Cornelia Prott

Ultralow biased widely continuously tunable fabry-Perot filter

Optical filters capable of single control parameter-based wide tuning are implemented and studied. The surface micromachined Fabry-Perot filter consists of two InP-air-gap distributed Bragg reflectors and shows a wavelength tuning of more than 140 nm using only a single voltage of up to 3.2 V at currents below 0.2 mA. The membrane-based filter is designed to block all wavelengths in the whole range of 1250-1800 nm apart from its transmission wavelength.

Modeling of ultrawidely tunable vertical cavity air-gap filters and VCSELs

Tunable vertical cavity devices including an air-gap integrated in the cavity have been designed, fabricated, and investigated. The ultrawide wavelength tuning is realized by micromechanical actuation of Bragg mirror membranes. Based on optical and mechanical model calculations, the air-gap filters and vertical cavity surface emitting lasers (VCSELs) are designed for investigating mainly the optical tuning efficiency. In our research, we focus on two different mirror material systems, dielectric Si/sub 3/N/sub 4//SiO/sub 2/ and InP/air-gap Bragg mirrors and on two tuning concepts, respectively. For the dielectric mirrors, continuous tuning is achieved by thermal actuation of the Si/sub 3/N/sub 4//SiO/sub 2/ mirror membranes, and for InP/air-gap mirrors, electrostatic actuation of the InP membranes is used. To verify the optical and mechanical simulations, InP/air-gap filters are characterized by measuring reflectance spectra and the tuning behavior. The measured results agree with the simulations used to optimize the micromechanical and optical characteristics of air-gap filters and VCSELs for optical communication applications.

Tuning efficiency and linewidth of electrostatically actuated multiple air-gap filters

We investigated the tuning efficiency of electrostatically actuated multiple air-gap filters fabricated in InP for dense wavelength division multiplex applications by comparing measured tuning curves with the results of optical and mechanical simulations. These filters exhibit a record tuning range of 127 nm at 7.3 V tuning voltage. The filters were measured in reflection using standard single mode fiber. The subsequent analysis is based on a one-dimensional electromechanical and optical model providing a reasonable estimation for the pull-in voltage. Optical simulations show that the filter linewidth does not affect the tuning efficiency.

A novel low-cost tunable dielectric air-gap filter

Dense wavelength division multiplex (DWDM) systems is a promising technology for long-haul networks using the established fiber networks. Tunable devices such as optical filters, highly selective photodetectors, as well as lasers are considered to be key components for dynamic WDM systems. A novel low-cost tunable dielectric filter consisting of an air-gap cavity embedded by two DBRs is presented. A FWHM of 8 nm and a tunability of 15 nm/mA at 2 k/spl Omega/ heating resistance is obtained.

Potential of micromachined photonics: miniaturization, scaling, and applications in continuously tunable vertical air-cavity filters

In technology and nature, tailored scaling represents a principle of success which allows the effectiveness of physical effects to be enhanced. For our optical microsystems, we state that appropriate miniaturization increases the mechanical stability and the effectiveness of spectral tuning by electrostatic and thermal actuation since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55μm vertical-resonator-based filters, capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps which are part of a vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (I) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (II)Δn=0.5 for Si3N4/SiO2 DBR’s (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >9% of the absolute wavelength is obtained. Varying the reverse voltage (U=0 .. 3.2V) between the membranes (electrostatic actuation), a tuning range up to 142nm was obtained. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. Principles of III/V semiconductor micromachining and the detailed technological fabrication process of our filters are focused.

