comparison on performance of acousto-optically q-switched nd:gdvo_4 and nd:yvo_4 lasers at high...

Comparison on performance of acousto-optically

Q-switched Nd:GdVO4 and Nd:YVO4 lasers at

high repetition rates under direct diode pumping

of the emitting level

Xudong Li*, Xin Yu, Fei Chen, Renpeng Yan, Jing Gao, Junjua Yu, Deying Chen

National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology,

150080, Harbin, China

*[email protected]

Abstract: We detail the comparison on laser performance of Nd:GdVO4

and Nd:YVO4 lasers at high repetition rates operated at 1.06µm under direct

diode pumping of the upper laser level. The results reveal that Nd:GdVO4

and Nd:YVO4 are efficient laser crystals for solid-state lasers under direct

pumping of the emitting level. However, Nd:YVO4 crystal, compared with

Nd:GdVO4 crystal, is a more favorable gain medium when higher repetition

rates and shorter pulse width are desired, owning to its larger stimulated

emission cross-section.

©2009 Optical Society of America

OCIS codes: (140.3480) Laser, diode-pumped; (140.3530) Laser, neodymium; (140.3540)

Laser, Q-switched

References and links

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#110131 - $15.00 USDReceived 14 Apr 2009; revised 13 May 2009; accepted 17 May 2009; published 21 May 2009

(C) 2009 OSA 25 May 2009 / Vol. 17, No. 11 / OPTICS EXPRESS 9468

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1. Introduction

High repetition rates Q-switched solid-state lasers with short pulse width have a variety of

applications such as remote sensing, ranging, micro-machining, marking and so on [1–3].

They can be realized by active Q-switching, such as acousto-optically (A-O) Q-switching,

which takes the advantages of low modulation voltage, low insertion losses, high repetition

rate and short pulse width. Short pulse width is benefit in obtaining high peak power for high

repetition rates Q-switched lasers. Both Nd:GVO4 and Nd:YVO4, which are widely

researched and used for their excellent physical and optical properties [4–6], are the favorable

gain medium when short width and high repetition rates are desired, owing to its high gain

and limited upper-state lifetime. A large stimulated emission cross section and effective

absorption coefficient can provide higher gain which enhances Q-switching at high repetition

rates. Meanwhile, their modest upper-state lifetime leads to the faster building up of the pulses

to achieve short pulse width. Therefore, both Nd:GdVO4 and Nd:YVO4 crystals were regarded

as excellent laser medium for Q-switching operation at high repetition rates with short pulse

width [7,8].

Many researches on solid-state lasers with high repetition rates have been reported by

making use of Nd:GdVO4 and Nd:YVO4 crystals [9–14]. For Nd:GdVO4 lasers, Li et al

reported a 100kHz Nd:GdVO4 laser under 879nm diode-laser pumping, an average output

power of 12.1W and a pulse width of 20.3ns were obtained in A-O Q-switched operation [9].

A. Minassian reported a diode-pumped TEM00 Nd:GdVO4 MOPA system and obtained an

average power of 101W and a pulse width of about 20ns from 100~600kHz [11]. To the best

of our knowledge, it’s the highest pulse repetition rate making use of A-O Q-switching

Nd:GdVO4 laser. For Nd:YVO4 lasers, J. H. García-López et al reported a high power

Nd:YVO4 slab laser with repetition rate up to 500kHz, which had a pulse width of 15ns and a

average power of 15.9W at 200kHz [13]. Liu et al demonstrated a 850kHz A-O Q-switching

diode-pumped MOPA Nd:YVO4 laser with average power of 183W and a pulse width of 72ns

[14]. X. Yan reported a 2.2MHz A-O Q-switching Nd:YVO4 laser with a pulse width of 31ns.

It is the highest pulse repetition rate ever reported based on A-O Q-switching Nd:YVO4 laser



[15]. It’s forecasted that short pulse width solid-state lasers with higher and higher repetition

rates will be desired and favored. Choosing a favorable crystal as the laser medium is benefit

to shorten the pulse width and improve the performance of lasers with high repetition rates.

