Ultra-long-period fiber grating: a novel ultrafast optical device

Fig. 1 Preparation and characterization of ultra-long-period fiber grating: (a) Schematic diagram and photograph of samples. (b) Transmission spectrum and photograph of the sample.

Fig. 2 Experimental design and output characteristics of multi-wavelength ultrafast fiber laser: (a) Experimental setup, (b), (c) optical spectrum and pulse train of multi-wavelength ultrafast laser (taking three-wavelength as an example), (d) Output power and slope efficiency under different dispersion.

Ultrafast laser has the advantages of flexible design, short pulse width and high peak power, which has a very broad application prospect in the fields of micro-/nano-processing, biomedicine, national defense and military. In order to achieve ultrafast laser output, mode-locking technology is generally needed. To this end, researchers have proposed a variety of schemes. In 2003, Karen Intrachat and J. Nathan Kutz, Department of Applied Mathematics of University of Washington in USA, predicted theoretically that long-period fiber grating has pulse-shaping function, which can be used to construct ultrafast fiber lasers. In 2008, Abdullah S. Karar, Tom Smy and Alan L. Steele, department of electronics of Carlton University in Canada, conducted theoretical research on ultrafast fiber lasers with long-period fiber gratings. They found that this kind of laser can produce a variety of soliton pulses. However, there is a lack of experimental research on (ultra)-long-period fiber gratings as ultrafast optical devices by far.

Bo Guo's team from Harbin Engineering University found that the ultra-long-period fiber grating not only has passive mode-locking, but also has wavelength-filtering, which can realize the multi-wavelength ultra-fast fiber laser based on the ultra-long period fiber grating.The research results are published in Chinese Optics Letters, Vol. 19, No. 7, 2021 (Bo Guo et al., Ultra-long-period grating-based multi-wavelength ultrafast fiber laser).

In the experiment, the researchers used the fused taper method to fabricate the ultra-long -period fiber gratings (Fig. 1). Then, the researchers carried out the optical spectrum test and visible-light transmission experiment on the grating, and found that it has good transmission in the C + L band. Then, the nonlinear optical characteristics of the grating are studied, and it is found that the grating has a significant saturable absorption behavior.

Furthermore, the researchers introduced the grating into the ring cavity fiber laser and obtained three-, four-, five-, six- and seven-wavelength picosecond ultrafast pulses (Fig. 2); In addition, a novel hybrid pulse composed of four-wavelength pulses and conventional solitons is also observed. Finally, the influence of intracavity dispersion on multi-wavelength ultrafast laser, power characteristics and slope efficiency are discussed.

In this work, the passive mode-locking and wavelength-filtering characteristics of ultra-long-period fiber grating are found experimentally. Furthermore, by introducing it into the fiber laser, a variety of laser pulses are obtained. This kind of all fiber multi-wavelength ultrafast laser has the advantages of compact structure, easy integration, good beam quality and high transmission speed, which is expected to be applied in all-optical communication and microwave generation.

Through this work, the researchers believe that (ultra)-long-period fiber grating is an excellent all-fiber pulse-shaping device, which can be used in the construction of new ultrafast fiber lasers and the study of related soliton phenomenon, thus bringing new vitality to the development of ultrafast optics. In the future, researchers will further optimize the performance of this kind of ultrafast lasers and extend it to other types or waveband ultrafast lasers, such as dissipative soliton lasers, mid-infrared lasers and so on.

超长周期光纤光栅，为超快光学发展带来新活力

图1超长周期光纤光栅的制备和表征：（a）样品的示意图及照片；（b）样品的透射光谱及通光照片。

图2 多波长超快光纤激光器的实验设计及输出特性：(a) 实验装置；(b) , (c) 多波长超快激光的光谱及脉冲序列（以三波长为例）；(d) 不同色散时的输出功率和斜率效率。

超快激光器具有设计灵活、脉冲宽度短、峰值功率高等优点，在微纳加工、生物医疗和国防军事上有着十分广阔的应用前景，而实现超快激光输出一般需要锁模技术，为此，研究人员提出了多种方案。

2003年，美国华盛顿大学应用数学系Karen Intrachat和J. Nathan Kutz在理论上预言长周期光纤光栅具有脉冲整形功能，可用于构建超快光纤激光器。

2008年，加拿大卡尔顿大学电子系Abdullah S. Karar、Tom Smy和Alan L. Steele对含长周期光纤光栅的光纤激光器进行了理论研究。他们发现，这类激光器能够产生丰富的孤子脉冲，但关于（超）长周期光纤光栅作为超快光学器件的实验研究至今仍较匮乏。

哈尔滨工程大学郭波团队在实验中发现，超长周期光纤光栅不仅有被动锁模功能，还有滤波功能，进而实现了基于超长周期光纤光栅的多波长超快光纤激光器。这一研究成果发表在Chinese Optics Letters 2021年第19卷第7期上（Bo Guo et al., Ultra-long-period grating-based multi-wavelength ultrafast fiber laser）。

研究人员首先采用熔融拉锥法对单模光纤进行级联拉锥来制备超长周期光纤光栅（图1）。随后，对该光栅进行了光谱测试，发现它在C+L波段有着良好透射。同时，对该光栅的非线性光学特征的研究中发现其具有显著的饱和吸收行为。

基于上述结果，研究人员将该光栅引入环形腔光纤激光器中，获得了三、四、五、六、七波长皮秒量级的超快脉冲（图2）。同时，还观察到由四波长脉冲和传统孤子组成的新颖混合脉冲。此外，研究人员还讨论了腔内色散对多波长超快激光的影响、激光器的功率特性及斜率效率等。

该工作不仅发现了超长周期光纤光栅被动锁模和滤波两种特性，还发现将其引入光纤激光器中，能够获得多种激光脉冲。这种全光纤的多波长超快激光器具有结构紧凑、易于集成、光束质量好、传输速度高等优点，有望在全光通信、微波产生领域得到应用。

研究人员认为，（超）长周期光纤光栅是一种十分优良的全光纤脉冲整形器件，可用于新型超快光纤激光器的构建及相关孤子现象研究，为超快光学的发展带来新的活力。在未来工作中，研究人员将进一步优化这类超快激光器的性能指标，并将其拓展到其他类型或波段超快激光器中，比如，耗散孤子激光器、中红外激光器等。