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Abstract
We present an ultrasensitive enhanced fabrication-tolerance refractometer utilizing the polarimetric interference of a tapered PANDA-air-hole fiber (PAHF). To obtain high birefringence and unique group birefringence, the PAHF is specially designed by introducing double air holes into the cladding. Ultrahigh sensitivity can be achieved by reducing the group birefringence difference to zero, defined as birefringent dispersion turning point (BDTP). By modifying the diameter of PAHF, the birefringent dispersion can be effectively manipulated to reduce the group birefringence difference. In this way, the workable diameter range for realizing the ultrahigh sensitivity is twice as large as that of conventional microfibers. Additionally, the ultrasensitive wavelength band is dramatically expanded by at least 600 nm, enabling a compact structure and a flexible fiber-length design. Due to the tunable dispersion optimization, the distinctive properties of ultrahigh sensitivity, enhanced fabrication tolerance, and broadband operation can be achieved. We experimentally verified the ultrahigh refractive index sensitivity of 47223 nm/RIU around the BDTP, and the experimental results matched well with the simulations.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Salinity is a key indicator of water quality. Almost all of the water that people can access is saline water. Saline water cannot be directly used for many purposes, such as irrigation, drinking, and industrial uses, because salinity not only relates to human health but also seriously affects industrial operations and agricultural productions [1]. According to the concentration of dissolved salts, the water can be classified into freshwater, slightly saline water and moderate to high saline water [2]. In the past years, optical microfiber-based sensors with miniaturized structures and low costs have received substantial attention in the field of salinity measurement. The microfiber has a uniform diameter ranging from tens of nanometers to several micrometers, which can be fabricated by a flame tapering process [3]. The optical microfiber, as a combination of optics and nanotechnology, has been engaged in many fields, including optical sensing [4–6], ultrafast modulation for laser [7], quantum optics, atom trapping [8,9], and nonlinear optics [10]. Due to the subwavelength scale structure, the light propagating along the microfiber exposes more energy into the evanescent field, and thus the light-matter interaction is extremely enhanced in comparison with that of standard optical fiber. Therefore, the microfiber shows great potential in sensing applications because of the large evanescent field, structural flexibility, electromagnetic passivity, and small footprint. Over the past decades, various types of microfiber sensors have been reported, which can be classified into interferometer [5,11–13], resonator [6,14], and grating [4,15,16]. Among all the structures, the interferometer-based microfiber sensors have aroused a high level interest in the refractive index (RI) sensing due to the simple fabrication and exceptional sensing properties. For example, one reported a Mach-Zehnder interferometer (MZI) using a microfiber with the diameter of 2 µm as the sensing arm, and a RI sensitivity of 7159 µm/RIU was realized [17]. Nevertheless, the strict requirements on the reference arm limit its practical implementation. To further improve the sensitivity, a splicing point tapered fiber MZI is proposed to detect the salinity in seawater with sensitivities of 223.07pm/‰ [5]. Unfortunately, the MZI structure should be carefully designed and fabricated, which reduces the device repeatability. However, most reported MZI configurations exhibited low sensitivities, failing to meet the requirements of salinity detection.
