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Sensitivity to alcohols of a planar waveguide ring resonator fabricated by a sol–gel method
Sensors and Actuators B 120 (2007) 610–614
Sensitivity to alcohols of a planar waveguide ring resonator fabricated by a sol–gel method Fufei Pang a,b,∗ , Xiuyou Han a,b , Fenghong Chu a,b , Jianxin Geng a , Haiwen Cai a , Ronghui Qu a , Zujie Fang a a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 27 October 2005; received in revised form 16 March 2006; accepted 16 March 2006 Available online 12 May 2006
Abstract A planar waveguide ring resonator was fabricated by organic–inorganic hybrid sol–gel materials; its sensitivity to ethanol vapor was experimentally investigated. It was found that dips in the transmission spectrum of the device shifted to longer wavelengths with increasing the ethanol concentration, and its sensitivity showed a linear relation with the ethanol concentration, showing a coefficient of 1.13 pm/ppm. In addition, the transmission loss of the ring resonator decreased with increasing the ethanol concentration. The measured characteristics suggest that the device may be considered as one of the candidates of alcohol vapor sensors. © 2006 Elsevier B.V. All rights reserved. Keywords: Chemical vapor sensor; Integrated optics; Ring resonator
1. Introduction Integrated optical sensors have been used in chemical and biological analyte detection. Various methods, such as MachZehnder interferometers [1], birefringence interferometers of planar or channel waveguides [2,3], multimode interferometers [4] and planar waveguide ring resonators [5], have been proposed and investigated experimentally. Compared to the other schemes, ring resonators have distinctive advantages such as high wavelength selectivity and high sensitivity. Chemical vapor analyses are needed in many fields such as environmental assessments, breath diagnosis, freshness analysis, etc. High ethanol vapor concentration may cause inflammation of the nasal mucous membrane and conjunctiva, irritation of the skin, and even alcohol poisoning [6]. Absorption of hydrocarbon vapor by methyl- or phenyl-groups modified silica materials has been studied experimentally [7] by coating these materials around a fiber. In this paper, fabrication of a planar waveguide ring resonator by a sol–gel method is described, and its sensitivities to ethanol vapor are presented, indicating that the ∗
Corresponding author. E-mail address: [email protected] (F. Pang).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.03.031
planar waveguide ring resonator may be a promising candidate in ethanol sensing. 2. Principle of the device A scheme of the ring resonator is shown in Fig. 1(a). It comprised of a ring and a single straight waveguide, connecting through a waveguide coupler. Its transmission is written as follows [8]: T =
t 2 + L2S − 2tLS cos(nr klr ) 1 + t 2 L2S − 2tLS cos(nr klr )
(1)
where nr is the real part of the waveguide effective refractive index, lr is the perimeter of the waveguide ring, k is wave vector in vacuum and t is the optical field amplitude splitting ratio of the coupler with power splitting ratio of t2 :(1 − t2 ). LS = exp(–ni lr ) is the loss factor that the light has experienced after propagation of single trip in the ring, including material loss and scattering loss, and ni is the imaginary part of the waveguide effective refractive index. The transmission spectra are shown in Fig. 1(b), where LS = 1 is for a lossless ring, and LS < 1 is for a ring with loss. It is seen that in case of no loss the ring resonator is an all-pass filter, and in case of some loss the transmission shows resonated dips at
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Fig. 1. Ring resonator scheme and its transmission spectra.
certain wavelengths, which satisfy the condition of nr klr = 2mπ. The depth of the transmission dip is a function of the loss factor LS . Therefore, any changes of waveguide effective refractive index and/or loss in the ring caused by environmental vapor condition may result in changes of the transmission spectrum, and can be detected optically. From the resonant condition, the resonant wavelength can be derived as nr lr . (2) m It means that the resonant dips shift to longer wavelengths with an increase in effective refractive index. From formulae (1) and (2), the transmission at resonant wavelengths can be derived as
λres =
Tres =
(LS − t)2 . (1 − tLS )2
(3)
The resonant transmission will be equal to 0 only if LS = t, which is known as the critical coupling condition. For characterization, the contrast of transmission spectra can be defined as Tantires − Tres contrast = , (4) Tantires + Tres where Tantires = (LS + t)2 /(1 + tLS )2 is the transmission at antiresonant wavelengths defined as nr klr = (2m + 1)π. From formula (4), the relation of contrast on LS and t can be depicted in Fig. 2. It is shown that in critical case, LS = t, the contrast reaches its maximum and equal to 1; it increases with an increasing in LS for LS ≤ t, and decreases for LS ≥ t. The resonant wavelength and the contrast are the most important parameters for detecting
Fig. 2. Variation of the contrast with coupler coefficient and transmission loss.
