使用MCVD设备制取62.5μm大芯径、大数值孔径光纤预制棒的新工艺探索

总结了使用ＭＣＶＤ设备制备大芯径、大数值孔径光纤预制棒的新工艺。探索在不掺硼的情况下,在预制棒芯层逐层增加ＧｅＣｌ４的流量。在内包层掺氟以降低内包层折射率。解决了为达到较高折射率差Δｎ,在芯层掺锗过多而引起光纤预制棒在沉积后期和缩棒过程中由材料的热膨胀系数而导致的炸裂问题。并通过改变火焰平移速度,提高了沉积速率,缩短了制棒时间。所拉制的光纤、数值孔径ＮＡ高达０．３０。     

Time-resolved spectroscopy and optical gain of silica-based fibers co-doped with Bi, Al and/or Ge, P, and Ti

The optical properties of optical fibers based on silica glass doped with bismuth and co-doped with aluminum oxides and/or germanium, phosphorus, and titanium oxides are studied. The optical loss and luminescence spectra of optical fibers substantially depend on the core composition. The gain spectra of single-mode optical fibers are measured in the IR range. It is demonstrated that the phosphorus-germanium-silicate optical fiber doped with bismuth exhibits a broad gain band (1270–1520 nm) when pumped at a wavelength of 1230 nm. It is also shown that the bismuth-aluminosilicate optical fibers additionally doped with Ge or Ti at about 1 at % have the gain spectra that are significantly narrower than the IR luminescence spectra (in contrast to the fibers that do not contain Ge and Ti). The intensity decay curves of the IR luminescence in such fibers indicate the presence of both short-lived (with the lifetime τ≤4 μs) and long-lived (τ ～ 1 ms) energy levels in the bismuth active centers.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

矩形晶格高偏振、低损耗铋锗镓光子晶体光纤的结构设计及性能分析

非对称结构光子晶体光纤应用广泛。其良好的偏振特性、灵活的色散调控能力以及低限制损耗品质,对于优化与改善偏振光纤器件、非线性光学光纤、光通信光纤、光纤传感器等性能发挥着关键的作用。选用高折射率铋锗镓激光玻璃为材料,设计了八边形阵列、矩形晶格排列的光子晶体光纤,纤芯缺陷区包层及外包层均为圆形空气孔。模拟实验数据显示,结构参数为M=0.5,0.6时,在波长为1.55 μm处的双折射系数分别为1.16×10?2和1.33×10?2;在近红外波段短波区,矩形晶格结构光子晶体光纤的色散范围分别在±30 ps·nm?1·km?1之间及?18～32 ps·nm?1·km?1之间。色散斜率较低,曲线具有零色散点,展现了良好的连续谱调控能力;在1.00～1.90 μm波段内,当M=0.5,0.6时,光纤限制损耗稳定在10?7～10?9 dB·km?1之间;在1.55 μm处,限制损耗测量值分别为2.32×10?7和1.62×10?8 dB·km?1。     

Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression

We report on the recent design and fabrication of kagome-type hollow-core photonic crystal fibers for the purpose of high-power ultrashort pulse transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all kagome fibers) of 40 dB/km over a broadband transmission centered at 1500?nm. We show that the large core size, low attenuation, broadband transmission, single-mode guidance, and low dispersion make it an ideal host for high-power laser beam transportation. By filling the fiber with helium gas, a 74?μJ, 850?fs, and 40?kHz repetition rate ultrashort pulse at 1550?nm has been faithfully delivered at the fiber output with little propagation pulse distortion. Compression of a 105?μJ laser pulse from 850?fs down to 300?fs has been achieved by operating the fiber in ambient air.     

Bending insensitivity of fiber Bragg gratings in suspended-core optical fibers

This Letter presents simulation and experimental results that explore bending insensitivity of fiber Bragg gratings in suspended-core optical fibers. The implementation of thin silica bridge in the fibers enhances index contrast of the fiber core and reduces bending-induced strain transfer to the fiber core. This fiber design lead to a reduction of over 7 times in strain-induced fiber Bragg grating resonant peak shifts in the suspended-core fiber compared with that in standard telecommunication fiber, and an 0.14 dB bending loss at a bending radius of 6.35 mm.     

Transmission of Cerenkoverenkov radiation in optical fibers

Cerenkov radiation is generated as an unwanted background when optical fibers carrying signals pass through radiation fields. The angular dependence of the intensity of ? Cerenkov radiation transmitted in silica-core fibers was measured using 6 and 12 MeV electron beams from a Varian Clinic accelerator. These confirmed theoretical predictions that the angular variation of ? Cerenkov radiation transmitted along optical fibers depends only on the refractive index difference Dn between the core and the cladding, and that the peak intensity is proportional to the cube of the fiber core radius.     

