GB2325528A

GB2325528A - Optical grating generating apparatus

Info

Publication number: GB2325528A
Application number: GB9710108A
Authority: GB
Inventors: Chi-Wang Kuo; Chi-Kung Lee
Original assignee: Ahead Optoelectronics Inc
Current assignee: Ahead Optoelectronics Inc
Priority date: 1997-05-19
Filing date: 1997-05-19
Publication date: 1998-11-25
Also published as: GB9710108D0

Abstract

An optical grating generating apparatus, which can reproduce a grating according to an image or utilize a spot light source to draw a grating on a photosensitive plate. The optical grating generating apparatus is achieved by combining a computer image system, an optical image system and a precision machine control system to provided a grating generating process with high flexibility, high speed and low cost. The apparatus may include automatic focusing.

Description

OPTICAL GRATING GENERATING APPARATUS The present invention relates to an optical grating generating apparatus, which can reproduce a grating from an image or utilize a light spot to draw a grating on a photosensitive plate with high flexibility, high speed and low cost.

Nowadays, the need for the amount of grating elements is very huge in the optoelectronics industry. A grating can be applied to various fields such as optical integrated circuits, optical systems, holographic display and optical signal processing. There currently are several types of methods for generating a grating, for example, a transmission type holography, a reflection type holography, a precision machine machining process, an electronic beam etching process and a laser machining process. However, those conventional methods have respective disadvantages thereof in operation.

The transmission type holography is utilized in hologram artistical display and optical signal processing.

Grating elements generated by such method have low diffraction efficiency and cannot be coaxially used.

The reflection type holography is used in similar fields as the transmission holography. Although the diffraction efficiency of grating elements generated by the reflection holography is higher than those made by the transmission holography. However, the reconstruction frequency thereof is difficult to control, and the cost is high.

The precision machine machining process can generates grating elements with a certain level of quality. However, such method can only be utilized to generate linear or circular gratings.

The most commonly utilized method is the electronic beam etching process. However, such method is not suitable for generating a grating element with a large diffraction angle and a refraction/diffraction combination optical element. While the laser machining process is only suitable for generating the grating element with a large diffraction angle or the refraction/diffraction combination optical element.

Accordingly, each of the conventional methods has strict limits thereon, and therefore cannot be applied to generate various gratings The invention is then developed in order to solve the above problems.

We describe below how to provide an improved optical grating generating apparatus for generating a grating on a photosensitive plate. The optical grating generating apparatus is achieved by combining a computer image system, an optical image system and a precision machine control system to provided a grating generating process with high flexibility, high speed and low cost.

The specifically described optical grating generating apparatus reproduces a grating on a photosensitive plate according to a grating image.

The specifically described optical grating generating apparatus generates a grating on a photosensitive plate by a converging light spot drawing a pattern of the grating directly on the photosensitive plate.

In accordance with one aspect of the present invention, the optical grating generating apparatus comprising a light source module, a grating-image generating module, an optical image processing module, a position controlling module, a photosensitive plate and a control unit. The control unit instructs the grating-image generating module to generate a grating image with a specific grating pattern. The image processing module then reduces the grating image to a certain size. The photosensitive plate held by the position controlling module is exposed by the light source module according to the contracted grating image.

Preferably, the optical grating generating apparatus further comprises a converging light spot source, so that a grating can be generated by moving the photosensitive plate, which is held by the position controlling module, so that the light spot thereof functions as a pen to draw a pattern of the grating on the photosensitive plate.

Preferably, the grating-image generating module of the optical grating generating apparatus is provided with grating masks with various grating patterns, so as to create a spot-array of grating.

Suitably, the grating-image generating module of the optical grating generating apparatus is provided with a liquid crystal display, which not only provides the effect of the spot-array of grating, but also is adaptable for a computer generated hologram and a diffractive optical element.

Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

IN THE DRAWINGS: Fig. 1 is a schematic diagram showing an arrangement of an optical grating generating apparatus in accordance with the present invention; Fig. 2 shows an example of an optical image processing module of the optical grating generating apparatus in accordance with the present invention; Fig. 3 shows another example of the optical image processing module of the optical grating generating apparatus in accordance with the present invention; Fig. 4 shows an example of a grating-image generating module of the optical grating generating apparatus in accordance with the present invention; Fig. 5 shows another example of the grating-image generating module of the optical grating generating apparatus in accordance with the present invention; Fig. 6 shows a further example of the optical image processing module with Fourier transform; Fig. 7 shows an embodiment of the optical grating generating apparatus in accordance with the present invention comprising a monitoring module; and Fig. 8 shows an automatic focusing means module of the optical grating generating apparatus in accordance with the present invention.

