Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

Design of a 1 × 2 novel optical switch based on polarization converters and splitters

In this paper, a 1?×?2 optical switch based on two TE/TM polarization converters, one 1?×?2-polarization beam splitter and a hybrid 2?×?2-polarization beam splitter/combiner is designed and discussed. The novelty of this work resides in the design of a 2?×?2-hybrid polarization beam combiner/splitter, operating as a 2?×?2 polarization optical switch through the combining and the splitting of polarized signals issued from two TE/TM polarization controllers. The novel hybrid splitter/combiner can route an optical signal either to a bar or a cross port with an extinction ratio higher than 90 dB, thanks to the feature of polarization splitting used in this device to suppress undesired polarization states and minimize the polarization-dependent loss. We have used polarization beam converters to switch between two orthogonal modes in order to facilitate the routing of these signals through the 2?×?2-hybrid polarization splitter/combiner. We changed the polarization states of signals, in our simulation via OptiSystem, through polarization controllers, by modifying only their phase shifts between 0 rad and π rad. The proposed 1?×?2 optical switch presents an average insertion loss of 3.5 dB.

     

Polarization characteristics of index-guided surface-emitting lasers with tilted pillar structure

We report precise control of polarization states for index-guided surface-emitting lasers by tilted-etching of the laser pillar. Circular laser pillars were etched by tilting the substrate toward ~110 or ~11~0 direction with an angle of 15/spl deg/-30/spl deg/ using reactive ion beam etching. For the laser device with a diameter of 7-10 /spl mu/m, we observed selectivity of the polarization state. We found a dominant polarization with an electric field perpendicular to the tilted direction of laser pillar. The maximum orthogonal polarization suppression ratio was about 25 dB. The selectivity of polarization in the tilted laser pillar devices is interpreted to be originated from the difference in optical losses for the two waves polarized to ~110 and ~11~0 directions.     

Polarization independent demultiplexing in a polarization diversity nonlinear optical loop mirror

We present a novel technique to achieve polarization insensitive all-optical demultiplexing using standard dispersion shifted fibers in a nonlinear optical loop mirror. A polarization dependence of /spl plusmn/0.2 dB is reported with no power penalty due to change in polarization.     

SOA-Assisted Data-Polarization-Insensitive Wavelength Conversion in a PPLN Waveguide

We demonstrate a novel method of wavelength conversion in a periodically poled lithium niobate (PPLN) waveguide using polarization modulation and amplitude modulation of its pump. Polarization-insensitive, all-optical, intensity-to-polarization conversion in a semiconductor optical amplifier (SOA) is used to reduce the waveguide's polarization sensitivity from >30 dB to <1 dB. We demonstrate BER measurements of the converted signal at 2.5 Gb/s. We also demonstrate polarization modulation of the PPLN waveguide pump using cross-polarization modulation in highly nonlinear fiber (HNLF) at 10 Gb/s.     

Polarization stabilizer using liquid crystal rotatable waveplates

In standard single-mode fiber (SMF) transmission systems, a polarization stabilizer is useful for optimizing the performance of polarization dependent systems or devices. We have proposed and developed a polarization stabilizer which employs liquid crystal (LC) rotatable waveplates. It has a simple configuration, no moving parts, and endless-control characteristic. High polarization stability was confirmed under various state of polarization (SOP) fluctuations. The maximum response speed of the polarization stabilizer is about 900°/s, but this can be increased by improving the controller. We applied the polarization stabilizer to a semiconductor optical amplifier (SOA) having polarization dependence, and achieved good gain stabilization     

Polarization Rotation in Twisted Polarization Maintaining Fibers Using a Fixed Reference Frame

In previous studies addressing polarization evolution in optical fibers, the frame of reference in which polarization rotation in twisted optical fibers is described lacks the compatibility with the modeling of some fiber sensor applications. We therefore introduce a formulation of the polarization evolution in a twisted optical and birefringent fiber, which is based on a laboratory coordinate system. We employ coupled mode theory using the fundamental modes of a single mode fiber as a basis. We treat the birefringence of the polarization maintaining fiber and the twist induced birefringence as perturbations introduced into this mode system.      

