All-solid microstructured optical fibers with all-normal dispersion used to generate ultraflat, broadband, and highly coherent supercontinuum
Left: Schematic diagram of the proposed fiber; Right: spectrum profiles of pump pulses at propagation distances of 0, 5 and 100 cm from the top to the bottom, respectively. The pump pulses are 200-fs duration and 100-kW peak power at 1550 nm.
Comprising various dispersive and nonlinear effects, supercontinuum (SC) generation in fibers is a very active field of research that provides a high-brightness broadband optical source for a wide set of applications, such as ultrashort pulse generation and optical frequency metrology. The high flatness and high coherence properties of SC are essential for some applications.
In general, SC generation in fibers has been extensively explored using pump pulses in the anomalous dispersion regime. Soliton splitting and modulation instability can lead to bad coherence and fine structures in spectra. SC generation in all-normal dispersion fibers is mainly dependent on self-phase modulation (SPM) and optical wave-breaking (OWB). Both the effects can maintain smooth phase distribution as well as the integrity of the pump pulses in the time domain. However, it is a challenge to generate ultraflat SC spectra with broad pump pulses.
A research team from Shanghai Institute of Optics and Fine Mechanics, CAS propose an all-solid microstructured fiber composed only of hexagonal glass elements. The optimized fiber possesses a low and flat all-normal dispersion profile, covering a wide wavelength interval of approximately 1.55 μm. An SC spectrum spanning from approximately 1030 to 2030 nm (corresponding to nearly one octave) with flatness < 3 dB is numerically generated in the fiber pumped by 200-fs pulses with 100 kW peak power at 1.55 μm. Moreover, the SC pulses feature high coherence and a single pulse in the time domain, which can be compressed to 13.9-fs pulses with high quality even for simple linear chirp compensation. Related results are published in Photonics Research, Volume 6, No. 6, 2018 (C. Huang et al. Ultraflat, broadband, and highly coherent supercontinuum generation in all-solid microstructured optical fibers with all-normal dispersion).
Compared with results of previous papers, the fiber allows much longer pump pulse duration to generate ultraflat, broadband, and highly coherent supercontinuum. Researchers from this team believe that this study is of great importance due to the width at −3 dB of the maximum spectral intensity almost corresponding to the full width at half-maximum spectral intensity, and the fiber as the nonlinear media can potentially be used in all-fiber, ultraflat and highly coherent SC systems. The further work will focus on the realization of all-fiber SC sources and ultrashort pulse generation.
左图：设计的光纤结构示意图；右图：从上到下依次是抽运脉冲在该光纤中传输时，传输长度分别是0，5和100 cm处的光谱。抽运脉冲的参数为：脉冲宽度200 fs，峰值功率 100 kW，中心波长1550 nm。
中国科学院上海光学精密机械研究所的研究团队提出了一种全固态微结构光纤。该光纤的结构中只含有正六边形元素，结构优化后的光纤在1.55 μm波长附近的波长范围内具有绝对值小且平坦的正常色散特性。通过数值模拟的方法，该课题组采用宽度为200 fs、峰值功率为100 kW、中心波长为1.55 μm的脉冲抽运该光纤，能够产生高平坦、高相干性的超连续谱。在-3 dB强度范围内，超连续谱覆盖波长1030~2030 nm，接近于一个倍频程。理论上，仅仅通过线性啁啾补偿能够将该超连续谱脉冲压缩至13.9 fs（约为2.69个光学周期）。相关成果发表在Photonics Research 2018年第6卷第6期上(C. Huang, et al. Ultraflat, broadband, and highly coherent supercontinuum generation in all-solid microstructured optical fibers with all-normal dispersion)。