All-fiber bidirectional optical parametric oscillator for precision sensing.

We present the design and operation of an all-fiber, synchronously pumped, bidirectional optical parametric oscillator (OPO) for precision sensing applications. The fiber-based OPO (FOPO) generates two frequency combs with identical repetition rates but different carrier offset frequencies. A narrow beatnote was observed with full-width at half-maximum (FWHM) linewidth of <10 Hz when the two frequency combs were overlapped on a photodetector. The all-fiber design removes the need for free-space alignment and adjustment. In addition, an external delay line to overlap the two pulse trains in time on the detector is not needed since our unique design provides automatic delay compensation. We expect the novel FOPO to find important applications in precision measurements including rotation sensing with ultra-large sensing area and sensitivity.

On the higher-order Kerr effect in femtosecond filaments.

A recent experiment probing the electronic nonlinearity in the femtosecond filament indicated that the optical Kerr effect not only saturates but even changes its sign at high intensities and thus switches from self-focusing to a strongly defocusing regime. Here we examine, through simulations and experiment, some implications of such a behavior. We perform comparative simulations based on the standard model on one hand and on a model implementing the intensity-dependent Kerr effect on the other. Comparison with an experiment provides a strong indication that of these two Kerr-effect models the standard model is better in capturing the observed length of the filament. However, neither of the models can reproduce length and filament radius. Possible implications are discussed.

Mode-locked ring lasers for the backscattering measurement of mirrors.

A new method for measuring the backscattering produced by mirrors is presented. A mode-locked ring laser allows us to achieve more sensitivity than with any other technique. The present sensitivity to a minimum intensity backscattering coefficient (ratio of backscattered versus incident intensity) of 10(-15) can be enhanced through stabilization. We compare results realized with this technique with results issued from classical bidirectional reflectance distribution function measurements.

Time-resolved measurements of self-focusing pulses in air.

The spatial, spectral, and temporal properties of self-focusing 798-nm 100-fs pulses in air are experimentally measured with high-resolution, single-shot techniques at a set propagation distance of 10.91 m. The data, obtained with an initially collimated Gaussian beam, show significant evolution of spatial narrowing as well as temporal and spectral changes at intensities lower than those required for significant ionization of air.

Stabilization of femtosecond lasers for optical frequency metrology and direct optical to radio frequency synthesis.

We have actively stabilized the comb of frequencies from a mode-locked femtosecond laser using a Fabry-Perot reference cavity. This technique offers the ability to synthesize a comb of highly stable radio frequencies directly from optical transitions. The measured fractional frequency instability of the components of the frequency comb relative to the reference cavity was <5x10(-13) in 0.1 sec. The variation of the optical mode spacing versus frequency of the reference cavity was also directly measured using this technique.

Bidirectional, synchronously pumped, ring optical parametric oscillator.

We report the operation of a bidirectional femtosecond pulsed ring optical parametric oscillator based on periodically poled lithium niobate, pumped alternately with nonsimultaneous pulses from a Ti:sapphire mode-locked laser. A beat note between the two counterpropagating beams attests to a gyro response without dead band. The sensitivity of the device to differential phase changes is demonstrated by measurement of the nonlinear index of lithium niobate.

Bichromatic two-photon coherent excitation of sodium to provide a dual-wavelength guidestar.

A bichromatic sodium guide star based on two-photon coherent excitation followed by cascade emission is considered. Results of density matrix computations, including Doppler averaging, are presented for both a three-level model and the full hyperfine structure of the sodium 3S, 3P, and 4S levels. These results indicate that the proposed scheme for a bichromatic guide star is feasible.

Greater than 100% photon-conversion efficiency from an optical parametric oscillator with intracavity difference-frequency mixing.

