High-power Simultaneously Q-switched and Kerr-lens Mode-locked

Eye-safe Nd:YAP/YVO

Intracavity Raman Laser Based on Injection

Locking

Zaijun Chen

1,2

, Yumeng Liu

1,2

, Zhenqiang Chen

1,2

, Hao Yin

1,2

, Zhen Li

1,2

and Weidong Chen

Key Laboratory of Optoelectronic Information and Sensing Technologies,

Guangdong Higher Education Institutes, Guangdong, Guangzhou, 510632, China

Institute of Optoelectronic Engineering, Jinan University, Guangdong, Guangzhou, 510632, China

Key Laboratory of Optoelectronic Materials Chemistry and Physics, CAS, Fuzhou, 350002, China

Keywords: All-solid-state Laser, Raman Laser, Q-switched Kerr-lens Mode-locked.

Abstract: A multi-Watt, multi-GHz laser eye-safe laser was obtained by simultaneously Q-switched Kerr-lens mode-

locking and stimulated Raman scattering. The high power fundamental laser at 1342 nm was generated

efficiently with a side-pump Nd:YAP laser rod. The fundamental laser was captured by the intra-cavity

Raman cavity, and the mode beating of the fundamental laser and the Raman laser was maximized by

setting the suitable length relationship between the fundamental cavity and the Raman cavity. The output

performance of the Raman lasers with different output coupler were measured and analyzed. The maximum

average output power at 1526 nm was 3.5 W, the pulse duration was about 220 ps and the pulse repetition

rate was 0.64 GHz.

1 INTRODUCTION

Water has a large absorption coefficient in this

spectral region at the vicinity of 1.5μm, which can

prevent the energy from reaching the retina. High

power laser at eye-safe wavelength region are

widely used in medical treatment, remote sensing,

radar system and optoelectronic countermeasure.

Optical parametric oscillator (OPO) is one of the

most effective methods to generate high-power eye-

safe pulse laser (

Huang YP et al. 2009, Huang YJ et al.

2012, Chang HL et al. 2011, Huang JY et al. 2012).

Raman laser also has become another promising way

to explore laser at new wavelength region due to the

rapid development of crystalline Raman medium in

the recent several years. Generally, there are two

approaches to achieve Raman laser within the eye-

safe wavelength region, the laser operation on the

2nd or 3rd stokes pumped bylaser at 1.064 μm

(

Shpak et al. 2012, Sabella et al. 2011) from the





/

–I





/

transition of Nd

and the 1st stokes

pumped by laser at 1.3μm (Chang YT et al. 2009,

Chen XH et al. 2012) from the F





/

–I





/

transition. However, it's relatively difficult to obtain

a high-order stokes generation, because it has lower

conversion efficiency and strict requirements on

both of the mirror coating and the working

temperature control of the Raman crystals.

Kerr-lens mode-locking is a promising way to

obtain laser pulse with picoseconds or femtosecond

scale. Adding a Q-switch in the Kerr lens mode-

locking laser has dual functions: to trigger the Kerr-

lens effect (KLE) in the Raman crystal and to further

modulate the mode-locking pulse train. Q-switched

Kerr-lens mode locking (QML) is usually avoided in

the laser experiment because the high peak power it

generates may damage the elements in the resonate

cavity. However, the stable QML laser can be

achieved with critical resonator design. In 2013,

Chenlin Du (Huang GX et al. 2013) reported a

diode-end-pumped QML first-stokes laser operating

at 1175.9 nm. But rare stable diode-side-pumped

QML Raman laser publications were seen to the best

of the authors' knowledge.

With regard to the high power Raman laser, in the

recent two years, the diode side pumped modular

was applied popularly in the Raman laser. Zhang

X.Y. reported a diode-side-pumped nanosecond Q-

switched Nd:YAG/BaWO

(Shen et al. 2012) in

2014and Nd:Gd

/BaWO

high-order stokes

laser in 2013 (Shen et al. 2013). They also referred

157

Chen Z., Liu Y., Chen Z., Yin H., Li Z. and Chen W..

High-power Simultaneously Q-switched and Kerr-lens Mode-locked Eye-safe Nd:YAP/YVO4 Intracavity Raman Laser Based on Injection Locking.

