Integrated-Optic Polarization Controlling Devices Using Electro-Optic Polymers

Integrated-optic polarization controlling devices such as polarizers, polarization splitters, and polarization converters, are proposed and demonstrated in nonlinear optic polymers. Poling-induced birefringence in electro-optic polymers is exploited to fabricate the devices. The polymeric waveguide polarizers show low excess losses, and extinction ratios of 20.7 dB and 17.1 dB for TM-pass and TE-pass polarizers, respectively. The polymeric waveguide polarization splitters exhibit TE-TM mode splittings with crosstalk of 14.2 dB and 10.1 dB for TM and TE mode splittings, respectively. The polymeric waveguide polarization converters show successful TE/TM polarization mode conversion with conversion efficiencies of higher than 30 dB. The device employs poling-induced waveguides which have slowly rotating azimuth angle of optic axis along the light propagation direction. The novel polarization converter is insensitive to wavelength and easier to fabricate than the other polarization converters containing periodic structures.     

Poling-induced waveguide polarizers in electrooptic polymers

In order to demonstrate polymeric waveguide polarizers, photobleached polymer waveguides supporting both TE and TM modes are integrated with poling induced polymer waveguides that support either TE or TM modes. Fabrication parameters like UV exposures and poling voltages are controlled to reduce excess losses due to the mode profile mismatch between the connected waveguides. A TM-pass polarizer is realized by poling the polymer in the vertical direction. For TE-pass polarizer the polymer is poled horizontally by using four poling electrodes which enhance the uniformity of the poling field direction. The measured excess loss is less than 0.5 dB, and polarization extinction ratios for TM-pass and TE-pass polarizers are 20.7 dB and 17.1 dB, respectively.     

Polarization-insensitive transition between sol-gel waveguide and electrooptic polymer and intensity modulation for all-optical networks

An intensity modulation using a hybrid electrooptic (EO) polymer/sol-gel straight channel waveguide, useful in the 1550-nm wavelength regime is demonstrated without using Mach-Zehnder interferometric waveguide. The sol-gel waveguide is selectively buried so that a vertical transition into and out of an EO polymer coated on the sol-gel waveguide is arranged. The throughput ratio for transverse electric (TE) and transverse magnetic (TM) modes of the light coupled out of the hybrid waveguide is improved up to 0.9 dB with the help of reduced birefringence of the EO polymer after corona poling. We show that the fabrication process of such hybrid-type waveguides enables production of a phase modulator operating at 1550-nm wavelength. The fabricated straight channel waveguide modulator exhibits stable- and high-intensity modulation efficiency (82%) using a simple cross-polarization setup after the polarization dependence is reduced. We demonstrate an all wet-etching process to fabricate polymeric EO modulators.     

Fabrication of integrated-optic polarization controller usingZ-propagating Ti-LiNbO3 waveguides

The fabrication and performance of a LiNbO₃-based waveguide-type polarization controller with a Ti-indiffused waveguide along the z axis is reported. The device, designed for use at 1.55-μm wavelength and consisting of a TE/TM mode converter and a phase shifter in series, exhibited the half-wave voltages of 25 and 20 V for the TE/TM mode converter and the phase shifter, respectively. The modal birefringence and the behavior of the respective elements under applied fields are described in some detail. In addition, a preliminary performance test of the device was conducted by placing the device in a Mach-Zehnder interferometer     

TE-TM mode converter in a poled-polymer waveguide

A novel active TE-TM mode converter in a poled electrooptic polymer waveguide is demonstrated. The mode converter utilizes a 45°-off poling configuration, which can be obtained by arranging upper- and lower-stripe poling electrodes with a proper lateral displacement. The mode-conversion properties and electrooptic effect are demonstrated by using a vector-beam propagation method in a buried channel-type waveguide. Applications to integrated polymer-waveguide devices are also suggested     

Polarization dependent loss in polymeric rib channel waveguide

Large polarization dependent loss (PDL) as high as 15 dB was observed in the rib-type channel waveguide fabricated using a birefringent polymer. The PDL mechanism was explained by the mode-mode coupling between the TM mode of a rib and the TE mode of a slab waveguides. This TM-TE mode conversion is due to nonuniform off-diagonal component of the dielectric susceptibility tensor which is generated by a thermal stress during the curing process.     

