10 W量级高功率中红外超快光纤激光系统中色散管理的仿真设计

超快光纤激光器具有结构紧凑、可靠性高和光束质量好等优点,在科学研究和工业生产上有广泛的应用。2～5μm波段的中红外超快光纤激光器在气体探测、激光手术与中红外对抗中具有巨大的应用潜力,已成为超快光纤激光器领域的一个研究热点,尤其是利用掺杂铒离子的氟化物光纤作为增益光纤的光纤激光器,其可利用常见的980 nm泵浦激光产生2.8μm波段的超快激光,是研究最为广泛的中红外超快光纤激光器系统之一。然而,2.8μm波段的超快光纤激光器无论是在平均功率还是在单脉冲能量上,都与国际先进的近红外波段超快光纤激光器存在较大差距。前期报道的2.8μm超快光纤激光器输出的最高平均功率约为1 W,单脉冲能量约为30 nJ,这极大地限制了中红外超快光纤激光在高灵敏度气体测量等领域的应用。针对这一问题,本文设计了一套基于掺杂铒离子氟化物光纤的多级啁啾脉冲放大系统,并对其进行了数值模拟,此系统可将脉冲平均功率放大到10 W量级,从而获得超过250 nJ的单脉冲能量。此系统输出的高能量中红外脉冲具有约400 fs的超宽脉冲宽度,脉冲峰值功率可达450 kW。     

迅速发展的光纤激光源

本文介绍了以稀土离子掺杂光纤为基础的大功率光纤激光器和基于固有非线性及光纤色散锁模的高峰值功率（高脉冲能量）的光纤系统以及基于频率变换技术、光纤光源倍频技术的红外或可见光大功率光纤光源系统的最新进展。     

中红外锁模氟化物光纤激光器研究进展

中红外超短脉冲激光是国际研究热点,它在激光微创治疗、聚合物精细加工、高次谐波产生、强场激光物理、超快分子成像等领域具有重要的应用前景,而锁模是产生超短脉冲的重要技术手段。本文围绕氟化物光纤激光器,从稀土离子中红外激光激射过程出发,对该波段目前常用的三种锁模方式(包括材料可饱和吸收、非线性偏振旋转、频移反馈)的工作机理、发展现状以及存在问题进行了介绍、分析与总结,并对中红外锁模光纤激光器的发展趋势进行了展望。     

飞秒激光作用蓝宝石光纤产生超连续谱的研究

对飞秒激光作用蓝宝石光纤产生超连续谱进行了实验研究。选用脉冲宽度为50 fs的飞秒激光器作为泵浦光,9 cm蓝宝石光纤为非线性介质;当激光器中心波长分别为1200 nm、1300 nm、1400 nm、1500 nm时,在蓝宝石光纤输出端均可获得红外和可见波段的超连续光源;为了寻求可见波段更好的超连续光源输出,在实验装置中加入半导体可饱和吸收体（SESAM）,利用光纤端面和SASEM构成谐振腔,通过SESAM的反馈实现多次作用光纤。结果表明,在可见波段内能够获得460~780 nm平坦稳定的超连续光源输出。     

高功率全光纤中红外超连续谱激光源

报道了一个高功率全光纤结构的中红外超连续谱激光源,该光源由1.55μm纳秒脉冲掺铒光纤激光器、包层抽运掺铥光纤放大器以及单模ZBLAN光纤组成。首先利用单模光纤将1.55μm纳秒脉冲激光频移至2.0μm波段,然后利用掺铥光纤放大器对其进行功率放大,最后利用ZBLAN光纤使掺铥光纤放大器输出的光谱进一步向中红外长波长方向扩展。当掺铥光纤放大器输出功率为3.95W时,ZBLAN光纤产生了2.2W的中红外超连续谱激光输出,相应的光谱范围为1.9~3.75μm,10dB光谱带宽大于1600nm。此外,通过增加掺铥光纤放大器的平均输出功率,中红外超连续谱的输出功率得到了进一步提高,当耦合进单模ZBLAN光纤的平均功率为21W时,中红外超连续谱的平均输出功率达到了16.2W,相应的光谱范围为1.9~3.5μm。     

