Design and characteristics of high-power (>0.5-W CW)diode-pumped vertical-external-cavity surface-emitting semiconductorlasers with circular TEM00 beams

We describe the design, fabrication, and measured characteristics of the high-power optically pumped-semiconductor (OPS) vertical-external-cavity surface-emitting lasers (VCSELs). Using diode laser pumping, we have recently demonstrated operation of such lasers, which for the first time generate high (watt-level) power and a circular Gaussian beam directly from a semiconductor laser. These OPS-VECSELs have a strain-compensated multi-quantum-well InGaAs-GaAsP-GaAs structure and operate CW near λ~1004 nm with output power of 0.69 W in TEM ₁₁ mode, 0.52 W in TEM₀₀ mode and 0.37 W coupled to a single-mode fiber. With multiple pump and gain elements, OPS-VCSEL technology is scalable to the multiwatt power levels. Such lasers will prove useful in a variety of applications requiring compact and efficient sources with high-power output in a single-mode fiber or with diffraction-limited beam quality     

Diode-pumped neodymium lasers repetitively Q-switched by Cr4+:YAG solid-state saturable absorbers

We report on the experimental results of a continuously diode-laser pumped Nd:YAG laser, operating at 1064 nm and repetitively Q-switched by a Cr⁴⁺:YAG solid-state saturable absorber. End-pumping the Nd:YAG with a 10-W fiber-coupled diode-laser we could either optimize the energy or the average output power, depending on the choice of the saturable absorber and the output coupler. The maximum energy was ≈200 μJ in single TEM₀₀, 17 ns pulses at 6 kHz, whereas a maximum average power of ≈2 W with 32-ns pulses at 20 kHz was obtained. We also present preliminary results of a repetitively Q-switched Nd:YVO₄ laser at 1064 nm. The repetitive Q-switching operation is described by an improved model, which accounts for the behavior of both the active medium and the solid-state saturable absorber. The results of the model agree fairly well with the experimental data. Experimental results of second harmonic conversion are also reported and interpreted using a depleted pump model     

High-power scaling (>100 W) of a diode-pumped TEM/sub 00/ Nd:GdVO/sub 4/ laser system

We present results of an investigation to scale a diode-pumped A Nd:GdVO/sub 4/ laser system to high powers (>100 W) using a bounce amplifier configuration. A Nd:GdVO/sub 4/ laser oscillator with a bounce amplifier geometry, pumped by a single 40-W diode bar, gave 24 W of multimode output power (60% optical efficiency), and 20 W of TEM/sub 00/ output with M/sup 2/<1.05. Power scaling of the oscillator system with pumping by multiplexing two (nominally) 40-W diode bars gave 50.1 W of multimode output at 83-W diode pumping, 40 W of predominantly TEM/sub 00/ output from 81-W of diode pumping, and 34 W TEM/sub 00/ output (M/sup 2//sub x/=1.05,M/sup 2//sub y/=1.1) with an external slit spatial filter. Higher power scaling is achieved by using a master-oscillator power-amplifier (MOPA) configuration with the double-diode-pumped oscillator and a bounce amplifier pumped by a 3-bar diode stack. A multimode MOPA output of 100 W is achieved by single pass amplification with 145-W amplifier diode pumping and 104-W TEM/sub 00/ mode using a double bounce amplifier configuration with 180-W amplifier diode pumping.     

Synchronization of high-power broad-area semiconductor lasers

Semiconductor lasers offer significant operational advantages due to their compactness and high electrical-optical conversion efficiency. The major drawback in considering semiconductor lasers for many applications is the relatively small emission power that can be obtained from a single semiconductor laser. Synchronization of laser arrays provides a unique solution to this limitation. In this paper, we describe our recent research on the synchronization of high-power broad-area semiconductor lasers and laser arrays. We demonstrate experimental results on 1) simultaneous injection locking of multiple broad-area lasers to achieve single longitudinal/transverse mode beams and 2) synchronization and coherent beam combination of an integrated 19 broad-area laser array based on a scalable external cavity. A number of issues in the synchronization of broad-area lasers have been addressed in this paper. These include the effects of laser coupling on the array synchronization performance and the gigahertz complementary intensity oscillations occurring at different transverse modes of broad-area lasers subject to the optical injection.     