DOI: 10.5220/0005254301570162

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 157-162

ISBN: 978-989-758-093-2

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

to the Q-switched self-mode-locking phenomenon.

Admittedly, diode-side-pump module may have

lower combining efficiency, but it also has many

irreplaceable advantages. Laser rod with big

dimension can be employed and the side-pump

module can provide higher pump power and better

water cooler, which are beneficial for obtaining high

power laser. Nd:YAP is an excellent side-pump laser

material at 1.3 μm (Zhu et al. 2007, Zhu et al 2009).

It possesses high thermal conductivity, excellent

optomechanical coefficient, and large product value

(330 ×10





μs) of stimulated emission cross

section and fluorescence lifetime at 1342 nm (from

R2-X1 intense overlapped stark transitions), which

contribute to the low threshold and high power laser

output. Cerny et al. (2004) referred to a passively Q-

switched BaWO

first-stokes nanosecond laser

pumped by Nd:YAP at 1.5 μm in 2004.

In this paper, a Q-switched Kerr-lens mode-

locked Raman eye-safe laser was obtained. The

Raman laser cavity and the fundamental cavity were

separated, and the fundamental laser at 1342 nm was

captured by the Raman cavity for efficient and stable

mode-locked Raman laser generation when the

frequency of two cavities matched. The maximum

output power was 3.5 W, pulse duration was 220 ps,

and pulse repetition rate was 0.64 GHz. The output

performance of the QML laser was measured and

analyzed.

2 EXPERIMENT DESIGN AND

SETUP

A line-resonator is adopted and the experiment setup

is sketched as Figure 1. The high pulse repetition

rate laser can be generated in narrow cavity, so the

separated Raman cavity is beneficial for obtaining

high pulse repetition rate of the Raman laser. The

side-pumped module (GT Optics, Beijing) consisting

of three pump units arranged in a three-fold

symmetry around the laser rod can provide a

maximum pump power of 250 W at 808 nm. A c-cut

Nd:YAP (grown by Fujian institute of research on

the structure of matter, Chinese academy of sciences)

with a dimension of ∅3 × 65mm



and a Nd

doping concentration of 0.9 at. % is set in the side-

pump system and cooled by water flow directly, with

both facets AR (R<0.2%) coated at 1342 nm. The

acousto-optical Q-switch (Gooch and Housego, QS-

027) with anti-reflection coating on the both facets

at 1342 nm provides a maximum modulation

frequency of 50 kHz. A c-cut YVO

crystal with

dimension of 3×3×29mm



is applied as Raman

gain media. It is wrapped with indium foil and

mounted in water-cooled copper blocks. Both facets

of the YVO

are coated with anti-reflection film at

1342 nm and 1525 nm. The Nd:YAP laser crystal, Q-

switch and YVO

crystal are water cooled to be

23 °C, 19 °C, and 17 °C, respectively.

Figure 1: Actively Q-switched mode-locking 1525 nm

laser setup.

The fundamental resonator is a plane-concave

configuration, which consists of a plane mirror M

and a concave mirror M

with a curvature radius of

500 mm. The concave mirror M

is used to

compensate the thermal lens effect of the laser

rod.M

is directly coated with high-reflectivity

dielectric film (HR@1342 nm, 1525 nm, R>99.8%),

and anti-reflectivity (AR@1079.5 nm) to suppress

the high-gain spectral line of Nd:YAP at 1079.5 nm.

Output couple (OC) M

is coated with high-

reflection (HR@1342 nm) for the fundamental wave,

antireflection (AR@1079.5 nm), and part-reflection

for the first-stokes wave (PR@1525 nm) output.

Two OCs with different reflectivity are applied to

carry out the experiment and the output performance

is compared. The transmissivity of OC-1 and OC-2

is 5.1% and 2%, respectively. Another mirror M

which is coated for antireflection (AR@1342 nm)

for the fundamental wave, but high reflectivity of the

fundamental wave (HR@1525nm, R>99.9%) was

set between side-pump module and the YVO

crystal

to separate the two resonators. The distance between

and M

is 37.35 cm and the length of the Raman

cavity is 19.72 cm.

