Dual-wavelength synchronously mode-locked Tm-doped
bulk laser with terahertz frequency beating
Xinyang Liu (刘新阳)
1,†
, Xiaowen Li (李晓文)
2,†
, Shengzhi Zhao (赵圣之)
1
,
Kejian Yang (杨克建)
3,4,
*, Lei Guo (郭 磊)
1,3
, Tao Li (李 涛)
1
, Wenchao Qiao (乔文超)
1
,
Ming Li (李 明)
1,
**, Baitao Zhang (张百涛)
4
, Jingliang He (何京良)
4
, Lihe Zheng (郑丽和)
5
,
Liangbi Su (苏良碧)
5
, and Jun Xu (徐 军)
6
1
School of Information Science and Engineering, and Shandong Provincial Key Laboratory of Laser Technology and
Application, Shandong University, Qingdao 266237, China
2
Key Laboratory of Micro-Nano Measurement-Manipulation and Physics (Ministry of Education),
Department of Applied Physics, Beihang University, Beijing 100191, China
3
Shenzhen Research Institute of Shandong University, Shenzhen 518057, China
4
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China
5
Key Laboratory of Transparent and Opto-Functional Inorganic Materials, Artificial Crystal Research Center, Shanghai
Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
6
School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China
*Corresponding author: k.j.yang@sdu.edu.cn; **corresponding author: dabaojiu@126.com
Received April 11, 2019; accepted May 28, 2019; posted online July 29, 2019
A dual-wavelength synchronously mode-locked homogeneously broadened bulk laser operating at 1985.6 and
1989 nm is presented for the first time, to the best of our knowledge, which delivers a maximum output power
of 166 mW and a repetition rate of 85 MHz. The pulse duration was measured to be 16.8 ps by assuming a sech
2
pulse shape. The recorded autocorrelation trace showed frequency beating signals with an interval of 3.8 ps
and a full width at half-maximum duration of 2 ps, corresponding to an ultrahigh beating frequency of about
0.26 THz, which agrees well with the frequency difference of the emitted two spectral peaks. The results indicated
that such a kind of dual-wavelength mode-locked Tm:YAlO
3
laser could be potentially used for generating
terahertz radiations.
OCIS codes: 140.4050, 140.3580, 140.3070.
doi: 10.3788/COL201917.091401.
Dual-wavelength ultrafast lasers are gaining more and
more attention, owing to their exceptional properties of
synchronous emission of different-color beating signals
with ultrahigh beating frequency up to terah ertz (THz)
caused by optical beating. Such lasers are highly required
for applications needing very fast acquisition time such as
fluid dynamic measurements
[1]
. Additionally, pulsed lasers
with dual-wavelength output also play a vital role in gen-
erating THz waves, which have become a hot topic due
to their unique applications including interstellar and
intragalactic observation, T-ray imaging, and broad-band
secure communications
[2,3]
. Employing Ti
3þ
-doped sap-
phire and Nd
3þ
-doped crystals as gain media, dual-
wavelength synchronously mode-locked lasers operating
at ∼0.8 μm and ∼1 μm have been widely demonstrated.
Beating pulses with several hundred femtosecond pulse
duration and THz-level ultrahigh beating frequency have
been experimentally generated
[4–12]
. Compared with the
near-infrared laser emissions, ∼2 μm lasers are more strik-
ing due to their characteristics of eye safety and special
wavelength, which makes it gain numerous free space ap-
plications including coherent laser radar, gas sensing,
medical surgery, and pumping of optical parametric oscil-
lators (OPOs)
[13,14]
. Howeve r, very few reports on dual-
wavelength ultrafast pulsed lasers at 2 μ m have been
found. In 2015 and 2017, ultrafast beating pulses with
THz-level beating frequency were delivered from dual-
wavelength mode-locked Tm∶CaYAlO
4
(Tm:CYA) and
Tm,Y∶CaF
2
lasers, respectively
[15,16]
. Tm:CYA and
Tm,Y∶CaF
2
were disordered crystals, in which the fluo-
rescence spectra were inhomogeneously broadened and
therefore beneficial for synchronous dual-wavelength
operation. However, the disordered structure has detri-
mental impact on the thermal property of the crystals,
as described in Ref. [
17]; thus, mass disorder would
increase the thermal resistivity. For example, in our
previous work based on Tm,Y∶CaF
2
, a low damage
threshold of several kilowatts per centimeter (kW/cm
2
)
was observed
[18]
, which meant the broad emission spectra
were at the cost of decreasing the thermal conductivity. So
much more efforts should still be paid on exploring novel
dual-wavelength ultrafast 2 μm laser systems, especially
based on the crystals with excellent mechanical and ther-
mal properties.
A Tm-doped YAlO
3
(Tm:YAP) crystal, a widely inves-
tigated gain material, has many favorable features includ-
ing structural anisotropy, long Y-O bonds, and natural
birefringence, which makes it free of the harmful thermally
induced depolarization phenomenon
[19–21]
. Compared
with the Tm∶Y
3
Al
5
O
12
(Tm:YAG) crystal, the Tm:YAP
COL 17(9), 091401(2019) CHINESE OPTICS LETTERS September 2019
1671-7694/2019/091401(4) 091401-1 © 2019 Chinese Optics Letters