High Sensitive Refractive Index Sensor Based on Cladding Etched Photonic Crystal Fiber Mach-Zehnder Interferometer

A high sensitive refractive index sensor based on the cladding etched photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) is proposed, which is spliced a section of photonic crystal fiber between two single modes fibers (SMFs).The interference fringe of the MZI shifts with the variation of the ambient refractive index (RI). It is found that the RI sensitivity slightly decrease with an increase in the interference length. The sensitivities of MZI with 35 mm PCF, 40 mm PCF, and 45 mm PCF are 106.19 nm/RIU, 93.33 nm/RIU, and 73.64 nm/RIU, respectively, in the range of 1.333 to 1.381. After etched, the RI sensitivity of the MZI could be improved obviously. The RI sensitivities of the MZI with 35 mm PCF are up to 211.53 nm/RIU and 359.37 nm/RIU when the cladding diameter decreases to 112 μm and 91 μm, respectively. Moreover, the sensor is insensitive to temperature, and the measured sensitivity is only 9.21 pm/°C with the range from 20°C to 500°C. In addition, the sensor has advantage of simple fabrication, low cost, and high RI sensitivity.


Introduction
Refractive index sensors can be used to measure the concentrations of gases and liquids, and are widely used in the biological and chemical environmental detection [1,2]. There are two optical methods, the optical path measurement method and optical fiber measurement method, which are used to measure the refractive indices of liquids [3]. The optical path structure is complex, and the optical path is not stable. Optical fiber is widely used in the liquid refractive index test because of its light weight, small volume, strong electromagnetic interference resistance, and simple design [4]. Up to now, a number of optical fiber microstructures have been proposed for refractive index sensors, including long period fiber grating (LPFG) [5,6], fiber Bragg grating (FBG) [7], Fabry-Perot interferometer (FPI) [8], and Mach-Zehnder interferometer (MZI) [9,10]. These structures have high sensitivity in refractive index sensing. However, a serious disadvantage for the above structures is high sensitivity to the environmental temperature [11,12]. Unlike traditional polarization maintaining photonic crystal fibers (PM-PCF), which contain at least two different glasses each with a different thermal expansion coefficient, the photonic crystal fiber birefringence is highly insensitive to the temperature [4,13]. The optical fiber sensor based on the MZI with the PCF structure has the advantages of ease to fabricate and convenience to measure, which is a common method in the research of refractive index sensors. In recent years, many novel in-line MZIs with the PCF structure have been proposed and studied. Y. Zhao et al. [14] proposed a PCF interferometer structure with up-tapered joints for RI sensing, which achieved a high RI sensitivity of 252 nm/RIU in the RI range of 1.333 -1.379. D. Wu et al. [15] proposed an RI sensor based on a tapered PCF MZI structure with an RI sensitivity of 51.902 nm/RIU in the range 1.3411 -1.3737. Y. Zhao et al. [16] reported a novel refractive index (RI) sensor based on the PCF-MZI which is fabricated by cascading a section of PCF with half-taper collapse regions (HTCRs) between two single mode fibers (SMFs) and investigated the RI sensitivity of 181.96 nm/RIU when the RI varied from 1.3333 to 1.3574. Dnyandeo Pawar et al. [17] proposed a tapered polarization maintaining photonic crystal fiber (PM-PCF) MZI sensor, and it had a sensitivity of 20.53 nm/RIU within the RI range from 1.33 to 1.37. These special structures of the MZI including tapered and cascading structure are complex to fabricate, time-consuming, and with low sensitivity. In order to improve the sensitivity of optical fiber sensors, many novel methods are proposed, such as chemical etching and coating on the optical fiber surface. Y. C. Tan et al. [18] presented a carbon nanotubes (CNT) deposited PCF featuring a Mach-Zehnder interferometer configuration for refractive index (RI) sensing. L. Melo et al. [19] reported an in-line MZI coated with hafnium oxide by atomic layer deposition to increase the RI sensitivity which was 1307 nm/SRI within the range of 1.3327 -1.3634. However, the deposition techniques used usually involve highly precise equipment or a long fabrication duration due to the extensive amounts of chemical processes involved.
In this paper, we present a PCF MZI refractive index sensor, which is realized by splicing a piece of PCF between two standard single mode fibers with a fusion splicer. The result shows that the effective RI of high order modes of the PCF cladding is more sensitive and more linear to surrounding refractive index changing, so we select high order mode spectra as the experimental measurement data. With an increase in the PCF length (30 mm, 35 mm, and 40 mm), the RI sensitivity decreases gradually (106.19 nm/RIU, 93.33 nm/RIU, and 73.64 nm/RIU) within the RI range of 1.333 -1.381. Afterwards, we adopt a novel method of etching the surface of photonic crystal fiber (the PCF length of 35 mm) by using hydrofluoric acid (HF) to enhance the RI sensitivity. The refractive index sensitivity increases to 359.37 nm/RIU when the cladding diameter decreases to 91 μm. With a decrease in the cladding diameter, the sensitivity of refractive index will be improved obviously. In addition, the PCF-MZI has advantages of temperature insensitivity, simple fabrication, low cost, and high RI sensitivity.

