Demultiplexer/Photodetector Integrated System Based on a-SiC:H Multilayered Structures
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Demultiplexer/photodetector Integrated system based on a-SiC:H multilayered structures P. Louro1,2, M. Vieira1,2,3, M. A. Vieira1,2, J. Costa1,2, M. Fernandes1,2, M. Barata1,2 1 Electronics Telecommunications and Computer Dept, ISEL, Lisbon, Portugal. 2 CTS-UNINOVA, Lisbon, Portugal. 3 DEE-FCT-UNL, Quinta da Torre, Monte da Caparica, 2829-516, Caparica, Portugal ABSTRACT In this paper we present results on the use of multilayered a-SiC:H heterostructures as an integrated device for simultaneous wavelength-division demultiplexing and measurement of optical signals. These devices are useful in optical communications applications that use the wavelength division multiplexing technique to encode multiple signals into the same transmission medium. The device is composed of two stacked p-i-n photodiodes, both optimized for the selective collection of photo generated carriers. The generated photocurrent signal using different input optical channels was analyzed at reverse and forward bias and under steady state illumination. A demux algorithm based on the voltage controlled sensitivity of the device was proposed and tested. An electrical model of the WDM device is presented and supported by the solution of the respective circuit equations. Other possible applications of the device in optical communication systems are also proposed. INTRODUCTION Wavelength division multiplexing (WDM) devices are used when different optical signals are encoded in the same optical transmission path, in order to enhance the transmission capacity and the application flexibility of optical communication and sensor systems. The use of WDM technologies not only provides high speed optical communication links, but also offers advantages such as higher data rates, format transparency, and self-routing. For these reasons WDM devices are crucial in fiber-optics sensing systems and in optical communication systems. Various types of available wavelength-division multiplexers and demultiplexers include prisms, interference filters, and diffraction gratings. Currently modern optical networks use Arrayed Waveguide Grating (AWG) as optical wavelength (de)multiplexers [1] that use multiple waveguides to carry the optical signals. In this paper we report the use of a monolithic WDM device based on an a-Si:H/aSiC:H multilayered semiconductor heterostructure that combines the demultiplexing operation with the simultaneous photodetection of the signal. The device makes use of the fact that the optical absorption of the different wavelengths can be tuned by means of electrical bias changes or optical bias variations. This capability was obtained using adequate design of the multiple layers thickness, absorption coefficient and dark conductivities [2, 3]. The device described herein operates from 400 to 700 nm which makes it suitable for operation at visible wavelengths in optical communication applications. DEVICE CONFIGURATION The device is a multilayered heterostructure based on a-Si:H and a-SiC:H produced by PECVD at 13.56 MHz radio frequency. The
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