Comparative experimental investigation of thermal loading in continuous-wave cr(4 +):forsterite lasers

Results of a detailed experimental study aimed at reducing the thermal loading effects in room-temperature continuous-wave Cr(4+):forsterite lasers are presented. By using a Nd:YAG pump laser operated at 1.06 mum, the effect of the absorption coefficient and crystal cross-sectional area on the power performance of three crystals was compared between 12 and 36 degrees C. Experiments indicated that a low differential absorption coefficient significantly reduces the pump-induced thermal effects and cavity losses that would otherwise give rise to inefficient operation and increased temperature sensitivity. In particular, a Cr(4+):forsterite crystal with an absorption coefficient of 0.57 cm(-1) yielded as much as 900 mW of output power at 1.26 mum and a crystal temperature of 15 degrees C with an incident pump power of only 7.6 W. To the author's knowledge, the demonstrated slope efficiency of 30% represents the highest continuous-wave power performance reported to date from this laser system at elevated temperatures.     

High-power continuous-wave cr(4+):forsterite laser

The temperature and pump power dependence of infrared luminescence of Cr(4+):forsterite are measured. It is demonstrated that temperature-dependent fluorescence in Cr(4+):forsterite is the major reason for saturation of the output power of a continuous-wave laser. At higher pump intensities the temperature rise inside the crystal becomes significant, and even outside cooling does not help to prevent significant reduction of the laser performance. These measurements serve as a guideline to construct a high-power continuous-wave Cr(4+):forsterite laser.     

Tunable diode-pumped and frequency-doubled Cr:LiSAF lasers

The performance of a diode-pumped Cr:LiSAF laser and its intracavity frequency-doubled operation is reported. Close to 1 W of quasi-cw power at 870 nm was obtained in multimode operation. Power in excess of 20-mW cw of TEM(00) second-harmonic power at 435 nm has also been obtained by using a lithium triborate crystal with an infrared-to-visible conversion efficiency of 33%. This result is believed to be the highest yet reported for a true cw blue output from a diode-pumped Cr:LiSAF system. However, the maximum attainable output power was limited by thermal fluorescence quenching. The effect of the pump size on the temperature rise and on the small-signal gain is investigated.     

Efficient quasi-continuous-wave and passively Q-switched laser operation of a Nd3+:BaGd2(MoO4)4 cleavage plate

Pumped by a Ti:sapphire laser at 807 nm, efficient quasi-cw and passively Q-switched laser operations have been realized from a 1.1 mm thick, 1.2 at. % Nd(3+):BaGd(2)(MoO(4))(4) cleavage plate in a plano-plano cavity. When the absorbed pump power was 1060 mW, the achieved maximum quasi-cw output power was 580 mW. The slope efficiency and absorbed pump threshold power were 60% and 50 mW, respectively. Using a Cr(4+):YAG crystal with an initial transmission of 85% as the saturable absorber, a pulse laser with 3.0 microJ energy, 30 ns duration, and 52 kHz repetition rate has been obtained when the absorbed pump power was 1060 mW. The polarization characteristic and laser spectra of both the quasi-cw and passively Q-switched lasers have been measured.     

Chromium-doped forsterite laser with 1.1 W of continuous-wave output power at room temperature

We demonstrate stable continuous-wave operation of a Cr:forsterite laser pumped by a cw Nd:YAG laser at 288 K. We use no choppers to limit the duty cycle of the cw system. The proper adjustment of the pump- and cavity-mode overlap based on the ABCD concept simulation eliminates the output power decrease at the high-level pump power. An output power of 1.1 W and slope efficiency of 26% are derived.     

Passively Q-Switched Yb:YAG Laser with Cr(4+):YAG as the Saturable Absorber

By using a continuous-wave Ti:sapphire laser as a pumping source, we demonstrated a passively Q-switched Yb:YAG laser at room temperature with Cr(4+):YAG as the saturable absorber. We achieved an average output power of as much as 55 mW at 1.03 mum with a pulse width (FWHM) as short as 350 ns. The initial transmission of the Cr(4+):YAG has an effect on the pulse duration (FWHM) and the repetition rate of the Yb:YAG passively Q-switched laser. The Yb:YAG crystal can be a most promising passively Q-switched laser crystal for compact, efficient, solid-state lasers.     

Efficient continuous-wave frequency doubling of a tunable CO(2) laser in AgGaSe(2)

Second-harmonic output at 4.6-5.5 mum of the order of 6 mW with a 0.12% external conversion efficiency has been obtained by pumping a AgGaSe(2) crystal with a low-power cw CO(2) laser. The surface damage threshold of AgGaSe(2) for cw radiation was found to be inside the limit of 33-45 kW/cm(2) in the 9.2-10.8-mum wavelength region. Another important limitation of the pump power connected with a thermal lensing effect in crystal was determined experimentally. A comparison was made of AgGaSe(2) and ZnGeP(2) crystals as materials suitable for the efficient generation of the second harmonic of cw CO(2) laser radiation.     

