Single-mode fiber polished into the core as a sensor element

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ELSEVIER

Sensors and Actuators A 64 (1998) 209-212

PHYSICAL

S i n g l e - m o d e fiber polished into the core as a sensor element Andrey Tz. Andreev *, Blagovesta S. Zafirova, Elka I. Karakoleva Institute of Solid State Physics, 72 Tsarigradsko Chaussee Blvd, Sofia, 1784, Bulgaria Received 4 I~arch 1997; revised 26 May 1997; accepted 10 June 1997

Abstract The light transmittance of a single-mode fiber, polished into the core, at different polishing depths is investigated experimentally. New results concerning amplitude, polarization and wavelength characteristics of such waveguide structure are presented, revealing the feasibility of this structure to act as a simple fiber-optic sensing element. © 1998 Elsevier Science S.A. Keywords: Optical fibers; Single-mode fibers; Sensors

1. Introduction Many fiber-optic components, such as fiber.-optic couplers [ 1 ], polarizers [ 2 ], amplifiers [ 3 ], distribute d single-mode fiber-to-planar waveguide coupler [ 4], etc., h~ ve been developed using side-polished single-mode fibers. The possibility of modifying the waveguide fiber's mode fielt as a result of the proximity of the polished surface to the fiber core allows these components to be used as sensor elements. This can be realized by bringing into contact the side-polished fiber surface with the overlay medium, the characteristics of which are to be measured directly or in combination with a metal layer (as a superstrate) or a planar waveguide [ 5 - 8 ] . Waveguide structures containing a side-polished optical fiber could be used for an environmental parameter mea,mrement, such as measurement of refractive index, absorption (an extinction coefficient), optical anisotropy, etc. In most applications of this structure, the optical fibers are side-polished without reaching the fiber core. But an optical fiber polished into the core exhibits an interesting property. Due to the broken waveguide geometry, an output optical loss has been observed, the value of which could have a strong dependence on the superstrate refractive index ns [9], i.e., the index of the medium over the polished fiber surface. The values of the refractive indices n~, for which these dependencies are observed, are smaller than the effective refractive index of a waveguide mode, i.e., n~ < 1.45. In this interval are included the refractive indices of water, alcohol, and biological solutions. This is the reason to investigate the character* Corresponding author. Tel.: + 359 2 7431 524. Fax: + 359 2 975 36 32. E-mail: [email protected] 0924-4247/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PII S 0 9 2 4 - 4 2 4 7 ( 9 7 ) 0 1 6 2 4 - 5

istics of a single-mode fiber polished into the core, revealing its feasibility to act as a simple sensing element. In this paper experimental results concerning amplitude, polarization and spectral characteristics of the side-polished single-mode optical fibers with disturbed fiber cores are presented.

2. Experimental results The experiments are carried out using a single-mode fiber with the following parameters: a core diameter of 3.9 p~m, a maximum core--cladding refractive-index difference An = 0.009 and a cut-off wavelength of 0.77 p,m. The optical fibers are glued to a fused quartz rectangular block, inside a curved groove of 50 cm radius. We use the method suggested in Ref. [9] for the determination of the moment when the fiber core is reached in the polishing process. The polishing of the fiber just to the core--cladding interface results in an attenuation on the order of a few tenths of 1 dB. The observation of a throughput increase with a change of the refractive index of the medium above the polished fiber surface, n~, from the value n~ = 1 (air) to the value n~ equal to the cladding refractive index is indication that the core has been reached. The polishing depths So (i.e., the minimum distance between the fiber core and the polished surface, Fig. 1 ) are approximately evaluated from geometrical considerations by measuring the change of the oval section length of the polished fiber surface. Three samples polished into the core with polishing depths So= - 0 . 1 txm, = - 0 . 3 Ixm and = - 0 . 7 ~m are used in the experiments.

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A.Tz. Andreev et al. / Sensors and Actuators A 64 (1998) 209-212

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and for sample 3 about 1.45. From the measurements it can be seen that by changing the polishing depth not only the slope of the curve but also the position of the maximum can be influenced. The deeper polishing of the fiber core shifts the steep region of the curves to higher values of the overlay refractive indices. This property can be used to select the region of the maximum sensitivity of the structure, when it is used as a refractometer.

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2.2. Polarization sensitivity of the single-mode fiber polished into the core

Fig. I. Longitudinal view of side-polishing into the optical fiber core.

