Light transmission characteristics of silica capillaries

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1 Light transmission characteristics of silica capillaries M. Borecki 1), M. Korwin-Pawłowski 2), M. Bebłowska 1) 1) Institute of Microelectronics and Optoelectronics WUT, Warsaw, 75 Koszykowa Str. 2) Centre de recherche en photonique Universite du Quebec en Outaouais, 101 rue Saint-Jean-Bosco, Gatieneau, QC J8X 3X7, Canada. ABSTRACT The paper discusses certain aspects of the light guidance in capillary tubing and the possibilities of its utilization in fibre optic sensor's heads. Keywords: Silica capillaries. 1. INTRODUCTION Optical capillaries are widely used in capillary gas- and liquid chromatography, capillary electrophoresis, absorbance spectroscopy, Raman spectroscopy etc. These micro-fluidic methods find applications in biotechnologies [1], medical diagnostic [2], development of new pharmaceutical technologies [3], and environmental sciences [4-7]. It is worth mentioning that in view of the achievements of recently developed modern technologies (e.g. MEMS structures [8]) new application fields will be found for the capillary technique such as e.g. instrumentation in e.g. experimental physics [9], components used in nonlinear optics [10] etc. The most popular application of optical capillaries is absorbance spectroscopy, which consists of the measurements of a transmitted or evanescent light power spectrum. In such a spectroscopic apparatus, a fused silica or teflon capillary is used for carrying the liquid sample. This means, that once the central hole is filled with the substance other than air, the light propagation parameters change. Thus it should be possible to design a sensor s head that signalises the presence of the monitored substance even when one specific wavelength of light. The evaluation of this possibility was the main goal of our experiments with silica capillaries. In our investigations we used thin wall silica capillary tubes produced by a research group guided by Professor Jan Dorosz at the Bialystok University of Technology, Poland [11]. We found that, to understand fully the light transmission mechanism in the capillary tubing, it is important to take into account the modes transmitted through the air hole of these waveguides. 2. PHOTONIC FIBRES AND OPTICAL CAPILLARIES Photonic Fibres and Optical Capillaries are new light waveguides developed at the end of the past century. They differ as well in structure as in guiding mechanism. The classification of these modern lightguides is shown in Fig. 1. Photonic Fibers Optical Air-guided PF (PBGF) Tubing Solid core PF cells Planar capillary structures Fig. 1. Classification of photonic fibres and optical capillaries

2 Figs. 2 and 3 show cross sections of the lightguides of Fig. 1. Photonic Bandgap Fibers (PBGF or Holow Bandgap Fibers) with the central hole filled with air are divided into Bragg Fibres (BF a multilayer cylindrical Bragg reflectors) and Photonic Crystal Fibers - PCFs characterised by two-dimensional periodically arranged air holes that run down the length of the fiber. The Photonic Bandgap guidance effect strongly depends on the wavelength, so that only certain frequency range can propagate in this defected of structure, which constitutes the apparent core of the fibre. Bragg Fiber Hollow-core Photonic Crystal Full-core Photonic Crystal Fiber Bragg mirror Centaral hole Periodic hole configuration Solid core Fig. 2. Examples of Photonic Crystal Fibers structures. In full-core Photonic Fibers (also known as Holley Fibers HF), the light transmission is driven either by the physical mechanism known as the index guiding mechanism or by the PBG effect if its structure periodicity is on the scale of the optical wavelength, [12, 13]. Fig. 3 shows examples of the optical capillary structures. The light propagation properties of such empty waveguides are assumed to depend on the proportion of the central hole diameter to the capillary wall thickness. Flexible capillary tubing cell Planar capillary structure Optical waveguide Coating Inner Diameter (2-700 m) µ (5-25 m) µ Illumination Sample flow capillary Detection Glass substrate Outer Diameter ( m) µ Fig. 3. Examples of optical capillary structures. 3. GEOMETRY OF THE CAPILLARY WAVEGUIDES Flexible Fused Silica Tubings are produced by the Polimicro Technologies [14] and the Silica Physics [15], specialized in Heavy Wall capillaries and Platinum or Gold coated capillaries. These capillaries are useful at extreme temperatures (to 750 deg C) and are resistant to (resis) most chemical substances. The Polimicro offers 41 Standard Products TSP Standard Polyimide Coating, TSG High Temperature Polyimide Coating, TSU UV Transparent Coating and Thick Wall Flexible Fused Silica Tubings designed for capillary liquid chromatography columns. The inner diameter of silica tubings ranges from 2 to 700 µm and the outer diameter from 150 to 850 µm (see Fig. 4). The inner diameters of capillaries used e.g. in capillary spectroscopy are usually within the range from 50 to 250µm, whereas the length of waveguides varies from 2 to 500cm. Greater holes and thinner tube walls are also in use. Wall thickness is at least 50µm and sometimes exceeds 250µm. 2

