Initially it was thought that the extremely high mirror reflectivity of VCSELs would reduce their susceptibility to external feedback, making them robust ultra-stable lasers for communication applications. This, however, turned out not to be the case because the high reflectivity is counteracted by the short cavity-round-trip time. The sensitivity to external feedback turned out to be similar to edge emitting lasers, including polarization switching, mode jumping and coherence collapse. While this poses some problems in communication applications, it opens up new applications in sensing. Sensitivity to outside factors is exactly what one needs in a sensor. Compared to the conventional configuration where the system being sensed is decoupled from the laser cavity, in this case it becomes part of the laser cavity causing a shift in spectral, spatial or polarization state of the laser beam. A number of VCSEL-based sensors for displacement, velocity and trace gases have been previously demonstrated.
In this work, Shogo Ura et al., extend this concept to angular position detection. They demonstrate that the polarization state of a 850nm VCSEL can be controlled by an external polarization rotator in a very predictable manner. As the external rotator is turned on its axis, the optical power distribution between the two degenerate spatial modes oscillated in a clear sinusoidal manner. Using a beam splitter and a couple of polarizers the power distribution between the two modes can be measured, which could then be used to determine the angular position of the rotator. This is a micro-optic rotation position sensor. Such an encoder has applications ranging from data storage discs and gyroscopes to motor shafts and machines in harsh environments. Compared to currently used optical rotary encoders, which consist of discrete reflectors placed around the circumference of a rotating disc, this scheme can record angular position continuously and under high speed. Furthermore, the rotating element can be in a remote location coupled through a polarization maintaining fiber.
This paper is written such that even those without an in-depth understanding of VCSEL structures can follow easily. The experimental setup and the data are clear and convincing, and the applications are potentially significant. A company or even an entrepreneur individual should be able to take this idea directly from paper and build a prototype or integrate into an existing system in a relatively short time using commercially available single mode VCSELs.
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