High-Power Laser Performance of Yb:YAl3(BO3)4 Crystals Cut Along the Crystallographic Axes

High-power continuous-wave laser operation near 1 mum was demonstrated at room temperature with c-cut and a-cut Yb:YAl₃(BO₃)₄ crystals end-pumped by a fiber-coupled diode laser. Using a 2-mm-thick c-cut crystal, 10.6 W of output power was generated with an optical-to-optical efficiency of 65% and a slope efficiency of 72%. The complex polarization state of the generated laser radiation was also studied. With a 2-mm-thick a-cut crystal, sigma-polarized laser oscillation was obtained, producing a maximum output power of 8.1 W; the optical-to-optical and slope efficiencies were 56% and 61%, respectively. Complete absorption and emission cross section spectra of the Yb:YAl₃(BO₃) ₄ crystal were also derived, revealing very strong anisotropy of the spectroscopic properties     

Theoretical and experimental investigations of a diode-pumpedquasi-three-level laser: the Yb3+-doped Ca4GdO(BO3)3 (Yb:GdCOB) laser

We present a theoretical and experimental analysis of a diode-pumped Yb³⁺-doped Ca₄GdO(BO₃)₃ (Yb:GdCOB) laser. A new model for a diode-pumped quasi-three-level laser is described. The effects of absorption saturation, temperature profile, and the beam quality M² factor of the pump diode have been taken into account, for the first time to our knowledge. We have obtained a good agreement between experimental measurements and theoretical calculations with two different pump wavelengths, 902 and 976 nm. Our model has given good predictions of the laser performances for different crystal temperatures and different M² factors of the pump beam. As much as 440 mW of output power (at 1082 nm) have been achieved for 640 mW of absorbed pump power at 976 nm, corresponding to one of the highest slope efficiencies (81%) ever obtained with Yb-doped lasers     

Improved second-harmonic generation by selective Yb ion doping in anew nonlinear optical crystal YCa4O(BO3)3

We describe experiments characterizing a new nonlinear optical crystal, YCa₄O(BO₃)₃ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (~3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa₄ O(BO₃)₃, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa₄O(BO₃)₃ (RE=La³⁺, La ³⁺, Y³⁺, Sm³⁺, Gd³⁺, Er³⁺, and Nd³⁺). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb³⁺, which has larger mass, but approximately the same atomic size as Y³⁺ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa₄O(BO₃)₃ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity     

Crystal growth, frequency doubling, and infrared laser performanceof Yb3+:BaCaBO3F

Yb:BaCaBO₃F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb³⁺ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1×10^-20 cm² with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3×10^-20 cm² with a transition bandwidth of 24 nm. The measured emission lifetime of Yb³⁺ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD ⁺P for a relative measure of the second-harmonic generating potential, yielding d_eff(BCBF)~0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 μm light; the predicted angle is 37° from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported     

Spectroscopic properties and efficient diode-pumped laser operationof neodymium-doped lanthanum scandium borate

We report on the spectroscopic properties and laser performance of Czochralski-grown LaSc₃(BO₃)₄ (LSB) crystals with high neodymium concentrations up to 2.5·10²¹ cm^-3. The low-concentration quenching of the upper laser level of neodymium and the polarization dependence of the spectra indicate that LaSc₃(BO₃)₄ crystalizes in a huntite-type structure like Nd³⁺:YAl₃(BO₃)₄ (NYAB). With diode laser pumping at 808 nm, a multimode Nd³⁺:LaSc₃ (BO₃)₄ (NLSB) laser at 1063 nm is demonstrated. Optical slope efficiency was 64% with respect to absorbed pump power. The possibility of second-harmonic generation in the laser crystal is discussed     

Room-temperature CW laser operation at ~1.55 μm (eye-safe range)of Yb:Er and Yb:Er:Ce:Ca2Al2SiO7crystals

Room-temperature CW laser operation at 1.55 μm of Yb:Er:Ca₂Al₂SiO₇ (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, Ce³⁺, decreases the Er ³⁺⁴I_11/2 excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of ~5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented     

Photon avalanche upconversion in Er3+:YAlO3

The results of an investigation of photon avalanche upconversion pumping in Er³⁺:TiAlO₃ are reported. Five pump wavelengths corresponding to transitions from the metastable ⁴I_13/2 state to the ²H_11/2 state generated upconversion laser emission at 549.8 nm. The dependence of the laser output power on pump power near laser threshold is discussed in terms of a four-level kinetics model and is shown to reflect the threshold power requirement for photon avalanche. The maximum output power at 7 K was 33 mW, giving an optical conversion efficiency of 3.5% and a conversion efficiency of 28% based on absorbed power. Pumping Er:YALO by cross relaxation energy transfer produced 166 mW of laser output with an optical conversion efficiency of 17%     

