Silicon photonic micro-ring resonators to sense strain and ultrasound

Abstract

We demonstrated that photonic micro-ring resonators can be used in micro-machined ultrasound microphones. This might cause a breakthrough in array transducers for ultrasonography; first because optical multiplexing allows array interrogation via one optical fiber and second because the silicon-on-insulator technology allows cost-effective fabrication. To understand this microphone, all of its components were studied: fundamental theory of the photonic resonators, experimental characteristics of the resonators, and the effect of a static deformation of the resonator. The most familiar use of ultrasound is observing unborn children but ultrasonography is widely used in medical and industrial applications. Today's clear ultrasonic images are made by digital focusing which requires an array of transducers that records the sound at a number of positions spaced less than a wavelength. Typical sound frequencies are 1-40 MHz with corresponding wavelengths of 0.04-1.5 mm in water. Conventional ultrasound transducers employ piezo-electric material to convert sound pressure to an electronic signal. An array requires individual fabrication, placement and wiring of these transducers. Last decades, micro-machined ultrasound transducers (MUTs) have received large interest. Array MUTs are fabricated and wired directly as a single silicon "chip". This micro-machining technology leverages the cost-effective wafer-scale CMOS technology that was developed by the semiconductor industry. A MUT consists of a flexible membrane that is sensitive to ultrasonic pressure waves, like a drumhead. The membrane deformation is most commonly measured by recording the electrical capacitance between the membrane and a fixed bottom plate. Unfortunately, these MUTs do not meet the sensitivity of piezo-electric transducers. Moreover, electrical transducers normally require a coaxial wire for each array element. The size of this wire bundle is, for example, problematic in medical intravascular ultrasonography (IVUS), where atherosclerosis is diagnosed from a high-resolution ultrasonograph of the artery wall that is obtained by catheter which is brought inside the blood vessel. We propose a new type of ultrasound microphone that consists of a silicon photonic micro-ring resonator integrated in the membrane of a MUT. Incident ultrasonic pressure waves deform the membrane and thus deform the resonator, thereby shifting its optical resonance frequencies. This shift is accurately recorded by an external interrogation system. Next to the resonators, it is possible to integrate tiny optical multiplexers on the same chip so that many resonators can be simultaneously interrogated via a single optical fiber. Moreover, the all-optical sensor can be used in MRI scanners. We proved the operation principle of this new ultrasound microphone. The designed, fabricated and characterized microphone consists of a photonic racetrack-shaped resonator (footprint 50 ?m by 10 ?m, height 0.220 ?m) that is integrated in an acoustically resonant silicon-dioxide membrane (diameter 0.124 mm, height 2.5 ?m). Fabrication of the microphone demonstrated successful integration of silicon photonic circuits in silicon micro-mechanical systems. First the photonic circuit was fabricated in a semi-industrial CMOS line. Second the membrane was fabricated by etching a hole from the back-side of the wafer using a Bosch etch process. The photonic micro-ring resonator was interrogated using a laser and a photo-receiver, providing a minimal detectable wavelength shift of 36 fm. We measured an ultrasonic minimal detection level (noise equivalent pressure) below 1 Pa which is on the same order of magnitude as the state-of-the-art of PZT piezo-electric based transducers. The microphone showed an acoustical resonance around 0.75 MHz with a -6 dB bandwidth of 20%. We only studied the most simple configuration of this microphone and there is a lot of room for improvement. The relation between a deformation of the micro-ring resonator and the shift in the resonance wavelengths was studied in a well-defined static mechanical setup. Depending on the width of the waveguide and the orientation of the silicon crystal, the linear wavelength shift per applied strain varies between 0.5 and 0.75 pm/microstrain for infrared light around 1550 nm wavelength. The influence of the increasing ring circumference is about three times larger than the influence of the change in the propagation speed of the light through the waveguide (effective index), and the two effects oppose each other. The strong dispersion in silicon sub-wavelength waveguides (400 nm by 220 nm) accounts for a decrease in sensitivity of about a factor two. The optical characteristics of the micro-ring resonators and their components were extensively studied. Different methods to characterize directional couplers (direct and in ring-resonators) gave similar results. An interesting observation was that directional couplers introduce a large coupling-induced phase delay when nearly all light couples from one waveguide to the other. Most properties of silicon ring resonators and their components can be computed using approximate analytical theories. Many theories on integrated optics were originally derived for low-index-contrast waveguides like optical fibers (?n < 0.1). We reviewed and revised those theories for application to silicon-on-insulator waveguides which have a very high index contrast (?n ~ 2). This work is formulated such that it can be used in a university course with only basic theory of electrodynamics as prerequisite. Analytical theories provide insight and allow fast computation of the behavior of photonic devices and circuits. In conclusion, we studied silicon photonic micro-ring resonators and their application in mechanical sensing. Application of these sensors in micro-machined ultrasound transducers opens new opportunities for ultrasonic array technology.