J. Plasma Fusion Res. SERIES, Vol. 8 (2009)

Development of high resolution LIF spectroscopy
with saturated absorption spectrum
Shuzo ETOH, Mitsutoshi ARAMAKI 1) , Kohei OGIWARA, Shinji YOSHIMURA2)
and Masayoshi Y. TANAKA3)
1)

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
Department of Electrical Engineering and Computer Science, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
2)
National Institute for Fusion Science, Toki 509-5292, Japan3)
Department of High Energy Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580, Japan(Received: 18 September 2008 / Accepted: 26 December 2008)

High resolution flow velocity measurement of neutrals has been performed by using a laser induced fluorescence (LIF) system installed with a saturated absorption spectroscopy unit. The improvement of accuracy isachieved by utilizing Lamb dip as the reference of laser wavelength. The newly-developed LIF system can determine neutral flow velocity with an accuracy of 2 m/s, which is one order of magnitude higher than the velocity
resolution of LIF systems with ordinary calibration technique utilizing iodine absorption spectrum.
Keywords: saturated absorption spectroscopy, laser induced fluorescence, laser wavelength calibration,neutral flow, high resolution velocity measurement, ECR plasma.

1. Introduction

performance and LIF measurements are presented in Sec.
3. The concluding remarks are given in Sec. 4.

Recently, it has been found that neutral particles play
an important role in phenomena of high density plasmas.
Neutral heating and neutral depletion have been observedin a high density plasma for chemical vapor deposition,
and affect the characteristic behavior of the plasma. [1, 2]
It is also found that a vortex structure, which rotates in
the opposite direction to E × B drift, occurs in a highdensity argon plasma produced by electron cyclotron resonance (ECR) heating. These phenomena are considered tobe caused by momentum transfer through charge exchangeinteraction between the neutral particles and ions. [3, 4]
The measurement of neutral flow velocity is becoming animportant issue and demands a high velocity resolution.
Although the role of neutral flow on dynamical behaviorof ions commonly has been attracting much attention, theflow velocity measurement of neutrals has not been carriedout so far except for a fast flowing case. [5]Laser induced fluorescence (LIF) method [6] is usually adopted to measure ion flow velocities of the order ofseveral km/s. For neutral flow measurement, on the other
hand, a high resolution LIF system is needed to measureslow neutral flow velocities of the order of 10 m/s, which
has not been developed so far because system stability andprecise calibration method are needed to achieve very highperformance in velocity resolution.
We have developed a high resolution LIF spectroscopy system utilizing saturated absorption spectroscopy and measured a neutral flow velocity with an accuracy of 2 m/s, demonstrating that the newly-developed
LIF spectroscopy system is a very powerful tool for studying the interaction of ion and neutral flow.
In Sec. 2, the experimental apparatus and the newlydeveloped LIF system are shown. The results of system

2. Experimental setup and LIF system
The experiments have been performed in the high density plasma experiment (HYPER-I) device at National Institute for Fusion Science, which is shown in Fig. 1. [7]The vacuum vessel is 30 cm in diameter and 200 cm inaxial length. The magnetic field line is weakly diverging
and ECR point is located 120 cm away from the open endof the device connected to the waveguide. The plasma is
produced by electron cyclotron resonance heating using a
microwave of 2.45 GHz frequency. The filling gases pressure is 10 mTorr (Ar) and input microwave power is 250
W. The typical electron density is 4.8 × 1010 cm−3 and theelectron temperature is 3 eV. The position of neutral flow
measurement is located more than 20 cm away from the
ECR region.

Fig. 1 Schematic of HYPER-I device.

