High-power diode-end-pumped Nd:YVO4 laser: thermally induced fracture versus pump-wavelength sensitivity

We report two kinds of compact and efficient diode-end-pumped TEM₀₀ lasers with output power >25 W at ≈50 W of incident pump power. One laser consists of a single 0.3 at. % Nd:YVO₄ crystal in a V-type cavity, the other laser includes two 0.5 at. % Nd:YVO₄ crystals in a linear cavity. Experimental results show that lowering Nd³⁺ concentration can be beneficial in extending the fracture-limited pump power but it also increases the sensitivity of the pump wavelength due to the overlapping efficiency. Received: 19 February 2000 / Revised version: 30 May 2000 / Published online: 20 September 2000     

Small signal gain measurements in a small scale HF overtone laser

The overtone gain medium of a small-scale HF overtone laser was probed using a sub-Doppler tunable diode laser. Two-dimensional spatially resolved small signal gain and temperature maps were generated for several ro-vibrational transitions in the HF (v=2→v=0) overtone band. Our results compare well with previous measurements of the overtone gain in a similar HF laser device. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-505/846-4807, E-mail: kevin.hewett@kirtland.af.mil     

Two-wavelength self-seeded pulsed tunable laser

A compact self-seeded pulsed tunable laser is described. Its optical cavity comprises a diffraction grating, operating at a grazing angle of incidence as a spectral selector and narrow-band spectral gate. The grating couples two partially overlapping laser channels: a highly selective master laser and a nonselective slave laser. Due to the implemented efficient intracavity self-seeding the laser emits spectrally pure single-longitudinal-mode radiation at two independently tunable wavelengths, with an efficiency and output energy that are typical for nonselective lasers. Results of the experimental investigation of the laser’s output characteristics are presented. Received: 12 March 2001 / Revised version: 10 December 2001 / Published online: 7 February 2002     

Field-induced ionization and Coulomb explosion of nitrogen

Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used at intensities varying from 5×10¹³ to 2×10¹⁵ W/cm². Strong N₂ ⁺, N₂ ²⁺, N⁺, N²⁺ and N³⁺ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses, only N₂ ⁺ and N₂ ²⁺ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone. The atomic ions N⁺, N²⁺ and N³⁺ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser polarization vector. In contrast to the atomic ions, N₂ ⁺ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic, with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization mechanism is dominant under our experimental conditions. Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002     

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams. Received: 10 July 2002 / Revised version: 5 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-774/955206, E-mail: jhsi@photon.jst.go.jp     

Frequency stabilization of a diode laser on the Cs D2 resonance line by the Zeeman effect in a vapor cell

Hyperfine optical pumping of a Cs-vapor cell through a diode laser in frequency standards requires a high purity of the laser emission spectrum, and a high stability of the emission frequency. We present here a frequency-locking scheme that is able to produce a discrimination signal without any modulation, by using the dichroism induced by a magnetic field on the atomic vapor. The larger line-width of the reference signal (Doppler-broadened) is compensated by its larger amplitude, when compared with the saturated absorption signal. As a final result, the slope is similar, and the large line-width warrants a large locking range, making robust the lock against external perturbations. This error signal is used to lock the frequency of an external cavity diode laser, with a grating reflector in Littman configuration. A fast correction band is obtained by changing the cavity length through an intra-cavity electro-optic modulator. The possible sources of instabilities of the locking point are discussed and estimated through a simulation computer program. Received: 28 November 2000 / Revised version: 21 May 2001 / Published online: 18 July 2001     

Solid-state active media of tunable organic- compound lasers pumped with a laser. II. A copper vapor laser

The lasing properties of organic compounds in a polymethylmethacrylate matrix pumped by a copper vapor laser are studied. The results demonstrate that the transverse pumping scheme of solid-state laser-active media with the copper vapor laser is promising compared to the longitudinal pumping scheme from the viewpoint of the lifetime parameters. Received: 6 February 2002 / Published online: 2 May 2002     

Operation characteristics of the Au-II 690-nm laser transition in a segmented hollow-cathode discharge

Single-mode operation at the Au-II 690-nm transition was obtained in a segmented hollow-cathode discharge laser without the use of any additional frequency-selective device. The pressure of the helium buffer gas, which is responsible for the significant homogeneous broadening of the laser line, was varied between 10 and 20 mbar. The discharge was excited with rectangular current pulses (up to 3 A) six times exceeding the threshold value. The time dependence of the laser output during the 1-ms-long discharge pulses is explained on the basis of the temperature and pressure changes in the tube. The highest small-signal gain at optimal discharge conditions was 11%m⁻¹. Received: 1 July 1999 / Revised version: 4 November 1999 / Published online: 23 February 2000     

Output power optimisation of slab-waveguide laser with Gaussian output mirror

We present an approximate analysis of the nonlinear operation of the hollow slab-waveguide laser with Gaussian reflectivity profile output mirror, including gain saturation and longitudinal as well as transverse field distribution of the laser mode. The model presented is general and can be applied to the study of arbitrary configuration of the slab-waveguide laser. In particular, the laser characteristics obtained reveal the influence of the position of the output mirror and the Gaussian mirror parameter on the power efficiency of the laser system. Received: 6 January 2000 / Published online: 8 November 2000     

The spatial distribution of small signal gain in a segmented hollow cathode discharge laser

Received: 5 February 1997/Revised version: 30 April 1997     

Threshold and saturation properties of a solid-state XeF (C-A) excimer laser

A dielectric laser cavity of 1 cm length has been optimized for high gain (7 cm^–1) operation which is achieved in XeF doped Ar crystals. Mode structures on the C-A spectral distribution around 540 nm and far field transverse mode patterns are reported. Photchemical gain burning is observed in the spectral mode structures. The dependence of the laser threshold on pump energy, pumped length and on cavity losses is studied. XeF densities of 7×10¹⁷ cm^–3 and distributed losses of 1.2 cm^–1 are derived. The measured quantum efficiency of 14% and the saturation behaviour are consistently described. Losses by transient aborption and two photon absorption are discussed.     