同步脉冲诱导的高功率中红外差频产生技术（特邀）

提出并实验探究了基于同步脉冲诱导的中红外差频产生技术,利用高速光电探测器将泵浦光脉冲转换为超短电信号,使其驱动宽带的幅度调制器,作用于可调谐连续激光器上,从而实现双色脉冲的稳定时域同步。利用了同步脉冲诱导的非线性差频过程,有效降低了光参量下转换的泵浦阈值,能够获得瓦量级的中红外超短脉冲输出,最大泵浦光转换效率达60%,且中心波长在3000~3175 nm范围内可调谐。得益于全保偏光纤架构,平均功率的不稳定度(STD/MEAN)在1 h内低至0.07%,展现了优异的长期稳定性。此外,该方案利用光-电-光高速调制实现高精度脉冲同步,免除了复杂的反馈电路,具有结构简单、即插即用、鲁棒性强的特点,为拓展中红外光源在野外的应用奠定了基础。     

用于多光子显微镜超短脉冲激光器的发展动态

介绍基于飞秒脉冲产生机理用于多光子显微镜的先进激光技术。讨论产生多色可调谐光脉冲的几种方法。集成超快光纤激光器被认为是显微技术中的第二代飞秒激光器。利用纤芯和色层中的折射率分布实现光传输控制等先进技术被认为是第三代显微学中的激光技术。光子等的光纤结构可在1m长的光纤中产生超连续光谱。     

多光子显微镜用超短脉冲激光器动态

介绍基于飞秒脉冲产生机理的多光子显微镜的先进激光技术。讨论产生多色可调谐光脉冲的几种方法。集成超快光纤激光器被认为是显微技术中的第二代飞秒激光器。利用纤芯和包层中的折射率分布实现光传输控制等先进技术被认为是第三代显微学中的激光技术。光子等的光纤结构可在１ｍ长的光纤中产生超连续光谱。     

超快光纤激光器提供灵活解决方案

利用超快光纤激光器产生皮秒或飞秒光脉冲是当今世界最活跃的研究领域之一.尽管人们已经成功研制出了利用超快锁模激光器产生皮秒和飞秒光脉冲的技术,但这项技术仍局限于实验室和高端应用.     

基于差频技术的宽谱中红外飞秒激光的产生

飞秒差频产生器(DFG)是一种获得宽谱中红外激光的有力工具。为了利用DFG产生更高瞬时带宽的中红外激光,可以使用窄带泵浦光、宽带信号光结合大信号光相位匹配带宽的非线性晶体或使用宽带泵浦光、窄带信号光结合大泵浦接受带宽的非线性晶体。研究表明,对于PPLN晶体,当泵浦光波长为1050 nm,闲频光波长在3.4μm附近时,非线性晶体具有较大的泵浦接受带宽,仅使用均匀极化周期PPLN晶体即可获得宽谱中红外激光。基于高重复频率的掺镱光纤激光放大器系统,通过引入自相位调制效应,获得了中心波长为1050 nm的宽谱光源,将其作为DFG系统的泵浦源。利用飞秒脉冲在负色散光子晶体光纤中的拉曼效应,产生了中心波长为1525 nm的超短脉冲,将其作为DFG系统的信号源。在长度分别为1 mm和3 mm的PPLN晶体中,都获得了宽谱中红外闲频光输出,其-10 dB光谱覆盖范围分别为2.72～4.15μm和2.87～4.08μm。     

Supercontinuum generation in non-silica fibers

The development of super continuum sources is driven by the requirements of a wide range of emerging applications. This paper points out how non-silica fibers are of benefit not only because their broad mid-IR transparency enables continuum generation in the 2–5 μm region but also since the high intrinsic nonlinearity of the glasses reduces the power-threshold for devices at wavelengths below 2 μm. For these glasses, the material zero-dispersion wavelength is typically shifted to long wavelengths compared to silica so dispersion tailoring is key to creating sources based on practical, near-IR, solid state pump lasers. We show how modeling work has produced fiber designs that provide flattened dispersion profiles with high nonlinearity coefficients and zero-dispersion wavelengths in the near-IR. Building on this flexibility, modeling of the pulse dynamics then demonstrates how coherent mid-IR supercontinuum sources could be developed. We also show the importance of the second zero-dispersion wavelength using bismuth fibers as an example. Nonlinear mode-coupling is shown to be a factor in larger core fibers for high-power applications. Demonstrations of supercontinuum in microstructured tellurite fibers, all-solid lead–silicate (SF57) fibers and in bismuth fibers and tapers are then reported to show what has been achieved experimentally using a range of materials and fiber geometries.     

