November 10, 2008 / Vol. 6, No. 11 / CHINESE OPTICS LETTERS 841
Laser pulse spectral shaping based on
electro-optic modulation
Yanhai Wang (
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, Jiangfeng Wang (
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, You’en Jiang (
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Yan Bao (
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, Xuechun Li (
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, and Zunqi Lin (
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National Laboratory of High Power Laser Physics, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800
2
College of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018
Received November 26, 2007
A new spectrum shaping method, based on electro-optic modulation, to alleviate gain narrowing in chirped
pulse amplification (CPA) system, is described and numerically simulated. Near-Fourier transform-limited
seed laser pulse is chirped linearly through optical stretcher. Then the chirp ed laser pulse is coupled into
integrated waveguide electro-optic modulator driven by an aperture-coupled-stripline (ACSL) electrical-
waveform generator, and the pulse shape and amplitude are shaped in t ime domain. Because of the direct
relationship between frequency interval and time interval of the linearly chirped pulse, the laser p ulse
spectrum is shaped correspondingly. Spectrum-shaping examples are modeled numerically to determine
the spectral resolution of this technique. The phase error introd uced in this method is also discussed.
OCIS codes: 140.3300, 300.6530, 350.2660.
doi: 10.3788/COL20080611.0841.
Chirped-pulse amplification (CPA) technique has been
used to create damage-free amplification of short-
duration optical pulses to terawatt and even petawatt
laser levels
[1]
. Successful amplification of ultra-
broadband optical pulses is limited by two effects: high-
order phase err ors in the amplification chain and gain
narrowing in the high-gain amplifying medium. The dis-
persive optical systems that are capable of controlling dis-
persion up to the fifth o rder have been demonstrated
[2]
.
In picosecond high power laser systems, the extent of
sp e ctral narrowing associated with gain na rrowing is of
the highest importance in pre-amplifiers, which typically
possess the highest gain in the laser system. Becaus e of
the direct relationship between frequency interval and
time interval of the linearly chirped pulses, gain narrow-
ing results in laser temporal narrowing. The broadba nd
seed pulses after spectral shaping can reduce time and
sp e ctrum narrowing in the regenerative amplifier, and
help extract more energy in the power amplifier. In
femtosecond hig h-peak-power laser systems, the spec-
tral s haping helps produce shortes t- duration hig h-peak-
power laser pulses.
Spectr al shaping techniques, such as intracavity
etalon, birefringent crystals, a nd prism-waveguide co u-
pler (PWC), have been adopted widely to broaden the
bandwidths of the amplified pulses effectively in high-
power pulsed lase r systems
[2−4]
. PWC spectral shaping
exp eriments have been demonstrated
[5,6]
. In this pa-
per, we report a new s pectral shaping scheme based
on electro-optic modulation of linearly chirped ultra-
short pulses. In a CPA system, because the seed optical
pulses are linearly chirped, there exists a one-to-one cor-
respondence between time duration and linearly chirped
frequency. So, if the temporal shapes of seed laser pulses
are modulated in time domain, their spectral shapes are
also changed correspondingly in fre quency domain. The
new spectral shaping scheme, put forward in this paper,
is base d on this one-to-one cor respondence. We focus on
the theory research and numerical simulation here.
The spectral shaping scheme is illustrated in Fig. 1.
Firstly, the seed laser pulses emitted from a mode-locked
Nd:glass laser are chirped linearly and stretched from
about 200 fs to 1 − 3 ns through a stretcher. So the
laser pulse freq uency is almost a linear function of time.
Secondly, the s tretched optical pulses are coupled into an
optical fiber, so that a single pulse is picked out in the
sampler which consists of a gate electr ical pulse genera-
tor and an integrated electro-optic waveguide modulator.
Thirdly, the single chirped optical pulse is divided into
two pulses by a fiber splitter. The s maller power pulse,
after amplification, is used to create a triggering elec-
trical pulse by the Si pho toconductive switch, to trigger
the electrical pulse generator. The other optical pulse
is sent to the second electro-optic waveguide modulator.
This optical path arrangement assures the high-precision
synchronization of the linearly chirped optical pulse
and the shaped electrical pulse (the root-mean-square
(RMS) time jitter is less than 5 ps)
[7]
. Finally, the lin-
early chirped pulse is shaped in the s e c ond waveguide
electro-optic modulator driven by an aperture-coupled-
Fig. 1. Spectral shaping scheme.
1671-7694/2008/110841-04
c
2008 Chinese Optics Letters