Optics Communications 230 (2004) 175–180

www.elsevier.com/locate/optcom

Comparison between a-cut and off-axially cut Nd:YVO4

lasers passively Q-switched with a Cr4þ:YAG crystal

Hao Chen a,c,*, E Wu a, Heping Zeng a,b,*

a Key Laboratory of Optical and Magnetic Resonance Spectroscopy, and Department of Physics, East China Normal University,

Shanghai 200062, PR Chinab School of Chemistry, The University of Sydney, NSW 2006, Australia

c Department of Physics, Xuzhou Normal University, Xuzhou 221009, PR China

Received 10 July 2003; received in revised form 12 September 2003; accepted 6 November 2003

Abstract

We experimentally compared pulsed lasers from off-axially cut and a-cut Nd:YVO4 crystals passively Q-switched by

Cr4þ:YAG crystals as the intracavity saturable absorbers. As compared to the a-cut Nd:YVO4 laser, the 45� off-axiallycut Nd:YVO4 laser could be Q-switched passively to output much shorter pulse width with a lower repetition rate.

Laser pulses from the off-axially cut crystal were stable in the output polarization states.

� 2003 Elsevier B.V. All rights reserved.

PACS: 42.60.Gd; 42.70.Hj; 42.55.Xi

Keywords: Q-switched; Off-axially cut; Nd:YVO4; Cr4þ:YAG

1. Introduction

Pulsed lasers in the near infrared region are

quite suitable for a wide range of applications[1–3], such as microsurgeon, laser ranging, injec-

tion seeds for amplifier, and remote sensing. In

recent years, Nd3þdoped crystals, such as

Nd:YVO4, Nd:GdVO4, and Nd:YAG, have been

extensively studied as gain media at 1064 nm for

all solid-state lasers pumped by laser diodes [4–6].

* Corresponding authors. Tel.: +862162232108; fax:

+862162232056.

E-mail addresses: chenhao7605@sohu.com (H. Chen),

hpzeng@phy.ecnu.edu.cn (H. Zeng).

0030-4018/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.optcom.2003.11.010

The corresponding lasers can be conveniently op-

erated in passive Q-switches by using intracavity

Cr4þ:YAG crystals to compose all-solid-state,

compact, simple and low-cost pulsed lasers, sinceCr4þ:YAG crystal has a saturable absorption band

from 0.9 to 1.2 lm, which makes it a suitable

saturable absorber to obtain passively Q-switched

lasers at 1064 nm without the need for high-volt-

age or RF drivers [7–9]. Among these Nd3þdoped

crystals, Nd:YVO4 is the most widely used laser

active material for laser diode pumping due to its

large stimulated emission cross-section and broadabsorption bandwidth. The stimulated emission

cross-section at 1064 nm parallel to the c axis

(a-cut), rk ¼ 25� 10�19 cm2, is about 5 times

ed.

176 H. Chen et al. / Optics Communications 230 (2004) 175–180

higher than that of Nd:YAG. In order to improve

the output power and slope efficiency, a-cut

Nd:YVO4 crystals are usually chosen to achieve

low pump-thresholds.

However, the large stimulated emission cross-

section is not preferable for passive Q-switch, sincethe second threshold condition requires saturation

of the absorber before the gain saturation in the

laser crystal. In a laser cavity composed of an

output mirror with the reflectivity R, a gain me-

dium with the stimulated emission cross-section r,and an intracavity saturable absorber with the

initial transmission T0 and the ground-state ab-

sorption cross-section rgs, the criterion for a goodpassive Q-switch can be deduced by the analysis of

the coupled rate equations [10]

lnð1=T 20 Þ

lnð1=T 20 Þ þ lnð1=RÞ þ L

rgs

rAAs

� c1� b

; ð1Þ

where L is the nonsaturable intracavity round-trip

dissipative optical loss, A=As is the ratio of the

effective area in the gain medium to that in the

saturable absorber, c is the inversion reductionfactor (c ¼ 1 and c ¼ 2 correspond to four-level

and three-level systems) [11], and b is the ratio of

the excited-state absorption cross-section to that

of the ground-state absorption in the saturable

absorber. From the above criterion, we can find

A=As and rgs=r are important parameters for

passive Q-switch. The former depends on the

geometric structure of the laser cavity and thelatter depends on the properties of the laser active

material and the saturable absorber.

