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Cavity-stabilized ultra-stable optical oscillators are one of the core ingredients in the ground-based or spaceborne precision measurements such as optical frequency metrology, test of special relativity, and gravitational wave observation. We report in detail the development of two ultra-stable systems based on 1064-nm neodymium-doped yttrium aluminum garnet lasers and 20-cm optical cavities. The optical cavities adopt ultra-low-loss silica mirrors with compensating rings. An electro-optic crystal with a wedged angle is used to reduce the residual amplitude modulation. Using two-stage thermal control, long-term stabilities of 100 µK are achieved for the outer wall of the vacuum chamber housing the optical cavity. Two additional thermal shields increased the time constant of the optical cavities to 70 h. By operating the optical cavity at the temperature of zero coefficient of thermal expansion, the frequency stability reaches 2.5 × 10

Residual amplitude modulation is one of the major sources of instability in many precision measurements using frequency modulation techniques. Although a transverse and inhomogeneous distribution of residual amplitude modulation has long been observed, the underlying mechanism is not well understood. We perform measurement and analysis of this spatial inhomogeneity using several electro-optic crystals of different types. Two distinct components are identified in the spatial distributions, and their detailed properties, some of which are previously unnoticed, are mapped out and analyzed, showing that the spatial inhomogeneity can be explained by acousto-optic interaction inside the crystal. Moreover, this spatial inhomogeneity can be further suppressed, improving the 1000-s stability of residual amplitude modulation to 3×10−7 (8×10−8) at modulation frequency of 11 MHz (120 kHz), corresponding to a frequency instability of 1×10−17 (3×10−18), estimated for a cavity-stabilized laser using a Pound-Drever-Hall discrimination slope of 1×10−4 V/Hz.

In order to diagnose the electron cyclotron resonance (ECR) plasma, a high-efficiency collimation system has been developed at the Institute of Modern Physics, and the bremsstrahlung spectra in the range of 10 keV-300 keV were measured on a third generation superconducting ECR ion source, SECRAL-II, with a CdTe detector. Used as a comparative index of the mean energy of the high energy electron population, the spectral temperature, Ts, is derived through a linear fitting of the spectra in a semi-logarithmic representation. The influences of some main source parameters, such as the neutral gas pressure, extraction voltage, microwave power, and bias disk voltage, on the high energy electrons are systemically investigated.

We develop a rotating-optical-cavity apparatus to test Lorentz invariance in the photon sector of standard model extension. The system adopts a precision air-bearing turntable on which two 1064-nm lasers are referenced to two orthogonal 10-cm optical cavities embedded in a single block of glass. The heterodyne beat frequency of the two locked lasers is probed as the table rotates, and valid data of 119 days are accumulated. By fitting the data to the predication of the test model, eight Lorentz violating parameters ( β ⊕ κ o+XY, β ⊕ κ o+XZ, β ⊕ κ o+YZ, κe-ZZ, κe-XX-YY, κe-XY, κe-XZ, κe-YZ) are constrained with precision at the level of 10−17. Uncertainties of the experimental values of five κe- parameter show a 1/ t decreasing trend with the measurement time. Influences of systematic effects on the test accuracy are investigated. The system is planned to run for more than one year, allowing for improved precision.

Residual amplitude modulation is one of the major stability-degrading factors in many precision measurements. Using an electro-optic (EO) crystal with wedged input and output surfaces is an effective way to suppress residual amplitude modulation. Here the mechanism of residual amplitude modulation in this approach is investigated. The residual amplitude modulations measured in standard and wedged EO crystals bear similarities in their temperature and polarization dependences, implying that a mixture of the two orthogonal polarizations in the extraordinary light is responsible for the residual amplitude modulation in the wedged EO crystal. Similar to a standard EO crystal, a non-uniform spatial distribution of residual amplitude modulation is also observed in the extraordinary light emerging from the wedged EO crystal. The optical isolator after the EO crystal is replaced by a Faraday rotator, and an improvement in the long-term stability is observed. With the wedged-crystal approach, residual amplitude modulation as low as 2×10