Although some paper have shown that both Nd:GdVO4 and Nd:YVO4 are promising crystals

to operate at high repetition rates, comparatively little research has been carried out on the

comparison of their pulse performance, especially under direct diode pumping of the emitting

level.

A major limitation in the scaling of a solid-state laser to high power is the quantum defect

between the pump and the laser emission wavelengths, which has a major contribution to the

heat generation in the laser material. The reduction of the quantum defect is an important

issue in diminution of heat, and for Nd3+

laser materials this can be accomplished by direct

pumping of the emitting level 4F3/2 [16,17]. Maik Frede et al reported an end-pumped

Nd:YAG laser with direct pumping into the upper laser level [18], the maximum output power

was 250W with an optical-optical efficiency of 57%. Y. Sato realized a near quantum-defect

slope efficiency in Nd:YVO4 laser under direct diode pumping [19], 80% and 75% slope

efficiency were obtain under Ti:sapphire and LD pumping at 880nm, respectively. An

efficient A-O Q-switched Nd:GdVO4 laser under 879-nm pumping was also reported, a

maximum average output power of over 4W was obtained at 100 kHz [7].

In this paper, we do in detail some comparative studies on the performance of A-O Q-

switching Nd:GdVO4 and Nd:YVO4 lasers at high repetition rates operated at 1.06µm under

direct diode pumping of the emitting level. The experimental results demonstrate that both

Nd:GdVO4 and Nd:YVO4 are efficient and promising laser crystals for diode pumped solid-

state lasers under direct pumping of the upper laser level. When higher repetition rates and

shorter pulse width are desired, Nd:YVO4 shows superior laser pulse performance to

Nd:GdVO4, which indicates that Nd:YVO4 crystal is a more favorable gain medium than

Nd:GdVO4 crystal, owning to its larger stimulated emission cross-section.

2. Theoretical analysis

Nd:GdVO4 and Nd:YVO4 are isomorph and have the same crystal structure. Table 1 shows

the thermal and laser properties of Nd:GdVO4 and Nd:YVO4 at room temperature. It’s noted

that there are some controversies in the literatures surrounding the relative thermal

conductivities of Nd:GdVO4 and Nd:YVO4 [20–22]. Here we just cited the data obtained by

Yoichi Sato et al. They has measured the thermal conductivity by quasi-one-dimensional flash

method and proved that there is no remarkable difference on thermal properties of YVO4 and

GdVO4. However, the stimulated emission cross-section is greatly different between

Nd:GdVO4 and Nd:YVO4. That of Nd:YVO4 is two more times greater than that of

Nd:GdVO4. In the pulse operation, σem·τ is an important parameter for lasers with high

repetition rates. σem is the stimulated emission cross-section at 1.06µm and τ is the upper-state

lifetime. At the same repetition rates, the higher product of σem·τ, the higher gain of each

pulse. The σem·τ of Nd:YVO4 is two more times greater than that of Nd:GdVO4. According to

the simulation theoretically, we find that Nd:YVO4 laser can obtain shorter pulse width than

Nd:GdVO4 laser operated at high repetition rates, owning to larger emission cross-section

related to the higher single-pulse gain.

From the theory of the continuously pumped and repetitively Q-switched system pulse

width ∆tp can be calculated by the following formulas [23]:

( )2 '

( ) [1 ln( / )]

i f

p

i t i t

n nLt

c T L n n n n

−∆ = ⋅

+ − + (1)

Where c is the velocity of light, L’ is the optical length of the cavity, T is the transmissivity of

the output coupler, and L is the other loss of the cavity. ni, nf and nt are the initial population



inversion density, the final population inversion density, and the population inversion density

at threshold, respectively.

