Mira Caldararu

Photonic circuits integrated with CMOS compatible photodetectors

Abstract This paper presents the integration of photodetectors and photonic circuits (waveguides and interferometers, coupling elements and chemo-optical transducing layer) on one silicon chip. Different materials: silicon, doped or undoped silica, SiOxNy, polymers, and different technologies: LPCVD, APCVD, sol–gel, spinning, micromachining have been used to realize the photonic and micromechanical components and the transducers. Also, MOS compatible processes have been used for optoelectronic circuits. The attention was focused on the matching of all the involved technologies, to allow the monolithic integration of all components, and also on the design and fabrication of special structures of photodetectors. Two types of high responsivity photodetectors, a photo-FET and a bipolar NPN phototransistor, with modified structures that allow the optical coupling to the waveguides have been designed and experimented. An original 3-D model was developed for the system: opto-FET-coupler-waveguide. A test circuit for sensor applications was experimented. All the components of the test circuits, photodetectors, waveguides, couplers, were obtained using CMOS-compatible processes. The aim of our research activity was to obtain microsensors with optical read-out.

Autonomous system for real time air pollution monitoring using semiconductor toxic gas sensors

The measurement concentration range of SnO/sub 2/ semiconductor sensors for toxic and flammable gases (TGS) was extended for use in air pollution monitoring. A real time autonomous system for air pollution monitoring (NH/sub 3/, CO, R113 and R22 freons) using TGS sensors and an 80C552 INTEL microcontroller for data acquisition and management was tested. The air pollution concentrations measured in the industrial and residential areas of Bucharest fulfil the Romanian air pollution standards requirements as well as the US EPA ones. A simulator (SIMPA) for real time air pollution measurements was developed for the environment protection laboratories of scientific and technical universities.

Silicon optoelectronic integrated circuits for MOEMS

We realized different types of optoelectronic integrated circuits by integrating on the same silicon chip: photo detectors, linear or logic electronic circuits, waveguides, coupling elements. This paper present the design, modeling and experimental realization of these components, underlining the original approaches and results. Special structures of photo detectors were designed, in order to allow optical coupling with waveguides and monolithic integration with electronic and photonic circuits. Original models for these photo detectors were developed. The electronic circuits we realized, unlike those reported in literature, can operate at very low input currents. Also new materials and processes were studied and experimented in order to improve the component performance. Specific technologies for optoelectronic circuits, compatible with either CMOS or bipolar processes, were established by analyzing the relationships between the technological parameters and circuit characteristics. Also the matching with waveguides and micro mechanical structures technologies was analyzed and experimented, as the aim of our research activity was to realize different types of micro-electro- mechanical systems for sensor applications.

The far to equilibrium time evolution of the ozone layer: steady-state and critical behaviour

Abstract The ozone layer stability in the far equilibrium conditions was studied in the framework of the irreversible thermodynamics. Following the Prigogines procedures concerning the Brusselator model, the phenomenological model of the ozone layer leads to a system of non-linear differential equations. The steady-state solution of the system outlines the fact that the ozone layer can reach a steady state in the far equilibrium conditions for any pollutant concentration values. Applying the Lyapounovs theorem of stability, it is demonstrated that all the steady-state solutions for the ozone layer are stable ones. The natural limit of the ozone concentration is calculated, as well as that of the pollutant concentrations (the upper limits). This procedure suggests a way of assessing the effect of the anthropogenetic activities. The transmission coefficients of the UV radiation are calculated establishing the lower limits of the “permitting” pollution. The error analysis shows that the prediction of the stability of the steady state of the ozone layer does not depend on the errors of the input data.

Ozone distribution in the stratosphere-troposphere system using the interdisciplinary physics modelling

A theoretical ozone distribution in the stratosphere-troposphere system is provided using the physical analogy between the stratosphere-troposphere system and the well-known illuminated semiconductor p-n junction. The equations of a 2D model of the illuminated p-n junction were rewritten for the ozone and pollutant particles in the altitude-latitude co-ordinates. A comparison between the calculated curves and the recent experimental achievements is also given.

Monitoring of volatile organic compounds using a single tin dioxide sensor

The paper describes a new and simple method for monitoring volatile organic compounds resulting from a specific industrial indoor process. The method is based on the characteristics of tin dioxide sensors to be sensitive and non-selective to a large variety of gases. We use these characteristics to determine an equivalent concentration of the volatile organic compound mixture, a method which has not been used in other studies or experiments. The value of the equivalent concentration shows the accuracy of the process and the air purity of an industrial environment. A system including a tin dioxide sensor with its conditioning circuits and a temperature compensation circuit, a microcontroller and a graphical interface, was built in order to obtain a versatile, online and user friendly monitoring tool. The system was used for monitoring the volatile organic compound concentrations resulting from leather finishing processes in the Romanian industry.

Hardware simulator for design and characterization of MOEMS for environmental applications

The MOEMS for enviromnental applications have a complex structure architecture: a sensitive layer for environment interface, optical devices for radiation beam processing (waveguides on semiconductor) and photodiodes. The sensor includes an electronic module, used as transimpedance amplifier and phase shift detector and a radiation source coupled to the waveguide. Each of these sensor modules have specific parameters which must be optimized in order to obtain the best sensitivity for MOEMS, as well as to determine the appropriate concentration domain. The optimization process involves a great number of parameters and boundary conditions, so that a mathematical model is not sufficient. In this paper a hardware simulator for MOEMS design and characterization is presented. The structure and dynamics of environmental signal, the beani radiation processing and the power transfer from the environmental signal to the photogenerated current are physically simulated. An application of this simulator for design and characterization of aimnonia measurement MOEMS is presented.

Chemo-optical sensor for toxic gases detection

Many applications in the field of toxicology and laboratory medicine as well as some air pollution applications require low concentrations levels for toxic gas detection and its real-time monitoring. The authors have designed and manufactured an experimental integrated chemo-optical sensor on silicon containing a sol-gel layer, a silicon nitride waveguide and a silicon photodiode. The first measurements realized on an experimental structure show a good correlation with the requirements of the Romanian standards in the fields as well as EPA standards

Natural and anthropogenetic pollution limits of the stratospheric ozone

Starting from the irreversible thermodynamics stability analysis of the ozone layer in the far equilibrium conditions, an analytical procedure for separating the natural and anthropogenetic ozone pollution channels was developed. In this way the upper and the lower limits of the ozone layer concentrations were calculated, as well as, the lower (natural) limits and the upper limits (permitted pollution) of the pollutants that destroy ozone. The results are important in practical applications allowing one to include the ozone layer depletion as statements on the NO/sub 2/ and freons F11 and F12 in the air quality standards.