Fiber-Laser-Pumped Er:YAG Lasers

Hybrid fiber-laser-pumped solid-state lasers exploit high-power cladding-pumped fiber lasers for direct (in-band) pumping of a crystal-based solid-state laser to reduce heating in the laser crystal, and hence allow scaling to higher power in both continuous-wave (CW) and pulsed modes of operation. In this paper, we briefly review the attractions of the hybrid laser approach for generation of output in the ~ 1.6 mum wavelength regime and consider the main design considerations for efficient operation of hybrid lasers based on Er:YAG in both CW and pulsed modes of operation. Examples of hybrid Er:YAG lasers, pumped by Er,Yb codoped fiber lasers at 1532 nm, with CW output powers up to 60 W at 1645 nm and 31 W at 1617 nm and slope efficiencies of 80% and 47% with respect to incident pump power, respectively, are described. In Q-switched mode of operation, pulse energies up to 30.5 mJ were obtained, limited by coating damage. Finally, the prospects for further increase in output power and improvement in overall performance in CW and Q-switched modes of operation will be discussed.     

Influence of thermal effect on output power optimization infiber-coupled laser-diode end-pumped lasers

A general model has been developed for the output power optimization of fiber-coupled laser-diode end-pumped lasers by including the thermal effect into the analysis. The optical path difference (OPD). Distribution has been derived as a function of the pump-beam quality, focus position of pumping light, and pump radius at the focal plane under the assumption that the end faces of the crystal are thermally insulated. With the derived OPD, the diffraction losses arising from thermal-induced spherical aberration have been estimated by the Strehl intensity ratio. The practical example of an Nd:YVO₄ laser pumped by a 1.2-W fiber-coupled laser diode is considered to illustrate the utility of the present model. Experimental results have shown a fairly good agreement with the theoretical predictions     

The metal-organic chemical vapor deposition growth and propertiesof InAsSb mid-infrared (3-6-μm) lasers and LEDs

We describe the metal-organic chemical vapor deposition (MOCVD) growth of AlAs_1-xSb_x cladding layers and InAsSb-InAs multiple-quantum well (MQW) and InAsSb-InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. The AlAs_1-xSb_x cladding layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. By changing the layer thickness and composition of SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 μm. We have made gain-guided injection lasers using undoped p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb-InAs MQW and InAsSb-InAsP SLS active regions. The lasers and light emitting diodes (LEDs) utilize the semi-metal properties of a GaAsSb(p)-InAs(n) heterojunction as a source for electrons injected into active regions. A multiple-stage LED utilizing this semi-metal injection scheme is reported. Gain-guided, injected lasers with a strained InAsSb-InAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8-3.9 μm and a characteristic temperature of 29-40 K. We also present results for both optically pumped and injection lasers with InAsSb-InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7-μm strained-layer superlattice (SLS) laser was 240 K. An SLS LED emitted at 4.0 μm with 80 μW of power at 300 K     

An investigation into the temperature sensitivity of strained andunstrained multiple quantum-well, long wavelength lasers: new insightand methods of characterization

