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Abstract
To balance the tradeoff between the high sensitivity and large dynamic range, a fiber optic refractive index sensor based on the anti-resonant reflecting optical waveguide (ARROW) and mode interference has been proposed and experimentally demonstrated. A double-layered ARROW was formed in a hollow core fiber, and a mode interference was also generated in the fiber skeleton using offset splicing. The proposed fiber optic refractive index sensor possesses both high sensitivity and large dynamic range due to the different refractive index sensitivities of the ARROW and mode interference. The experimental results show that a high refractive index sensitivity of 19014.4 nm/RIU for mode interference and a large dynamic range from 0.04 RIU for ARROW can be achieved simultaneously. The proposed fiber optic refractive index sensor can be used in chemical and biological applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber refractive index sensors have been widely used in the detection of biochemical reactions due to their unique advantages of high sensitivity, compact size, and remote sensing ability [1]. Many fiber refractive index sensors have been proposed based on various schemes. Fiber gratings, such as the fiber Bragg grating (FBG) [2], titled FBG [3], phase-shift FBG [4], long period grating [5], and polarization-maintaining FBG [6], could generate evanescent fields that interact with an ambient liquid to modulate the resonance wavelength of the fiber grating. Besides, a variety of fiber interferometers have been investigated with respect to the detection of refractive index (RI), such as the Fabry-Perot interferometer [7,8], the Mach-Zehnder interferometer [9,10], and the Michelson interferometer [11]. Moreover, optical fiber surface plasma resonance (SPR) is also a useful type of RI sensor that can measure RI with ultrahigh sensitivity using the surface bound electromagnetic waves formed at the interface between a metal and a dielectric [12].
The hollow-core fiber (HCF) contains inherent hollow holes inside the fiber, providing a natural in-line microfluidic channel for the liquid sample, which could simplify the complexity of the RI measurement significantly. Many mechanisms of HCF based RI sensor have been presented, such as interferometers, surface plasmon resonance, gratings, anti-resonant reflecting guidance (ARROW) [5,13,14]. Especially for the ARROW, the guided light at resonance wavelength is leaked into the cladding of the HCF, which could interreact with the liquid sample in the microfluidic channel strongly. R. Gao et al. demonstrated a dual-optofluidic waveguide in-line fiber biosensor using ARROW [15], S. K. Mishra’s group presented a RI sensor using hollow core anti resonance fiber [16], and Q. Wang et al. realized a label-free guided-mode resonant optical biosensor with ARROW [17]. Moreover, many other physical parameters can be also detected by using the ARROW mechanism, including temperature [18], bending [19], pressure [20], magnetic field [21], and so on. The ARROW based fiber sensors possess high sensitivity, low cost, and remote sensing, which have been attracted a great deal of interest over past decades.
One long-standing challenge for optical fiber RI sensors is to achieve both a high sensitivity and a large dynamic range. Due to the limitations on the sensing mechanism and the range of measurement of the device (such as light source or optical spectrum analyzer), it is very hard to achieve both characteristics simultaneously. For example, in Ref. [9], the multi-mode interferometer is realized to offer a ultra-high sensitive RI detection at 10675.9 nm/RIU, but the measurement range of the sensor is only 1.4484 - 1.4513. In contrast, although the planar multimode waveguide is limited to measure the RI with a low sensitivity of 151.29 nm/RIU, a large dynamic range of 1.345 - 1.405 can be obtained [10]. In order to balance the trade-off between sensitivity and dynamic range, several methods have been proposed. Two weak composite fiber Fabry-Perot cavities were fabricated, and a high sensitivity and large dynamic range were achieved simultaneously through the use of different wavelengths [22]. However, the measurement accuracy would be decreased by interrogating the light intensity due to the laser power fluctuates, and the end surface of the fiber may be contaminated. Some other methods are based on the multi-cavity or multi-mode in one fiber sensor [23]. A cavity with lower sensitivity can guarantee a large dynamic range, while another with higher sensitivity contributes to enhanced resolution. However, most of these schemes are designed for the measurement of temperature [23,24], strain [25] or gas [26].
