Manjusha Mehendale

Method for single-shot measurement of the carrier envelope phase of a few-cycle laser pulse.

The interference between different harmonics of a few-cycle optical pulse in the region of the spectral overlap is sensitive to the phase of the optical carrier inside the pulse envelope. Near-surface third-harmonic generation from Si(001) combined with second-harmonic generation in a 10-mum -thick beta-barium borate crystal produces sufficiently strong harmonic emission for single-shot measurement. We propose using this technique to measure the carrier envelope phase of high-energy 5-fs pulses.

Stimulated Brillouin scattering in Raman-pumped fibers: a theoretical approach

We develop a theoretical approach to describe stimulated Brillouin scattering (SBS) in Raman-pumped optical fibers. We derive the condition for the SBS threshold as a function of fiber parameters and the input power of forward and/or backward pump. A particular emphasis is given to the effect of the fiber length and unequal absorption coefficients for pump and signal wavelengths on the SBS threshold. Simple, yet accurate, analytical expressions for the SBS threshold in pumped fibers are also obtained. We show that in pumped fibers, the SBS threshold power is inversely proportional to the path-average integral of Raman gain. Validity ranges of derived formulas are considered in detail. The theoretical predictions are verified experimentally. The calculated values of the SBS threshold powers are in a good agreement with the measured threshold power in Raman-pumped dispersion-compensated fibers.

Molecular science with strong laser fields

The electric field of a laser pulse exerts a force on charged particles which can be on the order of (or exceed) the forces that bind electrons to atoms, molecules, solids or that bind atoms together in molecules or solids. With modern laser technology, this force can be applied with almost 1 fs, 1 µm precision.Even if the field is lower than the field required to ionize atoms or molecules, large nonresonant Stark shifts can be achieved. The Stark shift gives us a means to control molecules. The dependence of the Stark shift with respect to the intensity profile of the laser focus determines the spatial force exerted on the molecule. The dependence of the Stark shift on the orientation of the molecule with respect to the laser polarization determines the torque exerted on the molecule. Through these forces we can control position, orientation, and linear and/or angular velocity.The Stark shift also depends on the internuclear co-ordinates, giving us some degree of control over the structure of the potential energy surface in molecules. The ability to control these basic forces with precision depends on our ability to control optical pulses. Progress towards producing high power pulses with almost arbitrary time-dependent infrared fields will be discussed.In even stronger fields, where ionization occurs, the shifting and mixing of states becomes extreme. Measurement in this complex spectroscopic environment is difficult. Intuition based on perturbation theory is of limited value. Yet strong field probing allows us to supply a lot of electronic energy to a molecule very rapidly and to localize measurements in space and time. We illustrate molecular alignment and strong field probing together in one experiment where we study femtosecond dissociative ionization.

Conversion of high-power 15-fs visible pulses to the mid infrared

We measure the efficiency of converting high-power 15-fs 0.8-mum pulses to the mid infrared in GaAs and GaSe as well as the pulse duration and the spectrum of the infrared radiation that is produced. Free-carrier production limits the conversion efficiency in GaAs to approximately 5x10(-7) , allowing us to produce 2.5-pJ, 30-fs pulses spanning the spectral range from 6 to 14 mum . In GaSe we obtain, in a moderately saturated regime, a conversion efficiency of 7.5x10(-5) , limited by two-photon absorption, allowing us to produce pulses of 100-fs duration containing 10 nJ of energy.

Towards an anthrax detector using the femtosecond adaptive spectroscopic technique for coherent anti-Stokes Raman Spectroscopy: Coherent anti-Stokes Raman spectroscopy signal from dipicolinic acid in bacterial spores

Abstract We present experimental evidence of a resonant coherent anti-Stokes Raman spectroscopy (CARS) signal from dipicolinic acid, a marker molecule in anthrax spores. We also show some evidence of resonant CARS signal associated with the 1000 cm−1 resonance from Bacillus Globigii spores which are similar to anthrax spores. These results constitute a first step towards developing a rapid identification technique for bacterial spores.

All-ultraviolet time-resolved coherent anti-Stokes Raman scattering

We report all-UV coherent anti-Stokes Raman scattering (CARS) in calcite with 250-280 nm pump, Stokes, probe, and anti-Stokes light. UV CARS efficiency is approximately 7x higher than for comparable scattering in the visible, 480-540 nm. Time-resolved UV CARS reveals lengthening of the dephasing time of 1086 cm(-1) CO3(2-) internal vibrations from 4 to 7 ps with increasing vibrational excitation, consistent with a phonon depletion model.

Photosensitivity in glasses: comparing ultrafast lasers with vacuum-ultraviolet lasers

Summary form only given. Laser microfabrication technology is a promising photonics processing approach with parallels to the current use of lasers in semiconductor lithography, trimming, repair, and inspection. To this end, our groups are exploring two extreme forefronts of laser technology - ultrafast (UF) and deep-ultraviolet (UV) lasers - to drive strong interactions in transparent materials for shaping photonic structures. We recently provided head-to-head comparisons of F/sub 2/-laser and 1-ps UF-laser approaches in smooth surface microsculpting of optical glasses, and introduced a new UF-laser processing mode called burst machining that offers crack-free ablation. In this paper, we present an extension to more subtle laser-glass interactions that drive internal refractive-index changes. Photosensitivity processing rates, spatial resolution, and processing windows for both laser types are discussed together with the prospects for printing and trimming of optical waveguides and circuits.

Stimulated Brillouin scattering in Raman-amplified dispersion compensating fibers

In conclusion we performed experiments to study SBS in Raman amplified dispersion compensating fibres (DCFs) of different lengths. We developed a theory which estimates the SBS thresholds for unpumped, forward and backward Raman pumped DCFs of short length as a function of pump powers and fiber parameters. Theory and experiments both indicate significant alteration of SBS thresholds if Raman amplification is present inside a DCF depending on the power of the Raman pump. The results also imply that forward pumping of the DCF can be more detrimental to the transmission than backward pumping since it brings down the SBS threshold much faster than backward pumping does for the same length of the fiber.

Optimum pre-emphasis in ultra-long-haul networks

We investigate experimentally and theoretically the shape of the optimum spectral pre-emphasis for ultra-long-haul networks and systems in the presence of optical nonlinearities. In a 40×10 Gbits/s×2240 km system, we experimentally show the advantages of a pre-emphasis scheme that ensures minimum variations of path-averaged power among the wavelength-division-multiplexing channels between the cleanups performed by dynamic gain-flattening filters. This is achieved by enforcing a straight-line spectrum near the middle of the link between two consecutive gain-flattening nodes. We explain these results by a simple model, in which we derive the conditions for optimum pre-emphasis in the presence of either distortionlike (e.g., self-phase modulation) or noiselike (e.g., four-wave mixing) nonlinearities. We show that, for relatively small gain ripple, both of these optimum pre-emphasis techniques are equivalent to equalizing the optical signal-to-noise ratio for all channels and producing a flat power spectrum near the middle of the link. Hence, enforcing the straight-line spectrum in the middle of the link is the closest practical approximation to the optimum pre-emphasis.

Acousto-optic femtosecond pulse shaping in the ultraviolet

A simple method for ultraviolet acousto-optic pulse shaping of spectral phase and amplitude is described. The technique eliminates complications associated with the parametric transfer of the spectral phase of near-infrared pulses through nonlinear processes.