A dual-wavelength fiber ring laser based on multimode fiber-polarization maintaining fiber Bragg grating-multimode fiber (MMF-PMFBG-MMF) filter for simultaneously axial strain, temperature and refractive index (RI) sensing is proposed and experimentally demonstrated. In the ring laser, stable dual-wavelength lasing is determined by the MMF-PMFBG-MMF filter with two polarization states. The fiber birefringence affected by axial strain is far less than the effect of the temperature. Through monitoring the variations of each wavelength shift and output power, the simultaneous measurement for the axial strain, temperature and RI is realized. In our experiment, the proposed fiber laser sensor exhibits an axial strain sensitivity of 1.16 × 10−3nm/με and an RI sensitivity of 81.2dB/RIU. Meanwhile, the temperature sensitivities of two wavelengths are experimentally measured to be 9.74 × 10−3nm/°C and 9.2 × 10−3nm /°C, respectively.
© 2017 Optical Society of America
Due to high sensitivity, compact structure, immunity to electromagnetic interference, potential low cost, and multiplexing capability, multi-parameter measurement based on optical fiber sensor is attracting extensive attention in a range of application, include refractive index (RI) , strain , temperature , curvature , and so on. Particularly, axial stain, temperature and RI sensing are of the most importance for applications in safety performance, biomedical and environmental science [5–7]. Some fiber structures such as the in-line fiber Mach-Zehnder interferometer (MZI) , structured polarization-maintaining chirped fiber Bragg grating , Fabry-Perot interferometer based on pendant polymer droplet  and multimode microfiber interferometer  are investigated for simultaneous dual-parameter or three-parameter measurement. S. Wang et. al. reported a cascaded dual-pass MZI and a Sagnac interferometer for simultaneous measurement of strain and lateral stress . J. Li et al. proposed a cascaded long-period fiber grating and an S taper MZI for simultaneous measurement of temperature and RI . However, these structures have characteristics of relative complex, low optical signal to noise ratio (OSNR) and large 3dB bandwidth.
Fiber laser applied to sensor [14–16] is investigated extensively duo to their excellent performance of high OSNR, low insertion loss, low error and narrow 3dB bandwidth. Fiber laser sensors are mainly divided into three types based on the monitored characteristic parameters of laser system, which includes the varied lasing wavelength, output power, and beat frequency. X. P. Zhang et. al. reported an RI sensor based on a simple fiber ring laser incorporating a bent fiber filter . The measured RI sensitivity of about 60nm/RIU from 1.3259 to 1.3730 was obtained. J. F. Zhao et. al. proposed an RI fiber laser sensor based on a fiber Bragg grating (FBG) integrated with a section of no-core fiber . The measured RI sensitivity was 113.73dB/RIU with a range of 1.333-1.4076. S. Wang reported and demonstrated a highly-sensitive RI sensor based on a linear-cavity dual-wavelength erbium-doped fiber laser . Due to better stability of laser, the relative power difference at dual-wavelengths exhibited a higher sensitivity of −273.7dB/RIU. However, these laser sensors only realized measurement of single physical parameter.
In this paper, combining the features of fiber laser sensor and multi-parameter sensor, we proposed and experimentally demonstrated a dual-wavelength fiber ring laser based on multimode fiber-polarization maintaining fiber Bragg grating-multimode fiber (MMF-PMFBG-MMF) filter for axial strain, temperature and RI sensing, simultaneously. Through monitoring the variations of lasing wavelengths shift and output power, the proposed fiber laser sensor demonstrated an axial strain sensitivity of 1.16 × 10−3nm/με and a RI sensitivity of 81.2dB/RIU. Meanwhile, the temperature sensitivities of each lasing wavelength with 9.74 × 10−3nm/°C and 9.2 × 10−3nm /°C are experimentally obtained, respectively. To our best knowledge, it is the first time that the fiber laser sensor is applied for RI, temperature and axial strain simultaneous measurement. Such a simple and compact fiber laser sensor features low insertion loss, high OSNR, low error and narrow 3dB bandwidth.
The structure of sensing head which is composed of a PMFBG embedded into a bandpass filter (BPF) based on multimode fiber-PMF-multimode fiber (MPM) structure is fixed straightly along the platform, as shown in Fig. 1. The MPM structure is fusion spliced by a common commercial fiber fusion splicer (FSM-60s, Fujikura). Although the fabrication of the MPM structure is relatively easy, the length of MMF and PMF needs precisely to be controlled and carefully fusion spliced to ensure a desired transmission spectrum of the BPF. Because of the mode mismatch, a part of the core mode beam is coupled to cladding modes. Multimode interference by each excited mode with different sensitivity affects the sensing performance and spectral shape. Thus, it is necessary to control the number of generated cladding modes. In previous reference , the transmission characteristics of the different lengths of the middle PMF and the MMF are analyzed in detail. Thus, in order to obtain a better transmission spectrum and sensing performance, we choose 2.2cm PMF and two MMFs with an identical length of 1mm in our experiment. Through utilizing a 14cm long uniform phase mask with a period of 1068nm and position exposure of a 248nm KrF excimer laser, a PMFBG with 1cm length is directly written in a 12-day hydrogen-loaded (10Mpa; at room temperature) germanium-doped PMF. Figure 2 depicts the measured transmission spectrum of the fabricated MMF-PMFBG-MMF filter by utilizing a broad band source and an optical spectrum analyzer (OSA, AQ6375, resolution 0.05nm) in the range from 1500nm to 1590nm and the measured transmission spectrum of PMFBG. As can be seen, the two center wavelengths of PMFBG are 1545.12nm and 1545.32nm, respectively.
