具有四模式的低串扰及大群时延多芯微结构光纤的设计

针对光纤空分复用及模分复用传输系统中大容量和耦合串扰问题, 本文提出了一种具有四模式特性低串扰及大群时延的大容量多芯微结构光纤, 通过有限元法计算该光纤电磁场分布进而对其他参数进行分析. 结果表明: 合理的选定光纤结构参数, 可使得该光纤在C+L波段内同时实现19芯的LP₀₁, LP₁₁, LP₂₁, LP₀₂四个偏振模式的传输. 同时, 利用空气孔对电磁场较好的隔离作用来优化芯间串扰并得到较大的模式差分群时延及较为平坦的色散. 此外, 这种结构的光纤制作简单, 在短距离大容量的信息传递系统中具有重要应用.     

一种基于三芯光子晶体光纤的宽带模分复用器的设计与研究

本文提出了一种基于非对称三芯光子晶体光纤的宽带模分复用器.该器件主要是由位于光纤中心的可提供基模和高阶模传输的中心纤芯和分别位于中心纤芯两侧的可提供基模传输的2个旁芯构成.根据光耦合理论,在输入端对3个纤芯分别输入LP₀₁模式的光,在传输过程中左旁芯的LP₀₁模式的光将逐步向中心纤芯耦合并转换为LP₂₁模式传输,而右旁芯中的LP₀₁模式的光则逐步耦合并转换为中心纤芯中的LP₃₁模式来传输.通过对光纤结构的优化设计和光纤长度的选择,使得在输出端同时完成旁芯LP₀₁模向中心纤芯LP₂₁和LP₃₁模的最佳转换,从而实现LP₀₁、LP₂₁和LP₃₁ 3种模式的光在中心纤芯中的复用.反之,若将该器件的输出端用作输入端则可以实现中心纤芯中3种模式的光向3个纤芯的解复用.本文利用有限元法和光束传播法进行了优化设计和仿真,并将光耦合理论与超模理论相结合进行了分析计...     

基于少模光纤的全光纤熔融模式选择耦合器的设计及实验研究

提出了三种基于少模光纤的全光纤熔融模式选择耦合器. 根据模式匹配原理采用单模光纤与少模光纤熔融连接方式, 运用耦合模理论及光束传播法模拟分析了模式选择耦合器的结构参数对模式选择及耦合特性的影响, 实现了单模光纤中基模到少模光纤中不同阶模式的转换, 以满足不同的应用需求. 实验上以2× 2熔融光纤耦合器为例, 采用对称和非对称熔融拉锥方式, 分别实现了从基模到LP₁₁, LP₂₁模式的转换. 实验结果表明所得到的LP₁₁, LP₂₁模式在1530–1560 nm的波长带宽范围内均有较高的模式纯净度, 且模式耦合效率高于80%, 与理论模拟结果基本一致.     

少模光纤的弯曲损耗研究

随着光纤通信容量的不断增加, 基于少模光纤的模分复用技术由于其多信道复用、 高频谱效率及低非线性效应成为目前提高光纤通信容量的研究热点. 本文推导得到了适用于少模光纤中高阶模式弯曲损耗的计算公式, 系统研究了下陷层辅助弯曲不敏感抛物线型少模光纤的主要参数(包括芯层半径、芯层到下陷层距离、下陷层宽度及下陷层折射率差)对其弯曲损耗特性的影响. 研究表明: 对于少模光纤, 模式阶数越高, 光纤的弯曲敏感性越高; 随纤芯与下陷层间距离的变化, 光纤各阶模式的弯曲损耗均存在一个最小值. 本文结论对弯曲不敏感少模光纤的设计、制造及少模光纤弯曲性能优化具有指导意义.     

Stimulated four-photon mixing and efficient polychromatic visible light generation in SiO2-based single-mode optical fibers pumped at 1.319 μm

The generation of near-infrared and intense visible light through stimulated multi-wave mixing processes in single-mode silica-based optical fibers pumped by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm is described. The experimental results show that intense infrared light around 1.2 μm is produced via selp-hase-matched four-photon mixing at the minimum group velocity dispersion region of pure SiO₂-core and P₂O₅-doped silica fibers. In the visible spectral region, from 580 nm to 600 nm, 20 W peak power 100-ps pulses were generated by pumping single spans of single-mode P₂O₅-doped and undoped SiO₂-core fibers with 1.319-μm laser pulses. The signal light generated in such fibers propagated in the LP₀₂ fiber mode and exhibited a threshold power that depended upon the fiber length and a critical length that was power dependent. Also, it exhibited an asymmetrical spectrum of a few nanometers bandwidth, with a long tail toward high frequencies. For GeO₂-doped silica-based fibers, a multiple-wavelength visible signal propagating in several high-order fiber modes was generated.     

