Microdroplet-etched highly birefringent low-loss fiber tapers

We use hydrofluoric acid microdroplets to directly etch highly birefringent biconical fiber tapers from standard single-mode fibers. The fiber tapers have micrometer-sized cross sections, which are controlled by the etching condition. The characteristic teardrop cross section leads to a high group birefringence of B(G)≈0.017 and insertion losses <0.7 dB over waist lengths of about 2.1 mm.     

Effect of photonic crystal structure on the nonlinear optical anisotropy of birefringent porous silicon

Anisotropic photonic crystal structures consisting of birefringent porous silicon layers with alternating porosity were fabricated. The in-plane birefringence formed as a result of anisotropic etching in Si(110) results in unique multilayered structures with two distinct photonic bandgaps for orthogonal light polarizations. Nonlinear optical studies based on the third-harmonic generation from these structures demonstrate variation in the symmetry of the nonlinear optical response.     

Methods for increasing the efficiency of nonlinear optical interactions in nanostructured semiconductors

Methods for increasing the efficiency of the optical second-and third-harmonic generation in gallium phosphide and silicon nanostructures formed by electrochemical etching of crystalline semiconductors are discussed. The efficiency of nonlinear optical interactions can be increased by using phase matching in anisotropic nanostructured semiconductors that exhibit form birefringence or by increasing the local field, as in scattering in macroporous semiconductors. The efficiencies of third-harmonic generation in porous silicon and of second-harmonic generation in porous gallium phosphide are found to increase by more than an order of magnitude.     

直拉法生长的YAG单晶体中组分过冷引起的针状应力区和位错

本文研究了掺钕的YAG(Y₃Al₅O₁₂)晶体在组分过冷条件下,固液界面的形态演变以及产生的钕离子的富集区。该富集区在正交偏光下表现为针状应力区。利用光弹方法确定了该应力为压应力,且应力分布具有圆柱对称性。通过浸蚀法和双折射貌相法观察到在钕富集区边界处存在位错列,并测定了位错的线密度。其结果与根据钕的浓度差值估计的密度大体符合。故认为这些位错是钕富集区边界处点阵失配所引起的。 关键词：     

暗视场及相衬显微术及其对Nd:YAG晶体中缺陷研究

暗视场及相衬显微术的分辨率可达10～500(?),采用这两种技术可以十分清淅地观察到用引上法及温梯法生长的Nd:YAG晶体中的点缺陷集团,小尺寸位错环,蜷线位错,片状缺陷时精细结构.特别是观察到非缀饰刃位错及混台位错的图像.对非缀饰位错的观察已用化学腐浊,光双折射貌相术及X射线形貌术作了证明.     

Design and numerical analysis for low birefringence silica on silicon waveguides

A new fabrication technology for three-dimensionally buried silica on silicon optical waveguide based on deep etching and thermal oxidation is presented.Using this method,a silicon layer is left at the side of waveguide.The stress distribution and effective refractive index are calculated by using finite element method and finite different beam propagation method,respectively.The results indicate that the stress of silica on silicon optical waveguide fabricated by this method can be matched in parallel and vertical directions and stress birefringence can be effectively reduced due to the side-silicon layer.     

Femtosecond laser written stress-induced Nd:Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (Nd:YAG) channel waveguide laser

Single tracks and pairs of tracks were written into undoped and Nd-doped YAG crystals using a commercial femtosecond laser system delivering pulses with pulse duration of 140 fs and pulse energies up to 10 μJ. The pulses were focused by a 50× microscope objective below the surface of the crystals. Due to the elasto-optical effect, stress-induced birefringence was observed in domains surrounding the single tracks and between the pairs of tracks. Waveguiding was demonstrated in certain channels in these domains. To investigate the underlying guiding mechanism highly selective chemical etching of the modified material was performed with etching rates up to 5 μm/h. Pumped at 808 nm, laser operation at a wavelength of 1064 nm was achieved. The maximum output power was 25.5 mW at 261 mW of launched pump power with a slope efficiency of 23%.     

Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as SiO2, the birefringence modification range can be as large as 10(-3). In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from 1.2 x 10(-3) (without the upper cladding) to 4.5 x 10(-5) when a 0.8-microm oxide upper cladding with a stress of -320 MPa (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core-cladding index contrast, the associated mode mismatch loss is negligible.     

Compensation of the photoelastic birefringence of a polymer by doping with an anisotropic molecule

We report on a method to compensate the photoelastic birefringence of a polymer. In this method, a rod-like molecule that has a polarizability anisotropy was chosen and doped in a polymer. We demonstrated this method by compensating the negative photoelastic birefringence of poly(methylmethacrylate) at a wavelength of 633 nm. Homogeneous doping with 2.2 wt. % of trans-stilbene almost eliminated the photoelastic birefringence of the polymer. The photoelastic coefficient of the synthesized zero-photoelastic birefringence polymer was 0.057×10^-12 Pa^-1. We found that the photoelastic birefringence of poly(methylmethacrylate) was compensated by the motion of trans-stilbene in the polymer by the analysis of the infrared absorption spectrum. PACS 42.70.-a; 42.70.Jk; 78.30.Jw     

Sensitive birefringence measurement in a high-finesse resonator using diode laser optical self-locking

We demonstrate a new method to measure weak birefringence of dielectric mirrors with excellent spatial resolution and sensitivity (<10^-7 radians). We exploit a well-known optical feedback scheme for line-width narrowing and frequency locking of a diode laser to a high-finesse cavity. Feedback comes from the intracavity field which builds up at resonance, selected by its change in polarization with respect to the incident field. This change, due to the residual birefringence of the cavity mirror coatings, was already exploited for birefringence measurements using an active laser-locking scheme. Here we measure the optical feedback rate as a function of rotation angle of one of the cavity mirrors (around the cavity axis). A stable feedback signal is obtained since the laser, as soon as it locks to a cavity resonance, effectively behaves as a monochromatic source. By fitting the data with a theoretical expression, we determine quantitatively the local birefringence vectors of both mirrors, which are around 10^-6 radians. Our scheme is simple, works with cavities of very high finesse (F∼10⁵), and is promising for measuring birefringence in gases induced by external fields. Received: 18 July 2001 / Final version: 14 March 2002 / Published online: 8 May 2002     

A polymer photonic crystal fiber with high and flattened birefringence

A novel high birefringence polymer photonic crystal fiber (PCF) is proposed in this work. This PCF is composed of a polymer core and a cladding with elliptical air holes and squeezed triangular lattice. The high birefringence is introduced on the combined effect of elliptical air holes and the squeezed lattice. Our numerical results based on the supercell lattice method indicate that the birefringence can reach as high as 0.0018 at 650 nm wavelength with a properly designed cladding structure. We also analyze the dependence of the birefringence on structure parameters. And we design a PCF that has high and flattened birefringence.     

Reflectometric measurement of birefringence rotation in single-mode optical fibers

We present a novel reflectometric technique for the measurement of orientation and modulus of the linear birefringence vector in single-mode optical fibers. The technique provides information also on circular birefringence, although this component, if present, appears as a rotation of the linear birefringence. A detailed theoretical analysis is reported and validated by experimental results.     

Mechanisms of polarization switching in single-transverse-mode vertical-cavity surface-emitting lasers: thermal shift and nonlinear semiconductor dynamics

We analyze polarization switching in vertical-cavity surface-emitting lasers, taking into account a proper semiconductor frequency-dependent complex susceptibility and spin-flip processes. Thermal effects are included as a varying detuning, and gain differences arise from birefringence splitting. We find that, for large birefringence, gain differences between the two linearly polarized modes are preponderant, and switching occurs owing to thermal shift. For small birefringence polarization switching from the high- to the low-gain mode occurs owing to the combined effect of birefringence and semiconductor phase-amplitude-coupled dynamics for a finite value of the carrier spin-flip rate.     

The role of birefringence in modulation instability fiber ring resonators

We considered a high birefringence fiber ring cavity, which can be represented by a system of two incoherently coupled nonlinear Schrödinger equations with periodic boundary conditions. The stability of the system against time domain periodic perturbations could be strongly conditioned by fiber birefringence. We apply both a perturbative approach and a mean field approximation, in order to highlight the dependence of modulation instability on birefringence and ring detuning. We extend the study of the dynamical properties of the system by means of a phase matching interpretation and a selection of numerical solutions of the governing equations.     

