Cancellation of laser dither modulation from optical frequency standards

Generation of Long-Term Stable Squeezed Vacuum States Using Dither-Locking Technique

We report the generation of long-term stable squeezed vacuum states at 1064 nm using a degenerate optical parametric amplifier (DOPA) with a periodically poled KTiOPO4 crystal (PPKTP). The OPA is pumped by a 532 nm light produced by frequency doubling the fundamental light with a bow-tie enhancement second harmonic generator (SHG). When the DOPA and relative phases are locked using a dither-locking method, the squeezed vacuum states are stably measured over 2 h at 11 MHz. The highly compact and simple squeezed light source is suitable for applications in quantum optics experiments.

Ultrastable offset-locked frequency-stabilized heterodyne laser source with water cooling

An ultrastable frequency-stabilized He-Ne laser with a water-cooling structure has been developed for a high-speed and high-accuracy heterodyne interferometer. To achieve high frequency stability and reproducibility, a two-mode He-Ne laser was offset locked to an iodine-stabilized laser. An improved synchronous multi-cycle offset frequency-measurement method with a gate time of an integer multiple of the modulation period was employed to remove the frequency-modulation effect on the offset-frequency counter. A water-cooling structure based on the double-helix structure was established to provide a stable and low-temperature working environment. This structure can remarkably reduce the frequency instability arising from the environmental temperature variation and the thermal pollution released from the laser to the environment. The experimental results indicate that the frequency stability according to the Allen variance is better than 2.3×10^-11 (τ=10  s) and the frequency reproducibility is better than 4.5×10^-10.

Absolute refractometry of dry gas to ±3 parts in 10⁹

We present a method of measuring the refractive index of dry gases absolutely at 632.8 nm wavelength using a Fabry-Perot cavity with an expanded uncertainty of <3×10⁻⁹ (coverage factor k=2). The main contribution to this uncertainty is how well vacuum-to-atmosphere compression effects (physical length variation) in the cavities can be corrected. This paper describes the technique and reports reference values for the refractive indices of nitrogen and argon gases at 100 kPa and 20 °C with an expanded uncertainty of <9×10⁻⁹ (coverage factor k=2), with the additional and larger part of this uncertainty coming from the pressure and temperature measurement.

A novel way for wavelength locking with acousto-optic frequency modulation

A novel wavelength locking scheme called acousto-optic frequency modulation (AOFM) is put forward and demonstrated experimentally. The laser frequency is modulated by an acousto-optic modulator, rather than direct dithering on the laser resonator. A new optical configuration is proposed to compensate the angular deflection of the acousto-optic modulator, which is driven by a Direct Digital Synthesis generator with a frequency stability and precision on the order of Hz. This locking scheme is relatively simple and economical, and it can avoid extra frequency and intensity noise due to the direct frequency dither on the laser resonator in the conventional saturation absorption locking scheme. Our scheme provides a new way to improve the noise and stability of the tunable laser.

Three-longitudinal-mode He-Ne laser frequency stabilized at 633 nm by thermal phase locking of the secondary beat frequency

We present a simple scheme for thermal frequency stabilization of a three-longitudinal-mode He-Ne laser at 633 nm with improved short-term (5 x 10(-11) at 1-s average time) and long-term (124-kHz standard deviation for 10 h) frequency stability. A stabilized output power of 3 mW was readily obtained from the central mode by polarization-mode selection. The optical frequency of the central mode could be precisely tuned by an external frequency reference over 160 MHz with high precision or could be optically phase locked to a reference laser by use of a thermal frequency-stabilization circuit and an external acousto-optic modulator. The laser will be useful in various applications for which a laser source with high power and frequency stability is necessary.