Generation of sub-150-fs, 100 nJ pulses from a low-cost
cavity-dumped Cr:LiSAF laser
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Citation
Umit Demirbas et al. "Generation of sub-150-fs, 100 nJ pulses
from a low-cost cavity-dumped Cr:LiSAF laser" Proceedings of
the Conference on Lasers and Electro-Optics (CLEO) and
Quantum Electronics and Laser Science Conference (QELS),
2010. © Copyright 2010 IEEE
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Detailed Terms
OSA / CLEO/QELS 2010
a1508_1.pdf
CMNN2.pdf
Generation of Sub-150-fs, 100 nJ Pulses from a Low-cost
Cavity-dumped Cr:LiSAF Laser
Umit Demirbas, Kyung-Han Hong, James G. Fujimoto, Alphan Sennaroglu, and Franz X. Kärtner
1
Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics,
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
jgfuji@mit.edu and kaertner@mit.edu
Abstract: We report a low-cost, cavity dumped Cr:LiSAF laser, generating 135-fs pulses at 825 nm, with 105 nJ
pulse energies and ~0.78 MW of peak power at 10 kHz, using only 600 mW of pump power.
2010 Optical Society of America
OCIS codes: (140.3460) Lasers; (140.3480) Lasers, diode pumped; (140.4050) Mode-locked lasers
Several application areas of ultrafast laser technology such as white-light generation, micromachining, and deep
multiphoton microscopy imaging, require high peak power laser sources. Compared to multipass cavity lasers or
complex amplifying schemes, cavity dumping is a relatively simple technique which can be used to scale up the
available peak powers from mode-locked laser oscillators. Cavity dumping has been successfully applied to increase the
pulse energies of Ti:Sapphire lasers [1-3], ytterbium-doped lasers [4] and neodymium-doped lasers [4]. However,
Ti:Sapphire lasers can not be directly diode-pumped, which increases the cost and complexity of the system. Also, for
the diode-pumped ytterbium-doped and neodymium-doped lasers, the obtainable pulsewidths are limited to hundreds of
femtoseconds to picoseconds [4]. As an alternative, Cr3+-doped colquiriite gain media can be directly diode pumped
around 650 nm, and have broad emission bandwidths around 800-850 nm, enabling 10-fs long pulse generation [5].
Moreover, direct-diode pumping of Cr:Colquiriite lasers also allows high electrical-to-optical conversion efficiencies
(~10%), compactness, and ease of use [5]. Pumping Cr:Colquiriites with four single-mode diodes, ~50-100 fs pulses
with ~1-2.5 nJ pulse energies and ~20 kW peak powers have been generated from standard ~100 MHz cavities [5],
where the obtained peak powers were limited by the total available pump power (~600 mW). Slightly higher peak
powers can be obtained with multimode diode pumping (~40 kW) [6], at the expense of increased complexity.
In this work, we report the first cavity-dumping experiments with a simple, low-cost, single-mode diode-pumped
Cr:Colquiriite laser. As the gain medium, we have chosen Cr:LiSAF among the Cr:Colquiriite family, since it has a
higher emission cross-section. This increases the gain and reduces Q-switching instabilities. The crystal was pumped by
four ~150-mW single-mode laser diodes at 660 nm, each costing only $150. A semiconductor saturable absorber mirror
(SESAM) [7] (also referred as saturable Bragg reflectors (SBR) [8]) was used for initiating and sustaining modelocking, making stable turn-key mode-locked operation possible. By cavity dumping at 10 kHz repetition rate, the laser
generated ~135 fs pulses at ~825 nm, with 105 nJ of pulse energy and ~0.78 MW of peak power. At higher dumping
rates approaching 1 MHz, the pulse energy was reduced to 62 nJ, due to the limitations imposed by Q-switching
instabilities. This study demonstrates that low-cost Cr:Colquiriite lasers have the potential to generate ~MW level peak
powers with very modest pump requirements.
HR
HR
D4
D2
f=75
HR
M1
M2
f=75
PBS
D1
PBS
GTI
D3
DCM
DCM
Output
DCM
Dumping rate = 10 kHz
HR
Cr:LiSAF
SESAM/SBR
100 ms
DCM
DCM
(a)
(b)
(c)
Fig. 1. (a) Schematic of the cavity dumped, single-mode diode-pumped Cr:LiSAF laser. PBS: polarizing beam splitting cube. (b) Measured dynamics
of intracavity pulse train at a dumping rate of 10 kHz. (c) Contrast ratio between the dumped pulse and neighboring pulses (>20:1).
