We experimentally demonstrate an interrogation procedure of a ring-resonator ultrasound sensor using a fiber Mach-Zehnder interferometer (MZI). The sensor comprises a silicon ring resonator (RR) located on a silicon-oxide membrane, designed to have its lowest vibrational mode in the MHz range, which is the range of intravascular ultrasound (IVUS) imaging. Ultrasound incident on the membrane excites its vibrational mode and as a result induces a modulation of the resonance wavelength of the RR, which is a measure of the amplitude of the ultrasound waves. The interrogation procedure developed is based on the mathematical description of the interrogator operation presented in Appendix A, where we identify the amplitude of the angular deflection Φ on the circle arc periodically traced in the plane of the two orthogonal interrogator voltages, as the principal sensor signal. Interrogation is demonstrated for two sensors with membrane vibrational modes at 1.3 and 0.77 MHz, by applying continuous wave ultrasound in a wide pressure range. Ultrasound is detected at a pressure as low as 1.2 Pa. Two optical path differences (OPDs) of the MZI are used. Thus, different interference conditions of the optical signals are defined, leading to a higher apparent sensitivity for the larger OPD, which is accompanied by a weaker signal, however. Independent measurements using the modulation method yield a resonance modulation per unit of pressure of 21.4 fm/Pa (sensor #1) and 103.8 fm/Pa (sensor #2).

Silicon photonic platforms offer relevance to large markets in many applications, such as optical phased arrays, photonic neural networks, programmable photonic integrated circuits, and quantum computation devices. As one of the basic tuning devices, the thermo-optic phase shifter (TOPS) plays an important role in all these applications. A TOPS with the merits of easy fabrication, low power consumption, small thermal time constant, low insertion loss, small footprint, and low crosstalk, is needed to improve the performance and lower the cost of the above applications. To meet these demands, various TOPS have been proposed and experimentally demonstrated on different foundry platforms In this paper, we review the state-of-the-art of TOPS, including metal heater, doped silicon, silicide, with silicon substrate undercut for heat insulation, folded waveguide structure, and multi-pass waveguide structure. We further compare these TOPSs and propose the directions of the future developments on TOPS. Graphical abstract

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We present a compact integrated photonics interrogator for a ring-resonator (RR) ultrasound sensor, the so-called MediGator. The MediGator consists of a special light source and an InP Mach-Zehnder interferometer (MZI) with a 3 × 3 multi-mode interferometer. Miniaturization of the MZI to chip size enables high temperature stability and negligible signal drift. The light source has a −3 dB bandwidth of 1.5 nm, a power density of 9 dBm/nm and a tuning range of 5.7 nm, providing sufficient signal level and robust alignment for the RR sensor. The mathematical procedure of interrogation is presented, leading to the optimum MZI design. We measure the frequency response of the sensor using the MediGator, giving a resonance frequency of 0.995 MHz. Further, high interrogation performance is demonstrated at the RR resonance frequency for an ultrasound pressure range of 1.47 − 442.4 Pa, which yields very good linearity between the pressure and the resulting modulation amplitude of the RR resonance wavelength. The measured signal time traces match well with calculated results. Linear fitting of the pressure data gives a sensor sensitivity of 77.2 fm/Pa. The MediGator provides a low detection limit, temperature robustness and a large measurement range for interrogating the RR ultrasound sensor. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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