The magnitude of the temperature rate of change of the threshold current density (J_th) is examined with respect to J_th , for a variety of unstrained and strained, long wavelength multiple quantum-well (MQW) lasers. A strong correlation is found between these parameters, and a new relationship describing the J_th -T relationship for these lasers is arrived at in terms of two new essentially temperature and length independent constants. A third constant, T_max, also appears which estimates the theoretical maximum operating temperature of the laser. It is proposed that these constants may prove to be more useful in characterizing the temperature sensitivity of semiconductor lasers than the conventional parameters T ₀ and I₀ which exhibit both a length and temperature dependence. Furthermore, an expression is found which relates the magnitude of T_max to adjustable device structural and material parameters, such as: the cavity length, L; facet reflectivity, R; transparency current density, J_tr; and, the modal gain coefficient, β. It is revealed that a close examination of this relationship may provide new insight into the physics of semiconductor lasers as well as a means for optimizing device design to obtain a high maximum operating temperature in order to eliminate the need for thermoelectric coolers in device packaging. Finally, the measured T_max, versus L characteristics of six different strained and unstrained MQW laser structures are presented     

High-power and high-efficiency operation of a CW-diode-side-pumpedNd:YAG rod laser

We have demonstrated high-power and high efficiency performance of a continuous-wave (CW) Nd:YAG laser with a simple and scalable side-pumping configuration. The maximum output power of 147 W was obtained in low brightness operation of M²=45. The corresponding electric efficiency is 14.8%. To our knowledge, this is the highest value reported for diode side-pumped Nd:YAG lasers. By using a rate and photon transport calculation, we have estimated the pumping efficiency of 72%. High brightness operation was also carried out by applying bifocusing compensation of the Nd:YAG rod. The brightness of 272 MW/cm² sr with beam quality of M²=5.9 and output power of 107 W was obtained at the electric efficiency of 11.6%. The brightness and the electric efficiency are comparable with those of industrial high-power CO₂ lasers that have been the first option for industrial applications     

Cryogenic Yb3+-Doped Solid-State Lasers

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO₄, and Y₂O₃. Spectroscopic properties of Yb:YAG and Yb:LiYF₄ (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M² 1.4.     

Power scalable TEM/sub 00/ CW Nd:YAG laser with thermal lens compensation

We present finite-element analysis and experimental results to validate our approach for building high-power single-mode Nd:YAG lasers. We show that the thermooptical and thermomechanical properties of a slab laser can be controlled. This is essential for the use of the proposed unstable resonator, We include demonstration of an efficient subscale laser operating at 20 W TEM/sub 00/.     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

Resonantly pumped eyesafe erbium lasers

The viability of high-power and high-energy, direct eyesafe emission from bulk erbium lasers has recently been demonstrated. In this paper, we present a review of eyesafe erbium lasers that are resonantly pumped by both fiber and diode lasers. High brightness pumping with a 1.53-/spl mu/m erbium fiber laser has yielded 60 W of continuous wave (CW) output, 10 W of repetitively Q-switched output, and as much as 16 mJ of pulse energy. Diode laser pumping has yielded 38 W of quasi-CW output and >40 mJ of Q-switched output.     

CW solid-state ultraviolet laser for optical disk masteringapplication

We have built a prototype laser head that emits 355-nm ultraviolet light in CW mode. The system is based on nonlinear sum-frequency mixing using a β-BaB₂O₄ (BBO) crystal in an external resonator in which two input emissions at 1064 and 532 nm are resonantly enhanced simultaneously. The output exhibits promising characteristics as the master recording laser source, in terms of beam shape and intensity stability. The result of reliability testing of the system in 1000-h real-time operation is also discussed     

CW PM Multiwatts Yb-Doped Fiber Laser Directly Emitting at Long Wavelength

We study the impact of standard active double-clad (DC) fiber characteristics and laser architectures on the generation of long-wavelength Yb-doped high-power fiber lasers around 1154 nm. Both theoretical and experimental works have been performed highlighting that Yb-doped fiber lasers can emit efficiently in multiwatts operation at wavelengths higher than 1150 nm. This study has been focused on the use of standard all-fiber active and passive components as well as on the use of standard DC Yb-doped fibers without any spectral filtering except cavity mirrors.      