In this paper, a high sensitivity fiber RI sensor with a large dynamic range based on an anti-resonant reflecting optical waveguide (ARROW) and a mode interference is proposed and experimentally demonstrated. An microfluidic channel was fabricated inside a HCF to form a double-layered ARROW. Besides, mode interference is also formed through the offset splicing between the single mode fiber (SMF) and HCF. Due to the large difference in RI sensitivity, high sensitivity and large dynamic range can be achieved simultaneously in a single optical fiber RI sensor.
2. Fabrication of the anti-resonant reflecting optical waveguide and mode interference
The silica between the air core and air ring is defined as fiber skeleton, which is used to divide the HCF into three parts: an air core, an air ring with eight hollow holes, and a solid silica section, as shown in Fig. 1(a). The side length of the air octagon core is 18 µm, the inner diameter of the hollow hole inside the air ring is 35 µm, and the diameter of the outer cladding is 190 µm.
Fig. 1. (a) Cross section of the HCF; schematic (b) and close-up view (c) of offset splicing between SMF and HCF.
Firstly, one end of the HCF is spliced to a lead-in SMF with a lateral offset along the radial direction of the HCF (gap of 12 µm, arc power of 20 mA, and arc duration of 120 ms). The core of the SMF covers both the cross section of the fiber skeleton and the air core of the HCF, as shown in Fig. 1(b) and (c). In this way, the guided light is coupled to both the fiber skeleton and the air core of the HCF. The splicing loss with a lateral offset between the SMF and the HCF is ∼8dB, which is higher than that of aligned splicing (3.2dB) [27]. The splicing with a lateral offset may increase the light back-reflections, and the distance between the centre of the SMF and the HCF could affect the mode excitation significantly. A large distance may only excite the mode interference, while a small distance may only excite the ARROW. Hence the distance of the lateral offset should be optimized, which is discussed in section 4. Moreover, all splicing process between the SMF and the HCF were accomplished by using the fusion splicer (FSM-28S). Hence once the the splicing parameter of gap, arc power, and arc duration are fixed, the splicing reproducibility with a lateral offset can be achieved. Then at the other splicing point, a lead-out SMF is also spliced with the HCF with the same lateral offset as the input side. The splicing region is tapered slightly but air holes are still existed, as shown in Fig. 1(c). The length of the HCF is 6cm.
Fig. 2. (a) Microchannel in the HCF; (b) schematic of in-line microfluidic channel in the HCF ; (c) fiber stage.
After that, two microchannels are then drilled through the cladding of the HCF using femtosecond laser micromachining, as shown in Fig. 2(a). These two microchannels, the inlet and outlet, are aligned with a hole in the air ring to allow the liquid sample to flow. A microfluidic channel was formed based on this hole in the HCF, as shown in Fig. 2(b). Finally, the SMF and HCF are slightly pre-stretched and fixed in a micro-groove in a stage, as shown in Fig. 2(c). This can prevent the HCF from bending during the experiment. Two channels were fabricated in the stage to allow the liquid to flow into two microchannels of the HCF.
3. Principle of the fiber optic RI sensor
The principle of the fiber optic RI sensor is based on the ARROW effect [15]. Due to the small RI of the air core, the guided light is reflected at both the inner and outer surfaces of the HCF, as shown in Fig. 3(a). The cladding of the HCF can therefore be described as a double-layered Fabry-Perot etalon, which consists of the liquid sample and the silica. The guided light is reflected at the anti-resonance wavelength (as shown in Fig. 3(b), while other light at the resonance wavelength is transmitted out of the HCF (as shown in Fig. 3(c)).
Fig. 3. (a) The principle of the ARROW; optical intensity distribution of the ARROW (b) at the anti-resonance wavelength and (c) at the resonance wavelength; (d) principle of mode interference; Inset shows the suspended triangular column with three thin struts. (e) optical distribution of mode interference; closed view of (f) fundamental mode; and (g) high-order mode.
