A micro-cavity fiber Fabry-Perot interferometer based on dual capillaries is proposed and demonstrated for gas pressure measurement. Such a device is fabricated by fusion splicing of a tiny segment of a main-capillary with a feeding-capillary on one end, and a single mode fiber on the other, to allow gas enters the main-capillary via the feeding-capillary. The reflection spectrum of the interferometer device shifts with the variation of gas pressure due to the dependence of gas refractive index on the pressure applied. During the device fabrication process, a core-offset fusion splicing method is adopted, which turns out to be highly effective for reducing the detection limit of the sensor. The experimental results obtained show that the proposed device exhibits a high gas pressure sensitivity of 4147 pm/MPa, a low temperature cross-sensitivity of less than 0.3 KPa/°C at atmospheric pressure, and an excellently low detection limit down to ~4.81 KPa. The robust tip structure, ultra-compact device size and ease of fabrication make the device an attractive candidate for reliable and highly sensitive gas pressure measurement in a precise location.
© 2015 Optical Society of America
Optical fiber pressure sensors are of great interests owing to their many advantages, such as compact size, immunity to electromagnetic interference and easy of signal detection. Various types of optical fiber pressure sensor configurations have been explored, including π-phase-shifted fiber Bragg grating on side-hole fiber , side-hole dual-core photonic crystal fiber , and Polymethyl methacrylate (PMMA) birefringent fiber , etc. Because of the miniature size and high sensitivity, Fabry–Pérot interferometers (FPIs) play an important role in pressure sensing [4–9]. There are two types of FPI pressure sensors: based on closed cavity and open-cavity respectively. In the closed cavity configuration, the FP cavity length varies with the pressure change. For instance, a hollow-core fiber based FPI exhibits a pressure sensitivity of 23.4 pm/MPa . The fiber-tip micro-bubble based FPI shows a pressure sensitivity of 315 pm/MPa . Liao et al. optimized the fabrication process and enhanced the pressure sensitivity up to 1036 pm/MPa . An efficient way to improve the pressure sensitivity is to decrease the thickness of the FP cavity wall or the diagram and as a result, ultrahigh pressure sensitivities of 39.4 nm/KPa  and 70.5 nm/KPa  have been obtained. However, the measurement range of the device is limited and the mechanical strength is rather poor as the thin diaphragm attached at the fiber end is easily to be cracked. Wang et al demonstrated FPIs with interesting structures that provide ultrahigh pressure sensitivity of even > 1000 nm/KPa  however, the sensing system is complicated and the measureable pressure range is limited. The open-cavity-based FPIs have extended pressure measurement range owing to their different principle of operation: the gas refractive index (RI), instead of the cavity length, varies with the pressure change. A number of open-cavity FPIs have been reported [11–20]. Xiao et al. utilized a bonding technique to demonstrate a simple cavity FPI , in which different materials were used to construct the sensor head, hence a large temperature cross-sensitivity was produced due to the large thermal-expansion coefficient of the glue used in the structure. Coelho et al. reported a hybrid FPI for simultaneous measurement of the partial pressure of O2 and CO2 , which exhibited a superimposed interference spectrum that made the signal demodulation complicated. Duan et al. demonstrated an open-cavity FPI by introducing a large lateral offset of 67.5 μm during splicing and used it as a gas refractometer . The large lateral offset degraded the mechanical strength of splicing point and led to the device fragility. The open-cavity FPIs fabricated by selectively etching the specially designed P2O5-doped fibers were also reported [14, 15], with poor robustness. Recently, femtosecond (fs) laser micromachining [16–18] and focused two beam FPI spectrum and hence supporting easy demodulation, a higion beam (FIB) milling [19, 20] techniques have been used for open-cavity FPI fabrication. However, such systems require high-quality laser/FIB source and bulky optics, as well as high-resolution monitoring setups, rendering the device fabricated highly expensive and fragile due to partly removed fiber material. Wang et al. reported a hybrid fiber FPI structure consisting of two capillaries with different inner diameters for gas refractive index (RI) sensing . The sensor is easy to be fabricated however, its output fringe spectrum is a three-beam interference pattern, i.e. a superposition of fringes corresponding to different cavities formed by three interfaces including the free end face of the feeding-capillary. The signal demodulation for such a system is rather complicated.
