Power scale-up of the diode-pumped actively Q-switched Nd : YVO4 Raman laser with an undoped YVO4 crystal as a Raman shifter

DOI: 10.1007/s00340-007-2648-0 Appl. Phys. B 88, 47–50 (2007)

Lasers and Optics

Applied Physics B

k.w. su

y.t. chang

y.f. chen

Power scale-up of the diode-pumped actively

Q-switched Nd:YVO

₄

Raman laser with

an undoped YVO

₄

crystal as a Raman shifter

Department of Electrophysics, National Chiao Tung University, F353, Engineering Building VI, No 1001, Ta Hsueh Rd, Hsinchu 300, Taiwan

Received: 18 December 2006/ Revised version: 9 February 2007

Published online: 15 May 2007 • © Springer-Verlag 2007

ABSTRACT

With an undoped YVO

crystal as a Raman shifter,

we substantially improved the reliability and the output

per-formance of an actively Q-switched 1176-nm Nd:YVO

Raman

laser. With an incident pump power of 18

.7 W, the average

power is greater than 2

.6 W at 80 kHz. The pulse width of the

pulse envelope is shorter then 5 ns with mode-locked

modula-tion. With an incident pump power of 12

.7 W, the pulse energy

and peak power is higher than 43

µJ and 14 kW at 40 kHz.

42.55.Ye; 42.55.Xi; 42.60.Gd

1

Introduction

Since the development of new Raman crystals

in the last decade [1–12], the stimulated Raman

scatter-ing (SRS) in crystal [13] has provided solid-state lasers

with an important way of operating in different

wave-lengths. The most commonly known materials for SRS are

Ba(NO

[14], LiIO

[15], KGd(WO

[16], PbWO

[17],

and BaWO

[5, 6, 18–22]. Moreover, for self-Raman lasers,

the materials such as Yb:KGd(WO

[23], Nd:KGd(WO

[24–31],

Nd:PbWO

[32],

Nd:GdxY

VO

[33, 34],

Nd:PbMoO

and Nd:SrMoO

[11, 12] have been reported.

A combination of their laser-emission and SRS properties

made these crystals appealing self-Raman media.

Neverthe-less, the Raman scattering was generated by host material.

Lasers could offer a number of advantages if the SRS could

be transferred from self-Raman media to additional undoped

crystals.

Nd-doped YVO

and GdVO

crystals, the acknowledged

useful gain media, were used in passively Q-switched (PQS)

and actively Q-switched (AQS) self-Raman lasers [34–39].

For instance, an AQS Nd:YVO

self-Raman laser

demon-strated the average power, pulse width and peak power

of 1

.5 W, 18 ns and 4.2 kW for the Stokes wavelength of

1176 nm

[36]. However, the issue of the filed-induced

crys-tal damage usually restricted the output powers in self-Raman

Q-switched lasers [39].

u Fax: +886-35 725230, E-mail: [email protected]

In this work, to our knowledge, we report a new design

of a diode-pumped AQS 1176-nm Nd:YVO

Raman laser

to increase the average power, repetition rate, peak power,

and damage threshold comprehensively. An undoped YVO

crystal is employed to be an intracavity Raman shifter in

a Nd:YVO

AQS laser. At an incident pump power of 18

.7 W,

the AQS Raman laser produces an average power greater than

2

.6 W with a pulse repetition rate of 80 kHz. The output pulses

noticeably display a mode-locking phenomenon that leads to

the maximum peak power to be higher than 14 kW.

2

Experimental setup

Figure 1 shows the experiment configuration for

AQS Nd:YVO

1176-nm

Raman laser which differs from

self-Raman laser. The pump source was an 808-nm

fiber-coupled laser diode with the core diameter of 800

µm, the

numerical aperture of 0.16, and the maximum output power

of 25 W. A focusing lens unit with a 85% coupling

effi-ciency was used to reimage the pump beam into the gain

medium with a pump spot radius of 400

µm. The gain

medium, a 9-mm-long a-cut Nd:YVO

crystal with low

con-centrations, 0

.25 at. %, was used to reduce thermally induced

fracture [36]. Both sides of this laser crystal were coated

for antireflection (AR) at 1

.06 µm (R < 0.2%). The Raman

crystal was a 9.6-mm-long a-cut undoped YVO

crystal.

These two crystals were both wrapped with indium foil and

mounted in water-cooled copper blocks individually. The

30-mm-long acousto-optic (AO) Q switch (NEOS

Technolo-gies) had AR coating at 1064 nm on both faces and was

driven at a 27

.12-MHz center frequency by 15 W of RF

power. The resonator was a plano-concave configuration.

Front mirror, a 500-mm radius-of-curvature concave mirror,

was coated with AR coating at 808 nm (R

< 0.2%) on the

en-trance face, and with high-reflection (HR) coating at 1064 nm

(R

> 99.8%) and high-transmission (HT) coating at 808 nm

(T

> 90%) on the other face. The coating of front mirror at

1176 nm

was high-reflection, too. The output coupler (OC)

was a flat mirror with HR coating at 1064 nm (R

> 99.8%) and

partial-reflection (PR) coating at 1176 nm (R

= 51%). The

cavity length was around 115 mm and depended on

pump-ing power. The spectrum of laser output was monitored by

an optical spectrum analyzer (Advantest Q8381A,

includ-48 Applied Physics B – Lasers and Optics

FIGURE 1 Schematic of a diode-pum-ped actively Q-switched Nd:YVO4

Ra-man laser at 1176 nm

ing a diffraction lattice monochromator) with a resolution

of 0

.1 nm. The temporal behaviors for fundamental and

Ra-man pulses were recorded by a LeCroy digital oscilloscope

(Wavepro 7100, 10 Gs

/s, 1-GHz bandwidth) with two fast

p-i-n photodiode ap-i-nd ap-i-n ip-i-nterferep-i-nce filter allowip-i-ng trap-i-nsmissiop-i-n

only at 1064 nm.

