# Circular Array Fiber-Optic Sub-Sensor for Large-Area Bubble Observation, Part I: Design and Experimental Validation of the Sensitive Unit of Array Elements

## TL;DR

A new buoy-type bubble sensor is designed to measure microbubbles in the ocean using a fiber-optic sub-sensor with a circular array.

## Contribution

The design and experimental validation of a circular array fiber-optic sub-sensor for bubble detection is presented.

## Key findings

- The optimal cone angle for the quartz fiber-optic sensitive unit ranges from 45.2° to 92°.
- A prototype with a 90° cone angle showed reflected optical power differences of an order of magnitude between gas and liquid phases.
- Dynamic sensing of microbubbles showed reflected optical power between 13.4 nW and 29.3 nW.

## Abstract

For large-scale measurement of microbubble parameters on the ocean surface beneath breaking waves, a buoy-type bubble sensor (BBS) is proposed. This sensor integrates a panoramic bubble imaging sub-sensor with a circular array fiber-optic sub-sensor. The sensitive unit of the latter sub-sensor is designed via theoretical modeling and experimental validation. Theoretical calculations indicate that the optimal cone angle for a quartz fiber-optic-based sensitive unit ranges from 45.2° to 92°. A prototype array element with a cone angle of 90° was fabricated and used as the core component for feasibility experiments in static and dynamic two-phase (gas and liquid) identification. During static identification, the reflected optical power differs by an order of magnitude between the two phases. For dynamic sensing of multiple microbubble positions, the reflected optical power varies from 13.4 nW to 29.3 nW, which is within the operating range of the array element’s photodetector. In theory, assembling conical quartz fiber-based sensitive units into fiber-optic probes and configuring them as arrays could overcome the resolution limitations of the panoramic bubble imaging sub-sensor. Further discussion of this approach will be presented in a subsequent paper.