Sapphire Mode-locked Laser Research Articles

Experimental Evidence for Soliton Explosions

We show, experimentally and numerically, that Ti:sapphire mode-locked lasers can operate in a regime in which they intermittently produce exploding solitons. This happens when the laser operates near a critical point. Explosions happen spontaneously, but external perturbations can trigger them. In stable operation, all explosions have similar features, but are not identical. The characteristics of the explosions depend on the intracavity dispersion.

Spectral purity and sources of noise in femtosecond-demodulation terahertz sources driven by Ti:sapphire mode-locked lasers

Direct measurements of the spectral purity in terahertz femtosecond-demodulation sources are reported and compared to theory. Because these sources operate at very high harmonics (/spl sim/102-10/sup 4/) of the mode-lock frequency, a high spectral purity source is very dependent on a low-jitter femtosecond laser. Conversely, the spectral content of the terahertz sources provides detailed information about timing jitter and stringent tests of models used to describe the jitter. We find that both the behavior of the central core, and the noise skirts of the power spectrum of our sources, can be quantitatively related to measured ripple and continuum amplitude noise on the Ar/sup +/ pump laser by use of modulation theory.

Bidirectional, synchronously pumped, ring optical parametric oscillator

We report the operation of a bidirectional femtosecond pulsed ring optical parametric oscillator based on periodically poled lithium niobate, pumped alternately with nonsimultaneous pulses from a Ti:sapphire mode-locked laser. A beat note between the two counterpropagating beams attests to a gyro response without dead band. The sensitivity of the device to differential phase changes is demonstrated by measurement of the nonlinear index of lithium niobate.

Dispersion-managed mode locking

The pulse shape and spectrum of ultrashort-pulse Ti:sapphire mode-locked lasers are explained by a model akin to dispersion-managed pulse propagation of optical communications. The pulse is characterized as a nonlinear Bloch wave in a periodic structure. The ultimate spectral width is limited by dispersion and mirror bandwidth and less strongly by gain filtering. The main role of Kerr-lens mode locking is to provide stability against noise buildup. One-dimensional computer simulations are sufficient to explain the major pulse-shaping dynamics. Results are compared with experiment.

Ion Dynamics of Plasmas Induced in Elemental Targets by Femtosecond Laser Irradiation

The energies and number densities of ions emitted from laser-induced plasmas were studied by means of a simple time-of-flight technique. The plasmas were formed on solid elemental targets exposed to focused pulses from a Ti: Sapphire mode-locked laser and regenerative amplifier. The pulse energy and length were 1.0 mJ and 100 fs, respectively, with irradiance at the target of 2 × 1013 W/cm2. Ion kinetic energies, number densities, and mass removal rates were measured as functions of atomic mass, repetition rate, and angular orientation of the target with respect to the detector. Ion velocities varied within the 5.0–30.0 km/s range, corresponding to an average kinetic energy of 20–50 eV. Observed angular distribution of ion energy and number density obeyed the simple cosine function, which varied slightly for different elements. The total number of atoms removed per pulse was about 2 × 1013, while the ion fraction varied in the range of 3.0–5.0% for the majority of studied elements. The estimated total ion kinetic energy per pulse was ∼ 1014 eV, which corresponds to ∼ 1.5% of laser pulse energy. Mass removal rates demonstrated a direct relationship to transport properties of free electrons.

Imaging through flesh tissue using fs electronic holographic gating method

The experimental results of imaging through flesh tissue using fs electronic holographic gating method is reported. In the experiment, Ti: sapphire mode-locked laser is used as light source, of which the repetition rate is 100 MHz, central wavelength 800 nm, duration of pulse 20 fs, output power 80 mW. Tissue is a 7 mm thick chicken slice, and the imaged object is a metal wire with diameter of 0.5 mm. A general CCD is used to record holograms and a clear image of metal wire is obtained. Several relevant problems are discussed.

Construction of a two-photon microscope and optimisation of illumination pulse duration.

The construction of a two-photon/confocal microscope system is described in detail. For two-photon illumination, a Ti:sapphire modelocked laser generating 62-fs pulses at 715 nm was used. The effect of the optical train on illumination pulse width was examined and the observed increase in pulse duration was almost completely removed by the addition/adjustment of a prism compressor system. The imaging capabilities of the two-photon microscope are demonstrated and it is shown that the imaging performance of the two-photon microscope is similar to that of a conventional confocal microscope. With two-photon illumination, the resolution (full width at half-maximum intensity) was 0.42 microM (x-y) and 0.81 microM axially, while with single-photon illumination (at 488 nm in the same instrument with a confocal pinhole detector) the resolution was 0.3 microM (x-y) and 0.75 microM axially. The results are discussed with regard to the general problem of femtosecond pulse distortion in an optical system and a simple procedure for optimal pulse restoration is described.

100 femtosecond/100 nanometer near-field probe

Cross-correlation by upconversion has been used to measure pulse widths below 100 fs emitted from an aluminum-coated fiber near-field tip of the variety reported by Betzig et al. [Science 251 (1991) 1468]. The pulses are 0.8 μm center-wavelength with 76 MHz repetition rate generated by a Ti: sapphire mode-locked laser. The size of the probe is estimated at 100 nm by SEM micrographs and previous experience with similar probes used in our continuous wave NSOM systems. We measure average tip output powers of ∼ 1 nW for average input powers of ∼ 500 μW.

Defect processes induced by electronic excitation at halide crystal surfaces

Atomic force microscopy (AFM) provides a new topographic view of radiation effects near surfaces, complementing earlier electron microscopy as well as desorption and spectroscopic studies. Potassium iodide crystal surfaces have been imaged by AFM while undergoing exposure to ultraviolet light in the exciton absorption spectrum. Pronounced erosion and modification of the surface by ultraviolet radiation has been observed with resolution down to ∼ 50 nm. Prospects for extending such measurements to atomic resolution are considered. In related studies of KI under illumination in the exciton region, photoelectron spectra reveal photoinduced gap states which appear attributable to F centers and alkali clusters. In both of these studies, fourth harmonic radiation from a Ti:sapphire mode-locked laser has proven especially useful because of its tuneability through the strongly absorbing exciton region of alkali iodides, producing near-surface excitation densities comparable to electron beams.