Novel low-cost and simple fabrication technology for tunable dielectric active and passive optical air-gap devices

A novel low cost technology for fabrication of micro-opto-electro-mechanical devices based on plasma enhanced chemical vapor deposition (PECVD) of dielectric materials is presented. Applying surface micromachining, we produce suspended dielectric membranes and cantilevers by involving a common photo resist as sacrificial layer. The intrinsic stress in the layers is adjusted using an interlacing of high (13.56MHz) and low (130kHz) plasma excitation frequencies in the PECVD. A diffraction image method and microstructures are used for the homogeneous stress evaluation. The stress of silicon nitride can be varied in a wide range between +850MPa compressive and −300MPa tensile and no dependence of the frequency on silicon dioxide intrinsic stress is noticed. Depending on lateral design and gradient stress variation, Fabry-Perot filter membranes with radius of curvature (ROC) between −1.7mm and 51mm as well as cavity lengths between 2.3μm and 13.5μm are implemented. Thus, convex, concave and plane membranes are produced. Furthermore, a thermally tuned air-gap Fabry-Perot filter with 8nm FWHM and a tunability of 15nm/mA is fabricated. Strategies of combining these filters with organic laser materials are developed. For this purpose, molecular glasses capable of amplified spontaneous emission (ASE) are chosen, e.g. the molecular glass 4-Spiro which shows an amplified spontaneous emission line at a low threshold of 3.2μJ/cm2 pump laser power density.

Continuously tunable air gap micro-cavity devices for optical communication systems

We present ultra-widely tunable micro-cavity devices realized by micro-opto-electro-mechanical system (MOEMS) technology. We modeled, fabricated and characterized 1.55μm micromachined optical filter and VCSEL devices capable of wide, monotonic and kink-free tuning by a single control parameter. Our vertical cavity devices comprise single or multiple horizontal air-gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air-gaps are implemented. Due to the high refractive index contrast between air (n=1) and InP (n=3.17) only 3 periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500nm. Unlike InGaAsP/InP or dielectric mirrors they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of outmost importance for the fabrication of MOEMS devices. By controlling the stress we are able to fabricate InP membranes which are extremely thin (357nm thickness) and at the same time flat (radius of curvature above 5mm). Micromechanical single parametric actuation is achieved by both, thermal and electrostatic actuation. Filter devices with a record tuning over 127nm with 7.3V are presented.

Theoretical model calculations of long-haul edge-emitting communication lasers: comparison of fractally mixed, fine-pitched and continuously chirped DFB gratings

We have studied index-coupled semiconductor distributed feedback (DFB) lasers and compared three types including continuously chirped gratings, fractally mixed gratings and fine-pitched gratings. Using transfer matrix model calculations self-consistently combined with the set of rate equations, important static and dynamic laser properties are compared and it is found that they can be designed to be nearly identical above laser threshold for the three types of lasers. Therefore, the grating configuration involving the lowest effort and production cost can be selected: continuously chirped DFB lasers implemented by bent waveguides. These gratings are very attractive since they show high performance and have the potential of production at lower cost.

Wide and continuous wavelength tuning of microcavity devices for optoelectronic applications

Ultra-widely tunable microcavity devices implemented by surface micromachining are studied. We model, fabricate, and characterize 1.55-μm vertical-resonator-based optical filters and vertical cavity surface emitting lasers (VCSELs) capable of wide, monotonic, and kink-free tuning by a single control parameter. Our devices are comprised of single or multiple horizontal air gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air gaps are implemented. Due to the high refractive index contrast between air (n = 1) and InP (n = 3.17), only three periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500 nm. Unlike InGaAsP/InP or dielectric mirrors, they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of utmost importance for the fabrication of these devices. By controlling the stress, we are able to fabricate InP membranes that are extremely thin (357 nm thick) and at the same time flat (radius of curvature above 5 mm). Micromechanical single parametric actuation is achieved by both thermal and electrostatic actuation. Filter devices with a record tuning more than 142 nm with 3.2 V are presented.

Record wavelength tuning of 127 nm for vertical cavity Fabry-Perot filter

Demonstrated micromechanically tunable Fabry-Perot filters with record electromechanical and optical performance. Wavelength tuning of 127nm applying only 7.3V is achieved. The stop-band of these filters is extremely wide (1250nm - 1800nm). These results open new application fields for micromechanically tunable filters, such as spectrometry for environmental and process analysis, medical diagnostics and colorimetry.