We validate a miniaturized pulsed laser source for use in time-domain (TD) diffuse optics, following rigorous and shared protocols for performance assessment of this class of devices. This compact source (12×6 mm2) has been previously developed for range finding applications and is able to provide short, high energy (∼100 ps, ∼0.5 nJ) optical pulses at up to 1 MHz repetition rate. Here, we start with a basic level laser characterization with an analysis of suitability of this laser for the diffuse optics application. Then, we present a TD optical system using this source and its performances in both recovering optical properties of tissue-mimicking homogeneous phantoms and in detecting localized absorption perturbations. Finally, as a proof of concept of in vivo application, we demonstrate that the system is able to detect hemodynamic changes occurring in the arm of healthy volunteers during a venous occlusion. Squeezing the laser source in a small footprint removes a key technological bottleneck that has hampered so far the realization of a miniaturized TD diffuse optics system, able to compete with already assessed continuous-wave devices in terms of size and cost, but with wider performance potentialities, as demonstrated by research over the last two decades.
Fundamental mode, ~100 ps, ~40 W optical pulses are demonstrated from a laser diode with a strongly asymmetric
waveguide structure and a relatively thick (~0.1 μm) active layer driven with ~15 A, ~1.5 ns injection current pulses
produced by a simple avalanche transistor circuit. Using this compact laser source, pulsed time-of-flight laser
rangefinding measurements were performed utilizing a single-photon avalanche detector. The results show the feasibility
of a very compact overall device with centimeter-level distance measurement precision and walk-error compensated
accuracy to passive targets at tens to hundreds of meters in a measurement time of about ten milliseconds.
In time-of-flight laser distance measurement a nanosecond-class laser pulse is reflected off a target, and the distance to
the target is calculated from the flight time of the pulse. The distance measurement precision is directly proportional to
the jitter of the pulse (i.e. the uncertainty of the arrival time of the pulse due to noise). In this work, the effect of signal
quantum shot noise on the jitter of detected laser pulses was researched. It was discovered that signal quantum shot noise
drives the optimal detection level of the pulse lower because shot noise increases along with received pulse power. The
effect is more significant with an AP-diode receiver than with a PIN diode receiver due to the avalanche multiplication
of shot noise in an AP-diode. This jitter phenomenon was modeled in Matlab, and the result was verified by