide measurement range without a bulky cryogenic apparatus.

Digital long-term laser frequency stabilization with an optical frequency comb.

Laser frequency stabilization plays an important role in high-precision spectroscopic measurements. Since high-accuracy commercial wavemeters became available, wavemeter-based frequency stabilization has found a broad application due to its convenience, flexibility, and wide applicability. However, such stabilization schemes frequently suffer from long-term drift, since the accuracy of the wavelength measurement of a wavemeter is affected by ambient temperature fluctuation. In this work, we demonstrate that such long-term drift can be suppressed by regularly calibrating the frequency of a wavemeter-locked laser utilizing an optical frequency comb, which has much better long-term stability. Under this dual-referenced locking scheme, the Allan deviation is reduced to 3.5 E-12 at 4000 s for a fiber laser operated at 548 nm, which when used in the optical Ramsey spectroscopic measurement of 7Li+, reduces the standard deviation by as much as 40%, compared to the case when only wavemeter locking is applied.

Digitally controlled laser frequency stabilization for a ring laser using saturated absorption.

A digital system for controlling the frequency of a continuous wave (CW) ring laser is described. The system utilizes Doppler-free absorption to steer the laser onto a resonance peak within a vapor cell and can hold the laser at this frequency for long periods of time using active feedback. The vapor cell is immersed in a sinusoidally varying magnetic field that produces a feedback signal by exploiting the Zeeman effect so that the laser frequency does not need to be dithered to achieve lock. A bias field can also be applied to adjust the frequency over several MHz while maintaining lock. This is advantageous for cold atom studies that require the laser to be red-detuned from resonance. Signals from the absorption cell and magnetic field are digitized and fed to a dedicated microcontroller that calculates and produces the feedback signal. The digital control system is described, and measurements are presented where the locked frequency is compared to data obtained from a commercial wavemeter. The locked frequency standard deviations ranged from 250 to 450 kHz within a 1 h period. The Allan deviations of the locked frequency had the best stability at a value of 6 × 10-11 in a 10s averaging period. The locking system described here has application in experiments that require very stable laser frequency control and can also be used for regular recalibration of wavemeter parameters. The design can easily be adapted to control different CW laser systems and hence can be used for a range of atomic targets.

High stability laser locking to an optical cavity using tilt locking.

This paper describes, to our knowledge, the first demonstration of high performance tilt locking, a method of stabilizing laser frequency to an optical reference cavity using a spatial-mode readout technique. The experiment utilized a traveling wave cavity with a finesse of approximately 10,000, housed in a thermally controlled vacuum chamber. The tilt locking method in a double pass configuration has promising performance in the 100 µHz-1 Hz band, including surpassing the Gravity Recovery and Climate Experiment (GRACE) Follow-On laser ranging interferometer requirement. Tilt locking offers a number of benefits such as high sensitivity, low cost, and simple implementation and therefore should be considered for future applications requiring high performance laser locking, such as future laser-based satellite geodesy missions and the Laser Interferometer Space Antenna.

Frequency stabilization of a quantum cascade laser by weak resonant feedback from a Fabry-Perot cavity.

Frequency-stabilized mid-infrared lasers are valuable tools for precision molecular spectroscopy. However, their implementation remains limited by complicated stabilization schemes. Here we achieve optical self-locking of a quantum cascade laser to the resonant leak-out field of a highly mode-matched two-mirror cavity. The result is a simple approach to achieving stable frequencies from high-powered mid-infrared lasers. For short time scales (<0.1ms), we report a linewidth reduction factor of 3×10-6 to a linewidth of 12 Hz. Furthermore, we demonstrate two-photon cavity-enhanced absorption spectroscopy of an N2O overtone transition near a wavelength of 4.53 µm.

Tracing method based on heterodyne interferometer for scanning electron microscopes

In this study, a tracing method for a scanning electron microscope was developed based on a heterodyne interferometer, which can directly trace the measured values to the laser wavelength and then to the internationally defined meter length reference. In this method, a sample scanning motion is introduced, and the measuring mirror of the interferometer is fixed on the sample stage. The movement and displacement of the sample stage is recorded by the laser interferometer displacement measurement system, which synchronously collects the secondary electrons or backscattered electron signals excited on the sample surface. The feature size of the sample is measured, and the value can be traced directly to the international unit of length using the frequency-stabilized laser reference, which proves that the proposed method is an absolute measurement method. This study analyzed the specific technology implementation methods and experimental results, including positioning control of the displacement system and its measurement and control during the measurement process. Measurement uncertainty budgets are also discussed. Experimental measurements were carried out and their accuracy was verified. Results showed that this method could effectively obtain the performance parameters of the feature size intuitively and significantly improve the measurement accuracy.

