Frequency Stabilization Scheme Research Articles

Sensor noise in LISA Pathfinder : Laser frequency noise and its coupling to the optical test mass readout

The (LPF) mission successfully demonstrated the feasibility of the technology needed for the future space borne gravitational wave observatory LISA. A key subsystem under study was the laser interferometer, which measured the changes in relative distance in between two test masses (TMs). It achieved a sensitivity of 32.0−1.7+2.4 fm/Hz, which was significantly better than the prelaunch tests. This improved performance allowed direct observation of the influence of laser frequency noise in the readout. The differences in optical path lengths between the measurement and reference beams in the individual interferometers of our setup determined the level of this undesired readout noise. Here, we discuss the dedicated experiments performed on LPF to measure these differences with high precision. We reached differences in path length difference between (368±5) μm and (329.6±0.9) μm which are significantly below the required level of 1 mm or 1000 μm. These results are an important contribution to our understanding of the overall sensor performance. Moreover, we observed varying levels of laser frequency noise over the course of the mission. We provide evidence that these do not originate from the laser frequency stabilization scheme which worked as expected. Therefore, this frequency stabilization would be applicable to other missions with similar laser frequency stability requirements. Published by the American Physical Society 2024

Stability improvement of a dual-loop optoelectronic oscillator based on self-phase locking.

In this study, we experimentally demonstrate what we believe is a novel dual-loop optoelectronic oscillator (OEO) with a self-phase locking loop (SPLL). Using this feedback loop, the phase of the oscillating signal is locked to its past by adjusting the DC bias voltage of the used Mach-Zehnder modulator. In this way, the oscillating frequency stability can be improved. Proving experiments on a 10-GHz OEO are performed. The results show that more than 20 dB improvements in the phase noise at low offset frequencies are achieved, the frequency fluctuation is reduced to less than 4 kHz in an hour, and the corresponding Allan deviation is decreased by 1 order of magnitude owing to the SPLL. This frequency stabilization scheme without use of any external reference sources has a great prospect in an integrated OEO.

Development of a Laser Frequency Stabilization and an Optical Transmission System for the Francium Electric Dipole Moment Search

Abstract We developed a laser frequency stabilization and an optical fiber transmission system for the the francium electric dipole moment search. The absolute accuracy of a laser frequency stabilization scheme using a state-of-the-art commercial wavelength meter was 0.48 MHz at ±2 nm and -1.33 MHz at ±200 nm from calibration wavelength, respectively, and the frequency instability is below 10-9 with a standard deviation of 0.56 MHz over 60 hours. We also demonstrated that a 400 m long fiber laid between laboratories can transmit 30 mW of trapping laser light, which is sufficient for a magneto-optical trapping of francium. The polarization crosstalk in the fiber was stable at -25 dB over 12 hours of measurement.

A comparative performance analysis of frequency stabilization schemes for multi-area multi-source deregulated power system

A comparative performance analysis of frequency stabilization schemes for multi-area multi-source deregulated power system

Design and characteristics of a cavity-enhanced Fourier-transform spectrometer based on a supercontinuum source.

We report the in-house fabrication of a high-resolution Fourier-transform spectrometer (FTS) for the spectroscopy of molecules in the gas phase at resolutions down to 0.002 cm-1 working in the spectral range from 5880 cm-1 (1.7 μm) to 15 380 cm-1 (650 nm). The FTS employs a supercontinuum as a broadband light source and a He:Ne laser with a homemade frequency-stabilization scheme as the spatial reference for the sampling of the interferogram on a constant optical path difference (OPD) grid. The sampling of the two lasers is performed at constant time intervals, and the resampling process is performed at the software level. The resampling of the interferogram on a constant OPD grid relies on cubic approximations of the He:Ne interference pattern to determine its zero-crossings. The use of an invariant in the sampling process allows us to perform on-the-fly data treatment. Both the hardware aspect and the data processing are described with, in each case, an original approach. We also report the successful coupling of the FTS with a high finesse optical cavity with effective mirror reflectivities of 99.76%, allowing us to reach sensitivities down to 6.5 × 10-8 cm-1 with a root-mean-square accuracy of 0.0017 cm-1 on the position of the Doppler-broadened transitions with a mean transition width of 0.046 cm-1 for spectra recorded at a spectral resolution of 0.015 cm-1. The sensitivity of the instrument per spectral element, once normalized, represents the best sensitivity reported in the literature for Fourier-transform incoherent broadband cavity-enhanced absorption spectroscopy with a supercontinuum light source.