Recently, microfiber-based multimodal interferometers operating near the dispersion turning point (DTP) have been investigated and served as promising candidates for liquid sensing, due to their ultrahigh sensitivity [11,18–23]. Through decreasing the microfiber diameter, the group effective RI difference between the interfering modes is mitigated, and the RI sensitivity can be dramatically improved to 104 nm/RIU or even higher. The wavelength where the value of group effective RI difference equal to zero is defined as DTP. Guided by this principle, several microfiber-based liquid sensors working at the DTP have been reported. For example, one presented microfiber coupler-based sensors [21,24,25], and the sensitivity of 39541 nm/RIU has been achieved within the near-infrared range. Researchers also have tried to explore more flexible and simpler configurations, and the SMF-based in-line interferometers have been proposed. The standard SMF was tapered into micron size, and the RI sensitivity exceeding 126000 nm/RIU was obtained [11,19,20,22]. However, the higher-order mode cannot be always excited unless the taper parameters meet the non-adiabatic conditions [26]. Additionally, the large difference in power between the two interfering modes results in the low spectral extinction ratios [27], which deteriorates the measurement accuracy. Another well-known configuration is based on the highly birefringent microfiber polarization interferometer [18,28,29], which can realize a large extinction ratio. One fabricated an elliptic highly birefringent microfiber and observed the sensitivity of 195348 nm/RIU in a Sagnac loop [18]. However, a special CO2 laser machining system was required to precisely control the ellipticity of the microfiber cross-sections, which greatly increased the fabrication cost and decreases the device repeatability. Although multifarious microfiber-based interferometers working near the turning point have been studied, the microfiber diameter should be strictly controlled within a small range, only several hundred nanometers, to possess high sensitivity. Besides, all reported configurations have narrow ultrasensitive wavelength band, which indicated that the operation band of optional probing wavelength is narrow and the liberal fiber-length condition on fiber fabrication is severe. To address the detection difficulties and ease the strict requirement for fabrication, researchers have proposed modified flame-drawing methods to control the structure parameters to possess high sensitivities [19,20]. Nevertheless, an effective method to fundamentally enlarge the workable diameter range and broaden the operation band is vital to improve the fabrication tolerance as well as the detection efficiency.
In this paper, we demonstrate an ultrasensitive high fabrication-tolerance refractometer based on a tapered PANDA-air-hole fiber (PAHF) operating near the group birefringence turning point (BDTP), which has a flexible and scalable fabrication and broadband operation. We optimize the birefringent dispersion profiles by using the PAHF-based microfiber which is specially designed by introducing the double air holes into the cladding. Due to the tunable birefringent dispersion, the diameter dependence of the group birefringence difference is greatly reduced by nearly one order of magnitude, compared with all the reported microfiber. In this way, the PAHF-based microfiber has a more scalable diameter range on fiber fabrication and a broader ultrasensitive wavelength band. More precisely, compared with the all the reported microfibers [18,19,21], the workable diameter range is doubled, indicating larger fabrication tolerance and higher device repeatability. Besides, the ultrasensitive bandwidth of PAHF-based microfiber (800 nm) is dramatically enhanced for at least 600 nm. Notably, the high extinction ratios can be easily obtained by polarimetric interferometer, overcoming the difficulty of controlling taper parameters on microfiber fabrication. Then, we experimentally demonstrated the sensitivity of 24841 nm/RIU in a Sagnac loop, and both the positive and negative sensitivities were observed due to the broadband operation. The sensitivity of 47223 nm/RIU can be obtained by tracing the separation between the oppositely shifted twin dips. The extinction ratios are over 20 dB, nearly 15 dB larger than that of the multimode interference-based microfibers, ensuring higher measurement accuracy [11,19]. Due to the tunable dispersion provided by the special air-hole microstructure, the PAHF-based microfiber exhibits advantages of ultrahigh broadband sensitivity, scalable simple fabrication with large tolerance, and high extinction ratios, which shows considerable promise in salinity measurement or other micro components detection.
2. Sensing principle and simulations
2.1 Expression of PAHF-based microfiber
The specially-designed PAHF has an elliptic core with a major axis length of 8 µm and a minor axis length of 6 µm. The cladding diameter is 125 µm, and two non-circular air holes are introduced on both sides of the core. Figure 1(a) shows the cross-section of the PAHF. The configuration of PAHF-based sensor which is composed of the lead-in SMF, the transition regions, a segment of the tapered PAHF, and the lead-out SMF. A section of PAHF is sandwiched into the lead-in and lead-out SMFs. Subsequently, the sensor is fabricated by tapering the PAHF section down to the micron scale. Light launches into the SMF in the form of the fundamental mode and then propagates along the PAHF-based microfiber. As the fiber diameter decreases, the original core mode will spread out into the cladding, and thus the birefringence may be increased. Here, we assume that the taper slopes of the transition regions are adiabatic, so that the fundamental mode evolves into the fundamental mode without exciting higher-order modes. We only consider polarimetric interference between the two orthogonal guided modes, $HE_{11}^x$ and $HE_{11}^y$, propagating along the microfiber and neglect the higher-order modes which may be excited in the SMF-PAHF splicing point or the transition region.