author’s e-mail: syu-eto@aees.kyushu-u.ac.jp

20©2009 by The Japan Society of Plasma
Science and Nuclear Fusion Research

S. Etoh et al., Development of High Resolution LIF Spectroscopy with Saturated Absorption Spectrum

The LIF spectroscopy system used in the present experiment is shown in Fig. 2. A narrowband tunable diodelaser (696.73 nm) is used to pump Ar metastable atoms(4s[3/2]o2 ) to an upper energy level (4p� [1/2]1 ). The fluorescence photons of 826.67 nm in the 4s� [1/2]1 −4p� [1/2]1transition are collected by a lens, detected by a photo
multiplier tube (PMT), and amplified by a lock-in amplifier. The similar excitation scheme of Ar (4s[3/2]o2 −4p[3/2]1 ) is possible using a 772.59 nm diode laser. [8]The wavelength of the fluorescence photons is 810.59 nm(4s[3/2]o2 − 4p[3/2]1 ) and is very close to another Ar I line
(4s[3/2]o1 − 4p[3/2]1 ), located at 811.75 nm. [9] The distinction between these two lines is difficult when the fluorescence photons are detected by a PMT. On the other
hand, in the scheme adopted in the present experiment, thefluorescence photons are emitted in a wavelength of 826.67
nm, around which there is no Ar I line. [10] As a result, thepresent LIF scheme will provide a higher signal to noise
ratio.
The spectral bandwidth of the laser is 5 fm and the
maximum output power is 15 mW. The spatial measurement is performed by moving the collection optics along
the radial direction. A narrow filter with the center frequency of 825 ± 5 nm is set in front of the PMT. Since the
LIF spectrum is proportional to the velocity distributionfunction of neutral particles, the neutral flow velocity canbe determined by the Doppler shift of the LIF spectrum.
To obtain the exact value of Doppler shift, the laser wavelength must be precisely calibrated. For this purpose, theabsolute laser wavelength is determined by using a Fabry
Perot resonator and saturated absorption spectrum. The
laser beam is divided into two sub beams, one is introduced
into the Fabry Perot resonator. The output fringes, which
consist of series of pulse with constant frequency interval(Δ f = 294 MHz), is used for the reference of wavelengthscale when scanning the laser frequency. Another beam isintroduced to the vacuum vessel and the transmitted beamis reflected by a mirror. The reflected laser beam reenteredalong the same optical path and is detected to obtain thesaturated absorption spectrum.

Saturated absorption spectrum is a Doppler broadened spectrum superposed with a natural broadened spectrum. [11] So-called Lamb dip appears on the absorption
spectrum when the laser frequency is exactly the same as
the resonant frequency of the medium at rest. When thelaser frequency ω is not equal to resonance frequency ω0 ,the incident wave is absorbed by the atoms with a velocity v = (ω − ω0 )/k, and the reflected wave by the atoms
v = −(ω−ω0 )/k, where k is wavenumber of the laser beam.
When ω = ω0 , the atoms with v = 0 are excited for both theincident and reflected beams. Since the lower level atoms
is excited to an upper level by the incident laser, “burnout”
of lower level atoms takes place for the reflected beam(probe beam) to be absorbed. Then, the absorption coefficient reduces for the reflected beam, indicating a sharp
peak at frequency ω = ω0 , which is called Lamb dip. The
saturated absorption coefficient α s (ω) is given by����
(1 + S )(γ/2)2
α s (ω)
S
1+
,
= 1−
α(ω)
2
(ω − ω0 )2 + (1 + S )(γ/2)2(1)
where α(ω), S , γ are the absorption coefficient, the saturation parameter and the natural linewidth, respectively. The
profile of saturated absorption spectrum depends on laser
intensity since S is a function of laser intensity and resonance frequency. The quantity α(ω) is a Maxwellian distribution function corresponding Doppler broadening due
to thermal motion.
When the intensity of reflected beam is weak (S � 1),
α s (ω) profile becomes��(γ s /2)2
α s (ω)
S0
, (2)
= 1−
α(ω)
2 (ω − ω0 )2 + ((γ s + γ)/4)2√
where γ s is given by γ s = γ 1 + S . [12] The α s (ω) profile
is approximated by a convolution of Maxwellian distribution function and Lorentz distribution function. The full
width at half maximum (FWHM) of Lamb dip is much narrower than that of Doppler broadening. Therefore, Lambdip is an useful reference for calibrating the laser wavelength. An absorption line of iodine is frequently usedas the wavelength reference, however, it is subjected to
Doppler broadening due to iodine temperature, which isabout two order of magnitude wider than the width of
Lamb dip ( ∼ 0.1 pm ). The reflected laser beam is attenuated to 1 % of incident beam by a neutral density (ND)
filter in the present experiment.

Fig. 2 LIF spectroscopy system.

The electron density profile is measured by a Langmuir probe. The diameter and length of the tip are 1.5 mm
and 0.1 mm, respectively. The probe is movable in the radial direction (-15 cm < x < 15 cm).

21

S. Etoh et al., Development of High Resolution LIF Spectroscopy with Saturated Absorption Spectrum

3. Experimental Results
A typical example of LIF and saturated absorption
spectra are shown in Fig. 3. There are three sharp peaksin the absorption spectrum (Lamb dip). The FWHM ofabsorption spectrum is much larger than that of Lambdip. The stark broadening, pressure broadening and power
broadening are considered to be negligible. [13] The widthof Lamb dip is due to natural broadening. The three components of Lamb dip are attributable to the anomalous Zeeman Effect. It is emphasized that the Lamb dip in theabsorption spectrum occurs at the fixed wavelength corresponding the resonant frequency (ω0 ) in the rest frame
regardless of the experimental conditions, and is highly
suitable as a wavelength standard. On the other hand,
the LIF spectrum is shifted due to average motion of neutrals (Doppler shift), from which the flow velocity is determined. The LIF spectrum exhibits a Maxwellian distribution with a temperature of 0.033 eV.

