A novel differential intensity-measurement high-sensitivity refractive index (RI) sensor based on cascaded dual-wavelength fiber laser and single-mode-no-core-hollow-core-no-core-single-mode (SNHNS) structure is proposed and demonstrated. The sensing unit consists of one uniform fiber Bragg grating (FBG) and an SNHNS structure as all-fiber interferometer filter. The dual-wavelength fiber laser has a ring cavity composed of two FBGs with central wavelengths of 1550.10nm and 1553.61nm. Through monitoring the wavelength shift and the output power difference of the dual-wavelength fiber laser, the simultaneous measurement for RI and temperature is realized. In our experiment, the proposed fiber laser sensor exhibits high RI sensitivities of −193.1dB/RIU and 174.8dB/RIU in the range of 1.334-1.384. The relative variation of output power at the two FBG wavelengths shows a higher RI sensitivity of −367.9dB/RIU with better stability, which is greater than the traditional modal interferometer structure. Meanwhile, the temperature sensitivity of the proposed sensor is 8.53 × 10−3nm/°C, and the changes of laser output power caused by temperature are −0.223dB/°C and 0.215dB/°C.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Refractive index (RI) and temperature have become the important research parameters and attracted great attention due to their wide applications in chemical and biological fields such as food quality and safety, environmental monitoring, medical diagnostics, and so on [1,2]. Due to high sensitivity, larger measurement range, compact structure, potential low cost, and immunity to electromagnetic interference, optical fiber sensing for RI or temperature has attracted wide investigations with many different ways and structures . On the basis of signal demodulation mode, these optical fiber sensors for RI or temperature measurement can be categorized as follows: (1) Wavelength demodulation modes including different types of interferometers and fiber gratings. The sensors based on ordinary FBG  and LPG  own the simple and compact structure, but require complicated device fabrication processes and need written in special optical fibers  or work in modal transition region near the dispersion turning point to realize high sensitivity . The sensors based on interferometers such as Fabry–Perot interferometers (FPIs) [8,9] and Mach–Zehnder interferometers (MZIs) [10,11], Sagnac interferometers  and multi-mode interferometers (MMIs)  can reach high sensitivity, but need tapering, etching or special micro-structure fiber, which can influence the mechanical strength and the stability of the structure. The sensors based on surface plasmon resonance (SPR) [14,15] belong to great technology can achieve high sensitivity and extended application, but depends on the use of metal films or 2D materials which can result in the factors of instability and high loss in the optical fiber configuration. (2) Frequency demodulation methods by utilizing beating frequency of dual wavelength and polarization [16,17]. These sensors have the advantages of high sensitivity, good stability and reliability. However, beating frequency detection relies on the complex system and elaborate designed parameters of two filters. (3) Intensity demodulation modes based on different types of interferometers  and fiber gratings . These sensors are able to show good performances of relative high sensitivity, simple structure, and real-time response. Meanwhile, cost-effective power detection system is more suitable for practical application. But the measurement range is limited by the transmissivity of fiber grating and the extinction ratio of interferometer .
Recently, owing to the advantages of high optical signal to noise ratio (OSNR), narrow bandwidth, low insertion loss, and good stability, fiber laser applied to sensing field [21–23] is widely researched with different demodulation methods. L. Liang et. al reported an RI and temperature fiber ring laser sensor based on tapered multi-mode interference structure. The RI and temperature sensitivities were 163.80nm/RIU (1.333-1.399) and 10.8pm/°C (8°C-80°C) by utilizing wavelength demodulation mode, respectively . J. Zhang et. al presented a dual-wavelength RI fiber linear-cavity laser sensor based on microfiber Fabry-Perot (FP) interferometer. The RI sensitivity of 911MHz/RIU by measuring the beat-frequency signal was acquired with frequency demodulation method . S. Wang et.al proposed and demonstrated a high-sensitivity dual-wavelength RI fiber laser sensor. The RI sensitivity of −273.7dB/RIU in range from 1.300 to 1.335 was obtained by intensity demodulation mode .
