Nonlinear-Optical-Loop-Mirror-Based Mode-Locked Fiber Laser Sensor for Low-Temperature Measurement

Abstract

A temperature-sensing scheme is realized by a passively mode-locked Yb-doped fiber laser based on the nonlinear optical loop mirror (NOLM). The ambient temperature can be measured by detecting the pulse repetition frequency of the mode-locked fiber laser by an oscilloscope. When the ambient temperature increases from −40 °C to 6 °C, the pulse repetition frequency decreases linearly with a temperature sensitivity of 72.548 Hz/°C. The experimental results prove the feasibility of the mode-locked laser sensor operating in a low-temperature environment.

1. Introduction

Fiber-optic temperature sensors have important applications in industry, aerospace, biomedicine, and national security with the advantages of small size, high sensitivity, corrosion resistance, strong anti-electromagnetic interference, intrinsic safety, and long-distance detection [1,2,3,4,5]. According to the sensor type, traditional fiber-optic temperature sensors are mainly based on fiber Bragg gratings (FBGs) and fiber interferometric structures, such as Mach Zehnder interferometers (MZIs), Michelson interferometers (MIs), and Fabry–Perot interferometers (FPIs) [6,7,8,9]. Using the spectral demodulation technology, the temperature of the environment can be measured by detecting changes in transmitted or reflected spectral signals. However, such schemes rely on the expensive optical spectrum analyzer (OSA) with relatively low sensitivity. In recent years, fiber laser sensing schemes have emerged as a promising alternative for high-precision fiber temperature sensing with the development of Er³⁺ or Yb³⁺ ion-doped fiber lasers, with advantages such as compact size, narrow bandwidth, high signal-to-noise ratio (SNR), and good beam quality.

Compared with spectral demodulation technology, radio-frequency (RF) demodulation based on fiber lasers has the advantages of high speed, high resolution, and low cost [10]. The generation of multiple longitudinal modes is essential for these fiber laser sensors, typically achieved using a continuous-wave laser. A beat frequency signal (BFS) is generated between any two longitudinal modes, which is received by the photoelectric detector (PD) and converted into an electrical signal for display on the frequency spectrum analyzer (FSA). The BFS varies linearly with temperature, and temperature sensing is realized by measuring the frequency shift. Zuowei Yin et al. presented a novel multilongitudinal-mode (MLM) fiber ring laser sensor for temperature measurement with sensitivities of 10.24 kHz/°C @ 1581.7 MHz between 21 °C and 50 °C, 7.42 kHz/°C @ 1175.6 MHz between 22 °C and 240 °C, and 8.48 kHz/°C @ 1480.5 MHz between 100 °C and 1000 °C [11]. Long Huang et al. proposed an MLM fiber laser sensor with a sensitivity of −4.22 kHz/°C @ 1004.8 MHz and −16.78 kHz/°C @ 404l.2 MHz [12]. Xiujuan Yu et al. proposed a polarimetric MLM fiber laser with a sensitivity of −25.53 kHz/°C between 20 °C and 90 °C [13]. Xingxing Tong et al. proposed an MLM laser sensor system based on the neural network (NN) algorithm, demonstrating a single beat frequency sensitivity of 5.204 kHz/°C [14]. Generally, the higher the frequency of the beat signal, the higher the measurement accuracy. However, it is challenging for conventional lasers to generate a large number of longitudinal modes with a high SNR. Mode-locked lasers can generate plenty of longitudinal modes with equal frequency intervals, which have great potential for temperature, strain, and humidity monitoring in harsh environments [15,16,17,18]. Recently, Jian Luo et al. proposed a temperature-sensing scheme based on a passively mode-locked erbium-doped fiber (EDF) laser [19]. At a beat frequency of 10 GHz, the temperature sensitivity reaches up to −44 kHz/°C between 20 °C and 80 °C. Shaonian Ma et al. presented an optomechanically mode-locked EDF laser sensor in the temperature range of 20 °C to 65 °C, yielding a temperature sensitivity of −13.3 kHz/°C @ 1.917 GHz with a high SNR of 58 dB [20]. When the mode-locked fiber laser sensor is used outdoors, it is an important prerequisite for temperature sensing that mode-locked laser can work normally in high- and low-temperature environments. However, there are no reports on the temperature sensing characteristics of mode-locked fiber lasers in low-temperature environments.

