Supercontinuum fiber laser-based coherent anti-Stokes Raman scattering microscopy for label-free chemical imaging

Coherent anti-Stokes Raman scattering(CA RS)microscopy can resolve the chemical compo-nents and distribution of living biological systems in a label-firee manner and is favored in several disciplines.Current CA RS microscopes typically use bulky,high-performance solid-state lasers,which are expensive and sensitive to environmental changes.With their relatively low cost and environmental sensitivity,supercontinum fiber(SF)lasers with a small footprint have found increasing use in biomedical applications.Upon these features,in this paper,we homebuilt a low-cost CARS microscope based on a SF laser module(scCA RS microscope).This SF laser module is specially customized by adding a time synchronized seed source channel to the SF laser to form a dual-channel output laser.The performance of the scCARS microscope is evaluated with dimethyl sulfoxide,whose results confirm a spatial resolution of better than 500nm and a detection sensitivity of millimolar concentrations.The dual-color imaging capability is further demonstrated by imaging different species of mixed microspheres.We finally explore the potential of our scCARS microscope by mapping lipid droplets in different cancer cells and corneal stromal lenses.

A novel 2-μm pulsed fiber laser based on a supercontinuum source and its application to mid-infrared supercontinuum generation

A new method to achieve 2-μm pulsed fiber lasers based on a supercontinuum （SC） is demonstrated. The incident pump light is a pulsed SC which contains a pump light and a signal light at the same time. The initial signal of the seed laser is provided by the incident pump light and amplified in the cavity. Based on this, we obtain a 2-μm pulsed laser with pulse repetition rate of 50 kHz and pulse width of 2 ns from the Tm-doped fiber laser. This 2-μm pulsed laser is amplified by two stages of fiber amplifiers, then the amplified laser is used for mid-infrared （mid-IR） SC generation in a 10-m length of ZrF4-BaF2-LaF3-AIF3-NaF （ZBLAN） fiber. An all-fiber-integrated mid-IR SC with spectrum ranging from 1.8 ~tm to 4.3 μm is achieved. The maximal average output power of the mid-IR SC from the ZBLAN fiber is 1.24 W （average output power beyond 2.5 μm is 340 mW）, corresponding to an output efficiency of 6.6% with respect to the 790-nm pump power.

Supercontinuum generation of highly nonlinear fibers pumped by 1.57-μm laser soliton

Highly nonlinear fibers(HNLFs) are crucial components for supercontinuum(SC) generation with laser solution.However, it is difficult to exactly estimate the structure of produced SC according to material parameters. To give a guideline for choosing and using HNLFs for erbium-fiber-based optical applications, we demonstrate SC generation in five types of HNLFs pumped by 1.57-μm laser solitons. All five fibers output a SC exceeding 1000 nm. Three different SC formation processes were observed in the experiment. By comparing optical parameters of these fibers, we find the zero dispersion wavelength(ZDW) of fiber has an important influence on the SC structure and energy distribution for a given pump source.

Generation of a compact high-power high-efficiency normal-dispersion pumping supercontinuum in silica photonic crystal fiber pumped with a 1064-nm picosecond pulse

Broadband normal dispersion pumping supercontinuum （SC） generation in silica photonic crystal fiber （PCF） is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.

Effect of pulse width on near-infrared supercontinuum generation in nonlinear fiber amplifier

The effect of pulse width on near-infrared supercontinuum generation in nonlinear fiber amplifier is investigated theoretically and experimentally. The complex Ginzburg–Landau equation and adaptive split-step Fourier method are used to simulate the propagation of pulses with different pulse widths in the fiber amplifier, and the results show that a longer pulse is more profitable in near-infrared supercontinuum generation if the central wavelength of the input laser lies in the normal dispersion region of the gain fiber. A four-stage master oscillator power amplifier configuration is adopted and the output spectra under picosecond and nanosecond input pulses are compared with each other. The experimental results are in good accordance with the simulations which can provide some guidance for further optimization of the system.

Effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier

Theoretical and experimental research on the effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier is carded out. The complex Ginzburg-Landau equation is used to simulate the propagation of the pulse in the fiber amplifier and the results show that pulses with negative initial chirp produce the widest supercontinuum and pulses with positive initial chirp produce the narrowest supercontinuum when the central wavelength of the pump lies in the normal dispersion region of the gain fiber. A self-made line width narrowing system is utilized to control the initial chirp of the nanosecond pump pulse and a four-stage master oscillator power amplifier configuration is adopted to produce a high power near-infrared suppercontinuum. The experimental results are in good agreement with simulations which can provide some guidance on further optimization of the system in future work.

