Over Ten-millijoule Eye-safe Laser Generation by Extra-cavity

Optical Parametric Oscillator Driven with a Diode-pumped

Nd:YAG/Cr

:YAG Q-Switched Laser

Y. P. Huang

, Y. J. Huang

and Y. F. Chen

2,3

Department of Physics, Soochow University, Shih Lin, Taipei 11102, Taiwan

Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan

Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

Keywords: Optical Parametric Oscillator, Eye-safe Laser, Q-switched, Nd:YAG Laser.

Abstract: An efficient diode-side-pumped Nd:YAG/Cr4

:YAG Q-switched laser with a convex-concave resonator is

employed to develop a high-pulse-energy eye-safe laser. We utilize a monolithic KTP crystal to be the

optical parametric oscillator (OPO) crystal, and to form an extracavity OPO configuration. Based on the

efficient Nd:YAG laser oscillator at 1064 nm carrying a pulse energy of 30 mJ, the OPO energy at 1573 nm

of 13.3 mJ is obtained with a pulse width of 6 ns, corresponding to an OPO conversion efficiency of 44.3%.

1 INTRODUCTION

The high gain of Nd:YAG crystal makes it preferred

for use in pulsed solid-state lasers and nonlinear

wavelength conversions. Laser resonators with a

large mode volume, efficiently utilizing the stored

energy in the laser medium, are highly desirable for

generating the high-energy laser oscillators. Chesler

and Maydan (1972, p. 2254) reported that convex-

concave resonator has the advantages of

compactness, high efficiency, and insensitivity to

perturbations. Therefore, it is valuable to develop a

highly efficient laser oscillator with millijoule and

nanosecond laser pulses based on a compact convex-

concave resonator providing a large mode volume.

Thermal lensing of the laser rod dominantly

affects the laser mode volume, stability of laser

resonator, and output performances. Quasi-

continuous-wave (QCW) laser bars and stacks,

giving significantly weaker thermally induced

lenses, have been frequently used to achieve multi-

millijoule Q-switched neodymium-doped lasers and

optical parametric oscillators (OPOs) (Agnesi et al.,

2006; Zendzian, Jabczynski and Kwiatkowski, 2008;

Huang et al., 2011; Schilling et al., 2006) .

In this work, we employ a convex-concave

resonator, providing a large mode volume, to

develop a high-pulse-energy diode-side-pumped

passively Q-switched Nd:YAG/Cr

:YAG laser

oscillator. At a diode pumped energy of 227 mJ, the

output laser pulse reaches 30 mJ with a pulse width

of 6 ns. The optical-to-optical conversion efficiency

is 13.2%. With the developed Nd:YAG laser

oscillator, the OPO is further investigated with a

monolithic KTP crystal in an extracavity

configuration. With the 1064-nm input energy of 30

mJ, the OPO energy at 1573 nm is found to be 13.3

mJ. The OPO conversion efficiency is 44.3%,

illustrating the excellent performance of eye-safe

OPO based on the efficient 1064-nm laser oscillator.

2 EXPERIMENTAL SETUP

Figure 1 (a) depicts the experimental setup for the

QCW diode-side-pumped passively Q-switched

Nd:YAG/Cr

:YAG/KTP eye-safe laser. The pump

source was two QCW high-power diode stacks.

Each diode stack consisted of six 10-mm-long diode

bars generating a maximum output power of 90 W

per bar at the central wavelength of 808 nm. The

diode stack was constructed with 400 μm spacing

between the diode bars so that the whole emission

area was approximately 10 × 2.4 mm

. The full

divergence angles in the fast and slow axes are

approximately 35° and 10°, respectively. The radii

of curvature of cavity mirrors are chosen as R

= -

500 mm and R

= 600 mm for the M

and M

172

Huang Y., Huang Y. and Chen Y..

Over Ten-millijoule Eye-safe Laser Generation by Extra-cavity Optical Parametric Oscillator Driven with a Diode-pumped Nd:YAG/Cr4+:YAG Q-Switched

Laser.