Table 1. Thermal and laser properties of Nd:GdVO4 and Nd:YVO4 at 25°C

Crystal (1.0at.%) Nd:GdVO4 Nd:YVO4

a-axis c-axis a-axis c-axis

Thermal expansion coefficient, a ( × 10−6/K) [20] 1.14 7.89 1.69 8.19

Thermal diffusivity, (mm2/s) [20] 3.63 4.43 3.78 5.08

Thermal conductivity, K (W/m·K) [21] 8.6 10.5 8.9 12.1

Thermal-optic coefficient, dn/dT ( × 10−6 /K) 6.9 4.7 8.6 3.0

Laser emission wavelength, λem (nm) [4,24] 1062.9 1064.3

Upper-state lifetime, τ (µs) [23] 100 100

Stimulated emission cross-section at 1.06µm, σem ( × 10−19cm2) 7.6 15.6

Absorption wavelength, λab (nm) [25] 879.0 879.8

Absorption coefficient (π-polarization), (cm−1) [25] 22.2 36.1

0 20 40 60 80 1000

10

20

30

40

50

60

70

80

Nd:GdVO4

Nd:YVO4

Pu

lse

Wid

th (

ns)

Repetition Rate (kHz)

Fig. 1. Theoretical pulse width versus repetition rates at absorbed pump power of 20W

0 5 10 15 20 25 30

60

70

80

90

100

110

120

130

140

150

f=100kHz

Nd:YVO4

Nd:GdVO4

Puls

e W

idth

(ns)

Absorbed Pump Power (W)

Fig. 2. Pulse width versus absorbed pump power at the repetition rate of 100kHz



We calculated theoretically ∆tp for various repetition rates at the absorbed pump power of

20W for Nd:GdVO4 and Nd:YVO4 lasers, respectively. Figure 1 shows the theoretical pulse

width as a function of repetition rate for Nd:GdVO4 and Nd:YVO4 lasers. As shown in Fig. 1,

at relative lower repetition rates, such as lower than 30kHz, the difference of the pulse width

between Nd:GdVO4 and Nd:YVO4 lasers is not obvious. As the repetition rate increases, the

pulse width of Nd:YVO4 laser is gradually shorter than that of Nd:GdVO4 laser. The pulse

width as a function of absorbed pump power at the repetition rate of 100kHz is also simulated

for Nd:GdVO4 and Nd:YVO4 lasers, respectively. As shown in Fig. 2, it is easy to observe the

difference of pulse width between two lasers. As shown above, it’s forecasted that the pulse

performance of Nd:YVO4 laser will be superior to that of Nd:GdVO4 laser at very high

repetition rates according to the theoretical simulation.

3. Experimental setup

Fig. 3. Experimental setup of A-O Q-switching operation

The experimental setup of A-O Q-switching operation is shown schematically in Fig. 3.

The laser crystals used in our experiments were the Nd:GdVO4 and Nd:YVO4, which were

grown at Beijing Ke-Gang Electro-optics company in China by the Czochralski technique and

had a comparable crystal quality. Nd:GdVO4 and Nd:YVO4 crystals were both polished and

antireflection-coated at both the pump and laser wavelength on two facets of each crystal. The

crystal was wrapped with indium foil and mounted in a copper heat-sink cooled by flowing-

water at 18°C, with good thermal contact between crystal and heat-sink. The a-cut cuboid

Nd:GdVO4 and Nd:YVO4 laser crystals employed had a same Nd3+

ion concentration of

0.5at.% and had a same dimensions of 4mm × 4mm × 8mm. The crystal was placed in a about

100mm long flat-flat resonator with an output coupler with a 35% transmissivity. The mirror

M1 coated with antireflection at 879nm and high reflection at 1064nm and served as the front

cavity mirror. A 879nm pumping source used in our experiments was a commercially

available high-power fiber-coupled diode-laser (NL-LDM-120-879, made by nLIGHT Inc.),

which had a top-hat intensity distribution, the FWHM of pumping radiation was less than

3nm. The pump light of diode-laser was imaged a spot of about 533µm into the crystal

through two aplanatic lenses. The A-O Q-switch (39041-50DSFPS, made by Gooch and

Housego Inc.) had antireflection-coating at 1064nm on both facets and the power of the radio-

frequency driver was 50W at 41MHz.

