Laser Frequency Stabilization Research Articles

Linewidth narrowing and frequency stabilization of a coin-sized laser module

We demonstrate linewidth narrowing and frequency stabilization of a coin-sized laser module using both a short imbalance path length Michelson fiber interferometer and Doppler-free spectroscopy of molecular iodine. The linewidth of the coin-sized laser is reduced from 2 MHz to 6 kHz, which is now narrower than the Doppler-free spectral linewidth (670 kHz) of molecular iodine. Laser frequency stabilization based on the Doppler-free iodine signal is carried out by directly controlling the imbalance path length of the fiber interferometer. The frequency stability of the hybrid-locked coin-sized laser is 9.8 × 10−13 at 1-s averaging time and reaches 6.8 × 10−14 at 400 s. The hybrid-locked coin-sized laser with linewidth narrowing and frequency stabilization has a long coherence time and known absolute frequency and can be used for precision measurements in either fundamental science or industrial applications.

High-Bandwidth Heterodyne Laser Interferometer for the Measurement of High-Intensity Focused Ultrasound Pressure.

As a high-end medical technology, high-intensity focused ultrasound (HIFU) is widely used in cancer treatment and ultrasonic lithotripsy technology. The acoustic output level and safety of ultrasound treatments are closely related to the accuracy of sound pressure measurements. Heterodyne laser interferometry is applied to the measurement of ultrasonic pressure owing to its characteristics of non-contact, high precision, and traceability. However, the upper limit of sound pressure measurement is limited by the bandwidth of the interferometer. In this paper, a high-bandwidth heterodyne laser interferometer for the measurement of high-intensity focused ultrasound pressure is developed and tested. The optical carrier with a frequency shift of 358 MHz is realized by means of an acousto-optic modulator. The selected electrical devices ensure that the electrical bandwidth can reach 1.5 GHz. The laser source adopts an iodine frequency-stabilized semiconductor laser with high-frequency spectral purity, which can reduce the influence of spectral purity on the bandwidth to a negligible level. The interference light path is integrated and encapsulated to improve the stability in use. An HIFU sound pressure measurement experiment is carried out, and the upper limit of the sound pressure measurement is obviously improved.

Single-Element Dual-Interferometer for Precision Inertial Sensing: Sub-Picometer Structural Stability and Performance as a Reference for Laser Frequency Stabilization.

Future GRACE-like geodesy missions could benefit from adopting accelerometer technology akin to that of the LISA Pathfinder, which employed laser interferometric readout at the sub-picometer level in addition to the conventional capacitive sensing, which is at best at the level of 100 pm. Improving accelerometer performance holds great potential to enhance the scientific output of forthcoming missions, carrying invaluable implications for research in climate, water resource management, and disaster risk reduction. To reach sub-picometer displacement sensing precision in the millihertz range, laser interferometers rely on suppression of laser-frequency noise by several orders of magnitude. Many optical frequency stabilization methods are available with varying levels of complexity, size, and performance. In this paper, we describe the performance of a Mach-Zehnder interferometer based on a compact monolithic optic. The setup consists of a commercial fiber injector, a custom-designed pentaprism used to split and recombine the laser beam, and two photoreceivers placed at the complementary output ports of the interferometer. The structural stability of the prism is transferred to the laser frequency via amplification, integration, and feedback of the balanced-detection signal, achieving a fractional frequency instability better than 6 parts in 1013, corresponding to an interferometer pathlength stability better than 1pm/Hz. The prism was designed to host a second interferometer to interrogate the position of a test mass. This optical scheme has been dubbed "single-element dual-interferometer" or SEDI.

