Q-switched mode-locked multimode fiber laser based on a graphene-deposited multimode microfiber

Fig. 1 The characteristics of the Q-switched mode-locked pulse. (a-b) The pulse-train; (c) the radio-frequency spectrum; (d) the beam profile.

Nowadays, the multimode fiber (MMF), which was overlooked for decades, is making a strong comeback since it not only has broad applications in fields of optical communications, imaging, metrology, spectroscopy, and three-dimensional nonlinear phenomena, but also is served as the medium for new high-energy pulse laser technology. The MMF has a large core diameter and therefore can withstand higher nonlinearity. Compared with the conventional single-mode fiber laser, the mode-locked MMF laser is able to improve the single-pulse energy by more than two orders of magnitude in theory, which brings new opportunities for high-energy laser technology and will be important for both fundamental research and practical applications.

At present, the investigation on MMF lasers is still in its infancy, mainly focusing on the generation of continuous mode-locked pulses. In fact, there are two other types of pulses in lasers, namely Q-switched and Q-switched mode-locked (QML) pulses. Compared to continuous mode-locked pulses, Q-switched pulses and QML pulses have the advantage of higher pulse energy. Based on this feature, the Q-switched or QML technology is expected to achieve the higher-energy pulse in the MMF lasers. In addition, since Q-switched and QML pulses have the advantages of tunable and low repetition-rate, they can alleviate the heat accumulation better in practical applications.

The research group led by Prof. Ai-Ping Luo from South China Normal University reported the generation of QML pulses in a MMF laser with an all-fiber structure. The results are published in Chinese Optics Letters, Volume 19, Issue 12, 2021, (J. Wu et al., Q-switched mode-locked multimode fiber laser based on a graphene-deposited multimode microfiber).

The characteristics of the Q-switched mode-locked pulse are shown in Figure 1. In this work, a self-made graphene-deposited multimode microfiber (GMM) device was utilized as a saturable absorber to generate multimode QML pulses. This device makes full use of the unique characteristics of the evanescent field of the multimode microfiber, which increases the interaction length between the light and the saturable absorption material, and thus leads to a higher damage threshold. In addition, the modulation depth and non-saturable loss of the device are large, which are favorable for the generation of QML pulses. Meanwhile, benefiting from the filtering characteristics of the device and the inherent multimode interference filtering effect in the laser cavity, tunable single-wavelength and dual-wavelength high-energy QML pulses are achieved in the laser. This flexible and tunable laser has potential applications in the fields of laser processing, optical sensing, and measurement.

The GMM device proposed in this work is a key component to generate high-energy QML pulses. It has advantages of the simple and compact structure, low cost, good saturable absorption characteristics and controllable nonlinearity, etc. In the MMF lasers, it can be employed to generate not only QML pulses, but also Q-switched pulses, continuous mode-locked pulses, and even supercontinuum.

In the future work, it is possible to engineer the nonlinearity by designing the structure of the multimode microfiber and the deposition amount of graphene. We could make full use of its nonlinear effect to further improve the MMF laser performance, such as the Kerr self-cleaning effect in the MMF, which can promote the generation of QML pulse with high beam quality. Meanwhile, owing to the broadband saturable absorption characteristics of the graphene, the GMM can be applied in the MMF lasers in other wavebands to generate high-energy QML pulses. Besides, the device will also be placed in the MMF lasers to generate spatiotemporal mode-locked pulses with high beam quality and manipulate its spatiotemporal properties.

新型调Q锁模多模光纤激光器

图 1调Q锁模脉冲特性图：（a-b）脉冲序列；（c）射频谱；（d）光斑。

近年来，多模光纤正在强势回归，因其不仅在光通信、成像、计量学、光谱学、三维非线性现象研究等领域有广泛的应用前景，而且可以作为新型高能量激光脉冲技术的介质。由于多模光纤具有较大的纤芯直径，能够承受更高的非线性，与单模光纤激光器相比，多模锁模光纤激光器在理论上能够将单脉冲能量提高至少两个量级，为高能量激光技术的发展带来了新的机遇，具有重要的研究意义和实际应用价值。

目前，对多模光纤脉冲激光器的研究还处于起步阶段，且主要集中在连续锁模脉冲上。但实际上，除了连续锁模脉冲以外，在激光器中还存在着另外两种类型的脉冲，即调Q脉冲和调Q锁模脉冲。相比于连续锁模脉冲，调Q脉冲和调Q锁模脉冲具有输出脉冲能量高的优点。因此，在多模光纤激光器中采用调Q或调Q锁模技术有望实现更高脉冲能量的输出。此外，由于调Q和调Q锁模脉冲具有重频可调谐且重频较低的特点，在实际应用中还能较好地缓解热积累问题。

近期，华南师范大学罗爱平研究员课题组采用自制的石墨烯包覆多模微纳光纤器件在多模光纤激光器中实现了调Q锁模脉冲的产生。相关成果发表在Chinese Optics Letters，2021年第19卷第12期上（J. Wu et al., Q-switched mode-locked multimode fiber laser based on a graphene-deposited multimode microfiber）。

该器件充分利用了多模微纳光纤倏逝场的特性，使得光与可饱和吸收材料具有更长的相互作用长度，从而具备较高的损伤阈值。此外，该器件的调制深度和非饱和损耗较大，有利于调Q锁模脉冲的产生。同时，得益于该器件的滤波特性以及激光腔中固有的多模干涉滤波效应，该激光器还实现了波长可调谐的单、双波长高能量调Q锁模脉冲输出，这种灵活可调的激光器在激光加工、光传感和测量等领域具有潜在的应用价值。图 1为激光器调Q锁模脉冲特性图。

该课题组提出的石墨烯包覆多模微纳光纤器件是产生高能量调Q锁模脉冲的关键器件，具有结构简单紧凑、制备成本低、良好的可饱和吸收特性和非线性可控等优点，在多模光纤激光器中不仅可以用来产生调Q锁模脉冲，也可以用于产生调Q脉冲和连续锁模脉冲，甚至可以用于超连续谱的产生。

未来，还可以通过设计多模微纳光纤的结构和石墨烯的沉积程度来实现对非线性的调控，充分利用其非线性效应来进一步提升多模光纤激光器的性能，例如：利用多模光纤中的克尔自清洁效应可获得高光束质量调Q锁模脉冲输出。同时，由于石墨烯具有宽带可饱和吸收特性，可应用到其它波段的多模光纤激光器中产生高能量调Q锁模脉冲。此外，该器件也能应用在多模光纤激光器中产生具有高光束质量的连续时空锁模脉冲并对其时空锁模特性进行调控。