22P117-P
Development of two color laser interferometer/polarimeter
２波長レーザー干渉・偏光計測の開発
Kazuo Kawahata1, Tsuyoshi Akiyama1, Kazuya Nakayama2, Shigeki Okajima2
川端一男1, 秋山毅志1，中山和也2，岡島茂樹2
1

National Institute for Fusion Science322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2Chubu University
Matsumoto-cho, Kasugai-shi, Aichi 487-8501, Japan
中部大学 〒487-8501 愛知県春日井市松本町
Two color laser diagnostics using THz laser sources are under development for a high performance
operation of the Large Helical Device and for future fusion devices such as ITER. So far, we haveachieved high power laser oscillation lines simultaneously oscillating at 57.2 μm and 47.7 μm byusing a twin optically-pumped CH3OD laser, and confirmed the original function, compensation ofmechanical vibration, of the two color laser interferometer. In this article, recent progress of the twocolor laser polari-interferometer will be presented.

1. Introduction
Measurement of the plasma current density
profile is indispensable for the so-called advanced
modes of tokamak operation. In ITER, a poloidal
polarimeter based on the Faraday effect of a
far-infrared laser beam passing though the plasma
has been designed1 to measure the profile. A
118.8-μm CH3OH laser line is proposed as a
probing light source for the polarimeter, since theCH3OH laser line is the shortest and high power far
infrared laser oscillation line among many
oscillation lines applied on fusion devices so far.
The Faraday rotation angles expected in ITER arelarge enough to be measured accurately. However,
the Cotton-Mouton effect is also large, and thenshould be taken into account for evaluation of therotation angle. Therefore, we have been developing
shorter wavelength laser oscillation lines2 around 50
μm, since the Faraday rotation angle is still large
enough and the Cotton-Mouton effect is smallenough. Another advantage of adapting 50 μm is toreduce the beam bending effect to a quarter of thatat 118.8 μm. Here, development of a two color laserinterferometer/polarimeter is described.
2. Two Color Laser Interferometer/Polarimeter
Figure 1 shows a schematic drawing of the twocolor laser interferometer3,4 combined with apolarimeter. The system consists of a twin
terahertz laser which simultaneously oscillates at
57.2 μm and 47.6 μm, beam splitters (BS), beamcombiners (BC), and phase/rotation detection

systems. In the phase detection part, Ge:Ga
photoconductors (D1 and D2) detect two color
beat signals having different beat frequencies.
Figure 2 shows the frequency spectrum of twocolor beat signals at 1.2 MHz for 57.2 μm and0.55 MHz for 47.6 μm. It can be seen in this figurethat the signal-to-noise ratio is excellent to beabout 40 dB. The interference signals detected areseparated electronically, and then introduced into
phase comparators for phase measurement. In theFaraday rotation detection part, one wavelength isselected by using a Fabry-Perot interferometer(Filter), and its polarization rotation is detectedwith a dual photo-elastic modulators (PEMs)5
made of highly resistive silicon.

Fig.1. Schematic drawing of a combined system of thetwo color laser interferometer/polarimeter.

(a)057 μm
48 μm(c)Fig. 2 Two color beat signals detected by the Ge:Ga
photoconductor. The spectrum analyser trace shows twocolor beat signals corresponding to a 57.2 μm beat of 1.2
MHz and a 47.6 μm beat of 0.55 MHz.

3. Simultaneous Measurements of A Phase Shiftand A Rotation Angle
In order to simulate a phase shift caused byplasma, we have applied a gas cell as is shown inFig. 3. The gas cell of 24 cm in length can bepumped down, and filled with air or other gasses
with known indices of refraction. Figure 4 showstime behaviors of pressure of a gas cell (a), phaseshifts of two color laser interferometers (b), andpolarization angle (c). (d) and (e) are the expanded
scale plots of the polarization angle and the phaseshift. The magnitude of the phase shift is close tothe value estimated by using the refractive index ofair (n = 1.0002726 in the condition of theatmospheric pressure and 18 degrees). Large
amplitude signals are seen in the traces (d) and (e),which are considered to be due to a nonlinear effectof the silicon windows of the gas cell caused by thepressure difference.

Polarization angle (deg.) Polarization angle (deg.)

10500 kHz/div.

(b)

00100

200

Time (s)1(d)

010-150607080Time (s)1(e)

100-1140

150
Time (s)160170Fig. 4. Time behaviors of (a) pressure of a gas cell, (b)phase shifts of two color laser interferometers, and (c)polarization angle. (d) and (e) are expanded scale plots ofthe polarization angle and the phase shift.

Acknowledgments
This work was supported in part by aGrant-in-Aid for Science Research from theJapanese Ministry of Education, Culture, Sports,
Science and Technology (Grant No. 22360393) andalso by the NIFS budget code of NIFS10ULHH004.
References[1] A. J. H. Donne, M. F. Graswinckel, M. Cavinato, L.
Giudicotti, E. Zill, G. Gil, H. R. Koslowski, P.
McCarthy, C. Nyhan, S. Prunty, M. Spillane, and C.
Walker, Rev. Sci. Instrum. 75 (2004)4964.
[2] S. Okajima, K. Nakayama, H. Tazawa, K. Kawahata,
K. Tanaka, T. Tokuzawa, Y. Ito, and K. MIzuno, Rev.
Sci. Instrum. 72 (2001)1094.
[3] K. Kawahata, T. Akiyama, K. Tanaka, S. Okajima,
and K. Nakayama, Rev. Sci. Instrum. 77,(2008)10E707.
[4] K. Kawahata, T. Akiyama, R. Pavlichenko, K.
Tanaka, T. Tokuzawa, Y. Ito, S. Okajima, K.
Nakayama, and K. Wood, Rev. Sci. Instrum. 77,(2006)10F132.
[5] Akiyama, K. Kawahata, S. Okajima, K. Nakayama,
and T. C. Oakberg, J. Plasma Fusion Res. 2 (2007)
S1113.

Fig. 3. A photograph of the bench testing of the twocolor laser interferometer/polarimeter.

Phase shift (fringe)

Polarization angle Phase shift(deg.)(fringe)-100
321200Phase shift (fringe)

Pressure(kPa)100