4. Experimental results and discussion

Experiments for Nd:GdVO4 and Nd:YVO4 were carried out under the same conditions,

respectively. End-pumped very highly efficient continuous-wave (CW) lasers system for

Nd:GdVO4 and Nd:YVO4 were obtained under direct diode pumping of the emitting level by

removing the A-O Q-switch from the laser resonator. The Nd:GdVO4 and Nd:YVO4 crystals

used in our experiment did not absorb efficiently the pump radiation. The amount of absorbed

pump power was determined by monitoring the transmitted pump power behind the laser

crystal. The absorption efficiency to pump radiation was about 55.0% for Nd:GdVO4 crystal

and 58.8% for Nd:YVO4 crystal, respectively. Figure 4 shows the CW output power as a



function of the incident pump power. The maximum multi-mode CW output powers were

22.2W and 23.5W for Nd:GdVO4 and Nd:YVO4 lasers, respectively. The beam quality factors

were measured with a beam propagation analyzer (M2-101, made by Spiricon Inc.). The M

2

factors at the maximum CW output power were measured as M2

x = 2.27, M2y = 2.30 for

Nd:GdVO4 laser and M2x = 2.28, M

2y = 2.32 for Nd:YVO4 laser, respectively. For Nd:GdVO4

laser, the maximum optical-to-optical efficiency and slope efficiency in the range of linear

output with respect to incident pump power are 38.1% and 60.7%, respectively. For Nd:YVO4

laser, they are 40.5% and 66.5%. The CW and average output power as a function of absorbed

pump power were also measured. As shown in Fig. 5, the optical-optical efficiency of CW

output power to absorbed pump power was about 69% and the slope efficiency achieved

about 75% for both Nd:GdVO4 and Nd:YVO4 lasers. The differences in laser performance of

Nd:GdVO4 and Nd:YVO4 crystals would be likely attributed to the different absorption

coefficient to pump light. It’s concluded that the CW laser performance of Nd:YVO4 crystal is

slightly superior to that of Nd:GdVO4 crystal under direct diode pumping of the emitting

level. However, both Nd:GdVO4 and Nd:YVO4 are efficient laser crystal for diode pumped

solid-state lasers under direct pumping of the emitting level.

10 20 30 40 50 600

5

10

15

20

25

Nd:GVO4 ηs∼60.7%

Nd:YVO4 ηs∼66.5%

CW

Ou

tpu

t P

ow

er

(W)

Incident Pump Power (W)

Fig. 4. CW output power versus incident pump power

0 5 10 15 20 25 30 350

5

10

15

20

25

0 5 10 15 20 25 30 350

5

10

15

20

25

b)


Avera

ge O

utp

ut P

ow

er

(W)

CW

Outp

ut

Pow

er

(W)


a)

Nd:GdVO4

Nd:YVO4

Linear Fit of Data_Nd:GdVO4

Linear Fit of Data_Nd:YVO4

Nd:GdVO4

Nd:YVO4



ηs~75%

ηs~67.6%

Fig. 5. Output power versus absorbed pump power a) CW output power b) average output

power



Stable Q-switched mode operations for Nd:GdVO4 and Nd:YVO4 were accomplished

with the A-O Q-switcher inserted into the resonator. In our experiments, the highest

repetition rate was only up to 100kHz due to the restriction of A-O Q-switch. At the repetition

rate of 100kHz, the comparison of average output power as a function of absorbed pump

power is also shown in Fig. 5. More than 20W average output power were obtained and the

slope efficiencies of average output power were nearly equal and above 67.6% for both

Nd:GdVO4 and Nd:YVO4 lasers. Meanwhile, the output ratios of Q-switching to free running

at 100 kHz were higher than 91% for both. It’s illuminated that Nd:GdVO4 and Nd:YVO4

were excellent laser crystals for Q-switching operation. Although the stimulated emission

cross-sections of two crystals are different, it has had little influence on their CW and average

output powers. We can see clearly that there is almost no remarkable difference on CW and

average output power between Nd:GdVO4 and Nd:YVO4 lasers. The optical-optical

efficiencies and the slope efficiencies to absorbed pump power for Nd:GdVO4 and Nd:YVO4

lasers are nearly equal.

