J. Pfeiffer

III–V Compound semiconductor micromachined actuators for long resonator tunable fabry-pérot detectors

Abstract Modern telecommunication demands tunable detectors that are needed for optical demultiplexing of fibre-transmitted data. The receiver therefore consists of a wavelength-selective detector sensitive around 1550 nm. Since detectors for this wavelength are typically fabricated on gallium arsenide (GaAs) or indium phosphide (InP), a monolithic integration with the filter provides additional functionality and reduces coupling problems. This article presents a long-resonator Fabry-Perot filter that has been fabricated by bulk micromachining and can be tuned by deflection of one or both of the Bragg mirrors that are realized as floating membranes. Thermal actuation of the membrane yields mechanical sensitivities up to 153 nm mW −1 under vacuum conditions and 13 nm mW −1 under normal pressure. This is high enough to tune the free spectral range with less than 5 mW.

Modal Behaviour of Passive, Stable Microcavities

Tunable active and passive components for wavelength division multiplex (WDM) systems like vertical cavity surface emitting lasers (VCSELs) and filters show advanced optical performance at reduced production costs if the cavity structure is of stable, half symmetric type. In this paper we report about the mathematical modelling of the power of sidemodes which appear if beam and cavity geometry do not match. We present experimental data which show that the sidemodes are filtered out up to a certain degree by the fiber coupling optics. A limit for the sidemode suppression even for perfect cavity and beam geometry match was also observed. These aspects raise the question on the influence of spherical aberrations on the performance of fiber coupled devices.

Mechanical-optical analysis of InP-based Bragg membranes for selective tunable WDM receivers

Monolithically-integrated wavelength-selective receivers are needed for dense wavelength-division multiplex transmission in the 1.55 micrometers wavelength regime. We present a novel concept for tunable optical Fabry-Perot filters with long resonant cavities of about 30 micrometers . III-V semiconductor technology compatible to PIN detector integration is applied to fabricate bulk-micromachined movable membrane Bragg mirrors. The initial membrane curvature as well as the actuation-induced bending are analyzed using a white light interferometer. Continuous filter tuning achieved by thermal or electrostatical actuation is analyzed in the optical as well as in the mechanical regime. Opto-mechanical constraints of the realized filters are discussed in view of novel epitaxial demands and optimum design for micro-opto- electro-mechanical receiver systems.

Micromachined two-chip WDM filters with stable half symmetric cavity

Wavelength Division Multiplexing has become a leading technology for long haul transmission systems which operate at 1550 nm wavelength. One of the key components of such systems are tunable filters. Beside low insertion loss, polarization insensitivity and large tuning range there is a strong demand for cost effectiveness and reliability. Two-chip micromachined filters are very promising candidates to fulfill these demands. Two Bragg mirrors are deposited on distinct chips. One of them is engineered as actuable membrane. The Fabry- Perot cavity is created by proper adjustment of the two chips one against the other. Modifying the cavity length by thermal induced heating of the membrane mirror or by applying an electrostatic force provides tunability of the transmission function. Tuning stability and insertion loss can be considerably improved if a stable half symmetric cavity containing a bend membrane instead of a flat one is used. This also helps to overcome some severe fabrication problems. On the other hand the half symmetric cavity is more sensitive to mismatch between filter geometry and phase fronts of the existing Gaussian beam. This aspect and the tolerances which can be accepted are discussed in this paper in detail.

InP-based bulk-micromachined tunable filter applications for WDM systems

A micromechanically wavelength-tunable optical filter with an integrated pin-photodetector for the wavelength band around 1.55 micrometer is demonstrated. The micromechanical modification of the resonator length realized by either thermal or electrostatic actuation of micromachined Bragg reflectors is used as tuning principle. The maximum tuning of the filter can be determined by its free spectral range (FSR) in the order of 40 nm, according to a resonator length around 30 micrometer. The required micromechanical displacement of the movable Bragg mirror of 800 nm is observed for an actuation voltage of 32 V utilizing capacitive actuators, while a heating power of 1.3 mW is required for electrothermally actuated membranes. Epitaxial (InAlGaAs/InAlAs) as well as dielectric (SiO2/Si3N4) material compositions are used for the Bragg reflectors to meet the mechanical and optical demands of the filter. The experimental full width at half maximum (FWHM) of the tunable wavelength division multiplexing (WDM) filter is 0.24 nm corresponding to a finesse of F equals 180. The insertion loss at resonance wavelength is 2.8 dB, whereas the contrast between maximum and minimum transmission exceeds 40 dB. The integration of an InGaAs/InP photodiode and a bulk- micromachined Bragg mirror allows the realization of a wavelength-selective pin photodetector. We report on bulk- micromachined thermally actuated highly selective photodetectors with a maximum tuning range of 35 nm, a FWHM around 0.4 nm and a tuning sensitivity of 20 nm mW-1. The technology discussed in this paper will be compatible to opto-electronic integrated circuit (OEIC) fabrication processing based on the InP-material system and therefore will enable the realization of receiver front ends with higher functionality for future dynamic WDM systems.

Integration of InGaAs/InP Pin diodes with Fabry-Pérot filters for wavelength selective receivers

We have fabricated InGaAs/InP PIN diodes with a coplanar waveguide design. The diodes have been integrated in a two- chip wavelength-selective Fabry-Perot filter for the use as wavelength-selective PIN receivers for dense wavelength division multiplex systems.