Refractive index sensing characterization of a singlemode–claddingless–singlemode fiber structure based fiber ring cavity laser Zhi-bo Liu,1,2,* Zhongwei Tan, Bin Yin, Yunlong Bai, and Shuisheng Jian 1
Key Lab of All optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing, 100044 China. 2 Institute of Lightwave Technology, Beijing Jiaotong University, Beijing, 100044, China * [email protected]
Abstract: This paper firstly demonstrated the refractive index (RI) characteristics of a singlemode-claddingless-singlemode fiber structure filter based fiber ring cavity laser sensing system. The experiment shows that the lasing wavelength shifts to red side with the ambient RI increase. Linear and parabolic fitting are both done to the measurements. The linear fitting result shows a good linearity for applications in some areas with the determination coefficient of 0.993. And a sensitivity of ~131.64nm/RIU is experimentally achieved with the aqueous solution RI ranging from 1.333 to 1.3707, which is competitively compared to other existing fiber-optic sensors. While the 2 order polynomial fitting function, which determination relationship is higher than 0.999, can be used to some more rigorous monitoring. The proposed fiber laser has a SNR of ~50dB, and 3dB bandwidth ~0.03nm. ©2014 Optical Society of America OCIS codes: (280.0280) Remote sensing and sensors; (280.3420) Laser sensors.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
N. Chen, B. Yun, and Y. Cui, “Cladding mode resonances of etch-eroded fiber Bragg grating for ambient refractive index sensing,” Appl. Phys. Lett. 88(13), 133902 (2006). V. Bhatia and A. M. Vengsarkar, “Optical fiber long-period grating sensors,” Opt. Lett. 21(9), 692–694 (1996). J. Zhang, Q. Sun, R. Liang, J. Wo, D. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett. 37(14), 2925–2927 (2012). X. Wu, J. Zhang, J. Chen, C. Zhao, and Q. Gong, “Refractive index sensor based on surface-plasmon interference,” Opt. Lett. 34(3), 392–394 (2009). Z. Tian, S. S.-H. Yam, J. Barnes, W. Bock, P. Greig, J. M. Fraser, H. P. Loock, and R. D. Oleschuk, “Refractive index sensing with Mach-Zehnder interferometer based on concatenating two single-mode fiber tapers,” Photonics Technology Letters, IEEE. 20(8), 626–628 (2008). J.-F. Ding, A. P. Zhang, L.-Y. Shao, J.-H. Yan, and S. He, “Fiber-taper seeded long-period grating pair as a highly sensitive refractive-index sensor,” Photonics Technology Letters, IEEE. 17(6), 1247–1249 (2005). S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode-multimode-single-mode structures,” Lightwave Technology, Journalism 27, 2348–2356 (2009). A. Mehta, W. Mohammed, and E. G. Johnson, “Mutimode interference-based fiber-optic displacement sensor,” Photonics Technology Letters, IEEE. 15(8), 1129–1131 (2003). D. Ðonlagić and M. Završnik, “Fiber-optic microbend sensor structure,” Opt. Lett. 22(11), 837–839 (1997). E. Li, X. Wang, and C. Zhang, “Fiber-optic temperature sensor based on interference of selective higher-order modes,” Appl. Phys. Lett. 89(091119), 1–3 (2006). Q. Wang, Y. Semenova, P. Wang, and G. Farrell, “High sensitivity SMS fiber structure based refractermeteranalysis and experiment,” Opt. Express 31, 7937–7944 (2011). X. Lan, J. Huang, Q. Han, T. Wei, Z. Gao, H. Jiang, J. Dong, and H. Xiao, “Fiber ring laser interrogated zeolitecoated singlemode-multimode-singlemode structure for trace chemical detection,” Opt. Lett. 37(11), 1998–2000 (2012). W. S. Mohammed, A. Mehta, and E. G. Johnson, “Wavelength tunable fiber lens based on multimode interference,” Lightwave Technology, Journalism 22, 469–477 (2004). W. S. Mohammed, P. W. E. Smith, and X. Gu, “All-fiber multimode interference bandpass filter,” Opt. Lett. 31(17), 2547–2549 (2006).