The average output power (AOP) was measured

by a power meter (from Physcience Opto-

Electronics, Beijing). The temporal pulse profile of

lasers are received by a PIN photo detector with a

rising time 25 ps (EOT ET-3500) and displayed by a

oscilloscope with a bandwidth of 12 GHz and a

sample rate of 40 GHz/s (Agilent DSA91304A). The

emission spectra of the laser were monitored and

measured by a grating spectrum meter (Zolix Omni-

λ300) of a spectra range from 300 to 1700 nm.

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

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3 EXPERIMENT RESULT AND

ANALYSIS

3.1 Kerr-lens Mode Locking and

Injection Locking

The Q-switched mode-locked laser was generated by

modulating the fundamental Nd:YAP laser with the

Q-switch and the Kerr-lens effect of the Nd:YVO

The Raman laser was generated by capturing and

converting the energy from the fundamental cavity

to the Raman cavity and Raman laser. The Q-

switched laser pulse transformed into QML at the

suitable modulation frequency. In the generation of

the Q-switched pulse, the lower modulation

frequency leads to a narrower Q-switched pulse and

higher peak power. The KLE of the crystals

depended on the power intensity and the intensity-

dependent nonlinear index of refraction (Huang et al.

1992) in the Gaussian field is given by













πω



exp[−2







] (1)

where n



is the linear refractive index, n

is the

nonlinear index, P is the power, ω is the 1/e

amplitude beam radium. Nd:YVO

has a large

nonlinear refraction index (n

=4.7×10

−18

/W), and

is able to introduce a stronger self focusing of the

beam (Haus et al. 1992).The Kerr lens effect

affected by the power intensity generates a soft

aperture to modulate the Q-switched fundamental

laser pulse. The soft aperture has the function to

suppress the modes whose perturbation intensity is

too low. Thus, when repetition rate of the pulse

which is modulated by the KLE matches the

frequency separation of the resonator, QML laser

can be achieved. The QML Laser with OC-1 is

obtained at the modulation frequency range within

4.5 kHz to 16 kHz. The mode locking modulation

depth varies with the changing modulation

frequency, and reaches maximum at the modulation

frequency of 13.5 kHz. The laser with OC-2

transforms into QML with the modulation frequency

within 9 kHz to 12 kHz.

According to the properties of the Fabry-Perot cavity,

the frequency separation ∆ of the longitudinal

modes in the resonator is given by Haus (1978)

∆ =







(2)

Where 



is the optical length of the

resonator.And the oscillating waves in the cavity is a

series of eigenmodes with specific frequencies=

∆, where n is natural number n = 0,1,2,3⋯. The

Raman line width of YVO

is 2.6 cm

-1

(Piper et al,

2007), so the frequency gain bandwidth is 7.8 ×



Hz. The frequency separation of the Raman

resonator is 0.64 GHz, which indicates that every

fundamental longitudinal mode whose intensity is

higher than the Raman threshold will generate 121

first-stokes laser longitudinal modes. The

transmission peaks of the F-P resonator also has a

gain bandwidth, and its full width at half-maximum

(FWHM) is given by (Haus et al. 1992)

1/ 2

(1 )

opt

−

(3)

The FWHM of the transmission peak of the

Raman cavity at the output coupler (R=95%) is 2.0×



Hz, which is two degrees lower than the Raman

gain bandwidth. In this case, every longitudinal

mode whose power is over threshold will generate a

Raman sideband and the oscillating frequencies

depends on the characteristic frequency of the

Raman cavity. but the injection locking and pulling

occur in the process when the Raman cavity captures

the fundamental laser. The length relationship

between the two cavities is a crucial parameter for

the mode beating of the two cavities in the transient

process. The optical length of the fundamental cavity

is two times longer than the length of the Raman

cavity and after small adjustment was added in the

position of the mirror M2, the fundamental laser can

be damped by the Raman cavity efficiently and the

modulation depth increased.

Figure 2: Oscilloscope traces of the performances of the

fundamental laser and the first-stokes laser:(a) the first-

stokes laser at 1525 nm at PRF of 13.5 kHz with the time

span of 20 ns; (b) the first-stokes laser at 1525 nm at PRF

of 13.5 kHz with the time span of 2ns.

