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Abstract
In this paper, a multi-wavelength fiber ring laser (MWFRL) based on a hybrid gain medium and Sagnac interferometer (SI) used for temperature measurement has been proposed and experimentally demonstrated. Experiments have been carried out with polarization maintaining fibers (PMF) of different lengths, which are incorporated in the SI as sensing elements. Stable multi-wavelength oscillation at 1560 nm band is successfully achieved with the wavelength instability of ±0.08 nm and the signal-to-noise of 42 dB. The experimental results show that the wavelength change of the MWFRL with temperature variation has a good linear response and the temperature sensitivity of 1.8063 ± 0.00933 nm/°C is obtained when the length of the PMF is 1.7 m. As the length of PMF increases, the sensitivity can be improved.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past decade, fiber-optic sensors have been widely studied due to their unique advantages such as high flexibility, low fabrication cost, high accuracy, simultaneous sensing ability, and immunity to electromagnetic interference [1]. In addition, most of current fiber-optic sensors are passive and employed by a broadband light source, and the reflected or transmitted sensing signal are detected to monitor the changes in external physical parameters, like refractive index (RI), pressure, temperature, strain, and so on [2–7]. However, the sensing signal would suffer from a low extinction ratio and small signal output [8]. It will make this type of sensor unsuitable for some specific applications.
In recent years, researchers have proposed various methods, like cascaded interferometers or fiber core doped with lead sulfide (PbS) [9,10], to improve the sensitivity of fiber-optic sensing systems. Meanwhile, one of the most remarkable and efficient way is to combine fiber lasers with fiber-optic sensors. Due to the instinct advantages of high signal-to-noise ratio (SNR), good stability and narrow bandwidth of fiber lasers [11], the fiber laser sensors would provide a new research direction for the field of sensing [12–16]. For instance, A. H. Sulaiman et al. had revealed dual-wavelength fiber laser temperature sensor (DWFL) based on bidirectional Lyot filter and semiconductor optical amplifier (SOA) with sensitivity of 1.35 ×10−3 nm/°C when the 10.6 m PMF used [17]. Chunran Sun et al. had presented a liquid level and temperature sensor based on a fiber laser ring cavity configuration, which achieved sensitivity of 10.3 pm/°C and 13.8 pm/°C with the temperature ranging from 0 °C to 90 °C corresponding to the two lasing wavelengths selective by the multimode interference (MMI) filter and FBG, respectively [18]. Wei He et al. had demonstrated a fiber ring laser sensor setup utilizing two fiber Bragg gratings (FBGs) for simultaneous measurement of ambient temperature and relative humidity (RH), the RH sensitivity was 3.6 pm/%RH and temperature sensitivities were 9.6 pm/°C and 12.15 pm/°C corresponding to the bare FBG and the PI-coated FBG respectively [19]. M. A. Gonzalez-Reyna et al. had reported a single wavelength fiber laser temperature sensor with sensitivity of 18.8 pm/°C, which was based on a fiber Bragg grating (FBG) and an in-line Mach–Zehnder interferometer (MZI) [20]. S. Diaz et al. had proposed a multi-wavelength single-longitudinal-mode (MW-SLM) fiber laser for remote temperature sensing with a length of 25 km single mode fiber (SMF), where the sensing element is an array of four fiber Bragg gratings [21]. Moreover, Bin Yin et al. had demonstrated a dual-wavelength fiber laser sensor incorporating a FBG and a Polyvinyl-alcohol (PVA) film-coated long-period grating (LPG) as sensor elements for simultaneous detection of relative humidity (RH) and temperature [22]. In order to further improve the performance of such sensors, various structures of fibers are inserted into these fiber lasers, like single-mode-tapered-claddingless-single-mode (STCS) fiber [23], single-mode-suspended-core fiber [24]. More recently, the SI-based fiber laser temperature sensor has been obtained used for remote sensing [25]. Since the PMFs used in SI have higher birefringent coefficient and thermal optics coefficient in comparison with SMFs, the sensitivities are improved largely. However, this scheme cannot avoid multi-wavelength competition due to the multi-channel filtering characteristic of SI and the homogeneous broadening of erbium-doped fiber (EDF) at room temperature.
