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Highly Sensitive Refractive Index Optical Fiber Sensors Fabricated by a Femtosecond Laser Volume 3, Number 6, December 2011 Jinpeng Yang Lan Jiang Sumei Wang Qianghua Chen Benye Li Hai Xiao, Senior Member, IEEE

DOI: 10.1109/JPHOT.2011.2176535 1943-0655/$26.00 ©2011 IEEE

IEEE Photonics Journal

Highly Sensitive RI Optical Fiber Sensors

Highly Sensitive Refractive Index Optical Fiber Sensors Fabricated by a Femtosecond Laser Jinpeng Yang, 1 Lan Jiang, 1 Sumei Wang, 1 Qianghua Chen, 1 Benye Li, 1 and Hai Xiao,2 Senior Member, IEEE 1

Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China 2 Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65409 USA DOI: 10.1109/JPHOT.2011.2176535 1943-0655/$26.00 Ó2011 IEEE

Manuscript received September 29, 2011; revised November 9, 2011; accepted November 11, 2011. Date of publication November 18, 2011; date of current version December 6, 2011. This work was supported by the National Natural Science Foundation of China under Grant 90923039, Grant 51025521, and Grant 51005022. Corresponding author: L. Jiang (e-mail: [email protected]; [email protected]).

Abstract: Highly sensitive and robust refractive index (RI) fiber sensors are proposed, which are based on concatenating two micro air-cavities. The micro air-cavities embedded in the fiber along the core axis are fabricated by combining the techniques of femtosecond laser micromachining and arc fusion splicing. The full-width at half-maximum (FWHM) of an attenuation peak of a sensor in air is 0.048 nm, and the extinction ratio is 15 dB. When the diameter of the cavity is slightly larger than the fiber core diameter, the sensor is highly sensitive to external RI changes. A high sensitivity of 172.4 nm/refractive index unit (RIU) around the RI range of 1.333–1.365 is achieved, which is about six times higher than longperiod fiber grating (LPFG)-type and taper-type RI sensors in the same RI range. Index Terms: Fiber optics systems, sensors, ultrafast lasers.

1. Introduction Optical fiber sensors for refractive index (RI) sensing have attracted increasing research interests for their low power consumption, immunity to electromagnetism, and broad applications in biomedical, chemical, and industrial processes. Various RI fiber sensors have been developed. RI fiber sensors based on a microresonator by utilizing a whispering gallery modes sensing mechanism usually have ultrahigh resolution to external RI changes and ultrahigh quality factor [1]–[3], yet they also have some drawbacks, such as assembly difficulties and low reliability. An ultracompact fiber Mach–Zehnder interferometer (MZI) fabricated by a femtosecond laser with very high sensitivity to external RI changes has been reported [4], [5], but the very fragile structure limits its practical applications, and the structure fabrication by femtosecond laser is also time-consuming and expensive. Fiber gratings are also useful devices for RI sensing, such as fiber Bragg gratings (FBGs) [6], [7] and long-period fiber gratings (LPFGs) [8]–[10]. Some special postprocesses, such as etching, are required for FBGs applications in RI sensing, which leads to a reduction in the sensor’s mechanical strength. Tilted FBGs are also utilized to measure external RI but with a least detection resolution of 10-4 [11]. LPFGs RI sensors usually have many advantages, such as low losses, high extinction ratio, and robustness, but single LPFG directly used for RI sensing has a small sensitivity of 20–40 nm/refractive index unit (RIU) in the RI range of 1.33–1.38 [8]. Fiber MZI constructed by a

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Fig. 1. (a) Schematic diagram of the microcavity-based fiber sensor. (b) Experimental setup for fabrication of a microhole in the core of the fiber. (c) Microscopic image of the microhole with the diameter of  8 m. (d) Splicer panel view of the micro air-cavity.

