INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 3, ISSUE 7, JULY 2014 ISSN 2277-8616 314 IJSTR©2014 www.ijstr.org Multimode Interference Biosensor Working With Multiple Wavelengths And Two Polarizations Moisi Xhoxhi, Partizan Malkaj, Tatjana Mulaj, Alma Dudia, Aurel Ymeti Abstract: In this paper is presented a new design for Young Interferometer (YI) biosensor and the analysis of output power of a MMI waveguide that will be used in it. The waveguides are simulated with OptiBPM software, which is a Waveguide Optics Modeling software from Optiwave Coorporation. Output power and excess loss is analysed for 15 and 110 planar MMI waveguides for different wavelengths with TE and TM polarization. It is demonstrated that output power decreases exponentially with the increase of the wavelength for both polarizations. The evaluation of the excess loss shows that it is higher for a TM polarized field for all the wavelengths and periodicities considered. Power imbalance seems to have small values suggesting the use of MMI waveguides as good power splitters. A comparison of the excess loss between 15 and 110 MMI waveguides shows that it remains almost the same for both polarizations for the optimum wavelength ( = 647 nm). Index Terms: biosensor, integrated optics, MMI structures, waveguides, multichannel interferometer, multiple wavelength, two polarizations, virus detection ———————————————————— 1 INTRODUCTION BIOSENSORS are a new technology in medical diagnostics that have had a big impact and have accelerated the investigation in many research fields where the most important is biomedical analysis. They are used to detect and measure the presence of a specific analyte (microorganism or chemical substance that is subject to analysis) in a specific sample (blood, serum, beverage etc) by means of a recognition system. Hundreds of different biosensors exist nowadays depending on their design and working principle. Most of the research in this field until now has been focused on improving their sensitivity, selectivity, and stability, while little research is done to reduce their high cost of production and increase their measurement capacity. In this paper is presented a new design for the integrated optics Young Interferometer (YI)[1] that addresses the two latter issues. The original biosensor, as shown in Fig. 1, is based on Young’s Interferometer (YI) and combines the optical output from four channels to form interference fringes on a CCD camera. Sensing is accomplished by measuring the shift in phase of the output beams due to the changes in the binding surface. The new design that we propose is composed of two MMI waveguides one after another, as shown in Fig. 2b. This design offers more output channels for simultaneous measurements, in this way increasing its measurement capacity. Because MMI waveguides have excellent properties as compact size, low loss and large fabrication tolerances [4] they are suitable to be used in biosensors and bioassays to reduce their cost. The main component of the new design is the first MMI waveguide, shown in Fig. 4, which is the subject of our analysis. The thickness of the core layer, height of the channel ridge, thickness of the substrate and cover layer are the same as the Young Interferometer [1]. Table 1 presents all the parameters for the first MMI waveguide. We report on the output power and excess loss when changing different parameters such as wavelength, polarization and periodicity of replicas of the first MMI waveguide because it directly affects the quality of measurements in the biosensor. Fabrication tolerances are independent from the number of output channels, N, but are proportional to the output channel separation [2]. Therefore, when increasing the number of output channels, we expect less fabrication tolerances. Standard splitters based on X- and Y- junction design suffer from high reflection and radiation losses due to branching complexity [3]. On the other hand, there has been a growing intereset in the application of the multimode interference (MMI) waveguides in integrated optics [4], [5], due to their excellent properties and ease of fabrication. They are quite easy to design and are compatible with both weakly guided and strongly guided structures [5]. Fig. 1. Schematic representation of the four-channel integrated optical YI biosensor: 1, 2 and 3 indicate the measuring channels, and 4 is the reference channel. Adapted from ―Fast, Ultrasensitive virus detection using a Young Interferometer sensor‖ by A. Ymeti et al., 2007, Nano Letters, 7, p. 394-397. Copyright 2007 by the American Chemical Society.