Continuous-wave Laser Operation Research Articles

Continuous wave laser operation of Tm and Ho co-doped CaYAlO4 and CaGdAlO4 crystals

The growth, spectroscopy and lasing performance of Tm/Ho: CaYAlO4 and Tm/Ho: CaGdAlO4 crystals are reported. The crystal exhibited a broad emission band from 1600nm to 2200nm. Single- and multi-wavelength operations of Tm/Ho:CaYAlO4 and Tm/Ho:CaGdAlO4 lasers have been investigated in detail, respectively. The maximum output powers of 376mW for Tm/Ho:CaGdAlO4 crystal and 582mW for Tm/Ho:CaYAlO4 crystals have been obtained, respectively. Red-shift of the emission wavelengths was obviously observed with the increasing of the absorbed pump power.

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.

High power laser heating of low absorption materials

A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500 μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500 μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m⋅ tan−1(t/m) in the transition region between the two.

Dynamical electrical tuning of a silicon microsphere: used for spectral mapping of the optical resonances.

In this work, electrical square pulses at various duty cycles are applied to a silicon microsphere resonator in order to continuously tune the refractive index of a silicon microsphere and to map the optical resonance in the time domain. A continuous-wave semiconductor diode laser operating in the L-band is used for the excitation of the silicon microsphere optical resonances. The 90° transverse magnetically polarized elastic scattering signal is used to monitor the silicon microsphere resonances. We show that at a constant input laser wavelength, up to five high-quality-factor optical resonances can be scanned by dynamical electrical tuning of the silicon microsphere cavity.

Power-scaling of a Pr:YAlO3 microchip laser operating at 747 nm wavelength at room temperature

We report efficient continuous wave laser operation of a laser diode both-side pumped Pr:YAlO3 crystal at room temperature in a microchip geometry arrangement. The microchip was formed by a 5 mm long 0.6 at.% doped Pr:YAlO3 crystal having dielectric mirrors directly deposited on its facets. As pump sources, two InGaN laser diodes with maximum output power of 1 W each were used. 490 mW of output power at 747 nm with an oscillation threshold of about 300 mW has been demonstrated. The corresponding slope efficiency related to the absorbed pump power was 45%.

Spectroscopic properties and continuous-wave laser operation of Yb:Bi₄ Si₃ O₁₂ crystal.

Yb3+:Bi4Si3O12 single crystal with Yb3+ concentration of 5.7 at.% has been grown successfully by the Czochralski method. The energy level positions of Yb3+ in Bi4Si3O12 crystal were determined based on the absorption and fluorescence spectra. The peak absorption cross-section is 0.98 × 10−20 cm2 at 976 nm and the peak emission cross-section is 0.57 × 10−20 cm2 at 1035 nm. The fluorescence lifetime of the excited multiplet is 1.26 ms. Diode-pumped continuous-wave laser operation around 1038 nm has been demonstrated in the Yb3+:Bi4Si3O12 crystal with a slope efficiency of 27% and maximum output power of 240 mW.

All-Electrical Thermal Monitoring of Terahertz Quantum Cascade Lasers

A key limitation, especially for the continuous-wave operation of terahertz quantum cascade lasers, is the large amount of heat dissipated in the active region. We demonstrate an all-electrical technique for monitoring the lattice temperature and characterizing the thermal properties of the active region, using the waveguide of the device as a temperature sensor. We report a measured temperature difference between the heat sink and top waveguide layer of up to 27 K during the continuous-wave operation of GaAs/Al 0.15 Ga 0.85 As-based devices, lasing at 2.4 THz. A thermal model of the devices is used to determine the thermal conductivity of the active region perpendicular to the semiconductor interfaces to be 7.3 W/(m·K).

Crystal growth, optical spectroscopy, and continuous-wave laser operation of co-doped (Ho,Tm):KLu(WO_4)_2 monoclinic crystals

We report on the crystal growth, optical spectroscopy, and room-temperature continuous-wave laser generation around 2 μm with co-doped (Ho,Tm):KLu(WO4)2 monoclinic crystals. Co-doped crystals at different doping levels were grown by the top-seeded solution growth slow-cooling method. We characterized the spectroscopy of Ho and Tm ions in KLuW in terms of polarized optical absorption, photoluminescence, and lifetime measurements. Continuous-wave laser operation at room-temperature at a wavelength of 2.06 μm was achieved with several crystals at different co-doping levels, establishing that the optimum doping ratio is 5.29×1019 at./cm3 for Ho and 3.10×1020 at./cm3 for Tm (0.5 at. %Ho, 5.0 at. %Tm:KLuW), reaching a maximum output power of 145 mW and a slope efficiency of 12.9%.