Design of MZI
The schematic of the sensor structure is shown in Fig. 1. The sensor is composed of SMFs and one segment of PCF (NKT Photonics LMA-10). The PCF is sandwiched to SMFs (Corning SMF-28). They are spliced together directly by a commercial fusion splicer (Fujikura 80S), and the splicing loss is about 1 dB. Figure 1(b) shows the scanning electron micrograph (SEM) of the cross section of the PCF. The PCF has an outside diameter of 125 μm, a core diameter of 10 μm, a hole diameter of 2.85 μm, and a hole pitch of 7 μm. The standard single mode fiber has an outside diameter of 125 μm and a core diameter of 9 μm. Figures 1(a) and 1(c) show the schematic diagram and optical micrograph of the MZI with the PCF structure. Through many experimental results, we find that a high fringe contrast suitable for fusion parameters can be obtained by tuning the arc power and arc time are + 60 bit (the discharge intensity of standard is 0) and 380 ms, and the overlap value is 20 μm, respectively. From the collapsed region in Fig. 1(c), we can know that the air holes of the PCF have slight collapse. However, the collapse of air holes will increase the mode field diameter and make the energy between the two modes of interference more balanced, which is more conducive to the formation of interference and increase the contrast of spectral fringes [20,21]. A light source and an optical spectrum analyzer are connected to monitor the spectrum change during the fabrication.  Figure 1 shows the in-line MZI with the PCF structure. A piece of PCF is spliced between two standard single mode fibers directly. When light travels from the SMF to the PCF, the fundamental mode of the SMF begins to diffract. In the first collapsed region, both the core and cladding modes with different propagation constants are excited. The interference spectrum can be expressed using the following two-beam optical interference equation [14,22]

Principle of the MZI
where I is the intensity of the total interference signal, and I 1 and I 2 are the intensities of the core and cladding modes, respectively. The phase difference β between the core and clad modes of an MZI is expressed as where L is the interference length, λ is the wavelength of propagating light, and eff n Δ is the difference between the effective refractive indices of core and cladding modes, When the difference between the cladding mode and core mode equals to (2m + 1)π, the m-order interference valley is shown as follows: The free spectrum range (FSR) of the interference spectrum can be defined as follows: The wavelength of the m-order interference valley varies along with the surrounding refractive index, and the variation is shown as where ∆n is the variation of the effective refractive index of the photonic crystal fiber cladding along with surrounding refractive index changing.

Experimental measurement method
The schematic diagram of our experimental measurement is shown in Fig. 2. A broadband light source (C + L, Shanghai Hanyu) with a wavelength range of 1528 nm -1602 nm is launched into the MZI structure. There is an optical path difference when light transmits through the sensor, and the interference will happen at the second collapsed region. The interference spectrum can be monitored by using an optical spectrum analyzer (OSA, Agilent 86142B, 600 nm -1700 nm). The sensor is attached to the bottom of the glassware during testing the RI changes.

