Diffraction-based sensing of mirror position
Scanning apparatus includes a transmitter, which is configured to emit a beam comprising pulses of light, and a scanning mirror, which is configured to scan the beam over a scene. A receiver is configured to receive the light reflected from the scene and to generate an output indicative of the pulses returned from the scene. A grating is formed on an optical surface in the apparatus and is configured to diffract a portion of the beam at a predetermined angle, so as to cause the diffracted portion to be returned from the scanning mirror to the receiver. A controller is coupled to process the output of the receiver so as to detect the diffracted portion and to monitor a scan of the mirror responsively thereto.
1. Scanning apparatus, comprising:
a transmitter, which is configured to emit a beam comprising pulses of light;
a scanning mirror, which is configured to scan the beam over a scene;
a receiver, which is configured to receive the light reflected from the scene and to generate an output indicative of the pulses returned from the scene;
a grating, which is formed on an optical surface in the apparatus and is configured to diffract a portion of the beam at a predetermined angle, so as to cause the diffracted portion to be returned from the scanning mirror to the receiver; and
a controller, which is coupled to process the output of the receiver so as to detect the diffracted portion and to monitor a scan of the mirror responsively thereto.
2. The apparatus according to claim 1 , wherein the grating is formed on the mirror.
3. The apparatus according to claim 2 , wherein the grating is formed on the mirror by photolithography.
4. The apparatus according to claim 3 , wherein the scanning mirror is produced by applying a photolithographic process to a substrate and then etching the substrate to define the mirror and spindles connecting the mirror to a support, thereby defining an axis of rotation of the mirror relative to the support.
5. The apparatus according to claim 1 , wherein the receiver is positioned to receive the pulses reflected from the scene after reflection of the reflected pulses from the scanning mirror.
6. The apparatus according to claim 1 , wherein the controller is configured to calibrate the scan of the mirror responsively to detection of the diffracted portion.
7. The apparatus according to claim 6 , wherein the grating generates multiple diffraction lobes at different respective angles, and wherein the controller is configured to calibrate an angular scale and speed of the scan by monitoring a spacing between the lobes, responsively to the detection of the diffracted portion.
8. The apparatus according to claim 1 , wherein the controller is configured, upon failing to detect the diffracted portion at an expected time, to determine that the apparatus has malfunctioned.
9. The apparatus according to claim 8 , wherein the controller is configured to turn off the beam from the transmitter upon determining that the apparatus has malfunctioned.
10. The apparatus according to claim 1 , wherein the controller is configured to process the output of the receiver during the scan so as to generate a three-dimensional map of the scene.