Expansion of the measuring range of an optical angle sensor based on laser autocollimation with a light source having multiple longitudinal modes

An optical angle sensor based on laser autocollimation employing a laser source with multiple longitudinal modes (multi-mode laser), as well as a multi-element photodiode (PD array), is newly proposed. In the newly proposed optical angle sensor, the multi-mode laser reflected by a measurement target is spectrally dispersed by a diffraction grating, focused by an objective lens, and then detected by the PD array. The spacing of the neighboring focused modes is set appropriately with respect to the element spacing of the PD array. This optical configuration is expected to expand the measuring range of the optical angle sensor by stitching the readouts obtained from the PD array while maintaining a good measurement resolution. At first, the feasibility of the proposed method is theoretically investigated by numerical calculations. A prototype optical setup employing a laser diode having four independent longitudinal modes is then designed and constructed. Finally, the resolution and measuring range of the constructed optical angle sensor are evaluated through experiments to verify the feasibility of the proposed method with the multi-mode laser and the PD array.