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5037
15. L. Cheng, J. Han, Z. Guo, L. Jin, and B.-O. Guan, “Faraday-rotation-based miniature magnetic field sensor using polarimetric heterodyning fiber grating laser,” Opt. Lett. 38(5), 688–690 (2013). 16. Z. Yin, L. Gao, S. Liu, L. Zhang, F. Wu, L. Chen, and X. Chen, “Fiber ring laser sensor for temperature measurement,” Lightwave Technology, Journalism 28, 3403–3408 (2010).
1. Introduction Refractive index (RI) is an important parameter in chemical and biotechnological industries. All-fiber RI sensors have been widely used due to their distinct advantages such as high sensitivity, compact size and immunity to electromagnetic interference. Many ways, such as fiber Bragg gratings (FBGs) [1], long period fiber gratings (LPFGs) [2], Fabry-Perot interferometry [3], fiber surface plasma technology [4], fiber taper technology [5] and the combination of two or more techniques above [6], have been demonstrated in RI measurement. However, the fabrication of LPFG and FBG (generally require phase mask and UV laser) is complicated and expensive. While the taper based fiber device is fragile and need special splicing program. Recently, singlemode-multimode-singlemode (SMS) fiber structure based optical devices [7–13] have been investigated for various applications benefits from their unique advantages, ease of fabricated and low cost, for example. The structure is fabricated by using a section of multimode fiber (MMF) sandwiched splicing into two singlemode fibers (SMF) leads. Several applications such as the measurement of axial strain [7], displacement [8], micro-bend [9], temperature [10], RI [11], and chemical vapor concentration [12] have been demonstrated and show a great performance. And some researchers studied the SMS characteristics as fiber lens [13] and band-pass filter [14]. It is worth noting that the ASE C-band light source is used in the conventional SMS fiber structure based sensing systems [7–11], which intensity is low. And thus causes the transmission spectrum of the sensing system relatively low. Another problem existed in the SCS based sensing systems is that the 3dB bandwidth of the transmission spectrum is large, and thus these systems have a lower resolution. All those problems will cause the measurements inaccuracy, which should be restrained in practical applications Fiber laser sensor [12,] [15], [16] is investigated extensively for their excellent performance of high intensity and narrow 3dB bandwidth. Fiber laser sensors is conventionally divided into two types according to the characteristic parameters monitored in the system, one is based on the varied lasing wavelength, while another monitors the changed beat frequency because of ambient disturbance. Though the fiber laser sensors have been widely demonstrated, its applications in measurement of ambient RI have not been further discussed. Lan [11] demonstrated fiber vapor sensor using an fiber ring laser combined with a SMS fiber structure with coating. The scheme measured the concentration of the chemical vapor using refractive characteristic of the coated zeolite when the film is exposed to chemical vapor, which is complicated in operation. The coating can be considered as the cladding of the MMF for the origin cladding is etched. And thus difficult ensure the consistent of the sensor. In this paper, we proposed and demonstrated a RI sensor using fiber ring laser interrogated with a singlemode-claddingless-singlemode (SCS) fiber structure. The SCS structure in this laser sensing system is used not only as the filter but as the sensing head. The proposal has the additional advantages of higher OSNR and narrower 3dB bandwidth than the conventional sensing system based on SCS fiber structure. The linear characteristic of the sensor is well enough in small range measurement applications. The measurements has also been fitted using 2 order polynomial fitting, the coefficient of determination is higher than 0.999, shows that the proposal can be applied in measurement of RI after well calibrated, though the calculation is a little complex than that linear result. 2. Principle and Experiment setup 2.1. RI characteristics of the SMS fiber structure The basis of this paper is the concept of multimode interference (MMI) effect in the MMF section when the light is injected from a SMF leads. Assuming that the spliced point between
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5038
SMF and MMF is ideally aligned and the MMF has circular cross, thus only linear polarized radial modes will be excited and transmitted in the MMF section. The power distribution in the MMF is determined by the coupling coefficient from LP01 to LP0m mode. The coupling coefficient, according to coupling mode equations, can be expressed by cn
=
∞
0 ∞
0
E s ( r )Φ n ( r ) rdr Φ n ( r )Φ n ( r ) rdr
(1)
Where Es(r) and Φn(r) are the eigenmode of the input SMF and the nth eigenmode of the MMF. The field distribution after propagate a distance z can be expressed as N
E ( r , z ) = cn Φ n ( r ) exp( j β n z )
(2)
n =0
Where βn is the longitudinal propagate constant of nth mode within the MMF. N is the total mode number supported by the MMF. When the ambient RI changes, the propagation constant changes, and hence causes the transmission spectrum changes. In the other word, the pass-band of the SCS fiber structure filter is varied when there is a variation of ambient RI, for the reason that the ambient aqueous solution act as the cladding of the claddingless fiber. At the junction between MMF and output SMF leads, the higher order modes are filtered. According to the MMI theory, the wavelength spacing can be expressed as follows
λwp =
16neff a 2
(3) ( m − n )[2( m + n ) − 1]L Where neff is the effective refractive index of the wavelength, a and L are the radius and length of MMF, m and n are the mode order. Therefore the peak wavelength of the filter will change when there is a variation of the ambient solution. 2.2 Experiment setup The conventional sensing system based on SCS fiber structure is established according to the configuration shown in Fig. 1. The homemade claddingless fiber is spliced between two SMF28s using a commercial splicer (FSM-60s, Fujikuwa). The diameter of the homemade claddingless fiber is 104μm. The optical spectrum analyzer (OSA, YOKOGAWA AQ6375) is used to record the varied spectra when the ambient RI is changed.
