Efficient and accurate vector mode calculations by beam propagationmethod

An in-depth study of the beam propagation method for the vector mode calculation of optical waveguides is presented. It is established that the imaginary-distance propagation method may be generalized to vectorial modes for arbitrary structures by combining it with the finite-difference vector beam propagation method (FD-VBPM). A simple, unified theory and analysis are developed. An assessment of the accuracy and efficiency is carried out by comparing the simulated results with the analytical solutions for well-known structures     

Study of form birefringence in waveguide devices using the semivectorial beam propagation method

Waveguide devices based on nonbirefringent substrates may still have form birefringence if index discontinuities exist. It is demonstrated that the semivectorial beam propagation method can be used to study the polarization dependence in waveguide devices. It is shown that the calculated coupling lengths of planar directional couplers match within 3.5% of analytical solutions. Results on polarization dependencies of coupling lengths and bending losses of rib waveguide devices are presented. Results of rib waveguide directional couplers indicate that the form birefringence can be significant. Losses of S-shaped bends are weakly polarization dependent in the low-loss regime.<>     

Highly stable optical add/drop multiplexer using polarization beam splitters and fiber Bragg gratings

We demonstrate a novel wavelength-division add/drop multiplexer employing fiber Bragg gratings and polarization beam splitters. The multiplexer is easy to fabricate without any special technique such as UV trimming, and yet shows very stable performance with less than 0.3-dB crosstalk power penalty in a 0.8-nm-spaced, 2.5-Gb/s-per-channel wavelength-division multiplexing (WDM) transmission system.     

Pulsewidth compression in optical components with polarization modedispersion using polarization controls

An algorithm capable of finding the input and output states of polarization for pulsewidth narrowing at the output of an optical fiber with polarization mode dispersion (PMD) is analytically presented and numerically solved. It has been demonstrated theoretically and experimentally that the output pulses can he narrower than the input pulses when PMD is compensated by controlling both the input polarization and the receiver polarization states. The cost of this improvement is a reduction of the optical power in the output pulse     

Sensitive measurement of optical nonlinearities using a single beam

A sensitive single-beam technique for measuring both the nonlinear refractive index and nonlinear absorption coefficient for a wide variety of materials is reported. The authors describe the experimental details and present a comprehensive theoretical analysis including cases where nonlinear refraction is accompanied by nonlinear absorption. In these experiments, the transmittance of a sample is measured through a finite aperture in the far field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinear refraction are easily deduced from such a transmittance curve (Z-scan). Employing this technique, a sensitivity of better than λ/300 wavefront distortion is achieved in n ₂ measurements of BaF₂ using picosecond frequency-doubled Nd:YAG laser pulses     

Anomalous in-plane polarization dependence of optical gain in compressively strained GaInAsP-InP quantum-wire lasers

In-plane polarization anisotropy of optical gain in compressively strained GaInAsP-InP quantum wire (Q-wire) lasers including elastic strain relaxation induced band mixing is studied. The interaction between two-dimensional (2-D) quantum confinement and elastic strain relaxation effects is found to be complex depending qualitatively also on the wire width. Additional valence band mixing due to strain relaxation has a strong influence on the polarization dependence of optical gain. In the absence of elastic strain relaxation, gain is the maximum for tranverse electric (TE) polarization with the electric field parallel to the wire axis (TE/sub /spl par//), in agreement with the existing theory. On the other hand, when strain relaxation is strong, contrary to the existing theory, valence band mixing causes the gain to be the maximum in TE polarization with the electric field normal to the wire axis (TE/sub /spl perp//). Moreover, Q-wire lasers without suppression of strain relaxation are more likely to exhibit ground-state lasing for TE/sub /spl perp// polarization. These results suggest that in the presence of strong strain relaxation, a laser cavity parallel to the wire axis would provide higher gain. Therefore, the appropriate orientation of the laser cavity in strained GaInAsP-InP Q-wire lasers should be decided after carefully studying the polarization dependence of gain. Our calculation also shows that strong strain relaxation causes in-plane polarization anisotropy to show complex, nonmonotonic dependence on the wire width. Consequently, in such structures, in-plane polarization anisotropy may not be regarded as a direct measure of 2-D confinement effects.     