Referring to Fig. 1, an optical grating apparatus in accordance with the present invention comprises a light source 1, a grating-image generating module 2, an optical image processing module 3, a photosensitive plate 4, a position controlling module 5 and a control unit 6. The light source 1 provides the light necessary for generating a grating image and for exposing the photosensitive plate 4.

The grating-image generating module 2 is used for generating a grating image. The light source 1 and the grating-image generating module 2 thus can be referred to as an "image source". The grating-image processing module 3 receives the grating image generated by the grating-image generating module 2 , contracts the image into a certain size, and filters noises in the image. The photosensitive plate 4 is used for recording the processed grating image. The position controlling module 5 holds the photosensitive plate 4 and controls the position thereof. The control unit, which can be a computer or a microprocessor, provides all the control signals for respective modules.

The light projected from the light source 1 is used as a back light for the grating-image generating module 2, which generates a grating image. The grating image is transmitted to the optical image processing module 3 to be sub ject to an optical image processing. The processed image is then formed on the photosensitive plate 4, and the photosensitive plate 4 is exposed. By such manner, hundreds of um square of grating images can be recorded on the photosensitive plate 4 at a time. The position of the photosensitive plate 4 is controlled by the position controlling module 5.

Accordingly, various grating images, each of which has an area of hundreds of ym square, can be integrated and arranged to form a grating with a large size. The light source 1, the grating-image generating module 2 and the position controlling module 5 are controlled by the control unit 6.

The wavelength of the light from the light source 1 shown in Fig. 1 is sensitive to the photosensitive plate 4.

In addition, the time and intensity that the light enters into the optical image processing module 3 must be controllable to ensure a proper exposure. Accordingly, the light source can be a mercury lamp or a laser. A shutter (not shown) can be added thereto in order to directly control the exposure time. Preferably, the light source 1 is an LED or a semiconductor laser, whereby the time and intensity of the light can be controlled directly by the control unit 6, without use of a shutter.

The photosensitive plate 4 is made by coating a photosensitive material on a substrate to record the energy and intensity of the light. The normally used photosensitive materials include silver halide, photopolymer and photoresist. The substrate can be made of glass, metal or other materials.

The position controlling module 5 is used for holding the photosensitive plate 4 and can move the photosensitive plate 4 with a stepping or continuous manner in various directions. The position controlling module 5 can be a translation stage used in the operation of coherent light interferometry, or a translation stage used in the process of the integrated circuit fabrication, depending upon the number of axes to be controlled in moving. If the grating is produced by reproduction from the grating image generated by the grating-image generating module 2, it is necessary to control the transition on an X-axis and transition on a Yaxis. On the other hand, if the grating is produced by the light spot drawing directly on the photosensitive plate 4, then it is preferable to control the transition on the Xaxis, the transition on the Y-axis, and the rotation on a Zaxis; in addition, the movement is preferably in a continuous manner in this case.

The image processing module 3 can be an.optical reduction means, which utilizes various lenses to form a reduced grating image from the grating image generated by the grating-image generating module 2, in front of the position controlling module 5. If the photosensitive plate 4 is now placed on the position controlling module 5, the reduced grating image is recorded on the photosensitive material 4.

As shown in Fig. 2, the optical image processing module 3 can simply consist of a single reduction lens 3a, alternatively, another type of lens, or a lens combination to reduce the image to a certain size.

For example, referring to Fig. 3, the optical image processing module 3 comprises a parallelizing lens 3b, and a focusing lens 3c. Each of the lenses can be a single lens or a lens combination. The parallelizing lens 3b is used to transform divergent spherical light, which is radiated from any arbitrary spot light source sent by the grating-image generating module 2, into parallel light, while the focusing lens 3c is used to converge parallel light and focus the converged light on the photosensitive plate 4. The clarity of a focus point on the photosensitive plate 4 due to the above process depends upon the quality of the lenses.

Since the light transmitted between the parallelizing lens 3b and the focusing lens 3c is parallel, the distance between the lens 3b and 3c is irrelevant to the quality of the image formed on the photosensitive plate 4.

However, the quality of the image may be degraded due to a rough surface of the photosensitive plate 4 or vibration of the system. It is preferable to add an automatic focusing means in the optical image processing module 3, as shown in Fig. 8. The automatic focusing means comprises a laser diode 70, two sensors 71, 72, and a mirror 73, which is disposed on the light path between the parallelizing lens 3b and the focusing lens 3c. A laser beam is emitted from the laser diode 70 to the mirror 73 to be reflected to the photosensitive plate 4, and the return laser beam returns back along the same path to be received by the two sensors 71, 72. The two sensors 71, 72 determine whether the focusing on the photosensitive plate 4 is precise according to the received laser light. If the answer is negative, then the position of the focusing lens 3c is adjusted to achieve precise focusing. Since the automatic focusing is conventional, the detail description thereof is omitted herein.

