High power, microjoule-level diffraction-limited
picosecond oscillator based on Nd:GdVO
4
bulk crystal
JIE GUO,
1
WEI WANG,
1,2
HUA LIN,
1
AND XIAOYAN LIANG
1,
*
1
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences,
Shanghai 201800, China
2
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author: liangxy@siom.ac.cn
Received 7 January 2019; revised 14 February 2019; accepted 14 February 2019; posted 15 February 2019 (Doc. ID 356815);
published 19 March 2019
1.2 μJ pulses with average power of 9 W were directly generated from a passively mode-locked picosecond
oscillator based on a Nd:GdVO
4
bulk crystal. Short cavity operation in continuous wave and mode-locking re-
gimes was conducted first to confirm the resonator performance and proper alignment. With a carefully calibrated
q-preserving multi-pass cell inserted into the laser cavity, the cavity length of the original short cavity was
extended while the mode-matching condition was maintained fairly well. Compared with the short cavity, nearly
fivefold energy enhancement was achieved while the diffraction-limited beam quality was undisturbed. To the
best of our knowledge, this is the highest output power ever produced from a mode-locked oscillator based on a
single bulk crystal at a repetition rate below 10 MHz without cavity dumping.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000452
1. INTRODUCTION
Solid-state oscillators with repet ition rates on the order of several
megahertz possess numerous advantages over general oscillators
with 50–100 MHz repetition rat es [1–3]. Additional pulse pick-
ers or intracavity modulators can be applied to reduce repetition
rate for the latter, but they add complexity and cost. Without
resorting to any high-speed or high-voltage elements, i nsertion
of a multi-pass cell (MPC) offers yet a straightforward, cost-
effective alternative to increase cavity length and thus reduce rep-
etition rate [4–7]. I t also provides a simple method to scale up
pulse energy, hence suppressing Q-switching instabilities and
enhancing Kerr nonlinearity for low-average-power laser systems
[8–11]. If high average power is obtained at the same time, pulse
energy at the microjoule level can be achieved, resulting in much
higher peak power, which is also b eneficial for applications in non-
linear optics [12–15]. This type of MPC has already been success-
fully implemented in mode-locked thin-disk lasers. Sp aring the
complexity of further amplification, a pulse energy as high as
80 μJ was generated at a repetition rate of 3.03 MHz in a vacuum
environment [16]. Picosecond oscillators with enhanced pulse
energy up to the microjoule level can be directly used for some
industrial micromachining and dental ablation investigations
[17–19]. They can also serve as energetic seeds to simplify
amplifier systems such as eliminating bifurcation phenomena in
high-repetition-rate regenerative amplifiers [20], as well as facili-
tate direct energy extraction in multi-pass amplifiers [21].
With excellent optical and mechanical properties as well as
capacity to conveniently be pumped with laser diodes, neodym-
ium-doped vanadate laser crystals are one of the most ideal
series of gain media for compact solid-state picosecond lasers
with linearly polarized output. Such sources could be obtained
by dual-crystal configurations [22,23 ], but much more com-
plexity would be added. In practice, an oscillator built with
a single crystal is much more preferred as the robust seed of
a laser system. In 2003, three modes of repetition rates were
realized by a flexible extra-long MPC to produce 4.1 W at
4.1 MHz and 3.5 W at 1.5 MHz with a bulk Nd:YVO
4
crystal
[12]. By implementing a dual mode-locking technique involv-
ing both quadratic polarization switching and a semiconductor
saturable absorber mirror (SESAM), 1.5 W output power at
3.95 MHz was produced by a bulk Nd:GdVO
4
crystal in
2006 [24]. To seed a regenerative amplifier, an oscillator based
on a bulk Nd:LuVO
4
crystal with 7 W maximum output
power at 11.5 MHz was produced in 2016 [20]. Nd:GdVO
4
is an attractive applicant for high-power lasers because of its
considerably higher thermal conductivity, which makes it an
ideal combination by merits of Nd:YVO
4
and Nd:YAG [25].
In this work, we demonstrate a high-power microjoule-level
passively mode-locked picosecond oscillator with a single end-
pumped Nd:GdVO
4
crystal combined with a SESAM and a
q-preserving MPC. Based on the designing principles for large-
volume fundamental mode resonators [26], a mode-matching
452
Vol. 7, No. 4 / April 2019 / Photonics Research
Research Article
2327-9125/19/040452-05 Journal © 2019 Chinese Laser Press