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

Li Qinan，Ding Yuhang，F(xiàn)eng 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，F(xiàn)eng 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．

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采用885 nm直接泵浦Nd：Sr3Y2（BO3）4晶體產(chǎn)生1.06 μm激光

李奇楠1，丁鈺航1，馮露1，張韜1，祖寧寧1，李瑞1，潘忠奔2，3，于浩海2，3

（1. 齊齊哈爾大學(xué) 理學(xué)院，黑龍江 齊齊哈爾 161006；山東大學(xué) 2. 晶體材料國(guó)家重點(diǎn)實(shí)驗(yàn)室，3. 晶體材料研究所，山東 濟(jì)南 250100）

介紹了一種由885 nm直接泵浦Nd：Sr3Y2（BO3）4晶體產(chǎn)生1.06μm光的激光器．Nd3+離子從基態(tài)和熱激發(fā)態(tài)的4I9/2能級(jí)激發(fā)到4F3/2的上激光能級(jí)，直接產(chǎn)生1.06μm的激光．實(shí)驗(yàn)測(cè)得最大輸出功率為237mW，斜率效率約為0.8%．通過885 nm的直接泵浦，降低了Nd：Sr3Y2（BO3）4晶體的熱效應(yīng)，得到了高質(zhì)量的基橫模．

885 nm直接泵浦；Nd：Sr3Y2（BO3）4晶體；熱效應(yīng)

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