changes of the refractive index nr and the loss factor LS that may be caused by absorption of chemical vapor. 3. Fabrication of the planar waveguide ring resonator Since sol–gel materials are usually synthesized through alkoxides and somewhat porous in microscopic scale, it is reasonable to expect that they will absorb some kinds of chemical vapor molecules and show chemical sensitivities. In this work, three precursor materials, phenyltriethoxysilane (PHTES), methyltriethoxysilane (MTES) and tetraethylorthosilicate (TEOS), were used to synthesize sol–gel waveguide films. The molar fraction of the three precursors were taken as 0.65:1 for total silicon species to ethanol, 0.7:1 for water to ethoxide groups, following the published data [9]. They were mixed with acidified water (0.004 mol/L, hydrochloric acid) and ethanol, and magnetically stirred at 50 ◦ C for 24 h in a sealed flask under refluxing conditions to form a sol. The waveguide film was made by spin-coating on a silicon wafer with a thick layer of thermally oxidized silica, and then the wafer was annealed to drive off residual water, ethanol and un-reacted monomers to some extent, and to enhance condensation of silanol groups [9]. Two samples with different molar ratios were synthesized and deposited onto the wafer as core and cladding layers, respectively. The molar ratio for the core layer was PHTES:MTES:TEOS = 9:0:1, and its refractive index was obtained to be 1.550 by m-line measurement. The molar ratio for the cladding layer was PHTES:MTES:TEOS = 2:6:2, and its refractive index was measured to be 1.478. The waveguide ring resonator could then be fabricated on the wafer with the sol–gel films. A reversed ridge waveguide was adopted to form strip waveguide structure whose fabrication procedure is shown in Fig. 3. Firstly, a ring resonator pattern was etched by inductively coupled plasma (ICP) technique. The flow rates of reactive gases were 200 sccm (standard cubic centimeters per minute), 20 and 5 sccm for trifluoromethane (CHF3 ), argon and oxygen, respectively. Then the core sol was spincoated onto the grooved wafer. After the first step annealing, the cladding sol was spin-coated on the top, followed by the second step annealing. The advantage of using the reversed ridge waveguide is that it can avoid attack to the sol–gel layer by developer and cleaning process when the ridge waveguide is made on sol–gel surface by photolithography, and it is much easier to clean the patterned grooves on the silica layer than the ridge
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Fig. 3. The cross-section profile of the ridge waveguide in processing.
Fig. 4. Top-view micrograph of the ring resonator (a) and a magnified view at the coupling part (b).
waveguide on the sol–gel surface. Fig. 4 shows a microscopic photo of the device, and the detailed view at the coupler shows a satisfactory processing. The wafer was then cleaved and coupled with a pair of lensed single mode fiber (SMF) as the optical input and output ports. 4. Measurement of the sensitivity to ethanol of the device The fabricated ring resonator was sealed into a chamber to investigate its sensitivity to chemical vapors, as shown in Fig. 5, where amplified spontaneous emission (ASE) was the light source, and optical spectrum analyzer (OSA, type: AQ6317C by ANDO) was used to measure the transmission spectra. Ethanol vapor was taken to be the analyte in the experiment. In the measurement pure nitrogen was first flowed through the sealed chamber to purge the air and unwanted water vapor, and the spectrum was monitored and recorded, as shown in Fig. 6. The free spectral range (FSR) was read to be 0.182 nm and the refractive effective index could be calculated to be 1.528891. Then the nitrogen flow was stopped and a certain amount water-free ethanol was injected into the sealed chamber by a micro syringe and was evaporated by a small fan inside the chamber. The spectrum was measured at the same time, as shown in Fig. 6. From the injected ethanol amount and the volume of the chamber, which was about 4.8 L, the ethanol vapor concentration could be calculated. The
Fig. 5. The measurement setup.
