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.     

Visible continuum generation using a femtosecond erbium-doped fiber laser and a silica nonlinear fiber

Supercontinuum extending to visible wavelengths is generated in a hybrid silica nonlinear fiber pumped at 1560 nm by a femtosecond, erbium-doped fiber laser. The hybrid nonlinear fiber consists of a short length of highly nonlinear, germano-silicate fiber (HNLF) spliced to a length of photonic crystal fiber (PCF). A 2 cm length of HNLF provides an initial stage of continuum generation due to higher-order soliton compression and dispersive wave generation before launching into the PCF. The visible radiation is generated in the fundamental mode of the PCF.     

All-solid silica-based photonic crystal fibers

An index-guiding all-solid photonic crystal fiber (PCF) composed entirely of silica material is proposed in this paper. The core of this optical fiber is composed of pure silica, and the cladding consists of doped silica rod in the background of pure silica. The dependence of confinement loss on the diameter of the doped rods, the number of doped-rod rings, and the doping level is investigated numerically. In addition, the proposed fiber possesses a shorter cutoff wavelength as compared with the air/silica PCF, which is directly confirmed by the V parameter, and explained based on a scalar approximation method. Furthermore, the bending loss for the fiber is predicted. A low-loss single-mode all-solid silica-based PCF with a large-mode-area is possible by the appropriate selection of configuration parameters.     

Photosensitive, all-glass AgPO3/silicaphotonic bandgap fiber

Photonic bandgap (PBG) guidance is observed in a solid core photonic crystal fiber (PCF) consisting of silver metaphosphate (AgPO(3)) glass embedded into a silica cladding, realized by vacuum-assisted infiltration of the molten glass into the hollow channels of a commercial silica PCF. Morphologic analysis of the cladding microstructure by optical and scanning electron microscopy reveals the formation of highly homogeneous glass strands along the PCF length. The characteristic transmission spectrum of the fiber shows PBG guidance in the range between 350 and 1650 nm. The exposure of the cladding glass matrix, using 355 nm, 150 ps laser irradiation, allows photo-induced enhancement of the transmission-to-stop-band extinction ratio by ～60 dB/cm and bandwidth tuning. Numerical calculations of the transmission pattern of the fabricated AgPO(3)/silica bandgap fiber are in good agreement with experiments.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

Raman-resonance-enhanced composite nonlinearity of air-guided modes in hollow photonic-crystal fibers

Coherent anti-Stokes Raman scattering (CARS) is used to measure relations between the resonant (Raman) and nonresonant (Kerr-type) optical nonlinearities of air-guided modes in a hollow-core photonic-crystal fiber (PCF). We demonstrate that, due to its interference nature, CARS provides a convenient tool for measuring the contribution of the fiber cladding to the total nonlinearity sensed by air-guided modes in hollow PCFs. On a Raman resonance with molecular vibrations in the gas that fills the fiber core, a two-color laser field is shown to induce optical nonlinearities that are several orders of magnitude higher than the nonresonant Kerr-type nonlinearities typical of air-guided PCF modes.     

用157 nm激光制作的光子晶体光纤法布里-珀罗传感器

157nm准分子激光用于微加工具有单光子能量高,峰值功率高,材料吸收系数高,分辨率高等优点。利用157nm激光微加工的方法,在光子晶体光纤上融切出微小矩形孔,从而构成腔长为45.6μm的微光纤法布里-珀罗干涉腔,得到的干涉条纹平滑,衬比度约为26dB,并从激光与石英材料的相互作用上分析了形成较好干涉条纹的原因。把这种微腔应用于应变测量,在550μm范围内,腔长增量相对于应变的灵敏度为0.32nm/μm,线形度达0.9994。实验证明该微腔对温度不敏感,800℃范围内腔长变化仅20nm。157nm准分子激光加工光纤法布里-珀罗腔方法简单,一次成型,具有较高的加工效率和精度,有望实现光纤法布里-珀罗腔的规模化批量制造,具有较好的应用前景。     

Enhanced nonlinearity in photonic crystal fiber by germanium doping in the core region

Germanium doping in silica can be used as a method for nonlinearity enhancement.Properties of the enhanced nonlinearity in photonic crystal fiber(PCF)with a GeO2-doped core are investigated theoretically by using all-vector finite element method.Numerical result shows that the nonlinear coefficient of PCF is greatly enhanced with increasing doping concentration,furthermore,optimal radius of the doped region should be considered for the desired operating wavelength.     