Intracavity frequency-doubled femtosecond cr(4+):forsterite laser

The generation of femtosecond optical pulses centered at ~620 nm directly from an all-solid-state laser oscillator is reported. Red pulses with pulse widths of the order of 170 fs were obtained with 24-mW average power at an 81-MHz repetition rate. They were achieved by intracavity frequency doubling of a mode-locked Cr(4+):forsterite laser with a 1-mm-thick beta-BaB(2)O(4) crystal. The process of laser mode locking was modified by surface coating the doubling crystal.     

Continuous-wave laser performance of Nd:LuVO4 crystal operating at 1.34 microm

A laser-diode-array end-pumped Nd:LuVO4 crystal continuous-wave (cw) laser operating at 1.34 microm has been demonstrated. The maximum cw output power of 1.85 W was obtained at the incident pump power of 17 W for a c-cut 0.5 at. % Nd-doped LuVO4 crystal sample, giving the corresponding optical conversion efficiency of 10.88% and a slope efficiency of 13.5%. Laser experiments of Nd3+ concentration of 0.5 and 0.9 at. % a-cut crystal LuVO4 samples were also investigated; due to the strong excited-state absorption of Nd:LuVO4 at 1.34 microm, the output power was limited.     

Semianalytical thermal analysis on a Nd:YVO(4) crystal

Based on analytical theory of anisotropy, distributions of thermal distortion and temperature field within a diode-end-pumped rectangular Nd:YVO(4) laser crystal are investigated. A thermal model that matches the actual working state of the laser crystal is established by analyzing the working characteristics of the Nd:YVO(4) laser crystal. A novel method, to the best of our knowledge, is adopted to solve the heat conduction equation of the anisotropic medium. General solutions of the temperature field, thermal strain field, and thermal distortion field of the Nd:YVO(4) crystal are obtained. The effect of anisotropic thermal parameters on the thermal strain field of the Nd:YVO(4) laser crystal is also analyzed quantitatively. Research results show that a maximum temperature rise of 244.9 degrees C and a maximum thermal distortion of 1.99 mum can be obtained in the center of the pump face when the Nd:YVO(4) laser crystal doped with 0.5 at. % Nd(3+) is diode end pumped in the center of the front end face with 15 W output power. This method can be applied to other thermal analyses of laser crystals and offers a theoretical basis to solve thermal problems effectively in the laser system.     

808-nm diode-pumped continuous-wave Tm:GdVO4 laser at room temperature

A high-quality gadolinium vanadate (GdVO4) crystal with 7-at. % thulium as the starting material was grown by the Czochralski technique. The measured absorption spectra exhibited sufficient absorption coefficients for laser diodes (LDs) for neodymium laser pumping: 6.0 cm(-1) for pi polarization and 6.2 cm(-1) for sigma polarization at 808 nm. Laser oscillation was carried out with single-stripe 808-nm LDs in an end-pumping configuration. A slope efficiency of 28% and a threshold of 750 mW were exhibited with respect to the absorbed pump power. An output power of 420 mW was achieved at an absorbed power of 2.4 W. It was demonstrated that Tm:GdVO4 is a useful material for 2-microm lasers, particularly in a compact LD-pumped system.     

Diode-Pumped Cr:forsterite Laser Mode Locked by a Semiconductor Saturable Absorber

We report the passive mode locking of a diode-pumped Cr:forsterite laser using a semiconductor saturable absorber. The laser generates 10-pJ pulses as short as 1.5 ps in duration with ~650 mW of absorbed pump power. We measured the transient reflectance of the saturable absorber mirror under different incident energy fluences by using a standard pump-probe method. The performance of this laser is also compared with a high-power fiber-laser-pumped femtosecond forsterite laser mode locked with the same absorber.     

Continuous wave thermal loading in saturable absorbers: theory and experiment

Theoretical and experimental results of a study that investigates cw thermal loading in solid-state saturable absorbers with low heat conductivities are presented. In addition to the temperature dependence of the refractive index, the proposed model considers the temperature dependence of the fluorescence lifetime to account for the local variations in the saturation intensity resulting from thermal gradients. In the calculations an iterative scheme is employed to calculate first the temperature distribution produced by the pump beam subject to saturable absorption with a constant saturation intensity and then the resulting modifications in the propagation parameters that are due to the presence of the calculated temperature distribution. Excellent agreement is obtained between the numerically calculated results and experimentally measured cw transmission data obtained with use of a Cr:YAG saturable absorber. Because the absorption cross section of the medium is used as one of the fitting parameters to yield the best fit between theory and experiment, the model further offers an accurate method whereby the cw power transmission data can be used to determine the absorption cross section of a saturable absorber subject to thermal loading.     