Polarization mode dispersion probability distribution for arbitrary distances

The probability distribution of the differential group delay (DGD) at any fiber length is determined by use of a physically reasonable model of the fiber birefringence. We show that if the fiber correlation length is of the same order as or larger than the beat length, the DGD distribution approaches a Maxwellian in roughly 30 fiber correlation lengths, corresponding to a couple of kilometers in realistic cases. We also find that the probability distribution function of the polarization dispersion vector at the output of the fiber depends on the angle between it and the local birefringence vector on the Poincaré sphere, showing that the DGD remains correlated with the orientation of the local birefringence axes over arbitrarily long distances.     

Birefringence and anisotropic optical absorption in porous silicon

The refractive indices and the coefficients of optical absorption by free charge carriers and local vibrations in porous silicon (por-Si) films, comprising nanometer-sized silicon residues (nanocrystals) separated by nanometer-sized pores (nanopores) formed in the course of electrochemical etching of the initial single crystal silicon, have been studied by polarization-resolved IR absorption spectroscopy techniques. It is shown that the birefringence observed in por-Si is related to the anisotropic shapes of nanocrystals and nanopores, while the anisotropy (dichroism) of absorption by the local vibrational modes is determined predominantly by the microrelief of the surface of nanocrystals. It is demonstrated that silicon-hydrogen surface bonds in nanocrystals can be restored by means of selective hydrogen thermodesorption with the formation of a considerable number of H-terminated surface Si-Si dimers.     

Observation of elliptical polarization rotation in a long twisted fiber

The intensity-dependent polarization rotation caused by the self-induced nonlinear birefringence in a long twisted fiber is studied numerically and experimentally. By using a 1 km length of twisted fiber, we show that the effect of ellipse rotation accumulates monotonically as if the fiber were perfectly isotropic. Such behavior cannot be observed in a typical nontwisted fiber with randomly varying birefringence and is a unique feature of the twisted fiber, in which the large circular birefringence plays an essential role of preserving the handedness of elliptical polarization. We demonstrate a novel Faraday-mirror configuration to ensure automatic polarization alignment, which is directly applicable to achieving a simple and stable intensity discriminator.     

高数值孔径双芯光纤的双折射研究

具体分析了具有高数值孔径的双芯光纤的双折射特性。首先利用超格子正交函数法和耦合模理论分析了双芯光纤的几何双折射,并将两种方法计算的几何双折射进行了比较分析。数值计算结果表明双芯光纤在两纤芯非常接近的情况下,几何双折射仍较小,只能到10-5量级。利用超格子正交函数法计算了双椭圆芯光纤的双折射,改变结构参量可使几何双折射达到10-4量级。高的数值孔径需要高的掺锗量,理论上分析了高数值孔径时双芯光纤功率集中区域的应力双折射,应力双折射接近10-4量级。设计制作出了具有良好保偏性能的双芯掺铒光纤,测试、分析了它的几何参量和折射率分布;双芯光纤双折射系数达到了8.4×10-5。双芯掺铒光纤可以作为保偏掺铒光纤,应用到制作具有稳定的单一偏振态输出的光纤激光器。     

Birefringence-induced splitting of the zero-dispersion wavelength in nonlinear photonic crystal fibers

We investigate how the strongly wavelength-dependent birefringence in nonlinear photonic crystal fibers leads to a splitting in the zero-dispersion wavelength for the two polarizations. We translate the requirements for the maximum splitting of the zero-dispersion wavelength to requirements for transverse structural uniformity by adopting a simple effective-index approach in which the birefringence is calculated in a step-index fiber with an elliptical core. We find that to reduce the splitting to less than 1 nm the birefringence should be less than 2 x 10(-5), resulting in a transverse uniformity requirement of 1-3%, depending on the index step from the core to the cladding.     

Method for improving the resolution and accuracy against birefringence dispersion in distributed polarization cross-talk measurements

We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.