The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999
~Evaluation of Optical Resonator for the JAERI Far-Infrared Free Electron Laser

Ryoji NAGAI, Masaru SAWAMURA, Ryoichi HAJIMA, Nobuhiro KIKUZAWA, Nobuyuki NISHIMORI,Toshiyuki SHIZUMA and Eisuke MINEHARA
Free-Electron Laser Laboratory, Advanced Photon Research Center, JAERI
2-4, Shirakata-Shirane, Tokai, lbaraki 319-1195, Japan
iterative calculation procedure used by Fox and Li[7].

Abstract

The performance of optical resonator withoutput coupling for the JAERI far infrared free electronlaser is evaluated by numerical calculating of mode profile,diffraction loss and output coupling efficiency. The
optical resonator is in near concentric geometry and 14.4 m
length. The evaluated configurations are output coupled
by a center hole and ring-shaped mirror at downstream side
mirror of optical resonator.
It is found that thering-shaped mirror output coupler is more efficient than thecenter hole, mode degeneracy in both of them is not somuch and the output coupler can provide satisfactory
performance at JAERI far-infrared free-electron laser.

1.

Introduction

A free electron laser based on a superconducting rflinac has been lased in far-infrared region and achieved
100 W class lasing at JAERI (Japan Atomic EnergyResearch Institute)[!]. Improvements for the 1 kW classlasing of the FEL system are under progress.
To realize
a high power and high efficiency FEL system, output
coupling of the optical resonator is done efficiently. In
infrared region, FEL optical beam is output coupled by
cutting away of a part of the optical beam of inside theresonator. Diffraction loss may rise up due to cut away ofthe optical beam. For the high power far-infrared region
FEL, diffraction loss of the optical resonator is one of themost important parameters. The FEL system performanceis degraded by the diffraction loss at the mirror edges andapertures. Dominant part of the loss is based on distortionby the output coupling. So, reducing of the distortion by
the output coupling has an effect on improvement of theFEL system performance.
In the first lasing experiment at JAERI FEL[2-5], theoptical resonator system of the FEL was near concentricgeometry with an insertable scraper mirror output coupler.
This coupler allows easy adjustment of the output coupling
but the mode quality of inside the resonator can be poor.
In case of over a few percent output coupling, there is a lotof mode degeneracy[6]. So, this coupler is not adapted
for high power and high efficiency FEL system. Then acenter hole and a ring-shaped mirror output coupling
configurations are investigated in this study.
In the output coupling configurations, the mode
profile of inside the resonator and diffraction loss at themirror edges and apertures are calculated by using the2. Calculations and results

A dominant eigenmode in the JAERI FEL opticalresonator is calculated using Fox-Li procedure at optical
wavelength 30 ~-tm. The total loss per round trip andoutput coupling efficiency are also evaluated. The total
loss means an amount of the diffraction and output
coupling, not contains reflection loss of the mirror. The
output coupling configurations and the nominal
configuration of the JAERI FEL optical resonator are
shown schematically in fig.l and 2. The Calculation
model contains two perfectly refrecting mirrors and fourapertures. The effect of the undulator duct and bending
magnet ducts is taken into account by the four apertures atboth end of the undulator and outer side of the bending
magnet ducts. The optical resonator for the JAERI FELisin near concentric configuration, with the parameters listed
in table 1.

-379-

OutputCenter hole(a) Center hole output coupling(b) Ring-shaped mirror output coupling

Fig. 1. Output coupling configurations.

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

R=lmm

14.4m

R=Smm

R=10mmFig. 3. Typical results of the transverse intensity
profile at the output coupling side mirror for center hole
output coupling resonator.

Aperture of the bending duct

Fig. 2. The nominal configuration of the JAERI FEL
optical resonator.

