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 RIU, making this device the most sensitive multimode interference sensor reported so far.
Highlights
The early detection and diagnosis of diseases is of vital importance, both to manage patients’ conditions and to provide them with adequate treatment
The recent COVID-19 global health crisis showed the relevance of early detection, since it has been crucial for containing the rapid spread of the disease [2,3]
As the aforementioned pandemic has shown, social distancing and confinement play an important role in flattening the epidemic curve, but thorough tracing and massive population testing should be a priority as well [4]
Summary
Among these various sensor classes, which present desirable characteristics for supporting the early detection and diagnosis of diseases, the optical sensors correspond to a potential technology that can be used to achieve the goal They can be designed to be compact, rapid/real-time, very sensitive, and label-free detection devices [4,15,16,17,18]. Several works have been presented, with the aim of developing the original device by means of utilizing a different set of modes in the interferometer (the trimodal waveguide interferometer (TriMW)) [17,28] and by improving the efficiency of light-coupling into the chip itself [16] or the desired waveguide modes [18,25,29] These works contributed to the development of the sensor, adding incremental improvements to sensitivity and signal-to-noise ratio. The sections of this paper were organized by each original contribution: first, the generalization of the concept of bimodal and trimodal sensors by using higher-order modes, how it can benefit the device quality, and the bulk sensitivity calculations (comparing with previous works results) are presented, as well as an optimized design for a 4th order sensor with an estimated limit of detection of 1.9 × 10−7 RIU; second, a novel hybrid method for excitation of the higher-order modes was developed, characterized by efficiently and selectively exciting the TE00 and TE04 modes; third, because the interferometer has a different electromagnetic field (EM) distribution compared to previous works reported in the literature, a new mechanism for detection is presented; we draw the conclusions and make the final remarks of the work
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