When a Nd:YVO4 crystal is cut along the c (i.e. c-

cut) and a (i.e. a-cut) axis, the effective stimulated

emission cross-section is dominated by r? and rk,

respectively. Because of the crystallographic struc-

ture of Nd:YVO4 crystal, the stimulated emission

cross-section parallel to the c axial is about 4 timeslarger than that orthogonal to the c axis [12]. In an a-cut Nd:YVO4 crystal, large value (rk) of the effective

stimulated emission cross-section, which is compa-

rable to the rgs value of the Cr4þ:YAG crystal, is

disadvantageous for a passively Q-switched laser.

The a-cut Nd:YVO4 lasers generally produce longer

pulse widths with lower peak powers, usually less

than 1 kW, by using Cr4þ:YAG crystal as the satu-rable absorber. The experimental comparison

betweena-cut and c-cutNd:YVO4 laserspassivelyQ-

switched with a Cr4þ:YAG has been reported by

Chen and Lan [12]. A similar experimental compar-

ison between a-cut and c-cut Nd:GdVO4 lasers pas-

sively Q-switched with a Cr4þ:YAG has been

reported by Liu et al. [13]. Due to the lower emissioncross-section of the c-cut Nd:YVO4 crystal, the pas-

sive Q-switching effect was enhanced to output a

peak power 10 times larger than that obtained with

the a-cut Nd:YVO4 laser, which clearly revealed that

c-cut Nd:YVO4 is a very competitive material to

produce short-pulse lasers with pulse duration down

to subnanosecond regime and high peak powers up

to 10 kW. Nevertheless, as is well known that asmaller stimulated emission cross-section normally

results in a larger pumping threshold. It should be

good to compromise small pumping threshold and

high peak output power in a passively Q-switched

laser. This can be realized by cutting the Nd:YVO4

crystal off axially to control the effective stimulated

emission cross-section. When passively Q-switched

by a Cr4þ:YAG crystal, an a-cut Nd:YVO4 lasergenerally produces pulses with pulse widths larger

than 10 ns, and a c-cut Nd:YVO4 laser generally

produces subnanosecond pulses, while off-axially cut

Nd:YVO4 laser may be passively Q-switched to

produce pulses of a few nanoseconds, and the output

pulses can be controlled by using different off-axially

cut crystalwith different effective stimulated emission

cross-section. The effective stimulated emissioncross-section of an off-axially cut Nd:YVO4 crystal

can be controlled to take a value between r? and rkaccording to the crystal cut angle, which provides a

potential possibility to achieve a good passively Q-

switched laserwith a controllable output pulsewith a

Cr4þ:YAGcrystal as the saturable absorber. The off-

axially cut Nd:YVO4 crystal also has a large ab-

sorption cross-section and a wide absorption band,which can be efficiently pumped by a laser diode. In

this paper,we experimentally compared the passively

Q-switched lasers from the off-axially cut and a-cut

Nd:YVO4 crystal.

2. Experiments and results

The experimental setup of the laser cavity

structure is schematically given in Fig. 1. The laser

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

CW

outp

utpo

wer

(W)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 2. The cw laser output power of the a-cut and off-axially

cut Nd:YVO4 lasers as a function of the incident pump power.

0.2 0.4 0.6 0.8 1.0 1.2 1.4

0

20

40

60

80

100

120

Ave

rage

outp

utpo

wer

(mW

)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 3. The average output power of the a-cut and off-axially

cut Nd:YVO4 passively Q-switched lasers with a saturable ab-

sorber of T0 ¼ 0:85.

Fig. 1. Experimental setup for the passively Q-switched

Nd:YVO4 laser.