3 Silica (n2) Air hole (n1) Doped silica (n3) Polyimide coating Teflon or silica (n2) Fig. 4. Cross sections of the optical capillary tubings. The hole dimensions in Thick Wall FFSCT are within the range from 150 to 300µm and outer diameters are 665µm. The Polimicro also offers square cross-section, flexible fused silica capillary tubes with a durable polyimide coating. They are characterized by a larger effective internal surface area (i.e. greater volume per unit length). In new 3-layer s structures, the cladding or core are doped adequately. For special applications (e.g. water quality monitoring) capillaries made of teflon or teflon- AF coated capillaries are proposed. The index of refraction (IR) of teflon is of the order of 1.30, whereas IR of water is approximately EXPERIMENTAL SETUP Measurements of the light transmission along the capillary and the estimation of the light power introduced into the capillary were done in a set-up from Fig. Source of collimated light focus distance Optical capillary Detection unit: 1) power meter 2) camera+microscope 3) photo-diode with sensitive area 3 3mm+high resolution opto-electronic interface Support Fig. 5. Experimental setup for the measurements of light transmission. As a light source Laser Diode (λ=670nm) equipped with a collimator was used. The emitted light power was equal 1,4mW. The focus of a collimating lens system equals 310mm and a focus diameter -2mm. Tested silica capillaries are produced by a research group guided by Professor Jan Dorosz at the Bialystok University of Technology [17], have inner diameters 200 and 260µm and a wall thickness -20µm. The length of samples was about 200mm. As a detector unit Light Power Meter (from Ando) or optoelectronic interface elaborated in our laboratory were applied. The capillaries tip was observed with black & white camera of 700 horizontal lines that is mounted on PZO microscope. 5. PHYSICAL MECHANISM OF THE LIGHT PROPAGATION IN A THIN-WALL HOLLOW-CORE GLASS CAPILLARY Large core fibre Air hole (n1=1) wall Cladding Fig. 6. TIR phenomenon of light guiding in an optical capillary. 3

4 In order to couple light into the capillary and to the detector unit, conventional optical fibres are sometimes attached to the light guide ends. The simplest theory of the guiding phenomena in optical capillaries assumes that light propagates through the cylindrical region of the highest refractive index in a multimode manner way (Fig. 6), [16]. We examined thin wall capillaries. The air-core cross section surface area was approximately twice as large as that of the glass wall. In our opinion, the high ratio of the diameter of the air hole to the wall thickness affects the guiding phenomena and this fact should be taken into account in an analysis of the capillary transmission properties. Fig. 7 is a schematic illustration of the light penetration into the capillary interior. The light power introduced into the capillary is divided into the modes guided in high-index tube and the modes transmitted in the air hole. The modes propagating in the capillary near its input end are ballistic modes and the modes reflected from the inner capillary wall (Fresnel reflections). Along the whole tubing length, evanescent modes are injected from the glass to the central hole. The light power of the air-hole modes decrease rapidly because of the light path irregularities and the disappearance evanescence of the modes that propagate from the hole through the glass cylinder to the outside. Evanescent modes modes Reflected modes Hole modes Balistic modes Evanescent modes n3=n1=1 Fig. 7. Modes propagating in the glass wall and in the air hole. In the case when the surface of the air-core is equal or greater than the cross section surface area of the capillary wall, there exists a certain characteristic length of the capillaries below which the light chiefly propagates through the air and not through the solid glass. This length depends on the intensity of light introduced into the waveguide. In our experiment, this length appeared to be 50 mm. In longer cylinders, the proportion of the light power transmitted through the silica wall and the hollow core undergoes changes. This means that the observations at the capillary output end give information about the power distribution. The presence of the high power ballistic modes and the reflected hole modes results in a bright circle occurring in the central part of the fibre front face, whereas in long capillaries the glass wall is intensively bright. Photographs of these effects (discussed states) are shown in Fig. 8. Fig. 8. Photographs of the optical capillaries (i.d.=260µm, o.d.=300µm) output ends with various lengths : a 50mm, b- 300mm. We injected a small volume of a transparent liquid into a short silica capillary (e.g. a drop of water or of glycol ethylene) near its input end. The index of refraction of the capillary was about 1.46 and that of the liquid was smaller. The presence of the liquid appeared not to disturb significantly the light propagation in the solid wall, but affected the hole modes. This was so since the liquid drop formed a dispersive lens inside the capillary, so that the ballistic and reflected hole modes were scattered by it (Fig.9). 4

5 L1 L2 Drop of liquid Photodetector Fig. 9. The effect of hole modes scattering due to the presence of a liquid lens. The light power measured at the output of a capillary tube diminished dramatically. The observed change of the electric signal may be expected to be greater with a capillary of a larger internal diameter. The results of the experiment are shown in Fig Light power [a.u.] Air hole light power Residual hole modes Inner diameter [ µ m] Fig. 10. Light power propagated in a hollow core capillary and in a capillary with a liquid lens inside the hole, ID - inner diameter (µm). As the liquid drop moves toward the photodetector, the scattered light can be collected and the electric signal, proportional to light power, increases. Fig. 11 shows evidently that the effect described above takes place. The length of the capillary tubing in this experiment was 200 mm Light power [nw] L2 from Fig. 9. [mm] Fig. 11. Change of the light power transmitted in an optical capillary as a function of the liquid drop position. The above effects seem to be another evidence of the existence of the air-hole modes and of their significant role played in the power distribution in thin-wall large air-core capillaries. 5