Fig. 1. (a) Schematic illustration of the PAHF-based sensor, and the optical image of the standard PAHF cross section. (b) Effective RIs versus waist diameter for $HE_{11}^x$ and $HE_{11}^y$ modes, respectively. Inset: mode profiles of $HE_{11}^x$ and $HE_{11}^y$ modes, respectively. (waist: 3.6 µm) (c) The birefringence B as a function of the microfiber diameter at 1550 nm (SRI: 1.3324).
A finite element method is applied to calculate the effective RIs of the two orthogonally polarization states of the fundamental mode against wavelength via commercial software (COMSOL Multiphysics 5.4). Effective RIs versus waist diameter for $HE_{11}^x$ and $HE_{11}^y$ modes are shown in Fig. 1(b). Notably, each mode has a cut-off diameter, which is defined as the fiber diameter corresponding to the mode cut off. When the diameter of the PAHF-based microfiber is below the cut-off diameter, such a mode will not be supported in the microfiber. It can be seen that the value of the $HE_{11}^x$ mode is 3 µm, and that of the $HE_{11}^y$ mode is 3.05 µm. The mode fields of $HE_{11}^x$ and $HE_{11}^y$ modes for the PAHF-based microfiber with a diameter of 3.6 µm are presented in the inset of Fig. 1(b). Obviously, the evanescent field of the guide modes leaks out of the microfiber and can interact with the surrounding molecules in the water. Compared with the $HE_{11}^x$ mode, the $HE_{11}^y$ is more sensitive to the change of surrounding refractive index (SRI) because more guided power diffuses out of the microfiber. Therefore, the variation of the salinity could affect the optical path difference between the two polarizations, and the phase difference can be detected by the wavelength shifts.
The well-designed PAHF at the micrometer scale has the potential for achieving high birefringence and unique group birefringence. Figure 1(c) plots the birefringence $B\textrm{ = }n_{eff}^x - n_{eff}^y$ as a function of the fiber diameter at 1550 nm (SRI: 1.3324), where the $n_{eff}^x$ and $n_{eff}^y$ are the effective RIs of the $HE_{11}^x$ and $HE_{11}^y$ modes, respectively. The birefringence becomes larger with a decrease of the fiber diameter, mainly due to the mode extending into the cladding towards the evanescent field regime. The PAHF-based refractometer can be realized in a Sagnac loop to form the polarimetric interference between the two orthogonally polarized optical beams without the polarimetric detection [21,25,30]. The Sagnac interferometer consists of a 3 dB fiber coupler and the PAHF-based microfiber. Light launches into the 3 dB coupler and then is split equally into two counter-propagating waves. The separated beams propagate along the PAHF-based microfiber severally. The beating between the two orthogonal polarizations occurs when they recombine at the 3 dB coupler. The transmission ratio of the optical intensity injected into the Sagnac interferometer in terms of the phase difference $\varphi $ can be expressed as
The wavelength positions of the ${N_{th}}$ dip ${\lambda _N}$ in the interference spectrum satisfies
2.2 Enhanced fabrication tolerance of PAHF-based microfiber
For the microfiber-based sensors, the sensing performances are highly sensitive to the fiber parameters such as fiber length, diameter, and taper region. Therefore, the development of sensors with high fabrication tolerance is indispensable. To achieve an ultrahigh RI sensitivity measurement, the waist diameter should be carefully designed, so that the DTP can fall within the light source bandwidth. Actually, for all the reported DTP theory-based microfiber, the workable diameter range is just only several hundred nanometers [11,12,19,24,31], which requires high fabrication precision. Additionally, considering the spectral width of the interferometric dips, the fiber length should be carefully controlled so that the dips can be observed within the high-sensitivity region. Besides, taper parameters need to be deliberately modified to obtain the high extinction ratio, ensuring the measurement accuracy [19,22]. Though researchers have proposed optimized flam-drawing methods to decrease the fabrication error, an effective method to fundamentally promote the fabrication tolerance remains unexplored to date.