Further, the sensing principle of cascaded PMFBG and MZI based on multimode interference is analyzed. Due to the slight difference of effective RI for the two orthogonal polarization modes, the FBG writing in a PMF will generate two resonance peaks with different wavelengths, which can be written as
From analysis of previous references [9, 21] and the material constants of the PMF, we estimate that the difference between the strain sensitivities of the two PMFBG wavelengths is less than 1 × 10−6nm/με, which is far less than the strain sensitivity of each PMFBG wavelength. Thus, . The Eq. (2) can be simplified to
As the resonance wavelength of PMFBG is independent on the RI, , the change of each PMFBG wavelength can be further expressed by
The output power variation of the laser caused by RI, temperature and axial strain is determined by the change of multimode interference. The propagation constants of the interfering modes are caused by the applied RI, temperature and axial strain, consequently leading to a detectable wavelength shift or power variation of certain wavelength . It can be described by
Thus, simultaneous measurement of axial strain, temperature and RI can be achieved by monitoring each wavelength and output power of dual-wavelength laser. The equations of the proposed laser sensor for three parameters sensing can be jointly expressed by
Further, the output power variation of fiber laser influenced by temperature and axial strain are analyzed. The free spectrum range (FSR) of MSM structure is approximately as 
The change of temperature and axial strain will lead the variation of both the effective index difference and the sensing fiber length, according to the following eqs [23,24]:
For the temperature effect, the transmission spectrum of MSM and PMFBG shifts uniformly to the longer wavelength with the increasing temperature. Thus, the power change of fiber laser influenced by MSM and PMFBG can be partly offset. According to the parameters of our experimental sensor, the difference value of wavelength sensitivities of MSM and PMFBG is less than 25pm/°C, corresponding to the power variation of fiber laser less than 1 × 10−2dB/°C based on the FSR and OSNR of MSM structure. For the axial effect, although the transmission spectrum of MSM and PMFBG shift the opposite wavelength direction with the increasing axial strain, the difference value of wavelength sensitivities of MSM and PMFBG is less than 2pm/με, corresponding to the power variation of fiber laser less than 8 × 10−4dB/με. Thus, the influence on laser output power by axial strain and temperature can be ignored in our experimental range.
The experimental setup of the proposed fiber laser multi-parameter sensor based on MMF-PMFBG-MMF filter is shown in Fig. 3. It consists of a 976nm laser pump diode (maximum output power 700mW) which was fusion spliced to a 980/1550nm wavelength division multiplexer(WDM), a 1.5m long Er-doped fiber (EDF) as the gain medium, an optical circulator (OC), a polarization controller (PC) and an isolator (ISO) assuring the unidirectional operation. The OC is employed to implement narrow-band filter and sustain the unidirectional oscillation in the ring laser cavity. The MMF-PMFBG-MMF filter spliced to the 2-port of OC is used to realize the stable dual-wavelength operation. The output spectrum of the laser sensor system is measured by an OSA via the other port of the MMF-PMFBG-MMF filter.
3. Experiment results and discussion
In our experiment, the pump threshold of the proposed dual-wavelength fiber ring laser is about 200mW. Through adjusting PC to balance gain and loss of the resonant cavity, the stable dual-wavelength laser output is achieved at the room temperature. The measured two center wavelengths are 1545.12nm and 1545.32nm corresponding to the resonant wavelengths of PMFBG, respectively. The OSNR of each emission wavelength is more than 40dB and the 3dB bandwidth is approximately 0.025nm. In order to investigate the stability of the proposed dual-wavelength laser sensor, the optical spectra with 3-min interval during half an hour are measured, as shown in Fig. 4. Further, Fig. 5 shows that the fluctuations of each laser peak power and center wavelength at dual-wavelength operation are less than 0.45dB and 0.01nm, which means dual-wavelength laser works at stable operation. Thus, the stable dual-wavelength laser output will provide guarantee for our sensing experiment.