Many precision measurements using rotating optical cavities have tight requirement on the level of a rotating platform on which the cavities are installed. We develop a single-stage tilt control system as one part of the experimental setup for testing Lorentz invariance using two optical cavities that are perpendicular to each other. Home built voice-coil actuators, together with a tilt sensor and control electronics, are adopted to simultaneously suppress the tilt of the rotating platform in two orthogonal directions. The large cross coupling between two orthogonal axes is observed, and its mechanism is examined. By adding a compensating weight, the loop instability resulting from the cross coupling is effectively removed. With the active control, the variation of the tilt around each of the two axes is reduced from the free-running value of ±30 µrad to within ±0.2 µrad, independently measured by using a second tilt sensor. This residual tilt has a component at the rotational frequency with an amplitude of 0.1 µrad, which contributes a systematical offset of 1.8 × 10−17 to the Lorentz violating parameter κe−ZZ, estimated by assuming that the tilt sensitivity of the optical cavity is 1 × 10−16/μrad. Further improvement is still possible by using piezo-electric actuators that exhibit higher resonant frequencies, a different approach that will suppress the residual variation of the tilt to within ±0.01 µrad and allow a reduced systematical offset of 1.8 × 10−18 for κe−ZZ.

Cavity-stabilized ultra-stable optical oscillators are one of the core ingredients in the ground-based or spaceborne precision measurements such as optical frequency metrology, test of special relativity, and gravitational wave observation. We report in detail the development of two ultra-stable systems based on 1064-nm neodymium-doped yttrium aluminum garnet lasers and 20-cm optical cavities. The optical cavities adopt ultra-low-loss silica mirrors with compensating rings. An electro-optic crystal with a wedged angle is used to reduce the residual amplitude modulation. Using two-stage thermal control, long-term stabilities of 100 µK are achieved for the outer wall of the vacuum chamber housing the optical cavity. Two additional thermal shields increased the time constant of the optical cavities to 70 h. By operating the optical cavity at the temperature of zero coefficient of thermal expansion, the frequency stability reaches 2.5 × 10−16 at 10 s averaging time and remains below 5 × 10−16 with an extended time of 1000 s after removing the first- and second-order drifts. The dependence of the laser linewidth on the measurement time is tested against a simplified theoretical model.

Due to their excellent noise performance and technical maturity, ultra-stable continuous-wave neodymium-doped yttrium aluminum garnet lasers are one of the major light sources in a series of precision measurements such as ground and space-based gravitational wave detection, inter-satellite laser ranging and coherent optical communication. As the first step of developing an ultra-stable spaceborne laser, we carried out the design, development and environmental test of an all-solid-state Nd:YAG NPRO spaceborne laser. The laser deliveries 11.48 mW of optical power at 1064.405 nm, exhibits intensity and frequency noises less than 1 × 10[Formula: see text]/[Formula: see text]Hz and 1 MHz/[Formula: see text]Hz (Fourier frequencies [Formula: see text] 10 mHz), respectively, passed environmental tests and has been onboard the first satellite of the Taiji program for one year. The work laid a solid foundation for realizing ultra-stable lasers for the Taiji program as well as being used in many space applications.

A high-temperature oven based on the inductive heating technology was developed successfully at the Institute of Modern Physics in 2019. This oven features a durable operation temperature of over 2000 °C inside the tantalum susceptor. By carefully designing the oven structure, the material compatibility issue at high temperature has been successfully solved, which enables the production and routine operation of refractory metal ions with SECRAL-II (Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou No. 2). To further apply this type of oven to the room temperature ECR ion sources LECR4 and LECR5 (Lanzhou Electron Cyclotron Resonance ion source No. 4 and 5), a mini-inductive heating oven has been fabricated and tested in 2020. By directly evaporating calcium oxide, some high charge state calcium beams have been produced successfully, such as 52 euA of 40Ca16+, 30 euA of 40Ca17+, and 12 euA of 40Ca18+. The detailed design and testing results will be presented and discussed.