Continuous-Wave Frequency-Tunable Terahertz-Wave Generation From GaP

Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon–polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69–2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz.     

UV-Inscribed Optical Fiber Gratings in Mid-IR Range and Their Laser Applications

We have UV-inscribed fiber Bragg gratings (FBGs), long-period gratings (LPGs), and tilted fiber gratings (TFGs) into mid-IR 2m range using three common optical fiber grating fabrication techniques (two-beam holographic, phase mask, and point-by-point). The fabricated FBGs have been evaluated for thermal and strain response. It has been revealed that the FBG devices with responses in mid-IR range are much more sensitive to temperature than that in near-IR range. To explore the unique cladding mode coupling function, we have investigated the thermal and refractive index sensitivities of LPGs and identified that the coupled cladding modes in mid-IR range are also much more sensitive to temperature and surrounding medium refractive index change. The 45 tilted fiber gratings (45-TFGs) as polarizing devices in mid-IR have been investigated for their polarization extinction characteristics. As efficient reflection filters and in-cavity polarizers, the mid-IR FBGs and 45-TFGs have been employed in fiber laser cavity to realize multi-wavelength 2 m Tm-doped CW and mode locked fiber lasers, respectively.     

小型化宽调谐光纤型DFG中红外激光源的研制

研制了一种小型化宽调谐光纤型DFG/QPM中红外光源。该光源的基频光采用分段调谐的方案,以3个不同工作波段的YDFL作为泵浦源,3个不同工作波段的EDFL作为信号源,并通过光开关实现各波段间的快速切换。研制了DFG中红外光源样机,其尺寸为440×290×500mm3,测试结果表明,泵浦/信号光可在1050~1110nm和1535～1600nm范围内宽带连续调谐,经差频过程最终中红外激光实现3054.5~4009.1nm波段内宽带无缝调谐。     

PPLN-Based Flexible Optical Logic and Gate

All-optical flexible 20-Gb/s logic and gate based on cascaded sum- and difference-frequency generation in a periodically poled lithium niobate waveguide is proposed and experimentally demonstrated. The theoretical analyses further indicate that 40-, 80-, and 160-Gb/s ultrahigh-speed logic and operations can potentially be performed. Moreover, it is expected that the and output can be tuned in a wide wavelength range (67 nm) with slight fluctuation of the Q-factor and extinction ratio.     

Modulation instability initiated high power all-fiber supercontinuum lasers and their applications

High average power, all-fiber integrated, broadband supercontinuum (SC) sources are demonstrated. Architecture for SC generation using amplified picosecond/nanosecond laser diode (LD) pulses followed by modulation instability (MI) induced pulse breakup is presented and used to demonstrate SC sources from the mid-IR to the visible wavelengths. In addition to the simplicity in implementation, this architecture allows scaling up of the SC average power by increasing the pulse repetition rate and the corresponding pump power, while keeping the peak power, and, hence, the spectral extent approximately constant. Using this process, we demonstrate >10 W in a mid-IR SC extending from ～0.8 to 4 μm, >5 W in a near IR SC extending from ～0.8 to 2.8 μm, and >0.7 W in a visible SC extending from ～0.45 to 1.2 μm. SC modulation capability is also demonstrated in a mid-IR SC laser with ～3.9 W in an SC extending from ～0.8 to 4.3 μm. The entire system and SC output in this case is modulated by a 500 Hz square wave at 50% duty cycle without any external chopping or modulation. We also explore the use of thulium doped fiber amplifier (TDFA) stages for mid-IR SC generation. In addition to the higher pump to signal conversion efficiency demonstrated in TDFAs compared to erbium/ytterbium doped fiber amplifier (EYFA), the shifting of the SC pump from ～1.5 to ～2 μm is pursued with an attempt to generate a longer extending SC into the mid-IR. We demonstrate ～2.5 times higher optical conversion efficiency from pump to SC generation in wavelengths beyond 3.8 μm in the TDFA versus the EYFA based SC systems. The TDFA SC spectrum extends from ～1.9 to 4.5 μm with ～2.6 W at 50% modulation with a 250 Hz square wave. A variety of applications in defense, health care and metrology are also demonstrated using the SC laser systems presented in this paper.     