Fig. 1. Configuration of conventional SCS based RI sensing system. Inset: schematic of the SCS fiber structure.
The proposed fiber ring cavity laser is also founded in accordance with the schematic shown in Fig. 2 (a).
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5039
The fiber ring cavity is formed by a section of claddingless fiber with length of 23.3cm, an isolator, a wavelength division multiplexer (WDM) of 980nm and 1550nm, and a section of homemade Er-doped fiber (EDF). A 976nm pump light source and OSA are also used in this configuration. From the Eq. (3), it can be easily obtained that the wavelength spacing is inversely proportional to the length of MMF and the function relationship between the length of claddingless fiber and the FSR of the filter is shown is Fig. 2 (b). Thus, the length of the MMF is either too short for large wavelength spacing or too long for complicated operation. As such, in the experiment setup, the length of the MMF is chosen to be ~25cm, at which the wavelength spacing is less than 20nm. While after splicing the claddingless fiber between the two SMFs with low insertion loss (
Key Lab of All optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing, 100044 China. 2 Institute of Lightwave Technology, Beijing Jiaotong University, Beijing, 100044, China * [email protected]
Abstract: This paper firstly demonstrated the refractive index (RI) characteristics of a singlemode-claddingless-singlemode fiber structure filter based fiber ring cavity laser sensing system. The experiment shows that the lasing wavelength shifts to red side with the ambient RI increase. Linear and parabolic fitting are both done to the measurements. The linear fitting result shows a good linearity for applications in some areas with the determination coefficient of 0.993. And a sensitivity of ~131.64nm/RIU is experimentally achieved with the aqueous solution RI ranging from 1.333 to 1.3707, which is competitively compared to other existing fiber-optic sensors. While the 2 order polynomial fitting function, which determination relationship is higher than 0.999, can be used to some more rigorous monitoring. The proposed fiber laser has a SNR of ~50dB, and 3dB bandwidth ~0.03nm. ©2014 Optical Society of America OCIS codes: (280.0280) Remote sensing and sensors; (280.3420) Laser sensors.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
N. Chen, B. Yun, and Y. Cui, “Cladding mode resonances of etch-eroded fiber Bragg grating for ambient refractive index sensing,” Appl. Phys. Lett. 88(13), 133902 (2006). V. Bhatia and A. M. Vengsarkar, “Optical fiber long-period grating sensors,” Opt. Lett. 21(9), 692–694 (1996). J. Zhang, Q. Sun, R. Liang, J. Wo, D. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett. 37(14), 2925–2927 (2012). X. Wu, J. Zhang, J. Chen, C. Zhao, and Q. Gong, “Refractive index sensor based on surface-plasmon interference,” Opt. Lett. 34(3), 392–394 (2009). Z. Tian, S. S.-H. Yam, J. Barnes, W. Bock, P. Greig, J. M. Fraser, H. P. Loock, and R. D. Oleschuk, “Refractive index sensing with Mach-Zehnder interferometer based on concatenating two single-mode fiber tapers,” Photonics Technology Letters, IEEE. 20(8), 626–628 (2008). J.-F. Ding, A. P. Zhang, L.-Y. Shao, J.-H. Yan, and S. He, “Fiber-taper seeded long-period grating pair as a highly sensitive refractive-index sensor,” Photonics Technology Letters, IEEE. 17(6), 1247–1249 (2005). S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode-multimode-single-mode structures,” Lightwave Technology, Journalism 27, 2348–2356 (2009). A. Mehta, W. Mohammed, and E. G. Johnson, “Mutimode interference-based fiber-optic displacement sensor,” Photonics Technology Letters, IEEE. 15(8), 1129–1131 (2003). D. Ðonlagić and M. Završnik, “Fiber-optic microbend sensor structure,” Opt. Lett. 22(11), 837–839 (1997). E. Li, X. Wang, and C. Zhang, “Fiber-optic temperature sensor based on interference of selective higher-order modes,” Appl. Phys. Lett. 89(091119), 1–3 (2006). Q. Wang, Y. Semenova, P. Wang, and G. Farrell, “High sensitivity SMS fiber structure based refractermeteranalysis and experiment,” Opt. Express 31, 7937–7944 (2011). X. Lan, J. Huang, Q. Han, T. Wei, Z. Gao, H. Jiang, J. Dong, and H. Xiao, “Fiber ring laser interrogated zeolitecoated singlemode-multimode-singlemode structure for trace chemical detection,” Opt. Lett. 37(11), 1998–2000 (2012). W. S. Mohammed, A. Mehta, and E. G. Johnson, “Wavelength tunable fiber lens based on multimode interference,” Lightwave Technology, Journalism 22, 469–477 (2004). W. S. Mohammed, P. W. E. Smith, and X. Gu, “All-fiber multimode interference bandpass filter,” Opt. Lett. 31(17), 2547–2549 (2006).
