WO2002046800A2

WO2002046800A2 - Optical configuration for focus sensing

Info

Publication number: WO2002046800A2
Application number: PCT/US2001/044926
Authority: WO
Inventors: Newell Convers Wyeth; Edward Alan Phillips
Original assignee: Science Applications International Corporation
Priority date: 2000-12-08
Filing date: 2001-11-30
Publication date: 2002-06-13
Also published as: WO2002059667A1; AU2002227054A1; WO2002046800A3

Abstract

The present invention makes use of integrated optical or fiber optical systems in focusing scanner applications more practical by utilizing an all-waveguide system design (50) without the need for a conventional, discrete optic focus-sensing subsystem. In addition, the invention uses the limited light acceptance properties of single-mode waveguides (20, 30) to eliminate interfering signals from reflections produced by other light spots scanning nearby areas of the surface in parallel ('crosstalk') or from ambient light (noise). The invention produces focus-error feedback signals necessary to maintain a focused light spot that is scanned over an area of interest. The invention may be used with scanner designs that use single-mode optical fibers or integrated optical waveguides (10, 20, 30) as the sources imaged as spots for scanning.

Description

OPTICAL CONFIGURATION FOR FOCUS SENSING

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applications 60/251,878 and 60/251,863 filed December 8, 2000 and U.S. Patent Application Serial Nos. / filed November 29, 2001, entitled "Optical Configuration for Improved Lens Performance" and 09/942,591, filed August 31, 2001 entitled "Optical Configuration for Focus Sensing" which are incorporated herein by reference in their entireties. The following United States Patents and patent applications are herein incorporated by reference: Multiple Parallel Source Scanning Device (U.S. Patent No. 6,137,105); Multiple Channel Data Writing Device (U.S. Patent No. 6,166,156); Multiple Channel Scanning Device Using Optoelectronic Switching (U.S. Patent No. 6,246,658); Method and Apparatus for Controlling the Focus of a Read Write Head for an Optical Scanner (U.S. Application No. 09/088,781).

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates generally to optical scanning. More particularly, the present invention is related to optical configurations for achieving focus-sensing within an optical scanning system.

Description of the Related Art

Many optical devices operate by maintaining one or more focused spots on a surface. For example, optical scanning of a surface is generally done with a focused spot to provide localized interaction with a small area of the surface at any one time. Optical scanners have been developed for a variety of uses including optical data storage, bar code readers, image scanning for digitization or xerography, laser beam printers, inspection systems, densitometers, and 3-dimensional scanning (surface definition, surface characterization, robotic vision). In these scanning applications, the light spot is scanned over a surface for either "reading" the surface (i.e., producing a return signal that carries information about some local property of the surface) or "writing" to the surface (i.e., causing a localized change in a property of the surface material).

The reading of optically stored data is a prime application example of optical devices that must maintain a focused spot during operation. Commercial read write heads for optical data storage systems scan with a diffraction-limited light spot typically produced by focusing a collimated laser beam with a fast objective lens system. The head assembly must include focus and track (transverse) adjustments and sensors to provide information for use in controlling those adjustments to an accuracy of less than 1 micron.

Light reflected from the surface contains focus information. A typical compact disk (CD) reader uses an astigmatic lens and quadrant detector to sense focus errors. The sum of the four detector outputs (a+b+c+d) is the data signal, and the difference (a+c-b-d) gives the focus error signal. The curve plotting focus error signal versus position crosses zero at the point where the spot is in focus. The servo system that drives the focus adjustment works to maintain the focus error signal as close to zero as possible.

Integrated optics or assemblies of optical fibers can be used to direct multiple light sources through parallel, single-mode waveguides toward the same focusing lens, which images them to separate spots on the recording medium for parallel reading or writing. Re-imaging returns the light reflected from each spot to the waveguide from which it originated. However, astigmatic focus sensing or other designs using split detectors cannot be used with a single-mode waveguide collecting the return signal because details of the out-of-focus change in the reflected ray pattern are lost in coupling to the single propagating mode.

The spot from a waveguide in an integrated optics or fiber system is in focus when the re-imaged return signal in the waveguide is at a maximum. Positioning the focus at that maximum using only the return signal amplitude is not easy, since control circuitry normally works best when it is set to find a zero (as in the CD reader) rather than a maximum. One approach is to dither (vary periodically) the focus and measure the corresponding variation of the return signal. The spot will be in focus when the return signal change per dither amount (i.e., the slope of the signal) is zero. The dither frequency must be high compared to the expected servo bandwidth, moderate compared to the focus actuator mechanical response, and low compared to the data signal modulation frequency. In addition, the dither amplitude must be large enough to cause a significant change in the return signal but small enough that the spot size remains acceptable for its intended use. This set of conditions cannot all be achieved without severe compromises.