Continuous-Wave and Passively Mode-Locked Operation of a Cunyite (Cr(4+):Ca(2)GeO(4)) Laser

Continuous-wave and mode-locked Cr(4+):Ca(2)GeO(4) lasers that use a fiber laser pump source were demonstrated. The continuous-wave Cr(4+):Ca(2)GeO(4) laser yielded a maximum output power of 415 mW at 1420 nm and a tuning range of 1335-1492 nm. With a saturable-absorber mirror, 60-ps pulses and 110-mW maximum output power were generated from a passively mode-locked Cr(4+):Ca(2)GeO(4) laser.     

Passive Q switching of a solar-pumped Nd:YAG laser

Passive Q switching is a preferable choice for switching the Q factor of a solar-pumped laser because it requires neither a driver nor an electrical power supply. The superior thermal characteristics and durability of Cr(4+):YAG single crystals as passive Q switches for lamp and diode-pumped high-power lasers has been demonstrated. Here we report on an average power of 37 W and a switching efficiency of 80% obtained by use of a solar-pumped Nd:YAG laser Q switched by a Cr(4+):YAG saturable absorber. Concentration of the pumping solar energy on the laser crystal was obtained with a three-stage concentrator, composed of 12 heliostats, a three-dimensional compound parabolic concentrator (CPC) and a two-dimensional CPC. The water-cooled passive Q switch also served as the laser rear mirror. Repetition rates of as much as 50 kHz, at pulse durations between 190 and 310 ns (FWHM) were achieved. From the experimental results, the saturated single-pass power absorption of the Cr(4+):YAG device was estimated as 3 ? 1%.     

1.86 W cw single-frequency 1319 nm ring laser pumped at 885 nm

A 1.86 W cw single-frequency 1319 nm laser was produced by using an 885 nm-pumped Nd:YAG crystal with a compact four-mirror ring cavity, for the first time to our knowledge. The Nd:YAG produced a slope efficiency of 21% and an optical-to-optical efficiency of 18% with respect to the absorbed diode pump power. A near-diffraction-limited beam with M(2)=1.2 was achieved under the maximum output power.     

Optical Kerr coefficient measurement in coiled high-birefringent fibers

The optical Kerr effect in coiled high-birefringent fibers was measured based on a double-beam polarimetric method. A Q-switched Nd:YAG laser, operating at 1.064 mum (FWHM of 80 ns at 1 kHz), was used as the pump beam and a cw 0.633-mum He-Ne laser was used as the probe beam with its polarization fixed at 45 degrees with respect to the birefringent axis whereas orientation of the linearly polarized pump light varied. The phase shifts induced by an intense pump beam in a short bow-tie high-birefringent fiber were determined for different fiber lengths coiled into 15- and 30-cm-diameter drums. It was found that the induced phase shift changes drastically with the state of polarization of the pump light. A strong dependence of the phase shift on orientations of linear pump polarization is attributed to differential losses of eigenmodes peculiar to birefringent axes. Therefore, optical Kerr coefficients remain unchanged regardless of the dependence of the nonlinear response of the coiled high-birefringent fibers on pump polarization.     

Passively Q-Switched Diode-Pumped Continuous-Wave Nd:YAG-Cr(4+):YAG Laser with High Peak Power and High Pulse Energy

A high peak power and high pulse energy passively Q-switched diode-pumped cw Nd:YAG laser at 1.064-mum wavelength has been demonstrated with Cr(4+):YAG crystal as the saturable absorber. The average output power of 7-12 W and pulse duration of 100-250 ns was obtained with kilohertz repetition rates. The highest peak power and pulse energy obtained were 30 kW and 3.4 mJ, respectively. All the output resulted from the TEM(00) mode with M(2) < 1.1. The thermal lensing effect of the saturable absorber was investigated, demonstrating that it played an important role in optimization of the output.     

Mid-Infrared-Wavelength Generation in 2-mum Pumped Periodically Poled Lithium Niobate

A periodically poled lithium niobate optical parametric oscillator pumped by a Tm:YAG laser at 2.0124-mum wavelength has been demonstrated. A pump pulse energy of 5.1 mJ generated 0.65 mJ of signal and idler pulse energy at a 50-Hz repetition frequency with a 27.8-mum domain-period-length grating. The lithium niobate crystal at a temperature of 180 degrees C yielded 3.61- and 4.55-mum signal and idler wavelengths, respectively. Wavelength tuning over a wide range was achieved with domain-period lengths from 25.5 to 28.2 mum and crystal temperature tuning from 50 to 180 degrees C. Signal wavelengths of 3.26-3.76 mum and idler wavelengths of 4.33-5.34 mum were generated.     

Efficient laser operation of an Yb:S-FAP crystal at 985 nm

We have obtained three-level cw laser operation at 985 nm with a Yb-doped S-FAP bulk crystal pumped by a Ti:sapphire laser. An output power of 250 mW for an incident pump power of 1.45 W has been achieved, which is the highest cw output power ever obtained, to our knowledge, at this wavelength with a Yb-doped crystal. The experimental results are in good agreement with the numerical model that we have developed.