CW Output Research Articles

Ho:KLuW microchip laser intracavity pumped by a diode-pumped Tm:KLuW laser

A compact intracavity-pumped microchip Ho laser is realized using stacked Tm:KLuW/Ho:KLuW crystals pumped by a laser diode at 805 nm; both crystals are cut for light propagation along the N g optical indicatrix axis and emit with polarization along the N m axis. Maximum CW output power of 285 mW is achieved at a wavelength of 2080 nm for 5.6 W absorbed pump power in the Tm:KLuW crystal with a maximum slope efficiency of 8.3 %. Maximum total (Tm3+ and Ho3+ emission) output of 887 mW with a slope efficiency of 23 % is achieved. Laser operation is obtained in the 1867–1900 nm spectral range corresponding to the Tm emission, while Ho emits at 2078–2100 nm, depending on the output coupling. The microchip Ho laser generates a near-circular output beam with M 2 < 1.1. The compact laser setup with plane–plane cavity provides automatic mode-matching condition for the Tm and Ho laser modes.

Gyrotron development at KIT: FULGOR test facility and gyrotron concepts for DEMO

At the Karlsruhe Institute of Technology (KIT), theoretical and experimental foundations for the development of future gyrotrons for fusion applications are being laid down. This includes the construction of the new Fusion Long Pulse Gyrotron Laboratory (FULGOR) test facility as well as physical design studies towards DEMO-compatible gyrotrons. Initially FULGOR will comprise of a 10MW CW power supply, a 5MW water cooling system (upgradeable to 10MW), a superconducting 10T magnet, one or two 2MW ECRH test loads and a new control and data acquisition system for all these elements. The test facility will then be equipped to test the conventional 1MW or coaxial 2MW gyrotrons for DEMO, currently under design, as well as possible upgraded gyrotrons for W7-X and ITER. The design of the new high voltage DC power supply (HVDCPS) is flexible enough to handle gyrotrons with 4MW CW output power (conceivably up to 170GHz), but also test gyrotrons with higher frequencies (>250GHz) which, due to physical limitations in the gyrotron design, will require less power but have more stringent demands on voltage stability.

High-power continuous-wave interband cascade lasers with 10 active stages.

We report the pulsed and continuous wave (cw) performance of 10-stage interband cascade lasers (ICLs) emitting at both λ ≈3.2 μm and λ ≈3.45 μm. The slope efficiency is higher while the external differential quantum efficiency per stage remains about the same when comparison is made to earlier results for 7-stage ICLs with similar carrier-rebalanced designs. At T = 25°C, an 18-μm-wide ridge with 4.5 mm cavity length and high-reflection/anti-reflection coatings emits up to 464 mW of cw output power with beam quality factor M(2) = 1.9, for higher brightness than has ever been reported previously for an ICL. When the cavity length is reduced to 1 mm, both the 10-stage and 7-stage devices reach 18% cw wallplug efficiency at T = 25°C.

Diode-pumped continuous-wave and femtosecond Cr:LiCAF lasers with high average power in the near infrared, visible and near ultraviolet.

We demonstrate continuous-wave (cw), cw frequency-doubled, cw mode-locked and Q-switched mode-locked operation of multimode diode-pumped Cr:LiCAF lasers with record average powers. Up to 2.54 W of cw output is obtained around 805 nm at an absorbed pump power of 5.5 W. Using intracavity frequency doubling with a BBO crystal, 0.9 W are generated around 402 nm, corresponding to an optical-to-optical conversion efficiency of 12%. With an intracavity birefringent tuning plate, the fundamental and frequency-doubled laser output is tuned continuously in a broad wavelength range from 745 nm to 885 nm and from 375 to 440 nm, respectively. A saturable Bragg reflector is used to initiate and sustain mode locking. In the cw mode-locked regime, the Cr:LiCAF laser produces 105-fs long pulses near 810 nm with an average power of 0.75 W. The repetition rate is 96.4 MHz, resulting in pulse energies of 7.7 nJ and peak powers of 65 kW. In Q-switched mode-locked operation, pulses with energies above 150 nJ are generated.

The Development of 460 GHz gyrotrons for 700 MHz DNP-NMR spectroscopy

Two demountable gyrotrons with internal mode converters were developded as sub-THz radiation sources for 700 MHz DNP (Dynamic Nuclear Polarization) enhanced NMR spectroscopy. Experimental study on the DNP-NMR spectroscopy will be carried out in Osaka University, Institute for Protein Research, as a collaboration with FIR UF. Both gyrotrons operate near 460 GHz and the output CW power measured at the end of transmission system made by circular waveguides is typically 20 to 30 watts. One of them named Gyrotron FU CW GVI (we are using “Gyrotron FU CW GO-1” as an official name in Osaka University) is designed to have a special function of high speed frequency modulation δf within 100 MHz band. This will expand excitable band width of ESR and increase the number of electron spins contributing to DNP. The other gyrotron, Gyrotron FU CW GVIA (“Gyrotron FU CW GO-II”) has a function of frequency tunability Δf in the range of wider than 1.5 GHz, which is achieved in steady state by changing magnetic field intensity. This function should be used for adjusting the output frequency at the optimal value to achieve the highest enhancement factor of DNP.

