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Abstract
In this paper, an up-tapered fiber (UTF) cascaded with a single-mode-fiber (SMF)-formed droplet-like multimode interferometer (DLMI) for simultaneous three-parameter measurement is proposed. In our setup, a segment of coating-stripped SMF is bent into a droplet-like structure. A UTF is introduced in front of a leading-in SMF of DLMI to enhance the excitation of high-order cladding modes in SMF due to its much bigger divergence angle like a convex lens. As a result, three sets of main interference between fiber core mode and cladding modes appear and then are recovered respectively by use of Fourier-transform-interrogation-based technique. Due to the fact that each recovered interference component of the sensor responses differently to the changes in liquid level, surrounding refractive index (RI) and temperature, simultaneous liquid level, surrounding RI and temperature measurement is achieved, with maximal sensitivity of −154.00 pm/mm, −132.47 nm/RIU, 238.97 pm/°C respectively. The all fiber sensor we proposed is simple, easy fabrication and low cost, especially with better mechanical strength and suitable for practical applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber sensor for multi-parameter simultaneous measurement has extensive application in monitoring of complex environment such as ocean monitoring [1,2], biosensing [3,4], chemical sensing [5,6], which can effectively overcome cross-sensitivity among multiple parameters in fiber interferometer sensing. Hybrid interferometers applied for simultaneous measurement of multiple parameters is a feasible solution to this issue compared with compensation technique [7]. For example, those based on fiber grating embedded interferometer [8–13] in which fiber grating and the interferometer are usually affected by ambient environment in different manner, microstructures designed in fiber to form hybrid interferometers such as two Mach-Zehnder interferometers (MZIs) [14] or Fabry-Perot interferometers (FPIs) with different sensitivities to surrounding [15–18] or hybrid structure of Michelson interferometer [MI] combined MZI [19], modal interferometers constructed by some special fibers such as non-zero dispersion-shifted Fiber (NZDSF) [20], multi-core fiber [21] or photonic crystal fiber (PCF) [22]. For all of these sensors, each parameter can be extracted respectively through establishing the resolution matrix for multi-parameter sensing. These hybrid structures reported provide the solutions to the cross-sensitivity in sensing, however simultaneous measurement of more than two parameters is difficult. Although there are a few reports of three- or four-parameter simultaneous measurement [23,24], which is in favor of the comprehensive monitoring in complex environment, while the sensor is either complicate in structure [23] or difficult to overcome the cross sensitivity to multiple parameters very well due that each measurement wavelength dip has similar response to different environment parameters [24].
Recently, a kind of sensors based on bent-fiber-formed interferometers [25,26] as well as droplet-like fiber interferometers [27–31] have been employed due to their simple and compact structure, and high sensitivity to the ambient environment. This type of sensor works on modal interferences between the core mode and cladding mode owing to the light in the fiber core leaked into the cladding and then recoupled back into the fiber core in bent section of fiber. However, two-parameter simultaneous measurement is difficult to realize for this kind of interferometer except additional component such as FBG, long period grating (LPG) or Sagnac loop mirror inserted into [27–29]. It is because only one dominant cladding mode can be excited with relative high intensity while other high order cladding modes is very weak by just bending SMF. In addition, inhomogeneous strain from bent coating usually at the same time bring additional perturbation to the interference pattern, which make the output spectrum complicated [30,31].
In this paper, we propose an up-tapered fiber (UTF) cascaded with a single mode fiber (SMF)-formed droplet-like multimode interferometer (DLMI) for simultaneous three-parameter measurement. In our setup, a piece of SMF is bent into a droplet-like structure. To improve modal interferences between core mode and cladding modes in DLMI, we firstly strip off the coating of the bent SMF to keep the cladding modes from getting in, then a prefixal UTF is introduced in front of the leading-in SMF of DLMI to enhance the excitation of high-order cladding modes due to its much bigger divergence angle like a convex lens. As a result, three sets of main interference between core mode and cladding modes appear and then are recovered respectively by use of Fourier-transform-interrogation-based technique. Simultaneous liquid level, refractive index (RI), and temperature is achieved due to the fact that each recovered interference component responses differently to the changes in liquid level, RI and temperature. Maximal sensitivity of −154.00 pm/mm, −132.47 nm/RIU, 238.97 pm/°C are achieved respectively. The sensitivities for the proposed sensor are moderate among those of other multiple-parameter sensors [9,13,21,27,28]. Compared with high sensitive sensor reported in [16] which is fragile in structure, the all fiber sensor we proposed is simple, easy fabrication and low cost, especially with better mechanical strength and suitable for practical applications.