Compact modulation transfer spectroscopy module for highly stable laser frequency

A compact modulation transfer spectroscopy (CMTS) module with high stability in frequency and robustness is realized for a transportable 87Rb atomic gravimeter. The module is fabricated in a 91 mm×169 mm×60 mm box. The frequency instability achieved with CMTS is below 1.3×10−13/τ in the short term, not exceeding 5×10−12 until 1×105s, which is the best performance of 87Rb recorded so far in the literature. The achievement of high frequency stability with CMTS is discussed, analyzing the factors that impact frequency stability, with the Allan deviation measured under various conditions. This study offers a novel and efficient choice for applications employing the stabilized laser frequency and transportable system.

Numerical Simulations of Arm-locking for Taiji Space Gravitational Waves Detection

The laser frequency stabilization is one of the most important key technologies for the interferometer measurement system of space gravitational waves detection. As a proposed frequency stabilization technique, the scheme of arm-locking is to convert the stability of interferometer arm-length into the stability of laser frequency. Some numerical simulations of arm-locking for Taiji mission were investigated in the paper. Meanwhile, an innovative controller consisted of a compensation filter and two-stage integrators in parallel was presented to suppress the laser frequency noise without increasing gain and prevent the high gain from suppressing the gravitational waves signal. The single arm-locking simulation results showed that the laser noise of closed loop was lower than 3.19 μm/√Hz@10 mHz only in the frequency range of 0.1 mHz – 0.03 Hz. But the dual arm-locking simulation results showed that the laser noise of closed loop was lower than 3.19 μm/√Hz@10 mHz in the full frequency range of 0.1 mHz – 1 Hz, meeting the requirement of Taiji mission. Preliminary results represented the feasibility and effectiveness of arm-locking on laser frequency stabilization for the Taiji mission.

Study and simulation of heterodyne optical phase-locked loop for inter-satellite laser interferometer

Coherent transmission of optical signal is an important module of inter-satellite laser interferometer. Through heterodyne optical phase-locking technology, the coherent transmission of laser signals from the master spacecraft can be achieved. And in the case of ensuring the stability of the local laser frequency, amplifying of the optical output power is realized. This article introduces and analyzes the principle, modeling and simulation of heterodyne optical phase-locking technology, which provides a reference for the realization of laser signal’s coherent transmission from slave satellite.

Spectral filtering of dual lasers with a high-finesse length-tunable cavity for rubidium atom Rydberg excitation* *Project supported by National Key Research and Development Program of China (Grant No. 2016YFA0302800), the National Natural Science Foundation of China (Grant Nos. U20A2074 and 12074391), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB 21010100), the Youth Innovation Promotion Association

We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity (HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound–Drever–Hall (PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 kHz, meanwhile the obtained lasers’ phase noise around 750 kHz are suppressed about 30 dB. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of (67 ± 37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ∼3.3 kHz and the maximum long-term laser frequency drift of a single laser is ∼46 kHz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.

Frequency and amplitude modulation of a cw-dye laser for measuring hyperfine frequency separations in optogalvanic spectra of 139La I.

This paper describes frequency modulation of a cw-dye laser to generate a frequency-discriminant dispersive signal by phase-sensitive detection of the optogalvanic signal in a lanthanum hollow cathode lamp. The frequencies of the six major hyperfine components in 5d26s4F9/2 (4121.572cm-1)----→579.13nm⁡5d26p4F9/2(21384.0cm-1) transition in 139La I were measured with an accuracy of 50 MHz, using a calibrated wavemeter after locking the cw-dye laser frequency to the dispersive error signal of the constituent hyperfine component. The measured hyperfine separations are found to be consistent with separations measured using the standard amplitude modulation technique and also with earlier reported results. The relative ease of the optical setup for locking of the cw-dye laser to optogalvanic spectra can be adapted to atomic physics applications for stabilization of laser frequency in the visible region.