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er3+:LiLuF4. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er3+:LiLuF4 crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10mW of output power at 553nm.

Dynamics of solid-state lasers pumped by mode-locked lasers.

We analyze the dynamics of mode-locked pumped solid-state lasers focusing on the transition between mode-locked and CW behavior. Where the ratio of the pump and laser cavity lengths is a rational number, 'rational-harmonic mode-locking' is obtained. When the cavity length is detuned away from such resonances, modulated continuous output is generated. The transition from mode-locked to modulated CW operation is explored experimentally for a Ce:LiCAF laser operating at 290 nm and pumped by a 78.75 MHz mode-locked frequency quadrupled Nd:YVO(4) laser. Both CW output and mode-locked output with pulse repetition rates up to 1.1 GHz were achieved. A rate equation model is developed to predict optimum cavity lengths for achieving CW output with minimized modulation.

High average power Q-switched 1314-nm two-crystal Nd:YLF laser.

A 1314 nm two-crystal Nd:YLF laser was designed and operated in both CW and actively Q-switched modes. Maximum CW output of 26.5 W resulted from 125 W of combined incident pump power. Active Q-switching was obtained by inserting a Brewster-cut acousto optic modulator. This setup delivered an average power of 18.6 W, with a maximum of 5.6 mJ energy per pulse with a pulse duration of 36 ns at a pulse repetition frequency of 500 Hz.

Development of Surface Grating Distributed Feedback Quantum Cascade Laser for High Output Power and Low Threshold Current Density

We report on the room-temperature cw operation of a surface grating distributed feedback (DFB) quantum cascade laser (QCL) at λ ∼4.7 μm. Both grating design and material optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4-mm-long laser, high wall-plug efficiency of 6% is obtained at 20°C from a single facet producing over 1 W of cw output power. The threshold current density of DFB QCL is as low as 1.13 kA/cm2 at 10°C and 1.34 kA/cm2 at 30°C in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in the working temperature range of 20–50°C.

An 11.6 W output, 6 kHz linewidth, single-polarization EDFA-MOPA system with a (13)C(2)H(2) frequency stabilized fiber laser.

We demonstrate a record high CW output power of 11.6 W and an ultra-narrow linewidth of 6 kHz in an all-fiber master oscillator and power amplifier (MOPA) fiber laser system. The master oscillator is a (13)C(2)H(2) frequency-stabilized single-polarization fiber laser with a 100 mW output. The power amplifier section consists of a core-pumped polarization-maintained erbium-doped fiber pumped by a 1480 nm cascaded Raman fiber laser. A total electric-to-optical conversion efficiency with a record high value of 12% was achieved with an all-fiber configuration.

High-power pump combiners for Tm-doped fibre lasers

AbstractIn this paper our results of investigation on a pump power combiner in a configuration of 7×1 are presented. The performed combiner, with pump power of 80–85% transmission level, was successfully applied in a thulium doped fibre laser. The performed all-fibre laser setup reached a total CW output power of 6.42 W, achieving the efficiency on a 32.1% level.

Dual-Wavelength Passively Q-Switched Nd,Mg:LiTaO3 Laser With a Monolayer Graphene as Saturable Absorber

Compact LD pumped, CW, and passively Q-switched Nd,Mg:LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> lasers with a stable, synchronous dual-wavelength operation near 1082 and 1092 nm were demonstrated for the first time. Under an incident pump power of 7.69 W, a maximum CW output power of 464 mW was obtained, corresponding to a slope efficiency of 7.6%. By using a monolayer graphene as saturable absorber, the steadily Q-switched Nd,Mg:LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> laser with dual-wavelength oscillation was achieved. The maximum output power of the Q-switched laser was 365 mW with a repetition rate of 133 kHz. The minimum pulse duration was measured to be 176 ns. The corresponding maximum pulse energy and peak power were 2.75 μJ and 15.7 W, respectively.

All-fiber DFB laser operating at 2.8 μm.

Single-frequency laser emission from a distributed feedback all-fiber laser operating in the vicinity of 3μm is demonstrated. The laser cavity was made of a 30mm long π-phase-shifted fiber Bragg grating inscribed in a heavily erbium-doped fluoride fiber using infrared femtosecond pulses and the dithering phase-mask technique. A maximum CW output power of 12mW was obtained at 2794.4nm by using a multimode pumping scheme. The narrow linewidth was characterized to be lower than 20kHz using a heterodyne technique. This achievement represents a significant step toward the development of active frequency references operating in the mid-infrared.

