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Abstract
A single-multi-single mode (SMS) fiber structure with spiral microgroove, fabricated by femtosecond laser inscription has been proposed and successfully employed for refractive index (RI) sensing. The multimode interference in the SMS structure is effectively affected by the external perturbation due to the microgroove, which goes deep into the core of the multimode fiber (MMF). Experimental results show that this femtosecond-induced spiral micro-structured SMS (FISM-SMS) fiber structure exhibits a linear response to eternal liquid refractive index in a large RI range of 1.3373–1.4345. The maximum sensitivity of the structure can reach to 2144 nm/RIU and can be further improved by increasing the depth of the spiral micro-grooves.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber optical sensors have been researched and widely applied in the past few decades for their compact size, fast response, high resolution, usage in harsh environments and immunity to electromagnetic interference. Various senor probes including fiber Bragg gratings (FBG) [1,2], long period gratings (LPG) [3–5], D-shaped optical fiber [6], surface plasmon resonance (SPR) [7, 8] and interferometers [9, 10] have been presented and successfully applied in different fields. Among them, sensors based on single-multi-single mode (SMS) fiber structures have aroused general interest in sensing field for the ease of fabrication and low cost, and are widely applied on measurement of different parameters such as curvature [11, 12], strain [13], displacement [14, 15], temperature [16] and refractive index [17–19]. In term of sensing of refractive index, polishing [20], tapering [21] and chemical etching methods [22] are employed to make the multimode interference within the multimode fiber (MMF) influenced by the change of external liquid. Femtosecond laser have precise micromachining accuracy in fabrication and it results in minimal mechanical and thermal deformation on the surface of materials [23, 24]. All these advantages make laser processing widely employed to machine microstructures on fiber sensors.
In this paper, we present and demonstrate a refractive index (RI) sensor based on ablating spiral microgroove on the multi-mode section of the SMS structure by femtosecond laser. The multimode interference in the micro-structured SMS sensor is affected by the external perturbation through two ways: For the higher order modes that pass through the microgrooves, the optical path length are determined directly by the RI of the tested liquid because the liquid in the microgroove can be considered as part of the waveguide; For the lower order modes that always travel in the silica MMF, the effective RI of the modes are affected by the RI of the liquid through the evanescent field. The combine effect of the two factors is that the proposed structure has a linear RI response in a large RI range. Compared with previous works based on multimode interference whose RI responses are exponential [25, 26], the linear response of the present work provides great convenience for practical application. The linear RI response can also be realized by drilling a micro-cavity in the MMF of the SMS structure to form a Mach-Zehnder interferometer [27]. The sensor has an ultrahigh sensitivity (9756.75 nm/RIU). However, for high RI that close to the RI of the fiber material, the FSR of the interferometer can reach several hundred nanometers, which limits its application in high RI measurement. In the present work, experimental investigations show that the refractometer based on the femtosecond-induced spiral micro-structured SMS (FISM-SMS) can keep a constant RI sensitivity in the RI range of 1.3373-1.4345 with a reliable linearity of 99.21%. RI sensitivity of this device can be enhanced significantly with the increase of the laser power. This proposed RI sensor with high sensitivity and large measuring region has potential application in chemical and biotechnology due to the large contact area caused from the spiral microstructure of the fiber.
2. Working principle
The schematic diagram of the femtosecond-induced micro-structured SMS fiber structure is shown in Fig. 1. The MMF is sandwiched in between two single mode fibers (SMF). The spiral microgroove in the MMF are characterized by the parameters of pitch-p, microgroove depth-d, microgroove width-w and its axial component-l, angle-θ, as illustrated in Fig. 1.
