利用反饱和吸收增强宽频噪声样脉冲

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"Using reverse saturable absorption to boost broadband noise-like pulses" 这篇研究论文深入探讨了如何利用光子晶体光纤中的反饱和吸收效应来增强宽带噪声样脉冲（Noise-Like Pulses，NLPs）。NLPs是一种具有广泛应用前景的光脉冲类型，尤其在光通信、光学参量振荡器以及量子信息处理等领域。本文的核心在于通过这种反饱和吸收机制，不仅能够拓宽脉冲的光谱范围，还能改善光谱的平坦度，从而生成超宽带的NLPs。首先，文章介绍了光子晶体光纤（Photonic Crystal Fiber, PCF）的基本原理。光子晶体光纤是一种特殊的光纤，其内部结构包含周期性排列的空气孔，这些孔洞对光的传播特性产生显著影响，使得PCF在非线性光学效应方面表现出优异的性能。在本研究中，PCF的非线性效应被用来产生并拓宽NLPs的光谱。接着，作者们通过数值模拟（Numerical simulations）展示了如何利用PCF中的反饱和吸收来优化NLPs的特性。反饱和吸收是一种非线性吸收现象，在高功率光脉冲作用下，材料的吸收会因光强增加而降低，这有助于拓宽脉冲的光谱并提高光谱的平坦度。根据文中提到，通过这种方式，他们成功地得到了具有290纳米3-dB带宽的宽带光谱，这是迄今为止所知的NLPs所能达到的最宽光谱带宽。然而，文章也指出，尽管取得了显著的成果，但NLPs的光谱退化问题依然存在。这些退化可能源于多种因素，包括光纤内的非线性效应、光纤的几何结构不均匀以及系统的稳定性等。作者对这些现象进行了观察和讨论，并可能提出了相应的解决方案或改进建议，以克服这些问题，进一步提升NLPs的性能。总结起来，这篇论文的贡献在于提出了一种利用反饱和吸收增强光子晶体光纤中宽带噪声样脉冲的方法，实现了前所未有的光谱带宽，并对由此产生的光谱退化进行了分析。这项工作对于理解非线性光学现象以及开发高性能的光纤激光器有着重要的理论和实际意义。同时，它也为未来进一步优化NLPs的性能提供了新的研究方向和思路。

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2020.2977272, Journal of

Lightwave Technology

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Abstract—Utilizing the nonlinear effects in a photonic crystal

fiber to broaden the spectrum and reverse saturable absorption

to further improve the flatness of the spectrum, ultra-broadband

noise-like pulses (NLPs) were produced in a fiber laser. Guided

by numerical results, a broadband spectrum with 290 nm 3-dB

bandwidth was obtained, which, to the best of our knowledge, is

the broadest bandwidth achieved for NLPs. Spectral

degradations of the NLPs have also been observed, and are

discussed.

Index Terms—Broadband spectrum, fiber laser, noise-like

pulse, reverse saturable absorption.

I. I

NTRODUCTION

B-DOPED fibers (YDFs) have potential for use over a

wide spectral range from 930 to 1200 nm. Because of

their wide wavelength gain response, high pump efficiency of

YDFs and their normal material dispersion at 1-μm

wavelength, YDF lasers are highly suitable for producing

ultrafast optical pulses with high power and a broadband

spectrum [1], [2]. Mode-locked YDF lasers have been shown

to produce 20-fs pulses with spectral bandwidths of up to 200

nm at the base of the spectrum [3]. Recently Li et al. showed

numerically that a single pulse with a 3-dB spectral width of

186.6 nm could be produced through nonlinear spectral

broadening in a segment of single-mode photonic crystal fiber

(PCF) and it was preferentially extracted before other

Manuscript received xx, xxxx; revised xx, xxxx; accepted xx, xxxx. This

work was supported by grants from the National Natural Science Foundation

of China (NSFC) (61605040, 11374089); Natural Science Foundation of

Hebei Province (NSFHP) (F2017205162, F2017205060, F2016205124);

Program for High-Level Talents of Colleges and Universities in Hebei

Province (PHLTCUHP) (BJ2017020).

