CW Operation Research Articles

Self-Q-switched Cr:LiCAF laser

We report self-Q-switched operation of a Cr:LiCAF laser for the first time to our knowledge. Self-Q-switching (SQS) refers to the generation of a periodic train of Q-switched pulses from a laser cavity containing only the gain medium. Since SQS does not require any additional elements such as saturable absorbers or active modulators, it is far simpler and lower cost in comparison with other Q-switching methods. In the experiments, SQS operation was observed by using an x-shaped, astigmatically compensated laser cavity which contained only the Cr:LiCAF gain medium. A 140 mW, single-mode continuous wave (cw) diode at 660 nm was used as the pump source. In typical cw operation, the Cr:LiCAF laser produced output powers as high as 50 mW with about 50% slope efficiency. The laser had a diffraction-limited output and had a spectral width of about 0.5 nm near 795 nm. SQS operation could be initiated by fine tuning of the separation between the curved mirrors of the cavity and occurred at several discrete separations of the curved mirrors within the stability range of the resonator. Pulsed pumping of the pump diode, active cooling of the gain medium, and/or misalignment of the cavity end mirrors was not necessary to initiate SQS operation. In the SQS regime, the Cr:LiCAF laser produced about 5 μs wide pulses at repetition rates between 10 and 30 kHz. The corresponding pulse energies and peak powers were as high as 3.75 μJ and 590 mW, respectively. SQS operation was further accompanied with (i) a decrease in the output power to the 30–45 mW range, (ii) an increase of the spectral bandwidth up to 10 nm (full width at half-maximum), and (iii) a switching of the laser output from pure TEM00 to a structured beam containing higher-order spatial modes. We present detailed experimental data describing the temporal, spectral, and spatial characteristics of the SQS Cr:LiCAF laser, as well as the effect of curved mirror separation on SQS. The power-dependent repetition rate data were further analyzed to estimate the effective small-signal loss coefficient of the saturable absorber action.

Development of the superconducting rf 2-cell cavity for cERL injector at KEK

An injector cryomodule for the compact energy recovery linac (cERL) is under development at KEK. This injector cryomodule has 3 L-band 2-cell superconducting rf cavities. The cERL is required to accelerate a 10-mA CW electron beam to 5MeV. The required accelerating gradient per cavity is 7.5–12.5MV/m at ∼30kW input power to the cavity and the beam. The operational frequency is 1300MHz at 2K and the mode of operation is CW. In this application, the critical hardware components are not the cavities, but the rf input couplers and higher-order-mode (HOM) dampers. Initially, a TESLA-style coaxial HOM coupler was chosen for HOM damping of the injector cavities. However, this HOM coupler had a heating problem at low gradients (a few MV/m) in CW operation. The components heated in the accelerating mode were the HOM body and the feedthrough that extracts HOM power from the cavity. To control the heating problem, a new HOM coupler was designed based on a TESLA-style coaxial HOM coupler, and the feedthrough was also modified based on a Kyocera N-R type connector to have better thermal conductivity. A prototype 2-cell cavity and 3 other 2-cell cavities with 5 new HOM couplers for actual operation were fabricated through May 2011. Vertical tests of these cavities were carried out after standard surface preparation at the KEK Superconducting Accelerator Test Facility (KEK-STF) through March 2012. The accelerating gradient achieved exceeded 50MV/m without quenching during the vertical test using the prototype 2-cell cavity and feedthroughs. The magnetic field at the cell equator was 2127Oe. Three 2-cell cavities passing the criteria of the High Pressure Gas Safety Institute of Japan exceeded 25MV/m without field emissions. The cavities with the best performance were prepared in March 2012 for the cERL injector. The designs of the HOM couplers and feedthroughs and the results of the vertical tests to evaluate their performance are reported here.

Semiconductor A3B5 nanostructures for infrared femtosecond lasers

Two techniques were suggested and tested for the recovery time shortening of saturable absorbers on a base of A3B5 compounds including quantum wells. The first one, proposed by authors, is the sample post-growth treatment by UV laser radiation; it implied generation of point defects, which, in its turn, led to electron-hole recombination acceleration and to recovery time shortening by an order of magnitude and more. Another technique based on special design of barriers gave promising results for the fast saturable absorbers. Semiconductor mirrors designed for Yb3+:KY(WO4)2 infrared laser mode locking led to 115 fs stable mode-locking regime with average power close to CW operation. Results on fast saturable absorbers for spectral region of 1500 nm are also presented.

