全波段正常色散光子晶体光纤中超连续谱的产生

设计了一种铅硅酸盐SF57 材料的光子晶体光纤, 利用有限元法数值模拟了该光纤的 色散特性. 研究结果显示在整个透明波段光纤具有正常色散. 利用自适应分布傅里叶法求解非线性薛定谔方程, 对中心波长为1550 nm, 初始脉宽为150 fs 的脉冲在该光纤中传输进行了模拟, 获得了关于入射脉冲中心波长对称的展宽范围超过了600 nm 的超平坦连续光谱, 并且光谱具有极其稳定和 相干的特性. 关键词： 光子晶体光纤 超连续谱产生 正常色散     

基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic-crystal fiber (PCF) pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.     

基于全正色散光子晶体光纤的超连续谱光源

利用预估校正分步傅里叶法数值求解非线性薛定谔方程, 模拟超短激光脉冲在全正色散光子晶体光纤中传输时的演化情况, 分析了不同脉宽和能量的脉冲对产生的超连续谱的影响. 结果表明: 无啁啾高斯脉冲在此全正色散光子晶体光纤中传输时, 始终保持单个脉冲特性, 提高脉冲峰值功率可进一步展宽获得的超连续谱.模拟结果同时表明, 利用中心波长为1060 nm, 脉宽和能量分别为50 fs, 15 nJ的脉冲抽运此光纤, 当传输12 cm 后便可获得具有较好的光谱连续性和光谱平坦度的超连续谱. 进一步模拟结果表明, 采用棱镜对对其进行脉冲压缩, 可获得脉宽约15 fs, 谱宽约700 nm的理想超连续谱光源. 关键词： 超连续谱 光子晶体光纤 全正色散     

Endlessly single-mode photonic crystal fiber

We made an all-silica optical fiber by embedding a central core in a two-dimensional photonic crystal with a micrometer-spaced hexagonal array of air holes. An effective-index model confirms that such a fiber can be single mode for any wavelength. Its useful single-mode range within the transparency window of silica, although wide, is ultimately bounded by a bend-loss edge at short wavelengths as well as at long wavelengths.     

Supercontinuum generation in standard single-mode fiber pumped by a nanosecond-pulse laser

We have experimentally investigated supercontinuum generation in a conventional single-mode fiber pumped with a nanosecond pulse source. The experimental results show that, when pump power increases, the spectral flatness is improved obviously and the spectral broadening only occurs in a red-shifted radiation rather than a blue-shifted radiation. A supercontinuum source is experimentally reported with a flatness of 4.7 dB over 180 nm (ranging from 1555 to >1735 nm) at pump power P _R = 5 W and is predicted to have the flatness of less than 1 dB at P _R > 8 W. The cascade of stimulated Raman scattering (SRS) together with soliton fission plays the key roles in supercontinuum generation.     

单偏振单模聚合物光子晶体光纤设计

 设计了一种聚甲基丙烯酸甲酯(PMMA)基的单偏振单模(SPSM)微结构聚合物光纤(MPOF)。利用全矢量有限元法和光束传播法相结合分析了这种光纤的偏振特性和约束损耗。通过优化光纤结构参数,发现在0.51 μm～0.62 μm的可见光波长范围,由于基模两个正交偏振模的截止波长不同,这种微结构聚合物光纤只能传输基模中的一个偏振模,从而实现单偏振单模运转。该11圈圆空气孔六角排列光纤结构的传导偏振模在0.57 μm波长处约束损耗仅为1.13 dB/m,这种低损耗的单偏振单模微结构聚合物光纤可有效消除传统保偏光纤固有的偏振串扰和偏振模色散。     

Supercontinuum generation in photonic crystal fiber using femtosecond pulses

Supercontinuum (SC) generation in photonic crystal fiber (PCF) is demonstrated using an amplified femtosecond stretched pulses. The stretched pulse is obtained from a mode-locked Erbium-doped fiber laser and operates at 1564 nm with a repetition rate of 8.27 MHz and a pulse width of 340 fs. Using a 50 m long PCF, broad SC spectra are observed starting from 1220 and 1050 nm for the corresponding 5.1 and 177 kW pump and spanning a wavelength region of more than 1750 nm. At a maximum peak pump power of 177 kW, flat SC which extends over bandwidths of 660 and 486 nm are obtained using 50 and 100 m piece of PCF respectively. However, the output power level is higher for the 100 m PCF especially at longer wavelength region.     