A 168-km ultra-long-range OTDR measurement at a wavelength of 1.55 μm

A Rayleigh backscattered signal and Fresnel reflection over a 167.6-km and a 251.2-km long pure silica-core fiber (PSF) with fiber loss 0.176 dB/km, and a 138.6-km and a 206.5-km long Ge-doped core dispersion shifted fiber (DSF) with fiber loss 0.218 dB/km have been observed with ultra-long-range optical time domain reflectometry (OTDR) using a 1.55-μm high-power semiconductor laser (LD) and InGaAs-APD with a low dark current at room temperature. The Rayleigh backscattered signal level and fiber loss are strongly dependent on the fiber dopant materials and fiber structural parameters. DSF gives a better SNR for short length fibers because the Rayleigh backscattered signal level is larger than PSF, but because the DSF loss is also larger, PSF gives a better SNR for fibers longer than the critical length. Accordingly, the dynamic range of OTDR in the different fiber types such as PSF and DSF becomes different. One-way backscattered dynamic ranges of 29.5 dB for the PSF and about 30.2 dB for the DSF at room temperature have been achieved.     

A 168-km ultra-long-range OTDR measurement at a wavelength of 1.55 μm

A Rayleigh backscattered signal and Fresnel reflection over a 167.6-km and a 251.2-km long pure silica-core fiber (PSF) with fiber loss 0.176 dB/km, and a 138.6-km and a 206.5-km long Ge-doped core dispersion shifted fiber (DSF) with fiber loss 0.218 dB/km have been observed with ultra-long-range optical time domain reflectometry (OTDR) using a 1.55-μm high-power semiconductor laser (LD) and InGaAs-APD with a low dark current at room temperature. The Rayleigh backscattered signal level and fiber loss are strongly dependent on the fiber dopant materials and fiber structural parameters. DSF gives a better SNR for short length fibers because the Rayleigh backscattered signal level is larger than PSF, but because the DSF loss is also larger, PSF gives a better SNR for fibers longer than the critical length. Accordingly, the dynamic range of OTDR in the different fiber types such as PSF and DSF becomes different. One-way backscattered dynamic ranges of 29.5 dB for the PSF and about 30.2 dB for the DSF at room temperature have been achieved.     

Effect of radiation-induced mean wavelength shift in optical fibers on the scale factor of an interferometric fiber optic gyroscope at a wavelength of 1300 nm

In order to analyze the effect of wavelength-dependent radiation-induced attenuation (RIA) on the mean trans- mission wavelength in optical fiber and the scale factor of interferometric fiber optic gyroscopes (IFOGs), three types of polarization-maintaining (PM) fibers are tested by using a 60 Co γ-radiation source. The observed different mean wave- length shift (MWS) behaviors for different fibers are interpreted by color-center theory involving dose rate-dependent absorption bands in ultraviolet and visible ranges and total dose-dependent near-infrared absorption bands. To evaluate the mean wavelength variation in a fiber coil and the induced scale factor change for space-borne IFOGs under low radiation doses in a space environment, the influence of dose rate on the mean wavelength is investigated by testing four germanium (Ge) doped fibers and two germanium-phosphorus (Ge-P) codoped fibers irradiated at different dose rates. Experimental results indicate that the Ge-doped fibers show the least mean wavelength shift during irradiation and their mean wavelength of optical signal transmission in fibers will shift to a shorter wavelength in a low-dose-rate radiation environment. Finally, the change in the scale factor of IFOG resulting from the mean wavelength shift is estimated and tested, and it is found that the significant radiation-induced scale factor variation must be considered during the design of space-borne IFOGs.     

Optimization of microstructured fibers with germanium-doped core for broad normal dispersion range

We have numerically studied different designs of technologically feasible microstructured fibers with a germanium-doped core in order to obtain normal dispersion reaching possibly far in the mid infrared. Hexagonal, Kagome and the combination of both geometries were numerically examined with respect to different constructional parameters like pitch distance, filling factor of air holes, number of layers surrounding the core, and level of germanium doping in the core. Our analysis showed that the broadest range of normal dispersion reaching 2.81?μm, while keeping an effective mode area smaller than 30?μm², was achieved for a hexagonal lattice and a 40?mol% GeO₂ doped core. The proposed fibers designs can be used in generation of a normal dispersion supercontinuum reaching the mid-IR region.     

Design of large mode area multicore photonic crystal fiber with a flat-top mode

A large mode area multicore photonic crystal fiber with a flattened fundamental mode is proposed in this paper. Another two kinds of single core fibers are also presented as comparison. The modal characteristics such as effective mode area and confinement loss are investigated using the finite element method. Numerical results show that a combination of flattened fundamental mode, large mode area and ultralow loss is obtained in the multicore fiber by introducing higher refractive index in the core region.     