A Compact Laser Pumped 4He Magnetometer with Laser-Frequency Stabilization by Inhomogeneous Light Shifts

We propose a compact 4He magnetometer realizing magnetic field measurement and laser-frequency stabilization simultaneously in a single 4He atomic cell. The frequency stabilization scheme is based on the asymmetric line shape of magnetic resonance which is induced by spatially inhomogeneous light shifts. We investigate the asymmetric line shape of the magnetic resonance signal theoretically and experimentally in laser pumped 4He magnetometer with the magneto-optical double-resonance configuration. Notice that, due to the asymmetric line shape, the in-phase component of the magnetic resonance signal is shown to have a linear dependence with respect to the laser frequency detuning and is used to actively lock the laser frequency to the resonant point. The method reduces the complexity of the system and improves the stability of the magnetometer, making the laser-pumped 4He magnetometer more compact and portable.

Steady-State Analysis of the Effects of Residual Amplitude Modulation of InP-Based Integrated Phase Modulators in Pound–Drever–Hall Frequency Stabilization

Residual amplitude modulation of the phase modulator deployed in Pound-Drever-Hall frequency stabilization is an effect known to cause instabilities to the absolute wavelength of the stabilized laser. We present measurements and analysis of the residual amplitude modulation in an InP-based waveguide electro-optic phase modulator. The modulator is monolithically integrated in the output waveguide of a tuneable laser. The effects on the frequency stabilization of such a laser system to a reference etalon using a Pound-Drever-Hall frequency stabilization scheme are quantified. Frequency offset values in the stabilization point from the reference Fabry-Perot etalon resonance caused by the amplitude modulation are predicted and optimum operating points to minimize residual amplitude modulation are discussed. By operating an electro-refractive phase modulator at the proper bias point we show that frequency offsets corresponding to less than 3×10 ^{-3}$ of the reference cavity full-width half-maximum can be achieved.

Laser frequency stabilization in sub-nanowatt level using nanofibers

We introduce a novel scheme which combines conventional Doppler dichroic atomic vapor laser lock (DAVLL) and nanofiber techniques for realizing frequency stabilization with sub-nanowatt laser power. The dependences of DAVLL signal on the total incident power of probe light and the quantification magnetic field amplitude indicate that the power for frequency stabilization could be minimized to only 15 pW. To evaluate the frequency stability of the locked laser, we calculate the Allan standard deviation, which shows that the relative frequency stability could reach 10−10 level at 10 s. This frequency stabilization scheme paves the road for future quantum optical device integration.

Adaptive Dual-Comb Spectroscopy With 1200-h Continuous Operation Stability

Dual-comb spectroscopy (DCS), without moving mirrors, enables fast optical sampling of molecular vibrations and results in high-resolution and high-accuracy Fourier-transform spectra. This motionless technique holds much promise in gas sensing and environmental monitoring. However, in many cases, these applications require a mature device continuously operating for days or even months, thus posing a challenge to long-term stability of this delicate technique. In this paper, we demonstrate the feasibility of DCS for long-term routine spectral monitoring. A compact dual-comb spectrometer is built based on adaptive sampling and simple frequency stabilization schemes. The spectrometer is ceaselessly recording and displaying, in real time, transmission spectra of a phase-shifted fiber Bragg grating and of gas-phase samples for over 1200 h (50 days). We spectroscopically validate the system by measuring absorption lines of ν1 + ν3 band of C2H2 and of 2ν 3 band of CH4 and comparing the experimental data with HITRAN database.

Electronic-Tuning Frequency Stabilization of a Terahertz Gyrotron Oscillator

Gyrotron oscillator, as one kind of efficient terahertz (THz) source, is developed with the capability of frequency and power tuning to meet diverse application demands. Due to system nonlinearity, it is quite difficult to achieve high frequency stabilization (FS) during power tuning without using external auxiliary components. This paper proposes an FS scheme with simultaneous power tuning in gyrotron backward-wave oscillator state. This is realized by compensating the detuning electron cyclotron frequency during the electronic tuning process. The required quasi-linear relationship between the electron-beam pitch factor and the accelerating voltage is numerically verified by using the SLAC EGUN code. The investigation focuses on a Watt-level open-cavity gyrotron oscillator. The tens-of-Watts-level output power can be tuned by over 40% at the FS point, and especially the self-adaptive FS reaches the accuracy of about 10-MHz level. The proposed scheme would be attractive for the high-stability THz-wave heating in material sciences and biomedicines.