An active method for coupling laser with a high-finesse Fabry–Pérot cavity in ultra-stable lasers

An active method for coupling a laser with a high-finesse Fabry–Pérot cavity in ultra-stable lasers is proposed. This method uses the centroid of the reflected laser spot and the power ratio of the resonant modes to detect the misalignment between the laser and the Fabry–Pérot cavity. Subsequently, four actively rotating wedge prisms are used to compensate for the misalignment. Based on ray tracing and table lookup, the detection and compensation systems can form a closed loop for actively recoupling the laser into the Fabry–Pérot cavity when a misalignment occurs. Through experiments, the laser can be re-coupled with the Fabry–Pérot cavity by four 0.1° wedge prisms within a 0.9 mm decentration or a 0.1° tilt. The re-alignment of the laser and Fabry–Pérot cavity by active optical coupling may provide an alternative solution for the issue of support and vibration sensitivity of the Fabry–Pérot cavity. This solution is essential to improve the frequency stability of transportable and space-borne ultra-stable lasers.

Palm-sized, vibration-insensitive, and vacuum-free all-fiber-photonic module for 10−14-level stabilization of CW lasers and frequency combs

Compact and robust frequency-stabilized laser sources are critical for a variety of fields that require stable frequency standards, including field spectroscopy, radio astronomy, microwave generation, and geophysical monitoring. In this work, we applied a simple and compact fiber ring-resonator configuration that can stabilize both a continuous-wave laser and a self-referenced optical frequency comb to a vibration-insensitive optical fiber delay-line. We could achieve a thermal-noise-limited frequency noise level in the 10 Hz–1 kHz offset frequency range for both the continuous-wave laser and the optical frequency comb with the minimal frequency instability of 2.7 × 10−14 at 0.03-s and 2.6 × 10−14 at 0.01-s averaging time, respectively, under non-vacuum conditions. The optical fiber spool, working as a delay reference, is designed to be insensitive to external vibrations, with a vibration sensitivity of sub-10−10 (1/g) and a volume of 32 ml. Finally, the ring-resonator setup is packaged in a palm-sized aluminum case with 171-ml volume with a vibration-insensitive spool, as well as an even smaller 97-ml-volume case with an ultracompact 9-ml miniaturized fiber spool.

Characterization of Laser Frequency Stability by Using Phase-Sensitive Optical Time-Domain Reflectometry

A new method to measure laser phase noise and frequency stability based on the phase-sensitive optical time-domain reflectometry is proposed. In this method, the laser under test is utilized in a phase-sensitive optical time-domain reflectometer, which employs phase-modulated dual pulses and acts as an optical frequency discriminator: laser frequency fluctuations are deduced from the analysis of the reflectometer data corresponded to phase fluctuations along the vibration-damped and thermally insulated fiber spool. The measurement results were validated by comparison with direct optical heterodyning of the tested and more coherent reference lasers. The use of dual pulses generated by an acousto-optic modulator makes it easy to adjust the time delay during measurements, which distinguishes favorably the proposed method from standard optical frequency discriminators. The method is suitable for testing highly coherent lasers and qualifying their parameters such as linear drift rate, random frequency walk rate, white frequency noise (which is directly related to laser instantaneous linewidth), and flicker noise level.

An ultra-stable laser based on molecular iodine with a short-term instability of 3.3 × 10−15 for space based gravity missions

Many space based gravity missions require frequency stabilized lasers with stringent requirements. Toward those requirements, we develop a compact frequency-stabilized laser which is referenced to the R(56)32−0: a1 transition of molecular iodine based on the modulation transfer spectroscopy technique. The stability of the laser is limited by the beam pointing noise, the electronic servo noise, and the residual amplitude modulation (RAM) noise. To improve the beam pointing stability, the system is constructed by gluing most components of the optical system on an ultra-low expansion glass base. We use a pre-amplifier to suppress the electronic servo noise, and use a wedged electro-optic phase modulator to suppress the RAM noise. The fractional frequency instability of the system is evaluated to be 3.3 × 10−15 at 2 s and 4 s averaging time, and is lower than 6 × 10−15 at averaging times from 1 s to 10 000 s. To our knowledge, this is the best short-term (1–4 s) instability reported so far for an iodine stabilized laser. The stability fully meets the requirements of next generation gravity mission and laser interferometer space antenna mission.

