The 1.06 μm laser of Nd：Sr3Y2（BO3）4 with a 885 nm direct pumping

Li Qinan，Ding Yuhang，Feng Lu，Zhang Tao，Zu Ningning，Li Rui，Pan Zhongben，Yu Haohai

The 1.06 μm laser of Nd：Sr3Y2（BO3）4with a 885 nm direct pumping

Li Qinan1，Ding Yuhang1，Feng Lu1，Zhang Tao1，Zu Ningning1，Li Rui1，Pan Zhongben2，3，Yu Haohai2，3

（1. School of Science，Qiqihar University，Qiqihar 161006，China；2. State Key Laboratory of Crystal Materials，3. Institute of Crystal Materials，Shandong University，Jinan 250100，China）

A 885 nm direct pumping Nd：Sr3Y2（BO3）41.06 μm laser is demonstrated．The Nd3+ion is excited from levels of4I9/2that the ground lever and the thermally excited ground lever to topmost position of4F3/2directly to product 1.06 μm laser．The maximum output power of 237 mW is obtained and the slope efficiency equal approximately 0.8%．The thermal effect of Nd：Sr3Y2（BO3）4crystal is reduced by 885 nm direct pumping，which generate a high-quality fundamental transverse mode．

885 nm direct pumping；Nd：Sr3Y2（BO3）4crystal；thermal effect

Ultrafast laser pulses are widely used in precision processing，scientific research，military and other fields．The most effective way to generate ultrafast laser pulses is to use mode-locked lasers，which requires that the laser gain materials must have broad emission spectra. Because Nd3+-doped crystals are disordered，they have attracted much attention as a kind of ultrafast laser material．The greatest advantage of this type of crystal is that disordered structure makes the absorption and emission spectra broaden inhomogeneous strongly．This broadened spectral characteristic makes the crystal particularly suitable for diode pumping and production of ultrashort pulses[1-4]．

A typical example of Nd3+-doped disordered laser gain media is Nd：M3Re2（BO3）4crystal，where M=Ca，Sr， or Ba and Re=Y，La，or Gd．Some researchers have reported these crystals[5-10]．The experiment results show that the broad absorption and fluorescence spectra of these disordered crystals thus favor efficient diode pumping and ultrashort pulse generation[11-12]．

The above reports on Nd：M3Re2（BO3）4lasers all use the traditional 808 nm light as the pumping light source．Nd ions absorb 808 nm pumping light strongly，which is conducive to improving the output power of the laser．However，the weakness of 808 nm pumping is also very obvious，that is，a large amount of heat accumulated on the gain crystal，resulting in the instability of the laser．In comparison with Nd：YAG crystal，the thermal conductivities of Nd：M3Re2（BO3）4are only between one-tenth and one-twentieth of that of Nd：YAG（0.13 W/cm·oC），so the thermal effect is more serious．

The direct pumping scheme of 885 nm for Nd-doped materials is different from the old-fashioned 808 nm pumping scheme．It can pump Nd3+ions from the ground state and the thermally excited levels to upper lasing levels of4F3/2instead of levels of4F5/2．Under the 885 nm direct pumping，the relaxation process from4F5/2to4F3/2does not exist and the Stokes shift between the pump and lasing photon is reduced．Therefore，the direct pumping scheme generates less heat in the Nd-doped crystal and leads to thermally induced effects reduce owing to the higher quantum efficiency．This advantage of direct pumping at 885 nm for Nd-doped crystal over traditional 808 nm pumping scheme has been confirmed by relevant research reports[13-26]．

A 1.06 μm CW Nd：Sr3Y2（BO3）4laser pumped at 885 nm，direct entry into the4F3/2emission level of Nd3+ion，is demonstrated in this paper．The maximum output power is 237 mW. The slope efficiency is about 0.8%．Because of the reduction of thermal effect for Nd：Sr3Y2（BO3）4crystal by 885 nm direct pumping，a high-quality fundamental transverse mode can be observed

1　Experimental setup

The Nd：Sr3Y2（BO3）4crystal used in our experiment is grown by the Czochralski method，which under a nitrogen atmosphere containing 2% oxygen in an indium crucible and the Nd3+ion atomic percentage is 0.5at.%．