To decrease fabrication error sensitivity, we manipulate and optimize the birefringent dispersion by a specially designed PAHF. By introducing double air holes into the cladding, the material index of the PAHF is flexibly engineered to reduce the diameter dependence of the group birefringence G. More specifically, for the diameter ranging from 3 to 8 µm, the variation of G drops nearly one order of magnitude than that of the SMF-based microfiber reported in [19]. The comparison is presented in Fig. 2(a). In this way, the workable diameter range can be effectively broadened. It is worth mentioning that G only exists when two interfering mode is supported. Therefore, the value of cut-off point of G equals the minimum value of the cut-off diameter of the modes. Subsequently, we analyze the RI sensitivity as a function of the diameter according to the working principle of BDTP. The calculated results are illustrated in Fig. 2(b). The workable diameter ranges for PAHF-based microfiber and SMF-based microfiber are marked in light and dark gray, respectively. Obviously, the workable diameter range of PAHF-based microfiber is twice as large as that of SMF-based microfiber, allowing a more flexible diameter design.
Fig. 2. (a) Value of G for the PAHF and SMF with different diameter, respectively (SRI: 1.3324). (b) Sensitivity SSRI for the PAHF and SMF with different diameter, respectively (SRI: 1.3324). The high-sensitivity regions are marked in grey, where the value of |SSRI|>104nm/RIU. (c)–(d) RI sensitivity regions and interference cutoff regions of PAHF and SMF, respectively.
The operation bandwidth of high-sensitivity region is also a significant characteristic to realize the sensors with fabrication error insensitivity. Considering the spectral width of the interferometric dips, a broader operation band can afford a route for a liberal waist-length condition, a compact structure, and high measurement efficiency. To obtain the bandwidth of the high-sensitivity region, firstly, we numerically analyze the group birefringence G as a function of wavelength for the PAHF-based microfibers. Then, the RI sensitivity SSRI as a function of wavelength can be calculated through Eq. (3). In this way, the high sensitivity regions for PAHF- and SMF-based microfibers of the same diameter range is obtained, as plotted in Fig. 2(c) and Fig. 2(d), respectively. Apparently, the broadband operation of PHAF-based microfiber can be achieved, which is over 600 nm greater than that of SMF-based microfiber [19]. Thereby, the design of taper length can be more flexible, suggesting fabrication tolerant structures. Besides, a more compact microfiber-based structure can be achieved. Notably, the sensitivity regions can be divided into positive sensitivity region and negative sensitivity region by the line of BDTP. As presented in Fig. 2(c), with the increase of diameter, the whole wavelength band experiences redshift. The positive high-sensitivity region is a limited area near the mode cut-off line. From the above, by implementing the well-designed PANDA air-hole microstructure, the birefringent dispersion can be effectively optimized. The exceptional tunable birefringent dispersion allows the realization of ultrasensitive compact structures with enhanced fabrication tolerance and a broadband operation.
