Tunable Mach-Zehnder polarization splitter using height-tapered Y-branches

A tunable TE/TM polarization splitter, based on a Mach-Zehnder interferometer with an electrooptic switch, is demonstrated in GaAs-AlGaAs. Symmetric and asymmetric Y-branches employing height-tapered waveguides are used to achieve power splitting and mode sorting, respectively, in the interferometer. The device has an extinction ratio of /spl sim/20 dB and an excess loss less than 1.5 dB for both TE and TM polarized light. The device can be reconfigured by tuning the switching voltage for operation at both 1.3- and 1.55-pm wavelengths.     

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.     

Substrate removed GaAs-AlGaAs electrooptic modulators

A novel GaAs-AlGaAs electrooptic modulator utilizing push-pull metal electrodes was fabricated using substrate removal techniques and polymer integration. The scheme enables a significant drive voltage reduction over previous designs and should enable ultrahigh-speed adjustable chirp operation     

0-1 GHz waveguide 10.6 μm GaAs electrooptic modulator

A GaAs electrooptic modulator for use in 10.6-μm laser frequency shifting applications is described. High-beam-quality output signals with a frequency tunability of 0.1-1.0 GHz were demonstrated. Thick epitaxial GaAs-AlGaAs waveguide structures were successfully fabricated into single-mode channel waveguides with excellent optical and electrooptic properties. A lumped-element modulator, with electrical and electrooptic characteristics in good agreement with theory, was operated at RF drive power levels up to the reverse breakdown voltage limit. The insertion loss of the modulator was quite high, due to absorption losses in the epitaxial material     

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     

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.     

Integrated-optic polarization transformer for reset-free endlesspolarization control

An electrooptic polarization transformer which employs a multiple cascade of alternating two-way TE-TM polarization-converter and TE/TM phase-shifter sections in a birefringent crystal, such as LiNbO₃ , and requires only three independent drive voltages is described. It is capable of transforming any arbitrarily varying input polarization into any desired output polarization without ever requiring a reset cycle. It accomplishes this by producing adjustable elliptical birefringence of constant total phase retardation in a single-mode waveguide     

A chopped quantum-well polarization-independent interferometricswitch at 1.53 μm

We have theoretically designed and realized a phase shifter for a low-loss Mach-Zehnder interferometric switch. The phase shifter is based on 0.85% tensile strained InGaAs-InP chopped quantum-well material. We realized a Mach-Zehnder interferometric switch with polarization-independent switching voltages as low as 3.3±0.05 V at 1525 nm for a switch with a 4-mm-long phase shifting section. The wavelength sensitivity of the switch is 0.036 V/nm for TE and 0.053 V/nm for TM polarization. Calculations of the electro-refraction in the -0.85% strained chopped quantum-well (QW) material based on the 4×4 Luttinger-Kohn Hamiltonian show that the electro-refraction due to the quantum-confined Stark effect (QCSE) for TM polarization is equal to the sum of the mutually comparable QCSE electro-refraction and the Pockels effect for TE polarization in waveguides along the [11¯0] axis. Our first-principle model for calculating the electro refraction is an accurate design tool for predicting device performance in complicated layer structures. The shortest possible phase shifter with a <-25 dB crosstalk penalty due to electro-absorption unbalance can be as short as 2.2 mm. This compact switch is predicted to have a 6-V switching voltage and a 15-nm window for polarization-independent switching with a <-25-dB crosstalk penalty. With a slight increase of the strain, this chopped QW material can be used for polarization independent switching around 1550 nm     

Electrooptic guided-to-unguided mode converter

Experimental demonstration of electrooptic polarization conversion from a guided TE wave to an unguided TM wave in a thin-film light guide is reported. In the specific device we fabricated utilizing Ta2O5film and LiTaO3substrate, we have observed a 40-percent modulation when 500 V were applied across two electrodes spaced 50 μm apart. We present here a ray-optics analysis of the conversion process. With the inclusion of the Goos-Haenchen shifts, the analysis agrees well with the experiment.     

Low-loss substrate-removed (SURE) optical waveguides in GaAs-AlGaAsepitaxial layers embedded in organic polymers

Low-loss single-mode semiconductor rib optical waveguides fabricated in GaAs-AlGaAs epitaxial layers are removed from GaAs substrates and bonded to transfer substrates using a benzocyclobutene organic polymer. Optical quality facets were obtained by cleaving through the transfer substrate. An average propagation loss of 0.39 and 0.48 dB/cm at 1.55 μm wavelength for TE and TM polarizations, respectively, were measured. This was on average 0.05 dB/cm greater than control guides fabricated in GaAs-AlGaAs epilayers on GaAs substrates with air as the top cladding. This demonstrates the feasibility of a process enabling semiconductor polymer integration and processing both sides of an epitaxial layer     

Push-pull polarization conversion using novel asymmetric coupled quantum-well structures

A novel type of polarization conversion modulator, employing multiple repetitions of two novel InGaAlAs-InAlAs asymmetric coupled quantum-well structures, is proposed. The novelty of the proposed design is that, via the electrorefraction effect, it allows for a change in the TM mode's effective refractive index, which is opposite in sign to the change in the TE mode's effective refractive index. Numerical simulations based on the effective-mass envelope-function approximation show that the modulator provides not only an electrorefractive response similar to other quantum-well modulators but can also provide a low-chirp push-pull polarization modulation.     