In this paper, we demonstrate a stable MWFRL based on hybrid gain medium and SI used for temperature sensing. Compared with single wavelength fiber laser [26], the proposed fiber laser sensor enjoys multi-wavelength oscillation so that the subsequent data processing results are more accurate. When PMF with length of 1.7 m is utilized as the sensing element in the SI, the temperature sensitivity and resolution of the proposed sensor are estimated to be 1.8063 ± 0.00933 nm/°C and 0.0442 °C, respectively, which is higher than conventional fiber laser sensors attributed to the physical properties of SI itself. Furthermore, the experimental results also show that the proposed MWFRL sensing scheme is resistant to external disturbance. In addition, experiments are carried out with PMF of different lengths. Considering the practical applications, the range of temperature measurement is expanded by using data processing scheme. The proposed temperature sensing scheme enjoys a variety of advantages such as easy implementation, good stability and high temperature sensitivity, and thus has good application prospect in circumstances which needs precise temperature control with a relatively small temperature measurement range, such as grain storage and wine brewing [27–30], etc. in food industry.
2. Operation principle and experimental setup
As schematically illustrated in Fig. 1, the SI usually consists of a polarization controller (PC), a section of PMF and a 3 dB optical coupler (OC). In our experiment, the length of PMF used for experiencing temperature is L’, which acts as the sensing head as well as introduces phase difference. The input light is split into two direction by a 3 dB coupler and the two counter-propagation beams are combined at the same coupler. In order to get the optimum transmission spectrum, the polarization of the light propagating in the SMF is carefully adjusted by an in-line PC.
Fig. 1. The Schematic diagram of Sagnac interferometer. PMF: polarization maintaining fiber. L: total length of PMF. L’: sensing length of PMF.
According to Joses matrix, if the fiber loss and the birefringence of the SMF in the Sagnac loop are negligible, the transmission spectrum of the SI is approximately a periodic function of wavelength which can be expressed as Eq. (1) [31]:
where $\alpha $ denotes the phase difference introduced by the PMF is simply given as: $B\textrm{ = }|{{n_f} - {n_s}} |$ is the birefringence, where ${n_f}$ and ${n_s}$ are effective refractive indices of the PMF at the fast and slow axes, respectively. L is the length of the PMF. The wavelength spacing of two adjacent peaks (named FSR, Free Spectrum Rang) is as follows: For a given wavelength λ and PMF, the FSR is changed only with the length of the PMF. At the output of the SI, the wavelength of destructive interference (dip point) must satisfy the following equation:In this relation, m denotes any integer. When the temperature applied to PMF changes, both values of B in birefringence and L in fiber length will be changed accordingly, resulting in wavelength shift, which can be expressed as [32]:
By substituting Eq. (4) into the above equation we can get:
If both sides of the above equation are divided by the temperature variation $\Delta T$, the relationship between the resonant dip wavelength shift $\Delta {\lambda _{D\textrm{ip}}}$ and the temperature change can be given as:
Obviously, the above formula shows that the temperature sensitivity is not only related to the temperature induced birefringence change $\frac{{\Delta B}}{{\Delta T}}$ of PMF (the coefficient to characterize temperature induced birefringence change for PMF), but also proportional to sensing length ($L^{\prime}$) of PMF. The PMF used in our experiment is provided by Fujikura Company (PANDA type, SM15-PS-U25D) and the birefringence is around $3.69 \times {10^{ - 4}}$ [33].
Figure 2 shows the experimental setup of proposed MWFRL based on hybrid gain medium and SI used for temperature measurement. A 980 nm pump laser is coupled into the laser cavity by a 980/1550 nm wavelength division multiplexer (WDM). A section of 0.65 m highly doped EDF (Liekki, EDFC-Er-110) with an absorption coefficient of 110 dB/m at 1530 nm is used as the gain medium. The SOA was driven by a current source with a maximum current of 160 mA, which introduced inhomogeneous gain and offered a saturation output power of 7 dBm. What’s more, the biased current of the SOA should be carefully controlled [34]. If it is too small, the gain in the cavity will not be enough to produce laser output. What’s more, if the SOA supplies too much gain, high noise and harmful nonlinear effects would further degrade the performance of the laser. The temperature control module (TCM) with temperature control accuracy ±0.01 °C is connected with a thermoelectric cooler (TEC). As a sensing head, most length of the PMF is covered in a cuboid container made of acrylic sheets to experience temperature changes, with only a small section PMF exposed in the air. In addition, the isolator and the circulator make the fiber ring laser operate unidirectionally preventing undesired effects such as the spatial-hole-burning (SHP) effect [35]. Furthermore, two fiber PCs including an in-line PC are used to optimize laser oscillation, and the laser output power is extracted from the cavity through the 10% port of a 10:90 OC. An optical spectrum analyzer (OSA, Hp-70004A, resolution 0.08 nm) connected to the 10% port is engaged to observe the laser output.
