Ultrafast-laser-induced backward stimulated Raman scattering for tracing atmospheric gases.

By combining tunable broadband pulse generation with the technique of nonlinear spectral compression we demonstrate a prototype scheme for highly selective detection of air molecules by backward stimulated Raman scattering. The experimental results allow to extrapolate the laser parameters required for standoff sensing based on the recently demonstrated backward atmospheric lasing.

Hollow-fiber compression of 6 mJ pulses from a continuous-wave diode-pumped single-stage Yb,Na:CaF2 chirped pulse amplifier.

Here, 200 fs 6 mJ pulses from a cw diode-pumped Yb,Na:CaF(2) amplifier are spectrally broadened in an Ar- or Ne-filled hollow-core fiber and recompressed to 20 fs (Ar) and 35 fs (Ne) using a prism pair. The results of spectral broadening and phase measurement are in excellent agreement with numerical modeling based on the generalized nonlinear Schrödinger equation. The longer laser wavelength of 1030 nm permits favorable energy scaling for the hollow-fiber technique compared to ultrafast amplifiers operating at 800 nm.

Picosecond absorbance difference spectra of the antenna of photosynthetic purple bacteria. The influence of exciton interactions and librations.

The influence of exciton interactions and pseudolocal librational modes on the relaxation of photo-induced absorption changes of the light-harvesting antenna of the photosynthetic purple bacterium, Rhodospirillum rubrum, was studied in the ps time domain. A hypothesis is put forward that the long-lived librational modes can occur in the circular aggregate of light-harvesting bacteriochlorophyll molecules of the core antenna. The model spectra obtained quantitatively fit the shape of the photo-induced picosecond absorbance difference spectra. The model predicts the decay of photo-induced spectra with the time constant within tens of picoseconds.

Spatiotemporal three-dimensional mapping of nonlinear X waves.

The spatiotemporal intensity profile of a 100-fs wave packet at the output of a X2 crystal, tuned for mismatched second-harmonic generation, is probed via sum-frequency generation with a compressed, 20-fs pulse, revealing the appearance of an X-type wave shape.

Scalable Yb-MOPA-driven carrier-envelope phase-stable few-cycle parametric amplifier at 1.5 microm.

Carrier-envelope phase-stable 4 microJ pulses at approximately 1.5 microm are obtained from a femtosecond Yb:KGW-MOPA-pumped two-stage optical parametric amplifier. This novel technology represents a highly attractive alternative to traditional Ti:sapphire front-ends for seeding multimillijoule-level optical parametric chirped-pulse amplifiers. For this task, we demonstrate stretching of the OPA output to approximately 40 ps and recompression to 33 fs pulse duration. As a stand-alone system, our tunable two-stage OPA might find numerous applications in time-resolved spectroscopy and micromachining.

Multi-mJ, 200-fs, cw-pumped, cryogenically cooled, Yb,Na:CaF2 amplifier.

Using a novel (to our knowledge) broadband Yb-doped Yb3+,Na+:CaF2 crystal cooled in a closed loop to 130 K we demonstrate a chirped pulse regenerative laser amplifier delivering the energy of up to 3 mJ at a repetition rate of 1 kHz and an average output power of 6 W at 20 kHz. The gain narrowing in the laser crystal is compensated by shaping the amplitude of the seed pulse spectrum. As the result, at the highest amplified pulse energy we obtain a 12 nm FWHM bandwidth supporting a 130 fs pulse duration, assuming ideal compression. Amplified pulses were recompressed from 250 ps to 195 fs with a 1700 lines/mm transmission grating compressor.

Multimillijoule chirped parametric amplification of few-cycle pulses.

The concept of optical parametric chirped-pulse amplification is applied to attain pulses with energies up to 8 mJ and a bandwidth of more than 100 THz. Stretched broadband seed pulses from a Ti:sapphire oscillator are amplified in a multistage noncollinear type I phase-matched beta-barium borate parametric amplifier by use of an independent picosecond laser with lock-to-clock repetition rate synchronization. Partial compression of amplified pulses is demonstrated down to a 10-fs duration with a down-chirped pulse stretcher and a nearly lossless compressor comprising bulk material and positive-dispersion chirped mirrors.

Accurate remote distance sensing by use of low-coherence interferometry: an industrial application.

An apparatus for accurate remote distance sensing based on fiber-optic low-coherence light interferometry has been designed for molten glass level measurement. We demonstrate operation of the meter in an adverse industrial environment with <20-mum resolution (standard deviation) within a 20-mm range with the sensing head placed in an oven at ~800 degrees C. In laboratory conditions we were able to measure with 3-mum resolution, which could be improved to submicrometer level by optimization of a reference arm of the interferometer and detection electronics.