LPFG pair can have a relatively large sensitivity, but at the same time, a longer dimension of the sensor is introduced [12]. Recently, core-cladding-mode interferometers used for RI sensing have been intensively studied [13]–[23]. This type of interferometer is operated based on the interference between the cladding modes and fundamental mode, because the cladding modes and fundamental mode will have a phase difference after propagating in a certain length as those modes have different effective refractive indices. Configurations of core-cladding-mode interferometers can be different, such as an LPFG pair [14], an abrupt taper pair [15]–[17], a core-offset attenuator pair [18], a combined abrupt fiber taper or core-offset attenuator with LPFG [19], [20] and a splicing singlemode optical fiber (SMF) with a section photonic crystal fiber or small core diameter fiber [21]–[23]. The fabrication of abrupt taper or core-offset attenuator based core-cladding-mode interferometer is simple and cost-effective. However, a taper-based interferometer is relatively fragile, and has low sensitivity to external RI changes (15–35 nm/RIU). The core-offset attenuator-based core-claddingmode interferometer is robust, but it gives rise to a different set of optical modes within the waveguides resulting from the asymmetrical orientation of connector offsets, and its sensitivity is also low. This study proposes highly sensitive RI fiber sensors by concatenating two micro air-cavities. The microcavity embedded in optical fiber is fabricated by combining femtosecond laser micromachine technique and fusion splicing technique. The full-width at half-maximum (FWHM) of an attenuation peak of a sensor in air is measured to be 0.048 nm, and the extinction ratio is 15 dB. A high sensitivity of 172.4 nm/RIU is achieved around the RI range of 1.333–1.365 by the proposed sensor, which is about six times higher than LPFG-type and taper-based fiber sensors. This study also investigates the impact of the cavity diameter on the performance of the proposed sensors. Furthermore, the proposed structure is more robust than taper-type and trench structure devices. With an advantage of higher RI sensitivity, it is nearly as strong as some other core-cladding-mode interferometers.

2. Fabrication and Principle 2.1. Fabrication The schematic diagram of the sensor is shown in Fig. 1(a). The two microcavities separated by a middle section are embedded inside an SMF, and their centers are aligned with the fiber core axis. Note that the diameter of the microcavity is slightly larger than the fiber core diameter in Fig. 1(a). In our experiments, an amplified Ti:sapphire laser (Spectra Physics, Inc.) of 800 nm and 35 fs at a repetition rate of 1 kHz is used to fabricate a microhole on the end of an SMF (Corning SMF-28). The cleaved fiber end held by fiber holder is mounted on a six-axial moving stage with a resolution of 1 m in the x - and y -directions and 0.5 m in the z-direction. The experimental setup for

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Fig. 2. (a) Transmission spectrum of the sensor-1. (b) Transmission spectrum of the sensor-2. (c) Transmission spectrum of the sensor-3. (d) Magnified images of peak A, peak B, peak C, and peak D, which are marked in (a) and (b).

fabrication of a microhole in the core of the fiber is shown in Fig. 1(b). The attenuated femtosecond laser beam is focused by a 20 objective lens (N.A. ¼ 0:45, Olympus), and the spot size is about 2 m. The single pulse energy before the objective lens is about 0.5 J. A cylindrical hole is fabricated on the fiber end by controlling the movement of the stage. An ablated microhole on a fiber-end is shown in Fig. 1(c). The diameter and depth of the hole are  8 m and 12 m, respectively. A micro air-cavity is formed by splicing a microhole fiber end with a normal cleaved fiber end using a conventional fusion splicer (model IFS-9, INNO INSTRUMENT, Inc.). After splicing by arc fusion, the cylindrical hole is changed into an ellipsoid cavity, as shown in Fig. 1(d). The arc power and arc time that used are 40% and 1200 ms, respectively. The equatorial diameter of the ellipsoid cavity is about 14 m, which is larger than the diameter of the hole ð 8 mÞ, and the polar diameter (longitudinal axis) is about 15 m, which is also larger than the depth of the hole ð12 mÞ. A similar micro air-cavity is fabricated about 15 mm apart from the first one along the fiber axis. The equatorial diameter and polar diameter of the second cavity are about 13 m and 15 m, respectively. A detection system (Agilent Technologies Inc.) consisting of a tunable laser (81980A) with a scanning range of 1465–1575 nm and an optical power meter (81636B) is employed to monitor the transmission spectrum. The transmission spectrum of this sensor (sensor-1) is shown in Fig. 2(a). There are many fringes in the spectrum, in which some outstanding peaks are much deeper than their lateral fringes. The second sensor (sensor-2) with a separated length of 20 mm is also fabricated. The equatorial and polar diameters of the first cavity are about 15 m and 17 m, respectively, while the second one are about 16 m and 17 m, respectively. The transmission spectrum of the second sensor (sensor-2) is shown in Fig. 2(b). For comparison, another sensor (sensor-3) with cavity diameter smaller than the fiber core, which is little different from Fig. 1(a), is also fabricated. The equatorial and polar diameters of the first cavity are about 5 m and 3 m,