There are various manners to realize the grating-image generating module 2. For instance, Fig. 4 shows an embodiment of the grating-image generating module 2, which is adaptable to generate a "grating array" utilized in the hologram display. The "grating array" is composed of various linear gratings with different directions and different pitches. In practice, it needs tens of grating angles and several grating pitches to create such a grating array. The image generating module 2 of Fig. 4 comprises a light diffusion plate 2a, grating masks 2b, 2c, 2d with different grating pitches, and an area control mask 2e. The light diffusion plate 2a is used to diffuse the incoming light and make the light intensity distribution more uniform.

The respective pitches of the grating masks 2b, 2c, 2d are set depending upon the desired grating pitch and the reduction multiple of the image. The grating masks 2b, 2c,2d are fixed on a translation stage (not shown), so that an appropriate one of the grating masks can be moved to a light axis X for the purpose of selecting a proper pitch. The respective grating masks 2b, 2c and 2d are rotatable by means of a motor (not shown) and the like to obtain a desired grating angle. The number of the grating masks can be increased or decreased as required. The translation stage and the motor are controlled by the control unit 6 (Fig. 1).

The area control mask 2e is used to control a size of exposure area of a reduced grating image. The diffusion plate 2a and the area control mask 2e are located on the light axis X, and the object distances of the grating masks 2b, 2c and 2d should be properly selected.

Fig. 5 shows another embodiment of the grating-image generating module 2. In this figure, a liquid crystal display as a liquid crystal projection plane 2f is used to substitute the grating masks 2b, 2c, 2d in Fig. 4. The liquid crystal projection plane 2f is controlled by the control unit 6 to generate arbitrary images as desired. In such case, the area control mask 2e can be eliminated.

Referring to Fig. 6, the optical image processing module 3 can be realized by utilizing optical Fourier Transform. In this embodiment, the optical image processing module 3 comprises a Fourier transform lens 3d, an inverse Fourier transform lens 3e and a filtering mask 3f. Each of the lenses 3d and 3e can be a single lens or a lens combira- tion. When the optical image processing module 3 of this embodiment is used in conjunction with the grating-image generating module 2 shown in Fig. 4 or 5, the diffusion plate 2a has to be eliminated, and the light source 1 must generate parallel coherent light with a proper wavelength and a controllable exposure.

The grating image generated from the grating-image generating module 2 is illuminated by the parallel coherent light, and is transformed by the Fourier transform lens 3d.

The filter mask 3f is placed at the position of the Fourier transform plane of the Fourier transform lens 3d to filter the spectral signals of the transformed grating image. The filtered grating image is then transformed by the inverse Fourier transform lens 3e, so that a final grating image is obtained at a back focal plane of the inverse Fourier transform lens 3e. Accordingly, the photosensitive plate z is placed at said back focal plane to record the final grating image. The final grating image can be reduced or enlarged by such arrangement, depending upon the proportional relationship between the focal lengths of the Fourier transform lens 3d and the inverse Fourier transform lens 3e. In the arrangement of Fig. 6, the automatic focusing module shown in Fig. 8 can be added therein.

Moreover, more than one set of lenses, which includes a Fourier transform lens and an inverse Fourier transform lens, can be used. A distance between every two sets of the lenses is preferably equal to a back focal length of the inverse Transform lens in a front set added to a front focal length of the Fourier Transform lens in the back set. There is a Fourier transform plane in each of the set. The filter mask 3f can be placed at any one of the Fourier transform planes.

It is noted that the image processing using Fourier transform is adaptable only for reproducing a grating from a grating image.

Returning to Figs. 1, 2 and 3, the light source 1 and the optical image processing module 2 can be substituted by a spot light source (not shown), that is, the "image source" is the light spot thereof. The light of the spot light source is converged by the optical image processing module 3 to form a very fine light spot on the photosensitive plate 4. The fine light spot can be used as a pen to draw an arbitrary pattern of a grating on the photosensitive plate 4 by the position control module 5 holding and moving the photosensitive plate 4 relative to the light spot so as to vary the relative positions therebetween and by controlling the intensity of the light of the spot light source.

There are various methods to generate a spot light source. For example, an ideal spot light source is obtained by focusing a laser beam and filtering the laser beam with a pinhole. An LED or a laser diode can be directly used as a spot light source. Alternatively, the spot light source is obtained by focusing the light of an ordinary lamp light source, and filtering the light on a focal plane thereof with a pinhole. It is noted that any selected light source should have a proper wavelength to be sensed by the photosensitive plate 4, and the intensity and exposure of the light spot must be controllable.