Circularly symmetric modal interferometers: Equalization wavelength and equivalent step determination for matched-cladding fibers

An LP₀₁-LP₀₂ modal interferometer fabricated from a standard matched-cladding telecommunication fiber is studied in the visible wavelength range. Its spectral response in the vicinity of the LP₀₁/LP₀₂ equalization wavelength is used to characterize the fiber by the determination of an equivalent step index profile.     

Mid-infrared supercontinuum and optical frequency comb generations in a multimode tellurite photonic crystal fiber

We numerically investigate the mid-infrared(MIR) supercontinuum(SC) and SC-based optical frequency comb(OFC) generations when the three optical modes(LP01, LP02, and LP12) are considered in a multimode tellurite photonic crystal fiber(MM-TPCF). The geometrical parameters of the MM-TPCF are optimized to support the multimode propagation and obtain the desired dispersion characteristics of the considered three optical modes. When the pump pulse with center wavelength λ = 2.5 μm, width T = 80 fs, and peak power P = 18 k W is coupled into the anomalous dispersion region of the LP01, LP02, and LP12 modes of the MM-TPCF, the-40-d B bandwidth of the generated MIR SCs can be up to2.56, 1.39, and 1.12 octaves, respectively, along with good coherence. Moreover, the nonlinear dynamics of the generated SCs are analyzed. Finally, the MIR SCs-based OFCs are demonstrated when a train of 50 pulses at 1-GHz repetition rate is used as the pump source and launched into the MM-TPCF.     

Cutoff wavelength of single-mode fibers: Definition, measurement, and length and curvature dependence

The cutoff wavelength λ_c as defined by the CCITT can be defined equivalently as that wavelength for which the attenuation of the second-order LP₁₁ mode is 19.34 dB higher than the attenuation of the fundamental mode. This equivalent definition of λ_c does not depend on the length and curvature of the fiber. Both measuring methods for λ_c, the bending method and the multimode reference method, are analyzed and the measurement conditions that lead to the CCITT cutoff wavelength are indicated. The bend attenuation of the second-order mode of a typical matched-cladding, step-index fiber is calculated numerically, and methods are described to determine the length and curvature dependence of the cutoff wavelength from the bend loss data.     

Tunable Optical Add/Drop Multiplexers Using Cascaded Mach Zehnder Coupler

A tunable add/drop filter based on Cascaded Mach Zehnder (CMZ) coupler using SiO₂/silicon oxinitride (SiON) is presented with its mathematical model. The increase of filtered width (i.e., free spectral response) and lowering of crosstalk for this filter are optimized using its mathematical model with truncated binomial coupler distribution. The number of wavelength channels with 100 GHz (0.8 nm) channel spacing for 5-stage (M = 9, r = 2) truncated binomial CMZ filter with index contrast ∼5% at -20 dB crosstalk and the bending loss of 0.1 dB per MZ section are obtained as ∼37 (free spectral range of 31 nm). It is seen that if during the fabrication process, waveguide core width w is increased or decreased by 0.1 μm (in percentage terms ∼±6.6%), the crosstalk is slightly increased by ∼7%. Thermal tuning for wavelength channels is achieved by thin film heater with low power thermooptic delay line structure which reduces the heating power by ∼1.58 times in comparison to the conventional structure.     

Continuous-wave and passively Q-switched Nd:GdVO4 lasers at 1.06 μm end-pumped by laser-diode-array

We demonstrated a diode-end-pumped continuous-wave and passively Q-switched Nd:GdVO₄ laser operating at 1.06 μm wavelength with a three-mirror folded resonator. The maximum continuous-wave output power of 8.18 W was obtained at the incident pump power of 22.5 W, with the corresponding optical conversion efficiency of 36.4%. For Q-switched operation, the maximum average output power was measured to be 4.64 W with the corresponding optical conversion efficiency of 25.8%, when the initial transmission of Cr⁴⁺:YAG crystal was 90%. The shortest pulse width of 83 ns, the largest pulse energy of 20.7 μJ and the highest peak power of 246.7 W were obtained when the Cr⁴⁺:YAG crystal with an initial transmission of 85% was used.     