BRIEF SUMMARY OF THE INVENTION This invention provides a method for using light from a pair of optical waveguides to provide focus-sensing information from a reflecting surface. The invention leads to a reliable, robust, manufacturable, low-cost component for optical devices used for applications such as, but not limited to, optical data storage, bar code readers, image scanning for digitization or xerography, laser beam printers, inspection systems, densitometers, and 3 -dimensional scanning (surface definition, surface characterization, robotic vision). This invention produces focus-error feedback signals necessary to maintain a focused light spot that is scanned over an area of interest. The invention may be used with scanner designs that use single-mode optical fibers or integrated optical waveguides as the sources imaged as spots for scanning. Thus, the present invention can replace several discrete optical components needed to implement the prior art (e.g., beamsplitter for separate optical path, astigmatic lens, split detector).

In an embodiment of the present invention, two return signals are compared in order to decide whether the lens is in focus. The present invention generates those two return signals by using two different waveguides. The two waveguides are set out of focus the same amount in opposite directions. By shifting the location of those waveguides to other focal distances than the ones that will be in focus on the tape, the waveguides will produce return signals that contain focus-error information.

According to another embodiment, the present invention makes the use of integrated optical or fiber optical systems in focusing scanner applications more practical by utilizing an all-waveguide system design without the need for a conventional, discrete optics focus-sensing subsystem. In addition, the invention uses the limited light acceptance properties of single-mode waveguides to eliminate interfering signals from reflections produced by other light spots scanning nearby areas of the surface in parallel ("crosstalk") or from ambient light (noise).

Other features, advantages, and embodiments of the invention are set forth in part in the description that follows, and in part, will be obvious from this description, or may be learned from the practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings. In the Figures:

FIG. 1 shows an embodiment of the present invention with the edge of the integrated optics chip and the waveguides cut at different lengths;

FIG. 2 shows an embodiment of the invention with the waveguides fabricated with different lengths in the integrated optics chip; and FIG. 3 shows an embodiment of the invention using fiber optics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

OF THE INVENTION The present invention applies to an integrated optics system or an optical fiber system.

While the following discussion, embodiments and figures use an integrated optics chip as an example, it is contemplated in at least one embodiment of the present invention that fiber optics may be used.

Figures 1 and 2 show integrated optics chips 50 having a set of waveguides 10 whose exit apertures 15 lie in a plane 40. These apertures 15 act as sources for a focusing lens system 70 and image as focused spots 85 on a surface 90. According to the present invention, the surface 90 may be any surface that reflects light, including but not limited to, tape, disks, cards, print media, and labels. The waveguides are driven by at least one optical source that could be a laser, an LED, or other appropriate source. According to an embodiment of the present invention, splitters and or couplers may be used in the integrated optics chips 50 so that several waveguides may be driven by a single laser source.

According to at least one embodiment of the present invention, as shown in Figures 1 and 2, a focus error signal is generated using two additional waveguides 20 and 30. The exit apertures 17 and 18 of these two waveguides are imaged by the focusing lens system 70 onto separate areas of the surface 90. In a preferred embodiment, the two waveguides for focus sensing 20 and 30 are very close to each other and illuminate regions on surface 90 that are similar in reflectivity. In another preferred embodiment, the area containing the spots from waveguides 20 and 30 is close to or within a region of the surface 90 where the spots from the other waveguides 10 are to be focused. Reflected light from each spot is re-imaged by the lens system 70 to provide a return signal into its respective waveguide.

In order to understand why two waveguides are needed to focus the system it is first important to understand what happens if everything is in focus. If the chip 50, the lens 70, and the surface 90 are correctly positioned, everything is focused. In this example, the light diverges from a waveguide, hits the lens, is focused by the lens so that it converges, and hits a spot on the surface 90, for example, of a tape. When the light bounces off the spot on the tape, it spreads out again, goes back through the lens, and is refocused back into the waveguide. Since everything is in focus in this example, the intensity of the light recollected in the waveguide should be at a maximum. The reality of optical read/write systems is that the surface 90 is not always in the correct position from the lens system. This is especially true for surfaces like tape that sweep by at meters per second. If the surface 90 (e.g. tape) moves closer to the lens 70, the spot that is being refocused back onto the waveguide aperture gets bigger. In other words, it spreads over the waveguide aperture resulting in less light being coupled back into the waveguide. If the surface 90 (e.g. tape) moves away from the lens 70, there is also a spreading of the reflected light spot back at the waveguide aperture. The loss due to the spreading translates into a loss of intensity within the waveguide. The amount of light collected in a waveguide is a measure of how well a spot is being refocused back on the waveguide entrance. Thus, in an embodiment of the present invention, the intensities of the spots from waveguides 20 and 30 being redirected back at the waveguides are compared. The results of the comparison are then used to adjust the system, if needed, to maintain focus. Two waveguides are used in the present invention for focus sensing to normalize for changes other than focus (e.g. reflectivity). In addition, analyzing the intensity of only one reflected spot will not provide enough information to determine whether the system is out of focus because the surface 90 has moved closer to the lens system or farther away.