2. Fabrication and measurement principle
The schematic diagram of a UTF cascaded with SMF-formed DLMI is shown in Fig. 1, which comprises an arc-induced UTF and a SMF-formed DLMI. The UTF is fabricated by a commercial fusion splicer (Furukawa FITEL S178C), as shown in Fig. 2(a)–(d). Firstly, a segment of coating-stripped SMF is fixed on the left holder, with the end of the fiber placed 135 μm on the right side of arc, as shown in Fig. 2(a). The up taper can be fabricated through changing the common fusion mode of manual fusion, where the duration time and power of discharging is set to be 750 ms and 230 bit respectively. After discharging, the end of fiber is spliced into an up taper as shown in Fig. 2(b). Then another segment of coating-stripped SMF is fixed on the right holder and finely adjusted to align the up taper on the left side in X and Y direction respectively, as shown in Fig. 2(c). Thus the UTF is formed after the second discharging with duration time, power and overlap this time set to be 750 ms, 100 bit and 15 μm respectively. The fabricated UTF is shown in Fig. 2(d), with the waist diameter of 262 μm and the length of 290 μm. The UTF is fabricated by the high precision fusion splicer, and specific parameters are set to realize the structure required in each fabrication step, which makes the fabrication of UTF repeatable. However, the small deviation of the fabrication is inevitable due to the minute mechanical error and the arc abrasion of the fusion splicer.
Fig. 1. (a) The schematic diagram of a UTF cascaded with SMF-formed DLMI, (b) detailed light paths for three main points of (i), (ii) and (iii) in Fig. 1(a).
Fig. 2. (a), (b), (c) and (d) The fabrication steps of UTF, (e), (f), (g), (h) the fabrication steps of DLMI
In order to fabricate the UTF cascaded with the DLMI, one end of the UTF is firstly penetrated into one of two metal tubes of 0.35 mm internal diameter adhering side by side tightly on glass substrate using UV glue, then out from the other, as shown in Fig. 2(e). Thus a preliminary SMF-formed DLMI is constructed, as shown in Fig. 2(f). To improve the sensing performance of The UTF cascaded with DLMI, two pieces of glass slide are put under the DLMI to guarantee the flatness of the structure, as shown in Fig. 2(g). Then the UTF is further packaged in a heat shrink tuber (HST) with two sides heated and then shrank to make sure the stability of the sensor during measurement. After adhering the HST to the glass substrate, the UTF cascaded with DLMI is finished, as shown in Fig. 2(h).
When there is no UTF inserted in front of the DLMI. The light launched into the DLMI will be partly leaked into cladding due to the decrease of the curvature radius, some cladding modes are then excited. After propagating through the bent section, these cladding modes will gradually couple back into the fiber core due to the increase of the curvature radius at the end of droplet-like interferometer and interfering with the residual core mode. The DLMI with size around D = 0.9 cm is designed to comprehensively considering bending induced loss, fringe visibility of interference. The transmission spectrum of DLMI of D = 1.0 cm, 0.9 cm, 0.8 cm without UTF are shown in blue, orange and yellow curves respectively in the inset of Fig. 3, with corresponding spatial frequency spectra (SFS) shown in Fig. 3. From Fig. 3 we can find that for each size of DLMI only one domain peak of SFS appear, located near 0.0122 nm−1, 0.0171 nm−1 and 0.0228 nm−1 respectively, corresponding to the interference between core mode and one excited relatively low-order cladding mode which is related the curvature radius of the bent section respectively.