H. Chen et al. / Optics Communications 230 (2004) 175–180 177

cavity was composed of a concave mirror M1

(R ¼ 30 mm) with anti-reflection (AR) coating at

808 nm on both sides and high-reflection (HR)

coating at 1064 nm on the concave side, and a plane

output coupler M2with a transmission of 5% at

1064 nm. The total cavity length was approxi-

mately 25 mm. The pump source was a 2-W cw

laser diode with the thermal regulation at 26 �C,and its emission wavelength matched the absorp-tion peak of the Nd:YVO4 crystal. The laser diode

emission was coupled to the crystal by a pair of

nonspherical lens. The 1-mm thick 2.0 at.% doped

Nd:YVO4 crystal, which was cut at 45� to its op-

tical axes (a axis and c axis), was AR coated on

both sides for 808 and 1064 nm light. The saturable

absorber (Cr4þ:YAG) with AR coating at 1064 nm

transmitted the 1064-nm light with the initialtransmission about 85%. An InGaAs Pin detector

(Newport 818-BB-30) photodiode and an oscillo-

scope (500 MHz, HP-54616C) were used to record

the output laser pulses. All lasers were operated at

TEM00 during all the experimental measurements.

Fig. 2 shows the cw output powers of a-cut and

off-axially cut Nd:YVO4 lasers. The cw laser op-

eration could be achieved in both lasers whenpump powers were near 20 mW. There was no

observable diversity of the pumping thresholds

between a-cut and off-axially cut Nd:YVO4 lasers.

This was quite different from the experimental re-

sults that distinctly different pumping thresholds

existed for the a-cut and c-cut Nd:YVO4 lasers.

The result indicated that the off-axially cut crystal

possesses some of the a-cut properties. The slopeefficiency of the a-cut Nd:YVO4 laser was about

42%, while that of the off-axially cut Nd:YVO4

laser was about 35%. The difference of the slope

efficiencies was resulted from the different emission

cross-sections, i.e., the emission cross-section of

the off-axially cut Nd:YVO4 crystal is smaller than

that of the a-cut one. In addition, we also found

the output laser beam of the off-axially cut laserwas well polarized. The polarization extinction

ratio was about 600:1.

The average output powers of the a-cut and off-

axially cut Nd:YVO4 passively Q-switched lasers

are shown in Fig. 3. Since the intracavity saturable

absorber induced additional loss in the laser cavi-

ties, the slope efficiencies of the pulsed lasers were

therefore lower than those in cw operations.The a-cut and off-axially cut Nd:YVO4 passively

178 H. Chen et al. / Optics Communications 230 (2004) 175–180

Q-switched lasers were operated with slope effi-

ciencies of 11.3% and 8.5%, respectively. The

pumping threshold of the a-cut Nd:YVO4 laser

was much lower than that of the off-axially cut

one. That is to say, they have notable distinction in

passive Q-switch operation.Figs. 4 and 5 illustrate the repetition rates and

single-pulse energies of the a-cut and off-axially cut

Nd:YVO4 lasers. The pulse repetition of the off-

axially cut Nd:YVO4 laser increased from 2.6 to

11.1 kHz when the incident pump power increased

from 615 to 1340 mW, while the a-cut Nd:YVO4

laser was Q-switched at a repetition rate from 7.5

to 28.4 kHz when the incident pump power in-creased from 248 to 1340 mW. Obviously, the

0.2 0.4 0.6 0.8 1.0 1.2 1.40.0

5.0

10.0

15.0

20.0

25.0

30.0

Rep

etiti

onra

te(k

Hz)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 4. The Q-switched pulse repetition rate varied with the

pump power for the a-cut and off-axially cut Nd:YVO4 pas-

sively Q-switched lasers.

0.2 0.4 0.6 0.8 1.0 1.2 1.4

1.0

2.0

3.0

4.0

5.0

6.0

Pul

seen

ergy

(µJ)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 5. The single-pulse energy of the a-cut and off-axially cut

Nd:YVO4 passively Q-switched lasers.

repetition rate of the off-axially cut Nd:YVO4

passively Q-switched laser was nearly half of that

of the a-cut one. This indicated that the off-axially

cut crystal could achieve higher pulse power than

the a-cut one. As is usually observed in passives Q-

switched lasers, the Q-switched output pulse rep-etition rates of both a-cut and off-axially cut

Nd:YVO4 lasers were not very stable. Slight fluc-

tuations of the repetition rate could be observed.

Form Fig. 4, we can see that the repetition rates of

both lasers were not linear with the incident pump

power. Especially, when the incident pump power

increased beyond 800 mw, the increases of the

repetition rates became slow for both a-cut andoff-axially cut Nd:YVO4 lasers. As shown in Fig. 5,

the single-pulse energy generally increased while

the incident pump power increased. The single-

pulse energy of the off-axially cut Nd:YVO4 laser

increased more rapidly than that of the a-cut one,

partly because the repetition rate of the off-axially

cut laser was smaller. The pulse energy fluctuated

during the passive Q-switch operation, mainlybecause of unavoidable fluctuation of the passively

Q-switched pulse repetition.

