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SOI-based microring resonators (MRRs) have attracted extensive attentions as ultra-compact sensors. Recently, a new
structure design combining a ring and a Mach-Zehnder interferometer (MZI) was proposed as sensors for biomedical
applications, and as modulators for communications applications. In this design, the MZI uses two identical couplers,
where one arm is formed by connecting the access waveguide of the couplers, while the other arm is part of the
microring. Such a device may have only one major resonance with a high extinction ratio in a very broad wavelength
span (quasi-free spectral range, quasi-FSR), which offers a very large measurement range for sensing applications. 2×2
multimode interference (MMI) couplers are used to couple the microring and the bus waveguides as MMI couplers have
broader wavelength responses. We present the first experimental demonstration of the MMI-coupled MZI racetrack
microrings for sensing applications. Two types of MMI-coupled MZI racetrack microrings are discussed: one with wire
waveguides, and the other using slotted waveguides. For the MZI racetrack microring using wire waveguides, we
achieve a quasi-FSR of 34.3 nm near the wavelength of 1520 nm. The corresponding major resonance of the MZI
racetrack microring demonstrates a high extinction ratio of ~22.4 dB with a full-width-half-maximum (FWHM) of 1.94
nm, and a quality factor Q of ~800. On the other hand, the quasi-FSR of the MZI racetrack microring with slot
waveguides is 23.2 nm near the wavelength of 1540 nm; and the extinction ratio of the major resonance is ~24.5 dB with
λFWHM=0.82 nm and Q=~1,900. To demonstrate the uses for sensing applications, we measure the resonance shifts corresponding to the concentration change of the ambient aqueous solutions of sucrose. DI water is used as the reference for calibration to avoid any other variations, e.g. temperature change. Experiments show that the sensitivities of the MZI racetrack microring sensors with wire and slot waveguides are 101.7 nm/RIU and 166.7 nm/RIU, respectively.
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