Theoretical and experimental analysis of the dependence of a signal's degree of polarization on the optical data spectrum

We show theoretically and experimentally the relationship between a signal's degree of polarization (DOP), all-order polarization mode dispersion (PMD), and the optical spectrum (and hence the data modulation format and pulse width), and that these effects must be taken into account when using the DOP for differential group delay (DGD) monitoring. We explain the theory behind how all-order PMD affects a signal's DOP, and observe the pulse-width dependence for 10-, 20-, and 40-Gb/s return-to-zero (RZ) systems as the duty cycle changes. We then analyze and show (via simulation and experimentation) the effects of different data modulation formats (RZ, carrier-suppressed RZ, alternate-chirped RZ, and differential phase-shift keying) on the DOP in a DGD monitor. We conclude that the measurable DGD range and DOP sensitivity in DOP-based DGD monitors are dependent on a signal's pulse width and the data modulation format. We also show the theory behind the effects of first- and second-order PMD on the maximum and minimum DOP.     

Origin of spurious modes in the analysis of optical fibre using the finite-element or finite-difference technique

An origin of spurious modes in the finite-element or finite-difference analysis of optical fibres is found. Such spurious modes are due to the ill-condition in the resultant matrix equation, which occurs whenever the propagation constant approaches the average between the permittivities of two consecutive node points employed.     

LD redundancy system using polarization components for a submarine optical transmission system

An LD redundancy system having one cold standby laser diode is proposed to improve reliability in submarine optical transmission systems. This system makes use of the intrinsic laser polarization and consists of two LD modules and a polarization coupler connected by polarization maintaining (PANDA) fibers. The optical power insertion loss of this LD redundant system is 5.5 dB, including coupling loss between the laser diode and PANDA fiber. This LD redundant system will be applied in submarine optical repeaters.     

Vectorial wave analysis of stress-applied polarization-maintaining optical fibers by the finite-element method

A vectorial wave analysis of stress-applied polarization-maintaining optical fibers is presented using a vectorH-field finite-element method. In this approach, the divergence-free constraint forHis imposed and the spurious, nonphysical solutions which are included in the solutions of earlier vectorial finite-element methods do not appear in a guided region. In order to verify the accuracy of solutions, numerical results for a step-index circular-core fiber are presented and compared with exact ones. We also propose an approximate method for calculating the splice loss between two optical fibers and show the normalized radiated power caused by transverse offset between two stress-applied optical fibers.     

Simulation of mode-locking by nonlinear polarization rotation in a semiconductor optical amplifier

We present a theoretical investigation of a mode locked laser that has a semiconductor optical amplifier (SOA) in its ring cavity. A mode-locked train of narrow pulses is obtained by combining nonlinear polarization rotation in the SOA and a polarization filter whose polarization axis is set such that the tail of optical pulses is removed in each cavity round-trip. The pulse narrowing process is demonstrated numerically and good qualitative agreement with experiments in our previous work is achieved. The pulse performance is largely determined by the ultrafast SOA gain dynamics and the cavity dispersion. Our simulation shows that the laser can produce a pulse train of subpicosecond pulsewidth at a repetition rate of 28 GHz for a moderate SOA current level. We observe that the laser can switch itself on or off depending on the initial pulse.     

光声信号与光束在样品上照射位置关系的研究

我们采用压电陶瓷作为光声探测器,研究了单晶硅在10.6μm的光吸收产生的光声信号与光束在样品上照射位置的关系。实验表明,当光束沿着圆形样品的直径扫描,光束照射在两侧时,光声信号随照射位置离开中心距离的加大而减小,与我们进行的理论计算符合得较好。     

The dependence of orientational optical nonlinearity in dye-doped liquid-crystal films on the polarization direction of the recording beams

We study, both experimentally and theoretically, the dependence of the absorbance and the two-beam coupling diffraction efficiency of dye-doped liquid-crystal films on the angle between the director vector of the liquid-crystal molecules and the polarization direction of the recording beams, which is shifted continuously from parallel to antiparallel corresponding to the director vector of the liquid-crystal molecules. The sample used in our experiment is the nematic liquid-crystal 5CB-doped with Disperse Red 1 (DR1). Also, there is no electronic field applied to it. The diffraction efficiency of the probe beam increases with the increase of the recording light absorption. Coupled-wave equations are developed to describe the laser-induced refractive index change, grating formation, and coherent wave-mixing effect. The analytical solutions of the recorded holographic grating and the diffraction efficiency of the probe beam are obtained for the case of taking losses into account. The results show that the diffraction efficiency can be improved by choosing suitable material parameters and recording parameters.     