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Fig. 3. Spatial frequency spectra of sensor-1 and sensor-3.

respectively, and those of the second one are also about 5 m and 3 m, respectively. The distance between the two smaller air cavities is about 15 mm. The transmission spectrum of this sensor (sensor-3) is shown in Fig. 2(c) with a maximum extinction ratio of 20 dB. Fig. 2(d) shows the magnified images of peak A, peak B, peak C, and peak D, which are marked in Fig. 2(a) and (b). The FWHM of peak A is 0.11 nm, the extinction ratio is 15 dB at the wavelength of 1492.75 nm, and that for peak B, C, and D are 0.06 nm, 17 dB, and 1556.17 nm; 0.048 nm, 15 dB, and 1483.75 nm; 0.079 nm, 13 dB, and 1505.54 nm, respectively, which are very suitable for sensing applications. The transmission spectra in Fig. 2(a) and (c) is Fourier transformed to obtain the spatial frequency spectra of the two interferometers and then to determine the number and power distribution of the coupling modes. The spatial frequency spectra are shown in Fig. 3. It can be seen from Fig. 3(a) that there are more than ten cladding modes participating in the interference. Thus, many fringes are shown in the spectrum of sensor-1. The power is primarily distributed in the fundamental mode and the first seven cladding modes, and the others make very small contribution to the interference pattern, except that they slightly increase some small fringes in the spectrum pattern. However, only a few cladding modes with low power distribution can be seen in the spatial frequency spectrum of sensor-3, as shown in Fig. 3(b).

2.2. Principle For the sensor with cavity diameter larger than fiber core, as illustrated in Fig. 1(a), when the detection light enters into the first micro air-cavity, it is split into two optical paths: one propagating through air as a leaky wave mode and the other guided along ring-shaped silica cladding. The leaky wave mode will be guided to the fiber core when it passes through the cavity and continues to propagate forward in the core. Parts of the cladding modes are coupled back to the core when there is a similar microcavity apart from the first one within a few millimeters. As the effective refractive indices of cladding modes are different from that of core mode, they will have different optical path lengths when they pass the second cavity. The number of fringes is partly determined by the size of the cavity. The larger the cavities are, the more the fringes are, and consequently, the FWHM of attenuation peaks will be smaller. Yet, larger cavities also introduce much more insertion loss to the detection system. In the spectrum, there are some outstanding peaks which are much deeper than their adjacent fringes. As aforementioned demonstration of light propagating process in this fiber sensor, an optical path length difference ðnL Þ is accumulated by the light propagating eff in the core and that in the cladding, and it can be approximated as nL ¼ ðnco  ncleff ÞL þ eff eff eff 2ðnair  ncleff ÞLC , where nco , ncleff , and nair are effective refractive indices of fiber core, cladding, and eff air, respectively. Generally, nair  1. L is the distance between the two cavities, and LC is the polar diameter of the microcavity. The wavelengths ðm Þ of the outstanding attenuation peaks are then expressed as m ¼ 2nL =½ð2m þ 1Þ

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where m is an integer. Thus, the separation ðm Þ of two adjacent outstanding peaks in the spectrum can be expressed as m ¼ m1  m ¼ m m1 =nL :

(2)