Visible upconversion lasers in praseodymium-ytterbium-doped fibers

We report on a ZBLAN-fiber-based praseodymium-ytterbium-doped upconversion fiber laser operating in the blue-green with diffraction-limited beam quality. cw output powers of more than 150 mW at 491 nm are achieved for several hours without degradation. The spectroscopic data of the active material and laser parameters including the amplitude noise are discussed. Received: 28 June 1999 / Revised version: 20 August 1999 / Published online: 20 October 1999     

Coherent hole-burnings induced by a bichromatic laser field

In a Doppler-broadened three-level Λ-type system driven simultaneously by a coupling laser and two equal-amplitude saturating laser fields with a frequency difference 2δ, the absorption spectrum of a weak probe laser exhibits multiple deep spectral holes through coherent hole-burning CHB with controllable numbers, widths, depths, and positions. More significant, changing δ or lasers directions, CHBs can degenerate into narrower and deeper spectral holes where the slope of the refractive index is very steep. The multiple narrow spectral holes in a single absorption profile are expected to have potential applications in high density storage, optical information processes, and slow-light.     

Electrical conductivity of long plasma channels in air generated by self-guided femtosecond laser pulses

e >10¹² cm^-3) in the filaments, which plays a crucial role in the interplay of diffraction and self-focussing, leading to the formation of long stable channels. These filaments may have applications for laser-triggered lightning. Received: 12 February 1999     

Lasing action of rhodamine B in polyacrylic acid films

 Rhodamine B (Rh-B) has been embedded in PolyAcrylic acid (PAA) films. The materials are studied for the laser action with nitrogen laser excitation. The lasing efficiency of Rh-B/PAA films is comparable to that of rhodamine 6G in methanol. There is a 50% reduction in the laser intensity of the films with 2600 pulses from the N₂ laser (2 Hz). The photostability of these materials is far superior to that of other polymeric materials and is comparable to that of sol–gel materials. Received: 22 May 1996 /Revised version: 10 July 1996     

Kinetics of Er3+-doped fluorozirconate optical fiber upconversion fluorescence and laser emission under 800 nm excitation

Applying single- and double-pulse excitation at 800 nm, the kinetics of the upconversion fluorescence in the green, as well as the upconversion laser at 543 nm was studied. No significant delay between the pumping pulse and the laser emission was found. In the erbium doped (1000 ppm) optical fiber, the mechanism responsible for the upconversion is purely of the excited state absorption (ESA) type. The double-pulse technique enables also a determination of the lifetime of the intermediate metastable state ⁴ I _11/2 (7±0.3 ms). Some other basic properties of the upconverted fluorescence and of the laser itself (fluorescence spectrum, optical gain, laser threshold) are also described. Received: 11 December 2000 / Revised version: 18 February 2001 / Published online: 18 July 2001     

Emission properties of a Tm30.1mm+:GdVO4 microchip laser at 1.9 μm

4 as a host for thulium has several advantages for diode pumping in comparison with other crystals. The absorption cross section of thulium in GdVO₄ is considerably stronger and broader than in YAG and YLF and the spectrum is shifted closer to the emission wavelength of commercially available AlGaAs laser diodes. In our paper, we report on the temporal and spectral emission properties of a monolithical Tm^30.01mm+([%at.]6.9):GdVO₄ microchip laser at 1.9 μm. The laser can be adjusted to emit either cw or in an oscillating regime. Slope efficiencies up to 47% are achieved. This value exceeds the Stokes limit of 42%. Received: 31 March 1998/Revised version: 02 June 1998     

Wide-bandwidth frequency locking of a 1083-nm extended-cavity DBR diode laser to a high-finesse Fabry–Pérot resonator

Received: 28 July 1997/Revised version: 27 November 1997     

A broad emitter diode laser system for lithium spectroscopy

7 Li atoms. With 70 mW of laser power, 10⁷ atoms have been loaded from a nearby thermal source. Received: 19 January 1998     

Gain dynamics of two-wavelength Ti:sapphire femtosecond laser

We report the theory and experiments of a two-wavelength bi-tuning femtosecond Ti:sapphire laser. The cross-mode-locked gain competition is described with numerical solution and operation of the laser is accomplished in which two trains of femtosecond pulses cross in the Ti:sapphire rod. For this reason, a cross-bearings technique is used. The laser resonator is a six-mirror bi-cavity, and two beams of pump lights cross in Ti:sapphire crystal. Two trains of sub-30 fs cross-mode-locked pulses have been generated with independent tunable wavelength when the pump power of the argon laser is approximately 5 W. As the design of the bi-cavity is fine, not only strong gain competition is overcome, but also stable output characteristics are performed. Received: 21 July 1998 / Revised version: 5 January 1999 / Published online: 30 July 1999