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.     

高温光子晶体光纤温度传感器

针对大于500℃的高温环境,提出了一种可用于高温温度测量的高温光子晶体光纤(PCF)温度传感器。在光子晶体光纤末端熔接一段纯石英无芯光纤构成外腔式光纤法珀腔(EFPI)结构。纯石英无芯光纤在高温下的热膨胀和热光效应使得EFPI的光学腔长发生变化。结合光纤白光干涉测量技术,通过测量EFPI的腔长得到被测温度。在不同温度环境下,对腔长为175μm的EFPI光纤温度传感器进行连续测量。测量结果显示,设计的高温光纤温度传感器在27~1100℃范围内,腔长-温度三阶拟合精度达到99.95%,腔长-温度灵敏度为(0.851+0.0023T-0.000000957T2)nm/℃,其中在1100℃时,温度测量分辨率为0.225℃。     

Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 μm

A robust design for a photonic crystal fiber (PCF) based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is presented.This design is suitable for flat broadband supercontinuum (SC) generation in the 1.55-μm region.The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W -1 ·km -1 at 1550 nm.Ultraflat dispersion with a value between -1.65 and -0.335 ps/(nm·km) is obtained ranging from 1375 to 1625 nm.The 3-dB bandwidth of the SC is 125 nm (1496–1621 nm),with a fiber length of 80 m and a corresponding input peak power of 43.8 W.The amplitude noise is considered to be related to SC generation.For practical fabrication,the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.     

Brillouin scattering spectrum in photonic crystal fiber with a partially germanium-doped core

The Brillouin scattering spectrum in a photonic crystal fiber (PCF) with a partially Ge-doped core is measured with a pump-probe technique at a wavelength of 1320 nm. One main peak and four subpeaks are observed. The main peak has a Lorentzian shape with the bandwidth deltanuB = 66 MHz. Its intensity is six times higher than that from a standard single-mode fiber measured under the same conditions, which is consistent with the ratio of (1/Aeff(PCF))/(1/Aeff(SMF)), where Aeff is the effective area of the fibers. The temperature coefficient for the main peak is 0.96 MHz/degrees C. We believe that the subpeaks are caused by an interaction between light-wave and guided modes of longitudinal acoustic waves in the graded-Ge-doped region, the silica region, and the microstructured cladding. An analysis of the guiding and antiguiding properties of the PCF for acoustic waves is presented.     

Deep-notch, ultracompact long-period grating in a large-mode-area photonic crystal fiber

An extremely short long-period grating (LPG) with strong resonance has been developed in a large-mode-area photonic crystal fiber (PCF) by use of the heat source of a CO2 laser. We believe that such a LPG in pure silica PCF is the first example to be obtained with the point-by-point technique. The fabrication method is simple and repeatable. The resulting LPG has 8 periods, written by a CO2 laser, within a 2.8-mm length of fiber, which yields a deep notch of core-cladding mode coupling of -31.5 dB at the telecommunication wavelength of 1529.2 nm, with a FWHM of approximately 0.7 nm. The principal advantages of this LPG are that it is practical, cost effective, and compact.     

Raman effects in a highly nonlinear holey fiber: amplification and modulation

We experimentally demonstrate that a short length of highly nonlinear holey fiber (HF) can be used for strong L(+) -band (1610-1640-nm) Raman amplification and ultrafast signal modulation. We use a pure silica HF with an effective area of just 2.85mum(2) at 1550 nm, which yields an effective nonlinearity ~15 times higher than in conventional silica dispersion-shifted fiber. Using a 75-m length of this fiber, we obtained internal Raman gains of more than 42 dB and a noise figure of ~6 dB under a forward single-pump scheme, and the Raman gain coefficient was experimentally estimated to be 7.6 chi 10(-14)m/W . Also, an 11-dB signal extinction ratio in a Raman-induced all-optical modulation experiment was achieved with the same fiber.     