Fig. 3. The SFS of the DLMI of D = 1.0, 0.9, 0.8 cm without UTF respectively, inset shows the corresponding transmission spectra
In order to simultaneously measure the three parameters by use of the DLMI, additional mechanism should be further introduced to the DLMI to excite other cladding modes. In our experiment a UTF with the function of divergence like a convex lens is inserted in front of the interferometer. Compared with the DLMI without UTF, this time the diverged light coupled into the lead-in fiber of the DLMI will be excited into high-order cladding modes because of their much bigger incident angles than those occurring in the interface of the core and the cladding in bent section of DLMI. The superimposed transmission spectrum of The UTF cascaded with DLMI is shown in the inset of Fig. 4(a), and corresponding SFS are shown in Fig. 4(a). In Fig. 4(a), besides the domain peak 1 with spatial frequency of 0.0228 nm−1 which is still originated from the interference between the core mode and cladding mode excited by the bent section of the DLMI as shown in Fig. 3, another two peaks, peak 2 and peak 3 with spatial frequency of 0.0633 nm−1 and 0.0786 nm−1 appear respectively, which are originated from the interferences between the core mode and higher order cladding modes excited by the UTF. The proposed UTF structure is simple, robust and easy fabrication for high-order modes excitation compared with those structures based on long period grating (LPG) or fragile tapered-fiber which are also excite several high-order modes [32,33].
Fig. 4. (a) The SFS of The UTF cascaded with DLMI, inset shows the corresponding transmission spectrum, (b) inverse Fourier transformed cosine wavelength spectra of peak 1, peak 2 and peak 3 respectively
The three peaks of SFS in Fig. 4(a) are filtered out by a finite impulse response filter (FIR) with the corresponding central frequency of 0.0228 nm−1, 0.0633 nm−1 and 0.0786 nm−1 respectively, and the 3 dB bandwidth of 0.0001 nm−1. The corresponding cosine wavelength spectra are recovered by means of inverse Fourier transforming (IFFT) the filtered spatial frequency peak, which are depicted in red, green and blue curves in Fig. 4(b) respectively.
When ignoring the interferences among cladding modes which are relatively weak as shown in Fig. 4(a), the total intensity I for multimode interference in DLMI can be expressed as [26]
3. Experiments and discussions
Experiment setup for simultaneous measurement of three parameters based on the UTF cascaded with DLMI is shown in Fig. 5. The sensing device is put into a glass of water. The light from supercontinuum broadband optical source (SBOS) (OYSL SC-5-FC) is launched into the UTF where experiences divergence, and then into the DLMI where interference occurs. The transmission spectrum is monitored by an optical spectral analyzer (OSA, ANDO, AQ6317B) with resolution of 0.1 nm.
Fig. 5. Experiment setup for simultaneous measurement of three parameters based on the UTF cascaded with DLMI
Liquid level response is tested by use of the up-tapered DLMI. L is varied from 2 to 12 mm, and the sensor is immersed completely into the liquid when L = 12 mm, with the liquid RI of 1.3388 at the ambient temperature of 30°C. The measured superimposed transmission spectrum will shift with the rise of liquid level. The recovered wavelength spectra from peak 1, peak 2 and peak 3 in Fig. 4(b) totally exhibit blue shift with the rise of liquid level, which are depicted in insets of Fig. 6(a), (b) and (c) with wavelength dip tested near 1636 nm, 1634 nm and 1639 nm respectively. This is because effective RI of cladding modes in the liquid is larger than that in the air, which bring negative sensitivity coefficients for the rise of liquid level as illustrated in Eq. (6). Relationship between the rise of liquid level and the corresponding wavelength shifts are shown in Fig. 6(a), (b) and (c), with different liquid level response sensitivity of −154.00 pm/mm, −79.43 pm/mm and −65.43 pm/mm for peak 1. Peak 2 and peak 3 respectively obtained by using linear fitting method.
