1550
Anal. Chem. 1987, 59, 1550-1554
Single-Mode and Multimode Optical Fibers for Light Introduction in Thermal Lens Spectrophotometry Kazuhiko Nakanishi, Totaro Imasaka, a n d Nobuhiko Ishibashi*
Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
When a muitlmode fiber is used for transmission of a laser beam, the coupling efficiency lo fair. But, the enhancement factor, the relative sensHivHy In comparison with conventional spectrophotometry, is low because of the poor beam quality of the transmltted laser. For the single-mode fiber the coupling efflclency is slightly lower, but the enhancement factor can be substantlally Improved slnce beam coherence Is retained during light transmission In the fiber. The achieved enhancement factor is 57% of the conventional thermal lens method. The performance of fibertoupled thermal lens spectrophotometry Is discussed on theoretical and experlmental bases, using the optical parameters for the slngiemode and multimode fibers.
Thermal lens spectrophotometry has a great advantage with respect to sensitivity in analysis of trace species (1, 2). However, optical alignment of the thermal lens system is more critical than with conventional absorption spectrometry. This prevents application of the thermal lens method to practical spectrometric work. The thermal lens effect is usually induced by the focused laser beam and is detected by the combination of a pinhole and a photodiode to measure the signal intensity at the beam center. In order to simplify the detection system, several researchers have used an optical fiber for monitoring the intensity at the beam center (3-6). This simplifies the detection system, and furthermore radio frequency interference can be reduced by placing the detection equipment far away from the noisy laser source. Only one report has described use of the optical fiber for transmission of the exciting laser, though ita performance is not discussed in detail (7). However, this approach of using an optical fiber to improve pointing stability of the pulsed nitrogen laser may have limited performance for sensitive detection of weak absorption. Since the damage threshold of the optical fiber by pulsed laser excitation is 1-10 J/cm2 (8),only a 0.&8 pJ input energy can be introduced to a 10-pm core single-mode fiber, resulting in a low enhancement factor. In this study we use single-mode and multimode optical fibers for transmission of the continuous wave laser. The optical fiber has low transmission loss and high flexibility and therefore allows transmission of the single transverse mode laser for a long distance, for example, from a monitoring station to a work place. First, we measure the coupling efficiency between the laser and the optical fiber. Second, we investigate beam properties of the laser transmitted from the optical fiber, since the sensitivity of fiber-coupled thermal lens spectrophotometry is strongly affected by this parameter. Finally, we compare the enhancement factor achieved for the fib x-coupled thermal lens method with that for the conventional method. EXPERIMENTAL SECTION Optical Fiber. The parameters of the optical fibers used in this study (Fujikura) are listed in Table I. The first two are single-mode fibers with stepped refractive index, which are de0003-2700/S7/0359-1550$01.50/0
Table I. Parameters of Optical Fibers Used in This Study"
code SM 6/ 125 SM 10/125 G 50/125 G 200/300 G 800/1100
core cladding specific diameter, diameter, wavelength, numerical fim nm aperture wm
6 10 50
200 800
125 125 125 300 1100
850
1300 850
0.2 0.2 0.2
"All the fibers are made of quartz and are protected by a polyimide coating. The diameters of the assembled fibers are 0.9 mm, except for G 800/1100 (1.7 mm). signed for use at 850 and 1300 nm, respectively. The latter three are graded index multimode fibers, whose refractive index is designed to have a parabolic distribution. The optical fiber was broken after carefully making marks with a glass cutter. For protection of the fiber it was covered with a stainless steel tube (16 or 17 gauge), which contained epoxy resin. The end of the fiber was first ground with emery paper (2000 mesh) and succeedingly polished with powder for optical fibers (Mitsubishi Rayon). The fiber end was further polished with very fine powder, which is used for polishing the window