chamber was purged by a pure nitrogen flow after every measurement to make the analyte desorbed. The resonant dip of the spectra varied with time is depicted in Fig. 7(a). It is shown in Figs. 6 and 7 that the dips shifted towards longer wavelengths and the contrast decreased with increasing the ethanol concentration. The measured results showed that the effective index of the waveguide increased, and the loss of the ring decreased, with increasing the ethanol concentration. By fitting the data to formulae (2) and (4), the effective index and loss factor can be deduced from the spectra, as shown in Table 1. The spectral curves given by the data fitting indicated that the condition of LS ≥ t was coincident with the case, resulting in inverse ratio relations between the contrast and the loss factor as shown in Fig. 2. Fig. 8 shows the variation of the dip wavelength and loss with the concentration, showing a linear relation with a slope of 1.13 pm (picometer)/ppm (part per million) and 0.0026 ppm−1 . The concentration changed from 0 to about 160 ppm in the experiment. Beyond 160 ppm the spectral contrast decreased to a too low level to be read out precisely, and the dip shift exceeded
Fig. 6. Transmission spectra changes with increasing the ethanol vapor concentration. (a) Initial spectrum; (b) 31 ppm; (c) 63 ppm; (d) 95 ppm; (e) 127 ppm; (f) 159 ppm.
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Fig. 9. Relationship between the resonant wavelength shift and the waveguide chip temperature.
Fig. 7. (a) Response of resonant wavelength shift to different ethanol vapor; (b) the magnified curve of the first period for clearness.
Fig. 8. Resonant wavelength shift and loss factor LS varying with increasing the ethanol vapor concentration increasing. Circle: experimental data; line: fitted linearly.
one period, that is, a phase shift of 2π. The device seems quite sensitive in the low concentration range, but its dynamic range needs to be increased. It is possible, but needs further work, for example, by using a thicker layer, different design of the ring, or using different composition. In addition, the sensitivity of the resonant wavelength may be detected easier and more precisely by spectroscopy than that of loss measurement. In Fig. 7(b), the first period curve is depicted and magnified to show the temporal response. The resonant wavelength variation can be fitted approximately by exponential functions of λ ∝ 1 − exp(–t/τ 1 ) for absorption course and λ ∝ exp(−t/τ 2 ) for desorption with the time constants of τ 1 = 2.14 min and τ 2 = 3.88 min, respectively. Time constants of other periods are also calculated as shown in Table 1, which indicates that the time constants become shorter with an increase in ethanol vapor concentration. It could be judged from Fig. 7 that the ethanol absorption for every concentration reached a saturated state, and the desorption after every measure was sufficient enough. It was also noticed in Fig. 7 that the background wavelength after desorption shifted a little towards longer wavelengths. This shift might be attributed to the temperature change. Generally organic–inorganic hybrid materials have high temperature coefficients, as shown in many publications [10]. The temperature sensitivity of the sol–gel waveguide device fabricated in this work was measured to be −154 pm/◦ C before packaging, as shown in Fig. 9. It is noticed that the coefficient is negative, similarly to the ordinary polymer materials. The variation of the background in Fig. 7 might indicate about 0.16 ◦ C cooling. There were some factors to make temperature shifting, such as nitrogen flow, ethanol evaporation, absorption and desorption. The experiment was carried out at room temperature of 25 ◦ C,
Table 1 Spectral contrast, ring loss factor, effective index and time constants at different ethanol concentration Concentration (ppm) Contrast (dB) Loss factor LS Effective index Time constant τ 1 (min) Time constant τ 2 (min)
0 0.169 0.51 1.528891 – –
31 0.142 0.56 1.528934 2.45 3.88
63 0.103 0.65 1.528962 1.77 3.26
95 0.078 0.72 1.529001 1.65 3.21
127 0.048 0.81 1.529025 1.42 3.14
159 0.026 0.90 1.529065 1.39 2.73
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which was also variable. The temperature effect of the devices is one of the issues to be investigated in future. One of the possible schemes is to design two rings in the devise; one of them is sensitive to the analyte, while the other is insensitive, but both of them have similar temperature responses. The experimental phenomena imply that ethanol absorption will increase the refractive index of the sol–gel film, and lower its loss. It is an interesting subject to probe its essence. It may be a too big topic for this paper to cover, but one may image that the tiny holes in the porous sol–gel film are filled by absorbed ethanol molecules, and make the material more dense than in the case of the empty holes [4], and also may weaken the scattering effect by the reduced index difference at the interfaces of the holes, resulting in a reduced loss. It is an interesting and important topic to investigate the properties of the material further by more methods and also by changing the material composition. The results presented in this paper may help to understand the internal mechanisms and to develop practical devices, but there is still much work to do. It was found in the experiments that the device had sensitivities also to water vapor and chemicals other than ethanol, such as methanol, benzene, toluene in some degree. Therefore the selectivity for different analytes is remaining as an unsolved problem, so is the method of avoiding the water vapor influence in practical applications. Experimental work on these points is being undertaken. 