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.
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。
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）。