基于少模长周期光纤叠栅的模式转换器

本文提出一种在同一段少模光纤上写入两个不同周期Λ1和Λ2的长周期光纤光栅构成叠栅的方法,实现了纤芯基模LP_(01)向高阶纤芯模LP_(11)模式转换的宽带宽的新型的全光纤模式转换器.利用有限元法和耦合模理论建立了模式转换器的理论分析模型.数值仿真分析了叠栅中两个子光栅周期间隔、光栅长度、耦合系数等光栅参数对模式转换器的影响.仿真分析和实验结果表明,通过改变两个子光栅的周期间隔来改变两个损耗峰的位置,形成一个损耗峰,从而可以实现宽带宽的模式转换器,其10dB带宽约是单栅的2倍.与传统的模式转换器相比,该转换器带宽宽、转换效率高,尺寸小、抗干扰能力强,可以在模分复用系统和光通信中得到广泛的应用.     

Growth and laser properties of Nd:Ca4YO(BO3)3 crystal

Nd:Ca₄YO(BO₃)₃ (Nd:YCOB) crystal was grown by the Czochralski method, and its structure was measured by using a four circle X-ray diffractometer. The transparent spectrum from 200 to 2600 nm was measured at room temperature. The fluorescence spectrum near 1.06 μm showed that the main emission wavelength of Nd:YCOB crystal was centered at 1060.8 nm. Laser output at 1.06 μm has been demonstrated when it was pumped by a Ti:sapphire laser at the wavelength of 794 nm, the highest output power was 68 mW under pumping power of 311 mW, the pumping threshold was 163 mW and slope efficiency was 46.9%. The self-frequency doubled green light has been observed when it was pumped by a Ti:sapphire or a laser diode (LD). A 14.5 mm Nd:YCOB crystal sample cut at (θ, φ)=(90°, 33°) was used for type I second-frequency generation (SHG) of the 1.06 μm laser pulse. The SHG conversion efficiency was 22%.     

基于包层模谐振的三包层石英特种光纤温度传感特性研究

提出了一种基于包层模谐振的光纤温度传感器. 它是通过将三包层石英特种光纤(TCQSF)两端分别与普通单模光纤(SMF)电弧熔接构成的SMF-TCQSF-SMF结构. 根据耦合模理论, 首先将TCQSF等效为三个同轴波导, 按各波导模场的分布特点标量计算其传输模式的色散曲线, 并深入研究其耦合长度与传输谱线之间的关系; 其次根据光纤的热光效应及热膨胀效应, 分析计算该传感器的温度灵敏度; 最后选取耦合长度为一个拍长时的传感器进行温度传感实验. 实验结果表明, 在35-95 ℃的温度变化范围内, 其温度灵敏度为73.74 pm/℃, 与理论计算结果一致. 因此, 该传感器具有结构简单、制备容易、灵敏度高、包层模激发可控等优点, 可用于工业生产、生物医学等温度传感领域.     

A pulsed laser source using stimulated Raman scattering and difference frequency mixing: Remote sensing of methane in air

Mid-infrared radiation at 3.43 μm is generated by difference frequency mixing a 1.064 μm Nd:YAG laser with a methane gas Raman shifted Nd:YAG (1.064 μm) laser at 1.54 μm in KTiOAsO₄ (KTA). Using this pulsed (7 ns), moderate energy (1 mJ) source we demonstrate the optical detection of methane in air and measure an absorption coefficient of 1.2 cm⁻¹atm⁻¹. Additional source characteristics include an intrinsic wavelength stability defined by the methane Raman vibrational frequency and a moderate linewidth (1.5 cm⁻¹).     