Novel single-mode and polarization maintaining photonic crystal fiber

In this paper, we present and propose a novel structure for improved birefringence and single-mode propagation condition photonic crystal fiber (PCF) in a broad range of wavelength. The birefringence of the fundamental mode and single mode property in such a PCF is numerically estimated by employing full vector finite element method (FVFEM) and anisotropic perfectly matched layers (APML). The simulation results illustrate that we can achieve a high birefringence and perfect single-mode condition by employing silica-filled into one-line elliptical air holes parallel to x-axis and rotated by an angle. Obviously, the proposed PCF is quite useful for optical devices.     

Supercontinuum generation in seven-core photonic crystal fiber pumped by a broadband picosecond pulsed fiber amplifier

We report a supercontinuum source generated in seven-core photonic crystal fibers(PCFs) pumped by a self-made all-fiber picosecond pulsed broadband fiber amplifier. The amplifier's output average power is 60 W at 1150 nm with spectral width of 260 nm, and its repetition rate is 8.47 MHz with pulse width of 221 ps. With two different lengths of seven-core PCF, different output powers and spectra are obtained. When a 10 m long seven-core PCF is chosen, the output supercontinuum covers the wavelength range from 620 nm to 1700 nm, with the output power of 11.7 W. With only 2 m long seven-core PCF used in the same experiment, the wavelength of the supercontinuum spans from 680 nm to 1700 nm,with the output power of 20.4 W. The results show that the pulse width is 385 ps in the 10 m long seven-core PCF and 255 ps in the 2 m long one, respectively, due to the normal dispersion of the PCF.     

Supercontinuum generation in the irregular point of hollow-core photonic crystal fiber

Broadband supercontinuums (SC) are generated by soliton self-frequency shift (SSFS) of hollow-core photonic crystal fiber (HC-PCF) in our laboratory. With the pump works at 810 nm when the pump power increase from 400 to 600 mW, the Raman Soliton shifts from 2089 to 2215 nm, the bandwidth of SC increases from 2213 to 2320 nm. The ultra-violet part of SC is below 180 nm, and the mid-infrared part of SC exceeds 2500 nm. Moreover, the influence of pump power on SC is also analyzed.     

Supercontinuum generation in square photonic crystal fiber with nearly zero ultra-flattened chromatic dispersion and fabrication tolerance analysis

This paper presents a simple index-guiding square photonic crystal fiber (SPCF) where the core is surrounded by air holes with two different diameters. The proposed design is simulated through an efficient full-vector modal solver based on the finite difference method with anisotropic perfectly matched layers absorbing boundary condition. The nearly zero ultra-flattened dispersion SPCF with low confinement loss, small effective area as well as broadband supercontinuum (SC) spectra is targeted. Numerical results show that the designed SPCF has been achieved at a nearly zero ultra-flattened dispersion of 0 ± 0.25 ps/(nm·km) in a wavelength range of 1.38 μm to 1.89 μm (510 nm band) which covers E, S, C, L and U communication bands, a low confinement loss of less than 10⁻⁷ dB/m in a wavelength range of 1.3 μm to 2.0 μm and a wide SC spectrum (FWHM = 450 nm) by using picosecond pulses at a center wavelength of 1.55 μm. We then analyze the sensitivity of chromatic dispersion to small variations from the optimum value of specific structural parameters. The proposed index-guiding SPCF can be applicable in supercontinuum generation (SCG) covering such diverse fields as spectroscopy applications and telecommunication dense wavelength division multiplexing (DWDM) sources.     

Supercontinuum generation in all-solid photonic crystal fiber with low index core

In this paper we report on the fabrication and characterization of an all-solid photonic band gap fiber with high contrast and low index core. The fiber cladding is composed of high index lead-silicate rods while borosilicate NC21 glass is used as a background glass. A 70 nm wide photonic band gap at 875 nm central wavelength is experimentally identified and compared with a numerical model. We also present a novel method for photonic band gap measurement using a femtosecond pulsed laser. The method is verified against standard one and discussed.     

Supercontinuum generation from dispersion-flattened photonic crystal fiber using picosecond pulses

We present the all-fiber system for supercontiuum (SC) generation with picosecond pulses. By launching1.6-ps pulses from pulsed erbium-doped fiber laser (EDFL) into a section of photonic crystal fiber (PCF),the spectral broadening is observed. The bandwidth of 237 nm (at 20 dB level) is achieved.     

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d_core=2.10 μm, d₁=0.90 μm, d₂=0.44 μm and d₃=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.     