Fig. 1 shows the schematic of the Cr:LiSAF laser. The 5-mm-long, 1.5% Cr:LiSAF crystal was pumped by four
linearly-polarized, AlGaInP single-mode diodes, and up to 600 mW of pump power was incident on the crystal. An
astigmatically-compensated, x-folded laser cavity, with curved dichroic mirrors (ROC=75 mm, ROC=radius of
curvature) (M1-M2 in Fig. 1(a)) was used in the laser experiments. A second Z-fold focus was created by use of 100
mm ROC mirrors, where we placed the ~3-mm thick, fused silica acousto-optic cavity dumper. The cavity dumper
(64380-SYN-9.5-2, Neos Technologies, Inc.) had a single-pass diffraction efficiency of ~30%, and was used in doublepass configuration to obtain 50-60% dumping efficiency. The dumped beam was picked up with a small metallic high
reflector after its second pass through the dumper. A 250 mm ROC curved mirror was used to focus onto the
978-1-55752-890-2/10/$26.00 ©2010 IEEE
OSA / CLEO/QELS 2010
a1508_1.pdf
CMNN2.pdf
SESAM/SBR, which initiated and sustained mode-locked operation. For soliton pulse shaping, negative dispersion was
introduced into the cavity with Gires–Tournois interferometer (GTI) and double-chirped mirrors (DCM). The estimated
total round-trip cavity dispersion was ~ -2250 fs2. We did not use any output coupler in the cavity in order to increase
the intracavity pulse energies. At an absorbed pump power of ~520 mW, the laser produced 135-fs pulses with an
average intracavity power of ~15 W at 80 MHz repetition rate (~190 nJ intracavity pulse energy).
Dumping
frequency
(kHz)
10
20
50
100
200
500
1000
Pulse
energy
(nJ)
105
100
93
83
74
64
62
Pulse
width
(fs)
~135
~135
~135
~137
~140
~146
~160
Average
power
(mW)
1.05
2.01
4.64
8.31
14.8
32
62
Peak
power
(kW)
778
741
689
615
548
427
354
Dumping
efficiency
(%)
55
53
49
44
39
34
33
Table 1: Summary of the cavity dumping results with the single-mode diode pumped Cr:LiSAF laser.
1
1
dumper off
SHG Intensity (au)
Intensity (au)
0.75
dumper off
200 kHz
500 kHz
1 MHz
0.5
0.25
0
815
820
825
Wavelength (nm)
830
835
0.75
200 kHz
500 kHz
1 MHz
0.5
0.25
0
-400
-200
0
200
400
Delay (fs)
Fig. 2. Measured optical spectra and second harmonic autocorrelation traces from the cavity dumped Cr:LiSAF laser at several dumping rates.
Table 1 summarizes the cavity dumping results. For repetition rates up to 100 kHz, dumping efficiencies of ~50%
and pulse energies of ~90-100 nJ could be obtained and the dumping had very little effect on laser dynamics. The
contrast ratio between the dumped output pulses and the neighboring pulses was greater than 20:1 (Fig. 1 (c)). The
highest pulse energy was 105 nJ, obtained at a repetition rate of 10 kHz. For this case, the pulse duration was ~135 fs,
corresponding to a peak power of 778 kW. Fig. 1 (b) shows the measured intracavity pulse train dynamics for a
dumping rate of 10 kHz, where we first see an overshoot of intracavity pulse energy which then relaxes back to steady
state within ~30 ms. At 50 kHz dumping rate (and above), the subsequent dumping event occurs even before the
transient from the current dumping has relaxed, and this requires the usage of a lower dumping rate (to prevent pulse to
pulse instability). Also, for dumping rates above 200 kHz, the pulse duration and spectrum also start to change
considerably because the dumping event is frequent enough to significantly change the intracavity laser dynamics (Fig.
2). Moreover, two photon absorption processes in the SESAM/SBR limited the obtainable pulse widths to ~135-fs, and
caused multiple-pulsing instabilities for shorter pulses. In summary, we have presented what is to our knowledge the
first demonstration of cavity dumping of a Cr:Colquiriite laser, demonstrated peak powers approaching ~MW level, and
discussed the limitations imposed by the SESAM/SBR-induced mode-locking dynamics.
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