Towards visible-wavelength passively mode-locked lasers in all-fibre format

锁模皮秒/飞秒光纤激光器具有小型化、光束质量好、稳定性佳、低成本且免维护等优点，然而，目前被动锁模光纤激光器工作波段仍主要局限在近红外1-2 μm光谱区域，在可见光波段（380-760 nm）却几乎未有进展。如何实现可见光被动锁模光纤激光器，直接产生小型化、低成本且高性能的可见光超快激光，是一直困扰超快激光研究领域的一个难题。罗正钱教授课题组通过数值求解金兹伯格-朗道方程，发现耗散孤子谐振机制利于可见光波段超大色散光纤腔被动锁模脉冲的稳定建立。基于数值模拟的结果，该研究成果是在可见光范围内向小型化超快光纤激光器迈出的重要一步。将为可见光超快光纤激光在精密光谱学、生物医学、显微成像、光通信、科学研究等领域的应用奠定基础，具有很好的研究潜力和应用价值。厦门大学为该论文的第一署名单位，电子科学与技术学院博士研究生邹金海为论文第一作者，罗正钱教授为论文通讯作者。Mode-locked fibre lasers (MLFLs) are fundamental building blocks of many photonic systems used in industrial, scientific and biomedical applications. To date, 1–2 μm MLFLs have been well developed; however, passively mode-locked fibre lasers in the visible region (380–760 nm) have never been reported. Here, we address this challenge by demonstrating an all-fibre visible-wavelength passively mode-locked picosecond laser at 635 nm. The 635 nm mode-locked laser with an all-fibre figure-eight cavity uses a Pr/Yb codoped ZBLAN fibre as the visible gain medium and a nonlinear amplifying loop mirror as the mode-locking element. First, we theoretically predict and analyse the formation and evolution of 635 nm mode-locked pulses in the dissipative soliton resonance (DSR) regime by solving the Ginzburg-Landau equation. Then, we experimentally demonstrate the stable generation of 635 nm DSR mode-locked pulses with a pulse duration as short as ~96 ps, a radio-frequency signal-to-noise ratio of 67 dB and a narrow spectral bandwidth of 1 nm) and modulated optical spectrum. This work represents an important step towards miniaturized ultrafast fibre lasers in the visible spectral region.This work was supported by the Major Research Plan of the National Natural Science Foundation of China (91750115), Equipment Pre-research Project of Equipment Development Department of Central Military Commission (61404140112), and Natural Science Foundation of Fujian Province for Distinguished Young Scientists (2017J06016).Prof. Zhengqian Luo acknowledges the Program for Young Top Notch Talents of Fujian Province and the Program for Nanqiang Young Top Notch Talents of Xiamen University.该研究受到国家自然科学基金、福建省杰出青年基金、福建省特支‘双百’青年拔尖人才项目以及厦门大学南强青年拔尖人才项目的支持

Theoretical and experimental investigation on backward-pumped Yb(3+)-doped double-clad fiber lasers

Based on the rate equations under the steady-state condition, strongly backward-pumped Yb(3+)-doped double-clad fiber (YDCF) lasers are analyzed numerically. The effects of laser cavity feedbacks and length of YDCF on the laser performance and the dependency of the output laser power on the pump power are investigated. Using a 975 nm laser diode (LD) as the pump source, a backward-pumped YDCF laser with a slope efficiency of 82% is demonstrated experimentally. The output laser power at the wavelength of 1088 nm is up to 85 W. The experimental results are in good agreement with those by numerical simulations

Switchable and tunable multiple-channel erbium-doped fiber laser using graphene-polymer nanocomposite and asymmetric two-stage fiber Sagnac loop filter

A high-performance multiple-channel erbium-doped fiber laser (EDFL) is proposed and experimentally demonstrated, using graphene-polymer nanocomposite as a multiwavelength equalizer and an asymmetric two-stage polarization-maintaining fiber (PMF) Sagnac loop as a flexible comb filter. At first, the filtering characteristics of the PMF Sagnac loop filter (SLF) are investigated. Both theoretical and experimental results show that it can provide a flexibly switchable and tunable comblike filtering. Then, the two-stage PMF SLF is inserted into a graphene-assisted EDFL cavity for generating multiwavelength oscillation. The extreme-high third-order optical nonlinearity of graphene is exploited to suppress the mode competition of the EDFL, and a stable multiple-channel lasing is observed. By carefully adjusting the polarization controllers in the two-stage PMF SLF, not only can the lasing-line number per channel be switchable between single and multiple wavelengths, but also the wavelength spacing in the triple-wavelength condition can be tunable. In the case of triple wavelengths per channel, up to 12 wavelengths with four channels stable oscillations can be achieved. The multiple-channel EDFL can keep a high extinction ratio of >40 dB and a narrow linewidth of <0.01 nm. (C) 2011 Optical Society of Americ

