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Abstract
A highly sensitive gas refractive index (RI) sensor based on hollow-core photonic bandgap fiber (HC-PBF) and Fourier transform white-light interferometry was experimentally demonstrated. HC-PBFs with lower loss than hollow silica tubes render a longer air cavity for the Fabry-Perot interferometers (FPIs) without a great deal of compromise to the fringe visibility of interference. Fourier transform phase demodulation method was employed in the experiment and a directly proportional relationship between the phase sensitivity and cavity length was demonstrated. For a cavity length of ∼24.9 mm, the sensor’s gas RI sensitivity reaches up to 50775.54 µm/RIU in an air RI range from 1.000 to 1.030. Considering the cavity length demodulation resolution of 0.06 µm achieved by this method, the sensor can detect gas RI change with a resolution of 10−6 RIU, which can meet the sensing demand for almost all the gases. Moreover, the gas RI sensitivity and measurement range can be improved further by lengthening the HC-PBF. The high sensitivity, large dynamic range and good linearity of the proposed sensor make it a good candidate for biosensing, monitoring of modern chemical industry or gas laser systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Precise determination of refractive index (RI) is of great significance in many fields such as label-free chemical and biomedical sensing [1,2]. Owning to their compact size, optical fiber RI sensors have been widely studied and reported, such as fiber surface Bragg gratings [3], microfiber Bragg gratings [4], long-period fiber gratings [5,6], fiber Mach-Zehnder interferometers [7,8], fiber Fabry-Perot interferometers (FPIs) [9], fiber Michelson interferometers [10], fiber surface plasmon resonance sensors [11,12] and liquid-modified photonic crystal fiber directional couplers [13]. However, developed for chemical and biological applications, the most reported optical fiber RI sensors are designed for aquatic environment, namely the RI measurement range are around 1.33 [14], which suffers an extremely low sensitivity for gas RI measurements. The measurement of gas RI, which is around 1.0, is of great significant in many fields such as chemical industries (i.e. the detection of flammable, explosion, and toxic gases [15,16]), modern optical instruments and gas lasing systems [17,18,19], where high-power laser beams propagate in and interact with gaseous media. Recently, several optical fiber gas RI measurement schemes have been reported, the most typical case is open-cavity FPIs [20], the RI sensitivities of which are around 1053.44 nm/RIU. By employing Vernier effect in the open-cavity FPIs, the RI sensitivities can be enhanced up to 30899 nm/RIU [21]. However, the sensitivity of an FPI can hardly be enhanced further, using the traditional wavelength demodulation method. Moreover, the wavelength demodulation method, characterized by tracking a peak or dip wavelength, is not an absolute but a relative measurement of RI, which needs to be calibrated to the standard RI.
Fourier transform white-light interferometry has been widely used to interrogate the absolute optical path or phase difference, which can be employed to determine the absolute RI of gas diffused into the interferometer. Phase detection is an ultrasensitive detection method that has been widely employed for probing extremely weak signals [22]. For FPI sensors, the phase sensitivity is in direct proportion to the cavity length due to the phase accumulating effects. However, all the aforementioned open-cavity FPI gas RI sensors suffer a limited cavity length due to the poor optical transmission properties of the FPI cavity. Thus, a low-loss hollow core fiber will be advantageous to a long cavity FPI. Hollow-core photonic bandgap fibers (HC-PBFs) can simultaneously confine the propagating lights and gas into the hollow core and the transmission loss is typically ∼1.2 dB/km [23], which enables long-distance interaction of light and gas [24], offering an excellent low-loss cavity for gas RI measurements. A minor change of gas RI will result in a relatively large optical phase change, which is proportional to the HC-PBF length.
Herein, we reported on the highly sensitive absolute measurement of gas RI based on HC-PBF open-cavity FPI. Fourier transform white-light interferometry was used in the experiments and the gas RI sensitivities can reach up to 50775.54 µm/RIU with a 24.9 mm-long HC-PBF cavity. Considering the OPD demodulation resolution of 0.06 µm by this algorithm [25], the sensor can detect gas RI change with a resolution of 10−6 RIU. A directly proportional relationship between the gas RI sensitivity and the cavity length of HC-PBF was demonstrated both theoretically and experimentally, namely the gas RI sensitivity can be improved further by employing a longer HC-PBF cavity. The proposed sensor is compact in size and free of electromagnetic interference, and hence may find vital applications in biosensing, chemical industry or modern gas laser systems monitoring as well as dynamic acoustic pressure detection.
