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Abstract
A kind of hybrid fiber interferometer consisting of a fiber Sagnac interferometer (FSI), a closed-cavity Fabry-Perot interferometer (FPI), and an open-cavity FPI is proposed for generating combined-Vernier-effect. Through adjusting the polarization-maintaining fiber (PMF) length of the FSI, the free spectral range (FSR) is tailored to be similar to that of the parallel-connected reference FPI for producing the first Vernier effect, of which the spectrum is used to match the sensing FPI spectrum for obtaining the second Vernier effect. Noticeable lower and upper spectral envelopes are achieved in the first and second Vernier effects, respectively, so called the combined-Vernier spectrum. Accessibly, the upper envelope is only sensitive to the refractive index (RI) owing to the characteristics of the open-cavity FPI, while the lower one is immune to the RI and employed to detect the temperature by taking advantage of the FSI. Most importantly, the sensitivities of RI and temperature can be significantly improved simultaneously without crosstalk. The experimental results show that the RI sensitivity is −19844.67 nm/RIU and the temperature sensitivity is −46.14 nm/°C, which can be used for high-precision temperature and RI simultaneous measurement.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Salinity and temperature are two essential parameters of ocean monitoring, which can be widely used in marine scientific research, ocean resource investigation and development, ocean environmental monitoring and protection, and other fields [1–3]. According to the traditional Conductivity-Temperature-Depth profiler, sensing resolution is required over 0.0004‰ for salinity measurements and 0.0001°C for temperature measurements. Currently, optical fiber sensing technology through measuring the refractive index (RI) is one of the promising methods to detect the seawater salinity [4], which is converted to the RI range of 1.33-1.34 [5]. The sensitivity of the optical fiber RI sensors is required to be greater than 13500 nm/RIU under the condition of a spectrometer resolution of 1 pm.
In the past few decades, optical fiber sensors have attracted increasing attention due to their advantages of EMI immunity, anti-corrosion, high sensitivity, and fast response. Many types of optical fiber sensors have been proposed for RI and temperature measurements, such as etched fiber Bragg gratings (FBGs) [6,7], fiber interferometers [8–10], fiber optic surface plasmon resonance (SPR) [11,12], and long-period gratings (LPGs) [13,14]. The FBGs sensor is simple to be manufactured, but its sensitivity is only several nm per RIU. Various interferometer sensors have been proposed, including core-offset sections, no-core fiber (NCF)-based sections, photonic crystal fiber (PCF)-based sections, in-line tapering, etcetera. They show sensitivities from tens to thousands of nm per RIU. Fiber SPR measurement of RI has a high sensitivity to meet the requirements of measurement of salinity in seawater. However, SPR sensors are difficult to be manufactured, and it is mainly at the stage of theoretical simulation. The LPGs sensor shows sensitivities from tens to thousands of nm per RIU but is also sensitive to the environmental strain and the bending simultaneously, leading to many limitations for meeting the measurement needs. It is of excellent need to develop optical fiber sensing technologies for highly sensitive dual-parameter detection.
In recent years, the Vernier effect in optical fiber sensors has been demonstrated as a successful method for improving sensitivity. The optical sensors are usually composed of two cascaded or parallel interferometers for using the Vernier effect. Their sensitivities can reach hundreds or even thousands of nm/RIU for RI measurement [15–17] and tens of nm/°C for temperature measurement [18–20]. Yao et al. proposed the FPIs filled with liquids for RI sensing in a parallel configuration [21]. The Vernier envelope achieved a RI sensitivity of 30801.53 nm/RIU between 1.33347 and 1.33733. Besides, a temperature sensor using parallel dual polarization-maintaining fiber Sagnac interferometers (PMF-SIs) was proposed and demonstrated [22]. The temperature sensitivity was improved from −1.646 nm/°C (single PMF-SI) to 78.984 nm/°C (parallel dual PMF-SIs), with a magnification factor of 47.9. It is no doubt that higher sensitivity could be obtained by employing the Vernier effect. However, those reported works based on the Vernier principle can only enhance the sensitivity of single-parameter detection. Consequently, it is interesting to develop the dual-parameter measurement with high sensitivity using the Vernier effect.
