Optical Analysis System For HOE Quality Appraisal

Abstract

This application discloses an automated system for measuring the quality of an HOE using incoherent light and a camera or image screen.

Claims

1. A system for analyzing quality of a holographic optical element comprising an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

2. The system of claim 1 where the optical transformation optics comprises a fiber optic cable.

3. The system of claim 1 where the optical transformation optics comprises one or more lenses.

4. The system of claim 1 where the optical transformation optics comprises a spatial filter.

5. The system of claim 1 where the optical transformation optics comprises a spatial filter, a fiber optic cable, one or more lenses, or a mixture thereof.

6. A method of measuring quality of a holographic optical element (HOE) comprising: (a) illuminating an active area of an HOE with incoherent light of a very narrow wavelength range, wherein the HOE diffracts the light into a camera or onto an image screen; and (b) detecting an image depicting quality or uniformity of the HOE.

7. The method of claim 6 comprising an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

8. The method of claim 7 where the optical transformation optics comprises a fiber optic cable.

9. The method of claim 7 where the optical transformation optics comprises one or more lenses.

10. The method of claim 7 where the optical transformation optics comprises a spatial filter.

11. The method of claim 7 where the optical transformation optics comprises a spatial filter, a fiber optic cable, one or more lenses, or a mixture thereof.

12. A system for analyzing quality of the focusing optical properties of a holographic optical element comprising an incoherent light source, optical transformation optics, a narrow band filter, one or more translation stages and a camera or an image screen.

13. The system of claim 12 where the optical transformation optics is fiber optic cable.

14. The system of claim 12 where the optical transformation optics is a telescope.

15. The system of claim 12 where the optical transformation optics is a spatial filter.

16. The system of claim 12 where the optical transformation optics comprises a spatial filter, a fiber optic cable, one or more lenses, or a mixture thereof

17. A method of measuring quality of the focusing optical properties of a holographic optical element (HOE) comprising: (a) illuminating an active area of an HOE with incoherent light of a very narrow wavelength range, wherein the HOE diffracts the light into a camera or onto an image screen; and (b) detecting an image depicting quality or uniformity of the HOE at different positions by means of one or more translations in space.

18. The method of claim 17 comprising an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

19. A method of assaying the quality of a population of holographic optical elements comprising illuminating an HOE with light having a very narrow wavelength range, wherein the light is diffracted back into a camera or onto an image screen, and detecting quality of an image for each member of the population of holographic optical elements.

20. The method of claim 19 comprising an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a graphical comparison of the incoherent light source (M625F2) vs the filtered light source as of the system described herein vs a laser (coherent light source).

[0010] FIG. 2 is a graphical comparison of the laser source temperature dependence.

[0011] FIG. 3 illustrates an image of the output of an Optical Analysis System.

[0012] FIG. 4 shows individual components of this system.

[0013] FIG. 5 shows a system in which the camera is translated.

[0014] FIG. 6 shows HOE measurement.

[0015] FIG. 7 shows astigmatism analysis.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0016] The present application relates to an apparatus and method for accurately measuring the quality of an HOE in a single or mass production environment. To accomplish this, the apparatus illuminates the active area of the HOE with incoherent light of a very narrow wavelength range, so as to simulate laser illumination of the HOE and the corresponding image without actually using a laser light. The HOE then diffracts this light and the diffracted beam can be imaged on a piece of paper, screen or with a camera sensor to provide an image that directly reflects the uniformity and quality of the HOE being tested.

[0017] One embodiment is a system for analyzing quality of a holographic optical element comprising an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen. The optical transformation optics can include a spatial filter, a fiber optic cable, one or more lenses, or a mixture thereof. In one embodiment, the optical transformation optics comprises a fiber optic cable. In another embodiment, the optical transformation optics comprises one or more lenses. In yet another embodiment, the optical transformation optics comprises a spatial filter. In still another embodiment, the optical transformation optics comprises a spatial filter, a fiber optic cable, one or more lenses, or a mixture thereof. In another embodiment, the system further includes one or more translational stages.

[0018] The individual components of this system, as shown in FIG. 1, can include:

[0019] Incoherent light source: A monochrome LED is an example of an incoherent light source having an output suited to fiber coupling (i.e. SMA connector) but a multicolor LED can be used also. An example part is the Thorlabs M625F2 that consists of a single LED with spectrum centered at 625 nm that is coupled to the optical fiber for a red illuminator. The emission spectrum is shown in Figure

[0020] Optical transformation optics: The optics transform the light source wavefront into a wavefront suitable to assess the holographic optical element. The transformation optics should be able to control the parameters of the output wavefront such as spot size, wavefront aberration, etc. In the case of a focusing holographic optical element, the suitable wavefront is a diverging source. An example of suitable transformation optics in this case is a fiber optics cable such as the Thorlabs M38L02. In this case, the diameter of the fiber optic cable determines the level of detail in the image produced by the system. It has been observed that as the diameter of the fiber increases, the image increases and highlights smaller-scale features.

