Frequency noise reduction of a diode laser by using the FM sideband technique

Phase regeneration of phase encoded signals by hybrid optical phase squeezer

We present a new method to perform phase regeneration of phase encoded signals. In our concept called "hybrid optical phase squeezer (HOPS)," a multilevel phase-quantized signal is synthesized through the coherent addition of a phase-conjugate copy of the signal and a phase harmonic of the signal with a frequency shifter. Unlike the conventional method by phase sensitive amplification, HOPS does not use any optical parametric gain such that only optical elements with low optical nonlinearity are necessary for optical phase quantization. In the proof-of-concept experiment, it is confirmed that a 2-level HOPS can perform quadrature squeezing with an extinction ratio of 40 dB. Simultaneous phase regeneration of two coherent wavelength-division-multiplexed 10.75-Gb/s binary phase-shift keyed signals is successfully demonstrated using a 2-level HOPS based on a semiconductor optical amplifier.

Diode laser-frequency stabilization by use of frequency modulation by a vibrating mirror

Modulation of a laser frequency with a negligibly small residual amplitude modulation component has been demonstrated with a piezoelectric transducer-driven mirror. The vibrating-mirror method has been successfully applied to high-resolution spectroscopy of saturated absorption and to laser-frequency stabilization. The bandwidth of the stabilization feedback loop as wide as 15 kHz has been achieved to suppress acoustic noise in ordinary laboratory environment.

High-frequency-stability laser at 1.5 microm using Doppler-free molecular lines

An extended-cavity 1.5-microm semiconductor laser was frequency stabilized to saturated-absorption lines of acetylene. Its long-term frequency stability is of the order of 10(-12), with a reproducibility of +/-10 kHz. Using the lines of C(2)H(2) or HCN, we could obtain such a high stability with the same laser at many wavelengths covering the 1.51-1.56-microm band.

Ultranarrow (13)C(2)H(2) saturated-absorption lines at 1.5 microm

The saturation spectroscopy of molecular transitions at 1.5microm is reported. We show that it is possible, with a low-power laser diode, to obtain 2-MHz-wide acetylene saturated-absorption lines, and we give what is to our knowledge the first experimental estimation of (13)C(2)H(2) saturation intensity. To our knowledge, these lines are the narrowest reported 1.5-microm frequency references and should be suitable for high-quality 1.5-microm laser frequency standards.

Ultrahigh-frequency stabilization of a diode-pumped Nd:YAG laser with a high-power-acceptance photodetector

We describe the ultrahigh-frequency stabilization of a diode-pumped Nd:YAG laser by use of a high-poweracceptance photodetector. The photodetector, which is constructed with an optical power divider and eight combined photodiodes, is developed to reduce shot-noise-limited FM noise. The laser frequency is locked to a Fabry-Perot cavity with a finesse of 25,600 by the Drever-Hall technique. By using the optical power of 47 mW at the photodetector, we reduce the relative FM noise at the error signal to the shot-noise limit of 4.2x10(-5) Hz/ radicalHz below the Fourier frequency of 500 Hz.

193-mHz beat linewidth of frequency-stabilized laser-diode-pumped Nd:YAG ring lasers

We describe frequency-stabilized laser-diode-pumped Nd:YAG ring lasers locked to a high-transmission and high-finesse reference Fabry-Perot cavity. The cavity has a resonant linewidth of 38.0 kHz, a finesse of 19,400, and a transmission efficiency of 79.4% for the TEM(00) mode. We measure the frequency stability by locking two lasers one free spectral range apart and observe the heterodyne beat signal. As a result, the spectral densities of frequency noise are reduced to 10 mHz/ radicalHz, and a beat linewidth of 193 mHz is obtained. The linewidth is 0.73 times the Schawlow-Townes limit.