5. Conclusion A planar waveguide ring resonator was fabricated by organic–inorganic hybrid sol–gel materials by using PHTES, MTES and TEOS precursors. The ring resonator showed resonant dips in its spectrum. The device was experimentally investigated as a sensor for ethanol vapor, due to the porous characteristic of the sol–gel material. It was found that the spectral dip shifted to a longer wavelength with increasing the ethanol vapor concentration; its sensitivity showed a linear relation between the wavelength shift and the ethanol concentration, and the coefficient was measured to be 1.13 pm/ppm. Additionally the absorption of ethanol vapor made the spectral contrast decline and the transmission loss decrease. Acknowledgment Authors would like to acknowledge Prof. Changhe Zhou and his colleagues for help with the ICP processing. References [1] R.G. Heideman, P.V. Lambeck, Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach-Zehnder interferometer system, Sens. Actuators B 61 (1999) 100–127. [2] G. Hanisch, R.P. Podgorsek, H. Franke, Origin of optical anisotropy in planar polymer waveguides, Sens. Actuators B 51 (1998) 348–354. [3] A. Klotz, A. Brecht, G. Gauglitz, Channel waveguide mode beat interferometer, Sens. Actuators B 38–39 (1997) 310–315. [4] K.R. Kribich, R. Copperwhite, H. Barry, B. Kolodziejczyk, J.-M. Sabattie, K. O’Dwyer, B.D. MacCraith, Novel chemical sensor/biosensor platform
[5] [6]
[7]
[8]
[9]
[10]
based on optical multimode interference (MMI) couplers, Sens. Actuators B 107 (2005) 188–192. E. Krioukov, D.J.W. Klunder, A. Driessen, J. Greve, C. Otto, Sensor based on an integrated optical microcavity, Opt. Lett. 27 (2002) 512–514. K. Mitsubayashi, T. Kon, Y. Hashimoto, Optical bio-sniffer for ethanol vapor using an oxygen-sensitive optical fiber, Biosens. Bioelectron. 19 (2003) 193–198. F. Abdelmalek, J.M. Chovelon, M. Lacroix, N. Jaffrezic-Renault, V. Matejec, Optical fibre sensors sensitized by phenyl-modified porous silica prepared by sol–gel, Sens. Actuators B 56 (1999) 234–242. A. Yariv, Universal relations for coupling of optical power between microresonators and dielectric waveguides, Electron. Lett. 36 (2000) 321–322. R.A. Graham, R.K. Maryla, S. Anna, Optical properties of an ormosil system comprising methyl- and phenyl-substituted silica, J. Non-Crystal. Solids 265 (2000) 210–220. E.-S. Kang, T.-H. Lee, B.-S. Bae, Measurement of the thermo-optic coefficients in sol–gel derived inorganic–inorganic hybrid material films, Appl. Phys. Lett. 81 (2002) 1438–1440.
Biographies FuFei Pang received his BS degree in Eletronics Engineering from Liaoning Normal University in 2001 and MS degree in Optical Communication in SIOM of Chinese Academy of Sciences in 2004. Now he is working for PhD candidate in Optical Communication in SIOM of Chinese Academy of Sciences. His primary area of research is design, fabrication, characterization and sensor application of novel planar waveguide components. Xiuyou Han received his BS and MS degree from Hebei Normal University in 2000 and 2003, respectively. Now, he is pursuing the PhD candidate at Shanghai Institute of Optics and Fine Mechanic, CAS. His current research interests are passive and active integrated photonic devices for optical communication and sensor systems. Fenghong Chu received his BS degree in Mechanical and Electronics Engineering from Yanshan University in 2002 and MS degree in Optical Engineering in Yanshan University in 2005. Now she is working for PhD candidate in Optical Engineering in SIOM of Chinese Academy of Sciences. Her primary area of research is sensor application of novel planar waveguide components. Jianxin Geng is the Senior Engineer of Shanghai Institute of Optics and Fine Mechanic, CAS. His current research interests is package of passive and active integrated photonic devices. Haiwen Cai received his BS degree from HuaZhong Science and Technology University in 1997. And he received PhD degree from Shanghai Institute of Optics and Fine Mechanics (SIOM) in 2002. He is currently an associate professor of SIOM. He has worked in most areas of fiber sensor technology and fiber commutation. Ronghui Qu received his BS degree from WuHan University in 1995. And he received his MS and PhD degree from Shanghai Institute of Optics and Fine Mechanics (SIOM) in 1998 and 2001, respectively. He is currently a professor of SIOM. He is a committee member of the Communication Society of China. His current research interests are novel fiber and waveguide devices for optical communication. Zujie Fang graduated from Physics Department of Fudan University, Shanghai, in 1964, and graduated from Graduate School in Semiconductor Physics in 1968. He jointed Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences in March 1968. Since then he has been working in the Division of Semiconductor Laser and Optoelectronics of the Institute. He is a Professor of SIOM, a member of the Academic Committee of SIOM. He is a committee member of the Communication Society of China. He worked as a visiting scholar in University of California at Berkeley for two more years and in University of Illinois at Urbana-Champaign for half a year. He has engaged in the researches on semiconductor lasers and photonic devices. His present research interests are in the area of fiber devices, optical waveguides and optical communications.