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5037
15. L. Cheng, J. Han, Z. Guo, L. Jin, and B.-O. Guan, “Faraday-rotation-based miniature magnetic field sensor using polarimetric heterodyning fiber grating laser,” Opt. Lett. 38(5), 688–690 (2013). 16. Z. Yin, L. Gao, S. Liu, L. Zhang, F. Wu, L. Chen, and X. Chen, “Fiber ring laser sensor for temperature measurement,” Lightwave Technology, Journalism 28, 3403–3408 (2010).
1. Introduction Refractive index (RI) is an important parameter in chemical and biotechnological industries. All-fiber RI sensors have been widely used due to their distinct advantages such as high sensitivity, compact size and immunity to electromagnetic interference. Many ways, such as fiber Bragg gratings (FBGs) [1], long period fiber gratings (LPFGs) [2], Fabry-Perot interferometry [3], fiber surface plasma technology [4], fiber taper technology [5] and the combination of two or more techniques above [6], have been demonstrated in RI measurement. However, the fabrication of LPFG and FBG (generally require phase mask and UV laser) is complicated and expensive. While the taper based fiber device is fragile and need special splicing program. Recently, singlemode-multimode-singlemode (SMS) fiber structure based optical devices [7–13] have been investigated for various applications benefits from their unique advantages, ease of fabricated and low cost, for example. The structure is fabricated by using a section of multimode fiber (MMF) sandwiched splicing into two singlemode fibers (SMF) leads. Several applications such as the measurement of axial strain [7], displacement [8], micro-bend [9], temperature [10], RI [11], and chemical vapor concentration [12] have been demonstrated and show a great performance. And some researchers studied the SMS characteristics as fiber lens [13] and band-pass filter [14]. It is worth noting that the ASE C-band light source is used in the conventional SMS fiber structure based sensing systems [7–11], which intensity is low. And thus causes the transmission spectrum of the sensing system relatively low. Another problem existed in the SCS based sensing systems is that the 3dB bandwidth of the transmission spectrum is large, and thus these systems have a lower resolution. All those problems will cause the measurements inaccuracy, which should be restrained in practical applications Fiber laser sensor [12,] [15], [16] is investigated extensively for their excellent performance of high intensity and narrow 3dB bandwidth. Fiber laser sensors is conventionally divided into two types according to the characteristic parameters monitored in the system, one is based on the varied lasing wavelength, while another monitors the changed beat frequency because of ambient disturbance. Though the fiber laser sensors have been widely demonstrated, its applications in measurement of ambient RI have not been further discussed. Lan [11] demonstrated fiber vapor sensor using an fiber ring laser combined with a SMS fiber structure with coating. The scheme measured the concentration of the chemical vapor using refractive characteristic of the coated zeolite when the film is exposed to chemical vapor, which is complicated in operation. The coating can be considered as the cladding of the MMF for the origin cladding is etched. And thus difficult ensure the consistent of the sensor. In this paper, we proposed and demonstrated a RI sensor using fiber ring laser interrogated with a singlemode-claddingless-singlemode (SCS) fiber structure. The SCS structure in this laser sensing system is used not only as the filter but as the sensing head. The proposal has the additional advantages of higher OSNR and narrower 3dB bandwidth than the conventional sensing system based on SCS fiber structure. The linear characteristic of the sensor is well enough in small range measurement applications. The measurements has also been fitted using 2 order polynomial fitting, the coefficient of determination is higher than 0.999, shows that the proposal can be applied in measurement of RI after well calibrated, though the calculation is a little complex than that linear result. 