Temperature Sensor Based on Fiber Ring Laser With Cascaded Fiber Optic Sagnac Interferometers

A new temperature fiber ring laser (FRL) sensor based on a cascaded Sagnac loops fiber structure is proposed and experimentally demonstrated. The optical FRL sensor consists of cascaded Sagnac loops by inserting two polarization-maintaining optical fibers (PMF) with slightly different lengths. PMF with length of 56 cm and 75 cm are used in Sagnac loops as a filter and sensing unit in laser cavity. The working principle of the sensor is based on the phase shift (θ) caused by birefringence between two principal polarization modes and enhance the sensitivity by constructing a Vernier-scale. In an appropriate temperature range (25 °C - 31 °C), the detection sensitivity of FRL sensor based on a cascaded Sagnac structure is significantly higher than other FRL sensors. Thanks to the laser sensing system, the sensitivity can be modulated by changing the free spectral range (FSR). The experimental results show that the temperature sensitivity of the cascade Sagnac structure sensor is - 4.031 nm / °C, which is five times higher than that of FRL sensor base on the single Sagnac structure.


Introduction
In the last few decades, fiber-optic sensors have attracted extensive attentions in engineering and scientific applications. Due to the advantages of their compact size, high resolution, immunity to electromagnetic interference and cost-effective deployment of the sensors. Lots of traditional optical fiber sensor schemes have been proposed over the last twenty years. Temperature sensors are designed and fabricated based upon the principles of fiber Bragg grating (FBG) [1]- [3], Mach-Zehnder interferometer (MZI) [4]- [6], and Fabry-Perot interferometers (FPI) [7]- [9]. Recently, there is a growing interest in all-fiber laser sensors, since it has the advantages of high sensitivity, good stability, low insertion loss and high signal-to-noise ratio. In general, fiber laser sensors can be classified into three types: distributed Bragg reflector(DBR) fiber laser [10]- [12], distributed feedback(DFB) laser [13]- [15] and fiber ring laser(FRL) [16]. It was observed that the tunable erbium-doped based FRL is a good candidate for temperature sensing since its spectrum is geared to the telecommunication wavelength window. Various FRL sensors have been extensively investigated to measure temperature [24]- [26], gas pressure [17], refractive index (RI) [18], strain [19], and curvature sensing [20]. Those sensors are useful in biomedicine, health monitoring, aviation, hydrostatic pressure [21]- [23], etc.
Contemplating the different sensing structures, performance enhancement of the sensitivity and stability of FRL temperature sensor is one of the research directions that has attracted extensive attention. A temperature FRL sensor based upon liquid filled photonic crystal fiber (PFC) with a sensitivity of -1.747 nm/°C was proposed by Yang et al. [24]. However, liquid evaporation has a great influence on the stability of the sensor. Zhao et al. used a core-offset Mach-Zehnder (MZ) interferometer to measure the refractive index and temperature simultaneously, with a temperature sensitivity is 0.049 nm/°C and a signal to noise ratio of 60dB [25]. An FPI is inserted into the FRL working as a temperature sensor through an optical fiber circulator [26]. The temperature sensitivity is 0.249 nm/°C with a signal to noise ratio of 52dB. Over recent years, substantial research efforts have been devoted to the development of FRL sensor based on Sagnac interferometer [27]- [29]. A Sagnac loop structure FRL sensor with high temperature sensitivity of 1.739 nm/°C, narrow 3-dB bandwidth of less than 0.05 nm, and high signal to noise ratio of 50 dB was proposed by Yao et al. [30]. The result shows that with the insertion of FBG, the temperature resolution has been improved to 10 −6°C [31]. However, limited by the intrinsic structure of the sensor, the temperature sensitivities obtained among these works are not high as traditional fiber sensors. Besides, cascaded interferometers can significantly enhance the spectral shift range through the Vernier effect. The use of cascaded interferometers as filters for interval-adjustable FRLs has been proposed in recent years [32]- [34]. However, to the best of our knowledge, FRL sensor operated under the principle of Vernier effect has not been widely proposed.
