Grating-based spatial mode filter for laser scanning
An optical device includes a laser light source configured to emit a collimated beam of light, and a scanning mirror, which is configured to reflect and scan the beam of light over a predefined angular range. The optical device further includes a volume holographic grating (VHG), which is positioned to receive and reflect the collimated beam emitted by the laser light source toward the scanning mirror by Bragg reflection at a predefined Bragg-angle, while transmitting the beam reflected from the scanning mirror over a part of the angular range that is outside a cone containing the Bragg-angle.
1. An optical device, comprising:
a laser light source configured to emit a collimated beam of light;
a scanning mirror, which is configured to reflect and scan the beam of light over a predefined angular range; and
a volume holographic grating (VHG), which is configured as a Bragg-grating so that it reflects only those beams that impinge on it at a predefined Bragg-angle, and which is positioned within the predefined angular range over which the scanning mirror scans the beam so that the VHG receives and reflects the collimated beam emitted by the laser light source toward the scanning mirror by Bragg reflection at the predefined Bragg-angle and so that the beam reflected from the VHG at the predefined Bragg-angle impinges on the scanning mirror along an axis that is contained within the predefined angular range over which the mirror scans the beam, while the beam reflected from the scanning mirror over a part of the predefined angular range over which the mirror scans the beam that is outside a cone containing the Bragg-angle is transmitted through the VHG toward a scene.
2. The optical device of claim 1 , and comprising:
a detector, which is configured to receive and sense the light that is returned from the scene through the VHG onto the scanning mirror, such that the returned light reflected by the scanning mirror reflects from the VHG by Bragg reflection toward the detector.
3. The optical device of claim 2 , and comprising control circuitry, which is coupled to drive the laser light source and the scanning mirror and to receive a signal output by the detector in response to the reflected light impinging on the detector, and which is configured to map the scene responsively to the signal.
4. The optical device of claim 3 , wherein the collimated beam of light comprises pulses of light, and wherein the control circuitry is configured to determine a distance to the scene responsively to a time-of-flight of the pulses.
5. The optical device of claim 1 , wherein an apex angle of the cone does not exceed 2°.
6. The optical device of claim 1 , wherein the VHG is formed on a substrate and extends over only an area of the substrate where the collimated beam impinges on the substrate.
7. The optical device of claim 6 , wherein the VHG is apodized.
8. A method for optical scanning, comprising:
directing a collimated beam of light from a laser source toward a volume holographic grating (VHG), which reflects the beam at a predefined Bragg-angle toward a scanning mirror; and
reflecting and scanning the collimated beam of light by the scanning mirror over a predefined angular range toward the VHG,
wherein the VHG is configured as a Bragg-grating so that it reflects only those beams that impinge on it at the predefined Bragg-angle, and is positioned within the predefined angular range over which the scanning mirror scans the beam so that the VHG receives and reflects the collimated beam from the laser light source toward the scanning mirror by Bragg reflection at the predefined Bragg-angle and so that the beam reflected from the VHG at the predefined Bragg-angle impinges on the scanning mirror along an axis that is contained within the predefined angular range over which the mirror scans the beam, while the beam reflected from the scanning mirror over the part of the predefined angular range over which the mirror scans the beam that is outside a cone containing the Bragg-angle is transmitted through the VHG toward a scene.
9. The method according to claim 8 , and comprising receiving and detecting the light that is returned from the scene through the VHG onto the scanning mirror, such that the returned light reflected by the scanning mirror reflects from the VHG by Bragg reflection toward a detector.
10. The method according to claim 9 , and comprising mapping the scene responsively to a signal output by the detector in response to the reflected light impinging on the detector.
11. The method according to claim 10 , wherein directing the collimated beam of light comprises emitting pulses of light, and wherein mapping the scene comprises determining a distance to the scene responsively to a time-of-flight of the pulses.
12. The method according to claim 8 , wherein an apex angle of the cone does not exceed 2°.
13. The method according to claim 8 , and comprising forming the VHG on a substrate, wherein the VHG extends over only an area of the substrate where the collimated beam impinges on the substrate.
14. The method according to claim 13 , wherein forming the VHG comprises apodizing the VHG.