In this paper, we propose a dual-parameter sensor composed of three interferometers by taking advantage of the combined-Vernier-effect. The PMF-SI, which is sensitive to the temperature, is connected in parallel with the referenced FPI (R-FPI) and then connected in series with the RI sensing FPI (S-FPI). The temperature and RI changing information can be obtained from the upper and lower envelopes of the combined-Vernier effect spectrum simultaneously, respectively, to solve the cross-sensitivity problem. The dual parameters have independent magnification effects using separate envelopes, and the sensitivities can be effectively enlarged.
2. Sensor structure and principle
There are mainly two connection types: parallel or cascaded configurations to produce the Vernier effect of combining two optical fiber interferometers. For the parallel configuration, the output light intensity is [23]:
In the cascaded interferometers, the total transmission can be derived as [24]: Here, ${I_R}$ and ${I_S}$ are the transmission intensities of the reference and sensing interferometers, respectively.The intensities of the two interferometers are required similar in the parallel configuration, resulting in a noticeable lower envelope spectrum. Therefore, deliberately adding extra loss to an interferometer is common to maintain the intensity balance. The cascaded configuration is used to obtain the upper envelope when the intensities of the two interferometers cannot remain similar. Based on the above, a coupler with an appropriate coupling ratio is used to ensure the intensity balance between the R-FPI and the PMF-SI. Noticeable lower spectral envelopes are achieved, which will be sensitive to temperature. The upper envelope is modulated to realize the simultaneous measurement of RI by cascading S-FPI.
Figure 1 is the schematic diagram of the RI and temperature measurement. It consists of a temperature sensing PMF-SI and a R-FPI. The PMF-SI is composed of a 3 dB coupler, PMF, and a polarization controller (PC). By the 3 dB coupler, the light entering the PMF-SI is divided into two opposite beams, transmitted in the fast and slow axes of the PMF, respectively, and then returned to the coupler. Because of the phase difference attributed to the fast and slow axes, the light will interfere after returning to the coupler, and the interference light intensity can be expressed as:
where ${\phi _1}\textrm{ = }2\pi B{L_1}/\lambda$ is the phase difference between the two waves, ${L_1}$ and $\lambda$ are the PMF length and the incident light wavelength, respectively, and $B = {n_{slow}} - {n_{fast}}$ is the birefringence of the PMF.
Fig. 1. Schematic diagram of the RI and temperature sensor using a FSI and two FPIs (SLD: Broadband light source, OSA: Optical spectral analyzer).
When the phase difference satisfies $\phi = 2m\pi$ (m is integer), ${I_{sagnac}}$ reaches the minimum value, and the ${m^{th}}$ dip wavelength in the interference spectrum of the PMF-SI can be written as:
The free spectral range (FSR) of the PMF-SI can be expressed as:As shown in Fig. 1, when the light enters the FPI from the circulator, it will be reflected by the first and second reflecting surfaces, and interference between the two reflected lights occurs. Afterward, the light intensity ${I_{FP}}$ is expressed as:
The ${m^{th}}$ dip wavelength in the interference spectrum of the R-FPI can be written as:
The FSR of the R-FPI is:According to Eq. (3), the simulated spectrum of the single SI is shown in Fig. 2(a) by setting $B\textrm{ = }5.844 \times {10^{\textrm{ - }4}}$ and ${L_1}\textrm{ = }118\textrm{ }cm$. According to Eq. (6), when $n\textrm{ = 1}$ and ${L_2}\textrm{ = 333 }um$, the simulated spectrum of the closed-cavity R-FPI is plotted in Fig. 2(b), while Fig. 2(c) shows the spectrum of the parallel structure.
Fig. 2. (a) R-FPI spectrum; (b) Temperature sensing SI spectrum; (c) PMF-SI and R-FPI parallel-connected spectrum.