DOI: 10.5220/0005254001720176

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 172-176

ISBN: 978-989-758-093-2

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

mirrors, respectively. The M

mirror was coated for

high reflection at 1064 nm on the convex surface.

The M

mirror was coated for partial reflection at

1064 nm on the concave surface. The gain medium

was a 1.0 at. % Nd:YAG crystal with a length of 25

mm, and cut with 2°-wedged end facets to avoid

etalon effects. Both end facets of the laser crystal

were coated for anti-reflection at 1064 nm, and the

pump face was coated for anti-reflection at 808 nm.

The diode stacks were placed close to the lateral

surface of the laser crystal to have good pump

efficiency, and were driven to emit optical pulses of

300 µs to match the upper-level lifetime of Nd:YAG

laser crystal. The Cr

:YAG crystal with the initial

transmission of 30%. The nonlinear crystal KTP

with a length of 25 mm was cut along x axis (θ = 90°

and φ = 0°) for type-II non-critically phase-matched

OPO. The pump face of the KTP crystal, acting as

the front mirror of the OPO cavity, was coated for

high transmission at 1064 nm and high reflection at

1573 nm. The other face of the KTP crystal was

coated for high reflection at 1064 nm and partially

reflection at 1573 nm, which enables a double pass

of the 1064-nm input light within the KTP crystal to

lower the threshold and to enhance the efficiency for

OPO conversion. Note that a slight misalignment of

the OPO cavity, replacing the need of an optical

isolator, was applied to prevent the feedback of

1064-nm input light into the laser oscillator. An anti-

reflection coated half-waveplate at 1064 nm and a

polarizer were combined to be a variable attenuator

for adjusting the input pulse energy at 1064 nm. All

crystals were wrapped with indium foil and mounted

in conductively cooled copper blocks. The pulse

temporal behavior was recorded by a LeCroy digital

oscilloscope (Wavepro 7100, 10 G samples/s, 1 GHz

bandwidth) with a fast InGaAs photodiode.

Figure 1: Experimental configuration for an extracavity

OPO with a monolithic KTP crystal pumped by the

passively Q-switched Nd:YAG/Cr

:YAG laser oscillator

with a convex-concave resonator.

3 EXPERIMENTAL RESULTS

First, the QCW free-running operation without the

:YAG crystal was performed to confirm the

reliability of the laser configuration. Figure 2 depicts

the experimental results of the output energy as a

function of the diode pump energy in the free-

running operation. The threshold pump energy is

approximately 64 mJ. With a diode pump energy of

298 mJ, the output energy at 1064 nm is 122 mJ,

corresponding to an optical-to-optical conversion

efficiency of 41%. The temporal shape of the laser

pulse, as showed in Fig. 3, reveals a train of spikes

caused by the relaxation oscillation. The overall

slope efficiency is found to be 54%. We then

inserted the Cr

:YAG crystal into the laser

resonator to implement the passively Q-switched

Nd:YAG/Cr

:YAG laser oscillator. The threshold

pump energy for the Q-switched operation is

measured to be about 227 mJ, and the output pulse

energy at 1064 nm is about 30 mJ, corresponding to

an optical-to-optical conversion efficiency of 13.2%.

Compared with the results of the passively Q-

switched Nd:YAG/Cr

:YAG laser with a QCW

diode side-pumping system to date (Zendzian,

Jabczynski and Kwiatkowski, 2008; Afzal et al.,

1997; Sauder, Minassian and Damzen, 2006), the

energy extraction efficiency is the highest to our

knowledge thanks to the superior cavity design of a

convex-concave resonator.

A typical temporal shape of the laser pulse, as

depicted in Fig. 4, displays the mode-locked

modulation resulted from the multi-longitudinal

mode beating. The separation of the mode-locked

pulses is verified to be correspondent with the cavity

round-trip frequency of 1.36 GHz. The Q-switched

pulse envelope is approximately 6 ns. The Q-

switched mode-locked pulses enhance the peak

power, which is estimated to be up to 6.3 MW. The

laser beam quality was measured employing the z-

scan method. The beam width was evaluated by the

scanning knife-edge method. Figure 5 shows the

experimental results of the beam widths in the

horizontal and vertical directions as a function of the

position along the propagation direction,

respectively. The beam quality factors M

are

estimated to be 5.0 × 3.0 (horizontal × vertical).