In this work, we proposed an ultra-high sensitivity FRL temperature sensor with cascaded Sagnac loops. Narrow 3-dB bandwidth less than ∼0.67 nm and a high signal to noise ratio of ∼30 dB is experimentally demonstrated. The theoretical model and experimental design of the proposed method are discussed in detail. The temperature sensitivity of the proposed sensor is about -4.031 nm/°C, significantly higher than previously reported FRL based sensors [24]- [26]. The sensitivity enhances ∼5 times based on the Vernier effect in the cascaded Sagnac interferometers than single Sagnac loop. Unlike traditional cascaded Sagnac loops fiber sensor [35]. The FRL sensor is able to demodulate the wavelength change directly, relaxing the need for transmission spectra calculation procedure. The stability and measurement error of the proposed sensor is also analyzed experimentally. The proposed temperature sensor can be used for several engineering applications including biomedical science and aviation monitoring.

Principles
The schematic diagram of the cascaded Sagnac loop interferometer is shown in Fig. 1. The whole system is consisted of two 3 dB couplers and two PMF with similar length. In the Sagnac interferometer, two polarization states can be represented by Jones matrix. The transmission of the electric field is described by Jones vector [36]: where E in is the input electric field. E x in and E y in are the two orthogonal polarization components of E in . The Jones matrix of the 3dB coupler can be described as [36]: where κ is the coupling coefficient of the optical coupler. The output electric fields of the optical coupler are described as: where E 1 and E 2 represent the two output electric fields of the optical coupler, respectively. The Jones matrix of the PMF can be expressed as [36]: where ϕ is the phase of the two orthogonal components of the electric field in the PMF, λ is the wavelength, n is the refractive index difference between the fast axis and the slow axis of the PMF. L is the length of the PMF. The output of the electric fields is denoted as E 3 and E 4 after propagating through PMF,which can be written as: Here θ is the phase shift of light propagating through PMF between two polarization modes. After transmission through the loop the output electric field of the Sagnac interferometer can be calculated as: From Eqs.1-8, we can calculate the transmission spectra of a single Sagnac loop interferometer. The transmission spectra of a single Sagnac loop interferometer with different lengths of PMF is shown in Fig. 3. The lengths of PMF we used were 56 cm and 75 cm, respectively. The wavelength interval between two adjacent transmission peaks, named free spectral range (FSR) can be calculated by the following equation [35]: where B is the birefringence of the PMF. When the temperature is changed by T, a corresponding wavelength shift λ will be introduced as [35]: where S is the temperature sensitivity of the proposed sensor. Thus, the real phase shift can be written as [36]: The spectra of a single Sagnac interferometer under different PMF lengths is obtained in Fig. 3. According to eq. (8), PMF with length of 56 cm and 75 cm are inserted into the cascaded fiber ring, respectively. To improve the sensitivity of the temperature sensor. We constructing a Vernierscale with two cascaded Sagnac interferometers. Vernier-scale is widely used in high-precision and short-distance ranging to enhance the accuracy of detection. It mainly uses the change of small measurement value to cause large-scale change of alignment graduation. Measure by overlapping two tick marks. The peak spacing of the spectrum is affected by the length of PMF, which produced different FSR. The Saganc loop of the sensing one in Fig. 1 shifts the wavelength without changing the FSR. The peak appears at the wavelength where two interference peaks overlap, and the height of each peak is determined by the overlap. The envelope period is given by [32]: f sr sensor * f sr filt er f sr senosr − f sr filt er (12) When the interference peaks of two independent Sagnac loops overlapping, the maximum peak wavelength appears in the cascaded loop. If the sensor loop shifts with temperature change, the peak will be transmitted to the adjacent peak. The magnification is: f sr filt er f sr senosr − f sr filt er (13) Fig. 1 shows how the Vernier-scale concept is applied to Sagnac loops. The interference signal of the first Sagnac loop is used as the input for the second sensing loop. The total transmission spectrum of cascaded Sagnac loops is the product of each Sagnac loop. The two interference peaks of each Sagnac loop are partially overlapped, and the height of each envelope will be determined by the overlap amount. In practical application, we can get the ideal FSR by controlling the length of PMF. The filter Sagnac ring is a good shield of the ambient temperature or strain changes, which is a fixed part of the Vernier scale. The second sensor Sagnac is more like the sliding part of the Vernier scale, because the change of temperature will cause the shift of the interference wavelength.

