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.
CMOS-compatible ring-based active devices have attracted significant attention for their ability to confine and
manipulate light on a compact SOI platform. Active modulation of a ring resonator is typically achieved by changing the
intensity response. As an alternative to intensity modulation, the phase modulation of the ring resonator can be converted
into intensity modulation of a Mach-Zehnder interferometer (MZI) by means of a ring-assisted Mach-Zehnder
interferometer (RAMZI) structure. We theoretically demonstrate an all-optical single resonance switching using a silicon
RAMZI by optically controlling the intracavity loss of the side-coupled silicon ring based on inverse Raman scattering
(IRS). The RAMZI structure improves the modulation robustness against fabrication deviations by relaxing the coupling
condition for the ring resonator, without compensating the modulation performance. In silicon, the IRS produces optical
loss with a bandwidth of 105 GHz at the anti-Stokes wavelength, which blueshifts 15.6 THz from the control light. For
our proposed RAMZI structure, the IRS induced loss is spectrally wider than the linewidth of the side-coupled ring, but
narrower than the free spectral range (FSR) of the ring, guaranteeing single resonance selectivity. When the control light
pulse of 200 ps switches from “off” (zero) to “on” (20pJ), the transmission of the anti-Stokes resonance transfers from
1.7% to 92.3%. The proposed structure provides the potential to multichannel all-optical routers on a CMOS compatible
platform.
Optical biosensors on silicon platforms have demonstrated their great potential in label-free detection and analysis tool.
The major challenge of ring resonator based optical biosensors is their high sensitivity to temperature variations. We
have designed a double-ring resonator biosensor using a vertical coupling method. Simulation results show that the
double-ring configuration effectively eliminates the temperature and environmental fluctuations by the resonant
wavelength shift correction induced from the reference ring. The coupling gap in the vertical coupling method can be
tuned by timing the deposition/growth rate of the space layer, without the need of any advanced lithography. The vertical
coupling method allows a low fabrication cost.
We demonstrate low-loss photonic wire waveguides, in both the straight and bent waveguide configurations, fabricated
by the LOCal Oxidation of Silicon (LOCOS) process, using the standard optical lithography. The oxidation in the
LOCOS process produces waveguides in submicron dimensions with ultra-smooth sidewalls. The Full-Width Half-
Maximum (FWHM) of the fabricated LOCOS wire waveguide is approximately 650 nm and the height is 280 nm. We
used the cut-back method to measure the propagation loss of the TE (x-polarized) mode. The average propagation loss
measured by the cut-back method was 8.78 dB/cm, while the minimum measured propagation loss achieved was 7.18
dB/cm for simple straight waveguides. The propagation loss is expected to be lower, as we include the scattering loss in
the measurements. The measured bending loss of the LOCOS wire waveguide with a bending radius of 5 um is as low as
0.0089 dB/90° bend for the TE mode. To the best of knowledge, this is the first direct measurement in propagation loss
and bending loss for LOCOS wire waveguides.
A silicon-based athermal double-ring resonator biosensor with a vertically coupled configuration is developed. We
present an optimal design of the sensor structure by specifying the radii of the reference and the sensing rings, the
vertical coupling offset, d, between the two rings and the bus waveguide, and the lateral offset, l, between the edges of
the rings and the bus waveguide. By using Lumerical software package, we demonstrate that the optimal vertical and
lateral offsets are d=325 nm and l=-80 nm, respectively. One major challenge faced by ring based biosensors is their
temperature dependent characteristics. In this study, the sensing ring is exposed to the biomaterial under test, while the
reference ring provides a temperature-insensitive reference to the sensing measurements. By assuming the biomaterial
medium has small variations in temperature, we conclude that the proposed biosensor device offers temperature
insensitive measurement, where the temperature effects are fully corrected by the reference ring response. The double-ring
sensors are proposed to be fabricated with the local oxidation of silicon process, without the need for advanced
lithography methods such as e-beam or deep UV lithography. In addition, the vertically coupled double-ring
configuration allows precise control of the critical coupling separation between the rings and the bus waveguide. The
proposed silicon double-ring biosensor can be used for highly sensitive and stable sensing for both biomedical and
environmental applications.
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