Using 100-ps Nd:YAG pump pulses, we synchronously pump an optical parametric oscillator with intracavity difference-frequency mixing (DFM) between the signal and the idler. The cavity is singly resonant at the signal frequency. The signal, idler, and difference wavelengths are near 1.5, 3.5, and 2.8microm , respectively. Periodically poled lithium niobate is used for both interactions. Results show an 80% enhancement in the idler power-conversion efficiency and an idler photon-conversion efficiency of 110% when the DFM interaction is phase matched. Backconversion of the pump is suppressed when the DFM interaction is phase matched, and pump depletion increases from 65% to 79% at full pump power.

Measuring intracavity phase changes by use of double pulses in a linear cavity.

We report the use of a mode-locked linear-cavity Ti:sapphire laser as a sensitive phasemeter. We were able to generate two pulses in the cavity by placing an absorbing-dye jet in the middle of the cavity. A voltage applied to an intracavity LiNbO(3) crystal modified the phase velocity for one of the two pulses at each of its passes. We could then measure accurately the change in optical cavity length by observing a beat frequency between the two pulses. We measured the ratio of the r(33) to r(13) electro-optic coefficients in LiNbO(3) to be 3.57+/-0.1 , in close agreement with the accepted value of 3.62.

Two-dimensional imaging and three-dimensional reconstruction of low reflectivity surfaces by using the range-gating upconversion second-harmonic method.

Three-dimensional images of objects with very low reflectivity are obtained through a nonlinear upconversion gating with amplified femtosecond laser pulses. The current sensitivity of 10(-10) of the incident pulse intensity can be improved by use of better nonlinear crystals and higher-intensity gating pulses. The intensity rejection ratio between two coherent pulses with a delay of a few millimeters between them is better than 2 orders of magnitude. The depth resolution is ~15 µm. The transverse resolution of 300 µm is mainly limited by the two-dimensional detector that was used.

Parametric oscillation and compression in KTP crystals.

We report effective pulse compression under conditions of giant-pulse generation in a synchronously pumped parametric oscillator pumped by the second-harmonic radiation of an actively-passively mode-locked Nd;YAG laser with passive negative feedback. A minimum pulse duration of 0.39 ps is achieved. The output radiation can be tuned from 0.614 to 4.16 microm.

Analysis of a ring-laser gyroscope with intracavity phase-conjugate coupling.

A ring-laser gyroscope with the opposite modes coupled by an intracavity phase-conjugate interaction is examined analytically and numerically. It is shown that the coupling mitigates cross-saturation-induced mode extinction, allowing a homogeneously broadened medium to be used as the gain for a laser gyroscope. It is also shown that the coupling can induce a bias beat frequency that can potentially be used as an all-optical dither. The phase conjugation is shown to affect the lock-in threshold only indirectly. The results are compared with pertinent experiments from the literature.

Imaging with femtosecond pulses.

An all-optical three-dimensional imaging system based on direct upconversion of depth-resolved images is described. Femtosecond gating of the signal provides a depth resolution in the microm range and a discrimination of the image against scattered light.

Femtosecond ring dye laser: a potential new laser gyro.

Measurements on a passively mode-locked dye laser demonstrate a sensitive gyro-type response, when Fresnel drag in air is used to simulate rotation. Such a response is unexpected for a homogeneously broadened ring laser.

Transversely pumped 11-pass amplifier for femtosecond optical pulses.

An off-axis near-concentric interferometer is used for multiple passages through a small (2-mm-diameter) cylindrical volume of amplifying dye. The gain volume is pumped transversely by a copper-vapor laser, in a geometry designed for uniform pump. Pulse-to-pulse reproducibility better than 1% is obtained by operating with full saturation and using a solvent with a vanishing temperature-induced change in index of refraction (dn/dT = 0).

Thermal nonlinear effects in exotic media: application to the study of nonlinear interfaces.

Nonlinearities reported as self-focusing in exotic media such as green Chinese tea are identified as thermal selfdefocusing. These large index changes can be used to study hysteresis, power-induced self-switching and cross-switching, the power-induced lensing effect, and other nonlinearities at interfaces.

Self-induced reflection at a saturable interface.

The reflection properties of an interface between a dielectric and a saturable dye solution are investigated. A simple phenomenological approach describes the switching behavior, including self-phase-modulation effects induced at the interface.