The extinction ratio, namely, the interferometric visibility, is another key parameter for sensor performance [27] and fabrication tolerance. A degraded extinction ratio will degrade the measurement accuracy. The extinction ratio depends on the similarity between the optical fields of the interfering modes. Namely, when there is any dissimilarity between the two modes, the extinction ratio will be correspondingly degraded. In the case of SMF-based microfiber which is relied on multimodal interferometer (MMI), the excitation of the higher-order mode is dependent on the non-adiabatic conditions. The control of higher-order mode excitation and power distribution of modes remain challenges, which is indeed correlated with the taper parameters in fabrication, such as drawing velocity and duration [3,19]. Thus, the excitation of the two interfering modes is difficult to be exactly the same [26,32]. Besides, most of the higher-order mode power is in the form of the evanescent wave, and apparently, the higher-order mode modes suffer a greater loss than the fundamental modes with the same propagation distance. Therefore, there is large dissimilarity between the two modes in coupling, resulting in low extinction ratios and thus measurement inaccuracy. To improve the extinction ratios, some researchers monitored the evolution of modes during fiber pulling through a CCD camera to carefully control the taper angle [19,20,22]. Nevertheless, the complex and strict manufacturing process inevitably increases the fabrication difficulty and cost. Compared with the SMF-based MMI, the PAHF-based microfiber relied on polarimetric interference allows a great improvement of the extinction ratios, overcoming the strict requirements of tapering and thus providing a higher device repeatability. The incident light is split equally by the 3dB coupler and two beams are made to follow the same path but in opposite directions. Then the polarization states of the two counter-propagating light beams are modified into x- and y-polarization by the polarization controller (PC), respectively. As illustrated above, the x- and y-polarized modes, $HE_{11}^x$ and $HE_{11}^y$, show similarity in mode field distribution, and the calculated attenuation constants of the two modes are approximately equal. On return to the 3 dB coupler, the two light beams of the same polarization state exit the loop and undergo interference. Therefore, the intensity ratio of the interfering modes can approximately approach 1, resulting in higher extinction ratios and thus more accurate measurements. Generally, the proposed PAHF-based microfiber exhibits excellent sensing performance of enhanced fabrication tolerance, broadband operations, and high extinction ratios, which demonstrates great potential for salinity sensing in freshwater.
2.3 Numerical analysis of the RI sensitivity around the BDTP
According to the working principle of BDTP discussed above, when the value of group birefringence G approaches zero, defined as BDTP, the sensitivity SSRI can be dramatically enhanced and even reach infinite. The value of G is mainly dominated by the diameter of the PAHF-based microfiber. Guided by this rule, an ultrasensitive RI sensor can be realized by optimizing the diameter of the PAHF-based microfiber to achieve the BDTP. We systematically study the RI sensitivity characteristics of the PAHF-based microfiber working around the BDTP. Firstly, we numerically analyze the group birefringence G as a function of wavelength for the PAHF-based microfibers with different diameters (SRI: 1.3324). The simulations are plotted in Fig. 3(a). The value of G monotonously increases from negative to positive with the increase of wavelength. Additionally, the BDTP shifts to a longer wavelength when the microfiber diameter is tuned from 3.2 to 4.4 µm. More precisely, with a small diameter variation of 0.4 µm, the position of BDTP experiences a redshift of about 80 nm.
Fig. 3. (a) Value of G as a function of wavelength for different waist diameters (3.2∼4.4 µm). (b) RI sensitivities around the BDTPs for different waist diameter (3.2∼4.4 µm).
Subsequently, we investigate the RI sensitivity SSRI as a function of wavelength, and the result is plotted in Fig. 3(b). As expected, the RI sensitivity SSRI is greatly enhanced and even reaches infinity when G approach 0, which is in good accordance with Eq. (3). It should be mentioned that the y-axis in Fig. 3(b) ranges from −5×105 to 5×105 nm/RIU. The operation bandwidth where |SSRI|>105 nm/RIU is marked in gray. When the probing wavelength is larger than BDTP, the negative SSRI is obtained, and likewise, the positive SSRI for shorter wavelength. Namely, the dips on the different sides of BDTP present different RI sensitivity. As the wavelength shifts away from the BDTP, the absolute value of SSRI falls sharply and then gradually flattens out. Consequently, for a certain probing wavelength, the SSRI can be flexibly tuned by carefully controlling the microfiber diameter.
In the discussion above, we only considered the uniform waist region and neglected the taper profile. The PAHF-based microfiber is composed of the lead-in SMF, the standard regions, the taper regions, a segment of the tapered PAHF, and the lead-out SMF, as illustrated in Fig. 4(a). To evaluate the influence of the taper profile, we construct geometric model firstly. Many mathematical models have been used to discuss the taper shape, such as the linear, raised cosine, and modified exponential taper [33]. In our modeling, we adopt the exponential taper profile to simulate the taper regions, considering the adiabaticity of the taper. Such taper profiles of the PAHF-based microfiber can be expressed in terms of the fiber diameter r(z) as
Fig. 4. (a) Schematic diagram showing the taper profile of the PAHF-based microfiber. (b) Step-like approximation method for the down taper region.
