We report a wavelength-locked 914 nm laser diode in-band pumped intra cavity efficient green laser, in our experiment, a wave-locked 914 nm laser diode was used as a pumping source, which improve pump uniformity and pump efficiency and reducing the thermal effect of the laser greatly, thus, a high beam quality 532 nm laser output is obtained. When the pump power is 18 W and the repitition rate is 130 kHz, a green laser output of 6.7 W is obtained, and the conversion efficiency is 37.2% for pumping power of 18 W, which corresponds to a conversion efficiency of 60% for absorbed pumping.

1. Wave-locked914 nm laser diode in-band pumped Nd:YVO4 /LBO
A-O Q switched green laser
Li Bin1,2*
Sun Bing2,3
Miao Yinping1
1
School of Electrical and Electronic Engineering, Tianjin University of Technology,
Tianjin 300384, China
2
Tianjin Maiman Laser Technology Co., Ltd., Tianjin 300111, China
3
Key Laboratory of Opto-Electronics Information Science and Technology, Ministry of Education,
College of Precision Instrument and Opto-Electronics Engineering, Institute of Laser and
Opto-Electronics, Tianjin University, Tianjin 300072, China
Abstract We report a wavelength-locked 914 nm laser diode in-band pumped intra cavity
efficient green laser, in our experiment, a wave-locked 914 nm laser diode was used as a pumping
source, which improve pump uniformity and pump efficiency and reducing the thermal effect of
the laser greatly, thus, a high beam quality 532 nm laser output is obtained. When the pump power
is 18 W and the repitition rate is 130 kHz, a green laser output of 6.7 W is obtained, and the
conversion efficiency is 37.2% for pumping power of 18 W, which corresponds to a conversion
efficiency of 60% for absorbed pumping.
Keywords: Wave-locked； Nd:YVO4； In-band pumping；frequency doubling； 532 nm
OCIS Codes：140.3460；140.3480；140.3515；140.3540；140.3580
1 Introduction
Thermal effect is one of the most important factors hindering the further improvement of the
performance of solid-state lasers. Severe thermal effects can lead to deterioration of the beam
quality of the laser, thermal saturation and even thermal cracking of the crystal [1]. In recent years,
people have been looking for ways to reduce the thermal effect of the laser. At present, the most
effective method is to use resonant pumping technology, In-band pumping, which uses pump light
of a specific wavelength to directly pump the ground state atoms to the upper level of the laser,
instead of Pumping to a higher excited state and then relaxing through the non-radiative transition
to the pumping level of the laser, the resonant pumping method eliminates the non-radiative
transition from the excited state to the upper level of the laser, effectively reducing the pump The
quantum loss between the puddle and the oscillating light reduces heat generation and improves

2. quantum efficiency. It can fundamentally solve the thermal effect of the laser, thus effectively
improving the performance of the laser. For the Nd:YVO4 laser resonant pump Pu mainly
concentrated on three pump wavelengths of 880nm, 888nm and 914nm [2-7], theoretically and
experimentally proved that Resonant pumping technology can achieve high power, high beam
quality laser output, but resonant pumping technology still has many shortcomings. First, the
absorption rate of crystals for these wavelengths is relatively low, especially for 914nm absorption
rate. Low, which is not conducive to the improvement of the overall light-to-light conversion rate.
Second, the Nd:YVO4 crystal has a narrow absorption linewidth for these wavelengths, and the
semiconductor laser will shift the emission spectrum with temperature, which is not conducive to
the laser. Adapt to changes in ambient temperature. In order to overcome these shortcomings, we
have adopted wavelength-locked resonance pumping technology, which can effectively overcome
the shortcomings of resonant pumping and further exploit the advantages of resonant pumping.
The so-called lock-wavelength resonance pumping technique is to lock the emission wavelength
of the pump source with a volume Bragg grating (VBG), so that its emission wavelength tends to
be stable, and the change of the external environment temperature has little influence on its
emission spectrum. At the same time, the pump Puyuan's emission spectrum is narrower and more
accurate, corresponding to the absorption spectrum of the working substance. In combination with
the above aspects, the locked-wavelength resonant pumping technique has obvious advantages
over the ordinary resonant pumping technology.
At present, there are not many researches on 914nm resonant pumping technology, and basically
use non-locking wavelength semiconductor lasers or 914nm lasers generated by solid-state lasers
as pump sources. In 2009, Damien Sangla et al reported using 914nm pumped Nd. : YVO4 crystal
laser, when absorbing 14.8W of pump light power, obtained 11.5W of 1064nm laser input
The corresponding light-to-light conversion rate of the absorbed pump light is 78.7%, and the
slope efficiency is 80.7% [8]. In 2013, Chen et al. used 914nm as the pump source of the
regenerative amplifier to reduce the heat load and achieve high The performance of the
regenerative amplifier, and 1nJ, pulse width 5.7ps, frequency 42.7MHz seed regenerative
amplification, and finally get 100kHz, 21.2W regenerative amplification output [9], the same year
Ding Xin et al. using end-pumped 914nm solid The laser is used as a pump source to pump
Nd:YVO4. The 20mm long, doped 2% Nd:YVO4 is used as the gain medium, and the 3.92W
1064nm laser output is obtained at 6.9W injection pumping power. - The light conversion
efficiency is 56.9% [10]. In 2016, Tanant Waritanant et al. used the 914nm pumped Nd:YVO4 to
achieve mode-locked laser output, and obtained a 6.7W, 87MHz mode-locked laser output,