Thus, two waveguides are used in the present invention for focus sensing in order to provide a level of differentiation to determine whether the system is in focus and, if not, in what direction the system is out of focus. In an embodiment of the present invention, the waveguides 20 and 30 are arranged to have different distances between the plane 40 and the lens system; one waveguide 20 is positioned in front of the plane 40 and one waveguide 30 is positioned behind the plane 40. With this arrangement, the return signals collected back into waveguides 20 and 30 are maximal at different focus positions on either side of the position where the other waveguides 10 are in focus. If the return signals from waveguides 20 and 30 are called g and h, respectively, the difference (g-h) gives the focus error signal according to an embodiment of the present invention. This error signal can be normalized to the sum signal: (g-h)/(g+h), thereby eliminating variations in their intensities due to factors such as reflectivity or some other background change such as variation in the optical source level. This focus error signal carries the focus sensing information necessary for a servo system to apply corrections to the focus adjustment. According to an embodiment, the system is designed so that this focus error signal is zero (i.e., g = h) when the spots from waveguides 10 are in focus. The servo system works to maintain the focus error signal as close to zero as possible. For first order, according to an embodiment, the in-focus position for waveguides 10 is hal way between those for waveguides 20 and 30.

According to the present invention, the focus error signal is fed to a servo system that attempts to correct out-of-focus motion by moving the lens, the scanning head, or other element. Once the two reflected signals from waveguides 20 and 30 are electronically captured, one signal is subtracted from the other. The lens, scanning head, or other element is then moved according to the results of the subtraction of the two signals. The sign of the difference between the two reflected signals from waveguides 20 and 30 indicates which way to move the lens, scanning head, or other element and how much. When the two signals are equal, and therefore their difference is zero, proper focus is achieved.

An embodiment of the present invention requires precise placement of the various waveguide exit apertures 15, 17 and 18. To maintain the focus accuracy to less than a micron, the two focal positions 87 and 88 of waveguides 20 and 30, respectively, should be a few microns

(e.g. 3-5 μm) different on the imaging space side of the lens system where the surface 90 exists. The corresponding difference between the exit aperture positions 87 and 88 of waveguides 20 and 30 is found by multiplying by the square of the lens system lateral magnification ratio. According to an embodiment, the lens system 70 may be designed to make the spots from the waveguides 10 closer together on the surface 90 than the lateral separation of the apertures 15 along the chip 50 edge. In this case the lens system will be de-magnifying, and the few microns difference of the waveguides 20 and 30 focal positions 87 and 88 at the surface can translate to many microns difference in the exit aperture positions 17 and 18 in the chip.

In an embodiment of the present invention, as shown in Figure 1, the edge of the chip 50 is cut so that the waveguides 10, 20, and 30 are at different lengths. Waveguide 20 is cut so that its exit aperture 17 extends beyond the plane 40, waveguide 30 is cut so that its exit aperture 18 lies in front of the plane 40 and waveguides 10 are cut so that the exit apertures 15 are along the plane 40. The integrated optics chip 50 may be cut or diced using any conventional technique, including, but not limited to, laser ablation, microtoming, or wafer sawing.

In another embodiment of the present invention, as shown in Figure 2, all of the waveguides 10, 20 and 30 terminate inside the integrated optics chip 50 cladding material. Light diverges from the waveguide ends, first inside and then outside the medium (i.e. the integrated optics chip cladding material), establishing the exit aperture locations 15, 17 and 18. Aberrations are added to the diverging beams from their passage through the cladding material between the waveguide aperture and the edge of the chip. However, these will be small and not significantly affect the focusing of the lens system if the distance traveled in the cladding is small compared to the lens system focal length. In this way, the waveguides used for focus sensing 20 and 30 can be made longer and shorter than the other waveguides 10 as part of the integrated optics chip 50 fabrication process using conventional fabrication techniques including, but not limited to, photolithography combined with etching, photopolymerization, and/or material deposition techniques. For example, in the case of photo-defined waveguides, this is accomplished by simply drawing the fabrication photomask to make the waveguides for focus sensing 20 and 30 longer and shorter respectively.