Monolithic all-fiber mid-infrared supercontinuum source based on a step-index two-mode As2S3 fiber

We demonstrate efficient supercontinuum generation extending into mid-infrared spectral range by pumping a twomode As2S3 fiber in the normal dispersion regime. The As2S3 fiber is fusion spliced to the pigtail of a near-infrared supercontinuum pump source with ultra-low splicing loss of 0.125 dB, which enables a monolithic all-fiber mid-infrared supercontinuum source. By two-mode excitation and mixed-mode cascaded stimulated Raman scattering, a supercontinuum spanning from 1.8 μm to 4.2 μm is obtained. Over 70% of the supercontinuum power is converted to wavelengths beyond2.4 μm. This is the first experimental report with respect to the multimode mid-infrared supercontinuum generation in a step-index two-mode chalcogenide fiber.

The 2-μm to 6-μm mid-infrared supercontinuum generation in cascaded ZBLAN and As2Se3 step-index fibers

Fiber-based mid-infrared(MIR)supercontinuum(SC)sources benefit from their spectral brightness and spatial coherence that are needed for many applications,such as spectroscopy and metrology.In this paper,an SC spanning from 2μm to 6μm is demonstrated in cascaded ZrF4-BaF2-LaF3-AlF3-NaF(ZBLAN)and As2Se3 step-index fibers.The pump source is a ZBLAN fiber-based MIR SC laser with abundant high-peak-power soliton pulses between 3000 nm and 4200 nm.By concatenating the ZBLAN fiber and the As2 Se3 fiber,efficient cascading red-shifts are obtained in the normal dispersion region of the As2 Se3 fiber.The spectral behavior of cascaded SC generation shows that the long-wavelength proportion of MIR SC generated in the ZBLAN fiber plays a critical role for further spectral extension in the As2 Se3 fiber.

Recent developments in high power near-infrared super-continuum generation based on photonic crystal fiber

High power supercontinuum generation has witnessed rapid developments during the past few years. The mecha- nism and the latest achievements in high power supercontinuum generation are reviewed both for the continuous wave pump regime and the pulsed pump regime. The challenges in scaling the average power of supereontinuum generation are analyzed. Some of our works on high power supercontinuum generation are summarized, and perspectives for the future development are discussed.

纳秒声光调Q光纤激光器产生超连续谱

根据现有生成超连续谱方案的不足,搭建了一台全光纤结构的纳秒声光调Q激光器。该激光器中心波长为1 064.3nm,重复频率为20kHz,脉冲宽度为250ns；经过一级放大得到的输出功率为13.02 W。将信号光耦合进自行研制的高功率模场适配器（MFA）,得到了10.7 W的信号光输出,耦合效率高达82.2%。将MFA与光子晶体光纤（PCF）熔接,得到了平均功率为5.43 W的超连续谱输出,光谱覆盖为900～1 700nm。由于实验采用的声光调制器（AOM）脉宽较宽,导致泵浦光峰值较低,非线性效应较弱,未能使超连续谱向可见光展宽。因此,建议采用较窄脉宽的AOM作为调Q元件来实现高峰值功率输出,以改善纳秒脉冲PCF产生的超连续谱特性。

70W全光纤超连续谱光源

超连续谱光源在很多领域具有广泛而重要的应用,过去40多年一直是国际研究热点之一。但一方面由于普通双包层光纤与光子晶体光纤模场不匹配会导致较高的熔接损耗和耦合损耗;另一方面受高质量超快光纤脉冲激光器输出平均功率的限制,目前超连续谱光源的最高输出平均功率只有50 W。报道了一种全光纤结构的超连续谱光源,输出平均功率为70 W。由于整个装置采用一种新的超连续谱形成机制,较好解决了普通双包层光纤与光子晶体光纤由于模场不匹配导致的较高熔接损耗和耦合损耗;降低了对脉冲泵浦源光谱质量的要求。

28W高功率超连续谱光纤激光光源

报道了一个全光纤结构的高功率超连续谱激光光源。利用自行搭建的环形腔掺镱脉冲光纤激光器作为种子源，采用三级MOPA功率放大，得到了平均功率为62 W，中心波长为1065 nm，3 dB谱宽15 nm，重复频率为118 MHz的皮秒锁模脉冲输出，将其耦合进零色散波长为1040 nm的光子晶体（PCF），最终得到平均功率为28 W，谱宽覆盖范围为600~1700 nm的超连续谱激光输出，超连续谱的光-光转换效率为45%。实验解决了高功率下大芯径掺杂光纤与PCF的耦合效率低的问题。

全光纤白光超连续谱实现563 W输出

受限于短脉冲泵浦源高峰值功率和高平均功率之间的矛盾,以及短脉冲泵浦源芯径与光子晶体光纤芯径的较大差异,现有的短脉冲白光超连续谱光源输出在200 W左右。基于短脉冲泵浦源工作频率和脉冲宽度的调节技术、高效的模场匹配技术等,实现了k W级全光纤短脉冲泵浦源的稳定产生,以及短脉冲泵浦源与光子晶体光纤纤芯之间的高效耦合,产生了平均输出功率为563 W的高功率超连续谱光源,为目前该领域公开报道的最高功率。

18.4W皮秒光纤激光器及其全光纤化超连续谱源

采用光纤非线性环形腔被动锁模方案,研制毫瓦级掺镒皮秒光纤激光器,对其进行3级主振荡功率放大(master oscillator power amplifier,MOPA),得到功率18.4 W,重复频率85 MHz,线宽5.7 nm的高质量皮秒激光输出.利用自主研发的模场适配器,实现了此高功率皮秒激光器对长度为50 m高非线性光子晶体光纤的高效全光纤化泵浦,研制了输出功率为3.6 W的全光纤化宽带超连续谱光源,其在1 700nm(500～2 200 nm)的带宽范围内具有10 dB的光谱平坦度.