Fig. 4 Lamb dip component (a) and the saturated absorption
spectrum (b).

Fig. 3 LIF and saturated absorption spectra. There are three
Lamb dips at the resonant wavelength (bottom of the absorption spectrum).

Figure 4 shows the magnified view of central Lambdip component. The profile of saturated absorption spectrum is well fitted by eq (2), as seen in Fig. 4(a). The
FWHM of Lamb dip is 0.056 pm and exhibits the naturalline width. The π transitions are not shifted from the central wavelength of an absorption spectrum and are linearlypolarized along the external magnetic field. The Doppler
broadening and the natural line width are 1.3 pm and 0.075
pm in the experimental condition, respectively. [14, 15]
The schematic of the excitation diagram is shown in
Fig. 5. Since the polarization of the laser beam is parallelto the external magnetic field, only the π component is considered in this scheme. As seen in the figure, the magnetic
quantum number of lower and upper levels are 3/2 and 1/2,
respectively, and thus the number of π component is three.
Consequently, three Lamb dips (peaks) appear in the saturated absorption spectrum. We take the center peak as
the wavelength reference to determine the absolute value

Fig. 5 Excitation diagram of 4s[3/2]o2 π component including
anomalous Zeeman effect.

of Doppler shift.
To realize high resolution Doppler spectroscopy, it isalso important to achieve stability of the system. We haverepeated the same LIF measurements under the constantexperimental conditions and confirm the reproducibility of
LIF spectrum. Figure 6 shows the LIF peak positions inwavelength measured during 5 hours. The magnitude of
deviation means the stability of the LIF spectroscopy system. The shadowed area in Fig. 6 shows the laser line
width and is the limit of accuracy of the system. The
LIF peak deviation determined by iodine absorption line
(696.74 nm) is plotted as reference data to be compared22

S. Etoh et al., Development of High Resolution LIF Spectroscopy with Saturated Absorption Spectrum

with. An iodine cell, which was heated up to 90 degrees
Celsius by silicon rubber heater, was used in the experiment.The LIF peak deviation corresponds to the total accuracy of the system, because the deviation is the sum offluctuations of the LIF spectrum and the error of referenceof laser wavelength. From Fig. 6(a), the LIF peak deviationis 5 fm for saturated absorption spectrum and 20 fm for iodine absorption spectrum. These deviations correspond tovelocity resolution of 2 m/s and 8 m/s, respectively. The
results show that the accuracy of the present system is 2
m/s and the system performance has been substantially improved by using the saturated absorption spectroscopy.

Fig. 7 Metastable temperature, density, radial flow and electron
density profile. The flow direction of metastable (neutral)
particles is inward. The metastable and electron density
are normalized at x = 0, respectively. The electron density
at x = 0 is about 4.8 × 1010 cm−3

causes Doppler shift of absorption spectrum, which is expected to be of the order of 1 m/s and is the same as thevelocity resolution of the system. On the other hand, theLamb dip in saturated absorption spectrum remains unchanged even when the experimental condition has beenchanged. The LIF peak deviation during 5 hours is within
5 fm, indicating long term stability (Fig. 6(b)).
The temperature, density and radial flow have beenmeasured with the newly-developed LIF system (Fig. 7).
Since the LIF spectrum is well fitted by a Maxwellian distribution, the temperature, density n∗ and flow are evaluated from FWHM, amplitude of the fitted Maxwellian distribution function and Doppler shift, respectively. In thepresent experimental condition, the component of ionizing

Fig. 6 Deviation of LIF peak position in wavelength for the repeated measurements under the same experimental condition. (a): short term; (b): long term. The referenceof laser wavelength is Lamb dip (open circles) or iodine
absorption spectrum (closed square).

The iodine absorption spectrum exhibits a complicated pattern because it consists of several hyper fine spectra within the single absorption line. Furthermore, the peakof iodine absorption spectrum changes depending on theiodine cell temperature since each hyperfine structure in
the absorption spectrum broadens due to thermal motion.
It is also emphasized that convection of iodine vapor also23

S. Etoh et al., Development of High Resolution LIF Spectroscopy with Saturated Absorption Spectrum

plasma is much larger than that of recombining component. [16] Thus, the plasma is considered to be in corona
equilibrium, in which almost all of the metastable atoms
are excited from the ground state level. [17] Therefore,
the simple evaluation of neutral density is possible bydividing the LIF intensity by the electron density, since
the metastable atoms is mainly excited by electron-neutral(ground state) collisions;I ∝ n∗ ∝< σven > nn ne ,cases, in which the electron density increases and theresultant collisional deexcitation reduces the number ofmetastable atoms, substantially. In these cases, further improvement of collection optics would be needed. We haveobserved the LIF signal in HYPER-I device up to 10 kW
input cases.
The relationship between ion temperature and neutral
temperature is important for estimating the effect of charge
exchange collisions because the ions transfer their thermalenergy to the neutral particles through ion-neutral collisions. The measurement of ion temperature is now undergoing.

