Proceedings of International Conference on Physics in ... - KEK

Figure 6: To reduce the laser waist size, 2-mirror cavity has to be concentric type. On the other hand, 4-mirror cavity is
confocal type.
However, in the 4-mirror cavity, effective focal length
(ft, fs) are different in tangential plane and sagittal plane,
and the difference causes astigmatism at the focal point. ft
and fs are expressed as
ft = ρ
cos θ (6)
2
fs = ρ
2 cos θ
where θ is reflection half angle <strong>of</strong> concave mirror. Because
<strong>of</strong> this astigmatism, laser pr<strong>of</strong>ile inside the 4-mirror cavity
will be ellipse.
To avoid the astigmatism, the cavity has to be three dimensional
configuration. 3D 4-mirror optical cavity generally
have a circular-polarization dependent property due to
the rotation <strong>of</strong> the image in the three-dimensional optical
path. A new method utilizing this property to obtain a differential
signal from the cavity resonance was proposed[2].
The differential signal can be used to lock an optical cavity
at a resonance peak.
We conducted experiment to obtain differential signal
using a 3D 4-mirror optical cavity testbench. The setup
is shown in Fig. 7. A linear polarization wave was injected
to the 4-mirror cavity. Then we measured the output <strong>of</strong> the
differential amplifier while scanning the cavity length. The
signal observed is shown in Fig. 8. The bottom line is the
signal <strong>of</strong> the photo-diode that measurred the transmission.
And it shows the points that the cavity’s resonance <strong>of</strong> right
and left-handed polarized light. The top and middle lines
are output <strong>of</strong> the differential amplifier. The signal crossed
zero at the point <strong>of</strong> resonace. It provides a good differential
signal for locking the cavity to one <strong>of</strong> the peaks <strong>of</strong>
resonance. Actually, we have succeeded to lock the optical
cavity on the resonance peak and switch the circularpolarization
peak. It means the polarization <strong>of</strong> the generated
photons by laser-Compton can be switched quickly.
Now, a 3D 4-mirror cavity are being designed in order to
be installed in the ATF during summer 2011. Then we will
cunduct the experiment <strong>of</strong> gamma-rays generation.
CONCULSION
In order to increase the number <strong>of</strong> generated photons by
laser-Compton scattering, it is necessary to reduce the laser
(7)
Figure 7: Setup to obtain the differential signal.
Figure 8: Signal <strong>of</strong> transmission and difference.
waist size. We performed gamma-rays generation experiment
using a 2-mirror cavity and observed 10.8 ± 0.1 photons/bunch.
The laser power enhanced by up to 760 times
and laser waist size is 30µm (1σ).
As the next step, a 4-mirror ring cavity with a threedimensional
(non-planar) configration is being constructed.
The cavity has unique property such that it only resonate
with circular polarized light. It allows us to utilize new
method to obtain feed back signal for the cavity stabilization
as well as fast polarization switching.
REFERENCES
[1] S. Miyoshi et al., Nucl. Inst. Meth. A 623 (2010) 576.
[2] Y. Honda et al., Opt. Commun. 282 (2009) 3108.