3. corresponding to the absorption of pump light. The slope efficiency is 77.1%, and the light-to-light
conversion efficiency is 60.7% [11]. The above studies all use ordinary semiconductor lasers or
solid-state lasers with non-locking wavelengths as pump sources, and all of them are for the study
of fundamental frequency light. In the case of a semiconductor laser as a pump source, since the
emission line width of the semiconductor laser is wide and the emission wavelength shifts with
temperature, the absorption efficiency of Nd:YVO4 to 914 nm pump light is low, resulting in
overall light. The light conversion efficiency is not high, much lower than the traditional 808nm
pumping method, and the use of the 914nm solid-state laser as the pump source adds complexity
and cost to the system.
In order to overcome the shortcomings of 914nm resonant pumping and take advantage of the
914nm resonant pumping technology, this paper first applied the locked-wavelength 914nm
resonant pumping technique to the Nd:YVO4 intracavity frequency doubled laser, using the pump
source as the center wavelength.
A 913.9nm lock-wavelength fiber-coupled-output semiconductor laser with a linewidth of 0.3nm,
using an acousto-optic Q-switch as a modulator, and a Class I critical phase-matched LBO as a
frequency-doubled crystal for high-frequency, high-beam The mass of 532nm laser output, when
the incident pump power is 18W and the modulation frequency is 130kHz, the highest 6.7W
532nm green light output is obtained, and the overall light-to-light conversion rate is 37.2%,
corresponding to the light-to-light conversion efficiency of the absorbed pump light. With 60%
pulse width and 52 ns, the experimental results show that the overall light-to-light conversion
efficiency can be compared with the traditional 808nm pumping method, and this method can
effectively reduce the thermal effect of the laser and improve the beam quality of the output laser.
2. Experimental device
The experimental device shown in Figure 1 is a 914nm wavelength-locked fiber-coupled
semiconductor laser with a maximum output power of 20W and a linewidth of 0.3nm. This narrow
linewidth can be well matched to the absorption bandwidth of the crystal. The resonance pump has
the disadvantage of narrow absorption band, and the pump source has good temperature stability.
When the heat sink temperature is changed from 10 °C to 40 °C, the center wavelength of the
emission is only moved by 0.6 nm, and the line width is basically maintained. This does not
change, which makes the use of the semiconductor laser as a pump source with extremely high
temperature stability. The transmission fiber has a diameter of 400μm and a numerical aperture
of 0.22. The Coupler is an optical coupling system. After the coupling system, the pump spot
diameter is about ~700μm, M1 is plated with a 1064nm high-reflection film, and the laser crystal