Sensitivity to alcohols of a planar waveguide ring resonator fabricated by a sol–gel method Fufei Pang a,b,∗ , Xiuyou Han a,b , Fenghong Chu a,b , Jianxin Geng a , Haiwen Cai a , Ronghui Qu a , Zujie Fang a a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 27 October 2005; received in revised form 16 March 2006; accepted 16 March 2006 Available online 12 May 2006
Abstract A planar waveguide ring resonator was fabricated by organic–inorganic hybrid sol–gel materials; its sensitivity to ethanol vapor was experimentally investigated. It was found that dips in the transmission spectrum of the device shifted to longer wavelengths with increasing the ethanol concentration, and its sensitivity showed a linear relation with the ethanol concentration, showing a coefficient of 1.13 pm/ppm. In addition, the transmission loss of the ring resonator decreased with increasing the ethanol concentration. The measured characteristics suggest that the device may be considered as one of the candidates of alcohol vapor sensors. © 2006 Elsevier B.V. All rights reserved. Keywords: Chemical vapor sensor; Integrated optics; Ring resonator
1. Introduction Integrated optical sensors have been used in chemical and biological analyte detection. Various methods, such as MachZehnder interferometers [1], birefringence interferometers of planar or channel waveguides [2,3], multimode interferometers [4] and planar waveguide ring resonators [5], have been proposed and investigated experimentally. Compared to the other schemes, ring resonators have distinctive advantages such as high wavelength selectivity and high sensitivity. Chemical vapor analyses are needed in many fields such as environmental assessments, breath diagnosis, freshness analysis, etc. High ethanol vapor concentration may cause inflammation of the nasal mucous membrane and conjunctiva, irritation of the skin, and even alcohol poisoning [6]. Absorption of hydrocarbon vapor by methyl- or phenyl-groups modified silica materials has been studied experimentally [7] by coating these materials around a fiber. In this paper, fabrication of a planar waveguide ring resonator by a sol–gel method is described, and its sensitivities to ethanol vapor are presented, indicating that the ∗
Corresponding author. E-mail address: [email protected] (F. Pang).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.03.031
planar waveguide ring resonator may be a promising candidate in ethanol sensing. 2. Principle of the device A scheme of the ring resonator is shown in Fig. 1(a). It comprised of a ring and a single straight waveguide, connecting through a waveguide coupler. Its transmission is written as follows [8]: T =
t 2 + L2S − 2tLS cos(nr klr ) 1 + t 2 L2S − 2tLS cos(nr klr )
(1)
where nr is the real part of the waveguide effective refractive index, lr is the perimeter of the waveguide ring, k is wave vector in vacuum and t is the optical field amplitude splitting ratio of the coupler with power splitting ratio of t2 :(1 − t2 ). LS = exp(–ni lr ) is the loss factor that the light has experienced after propagation of single trip in the ring, including material loss and scattering loss, and ni is the imaginary part of the waveguide effective refractive index. The transmission spectra are shown in Fig. 1(b), where LS = 1 is for a lossless ring, and LS < 1 is for a ring with loss. It is seen that in case of no loss the ring resonator is an all-pass filter, and in case of some loss the transmission shows resonated dips at
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Fig. 1. Ring resonator scheme and its transmission spectra.
