Jean-Claude Diels

Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy.

The performances of a tunable femtosecond dye laser are analyzed using accurate correlation techniques. The source is a passively mode-locked dye laser, of which both the frequency and frequency modulation are controlled by one or two intracavity prisms. Interferometric second-order autocorrelations, with a peak-to-background ratio of 8 to 1, are used simultaneously with the conventional intensity autocorrelation and the pulse spectrum to determine the pulse shape. The main advantages of the interferometric autocorrelations are that they provide phase information otherwise not available, and they are more sensitive to the pulse shape than the intensity autocorrelation. The phase sensitivity is demonstrated in an analysis of the Gaussian pulses with a linear frequency modulation. Analytical expressions for the envelopes of the interferometric autocorrelations of typical pulse shapes are provided for quick pulse shape identification. A numerical method is used to analyze the more complex pulse shapes and chirps that can be produced by the laser. A series of examples demonstrates the control of this laser over various pulse shapes and frequency modulations. Pulse broadening or compression by propagation through glass is calculated for the pulse shapes determined from the fittings. Comparisons of autocorrelations and cross correlations calculated for the dispersed pulses, with the actual measurements, demonstrate the accuracy of the fitting procedure. The method of pulse shape determination demonstrated here requires a train of identical pulses. Indeed, it is shown that, for example, a train of unchirped pulses randomly distributed in frequency can have the same interferometric autocorrelation than a single chirped pulse. In the case of the present source, a comparison of the pulse spectrum, with that of the second harmonic, gives an additional proof that pulse-to-pulse fluctuations are negligible.

Femtosecond ultraviolet laser pulse induced lightning discharges in gases

Ultraviolet pulses of 200 fs duration and low energy (/spl ap/0.2 mJ) have a sufficiently high peak power to ionize oxygen and nitrogen by three- and four-photon ionization, respectively. It is shown that the resultant ionization channel induces a lightning like discharge at half of the natural self-breakdown voltage in nitrogen or air. The laser triggered discharging process is studied by monitoring the voltage between two planar electrodes. The effects of oxygen on the induced breakdown is investigated. A complete theoretical model is presented to simulate: (1) the electron seeding; and (2) the evolution of the plasma of electron-ion in the applied field. The results of the theory verified by small scale experiments-are used to simulate the process of laser triggered lightning in atmosphere, and helps to define the parameters of a laser system for lightning protection. >

Dispersion measurements with white-light interferometry

White-light interferograms provide a simple, accurate, and physically intuitive picture of what happens to broadband optical pulses on transmission through, or reflection from, common optical materials. Quantitative measurement of group delay are made with an accuracy of ±0.1 fs and with high spectral resolution. This measurement accuracy is applied to the determination of d2n/dλ2 and d3n/dλ3 of fused silica with an accuracy of ±5 × 10−5μm−2 and ±1 × 10−3μm−3, respectively. Further applications are found in the measurement of the dispersion of broadband mirrors and a multiple-quantum-well structure.

Ultraviolet filamentation in air

Ultrashort UV pulses are seen to self-focus in air and form meter length filaments. Various measurements (spectrum, size, conductivity) indicate that the balance of the processes that produce the filament are different in the UV compared to the IR.

High-voltage electrical discharges induced by an ultrashort-pulse UV laser system

Using a femtosecond UV laser source (248.6 nm), we have conducted studies on large-gap laser-induced electrical discharges. Parameters such as pressure, lens selection, focal position and laser energy were investigated. We also present an experimental technique that allows the measurement of ionization cross-sections at atmospheric pressures. Spectroscopic analysis of the laser plasma and discharge are also presented. Prospects and conditions leading to a discharge over tens of metres, with dual UV and IR laser pulses, are discussed.

Generation of extended pulse trains of minimum duration by passive negative feedback applied to solid-state Q-switched lasers

Intracavity self-focusing in a thin two-photon absorber acts, in combination with pinholes, as an efficient power and energy limiter in flashlamp (pulsed) and arc lamp (CW) pumped actively passively mode-locked crystalline Nd-doped lasers. Power limiting by the intracavity two-photon absorber is exploited to create the condition for optimum pulse compression in the saturable absorber. Additional pulse shortening is due to the elimination of the pulse tail by self-defocusing in the nonlinear crystal. Extracavity and intracavity measurements, as well as three-dimensional computer simulations, give a complete understanding of the pulse evolution in the cavity, including the two mechanisms of pulse shaping and compression. >

Amplitude and phase recording of ultrashort pulses

The performance of a new method of measuring the amplitude and phase of ultrashort pulses is evaluated with simulated and real data.

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.

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 2.8 µm, 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.

Generation and measurement of 200 femtosecond optical pulses

Abstract Pulses as short as 0.2 ps have been obtained from a passively mode locked dye laser. The technique is simpler than those previously used, but one must pay the price of a loss of tunability. The pulse durations were determined from their second order correlation functions measured with a highly stable interferometer which is described.