Linear transformation of pulse chirp through a cascaded optical second-order process.

We have experimentally investigated the combination of chirps of two ultrashort light pulses through self diffraction induced by cascaded second-order frequency mixing in a beta-barium borate crystal. Interaction of two pulses with equal carrier frequencies and opposite chirps yielded diffracted pulses with enhanced chirp. Perfect compressibility of the diffracted pulse indicates that phase distortion by intensity-dependent effects was negligible.

Scanning delay line with a rotating-parallelogram prism for low-coherence interferometry.

We describe a fast scanning optical delay line for low-coherence interferometry that has good linearity, a high duty cycle, and a continuously adjustable scan range. The delay line consists of a rotating-parallelogram prism with the rotation axis tilted with respect to the incident beam and two motionless mirrors. The delay line is well suited for nearly simultaneous distance measurements at two different depths, which is useful for making absolute and differential distance measurements.

Adjusting pulse-front tilt and pulse duration by use of a single-shot autocorrelator.

We present a method of adjusting the pulse duration and eliminating the pulse-front tilt of an ultrashort pulse in real time by use of a specially configured single-shot autocorrelator. Pulse-front tilt, or a temporal delay across the pulse front, is a common ultrashort-pulse phenomenon when dispersive elements are being used. We show the design of an autocorrelator that can be used to eliminate the pulse-front tilt and simultaneously adjust the pulse duration in real time by adjustment of the pulse compressor of a chirped-pulse amplified laser system.

Fundamental-frequency pulse compression through cascaded second-order processes in a type II phase-matched second-harmonic generator.

We show that cascading of the second-order processes under large group-velocity mismatch can lead to the efficient compression of pulses at fundamental frequency in the type II phase-matched second-harmonicgeneration process. We demonstrate that any of three interacting pulses can be compressed if the proper ratio of intensities of the o- and e-polarized pump pulses is chosen on input. We experimentally achieved compression of o-polarized pulses from 1.3 ps down to 280 fs with energy conversion close to 50% when a conventional second-harmonic compressor with unbalanced intensities of the pump pulses was used.

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse.

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.

Nonlinear second-harmonic pulse compression with tilted pulses.

We demonstrate that the group velocities of interacting pulses can be adjusted by pulse tilting to optimize second-harmonic pulse compression for a variety of nonlinear crystals and wavelengths. As an experimental proof we present results of ninefold compression of 1.3-ps Nd:glass laser pulses in beta -barium borate crystal.

Near-degenerate cascaded four-wave mixing in an optical parametric amplifier.

Efficient generation of new spectral components owing to second-order cascading in a seeded broadband beta -barium borate typeI phase-matching parametric amplifier is demonstrated. One can vary the number and magnitude of these components by changing amplification bandwidth (wavelength) and phase-matching conditions. The phenomenon is treated theoretically by use of a formalism developed previously for the case of cascaded self-diffraction.

Generation of compressed 600-720-nm tunable femtosecond pulses by transient frequency mixing in a beta-barium borate crystal.

We demonstrate the feasibility of the compression of tunable pulses by means of sum-frequency generation under special group-velocity mismatch conditions. beta-Barium borate crystal has been shown to support the upconversion process, resulting in 160-fs pulses tunable within 600-720 nm when mixing 1.3-ps 1055-nm pulses from a Nd:glass laser with 0.95-ps pulses tunable within 1400-2200 nm from a traveling-wave parametric generator. A similar compression of ~150-fs Ti:sapphire pulses predicted for the sum-frequency tuning range of 520-590 nm.

Matching of group velocities by spatial walk-off in collinear three-wave interaction with tilted pulses.

We demonstrate that, for tilted pulses, the contribution to group velocity that is due to spatial walk-off can compensate for the group-velocity mismatch in three-wave interactions. With 100-fs, 0.4-Zmicrom pump pulses in beta-barium borate, we obtained efficient parametric generation of collinear superfluorescence tunable in the range from 0.456 to 3.25 microm.

Visible pulses of 100 fs and 100 µJ from an upconverted parametric generator.

Upconversion by sum-frequency and second-harmonic generation of pulses from a Ti:sapphire-pumped parametric generator provides 50-120 µJ, sub-100 fs, bandwidth-limited pulses at 470-700 nm.

Highly efficient parametric conversion of femtosecond Ti:sapphire laser pulses at 1 kHz.

Pulses with energies as high as 150 microJ and durations as low as 60 f(s) were generated from 1.1 to 2.6 microm by a traveling-wave parametric converter pumped by femtosecond pulses of a Ti:sapphire laser with chirped-pulse amplification.