Experimental Details
In the cascaded loops system, the first Sagnac loop works as the reference part of the Vernierscale, meanwhile also protecting the system from external interference. The second sensor is the sliding part of the Vernier-scale. As shown in Fig. 2. the experimental setup of the FRL sensor includes a laser pump source with a tunable output power from 150mW to 500mW, a wavelength division multiplexing (WDM), an isolator (ISO), two cascaded Sagnac loops and couplers. 1% of the output light energy was sent to OSA for analysis.
The pump light from a 980 nm laser source (PL-974-500-FC/APC-P-M) was launched into the FRL by a 980/1550 nm WDM. A section of 1.6m long EDF was adopted as the gain medium. Two PMFs (Changfei, PM 1550_125-18/250_Y) with lengths of 56 cm and 75 cm are used to construct Sagnac loop. The backscattered optical signal from the Sagnac loop was blocked by an optical isolator (PIIS1550DP01212) to prevent spatial hole-burning. Temperature change was applied by placing the sensing part of Sagnac loop inside a dry bath chamber. The output spectrum of the temperature sensing system is measured by an OSA (YOKOGAWA, AQ6370D) via a 1:99 coupler. The 1% output of the coupler is connected with OSA, and the other end is connected with WDM to couple the optical transmission back to the ring cavity. In this system, one Sagnac loop is used as a filter and the other working as a temperature sensor. The detection sensitivity is effectively enlarged by Vernier scale effect.
In order to verify the working principle of the sensor, a pre-experiment is carried out. By using supercontinuum light source. The spectral shifts of single Sagnac loop and cascade Sagnac loops were measured at different temperatures. The output transmission spectrum of the temperature sensor system composed of Sagnac loop is shown in Fig. 4. Besides, As shown in Fig. 5, the cascade Sagnac loop structure can significantly improve the detection sensitivity due to the Vernier effect. f sr senosr and f sr filt er were measured to be 6.31 nm and 4.92 nm, respectively, as shown in Fig. 3. Therefore, the calculated enhancement factor is 4.54. The sensing characteristics of the single and cascaded Sagnac loops were tested by placing the sensor in a dry bath while the temperature was tuned from 25°C to 31°C. The interval between each measurement is half an hour. As shown in Fig. 5, there is only one dominant transmission peak two sidelobes. The sidelobe suppression ratio is about 10 dB, demonstrating excellent a single-wavelength stability ability by cascaded Sagnac interferometers.
The pre-experimental results show that the wavelength has a blue-shifted with the increase of temperature. In Fig. 6, the temperature sensitivity of Sagnac structure sensor and cascade Sagnac IEEE Photonics Journal Temperature Sensor Based on Fiber Ring Laser  structure sensor is compared. Compared with the temperature sensitivity of a single Sagnac loop (-0.69 nm /°C), the temperature sensitivity of cascaded Sagnac loop is improved (-3.32 nm /°C). The R 2 of the sensor head is ∼0.999 and ∼0.996, respectively. Owing to the uncertainty measurement error of the envelope position in the spectrum. The final magnification of the experiment is ∼4.81, slightly larger than the theoretical value (∼4.54). Considering computed enhancement factor (see (11)), the sensitivity can be effectively increased by selecting two interferometers with small difference in FSR. However, in practical applications, there is a tread off between the sensitivity and measurement accuracy. Smaller difference in FSR will increase the envelope. Thus, the number of envelopes within the determined wavelength range is reduced. Therefore, it is difficult to track the offset of the envelope peak. Hence, a FRL temperature sensor based on cascaded Sagnac loops is proposed, which can effectively reduce the detection error caused by the shift of the tracking envelope peak through reading the spectral shift directly.