2. Sensor fabrication and working principles
The illustrative configuration of the proposed sensor is shown in Fig. 1(a). At first, a section of HC-PBF (NKT, HC-1550-02) was spliced with a lead-in single-mode fiber (SMF, Corning, SMF-28) with a commercial arc fusion splicer (Fujikura, FSM-60S) and optimized discharge parameters to avoid the collapse of hollow cores. The air core diameter of the employed HC-PBF is ∼10.9 µm and the insertion loss of a fusion joint is ∼2.6 dB. Then, the HC-PBF was cut off with a homemade microscope-assisted fiber cutter [26,27] to accurately control the length of reserved HC-PBF. A section of silica hollow core fibers (HCFs) was fusion spliced with the HC-PBF, and the other end of the HCF was sliced with a tilt angle to eliminate the parasitic interference generated from the end facet reflection. The inner diameter and outer diameter of the employed HCF are ∼2 µm and ∼125 µm, respectively. The cross-sectional SEM images of the employed HC-PBF and the HCF are shown in Fig. 2, respectively. The difference in the core diameters of HC-PBF and HCF enables a reflector and a microfluidic channel for gas into the HC-PBF cavity simultaneously. Thus, an open-cavity FPI was formed. It is worth noting that, HCFs with larger inner diameter can render a faster gas RI response while reducing the reflective intensity of the second reflector and thus the fringe contrast. Alternatively, the response time of the sensor can be improved by shortening the length of HCF. So HCFs with an inner diameter of ∼2 µm was employed in our experiments.
Fig. 2. (a) Cross-sectional SEM images of the employed HC-PBF and HCF and (b) locally enlarged view of the air holes in the centers of HC-PBF and HCF.
The low transmission loss of HC-PBFs enables an open long air cavity without a vast sacrificing of the visibility of fringe pattern. Four HC-PBF FPI sensors (S1, S2, S3, and S4) with different HC-PBF length, i.e. 107 µm, 1.1 mm, 2.1 mm and 12.3 mm, were fabricated, respectively. The reflection spectra of these sensors were measured with a broadband light source (BBS), a 3 dB fiber coupler and an optical spectrum analyzer (OSA) (Yokogawa, AQ6370C) with a resolution of 0.02 nm. As shown in Fig. 3, the free spectrum range (FSR) of the reflection spectra is narrowing with the increasing of HC-PBF length, which agree well with the relationship
where λ1, λ2 are the adjacent dip wavelength, n and L are the RI of air, the HC-PBF length, respectively. The decrease in fringe contrast with the cavity lengthening can be attributed to the transformation of light-guiding mechanism in the HC-PBF cavity. To be specific, when the HC-PBF is short enough, the photonic bandgap (PBG) guiding mechanism of the HC-PBF doesn’t work and the light transmits in the air cavity and is spatially coupled back to the lead-in SMF. Thus, the mode-mismatch-induced insertion loss of the splicing joint has little effect on the reflected light intensity of the second mirror. With the lengthening of HC-PBF, the PBG effect become dominant and the light suffers a mode-mismatch-induced splicing loss of ∼2.6 dB, resulting in a decreased fringe contrast compared with a shorter HC-PBF. When the HC-PBF is longer enough, the fringe contrast will not decrease significantly. As shown in Fig. 1(a), the proposed open-cavity FPI has two main reflection surfaces (Ι and ΙΙ). The reflectivity of the two surfaces R1 and R2 can be calculated byFig. 3. Reflection spectra of four fabricated sensors (S1, S2, S3, and S4) with different HC-PBF length, i.e. 107 µm, 1.1 mm, 2.1 mm and 12.3 mm, respectively.
Fig. 4. (a) Calculated OPDs of FPIs with different cavity length L as functions of the gas RI; (b) Calculated gas RI sensitivities as a function of FPI cavity length L.
Figure 5(a) shows the measured transmission spectrum of the employed HC-PBF with a length of ∼12.3 mm, where three typical regions in the wavelength range of 1250-1650 nm, named Region 1, 2 and 3, are clearly presented. In Region 1, which is beyond the photonic bandgap of HC-PBF, almost all of the optical energy is leaking out. By contrast, within the photonic bandgap of HC-PBF, i.e. Region 2 and 3, light propagates through HC-PBFs with low loss. Figure 5(b) shows the reflection spectrum of a fabricated HC-PBF FPI sensor (sample S4) with a cavity length of ∼12.3 mm and Fig. 5(c) exhibits an enlarged view of Fig. 5(b) at ∼1550 nm. It can be clearly seen that, within the photonic bandgap range (Region 2 and 3), the fringe visibility was ∼3.4 dB, whereas it decreased to less than 0.5 dB beyond the photonic bandgap of HC-PBF (Region 1). This is the reason why we choose HC-PBF to construct the open cavity of the FPI.
Fig. 5. (a) Measured transmission spectrum and (b) reflection spectra of the FPI with a 12.3 mm length HC-PBF; (c) Enlarged view of the FPI reflection spectrum at ∼1550 nm.