飞秒脉冲作用下两种多孔微结构光纤超连续谱的产生

报道了利用20 fs激光脉冲在两种多孔微结构光纤产生超连续谱的现象,光纤长度均为40 cm,连续谱展宽范围分别是420-980 nm和570-920 nm。实验上证明了不仅仅具有周期性结构的微结构光纤可以用于产生超连续谱,由无序填充气孔组成的多孔微结构光纤也可以出现类似的超强非线性和超连续谱展宽。分析表明,小模场面积和高非线性系数更有利于产生宽的超连续谱,不完全光子带隙的存在导致了光谱的分立结构。

基于八字腔锁模Yb光纤激光器的超连续谱激光源

报道了一种利用八字腔锁模的全光纤化结构的亚纳秒脉冲光纤激光器,其输出脉冲经过光纤化的一级衍射声光调制器实现了可变的脉冲选单以降低脉冲重复频率.从2.78MHz降低重频至93kHz后的激光脉冲经过第二级功率放大,放大斜率效率为68.1%,获得脉冲峰值功率超过25kW的激光输出.以此激励一段经优化后3m长的Bi离子掺杂的石英光纤和20m普通单模石英光纤,最终获得了波长覆盖1100～2200nm、光谱平坦度优于3dB的稳定超连续谱激光光源输出.

中红外超连续谱在氟化物光纤中的产生

为获得高功率全光纤中红外超连续谱，采用自制的掺Er锁模光纤激光器作为种子源，激光经过两级放大至1．67W，泵浦氟化物光纤，获得光谱覆盖1000~2400nm的超连续光谱；光谱宽度随着泵浦功率的增加而展宽，当输出功率达到1．21W时，转换效率为72％，并且产生的超连续谱被强烈的调制，在多个波长点处谱功率密度调制到0mW／nm，但调制波长与泵浦功率无关。

连续波泵浦的高功率全光纤化超连续谱光源

以6个输出功率为25 W的976 nm半导体激光器作为泵浦源,通过光纤合束器搭建双包层掺镱(Yb)光纤放大器,对中心波长为1 071.5 nm的10 W连续波掺Yb光纤激光器种子源进行主振荡功率放大,实现高功率输出.通过包层光剥离器及与系统双包层尾纤匹配的模场适配器,将放大系统的大模场双包层光纤与小芯径单模光纤进行模场匹配耦合,进而实现高功率连续波激光的单模输出.通过梯度折射率光纤熔接技术,将模场适配器的输出光纤与一段长度为200 m的高非线性光子晶体光纤进行高效率耦合,实现高功率连续波光纤激光器对高非线性光子晶体光纤的全光纤化泵浦,最终研制最大输出功率为36.5 W的全光纤化超连续谱光源,光谱范围覆盖990～1 700 nm,20 dB光谱范围达620 nm.

声光调Q纳秒脉冲全光纤超连续谱光源

基于声光调Q技术的纳秒脉冲光纤激光超连续谱光源系统为全光纤结构,使用普通的传能光纤作为超连续谱的产生介质,通过改变泵浦功率和调制频率,可获得不同的超连续谱输出。实验结果表明:泵浦功率越大,调制频率越低,传能光纤越长,光谱展宽越宽。当传能光纤为200m、调制频率为1kHz、占空比为1.0%、泵浦功率为4.68W时,获得谱线宽度超过700nm的超连续谱输出。该系统可作为超连续谱光源的一种参考方案。

光子晶体光纤超连续谱光源

介绍该课题组近两年在光子晶体光纤超连续谱方面的主要研究成果,包括基于连续波泵浦研制全光纤化超连续谱源,利用级联一段高非线性正常色散光纤,通过光纤的受激拉曼散射效应实现超连续谱的平坦化;基于皮秒锁模光纤激光器实现全光纤化5 W输出超连续谱源;拉制一段145 m的锥形光子晶体光纤,利用自制的纳秒光纤激光器与锥形光子晶体光纤熔接,制备输出功率2.2 W的宽带超连续谱源;利用自制的网状光子晶体光纤和全固态光子带隙光纤,分别研究亚微米薄壁上偏振相关的超连续谱产生,以及基于四波混频效应产生的超连续谱.