In addition to the use of ARROW in the HCF, a mode interferometer is also generated. Due to the lateral offset splicing, part of the guided light is also coupled into the fiber skeleton. The cross section of the fiber skeleton is a suspended triangular column with three thin struts, as shown in Fig. 3(d). The cross section can therefore be regarded as an embedded suspended core in the HCF [28] (Fig. 3(e)). Both the fundamental mode and the high-order modes are excited in the small area of the suspended triangular column, as shown in Fig. 3(f) and (g). Due to the different effective RIs of the fundamental mode and high-order modes, mode interference is also generated. Therefore, both ARROW and mode interference are formed in the HCF simultaneously, and the transmission spectrum can be expressed as [24]:
Figure 4(a) shows a simulated spectrum of the HCF. Assuming that ${I_{ARROW}}$, ${I_{core}}$, and ${I_{cladding}}$ are equal to each other, and the RI of the silica is set as 1.445. The spectrum is modulated with a large and short free spectrum range (FSR) corresponding to the ARROW and mode interference, respectively. Figure 4(b) shows the RI sensitivities of the ARROW and the mode interference according to Eq. (1). The effective RI of the high-order mode is simulated using Comsol software. The RI sensitivity of the ARROW is changed slightly over a range of RI from 1.33 to 1.45, while the RI sensitivity of the mode interference is increased significantly at an RI of 1.45. The higher RI sensitivity for larger RI is linked to the reduction of the RI contrast between the silica strut and its surrounding (the liquid-filled channel). The reduction of the RI contrast may cause the mode field distribution to spread towards the liquid channel, which cause the optical response of the mode interference to be more sensitive to its RI changes.
Fig. 4. (a) Simulated transmission spectrum; (b) RI sensitivities and ratio between ARROW and mode interference
The sensitivity ratio between the mode interference and the ARROW is also shown in Fig. 4(b). The lowest sensitivity ratio is 4.1 at a RI of 1.33. The sensitivity ratio increases with an increase of the RI, and reaches a peak at a RI of 1.45. The large gap of the RI sensitivity between the ARROW and the mode interference can be used for RI detection with high sensitivity and large dynamic range. The lower sensitivity of the ARROW gives RI detection with large range, while the higher sensitivity of the mode interference provides the high sensitive RI measurement.
4. Experiment and discussion
The experimental setup is shown in Fig. 5. A superluminescent diodes (SLD, Conquer Co., KG-SLD) with a wavelength ranging from 1450 to 1650 nm and a power of 5 mW was used as a light source. A polarization controller (PC) was placed between the SMF and the HCF to optimize the polarization. The liquid sample was pumped into the HCF using a syringe pump, and the transmission spectrum was monitored using an optical spectrum analyzer (OSA, Yokogawa, AD6370D). The sensitivity of the sensor is dependent of the resolution of the OSA, hence a high grade OSA is expected in this experiment.
The transmission spectrum of the HCF with different values for the lateral offset, defined as the distance between the centre of the SMF and the HCF, is shown in Fig. 6(a)-(d). Only the ARROW effect can be observed when the lateral offset is smaller than 18 µm, since the guided light in the the SMF is not coupled into the HCF skeleton. Conversely, the ARROW effect is not shown when the lateral offset is larger than 30 µm because all guided light was coupled into the HCF skeleton. On the other hand, both the ARROW and the mode interference are present in the modulated transmission spectrum when the guided light is coupled into both the fiber skeleton and air core of the HCF (22 and 18 µm). In the experiment, a lateral offset of 22 µm was chosen due to the balancing effect between the ARROW and the mode interference. The overall loss of the SMF-HCF-SMF is 32dB.
Fig. 6. Transmission spectral of the HCF with different offset of (a) 30 µm, (b) 22 µm, (c) 18 µm, and (d) 15 µm; (e) spatial frequency of Fig. 6(b); (f) filtered spectrum of mode interference and (g) ARROW.
Two bandpass filters were applied to extract the independent signal of the ARROW and mode interference from the modulated transmission spectrum. The spatial frequency of the modulated transmission spectrum was obtained using a Fourier transformation, as shown in Fig. 6(e). The peaks at 0.037nm−1 and 0.236nm−1 correspond to the ARROW and mode interference, respectively. There are other peaks in the spatial frequency due to other high-order modes generated in the embedded suspended core, but these can be neglected due to their low intensity. The bandpass of two filters for the ARROW and mode interference were 0.034∼0.040 nm−1 and 0.230∼0.242 nm−1, respectively.