Table 1. Resonator Parameters for JAERI FEL
Length of the resonator14.4 mMirror radius

120mm

Courverture of the mirror

7.34m

Undurator duct length2m

Aperture of the undulator duct

15X56 mm

Aperture of upstream bending duct

45.5X35.5 mm

Aperture of downstream bending duct

35.5 X 45.5 mm

Distance of the undulator aperture to
upstream bending aperture
Distance of the undulator aperture todownstream bending aperture
Distance of the bending aperture to
upstream mirror
Distance of the bending aperture todownstream mirror

R=40mm

R=15mm

R=25mmFig. 4. Typical results of the transverse intensity. profile at the output coupling side mirror for ring-shaped
mirror output coupling resonator.

80,.-..,*.s 70'-'.Oil

1.11 m]()1.25 m

I

5.09m

60

50
40.l..~"'0f.i 30"'"'.Q

4.95mc;J

Typical results of the transverse intensity profile
calculated for each output configuration are shown in fig.3
and 4 at the location of output coupling side mirror. As
shown in fig.3 and 4, the mode degeneracy due to output
coupling is not so much for the each output configuration.
In fig.S, the total loss per round trip and the amount ofpower coupled through the center hole on downstream side
mirror are shown as a function of the center hole radius.
In fig.6, the total loss per round trip and the amount ofpower coupled through the ring-shaped mirror in front ofdownstream side mirror are shown as a function of theinner radius of the ring-shaped mirror. The round trip loss
and the amount of output coupling are monotonouslyvaried in fig.S and 6. So, the mode degeneracy due tooutput coupling is not so much. If there is a lot of modedegeneracy, output coupling is discontinuously varied infig.S and 6.

I.....······•./_...··

20.............

~ 10//Output coupling...................··•···•

00

123456789 10

Radius of the center hole (mm)Fig. 5. Total loss per round trip and the amount ofpower coupled through the center hole on downstream sidemirror are shown as a function of the center hole radius.

-380-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

60

40

mirror output coupler optical resonator have shown that theresonator can provide satisfactory performance at JAERI
far-infrared free-electron laser. Because of there is not somuch mode degeneracy in the both case. It is found thatthe ring-shaped mirror output coupler is more efficient thanthe center hole.

30

References,..-._~.._.,.sOll

50'E.

5.§.
....u::s0"0§.s"'"'

[1]

20
[2]

<il~

10(3]
[4]

1015

20

25

30

3540
[5]

finer radius of the ring-shaped mirror (mm)Fig. 6. Total loss per round trip and the amount of power
coupled through the ring-shaped mirror in front ofdownstream side mirror are shown as a function of theinner radius of the ring-shaped mirror.

[6]

[7]

Output coupling efficiency defined to be the ratio of
the power through the output coupler to the total loss isshown as a function of the total loss in fig. 7. The output
coupling efficiency of the ring-shaped mirror coupler isbetter than the center hole coupler.

9080,..-._~.._.,>.
u·o~
1:5<l)

70
6050Oll.s'E.

40

8 30~

S<
20::s0

1000

1020

30

40

50

60

7080

Total loss (%)
Fig. 7. Output coupling efficiency defined to be theratio of the power through the output coupler to the total
loss is shown as a function of the total loss.

3. Conclusion
The performances of center hole and ring-shaped-381-

E. Minehara, et al.: Nucl. Instr. and Meth. A 429(1999) 9-11.
E. Minehara, et al.: Nucl. Instr. and Meth. A331
(1993) 276.
E. Minehara, et al.: Nucl. Instr. and Meth. A318
(1992) 127.
R. Nagai, et al.: Nucl. Instr. and Meth. A358 (1995)
403.
Minehara, et al.: Proceedings of the Particle
Accelerator Conference, Dallas, 1995, pp. 159-161.
R. Nagai, et al.: Proceedings of the 24th Linear
Accelerator Meeting in Japan, Sapporo, 1999, pp.
276-277.
G Fox and T. Li: The Bell System Technical Journal
40 (1961) 45.