Wolfgang Rudolph

Full-field characterization of femtosecond pulses by spectrum and cross-correlation measurements

We present a practical and accurate technique for retrieving the amplitude and the phase of ultrashort pulses from a nonlinear (second-order) intensity cross correlation and the spectrum that overcomes shortcomings of previous attempts. We apply the algorithm to theoretical and experimental data and compare it with frequency-resolved optical gating.

On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.

Characterization of sub-10-fs pulse focusing with high-numerical-aperture microscope objectives

Dispersion precompensation with a prism sequence and a third-order dispersion mirror resulted in negligible broadening of sub-10-fs pulses at subwavelength spot sizes when the pulses were focused with microscope objectives and moderate apertures. At larger apertures, lens chromaticity and spherical aberration led to an effective pulse broadening of up to 1.3x , depending on the aperture size and the detector position. The data suggest that intensities exceeding 10(14) W/cm(2) can be produced directly from femtosecond pulse oscillators.

Comparative study of confocal and heterodyne microscopy for imaging through scattering media

We investigate limits of the confocal microscope when applied to imaging through scattering layers and compare its performance with that of a correlation (heterodyne) microscope. Confocal laser scanning microscopy is shown to make possible imaging through scattering media owing to the spatial filtering of the signal back-reflected from the sample. Its performance is limited by the noise of the detection system and/or insufficient rejection of scattered light, depending on the sample under investigation. Correlation microscopy with narrow or broad bandwidth light can extend these limits by selective, optical amplification of the image information.

Impact of chromatic and spherical aberration on the focusing of ultrashort light pulses by lenses

Focusing of ultrashort light pulses with single lenses is analyzed by taking into account the unavoidable interplay between chromatic and spherical aberration simultaneously for the first time to our knowledge. The spatial intensity distribution is mainly affected by spherical aberration, whereas the temporal distribution is determined by both aberrations. The impact on second-harmonic generation for femtosecond pulse measurements is discussed. For example, the presence of spherical aberration allows one to record the correct autocorrelation of a 10-fs pulse even if chromatic aberration alone would cause a half-width of the autocorrelation function of 40 fs.

Modeling the effect of native and laser-induced states on the dielectric breakdown of wide band gap optical materials by multiple subpicosecond laser pulses

A model for the multiple-pulse laser-induced breakdown behavior of dielectrics is presented. It is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations involving transitions between band and mid-gap states (native and laser-induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the CB. The effect of native and laser-induced defects can be distinguished by their saturation behavior. Finally, measurements of ...

Femtosecond autocorrelation measurements based on two-photon photoconductivity in ZnSe

Two-photon photoconductivity in ZnSe is used to record femtosecond autocorrelation functions. This technique requires <100 muW of average power of a typical mode-locked femtosecond Ti:sapphire laser and distinguishes itself by a dynamic range over several decades and great conversion bandwidth, permitting the sensitive correlation of pulses of a few femtoseconds.

Noise sensitivity and accuracy of femtosecond pulse retrieval by phase and intensity from correlation and spectrum only (PICASO)

We investigate the accuracy of the femtosecond pulse-retrieval technique called phase and intensity from correlation and spectrum only (PICASO). Different versions of this technique that make use of balanced and unbalanced intensity and interferometric correlations are compared with respect to the rms phase and the intensity error. The effect of measurement noise on the phase and amplitude retrieval is studied, and the results are compared with other retrieval methods.

Hollow-core Optical Fiber Gas Lasers (HOFGLAS): a review [Invited]

The development of hollow core photonic crystal fibers with low losses over a broad spectral region in the near IR enabled the demonstration of a novel laser type - Hollow-core Optical Fiber Gas Laser (HOFGLAS). The laser combines attractive features of fiber lasers such as compactness and long interaction length of pump and laser radiation with those of gas lasers such as the potential for high output power and narrow line width. This paper summarizes recent developments and describes the demonstration of C2H2 and HCN prototype lasers. Avenues to extend laser emission further into the IR are discussed.

Mid-infrared gas filled photonic crystal fiber laser based on population inversion

We demonstrate for the first time an optically pumped gas laser based on population inversion using a hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with 12C2H2 gas is pumped with ~5 ns pulses at 1.52 μm and lases at 3.12 μm and 3.16 μm in the mid-infrared spectral region. The maximum measured laser pulse energy of ~6 nJ was obtained at a gas pressure of 7 torr with a fiber with 20 dB/m loss near the lasing wavelengths. While the measured slope efficiencies of this prototype did not exceed a few percent due mainly to linear losses of the fiber at the laser wavelengths, 25% slope efficiency and pulse energies of a few mJ are the predicted limits of this laser. Simulations of the lasers behavior agree qualitatively with experimental observations.