V型高双折射光子晶体光纤超连续谱产生的实验研究

高非线性高双折射光子晶体光纤是超连续谱产生的最有效介质之一, 因此本文选取V型光子晶体光纤作为研究对象. 通过多极理论数值模拟的结果, 确定V型光纤具有高双折射、高非线性等特性. 通过实验手段, 发现入射光中心波长和光纤的双折射效应对产生的超连续谱有很大的影响: 当入射光波长处于光子晶体光纤大的反常色散区时, 脉冲相对展得比较宽, 长轴要比短轴方向的超连续谱更宽, 频谱成分更加丰富. 在同一波长下光偏振方向越接近45°时, 超连续谱谱宽范围越大. 随着入射脉冲功率的增加, 超连续谱展得越宽, 但是当功率比较大时会达到功率饱和. 关键词： 光子晶体光纤 高双折射 多极理论 超连续谱     

Coherent and incoherent spectral broadening in a photonic crystal fiber

The coherence of the spectral broadening process is the key requisite for the application of supercontinua in frequency combs. We investigate the coherence of two subsequent supercontinuum pulses created in a photonic crystal fiber pumped by a femtosecond laser. We measure Young interference fringes from a Michelson-type interferometer at different wavelengths of the output spectrum and analyze their dependence on pump intensity and polarization. The visibility of these fringes is a direct measure of the coherence of the spectral broadening processes.     

Ultra-wideband single-polarization single-mode, high nonlinearity photonic crystal fiber

We report a novel design of photonic crystal fiber (PCF) with a rectangular array of four closely-spaced, highly elliptical air holes in the core region and a circular-air-hole cladding. The proposed PCF is able to support ultra-wideband single-polarization single-mode (SPSM) transmission from the visible band to the near infrared band. With the aid of the inner cladding formed by the central air holes, one polarization of the fundamental mode can be cut off at very short wavelengths and ultra-wideband SPSM propagation can be achieved. The inner cladding also suppresses the higher order modes and allows large air filling fraction in the outer cladding while the proposed fiber remains SPSM, which significantly reduces the mode effective area and the confinement loss. Our simulation results indicate that the proposed PCF has a 1540 nm SMSP range with <0.25 dB/km confinement loss and an effective area of 2.2 μm². Moreover, the group velocity dispersion (GVD) of the proposed PCF can also be tuned to be flat and near zero at the near infrared band (∼800 nm) by optimizing the outer cladding structure, potentially enabling many nonlinear applications.     

Numerical investigation of a broadband coherent supercontinuum generation in Ga8Sb32S60 chalcogenide photonic crystal fiber with all-normal dispersion

All normal dispersion (ANDi) and highly nonlinear chalcogenide glass photonic crystal fiber (PCF) is proposed and numerically investigated for a broad, coherent and ultra-flat mid-infrared supercontinuum generation. The proposed PCF consists of a solid core made of Ga₈Sb₃₂S₆₀ glass surrounded by seven rings of air holes arranged in a triangular lattice. We show by employing the finite difference frequency domain (FDFD) method that the Ga₈Sb₃₂S₆₀ PCF dispersion properties can be engineered by carefully adjusting the air holes diameter in the cladding region and ANDi regime is achieved over the entire range of wavelengths with a zero chromatic dispersion around 4.5?μm. Moreover, we demonstrate that injecting 50?fs width and 20?kW peak power laser pulses (corresponding to a pulse energy of 1.06?nJ) at a pump wavelength of 4.5?μm into a 1?cm long ANDi Ga₈Sb₃₂S₆₀ PCF generates a broad, flat-top and perfectly coherent SC spectrum extending from 1.65?μm to 9.24?μm at the 20?dB spectral flatness. These results make the proposed Ga₈Sb₃₂S₆₀ PCF an excellent candidate for various important mid-infrared region applications including mid-infrared spectroscopy, medical imaging, optical coherence tomography and materials characterization.     

Design of a single-polarization single-mode photonic crystal fiber double-core coupler

A simple single-polarization single-mode (SPSM) photonic crystal fiber (PCF) coupler with two cores is introduced. The full-vector finite-element method (FEM) is applied to analyze the modal interference phenomenon of the even and odd modes of two orthogonal polarizations and the power propagation within the two cores. Meanwhile, the SPSM coupling wavelength range and its corresponding coupling length for different structure parameters are numerically analyzed. The numerical results show that SPSM coupling can be realized with a broad range of wavelength, and the coupling length can be of millimeter magnitude. Moreover, the SPSM coupling wavelength range and the coupling length can be optimized by designing proper mirco-structure parameters of the coupler.