Proceedings of the 2nd International Conference on Natural and Environmental Sciences (ICONES) September 9-11, 2014, Banda Aceh, Indonesia ISSN 2407-2389 200 Eyes on the Sky: All-Optical Sensor Based on Nonlinear Modal Interferences Mohd Zubir Mat Jafri and Mehdi Tajaldini School of Physics, University Sains Malaysia, 11800 Penang, Malaysia Abstract. In this study, we propose an all-optical sensor based on consideration nonlinear effects on modal propagation and output intensity. The sensor can be tuned to highest sensitivity in the refractive index ranges sufficient to detect protein-based molecules, other water- soluble chemical, and air-pollutions. The nonlinear regimes show the capability to operate on any choice of materials for ridge waveguide. Crystalline polydiacetylene has high third-order susceptibility, thus could be best candidature for core material. This material in presence of nonlinear effect is studied in the multimode waveguide with NMPA method that promises to investigate the coupler in small lengths. The visible changes of field intensity at output facet in various surrounding layer refractive index show the high sensitivity to the refractive index of surrounding layer that is foundation of introducing a sensor. Also, the results show the high distinguished changes on modal propagation in various refractive index of surrounding layer. To the best of our knowledge this is the first time that a nonlinear MMI in a few micrometers is proposed as a robustness sensor. In fact, this paper brings a useful and powerful way to progress the all optical sensors based on MMI couplers. Keywords: MMI, NMPA, all-optical sensor, mode, waveguides. INTRODUCTION Multimode interference couplers [1] have recently become key elements of planar integrated photonics. These couplers have significant features such as low loss and crosstalk [2], high optical bandwidth [3], compact size [4], low sensitivity to input polarization [5], low sensitivity to operating wavelength [6], and tolerance to fabrication errors. MMI couplers have broad applications in photonic complex circuits, one of the important application is all- optical sensor especially when operated in the nonlinear regime. MMI coupler have been widely used for the variety of sensing proposes among them temperature, pressure and chemical/ biological sensors in which the change in refractive index is read out as change in output intensity [7-9].Some MMI sensors have been introduced with changes in the configuration of MMI coupler to have smaller length of self-imaging formation whereas they are modeled in long length and complicated structures [9]. The nonlinear MMI coupler as active device is more sensitive to input characterization and region environment that can be effective subjects to change the interference mechanism among the guided modes. Absolutely, nonlinear regimes might be more sensitive to the refractive index of surrounding and core layers due to the direct influence on refractive index. Therefore, nonlinear MMI coupler has the ability to show an accurate sensor. In this paper, nonlinear modal propagation analysis (NMPA) [10-16] method is studied in multimode interference waveguide as a novel and high potential approach for detecting the air-pollution, water-soluble materials, and any change in surrounding environment. Each change in surrounding layer lead to a change in refractive index or wavelength might predict different mode effective indices also interference mechanism that demonstrate the visible differences in output field profile. The high changes of output intensity that is created in effect of variety of surrounding layers prove the high performance of proposed sensor. Notably, this nonlinear device, could demonstrate reaction to others such as light, acoustic wave, and temperature, moreover, the changing in surrounding layer. NONLINEAR MODAL PROPAGATION ANALYSIS The MMI coupler is introduced to the photonic devices as the simplest structure. Although this device has broad applications in the integrated photonic circuits and telecommunications, these applications increase with the