Olivier Michel, Gagnon Marc-Daniel, Habel Joé
Centre d'optique, photonique et laser, Université Laval; Département de physique, Cégep Garneau;
Centre d'optique, photonique et laser, Université Laval.
J Vis Exp. 2016 Feb 28(108):53679. doi: 10.3791/53679.
When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.
当激光器被锁模时，它会以由激光腔长度决定的重复频率发射一系列超短脉冲。本文概述了一种全新且成本低廉的方法，用于在预调谐的非线性偏振旋转光纤激光器中实现锁模。该方法基于检测锁模发生时输出偏振态的突然变化。此变化用于控制腔内偏振控制器的对准，以找到锁模条件。更具体地说，当偏振控制器的角度被扫描时，第一斯托克斯参数的值会发生变化，而且当激光器进入锁模状态时，它会经历一个突然的变化。监测这种突然变化会提供一个实际且易于检测的信号，该信号可用于控制偏振控制器的对准，并驱使激光器进入锁模状态。这种监测是通过将一小部分信号馈入测量第一斯托克斯参数的偏振分析仪来实现的。当激光器进入锁模状态时，分析仪读出的该参数会发生突然变化。此时，将偏振控制器所需的角度保持固定。对准即完成。该方法为现有的自动化方法提供了一种替代方案，现有方法使用诸如光谱分析仪、射频频谱分析仪、连接到电子脉冲计数器的光电二极管或基于双光子吸收或二次谐波产生的非线性检测方案等设备。它适用于通过非线性偏振旋转锁模的激光器。它相对易于实现，所需成本低廉，尤其是在1550纳米波长时，并且与上述技术相比，它降低了生产和运营成本。