A compact linear cavity with a length of 25 mm is designed for our experiment．A fiber-coupled 885nm CW diode laser（JENOPTIK Laser GmbH）with the fiber diameter of 400 μm and the numerical aperture of 0.22 is used as the pump light source（See figure 1）．Collimating and focusing the 885 nm pump light on the Nd：Sr3Y2（BO3）4crystal after passing through two coupling lenses．The diameter of the focused spot is 200 m and ­the coupling efficiency is 90%．M1is a flat mirror coated with optical film that the transmittance is greater than 90% for 885 nm and the reflectance is greater than 99.9% for 1.06 μm．The Nd：Sr3Y2（BO3）4crystal used in our experiment is 6 mm in length．In order to reduce the loss of the laser resonant cavity and increase the absorption of the pump light for Nd3+ions，both ends of the Nd：Sr3Y2（BO3）4crystal are coated with optical films that the transmittance is greater than 99% at 885 nm pump light and greater than 99.8% at 1.06 μm laser．In order to reduce the heat generated in the gain medium，the Nd：Sr3Y2（BO3）4crystal is wrapped and fixed on a water-cooled heat sink with indium foil，and the water temperature is set to 10 ℃．M2is a flat concave output coupler mirror（OC）that has a concave surface，it′s radius of curvature is 250 mm and faces the interior of the cavity with the transmission of 2% at 1.06 μm．The output power of the laser is measured by a power meter placed behind the output mirror．The transverse mode of the output is recorded by a laser beam analyzer（OPHIR）．

Figure1Schematic of the Nd：Sr3Y2（BO3）41.06 μm laser

2　Experimental results and discussions

The relevant experimental results are shown by figure2～5．

Figure2pumping and direct pumping from either the ground level or thermally excited Stark components of the ground state for Nd3+ion

Figure3The fluorescence spectrum of the Nd：Sr3Y2（BO3）4crystal with the excitation wavelength of 885 nm at room temperature

Figurer4Output power of the 1.06 μm Nd：Sr3Y2（BO3）4laser as a function of the incident pump power

Figure5Power density distribution image of transverse mode of the 1.06 μm Nd：Sr3Y2（BO3）4laser

Figure2 shows the energy level comparison between the conventional 808 nm pump Nd3+ion laser and the direct pumping Nd3+ion laser．the direct pumping method products less heat in the medium and is advantageous for the stability of cavity．Because of the following two points：（1）the higher quantum efficiency；（2）the absence of relaxation from4F5/2to4F3/2manifold．

Figure3 shows the fluorescence spectrum of the Nd：Sr3Y2（BO3）4crystal from 1 000 nm to 1 140 nm with the excitation wavelength of 885 nm at room temperature．It can be found that the FWHM of the fluorescence band around 1.06 μm is about 28 nm and it is much broader than 0.5 nm of Nd：YAG crystal and 1.1 nm of Nd：YVO4crystal[27-28]．Thus，such a large bandwidth shows that Nd：Sr3Y2（BO3）4crystal may possibly produce femtosecond pulses in laser systems．

Figure4 shows the output power of 1.06 μm Nd：Sr3Y2（BO3）4laser as a function of the incident pump power．For the incident pump power of 885 nm， it′s threshold value is 0.9 W and the max-output power of 237 mW is obtained．The slope efficiency is about 0.8%．

The far-field beam space sections of the 1.06 μm laser is measured by using a laser beam analyzer（OPHIR） at a distance of 20 cm behind the OC．In the whole experiment，we can observe the Gauss distribution of intensity across the laser beam．

Figure 5 shows the typical CCD photo of transverse mode. At the pump power of 31 W，it′s output power is237 mW．This result indicates that the laser is oscillated in the fundamental transverse mode．Nd：Sr3Y2（BO3）4crystal by 885nm direct pumping，the high-quality fundamental transverse mode can be observed because of the reduction of thermal effect．