High-power mid-infrared supercontinuum sources: Current status and future perspectives

Mid-infrared (mid-IR) supercontinuum (SC) sources have recently gained much interest, as a key technology for such applications as spectral molecular fingerprinting, laser surgery, and infrared counter measures. However, one of the challenges facing this technology is how to obtain high power and broadband light covering a spectral band of at least 2–5 µm, especially with a very efficient output power distribution towards the mid-IR region. This directly affects their usage in the practical applications mentioned above. Typically, an SC is generated by pumping a piece of nonlinear fibre with high-intensity femtosecond pulses provided by mode-locked lasers. Although this approach can lead to wide continuum generation, the output power is limited only to the milliWatt level. Therefore, to achieve high-power SC light, other laser systems need to be employed as pump sources.This paper briefly reviews SC sources, restricted to those with an average output power of over 0.4 W and simultaneously with a long-wavelength edge of the continuum spectrum of over 2.4 µm. Firstly, the concepts of SC generation, including the nonlinear phenomena governing this process and the most relevant mid-IR fibre materials, are presented. Following this study, a review of the main results on SC generation in silica and soft-glass fibres, also including my experimental results, is presented. Emphasis is given to high-power SC generation with the use of different pump schemes, providing an efficient power distribution towards longer wavelengths. Some discussion and prospective predictions are proposed at the end of the paper.     

Wavelength conversions in quasi-phase matched LiNbO/sub 3/ waveguide based on double-pass cascaded /spl chi//sup (2)/ SFG+DFG interactions

Based on the cascaded nonlinear interactions (/spl chi//sup (2)/:/spl chi//sup (2)/) of sum- and difference-frequency generation (SFG+DFG), a novel all-optical wavelength conversion scheme is proposed for the first time in periodically poled LiNbO/sub 3/ (PPLN) waveguide, in which a double-pass configuration is introduced. The performance of this scheme is thus different from the previous single-pass SFG+DFG scheme. The concept of the "balance condition" is presented to optimize the power and frequencies of the two pump sources. Under this condition, the energy is transferred irreversibly from the pump waves to the SF wave during the forward propagation. The equations describing the SFG can be solved analytically under this condition. Subsequently, the DFG equations are solved under the assumption that the SF wave would be constant during the backward propagation. Theoretical expressions are derived and are found to be consistent with numerical calculations. Compared with the conventional converter based on the cascaded /spl chi//sup (2)/:/spl chi//sup (2)/ interactions of second-harmonic generation and difference frequency generation SHG+DFG, the same conversion efficiency can be achieved in our scheme by employing two pump sources with lower power, or conversely higher conversion efficiency can be reached using two pump sources similar to that used in SFG+DFG scheme. The profile of the conversion efficiency can be further improved by adjusting the wavelengths of the two pump sources. In addition, compared with the single-pass SFG+DFG scheme, the main advantage of this new scheme rests on the fact that the conversion efficiency can be enhanced significantly. The advantages of the double-pass SHG+DFG scheme and the single-pass SFG+DFG scheme are combined in this new design to a great extent.     

High-power difference-frequency generation at 4.4-5.7 µm in LiIO3

Efficient generation of high-power difference-frequency radiation, continuously tunable over the range of4.4-5.7 mum, has been achieved by mixing the Nd :YAG laser and Nd :YAG pumped infrared dye-laser outputs in LiIO3. Peak pulse powers as high as 550 kW with an average power output of 45 mW were obtained around 4.9μm.     

Tunable All-Optical Wavelength Conversion of 160-Gb/s RZ Optical Signals by Cascaded SFG-DFG Generation in PPLN Waveguide

We report, for the first time, tunable all-optical wavelength conversion of 160-Gb/s return-to-zero (RZ) optical signals based on cascaded sum- and difference-frequency generation in a periodically poled LiNbO₃ waveguide. The distorted signals due to limited phase-matching bandwidth during conversion were compensated by spectral reshaping. We achieved error-free tunable wavelength conversion with a bit-error rate of less than 10^-9 for 160-Gb/s RZ signals in a 23-nm tuning range over the C-band