陶小凡 -
单环掺铒光纤激光器的延时反馈—注入混沌同步
2007-03-06
确定波长间隔的多波长掺铒光纤激光器能应用于密集波分复用(DWDM)的光纤通信系统、光学传感器以及光学测量仪器中，近年来引起了众多学者极大的兴趣〔1～3〕. 然而，在光抽运下，室温下的掺铒光纤属均匀加宽的增益介质，在谐振腔内存在着强烈的模式竞争，因而激光振荡极不稳定，要实现多个波长的同时激射具有很大的难度. 众多研究者提出了不同的方法来实现掺铒光纤激光器多波长的同时振荡. 如对掺铒光纤进行液氮冷却，以降低均匀加宽的线宽，减少模式竞争〔4〕. 然而低温工作的掺铒光纤激光器不能满足实用的要求. Cowle等人〔5〕利用线性的掺铒光纤增益和非线性的Brillouin增益，实现了多波长振荡. Lim等人〔6〕将四波混频(FWM)与受激Brillouin散射(SBS)效应相结合，获得了34个波长的输出，然而两者都需要外加可调谐的DFB激光器抽运，使用成本较高. 本文提出一种结构新颖且简单的类似于F-P腔的掺铒光纤激光器，并采用多量子阱光波导(MQW)作为等效反射腔镜. 由于MQW结构的固有的各向异性，光波导的参数将随外界的注入光的偏振态而变化〔7〕，表现为高的非线性双折射特性，因而有助于增强腔内光的偏振非均匀性. 应用腔内的偏振烧孔效应，减少室温下掺铒光纤的均匀加宽的线宽，实现了稳定的腔内梳状滤波器，使多个波长同时振荡.
1 实验装置
图1所示是多波长振荡掺铒光纤激光器的结构示意图. 多量子阱光波导和由光纤耦合器、单模光纤以及光纤偏振控制器构成的Sagnac光纤环形镜组成了掺铒光纤激光器的两反射腔镜. MQW光波导对1 550 nm的光是透明的，它的后解理面镀有增反膜，反射率可达95%. 前解理面镀有部分反射膜. 在光纤激光器的腔内和光纤环形镜内放置有偏振控制器.掺铒光纤的长度为10 m，其芯径为5μm，数值孔径为0.18，截止波长约为945 nm.掺铒光纤由最大输出功率为60 mW的980 nm的半导体激光器通过980/1 550 nm的波分复用(WDM)光纤耦合器对其进行抽运.针对激射波长，WDM耦合器的2， 3端口间的耦合效率达90%. 光纤环形镜中单模光纤(SMF)的长度为1 133 m. MQW光波导是经过精心设计的带尾纤的MQW激光器件，在实验中它始终被偏置在阈值以下. 激光器输出的光谱由分辨率为0.2 nm的光谱分析仪作监测和分析.
图1 多波长振荡掺铒光纤激光器的结构示意图
2 实验结果及其讨论
当MQW光波导上不加偏置电流时，掺铒光纤激光器作不稳定的自由振荡，而且输出的光波长的位置随腔内光的偏振控制器的状态而异. 图2所示是当MQW光波导上的偏置电流在0～15 mA的范围内变化时，观察到激光器的单纵模调谐特性. 调谐范围被限制在一个纵模间隔内. 这表明，通过改变MQW光波导上的偏置电流，可实现激光器的可调谐. 这是由于MQW光波导的折射系数随着注入光波导的载流子的变化而变化所形成的. 由外加电流间接地引起折射系数的变化，一方面，造成激光场在腔内的往返振荡的光程变化；另一方面, 使得MQW光波导的等效反射系数发生变化，从而导致可调谐特性的产生. 实验中我们还发现，偏置电流在较低的范围内(<12 mA)变化时，激光器产生两纵模振荡，而且纵模间隔也随偏置电流而变化(如图3所示). 逐渐增大MQW上的偏置电流以及调节激光器中两偏振控制器的状态，观察到了激光振荡纵模数目及其波长位置的变化. 当偏置电流为22.8 mA，注入抽运功率为50 mW时，改变偏振控制器的状态，使入射至MQW光波导的光的偏振方向与量子阱层分别成90°, 0°和45°，激光器输出的光谱分别对应于图4(a)～(c). 在上述不同偏振方向的光入射至光波导的情况下，将在光波导内分别激励TM模、TE模或TM模和TE模共同传输. 通过调节偏振控制器，获得的线偏振光的偏振方向与波长是相关的，因而在掺铒光纤的光谱上产生了不同位置的偏振烧孔. 最终我们成功地获得了在1 559～1 569 nm波长范围内，间隔为0.9 nm的10个波长的输出.对多波长振荡(图4(c))的长时间实验监测表明，波长的间隔具有很好的稳定性，但各个纵模的光功率存在着差异. 一方面是掺铒光纤的增益不平坦；另一方面是谐振腔内建立的梳状滤波器的光功率传递函数对激射波长的不均衡. 而且模式竞争不能完全被抑制. 选择不同参数的MQW光波导作为等效反射腔镜，可改变多波长振荡的纵模间隔及数目(如图5所示).