The Space Environment Simulation and Research Infrastructure project, which uses various ion beams as irradiated materials in life science research, is being built at the Harbin Institute of Technology. A new room temperature electron cyclotron resonance ion source, the Lanzhou Electron Cyclotron Resonance Ion Source No. 5 (LECR5), has been designed and constructed. It is an intense, highly charged, heavy ion beam injector which generates ion beams from H to Bi, typically ∼50 eμA 209Bi32+. The LECR5 is designed to operate at microwave frequencies in the range of 14.5–18 GHz. The typical magnetic parameters are designed based on those optimized for SECRAL, which operates at 18 GHz. This paper presents the LECR5 ion source, its test bench, and the preliminary beam results.The Space Environment Simulation and Research Infrastructure project, which uses various ion beams as irradiated materials in life science research, is being built at the Harbin Institute of Technology. A new room temperature electron cyclotron resonance ion source, the Lanzhou Electron Cyclotron Resonance Ion Source No. 5 (LECR5), has been designed and constructed. It is an intense, highly charged, heavy ion beam injector which generates ion beams from H to Bi, typically ∼50 eμA 209Bi32+. The LECR5 is designed to operate at microwave frequencies in the range of 14.5–18 GHz. The typical magnetic parameters are designed based on those optimized for SECRAL, which operates at 18 GHz. This paper presents the LECR5 ion source, its test bench, and the preliminary beam results.

An all permanent magnet Electron Cyclotron Resonance ion source-LAPECR1U (Lanzhou All Permanent magnet ECR ion source no.1 Upgraded), has been built at IMP in 2017 to satisfy the requirements of LEAF (Low Energy intense-highly-charged ion Accelerator Facility) for first two years commissioning. LAPECR1U was designed to be operated at 14.5 GHz to produce intense low and medium charge state ion beams. LAPECR1U features a compact structure, small size, and low cost. A cone-shape iron yoke in injection side and an iron plasma electrode in extraction side were used to enhance the axial magnetic field strength. The typical parameters and the preliminary beam results of the source are given in this paper.

Two 1,064-nm Nd:YAG lasers frequency stabilized by high-finesse optical cavities are developed to investigate various noise mechanisms in ultra-stable optical oscillators. Active control of residual amplitude modulation using a separate sensing path is implemented and its effectiveness in the presence of a resonant optical cavity is theoretically analyzed and experimentally verified by measuring the rejection ratios in optical heterodyne beat between a perturbed laser and a stable reference. Laser frequency noises originated from vibration, residual amplitude modulation, quantum-limited shot noise, and electronic noise are experimentally analyzed. With active control, residual amplitude modulation is suppressed to below 1 x 10(-6) at 0.02-1,000 s, reaching a minimum of 2 x 10(-7) at similar to 2 s. A frequency stability of 2 x 10(-15) is obtained from 0.1 to 10 s, and the optical heterodyne beat of the two Nd: YAG lasers shows 1-Hz linewidth with a measurement time of 4.096 s. In addition, the experimentally determined linewidths agree well with the calculation according to a simplified relationship between the linewidth and the underlying flicker noise that modulates the laser frequency.

Residual amplitude modulation is one of the major stability-degrading factors in many precision measurements. Using an electro-optic (EO) crystal with wedged input and output surfaces is an effective way to suppress residual amplitude modulation. Here the mechanism of residual amplitude modulation in this approach is investigated. The residual amplitude modulations measured in standard and wedged EO crystals bear similarities in their temperature and polarization dependences, implying that a mixture of the two orthogonal polarizations in the extraordinary light is responsible for the residual amplitude modulation in the wedged EO crystal. Similar to a standard EO crystal, a non-uniform spatial distribution of residual amplitude modulation is also observed in the extraordinary light emerging from the wedged EO crystal. The optical isolator after the EO crystal is replaced by a Faraday rotator, and an improvement in the long-term stability is observed. With the wedged-crystal approach, residual amplitude modulation as low as 2×10−7 is observed, contributing a frequency instability of 8×10−18 (500 s) in Pound–Drever–Hall frequency stabilization with a discrimination slope of 1×10−4 V/Hz.