4. size is 3×3×20mm 3,1. % doped, crystals are wrapped in indium foil and placed in a copper heat
sink. M2 is plated with 914nm anti-reflection and 1064nm high-reaction film system, M3 plated
with 532nm anti-reflection and 1064nm high-reaction film system, M4 plated with 1064nm and
532nm
High-reaction film system, AO is acousto-optic Q switch, its ultrasonic frequency is 80MHz,
RF power is 20W, double-sided coated with 1064nm anti-reflection film, LBO is 3x3x15mm,
Class I critical phase matching method, cutting angle is θ=90 °, φ=11.2°, placed in a high-
precision temperature control box with a temperature control accuracy of ±0.03 °C, coated with
1064nm and 532nm anti-reflection coatings at both ends, and the distance from the right end of
M1 to Nd:YVO4 is 100mm, M2 to The distance from the left end of Nd:YVO4 is 20mm, and the
distance from M2 to M3 is 80mm.
The distance from M3 to M4 is 40mm, and M1, M2, M3, and M4 are flat mirrors.
Fig 1 setup and light path of wavelength-locked laser diode in-band
pumped Nd:YVO4 /LBO green laser
Figure 2 shows the spot size distribution in the cavity. It can be seen from the calculation
results that the fundamental mode spot diameter on the Nd:YVO4 crystal is 624μm, which has a
good pattern matching with the 700μm pump light. In addition, it is larger. The pump spot can
further reduce thermal effects while reducing the up-conversion effect of highly doped Nd:YVO4
crystals.

5. 0.5
M1
0.4 M4
0.3
0.2
0.1
Nd:YVO4 M2 M3
0
-0.1
-0.2
A-OQ sw itch
LBO
-0.3
-0.4
-0.5
20 40 60 80 100 120 140 160 180 200 220 240
Distance from M1/mm
Diameteroflaser/mm

6. Fig 2 Spot distribution in the cavity
3. Analysis of results
First, we tested the absorption of pump light by Nd:YVO4. When the crystal heat sink
temperature is 25 °C, 1% doping,
The 20mm long Nd:YVO4 absorbs 62% of the 914nm pump light. Figure 3 shows the
relationship between the output 532nm laser power and the incident pump power at different
modulation frequencies. It can be seen from the experimental results that when the pump power is
18W and the modulation frequency is 130kHz, a 532nm laser output of up to 6.7W can be
obtained, corresponding to the light-to-light conversion rate of the incident pump light.
37.2%, the light-to-light conversion rate corresponding to the absorbed pump light is 60%, and
the modulation frequency is 170 kHz, the green light output of 5.7W is obtained, and the light-to-
light conversion efficiency corresponding to the incident pump light is 31.7%, when the frequency
is gradually increased from 130 kHz. When the output is reduced, the output power and
conversion efficiency are gradually reduced. When the modulation is 90 kHz, 50 kHz and 20 kHz,
respectively, the maximum gain is 5.8 W.
The laser output of 5.2W and 3.2W, the corresponding light and light conversion efficiencies
are 32.2%, 26% and 17.7%, respectively. It can be seen that the Nd:YVO4 intracavity frequency
doubling laser pumped at 914nm is able to operate at high repetition rate. A relatively high
conversion efficiency is obtained. For a case where the repetition frequency is relatively low,
especially at a frequency of 20 kHz, the output power will have a similar thermal saturation effect
as the pump power increases. After the pump power exceeds 14 W, the output power is The slope
is obviously declining. Our analysis is mainly because the high-doping concentration crystal is
more serious when the inverter is running at low frequency, because the up-conversion rate can be
written as n2.
Where is the up-conversion coefficient, n is the inverse particle number density [12-16],
from which we can see that the up-conversion rate is proportional to the square of the inverse
particle number density, especially in the high When the doping concentration crystal is operated
at a low frequency and the pump power is high, the number density of the inverted particles is
high at this time, and the up-conversion and fluorescence quenching effects are further enhanced,
so that the light-to-light conversion rate is significantly lowered.

7. 8
20kHz
50kHz
6
90kHz
130kHz
170kHz
4
2
0
0 2 4 6 8 10 12 14 16 18 20
Incidence pump power (W)
Fig 3 Relationship between output power and pump power at different repetition rate
8
output pow er
Efficiency for absorbed pump pow er
6
0.8
0.7
0.6
4 0.5
0.4
2
0.3
0
0 20 40 60 80 100 120 140 160 180
Repetition Rate (kHz)
0.2
Fig 4 Relationship between output power , efficiency and repetition frequency
at pump power of 18W
Outputpowerof532nmOutputpower(W)
Efficiencyat.%