Polarization crosstalk dependence on length in silica-basedwaveguides measured by using optical low coherence interference

We describe a powerful method for precisely measuring polarization crosstalk dependence on length for birefringent waveguides which uses optical low coherence interference between excited and orthogonally coupled light waves. This method is applied to 10-m long silica-based waveguides with the total polarization crosstalks of 8.9×10^-3 and 7.5×10^-3. The spatial resolution is 10 cm and the measurement error for a waveguide part longer than 1 m is ⩽10%. A comparison of measured and theoretical crosstalk curves for the waveguides enables us to confirm that the bends in the waveguides are the main origin of the crosstalk. The polarization crosstalk per bent section is ~4×10^-5     

Analysis of integrated optical corner reflectors using a finite-difference beam propagation method

Integrated optical corner reflectors in III-V semiconductors are analyzed employing a finite-difference beam propagation method and propagating the beam in parallel with the etched semiconductor-air interface. For this choice of propagation direction, the effects of mirror roughness, rotation, and displacement of the mirror surface from its ideal position can be assessed very easily. The integrated reflector whose mode size is larger shows less dependence on the mirror displacement error. The loss due to mirror surface roughness depends weakly on the mode size and strongly on the mode polarization, being larger for the quasi-transverse-electric polarization. The loss due to rotational errors of the mirror surface is not a strong function of polarization, but increases as the waveguide width increases. However, for a rotation error smaller than 0.1 degrees , which should be achieved easily, the excess loss is smaller than 0.2 dB at 1.3 % mu m regardless of the waveguide width.<>     

Observation of polarization property in near-field optical imaging by a polarization-maintaining fiber probe

We fabricated an original near-field scanning optical microscopy (NSOM) fiber probe made of polarization-maintaining and attenuation-reducing (PANDA)-type polarization-maintaining optical fiber, and observed the polarization property of propagation light in a polymer optical waveguide. The distribution of the transmission coefficient in polarization angles through this NSOM probe showed that the linear polarization is maintained in the two crossing directions: the fast and slow axes. The polarization degree parallel to the slow axis decreases from 1000:1 to 2:1 by bending the fiber probe and the decrease is independent of the bending direction. Using this PANDA-type NSOM probe, we investigated the polarization property of periodic intensity modulation. It was found that the intensity modulation was observed clearly with the electric vector parallel to the radius direction of the waveguide, but was observed vaguely with the electric vector perpendicular to the radius direction.     

Novel finite-element formulation for vectorial beam propagationanalysis in anisotropic medium

A novel full-vectorial finite-element beam propagation method for dielectric anisotropic media is presented. This computationally efficient approach uses only the transverse magnetic field components and is free of spurious solutions. Comparisons with other numerical and experimental results validate the method     

Inverse scattering using the finite-element method and a nonlinearoptimization technique

A new spatial-domain technique for the reconstruction of the complex permittivity profile of unknown scatterers is proposed in this paper. The technique is based on a combination of the finite-element method (FEM) and the Polak-Ribiere nonlinear conjugate gradient optimization algorithm. The direct scattering problem is explicitly dealt with by means of the differential formulation and it is solved by applying the FEM. The inversion methodology is oriented to minimizing a cost function, which consists of a standard error term and regularization term. A sensitivity analysis, which is carried out by an elaborate finite-element procedure, results in the determination of the direction required for correcting the profile. Significant reduction of the computation time is obtained by introducing the adjoint state vector methodology. The efficiency of the presented inversion technique is validated by applying it to the inversion of synthetic scattered far-field measurements, which are corrupted by additive noise     

Shielding effectiveness in a two-dimensional reverberation chamber using finite-element techniques

A reverberation chamber is an enclosure consisting of metal walls with a metallic paddle wheel (denoted as a "stirrer" or "tuner") forming a high quality-factor (Q) cavity with continuously variable boundary conditions. Reverberation chambers are attaining an increased importance in determining the electromagnetic susceptibility of avionics equipment. Given the nature of the variable boundary condition, the ability of a given source to couple energy into certain modes and the passband characteristic due the chamber Q, the fields are typically characterized by statistical means. The emphasis of this work is to apply finite-element techniques at cutoff to the analysis of a two-dimensional structure to examine the notion of shielding-effectiveness issues in a reverberating environment. Simulated mechanical stirring is used to obtain the appropriate statistical field distribution. The shielding effectiveness (SE) in a simulated reverberating environment is compared to measurements in a reverberation chamber. A log-normal distribution for the SE is observed with implications for system designers. The work is intended to provide further refinement in the consideration of SE in a complex electromagnetic environment.     

基于保持光注入的半导体光放大器自偏振旋转性能的改善

研究了基于“保持光”（holding beam）注入（HBI）改善半导体光放大器（SOA）自偏振旋转（SPR）性能的方法。HBI虽然降低了SOA的偏振模式增益,但是缩短了对TE和TM模式的增益恢复时间,从而加速了SOA器SPR特性对超快信号的响应。以基于SOA的SPR特性的光功率均衡为例,我们发现,HBI可以明显抑制和...