        

Laser pulse chirped
from grating

Oscillator

Pulse stretcher

Pulse selector extracts
10Hz seed pulses thatare synchronized to a
10Hz pump laser

Pulse
selectorGain of 107
Amplified laser
pulse from OPCPA
Laser pulse
recompressed bygrating pair

Pump laser

Pump laser
pulse repeating
at 10Hz

1.0
Intensity (1=peak)

Laser pulse from oscillator
repeating at 40MHz

0.8
0.6
230fs

0.40.20.0

Preamplifier（OPCPA）

Pulse compressor               
At the JAEA Photo Medical Research Center (PMRC) weare developing a compact ion accelerator for cancer therapy
using a laser-driven plasma point source of ions. The highenergy ions needed to treat cancerous tissue at a typical depth
from the human body surface (~20cm) requires repeatedshort and intense laser pulses. Systems meeting theserequirements are typically of the “chirped pulse
amplification” (CPA) type which means that the pulses aretemporally stretched or ‘chirped’ to reduce intensity during
amplification and finally recompressed. High laser repetition
rates (100Hz or more) are desirable, in order to deliver
sufficient total dose within 10min. The limited ability of lasermaterials to dissipate heat has limited the acceleration of highenergy ions to large systems (~100m long) with low
repetition rates (a few shots a day). Also, though theserecompressed laser pulses are of short duration (< 1ps), they
may be preceded by lower power emissions (largely from
amplified spontaneous emission) that can reduce ion
acceleration, especially if preamplifiers are of theregenerative type (RA). A compact laser system that issuitable for cancer treatment has not yet been developed. To
increase repetition rate and reduce the prepulse energy, weare developing a new laser design (emission at 1030nm)
which will be pumped by high power 940nm laser diode(LD). As a first stage of this new development, we havedemonstrated the feasibility of a CPA system that uses a-400-2002000Time (fs)400            

different kind of preamplifier called an optical parametricchirped pulse amplifier (OPCPA).
Fig.11-5 is a diagram of our novel CPA laser system withwhich we have achieved amplification of laser pulses of shortduration at a wavelength of 1030nm. 10Hz low energy ‘seed’
pulses from an Yb doped oscillator source were amplified ina single pass through the OPCPA preamplifier unit to over
6.5mJ, representing a net energy gain of 107, achieved withcrystal of 56mm total length. Then, these high gain laser
pulses were recompressed to a 230fs duration as shown in
Fig.11-6. Formerly, under these conditions the recompressed
pulse amplitude using a regenerative preamplifier would havebeen limited to about 1.5-2 times larger than that of the laser
seed pulse from the oscillator, a well-known effect of gain
narrowing. For laser-driven ion acceleration the amplified
laser pulse from the OPCPA unit (not yet recompressed) will
be further amplified in the next stage in a thin Yb:YAGcrystal pumped by a high power LD at a 100Hz repetition
rate.
This work was supported by the Special Coordination
Fund (SCF) for Promoting Science and Technology managed
by the Ministry of Education, Culture, Sports, Science andTechnology of Japan (MEXT), and was done in collaboration
with Central Research Laboratory, Hamamatsu Photonics,
K.K..

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Suzuki, M. et al., Multi-Millijoule, Nonlinear Preamplifier for High Intensity Femtosecond Yb:YAG Chirped-Pulse Amplification Lasers at 1030 nm,Applied Physics B, vol.97, no.2, 2009, p.379-382, doi: 10.1007/s00340-009-3741-3.

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