The light from the lead-in single mode fiber (SMF) excites multiple modes in the MMF at the SMF-MMF interface. Because the existence of the micro-grooves across the MMF, the excited modes in the MMF will go through the MMF along two different ways: Some higher order modes will repeatedly pass through the spiral micro-grooves and the other lower order modes transmit in the MMF all along. Therefore, the field at the output end of the MMF can be written as [22]:
where E(r, 0) is the core mode of the input SMF, Ψm(r) and Ψn(r) are the modes those transmit in the MMF all along and pass through the micro-grooves, respectively, am, n are the excitation coefficients of each mode, M is the number of the modes kept in the MMF and N is the total mode number in the MMF. For the first item on the right of Eq. (1), the phase variation of the modes are caused by the change of the effective RIs of the lower order modes through evanescent field. While for the second item in Eq. (1), the phases of the higher order modes are determined by the RI of the measured liquid because these modes directly transmit through the liquid filled micro-grooves. Therefore, the multimode interference signal in the micro-structured MMF is disturbed by the two factors mentioned above.The output field intensity distribution of the micro-structured SMS configuration was simulated by using the beam propagation method to verify that the micro-grooves can affect the transmission field in the MMF. In the simulation, the diameter of the cladding and the core of the multimode fiber were set to 125 μm and 105 μm, respectively. The depth and pitch of the microgroove were set to be 15 μm and 120 μm, respectively, and the length of the MMF with microgrooves was set 1.5 cm. A piece of SMF with 200 μm length was connected to the output end of the MMF to collect the interference pattern of the micro-structured MMF. The microgroove of the MMF was supposed to be exposed into air and the operating wavelength is 1550 nm. Both the lead-in SMF and the lead-out SMF has a core diameter of 9 μm and the refractive indices of the core and cladding are 1.462 and 1.457, respectively. The refractive indices of the core and cladding of the MMF are 1.444 and 1.439, respectively. The calculation domains are x (−73.5 μm to 73.5 μm), y (−73.5 μm to 73.5 μm) and z (−100 μm to 15400 μm). The meshing size is 0.1 μm along x- and y- directions, and 2 μm along z-direction. The boundary condition was set to be transparent boundary condition. The calculated results are presented in Fig. 2, in which (a) and (b) are respectively the electric field amplitude distribution of the end face of the MMF and the output SMF of the SMS structure without microgrooves, and (c) and (d) are respectively the field distribution of the end face of the MMF and the output SMF of the SMS structure with microgrooves. Figure 2 (e) gives the light propagating along the micro-structured SMS structure. From Fig. 2 we can deduce that part of the excited modes transmitting into the spiral microgroove and back to the MMF, while the others propagate along interior of the MMF. The transmission behavior of these modes indicates that the microgrooves can enhance the influence of the surrounding RI on the field transmitting in the SMS structure.
Fig. 2 Simulated optical field profiles (a) in the end of the MMF for the sensor without spiral microgroove; (b) in the end of the lead-out SMF for the sensor without spiral microgroove; (c) in the end of the MMF for the sensor with spiral microgroove; (d) in the end of the lead-out SMF for the sensor with spiral microgroove; (e) of light propagating along the multimode fiber.
3. Preparation of the spiral micro-structured SMS fiber structure
To fabricate the FISM-SMS structure, a section of multimode fiber with a length of ~1.5 cm was fusion spiced in between two sections of SMF. The diameter of the cladding and the core of the MMF are 125 μm and 105 μm, respectively. Spiral microstructure was fabricated on the MMF section with ablation length of 1 cm by femtosecond system (IFRIT). A speed-controlled rotational jig was applied to control pitches of the thread. The sample was mounted on a high-precision three-axis stage which is driven by a computer. The repetition rate, central wavelength, and pulse width of the FS system are 1 kHz, 780 nm, and 180 fs, respectively. The laser beam was focused on the SMS section by an objective lens (Sigama, Koki, Japan) with a focal length of 60 mm. An optical attenuator and a diaphragm are applied to control the energy density irradiated on the fiber from 1% to 100%. The whole process of the laser ablation was monitored by a high resolution CCD camera in real-time. The SEM of the fabricated FISM-SMS structure is shown in Fig. 3.
The transmission spectrum changed greatly once laser ablation was applied to SMS structure. Figure 4 presents the transmission spectra of a SMS structure before and after laser ablation. The laser power was 20 mW, the pitch and depth of the micro-grooves was 120 μm and 14.4 μm, respectively. We can observe from the figure that the transmission dip almost disappears (shifts out of the spectrum range of the light source) after laser ablation. But the dip reappears when the sample is immerged in deionized water and accompanied with a large phase shift compared with the spectrum before laser ablation. Such phenomenon indicates that the refractive index of the medium filled in the micro-grooves has a great influence on the transmission spectrum, which makes the structure possible for liquid refractive index sensing.
4. Experimental results and discussion
A set of samples with different ablation parameters was fabricated to investigate the RI sensing characteristics of the FISM-SMS structure, where the schematic experimental setup for measuring the external RI is illustrated in Fig. 5. The samples were tested by immerging them in a series of NaCl solutions with the RI range of 1.3435 to 1.3676. The actual RI of the NaCl solutions were calibrated by an Abbe refractometer. The samples were pre-pulled and fixed on a glass strip by UV adhesive to decrease the measurement error such as bending and vibration. Light from a broadband light source (SLED) with a spectral range of 1450-1650 nm was launched into the FISM-SMS structure, and the transmitted interference spectrum of the sensor was monitored by an optical spectrum analyzer (OSA) with 0.02 nm resolution. The experimental results are listed in Table 1.
Table 1. Parameters of tested samples and its performance in NaCl solutions.