A scalable scanning transfer cavity laser stabilization scheme based on the Red Pitaya STEMlab platform.

Many experiments in atomic and molecular physics require simultaneous frequency stabilization of multiple lasers. We present a stabilization scheme based on a scanning transfer cavity lock that is simple, stable, and easily scalable to many lasers at minimal cost. The scheme is based on the Red Pitaya STEMlab platform, with custom software developed and implemented to achieve up to 100 Hz bandwidth. As an example demonstration, we realize simultaneous stabilization of up to four lasers and a reduction of long-term drifts to well below 1 MHz/h. This meets typical requirements, e.g., for experiments on laser cooling of molecules.

Artificial neural networks for laser frequency stabilization.

In order to stabilize a laser's emission frequency, absolute references such as molecular absorption lines are widely used. To automate the stabilization process, the desired absorption line needs to be identified reliably from a spectrum by a computer. We present an artificial neural network solving this task using the iodine spectrum as an example. The neural network is trained using only simulated data and subsequently tested using measured data. We show that this approach is robust against large variations of operating and environmental conditions.

Laser frequency stabilization based on Fano resonance in a microcylinder cavity.

We investigate the application of Fano resonance in microcylinder cavities for laser frequency stabilization. By combining Fano resonance and the differential subtraction method, we successfully reproduce the error signal of the traditional Pound-Drever-Hall (PDH) technique. The frequency noise of the laser, when locked to both microsphere and microcylinder cavities, approaches the thermal noise limit. The microcylinder cavity, with a high Q factor of ∼108, benefiting from its large mode volume, exhibits a significant reduction in frequency noise by one order of magnitude compared with a microsphere in the frequency range of 0.1 to 10 kHz, achieving a minimum noise of ∼2.25 Hz2/Hz at 10 kHz. As this approach eliminates the need for additional electronic circuits typically used in the PDH technique, it holds promise as a cost-effective and reliable solution for laser frequency stabilization.

Vibration sensitivity minimization of an ultra-stable optical reference cavity based on orthogonal experimental design

Abstract The ultra-stable optical reference cavity (USORC) is a key element for a variety of applications. In this work, based on the orthogonal experimental design method, we study the vibration sensitivity optimization of a classical USORC with a 100 mm length. According to a test of 4 levels and 3 factors, the L 16 (43) orthogonal table is established to design orthogonal experiments. The vibration sensitivities under different parameters are simulated and analyzed. The vibration sensitivities in three directions of the USORC are used as three single-object values, and the normalized sum of the three vibration sensitivities is selected as comprehensive object values. Through the range analysis of the object values, the influence degrees of the parameters on the three single objects and the comprehensive object are determined. The optimal parameter combination schemes are obtained by using the comprehensive balance method and the comprehensive evaluation method, respectively. Based on the corresponding fractional frequency stability of ultra-stable lasers, the final optimal parameter combination scheme A1B3C3 is determined and verified. This work is the first to use an orthogonal experimental design method to optimize vibration sensitivities, providing an approach to vibration sensitivities optimization and is also beneficial for the vibration sensitivity design of a transportable USORC.

Tri-comb generation with a dual-ring structure.

By introducing a third measurement comb with different repetition frequencies (Δ f r e p ), the tri-comb spectroscopy technique overcomes the ambiguity problem of the original dual-comb spectroscopy technique and eliminates physical delay stages in multidimensional coherent spectroscopy. Nowadays, tri-comb generation based on three frequency-stabilized comb lasers is overly complicated and costly for many potential applications. Previous research on single-cavity dual-combs inspired research on single-cavity tri-combs. However, the currently reported tri-comb structures cannot achieve independently controllable pulses. This paper shows a dual-ring tri-comb seed-source structure using wavelength-based multiplexing in one of the rings. The wavelength and power of the output pulse are independently controlled by using the dual-ring structure. The Δ f r e p of wavelength multiplexing-based dual-comb output can be tuned by adjusting the intra-ring polarization controller (PC). In the case of single-wavelength mode-locking, the PC can be adjusted to achieve a wavelength tuning range of nearly 20nm. The tri-comb source could offer an attractive alternative solution as a low-complexity light source for field-deployable multi-comb metrology applications.