Pulsed and continuous-wave laser operation of TGT-grown Nd,Y-codoped :SrF2 single crystal

In this letter we present laser properties of temperature gradient technique (TGT) grown Nd,Y : SrF2 crystals with Nd3+ concentrations of 0.4, 0.65 and 0.8 at% and Y3+ concentration of 10 at%. The noncoated crystal samples, 3.1 or 5 mm long, were pumped by the 796 nm laser diode matching the Nd,Y : SrF2 absorption peak. In the pulsed pumping regime (pulse-duration 2 ms, frequency 10 Hz), maximum average output power of 75 mW (corresponding to peak power of 3.75 W) was obtained with slope efficiency as high as 51% and optical-to-optical efficiency of 42% with respect to the absorbed pump power. The output beam spatial profile was nearly Gaussian in both axes, oscillations started at the wavelength of 1057 nm. At higher pumping levels, the second emission line at 1051 nm appears corresponding to our fluorescence measurements. Wavelength tuning using a birefringent filter from 1048 to 1070 nm is probably given by the crystal-field splitting of the 4F3/2 manifold in Nd3+. True-continuous-wave laser operation was also successfully obtained at lower pumping levels with maximum output power of 380 mW and slope efficiency of 28% at the wavelength of 1057 nm.

All-solid-state dual end pumped YVO4:Nd/LBO blue laser with 21.8 W output power at 457 nm

It is reported the efficient compact deep-blue laser at 457 nm generation by intracavity frequency doubling of a continuous wave (CW) laser operation of a diode-pumped YVO4:Nd laser on the (4) F (3/2) -> (4) I (9/2) transition at 914 nm. An LBO crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation (SHG) of the laser. With dual end pump configurations at total incident pump power of 60 W, as high as 21.8 W of CW output power at 457 nm is achieved with 20-mm-long LBO. The optical-to-optical conversion efficiency is up to 36.3%, and the power stability in 8 h is better than 2.36%.

Crystal growth, spectroscopic characterization and laser performance of Tm/Mg:LiNbO3 crystal

A Tm, Mg co-doped LiNbO3 crystal was grown by the traditional Czochralski method. The room-temperature absorption, photo-luminescence spectra and fluorescence lifetime of Tm3+ ions in the crystal have been investigated. The experimental results show that the co-doped of MgO can lead to the lengthening of the measured fluorescence lifetime of the upper Tm3+:3F4 level. Based on the Judd–Ofelt approach, the intensity parameters Ω2,4,6 of Tm3+ were calculated to be Ω2 (6.29 × 10−20 cm2), Ω4 (0.54 × 10−20 cm2) and Ω6 (0.79 × 10−20 cm2). Other spectroscopic parameters that relate to laser performance were also obtained. Non-photorefractive continuous wave laser operation with a Tm, Mg:LiNbO3 single crystal is demonstrated at room temperature for the first time. We obtained 1.026 W output power at 1.885 μm with a slope efficiency of near 14%, which, to the best of our knowledge, is the largest output power and the highest slope efficiency obtained for this crystal thus far. The output power was observed to be stable, and the crystal showed no sign of photorefractive damage.

Laser performance of diode-pumped Nd, Y-codoped CaF 2–SrF 2 mixed crystal

A disordered Nd, Y-codoped CaF2–SrF2 mixed crystal was obtained by the temperature gradient technique (TGT). The absorption and fluorescence spectra of the crystal were measured at room temperature. Diode-pumped continuous-wave (CW) and Q-switched laser operations were demonstrated at 1056 nm with a 0.65 at.% Nd, 10 at.% Y-codoped crystal, for the first time to our knowledge. The CW output power of 724 mW was obtained in a compact linear cavity. Also the Q-switched pulse characteristics of Nd, Y:CaF2–SrF2 laser crystal were reported based on Cr4+:YAG saturable absorbers in a folded cavity. The shortest pulse width of 110 ns and the highest peak power of 383 W were obtained when the initial transmission of the Cr4+:YAG crystals was 90%. The dependence of the operational parameters on the pump power was also investigated experimentally.

High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 μm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

Crystal growth of CW diode-pumped (Er3+,Yb3+):GdAl3(BO3)4 laser material

Phase relationships in the Er:YbxGd1−xAl3(BO3)4–K2Mo3O10–B2O3–(Gd,Er,Yb)2O3 (x=0–0.2, Er-1at%) system were studied in the temperature range from 1150 to 900°С. Multicomponent melts of 55mol%K2Mo3O10–45mol% B2O3 (I) and 55mol% K2Mo3O10–40mol% B2O3–5mol% (Gd,Er,Yb)2O3 (II) were suggested as reasonable fluxes for high-temperature solution growth of (Yb3+,Er3+):GdAl3(BO3)4 single crystals. Yb0.15Gd0.85Al3(BO3)4 solubility in the complex flux of II composition has been found as varying from 17 to 9.5mol% with decreasing temperature in the interval of 1000–800°C. As a result, (Er,Yb):GdAB single crystals with high optical quality and sizes up to 20×10×10mm were grown on dipped “point” seeds. Absorption and stimulated emission spectra, emission lifetimes, and efficiencies of energy transfer from Yb3+ to Er3+ ions were determined. Highly efficient continuous-wave diode-pumped laser operation of (Er,Yb):GdAl3(BO3)4 crystal was realized.