Fig. 6 indicates the change of pulse width with

increasing incident pump power. The pulse width

of the a-cut Nd:YVO4 laser dropped rapidly when

the pump power increased from the pumping

threshold to 700 mW, but decreased slowly when

the incident pump power increased beyond 800mW. However, the pulse width of the off-axially

0.2 0.4 0.6 0.8 1.0 1.2 1.4

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

Pul

sew

idth

(ns)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 6. The Q-switched laser pulse width of the a-cut and off-

axially cut Nd:YVO4 passively Q-switched lasers as a function

of the incident pump power.

pulse-width: 3.9nsfrequency: 11.1kHz

10 ns/div

Fig. 8. Pulse profile and Q-switched pulse train of the off-

axially cut Nd:YVO4 laser.

H. Chen et al. / Optics Communications 230 (2004) 175–180 179

cut Nd:YVO4 laser remained around 4.5 ns. It also

decreased when the incident pump power in-

creased from the pumping threshold to 1340 mW,

but very slowly and unnoticeably.

As shown in Fig. 7, the peak powers of these

two lasers are quite different. Obviously, the peakpower of the off-axially cut Nd:YVO4 laser was

several times higher than that of the a-cut one.

Furthermore, the peak pulse power of the off-ax-

ially cut Nd:YVO4 laser increased quickly when

the incident pump power increased from the

pumping threshold to 1340 mW. Limited by the

highest pump power of 1340 mW in our experi-

mental situation, we could only obtain a peakpower of 1.5 kW with a 3.9-ns pulse width and

11.1-kHz repetition rate. We believed that higher

peak power could be obtained with higher incident

pump power. Form Figs. 5 and 7, we can see that

the pulse energy and the peak powers of the off-

axially cut Nd:YVO4 laser fluctuated a little more

remarkably than the a-cut one. The reason was

that there exists polarization mode competitionbetween the a-polarization (parallel a axis) and c-polarization (parallel c axis) in the off-axially cut

Nd:YVO4 laser [14], which introduced perturba-

tion for the polarized output laser.

Fig. 8 represents the pulse width and pulse train

of the off-axially cut laser. On the contrary, the

peak power of the a-cut Nd:YVO4 laser increased

much more slowly and probably reached its high-est limit when the incident pump power increased

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

peak

pow

er(k

W)

Incident pump power (W)

a-cutoff-axially-cut

Fig. 7. The peak power of the a-cut and off-axially cut

Nd:YVO4 passively Q-switched lasers as a function of the in-

cident pump power.

beyond 1100 mW. The reason was that the pulserepetition and width of the a-cut Nd:YVO4 laser

were larger than those of the off-axially cut one.

These experimental observations confirmed that

the off-axially cut Nd:YVO4 crystal excelled the a-cut one to obtain passively Q-switched laser.

3. Conclusion

In summary, the 45� off-axially cut Nd:YVO4

has been confirmed better than the a-cut Nd:YVO4

for passively Q-switched laser. Higher peak power

and narrower pulse width can be obtained ad-

vantageously. The off-axially cut Nd:YVO4 pos-

sesses part of the a-cut and c-cut properties. Forthe cw lasers, the off-axially cut and the a-cutcrystals can both operate with the low pumping

thresholds. Furthermore, the light emitted from

the off-axially cut laser was polarized. For the limit

of our pump system, we could not get optimized

result about the output power. The off-axially cut

Nd:YVO4 laser may be passively Q-switched more

stably with increased pump power, and the output

efficiency may be evidently improved if operated ina microchip laser configuration.

Acknowledgements

This work was partly supported by Shanghai

Priority Academic Discipline, fund for young

teachers from the Ministry of Education of

180 H. Chen et al. / Optics Communications 230 (2004) 175–180

People�s Republic of China, National Key Project

for Basic Research (No. 1999075204), and Na-

tional Science Foundation (No. 10234030). H.Z.

thanks travel support from Australian Research

Council IREX (X00001736) and U2000 fellow-

ship at University of Sydney.

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