From (2), it can be seen that the separation of two adjacent outstanding peaks is determined by both of the size of cavities ðLC Þ and the length ðLÞ of the middle section fiber. Since the effective indices of the cladding modes depend on the external RI and that of core mode is independent to external RI, the function of the attenuation wavelength shift with variation of external RI can be deduced from (1) as    m L @ncleff m @n1 @n2 L2 LC d m =dnext ¼ 1 (3) nL @next nL @ @ eff eff  ncleff , n2 ¼ nair  ncleff , next is external RI, and  is the operating wavelength. where n1 ¼ nco By putting (1), i.e., m ¼ 2nL =½ð2m þ 1Þ, into (3), the function of the attenuation wavelength shift with variation of external RI can be expressed as    2L @ncleff m @n1 @n2 L2 LC : d m =dnext ¼ 1 (4) ð2m þ 1Þ @next nL @ @

From (4), it can be seen that RI sensitivity ðd m =dnext Þ of the fiber sensor increases with increasing of the distance ðLÞ between the two cavities.

3. Measurements and Discussion 3.1. RI Measurements The RI measurements of the proposed fiber sensors are carried out in a clean room with almost constant temperature in order to eliminate the effects caused by temperature fluctuations. The ends of the sensor are held by fiber holders to keep them upright. The test sample solution is put on a microscope glass slide, which is supported by a tunable stage right under the fiber sensor. The height of the stage is carefully adjusted to make sure that the sensor is totally immersed in the solution during the measurements. After each measurement, the stage is lowered, and the solution is removed. The glass and the fiber sensor are cleaned with distilled water and dried with a hairdryer. When a different RI solution is tested, the whole procedure is repeated. When the sensor-1 (with large cavities) is immersed in solution, there are fewer fringes, and the FWHM of the fringes become wider. Because higher order cladding modes have larger inject angles, and the RI of solution is larger than that of air; thus, some high order cladding modes transmit into water in the cladding-water interface and disappear after a certain propagation distance. As a result, fewer fringes and wider FWHM of attenuation peaks are shown in the transmission spectrum of the sensor in solution. The transmission spectrum of sensor-1 in 2 g sodium chloride per 100 g water solution (i.e., concentration of 1.96%) is shown in Fig. 4. The outstanding peaks, like peak A [marked in Fig. 2(a)], still exist, except they shift to a longer wavelength position. Peak B shifts to a far longer wavelength position, which is out of the wavelength scanning range. The sensor-1 is tested with sodium chloride (NaCl) solutions with various concentrations. The first group solutions are from 0–2 g sodium chloride per 100 g water with an interval of 0.2 g, (i.e., concentration from 0%–1.96%). The corresponding RIs are from 1.333 to 1.3365. The transmission spectrum in 1.96% sodium chloride solution is shown in Fig. 4. The peak A shifts to longer wavelength, as marked in Fig. 4. The spectra of the peak A in the first group NaCl solutions are shown in Fig. 5(a). High concentration sodium chloride solutions, from 2–6 g sodium chloride per 100 g water (the second group solutions with concentrations from 1.96% to 5.66%) with an interval of 1 g and from 2–22 g sodium chloride per 100 g water (the third group solutions with concentrations from 1.96% to 18.03%) with an interval of 4 g, are used to further investigate the performance of the sensor in higher RI medium. The corresponding RIs of the solutions are from 1.3365 to 1.3430

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Fig. 4. Transmission spectrum of sensor-1 in the solution of 2 g sodium chloride per 100 g water (concentration: 1.96%).

