22P087-R
Saturated absorption spectra of Balmer-alpha line of atomic hydrogenobserved using a independently-tuned pump laser
独立ポンプレーザーを用いて観測した水素原子バルマーアルファ線の飽和吸収スペクトル
Shusuke Nishiyama1 , Motoshi Goto2 , Nader Sadeghi3 , and Koichi Sasaki1 ,西山 修輔1 , 後藤 基志 2 , サデギ ナダー 3 , 佐々木 浩一 1
1

Division of Quantum Science and Engineering, Hokkaido University,Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan1
北海道大学大学院工学研究院量子理工学部門, 〒 060-8628 札幌市北区北 13 条西 8 丁目
2National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan2核融合科学研究所, 〒 509-5292 岐阜県土岐市下石町 322-6
3LIPhy, Universite Joseph Fourier & CNRS, Grenoble, France
3
ヨゼフフーリエ大学グルノーブル校およびフランス国立科学研究センター，フランス

We developed a saturation spectroscopy system using an independently-tuned pump laser to investigate the absorption spectrum in the whole region of the Doppler-broadened Balmer-α line of atomic
hydrogen with Zeeman splitting. The observed saturation spectrum, which was different from thatobserved in ordinary saturation spectroscopy, was interpreted and discussed.
1. Introduction
In Large Helical Device (LHD) at National Institute for Fusion Science, the particle balance in theplasma edge region is interested to study the highperformance confinement. In helium plasmas, the location of ionization was investigated by analyzing thespectra of He I emission lines [1]. Because the magnetic field strength and direction are vary with the position and known in the LHD plasma, the Zeeman splitting of spectral lines carries the information of the position of excitation, and the position of excitation isalmost the same as that of ionization.
In hydrogen plasmas, the emission spectrum of theBalmer-α line was investigated [2]. The analysis of theZeeman profile was complicated because the Balmerα line consists of several fine structure lines with alarger Doppler broadening. Therefore, we developeda system of saturation spectroscopy at the Balmer-αline of atomic hydrogen [3]. Saturation spectroscopy
achieves a Doppler-free spectral resolution. Our saturation spectroscopy system resolved the Zeeman split
fine structure of Balmer-α lines. However, the existence of a broadband offset component was difficult tobe explained by the theory of saturation spectroscopy.
A possible mechanism for this broadband offsetcomponent is the change in the velocity of atomic hydrogen or the transportation in the velocity space. In
this work, we developed a saturation spectroscopy system with an independently-tuned pump laser to investigate the effect of the pump laser beam to the whole
region of Doppler-broadened spectra.

2. Experiment
The experimental apparatus is schematicallyshown in Fig. 1. The plasma source was a linear machine with a uniform magnetic field of 300 G along
the axis. The helical antenna was connected to an rf
power supply at 13.56MHz. The total length of theplasma was 60 cm. The light sources for saturation
spectroscopy were two tunable cw diode lasers. Theprobe laser was scanned for a range of 50 GHz in 5 ms.
A part of the probe beam was picked up to a FabryPérot spectrum analyzer to measure the scanning relative frequency. For the pump laser beam, a handmade
tunable diode laser was used as a master oscillator andthe master oscillator beam was injected into a diodelaser amplifier. The amplified pump laser beam wasapproximately 80 mW (200 mW maximum). A partof the pump laser beam was picked up to a wavemeter to monitor the rough wavelength. The wavelengthof the pump laser beam was fixed. In addition, partsof the pump and probe laser beams were introduced
into an optical heterodyne interferometer to detect thefrequency difference between the two laser beams.
The probe and pump laser beams were injected
into the plasma source on the chord of the cylindricalaxis of the plasma from the counter directions. The
pump beam was collimated and the two beams wereoverlapped carefully. The probe laser beam transmitted through the plasma was picked up using a beam
splitter, and was detected using an avalanche photo
diode (APD). The beat signal of the optical heterodyne

1.2

Pump onPump offbeat signal(a)Antenna

APD
PUMPBEAM
APD

0.8
0.6
0.4

Collimator

Spectrum
analyzer

Diode laser
amplifier

Diode laser
oscillator

0.2

Isolator

λ/2
Isolator

PROBE
BEAM
Isolator

Absorbance αl10-10Diode laser
oscillator-5051015

Relative frequency [GHz]Wavemeter

0.1

Saturation spectrum
beat signal(b)0.08

Figure 1: Experimental apparatus.
∆αl/αlinterferometer was detected using a fast APD. The signals from two APDs and the spectrum analyzer wererecorded using a digital oscilloscope.

0.06
0.04
0.02
03. Results
Absorption spectra with and without the pumplaser beam are shown in Fig. 2(a). The horizontal
axis shows the frequency difference from 656.2819
nm. The vertical axis shows the absorbance αℓ which
is obtained by the Lambert-Beer law αℓ = − ln(It /I0 ),where It and I0 are the transmitted and the incident
probe laser beam intensities, respectively. The dashedcurve in Fig. 2(a) shows the absorbance without thepump laser beam. This smooth spectrum is understoodas the overlap of Doppler-broadened fine-structurecomponents of the Balmer-α line with Zeeman splitting. The solid curve shows the absorbance with thepump laser beam. This spectrum had several dips because the pump laser beam caused saturation of absorption. The dotted line shows the amplitude of thebeat signal of the optical heterodyne interferometer.
The peak of the beat signal indicated the wavelengthof the pump laser beam.
The saturation spectrum, which is the differencebetween the solid and dashed curves divided by thesolid one, is shown in Fig.2(b). This saturation spectrum was different from a spectrum of ordinary saturation spectroscopy [3] because the frequency of thepump laser was fixed. The narrow negative peaks
around 2 and −7 GHz were caused by the instabilityof the probe laser, and the distortion of the baseline
was caused by incomplete compensation of the temporal fluctuation of the probe laser intensity and plasma
density.

-0.02-10-5051015

Relative frequency [GHz]

Figure 2: (a) Spectra of absorbance with and without
the pump laser, and (b) spectrum of saturation spectroscopy.
quency of the transition line is the midpoint between
the peaks of the saturation spectrum and the pumplaser frequency. Under this interpretation, the two intense peaks in Fig.2(b) were assigned as the transitionof 22 P3/2 −32 D5/2 , consisting of the Zeeman split eight
σ components. The amplitude of the valley between
the two peaks was not negligible. There are π components of the same transition line at the valley, while
the probe laser beam along the magnetic field should
not interact with the π components. A possible reason
for the considerable amplitude at the valley would be
the transportation in the velocity space. On the otherhand, the broadband offset component was not clearlyobserved in this experiment. However, it is difficult todistinguish the offset component from the baseline distortion. For further study, the proper compensations of
the fluctuations of the laser and the plasma are necessary. The detailed interpretation of the saturation spectrum will be discussed by referring spectra observed atother plasma conditions and pump laser frequencies.
References[1] M. Goto, and S. Morita : Phys. Rev. E 65 (2002)
026401.
[2] A. Iwamae, M. Hayakawa, M. Atake, T. Fujimoto, M.
4. Discussion
Goto, and S. Morita : Phys. Plasmas 12 (2005)
Because the pump laser frequency is fixed, the042501.
saturation peaks arise at the opposite side of each [3] K. Sasaki, R. Asakawa, M. Goto, N. Sadeghi : 22P085,
This conference.
Doppler-broadened transition line. Therefore the fre-