3

Experimental results and discussion

As a stimulated Raman material, taking the place

of Nd:YVO

by undoped YVO

has advantages on

robust-ness and output properties. Figure 2 displays the experimental

result for optical spectrum of the laser output and Raman

scat-tering spectrum of the YVO

. The first Stokes wavelength

near 1176 nm was conversed from the fundamental

wave-length near 1064 nm by Raman peak at 890 cm

. The Raman

FIGURE 2 Optical spectrum of the actively Q-switched Raman output. The Raman scattering spectrum of an YVO4crystal showed in inset, which is

almost the same as it of Nd:YVO4crystal

shift of Nd:YVO

and undoped YVO

were almost the same,

came from the same periodic YVO

lattice. But crystals

with-out dopant had more perfect lattice, which brought on higher

damage threshold and more stable frequency conversion. So,

we can put the pure YVO

as a Raman crystal in the position

where the intensity is highest in the cavity, and still increase

the pumping power. Further, the reflectance of OC can be

lower (from 93% to 51%) to scale up average output power

due to lower lasing threshold at Raman wavelength. By using

lower reflection coating, we can narrow the pulse width. At the

same time, when we over drove current during the experiment,

the damage never happened in Nd:YVO

, but in Raman

crys-tal. That means the SRS was generated mainly in pure YVO

,

a more reliable and replaceable component in practical laser.

Figures 3 and 4 illustrate the output performance of AQS

1176-nm

Nd:YVO

Raman laser. The average output power

at the Stokes wavelength of 1176 nm with respect to the

inci-FIGURE 3 The average output power at the Stokes wavelength of 1176 nm with respect to the incident pump power at different pulse repetition rate from 20 kHz to 80 kHz

SUet al. Power scale-up of the Q-switched Nd:YVO4Raman laser 49

FIGURE 4 An oscilloscope trace with mode-locking effect for fundamental and Raman pulses

dent pump power for different pulse repetition rate of 20, 40,

60, and 80 kHz shown in Fig. 3. Because the thermal loading

of the end-pumped Q-switched Nd-doped laser increases with

decreasing repetition rate [24, 25], it can be seen that although

the pumping threshold is higher, increasing the pulse

repeti-tion rate can efficiently increase the maximum average output

power at 1176 nm and its maximum pump power (P

And for a certain repetition rate, to pump over P

will

get the unstable Raman conversion and fall the output power.

The average output power is up to 2

.61 W with an incident

pump power of 18

.7 W at a repetition rate of 80 kHz,

corres-ponding to the conversion efficiency of 14% and slope

effi-ciency of 40%. Comparing to results of 1176-nm self-Raman

laser by use of Nd:YVO

with lower dopant concentration of

0

.2 at. % [36], this Raman laser still has the increase of ratio in

average power of 74% and in conversion efficiency of 0

.7%.

It could be better if we were able to use 0

.2 at. % Nd:YVO

and correctly AR-coated c-cut [41] YVO

in this Raman laser.

On the other hand, the maximum pulse energy is generally

greater then 40

µJ at repetition rate from 20 to 60 kHz, and up

to 43

.5 µJ at 40 kHz with an incident pump power of 12.7 W.

The maximum pulse energy at repetition rate of 80 kHz is

32

.6 µJ.

The typical time traces for fundamental and Raman pulses

are shown in Fig. 4. The pulse width is always shorter then

5 ns, but the effective pulse width is much shorter due to

mode-locked shape. With the pulse energy of 43

.5 µJ, the pulse

width of the pulse envelop in Fig. 4 is 4

.5 ns, and the peak

power of the pulse seen as a Gaussian shape should be 9

.7 kW.

However, after curve fitting for the mode-locked shape, the

peak power is enhanced to 14 kW, 1.45 times the 9

.7 kW. In

other words, the effective pulse width is around 3

.1 ns which is

much shorter than 18 ns of self-Raman laser [36]. Comparing

to 1176-nm Nd:YVO

AQS self-Raman laser of 4

.2 kW [36],

the output peak power was enhanced to be more than two

times. The present peak-power level was close to the results of

the PQS Nd:GdVO

self-Raman lasers in which the average

power, pulse width and peak power were found to be 83 mW,

500 ps, and 9

.2 kW at 1174 nm in [37], or 140 mW, 750 ps,

and 8

.4 kW at 1176 nm in [38].

4

Conclusions

In conclusion, with an undoped YVO

as a

stim-ulated Raman crystal, we substantially improve the

dam-age threshold, repetition rate, averdam-age output power, pulse

width, and the peak power of AQS Nd:YVO

Raman lasers

at 1176 nm. With an incident pump power of 18

.7 W, the

average power is 2

.6 W at 80 kHz correspond to the

optical-to-optical conversion efficiency of 14%. Coming with the

mode-locked pulse shape, the effective cavity dump of intracavity

SRS leads to peak power at 1176 nm that is generally greater

than 10

.5 kW at repetition rate from 20 to 80 kHz. With an

incident pump power of 12

.7 W, the pulse energy and peak

power is higher than 43

.5 µJ and 14 kW at 40 kHz.

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