Time-Gated Optical Aquatic Sensors (TGOAS)

There is a need for more capable and affordable means to measure optical properties and biological constituents of marine and freshwater environments through the depth of the water. These measures are crucial for estimating the abundance of organisms such as those producing harmful algal blooms (HABs). The proposed research and development efforts combine proven indirect Time-of-Flight (iTOF) optical detectors with nanosecond pulsed light sources, to produce new high-sensitivity, low-noise optical sensing capabilities that can measure time-synchronized photons within an asynchronous ambient light field that includes scattered light (noise). Fluorescence produced by phytoplankton at a distance from the sensor will be determined by the time-of-flight and spatial distribution of the received photons from source-to-target-to-detector. The iTOF sensors will use on-chip capture and accumulation of the desired signal and ambient light “snapshots”. On-chip nano-second scale time-gated accumulation will improve signal-to-noise as compared to conventional instruments, which use photodiodes and off-chip processing. Further, the proposed designs and algorithms add the capability to measure multiple optical parameters simultaneously, several meters away from the instrument. The project will build and test prototypes using two types of iTOF optical detectors and mature the core iTOF technology and systems to produce commercially available water sensing systems. The proposed Phase II effort will produce and demonstrate prototype iTOF-enabled sensors as the next generation of optical water sensing instruments. Phase I, R&D demonstrated iTOF-based underwater optical detectors can have better performance, enhanced measurement capability, and can be made with lower system complexity and cost compared to today’s commercially available instruments. This effort will mature to commercial availability in-situ fluorometer instruments that have a unique depth-ranging capability. It is expected that the iTOF approach will provide range-resolved optical properties of the water column well beyond the nominal 1 cm range typical of today’s instruments. These results will aid coastal monitoring and modeling efforts in the detection of harmful algal blooms.