Laser characterization for 3D imaging sensor

Abstract

The master thesis focuses on the comprehensive characterization of the Time-of-Flight (ToF) module, which incorporates two Vertical Cavity Surface Emitting Laser (VCSEL) sources functioning as emitters. The primary objective of this characterization is to ensure that the VCSELs operate as expected and meet the required specifications for their intended applications.

The study involves four key measurements: waveform (WF) timing, power, far-field (FF), and spectrum. These measurements were conducted across 11 reflow boards, encompassing 11 ToF modules and 22 VCSEL sources). The reflow boards provide necessary electrical current to drive the ToF modules.

The Waveform (WF) timing measurements indicate that temperature has no significant impact on the waveform characteristics. However, pulse width asymmetry was observed between two channels.

In the power measurements, the peak power is maximized at an output temperature of approximately 25°C, as measured by the sensor on the reflow board. The power output meets the target specifications for input pulse widths greater than 8 ns, while short pulse widths result in lower peak power. Two issues were noted: power instabilities at 85°C for low pulse widths and power drops to zero on some boards at 3 A, 8 ns pulse width, and low temperatures.

The far-field (FF) measurements reveal that beam divergence increases with input pulse width and current, aligning with design specifications. The Percentage Power Through Aperture (PPTA) decreases with increasing pulse width. PPTA values fall within the specified range, with the worst-case scenario for laser safety occurring at a pulse width of 16 ns. Temperature also impacts the results: at low temperatures, the beam is more divergent but localized at the center whereas at high temperatures, the beam is less divergent and more localized at the edges, forming a donut shape.

In the spectrum measurements, the peak wavelength and bandwidth increase with input pulse width, with peak wavelengths ranging from 937 nm to 939 nm. However, the spectral width measurement is limited by the accuracy of the equipment.

The results of this characterization will be presented to other teams for further improvements to meet the design specifications.