Compact frequency-stabilization scheme for laser cooling of polar molecules

Laser cooling of polar molecules has made great progress in recent years. Due to the complicated energy levels with the vibrational and rotational states, more lasers are required compared with the laser cooling of atoms. The lack of vapor cell spectroscopy makes the frequency stabilization more complex. We have realized a versatile frequency-stabilization scheme to lock three lasers with one transfer cavity. The long-term drift due to the nonlinear response of the piezoelectric ceramics is carefully characterized. With the passive feedback of the environment of the cavity and zero-offset locking, the linewidth and long-term drift of the laser frequency is maintained to be less than 1 MHz, satisfying the stability required by the laser cooling experiment.

Laser frequency stabilization using a commercial wavelength meter.

We present the characterization of a laser frequency stabilization scheme using a state-of-the-art wavelength meter based on solid Fizeau interferometers. For a frequency-doubled Ti-sapphire laser operated at 461 nm, an absolute Allan deviation below 10-9 with a standard deviation of 1 MHz over 10 h is achieved. Using this laser for cooling and trapping of strontium atoms, the wavemeter scheme provides excellent stability in single-channel operation. Multi-channel operation with a multimode fiber switch results in fluctuations of the atomic fluorescence correlated to residual frequency excursions of the laser. The wavemeter-based frequency stabilization scheme can be applied to a wide range of atoms and molecules for laser spectroscopy, cooling, and trapping.

Optical Stabilization of a Microwave Oscillator for Fountain Clock Interrogation.

We describe an optical frequency stabilization scheme of a microwave oscillator that is used for the interrogation of primary cesium fountain clocks. Because of its superior phase noise properties, this scheme, which is based on an ultrastable laser and a femtosecond laser frequency comb, overcomes the frequency instability limitations of fountain clocks given by the previously utilized quartz-oscillator-based frequency synthesis. The presented scheme combines the transfer of the short-term frequency instability of an optical cavity and the long-term frequency instability of a hydrogen maser to the microwave oscillator and is designed to provide continuous long-term operation for extended measurement periods of several weeks. The utilization of the twofold stabilization scheme on the one hand ensures the referencing of the fountain frequency to the hydrogen maser frequency and on the other hand results in a phase noise level of the fountain interrogation signal, which enables fountain frequency instabilities at the 2.5 ×10-14 (τ/s)-1/2 level that are quantum projection noise limited.

A tunable Doppler-free dichroic lock for laser frequency stabilization

We propose and demonstrate a laser frequency stabilization scheme which generates a dispersion-like tunable Doppler-free dichroic lock (TDFDL) signal. This signal offers a wide tuning range for lock point (i.e. zero-crossing) without compromising on the slope of the locking signal. The method involves measurement of magnetically induced dichroism in an atomic vapour for a weak probe laser beam in presence of a counter propagating strong pump laser beam. A simple model is presented to explain the basic principles of this method to generate the TDFDL signal. The spectral shift in the locking signal is achieved by tuning the frequency of the pump beam. The TDFDL signal is shown to be useful for locking the frequency of a cooling laser used for magneto-optcal trap (MOT) for $^{87}Rb$ atoms.

Probe light-shift elimination in generalized hyper-Ramsey quantum clocks

We present a new interrogation scheme for the next generation of quantum clocks to suppress frequency-shifts induced by laser probing fields themselves based on Generalized Hyper-Ramsey resonances. Sequences of composite laser pulses with specific selection of phases, frequency detunings and durations are combined to generate a very efficient and robust frequency locking signal with almost a perfect elimination of the light-shift from off resonant states and to decouple the unperturbed frequency measurement from the laser's intensity. The frequency lock point generated from synthesized error signals using either $\pi/4$ or $3\pi/4$ laser phase-steps during the intermediate pulse is tightly protected against large laser pulse area variations and errors in potentially applied frequency shift compensations. Quantum clocks based on weakly allowed or completely forbidden optical transitions in atoms, ions, molecules and nuclei will benefit from these hyper-stable laser frequency stabilization schemes to reach relative accuracies below the 10$^{-18}$ level.