In order to experimentally investigate the RI sensing performance of proposed laser sensor based on MMF-PMFBG-MMF filter, the RI matching solution (glycerinum and purified water mixed by different radio) is utilized to carry out RI measurement experiment. The sensing head is totally immersed in RI matching solution which is kept at 25°C, and it is repeatedly cleaned by purified water and dried in air after each measurement. The measured output spectra of the laser sensor from 1.33303 to 1.44290 are depicted in Fig. 6(a). The output power of dual-wavelength laser increases as the RI of external surrounding goes up. From Fig. 6(b), the relationship between laser output power of each wavelength and RI is described. The RI sensitivities of both wavelengths are same as 81.2dB/RIU by fitting the experimental data based on linear regression (R2 approach to 0.99), when the RI changes from 1.333 to 1.4076. Compared with the conventional RI sensor based on wavelength shift of the FBG and interferometer, the fiber laser sensor by measuring output power has the advantages of higher accuracy and higher OSNR. Further, the wavelength shift influenced by RI is also investigated. Figure 6(c) shows that the wavelength shift is almost invariant (less than 0.02nm) as the RI increases from1.3303 to 1.44290. Thus, the variation of wavelength shift by the reason of the RI can be ignored. Theoretically, the RI resolution of the proposed sensor is limited by the optical power resolution of OSA. For the demonstrated fiber laser sensor, the optical power resolution of OSA is 0.1dB, which corresponds to a resolution of 0.0012RIU. But in practice, it is difficult to achieve such a high resolution due to the instability of the fiber laser and the power change by temperature and axial strain. According to the total power fluctuations of dual-wavelength fiber laser, the RI resolution of 0.015RIU in our experimental measurement has been calculated.
Temperature sensing experiment is realized by a temperature controlled chamber. Figure 7(a) shows the typical spectral evolution of dual-wavelength laser sensor when the temperature increases from 5 to 55°C with 10°C interval. The spectrum of the laser sensor shifts to higher wavelength as the temperature increases, but the difference of two laser emission wavelength shifts is decreasing. Figure 7(b) depicts the relationship between both emission wavelengths and temperature by plotting the experimental data and linear fitting (R2 approach to 0.998). The temperature sensitivities of both emission wavelengths are 9.74 × 10−3nm/°C and 9.2 × 10−3 nm/°C, respectively. Further, the relationship between the difference of two laser emission wavelengths and temperature is analyzed, as illustrated in Fig. 7(c). The temperature coefficient of emission dual-wavelength difference is −5.4 × 10−4nm/°C.
For the axial strain measurement experiment, the sensing head of MMF-PMFBG-MMF filter is fixed by fiber clamps with axial strain from 0 to 500με at room temperature. It is stretched by utilizing the high-precision translation stage with 0.01mm step and 20cm distance corresponding to axial strain of 50με. Figure 8(a) depicts the superimposed spectra of dual-wavelength laser sensor when the axial strain increases from 0 to 500με with 100με interval. The spectrum of the laser sensor shifts to higher wavelength as the axial strain increases, and the difference between two emission wavelengths is almost invariant as shown in Figs. 8(b) and 8(c). As can be seen from Fig. 8(b), the axial strain sensitivities of two emission wavelengths are equally 1.16 × 10−3nm/με by fitting the experimental data based on linear regression (R2 approach to 0.998). The experiment results verify that the axial strain sensitivity of emission dual-wavelength difference is far less than the axial strain sensitivities of each emission wavelength, which has been analyzed in section 2. Thus, the variation of dual-wavelength difference on account of the applied axial strain can be ignored.
Further, the axial strain and temperature influence not only the wavelength shift but also the output power of proposed laser. Figure 9 shows the measured variations of dual-wavelength laser output power with axial strain from 0 to 500με and temperature from 5 to 55°C. The change of axial strain and temperature are less than 0.35dB and 0.4dB within the measurement range, respectively. The corresponding maximum RI measurement error is about 4.3 × 10−3 and 4.9 × 10−3RIU, which indicates the variations of temperature and axial strain don’t have significant influence on the RI measurement. Thus, comparing with the effect of RI as shown in Fig. 6, the influence on laser output power by axial strain and temperature can be ignore in our experiment.
By analyzing the slopes of each wavelength shift and output power, the axial strain, temperature and RI applied to the MMF-PMFBG-MMF filter can be measured simultaneously. Thus, the coefficient Eqs. (6) between each wavelength shift and output power of dual-wavelength fiber laser sensor and the change of axial strain, temperature and RI are described as
In conclusion, we have proposed and experimentally demonstrated a simultaneous multi-parameter fiber-optic sensor based on a dual-wavelength fiber laser incorporating a MMF-PMFBG-MMF filter. By monitoring the variations of each wavelength and output power, the axial strain, temperature and RI were simultaneously measured. The axial strain sensitivities of each wavelength were measured to be equally 1.16 × 10−3nm/με, and the temperature sensitivities of each emission wavelength were 9.74 × 10−3nm/°C and 9.2 × 10−3nm/°C, respectively. Compared with the temperature dependence of wavelength difference with a sensitivity of −5.4 × 10−4nm/°C, the wavelength difference was insensitive to axial strain with a coefficient of less than 10−6nm/με. Meanwhile, the RI sensitivity of laser output power with 81.2dB/RIU was obtained.
The 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 Applied Research Fund of QingDao.
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