550 MHz carbon nanotube mode-locked femtosecond
Cr:YAG laser
Jun Wan Kim (金俊完)
1
, Sun Young Choi (崔善英)
2
, Won Tae Kim (金源泰)
2
,
Bong Joo Kang (姜奉周)
2
, Won Bae Cho (趙元培)
3
, Guang-Hoon Kim (金光勳)
1
,
and Fabian Rotermund (李相旻)
2,
*
1
Advanced Medical Device Research Division, Korea Electrotechnology Research Institute (KERI), Ansan 15588, Korea
2
Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea
3
BioMed Research Section, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Korea
*Corresponding author: rotermund@kaist.ac.kr
Received February 9, 2018; accepted April 19, 2018; posted online May 28, 2018
We demonstrate a femtosecond Cr:YAG laser mode-locked by a carbon nanotube saturable absorber mirror
(CNT-SAM) at a repetition rate of 550 MHz. By employing the CNT-SAM, which exhibits a modulation depth
of 0.51% and a saturation fluence of 28 μJ∕cm
2
at 1.5 μm, we achieved a compact bulk Cr:YAG laser with self-
starting mode-locked operation near 1.5 μm, delivering an average output power of up to 147 mW and a pulse
duration of 110 fs. To our knowledge, this system provides the highest repetition rate among reported CNT-SAM
mode-locked Cr:YAG lasers and the shortest pulse duration among saturable absorber mode-locked Cr:YAG
lasers with repetition rates above 500 MHz.
OCIS codes: 140.4050, 140.5680, 140.7090, 160.4236.
doi: 10.3788/COL201816.061404.
Femtosecond coherent sources with high repetition rates
operating near 1.5 μm have been widely investigated for
applications in optical communications, optical clocks,
analogue-to-digital conversion, electro-optical sampling,
and high signal-to-noise ratio nonlinear spectroscopy
[1–5]
.
Mode-locked fiber lasers can operate stably near 1.5 μm
in a compact cavity configuration
[6,7]
, but it is quite diffi-
cult to achieve a high average output power above
100 mW, a pulse duration as short as ∼100 fs, and a high
repetition rate above 500 MHz all at the same time be-
cause of the long cavity length and large nonlinearity of
the optical fiber. The Cr:YAG laser crystal, one of the rep-
resentative solid-state laser crystals emitting near the
1.5 μm wavelength, is able to simultaneously provide a
high average output power and a high repetition rate
with short pulse duratio n because of its broad gain band-
width, near 1.5 μm
[8,9]
. To date, most demonstrations have
been based on Kerr-lens mode locking, because the small
gain and low thermal conductivity of the Cr:YAG crystal
make the laser cavity susceptible to intracavity loss
[10–13]
.
There has only been one report of a high-repetition-rate
saturable absorber mode-locked Cr:YAG laser that
adopted a saturable Bragg reflector; it exhibited a rela-
tively long pulse duration of about 200 fs
[14]
.
Carbon nanotube saturable absorbers (CNT-SAs) have
been widely employed in a variety of bulk solid-state lasers
operating in the 0.8–2.1 μm spectral range because of their
broadband nonlinear optical characteristics, including low
saturation fluence, controllable modulation depth, and ul-
trafast recovery time
[15,16]
. For high-repetition-rate lasers,
the precision of these SA parameters is more important
because of the relatively low intracavity peak intensity
due to the short cavity length. Near the 1 μm wavelength
range, CNT-SAs with small modulation depths and
negligible nonsaturable losses turned out to be applicable
for high-repetition-rate mode-locked bulk solid-state
lasers
[17,18]
. Near the 1.5 μm wavelength range, a CNT-SA
that is inserted into the second intracavity of a Cr:YAG
laser also provides stable and self-mode-locked operation
at a repetition rate of 85 MHz and a pulse duration below
100 fs
[19,20]
. However, a CNT-SA mode-locked Cr:YAG laser
in a compact scheme with a high repetition rate of above
500 MHz has not been reported until now.
In this work, we demonstrate a CNT-SA mirror (CNT-
SAM) mode-locked femtosecond Cr:YAG laser above
500 MHz for the first time. By employing a CNT-SAM,
the stable and self-starting mode-locked laser delivers a
maximum output power of 147 mW and a 110 fs pulse du-
ration at 550 MHz. To our knowledge, this is the highest
repetition rate among CNT-SAM mode-locked Cr:YAG
lasers and the shortest pulse duration among saturable
absorber mode-locked Cr:YAG lasers with a repetition
rate above 500 MHz.
The fabrication process of the CNT-SAM is similar to the
one described in Ref. [
15]. We used commercially available
single-walled CNTs grown by employing the high-pressure
carbon monoxide (HiPCO) method. The CNT bundles dis-
tributed in the absorber layer show a broadband E
11
elec-
tronic transition located around 1.5 μm. As the first step,
the CNTs were dissolved in 1,2-dichlorobenzene (DCB)
with a concentration of 0.1 mg/mL and centrifuged. The
well-dispersed CNT solution was then mixed with poly-
methyl methacrylate (PMMA) at a volume ratio of 1∶1.
Finally, the CNT/PMMA mixture was directly spin-coated
onto a high-reflection plane dielectric mirror. At the end of
the fabrication process the prepared CNT-SAM was dried
COL 16(6), 061404(2018) CHINESE OPTICS LETTERS June 10, 2018
1671-7694/2018/061404(4) 061404-1 © 2018 Chinese Optics Letters