Inhomogeneous spatial distribution of residual amplitude modulation in optical phase modulation using a bulk electro-optic crystal.

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.

Suppressing residual amplitude modulation to the 10-7 level in optical phase modulation.

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.

High-efficiency 1064 nm nonplanar ring oscillator Nd:YAG laser with diode pumping at 885 nm.

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.

Ultra-stable 1064-nm neodymium-doped yttrium aluminum garnet lasers with 2.5 × 10-16 frequency instability.

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.

A two-axis tilt control system on a turntable for rotating-optical-cavity experiments.

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 κ e- 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 κ e- ZZ.

Developing a narrow-line laser spectrometer based on a tunable continuous-wave dye laser.

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 ((1)S0-(3)P0) in an Ytterbium optical lattice clock and perform high-resolution spectroscopy of iodine molecules trapped in the sub-nanometer 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 × 10(-15) (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.

Prototyping a compact system for active vibration isolation using piezoelectric sensors and actuators.

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.

Measurement and control of residual amplitude modulation in optical phase modulation.

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.