In recent times, there has been extensive research on fiber-optic imaging devices in order to enable imaging/sensing at a size scale inaccessible to other modalities. The design for fabrication of a highly sensitive fiber-optic ultrasound detector is proposed. The transducer employs a polymer Fabry–Pérot resonator for ultrasound detection. To enhance acoustic sensitivity, a method is proposed for fabricating a self-aligned polymer waveguide within the cavity to improve the resonator quality factor (Q-factor). Simulation studies were conducted to evaluate feasibility and quantify the improvement in Q-factor for different transducer configurations. Results show that with dielectric mirrors and a waveguide, Q-factor can approach the order of 10,000. Additional simulation studies are presented to analyze the effect of cavity shape on the device performance, drawing out the importance of a flat mirror for waveguided devices. Subsequently, results from optical testing of the first iteration of fabricated devices are presented, highlighting the main drawback of this method—the nonideal shape of the waveguiding pillar. Finally, initial results from the second iteration of devices that overcome this drawback are presented, demonstrating the feasibility of creating straight self-aligned polymer waveguides on gold-coated fibers, followed by a discussion on the implications of this work and future steps.
A highly sensitive fiber-optic Fabry-Perot photoacoustic transducer is proposed in this work. The transducer will consist of separate transmit and receive fibers. The receiver will be composed of a Fabry-Perot Ultrasound sensor with a selfwritten waveguide with all-optical ultrasound detection with high sensitivity. In previous work, we have shown an increase in resonator Q-factor from 1900 to 3200 for a simulated Fabry-Perot ultrasound detector of 45 μm thickness upon including a waveguide to limit lateral power losses. Subsequently, we demonstrated a prototype device with 30nm gold mirrors and a cavity composed of the photosensitive polymer Benzocyclobutene. This 80 µm thick device showed an improvement in its Q-factor from 2500 to 5200 after a selfaligned waveguide was written into the cavity using UV exposure. Current work uses a significantly faster fabrication technique using a combination of UV-cured epoxies for the cavity medium, and the waveguide within it. This reduces the fabrication time from several hours to a few minutes, and significantly lowers the cost of fabrication. We use a dip-coating technique to deposit the polymer layer. Future work will include the use of Dielectric Bragg mirrors in place of gold to achieve better reflectivity, thereby further improving the Q-factor of the device. The complete transducer presents an ideal solution for intravascular imaging in cases where tissue differentiation is desirable, an important feature in interventional procedures where arterial perforation is a risk. The final design proposed comprises the transducer within a guidewire to guide interventions for Chronic Total Occlusions, a disease state for which there are currently no invasive imaging options.
A highly sensitive Fiber-Optic Fabry Perot Ultrasound sensor with a self-written waveguide is presented in this work. A simulated device using Gold mirrors showed periodic resonance with Q-factor 1900 for 45 μm thick devices. Including a waveguide to limit lateral power losses resulted in improvement of Q-factor to 3200. Simulations also indicated greater improvement in Q-factor upon the introduction of waveguide with larger device thicknesses.
Subsequently, a prototype was fabricated on a single mode optical fiber. Benzocyclobutene was chosen as the cavity medium as it undergoes a refractive index change upon exposure to UV. The refractive index change in BCB upon UV exposure was studied using a phase grating. Upon confirming that 2-hour exposure produced a change of 0.004, a self-aligned waveguide was written into the cavity. A consequent increase in Q-factor from 2500 to 5200 was seen for an 80 μm thick device.
Simulation studies indicate further improvement when incorporating dielectric Bragg mirrors instead of Gold, with Q-factors of 6400 and 10200 with and without the waveguide. Therefore, the proposed design includes Dielectric Bragg mirrors as well as a self-aligned waveguide.
The fabrication techniques being fairly controlled and automated, this device is highly suitable for mass-manufacturing, making is possible to produce as an inexpensive, disposable device. A potential application is to integrate it within a commercial guidewire to create a smart guidewire capable of detecting vascular vessel walls in order to guide interventions for Chronic Total Occlusions, reducing risk of wall perforation, which is currently an unmet clinical need.