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 electrooptic BaTiO/sub 3/ thin film waveguide modulator

A strip-loaded electrooptic waveguide modulator based on an epitaxial BaTiO/sub 3/ thin film was fabricated and characterized for the first time. The strip-loaded waveguide structure greatly improves waveguide propagation and polarization-dependent loss performance. A propagation loss of 1.1 dB/cm and polarization dependent loss of 0.1 dB/cm were measured. The electrooptic waveguide modulator exhibited a half-wave voltage-interaction length product of 4.5 V /spl middot/ cm at a wavelength of 1542 nm. The measured effective electrooptic coefficient of the as-grown BaTiO/sub 3/ waveguide modulator was 38 pm/V. The experimental results indicate that a strip-loaded thin film waveguide modulator is suitable for photonic applications.     

Low-switching-voltage InGaAsP/InP waveguide interferometricmodulator for integrated optics

An InGaAsP/InP electrooptic Mach-Zehnder modulator consisting of two 3-dB Y-branch couplers and phase modulation arms is fabricated by the metal-organic vapor-phase epitaxy growth method. A schematic illustration and cross section of the interferometric modulator as well as the current-voltage characteristics of the arms are shown. The modulator is formed by a waveguide having almost the same cross-sectional configuration as a double-heterostructure laser diode. The switching voltage of 4.5 V applied to one arm of the modulator results in an excitation of about 7 dB     

Acoustooptic resonance in deep-etched GaAs-AlGaAs electrooptic modulators

Deep-etched GaAs-AlGaAs waveguide modulators are shown to generate acoustic waves that can cause elastooptic anomalies in the modulator response. The theory of acoustooptic interactions in linear electrooptic modulators is described and a one-dimensional (1-D) approximation of a deep-etched GaAs-AlGaAs waveguide modulator is developed. The 1-D theory is shown to predict the acoustooptic resonance phenomena that is seen in experimental devices.     

Electrooptic effects in GaAs-AlGaAs narrow coupled quantum wells

The linear and quadratic electrooptic coefficients in narrow single and strongly coupled GaAs-Al_xGa_1-xAs quantum wells have been measured. The quadratic electrooptic effect is enhanced over that of conventional square quantum wells for both TE and TM polarization in all the structures considered, by up to six times in the case of 2-nm-wide GaAs-Al_0.2Ga_0.8As strongly coupled quantum wells. The origin of the enhanced quadratic electrooptic effect was found to correlate with a larger red shift in the absorption edge exciton and strong Coulombic coupling of the bound exciton states with the quasi-continua     

Efficient electrooptic modulator in InGaAlAs/InP optical waveguides

A single heterostructure InGaAlAs/InP phase modulator utilizing the quadratic electrooptic effect (QEO) is reported for the first time. The calculated value of the QEO coefficient from the measurements is 3.7×10^-19 m²/V² at 80 meV below the band edge. In addition, the linear electrooptic effect (LEO) coefficient is estimated to be 1.2×10^-12 m/V, which is comparable to that of GaAs. The propagation loss of a single mode ridge waveguide is in the range of 1.5-1.7 dB/cm, which is better than the previously reported value in this material system. The measured single mode phase shifts are 5.5 and 2.8°/V mm for TE and TM polarizations, respectively. These values are the largest reported so far in an InGaAlAs system     

In-line optical fiber polarizer and modulator coated withLangmuir-Blodgett films

In-line optical fiber polarizer and modulator were fabricated by depositing Langmuir-Blodgett (LB) films on polished fiber surface with gold layer. The thickness of LB film was adjusted by changing the number of monolayers, which then optimized the performance of the fiber polarizer. Good agreement was observed between the theoretically predicted conditions for efficient coupling and the experimentally determined conditions for high TM/TE extinction ratio of the optical field. By applying an electric field between the LB film, the polarization state of the lightwave in the optical fiber can be modulated     

Analysis and design optimization of electrooptic interferometric modulators for microphotonics applications

Scaling the electrode layout (electrode gap, electrode length) down to microscale dimensions extends the application of electrooptic (EO) modulators to microphotonics. In this paper, design criteria are set up to minimize the switching voltage of microscale EO Mach-Zehnder interferometric modulators. Mach-Zehnder interferometric modulators under different directions of electric field with respect to the optic axis are analyzed. Three expressions of the intensity output characteristics are presented for various crystal classes and compared in terms of conditions of validity and design applications. The analysis in this paper suggests that the switching voltage is strongly related with the direction of the electric field relative to the optic axis. For the 4-mm BaTiO/sub 3/ (r/sub 33/=28 pm/V,r/sub 51/=820 pm/V) Mach-Zehnder modulators, r/sub 51/ is utilized when the electric field is applied normal to the optic axis. In this configuration, the thermal stability and polarization insensitivity improve but the extinction ratio becomes a function of the electrode length. The phase-retardation expression is useful to find a suitable modulator length and maximize the extinction ratio. Some of the discussions also apply to Fabry-Pe/spl acute/rot interferometric modulators.     

Polarization-independent waveguide modulators using 1.57-μmδ-strained InGaAs-InGaAsP quantum wells

We fabricated a multiple-quantum-well waveguide modulator incorporating two tensile S-strained layers, within a nominally lattice matched well, that act to provide bias-independent Stark shift for both the light and heavy holes. We report results for two waveguide geometries. One is a 2.3-μm-wide ridge single-mode waveguide that was deep-etched 0.7 μm below the active region to ensure that the confinement factor for both the TE and TM modes are nearly equal. The other was a broad-area (slab) multimode waveguide. For both geometries, transmission measurements indicate polarization insensitivity within 3 dB for 2.5 V of reverse bias over the 1.600-1.630-μm wavelength range