In this paper, a robust open-cavity FPI is proposed and demonstrated for gas pressure sensing, which has a fiber-tip structure, consisting of two tiny sections of fusion-spliced capillary tubes with different inner diameters. The device exhibits h sensitivity of 4147 pm/MPa within the pressure range from 0 to 1.52 MPa and an ultra-low temperature cross-sensitivity of less than 0.3 KPa/°C at atmosphere pressure. Moreover, by using core-offset fusion splicing technique during the fabrication process to narrow the fringe dips, the detection limit of the sensing device becomes as low as 4.81 KPa. The proposed sensor is also easy to fabricate and has an ultra-compact size of less than 150 μm. All these features make it especially suitable for highly sensitive gas pressure sensing at a precise location.
2. Device fabrication and operating principle
Figure 1 shows the schematic diagram and the microphotograph of the micro-cavity fiber Fabry–Perot interferometer (MFFPI) sensor tip. To fabricate the proposed MFFPI, two types of quartz capillaries from Polymicro Technologies are used. The inner diameters of the two capillaries are ~50 μm (TSP050150) and ~5 μm (TSP005150), named as C1 and C2, respectively. The outer diameters of both capillaries are ~150 μm. Firstly, a piece of C1 is spliced to a standard single-mode fiber (SMF, Corning, SMF28) by using a fiber fusion machine (Fujikura, FSM-45P), which is then cleaved by using a normal fiber cleaver to leave only a short section connected with SMF. Next, the cleaved C1 is fusion-spliced with a section of feeding-capillary C2 to form an FP cavity. Appropriate fusion splicing parameters including a relatively low level of arc power have to be adopted to avoid the collapse of the air holes of capillaries while ensuring the robustness of the device during the fabrication process. Table 1 lists the key parameters adopted in our experiments. It should be noticed that the capillary with larger inner diameter is always placed in a relatively large distance from the discharge electrodes to ensure the unbalanced heating and prevent the collapse of capillary holes. Moreover, the core-offset splicing technique is adopted to narrow the fringe dip, and hence decreasing the detection limit of the sensor. Finally, C2 is cleaved to create a short segment of capillary with slant end face to prevent the undesired Fresnel reflection from the glass/air interface. In our experiments, fs laser is used to implement cleaving to enable a more compact size, as shown in Fig. 1(b). The total length of C1 and C2 is less than 150 μm, and the cavity length of MFFPI is measured to be ~65μm.
When a wideband light source is incident from the left end of SMF, the reflecting beams from the two glass/air interfaces of the main micro-cavity C1 are combined, resulting in an FP interference spectrum. The output light intensity of the interferometer can be expressed as
When the gas pressure P increases, the RI of gas filling in the FP cavity also increases, and the interference spectrum of the MFFPI shifts accordingly. Assuming a constant cavity length, and by tracing the mth order interference fringe dip, , the gas pressure sensitivity of the MFFPI can be expressed as:22]:Equation (3) indicates that the wavelength shift of the interference spectrum has an approximately linear relationship with the pressure of air filling in the cavity, which is of practical importance. It also reveals that a longer operation wavelength provides a higher sensitivity and hence a broadband source covering the long wavelength range could be employed to improve the sensitivity of the sensor.
To evaluate the performance of the proposed sensor, the detection limit (DL) must be taken into account. DL is proportional to the full width at half-maximum (FWHM) value of the interference fringe, besides the other factors such as thermal noise and the OSA performance . For a FPI, the FWHM of fringe dips would be narrower with the increase of the reflection of cavity mirrors.
Additionally, according to Eq. (1), for a fringe dip, its extinction ratio (ER) can be evaluated quantitatively as
Figure 2 shows the calculated ER versus the intensity ratio of the two interference beams. With the increase of intensity ratio, ER increases accordingly. When the ratio is larger than 0.8, ER becomes greater than 25 dB. Due to the hollow core of C2, if the SMF, C1 and C2 are aligned in the direction of their central axis, the intensity of the beam reflected from the interface of C1/C2 is much lower than that from the interface of SMF/C1. Thus, the ER becomes low.