Cavity ring-down spectroscopy with a laser frequency stabilized and locked to a reference target gas absorption for drift-free accurate gas sensing measurements

A new gas sensor system with fast response and ultra-high sensitivity has been developed based on a combination of frequency modulation spectroscopy (FMS) and cavity ring-down spectroscopy (CRDS). The system consisted of two distributed feedback laser diodes (DFB-LDs) emitting at frequencies 6251.761 cm-1 (Laser-1) and 6257.762 cm-1 (Laser-2), respectively. A portion of Laser-1’s output was used by a frequency modulation spectroscopy technique to lock its frequency precisely at a CO2 absorption peak, while the rest of its output was coupled to an optical ring-down cavity, together with the Laser-2 output. The Laser-2 operated at a non-absorbing frequency for real-time correction of any baseline ring-down time drift caused by environmental changes (e.g., temperature, pressure). Laser frequency stabilization achieved a 5-fold improvement in CRDS detection sensitivity. This new system was able to make measurements at a data rate of 9 Hz. Based on Allan deviation analysis, the absorbance detection limit of the system was 4.4 × 10−11 cm-1 at an optimum averaging time of ∼5 s, whereas the time-normalized sensitivity at 1 s was 7.3 × 10−11 cm-1/Hz1/2. Measurements of atmospheric CO2 mole fraction were conducted and demonstrated its good performance and reliability. This sensor will be particularly suitable for making drift-free measurements over long periods, in the fields of environmental and industrial gas sensing.

System of Laser Interferometers with a Large Spatial Difference to Study the Seismic-Deformation Oscillations of the Ea

The results of unique experiments on synchronous recording of oscillations of earth’s surface with three laser interferometers–deformographs spaced at a distance of 6740 km, obtained during 2016–2020, are considered: two 100-m laser deformographs (Fryazino, Moscow oblast) and one 18-meter (Karymshina site, Kamchatka krai). It is shown that frequency-stabilized and thermocontrolled lasers, as well as systems for registering shifts of the compensation and modulation type interferogram provide an absolute instrumental resolution of 0.1–0.01 nm.

Ultra-narrow linewidth laser system based on intra-cavity electro-optic crystal frequency stabilization of external-cavity diode laser

An external-cavity diode laser based on an intra-cavity electro-optic modulator tuning is developed in this paper. Electro-optic crystal lithium niobate (LiNbO3), with no hysteresis effect and high response speed, is used to replace the traditional tunable part piezoelectric transducer (PZT). The linewidth of this laser at free-running mode is about 133 kHz. In addition, in the ultra-narrow linewidth laser system, the error signal is fed back to the driving voltage of LiNbO3 to realize laser locking, which can avoid the problem of laser output power fluctuation caused by feeding back to the laser injection current. The self-developed crystal-tuned laser has been successfully used in an ultra-narrow linewidth laser system, and the experimental results show that the linewidth of its beat frequency signal is about 1.5 Hz.

Field-programmable-gate-array-based digital frequency stabilization of low-phase-noise diode lasers.

We present the comparison of a field-programmable-gate-array (FPGA) based digital servo module with an analog counterpart for the purpose of laser frequency stabilization to a high-finesse optical cavity. The transfer functions of both the digital and analog modules for proportional-integral-derivative control are measured. For the lasers stabilized to the cavity, we measure the singe-sideband power spectral density of fast phase noise by means of an optical beat with filtered light transmitted through the cavity. The comparison between the digital and analog modules is performed for two low-phase-noise diode lasers at 1120 and 665 nm wavelengths. The performance of the digital servo module compares well to the analog one for the lowest attained levels of 30 mrad for the integrated phase noise and 10-3 for the relative noise power. The laser linewidth is determined to be in the sub-kHz regime, only limited by the high-finesse cavity. Our work exploits the versatility of the FPGA-based servo module (STEMlab) when used with open-source software and hardware modifications. We demonstrated that such modules are suitable candidates for remote-controlled low-phase-noise applications in the fields of laser spectroscopy and atomic, molecular, and optical physics.