Chalcogenide Glass-on-Graphene Photonics

Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

Visible-wavelength pulsed lasers with low-dimensional saturable absorbers

The recent renaissance in pulsed lasers operating in the visible spectral region has been driven by their significant applications in a wide range of fields such as display technology, medicine, microscopy, material processing, and scientific research. Low-dimensional nanomaterials as saturable absorbers are exploited to create strong nonlinear saturable absorption for pulse generation at visible wavelengths due to their absorption peaks located in visible spectral region. Here we provide a detailed overview of visible-wavelength pulsed lasers based on low-dimensional nanomaterials, covering the optical properties and various integration strategies of these nanomaterials saturable absorbers, and their performance from solid-state as well as fiber pulsed lasers in the visible spectral range. This emerging application domain will undoubtedly lead to the rapid development of visible pulsed lasers

Gestational weight gain and adverse pregnancy outcomes: a prospective cohort study

Objective To assess the associations of gestational weight gain (GWG) in early and late pregnancy with subsequent risks of adverse pregnancy outcomes in Chinese women.Design Prospective cohort study.Setting Shanghai, China.Participants We studied 2630 nulliparous singleton pregnant women with complete data on weight gain in early (≤17 weeks of gestation) and late (&gt;17 weeks) pregnancy in the Shanghai Birth Cohort.Methods GWG was standardised into z-scores by gestational age and categorised as low (z-score &lt;−1), normal (−1 to +1) and high (&gt;1). The adjusted relative risks (aRRs) and 95%CIs were estimated through log-binomial regression models. Interaction effects between GWG and some other adjustment factors were tested, further stratified analyses were performed separately where interaction terms were significant.Outcome measures Adverse maternal and neonatal outcomes.Results Independent from GWG in late pregnancy, higher GWG in early pregnancy was associated with higher risks of gestational diabetes mellitus (aRR: 1.66; 95% CI: 1.11 to 2.48), caesarean section (aRR: 1.21; 95% CI: 1.05 to 1.39) and prolonged hospitalisation (aRR: 1.56; 95% CI: 1.03 to 2.38). Higher GWG in late pregnancy was independently associated with higher risks of caesarean section (aRR: 1.24; 95% CI: 1.09 to 1.41), large for gestational age (aRR: 2.01; 95% CI: 1.50 to 2.7) and macrosomia (aRR: 1.90; 95% CI: 1.30 to 2.78). In addition, the risk of gestational hypertension increased significantly with increased total GWG (aRR: 1.78; 95% CI: 1.14 to 2.76). The effects of GWG in late pregnancy on maternal and neonatal outcomes were significantly different between the women bearing a female and the women bearing male fetus.Conclusion The GWG associations with adverse pregnancy outcomes differ at early and late pregnancy, and there may be effect modification by fetal sex in the association of GWG in late pregnancy with some pregnancy outcomes

COVID-19-related stressors and depression in Chinese adolescents: The effects of life history strategies and gender.

PURPOSE: The COVID-19 pandemic has changed the way people live, affecting both their physical and mental health. Adolescents are vulnerable to the stress of the pandemic, and may experience indicators of psychological distress, such as depression. This study aimed to examine the impact of COVID-19-related stressors on depression and the mediating role of life history strategies. METHODS: A two-wave longitudinal study was conducted with 1123 adolescents (51.20% girls, Mage&nbsp;=&nbsp;14.30) recruited from three junior high schools in the Northeastern province of China. Adolescents life history strategies, depressive symptoms, and demographic variables were assessed at Time 1 (November 2019) and Time 2 (August 2020), and adolescents experience of COVID-19-related stressors was assessed at Time 2. None of participants was infected by COVID-19 virus. RESULTS: COVID-19-related stressors were positively associated with depressive symptoms at Time 2 (β&nbsp;=&nbsp;0.08, p&nbsp;&lt;&nbsp;0.01), after controlling for gender, age, SES and depressive symptoms at Time 1. And life history strategies partially mediated the relation of pandemic stress to depression (indirect effect&nbsp;=&nbsp;0.02, p&nbsp;&lt;&nbsp;0.05, 95% CI [0.004, 0.034]). There were no gender differences in the relations between stress on depression. LIMITATIONS: The sample was from a district where the pandemic was not very severe, which may limit generalizability of the results. CONCLUSIONS: This study revealed that COVID-19-related stressors may have a long-term impact on adolescents, increasing depression through speeding up their life history strategies. Interventions should focus on life history strategies, particularly cognitive style, among adolescents during and after the pandemic