The authors are with the College of Physics, Hebei Normal University,

Shijiazhuang 050024, China (e-mail: lixingliangkaoyan@163.com;

zhangsm@hebtu.edu.cn; jmliu121@163.com; zjyang@vip.163.com)

(Corresponding author: Shumin Zhang).

This paper has supplementary downloadable material available at

http://ieeexplore.ieee.org, provided by the authors. It contains two video files.

Media 1. mp4 shows the spectrum with the recorded 290 nm of 3-dB spectral

bandwidth, high-speed and low-speed oscillograph traces, radio frequency

spectrum of NLP at a high pump power from OC

. Media 2. mp4 shows the

spectrum with the recorded 253 nm of 3-dB spectral bandwidth, high-speed

and low-speed oscillograph traces, radio frequency spectrum of NLP at a high

pump power from OC

. The total file size is 6.81 MB.

Color versions of one or more of the figures in this paper are available

online at http://ieeexplore.ieee.org.

elements with bandwidth limitation in a YDF laser [4].

Efficient spectral broadening can be implemented by pulse

propagation in gas filled hollow core fibers [5]–[7]. However,

these broad spectra are always accompanied by multipeak

spectral modulation which is a result of interference of same

frequency waves with different phases.

In fiber lasers, flat and broad spectra can also be produced

via noise-like pulses (NLPs) [8]–[13]. The spectral domain of

the NLPs has a wide wavelength envelope that exhibits

extremely variable wavelength emission intensity. A

pseudo-stable, low-coherence, flat and broadband spectral

information of NLP is provided by optical spectrum analyzer

which is the superimposed recording of hundreds of thousands

NLP spectra [14], [15]. However, these typical features of

NLPs do not prevent it widely applied in optical metrology

[16], low-coherence spectral interferometry [17], [18], spectral

domain optical coherent tomography [19], [20], optical

sensing and measurement [21], [22], laser-induced breakdown

spectroscopy and ablation threshold analysis [23],

micromachining [24], and supercontinuum [25]–[28].

Thus, a series of studies on the generation of NLPs, on their

detailed nature and their broadband spectrum in YDF lasers

has been undertaken by various authors. Zaytsev et al.

reported the generation of NLPs with a spectral bandwidth of

48.2 nm from a dispersion-managed (DM) YDF laser [29].

Suzuki et al. extended the spectrum of the NLPs to 131 nm by

careful management of cavity dispersion resulting in near zero

dispersion [19]. Both of the above methods for increasing the

spectral bandwidth can be attributed primarily to enhanced

self-phase modulation as a result of the high peak power pulse

propagation and compression inside the DM cavity. Li et al.

also showed that by using the Raman effect, broadband NLPs

could be generated [30]. Chen et al. demonstrated that

cascaded Raman scattering could boost the width of the NLP

spectrum to 165 nm in an all-normal-dispersion YDF laser

[31]. Recently, Li et al. demonstrated implicitly that the

reverse saturable absorption (RSA) of nonlinear polarization

rotation (NPR) could both enhance the spectral flatness and

further broaden the 3-dB spectral bandwidth of NLP [32]. It is

well known that the use of highly nonlinear fibers can increase

the spectral width of pulse. Therefore, how to combine the

highly nonlinear effect such as stimulated Raman scattering

with the RSA, and how to produce ultra-broadband NLPs by

using the combination and designing a optimal cavity structure

in fiber lasers should be further investigated, which formed the

initial motivation for this work.

Using Reverse Saturable Absorption to Boost

Broadband Noise-Like Pulses

Xingliang Li, Shumin Zhang, Jingmin Liu, and Zhenjun Yang

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