In this article, we propose and demonstrate a fiber optic sensor with higher sensitivity for RI measurement. The proposed sensor system includes a dual-wavelength fiber laser based on two FBGs and an SNHNS interferometer. In the ring fiber laser, variable optical attenuator (VOA) and two FBGs with similar reflectivity and 3dB bandwidth are utilized to realize the stable dual-wavelength output. The SNHNS interferometer as comb filter achieves the intensity modulation. The simultaneous measurement of RI and temperature are realized by monitoring the wavelength shift and output power difference of dual-wavelength fiber laser. In the experiment, the RI sensitivities of −193.1dB/RIU and 174.8dB/RIU are achieved ranging from 1.334 to 1.384, and the relative variation of output power at the two FBG wavelengths has a higher sensitivity of −367.9dB/RIU. At the same time, the temperature sensitivity of one lasing wavelength with 8.53 × 10−3nm/°C is experimentally obtained, and the variations of each laser output power caused by temperature are −0.223dB/°C and 0.215dB/°C. The proposed sensor with the characters of high sensitivity, narrow bandwidth, high OSNR and low error will be great for RI and temperature measurement in chemical or biochemical sensing fields.
2. Fabrication and operating principle
The schematic diagram of the proposed SNHNS interferometer is shown in Fig. 1. The structure includes a section of hollow core fiber (HCF) concatenated between two no-core fibers (NCFs) with length of 1mm. The inset of Fig. 1 shows the cross-section image of HCF, and the length and inner diameter of HCF are 0.8mm and 80μm, respectively. Two sections of NCF are coaxially spliced with two single-mode fibers (SMFs) by a commercial fusion splicer (Ericsson, FSU975). Then, the HCF is coaxially spliced with two sections of NCF with low loss by the set procedure and parameter. In order to avoid excessive collapse of the air hole and ensure suitable strength, the splicing parameter condition is modified. The length of NCF and HCF are precisely cut by vernier caliper and scale of fiber cutter. Furthermore, through utilizing a 14cm long uniform phase mask (OE-land, Canada) with a period of 1075nm and scan exposure of a 248nm KrF excimer laser, FBGs are directly written in a 14-day hydrogen-loaded (10Mpa; at room temperature) germanium-doped SMF. The accuracy control of FBG’s central wavelength is used by prestress which is applied in the fiber.
Due to the circular cross and symmetrical fusion splice among the SMF, NCF and HCF, only radial linear polarized modes LP0n will be excited and transmitted into the HCF. As the light propagates into the NCF from the lead-in SMF, a part of the fundamental core mode (LP01) beam will be coupled to the high-order modes due to mode field mismatch, then the beams are further launched into the HCF. A part of beam passes through the air core, and the other travels along the silica cladding. After propagating through the HCF, the two separated beams are recoupled back in the second NCF. At last, the fundamental core mode and high-order modes are recombined in the lead-out SMF exhibiting modes interference with each other due to the different effective RIs. The proposed SNHNS interferometer involves the interference of the fundamental mode and high-order modes. The transmission intensity is analyzed by using the two-mode interference model for simplification
According to Eq. (1), the intensity of the interferometer reaches its maximum peak when , where k is a random integer. In this case, the transmission peak wavelength is determined by
Due to the minor change of effective RI difference of adjacent order modes, the free spectral range (FSR) of the SNHNS interferometer can be written as
When an external perturbation is imposed upon the SNHNS interferometer, such as RI and temperature, the effective RI difference will be changed to result in wavelength shift of the interferometer transmission spectrum according to Eq. (3). Further, the wavelength shift leads to the output power change of dual-wavelength laser at the central wavelength (λ1 and λ2) of two FBGs according to Eq. (5). Due to intensity modulation between dual-wavelength fiber laser and SNHNS interferometer, each laser output power of the proposed sensor will vary as the RI and temperature changes. Figure 2 depicts the measured transmission spectrum of the fabricated SNHNS structure in the range from 1530nm to 1590nm by using a broadband light source and an optical spectrum analyzer (OSA, ANDO AQ6317B, resolution 0.01nm). As can be seen, the FSR of the SNHNS interferometer is about 3.5nm.
The differential intensity modulation principle of the sensor is shown in Fig. 3. The peak wavelength of the SNHNS interferometer is λ0. The central wavelengths of the two FBGs (λ1 and λ2) are chosen in the both sides of one peak of the SNHNS interferometer transmission spectrum, respectively. It assumes that the central wavelengths of FBGs should be always in the area of both sides and the area are approximate linear, when the spectrum shifts with the SRI. Because the bandwidths of the dual-wavelength laser are far smaller than that of the SNHNS interferometer, the output spectra functions of the laser which can be described as impulse functions. Assuming the expressions of the linear areas are A1(λ) and A2 (λ) (as shown in the black line in Fig. 3), the output powers of the dual-wavelength laser with SNHNS interferometer based on two FBGs (and ) can be written asEq. (5), it is obvious found that the output powers of the dual-wavelength laser are related to the SRI and have opposite changing trends. Thus, the intensity difference () is more sensitive to the SRI than the any single intensity measurement of the laser. Furthermore, because the dual-wavelength laser output has same gain media and resonant cavity, the intensity difference () of dual-wavelength laser to measure the SRI can reduce errors caused by the fluctuation of the laser.
