Wide range refractive index sensor using a twin-grating interferometer for intensity reference M. G. Shlyagin, Rodolfo Martínez Manuel Centro de Investigación Científica y de Educación Superior de Ensenada, Km.107 Carretera Tijuana a Ensenada, Ensenada, Baja California, 22860, México e-mail:
[email protected] O. Esteban Universidad de Alcalá, Departamento de Electrónica, 28871 Alcalá de Henares, Madrid, Spain. ABSTRACT. An optical fiber refractometer based on the Fresnel reflection in the fiber tip and twin FBG interferometer located near the fiber tip for light intensity referencing is reported. A wavelength scanning DFB diode laser was used for RI sensor interrogation. Signal processing in the Fourier domain allows intensity-independent accurate measurements for liquids −5
and gases with practically any RI. A resolution of 5 ⋅ 10 was demonstrated experimentally using different liquids including an anti-reflection index-matching liquid. Simple construction and the use of low-cost components make it interesting for many applications. Keywords: optical fiber refractometer, fiber Bragg gratings.
1. INTRODUCTION Refractometers find numerous applications in scientific laboratories and industry for quality control and technological process monitoring. Many applications require a miniature RI sensor which can be immersed into a small volume. Fiber-optic based RI sensors seem to be a very promising choice for such applications. Different types of optical fiber refractometers have been proposed and investigated. The most common approach consists of making the substance of interest to interact with the evanescent field of the fiber modes and measuring the effective propagation constants of the fiber modes. In order to get access to the evanescent field of the fiber guided modes, different structures such as long period gratings,1,2 etched or side polished fiber Bragg gratings,3 have been widely investigated. Usually, a precise measurement of resonant wavelengths in reflectance (transmittance) spectra is required in order to determine the surrounding refractive index. Wavelength encoded output of the sensor head is an advantage since it eliminates influence of light intensity variations on RI measurements. On the other hand, the need to perform accurate spectral measurements increases complexity and cost of such sensors. Typical operating range of such evanescent-wave refractive index sensors (1.0-1.45) is determined by the effective index of the silica glass optical fiber (1.450). A simple approach in RI sensing with optical fibers is measuring the Fresnel reflection coefficient at the tip of a rightangle cut single-mode optical fiber. When the fiber tip is submerged into liquid of interest, the refractive index of the sample can be calculated from the measured reflection coefficient using the Fresnel formula for normal incidence. However, uncertainty in determination of the reflection coefficient could be a problem because of instability of a light source and variable bent-induced losses in the lead fiber. To achieve acceptable measurement accuracy, a dynamic referencing of light power at the fiber tip has to be implemented. Different configurations of fiber-optic Fresnel refractometers with intensity referencing were reported.
20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75031J © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.835521 Proc. of SPIE Vol. 7503 75031J-1
Some of them use two different fibers: one fiber serves as the sensor head for RI measuring and another one for intensity referencing. Reflection signals from liquid–fiber interface of the measuring fiber are compared with reflection signals from air–fiber interface of the reference fiber. 4-7 In such a way, influence of light source instability on results of measurements can be cancelled. However, a special attention has to be paid to prevent errors caused by fiber bending during measurements. Another option for optical power reference in a point just before the fiber tip was recently demonstrated. 8,9 An additional partial reflector was formed in the fiber core near to the fiber tip. Two different configurations for the RI sensor head were proposed. One utilizes a very short FBG written in the core of single mode fiber as a partial reflector. 8 The second configuration is based on air-filled micro-cavity fabricated by using 157-nm laser micromachining. 9 The light waves reflected from the fiber tip thus interfere with the light waves reflected from the partial reflector. In such a configuration, a reflection spectrum of the interferometer consists of interference fringes. The external refractive index can be determined according to the maximum visibility of the spectral fringes in the reflection spectrum. Main sources of errors of this technique are related with a proper selection of spectral fringes representing the maximum visibility and with difficulty to measure low contrast of fringes at low light level. The need to use of a high performance optical spectrum analyzer for high resolution detection of spectral fringes at low contrast and light level makes it difficult to satisfy operational needs for many applications. In this work we present a further development of approach based on Fresnel reflection from the fiber tip for accurate measuring of refractive index.