This paper aims at two practical design issues encountered with electro-optic amplitude modulators, namely (1) the long-term optical power drift when a single electro-optic crystal is used and (2) the mechanical resonances of the crystal that limit the control bandwidth. We investigated an active control scheme using a single crystal to offset the drifts of different origins such as polarization, crystal birefringence, and electronics. The control system worked continuously for at least 15 h without interruption and a power instability of 8 x 10(-4) was achieved in a 3-h measurement. This technique can be employed in electro-optic applications where two matched crystals are usually used to compensate the birefringence-related drifts. Meanwhile, mechanical resonances in two crystals (LiNbO3 and KTiOPO4) were mapped out and their influences on the control bandwidth were quantitatively investigated. Residual noises of two intensity-stabilized lasers (a Helium-Neon laser and a dye laser) were analyzed in detail. (c) 2012 Elsevier Ltd. All rights reserved.

We experimentally demonstrate a monolithic single-frequency nonplanar ring oscillator (NPRO) laser pumped by diode lasers from a thermally excited ground level directly into a metastable level of Nd3+:YAG gain medium. Continuous-wave output power of 4.54 W is obtained at 1064 nm with 7.6 W input at 885 nm, and the slope efficiency is 76.9% at incident power above 2 W. Compared with the 50.7% slope efficiency obtained using 808 nm pumping for the same NPRO, this direct pumping shows a potential advantage in improving the energy efficiency. The observation is consistent with an analysis of the heat generation in both direct and traditional pumping schemes. The power fluctuation is measured to be within 0.8% at 3 W output during a 10 h period.

We present the development of a dye-laser-based spectrometer operating at 550-600 nm. The spectrometer will be used to detect an ultra-narrow clock transition (S-1(0)-P-3(0)) in an Ytterbium optical lattice clock and perform high-resolution spectroscopy of iodine molecules trapped in the subnanometer channels of zeolite crystal (AlPO4-11). Two-stage Pound-Drever-Hall frequency stabilization is implemented on the tunable continuous-wave dye laser to obtain a reliable operation and provide stable laser radiations with two different spectral linewidths. In the first-stage frequency locking, a compact home-built intracavity electro-optic modulator is adopted for suppressing fast frequency noise. With an acquisition time of 0.1 s the 670-kHz linewidth of the free-running dye laser is reduced to 2 kHz when locked to a pre-stabilization optical cavity with a finesse of 1170. When the pre-stabilized laser is locked to a high-finesse optical cavity, a linewidth of 1.4 Hz (2 s) is observed and the frequency stability is 3.7 x 10(-1)5 (3 s). We also measure and analyze the individual noise contributions such as those from residual amplitude modulation and electronic noise. The ongoing upgrades include improving long-term frequency stability at time scales from 10 to 100 s and implementing continuous frequency scan across 10 GHz with radio-frequency precision. (C) 2014 AIP Publishing LLC.

Being small in size and weight, piezoelectric transducers hold unique positions in vibration sensing and control. Here, we explore the possibility of building a compact vibration isolation system using piezoelectric sensors and actuators. The mechanical resonances of a piezoelectric actuator around a few kHz are suppressed by an order of magnitude via electrical damping, which improves the high-frequency response. Working with a strain gauge located on the piezoelectric actuator, an auxiliary control loop eliminates the drift associated with a large servo gain at dc. Following this approach, we design, optimize, and experimentally verify the loop responses using frequency domain analysis. The vibration isolation between 1 Hz and 200 Hz is achieved and the attenuation peaks at 60 near vibration frequency of 20 Hz. Restrictions and potentials for extending the isolation to lower vibration frequencies are discussed.

Residual amplitude modulation is one of the major sources of instability in ultra-sensitive optical detections based on frequency modulation. Using a MgO·LiNbO(3) electro-optic crystal, we systematically measure the temperature and polarization dependence of residual amplitude modulation and our experimental results are in good agreement with a previous theoretical analysis. After optical phase modulation, two independent arms including optical detection and frequency demodulation are employed to closely examine the instability of the residual amplitude modulation. Residual amplitude modulation below 25 ppm is obtained with an active cancellation scheme in which the crystal temperature is varied so as to zero the baseline drifts with different origins. Possible improvements for better suppression and stability are discussed.