[0021] Narrow band filter. A narrow-band filter reduces the light incident on the sample to a very narrow bandwidth (2-3 nm) as to simulate laser light without coherence self-interference. The filter should be of sufficient size as to encompass all of the light from the transformation optics. The preferred type of filter is a thin film stack on glass. An example filter is the Thorlabs FL632.8-3. This is a bandpass filter that transmits a well-defined wavelength band of light while rejecting unwanted radiation. The operating temperature of the filter is very wide starting from 50 C to 80 C without significant deviation of their spectral performance. This is in contrast to that of a laser source as shown in FIG. 2, which is a graph of laser source temperature dependence. A laser source is sensitive to operating temperature. The peak wavelength changes by up to 2 nm in a temperature range of 20-30 degrees C. In comparison, the operating temperature of the filter in the system described herein is very wide starting from 50 C to 80 C without significant deviation of their spectral performance.

[0022] Another component of this systems is a computer or a camera or an image screen: In the case of a camera, the sensor should have sufficient resolution to capture the details of the image required (i.e. 20482048). The camera or computer has a large sensor (0.50.5). An example of a suitable camera is the Edmund Optics EO-4010 Monochrome USB 3.0 Camera. In another embodiment, a diffusive sheet (i.e. Luminit 80 degree diffuser) can be used instead of a camera, as shown in FIG. 2, to capture an image from the HOE, which is displayed on the sheet.

[0023] The image in FIG. 3 shows an example HOE image from the quality inspection system described here. Any defects are clearly visible in the image as deviations from a flat, white background. Blemishes (left) and undesirable artifacts such as lines (right) are highlighted with arrows.

[0024] One example system was constructed using a selection of optical mounting hardware, which was attached to an optical breadboard. The mounting hardware was chosen to allow adjustments during development, but also the components could be mounted on custom-designed permanent hardware.

[0025] The system above can be used in a method of measuring the quality of a holographic optical element by:(a) illuminating an active area of an HOE with incoherent light of a very narrow wavelength range wherein the HOE diffracts the light into a camera or onto an image screen; and (b) detecting an image depicting quality or uniformity of the HOE.

[0026] One embodiment is method of measuring quality of the focusing optical properties of a holographic optical element (HOE) comprising: (a) illuminating an active area of an HOE with incoherent light of a very narrow wavelength range, wherein the HOE diffracts the light into a camera or onto an image screen; and (b) detecting an image depicting quality or uniformity of the HOE at different positions by means of one or more translations in space. The system for carrying out the method includes an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

[0027] Another embodiment includes a method of assaying the quality of a population of holographic optical elements comprising illuminating an HOE with light having a very narrow wavelength range, wherein the light is diffracted back into a camera or onto an image screen, and detecting quality of an image for each member of the population of holographic optical elements. The system for carrying out the method includes an incoherent light source, optical transformation optics, a narrow band filter, and a camera or an image screen.

[0028] The apparatus described above can be modified to measure the focal and optical aberration properties of a focusing HOE. To accomplish this, the apparatus illuminates the active area of the HOE with incoherent light of a very narrow wavelength range, so as to simulate laser illumination of the HOE and the corresponding image without actually using a laser light. The HOE then diffracts this light and the diffracted beam can be imaged on a piece of paper, screen or with a camera sensor to provide an image. By moving the camera in space, spot size, uniformity and quality of the HOE for different positions of the camera.

[0029] The individual components of another embodiment of this system, as shown in FIG. 4, can include:

Incoherent light source. The incoherent light source is a white LED.
Optical transformation optics: the optics transform the light source wavefront into a wavefront suitable to assess the holographic optical element. The transformation optics should be able to control the parameters of the output wavefront such as spot size, wavefront aberration, etc. of the incident illumination. The transformation optics is a single lens.
Narrow band filter. A narrow-band filter reduces the light incident on the sample to a very narrow bandwidth (2-3 nm) as to simulate laser light without coherence self-interference.
Camera or an image screen: In the case of a camera, the sensor should have sufficient resolution to capture the details of the image required.
Translation stages: The camera or screen is mounted on translation stages that allow to characterize the diffracted beam in one or more spatial dimensions.

[0030] FIG. 5 shows a system in which the camera is translated in one dimension using a translation stage with a translation axis parallel and colinear with the diffracted light from the HOE. In the diagram of FIG. 5, the minimum spot size (focal point) is found in position 2 while the degree of astigmatism is found by comparing the spot diameter in x and y along the range of translation.

[0031] The system above can be used to measure the focal spot size and optical aberrations of a holographic optical element by illuminating the active area of an HOE with incoherent light of a very narrow wavelength range wherein the HOE diffracts the light; measuring the light diffracted by the HOE into a camera or onto an image screen at multiple positions; detecting an image depicting quality or uniformity of the HOE at each camera position; and analyzing the differences between the images at different positions.

[0032] FIG. 6 shows images from HOE measurement for minimum diameter spot size in x (left), x and y (center) and y (right). FIG. 7 shows astigmatism analysis from images from camera translation.

[0033] Alternative embodiments of the subject matter of this application will become apparent to one of ordinary skill in the art to which the present invention pertains without departing from its spirit and scope. It is to be understood that no limitation with respect to specific embodiments shown here is intended or inferred.