Diode-pumped ytterbium-doped Sr5(PO4)3F laser performance

The performance of the first diode-pumped Yb³⁺-doped Sr ₅(PO₄)₃F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3×3×30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm ² for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5× higher than the intrinsic 2.2 J/cm ² saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0×10^-20 cm², obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm³ of stored energy density was achieved in a 6×6×44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to ~0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 μs pulses     

Tunable solid-state laser action in Ce3+:LiSrAlF6

Continuously tunable UV laser operation of cerium-doped LiSrAlF ₆ (Ce³⁺:LiSAF) between 285 and 297 nm is demonstrated. At the peak operating wavelength of 290 nm, output energies of 1.3 mJ with slope efficiencies of 17% have been obtained     

LiCaAlF6:Cr3+: a promising new solid-statelaser material

LiCaAlF₆:Cr³⁺ (Cr³⁺:LiCAF) exhibits an intrinsic (extrapolated maximum) slope efficiency of 67%. For comparison, the intrinsic slope efficiencies of BeAl₂O ₄:Cr³⁺ (alexandrite), Na₃Ga₂Li₃F₁₂:Cr³⁺ and ScBO₃:Cr³⁺ were found to be 65, 28, and 26%, respectively. The tuning range of LiCaAlF₆:Cr³⁺ was determined to be at least 720-840 nm. The conventional spectroscopic properties, such as the absorption, emission, and emission lifetimes as a function of temperature, are reported as well     

Continuous-wave laser action in Er3+:YLF at 3.41 μm

Continuous-wave laser emission at 3.41 μm from an erbium-doped LiYF₄ crystal (Er³⁺:YLF) at 77 K is demonstrated. Operation of this four-level laser is based on the Er^{3+ 4}F_9/2-⁴I_9/2 transition. An output power of 12 mW and a slope efficiency of 2% have been achieved     

LD泵浦的1.3at.%Er3+: CaF2中红外高功率固体激光器（特邀）

3 μm波段激光是高精度外科手术的理想光源,也可作为长波中红外光参量振荡器的有效泵浦源。LD直接泵浦Er3+掺杂晶体是获得2.7~3 μm波段中红外激光的有效技术途径,具有成本低、结构紧凑简单等优点。由于Er3+ 2.8 μm激光下能级阻塞问题,一般需要高浓度掺杂,但高浓度掺杂易引起强烈的光吸收,增强了激光晶体的热效应,从而阻碍了激光功率的提升。低声子能量的氟化钙晶体特有的萤石型结构使得三价稀土离子极易形成“团簇”,将低浓度Er3+掺杂到氟化钙晶体中即可获得高效率的中红外激光增益介质。笔者课题组使用温度梯度法成功生长了低浓度掺杂1.3at.%Er3+: CaF₂激光晶体,利用LD直接泵浦获得了2.2 W的中红外激光输出,这是目前利用LD端面泵浦同类晶体中的最高中红外激光输出功率。同时,文中还对上转换泵浦方式下该晶体的2.8 μm激光特性进行了研究。实验结果表明,低浓度掺杂的1.3at.%Er3+: CaF₂晶体是一类具有产业化前景的中红外激光材料,有望推动长波中红外激光器向着结构紧凑、成本低的方向发展。     

Raman Scattering and Nd3+ Laser Operation in NaLa(WO 4)2

The continuous-wave laser operation of Nd-doped tetragonal NaLa(WO ₄)₂ crystal is studied at room temperature by optical pumping in the spectral region overlapping AlGaAs diode laser emission. This crystal has inhomogeneously broadened optical bands. From the room-temperature spectroscopic parameters determined it is found that the optimum Nd concentration for the ⁴F_3/2rarr⁴I_J laser channels must be in the 3-5 at.% range. For J=11/2 and 13/2 channels (lambdaap1.06 and 1.3 mum) the most favourable polarization configuration is parallel to the crystallographic c axis, while for J=9/2 little polarization dependence of the laser efficiency is predicted. Laser operation was achieved with a 3.35 at.% Nd-doped sample grown by the Czochralski method. The laser operation was tested in an hemispherical optical cavity pumped by a Ti:sapphire laser. Stimulated emission at lambda=1056 nm was achieved for a wide spectral pumping range, lambda=790-820 nm. Stimulated Raman scattering was achieved in the picosecond regime with an efficiency similar to that of monoclinic KY(WO₄)₂ reference compound     

8.8 W diode-cladding-pumped Tm3+,Ho3+-dopedfluoride fibre laser

A maximum output power of 8.8 W was generated from a diode pumped Tm³⁺,Ho³⁺-doped ZBLAN glass fibre laser. The laser operated with a maximum slope efficiency of 36% with respect to the incident pump power. The wavelength was measured to be 2056 nm and some instability was observed in the output     

