A ring ZnGeP2 optical parametric oscillator pumpedby a Ho:LuAG laser

Ying-Jie Shen • Bao-Quan Yao • Zheng Cui •

Xiao-Ming Duan • You-Lun Ju • Yue-Zhu Wang

Received: 3 September 2013 / Accepted: 27 February 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract We have demonstrated that we believe to be the

first ring ZnGeP2 (ZGP) optical parametric oscillator

(OPO) pumped by a pulsed Ho:LuAG laser. The maximum

output power of the ring ZGP OPO laser was 5.51 W at

13.1 W incident Ho pump power, corresponding to a slope

efficiency of 59.0 %. The ZGP OPO laser produced 14 ns

mid-infrared pulses in the 3.72–4.01 and 4.37–4.75 lm

spectral regions simultaneously. In addition, the continuous

wave Ho:LuAG laser generated 26.5 W of linearly output

at 2,094.4 nm at the absorbed Tm pump power of 49.9 W.

1 Introduction

High-power mid-infrared (3–5 lm) lasers have various

important applications in earth monitoring and remote

sensing, and they are key components in directive systems

for countermeasures against infrared sensors. Due to no

practical, widely tunable high-power laser source in this

wavelength range, nonlinear optical conversion techniques

must be employed. The most attractive 3–5 lm sources are

ZnGeP2 (ZGP) based optical parametric oscillators (OPOs)

pumped by 2 lm, and it has been shown that over 50 %

conversion efficiency from 2 to 3–5 lm spectral band [1, 2]

can be achieved. In addition, ZGP has an optical trans-

mission range of 2–12 lm. With its excellent thermal

conductivity (0.36 W/cm K) [3] and high nonlinear figure-

of-merit (187 9 10-24 m2/V2) [4], ZGP is capable to result

in high efficient and high-power OPO operation. A ZGP

OPO based on rotated image singly resonant twisted rect-

angle cavity was reported at 2007 [5]; however, the con-

figuration of it was complicated and unavailable. A novel

V-shaped 3-mirror ring resonator which produced 22 W

output power was reported at 2010 [6]; however, the output

performance of the laser was limited to dimensions of the

ZGP crystal. A main challenge in this approach is to

develop an efficient Q-switched 2-lm pump laser with high

average output power and good beam quality.

Many Q-switched 2 lm sources [7, 8] suffer from

thermal problems, upconversion losses, and reduced laser

efficiency. Using a Tm3?-doped solid-state laser or a Tm

fiber laser as the pump source, this problem can be strictly

resolved. Resonantly pumped Ho lasers minimize the

quantum defect heating, leading to much improved quan-

tum efficiency, better beam quality, and higher average

power. Lutetium Aluminum Garnet (Lu3Al5O12, LuAG)

has an excellent mechanical properties similar to those of

YAG, while has a higher crystal field than YAG, which

result in a large manifold splitting and a low thermal

occupation for lower laser level. In our previous work,

efficient Ho:LuAG lasers end-pumped by 1.91 lm

Tm:YLF lasers have been demonstrated [9–11]. The

Q-switched Ho:LuAG laser could emitting at 2,094 or

2,100 nm [9], which can be employed as an excellent

pumping source of the ZGP OPO. The ZGP OPO resonator

was designed as a rectangle configuration consist of four

45� flat mirrors, which can be avoid the Ho:LuAG laser

from being influenced by feedback. To the best of our

knowledge, this is the first report of a ring ZGP OPO

pumped by a pulsed Ho:LuAG laser.

In this paper, we report a ring ZGP OPO laser pumped

by a Q-switched Ho:LuAG laser which has been double-

end-pumped by two diode-pumped Tm:YLF lasers at room

temperature. The maximum output power of the ring ZGP

Y.-J. Shen � B.-Q. Yao (&) � Z. Cui � X.-M. Duan � Y.-L. Ju �Y.-Z. Wang

National Key Laboratory of Tunable Laser Technology,

Harbin Institute of Technology, Harbin 15001, China

e-mail: yaobq08@hit.edu.cn

123

Appl. Phys. B

DOI 10.1007/s00340-014-5811-4

OPO laser was 5.51 W with a slope efficiency of 59.0 %

with respect to the incident Ho pump power. A beam

quality of M2 \ 4 was achieved. In addition, the maximum

output power of the continuous wave (CW) Ho:LuAG laser

of 26.5 W was achieved, corresponding to a slope effi-

ciency of 57.9 % and optical-to-optical conversion effi-

ciency of 53.1 %.