3. Air RI sensing response of the proposed HC-PBF FPI sensors
The air RI test setup and optical spectra acquisition device are schematically shown in Fig. 6(b). Four fabricated sensor samples S1-S4, which have a cavity length L of 107 µm, 1.1 mm, 2.1 mm and 12.3 mm, respectively, were placed into an air chamber. A commercial air pressure generator with a stability of ± 0.2 KPa was employed with a high-precision digital pressure meter (ConST-811). The chamber was kept at a constant temperature of 25 °C and was fitted with a feed-through and sealed by strong glue to extend the fiber pigtail outside the chamber for real-time measurements. The resolution of OSA was set to 0.02 nm. The air RI was increased from 1.00026 to 1.01056 in a step of 0.00132, corresponding to air pressure in the chamber elevated from atmosphere pressure (0.1 MPa) to 4.0 MPa in a step of 0.5 MPa according to Eq. (5) at 25 °C. At each measured point, the pressure was kept for 1 min and the temperature was controlled at 25 °C during the whole measurement process. After holding the air pressure for 1 min, the reflection spectra of the samples were recorded by the OSA. Each recorded reflection spectra was demodulated by the Fourier phase method as reported in Ref. [25] and [30]. Figure 6(a) presents a flow chart of the employed Fourier phase demodulation method. The OPD change of the HC-PBF FPI sensor samples S1, S2, S3, and S4 with respect to the air RI were illustrated in Figs. 7(a), 7(b), 7(c), and 7(d), respectively. The air RI sensitivities (measured from the linear fits of the OPDs as functions of air RIs, as shown in Fig. 7) of the four samples S1, S2, S3, and S4 were 208.245, 2276.091, 3839.792, 23364.642 µm/RIU, respectively. Note that the air RI sensitivity of the proposed sensors exhibits a positive correlation with the cavity length L.
Fig. 6. (a) Flow chart of the employed Fourier phase demodulation method. (b) Air RI test setup and optical spectra acquisition device (BBS: broadband light source, OSA: optical spectrum analyzer).
Fig. 7. Demodulated OPDs of four HC-PBF FPI sensor samples S1, S2, S3, and S4 as functions of the air RI in a range of 1.00026 to 1.01056. (a) S1 (L = 107 µm), (b) S2 (L = 1.1 mm), (c) S3 (L = 2.1 mm), and (d) S4 (L = 12.3 mm).
Furthermore, we investigated the air RI response of the proposed HC-PBF FPI sensor in a larger air pressure range from 0.1 to 10 MPa, corresponding to air RI from 1.00026 to 1.02640. Another four HC-PBF FPI sensor samples (S5, S6, S7, and S8) with cavity length L of 2.7, 6.7, 12.4 and 24.9 mm, respectively, were fabricated and tested by employing the same equipment and method as described above. The OPD demodulation results of S5, S6, S7, and S8 obtained by Fourier phase method were shown in Figs. 8(a), 8(b), 8(c), and 8(d), respectively, where linear fits were employed with high linearity of 0.999, 0.998, 0.999 and 0.999. It is obvious that the RI sensitivities of the samples S5-S8 are also of positive relevance with its cavity length L, which agrees well with the theoretical predictions shown in Fig. 3(a). It should be noted that, the maximum cavity length is limited by the wavelength resolution of the employed OSA. To be specific, the FSR of the sensor should be larger than the wavelength resolution of the OSA (∼0.02 nm). Therefore, the maximum cavity length in our case is calculated to be 60 mm. Figure 9 exhibits the measured air RI sensitivities of the eight HC-PBF FPI sensor samples (i.e. S1-S8) fabricated in this work as a function of their cavity lengths L. Theoretical calculations using Eq. (6) were also demonstrated in Fig. 9. It is clear that the experimental results agree well with the theoretical calculations. The small deviation may result from the measurement error in the cavity length L. Moreover, it is worth noting that, the proposed sensor can render a wider air RI measurement range, which was not explored in the experiments due to the limitation of our test equipment. Anyway, we believe that the proposed HC-PBF FPI air RI sensor could find vital applications in modern biochemical sensing, lasing industry and dynamic acoustic detection.
Fig. 8. Demodulated OPDs of four HC-PBF FPI sensor samples S5, S6, S7, and S8 as functions of the air RI in a range of 1.00026 to 1.02640. (a) S5 (L = 2.7 mm), (b) S6 (L = 6.7 mm), (c) S7 (L = 12.4 mm), and (d) S8 (L = 24.9 mm).
Fig. 9. The air RI sensitivity of eight HC-PBF FPI sensors (S1-S8) as a function of the cavity length L.
4. Conclusion
We experimentally demonstrated a highly sensitive gas RI sensor based on a long air-cavity HC-PBF FPI and optical phase demodulation method. A high gas RI sensitivity of 50775.54 µm/RIU in a RI range from 1.000 to 1.030 was achieved for the sensor with cavity length L of ∼24.9 mm. A positive proportionality between the gas RI sensitivity and cavity length L was proved both theoretically and experimentally. The proposed long air-cavity HC-PBF FPI gas RI sensor exhibits ultrahigh sensitivity, large dynamic range, excellent linearity and repeatability. Therefore, this novel type of gas RI sensor may find vital applications in the biochemical industry for noxious gas detect, pressure control and modern gas laser systems for gas nonlinearity research. Moreover, dynamic acoustic detection with the proposed gas RI sensor is also a promising research focus and will be studied in the future.
Funding
National Natural Science Foundation of China (61875128, 91860138, 61635007); Shenzhen Science and Technology Innovation Commission (JCYJ20180507182058432, JCYJ20160427104925452); Development and Reform Commission of Shenzhen Municipality.
Disclosures
The authors declare no conflicts of interest.
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