Quasi-continuous wave and continuous wave laser operation of Eu:KGd(WO4)2 crystal on a 5D0 → 7F4 transition

We report on the first demonstration of quasi-continuous wave (quasi-CW) and real CW room-temperature lasing on the 5D0 → 7F4 transition of Eu3+-doped material using a 25 at.%Eu3+ : KGd(WO4)2 crystal pumped into the 7F1 → 5D1 transition by a diode-end-pumped Nd3+ : KGd(WO4)2/KTP green laser at 533.6 nm. The maximum CW output power of this laser at 702.3 nm is 5.3 mW with 1.4% green-to-red conversion efficiency. In quasi-CW operation mode with a 10% duty cycle, the peak power of ms long pulses reaches ~54 mW, which corresponds to the optical conversion efficiency of 3.5%.

Spectroscopic properties and continuous-wave laser operation of Yb:LuPO₄ crystal.

For the first time, and to the best of our knowledge, we report a continuous-wave (cw) laser operation of Yb:LuPO₄ crystal, demonstrated at room temperature in a compact plano-concave resonator end pumped by a diode laser. 1.61 W of cw output power around 1039 nm were generated with 2.40 W of pump power at 976 nm absorbed in a 0.3 mm thick crystal, leading to an optical-to-optical efficiency of 67%, whereas the slope efficiency was 75%. Polarized absorption and emission cross-section spectra are also presented and discussed in detail.

Thermal modelling of high-power laser diodes mounted using various types of submounts

Using three-dimensional thermal modelling of a high-power laser diode with a stripe contact width of as an example, we analyse the thermal parameters of high-power laser diodes mounted using submounts. We consider a range of thermal conductivities of submounts that includes parameters of widely used thermal compensators based on AlN, BeO and SiC, as well as on CuW and CuMo composites and polycrystalline and single-crystal synthetic diamond with high thermal conductivity. Taking into account experimental overall efficiency vs. pump current data, we calculate the temperature of the active layer as a function of the width, thickness and thermal conductivity of the submount at thermal loads corresponding to cw output powers of 10, 15 and .

Single-transverse-mode near-IR superluminescent diodes with cw output power up to

A series of light-emitting modules based on single-mode quantum-well superluminescent diodes with centre emission wavelengths of about 790, 840, 960 and and a cw output power up to in free space is developed. A sufficiently long service life of these devices is demonstrated.

Efficient Q-switched Ho:GdVO₄ laser resonantly pumped at 1942 nm.

An efficient 2 μm room-temperature Q-switched Ho:GdVO4 laser end-pumped by a 1942 nm Tm-fiber laser is demonstrated. To our knowledge, this is the first report of Q-switched performance of Ho:GdVO4 crystal. A maximum CW output power of 6.85 W under the absorbed pump power of 24.1 W was obtained with a slope efficiency of 39.5% at a temperature of 17°C. With the same absorbed pump power, a maximum output energy per pulse of about 0.9 mJ and minimum pulse width of 4.7 ns were obtained at the pulsed repetition frequency (PRF) of 5 kHz, corresponding to a peak power of approximately 187.2 kW.

Orthogonally polarized dual-wavelength Nd:YAlO(3) laser at 1341 and 1339 nm and sum-frequency mixing for an emission at 670 nm.

We report a diode-pumped continuous wave (cw) orthogonally polarized dual-wavelength laser at 1339 and 1341 nm with a single b-cut Nd:YAlO(3) (Nd:YAP) crystal. By adjusting the tilt angle of the uncoated glass plate inserted in the laser cavity, we can control the cavity losses of two polarized directions. The output wavelengths are 1339 nm in a-axis polarization and 1341 nm in c-axis polarization, respectively, which are orthogonal to each other. At an incident pump power of 17.3 W, the cw output power obtained at 1339 and 1341 nm is 1.6 and 2.3 W, respectively. Furthermore, intracavity sum-frequency mixing at 1339 and 1341 nm was then realized in a KTiOPO(4) (KTP) crystal to reach the red range. To our knowledge, this is the first work realizing an orthogonally polarized dual-wavelength Nd:YAP laser based on the (4)F(3/2-)-(4)I(13/2) transition. Such a dual-wavelength laser would be especially valuable as a compact laser source to generate terahertz emission because the frequency difference between 1339 and 1341 nm is about 0.9 THz.

High beam quality in two directions and high efficiency output of a diode laser array by spectral-beam-combining.

Improvement is made in the traditional spectral beam combining structure by adding beam shaping element, namely Beam Transformation System (BTS). Spectral beam combination is performed in a horizontal direction by the external cavity after beam shaping. The effect of smile and the divergence of the slow axis are reduced. A standard semiconductor laser array is used in this experiment. A CW output power of 58.8 W and an electro-optic conversion efficiency of 51% are achieved. The spectral line-width is 12.7 nm. M(2) of 1.3 × 11.6 in horizontal and vertical directions are obtained. The beam quality of the output is close to that of a single emitter of the array in two directions.