2.1. Response of the single-mode fiber polished into the core to the superstrate refractive index Measurements of the output optical power as a function of the superstrate refractive index have been ;gerformed. A nonpolarized light source (LED and interference filter with wavelength 850 nm and halfwidth of l0 nm) is used. TEand TM-polarization states are measured separately at the fiber output. For the measurement of the output response via superstrate refractive index, a series of standard Cargille refractive-index liquids with n~ = 1.38 tc 1.46 (the superscript D signifies the 589 nm wavelength) are used. Fig. 2 shows a normalized output optical power :o refractive index variation of the material in contact with the polished surface of the fiber (at A=850 nm) for the three samples and TEpolarization. It is found that the input light: can be recovered for all the three samples by increasing the immersion refractive index. For the samples polished less in the core, the values of overlay refractive index at which the maximum input signal is recovered are smaller than the cladding refractive index. For sample 1 this value is about 1.38, for sample 2 about 1.42

The measurement of the output power for both TE- and TM-polarization states reveals the following interestingproperty. The throughput attenuation when decreasing the superstrate refractive index for the TM-polarization state is higher than that of the TE-polarization. This difference is greater for the samples polished deeper into the core. Fig. 3 shows the ratio of the TE- and TM-polarization signals of the output optical power PTE/PTM to the superstrate refractive index for sample 3. The value of this ratio for n~ = 1 (air) is 2.65 (the point absent from Fig. 3). From the results presented in Fig. 3 it can be concluded that the polarization state of the light transmitted through the side-polished fiber could be applied for sensing measurements. Comparing the results of Figs. 3 and 2, sample 3 shows that the change of the PTE/ PTM ratio is significant even for those values of ns at which the output optical power is changed very slightly (n~ < 1.38), i.e., the polarization measurements could be additional to the amplitude ones.

2.3. Spectral response of the single-mode fiber polished into the core The spectral response at a fixed overlay refractive index has been studied. The measurements are carried out with a

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Fig. 2. Fiber transmission Uo,,/~., for TE-polarization state, as a function of the refractive index n, of the semi-infinite medium above the polished fiber surface (for a drop of immersion liquid) and for different polishing depths: sample 1, So= - 0 . 1 Ixm; sample 2, sc~= - 0 . 3 ixm and sample 3, So -= - 0 . 7 Ixm. Experimentally measured points are presented as solid marks and the solid lines only connect experimental points.

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f. Fig. 4. Spectral response P,,,,/~n of optical fiber polished into the core for the three samples at different values of n~: (a) so = - 0.1 Ixm; (b) (c) so~ -0.7 txm. Experimentallymeasured points are ~resentedas solid marks and the solid lines only connect experimental points. halogen lamp and monochromator as a light source, giving a spectral halfwidth of 5 nm. Due to the low level of the optical signals, the measurements are performed without the polarization control of the fiber output light. The spectral measurements are presented in Fig. 4 ( a ) - ( c ) , for al] three samples and within the 770-1300 nm spectral region. The spectral dependences are similar to a longwave cut-off filter for all three samples. Decreasing the refractive index n~ from 1.46 to 1 (air), the filter's edge moves towards shorter wavelengths. For the samples 1 and 2 (polished less into the core) there exist wide spectral regions where the wariations of the output signal are minimal at the values of n, interesting for practical use. This feature of the waveguide structure can be used to create a measurement scheme supplied with two light sources, a reference and a signal one [ 10]. q'he ratio of the two output signals gives the ability to e l i m i l a t e additional

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losses, connector losses for example. Another conclusion which can be drawn from the experimental measurements presented above is that side-polished optical fibers can be used in sensor schemes with a wide bandpass spectral source ( L E D ) . Through the appropriate selection of the polishing depth and the signal light-source wavelength, a significant sensitivity can be achieved for a certain interval of refractive indices. From the experimental results it is clear that a single-mode fiber polished into the core can be used as a simply prepared fiber-optic refractometer. This refractometer can be used for the refractive-index measurement of liquids in the interval 1.30-1.45. The element studied above (polished into the optical fiber core) and completed with appropriate material as a superstrate (for example, polymers) can be used as a sensor element for another field measurement, such as temperature.