(3)

where I, σ, ven are LIF intensity, electron-neutral collisioncross section and relative velocity of electrons to neutral
particles, respectively.
There is a depletion in neutral density, and an inward
going flow due to the density gradient is present in theplasma ( Fig. 7(b),(c) ). The neutral temperature is slightly
higher than the room temperature and is about 400 K. The
experiment measurement been done 5 times at each radial
position. The error levels in Fig. 7 show the maximum
standard deviation and are small. The reason is that the
newly-developed LIF system has high stability and the reproducibility of HYPER-I plasma is also very good. In
addition, the signal to noise ratio is high enough so that
the LIF spectrum is well fitted by Maxwellian distribution,
because there is no spectral line in the vicinity of the observing fluorescence (826.67 nm).

Acknowledgment
The work is supported by the Ministry of Education,
Science, Sports and Culture, Grant-in-Aid for Scientific
Research (A), 19204057 and collaboration programs with
NIFS (NIFS06KOAP016). The author (S. E) would like tothank ECW group members for the useful discussions andtechnical advices.
[1] M. Hori, M. D. Bowden, K. Uchino, K. Muraoka
and M. Maeda, J. Vac. Sci. Technol. A, 14, 144(1996).
[2] H. Abada, P. Chabert, J. P. Booth and J. Robiche, J. Appl.
Phys., 92, 4223(2002).
[3] A. Okamoto, K. Hara, K. Nagaoka, S. Yshimura,
J. Vranjes, M. Kono and M. Y. Tanaka, Phys. Plasmas, 10,2211(2003).
[4] J. Vranjes, A. Okamoto, S. Yoshimura, S. Poedts, M. Kono
and M. Y. Tanaka, Phys. Rev. Lett, 89, 265002(2002).
[5] R. Engeln, S. Mazouffre, P. Vankan, I. Bakker and D. C.
Schram, Plasma Sources Sci. Technol. 11, 595-605(2001).
[6] R. A. Stern and J. A. Johnson III, Phys. Rev. Lett., 34, 15481551(1975).
[7] M. Y. Tanaka, R. Nishimoto, S. Higashi, N. Harada, T. Ohi,
A. Komori and Y. Kawai, J. Phys. Soc. Jpn., 60, 16001607(1991).
[8] D. O’ Connell, T. Gans, D. L. Crintea, U. Czarnetzki and
N. Sadeghi, J. Phys. D: Appl. Phys, 41, 035208(2008).
[9] J. E. Chilton, J. B. Boffard, R. S. Schappe and C. C. Lin,
Phys. Rev. A, 57, 267-277(1998).
[10] J. B. Boffard, G. A. Piech, M. F. Gehrke, L. W. Anderson
Phys. Rev. A, 59, 2749-2763(1998).
[11] W. Demtröder: “Laser Spectroscopy: basic concepts andinstrumentation” (Berlin, Springer-Verlag, 1996) p.484500.
[12] V. S. Letokhov and V.P. Chebotayev : “Nonlinear Laser
Spectroscopy”, Springer Ser. Opt. Sci., 4 (Springer, Berlin,
Heidelberg 1997).
[13] V. Bakshi and R. J. Kearney, J. Quant. Spectrosc. Radiat.
Transfer., 42, 405-413(1989).
[14] H. R. Griem, Phys. Rev., 128, 515-522(1962).
[15] G. Marr, “Plasma spectroscopy” (Elsevier, New York,1968), p.246.
[16] T. Fujimoto, J. Phys. Soc. Jpn., 47, 273(1979).
[17] H. R. Griem, “Principles of Plasma Spectroscopy”(McGraw-Verlag, berlin, 1981), p.158.

4. Conclusions
We have developed a high resolution neutral flow
measurement system utilizing LIF and saturated absorption spectroscopy. The accuracy of this system is about 2
m/s and is one order of magnitude higher than that withiodine absorption spectrum. This result shows that the
LIF Doppler velocimetry with saturated absorption spectroscopy is a very powerful tool for measuring slow neutralflow.
The temperature, density and radial flow have beenmeasured by using the newly developed LIF spectroscopy
system. The profile of neutral density is estimated from the
LIF intensity and the electron density. It is found that there
is a depletion in neutral density at the center of the plasma,
and also found that there is a radially inward flow of neutrals induced by the density gradient. A slow neutral velocity has been measured for the first time by using the LIF
system with saturated absorption spectroscopy. The considerations of other excitation and deexcitation processes
are needed for the accurate evaluation of the neutral density more accurately. The measurement of absolute neutral
density is left for future work.
It is worth mentioning that the error of velocity measurement may increase in a plasma under high input power24