A Planar Waveguide Nd:YAG Laser Using Active Q-Switching of a Hybrid Unstable Resonator

A planar waveguide laser operating in a negative branch unstable resonator is Q-switched by an acoustooptic modulator in a new configuration, providing effective, high-speed switching. The laser using a 200-mum Nd:YAG core, face pumped by 10 laser diode bars, has produced 100-W output in a good beam quality at 100-kHz pulse rate, and 4.5 mJ at lower frequency with 15-ns pulse duration.     

All solid-state continuous-wave frequency-quadrupled Nd:YAG laser

We report all solid-state, continuous-wave, high-power, deep ultraviolet generation. An intracavity frequency doubled Nd:YAG laser pumped by fiber-coupled diodes is used to generate the high-power green output. The significance of nonlinear coupling and spatial hole burning of the two-mode oscillations in the laser is discussed. When the incident 532 mn power on an external resonant doubler was 2.9 W, we generated 1.5 W of continuous-wave 266-nm radiation using a Czochralski-grown β-BaB₂O₄. To achieve stable external resonance, a novel voice coil motor actuator is employed with servo bandwidth as large as 50 kHz     

Double-pass fiber Raman laser-a powerful and widely tunable in theultraviolet, visible, and near-infrared fiber Raman laser for biomedicalinvestigations

We present the operating principle, general features, and advantages of a novel concept for the development of powerful and widely tunable fiber Raman lasers (FRL's), based on the high-power pumping of large-core low-loss multimode fibers using a simple double-pass laser arrangement with Littrow-prism-tuned emission. Basic experimental results obtained with various modification double-pass FRL schemes, which provide the generation of both discretely tunable emission in the UV/blue (360-493 nm) at λ_p=355 nm and continuously tunable emission in the visible/near-IR (0.54-1.01 μm) at λ _p=532 nm, are reported. The double-pass FRL schemes permit powerful and widely tunable spectral components to be generated, due to the double passing of nonlinear-converted laser emission through the optical fiber, maximum cavity feedback, the use of multimode fibers, and high-power pumping     

High-power mode-locked external cavity semiconductor laser usinginverse bow-tie semiconductor optical amplifiers

This paper presents experimental results of using an inverse bow-tie gain guided semiconductor optical amplifier (SOA) as the optical gain element in a high-power external cavity semiconductor laser. An average output power of 700 mW is demonstrated in continuous-wave (CW) operation while 400 mW of average power is obtained in both passive and hybrid mode-locked operation, with subsequent optical amplification in an identical SOA. The mode-locked laser operates at a repetition rate of 1.062 GHz, owing to the interplay between the gain and saturable absorber dynamics. Optical pulses are generated with a temporal duration of 5 ps, which implies a pulse energy of 376 pJ, and a peak power of 60 W. Further reduction of the optical pulsewidth to 1.3 ps is also achieved by using dispersion compensation techniques. These results show the promise of novel SOA devices for use as gain elements in external cavity semiconductor lasers. The generated output pulse characteristics from mode-locked operation is sufficient for use in novel three-dimensional data storage applications, and in large-scale commercial printing and marking applications     

Numerical modeling of the output and operations of semiconductor lasers subject to strong optical feedback and its dependence on the linewidth‐enhancement factor

Influence of the linewidth‐enhancement factor on the output and operations of InGaAs/InP pumping lasers emitting at a wavelength of 980 nm under strong optical feedback is investigated numerically. The investigations are performed based on intensive numerical integration of an improved time‐delay rate equations of semiconductor lasers over wide ranges of the linewidth‐enhancement factor and optical feedback strength. The results show that the semiconductor laser operates under strong optical feedback in continuous wave and pulsation at small values of the linewidth‐enhancement factor. Under large values of the linewidth‐enhancement factor, the laser happens to exhibit chaos and pulsation. We predict that semiconductor laser subjected to strong optical feedback exhibits much more stable pulsing operation under higher values of the linewidth‐enhancement factor, which indicates that the laser is locked at the external cavity frequency. Copyright © 2010 John Wiley & Sons, Ltd.