Liquid-crystal polarization stabilizers on fiber arrays

We have built polarization stabilizer arrays of ultrathin liquid-crystal (LC) polarization controller (PC) arrays and a tap-type photodetector array on a fiber array. The ultrathin (35-/spl mu/m-thick) LC cells are inserted into trenches cut across the fiber array, which is affixed to V grooves in a glass plate and are used to change the phase of 1.55-/spl mu/m lights from 0.5 to 3.9/spl pi/ with the application of less than 3 Vrms. Two cascaded ultrathin LC cells, where the directions of alignment of the LCs are rotated by 45/spl deg/ relative to each other, are capable of converting light with an arbitrary input polarization to the transverse electric (TE) or transverse magnetic (TM) polarization. We fabricated a multichannel TE- and TM-polarization stabilizer of the previously mentioned 16-channel LC PC array and a feedback system made up of a multichannel tap-type photodetector array, sheet polarizer, and a computer and analog-to-digital converters. Feedback control was applied to maximize and minimize the tapped light, to convert an arbitrary input polarization to TM or TE output polarization, and stabilize it in the given state.     

Probability density functions of optical polarization states:theory and applications

We introduce the concept of probability density functions of polarization states for a more systematical approach of depolarization and the evolution of polarization in optical fiber. This concept is useful when polarization is a random function of wavelength, time and/or distance, like in depolarizers and in low-birefringent optical fiber. We give three examples of such probability density functions and present three applications     

Memory effect for polarization of pump light in optically pumpedvertical-cavity semiconductor lasers

We report that the polarization of the emission of an optically pumped vertical-cavity surface-emitting laser (VCSEL) is sensitive to the polarization state of the pump light. By measuring this memory effect for circularly polarized pump light, we determine the normalized relaxation rate of the carrier spin, Γ_s, which is a vital parameter in current theoretical models of VCSEL polarization. We find Γ_s=300±150, a value which is significantly larger than previously estimated. We also observe a memory effect for the orientation of linearly polarized pump light. This signals that, apart from the carrier spin, the VCSEL polarization is also determined by the carrier momentum     

Polarization-insensitive planar lightwave circuit dual-rate Mach-Zehnder delay-interferometer

We demonstrate a silica waveguide Mach-Zehnder delay-interferometer that uses integrated switches to demodulate either 10- or 40-Gb/s differential phase-shift keyed signals. We achieved polarization insensitivity by using a half-wave plate and reducing polarization crosstalk in the couplers.     

Direct Detection of 107-Gb/s Polarization-Multiplexed RZ-DQPSK Without Optical Polarization Demultiplexing

We demonstrate a direct detection scheme for a 107-Gb/s polarization-multiplexed return-to-zero differential quadrature phase-shift keyed signal, featuring automatic polarization demultiplexing without polarization tracking. The technique is used to study the performance under various impairments, including tight optical filtering, polarization-mode dispersion, and single-channel transmission in a dispersion-managed optical link. The performance is also compared with optical polarization demultiplexing.      

The effect of thickness of delta-strained Layers in the design of polarization-insensitive semiconductor optical amplifiers

We have analyzed the effect of thickness of delta (/spl delta/)-strained layer in the design of polarization insensitive semiconductor optical amplifiers. We considered a system consisting of a single quantum well (QW) with several different locations and varying thicknesses of /spl delta/ layer. For its analysis, we used the self-consistent approach of solving the Poisson, Schro/spl uml/dinger, and 4 /spl times/ 4 Lu/spl uml/ttinger-Kohn equations. The many-body effects of bandgap renormalization, Coulombic scattering interactions, and a non-Markovian distribution were all included in the model. We have found that up to eight monolayers produce very good polarization insensitivity (<1 dB) provided that the /spl delta/ layer is properly positioned inside QW. For a given thickness of /spl delta/ layer, we also provide the location inside QW which gives the best polarization insensitivity.     

