基于REC-DFB可调谐激光器的无波长校正FBG传感系统

FBG Sensor System without Wavelength Correction Based on REC-DFB Tunable Laser

提出了一种无波长校正的光纤布拉格光栅(FBG)传感系统,可在未对基于重构等效啁啾(REC)技术制造的扫频DFB激光器进行波长线性校正的情况下,识别波长变化,并解调出光栅传感器的温度变化。所提无波长校正FBG传感系统可以在200℃范围内进行精确的温度检测,测得的波长与温度的线性系数1-R^(2)仅为0.004 4。该传感系统光源在1 kHz的锯齿波调制下,调谐范围可达2.5 nm,且测量过程中无需额外波长校正器件。

With the rapid development of optical fiber communication and optical fiber sensing,the Fiber Bragg Grating(FBG)sensor has become one of the fastest growing and most widely used optical fiber sensors.Owing to the FBG sensor has many advantages in terms of compact size,low cost,wavelengthencoded,anti-electromagnetic interference,easy multiplexing and so on,it has attracted great interests in the field of sensing and been widely used in medical healthcare,pressure detection,battery safety condition and building structural health monitoring.FBG sensors demodulate parameters such as strain,vibration and temperature by detecting the changes of FBG reflection wavelength,with the aid of core technology of laser source and demodulation method.However,the current Tunable Distributed Feedback(DFB)lasers for optical communication applications often have narrow tuning range and low tuning speed,and require additional auxiliary wavelength reference when directly used for FBG sensor demodulation system,which makes the system complex and affects its practicability.Therefore,there is still no tunable laser source with simple structure and low cost that is specifically suitable for FBG sensing systems.In this paper,a wavelength correction-free FBG sensing system was proposed to identify the wavelength changes and demodulate the temperature changes of the FBG sensor without wavelength correction of the wavelength-swept DFB laser based on Reconstruction Equivalent Chirp(REC)technique.As one of the most significant wavelength tunable laser sources,the DFB semiconductor laser has the characteristics of high reliability,low noise,fast response,high output power,good repeatability,and simple tuning scheme.Compared with many other tunable lasers with different realization schemes,the fabrication process of DFB laser is the best choice for both cost and applicability.The DFB laser used in this paper was designed and fabricated using a special process method-REC technique.By sampling the seed grating with uniform period,the low-cost submicron holographic exposure technique of laser chip fabrication can achieve the same wavelength accuracy as the high-cost nanometer electron beam lithography technique while retaining its intrinsic characteristics.When a tunable DFB laser is selected as the laser source for an FBG temperature sensing system,the temperature information can be demodulated by reading the voltage signal of the optical signal reflected by the FBG through the Photodetector(PD).In the wavelength-swept-based FBG sensing demodulation system,the only data that can be obtained directly from the Oscilloscope(OSC)is the optical intensity of the FBG reflection spectra and the corresponding time in the sweeping cycle of the DFB laser.In order to accurately analyze the external environment changes measured by the FBG sensor from these data,it is necessary to calibrate the relationship between wavelength and time during the wavelength-swept process of DFB laser in advance.Firstly,the instantaneous wavelength is calibrated by the calibration method based on Mach-Zehnder Interferometer(MZI),and the relationship between wavelength and time is obtained.Then in the FBG temperature sensing system based on tunable laser,the FBG sensor is placed in the thermostatic bath with a constant temperature accuracy of 0.1℃.In the experiment,the tunable REC-DFB semiconductor laser was used as the wavelength-swept laser source.After the optical signal passed through the circulator,it acted on the FBG sensor for temperature sensing,and then the FBG reflected optical signal was received by the PD and recorded by the OSC.By using the characteristic that the reflected optical signal reaches maximum when the laser wavelengthcoincides with the reflection central wavelength of the measured FBG, and then using the relationshipbetween wavelength and time in the laser sweeping cycle that has been calibrated, the wavelengthcorresponding to the maximum output signal of the FBG demodulation system is extracted. Finally, bydetecting the wavelength offset of the reflected FBG, the measured changes of external temperature canbe linearly demodulated even when the laser wavelength changes are nonlinear.Experimental results show that the proposed wavelength-correction-free FBG sensing system canperform accurate temperature detection in the range of 200 ℃, and the coefficient of determination (R^(2)) oflinear fitting relationship between wavelength and temperature is 0.995 6. The linearity of the measuredwavelength and temperature is good, with the 1-R^(2) of only 0.004 4. The laser source of sensing systemcan be tuned up to 2.5 nm with 1 kHz sawtooth wave modulation, and no additional wavelength correctiondevice is required for the measurement process.