of an excimer laser (Lambda Physik). The surface condition was examined with a microscope by irradiating the opposite end surface with a tungsten lamp. This procedure was useful for visualization of the core position and the surface roughness of the fiber. Optical Arrangement of the Thermal Lens System. Figure l a shows the optical configuration of the conventional thermal lens system. The exciting source of a He-Ne laser (Uniphase, Model 1103) or an argon ion laser (NEC,GLG3200) is modulated by a homemade chopper. The beam is focused into the sample cell by a lens (Asahi, focal length 25 cm). The intensity at the beam center is measured by the combination of a multimode optical fiber (G 800/1100)and a photodiode (Hamamatsu Photonics, S780-8BQ). The decay signal is recorded by a transient digitizer (Autnics, S210) equipped with a signal averager (Autnics, F601). These electronics are controlled by a microcomputer (Sord, M223 Mark 111) through a GP-IB interface. The decay curve was displayed by a plotter (Watanabe, WX4671). Figure 1b shows the thermal lens system using two optical fibers for transmission of the laser beam and detection of the thermal lens effect. An objective lens (Nikon, X10) is used to focus the laser beam into a single-mode optical fiber (SM 6/125). The positions and the directions of the optical fiber were carefully adjusted to the focal point and the direction of the laser. The optical fiber was mounted on a small optics holder (Shiguma Koki, 2-60-(2)), which can precisely moved in three rectangular directions and two angles. It is further mounted on a larger stage, which can be more precisely moved in three rectangular directions (Tokyo Kodenshi Kogyo, LST-501). The output beam from the optical fiber is focused into the 1-cm square quartz cell by an eyepiece lens (Nikon, X10). The thermal lens effect was observed by passing the laser beam through a multimode fiber (G 800/1100). When this multimode fiber with a large core diameter was used for transmission of the laser beam, a quartz lens (Shiguma Koki, focal length 7 cm) was used to focus the laser beam into the fiber. In this case the output beam was focused into the sample cell by a conventional lens (Asahi, focal length 25 cm). The thermal lens effect was observed by using a multimode optical fiber (G 25/125). For measurements of the laser beam profile and its diameter, a 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
into two components, that is, axial and perpendicular modes. Only the perpendicular mode with a phase difference of 2 7 for the round trip distance can be transmitted in the fiber. The number of modes, NM, in the single mode optical fiber can be expressed as follows by using a normalized frequency,
(a) Conventional System mChopper ,
Ha-Ne Laser
I
v (10)
NM = v 2 / 2
I
Y
NM =
(b) Fiber Coupled System Chopper Ha-Ne Laser
ObJecthreLens
%
Optical Fiber
V
Photodiode
Optlcai Fiber
Flgure 1. Block diagram of thermal lens spectrophotometer: (a) conventional thermal lens system using optical fiber for detection of signal intensity at beam center: (b) fiber-coupled thermal lens system.
homemade pinhole (50 pm) or a commercially available one (Graticules, Ltd., 5 pm) was used. Reagents. The standard sample used was iodine dissolved in carbon tetrachloride. The reagents were used without further purification. The water was doubly distilled and deionized.
RESULTS AND DISCUSSION Parameter of Optical Fiber. The structure of the optical fiber and light traveling pathway are mentioned elsewhere (9, 10) and briefly described here. In the optical fiber the refractive index of the cladding is designed to be slightly lower than that of the core (nz< nl). The light introduced at an angle of 8, is transmitted into the fiber at a slightly narrower angle of 8,, and is totally reflected a t the boundary between the core and the cladding. For total reflection (9) sin
el c (n12- nz2)1/2/nl
(1)
The incidence angle at the end of the fiber, Bo, is expressed by sin Bo = nl sin el c (nI2- n?P2 (2)
A numerical aperture, NA, is defined by the critical angle, B,, which is the maximum angle that the fiber can accept a laser beam for light transmission
NA = sin Bc = (nI2- nz2)1/2
(3)
The difference in the relative refractive indexes for the core and the cladding, A, is given by A = (nl - 4 / n l Since A
(4)