High-powerSimultaneouslyQ-switchedandKerr-lensMode-lockedEye-safeNd:YAP/YVO4IntracavityRamanLaser

BasedonInjectionLocking

159

The pulse of the QML Raman laser after adjustment

at the modulation frequency of 13.5 kHz was shown

in Figure 3. The repetition rate of the pulse beneath

the Q-switch envelop is 0.64 GHz, which accords

with the frequency separation of the Raman

resonator. However, the modulation depth cannot

reach 100% because there are some transverse

modes in the side pump laser system and the soft

aperture couldn’t suppress all these modes. The

pulse duration is 220 ps.

3.2 Output Power and Threshold

Pump Power

The output power of lasers with OC-1 and OC-2 at

different modulated frequency are measured and

shown as figure 3. The output power increases with

the increasing pump power and the slope efficiency

decreased at high pump region. The threshold of the

QML Raman laser with OC-1 and OC-2 is 78.3 W

and 63 W at 13.5 kHz, respectively.

Figure 3: AOP for the QML laser at 1525 nm with OC-1

as a function of the incident pump power at 808 nm at 5

kHz, 13.5 kHz and 16 kHz, AOP for the QML laser at

1525 nm with OC-2 at 13.5 kHz and 16 kHz.

According to the Raman threshold given by

Heuvel et al. (1992), The Raman threshold can be

reduced by the higher Raman gain coefficient,

higher pump laser beam quality, smaller effective

action area and larger effective length of gain

medium. The laser with OC-1 has higher power

threshold than that of laser with OC-2 because the

Raman laser cavity with OC-2 has lower loss than

that of cavity with OC-1, the higher transmissivity is

better for Raman laser intensity accumulation.

Though the threshold pump power of the Raman

crystal is the same, the laser with lower-reflectivity

OC has higher a Raman laser threshold.

The output power of the QML Raman laser with

OC-1 and OC-2 at 16 kHz, 13.5 kHz and laser with

OC-1 at 5kHz is shown in figure 3. The laser with

OC-1 has higher output power than that of laser with

OC-2. This is because the Raman conversion

efficiency does not vary too much in these two

cavities, but the transmissivity of OC-2 is too low

and large part of the laser power is confined in the

resonator after first-stokes generation. Thus, there is

a suitable transmissivity for obtaining higher output

power. The output power of laser with the same OC

also differs from the modulation frequency of Q-

switch. Under the same pump power, the output

power of the QML laser increases with the

decreasing modulation frequency because the

envelop of the Q-switched laser pulse is shorten

when the Q-switch modulation frequency decreases

(Chen et al. 2013), which leads to a higher peak

power of the fundamental laser and a higher Raman

conversion efficiency. The maximum average output

power of 3.5 W was obtained under the pump power

of 245 W at the Q-switch modulated frequency of 5

kHz.

3.3 Spectrum and Beam Profile

The spectrum of the output laser under low pump

power is shown in figure 4. The fundamental laser is

at 1342 nm and the Raman laser is at 1525 nm,

which reveals an 892 cm

-1

Raman shift. The output

laser at 1525 nm was received by a fluorescent

screen, and the picture of the beam spot is also

shown in figure 4.

Figure 4: Optical spectrums for the fundamental laser at

1342 nm and the Raman laser at 1525 nm and the beam

spot.

4 CONCLUSIONS

A stable LD side-pump narrow pulse laser

modulated by Q-switch and Kerr-lens effect was

achieved. A Nd:YAP laser rod is used to provide

high power fundamental laser at 1342 nm. The

fundamental resonator and the Raman resonator

were separated. The transmissivity of OC has

892 cm

-1

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

160

significant effect on the average output power. The

mode-locking modulation depth varies with Q-

switch modulating frequency changing. With fixed

Q-switch modulation frequency, the modulation

depth of the Raman laser depends on the frequency

beating of the two cavities.

In the further research, another mirror coated for

high reflectivity of the fundamental laser and anti-

reflectivity for the first stokes laser can be inserted

between the Raman crystal and the output coupler to

construct a longer Raman cavity. With appropriate

length relation between these two cavity, the

conversion efficiency can be increased.

ACKNOWLEDGEMENTS

This work was supported by a Grant from the

National Natural Science Foundation of China

(61475067,11404332)，Natural Science Foundation

of Guangdong Province (S2013040016819,

S2013040012601), and the technology innovation

foundation of educational commission of

Guangdong Province (2013KJCX0022).

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