Design of Broadband GaN HEMT Power Amplifier with Ku Band

A broadband power amplifier module based on GaN HEMT operating Ku band is designed. TGF2023-02 Chip of GaN HEMT from TriQuint is modeled first. And then the module consists of two stages amplifiers. The first stage amplifier is single-stage amplifier and the second is two-way combiner amplifier. Wilkinson power divider, DC bias circuits and microstrip matching circuits are simulated and designed carefully. Simulation results showed that the amplifier module exhibits a power gain of 7 dB, power added efficiency of 13.9%, and an output power of 16 W under Vds=28 V, Vgs=-3.6 V, CW operating conditions at the frequency of 15 GHz.

14 W high-efficiency diode-pumped cw Yb:KGd(WO4)2 laser with low thermo-optic aberrations

A comparative experimental study of thermal lensing effect was performed for diode-pumped Yb3+-doped Y3Al5O12 and Ng-cut KGd(WO4)2 crystals under cw (continuous wavelength) operation mode. For Ng-cut Yb:KGd(WO4)2, thermal lens is positive for rays lying in all meridional planes and at least seven times weaker than that of Yb:Y3Al5O12. This is associated with lower quantum defect under 980-nm pumping (in comparison with 940-nm pumping for Yb:Y3Al5O12) and mutual compensation of negative impact of temperature dependence of the refractive index and positive impact of thermal expansion effect to the thermal lens optical power. Principal meridional planes of thermal lens in Ng-cut Yb:KGd(WO4)2 contain Np and Nm axes of optical indicatrix; corresponding sensitivity factors M=dD/dPabs equal 1.38m−1 and 1.09m−1 (with respect to absorbed pump power). This near-athermal crystal was utilized to produce air-cooled diode-pumped cw laser with output power of 14W, impressive slope efficiency of 76% and excellent beam quality (M2<1.2).

Resonantly pumped high efficiency Ho:YAG laser

High-efficient CW and Q-switched Ho:YAG lasers resonantly dual-end-pumped by two diode-pumped Tm:YLF lasers at 1908nm were investigated. A maximum slope efficiency of 74.8% in CW operation as well as a maximum output power of 58.7W at 83.2W incident pump power was achieved, which corresponded to an optical-to-optical conversion efficiency of 70.6%. The maximum pulse energy of 2.94mJ was achieved, with a 31ns FWHM pulse width and a peak power of approximately 94.7kW.

Index-coupled surface porous grating distributed feedback quantum cascade laser

We report design of special low loss, index-coupled surface porous grating distributed feedback profile by using holographic lithography combined with electro-chemical etching technology. Room temperature continuous-wave (cw) operation of single-mode quantum cascade lasers emitting at λ ∼ 7.6 μm has been achieved. Due to attenuate interplay between the surface plasmon waves and the waveguide waves, the waveguide loss of the lasers is significantly reduced, when compared to conventional surface metal/semiconductor grating devices. This resulted in an improved overall laser performance, such as a reduction of the threshold current density and an increase in cw output power.

High single-spatial-mode pulsed power from 980 nm emitting diode lasers

Single-spatial-mode pulsed powers as high as 13 W and 20 W in 150 and 50 ns pulses, respectively, are reported for 980 nm emitting lasers. In terms of energy, single-spatial-mode values of up to 2 μJ within 150 ns pulses are shown. In this high-power pulsed operation, the devices shield themselves from facet degradation, being the main degradation source in continuous wave (cw) operation. Our results pave the way towards additional applications while employing available standard devices, which have originally been designed as very reliable cw fiber pumps.

High-power 2.8 W blue-violet laser diode for white light sources

High-power 2.8 W blue-violet InGaN LD was fabricated, applying AlN facet coating technology. The AlN was found to be crystallized on the facets and very stable even after 2200 h cw operation. Luminous flux over 380 lm is obtained with a phosphor-converted LD excitation white light source using just a single laser chip at 1A operating current.