2 Experimental setup details

2.1 Ho:LuAG laser

The schematic diagram of experimental setup is shown in

Fig. 1. Due to the absorption peaks of the Ho:LuAG are near

1.91 lm, two diode-pumped Tm:YLF lasers with emission

wavelength of 1.91 lm were employed as the pump source of

Ho:LuAG laser. Each of the two Tm:YLF lasers involved two

crystals which had a cross section of 3 9 3 mm2 and a length

of 12 mm. The maximum output power of each of the two

Tm:YLF lasers was 44 W, and the beam quality of M2 was less

than two. The Ho laser resonator, using L-shaped configura-

tion, consists of a plane mirror (M1) with T [ 99.8 % at

1.9 lm and R [ 99.5 % at 2.1 lm, a 45� dichroic mirror (M2)

with T [ 97.7 % at 1.9 lm and R [ 99.8 % at 2.1 lm. The

output coupler (M3) with 200 mm curvature radius used in our

experiment is T = 51 % at 2.1 lm. Two thin film polarizers

(TFPs) were employed in the experiment to avoid the two

Tm:YLF lasers from being influenced by each other. A

30-mm-long Brewster-cut acousto-optic Q-switch (AOM)

with an acoustic aperture of 1.8 mm was employed in the

cavity. The radio-frequency (RF) input power at frequency of

40.7 MHz is rated for 25 W.

A Ho3?-doping concentration of 0.8 at.% was chosen in

our experiment, which was employed to diminish the up-

conversion losses [12]. The dimension of the Ho:LuAG

crystal is 5 mm in diameter and 25 mm in length. The crystal

was wrapped in indium foil and mounted into a copper block,

both end faces of which were antireflection coated at the

pump and laser wavelengths. The Ho:LuAG crystal was hold

at 18�C by the water-cooled heat sink. The physical length of

resonator was 130 mm resulting in an estimated resonant

mode size of about 320 lm in the crystal. The simple tele-

scopic lens system (TLS) consisting of collimating and

focusing lenses was used to image the pump beam into

Ho:LuAG crystals. After passing the TLS, the Tm pump spot

size of about 350 lm in diameter was focused into the Ho

crystal at the Tm pump power of 44 W.

2.2 Four-mirror ring resonator ZGP OPO

The ZGP OPO with a rectangle configuration is also shown

in Fig. 1, which consists of four 45� flat mirrors. The ZGP

OPO ring cavity was formed by three plane mirrors (M5)

with T [ 99.4 % at 2.1 lm for the p-polarized component

and R [ 99.9 % at 3–5 lm for the s-polarized component

and a plane output coupler with T = 50 % at 3–5 lm.

After passing the TLS, the Ho:LuAG pump spot size of

approximately 806 lm in diameter was focused into the

ZGP crystal at the Ho:LuAG pump power of 2.4 W. A

18.0-mm ZGP crystal cut for Type I phase matching (55�to the c axis) was employed in the experiment. The crystal

was wrapped in indium foil and mounted into a copper

block, both end faces of which were antireflection coated at

the pump and laser wavelengths.

3 Results and discussion

3.1 Ho:LuAG laser

The absorption coefficient of the Ho:LuAG crystal was

measured to be 0.46 cm-1 at 1.91 lm at the pump power

of 1.0 W, implying the single-pass absorption efficiency of

63 % under no lasing. The Ho laser operation was achieved

at both CW and Q-switched modes. Under CW mode, the

slope efficiencies as a function of the absorbed pump

power are shown in Fig. 2. In dual-end-pumping, the

maximum output power of 26.5 W was obtained at the

absorbed pump power of 50.0 W, corresponding to a slope

efficiency of 61.8 % and an optical-to-optical efficiency of

53.0 %. While in single-end-pumping, the maximum out-

put power of 14.6 W was achieved at 27.0 W absorbed

pump powers, which corresponded to a slope efficiency of

57.9 % and an optical conversion of 54.1 %. When the

laser runs in the Q-switch mode, as shown in Fig. 3, a

maximum output power of 25.0 W in dual-end-pumping at

10 kHz was achieved, which corresponded to a slope

efficiency of 54.2 %. While in single-end-pumping, up to

2.9 mJ pulse energy at 5 kHz was achieved, corresponding

M3

M1 M2Telescopic lens system

AOM

Ho:LuAG

1.91 µmlaser

TFP 1.91 µmlaser

TFP

ZGP

M5 M5

M5 M6

M4

3-5 µm Laser output

Telescopic lens system

Telescopic lens system

Fig. 1 Schematic diagram of experimental setup

Y. Shen et al.

123

to a slope efficiency of 61.5 %. In addition, the beam

quality of M2 factor was 1.11 at the highest output power of

CW Ho:LuAG laser in dual-end-pumping.

3.2 Four-mirror ring resonator ZGP OPO laser

The published results demonstrated that if the OPO is

doubly resonant and pumped by a multi-mode beam, the

output energy has a significant maximum when the optical

length of the OPO matches that of the pump source, even if

this length is considerably longer than the shortest possible

for the OPO [13]. The output power of the ring ZGP OPO

is the combined sum of the signal and the idler. The

average 3–5 lm output power with different cavity length

was measured as a function of the incident Ho:LuAG pump

power. The results are shown in Fig. 4. The best mea-

surement result of the output power of the ring OPO was

the cavity with physical length of 124 mm, which the ratio

of the optical length of OPO to that of the pump source is

1/1. When we increased the physical length of the ring

OPO resonator to 134 mm, the slope efficiency of the OPO

decreased from 61.0 to 41.3 %.