Electrooptic guided-wave device for general polarization transformations

We propose and analyze a novel optical waveguide device which performs general polarization transformations under electrical control. By combining an electrooptic TEleftrightarrowTM mode converter and two variable phase shifters in a unique optical wavegnide circuit, an input signal with arbitrary elliptical polarization can be converted to any desired polarization. This device is well suited for active polarization control of an optical signal received from a long single-mode fiber.     

Temperature-dependent polarization characteristics of composite-resonator vertical-cavity lasers

We investigate the output characteristics from 10/spl deg/C to 160/spl deg/C of a monolithic dual-resonator vertical-cavity laser composed of three distributed Bragg reflector mirrors that separate two nominally identical optical cavities. The light output from the top ion-implanted cavity under forward bias is partitioned into two orthogonal polarizations of the fundamental transverse mode. A reverse bias of sufficient magnitude applied to the bottom oxide cavity causes the abrupt suppression of the dominant polarization and simultaneous emergence of the orthogonal polarization, consistent with wavelength dependent electroabsorptive loss in the reverse biased quantum wells of the oxide-confined cavity. We calculate the internal loss as a function of reverse bias and temperature, and characterize the polarization properties of the device based on the temperature dependence of the laser output. The polarization switching is consistent with increasing absorption with increasing temperature and decreasing absorption at longer wavelengths.     

Influence of ordering on the polarization characteristics of GaInP vertical-cavity surface-emitting lasers

We investigate the polarization characteristics of GaInP vertical-cavity surface-emitting lasers (VCSEL's) as a function of substrate misorientation and quantum-well strain. All the structures investigated show dominant polarization along the [110] direction. The polarization selectivity is found to be essentially independent of QW strain but increases significantly as the substrate misorientation is reduced from 10/spl deg/ to 3/spl deg/. These results provide clear experimental evidence that the major contribution to the polarization characteristics of GaInP VCSEL's is a consequence of ordering-induced gain anisotropy.     

Planar lightwave circuit polarization-mode dispersion compensator

In this letter, we fabricated an integrated polarization-mode dispersion compensator on a silica-based planar lightwave circuit that incorporated an endless polarization controller and a fixed polarization dependent delay line. The compensator consisted of polarization beam splitters, thermooptic phase shifters, tunable couplers, and polarization converters. We experimentally confirmed its operation at a data rate of 43 Gb/s     

Analysis of a reset-free polarization controller for fast automaticpolarization stabilization in fiber-optic transmission systems

We analyze the operation of a reset-free polarization controller comprising three endlessly rotatable wave plates: a first quarter-wave plate followed by a half-wave plate and a second quarter-wave plate, which is rotated synchronously with the first quarter-wave plate. We show that for any arbitrary angular offset between the two quarter-wave plates; the controller allows continuous, reset-free transformations from any varying general input state of polarization into any general output state. An integrated-optic realization of this scheme on z-propagation LiNbO₃ offers control speeds that well exceed the speed of natural polarization fluctuations in standard single-mode fibers, thus allowing fast automatic polarization stabilization in fiber-optic transmission systems     

Full polarization insensitivity of a 20 Gb/s strained-MQWelectroabsorption modulator

We report on a MQW electroabsorption modulator with tensile-strained wells. The device transmission is shown to be fully polarization insensitive, i.e. both in amplitude and phase. The modulation efficiency is over 20 GHz/V (bandwidth higher than 20 GHz and 1 V drive voltage) which is the highest figure of merit reported for any kind of polarization insensitive modulator. Full polarization independence is further demonstrated by 2.5 Gb/s transmission at 1.55 μm over 475 km of standard fiber without penalty at 10^-9 BER whatever the polarization     

38-krad/s 3.8-Grad Broadband Endless Optical Polarization Tracking Using LiNbO$_{3}$ Device

We demonstrate automatic endless optical polarization tracking over 3.8 Grad at up to 38-krad/s control speed with mean/maximum polarization errors of 0.068/0.185 rad. Without polarization fluctuations, mean/maximum polarization errors are 0.05/0.1 rad. Small-signal control time constant is about 2 $mu$s. Function is maintained over the wavelength range 1505–1570 nm.