可用来产生宽带、高平坦、高相干性超连续谱的正常色散全固态微结构光纤

光纤中产生的超连续谱包含多种色散效应和非线性现象,具有宽带、高亮度等特性,能够应用于许多领域,比如超短脉冲产生、光频率测量等。在有些应用中,高平坦度和相干性是十分重要的。

通常,当抽运脉冲波长处于光纤的反常色散区时可产生超连续谱。由于孤子分裂和调制不稳定等效应,产生的超连续谱相干性较差,光谱具有较多的精细结构。抽运脉冲波长处于光纤的正常色散区时,产生超连续谱主要依赖自相位调制和光波分裂效应,它们能够保持抽运脉冲的整体性和相干性。然而,在宽脉冲抽运条件下产生高平坦度的超连续谱是一个很大的挑战。

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)。

与已报道的光纤相比,该光纤能够满足更宽的脉冲抽运产生宽带高平坦高相干性超连续谱的需求。该团队的研究人员认为,光谱的-3 dB强度几乎对应于光谱的半高宽,这在应用中具有重要的意义;另外,采用该光纤作为非线性介质有望实现在宽脉冲抽运的情况下获得全光纤宽带高平坦高相干性超连续谱源。下一步的工作重点是实现全光纤的超连续谱源和超短脉冲压缩。

图注: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。

PR Cover Story:All-solid microstructured optical fibers with all-normal dispersion used to generate ultraflat, broadband, and highly coherent supercontinuum

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.

Graphic description: 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.