Biaxial tripod MEMS mirror and omnidirectional lens for a low cost ...

2 Optical Sensor ConceptIt is one of the goals of the MINIFAROS project to develop a laser scanner that can serve many different ADASapplications. An optical concept with an omnidirectional field of view was therefore choosen. The divergent exitbeam of a fibre coupled NIR pulse laser diode is collimated and then directed on a large MEMS mirror whichreflects and scans the beam on a circular trajectory along the input facet of an omnidirectional lens (see Figure 2).After passing several internal beam forming reflections the laser beam exits the omnidirectional lens in horizontaldirection. This arrangement allows to scan the beam in a horizontal plane within an angular range of 360 degrees.When the emitted pulse hits a target the laser pulse is partially reflected back and enters the omnidirectional lens.The MEMS mirror then reflects the return pulse to an avalanche photodiode. Two fundamentally different opticalconcepts are being investigated: A biaxial system and a coaxial system (Figure 3). In the biaxial system the opticalsender path and the receiver path are totally discoupled. While the sender path uses the front side of the MEMSmirror the back reflected laser pulse passes a second omnidirectional lens and then is reflected by the reverse side ofthe MEMS mirror. This configuration requires implementation of two different designs of omnidirectional lenses.The second system configuration, called the coaxial sensor, uses only one omnidirectional lens for both, sender andreceiver path. Both optical paths use the same side of the MEMS mirror in a coaxial alignment. These two laserscanner concepts have their specific advantages and drawbacks. The biaxial laser scanner is by nature less sensitiveto stray light than the coaxial laser scanner. Since the laser scanners operate with a highly sensitive detector, crosstalk between the channels must be reduced, as much as possible, in order to prevent saturation of the receiver. Thebiaxial laser scanner however results in a slightly larger system size and is expected to be more complex withrespect to alignment and the required alignment accuracy. So, finally a coaxial system approach was chosen.Fig. 2.Omnidirectional scanning concept based on omnidirectional lens and circle scanning MEMS mirrorAPDlaser diodelensreceiver lensMEMS mirrorsender lenslaser diodebiaxial configurationcoaxial configurationFig. 3.Optomechanical concept of the laser scanning sensor showing the biaxial concept (left) and the coaxial concept (right)