2. Principle and Experiment setup 2.1. RI characteristics of the SMS fiber structure The basis of this paper is the concept of multimode interference (MMI) effect in the MMF section when the light is injected from a SMF leads. Assuming that the spliced point between
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5038
SMF and MMF is ideally aligned and the MMF has circular cross, thus only linear polarized radial modes will be excited and transmitted in the MMF section. The power distribution in the MMF is determined by the coupling coefficient from LP01 to LP0m mode. The coupling coefficient, according to coupling mode equations, can be expressed by cn
=
∞
0 ∞
0
E s ( r )Φ n ( r ) rdr Φ n ( r )Φ n ( r ) rdr
(1)
Where Es(r) and Φn(r) are the eigenmode of the input SMF and the nth eigenmode of the MMF. The field distribution after propagate a distance z can be expressed as N
E ( r , z ) = cn Φ n ( r ) exp( j β n z )
(2)
n =0
Where βn is the longitudinal propagate constant of nth mode within the MMF. N is the total mode number supported by the MMF. When the ambient RI changes, the propagation constant changes, and hence causes the transmission spectrum changes. In the other word, the pass-band of the SCS fiber structure filter is varied when there is a variation of ambient RI, for the reason that the ambient aqueous solution act as the cladding of the claddingless fiber. At the junction between MMF and output SMF leads, the higher order modes are filtered. According to the MMI theory, the wavelength spacing can be expressed as follows
λwp =
16neff a 2
(3) ( m − n )[2( m + n ) − 1]L Where neff is the effective refractive index of the wavelength, a and L are the radius and length of MMF, m and n are the mode order. Therefore the peak wavelength of the filter will change when there is a variation of the ambient solution. 2.2 Experiment setup The conventional sensing system based on SCS fiber structure is established according to the configuration shown in Fig. 1. The homemade claddingless fiber is spliced between two SMF28s using a commercial splicer (FSM-60s, Fujikuwa). The diameter of the homemade claddingless fiber is 104μm. The optical spectrum analyzer (OSA, YOKOGAWA AQ6375) is used to record the varied spectra when the ambient RI is changed.
Fig. 1. Configuration of conventional SCS based RI sensing system. Inset: schematic of the SCS fiber structure.
The proposed fiber ring cavity laser is also founded in accordance with the schematic shown in Fig. 2 (a).
#202855 - $15.00 USD (C) 2014 OSA
Received 11 Dec 2013; revised 28 Jan 2014; accepted 13 Feb 2014; published 25 Feb 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005037 | OPTICS EXPRESS 5039
The fiber ring cavity is formed by a section of claddingless fiber with length of 23.3cm, an isolator, a wavelength division multiplexer (WDM) of 980nm and 1550nm, and a section of homemade Er-doped fiber (EDF). A 976nm pump light source and OSA are also used in this configuration. From the Eq. (3), it can be easily obtained that the wavelength spacing is inversely proportional to the length of MMF and the function relationship between the length of claddingless fiber and the FSR of the filter is shown is Fig. 2 (b). Thus, the length of the MMF is either too short for large wavelength spacing or too long for complicated operation. As such, in the experiment setup, the length of the MMF is chosen to be ~25cm, at which the wavelength spacing is less than 20nm. While after splicing the claddingless fiber between the two SMFs with low insertion loss (