In practice, calibration is necessary for the optical grating generating apparatus. Accordingly, it is preferable to add a monitoring function to the system. Referring to Fig. 7, a monitoring module 6, which can utilize the principle of microscope monitoring, is added into the system. The monitoring module 6 comprises a beam splitter 61, a CCD (charge coupled device) image receiver 62 and a monitor 63.

When the grating image is projected on the photosensitive plate 4, the diffused light from the surface of the photosensitive plate 4 returns back along the same path and is reflected by the beam splitter 61 to the CCD image receiver 62. The CCD image receiver 62 then converts the received light signals into electrical signals. The quality of the grating image can be observed on the monitor 63 and calibrated by analyzing the electrical signals. The monitoring by eyes, instead of CCD, can also be utilized according to the principle of microscope monitoring.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

1. An optical grating generating apparatus comprising: - an image source for providing an image; - an optical image processing module for processing said image to control a dimension thereof; - a photosensitive plate for recording the processed image being formed thereon; - a position controlling module for supporting said photosensitive plate and controlling the position thereof; and - a control unit for controlling said image source, said optical image processing module and said position controlling module.

2. The optical grating generating apparatus as claimed in Claim 1, wherein said image source comprises a grating-image generating module for generating a grating image, and a light source, which provides light necessary for generating said grating image and for exposing the photosensitive plate.

3. The optical grating generating apparatus as claimed in Claim 2, wherein said light source is a light-emitting diode (LED).

4. The optical grating generating apparatus as claimed in Claim 2, wherein said light source is a semiconductor laser.

5. The optical grating generating apparatus as claimed in Claim 2, wherein said grating-image generating module comprises a diffusion plate for diffusing light, at least one controllably rotatable grating mask and an area control mask for controlling an area of a grating image formed via said grating mask on the photosensitive plate.

6. The optical grating generating apparatus as claimed in Claim 5, wherein said grating-image generating module comprises a plurality of grating masks having various grating patterns with different pitches.

7. The optical grating generating apparatus as claimed in Claim 6, wherein a proper one of said grating masks is selected by the control unit to form the grating image on the photosensitive plate.

8. The optical grating generating apparatus as claimed in Claim 2, wherein said grating-image generating module comprising a light diffusion plate for diffusing light and a liquid crystal display.

9. The optical grating generating apparatus as claimed in Claim 1, wherein said image source is a spot light source for drawing a grating image with light spot thereof on the photosensitive plate.

10. The optical grating generating apparatus as claimed in Claim 9, wherein said spot light source draws the grating image on the photosensitive plate by the control unit controlling the position controlling module supporting the photosensitive plate to vary the position of the photosensitive plate.

11. The optical grating generating apparatus as claimed in Claim 9, wherein said spot light source is an LED (light- emitting diode).

12. The optical grating generating apparatus as claimed in Claim 9, wherein said spot light source is a semiconductor laser.

13. The optical grating generating apparatus as claimed in Claim 2 wherein said optical image processing module comprises a Fourier transform lens, an inverse Fourier transform lens and a filter mask.

14. The optical grating generating apparatus as claimed in Claim 13, wherein each of the lenses is a lens combination.

15. The optical grating generating apparatus as claimed in Claim 13, wherein said filter mask is disposed between said Fourier transform lens and said inverse Fourier transform lens.

16. The optical grating generating apparatus as claimed in Claim 1, wherein said optical image processing module is a reduction lens.

17. The optical grating generating apparatus as claimed in Claim 16, wherein the lens is a lens combination.

18. The optical grating generating apparatus as claimed in Claim 1, wherein said optical image processing module comprises a parallelizing lens for converting divergent light into parallel light, and a focusing lens.

19. The optical grating generating apparatus as claimed in Claim 18, wherein each of the lenses is a lens combination.

20. The optical grating generating apparatus as claimed in Claim 1, wherein said position controlling module is a programmably controllable translation stage.

21. The optical grating generating apparatus as claimed in Claim 1, wherein said control unit is a computer.

22. The optical grating generating apparatus as claimed in Claim 1, wherein said control unit is a microprocessor.

23. The optical grating generating apparatus as claimed in Claim 1, further comprising an automatic focusing module for automatically calibrating and adjusting a focus of the image formed on the photosensitive plate.

24. The optical grating generating apparatus as claimed in Claim 1, further comprising a monitoring module for monitoring a quality of the image.

25. An optical grating generating apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.