Results and discussion
Different parameters influence the properties and performances of the PCF interferometer, such as arc time, arc power, interference length, and the diameter of the PCF. These factors should be considered during the fabrication of the PCF interferometer. In this paper, the effects of the interference length on the interference spectrum and the sensitivity of RI sensing are investigated experimentally. Three kinds of PCF interferometer with different interference lengths of 30 mm, 35 mm, and 40 mm are made by using the same welding parameters. The transmission spectra of a PCF interferometer with different sensing lengths in air and water are shown in Fig. 3. From Fig. 3, we can see some interference peaks with the low fringe contrast when the sensor is immersed in air, which are caused by multimode interference. When the sensor is immersed in water, the wavelength is red-shift, and the interference peaks caused by multimode interference are decreased because the refractive index of water is higher than that of air, and the multimode interference is suppressed in water. The interference peaks produced by multi-mode interference have a small influence on sensing.
In air, m  The ability of the PCF interferometer to detect the change of RI in the liquid environment is studied. The refractive index solution is formed by mixing water with glycerol in a certain proportion, and it ranges from 1.333 to 1.381 in the experiments. Figures 4(a), 4(b), and 4(c) are the spectral change diagrams of three different lengths PCF interferometers in different refractive index liquids. With an increase in the refractive index, the wavelength of the interference spectrum appears red-shift. The relationship between the maximum wavelength shift amount and refractive index of three different lengths PCF interferometers are illustrated in Fig. 4(d)   In order to improve the refractive index sensitivity of the PCF interferometer, in this experiment, an HF etching method is used to corrode the PCF with a length of 35 mm because it has a better corresponding fitting degree, and the concentration of HF is 20%. Figure 5 shows the different diameters of the PCF under the microscope, and the diameter is 125 μm (no etching), 112 μm (etching for 10 min), and 91 μm (etching for 80 min), respectively. Figure 6 shows the transmission spectra of the MZI with different etching time in air. With an increase in the etching time, the wavelength of the interference spectrum appears blue-shift, and the transmission loss decreases gradually. Figures  7(a), (b), and 7(c) show the spectral change diagrams of the 35 mm MZI with different etching time when liquid RI varies. A red shift in the spectrum is observed when RI increases from 1.333 to 1.381, and total wavelength shift amounts of the PCF interferometer before corrosion, and the etching  Fig. 7(d).

Photonic Sensors 132
It can be seen that the RI sensitivities of the PCF interferometer are improved evidently with an increase in the etching time. The reason is that the PCF diameter is decreased by the HF etching method. When the surrounding RI is a constant, a PCF diameter decrease will result in an increase in ∆n eff as expressed in (5). That is to say, the interaction between the evanescent waves of the cladding modes and the ambient environment around the fiber will be enhanced, so the influence of the surrounding refractive index changing on the transmission characteristics of the cladding mode is more obvious.
Because the PCF has a micro-hole structure which has a small coefficient of thermal expansion, the main advantage of the PCF is that it is insensitive to the temperature. In order to prove that temperature has less influence on the refractive index measurement, a 35 mm PCF interferometer is heated from 20 to 500 ℃ . The transmission ℃ spectra of this PCF interferometer are shown in Fig. 8(a). The relationship between the wavelength shift and temperature is shown in Fig. 8(b), and the temperature sensitivity is 9.21 pm/ . As the ℃ environmental temperature change is about 10 , ℃ the corresponding wavelength shift of the PCF interferometer is less than 0.1 nm. Our experimental temperature is 20 ℃, and the range of temperature fluctuation is slight (within 2 fluctuation). This ℃ means that temperature has less influence on the refractive index measurement, which could be ignored compared with the RI sensitivity.
A comparison between the proposed MZI and related MZIs based on the splicing technology is listed in Table 1. Table 1 shows that the sensitivities of the proposed sensor have higher RI sensitivity than PCF with up-tapered joints (252 nm/RIU) [14], PCF-MZI with two HTCRS (181.96 nm/RIU) [16], a n d M M F ( M u l t i m o d e f i b e r ) -S M F -M M F (-37.93 nm/RIU) [25]. Because the diameter of the corroded PCF decreases, the interaction between the evanescent wave of cladding mode and the surrounding environment of the fiber is enhanced. However, the sensor we reported has lower sensitivity than the open-cavity MZI (-1364.343 nm/RIU), but the open-cavity MZIs usually have small RI measurement range (only 1.333 -1.3468) although they have ultrahigh RI sensitivity. Because the PCF is insensitive to the temperature, the temperature sensitivity of the proposed PCF interferometers (9.21 pm/ ) ℃ is almost the same with that of other PCF structure interferometers, but they are lower than that of the SMF structure MZIs, such as small offset with the SMF (49 pm/ ) [24] ℃ and open-cavity MZI [26].

Conclusions
The paper proposes and studies a high sensitive refractive index sensor based on the Mach-Zehnder interferometer sensor, which splices a section of photonic crystal fiber (PCF) between two SMFs. The RI sensitivity of the MZI with the 35 mm PCF is enhanced obviously by HF etching with different time. In the RI range of 1.333 to 1.381, the RI sensitivity is 93.33 nm/RIU non-corrosion, which is up to 211.53 nm/RIU and 359.37 nm/RIU after 10 min and 80 min HF corrosion, and the temperature sensitivity is 9.21 pm/℃ with the temperature range from 20 ℃ to 500 ℃. In addition, the PCF-MZI has advantages of temperature insensitivity, simple fabrication, low cost, and high sensitivity, which is widely used in the biological and chemical environmental detection.