certain wavelengths, which satisfy the condition of nr klr = 2mπ. The depth of the transmission dip is a function of the loss factor LS . Therefore, any changes of waveguide effective refractive index and/or loss in the ring caused by environmental vapor condition may result in changes of the transmission spectrum, and can be detected optically. From the resonant condition, the resonant wavelength can be derived as nr lr . (2) m It means that the resonant dips shift to longer wavelengths with an increase in effective refractive index. From formulae (1) and (2), the transmission at resonant wavelengths can be derived as
λres =
Tres =
(LS − t)2 . (1 − tLS )2
(3)
The resonant transmission will be equal to 0 only if LS = t, which is known as the critical coupling condition. For characterization, the contrast of transmission spectra can be defined as Tantires − Tres contrast = , (4) Tantires + Tres where Tantires = (LS + t)2 /(1 + tLS )2 is the transmission at antiresonant wavelengths defined as nr klr = (2m + 1)π. From formula (4), the relation of contrast on LS and t can be depicted in Fig. 2. It is shown that in critical case, LS = t, the contrast reaches its maximum and equal to 1; it increases with an increasing in LS for LS ≤ t, and decreases for LS ≥ t. The resonant wavelength and the contrast are the most important parameters for detecting
Fig. 2. Variation of the contrast with coupler coefficient and transmission loss.
changes of the refractive index nr and the loss factor LS that may be caused by absorption of chemical vapor. 3. Fabrication of the planar waveguide ring resonator Since sol–gel materials are usually synthesized through alkoxides and somewhat porous in microscopic scale, it is reasonable to expect that they will absorb some kinds of chemical vapor molecules and show chemical sensitivities. In this work, three precursor materials, phenyltriethoxysilane (PHTES), methyltriethoxysilane (MTES) and tetraethylorthosilicate (TEOS), were used to synthesize sol–gel waveguide films. The molar fraction of the three precursors were taken as 0.65:1 for total silicon species to ethanol, 0.7:1 for water to ethoxide groups, following the published data [9]. They were mixed with acidified water (0.004 mol/L, hydrochloric acid) and ethanol, and magnetically stirred at 50 ◦ C for 24 h in a sealed flask under refluxing conditions to form a sol. The waveguide film was made by spin-coating on a silicon wafer with a thick layer of thermally oxidized silica, and then the wafer was annealed to drive off residual water, ethanol and un-reacted monomers to some extent, and to enhance condensation of silanol groups [9]. Two samples with different molar ratios were synthesized and deposited onto the wafer as core and cladding layers, respectively. The molar ratio for the core layer was PHTES:MTES:TEOS = 9:0:1, and its refractive index was obtained to be 1.550 by m-line measurement. The molar ratio for the cladding layer was PHTES:MTES:TEOS = 2:6:2, and its refractive index was measured to be 1.478. The waveguide ring resonator could then be fabricated on the wafer with the sol–gel films. A reversed ridge waveguide was adopted to form strip waveguide structure whose fabrication procedure is shown in Fig. 3. Firstly, a ring resonator pattern was etched by inductively coupled plasma (ICP) technique. The flow rates of reactive gases were 200 sccm (standard cubic centimeters per minute), 20 and 5 sccm for trifluoromethane (CHF3 ), argon and oxygen, respectively. Then the core sol was spincoated onto the grooved wafer. After the first step annealing, the cladding sol was spin-coated on the top, followed by the second step annealing. The advantage of using the reversed ridge waveguide is that it can avoid attack to the sol–gel layer by developer and cleaning process when the ridge waveguide is made on sol–gel surface by photolithography, and it is much easier to clean the patterned grooves on the silica layer than the ridge
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Fig. 3. The cross-section profile of the ridge waveguide in processing.
Fig. 4. Top-view micrograph of the ring resonator (a) and a magnified view at the coupling part (b).
waveguide on the sol–gel surface. Fig. 4 shows a microscopic photo of the device, and the detailed view at the coupler shows a satisfactory processing. The wafer was then cleaved and coupled with a pair of lensed single mode fiber (SMF) as the optical input and output ports. 4. Measurement of the sensitivity to ethanol of the device The fabricated ring resonator was sealed into a chamber to investigate its sensitivity to chemical vapors, as shown in Fig. 5, where amplified spontaneous emission (ASE) was the light source, and optical spectrum analyzer (OSA, type: AQ6317C by ANDO) was used to measure the transmission spectra. Ethanol vapor was taken to be the analyte in the experiment. In the measurement pure nitrogen was first flowed through the sealed chamber to purge the air and unwanted water vapor, and the spectrum was monitored and recorded, as shown in Fig. 6. The free spectral range (FSR) was read to be 0.182 nm and the refractive effective index could be calculated to be 1.528891. Then the nitrogen flow was stopped and a certain amount water-free ethanol was injected into the sealed chamber by a micro syringe and was evaporated by a small fan inside the chamber. The spectrum was measured at the same time, as shown in Fig. 6. From the injected ethanol amount and the volume of the chamber, which was about 4.8 L, the ethanol vapor concentration could be calculated. The
Fig. 5. The measurement setup.