Fig. 5. Spectra of the peak A in different concentration sodium chloride (NaCl) solutions. (a) 0–2 g sodium chloride per 100 g water with an interval of 0.2 g, (b) 2–6 g sodium chloride per 100 g water with an interval of 1 g, and (c) 2–22 g sodium chloride per 100 g water with an interval of 4 g.

and from 1.3365 to 1.3649, respectively. The transmission spectra of peak A in the second group and third group solutions are shown in Fig. 5(b) and (c), respectively. The wavelength shifts of peak A with external RIs changes of the sensor in the first group solutions, and the second and third group solutions are shown in Fig. 6(a) and (b). The wavelength shifts of peak A almost linearly with the external RIs changes in the range of 1.333–1.365. The sensitivity (slope) of peak A by linear fitting in the RI range of 1.333 to 1.3365 is 177.1 nm/RIU, and the slope of is 172.4 nm/RIU

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Fig. 6. Wavelength shifts of peak A with RI changes. (a) The wavelength shifts of peak A in the first group sodium chloride solutions (0–2 g sodium chloride per 100 g water) and (b) the wavelength shifts of peak A in the second group (2–6 g sodium chloride per 100 g water) and third group (2–22 g sodium chloride per 100 g water) sodium chloride solutions.

Fig. 7. Transmission spectra of sensor-3 in different sucrose solutions and in air.

in the RI range of 1.3365 to 1.3649. The two slopes are highly consistent with each other, and the slightly difference between them may be caused by statistical and fitting errors. The RI sensitivity of the sensor is about six times higher than those of taper-type [15], [17] and LPFG-type fiber sensors [8], [9] in the same range of RI. The sensor-3 with smaller diameter cavities is also tested in different sucrose solutions with concentrations from 0%–30% with an interval of 5%. The corresponding RIs are from 1.333 to 1.3811. The transmission spectra in different sucrose solutions and in air are shown in Fig. 7. The attenuation peaks hardly shift in various concentration solutions, which means that sensor-3 with cavity diameter smaller than fiber core is insensitive to the external RI.

3.2. Discussion First, note that the attenuation peaks shift to longer wavelength with external RI increasing for the sensor-1 with cavity diameter larger than the fiber core, which is opposite from those previously reported normal taper-type [15], [17] and LPFG-type [8], [9] sensors. The reason is that d m =dnext in (3) is different for different cladding modes. Similar mechanism is demonstrated in sensitivity characteristic of LPFGs, where d m =dnext is negative for lower order cladding modes and positive

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for higher order cladding modes [24]. As for the proposed fiber sensor, higher order cladding modes are excited by the large cavity in the measured wavelength range and satisfy ðm =nL Þðð@n1 =@ÞL þ 2ð@n2 =@ÞLC Þ 9 1, as one can see in (3), which leads to a positive sensitivity. Thus spectra red-shift happens with the increasing of external RI. Second, although the fabrication processes of similar core-cladding-mode interferometers in [13], [15], [17], [18], [21], and [23] are relatively simple as compared with the proposed structure, their RI sensitivities are relatively 20–100 nm/RIU, which is much lower than that of the proposed fiber sensor. As reported in [24], the RI sensitivity is much greater for higher order cladding modes. Thus, high RI sensitivity can be obtained by sensor-1 as high order cladding modes are excited. As for LPFGs, the lower the order of the cladding mode, the smaller the sensitivity to external RI changes [24]. This mechanism is also applicable for sensor-3. As shown in Fig. 3(b), only a few cladding modes with low power distribution can be seen in the spatial frequency spectrum, which means these low order cladding modes and core mode participate in the interference pattern. Thus, the energy of cladding modes is tightly confined in the fiber, and few of the guided light can propagate outside the fiber through the evanescent wave, which makes it insensitive to surrounding RI. Additionally, the slightly difference of the spectra (see Fig. 7) between in solutions and in air is probably contributed to both different RIs and different pressure in solutions and in air.

4. Conclusion RI sensors with two micro air-cavities embedded in a normal single-mode fiber are proposed in this study. The micro air-cavity is formed by femtosecond laser fabrication and arc fusion splicing. When the cavity diameter is slightly bigger than the fiber core diameter, a high sensitivity of 172.4 nm/RIU is achieved by the proposed sensor in the RI range of 1.333–1.365, which is about six times higher than that of LPFG-type and taper-type RI sensors in the same RI range. When the cavity diameter is smaller than the fiber core, the sensor is insensitive to external RI, as no high order cladding modes are excited. Furthermore, the proposed structure is more robust than taper-based and trench structure fiber sensors because there is no reduction in fiber diameter direction.

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