It can be concluded from the above that the position of BDTP depends on the waist diameter of the PAHF-based microfiber. Actually, the taper regions and the standard regions also effect the birefringence of the device, and thus the spectral characteristics. For the taper regions where the diameter of fiber varies, the birefringence varies correspondingly, and Eqs. (2) and (3) are not applicable to this situation. Therefore, we adopt a generalized expression to calculate the phase difference between $HE_{11}^x$ and $HE_{11}^y$ modes accumulated along the standard regions, the tapered regions and the waist region [22,32].
We discuss the influence of the length of taper regions and standard regions on the position of BDTP. To demonstrate the BDTP property more vividly, the transmission is calculated with the phase difference. First, we analyze the influence of the taper length Lt. We assume that the waist diameter, Ls, and Lw are constant and fixed, while the unilateral taper length Lt/2 varies from 0 to 7 mm. The evaluation of spectra and corresponding phase difference are shown in Fig. 5(a). The BDTP is where the maximum value of phase difference is, which can be deduced by Eqs. (2) and (3) in the manuscript. It can be seen that with the increase of taper length, the BDTP moves to the longer wavelength, and the period interference fringes varies slightly around the BDTP. The broad dip at the BDTP becomes shallow and the whole spectrum exhibits a red-shift tendency. Likewise, we analyze the influence of the length of standard PAHF Ls. We suppose that the waist diameter, Lt, and Lw are constant and fixed, while the unilateral PAHF length Ls/2 varies from 0 to 7 mm. Figure 5(b) is the simulated spectra and the corresponding phase difference. As the PAHF length increases, the BDTP appears at a longer wavelength. Both the phase difference and the spectra present red-shift. As discussed above, the position of BDTP is not only attributed to the waist diameter, but also dependence to the length of standard regions and taper regions. With a longer taper length or standard length, the BDTP exhibits a red-shift tendency. Therefore, by carefully designing the taper parameters, the position of BDTP can be flexibly manipulated. We have added the influence of the length of taper regions and standard regions in the manuscript.
Fig. 5. (a)–(b) Calculated transmission spectra and phase differences for tapered optical fibers with different length of taper regions and standard regions, respectively.
3. Experiment results and discussion
We fabricated and experimentally demonstrated the ultrasensitive refractometer based on PHAF-based microfiber to verify the broadband BDTP theory. Firstly, a section of PAHF with the polymeric coating removed was sandwiched into two segments of SMFs, using a commercial fusion machine, Fujikura 100P+ fusion splicer. The splicing was carried out by facile splicing procedures in automatic central alignment mode, suggesting easy assembly. The arc power and arc time were set to 226 bit and 800 ms, respectively. The splicing points had no offset and bubbles, ensuring that the sandwich structure was robust and easy to be handled for the next tapering process. After the splicing process, we introduced one-step heating and drawing method to fabricate intermediate tapered fibers. The schematic diagram of the homemade tapering station is shown in Fig. 6(a). The critical factor of fabricating the microfiber with the desired diameter while maintaining the internal microstructure is the controlling of pulling time and pulling distance [3,35–37]. By increasing the pulling speed, the internal microstructure can be maintained by the gas pressure in the holes and the low glass viscosity. Besides, with a relatively fast pulling speed compared with that of non-adiabatic taper, the adiabaticity of the taper region can be obtained. The pulling distance determines the diameter of the uniform waist region. At a certain pulling speed, it can be found that the longer the pulling distance is, the thinner the diameter is. This can be explained that the total volume of the glass material remains constant during the drawing procedure. Additionally, a steady temperature is also an important factor, which contributes to the uniformity of diameter. The PAHF-based microfiber is fabricated by a flame stretching method mentioned above. A segment of SMF-PAHF-SMF configuration was held between two translation stages horizontally and then tighten up. The PAHF section was placed at the edge of the outer oxyhydrogen flame and heated adequately to reach the drawing temperature. Then the flame was moved along the fiber axis in the scanning heating method to evenly heat the fiber. Simultaneously, stages I and II were moved in opposite directions under the control of a motor. The pull speed was carefully controlled to minimize the time during which the glass had low viscosity so that the internal air-hole microstructure could be maintained by the gas pressure in the holes. The motor helped keep a steady temperature distribution during the drawing. We could obtain PAHF-based microfibers with diameters down to 2 µm and lengths up to tens of millimeters. By the auto one-step heating and drawing process, we could easily manufacture a series of PAHF-based microfiber with the desired length and uniform diameter, ensuring the repeatability of the device. Figure 6(b) shows the top-view optical image of the fabricated 6-mm-long PAHF-based microfiber with a diameter of 3.6 µm.