The RI response of the ARROW was investigated. Refractive index matching liquids (RIMLs) with the RI in the range from 1.33 to 1.45 were pumped into the microfluidic channel in the HCF. The transmission spectra for the ARROW (extracted from the modulated transmission spectrum) with different RIMLs are shown in Fig. 7(a). The resonance dips are shifted to longer wavelengths, and the relationship between the RI and the wavelength of a resonance dip at the original wavelength of 1456.4 nm is shown in Fig. 7(b). A sensitivity of 542.89 nm/RIU was achieved at the RI of 1.33. Hence the wavelength valley was shifted in the wavelength range of the SLD source and the OSA with the RI range from 1.33 to 1.45. However, in a practical application, the confusion can be arisen when the wavelength valley was shifted beyond one FSR. The FSR of the sensor is 22nm (from 1456.4 nm to 1478.4 nm). Therefore, the maximum dynamic RI range of 0.04 RIU ($0.04\textrm{RIU} = 22\textrm{nm}/542.89(\textrm{nm/RIU}).$) of the proposed sensor for ARROW can be achieved, indicating a good candidate for the RI detection of large dynamic range. The entire process of the RI measurement for ARROW is shown in Fig. 7(c). At each measurement, the RIML was injected into the HCF, and the wavelength valley was shifted to longer wavelength due to the high RI of the RIML. The injection process was kept until the wavelength shift was stable. Then the RIML was kept in the HCF for 5s, followed by rinsing in ultrapure water for eliminating the cross-talk of different RIMLs. The wavelength valley was shifted back to shorter wavelength due to the low RI of the water (1.33). The rising process was kept until the wavelength valley was shifted back to the baseline.
Fig. 7. (a) Transmission spectra for the ARROW with RIMLs; (b) relationship between wavelength at the dip and RI.(c) wavelength shift at different RI. (d) response time.
The time response of the sensor has been also analyzed. Figure 7(d) shows the closed view of the measurement at the RIML with the RI of 1.37. From 105 to 109 s (before the first blue dash line in Fig. 7(d)), the sensor was washed with ultrapure water, hence, the baseline was chosen as 1456.4 nm. Then the RIML was passed into the sensor from 109 to 116 s, and the wavelength valley was shifted as 28.2 nm, indicating that the response time is estimated as 7s (between the first and second blue dash lines in Fig. 7(d)). After that, the RIML was kept in the sensor for 5s (from 116 to 121 s). Finally, the ultrapure water was washed into the sensor from 121 to 130 s, and the wavelength valley was shifted back to the baseline, indicating that the washing time is estimated as 9s (between the third and fourth blue dash lines in Fig. 7(d)).
On the other hand, the sensitivity of the sensor for the mode interference was also investigated. Three RI measurements with different ranges from 1.33 to 1.34, 1.40 to 1.41, and 1.43 to 1.44 were carried out, respectively. Figure 8(a) shows transmission spectra (extracted from the modulated transmission spectrum) for the mode interference with the RI range from 1.33 to 1.34 with an interval of 0.002. The wavelength valley is shifted to a longer wavelength with the increase of the RI. The relationship between the wavelength valley at the original wavelength of 1452.2nm and the RI is shown in Fig. 8(b). A sensitivity of 1064.6 nm/RIU was achieved. Besides, wavelength shifts for the other two RI ranges are also shown in Fig. 8(b). For the RI range from 1.40 to 1.41, a new original wavelength valley of 1458.2nm was chosen because the previous wavelength valley (1452.2nm) with the RI range from 1.33 to 1.34 was already shifted out of the SLD wavelength range (1450-1650nm), and the sensitivity of 4946.2 nm/RIU is achieved. For the RI range from 1.43 to 1.44, an original wavelength valley of 1455.6 nm was chosen, and the sensitivity of 19014.4 nm/RIU can be achieved. The sensitivity ratios between the mode interference and ARROW are 3.7, 6.2, and 12.5 for three RI ranges from 1.33 to1.34, 1.40 to1.41, and 1.43 to 1.44, respectively, which are in a good agreement with the results of the simulations. All RI sensitivities of the mode interference at different RI ranges are much higher than that of ARROW in the same RI range, indicating that the mode interference is a good candidate for high sensitive RI detection. Noted that the value of the RIMLs are typically measured at the wavelength of 589 nm. Due to the dispersion, the value of the RIML $n(\lambda )$ at different wavelength is defined as (www.cargille.com)
where ${n_0}$ is the initial refractive index, and ${C_0}$ and ${C_1}$ are constants. Hence the value of the RIML would be decreased at the wavelength of 1550 nm, and a higher sensitivity of the mode interference can be achieved in reality.Fig. 8. (a) Transmission spectra for the mode interference with different RIMLs; (b) relationship between wavelength and the RI