chamber was purged by a pure nitrogen flow after every measurement to make the analyte desorbed. The resonant dip of the spectra varied with time is depicted in Fig. 7(a). It is shown in Figs. 6 and 7 that the dips shifted towards longer wavelengths and the contrast decreased with increasing the ethanol concentration. The measured results showed that the effective index of the waveguide increased, and the loss of the ring decreased, with increasing the ethanol concentration. By fitting the data to formulae (2) and (4), the effective index and loss factor can be deduced from the spectra, as shown in Table 1. The spectral curves given by the data fitting indicated that the condition of LS ≥ t was coincident with the case, resulting in inverse ratio relations between the contrast and the loss factor as shown in Fig. 2. Fig. 8 shows the variation of the dip wavelength and loss with the concentration, showing a linear relation with a slope of 1.13 pm (picometer)/ppm (part per million) and 0.0026 ppm−1 . The concentration changed from 0 to about 160 ppm in the experiment. Beyond 160 ppm the spectral contrast decreased to a too low level to be read out precisely, and the dip shift exceeded
Fig. 6. Transmission spectra changes with increasing the ethanol vapor concentration. (a) Initial spectrum; (b) 31 ppm; (c) 63 ppm; (d) 95 ppm; (e) 127 ppm; (f) 159 ppm.
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Fig. 9. Relationship between the resonant wavelength shift and the waveguide chip temperature.
Fig. 7. (a) Response of resonant wavelength shift to different ethanol vapor; (b) the magnified curve of the first period for clearness.
Fig. 8. Resonant wavelength shift and loss factor LS varying with increasing the ethanol vapor concentration increasing. Circle: experimental data; line: fitted linearly.
one period, that is, a phase shift of 2π. The device seems quite sensitive in the low concentration range, but its dynamic range needs to be increased. It is possible, but needs further work, for example, by using a thicker layer, different design of the ring, or using different composition. In addition, the sensitivity of the resonant wavelength may be detected easier and more precisely by spectroscopy than that of loss measurement. In Fig. 7(b), the first period curve is depicted and magnified to show the temporal response. The resonant wavelength variation can be fitted approximately by exponential functions of λ ∝ 1 − exp(–t/τ 1 ) for absorption course and λ ∝ exp(−t/τ 2 ) for desorption with the time constants of τ 1 = 2.14 min and τ 2 = 3.88 min, respectively. Time constants of other periods are also calculated as shown in Table 1, which indicates that the time constants become shorter with an increase in ethanol vapor concentration. It could be judged from Fig. 7 that the ethanol absorption for every concentration reached a saturated state, and the desorption after every measure was sufficient enough. It was also noticed in Fig. 7 that the background wavelength after desorption shifted a little towards longer wavelengths. This shift might be attributed to the temperature change. Generally organic–inorganic hybrid materials have high temperature coefficients, as shown in many publications [10]. The temperature sensitivity of the sol–gel waveguide device fabricated in this work was measured to be −154 pm/◦ C before packaging, as shown in Fig. 9. It is noticed that the coefficient is negative, similarly to the ordinary polymer materials. The variation of the background in Fig. 7 might indicate about 0.16 ◦ C cooling. There were some factors to make temperature shifting, such as nitrogen flow, ethanol evaporation, absorption and desorption. The experiment was carried out at room temperature of 25 ◦ C,
Table 1 Spectral contrast, ring loss factor, effective index and time constants at different ethanol concentration Concentration (ppm) Contrast (dB) Loss factor LS Effective index Time constant τ 1 (min) Time constant τ 2 (min)
0 0.169 0.51 1.528891 – –
31 0.142 0.56 1.528934 2.45 3.88
63 0.103 0.65 1.528962 1.77 3.26
95 0.078 0.72 1.529001 1.65 3.21
127 0.048 0.81 1.529025 1.42 3.14
159 0.026 0.90 1.529065 1.39 2.73
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which was also variable. The temperature effect of the devices is one of the issues to be investigated in future. One of the possible schemes is to design two rings in the devise; one of them is sensitive to the analyte, while the other is insensitive, but both of them have similar temperature responses. The experimental phenomena imply that ethanol absorption will increase the refractive index of the sol–gel film, and lower its loss. It is an interesting subject to probe its essence. It may be a too big topic for this paper to cover, but one may image that the tiny holes in the porous sol–gel film are filled by absorbed ethanol molecules, and make the material more dense than in the case of the empty holes [4], and also may weaken the scattering effect by the reduced index difference at the interfaces of the holes, resulting in a reduced loss. It is an interesting and important topic to investigate the properties of the material further by more methods and also by changing the material composition. The results presented in this paper may help to understand the internal mechanisms and to develop practical devices, but there is still much work to do. It was found in the experiments that the device had sensitivities also to water vapor and chemicals other than ethanol, such as methanol, benzene, toluene in some degree. Therefore the selectivity for different analytes is remaining as an unsolved problem, so is the method of avoiding the water vapor influence in practical applications. Experimental work on these points is being undertaken. 