High-power Simultaneously Q-switched and Kerr-lens Mode-locked
Eye-safe Nd:YAP/YVO
Intracavity Raman Laser Based on Injection
Zaijun Chen
, Yumeng Liu
, Zhenqiang Chen
, Hao Yin
, Zhen Li
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.
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
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
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
high-order stokes
laser in 2013 (Shen et al. 2013). They also referred
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
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 3×3×29mm
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
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
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.
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
] (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
/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)
∆ =
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
(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 )
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.
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
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
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
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
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.
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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