Electrooptic polymer modulators operating in both TE and TM modes incorporating a vertically tapered cladding

We propose a modified electrooptic (EO) PMMA polymer waveguide structure supporting both TE and TM modes in spite of the large birefringence induced by the poling. A vertically tapered structure is employed to connect a single-mode rib waveguide and a confinement-enhanced rectangular buried waveguide. This waveguide structure can be easily fabricated by a two-step reactive ion etching (RIE) process with a shadow mask. To demonstrate TE/TM confinement and single-mode operation, Mach-Zehnder intensity modulators are fabricated by incorporating the proposed waveguide structure. The extinction ratio is better than -15 dB, which proves the single-mode operation. Half-wave voltages for TM and TE modes are 6 and 24 V, respectively.     

Low-loss and thermally stable TE-mode selective polymer waveguide using photosensitive fluorinated polyimide

A low-loss, thermally stable TE-mode selective optical waveguide was fabricated using a photosensitive fluorinated polyimide. The polymer undergoes photocrosslinking under UV exposure, thus changing its refractive index. The photocrosslinking-induced refractive index change was utilized to form channel waveguides. The propagation losses of the photosensitive fluorinated polyimide waveguides were less than 0.3 and 0.5 dB/cm for TE polarization at wavelengths of 1.3 and 1.55 /spl mu/m, respectively. The measured polarization extinction ratio was higher than 29 and 28 dB at wavelengths of 1.3 and 1.55 /spl mu/m, respectively. The refractive index of fluorinated polyimide film remains almost constant after being stored at 150/spl deg/C for 600 min.     

Integrated optical waveguide polarizer based on photobleaching-induced birefringence in an electrooptic polymer

A TE-pass waveguide polarizer is fabricated by utilizing the photobleaching-induced birefringence at room temperature in an electrooptic polymer. The polarizer consists of the photobleached waveguide supporting only TE mode, which is integrated in the middle of the etched rib waveguide supporting both TE and TM modes. It has a simple structure and requires no high temperature process like poling. The measured polarization extinction ratio is about 21 dB at the wavelengths of 1.3 and 1.55 /spl mu/m, and the estimated excess loss is about 0.4 dB.     

Birefringent properties of GaAlAs multiple quantum well planaroptical waveguides

Planar GaAlAs optical waveguides in which the waveguide core region is composed of multiple quantum wells are considered. Calculations of optical waveguide dispersion are performed to determine variations in effective refractive indexes for TE and TM modes and in the mode birefringence for large ranges of total waveguide thickness, number, and refractive index of well and barrier layers, and the ratio of well and barrier layer thickness. Ranges of these parameters which yield optical waveguides having unusually high birefringence and optical waveguides supporting single polarization planar propagation only are identified     

Wavelength-tunable electrooptic polarization conversion inbirefringent waveguides

Wavelength-tunable, electrooptic polarization conversion in a birefringent waveguide accomplished with a multiple cascade of alternating TE←→TM mode converter and TE/TM phase shifter sections is analyzed. Such polarization converters have been demonstrated in LiNbO₃ and have been used as electrooptically tunable narrowband (Δλ⩽λ₀/1000) wavelength filters with tuning ranges of at least λ₀/200. It is shown that the polarization converter requires only two independent drive voltages and provides arbitrary polarization transformations from any general input polarization to any desired output polarization with simultaneous wavelength tunability. The device is characterized by its overall transfer matrix and the optical bandwidths and tuning ranges for various electrode geometries     

Passive polarization converter in SiON technology

A passive polarization converter has been realized in silicon oxynitride (SiON) technology. The device is a grating assisted codirectional coupler consisting of segments of asymmetrically etched ridge waveguides. By using a double-masking technique, the fabrication of the device is tolerant with respect to the alignment of the required masks. Conversion efficiencies up to 0.98 (TE→TM and TM→TE) and insertion losses of 3 dB/cm have been measured. Using 2-D beam propagation method simulations, an observed beat pattern in the converter could be explained as due to a leaky mode, which is captured in the grating structure     

过模弯曲圆波导模式耦合设计

基于耦合波理论,波导轴线采用常规圆弧弯曲和改进的正弦弯曲结构,对TE01—TM11模式变换器进行全面优化分析,计算中考虑了多模、反向波、金属壁所带来的欧姆损耗以及相位重匹配等因素.以正弦弯曲设计的Ka波段TE01—TM11模式变换器的转换效率达到99%,带宽超过32%,并得出常弯曲结构中波导半径、波导曲率、变换器长度和转换效率之间的关系.     