High-power continuous-wave yellow–green laser at 558 nm under in-band pumping

We report a yellow laser at 558nm generation by intracavity frequency doubling of a continuous-wave (cw) laser operation of a diode-end-pumped Nd:YAG laser on the R1→Y5 transition at 1116nm. By pumping the neodymium ions directly into the emitting level using an 885nm laser diode, the maximum output power of ~1.5W is achieved, with an optical to optical efficiency with respect to the absorbed pump power of ~6.9%. To the best of our knowledge, this represents the highest output at 558nm generated by the diode pumped frequency doubled Nd:YAG lasers. The short term power stability is ~2.5% at the maximum output.

Crystal growth, spectroscopic characterization, and laser operation of Tm/Mg: LiTaO_3 crystal

Tm, Mg co-doped LiTaO3 crystal is grown by the traditional Czochralski method. By analyzing the absorption and emission measurements of the Tm/Mg: LiTaO3 single crystal with the Judd-Ofelt analysis, the intensity parameters Ω2,4,6, transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections were calculated. Non-photorefractive continuous wave laser operation with a Tm/Mg: LiTaO3 single crystal is demonstrated for the first time. We obtained 1.51 W output power at 1.92 μm with a slope efficiency of near 38.5%, which, to the best of our knowledge, are the largest output power and highest slope efficiency obtained for this crystal thus far. The long-term photorefractive effect was also quantitatively analyzed.

Wide wavelength tunability and green laser operation of diode-pumped Pr^3+:KY_3F_10

We report on wide spectral tunability of a quasi-continuous wave Pr(3+):KY(3)F(10) laser under InGaN laser diode excitation at 445 nm. The total tuning range exceeded 100 nm in the visible spectral range on several intervals between 521 nm and 737 nm. The broadest continuously tunable region of almost 50 nm extended from 688 nm to 737 nm. Furthermore we present what is to the best of our knowledge the first demonstration of continuous wave laser operation in Pr(3+):KY(3)F(10) on three transitions in the green spectral region. The highest output power of 121 mW was achieved at an emission wavelength of 554 nm with a slope efficiency of 27%.

Crystal growth, optical spectroscopy, and continuous-wave laser operation of Ho:KLu(WO4)2 crystals

We present the crystal growth, optical spectroscopy, and room temperature continuous-wave (CW) laser operation of monoclinic Ho:KLu(WO4)2 crystals. Macro defect-free crystals of several dopant concentrations were grown by top-seeded solution growth slow-cooling method. The evolution of unit cell parameters with holmium doping level and temperature was studied using X-ray powder diffraction. The spectroscopic properties were characterized in terms of room- and low-temperature optical absorption and photoluminescence. From low-temperature optical absorption measurements, the energy of the Stark levels was determined. Calculation of the emission and gain cross sections is presented. CW laser action was realized for 3 and 5 at. % Ho-doped KLu(WO4)2 by in-band pumping using a Tm:KLu(WO4)2 pump laser. A maximum output power of 507 mW with a slope efficiency of ~38 % with respect to the incident power was achieved at 2,080 nm with the Ho:KLu(WO4)2 laser.

Thermal, spectral and laser characteristics of Nd doped La0.05Lu0.95VO4 crystal

A 0.25at% Nd doped La0.05Lu0.95VO4 mixed crystal was grown by the Czochralski method. The thermal properties including thermal expansion, specific heat, thermal diffusion, and thermal conductivity were systematically studied. Meanwhile the reasons for cracking of this mixed crystal were also discussed. The refractive index at wavelengths of 633nm and 1539nm were obtained with the prism coupling method. The polarized absorption and fluorescence spectra of the crystal were also measured at room temperature. It was found that the fluorescence lifetime of 4F3/2 manifold for Nd3+ in Nd:La0.05Lu0.95VO4 is about 90μs. Diode-pumped continuous-wave (CW) laser operations at 1.06μm with a- and c-cut crystals were demonstrated. For the a-cut crystal a maximum output power of 5.41W was obtained at an incident pump power of 12.9W. Different from the a-cut crystal, the laser spectrum of the c-cut crystal was found to be dual-wavelength.

Optimization of dopant concentration in Ho:KLu(WO4)2 laser achieving ∼70% slope efficiency

We report on optimization of the dopant concentration in Ho:KLu(WO4)2 lasers based on continuous-wave laser operation under resonant pumping by a home-made diode-pumped Tm:KLu(WO4)2 laser. The highest laser efficiency amounted to 67.6% and corresponded to a dopant level of 5 at.% Ho in KLu(WO4)2. The maximum output power reached 507 mW at a wavelength of 2080 nm.