2. PRINCIPLE OF OPERATION AND EXPERIMENT A configuration for the proposed refractive index sensor is schematically shown in Fig.1. A sensor head consists of a twin Bragg grating interferometer located at some distance, L10 (in this experiment, it was about 5 cm), from the fiber tip. The FBGs had nominally the same Bragg wavelength and very low reflectivity of 0.05%. The 2 mm long gratings were written in standard optical fiber (SMF-28) without hydrogen loading. The distance between gratings, L12 , was selected to be different with the distance L10 . The cleaved fiber tip and two FBGs form 3 interferometers with different optical lengths. Wavelength tuning DFB diode laser
Circulator Digital Oscilloscope
Photo detector
Bragg gratings
PC
/ Liquid
Figure 1. Experimental arrangement for refractive index sensor.
Figure 2. Reflection spectrum of the sensor when the fiber tip was immersed in sunflower-seed oil with n0 = 1.46165 ;
A DFB diode laser was used as a light source for the sensor head interrogation. Output wavelength (1534nm) of the DFB laser was periodically thermally tuned within a range of 0.8 nm at rate of 1 scan per second. By such a way, the reflection spectra were observed with a digital oscilloscope and could be captured for signal processing and RI evaluation. An amplitude reflection coefficient, r0 , for the fiber tip is given by
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r0 =
neff − n0 neff + n0
,
(1)
where neff is the fiber core effective refractive index and n0 is refractive index of the substance at the fiber tip. When the laser wavelength is scanning, λ ( t ) = λin + δλ ( t ) = λin + γ t , spectral fringes appear in the reflection spectrum of the sensor head as a result of the interference between 3 reflected light waves. An example of reflection spectrum is shown in Figure 2. For small amplitude reflection coefficients of the Bragg gratings, r1 ≈ r2 neff . Results for refractive index measurement of sunflower-seed oil are shown in Figure 5.
Figure 4. Phase shift of 180o was observed for fiber dipped in sunflower-seed oil.
Figure 5. Results of RI measurements for sunflower-seed oil Standard deviation is 4.3x10-5
3. CONCLUSIONS This work presented a fiber optic sensor to measure the refractive index of liquids and gases. The sensor is based on Fresnel reflection in the fiber tip and a low-reflective twin grating interferometer for intensity reference. Signal processing in the Fourier domain allows accurate measuring in a wide range of RI. Simple construction and the use of low cost components make it interesting for many applications. This work was partially supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT) of México through the research grant 84124.
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2 Dae Woong Kim, Yan Zhang, Kristie L. Cooper, and Anbo Wang, “In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement,” Applied Optics 44 pp.5764-5769 (2005). 3. K. Shroeder, W. Ecke, R. Mueller, R. Willsch, and A. Andreev, “A fibre Bragg grating refractometer,” Meas. Sci. Technol. 12, 757–764 (2001). 4. Chang-Bong Kim and Chin B Su, “Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter,” Meas. Sci. Technol. 15 pp.1683-1686 (2004). 5. S J Buggy, E Chehura, SW James and R P Tatam, “Optical Fibre Grating Refractometers for Resin Cure Monitoring,” Journal of Optics A: Pure and Applied Optics 9, S60-S65 (2007). 6. Chun Hua Tan, Xu Guang Huang, and Yu Ping Shi,” In situ measurements of the solubilities of salt–water systems by a fiber sensor,” Rev. Sci. Instrum. 80, 034103 (2009). 7. Hui Sua and Xu Guang Huang, “Fresnel-reflection-based fiber sensor for on-line measurement of solute concentration in solutions,” Sensors and Actuators B: Chemical 126 pp. 579-582 (2007). 8. Susana F. O. Silva, O. Frazão, Paul Caldas, Jose L. Santos, F. M. Araújo and Luis A. Ferreira, “Optical fiber refractometer based on a Fabry-Pérot interferometer,” Opt. Eng., v. 47, 054403, (2008). 9. Ran, Z. L.; Rao, Y. J.; Liu, W. J.; Liao, X.; Chiang, K. S. “Laser-micromachined Fabry-Perot optical fiber tip sensor for highresolution temperature-independent measurement of refractive index,” Optics Express 16, pp. 2252-2263, (2008).
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