3. Experiment results and discussions
The experimental setup for the proposed sensor is shown in Fig. 4. In the part of ring fiber laser, two FBGs (FBG1 λ1 = 1550.10nm, FBG2 λ2 = 1553.61nm) with similar reflectivity of about 98% and 3dB bandwidth of about 0.1nm are the key devices for introducing dual wavelength gain competition. Their transmission spectrum shown in Fig. 5 is measured by OSA before the FBGs were spliced in the laser cavity. The gain medium is a 3m long Er-doped fiber (EDF, peak core absorption at 1532nm 36dB/m, mode field diameter at 1550nm 5.4 ± 0.7μm, cladding diameter 125 ± 1.0μm, core numerical aperture 0.23 ± 0.02) which is pumped by a 976nm laser diode (maximum output power 700mW) via a 980/1550nm wavelength division multiplexer (WDM). The optical circulator (OC) combined with two FBGs is used to implement narrow-band filter and guarantee unidirectional oscillation in the ring laser cavity. The polarization controller (PC) is utilized to tune the polarization of the laser cavity, and to suppress mode competition by adjusting gain and loss of the laser cavity. The isolator (ISO) is used to assure the unidirectional operation. The SNHNS interferometer spliced to FBG1 as an integrated sensing unit is utilized to realize the function for perceiving RI and temperature variation. If only a single SNHNS interferometer is used as the sensing unit, temperature will also lead to a change of laser output power and then have an impact on the refractive index measurement. Therefore, we choose cascaded one FBG and SNHNS interferometer as a whole sensing unit. In this way, the change of temperature can be known through the wavelength change of grating or laser output, and then the influence of temperature on refractive index measurement can be compensated. In order to achieve a stable dual wavelength state, we need to add a VOA between two FBGs to achieve the gain and loss balance of the two laser output wavelengths. When the stable dual wavelength of laser is achieved, the parameters of VOA can be fixed. If cascaded two FBGs and interferometer are selected as sensing unit which then are put into the temperature control box for our experiment, the VOA will also be put into the temperature control box. Its performance will change with the variation of temperature, which can result in the change of laser output power and then introduces the experimental error. Therefore, choosing one FBG and interferometer cannot only avoid the influence of VOA, but also realize simultaneous measurement of RI and temperature. The output spectrum of the proposed sensor system is measured by OSA via the other port of the SNHNS interferometer.
In our experiment, the stable dual-wavelength laser operation is realized by tuning PC and VOA to control gain and loss of the laser cavity. The pump threshold of dual-wavelength laser is about 85mW. Figure 5 shows the output spectrum of the proposed laser sensor via SNHNS interferometer or not under 150mW pump power, when the sensing unit is in air. Due to thermal effect of optical pumping, two lasing wavelengths are 1550.11nm and 1553.62nm which are slightly greater than the central wavelengths of two FBGs. The 3dB bandwidth and OSNR of dual-wavelength laser spectrum are about 0.02 nm and 48dB, respectively.
In order to investigate the dual-wavelength stability and fluctuation of the proposed laser sensor, the variations of laser spectrum via the SNHNS interferometer or not are measured with an interval of 3min at half one hour, as shown in Fig. 6(a) and Fig. 7(a). Further, the fluctuation of output power and center wavelength at λ1 and λ2 are recorded by scanning repeated 11 times with 3min interval. At first, the stability of the proposed dual-wavelength laser sensor without SNHNS interferometer is analyzed. From Fig. 6(b) and 6(c), the maximum laser power fluctuations at λ1 and λ2 are less than 0.28dB and 0.29dB, and the center wavelength fluctuations at λ1 and λ2 are less than 0.01nm, respectively. Then, the result of dual-wavelength laser sensor via the SNHNS interferometer is obtained as shown in Fig. 7(b) and 7(c). The measured output power fluctuations at λ1 and λ2 are less than 0.3dB and 0.33dB, which are higher than the proposed laser sensor without SNHNS interferometer. At the same time, the center wavelength fluctuations at λ1 and λ2 are also less than 0.01nm. Based on the above stability research, the stable dual-wavelength laser sensor has been proved, which guarantees for high-accuracy measurement in the experiment.