Experimental and theoretical study of finite-aperture tapered unstable resonator lasers

We present experimental and theoretical results of a novel finite-aperture tapered unstable resonator laser under pulsed and CW operations. At a current of 16 A an output power of about 10 W is reached. The beam parameter increases from M2 ≈ 10 at P = 1 W to M2 ≈ 20 at P = 10 W. Spatial–temporal simulations were performed to elucidate this experimental finding.

Continuous-wave wavelength conversion for high-power applications using an external cavity diamond Raman laser

We demonstrate continuous-wave (cw) operation of a diamond Raman laser at 1240 nm in an external cavity configuration. The output power increased linearly with pump power with a 49.7% slope efficiency and reached 10.1 W at the maximum available pump power of 31 W. The combination of resonator design with diamond provides a novel approach to power-scalable cw wavelength and beam conversion.

Nd:GdVO4 self-Raman laser using double-end polarised pumping at 880 nm for high power infrared and visible output

We report and evaluate a novel double-end polarised 880 nm pumping scheme for a Nd:GdVO4 self-Raman laser, aimed at efficiently generating high output powers in the near-infrared and visible. Compared to conventional single-end pumping, this pump scheme has significant benefits in terms of absorption efficiency, temperature effects in the crystal, and mode-matching between the pumping beam and TEM00 resonator mode. The maximum first-Stokes output powers were 4.1 W for CW operation and 5.63 W for quasi-CW (50 % duty-cycle) operation, with the diode-Stokes conversion efficiency of 11.2 % and 10.3 %, respectively. Visible emission was also realised by intra-cavity frequency-doubling (586.5 nm) or sum-frequency-generation (559 nm) using BBO or LBO crystals. For CW operation, the maximum output power was 3.46 W at 586.5 nm and 4.05 W at 559 nm, with diode-visible conversion efficiency of 10.7 % and 12.5 %; while for quasi-CW operation (50 % duty-cycle), the maximum output peak power was 6.5 W at 586.5 nm and 9.2 W at 559 nm, corresponding to 13.1 % and 18.9 % diode-visible conversion efficiency.

CW and mode-locked operation of Yb^3+-doped Lu_3Al_5O_12 ceramic laser

CW laser operation and first mode-locked laser operation of Yb:LuAG ceramic are reported. Efficient CW laser operation was obtained with maximum output power of 2.14 W and a 72% slope efficiency. Femtosecond mode-locked laser operation was achieved with pulse duration of 699 fs and a 200 mW average output power.

Continuous wave and mode-locked Yb^3+:Y_2O_3 ceramic thin disk laser

CW and mode-locked laser operation based on an Yb3+:Y2O3 ceramic thin disk is reported. In CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved. A higher slope efficiency of 70% was also obtained with a 50-W pump laser diode with more suitable emission characteristics. In mode-locked laser operation, pulses as short as 547 fs with an average power of 7.4 W were obtained. To our knowledge, this is the first demonstration of a high power CW and mode-locked laser operation based on Yb3+:Y2O3 ceramic thin disks.

Influences of spherical aberration on resonator’s stable zones and fundamental mode output power scaling of solid state laser oscillators

A parameter x is introduced to characterize the strength of thermal lens spherical aberration, whose influences on resonator's stable zones are analyzed theoretically. Some new and helpful results are obtained. For symmetrical plane-plane cavity, spherical aberration has just influence on the back edge of stable zone. For asymmetrical plane-plane cavity, spherical aberration has influence on the back edges of the two stable zones and the front edge of the second stable zone. Effects of transverse mode collapsing to TEM00 mode and stable zones separation of different order's transverse modes are pointed out, which is the foundation of TEM00 mode output power scaling for solid state laser oscillator. Influences of parameters such as resonator's long arm length, short arm length, and pump beam radius on the extent to which of stable zones separation of different order transverse modes are discussed. An experimental setup of a high power diodes dual-end pumped Nd:YVO4 TEM00 mode laser oscillator is built up and investigated experimentally. 51.2 W TEM00 mode output power in CW operation is achieved with an optical-to-optical efficiency of about 50% and beam quality factor M2 being 1.2.