The FSR of spectral of the parallel structure can be expressed as
For a single SI, the temperature-induced spectral shift is: The temperature change will cause the wavelength shift of the spectral envelope. The shift is compared with the single Saganc sensor and multiplied by the temperature magnification, written as: After the SI and the R-FPI are connected in parallel, the open-cavity FPI is connected in series to obtain the second Vernier effect. The spectrum of this complete sensing configuration is expressed as: The spectrum of the parallel structure as a referenced one is shown in Fig. 3(a). When the RI changes, the variant spectra of the single FPI and the whole system are shown in Fig. 3(b) and Fig. 3(c), respectively. The black and red lines indicate that the RIs of the filled solutions are 1.333 and 1.334, respectively.Fig. 3. The spectra of (a) the parallel-connected PMF-SI and R-FPI ; (b) RI sensing FPI; (c) the whole system.
The upper envelope period can be expressed as:
For a single FPI, the RI-induced spectral shift is: When the RI changes, there is a significant shift for the minimum point of the upper spectral envelope of the whole system, which can be used as the RI detection feature point. The shift amount is compared with the single FPI sensor and multiplied by the RI magnification, written as: The minimum point of the lower envelope under parallel connection is only sensitive to the temperature so that it can realize the combined-Vernier-effect. In this way, the temperature and RI can be measured simultaneously, and their sensitivities can be amplified.3. Sensor production and experimental results
The experiment setups for temperature and RI measurement are depicted in Fig. 4. A broadband light source (UBLS-1250-1650-FA-B) with a wavelength range of 1450-1650 nm is used as the input light, and the spectrum is measured by an optical spectrum analyzer (OSA, Yokogawa AQ6370D). The SI is composed of a 3 dB coupler, a PC, and a section of PMF (PM-1550-01 YOFC). The length of the used PMF is 118 cm. The FPI includes a ceramic ferrule and a ceramic sleeve. The cross-sectional diameter of the ceramic ferrule is 126 µm, slightly larger than the optical fiber diameter (125 µm). The fiber is inserted axially from the left end of the ceramic ferrule, and the right end is pasted with a mirror, which is fixed with corrosion-resistant epoxy glue. The circulator is used to connect the FPI to observe the interference spectrum. Through the observations of the FSR spectrum in the past experiments, we have demonstrated that the error of the length is less than 1%, which is tolerable. Moreover, this small error can be compensated by spectral calibration. The lengths of the R-FPI and the S-FPI are adjusted to 333.6 µm and 240 µm, respectively.
Fig. 5. (a) Spectrum drifts of S-FPI with the change of RI; (b) Spectrum drifts of the whole system with the change of RI; (c) Linear fitting curve of RI sensitivity of S-FPI; (d) Linear fitting curve of sensitivity of the whole system.
NaCl aqueous solutions of different concentrations are prepared to test the open-cavity FPI RI sensing performance; the RI of the aqueous solution ranges from 1.333-1.339, measured by a fully automatic refractometer (SGW-731, Shanghai, China). The solution is filled into the FPI cavity with a dropper. When the spectral contrast ratio decreases, it is considered that the solution has been injected into the cavity, and the data is recorded after the spectrum is stable. After the test, a cotton cloth is used to absorb the moisture. When the spectrum returns to the original one in air, it is considered that the FPI cavity has been dried. Under the conditions of different NaCl solutions, the measured RIs are shown in Fig. 5(a). It can be seen from Fig. 5(c) that in the RI range of 1.333-1.339, the open-cavity FPI unit achieves a RI sensitivity of 983.65 nm/RIU, which is similar to the previously reported result [25,26].