20 30 40 50 60 70 80 90 100 1108

9

10

11

12

13

14

15

16

Puls

e W

idth

(ns)

Repetition Rate (kHz)

Nd:GdVO4

Nd:YVO4



Fig. 6. Pulse width versus repetition rate at the absorbed pump power of about 33W

At the absorbed pump power of about 33W, we studied the pulse width as a function of

the repetition rate. The pulse width was detected by a high-speed silicon photo-detector

(DET210, Thorlabs) and shown by a digitizing oscillograph (TDS3032B,Tektronix). Fig. 6

shows the comparative results of Nd:GdVO4 and Nd:YVO4 lasers. As seen in Fig. 6, the pulse

width keeps on lengthening linearly as the repetition rate increases. It’s explained that the gain

for each pulse is reduced when the repetition rate is increased, leading to increased pulse

width. There is a remarkable difference between the Nd:GdVO4 and the Nd:YVO4 lasers. The

pulse width of Nd:YVO4 laser is obviously shorter than that of Nd:GdVO4 laser at the same

repetition rate from 30kHz to 100kHz. The minimum pulse widths at the repetition rate of

100kHz were 15.6ns for Nd:GdVO4 laser and 12.1ns for Nd:YVO4 laser, respectively. Figure

7 Shows the temporal single pulse profile of Nd:GdVO4 and Nd:YVO4 lasers at the repetition

rate of 100kHz.The experimental pulse width as a function of the repetition rate is difference

from the theoretical simulation. It can be resulted from the assumption that both the inversion

population density and the photon density remain uniform across the transverse section of the

laser crystal. This assumption limits the accuracy in the computation of pulse width [26].

However, it has no influence on the comparison of Nd:GdVO4 and the Nd:YVO4 lasers.



a)a)

b)b)

Fig. 7. Temporal single pulse profile at 100kHz a) Nd:YVO4 laser b) Nd:GdVO4 laser

To further demonstrate the difference of the pulse performance of Nd:GdVO4 laser and

Nd:YVO4 laser with high repetition rates, we studied the pulse width as a function of

absorbed pump power at the repetition rate of 100kHz. The experimental results are shown in

Fig. 8. The pulse width decays approximately exponentially as the absorbed pump power

increases for either Nd:GdVO4 laser or Nd:YVO4 laser. But at the same absorbed pump

power, Nd:YVO4 laser can obtained shorter pulse width than Nd:GdVO4 laser. As the theory

expected, we also observed the difference on pulse width of Nd:GdVO4 and Nd:YVO4 lasers

with high repetition rates in our experiments. The results reveal that Nd:YVO4 crystal has

more capability to obtain shorter pulse width and higher peak power at high repetition rates

than Nd:GdVO4 crystal, even the repetition rate is much higher than 100kHz.

0 5 10 15 20 25 30 350

20

40

60

80

100

120

140

160

180

Pu

lse W

idth

(n

s)


f=100kHz

Nd:YVO4

Nd:GdVO4

Fig. 8. Pulse width versus absorbed pump power at the repetition rate of 100kHz

5. Conclusion

We compare the performance of A-O Q-switching Nd:GdVO4 and Nd:YVO4 lasers at the

repetition rate of 100kHz operated at 1.06µm under direct diode pumping of the upper laser

level. There is no remarkable difference on property of power-output for both Nd:GdVO4 and

Nd:YVO4 lasers. Both of them are proved to be very efficient and promising laser crystals for

diode pumped solid-state lasers under direct pumping of the emitting level. But when operated

with high repetition rates, Nd:YVO4 laser can obtain shorter pulse width than Nd:GdVO4

laser. It’s concluded that Nd:YVO4 is a more favorable gain medium when higher repetition

rates and shorter pulse width are desired, owning to its larger stimulated emission cross-

section related to higher single-pulse gain. We believe that an efficient laser system with short



pulse width at much higher repetition rates will be further realized by Nd:YVO4 crystal, direct

pump scheme and A-O Q-switch.

Acknowledgments

This work was supported by program of excellent team in Harbin Institute of Technology.



comparison on performance of acousto-optically q-switched nd:gdvo_4 and nd:yvo_4 lasers at high...

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