Fig. 6. (a)Schematic diagram of heating-drawing method. (b) Top-view optical image of the fabricated PAHF-based microfiber with diameter of 3.6 µm.
Generally, the tapered microfiber with thin waist is fragile and sensitive to mechanical disturbances, such as fiber micro-bending. Hence, the PAHF-based microfiber was on-site packaged onto a well-designed chip to enhance the long term stability and portability. The chip ensured that the microfiber could be accessible to the liquid samples and maintain straight during the microfluidics injection. Then we covered the microchip with a piece of plastic film to prevent the dust from contaminating the microfiber. The experimental setup of the RI sensor in a Sagnac loop consisted of a 3 dB fiber coupler, a polarization controller, and the sensor chip, as presented in Fig. 7. The light from the broadband light source (BBS) launched into the 3 dB coupler and then was split equally into clockwise and anti-clockwise propagating waves. Polarimetric interferometer happened when the waves recombined at the 3 dB coupler, and an optical spectrum analyzer (OSA) was used to record the output spectrum. The polarization state of light was controlled by the polarization controller.
Fig. 7. Experimental setup of the Sagnac interferometer and the schematic diagram of the sensor chip.
In practice, freshwater is defined as water with less than 1000 parts per million (ppm) of dissolved salts, and typical good quality drinking water has less than 600 ppm concentration. Most of the dissolved inorganic salts in water are NaCl, KCl, MgSO4, and NaHCO3 [38]. The above elements present in the form of ions in water, and the refractive index of freshwater will change with the salinity. That is, the solution salinity can be obtained indirectly by detecting the refractive index of the solution. At a certain temperature, as the salinity increases, the refractive index also increases. The experiment was carried out in a cleanroom environment to keep the temperature constant (25 °C), ensuring measurement accuracy. We prepared a series of refractive index-matching liquid with different SRI ranging from 1.3324 to 1.3328 with a step of 0.0001 to simulate different water-quality situation (about 0∼ 3000 ppm of NaCl) [39]. The prepared liquid was slowly injected into the sensor chip, and the microfiber was completely immersed into the liquid integrally. Notably, before each measurement of liquid of different RI, we repeatedly cleaned the PAHF-based microfiber using the liquid under test until the measured spectra shown in OSA remained unchanged, and then we carried out the round of measurements five times.
Figure 8(a) presents the transmission spectra of the PAHF-based microfiber with the diameter of 3.6 µm surrounded by different SRI ranging from 1.3324 to 1.3328. The insertion loss of the tapered microfiber was about 15 dB, which mainly due to the splicing point and the surface roughness-induced scattering [40]. The extinction ratios were higher than 20 dB, about 15 dB greater than that of MMIs [19]. As the SRI increased, the interference dips of both sides of BDTP shifted towards the BDTP, presenting different drift trends. The transmission spectra were sinusoidal curves with some unexpected periodic fluctuations. This phenomenon was caused by the following two factors: first, higher-order modes may be excited in the SMF-PAHF splicing point due to the mode filed mismatch. Second, the imperfect packaging may lead to micro-bending of the PAHF-based microfiber, which induced the excitation of higher-order modes. These higher-order modes interfered with the fundamental modes, resulting in unique spectral fluctuations.
