High-Order Multimode Waveguide Interferometer for Optical Biosensing Applications

Abstract

A generalization of the concept of multimode interference sensors is presented here for the first time, to the best of our knowledge. The existing bimodal and trimodal sensors correspond to particular cases of those interference sensors. A thorough study of the properties of the multimode waveguide section provided a deeper insight into the behavior of this class of sensors, which allowed us to establish new criteria for designing more sensitive structures. Other challenges of using high-order modes within the sensing area of the device reside in the excitation of these modes and the interpretation of the output signal. To overcome these, we developed a novel structure to excite any desired high-order mode along with the fundamental mode within the sensing section, while maintaining a fine control over the power distribution between them. A new strategy to detect and interpret the output signal is also presented in detail. Finally, we designed a high-order sensor for which numerical simulations showed a theoretical limit of detection of 1.9×10−7 RIU, making this device the most sensitive multimode interference sensor reported so far.

Design of a novel hybrid multimode interferometer operating with both TE and TM polarizations for sensing applications

“…The larger the difference between the propagation constants ( ) for a given variation of the refractive index of the cladding the more sensitive the device. Thus, in order to improve device performance, it is necessary to select a pair of propagating modes that will be affected in the most distinct way when the sensing area is subject to the sample (Isayama and Hernández-Figueroa 2021 ).…”

“…With this exception, the rest of developments on MMI biosensors used modes exclusively with either TE polarization or TM polarization. The developments on excitation structures reported in (Isayama and Hernández-Figueroa 2021 ) allow not only the use of both polarizations within the MMI sensor, but also provide means to effectively and efficiently excite higher order modes as well. Using this excitation scheme, the device of Fig.…”

“… 2015 ). The bulk sensitivity may, then, be expressed as (Isayama and Hernández-Figueroa 2021 ) where is called intrinsic bulk sensitivity.…”

“…1999 ), and the first suggestion of its possible use as a biosensor, by Kribich et al (Kribich 2005 ), the planar MMI sensor has undergone several improvements, mostly within biosensing applications: increased sensitivity by use of higher order modes (Ramirez et al. 2015 , 2019 ; Isayama and Hernández-Figueroa 2021 ), better excitation structures for the propagating modes within the MMI (Liang et al. 2018 ; Ebihara et al.…”

“… 2018 ; Ebihara et al. 2019 ; Isayama and Hernández-Figueroa 2021 ), improved light in-coupling in the waveguides (Grajales et al. 2019 ), incorporation of metal under-cladding in the multimode waveguide structure (Dwivedi and Kumar 2018 ), and use of slow light enhancing the interaction between light and sample (Torrijos-Morán et al.…”