5. Conclusion A planar waveguide ring resonator was fabricated by organic–inorganic hybrid sol–gel materials by using PHTES, MTES and TEOS precursors. The ring resonator showed resonant dips in its spectrum. The device was experimentally investigated as a sensor for ethanol vapor, due to the porous characteristic of the sol–gel material. It was found that the spectral dip shifted to a longer wavelength with increasing the ethanol vapor concentration; its sensitivity showed a linear relation between the wavelength shift and the ethanol concentration, and the coefficient was measured to be 1.13 pm/ppm. Additionally the absorption of ethanol vapor made the spectral contrast decline and the transmission loss decrease. Acknowledgment Authors would like to acknowledge Prof. Changhe Zhou and his colleagues for help with the ICP processing. References [1] R.G. Heideman, P.V. Lambeck, Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach-Zehnder interferometer system, Sens. Actuators B 61 (1999) 100–127. [2] G. Hanisch, R.P. Podgorsek, H. Franke, Origin of optical anisotropy in planar polymer waveguides, Sens. Actuators B 51 (1998) 348–354. [3] A. Klotz, A. Brecht, G. Gauglitz, Channel waveguide mode beat interferometer, Sens. Actuators B 38–39 (1997) 310–315. [4] K.R. Kribich, R. Copperwhite, H. Barry, B. Kolodziejczyk, J.-M. Sabattie, K. O’Dwyer, B.D. MacCraith, Novel chemical sensor/biosensor platform
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Biographies FuFei Pang received his BS degree in Eletronics Engineering from Liaoning Normal University in 2001 and MS degree in Optical Communication in SIOM of Chinese Academy of Sciences in 2004. Now he is working for PhD candidate in Optical Communication in SIOM of Chinese Academy of Sciences. His primary area of research is design, fabrication, characterization and sensor application of novel planar waveguide components. Xiuyou Han received his BS and MS degree from Hebei Normal University in 2000 and 2003, respectively. Now, he is pursuing the PhD candidate at Shanghai Institute of Optics and Fine Mechanic, CAS. His current research interests are passive and active integrated photonic devices for optical communication and sensor systems. Fenghong Chu received his BS degree in Mechanical and Electronics Engineering from Yanshan University in 2002 and MS degree in Optical Engineering in Yanshan University in 2005. Now she is working for PhD candidate in Optical Engineering in SIOM of Chinese Academy of Sciences. Her primary area of research is sensor application of novel planar waveguide components. Jianxin Geng is the Senior Engineer of Shanghai Institute of Optics and Fine Mechanic, CAS. His current research interests is package of passive and active integrated photonic devices. Haiwen Cai received his BS degree from HuaZhong Science and Technology University in 1997. And he received PhD degree from Shanghai Institute of Optics and Fine Mechanics (SIOM) in 2002. He is currently an associate professor of SIOM. He has worked in most areas of fiber sensor technology and fiber commutation. Ronghui Qu received his BS degree from WuHan University in 1995. And he received his MS and PhD degree from Shanghai Institute of Optics and Fine Mechanics (SIOM) in 1998 and 2001, respectively. He is currently a professor of SIOM. He is a committee member of the Communication Society of China. His current research interests are novel fiber and waveguide devices for optical communication. Zujie Fang graduated from Physics Department of Fudan University, Shanghai, in 1964, and graduated from Graduate School in Semiconductor Physics in 1968. He jointed Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences in March 1968. Since then he has been working in the Division of Semiconductor Laser and Optoelectronics of the Institute. He is a Professor of SIOM, a member of the Academic Committee of SIOM. He is a committee member of the Communication Society of China. He worked as a visiting scholar in University of California at Berkeley for two more years and in University of Illinois at Urbana-Champaign for half a year. He has engaged in the researches on semiconductor lasers and photonic devices. His present research interests are in the area of fiber devices, optical waveguides and optical communications.