Er3+:YAlO3 upconversion laser

Upconversion lasing in Er:YAlO₃ is reported. Laser emission was produced using both sequential two-step pumping and cross-relaxation energy transfer. In addition, photon avalanche upconversion pumping was demonstrated. Selection among these pumping mechanisms is determined by the pump wavelength, and laser operation was obtained with excitation between 785 nm and 840 nm. The highest laser output power was achieved at 34°K, where 918 mW of pump power at 807 nm produced 121 mW of TEM₀₀ emission. The optical conversion efficiency was 13%. Repetitively Q-switched operation is reported, and the temporal- and temperature-dependence of the laser output is discussed     

Scaling of longitudinally diode-pumped self-frequency-doublingNd:YCOB lasers

We report the scaling of a new self-frequency-doubling laser, based on the crystal Nd³⁺:YCa₄O(BO₃) ₃, to higher powers. The power scaling is achieved by diode pumping using a novel technology of combining the output of up to four high-brightness laser diodes. Spectroscopic, thermo-mechanical, and laser properties are investigated for use in designing high-power self-frequency-doubling lasers. Using a method of angular-multiplexing individual laser diodes for pumping, we demonstrate output powers of more than 1.9 W of fundamental (1060 nm) radiation and 245 mW at the second harmonic. Experimental investigation rendered a thermal stress resistance figure-of-merit for this material to be between 210-280 W/m     

Tuning of a Tm3+:Ho3+: silica fiber laser at2 μm

Thermal tuning of a Tm³⁺:Ho³⁺:SiO₂ glass fiber laser was investigated. As a function of fiber length and temperature, the emission wavelength can be varied between 1960 and 2032 nm. The bandwidth of the laser emission is about 2 mm. Besides the wavelength also the change of threshold and slope efficiency with temperature and fiber length and the fiber absorption were measured     

Er3+/Yb3+共掺双包层光纤放大器实验研究

对1550nm高功率窄线宽光纤放大器进行了实验研究。该放大器采用双级放大(MOPA)结构, 其中第一级预放采用5 m长的掺Er3+光纤, 将种子光信号放大到约90 mW; 采用15 m长的Er3+/Yb3+共掺双包层光纤放大器作为二级放大, 抽运源采用2支工作波长为980 nm的大功率激光二极管(LD), 抽运阈值功率约1.3 W。 当抽运功率为10.8 W时, 得到放大激光输出功率为1.97 W, 光-光转换效率为18 %, 斜率效率为21%, 增益大于13 dB。所采用的种子光源为1550 nm单频窄线宽(DFB)LD, 输出功率为10 mW; 采用延迟自外差方法对种子源及放大器输出的线宽进行测量, 测量结果显示该种子源及放大后的激光输出的3 dB线宽均约为220 kHz, 在目前的实验条件下, 没有观察到放大后的激光线宽展宽现象。     

Evaluation of absorption and emission properties of Yb3+doped crystals for laser applications

The emission and absorption properties of numerous host crystals doped with Yb³⁺ ions have been studied. The hosts which have been selected include LiYF₄, LaF₃, SrF₂, BaF₂, KCaF₃, KY₃F₁₀, Rb₂NaYF₆, BaY₂ F₈, Y₂SiO₅, Y₃Al₅ O₁₂, YAlO₃, LuPO₄, Ca₅(PO₄)₃F, LiYO₂, and ScBO ₃. Spectral determinations have been made of the resonant absorption and emission cross sections between 850 and 1100 nm, and the emission decay times of the upper laser level have been measured. The emission cross sections have been evaluated using the absorption cross section and principle of reciprocity, and again using the Fuchtbauer-Ladenberg formula. Agreement between the two methods is within 20% for most materials. The results are discussed in the framework of requirements for an effective diode-pumped Yb³⁺ laser system. Ca₅(PO₄)₃F:Yb is predicted to exhibit the most useful laser properties and is expected to be far superior to Y₃Al₅O₁₂:Yb in many key microscopic parameter values     

Finite-element analysis of single-frequency silica-based Er3+-Yb3+ co-doped waveguide lasers

Continuous wave laser operations of silica-based Er³⁺-Yb³⁺ co-doped waveguides have been numerically analyzed by means of a finite-element method. The theoretical model, based on propagation-rate equations, describes uniform upconversion by a dipole-dipole interaction between Er³⁺ ions, and includes a pair-induced energy transfer process from Yb ³⁺-Er³⁺ Numerical results show that single-frequency operation with slope efficiency higher than 50% and threshold pump powers of few mW can be achieved in short and heavily doped waveguides equipped with input dielectric mirrors and output distributed Bragg reflectors