3　Conclusion

In this paper，the direct pumping manner is used to the Nd：Sr3Y2（BO3）4crystal to product 1.06 μm CW laser．The max-output power of 237 mW is obtained．Comparing with old-fashioned pumping manner of 808 nm for the Nd3+ion，a high-quality fundamental transverse mode can be observed owing to the reduction of thermal effect for Nd：Sr3Y2（BO3）4crystal by 885 nm direct pumping．

[1] Zhang Y Y，Zhang H J，Yu H H，et al．Synthesis，growth，and characterization of Nd-doped SrGdGa3O7 crystal[J]．J Appl Phys， 2010，108（6）：063534．

[2] Basiev T T，Es′kov N A，Karasik A Y，et al．Disordered garnets Ca3（Nb，Ga）5O12：Nd3+-prospective crystals for powerful ultrashort-pulse generation[J]．Opt Lett，1992，17（3）：201-203．

[3] Xie G Q，Tang D Y，Tan W D，et al．Subpicosecond pulse generation from a Nd：CLNGG disordered crystal laser[J]．Opt Lett， 2009，34（1）：103-105．

[4] Xie G Q，Tang D Y，Luo H，et al．Dual-wavelength synchronously mode-locked Nd：CNGG laser[J]．Opt Lett，2008，33（16）： 1872-1874．

[5] Zhang Y，Lin Z B，Hu Z S，et al．Growth and spectroscopic properties of Nd3+-doped Sr3Y2（BO3）4crystal[J]．J Solid State Chem， 2004，177：3183-3186．

[6] Wang Y，Tu C Y，You Z Y，et al．Optical spectroscopy of Ca3Gd2（BO3）4：Nd3+laser crystal[J]．J Mod Opt，2006，53（8）： 1141-1148．

[7] Pan Z B，Zhang H J，Yu H H，et al．Growth and characterization of Nd-doped disordered Ca3Gd2（BO3）4crystal[J]．Appl Phys B，2012，106（1）：197-209．

[8] Ma P，Chen J T，Hu Z S，et al．Structure of Ba3Y2（BO3）4crystal[J]．Mater Res Innovations，2005，9：63-64．

[9] Pan Zhongben，Cong Hengjiang，Yu Haohai，et al．Growth，thermal properties and laser operation of Nd：Ca3La2（BO3）4：A new disordered laser crystal[J]．Optics Express，2013，21（5）：6091-6100．

[10] Pan Zhongben，Cong Hengjiang，Yu Haohai，et al．Growth，morphology and anisotropic thermal properties of Nd-doped Sr3Y2（BO3）4crystal[J]．Journal of Crystal Growth，2013，363：176-184．

[11] Pan Zhongben，Yu Haohai，Cong Hengjiang，et al．Polarized spectral properties and laser demonstration of Nd-doped Sr3Y2（BO3）4crystal[J]．Appl Opt，2012，51（30）：7144-7149．

[12] Pan Zhongben，Cai Huaqiang，Huang Hui，et al．Growth，thermal properties and laser operation of a new disordered crystal： Nd-doped Sr3La2（BO3）4[J]．Journal of Alloys and Compounds，2014，607（15）：16-22．

[13] Lavi R，Jackel S．Thermally boosted pumping of neodymium lasers[J]．Appl Opt，2000，39（18）：3093-3098．

[14] Lupei V，Pavel N，Taira T．Laser emission in highly doped Nd：YAG crystals under4F5/2and4F3/2pumping[J]．Opt Lett，2001， 26（21）：1678-1680．

[15] Bjurshagen S，Koch R，Laurell F．Quasi-three-level Nd：YAG laser under diode pumping directly into the emitting level[J]．Opt Commun，2006，261（1）：109-113．

[16] Pavel N，Lupei V，Saikawa J，et al．Neodymium concentration dependence of 0.94-，1.06-and 1.34-μm laser emission and of heating effects under 809-and 885-nm diode laser pumping of Nd：YAG[J]．Appl Phys B：Lasers and Opt，2006，82（4）： 599-605．

[17] Lupei V，Pavel N，Taira T．Highly efficient continuous-wave 946-nm Nd：YAG laser emission under direct 885-nm pumping[J]．Appl Phys Lett，2002，81（15）：2677-2679．