8. Figure 4 shows the output power, light-to-light conversion efficiency (corresponding to
absorbed pump power) and repetition frequency for 18W pump power.
Relationship, it can be seen from the experimental results that when the repetition frequency is
gradually increased from 40 kHz to 130 kHz, the output power is also increased, and the light-to-
light conversion efficiency is also slowly increased. When the repetition frequency is 130 kHz, the
output power is up to 6.7 W. When the frequency is gradually increased from 130kHz to 170kHz,
the output power decreases. When the modulation frequency is 170kHz, the output power drops to
5.7W, but the output power and light-to-light conversion efficiency of the above two processes are
slowly changing, but when When the repetition frequency is reduced from 30 kHz to 15 kHz, both
the output power and the light-to-light conversion efficiency drop sharply. Experimental results
show that this
The 914nm resonant pumped high-doped Nd:YVO4 laser is more suitable for operation at high
repetition frequency. When the repetition frequency is too low, the high-doped crystal has a short
upper energy level lifetime and a high up-conversion coefficient, which leads to strong
spontaneous emission. And upconversion loss.
The experimental measurement shows that the pulse width is 52 ns at a pulse frequency of 130
kHz, and the corresponding peak power is 1 kW. We measure the beam quality of the laser with a
knife edge (90/10), and obtain the beam quality factor of the green light in the X direction and the
Y direction. The results are 1.3 and 1.2, respectively. The measurement results are shown in
Figure 5. The reason why the better beam quality output can be obtained is mainly because: First,
the 914nm pump reduces the heat generated by the quantum loss, thereby reducing the thermal
lens effect. The beam quality is improved; secondly, reasonable pattern matching is also an
important reason for obtaining good beam quality.

9. 图 4 给出了 18W 泵浦功率时输出功率、光光转换效率（对应吸收泵浦功率）与重复频率的
关系，由实验结果可以看出当重复频率由 40kHz-130kHz 逐渐增加时，其输出功率也增加，
光光转换效率也在缓慢增加，当重复频率为 130kHz 时，输出功率最高为 6.7W，当重复频
率由 130kHz-170kHz 逐渐增大时，输出功率呈下降趋势，当调制频率为 170kHz 时，输出功
率下降到 5.7W，但以上两个过程输出功率及光光转换效率都为缓慢变化趋势，但当重复频
率由 30kHz 下降到 15kHz 时，输出功率和光光转换效率都急剧下降，实验结果表明这
种
914nm 共振泵浦高掺杂 Nd：YVO4 激光器更适合在高重频下运转，当重复频率过低时由于
高掺杂晶体较短的上能级寿命和高的上转换系数会导致强烈的自发辐射和上转换损耗。
实验测得脉冲频率为 130kHz 时的脉冲宽度为 52ns，对应的峰值功率 1kW，我们采
用刀口发（90/10）对激光的光束质量进行测量，得到绿光的光束质量因子在 X 方向和 Y 方
向分别为 1.3 和 1.2，测量结果如图 5 所示，之所以能够获得比较好的光束质量输出，主要
是因为：其一，利用 914nm 泵浦减少了量子亏损产生的热，从而降低了热透镜效应，提高
了光束质量；其二，合理的模式匹配也是能够获得好的光束质量的重要原因。
0.25
0.2
X-direction
Ydirection
0.15
0.1
0.05
0
50 100 150 200 250 300
Distance (mm)
Fig 5 Beam quality factor measurement results
Radius(mm)

10. 4. Conclusion
For the first time, the 914nm resonant pumping technology is applied to the acousto-optic Q-
switched intracavity frequency doubling laser. In the experiment, Nd:YVO4 is used as the gain
medium, and the class I critical phase matching LBO is used as the frequency doubling crystal.
High repetition rate, high beam quality 532nm laser output. The laser has good performance at
high repetition frequency. When the repetition frequency is low (<30kHz), the doping
concentration of Nd:YVO4 is high, especially when high power pumping is up-converted and
spontaneous emission is compared. Serious, it will lead to a significant decline in light and light
conversion efficiency. When the pump power is 18W and the repetition frequency is 130kHz, the
multiplier light output of up to 6.7W is obtained, the light-to-light conversion rate corresponding
to the incident pump light is 37.2%, and the light-to-light conversion rate corresponding to the
absorbed pump light is 60%. The conversion efficiency can reach the level compared with the
traditional 808nm pumping method, and has a good application prospect.
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