Irina T. Sorokina

Ultrabroadband infrared solid-state lasers

Ultrabroadband infrared transition metal ion-doped solid-state lasers have come of age and are increasingly being used in trace gas monitoring, remote sensing, telecommunications, ophthalmology, and neurosurgery. Operating at room temperature, they are stable, versatile, and easy to handle successors to the color center lasers. They are becoming the critical components in optical frequency standards, space-based remote sensing systems, and may soon find application in femtochemistry and attosecond science. The article reviews the principles and basic physics of these types of lasers, which are distinguished by their ability to support the shortest pulses down to single optical cycle durations and the ultimately broad tuning ranges. The paper further reviews the state of the art in the existing diode-pumped sources of broadly tunable continuous wave, and ultrashort pulsed radiation in the infrared, and provides examples of their successful application to supercontinuum generation, trace gas measurements, and ultrasensitive intracavity spectroscopy. Developments in such lasers as Cr:YAG, Cr:ZnSe, Cr:ZnS, as well as the recently proposed mixed Cr:ZnS/sub x/Se/sub 1-x/ laser, are discussed in more detail. These lasers nearly continuously cover the infrared spectral region between 1.3 and 3.1 /spl mu/m. The gain spectra of these lasers perfectly match and extend toward the infrared spectra of such established ultrabroadband lasers, operating at shorter wavelengths between /spl sim/0.7-1.3 /spl mu/m, as Ti:sapphire, Cr:LiSAF/Cr:LiSGaF and Cr:forsterite. This opens up new opportunities for synthesis of single-cycle optical pulses and frequency combs in the infrared.

Broadly tunable compact continuous-wave Cr 2+ :ZnS laser

We report the development of a continuous-wave, room-temperature Cr(2+) ZnS laser that is compact and tunable over 700 nm. The laser is pumped by a diode-pumped Er-fiber laser and generates 0.7 W of linearly polarized radiation at 2.35microm , at up to 40% slope efficiency. Cr(2+) ZnS directly diode-pumped at 1.6microm yields polarized radiation that is tunable over 400 nm at up to 25 mW of output power. A comparison of Cr(2+) ZnS with Cr ZnSe (70 mW, 350 nm) in a similar setup is given. As opposed to Cr ZnSe, the Cr ZnS laser is intrinsically polarized. Finally, we observe sensitization of the output radiation by a few milliwatts of the visible (470-500-nm) and near-infrared (740-770-nm) radiation.

14-fs pulse generation in Kerr-lens mode-locked prismless Cr:LiSGaF and Cr:LiSAF lasers: observation of pulse self-frequency shift

Kerr-lens mode-locked Kr-laser-pumped Cr:LiSGaF and Cr:LiSAF lasers containing only two newly developed low-loss chirped mirrors instead of conventional dielectric resonator mirrors and generating pulses of widths as small as 14-fs with as much as 100 mW of average output power are reported. We report what is to our knowledge the first experimental observation of a pulse self-frequency shift in crystalline active media that are characteristic of such short pulses at megawatt intracavity peak powers. We also believe this phenomenon to be one of the significant limiting factors for pulse duration in femtosecond Cr:LiSAF-type lasers.

Multipulse operation and limits of the Kerr-lens mode-locking stability

Numerical analysis in combination with experimental data for Cr/sup 2+/:ZnSe and Ti:sapphire lasers reveal the following main mechanisms of multiple-pulse generation for Kerr-lens mode-locked solid-state lasers: 1) continuum amplification due to a spectral loss growth for ultrashort or chirped pulses and 2) a bounded perturbation rise for high-energy pulses. The role of such laser parameters as gain saturation and relaxation, saturable and unsaturable loss, self-phase modulation, Kerr-lensing, and pump intensity is analyzed. This analysis provides basic directions for single-pulse stability enhancement and for multiple-pulse generation control.

DIRECTLY DIODE-PUMPED TUNABLE CONTINUOUS-WAVE ROOM-TEMPERATURE CR4+:YAG LASER

We report what is to our knowledge the first directly diode-pumped tunable (over 120 nm) continuous-wave Cr(4+): YAG laser, operating near 1.5 microm . At room temperature the laser delivered as much as 200 mW of power at 4 W of absorbed pump power. We also report observation of slow laser output degradation and bleaching in highly concentrated crystals, which we suggest is due to a photorefractive phenomenon.

Tunable diode-pumped continuous-wave Cr2+:ZnSe laser

A midinfrared broadly tunable around 2.5 μm room-temperature continuous-wave diode-pumped Cr:ZnSe laser is reported. The laser is tunable over 350 nm and delivers up to 70 mW of the output radiation at 460 mW of the absorbed pump power and 17.5% slope efficiency. We observe the analog of the optical switching process, where the laser output is strongly modulated by only a few milliwatts of the visible radiation.

Crystalline Mid-Infrared Lasers

A survey of the well-established as well as newly emerging ion-doped crystalline lasers, operating in the mid-IR spectral range between 2 and 5 µm, is presented. The review includes rare-earth- and transition-metal-based ionic crystals, as well as color-center lasers. The emphasis is made on state-of-the-art compact all-solid-state room-temperature tunable sources, belonging to the broad class of vibronic lasers. The announcement of the efficient high-power room-temperature operation and super-broad tunability, as well as the possibility of generating ultrashort pulses from the novel class of chromium doped chalcogenide lasers led to a strong revival of research interest in vibronic laser systems, involving 3d n transition-metal ions. The review outlines the underlying physics of mid-IR lasers and sorts out the essential spectroscopic and laser characteristics, allowing assessment of the suitability of laser materials to serve as active media in diode-pumped laser systems.

Time-resolved Fourier transform intracavity spectroscopy with a Cr 2+ :ZnSe laser

Intracavity laser absorption spectroscopy (ICLAS) with an evacuated Cr2+:ZnSe laser is performed with a high-resolution time-resolved Fourier transform interferometer with a minimum detectable absorption coefficient equal to 4 x 10(-9) cm(-1) Hz(-1/2) in the 2.5 microm region. This represents the extreme limit currently reached in the infrared by ICLAS with Doppler-limited resolution. The broad gain band of the crystal allows a spectral coverage at most equal to 125 nm, wide enough to see entire vibration bands. Weak CO2 bands observed up to now only in the Venusian atmosphere are recorded for the first time, to our knowledge, in a laboratory. An H2O detection limit down to 0.9 parts per billion by volume is also demonstrated.

Femtosecond solid-state lasers using Nd 3+ -doped mixed scandium garnets

The mode-locking features of a series of Nd3+-doped solid solutions of aluminum and gallium garnets are investigated, and the influence of multiline gain spectra on mode-locking performance is discussed. With an additive-pulse nonlinear Michelson interferometer used to mode lock the laser passively, pulses as short as 260 fs are obtained. The effect of interferometer detuning on the pulse characteristics is studied.

Mechanisms of spectral shift in ultrashort-pulse laser oscillators

A number of factors that influence spectral position of ultrashort pulses in mode-locked lasers have been identified: high-order dispersion, gain saturation, reabsorption from the ground state, and stimulated Raman scattering. Using the one-dimensional numerical model for the simulation of the laser cavity, we analyze the relative contributions of different factors to the spectral position of the mode-locked pulses using the example of the Cr:LiSGaF laser. In this case the Raman effect provides the largest self-frequency shift from the gain peak (up to 60 nm), followed by the gain saturation (;25 nm), whereas the high-order dispersion contribution is insignificant (;5 nm). The results of the simulation are in good agreement with experimental data, confirming that stimulated Raman scattering is the dominant mechanism that causes the pulse self-frequency shift.