Fig. 8. (a) Variations of interference spectra of different SRI increasing from 1.3324 to 1.3328. (b) Wavelength shifts of dips around the BDTP. (c) Doubled sensitivities by tracing the distance between the twin dips on the opposite side of BDTP. (d) Comparison of the dip sensitivities of the measured results and the simulated results.
To clearly evaluate the sensing characteristics, we traced the interference dips, and the sensitivities of the dips are presented in Fig. 8(b). It could be found that all the dips presented linear response towards the RI variations. As expected, the closest dips near the BDTP showed the highest sensitivity than other dips, and the maximum sensitivity of 24841 nm/RIU could be obtained. Notably, both positive and negative sensitivity were experimentally observed due to the broadband operation properties. Additionally, the sensitivity were enhanced by tracing the separation between the twin dips which were positioned on opposite sides of the BDTP, and the sensitivity could be improved to twice that of a single dip, as shown in Fig. 8(c). The sensitivity of 47223 nm/RIU was achieved by calculating the distance between dip b and dip b’ (dbb’). The BDTP position was about 1410 nm for SRI of 1.3324, calculated by averaging the wavelength of dip b and dip b’. The wavelength difference between the calculation result (1395nm) and the experimental result was about 15 nm, which may arise from the measurement inaccuracy of the microfiber diameter using a microscope. As demonstrated in Fig. 8(d), the tendency of the measured sensitivity curve was similar to that of the simulated results, which well verified our proposed broadband BDTP theory. The sensitivity could be further promoted through controlling the taper length to obtain a desired free spectral range (FSR), so that the dips fell within the bandwidth of the higher sensitivity region (SSRI>105 nm/RIU). Table 1 shows the comparisons of the microfiber sensors based on the dispersion turning point.
Table 1. Microfiber sensor performances based on the dispersion turning point.
4. Conclusion
In summary, we proposed and experimentally demonstrated an ultrasensitive enhanced fabrication-tolerance refractometer based on tapered PAHF working at BDTP. Due to the tunable birefringent dispersion property, the diameter dependence of group birefringence difference for PAHF-based microfiber is significantly reduced than that of all the reported microfiber. Therefore, the PAHF-based microfiber can possess ultrahigh RI sensitivity with a larger workable diameter range. More specifically, the workable diameter range is about twice greater than that of reported SMF-based microfiber. Besides, the ultrasensitive wavelength band is prominently broadened. The bandwidth of the PAHF-based microfiber exceeds over 800 nm, nearly 600 nm larger than that of SMF-based microfiber. Additionally, the Sagnac interference provides high extinction ratios, overcoming the fabrication difficulty in tapering and the measurement inaccuracy. Consequently, by using the proposed PAHF, the birefringent dispersion can be effectively manipulated, and an ultrasensitive broadband refractometer with enhanced fabrication tolerance can be achieved. We experimentally fabricated a series of PAHF-based microfiber with different diameters and verified the ultrahigh broadband performance. By tracing the twin dips with different sensitivity trends, the maximum sensitivity of 47223 nm/RIU was achieved by a 6-mm-long PAHF-based microfiber with a diameter of 3.6 µm. In conclusion, the proposed PAHF-based microfiber refractometer presents the advantages of ultrahigh sensitivity, enhanced fabrication tolerance, and broadband operation, which suggests the great potential in monitoring the salinity in water.
Funding
Beijing Municipal Science and Technology Commission (Z191100004819012); National Natural Science Foundation of China (U1831110).
Acknowledgments
The authors acknowledge Prof. Perry Ping Shum (Southern University of Science and Technology), Prof. Lei Wei, and Dr. Yu Zheng (Nanyang Technological University) for assistance with measurements. The authors acknowledge Dr. Yiyang Luo and Dr. Wenjun Ni (Huazhong University of Science and Technology) for useful discussions.
Disclosures
The authors declare no conflicts of interest.
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