Single Mode-locked Laser Research Articles

Optical clockworks and the measurement of laser frequencies with a mode-locked frequency comb

Femtosecond laser-frequency comb techniques are vastly simplifying the measurement and synthesis of optical frequencies. A single mode-locked femtosecond laser, with its spectrum broadened by self-phase modulation in a microstructured or tapered nonlinear fiber, can produce millions of sharp laser lines in a precise evenly spaced grid spanning much of the visible and near-infrared spectrum. The absolute frequency of each line is determined by two observable radio-frequency signals. The pulse repetition rate gives the spacing of the comb lines and the rate at which the phase of the lightwave slips, relative to the intensity envelope from pulse to pulse determines the offset frequency by which each line is displaced from a precise integral multiple of the repetition frequency. This offset frequency can be measured most easily if the comb spans more than an optical octave so that one can observe a radio frequency beat note between the second harmonic of the infrared comb lines with the corresponding comb lines at the blue end. Such an optical-frequency synthesizer makes optical oscillations readily countable and provides the long-awaited compact optical clockwork for an all-optical clock.

Demonstration of time interweaved photonic four-channel WDM sampler for hybrid analogue-digital converter

An experimental demonstration of a novel photonic sampler for a hybrid analogue-digital converter based on time interweaved wavelength channels is described. The technique uses a single modelocked laser to produce multiple wavelength channels where the wavelength channels are time interweaved to form a pulsed optical carrier for signal sampling.

40 Gbit/s demultiplexing experiment with 10 GHz all-optical clock recovery using a modelocked semiconductor laser

A 40 Gbit/s time-division multiplexed signal was demultiplexed to 10 Gbit/s using a single modelocked semiconductor laser as the clock source. The pulses of the laser directly control an all-optical switch for demultiplexing.

Time resolution limits for two-color pump/probe spectroscopy

The characteristics of various cw mode-locked dye lasers were thoroughly explored, and the characteristics of a pump/probe apparatus employing two such lasers pumped by a single mode-locked pump laser. First, it was observed that the rf source used to drive the acousto-optic mode-locking element in the pump laser, while not affecting the average power or obtainable pulse width, had a profound effect on the noise characteristics of the pump laser, which are in turn passed along to the dye lasers. For a driver based on a tunable frequency synthesizer, pulse jitter on the order of 300 ps was observed, which represents ≊2.5% of the total interpulse time interval. This was deemed unacceptably high, and use of this mode-locker driver was discontinued. For a driver with a fixed frequency reference source (an oscillating quartz crystal), pulse jitter on the order of 5 ps was measured, a great improvement over the other driver. Next we studied various dye laser configurations pumped by this pump laser. We found that for time resolution of several picoseconds, a pair of synchronously pumped dye lasers employing birefringent filter tuning elements was an excellent source of independently tunable pulse trains, and that these pulse trains were well synchronized with each other despite the absolute timing jitter in the pump pulse train. To obtain better temporal resolution with independently tunable lasers was more problematic, however. A hybridly mode-locked laser (employing a saturable absorber within a synchronously pumped dye laser) can produce shorter pulses (≤100 fs has been reported), but the lack of temporal synchronization between two of these lasers limits the resolution of a two wavelength experiment to several picoseconds. An attempt was made to reduce this relative timing jitter by crossing two independently tunable hybrid lasers through a common intracavity saturable absorber, thereby temporally locking the pulse trains together. This mechanism failed to work well on time scales longer than a millisecond.

Rapid beam scanning using a mode-locked laser-driven laser array

A scheme for driving a laser-array with the spatially dispersed spectral components of a mode-locked laser to achieve very rapid (<or=1 ns) beam scanning is outlined. In the proposed scheme, the precisely-timed periodic pulse train generated by a mode-locked picosecond laser is used as the injection-locking source for the separate lasers of an uncoupled laser array. In a mode-locked picosecond laser, the time-varying gain or loss in the laser cavity forces the oscillating modes to be precisely equally-spaced spectrally and exactly phase-locked leading to a train of precisely-timed periodic short pulses in the output. The corresponding equally-spaced and phase-locked CW spectral components of a single mode-locked laser can be used as a source for locking the individual lasers oscillating in the fundamental spatial modes in an uncoupled laser-array for beam-scanning. The schematic of a conceptual proposal of such a system is presented, and the practical implementation of such a mode-locked-laser-driven laser array is considered.<<ETX>>

The picosecond kinetics of the spontaneous fluorescence of erythrosin B

We have measured the rise and decay times of spontaneous fluorescence of erythrosin B in solution with picosecond resolution in a single mode-locked laser shot. The rise of spontaneous followed the integrated intensity of the actinic light pulses and there were no oscillations in the decay curve. The lifetimes were 57 psec in water and 140 psec in methanol.