6 6. POSSIBLE USE OF CAPILLARIES IN FIBRE OPTIC INTENSITY SENSORS Depending on the substances introduced into the capillary interior, the light guiding properties can be modified. Liquidcore capillary waveguides play an important role in analytical chemistry. New instruments include capillary arrays and allow examining simultaneously several analytic samples. In general, the electronic units are multifunctional and rather complicated. Very often, e.g. in control systems, simple two-state sensors are needed. Their area of application includes medical diagnostic, food and drags production, water quality examination, etc. These sensors, which are small, easy in use, and cheap, can be utilised in monitoring blood gases, turbidity of liquids, drug insolubility, salts dissolved in seawater and the like. We found that the liquid lens effect mentioned above can be used for identifying certain liquids by testing samples of small volumes (less than 1µl). The samples can be examined by measuring the reduction of the light transmission, caused by introducing a specified volume of the tested liquid into the capillary tube (Fig. 12). Reduction of the light power [%] ,32 1,37 1,42 Index of refraction L1 from Fig. 9. = 9mm Fig. 12. Change of the light power due to the presence of a liquid in the capillary hole. Liquid volume 0,3 µl, liquid lens length -9mm, light wavelength- 670 µm. The light can be guided a specified length within and along the analyte channel. Transversal lighting was examined in e.g. ref. [8].We found that a capillary with a light incident perpendicularly to the tube axis can be treated as a cylindrical lens. Thus, depending of the liquid refraction, the lens focus will change. By measuring this change, we can identify the type of certain substances. As already shown, any change of the liquid drop position can easily be detected. This liquid lens movement can be provoked by e.g. deviating the supports from the horizontal position or by changing the gas pressure. We are in the course of studies on the design of such a sensor in which no optical alignment and microscopy are required. 7. CONCLUSIONS It is obvious that without optical capillaries, the microfluidic measuring methods known today could not exist. However, flexible capillary tubings are rarely used in intensity modulated sensors. It seems possible to construct sensor heads made of conventional optical fibres and capillary waveguides. Such a mixed structure can offer a new possibility of testing very small fluid samples in an apparatus reduced in size and cost. In short thin wall capillaries the power transmitted by the air modes plays a significant role, and its change can result in a perturbation of the light path. ACKNOWLEDGMENT We would like to thank Professor Jan Dorosz from the Bialystok University of Technology for make available thin wall optical capilares and for long valuable term cooperation. BIBLIOGRAPHY 1. C.W.Huk, G. Stecher, R. Bakry, G.K. Bonn, Recent Progress in High-Performance Bioseparations, Electrophoresis, 24, pp , D. Kieslinger, B. H. Weigh, Waveguide Optrodes for Medical Applications, Optical Review, V4, N4,, pp , C.-C. Lin, Y. T. Li, S. H. Chen, Recent Progress in Pharmacokinetic Applications of Electrophoresis, Electrophoresis, 24, pp ,

7 4. E. Dabek -Zlotorzynska, M. Piechowski, R. Aranda -Rodrigez, M. McGrath, E. P. C. Lai, Determination of Low- Molecular- Mass Carboxylic Acid in Atmospheric Aerosol and Vehicle Emission Samples by Electrophoresis, J. Chromatogr. A, 910, pp , E. Dabek -Zlotorzynska, M. Piechowski, R. Aranda -Rodrigez, K Keppel-Jones, Determination of Hydroxymethanesulfonic Acid in Environmental Samples Using Electrophoresis, J. Sep. Sci., 25, pp , E. Dabek -Zlotorzynska, R. Aranda Rodrigez, S. E. J. Buykx, Development and Validation of Electrophoresis for the Determination of Selected Metal Ions in Airborne Particulate Matter After Sequential Extraction, Anal. Biochem. Chem., 372, pp , E. Dabek -Zlotorzynska, H. Chen, L. Ding, Recent Advances in Electrophoresis and Electrochromatography of Pollutants A Review, Electrophoresis, 24, pp , Che-Hsin Lin, Gwo-Bin Lee, Chun-Che Lin, Microcapillary Electrophoresis chips integrated with burried SU- SOG optical waveguides for biomedical applications, 9. R.G. Dall, M.D. Hoogerland, Single-mode hollow optical fibers for atom guiding, Appl. Phys., B 74, pp , 2002,. 10. Nonlinear Optics in Waveguides and Waveguide Arrays, J. Dorosz, Optical fibers technology, Polish Academy of Science, Krakow, 2005 (in Polish). 12. T. Nasiłowski, R. Kotyński, F. Berghman, H. Thienpont, Photonic Crystal Fibers state of the and future perspectives, Lightguides and their applications II, Proceedings of SPIE, 5576, pp.1-12, 2004,. 13. P.J. Roberts, F. Couny, H. Sabert, Ultimate low loss of hollow-core photonic crystal fibres, Optics Express V13, N1, pp.236, J. Dragavon, Optical Waveguides, R. Romaniuk, J. Dorosz, Measurement techniques of tailored optical fibres, Proceedings of SPIE, Vol. 5064, pp ,

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