Furthermore, the RI response of the sensor is investigated by following the same steps above. The NaCl aqueous solution has the RI ranging from 1.333 to 1.339, while the temperature is kept at room temperature (21.3 °C). The tracking points are selected from both sides of the large period of the upper envelope, which has a larger magnification verified by Eq. (16). Fig. 5(b) shows the experimental spectrum, of which the minimum point of the upper envelope is used as the RI feature point. When the RI of the liquid in the cavity increases, the wavelength of the feature point blue shifts. In addition, the minimum point of the lower envelope is used as the temperature feature point. Since the temperature currently has not been changed, the temperature feature point does not drift. The relationship between the feature wavelength and the RI value is shown in Fig. 5(d). To further reveal the repeatability of the sensor, seven groups of NaCl solutions are successively filled inside the open cavity FPI, and the test for each group is repeated six times, as shown in Fig. 6. Moreover, the experimental sensitivity is −19844.67 nm/RIU. The most considerable wavelength fluctuation is around 3 nm. Therefore, the repeatability and stability of our RI sensor are reliable.
Then, we investigate the temperature sensing performance of the single PMF-SI. A water bath is used to control the ambient temperature around the PMF. The temperature characteristics are tested with the range from 27 °C to 30 °C. As shown in Fig. 7(a), when the temperature increases, the spectrum of PMF-SI has a blue shift. After the calculation in Fig. 7(c), the temperature sensitivity of −1.49 nm/RIU for the single PMF-SI is achieved within the range of 28-30 °C.
Fig. 7. (a) Spectrum drifts of PMF-SI with the change of temperature; (b) Spectrum drifts of the whole system with the change of temperature; (c) Linear fitting curve of temperature sensitivity of PMF-SI; (d) Linear fitting curve of temperature sensitivity of the whole system.
Moreover, the temperature sensing characteristic of the proposed system is investigated. The water bath temperature is detected by a precision quartz crystal transmitter (WBQY-J3). At the same time, another OSA (Anritsu MS9740A) is used to obtain the spectrum of the PMF-SI paralleled with FPI through the other interface of the 3 dB coupler. The lower envelope feature points of the spectra keep in the same position at all times, which is consistent with the theoretical analysis. The Vernier-effect spectrum has a blue shift when the temperature increases, as shown in Fig. 7(b). The wavelength of the temperature feature point has a linear relationship with the temperature in Fig. 7(d). Three tests are conducted under the same conditions to investigate the repeatability of the temperature measurement. The experimental results are shown in Fig. 8, revealing sensitivity of −46.13 nm/°C. Obviously, the cooling process and the heating process are not consistent completely. It may be caused by the Sagnac interferometer, which is easily affected by external factors. In order to obtain better repeatability, it may need to be fixed on a suitable carrier. Compared with the single Sagnac structure, the configuration of three cascaded interferometers dramatically improves the temperature sensitivity by 32.23 times.
Since our proposed sensor can measure the RI and the temperature simultaneously, more detailed performances comparisons of the dual-parameter-detection sensor are shown in Table 1. Our proposed sensor has an absolute advantage in the sensitivity of dual-parameter detection.
Table 1. Comparison of RI and temperature sensing performance of different sensor configuration
4. Conclusions
In conclusion, we have proposed and demonstrated a high-sensitivity RI and temperature hybrid fiber interferometer. The combined-Vernier effect was realized by connecting the PMF-SI and the R-FPI in parallel, which was then connected in series with the S-FPI. The upper envelope of the combined-Vernier spectrum was applied to detect the RI change, and the lower one was employed for the temperature sensing. The experimental results showed a RI sensitivity of −19844.67 nm/RIU and a temperature sensitivity of −46.14 nm/°C have been achieved. Compared with the single structure, the RI and temperature sensitivities were increased by 20.11 times and 30.96 times, respectively. The hybrid fiber interferometers have the advantages of high sensitivity, simultaneous measurement of RI and temperature, low cost, and easy production. In practical applications, the FPI cavity length and PMF length can be adjusted according to actual needs to obtain appropriate sensitivity. It is expected to be applied to high-precision ocean monitoring, medical and biochemical detection.
Funding
National Natural Science Foundation of China (61925501, 62175039, U2001601); the Science and Technology Project of Guangzhou (201904010243); Guangdong Province Introduction of Innovative R&D Team (2019ZT08X340); Guangdong Provincial Pearl River Talents Program (2019CX01X010).
Disclosures
The authors declare no conflict 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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