Based on the above investigation, we chose the physical

length of 124 mm to further study. The output power of the

ring resonator with the physical length of 124 mm is shown

in Fig. 5. The maximum output power of this resonator was

5.51 W at the pulse repetition frequency of 5 kHz, corre-

sponding to a slope efficiency of 59.0 %. The shortest pulse

width was 14 ns at the Ho incident pump power of 13.1 W,

the typical oscilloscope trace of which is shown in Fig. 6.

The ring ZGP OPO resonator operating at the pulse repe-

tition frequency of 10 kHz was also investigated, and the

slope efficiency of it was 57.5 %, which was similar to the

resonator with 5 kHz. The little difference of the slope

efficiency of the two pulse repetition frequencies can be

attributed to the lower energy per pulse and the larger pulse

instability of the Q-switched Ho:LuAG laser at 10 kHz. In

addition, the lasing threshold of the ring ZGP OPO was

about 0.2 J/cm2.

Figure 7 shows propagation characteristics of the ZGP

OPO beam at the output power level of 5.51 W. The beam

0 10 20 30 40 50 600

5

10

15

20

25

30 dual-end-pumped, ηs=57.9%

single-end-pumped, ηs=61.8%O

utpu

t pow

er (

W)

Absorbed pump power (W)

Fig. 2 CW output power of the Ho:LuAG laser with different

pumping

0 10 20 30 40 500

5

10

15

20

25 Single-end-pumping, 5 kHz, ηs=61.5%

Dual-end-pumping, 10 kHz, ηs=54.2%

Out

put p

ower

(W

)

Absorbed pump pwer (W)

Fig. 3 Output power of the Q-switched Ho:LuAG laser with different

pumping

7 8 9 10 11 12

0.5

1.0

1.5

2.0

2.5

3.0

3.5 L=124 mm, ηs=61.0%

L=134 mm, ηs=41.3%

Linear Fit of L=124 mmLinear Fit of L=134 mm

Out

put p

ower

(W

)

Incident Ho:LuAG pump power (W)

Fig. 4 Output power of the ring ZGP OPO with different cavity

length

4 6 8 10 12 140

1

2

3

4

5

6 5 kHz, ηs=59.0%

10 kHz, ηs=57.5%Linear Fit of 5 kHzLinear Fit of 10 kHz

Out

put p

ower

(W

)

Incident Ho:LuAG pump power (W)

Fig. 5 Output power of the ring ZGP OPO with the cavity length of

124 mm

A ring ZnGeP2 optical parametric oscillator

123

quality of the ring ZGP OPO was measured with the

combined sum of the signal and the idler. Using 90/10

knife-edge technique, we measured the 1/e2 beam radius at

several positions after passing a CaF2 lens with the focal

length of 100 mm. By fitting Gaussian beam standard

expression to these data, the fit yields M2 = 3.41 for the

output wavelength of 3.9 lm and M2 = 2.96 for the output

wavelength of 4.5 lm. The output spectrum of the ring

ZGP OPO was measured by a 300-mm WDG15-Z mono-

chromator and a HgCdTe detector, which is shown in

Fig. 8. We observed a broad spectrum envelop with a

FWHM of approximately 225 nm for the signal and

300 nm for the idler, which can be tuned by rotation of the

ZGP crystals.

4 Conclusion

In conclusion, a ring ZnGeP2 optical parametric oscillator

pumped by a Q-switched Ho:LuAG laser was demon-

strated. The maximum output power of the ring OPO res-

onator with 124-mm physical length was 5.51 W, which

corresponded to a slope efficiency of 59.0 % with respect

to the incident Ho pump power. A beam quality of M2 \ 4

was achieved. In addition, up to 2.9 mJ pulse energy of the

Q-switched Ho:LuAG laser at 5 kHz was achieved, cor-

responding to a slope efficiency of 61.5 %.

Acknowledgments This work was supported by National Natural

Science Foundation of China (No. 61308009), Science Fund for

Outstanding Youths of Heilongjiang Province (JQ201310) and Fun-

damental Research funds for the Central Universities (Grant No.

HIT.NSRIF.2014044).

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Fig. 6 Typical oscilloscope trace of expanded shape of a single pulse

0 30 60 90 120 150 1800.0

0.5

1.0

1.5

2.0

2.5

Bea

m r

adiu

s (m

m)

Distance from lens (mm)

M2=3.41, 3.9 μmM2=2.96, 4.5 μm

Fig. 7 Beam quality of the ring ZGP OPO laser

3750 3900 4050 4200 4350 4500 4650 48000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Inte

nsity

(ar

b. u

nits

)

Wavelength (nm)

Fig. 8 Output spectrum of the ring ZGP OPO

Y. Shen et al.

123