We then employ the developed Nd:YAG laser

oscillator to explore the performance of the

extracavity OPO. The dependence of the output

pulse energy at 1.57 m on the input pulse energy at

1.06 m is shown in Fig. 6. The OPO threshold

energy is approximately 6.0 mJ. With maximum

available input energy of 30 mJ at 1.06 m, the OPO

output energy of 13.3 mJ is obtained, leading to a

high slope efficiency of 54.8%. The OPO conversion

efficiency is also presented in Fig. 7 as a function of

the input pulse energy at 1.06 m. With increasing

OverTen-millijouleEye-safeLaserGenerationbyExtra-cavityOpticalParametricOscillatorDrivenwithaDiode-pumped

Nd:YAG/Cr4+:YAGQ-SwitchedLaser

173

the input energy at 1.06 m the OPO conversion

efficiency increases substantially; however, there is

a tendency toward saturation at the higher input

energy. The maximum OPO conversion efficiency

of 44.3% is obtained at the highest input energy of

30 mJ, in which the excellent performance is

attributed to the superior cavity design for the 1064-

nm laser oscillator. Figure 8 shows the temporal

shape of the OPO pulse for the maximum output

energy of 13.3 mJ, which exhibits a considerably

less pronounced modulation with the same beating

frequency as 1064-nm input pulses. The OPO pulse

width is similar to that of the 1064-nm input pulse of

approximately 6 ns. The corresponding OPO peak

power is calculated to be approximately 2.1 MW.

The optical spectrum of OPO, as shown in Fig. 9,

was measured with an optical spectrum analyzer

(Advantest Q8381A) which has the resolution of 0.1

nm. In addition, the beam quality M

factors of the

OPO beam are measured to be less than 3.0 in both

directions. The better beam quality than that of input

Figure 2: Output energy at 1064 nm with respect to the

diode pump energy at 808 nm in the QCW free-running

operation.

Figure 3: Temporal shape for the laser pulse at the

maximum diode pump energy of 298 mJ.

Figure 4: Temporal shape for the 1064-nm laser pulse.

Figure 5: Dependence of the beam width at 1064 nm in the

horizontal and vertical directions on the position along the

propagation direction.

Figure 6: Output pulse energy at 1.57 μm as a function of

the input pulse energy at 1.06 μm.

1064-nm beam results from the spatial cleaning

effect in the OPO conversion process. It is worth

noting that both the eyesafe pulse energy of 13.3 mJ

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

174

and the diode-to-signal conversion efficiency of

5.9%, obtained with the extracavity OPO driven by

the present side-pumped Nd:YAG laser oscillator,

are comparable to Schilling et al. (2006, p. 6607)

attained with an intracavity OPO scheme by end

pumping.

Figure 7: EOPO conversion efficiency as a function of the

input pulse energy at 1.06 μm.

Figure 8: Temporal behavior of the OPO pulse at 1.57

μm.

Figure 9: Optical spectrum measurement for the OPO.

4 CONCLUSIONS

A compact convex-concave resonator has been

demonstrated in a QCW diode-side-pumped

passively Q-switched Nd:YAG/Cr

:YAG laser

oscillator. The output pulse energy at 1064 nm

reaches 30 mJ with a pulse width of 6 ns. The

optical-to-optical conversion efficiency is as high as

13.2%. With the passively Q-switched Nd:YAG

laser oscillator, the extracavity OPO with a

monolithic KTP crystal is investigated and

performed. The maximum output energy at 1573 nm

of 13.3 mJ is obtained with a pulse width of 6 ns,

corresponding to an OPO conversion efficiency of

44.3%. Efficient extracavity wavelength conversions

validates that our compact convex-concave resonator

is potentially valuable for the laser oscillator with

high extraction efficiency and low beam divergence.

ACKNOWLEDGEMENTS

The authors thank the National Science Council for

their financial support of this research under Grant

No. NSC 102-2112-M-031-001-MY3.

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