基于高折射率液体填充的花瓣形微结构光纤可调滤模特性

提出一种新型的可调滤模光纤结构,利用纤芯模式与微结构包层形成的超模群之间的耦合实现选择性滤模,采用花瓣形包层结构使包层中传输的模式更容易产生高的泄漏损耗;提出以液体填充包层介质柱,使包层形成的超模群有效折射率区间可以通过环境温度来调节,从而达到可调选择性滤模目的.利用液体柱的LP11模所形成的超模群,有效增大了其工作带宽和温度调谐范围.数值模拟结果表明,采用长度仅为71.4 mm的滤模光纤,可以使特定的抑制模式损耗达到20 dB以上,而其他模式损耗均在1 dB以下.提出的光纤可以在少模光纤传输系统中作为滤模器使用,以降低模式转换器、复用器/解复用器以及光开关和光路由等的模式串扰.     

Range formulation for a staring electro-optical and imaging system incorporating the effects of atmospheric transmission, focal plane 1/f noise and fixed pattern noise

To increase the detection range in staring FPAs, commonly the integration time T_i is increased, as the range is assumed to increase as one-fourth power of T_i, (T_i^1/4). It is shown here that the range dependence on T_i is weaker than T_i^1/4, because of the effect of atmosphere. Since the atmospheric transmission coefficient decreases with increasing range, the effect of increasing T_i on the range is considerably reduced. It is also shown that when detector 1/f noise dominates over other noise sources, the dependence of range on T_i is much weaker than T_i^1/4, having a logarithmic dependence Calculations have been done by integrating equations involving spectral functions—photon flux, atmospheric transmission coefficient, optics transmission coefficient and responsivity—over wavelength, in the spectral range from 8 μm to 12 μm. LOWTRAN2 has been used for spectral atmospheric transmission coefficients, for different conditions of ambient temperature (−30°C to 55°C) and relative humidity (50–85%). The range formulation also distinguishes between distant targets (point sources) and nearby targets (extended sources). The analysis is applicable for terrestrial imaging, where the temperature difference of the target and the background is small. The effect of fixed pattern noise (FPN) in mercury cadmium telluride (MCT) FPAs has also been considered by modeling FPN in terms of a composition variation in the MCT. It is seen that range, both in the point source and the extended source cases, is not a sensitive function of FPN.     

Correlation between structural and optical properties of ion beam synthesized β-FeSi2 precipitates in Si

The structural and optical properties of β-FeSi₂ precipitates produced by ion beam synthesis have been investigated by transmission electron microscopy, photoluminescence (PL) analysis and near infrared transmission measurements. The PL spectrum of β-FeSi₂ precipitates in a dislocation free sample has been observed to consist of a sharp line at 1.54 μm and a weak peak at 1.46 μm. Optical transmission measurements showed a direct band gap about 0.8 eV smaller than in continuous β-FeSi₂ film. Calculation of the electronic bands of β-FeSi₂ for different values of the lattice parameters indicates that this reduction can be ascribed to band distortion provided by the lattice strain.     

Characterization and frequency measurement of a new far infrared laser line from CHin2Fin2

We present the discovery and frequency measurement of a new FIR laser line from CH₂F₂ optically pumped by a waveguide CO₂ laser. This new line has the same CO₂ laser pump line (9R32), offset (+5 MHz) and relative polarization of the strongest FIR laser line of CH₂F₂, namely the 184.3μm. The frequency is only 19,454 MHz apart from that of this line. We got a frequency measurement of 1,646,196.6 (10) MHz, which corresponds to a wavelength of 182.112 μm.     

Real-time observation of Bragg grating formation in As2S3 chalcogenide ridge waveguides

Real-time study of the formation of Bragg gratings in monolayer As₂S₃ ridge waveguides is presented. A transverse holographic method is used to record such gratings. The periodic photodarkening of the layer leads to the formation of a Bragg grating with resonant wavelength 1.55 μm. The experiment allowed to control in situ the grating parameters such as the reflectivity, the resonant wavelength (between 1500 and 1630 nm) and the bandpass at −3 dB.     

Transmission characteristics of multilayer films composed of electro-optic and dielectric materials

We describe the theoretical evaluation for the transmission characteristics of multilayer films composed of periodic electro-optic and dielectric materials in a variable electric field. The films are assumed to have cavity-type layer stacking structures, which work as a narrow bandpass filter, and their electro-optic material is uni-axial BaTiO₃, whose electro-optic coefficient r₅₁ is larger than 1300 pm/V. The evaluation indicates that films composed of BaTiO₃ and SiO₂ or MgO function as tunable optical bandpass filters, with a tunable range larger than 6 nm for an electric field of less than 4 V/μm.