[18] Lupei V，Aka G，Vivien D．Quasi-three-level 946 nm CW laser emission of Nd：YAG under direct pumping at 885 nm into the emitting level[J]．Opt Commun，2002，204（1）：399-405．

[19] Pavel N，Lünstedt K，Petermann K，et al．Multipass pumped Nd-based thin-disk lasers：continuous-wave laser operation at 1.06 and 0.9 μm with intracavity frequency doubling[J]．Appl Opt，2007，46（34）：8256-8263．

[20] ZONG Nan，ZHANG Xiaofu，MA Qinglei，et al．Comparison of Nd：YAG Ceramic Laser Pumped at 885 nm and 808 nm[J]．Chin Phys Lett，2009，26（5）：54211．

[21] Lavi R，Jackel S，Tal A，et al．885 nm high-power diodes end-pumped Nd：YAG laser[J]．Opt Commun，2001，195（5）：427-430．

[22] Pavel N，Lupei V，Taira T．1.34-μm efficient laser emission in highly-doped Nd：YAG under 885-nm diode pumping[J]．Optics Express，2005，13（20）：7948-7953．

[23] CAO Ning，LI QiNan，ZHAO YanYing，et al．Efficient Pumping Scheme by Direct Excitation of Upper Laser Level in Nd：CNGG[J]．Chin Phys Lett，2008，25（11）：4016-4018．

[24] Shi Yuxian，Li Qinan，Zhang Dongxiang，et al．Comparison of 885 nm pumping and 808 nm pumping in Nd：CNGG laser operating at 1061 nm and 935 nm[J]．Opt Commun，2010，283（14）：2888-2891．

[25] LI Qinan，ZHANG Tao，FENG Baohua，et al．An 885-nm Direct Pumped Nd：CNGG 1061nm Q-Switched Laser[J]．Chin Phys Lett，2014，31（7）：074209．

[26] LI Qinan，ZHAO Bin，ZHANG Tao，et al．Continuous wave and passively Q-switched Nd：Lu0.15Y0.85VO4laser with 885 nm direct pumping[J]．Optoelectronics Letters，2015，11（3）：199-202．

[27] Kenta Naito，Atsushi Yokotani，Takatomo Sasaki，et al．Efficient laser-diode-pumped neodymium-doped calcium-niobium-gallium-garnet laser[J]．Appl Opt，1993，32（36）：7387-7390．

[28] Sato Y，Taira T．Thermo-optical and mechanical parameters of Nd：GdVO4and Nd：YVO4[J]．IEEE J Sel Top Quantum Electron， 2005，11：613-620．

采用885 nm直接泵浦Nd：Sr3Y2（BO3）4晶体产生1.06 μm激光

李奇楠1，丁钰航1，冯露1，张韬1，祖宁宁1，李瑞1，潘忠奔2，3，于浩海2，3

（1. 齐齐哈尔大学 理学院，黑龙江 齐齐哈尔 161006；山东大学 2. 晶体材料国家重点实验室，3. 晶体材料研究所，山东 济南 250100）

介绍了一种由885 nm直接泵浦Nd：Sr3Y2（BO3）4晶体产生1.06μm光的激光器．Nd3+离子从基态和热激发态的4I9/2能级激发到4F3/2的上激光能级，直接产生1.06μm的激光．实验测得最大输出功率为237mW，斜率效率约为0.8%．通过885 nm的直接泵浦，降低了Nd：Sr3Y2（BO3）4晶体的热效应，得到了高质量的基横模．

885 nm直接泵浦；Nd：Sr3Y2（BO3）4晶体；热效应

1007-9831（2022）03-0042-05

TN248

A

10.3969/j.jssn.1007-9831.2022.03.009

date：2021-11-10

the Science and Technology Research Projects of Heilongjiang Province Education Department（Grant No.12531751）

Li Qinan（1975-），male，native of Qiqihar of Heilongjiang，professor，doctor，engaged in laser physics，applied spectroscopy and characteristics of solid state lasers research．E-mail：liqinan@163.com