Asymmetric Heterostructure With Reduced Distance From Active Region to Heatsink for 810-nm Range High-Power Laser Diodes

An asymmetric heterostructure design has been proposed to meet high-power laser diode (LD) requirements, such as a high catastrophic optical damage threshold, a low internal loss, low thermal and electrical resistances and a low vertical beam divergence. The asymmetry has been designed to shift the optical field in heterostructure waveguide toward the n-side, where losses are lower than those at the p-side. The main features of the design are: 1) a thin anti-guiding layer inserted between the “active” [containing quantum well (QW)] and the passive waveguides to elongate field penetration toward the n-side and to increase degree of freedom in design; and 2) reduced p-cladding layer thickness (thanks to the shift of the optical field toward the n-side) aimed at reducing the diode resistances due to shortened QW to heatsink distance. The characteristics of pulsed and CW operation of high-power LDs based on the asymmetric GaAsP/AlGaAs/GaAs heterostructure are presented. A lower temperature increase of the active region of asymmetric LDs compared with equivalent symmetric LDs has been evidenced by time-resolved spectra and thermovision characteristics.

Ultrawide-bandwidth, superluminescent light-emitting diodes using InAs quantum dots of tuned height

An ultrawide-bandwidth, superluminescent light-emitting diode (SLED) utilizing multiple layers of dots of tuned height is reported. Due to thermal effect, the superluminescent phenomenon is observed only under pulse-mode operation. The device exhibits a 3 dB bandwidth of 190 nm with central wavelength of 1020 nm under continuous-wave (cw) conditions. The maximum corresponding output power achieved in this device under cw and pulsed operation conditions are 0.54 mW and 17 mW, respectively.

High-performance uncooled distributed-feedback quantum cascade laser without lateral regrowth

We demonstrate, uncooled, room-temperature continuous-wave (cw) operation of single-mode distributed-feedback (DFB) quantum cascade lasers (QCLs) emitting around 4.6 μm without lateral regrowth. The effects of cavity length on device performance are studied. A record low threshold electrical power consumption of 2.3 W for the entire laser system with a 1.5-mm-long cavity is realized. For the 2-mm-long laser, high cw output power of 125 mW and very low threshold current density of 0.86 kA/cm2 are obtained. Our devices represent an important step towards using uncooled DFB QCLs in mid-infrared spectral range for practical applications.

Growth of flat p‐GaN contact layer by pulse flow method for high light‐extraction AlGaN deep‐UV LEDs with Al‐based electrode

AbstractImprovement of light extraction efficiency (LEE) is strongly demanded in an AlGaN‐based deep ultraviolet (DUV) light‐emitting diode (LED), because the LEE of AlGaN DUV LED is low (usually below 10%) due to the light absorption at around p‐GaN contact layer and p‐type electrode. We have demonstrated improvement of LEE of AlGaN DUV LEDs by using thin p‐GaN contact layer and using Al‐based high‐reflectivity p‐electrode. It was difficult to grow thin and flat p‐type GaN layers with heavy Mg‐doping on high‐Al‐content AlGaN layers. We demonstrated the growth of flat and thin p‐GaN contact layer by using ‘an NH3 pulse‐flow growth method’. We investigated the growth conditions for thin p‐GaN contact layer with high‐hole‐concentration using the NH3 pulse‐flow growth method. The surface roughness of the p‐GaN layer on high‐Al‐content p‐AlGaN was markedly improved by using an NH3 pulse‐flow growth method. We fabricated 250 nm‐band AlGaN quantum well (QW) DUV LEDs with thin p‐GaN contact layers grown by an NH3 pulse‐flow growth method. We observed single‐peaked operation spectrum for the 251 nm AlGaN MQW LED. The maximum output power and external quantum efficiencies (EQE) were 17.7 mW and 1.32%, respectively, measured under room temperature cw operation. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Eye-safe solid-state quasi-CW Raman laser with millisecond pulse duration

We demonstrate the first quasi-CW (ms-long pulses, pump duty cycle of 10%) end-diode pumped solid state laser generating eye-safe radiation via intracavity Raman conversion. The output power at the first Stokes wavelength (1524nm) was 250mW. A theoretical model was applied to analyze the laser system and provide routes for optimization. The possibility of true CW operation was discussed.