By setting a certain amount of core-offset during splicing of C1 and C2, the light leaking into the hollow core of C2 can be reduced or even eliminated, the ratio between I2/I1 would then be increased and hence the ER is improved. If the cavity length keeps as a constant with the increase of ER, as FSR would not change, the FWHM of the fringe dip would be reduced. Figure 3(a) shows the measured reflection spectra of the MFFPIs with different core-offset (δ) values. It can be found that the ER increases monotonically from ~8.5 to ~33 dB with the increase of core-offset from ~0 to ~6 μm, while FWHM decreases from 7.1 to 0.54 nm. If the core-offset is further increased, the ER and FWHM only slightly change. According to Eq. (5), the measured maximum ER value of ~31.4 dB has nearly achieved its theoretical limit, and the FWHM is ~0.53 nm in this case. The small wavelength shift of the resonance dips is due to the tiny angle between the two mirrors of the MFFPI.
The cavity length of the MFFPI is another factor affecting the ER. Figure 3(b) displays the interference spectrum of the MFFPIs with the same core-offset of ~8 μm but different cavity lengths. When the cavity length increases from 35 to 102 μm, the ER decreases rapidly from 28.8 to 11.5 dB at the wavelength of ~1565 nm, due to the increased beam divergence in the MFFPI. For cavity length of ~35 μm, the FWHM is ~0.9 nm.
Thus, for accurate measurement of dip wavelength shift and pursue of low detection limit of the proposed sensor, core-offset fusion splicing and short cavity length need to be adopted in our experiments.
3. Gas pressure and temperature experiments
The gas pressure sensing device based on dual capillaries with C1 length of ~35μm and core-offset of ~8μm was fabricated. The experimental setup used is shown in Fig. 4. An amplified spontaneous emission (ASE) light source with the wavelength range of 1450-1650 nm and an optical spectrum analyzer (OSA, Yokogawa, AQ6319) with a resolution of 0.05 nm were employed to operate the system and record the reflection spectrum. The device was placed in a gas chamber, where a commercial gas pressure regulator (Concoa) with the stability of ± 0.2 KPa was equipped with a pressure meter to measure the pressure in the chamber. The chamber was sealed by use of strong glue to support the fiber pigtail outside the chamber for real-time measurement and the gas used in the experiment is high purity nitrogen. The pressure in the chamber was increased from 0 to 1.52 MPa with a step of 0.04 MPa at room temperature, and stayed for 5 minutes at each step.
Figure 5(a) shows the reflection spectra of the sensor under different gas pressure conditions at 25°C. There are four dips within the scanning wavelength ranging from 1500 to 1610 nm. As the FSR is ~25.19 nm, the cavity length can be calculated to be ~50.1 μm, which agrees well with the measured C1 length of 50.5μm. With the increase of applied pressure, the dip shift toward longer wavelengths, resulting from the increase of optical path difference (OPD) due to the increase of RI of air, as shown in Fig. 5(a). The relationship between the fringe dip wavelength at ~1588 nm and the pressure is demonstrated in Fig. 5(b), where a good linear response with a high pressure sensitivity of ~4147 pm/MPa can be obtained. The sensitivities of dips at ~1565, ~1540 and ~1520 nm respectively, were also measured to be ~4137, ~4125 and 4116 pm/MPa, respectively. They are all slightly lower than that of the dip at 1588 nm, being consistent with our previous theoretic analysis in that a longer operation wavelength supports a higher sensitivity. The pressure-induced variation of optical path difference (OPD) can also be demodulated by taking the fast Fourier transform (FFT) analysis  as displayed in Fig. 5(b), where it can be observed that the sensor has a high OPD pressure sensitivity of 277nm/MPa.