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A.Tz. Andreev et al./ Sensors and Actuators A 64 (1998) 209-212

3. Conclusions The possibility of using single-mode fibers polished into the core as sensor elements has been experimentally investigated. The presented dependences of the output light transmittance on the overlay refractive index demonstrate a possibility for the structure to act as a fiber-optic refractometer. Changing the polishing depth, the slope of the curves and the position of their maximum values zan be influenced. It is demonstrated that the polarization state of the light transmittance could be applied for sensing measurements. The spectral dependence of this fiber-optic strt~cture is similar to that of a longwave cut-off filter and therefgre can be used in measurement schemes provided with a reference light source, and moreover with a wide ban@ass spectral source (LED).

Acknowledgements This work was supported by the National Science Fund, and the Ministry of Education, Science and Technologies, Bulgaria, under Contract F-612.

References [l] R.A. Bergh, G. Kotler and H.J. Shaw, Single-mode fibre optic directionalcoupler, Electron.Lett., 16 (1980) 260-261. [2] D. Grnchmann,K. Petermann,L. Staudigeland E. Weidel,Fiber-optic polarizers with high extinction ratio, Proc. 91h Eur. Conf. Optical Commun., Geneva, Switzerland, 23-27 Oct., 1983, North Holland, Amsterdam, 1983, p. 305. [3] W.V. Sorin,K.P. Jackson and H.J. Shaw, Evanescentamplificationin a single-modeoptical fibre, Electron. Len., 19 ( 1983) 820-822. [41 C. Millar, M. Brierleyand S. Mallinson,Expo~;ed-coresingle-modefiber channel-droppingfilter using a high-inde~overlay waveguide, Opt. Lett., 12 (1987) 284-286. [5[ W. Johnstone, G. Stewart, T. Hart and B. Culshaw, Surface plasmon polaritons in thin metal films and their role in fiber optic polarizing devices, IEEE J. LightwaveTechnol.,LT-8 (1990) 538-543. [6] D.G.Moodieand W. Johnstone,Wavelengthtur abilityof components based on the evanescentcouplingfrom a side-polishedfiberto a highindex overlay waveguide,Opt. Lett., 18 (1993) 1025-1027. [7] K. McCallion, S. Creaney, I. Madden and W. Johnstone, A tunable fiber-opticbandpassfilterbased on polished fiberto planarwaveguide coupling techniques,Opt. FiberTecbnol., 1 (1995) 271-277.

[8] W. Johnstone, G. Tbursby,D. Moodie and K. McCallion,Fiber-optic refractometer that utilizesmultimodewaveguideoverlaydevices,Opt. Lett., 17 (1992) 1538-1540. [9] M.J.F. Digonnet,J.R. Feth, L.F. Stokes and H.J. Shaw, Measurement of the core proximityin polished fiber substrates and couplers, Opt. Lett., 10 (1985) 463-465, [ 10] A. Andreev,G. Diankov,B. Zafirovaand A. Kebedjiev.Wavelengthdemultiplexed fiber-optic temperature-measurement instrument, Sensors and ActuatorsA, 30 (1992) 215-218.

Biographies Andrej Tz. Andreev was born in Sofia, Bulgaria, on November 14, 1951. He graduated from the Faculty of Physics, University of Sofia, in 1975. He received his Ph.D. degree from Lebedev Institute of Physics, Moscow, in 1981, in the field of optical-fiber loss measurements. Since 1981 he has been working as a research fellow in the Department of Optics and Spectroscopy at the Institute of Solid State Physics, Sofia. In 1994 he became an associate professor. His current research interests are in the field of fiber-optic elements and sensors. Blagovesta S. Zafirova was born in Kazanlak, Bulgaria, in 1952. She received the B.Sc. degree from the Faculty of Physics, University of Sofia, in 1975. Since 1976 she has been working as a research fellow in the Department of Optics and Spectroscopy at the Institute of Solid State Physics, Sofia. Her Ph.D. thesis was devoted to thin-film optics and its application in fiber-optic multi/demultiplexing. Her present activities include the development of and research on single-mode fiber-optic components and sensing systems. Elka 1. Karakoleva was born in Dupnitza, Bulgaria, in 1955. She received the B.Sc. degree from the Faculty of Physics, University of Sofia, in 1978. In 1979 she joined the Department of Nuclear Reactor Physics at the Institute of Nuclear Physics and Nuclear Energy. Since 1990 she has been working as a research fellow in the Department of Optics and Spectroscopy at the Institute of Solid State Physics, Sofia. Her current research interests are in single-mode fiber elements.