Ngai C. Wong

TERAHERTZ OPTICAL FREQUENCY COMB GENERATION AND PHASE LOCKING OF AN OPTICAL PARAMETRIC OSCILLATOR AT 665 GHZ

We have achieved efficient electro-optic phase modulation at high frequencies in a resonant modulator cavity. We enhance modulation by matching the phase velocities of the optical and microwave fields in the modulator substrate and by placing the modulator inside an optical cavity that is resonant for the input optical beam and the generated sidebands. An optical frequency comb with a span of 3 THz and at a spacing of 17 GHz is generated with 1 W of microwave power. The terahertz comb is utilized to phase lock an optical parametric oscillator at a signal-idler difference frequency of 665 GHz.

Tunable optical frequency division using a phase-locked optical parametric oscillator.

We report the experimental demonstration of a novel optical parametric oscillator approach to tunable optical frequency division. The beat frequency of the signal and idler subharmonic outputs of a tunable cw KTP optical parametric oscillator was phase locked to a microwave reference frequency source, which thus permitted precise determination of the output frequencies at approximately half the input pump frequency.

Optical Frequency Division Using an Optical Parametric Oscillator

A novel method of frequency division based on optical parametric oscillation is proposed. This scheme converts with high efficiency an input signal into two intense, coherent subharmonic outputs whose frequencies are tunable and whose linewidths are essentially limited by the input pump linewidth. By locking their difference frequency to a microwave, a millimeter-wave, or an infrared reference source, the output frequencies are precisely determined. The proposed frequency dividers can be operated in series or in parallel to measure, compare, and synthesize frequencies from optical to microwave. A line-narrowing effect for the generation of ultrastable radiation is discussed.

Stabilization and tuning of a doubly resonant optical parametric oscillator

We present experimental results of frequency tuning and stabilization of a type-II phase-matched potassium titanyl phosphate (KTP) doubly resonant optical parametric oscillator. Four tuning elements were employed to control the stability and tuning of the parametric oscillator. Discrete frequency tuning of a nearly degenerate optical parametric oscillator over a range of ~3 THz was obtained by crystal angle tuning and cavity-length scanning. We achieved continuous frequency tuning over a 0.5-GHz range through the use of temperature and electro-optic tuning of the KTP crystal. Using these frequency-control techniques, we phase locked the signal–idler beat frequency to an external microwave frequency source, thus demonstrating tunable optical frequency division. The power spectral density of the residual phase noise of the phase-locked signal–idler beat note was measured to be 0.3mrad/Hz. Characteristics of two different cavity designs, their operations, tuning behavior, and stability issues are examined.

Optical frequency counting from the UV to the near IR.

A method for measuring optical frequencies from the UV to the near IR relative to a microwave frequency standard is proposed. The concept is to measure the frequency difference of two known ratios ((1/2) and ?) of an optical frequency f relative to the cesium clock. By employing optical parametric oscillators and wideband modulators to link the ((1/2))f and (?)f frequencies, a precise and accurate optical frequency comb can be provided in the ~1-2-microm wavelength region. By using this comb, most optical frequencies from the UV to the near IR can be measured or synthesized. A configuration for measuring the frequency of the hydrogen 1S-2S transition is described.

Observation of two distinct phase states in a self-phase-locked type II phase-matched optical parametric oscillator

We demonstrate self-phase locking in a type II phase-matched optical parametric oscillator by mutual injection locking. An intracavity quarter-wave plate provides polarization mixing between the orthogonally polarized signal and idler that induces signal-idler self-phase locking when their frequency difference is within the capture range. We observed two distinct phase states that differ in their oscillator thresholds and their signal-idler phase differences, in good agreement with theory.

Experimental implementation of optical clockwork without carrier-envelope phase control

We demonstrate an optical clockwork without camer-envelope phase control using sum-frequency generation between a CW optical parametric oscillator at 3.39 μm and a modelocked Tisapphire laser with dominant spectral peaks at 834 and 670 nm.

OPTICAL FREQUENCY DIVISION BY 3 OF 532 NM IN PERIODICALLY POLED LITHIUM NIOBATE WITH A DOUBLE GRATING

Optical frequency division by 3 of 532 nm is demonstrated by back-to-back difference-frequency generation in a periodically poled lithium niobate crystal with a double grating. The first grating generates 1596-nm light from 532- and 798-nm inputs, and the second grating mixes the 798-nm input and the 1596-nm output from the first grating to produce a second 1596-nm output. The beat signal between the two 1596-nm outputs is detected and frequency stabilized to yield the 3:1 frequency ratio.

Quantum correlation and absorption spectroscopy in an optical parametric oscillator in the presence of pump noise

We present a linearized quantum analysis of the optical parametric oscillator that includes the effects of pump noise. We show that excess pump noise reduces the intensity correlation between the signal and idler at low frequencies, which explains the low-frequency spectrum of current experimental observations. Its dependence on the cavity loss mismatch permits the possibility of ultrasensitive intracavity absorption spectroscopy, even in the absence of nonclassical correlation observation.

Nonclassical intensity correlation from a type I phase-matched optical parametric oscillator

Signal and idler beams from a type I phase-matched nondegenerate optical parametric oscillator are separated by a Mach-Zehnder interferometer. Broadband nonclassical intensity correlation is observed, in agreement with a theory that includes pump noise. The maximum observed correlation, which occurs at 1.1 MHz, yields a noise level 2.8 dB below the shot noise.