Determine the wavelength spacing multi-wavelength erbium-doped fiber lasers can be used in dense wavelength division multiplexing (DWDM) fiber-optic communication systems, optical sensors and optical measurement instruments, many scholars in recent years has aroused great interest〕 〔1 ~ 3. However, in the optical pumping, the room temperature of erbium-doped fiber is a homogeneous broadening of the gain medium, in the resonant cavity mode of the existence of strong competition, and therefore very unstable laser oscillation, it is necessary to achieve the multiple-wavelength lasing at the same time with very difficult. many researchers proposed different ways to achieve the erbium-doped fiber laser multi-wavelength oscillation at the same time. such as erbium-doped fiber for liquid nitrogen cooling to reduce the uniform widening of the linewidth to reduce mode competition 〔4〕 . However, the work of low-temperature erbium-doped fiber lasers can not meet the practical requirements. Cowle et al 〔5〕 using erbium-doped fiber linear and nonlinear gain of the Brillouin gain, achieved multi-wavelength oscillation. Lim, who will be 〔6〕 four-wave mixing (FWM) and stimulated Brillouin scattering (SBS) effect a combination of access to the 34-wavelength output, but both the need for additional tuning of the DFB laser can be pumped, the use of higher cost. In this paper, a novel and simple structure similar to FP cavity erbium-doped fiber lasers, and optical waveguide multi-quantum well (MQW) as the equivalent reflectivity mirrors. MQW structure as a result of the inherent anisotropy, optical waveguide with the parameters to the outside world into the light polarization change 〔7〕, high performance characteristics of the non-linear birefringence, thereby helping to enhance the cavity polarization of light non-uniformity. the application of cavity polarization hole burning effect, reducing the room temperature, erbium-doped optical linewidth of the homogeneous broadening, the realization of a stable cavity comb filter, so that multiple-wavelength oscillation at the same time.
1 Experimental setup
Shown in Figure 1 is a multi-wavelength erbium-doped fiber laser oscillation of the structure diagram. MQW optical waveguide and the optical fiber coupler, single mode fiber and polarization controller consisting of fiber-optic Sagnac fiber loop mirror composed of erbium-doped fiber laser has two reflective mirrors. MQW optical waveguide of the 1 550 nm light and transparent, and its cleavage surface after the plating to increase the anti-film, 95% reflectivity. before the cleavage of some surface-plated reflective film. in Fiber Lasers fiber ring cavity and mirrors are polarization controller placed. erbium-doped fiber length of 10 m, its core diameter is 5μm, numerical aperture of 0.18, cutoff wavelength of about 945 nm. erbium-doped fiber by the maximum output power of 60 mW the 980 nm semiconductor laser through the 980 / 1 550 nm of wavelength division multiplexing (WDM) optical fiber coupler pumping it. for the lasing wavelength, WDM coupler 2, 3-port between the 90% coupling efficiency. fiber loop mirror in the single-mode fiber (SMF) of length of 1 133 m. MQW waveguide is designed with a pigtail of the MQW laser devices, in experiments it has always been bias in the following threshold. laser output spectrum a resolution of 0.2 nm from the optical spectrum analyzer for monitoring and analysis.
Figure 1 Multi-wavelength erbium-doped fiber laser oscillation of the structure diagram
2 Experimental results and discussion
When the MQW waveguide without the bias current, the erbium-doped fiber laser for the instability of the free oscillation, and output the location of the optical wavelength of light with the cavity of the state of polarization controller to another. As shown in Figure 2 when MQW optical waveguide on the bias current 0 ~ 15 mA in the framework of changes, observed in the single longitudinal mode laser tuning characteristics. tuning range is limited to a longitudinal mode interval. This shows that by changing the bias on the MQW waveguide Current home to the realization of a tunable laser. This is because the refractive index of MQW optical waveguide into the waveguide with the carrier changes formed. plus current indirectly by causing changes in refractive index, on the one hand, caused by the laser field oscillation in the cavity between the optical path change; On the other hand, makes the equivalent waveguide MQW reflection coefficient changes, resulting in the emergence of tunable properties. experiment we also found that at a relatively low bias current the extent (<12 mA) changes, have a two-longitudinal-mode laser oscillation, and the longitudinal mode spacing also varies with the bias current (as shown in Figure 3). MQW on increasing the laser bias current, as well as regulation of two polarization controllers, and observed that the longitudinal mode of laser oscillation wavelength of the number and position changes. When the bias current of 22.8 mA, pumped into the power of 50 mW, the change in the state of polarization controller so that the incident light to the MQW guide the direction of light polarization and quantum-well layer, respectively, as 90 °, 0 ° and 45 °, the spectra of laser output, respectively, corresponding to Figure 4 (a) ~ (c). in the light of different polarization direction of the incident to the waveguide cases, respectively in the optical waveguide TM mode excitation, TE mode or TM mode and TE mode transmission together. by adjusting the polarization controller, the line access to the polarization direction of polarized light with the wavelength are related, so in the erbium-doped fiber produce spectra at different positions of the polarization hole burning. eventually, we succeeded in 1 559 ~ 1 569 nm wavelength range, the interval of 0.9 nm output wavelength 10. of multi-wavelength oscillations (Figure 4 (c)) Experimental monitoring of the long period of time shows that the wavelength interval with a good stability, but each longitudinal mode of the existence of differences in optical power. On the one hand, the gain of erbium-doped fiber uneven; resonant cavity on the other hand, the establishment of the comb-like filter transfer function of the optical power of the lasing wavelength is not balanced. and mode of competition can not be completely suppressed. choice of different parameters as the equivalent of the MQW waveguide mirror reflection can change the multi-wavelength oscillation of the longitudinal mode spacing and number (such as as shown in Figure 5).