Wavelength Division Multiplexing has become the leading technology for optical transmission systems which operate at 1550 nm. One of the key components of such systems are tunable and wavelength selective receivers. In this paper we present a fibre-coupled two-chip receiver front end, which is highly wavelength selective and tunable over a wide wavelength range. The device is a bulk-micromachined Fabry-Perot pin-photodiode, which features a high finesse of more than 220 with a sufficient tuning range (> 40 nm) to cover wide wavelength region. The bandwidth (full-width half maximum) of the device is < 0.2 nm (25 GHz). The photocurrent crosstalk from an adjacent channel (100 GHz spaced apart) is below -30 dB. The wavelength tuning is achieved by a change in the resonator length, formed by the two chips. This is realized by current induced thermal heating on top of the membrane mirror suspensions, which deflects the membrane. The optical-electrical conversion takes place in the pin-photodiode. This integration reduces the need for any additional components. Fiber-coupling is achieved with a fiber-coupled lens that tailors the Gaussian beam to match with the Fabry-Perot cavity. The alignment process of the two-chip structure, forming the wavelength selective cavity, has been simplified to the point where a simple place-and-fix strategy can be applied.
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, polarisation insensitivity and large tuning range there is a strong demand for cost effectiveness and reliability. Two-chip micromachined filters are very promising candidates to fulfil these demands. In this paper we present and discuss a tunable optical filter structure which uses a simple bulk-micromachining process based on low-cost dielectric Bragg mirrors. The tuning is achieved by current induced thermal heating of the membrane suspensions. Common micromachined tunable optical filters either employ semiconductor Bragg mirrors with current induced heating or dielectric membrane mirrors with electrostatic actuation. The new concept combines the advantages of both types, the low-cost dielectric material and the simple actuation principle by current flow to create a best-of-breed two-chip solution. The alignment process of the two-chip cavity has been simplified to the point where a simple place-and-fix strategy can be applied. By matching the exciting Gaussian input beam to the stable half-symmetric cavity a fiber coupled and packaged tunable optical filter has been realized based on this concept. These micromachined tunable membranes are in general applicable to a wide variety of tunable components for wavelength division multiplexing systems, such as tunable optical filters, receivers and vertical cavity surface emitting lasers (VCSEL).
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.
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 (SiO<SUB>2</SUB>/Si<SUB>3</SUB>N<SUB>4</SUB>) 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<SUP>-1</SUP>. 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.