Fig. 2. The experimental setup of proposed MWFRL based on hybrid gain medium and SI used for temperature measurement. WDM: wavelength division multiplexer. SOA: semiconductor optical amplifier. OC: optical coupler. TEC: thermoelectric cooler. TCM: temperature control module.
3. Experimental results and discussions
The fiber lasers can greatly improve the performance of the fiber-optic sensors on account of their high SNR, high stability and narrow bandwidth. In our experiment, firstly we investigate the sensing performance of the proposed MWFRL based temperature sensors with the PMF length of 1.7 m.
Figure 3(a) shows the optical spectrum of the MWFRL output at the pump power of 180 mW and the SOA biased current of 70 mA. By adjusting the PC carefully, stable multi-wavelength lasing is achieved. The SNR ratios are larger than 42 dB around 1560 nm. It should be noted that, in our experiment, laser output at the wavelength of 1560 nm are employed for temperature sensing instead of 1600 nm. There are two main reasons: (1) the highly Er-doped fiber used in our experiment is much more suitable for C-band, and thus that the oscillation in the 1600 nm band is mainly stemmed from the gain provided by the SOA. So, once the temperature changes, the light in this band becomes unstable, which is unbeneficial for sensing applications. (2) the processing of optical signal at 1600 nm band in optic-fiber sensing is limited by the higher cost and complexity of the optical components in comparison with the 1560 nm band.
Fig. 3. The output characteristics of the proposed MWFRL when the length of PMF is 1.7 m. (a) Optical spectrum of the laser output within 60 nm span. (b) The wavelength and power stability and (c) the repeatedly scanned spectrum of the fiber laser within 60 minutes with a time interval of 5 minutes.
As shown in Fig. 3(b) and (c), the output spectra of the MWFRL are recorded every 5 minutes for 60 minutes at room temperature. The wavelength fluctuation is within ±0.08 nm, and the relative change of amplitudes are smaller than ±0.3, ±1 and ±0.8 dB at 1563.423 nm, 1567.328 nm and 1571.233 nm, respectively.
During the temperature sensing experiment, about 1.3 m of PMF (total length ∼1.7 m) as the sensing element is fixed into the cuboid container to experiencing temperature variation. Figure 4(a) reveals the optical spectrum evolution at 1567.328 nm when the temperature varies wavelength shifts toward shorter wavelength as the temperature rises. In order to clearly depict the linear relationship between three wavelengths change and the temperature variation, the test result is illustrated in Fig. 4(b). One can see that the range of temperature measurement is of about 2.2 °C, which is limited by the SI’s FSR. Due to the periodic feature of SI, the laser output spectra will overlap if the temperature variation is beyond 2.2 °C, which results in that the same wavelength corresponds to multiple temperatures. For practical applications, this limitation needs to be alleviated by increasing the range of temperature measurement.
Fig. 4. (a) Optical spectrum evolution at 1567.328 nm when temperature varies from 30 °C to 32.2 °C with a step of 0.2 °C in one FSR. (b) Optical spectrum variation of three wavelengths (1563.423 nm, 1567.328 nm, 1571.233 nm) when temperature varying from 24 °C to 34 °C with a step of 0.2 °C. (c) The relationship between wavelength and temperature with a 1.7 m long PMF.