Compact Variable ${rm TE}$-${rm TM}$ Polarization Splitter Based on Simplified Coherent Coupling in Lithium Niobate

A compact-sized electrically tunable ${rm TE}$- ${rm TM}$ mode splitter composed of a mode converter and an asymmetric Y-branch structure is presented. The asymmetric Y-branch consists of a straight and a bent waveguides to split two polarization modes based on the mode-sorting effect. To shorten the device length, a simplified coherently coupled-bending structure is utilized for the bent waveguide. Experimental results show that the device length is reduced about 52%, extinction ratios of both ${rm TE}$ and ${rm TM}$ modes are higher than 25 dB, yet the applied voltage is not significantly increased.      

Design of optical strip-loaded waveguides with zero modalbirefringence

Integrated optic devices are usually sensitive to the polarization state of light, because the two polarized modes of the waveguides that form the devices in general have different propagation constants and may also suffer from different losses. The performance of such devices becomes unstable when they are connected to single-mode fibers, as the polarization state of the output light from a practical fiber link usually fluctuates in an unpredictable manner. A simple solution could be provided by using waveguides in which the two polarized modes are degenerate, i.e., have equal propagation constants. In this paper, it is shown theoretically with the spectral index method that the quasi-transverse electric (TE) and quasi-transverse magnetic (TM) modes of a properly designed optical strip-loaded waveguide can have equal propagation constants. The conditions for achieving mode degeneracy, or zero modal birefringence, are presented and discussed. Strip-loaded waveguides with degenerate polarized modes can be used potentially for forming polarization-insensitive optoelectronic devices     

Stress-induced polymer waveguides operating at both 1.31 and 1.55 /spl mu/m wavelengths

Metal-defined polymer optical waveguides have been demonstrated for the first time. A metal strip patterned on top of a polymer slab waveguide causes a stress-induced refractive index change, providing lateral optical mode confinement within the core layer. Fabricated waveguides exhibit low propagation loss values of 1.1 dB/cm at 1.31 /spl mu/m and 1.3 dB/cm at 1.55 /spl mu/m for both TE and TM polarisations.     

TM-pass polymer modulators with poling-induced waveguides and self-aligned electrodes

Transverse magnetic (TM) mode selective polymer Mach-Zehnder modulators with self-aligned electrodes were fabricated for the first time in the electro-optic polymer material APC-CPW1 by employing a poling-induced (PI) writing method. The waveguides support only a TM polarisation, when an arbitrarily polarised optical source is incident. Modulators with a single 3 cm long driving electrode show a V/sub /spl pi// of 3.5 V at 1.55 /spl mu/m.     

Analysis and measurement of polarization conversion in a periodically loaded dielectric waveguide

The full-vectorial beam-propagation method with the improved finite-difference formula is applied to the analysis of a TE/TM mode converter based on an asymmetric periodic loaded waveguide. The polarization conversion behavior is measured at a microwave frequency. It is demonstrated that the calculated complete conversion length is in agreement with the experimental result.     

Stress analysis method considering piezoelectric effects and itsapplication to static strain optic devices

A stress analysis method considering piezoelectric effects based on the finite-element method (FEM), which can be applied to arbitrarily anisotropic material-based optical waveguide devices, is newly formulated. To produce a two-step analysis of static strain optic (SSO) and electrooptic (EO) modulations of optical waveguide devices, this stress analysis is linked to the guided mode analysis and the beam propagation analysis taking into account the refractive index changes. Numerical examples are shown for strain-induced optical waveguides and strain-induced polarization mode converters on LiNbO₃ substrates     

Polymeric waveguide polarization splitter with a buriedbirefringent polymer

A waveguide polarization splitter is demonstrated based on a low-loss polymer waveguide and a birefringent polyimide. Crosslinkable fluorinated polymers with an excellent stability and a low absorption loss are utilized for the device. The polyimide is buried under one branch of the Y-branch waveguide to enhance the birefringence between the TE and TM modes. By the adiabatic mode evolution, the TE mode is coupled to the branch with the polyimide strip, while the TM mode propagates through the other branch without the polyimide. For the device with a branch angle of 1/400 rad, we obtained a crosstalk less than -20 dB and a fiber-to-fiber insertion loss of 3.8 dB