采用光纤光栅级联结构实现光纤延迟线

在使用光纤光栅实现皮秒级别时延的基础上，提出一种光纤光栅与单模光纤相结合的微秒级别级联结构，该结构可以实现中心波长1 550~1 553 nm范围内，间距为1 nm的窄波长反射型时延线，共1，1.5，2和2.5 μs四种不同的时延。将单波长反射的啁啾布拉格光纤光栅与103 m单模光纤连接构成延迟单元，再利用光环形器将4个延迟单元级联并使用内半径为3 cm的光纤绕线盘，将四种延时单元的传输光纤进行整合。借助光纤光栅的反射镜作用，控制不同波长光信号通过不同的传输距离，从而达到时延目的。本文通过对啁啾布拉格光纤光栅的反射谱进行仿真分析，发现相邻反射谱的旁瓣会出现交叠现象，因此使用六个切趾函数对旁瓣滤除。结果显示：不同切趾函数的滤除效果也不同，能够完全滤除旁瓣并且对反射谱包络影响最小的是柯西切趾函数，经柯西切趾后能使不同波长光信号在对应中心波长1 nm范围内反射率达到1，而其他位置均为0。由于使用光纤绕线盘整合延迟单元传输光纤会产生一定损耗，因此对弯曲损耗进行仿真分析，结果表明：弯曲半径相同时，损耗与工作波长成正比；工作波长相同时，弯曲损耗与弯曲半径成反比。当弯曲半径大于2.9 cm时，弯曲损耗曲线变化平缓并趋于0，因此当光纤绕线盘内半径为3 cm时保证了在减小延迟模块体积的同时又不会有过大的损耗。通过TDS784D型示波器对频率为2 000 Hz的信号经不同传输距离后的波形进行测试，结果显示经3 m和5 km传输线后信号的各项参数基本保持不变，经过长距离传输后，依然能保持原信号特性，因此使用103 m传输线可达到延迟目的。使用W-GGL型光功率计对不同频率下的输出功率进行测量，与直光纤的输出功率相比，当弯曲半径为2~3 cm时偏差较大，等于3 cm时偏差为0.18 dBm，大于3 cm时则无限趋近，因此设置绕线盘内半径为3 cm符合光纤延迟线的损耗范围。

Using Fiber Grating Cascade Structure to Realize Fiber Delay Line

Based on fiber grating to achieve picosecond-level delay, a microsecond-level cascade structure combining fiber grating and single-mode fiber is proposed. This structure can achieve a narrow wavelength with a center wavelength of 1 550~1 553 nm and a spacing of 1 nm. The reflective delay line has four different delays: 1, 1.5, 2 and 2.5 μs. The single-wavelength-reflected chirped Bragg fiber grating is connected with a 103 m single-mode fiber to form a delay unit, and then an optical circulator cascades the four delay units and uses a fiber reel with an inner radius of 3 cm to integrate the transmission fibers of the four delay units. With the help of the mirror function of the fiber grating, the optical signals of different wavelengths are controlled to pass through different transmission distances to achieve the purpose of corresponding time delay. In this article, through the simulation analysis of the reflection spectrum of the chirped fiber Bragg grating, it can be found that the side lobes of the adjacent reflection spectrum will overlap. Therefore, six apodization functions are used to filter the side lobes. The results show that: the apodization function has different filtering effects on the side lobes of the reflection spectrum. The Cauchy apodization function can filter the side lobes and has the least impact on the reflection spectrum envelope. After Cauchy apodization, the reflectance of optical signals of different wavelengths can reach 1 in the range of the corresponding center wavelength of 1 nm, and the reflectance in other ranges is 0. Because the use of fiber reel to integrate the delay unit transmission fiber will produce a certain loss, the bending loss is simulated and analyzed. The results show that when the bending radius is the same, the loss is proportional to the working wavelength; when the working wavelength is the same, the bending loss is inversely proportional to the bending radius. When the bending radius is greater than 2.9 cm, the bending loss curve changes smoothly and tends to zero. Therefore, when the inner radius of the optical fiber winding reel is 3 cm, it is ensured that the volume is reduced without excessive loss. The waveforms of signals with a frequency of 2 000 Hz after different transmission distances are tested by a TDS784D oscilloscope. The results show that the signal parameters remain unchanged after 3 m and 5 km transmission lines. After long-distance transmission, the original signal characteristics can still be maintained. Therefore, the use of a 103 m transmission line can achieve the delay. The W-GGL optical power meter measured the output power at different frequencies. Compared with the output power of straight fiber, when the bending radius is 2~3 cm, the deviation is large, when the bending radius is equal to 3 cm, the deviation is 0.18 dBm, and when the bending radius is greater than 3 cm, it will approach infinitely. Therefore, the inner radius of the winding reel is set to 3 cm conforming to the loss range of the optical fiber delay line.