In this paper, we propose a temperature-sensing scheme for low-temperature applications using a passively mode-locked fiber laser based on the nonlinear optical loop mirror (NOLM). A platinum resistance thermometer is employed in the heating device to monitor temperature variations. The temperature sensing demodulation system is a common digital oscilloscope. The variation in the pulse repetition frequency can reflect corresponding temperature changes. The experimental results demonstrate that the pulse repetition frequency decreases linearly with increasing temperature with a temperature sensitivity of 72.548 Hz/°C. Here, the pulse repetition frequency of the mode-locked laser corresponds to the fundamental frequency of the FSA. If the frequency spectrum analyzer with GHz bandwidth is used, the temperature sensitivity can be comparable to the current report.

2. Experimental Setup

Figure 1 shows the experimental setup of the mode-locked fiber laser used for temperature sensing. The pump source is a single-mode 980 nm laser diode (LD) with a maximum output power of 500 mW. A 980/1064 nm wavelength division multiplexer (WDM) connects the pump source to the resonator through the 980 nm port. The pump light is coupled into the upper unidirectional ring and propagates clockwise through the WDM. The gain medium is an 80 cm polarization-maintaining ytterbium-doped fiber (PM-YDF) (PM-YSF-HI-HP, Nufern, East Granby, CT, USA). After the gain fiber, an isolator (ISO) is used to ensure unidirectional transmission of the laser in the upper ring. In order to suppress the radiation around 1030 nm, a 1064 nm bandpass fiber filter with a 2 nm bandwidth is used. A 10/90 fiber coupler (OC2) connects the upper unidirectional ring cavity with the lower bidirectional Sagnac ring. To reduce the mode-locking threshold, a phase shifter (PS) is used to provide an initial phase bias of π/2. Using an asymmetrical fiber coupler and a polarization-maintaining single-mode fiber on the lower side, an NOLM-based passively mode-locked fiber laser is successfully constructed with a figure-8 cavity [21]. Another fiber coupler (OC1) has a coupling ratio of 30/70. The 30% port of OC1 is used for the laser output, while the 70% port is connected inside the cavity to maintain its normal operation. The total cavity length is 27 m, and the length of the lower bidirectional Sagnac ring is 13 m. The waveform of the mode-locked pulse is monitored in real-time using a digital oscilloscope (InfiniiVision DSOX6002A, KEYSIGHT, Santa Rosa, CA, USA).

3. Experimental Results

Firstly, the output characteristics of the mode-locked Yb-doped fiber laser is studied. The relationship between the average output power and the incident pump power is depicted in Figure 2. When the pump power is lower than 130 mW, the laser operates in the Q-switched mode-locking (QSML) regime. As the pump power continues to increase, continuous-wave mode-locking (CWML) is self-started with a center wavelength of ~1063 nm. The maximum average output power of 16 mW is obtained under an incident pump power of 400 mW. The pulse repetition frequency as a function of the incident pump power is plotted in Figure 3. Once the laser achieves stable mode-locking, the repetition frequency remains at 7.6562 MHz. Figure 4 illustrates the temporal pulse train of the mode-locked laser with a pump power of 220 mW. And the pulse repetition frequency is stable with a fluctuation of a few tenths of Hz. This indicates that the laser is suitable for further temperature sensing experiments.

Subsequently, the mode-locked fiber laser is placed in a high- and low-temperature test chamber (ESS-KWGDS705 II, Chongqing Yinhe Experimental Equipment Co., Ltd., Chongqing, China). The temperature change range of the test chamber is from −70 °C to 150 °C, and the temperature change rate is ≤5 °C/min. The core device of the temperature sensor in our work is a passively mode-locked fiber laser based on the NOLM. By measuring the change in the pulse repetition frequency, the environment temperature can be accurately detected. In a mode-locked laser, the pulse repetition frequency for the ring fiber cavity is $f=c/nL$, where c, n, and L are the speed of light in vacuum, the refractive index, and the laser cavity length, respectively [22]. The temperature variation (∆T) will affect the fiber length (L) and the refractive index (n) of the fiber. The derivative of the repetition frequency with respect to temperature is expressed as

$${\displaystyle \frac{df}{dT}}=-c{\displaystyle \frac{1}{nL}}\left({\displaystyle \frac{1}{L}}{\displaystyle \frac{dL}{dT}}+{\displaystyle \frac{1}{n}}{\displaystyle \frac{dn}{dT}}\right)=-f_r\left({\displaystyle \frac{1}{L}}{\displaystyle \frac{dL}{dT}}+{\displaystyle \frac{1}{n}}{\displaystyle \frac{dn}{dT}}\right)$$

