DOE defect monitoring utilizing total internal reflection

1. Optical apparatus, comprising:

a primary radiation source, which is configured to emit first radiation;

a diffractive optical element (DOE), comprising:

at least one optical surface;

a side surface, wherein the side surface is not parallel to the at least one optical surface of the DOE; and

a grating, which is formed on the at least one optical surface so as to receive and diffract the first radiation from the primary radiation source that is incident on the grating;

at least one secondary radiation source, which is configured to direct second radiation to impinge on a first location on the side surface, causing at least part of the second radiation to propagate by multiple occurrences of total internal reflection within the DOE while diffracting internally from the grating and to exit through at least one second location on the side surface;

at least one radiation detector, which is positioned in proximity to the at least one second location so as to receive and sense an intensity of the second radiation that has exited through the side surface; and

a controller, which is coupled to receive at least one signal from the at least one radiation detector that is indicative of the sensed intensity of the second radiation, and to inhibit operation of the primary radiation source when the at least one signal is outside a predefined range.

2. The apparatus according to claim 1 , wherein the side surface is perpendicular to the at least one optical surface of the DOE.

3. The apparatus according to claim 1 , wherein the controller is configured to monitor a performance of the DOE responsively to the at least one signal.

4. The apparatus according to claim 3 , wherein the at least one radiation detector comprises at least first and second radiation detectors, and wherein the controller is coupled to receive first and second signals respectively from the first and second radiation detectors, and is configured to monitor the performance responsively to a difference between the first and second signals.

5. An optical method, comprising:

positioning a diffractive optical element (DOE), having at least one optical surface on which a grating is formed and a side surface, wherein the side surface is not parallel to the at least one optical surface, to receive and diffract first radiation that is emitted from a primary radiation source and is incident on the grating;

directing second radiation to impinge on a first location on the side surface, causing at least part of the second radiation to propagate by multiple occurrences of total internal reflection within the DOE while diffracting internally from the grating and to exit through at least one second location on the side surface;

receiving and sensing an intensity of the second radiation that has exited through the side surface so as to monitor the DOE; and

inhibiting operation of the primary radiation source when at least one signal indicative of the sensed intensity is outside a predefined range.

6. The method according to claim 5 , wherein the side surface is perpendicular to the at least one optical surface of the DOE.

7. The method according to claim 5 , wherein receiving and sensing the intensity comprises monitoring a performance of the DOE responsively to the at least one signal.

8. The method according to claim 7 , wherein the at least one signal comprises at least a first and a second signal, and wherein monitoring the performance of the DOE comprises monitoring the performance responsively to a difference between the first and second signals.

9. The method according to claim 1 , wherein the first radiation and the second radiation are emitted at different, respective wavelengths.

10. An optical method, comprising:

positioning a diffractive optical element (DOE), having at least one optical surface on which a grating is formed and a side surface, wherein the side surface is not parallel to the at least one optical surface, to receive and diffract first radiation that is incident on the grating;

directing the first radiation from a primary radiation source toward the at least one optical surface of the DOE, wherein the grating is configured to direct the first radiation into multiple orders of diffraction;

directing second radiation to impinge on a first location on the side surface, causing at least part of the second radiation to propagate within the DOE while diffracting internally from the grating and to exit through at least one second location on the side surface;

monitoring a performance of the DOE responsively to at least one signal indicative of an intensity of the second radiation that has exited through the side surface,

wherein a change of the at least one signal is indicative of an increase of an intensity of a zero order of the diffraction; and

controlling an operation of the primary radiation source responsively to the monitored performance,

wherein controlling the operation of the primary radiation source comprises inhibiting the operation when the change exceeds a predefined threshold.