Self-mixing displacement sensing using the junction voltage variation in a GaN laser

The self-mixing (SM) laser sensing technique allows a simple, self-aligned and robust system for measuring displacement. Low-cost blue emitting GaN laser diodes have recently become available due to the high volume requirements for Blu-ray Disc devices such as high-definition video players and gaming consoles. These GaN lasers have a significantly shorter wavelength (around 405 nm) compared to other semiconductor lasers (generally around 800 nm for SM sensors). Therefore, if used in SM displacement sensors, they provide significantly higher resolution. Further to that, the measurement resolution is affected by the ability of the system to properly interpret the movement corresponding to the fraction of the half wavelength of the laser resulting in an incomplete fringe. Doubling the number of fringes will reduce the global error when a fringe is not properly detected. In this paper we report the world's first SM displacement measurement system based on junction voltage variation in blue emitting semiconductor lasers. Instead of monitoring the SM signals using a photo-diode, the signal is obtained via direct sensing of the laser junction voltage variation. This removes the need for a photo-diode, providing a cost reduction and increasing the reliability of the system. The sensitivity and precision of this system is evaluated and compared against the performance of systems using red (650 nm) and near IR (780 nm) laser based sensors with all three sensors sharing the same optical and electronic hardware.