一种新型宽带光纤拉曼放大器的理论研究

主要比较分析了光子晶体光纤在设计宽带光纤拉曼放大器上的优越性。简化了多波长宽带光纤拉曼放大器功率耦合方程,分两步来确定各泵浦波的频率及输入功率的大小。首先通过模拟煺火算法迭代出满足条件的泵浦波频率,再利用平均功率分析方法,采用四阶阿当迭代方法计算出各泵浦波输入功率的大小,设计出了具有较宽平坦增益带宽的光纤拉曼放大器。分别对光子晶体光纤和色散位移光纤进行了模拟计算,仿真结果表明光子晶体光纤可以用来设计成短长度、高效的光纤拉曼放大器。     

Infrared supercontinuum from a large mode area PCF under extreme picosecond excitation

Evolution of the long-wavelength edge of supercontinuum (SC) in a large mode-area photonic crystal fiber (LMA PCF) under extreme (close to the fiber damage threshold) picosecond excitation is analyzed for the first time to our knowledge. The obtained results are interpreted on the basis of the numerical simulations explaining both spectral and temporal characteristics of SC in dependence on the pulse power and the fiber length. An existence of the minimum LMA PCF length providing the broadest spectrum under the highest picosecond excitations is predicted. The multimode cross-modulation inside the LMA PCF is analyzed, as well.      

Modeling photonic crystal fiber for efficient soliton pulse propagation at 850 nm

We numerically investigate the dynamics of soliton propagation at 850 nm in chloroform filled liquid core photonic crystal fiber (LCPCF) by using both finite element method (FEM) and split step Fourier method (SSFM). We propose a novel chloroform filled PCF structure that operates as a single mode at 850 nm featuring an enhanced dispersion and nonlinearity for efficient soliton propagation with low input pulse energy and low loss over small distances. We adopt the projection operator method (POM) to derive the pulse parameter equations which clearly describes the impact of fourth order dispersion on the pulse propagation in the proposed PCF. To analyse the quality of the pulse, we perform the stability analysis of pulse propagation numerically and compare our results of the newly designed chloroform filled PCF with that of standard silica PCF. From the stability analysis, we infer that the soliton pulse propagation in modified chloroform filled PCF is highly stable against the perturbation.     

Characterization of specially designed polarization maintaining photonic crystal fiber from far field radiation patterns

We report the development of theory and experiment for the characterization of polarization maintaining photonic crystal fiber (PM PCF) from far field intensity measurements. In this specially designed photonic crystal fiber, the air hole diameters along orthogonal axes adjacent to the core region are different, and hence create an effective index difference between the two orthogonal polarization modes. From the experimental measurements of the far field radiation pattern, we obtain transmission characteristics of PM PCF in terms of mode field diameters (MFD), V-values along major and minor axes, mode field area, birefringence, core radius, effective refractive index of cladding and numerical apertures along major and minor axes of PM PCFs. It is shown that the experimentally obtained geometrical and wave guiding parameters of the PM PCF match with the manufacturer/simulation/scanning electron microscope (SEM) data within the experimental limits.     

占空比对光子晶体光纤激光器性能的影响

 在传统光纤激光器工作原理的基础上,考虑光子晶体光纤(PCF)模场分布特征,给出了连续泵浦情况下单模PCF激光器的速率方程和功率传输方程。利用该方程对掺镱单模PCF激光器的性能进行了数值模拟研究。结果表明：虽然空气占空比大小对PCF激光器的输出功率、泵浦阈值和斜率效率等影响不显著,但对拉曼非线性阈值影响却很大。当泵浦功率小于拉曼非线性阈值时,激光器主要输出信号激光;当超过拉曼阈值且在较宽的范围内,激光器同时输出功率相近的信号激光和拉曼光。基于这种效应,提出一种由泵浦功率控制的双波长光纤激光器的新思路。考虑到PCF非线性系数的可调控特性,采用不同光纤有可能在较宽的功率范围获得双波长激光振荡。     

用于液体传感的高双折射新型光子晶体光纤结构

提出一种用于液体传感的光子晶体光纤的结构和设计方法。将六边形光子晶体光纤的纤芯区域设计成2个椭圆空气孔,分别填充水和乙醇,进而比较两种光子晶体光纤结构的传播特性。采用全矢量有限元法对所设计光子晶体光纤结构的双折射、相对灵敏度、限制损耗、非线性系数进行数值分析,对光纤各项参数的优化及纤芯椭圆结构参数进行调整,结果表明:当椭圆率为0.6时,纤芯填充乙醇的光子晶体光纤PCF2在波长1.55 nm处相对敏感度可达到72.506 7%,同时限制损耗可以降至10?8级。所设计的模型可用于传感和生物传感研究及应用。