The influence of temperature on the fiber sensor was investigated by placing the device into an electrical oven and gradually increasing the temperature from 16 to 48 °C with a step of 2°C. The temperature was maintained for 10 minutes at each step. Figure 6(a) shows the reflection spectra under different environmental temperatures. The spectrum exhibits no obvious shift. For the resonance dip at ~1588 nm, its wavelength variation with temperature is revealed in Fig. 6(b). By linear fitting, a very low sensitivity of ~1.18 pm/°C is achieved. For an FPI, ignoring the RI variation of the medium filled in the cavity, the change of cavity length, and the wavelength shift of the mth interference dip at the specific mth interference fringe satisfy the relation : . Thus, the theoretic sensitivity of the thermally induced interference fringe dip shift is calculated to be ~0.92 pm/°C at 1588 nm, which is very close to our experimental results while the margin between them may be resulted from the thermal-optic effect of air. The measured temperature response demonstrates that the temperature sensitivity of such a MFFPI is mainly determined by the thermal expansion effect of the capillary, and has a very small pressure-temperature cross-sensitivity of less than 0.3 KPa/°C at atmosphere pressure. Being limited by our lab conditions, the temperature responses for different pressures have not been tested. However, it is believed that the temperature sensitivity and the temperature cross-sensitivity both increase with the increase of pressure due to the increased RI of gas in FPI cavity. In that case, the thermo-optic effect of gas in the cavity should be considered, and the temperature sensitivity can be described by 18]27]. Thus the device supports a precise and reliable pressure measurement.
Comparing with the previously reported fiber pressure sensors, such as π-phase-shifted FBG (2.9 pm/MPa) , side-hole dual-core photonic crystal fiber (32 pm/MPa) , fiber tip micro-cavity (315 pm/MPa) , our dual capillaries-based MFFPI shows a much higher sensitivity of ~4147 pm/MPa and more compact structure. Although the sensitivity is lower than that of the fiber in-line Mach–Zehnder interferometric sensor based on an inner air-cavity with open micro-channel (~8.239 nm/MPa and ~9.6 nm/MPa) reported recently [27, 28], the sensor presented here has a very low temperature cross-sensitivity.
The detection limit (DL) is determined by the smallest detectable wavelength shift of the reflection spectrum (R) and the sensitivity (S) by DL = R/S. R can be assumed to be the standard deviation of the total noise of the output signal resulting from amplitude noise, thermal noise and OSA spectrum resolution, which can be expressed as23]. By assuming a SNR of 50 dB, is calculated to be 6.6 pm. Following Ref , the typical values of and can be assumed to be 10 fm and 0.5 pm, respectively. Both of them are very small compared to . That is to say, dominates the total noise of the output signal. Substituting the three parameters into Eq. (8), R can be calculated to be ~19.95 pm, and hence the detection limit DL is ~4.81KPa, which is much lower than that of the above-mentioned sensors, owing to the two techniques adopted, i.e., the core-offset fusion splicing and short cavity length, which significantly reduce the FWHM value.
Some other factors may also affect the performance of the pressure sensor proposed. Firstly, the gas composition, such as water vapor and Carbon dioxide, in the cavity would influence the gas RI [29, 30]. Thus the proposed sensor is suitable for sensing the gas with invariable composition during the measuring process. Secondly, for the main-capillary, the axis strain and lateral stress would be induced while the gas pressure increases. For the former, it would hardly affect the cavity length of FPI due to the large Young's modulus of the fiber optical (~7.29 × 104MPa). For the latter, it is applied vertically on the lateral walls of the FPI. So the lateral stress would not affect the cavity length of the MFFPI, and then it would not affect the interference spectrum.
In summary, we have demonstrated a compact and robust fiber FPI for gas pressure measurement, fabricated by splicing two tiny segments of fiber capillaries with different inner diameters to a standard SMF end face. Such a device exhibits a high sensitivity of 4147 pm/MPa within the range from 0 to 1.52 MPa, has a linear response and a very low temperature cross-sensitivity of less than 0.3 KPa/°C at atmosphere pressure, and the total length of the device is only less than 150 μm. Moreover, owing to the core-offset fusion splicing technique and short cavity length adopted, the detection limit of the device achieved is down to ~4.81 KPa. All these features make it an excellent candidate for gas pressure detection in environmental monitoring or industrial applications, especially for the reliable sensing in a precise location.
This work was supported by the National Natural Science Foundation of China (Nos. 61405184, 61377094 and 61290313).
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