To extrapolate the RI response characteristics of the fabricated sensor, the sensor unit including cascaded SNHNS interferometer and FBG1 is fixed in a plastic container and keep straight. The enough water-glycerin RI matching solutions are added into the plastic container to immerse the whole sensor unit which is hold at 20°C. The sensor unit is repeatedly cleaned by deionized water and dried after each measurement, and the RI change is realized by mixing different concentrations of water and glycerin. Figure 8(a) shows the measured output spectral evolution of the proposed sensor in the range from 1.334 to 1.384. As the external RI change from 1.334 to 1.384, the output power variations of dual-wavelength λ1 and λ2 are opposite. Meanwhile, two lasing wavelengths basically remain constant, as shown in Fig. 8(d). In order to further investigate the relationship between RI and the spectrum of laser sensor, RI responses are linearly fitted in Fig. 8(b). We can find that the RI sensitivity of −193.1dB/RIU and 174.8dB/RIU at λ1 and λ2 (R2 approach to 0.989 and 0.977) are achieved in the RI range of 1.334 to 1.384. Due to the same pump light jitter and experimental environment variation, the factors of the dual-wavelength laser instability can be offset by utilizing the method of measuring output power difference. The high RI sensitivity of −367.9dB/RIU (R2 approach to 0.998) is acquired by differential method, as shown in Fig. 8(c). According to the total power difference fluctuations of dual-wavelength fiber laser, the RI resolution of 0.0016RIU based on intensity difference in our experimental measurement has been calculated.
Then, the temperature response characteristics of the proposed sensor is researched by fixing the manufactural sensor unit (cascaded SNHNS interferometer and FBG1) into a temperature control box from 15 °C to 60 °C with a step of 5 °C. Figure 9(a) shows the measured output spectral evolution of the proposed sensor in the range from 15 °C to 60 °C. As the external temperature is increased, the output power variations of dual-wavelength λ1 and λ2 are opposite. Meanwhile, the center wavelength of λ1 shifts long wavelength direction, and the center wavelength of λ2 basically remains constant, as shown in Fig. 9(b). The temperature sensitivity of the proposed sensor at wavelength of λ1 is 8.53 × 10−3nm/°C. In order to further investigate the relationship between temperature and the output power of laser sensor, the temperature responses are linearly fitted in Fig. 9(c) and 9(d). It can be seen that the temperature sensitivities of −0.223dB/°C and 0.215dB/°C at λ1 and λ2 (R2 approach to 0.995 and 0.996) are obtained in the range from 15 °C to 60 °C. Meanwhile, the temperature sensitivity of −0.437dB/°C (R2 approach to 0.998) based on the differential method is calculated.
On basis of the above analysis, the proposed sensor responses to the RI and temperature with different sensitivities. When RI and temperature are applied on the proposed sensor simultaneously, the wavelength shifts and the differential output power can be derived by the following equations
In addition, the various high-sensitivity RI fiber sensors based on intensity demodulation were compared, as shown in Table 1. It’s seen that the RI sensitivity of cascaded structure is higher than that of the single structure from the comparison results, but the structure is relatively complex. Meanwhile, the differential intensity demodulation based on two FBGs is also larger than one FBG. Thus, the high sensitivity based on differential intensity modulation of two FBGs and the advantage of fiber laser sensor with narrow 3dB bandwidth, low insertion loss, high OSNR, and low error are combined in our proposed sensor.
In conclusion, we have proposed and experimentally demonstrated a novel high-sensitivity RI optical fiber sensor utilizing cascaded dual-wavelength fiber laser and SNHNS interferometer. The theoretical analysis and experiments of the sensing characteristics have been provided. The simultaneous measurement of RI and temperature has been achieved by monitoring the wavelength shift and output power difference of dual-wavelength fiber laser. Differential output power measurement for the two laser wavelengths has been carried out to realize a total high RI sensitivity of −367.9dB/RIU with a power fluctuation <0.33dB from 1.334 to 1.384. Meanwhile, the temperature sensitivity of 8.53 × 10−3nm/°C in the range from 15°C to 60°C is obtained, and the variations of each laser output power caused by temperature are −0.223dB/°C and 0.215dB/°C. The proposed sensor with the characters of high sensitivity, narrow bandwidth, high OSNR and low error will be great for RI and temperature measurement in chemical or biochemical sensing fields.
National Key Research and Development Program of China (No.2016YFC140090), the National Natural Science Foundation of China (No. 61475015, 61775015, 41471309, 41375016), the Postdoctoral Science Foundation of China (No. 2017M612350), and the Postdoctoral Creative Funding in Shangdong Province.
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