Fig. 6. Relationship between the rise of liquid level and the corresponding wavelength shift for the recovered wavelength spectra from (a) peak 1, (b) peak 2 and (c) peak 3, insets show the corresponding recovered wavelength spectra respectively
The liquid RI response is also tested by the proposed sensor, the UTF cascaded with DLMI is immersed into the index-matching liquid (surrounding) with the RIs of 1.3388, 1.3488, 1.3511, 1.3575, 1.3640, 1.3707, corresponding to the mixing of glycerol and water with proportion of 5%, 10%, 20%, 25%, 30%, 35% respectively. The effective RIs of each cladding modes increase with liquid RI, which will lead to blue shift of transmission spectrum due to the negative sensitivity coefficients of effective RI illustrated by Eq. (7). The recovered wavelength spectra from peak1, peak 2 and peak 3 are shown in the insets of Fig. 7(a), (b) and (c) respectively. The corresponding liquid RI sensitivities of −132.47 nm/RIU, −109.81 nm/RIU and −17.56 nm/RIU are obtained as shown in Fig. 7(a), (b) and (c) respectively.
Fig. 7. Relationship between the increase of RI and the corresponding wavelength shift for the recovered wavelength spectra from (a) peak 1, (b) peak 2 and (c) peak 3, insets show the corresponding recovered wavelength spectra respectively.
To investigate the temperature response of the sensor, water bath is adopted to change the ambient temperature of the sensor, the water bath from cold to hot and the temperature adjusting range is from 30 °C to 55 °C with a step of 5 °C and maintained for 5 minutes at each step, monitored by a thermometer in real time. The sensor is immersed completely into the liquid with liquid RI of 1.3707. The rise of temperature will lead to the increase of effect RI of core mode as well as cladding modes due to thermo-optic effect (the effective RI of the core mode shows a larger increase than that of the cladding modes in the commercial SMF), meanwhile cause expansion of total interference length L. The final wavelength shift is the combined action of these two effects, which is illustrated by Eq. (8). The recovered wavelength spectra from peak1, peak 2 and peak 3 exhibit red shift with the rise of temperature as shown in the insets of Fig. 8(a), (b) and (c) respectively. The corresponding temperature sensitivities of 119.66 pm/°C, 104.91 pm/°C and 238.97 pm/°C are obtained as shown in Fig. 8(a), (b) and (c) respectively.
Fig. 8. Relationship between the rise of temperature and the corresponding wavelength shift for the recovered wavelength spectra from (a) peak 1, (b) peak 2 and (c) peak 3, insets show the corresponding recovered wavelength spectra respectively.
Due to different sensitivities among peak 1, peak 2 and peak 3 for each of variations of liquid level, RI and temperature as shown in Fig. 6, Fig. 7 and Fig. 8 respectively, it is possible to simultaneously measure liquid level, RI and temperature with the help of the matrix method. The relationship between wavelength shift of each peak and the changes of liquid-level, RI and temperature can be expressed in matrix form as:
By further transformation towards the matrix, the performance of simultaneous measurement of liquid-level, RI and temperature for the UTF cascaded with DLMI is estimated by resolution matrix.
Fig. 9. Sensor output as determined by Eq. (10) for the applied liquid level at the constant liquid RI and temperature, applied liquid RI at the constant liquid level and temperature and applied liquid temperature at the constant liquid level and RI respectively
4. Conclusion
In summary, the proposed UTF cascaded with DLMI can be used for simultaneous measurement of liquid level, surrounding RI and temperature. The sensor can be fabricated by bending a segment of coating-stripped SMF into a DLMI. A UTF is introduced in front of the leading SMF of DLMI to enhance the excitation of high-order cladding modes, as a result, three sets of interference between code mode and cladding modes occur for the proposed UTF cascaded with DLMI. Owing to the feature of different interference components of DLMI exhibiting different responses to the changes of liquid level, surrounding RI and temperature, these three parameters can be simultaneously demodulated by use of matrix method. The proposed sensor is simple, easy fabrication and low cost, especially with better mechanical strength and suitable for practical applications.
Acknowledgment
The authors would like thanks Dr. Yongfeng Wu from School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology China, Dr. Minggui Wan and Chengyun Zhang from School of Physics and Electronics Engineering, Guangzhou University, China, and Prof. Xiaozhong Qiu from Provincial Key Laboratory of Tissue Construction and Detection in Tissue Engineering, School of Biomedical Engineering, Southern Medical University, China, for helpful advice.
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