To extend the range of temperature measurement, some data processing is necessary. In this paper, the method used is similar to that in [26], which adds the appropriate multiples of the FSR at the edge of temperature measurement range. The proposed fiber laser sensor has the advantage of multi-wavelength oscillation so that the added multiples of the FSR is more accurate. This will make the data processing result more precise and thus better used for temperature sensing. Unlike the single-wavelength temperature sensor in Ref. [26], in which there is dual-wavelength oscillation at the edge of the measurement range, caused by EDF homogeneous broadening effect at room temperature. However, the proposed hybrid gain medium based laser can achieve stable multi-wavelength output, so the process of wavelength change with temperature can be clearly distinguished. As shown in Fig. 4(c), the relationship between wavelength shifts and temperature changes can be fitted linearly with a R2 of 0.99869. The wavelength shifts from 1563.4518 nm to 1545.5411 nm when the temperature changes from 24 °C to 34 °C, corresponding to a temperature sensitivity of 1.8063 ± 0.00933 nm/°C. The measurement resolution is estimated to be ∼0.0442 °C which is limited by the resolution of our OSA. It is clearly to see that the temperature measurement range has been expanded about 4.5 times after data processing. In practice, it should be noted that the proposed laser temperature sensor is especially suitable for some scenarios with explicit temperature dynamic range, but not suitable for detecting the temperature with unknown range. Moreover, if the wavelength stability of the MWFRL is improved further and the OSA with higher resolution is used, the wavelength spacing among multiple wavelengths can be recorded more accurately, which makes it possible to use FSR or B*L demodulation method to identify the temperature range of unknown environment [36].
On the one hand, there is a trade-off between temperature sensitivity and the temperature measurement range, therefore when the sensing length is shorter than instead of the entire PMF, the temperature measurement range will be improved without significantly reducing its sensitivity. What’s more, since the temperature sensitivity is determined by the length and the birefringence of the PMF according to Eq. (8), the MWFRL based temperature sensing system with higher sensitivity can be realized with longer length of sensing PMF. Experiments are carried out by selecting the PMF with the length of ∼1.22 m and ∼2.96 m, respectively.
The optical spectra of the multi-wavelength laser oscillation are depicted in Fig. 5(a) and (b) when the length of the PMF are 1.22 m and 2.96 m, respectively. Both of them have an SNR more than 40 dB. As shown in Fig. 5(c) and (d), the wavelengths instability are ±0.18 nm and ±0.09 nm, respectively. Power fluctuation (less than ±1 dB) has been observed in the experiment, which is acceptable since in our sensing scheme wavelength demodulation has been adopted, and the SNR of the wavelength used for wavelength demodulation is greater than 40 dB and no other frequency components appear at the peak wavelength.
Fig. 5. The output characteristics of the proposed MWFRL and the fluctuation repeated scanning spectrum of the fiber laser within 60 minutes with a time interval of 5 minutes, when the length of PMF are 1.22 m (a)(c) and 2.96 m (b)(d), respectively
The output optical spectra of the MWFRL by using PMF with the length of 1.22 m (sensing length: 1.2 m) and 2.96 m (sensing length: 2.9 m) have been measured, when the temperature changes from 23 °C to 33 °C with a step of 0.2 °C and 0.1 °C, respectively. The measured results are portrayed in Fig. 6(a) and (b), and the temperature measurement ranges are estimated to be 2.8 °C and 1.1 °C, respectively. As depicted in Fig. 6(c) and (d), through the data analysis, the temperature sensitivities by using PMF with the sensing length of 1.2 m and 2.9 m are estimated to be −1.7915 ± 0.00751 nm/°C and −1.9073 ± 0.00658 nm/°C, respectively. One can see that the sensitivity is related to the length of PMF, longer PMF enjoys the higher sensitivity.
Fig. 6. Optical spectrum variation of two-wavelength (1565.45 nm and 1570.89 nm) (a) and four-wavelength (1564.825 nm, 1567.078 nm, 1569.456 nm and 1571.708 nm). (b) when temperature varying from 23 °C to 33 °C with a step of 0.2 °C and 0.1 °C, respectively; the relationship between wavelength and temperature by using PMF with the length of 1.22 m (c) and 2.96 m (d), respectively.
In this way, the sensitivity of this sensing scheme can be tailorable by utilizing PMFs with different lengths as the sensing elements. What’s more, by comparing the results of using PMF with the lengths of 1.7 m and 1.22 m, one can also see that when the total length of PMF is longer but the sensing length of PMF are close to each other, the temperature measurement range will be reduced but the sensitivity is similar. This is in good accordance with the theoretical analysis.