Ethanol solutions with different concentrations (0%, 10%, 20%, 30% and 40%) were pumped into the microfluidic channel, and glycerol solutions with different concentrations (50%, 52%, 54%, 56% and 58%) were then added to the same microfluidic channel. Due to the large difference between the RIs of ethanol (∼1.34) and glycerol solution (∼1.43), the performance of the sensor in terms of sensitivity and dynamic range can be investigated. Wavelength shifts of the ARROW and mode interference are shown in Fig. 9. The wavelength valley of the ARROW at the original wavelength of 1452.2nm was shifted to a longer wavelength slightly with different concentrations of ethanol solutions. However, the wavelength valley experiences a large shift when the glycerol solution was pumped into the microfluidic channel, following the same small wavelength shift with different concentrations of glycerol solutions. For two solutions with large RI difference, the original wavelength valley of 1452.2nm was shifted within the wavelength range of SLD and OSA. Therefore, the ARROW shows a large RI measurement dynamic range. On the contrary, the wavelength valley of the mode interference at the original wavelength of 1456.4nm was shifted to a longer wavelength significantly with different concentrations of ethanol solutions, showing a high sensitivity for different RI. When the glycerol solution was pumped into the microfluidic channel, a new wavelength valley of 1456.4nm for the mode interference was chosen as the original wavelength. The wavelength valley experiences a same large shift with different concentrations of glycerol solutions. Therefore, the proposed RI sensor possesses both high sensitivity for the mode interference and large dynamic range for the ARROW. Noted that in this experiment, the injection sequence is from low RI solution to high RI solution (from low to high concentration in each solution, from ethanol solution to glycerol solution). Although the solution with low RI already inside the sensor was not cleared out, it can be pumped out of the sensor in each measurement by using the followed solution with high RI without cross interference. In order to guarantee all solution with low RI to be pumped out of the sensor, the process of each measurement was kept until the wavelength shift was stable, indicating that all space of the hollow hole inside the HCF was filled with the injection solution with high refractive index, as shown in Fig. 9.
In the experiment, the visibility of the transmission spectrum is affected with the light polarization significantly because of the asymmetry structure of the liquid sample infiltrated HCF. Hence the polarization of the light source must be optimized by using the PC. For the analysis of polarization of the proposed sensor, we measured the polarization dependent loss (PDL) of the HCF, which is defined as the maximum change in the transmitted power for different polarization. A tunable laser diode (81980A, Agilent Technologies) with wavelength covering 1450 nm-1750 nm was used as the light source, and the output power was 20 mW, of which the polarization was adjusted by using the PC. The transmission intensity of the sensor was detected by using a powermeter (PM20, Thorlabs) with a resolution of 0.01 dB, as shown in Fig. 10. The HCF shows a dramatically PDL value (from 1.39 to 7.29 dB). Therefore, before the RI measurement, the polarization of the light source was adjusted by rotating the PC until the highest visibility of transmission spectrum was achieved.
5. Conclusion
In conclusion, we have proposed and experimentally demonstrated a fiber optic RI sensor with high sensitivity and large dynamic range simultaneously. An HCF is combined with both an ARROW and mode interference in an embedded suspended core. The proposed scheme has both a high sensitivity and a large dynamic range due to the different RI sensitivities of the ARROW and mode interference. The experimental results show that a high RI sensitivity of 19014.4 nm/RIU for mode interference and a large dynamic range from 0.04 RIU for ARROW can be achieved simultaneously. The proposed fiber optic refractive index sensor can be used in many fields such as chemical and biological applications.
Funding
Natural Science Foundation of China (62022016); National Key Research and Development Program of China (2019YFA0706300); Open Fund of IPOC (BUPT) (IPOC2020A006, IPOC2020A007); China Postdoctoral Science Foundation (2020M680384, 2020M680385).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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