Long-Term Frequency Stabilization of 10-GHz Quantum-Dash Passively Mode-Locked Lasers

This paper reports on the repetition frequency stability of InAs/InP quantum-dash-based mode locked lasers emitting in the 1.55-μm telecom window and of a long-term frequency stabilization schemes. We discuss the main noise sources that affect the frequency stability. In free running operation, quantum-dash-based mode locked lasers demonstrate an initial long-term stability of $\sim 10^{-7}$ . In order to improve it, we developed different approaches based on electrical feedback and optical feedback loops. The stabilization systems exploit the bias-dependent frequency drift and the optical feedback sensitivity to keep repetition frequency in lock for an extensive period of time with a stability of $1.5 \times 10^{-9}$ .

Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser.

We demonstrate an active phase-nulling scheme for terahertz (THz) frequency quantum cascade lasers (QCLs) under optical feedback, by active electronic feedback control of the emission frequency. Using this scheme, the frequency tuning rate of a THz QCL is characterized, with significantly reduced experimental complexity compared to alternative approaches. Furthermore, we demonstrate real-time displacement sensing of targets, overcoming the resolution limits imposed by quantization in previously implemented fringe-counting methods. Our approach is readily applicable to high-frequency vibrometry and surface profiling of targets, as well as frequency-stabilization schemes for THz QCLs.

An atomic optical filter working at 15 μm based on internal frequency stabilized laser pumping

An excited state Faraday anomalous dispersion optical filter (ES-FADOF) working at the optical communication wavelength (1.5 μm) is realized. Unlike the usual ES-FADOF schemes using an external frequency stabilization, an internal frequency stabilization scheme is proposed and the working atoms inside the filter are adopted as the reference. A particular cross line of multiple transitions is used for the frequency stabilization for the pump laser and thus, a higher pump efficiency is achieved. For example, compared with previous ES-FADOF schemes, this method can increase the transmittance from 10% to 60% at 100 °C. Moreover, in this scheme, the external frequency stabilization is not necessary and the volume of the atomic filter can be reduced. This simplifies the whole structure and a compact ES-FADOF can thus be realized.

Long-Term Repetition Frequency Stabilization of Passively Mode-Locked Fiber Lasers Using High-Frequency Harmonic Synchronization

Long-term repetition frequency stabilization of passively mode-locked (ML) fiber lasers using high-frequency harmonic synchronization is investigated. First, the standard cavity length controlling-based stabilization scheme is studied mathematically, and its disadvantages in high-frequency harmonic synchronization are analyzed. Theoretical studies are then carried out to prove that by modulating the pump power of lasers, the disadvantages can be overcome, and high-stability stabilization with low noises can thus be achieved. Based on the studies, an improved frequency stabilization scheme for passively ML fiber lasers is proposed. Its performances are evaluated by synchronizing a high-frequency harmonic of an ML laser with a 3.035-GHz reference microwave signal. Results show that the residual phase noise for the stabilization (synchronization) reaches around ${-}{100}~{\rm dBc}/{\rm Hz}~({-}{\rm 120}~{\rm dBc}/{\rm Hz})$ at 3-Hz (10 KHz) offset frequency, which results in 14.9 fs (21.2 fs) timing jitter integrated from 1 Hz to 0.1 MHz (1 MHz). The long-term (2 h) phase drift is less than 12 fs for in-loop measurement. For out-of-loop measurement, the drift is ${\sim}{\rm 74}~{\rm fs}$ , while the measurement setup itself brings a drift of ${\sim}{\rm 55}~{\rm fs}$ . The research provides deep studies for frequency stabilization of passively ML fiber lasers and can benefit their applications in various areas.

Frequency stabilization of a Q-switched Nd:YAG laser oscillator with stability better 300 kHz following an rf-sideband scheme

A frequency stabilization scheme following the Pound-Drever-Hall technique modified with a sample and hold circuit has been applied to a Q-switched diode-pumped Nd:YAG ring oscillator. The high-power ring is injection seeded by a monolithic non-planar ring laser oscillator (NPRO). The slave oscillator emits pulses of 23 ns duration and 20 mJ pulse energy with almost diffraction limited beam quality ( M 2 = 1.2) at a repetition rate of 400 Hz. The short-term fluctuation of the center frequency from pulse to pulse is 290 kHz. The oscillator is designed for applications within lidar measurements.