Stabilizing Frequency of a Diode Laser to a Reference Transition of Molecular Iodine through Modulation Transfer Spectroscopy

We report the frequency stabilization of an external cavity diode laser (ECDL) to a reference molecular iodine (I2) transition at 13,531.18 cm−1 (739.03382 nm). Using the Modulation Transfer Spectroscopy (MTS) method for the highly sensitive detection of weak absorption signals, the Doppler-free absorption peaks of I2 corresponding to the hot band transition R(78) (1–11) are resolved. The ECDL’s frequency is stabilized with respect to one of the lines lying within the reference absorption band. For this, the iodine vapor cell is heated to 450 °C and the corresponding circularly polarized pump and probe beam powers are maintained at 10 mW and 1 mW, respectively, to avoid power broadening. The short (100 ms) and long-term (50 h) linewidths of the frequency stabilized laser are measured to be 0.75(3) MHz and 0.5(2) MHz, respectively, whereas the natural linewidth of the specific I2-transitions lie within a range of tens of MHz.

Levitating the noise performance of ultra-stable laser cavities assisted by a deep neural network: the non-intuitive role of the mirrors.

The most precise measurand available to science is the frequency of ultra-stable lasers. With a relative deviation of 4 × 10-17 over a wide range of measuring times between one second and 100 seconds, the smallest effects in nature can thus be made measurable. To enable cutting-edge precision, the laser frequency is stabilized to an external optical cavity. This complex optical device must be manufactured to the highest standards and shielded from environmental influences. Given this assumption, the smallest internal sources of perturbation become dominant, namely the internal noise of the optical components. In this work, we present the optimization of all relevant noise sources from all components of the frequency-stabilized laser. We discuss the correlation between each individual noise source and the different parameters of the system and discover the significance of the mirrors. The optimized laser offers a design stability of 8 × 10-18 for an operation at room temperature for measuring times between one second and 100 seconds.

Sub-kHz high-order mode Brillouin random fiber laser based on long-period fiber grating and distributed Rayleigh scattering in a half-open linear cavity.

We demonstrate a narrow-linewidth high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on a long-period fiber grating (LPFG) and distributed Rayleigh random feedback in a half-open linear cavity. The single-mode operation of the laser radiation with sub-kilohertz linewidth is achieved thanks to distributed Brillouin amplification and Rayleigh scattering along kilometer-long single mode fibers whilst a few mode fiber-based LPFGs enable the transverse mode conversion among a broadband wavelength range. Meanwhile, a dynamic fiber grating (DFG) is embedded and incorporated to manipulate and purify the random modes, which hence suppresses the frequency drift resulting from random mode hopping. Consequently, the random laser emission with either high-order scalar or vector modes can be generated with a high laser efficiency of 25.5% and an ultra-narrow 3-dB linewidth of 230 Hz. Furthermore, the dependence of the laser efficiency and frequency stability on the gain fiber length are also experimentally investigated. It is believed that our approach could provide a promising platform for a wide range of applications such as coherent optical communication, high-resolution imaging, highly sensitive sensing, etc.

A simple and low-cost setup for part per billion level frequency stabilization and characterization of red He-Ne laser

This work describes the frequency stabilization of a dual longitudinal mode, red (632.8 nm) He-Ne laser, implemented using an open source low-cost microcontroller (Arduino Uno) and its performance characterization using a simple interferometric method. Our studies demonstrate that frequency stability up to 0.42 MHz (3σ, 17 h) can be achieved using this setup. This simple and low-cost system can serve as an excellent part per billion level frequency reference for high-resolution spectroscopy based applications.