We have also carried out experiment to study the influence of vibration on the laser’s output, as polarization noise can be caused by vibrations and external disturbances on the fiber link [37]. In this experiment, the length of EDF is 0.3 m and the length of PMF is 1.7 m, and the external vibration has been introduced by constantly hitting the optical platform with a weight, while 30 group of data has been recorded at the same time. The measurement results are analyzed, and the wavelength and power of the laser output with external vibration applied is shown in Fig. 7. It can be seen clearly that both of the wavelengths fluctuation are within ±0.06 nm (which is at the same level of laser instability measurement without vibration), and the relative change of amplitudes are smaller than ±1.45 and ±0.53 dB at 1560.3567 nm, 1564.0801 nm, respectively. This inapparent influence is attributed to the shorter length of fiber in the laser cavity in our experiment, which in [37], it is a very long fiber link.
As for the resolution of the proposed temperature sensing scheme, it is equal to the resolution of OSA divided by the temperature sensitivity, and the resolution of the detection instrument has influences on the resolution of the sensing scheme, but not its sensitivity. In our experiment, the resolution of OSA is 0.08 nm, so the resolution of the proposed temperature sensor is calculated to be 0.0442 °C, when the length of the sensing PMF is ∼1.3 m. If an OSA with resolution of 0.01 nm is used, higher resolution of 0.0055 °C can be achieved. It should be noted that in our experiment, an OSA has been adopted for wavelength demodulation, whose resolution is not high, and also it is an expensive demodulation method. However, in practical applications, other interrogation techniques can be employed, such as the scanning Fabry-Perot (FP) interferometer, whose resolution is in ∼MHz order of magnitude and already becomes a commonly used commercial equipment for linewidth measurement of narrow linewidth or single-frequency laser [38]. In this way, the system cost can be greatly reduced with enhanced resolution of the sensing scheme.
Compared with FBG based temperature sensors, whose temperature sensitivity is about 10 pm/°C [39], our proposed temperature sensing scheme has higher temperature sensitivity (∼1.8 nm/°C). The coefficient of thermal expansion and thermos-optic of fused silica at room temperature is on the order of 10−6/°C [40–42], which leads to the low sensitivity of FBG based temperature sensors. As for our proposed SI based MWFRL sensing scheme, its sensitivity is closely related to the length of sensing PMF. Although for commonly used PMF, the temperature induced birefringence coefficient change $\frac{{\Delta B}}{{\Delta T}}$ is generally in the order of magnitude of 10−7/°C, since the length of sensing PMF $L^{\prime}$ is in units of meter, high temperature sensitivity can be achieved as illustrated in Eq. (8).
4. Conclusions
In summary, we have demonstrated a temperature sensor utilizing MWFRL based on hybrid gain medium and SI, which is especially suitable for some scenarios with explicit temperature dynamic range. Due to the high SNR (>42 dB) and narrow 3 dB bandwidth (<0.08 nm) of proposed fiber laser sensor, we obtain high temperature sensitivity of 1.8063 ± 0.00933 nm/°C and resolution of 0.0442 °C when the length of the PMF in the SI is 1.7 m. What’s more, we also demonstrate that when the length of PMF are 1.22 m and 2.96 m, the sensitivities of the sensors are −1.7915 ± 0.00751 nm/°C and −1.9073 ± 0.00658 nm/°C, respectively. The experimental results show that the temperature measurement range will be improved without significantly reducing its sensitivity when the sensing length is shorter than instead of the entire PMF and the longer PMF enjoys the higher sensitivity. Moreover, the proposed sensing scheme also enjoys the characteristics of higher SNR and narrow linewidth (which means better resolution). Therefore, the proposed temperature sensing scheme exhibits the advantages of easy implementation, good linearity, high sensitivity, SNR and resolution, which shows good application potentials in certain scenarios which needs precise temperature control with a relatively small temperature measurement range, e. g. in food industry such as grain storage and wine brewing, etc.
Funding
National Natural Science Foundation of China (61975167, 61575166); Xiamen Science and Technology Planning Project (3502Z20183003).
Acknowledgments
The authors thanks Mr. Hang Wang and Mr. Qiujun Ruan for their kind help.
Disclosures
The authors declare no conflicts of interest.
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