Figure 6 shows the transmission dip shift of the FISM-SMS structure with pitch 120 μm under different laser power. The sensitivities of samples s-3, s-6, s-7 are 551.75, 1113.2, 2144.8 nm/RIU, while the standard SMS without spiral microstructure does not show any response to the external RI changing. Such a sensitivity level is comparable with that of photonic crystal fiber interferometer [28] and smaller than that of microfiber RI sensors [29,30]. The merits of the micro-structured SMS sensor is the linear RI response in a large RI range. It is apparent that the sensitivity is significantly improved by the increase of the laser power. The width and depth of the V-shape microgroove are expanded with the increase of the laser power. When the fabricated spiral microgroove of this FISM-SMS structure goes deep into the interior of the core of the MMF, part of the higher order modes in the MMF repeatedly pass through the solution filled in the spiral microgroove. When the laser power is enhanced, the total optical path length (TOPL) in the solution of these modes is increased. Besides, the effective RIs of more and more modes that do not directly exposed to the solution are influenced accordingly. Both the phase of the two types of the light beams were changed drastically, and they interfere with each other, resulting in RI sensitivity increasing sharply.
The relationship between the wavelength shift and pitch of the spiral microgroove were also experimentally investigated. Four samples s-1, s-2, s-3 and s-4 with different pitches were fabricated with same laser power. The responses of the samples to the external RI are shown in Fig. 7. From the figure, it can be seen that although the pitch varies from 60 μm to 150 μm, the RI sensitivity fluctuate around 600 nm/RIU and do not show an obvious trend. In this situation, as the depth of the microgroove is constant, the influences to the interference modes whether pass the liquid filled microgroove or not are similar for the SMS structure with different pitches. This can be illustrated as follows: For a SMS structure with ablating MMF length of L, the numbers of thread circle is n = L/p, the total length of one light beam transmitting through the solution can be written as:
In Eq. (2), the total ablation length L and the microgroove width-w are constant. So Ls is determined by the pitch-p and the angle-θ. As θ is proportional to p, once pitch is increased or decreased, θ will be increased or decreased accordingly, making pcosθ fluctuate in a small range. Therefore, variation of pitch makes the total length of the light transmitting in the solution changed slightly, resulting in the total optical path length of these light beams almost constant. Therefore, the sensitivity is nearly unchanged.
It is worth noting that large measuring range is another superiority to other high sensitivity RI sensors. In our experiment, we chose a series of glycerin solution with RI range from 1.3373 to 1.4345 to investigate its performance in a large RI range and compared the test results with its RI response in NaCl solutions. Because the wavelength shift of the sample s-7 with the highest sensitivity will exceed the spectrum range of the light source, the sample s-3 was employed for test. The results are shown in Fig. 8. From the figure, it can be seen that the transmission dip in the transmission spectrum shifts toward longer wavelength with RI sensitivity of 553.75 nm/RIU and 610 nm/RIU in the NaCl solution and the glycerin solution, respectively. Within the tolerance of 10%, we can consider that the RI sensitivities of the two sets of experiments are consistent. Compared with the behavior of exponential growth of RI response of other sensors, the proposed FISM-SMS structure has a significant advantage that the shift trend of the transmission dip has a good linearity in the large RI range. It can also be seen that the transmitted power of the interference spectrum varies with the surrounding refractive index. Such a phenomenon is due to the transmission path variation of the higher order modes with the change of the surrounding refractive index.
Fig. 8 Performance of refractometer with processing parameters of pitch 60 μm and power 20mW—(a) in NaCl solutions; (b) in glycerin solutions.
Temperature response of the sample s-3 was also investigated. This FISM-SMS based refractometer was placed into a temperature water bath, which was gradually heated from 30 °C to 60 °C with temperature interval of 5 °C. The relationship between the location of transmission dip and temperature is given in Fig. 9. The dip shift toward the short wavelength direction with a good linear temperature response of 91.1 pm/°C. If temperature compensation is not applied in practical measurements, the RI measurement error caused by temperature variation is 1.49 × 10−4 RIU/°C, which is mainly determined by the thermo-optical effect of the fiber materials. To solve this crosstalk resulted by temperature fluctuation, temperature calibration can be achieved by cascading the FISM-SMS structure with a fiber Bragg grating (FBG) which is insensitive to eternal refractive index. The mechanism is similar to the work in [27].
Fig. 9 Wavelength shift of the refractometer along with the temperature increasing from 30 °C to 60 °C.
5. Summary
In conclusion, we propose a high-sensitivity RI sensor with large RI measuring range. Experimental results show that the laser power, which determines the depth and width of microgroove can influence the sensitivity of refractometer significantly, while the pitch of the microgroove does not affect RI sensing obviously. The sensitivity of the refractometer with a process parameter of power 30 mW and pitch 120 μm is as high as 2144 nm/RIU. The sensitivity can be further improved by increasing the laser power. The significant superiority of this refractometer to other RI sensors is the linear shift of the transmission dip with the increase of liquid RI in a wide range of 1.3373 to 1.4345. This refractometer has a good potential application in liquid RI sensing for its excellent performance.
Funding
Project of National Natural Science Foundation of China, NSFC (Number: 61275087, 61475121); Fundamental Research Funds for the Central Universities.
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