Brillouin gain spectrum characterization in Ge-doped large-mode-area fibers

The dependence of Brillouin gain spectrum (BGS) characteristics, including the Brillouin frequency shift (BFS) and the BGS bandwidth, on germanium concentration in large-mode-area Ge-doped passive fibers is investigated theoretically and experimentally. The simulation results show that the BFS is inversely proportional to GeO₂ concentration, and the BGS bandwidth initially increases with the augment of GeO₂ concentration, and then decreases. The BGSs of four fibers with core diameters of 10 μm and 20 μm for different GeO₂ concentrations are compared experimentally. Experimental results demonstrate that with the same core diameter, the variations of BFS and BGS bandwidths with GeO₂ concentration accord with the simulation results. Additionally, the BGS characteristics of three large-mode-area passive fibers with diameters of 10 μm, 25 μm, and 30 μm are measured, which confirm that the increasing of the fiber diameters will cause the BGS bandwidth to broaden. We believe that these results can provide valuable references for modulating the high-power narrow-linewidth fiber lasers and Brillouin fiber amplifiers.     

低损耗Ge-As-Se-Te硫系玻璃远红外光纤的性能分析

中、远红外光学领域的发展,离不开低损耗光波导材料的发展,因此近年来远红外低损耗光纤一直是光学领域的热点之一.本论文在国内首次报道了一种基于挤压法的低损耗远红外光纤制备技术,获得了具有完整结构的远红外光纤,其损耗为:0.46 d B/m@8.7μm,1.31 d B/m@10.6μm,整体低于1 dB/m@7.2—10.3μm.在实验过程中,首先采用传统的熔融淬冷法和蒸馏纯化工艺制备了Ge-As-Se-Te玻璃样品.利用X射线衍射仪和热膨胀仪等测试了玻璃的结构和物理性质,分析了Ge对玻璃热学性质的影响;利用分光光度计、红外光谱仪等研究了玻璃的光谱性质;综合比较了还原剂铝、镁的除氧效果.最后采用挤压法制备了芯包结构光纤.实验结果表明:镁的除氧效果佳,新型挤压制备工艺和有效提纯技术共同推进了硫系光纤损耗的降低,所获得的Ge-As-Se-Te光纤具有远红外广谱应用的潜能(其透光波长接近12μm).     

All-fiber Kerr cell

An all-fiber nanosecond Kerr light gate is described that was constructed using microstructured fibers. The switching voltage for a 20?cm long device is as low as Vπ～85 V at a 1.06?μm wavelength. The device is fully spliced. The active element is a three-hole fiber provided with internal electrodes in the side-holes and a liquid core of nitrobenzene, which is fully enclosed. This work allows the exploiting of electrically driven liquid-core fibers and demonstrated the removal of the major limitations of Kerr cells in the past, allowing for integration, safe use, and relatively low switching voltage.     

Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation

Photodarkening of Yb-doped aluminosilicate fibers by continuous wave 488 nm irradiation was investigated. The irradiation induced significant excess loss in the UV-visible spectroscopy (VIS) region in Yb-doped aluminosilicate fibers while pure aluminosilicate fibers showed negligible induced loss. Ultraviolet-VIS-near-infrared spectroscopy revealed an absorption peak at 220 nm in unexposed Yb-doped aluminosilicate fiber preforms. The observed peak was attributed to Yb-associated oxygen deficiency centers (ODCs) and proposed as a precursor of the photodarkening. The proposed model was supported by measurements on oxygen-loaded Yb-doped aluminosilicate fibers. In these, the photodarkening could be significantly reduced, which we attribute to a smaller number of ODCs following oxygen loading.     

Residual stress relaxation in the core of optical fiber by CO(2) laser irradiation

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.     

Nineteen-port photonic lantern with multimode delivery fiber

We demonstrate efficient multimode (MM) to single-mode (SM) conversion in a 19-port photonic lantern with a 50 μm core MM delivery fiber. The photonic lantern can be used within the field of astrophotonics for coupling MM starlight to an ensemble of SM fibers in order to perform fiber-Bragg-grating-based spectral filtering. An MM delivery fiber spliced to the photonic lantern offers the advantage that the delivery fiber guides the light from the focal plane of the telescope to the splitter. Therefore, it is no longer necessary to have the splitter mounted directly in the focal plane of the telescope. The coupling loss from a 50 μm core MM fiber to an ensemble of 19 SM fibers and back to a 50 μm core MM fiber is below 1.1 dB.     

复折射率光纤传输特性分析的Davidenko方法

增益或损耗对光纤的传输特性影响很大。使用Davidenko方法对复折射率光纤的传输特性进行了分析。研究了复折射率纤芯或复折射率包层阶跃光纤,通过比较发现,使用Davidenko方法得到的解与精确解符合得很好。对于芯区为复折射介质的光纤,HE11模与LP01模增益值偏差约为0.6%;对于包层区为复折射率介质的光纤,HE11模与LP01模增益值偏差约为2%。实际研究工作中,为了得到更精确的结果,应该求解全矢量的复本征方程,尤其是包层具有增益或损耗的光纤。