Development of Femtosecond Sources

Various types of femtosecond oscillators (ring dye laser, antiresonant ring, Ti:sapphire linear and ring) have been built and developed over the years in this group. The lasers were intended as well as a research object (to investigate various mechanisms of pulse formation and pulse compression) as tools for various experiments.

Compact Oscillators

This group was one of the first to recognize the essential role of dispersion in femtosecond laser cavities, and to achieve intracavity pulse compression with prisms [W. Dietel, J.J. Fontaine and J.-C. Diels, Optics Lett. 8, 4-6 (1983) ]. We have now developed accurate instrumentation to measure the complete phase and amplitude response of all intracavity elements, including coatings and Multiple Quantum Wells (MQW). This instrumentation is applied to the design of mirrors and MQW with predetermined dispersion, with the overal goal of constructing compact lasers generating ultrashort pulses with a minimum of intracavity elements.

Compact High Power Sources

A combination of Ti:sapphire oscillator at 744 nm, pulse stretcher, regenerative amplifier, post amplifier, pulse compressor, frequency trippling to 248 nm, and an excimer amplifier provides us simultaneously with fs pulses of millijoule energy at several wavelengths. While being the main workhorse for a large variety of experiments, such a laser system is complex, uneconomical, and takes a large space in the laboratory. We are presently developing alternate sources based on the traditional flashlamp pumped Nd:YAG laser, and parametric oscillators and amplifiers. The Nd:YAG laser, stabilized by passive negative feedback, provides pulses of 10 ps duration. Synchronous pumping of Optical Parametric Oscillators (OPO) can generate tunable femtosecond pulses [A. Umbrasas, J. C. Diels, J. Jacob and A. Piskarskas, "Parametric oscillation and compression in KTP crystals", Opt. Lett. 19: 1753-1755 (1994)]. To further amplify this tunable radiation, we have extracted a single pulse from the Nd:YAG laser, and frequency doubled it to generate green pulses of 400 fs duration (our goal is to improve this figure to 200 fs) of several mJ, through a simple saturated nonlinear conversion in long (6 cm) KDP crystals [A. Umbrasas, J. C. Diels, G. Valiulis, J. Jacob and A. Piskarskas, "Generation of femtosecond pulses through second harmonic compression of the output of a Nd:YAG laser", Opt. Lett., 20: 2228-2230 (1995)].

Stabilized Femtosecond Lasers

We have shown that the spacing between modes of a fs laser is rigourously constant over a bandwidth eceeding 250 nm. Therefore, linear and ring femtosecond lasers are being locked to stable reference cavities and to atomic lines. Applications range from wavelength standard, to accurate atomic clocks, to high precision gyroscopic measurements, to the measurements of small magnetic fields and femtosecond resolved measurements of small (10-9) indices of refraction. We are developing methods to lock the repetition rate of the laser to an integer number of optical cycles.

The mode-locked atomic clock will provide signals in various time ranges: the optical frequency (wavelength standard), the cavity repetition rate (time standard), and the beat note between the counter-rotating beams of a ring cavity (another time standard). In addition, the error signal will provide rotation sensing with hitherto unprecedented sensitivity.

Sensors and Laser Gyros

Ultrashort pulse ring lasers can be viewed as active differential interferometers: the two oppositely travelling wave can have their repetition rate locked, but their phase uncoupled. Applications range from the laser gyro without dead band, to measuring displacements of less than 0.001 Å, to measuring changes in index of refraction of less thant 10-9. A practical laser gyro should be compact and economical. We are investigating solid state diode pumped lasers, fiber lasers, and synchronously pumped ring optical parametric oscillators. A totally different approach uses multiple quantum well lasers to create a pair of unidirectional (uncoupled) ring lasers operating in opposite direction. Such type of ring lasers have also been demonstrated in our laboratory.

Diagnostic Methods

All the source development cited above makes little sense without accurate diagnostic method to analyze the amplitude and phase of the pulses being generated. We have developed over the years numerous types of diagnostic methods, starting from the interferometric autocorrelation [J.-C. Diels, J. J. Fontaine, I. C. McMichael, and F. Simoni, Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy. Applied Optics: 1270--1282 (1985)], to the single shot ``femtonitpicker'' [J.-C. Diels, J. J. Fontaine, N. Jamasbi, Ming Lai and J.Mackey, "The Femtonitpicker", Conf. on Lasers and Electro-optics, Baltimore, June 1987, (1987)], to a diagnostic method that does not use any nonlinear optics [Steffen Prein, Scott Diddams and Jean-Claude Diels, "Complete characterization of femtosecond pulses using an all-electronic detector", Optics Comm., 123: 567-573 (1996) ].

Laser Induced Discharges

We have demonstrated laser guiding across 50 cm gaps in air, at less than 1/4 the self-breakdown voltage, with ultrashort UV laser pulses of less than 50 mJ energy. We are investigating a scaling up of these experiment across larger gaps. An application being investigated is the triggering and guiding of lightning.

Self-filamentation in Air

The latter topic led to the observation of self-filamentation in air: the laser pulses creates its own waveguide, and propagate without diffraction over tens of meters. We are investigating comparatively these effect at various wavelength ranging from the near IR to the UV. The mechanism of formation of and stabilization of these filaments as well as associated phenomena are totally different at 800 nm and 248 nm.

Femtosecond Communication

Femtosecond pulses of less than 100 fs should enable digital communication at a 10 THb rate. We have initiated experiments of time multiplexing at the emitter, and demultiplexing at the receiver. Fast electro-optic modulator arrays (32 channel each) are being tested for phase and amplitude modulation, in a pulse shaper that transforms a single pulse of 100 fs in a train spanning less than 10 ps, at a replenishment rate of 100 MHz. Femtosecond communication requires accurate synchronization between a laser at the emission and at the reception. We are investigating optical synchronization scheme for which we expect fs jitter, requiring only picowatt injection.

Femtosecond Manufacturing

New techniques are being investigated to manufacture three dimensional waveguide and modulators required for the high speed communication project cited above. Such modulator could also be used to provide wavefront correction with ns response. Femtosecond pulses offer the possibility of writing waveguides in the bulk of transparent materials. Femtosecond UV irradiation is also used for machining grooves and holes with large aspect ratio.

Guide Star

Coherent interactions properties are being exploited to excite sodium to the 4s level of sodium (or higher), using bichromatic pulses with a fluence of only a few mJ/cm2. Calculations show that properly shaped pulses can achieve complete (i.e. > 95%) population of the upper state, leaving the intermediate states completely unexcited.

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