Mass stranding of live whales has been explained by proposing many natural or human-related causes. Recent necropsy
reports suggest a link between the mass stranding of beaked whales and the use of naval mid-frequency sonar.
Surprisingly, whales have experienced symptoms similar to those caused by inert gas bubbles in human divers. Our goal
is to develop a compact optical sensor to monitor the consumption of the oxygen stores in the muscle of freely diving
whales. To this end we have proposed the use of a near-infrared phase-modulated frequency-domain spectrophotometer,
in reflectance mode, to probe tissue oxygenation. Our probe consists of three main components: radiofrequency (RF)
modulated light sources, a high-bandwidth avalanche photodiode with transimpedance amplifier, and a RF gain and
phase detector. In this work, we concentrate on the design and performance of the light sensor, and its corresponding
amplifier unit. We compare three state-of-the-art avalanche photodiodes: one through-hole device and two surface-mount
detectors. We demonstrate that the gain due to the avalanche effect differs between sensors. The avalanche gain near
maximum bias of the through-hole device exceeds by a factor of 2.5 and 8.3 that of the surface-mount detectors. We
present the behavior of our assembled through-hole detector plus high-bandwidth transimpedance amplifier, and
compare its performance to that of a commercially available module. The assembled unit enables variable gain, its phase
noise is qualitatively lower, and the form factor is significantly smaller. Having a detecting unit that is compact, flexible,
and functional is a milestone in the development of our tissue oxygenation tag.