Results
The measured spectra showed multiple envelopes in the C+L band as shown in Fig. 7. However, we found that the single-wavelength laser could oscillate in the main lobe at a certain range. Hence, we found that in order to realize the accurate measurement of the temperature sensor, the dry bath temperature should be controlled in the range of 25°C to 31°C. When the temperature exceeds this range, since the erbium-doped fiber concentrated in the C + L band, the frequency hopping will occur. As shown in Fig. 7, when the temperature is 24°C and 32°C, the output laser wavelength overlaps at 1536 nm. Besides, the side lobe can also provide energy for the corresponding wavelength in the process of laser building-up. Therefore, the laser spectrum will be similar to that of cascaded Sagnac rings in SLED. The main lobe begins to get energy to produce the laser, and the side lobe also gets part of the energy. This may be the reason why the spectrum of the sensor is wider than that of the ordinary FRL sensor. Fig. 7 depicts the measured output spectra of the cascaded Sagnac fiber ring structure in the scanning wavelength range of 1500 nm to 1580 nm: broadband light source (orange Line) and fiber ring laser (blue line). The peak wavelength of cascade Sagnac ring structure in FRL is consistent with that in broadband light source. It is worth noting that the fiber ring laser sensor has high SNR and narrow 3dB bandwidth. The SNR of the proposed FRL sensing system is more than ∼30dB, which is 3-5 times larger than that of the traditional sensing system. Besides, the 3dB bandwidth (∼0.67 nm) is much smaller than the traditional sensing system (∼10 nm).
The output spectra of the sensing system are measured in Fig. 8 and Fig. 9 with the temperature controlled from 26°C to 32°C. The peak wavelength was shifted to a shorter wavelength accordingly. (i.e., the laser wavelength has a blue-shift with the increase of temperature). Moreover, With the change of temperature, the birefringence of PMF in Sagnac loop changes, and the transmission spectrum of Sagnac loop shifts to a shorter wavelength. Besides, the response of the single Sagnac loop structure to temperature variations has also been analyzed. The FRL sensor based on individual Sagnac loop is placed into a temperature-controlled dry bath. The wavelength shift varies with ambient temperature from 25°C to 31°C, as shown in Fig. 8. Fig. 10(a) and Fig. 10(b) show the lasing wavelength as a function of the temperature. When temperature is varied from 25°C to 31°C, the linear fitting is 0.998 and 0.999, respectively. The result proves that the relationship between the wavelength of laser and temperature is linear. Besides, the peak intensity of the laser is shown in Fig. 11. where the peak intensity is between Fig. 8. Transmission spectra of the fiber laser temperature sensor based on single Sagnac loop system when T changes from 25°C to 31°C with the steps of 1°C. Fig. 9. Transmission spectra of the fiber laser temperature sensor based on cascaded Sagnac loops system when T changes from 25°C to 31°C with the steps of 1°C. −47.12 dBm and −48.22 dBm. The wavelength fluctuation range is 1560.37 nm to 1561.23 nm at 3 hours which shows that the proposed laser sensor system in this work is highly stable after long-time of work. The central wavelength shifts from 1564.41 nm to 1540.22 nm when the temperature changes from 25°C to 31°C, corresponding to a temperature sensitivity of ∼-4.03 nm/°C and the level of temperature resolution is 10 -3°C . According to Fig. 10 (a), the corresponding temperature sensitivity of a single Sagnac ring temperature sensor is ∼-0.826 nm/°C. Therefore, the experimental enhancement factor is ∼4.879, which is similar to the previous amplification factor (4.81). Moreover, better results can be obtained by choosing the length of erbium-doped fiber properly. The spectral broadening of cascaded Sagnac structure relative to single Sagnac loop structure may be caused by the change of the free spectral range which was described in [37].    Table 1 shows the performance comparison (sensitivity, measurement range and linearity) of the FRL temperature sensors based on different kinds of structure. To the best of our knowledge, our sensor shows the best sensitivity performance. The sensor is composed of cascaded Sagnac loop with uniform polarization maintaining fiber based on the Vernier effect. Furthermore, it has the characteristics of high temperature sensitivity and remote temperature detection by extending the length of Sagnac ring. Moreover, it has broad application prospects in the field of environmental monitoring.

Conclusion
In summary, an ultra-high sensitivity FRL temperature sensor based on cascaded Sagnac loops structure is proposed and experimentally demonstrated. PMF lengths of 56 cm and 75 cm were selected to construct Sagnac loop. Moreover, the cascaded loops work as a filter and the sensing unit inside the fiber laser ring cavity. The sensitivity of the sensor is improved by Vernier effect. In conclusion, an ultra-high temperature sensitivity of ∼-4.031 nm/°C is realized. The results show that the system can significantly improve the sensing performance by ∼4.879 times. Finally, the sensitivity is increased to 4.54 times by theoretical calculation, which is very close to the experimental results. The error may be due to the inaccurate selection of envelope. This technique could be explored to measure harsh environment.