(1)

$$∆f=-f_r\left({\displaystyle \frac{1}{L}}{\displaystyle \frac{dL}{dT}}+{\displaystyle \frac{1}{n}}{\displaystyle \frac{dn}{dT}}\right)∆T$$

(2)

where ${\displaystyle \frac{1}{L}}{\displaystyle \frac{dL}{dT}}$ is the thermal expansion coefficient and $dn/dT$ is the thermo-optic coefficient of the optical fiber [23,24]. The laser repetition frequency at 0 °C is used as the reference frequency $f_r$. According to Equation (2), the pulse repetition frequency decreases linearly with the increase in temperature. The sensitivity of the temperature-sensing scheme is defined as the amount of change in the repetition frequency when the temperature changes by 1 °C. This sensitivity is defined as S (Hz/°C); thus, Equation (2) can be rewritten as

$$S={\displaystyle \frac{∆f}{∆T}}=-f_r\left({\displaystyle \frac{1}{L}}{\displaystyle \frac{dL}{dT}}+{\displaystyle \frac{1}{n}}{\displaystyle \frac{dn}{dT}}\right)$$

(3)

where the unit of $f_r$ is MHz. It can be observed that the temperature sensitivity is related to the pulse repetition frequency, the thermal expansion coefficient, and the thermo-optic coefficient. Therefore, the sensitivity of the temperature sensor can be enhanced by increasing the repetition frequency of the mode-locked laser and using fibers with a high linear expansion coefficient.

In order to investigate the effect of environmental temperature variation on the pulse repetition frequency, the incident pump power is set to 220 mW. Considering the error between the actual temperature and the set temperature of the test chamber, a PT1000 platinum resistance thermometer with a resolution of 0.001 °C is used to measure the actual temperature value. As shown in Figure 5a,b, the relationship between the laser repetition frequency and the temperature as a function of time is recorded when the chamber temperature is set to −40 °C and 0 °C. It is clear that there is a highly negative correlation between laser pulse repetition frequency and temperature. At the same time, it also shows that the influence of ambient vibration on pulse frequency can be neglected in this experiment.

Figure 6a shows the relationship between the pulse repetition frequency and the temperature over time at low temperature. Since the temperature of the test chamber fluctuates greatly during the cooling process, we first lower the temperature to −40 °C in the experiment and then test the heating process. For the purpose of reducing the hysteresis effect of temperature measurement, the temperature rise rate of the test chamber should not rise too fast. As the temperature increases from −40 °C to 6 °C, the repetition frequency of the mode-locked laser gradually decreases. The repetition frequency as a function of temperature is plotted in Figure 6b with a linear decreasing trend. The temperature sensitivity is 72.548 Hz/°C by linear fitting. If the beat frequency demodulation system is used, a higher temperature sensitivity will be obtained.

To further verify the temperature resolution of the mode-locked fiber laser temperature sensor, the sensor is placed outdoors for experiments. As can be seen from Figure 7, the room temperature rises slowly in the morning over a period of 50 s. When the ambient temperature changes little, the temperature measurement value of the platinum resistance thermometer increases stepwise, while the repetition frequency of the mode-locked fiber laser decreases linearly and smoothly with the slow increase in temperature. Compared with the platinum resistance thermometer, it has a higher temperature measurement resolution and higher sensitivity.

4. Conclusions

In summary, a novel fiber temperature-sensing scheme for low-temperature measurement is demonstrated based on a passively mode-locked fiber laser. The sensor is a 1063 nm mode-locked Yb-doped fiber laser with NOLMs. When the ambient temperature increases from −40 °C to 6 °C, the repetition frequency of the mode-locked laser gradually decreases with the temperature sensitivity of 72.548 Hz/°C. At the same time, this type of temperature sensor also shows high-temperature measurement resolution and high sensitivity. Compared to conventional fiber-optic temperature sensors, this approach enhances both the sensitivity and SNR of temperature-sensing technology. Furthermore, as an innovative temperature-sensing method, it possesses substantial potential for future advancements.