While the above examples and figures show an integrated optics chip that uses waveguides for focus sensing, in at least one embodiment of the present invention, fiber optics are used in lieu of the waveguides. Figure 3 shows a set of fibers 100 whose exit apertures 150 lie in a plane 40. These apertures 150 act as sources for a focusing lens system 700 and image as focused spots 850 on a surface 900. The two fibers used for focus sensing 200 and 300 are arranged so that one fiber 200 extends beyond the plane 40 and one fiber 300 lies in front of the plane 40. According to this embodiment, the difference between the return signals from the focus sensing fiber optics 200 and 300 gives the focus error signal. The focus error signal can be normalized according an embodiment, by dividing the difference of the return signals by the sum of the return signals. The focus error signal carries the focus sensing information necessary for a servo system to apply corrections to the focus adjustment.

Other embodiments and uses of the present invention will be apparent to those skilled in the art from consideration of this application and practice of the invention disclosed herein. The present description and examples should be considered exemplary only, with the true scope and spirit of the invention being indicated by the following claims. As will be understood by those of ordinary skill in the art, variations and modifications of each of the disclosed embodiments, including combinations thereof, can be made within the scope of this invention.

Claims

CLAIMSWhat is claimed is:

1. An optical system and configuration for focus sensing, comprising: one or more structures capable of guiding optical energy, wherein each structure has a front end and wherein the front ends of the one or more structures are coplanar; a first focus sensing structure having a front end, wherein the front end of the first focus sensing structure terminates beyond the front ends of the one or more coplanar structures; a second focus sensing structure having a front end, wherein the front end of the second focus sensing structure terminates before the front ends of the one or more coplanar structures; at least one lens, wherein optical energy from the one or more coplanar structures, the first focus sensing structure and the second focus sensing structure passes through the at least one lens; and a surface that at least partially reflects the optical energy that passes through the at least one lens, wherein the difference between the reflected optical energy that is captured by the first focus sensing structure and the reflected optical energy that is captured by the second focus sensing structure is a focus error signal and wherein the focus error signal is used to bring the optical system into focus.

2. The system of claim 1, wherein the focus error signal is divided by the sum of the reflected optical energy captured by the first focus sensing structure and the optical energy captured by the second focus sensing structure to produce a normalized focus error signal, wherein the normalized focus error signal is used in place of the focus error signal to bring the optical system into focus.

3. The system of claim 1 , wherein the distance the front end of the first focus sensing structure terminates beyond the front ends of the one or more coplanar structures is substantially equal to the distance the second focus sensing structure terminates before the front ends of the one or more coplanar structures.

4. The system of claim 1 , wherein the one or more structures, the first focus sensing structure and the second focus sensing structure are optical waveguides.

5. The system of claim 4, wherein the optical waveguides are integrated into an optics chip.

6. The system of claim 5, wherein an edge of the integrated optics chip is cut so that the edge of the integrated optics chip corresponds to the front ends of the one or more structures, the front end of the first focus sensing structure and the front end of the second focus sensing structure.

7. The system of claim 5, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more structures, the front end of the first focus sensing structure and the front end of the second focus sensing structure terminate within the cladding material.

8. The system of claim 5, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more structures and the front end of the second focus sensing structure terminate within the cladding material and further wherein the front end of the first focus sensing structure terminates at the edge of the integrated optics chip.

9. The system of claim 1 , wherein the one or more structures, the first focus sensing structure and the second focus sensing structure are fiber optics.

10. A method for bringing an optical system into focus, wherein the optical system includes one or more structures capable of guiding optical energy, wherein each structure has a front end and wherein the front ends of the one or more structures are coplanar; a first focus sensing structure having a front end, wherein the front end of the first focus sensing structure terminates beyond the front ends of the one or more coplanar structures; a second focus sensing structure having a front end, wherein the front end of the second focus sensing structure terminates before the front ends of the one or more coplanar structures; at least one lens, wherein optical energy from the one or more coplanar structures, the first focus sensing structure and the second focus sensing structure passes through the at least one lens; and a surface that at least partially reflects the optical energy that passes through the at least one lens, the method comprising: measuring the reflected optical energy captured by the first focus sensing structure; measuring the reflected optical energy captured by the second focus sensing structure; calculating the difference between the reflected optical energy captured by the first focus sensing structure and the reflected optical energy captured by the second focus sensing structure, thereby producing a focus error signal; feeding the focus error signal to a control system which uses the focus error signal to adjust at least one optical system component, wherein the sign of the focus error signal determines the direction the at least one optical system component is adjusted.

11. The method of claim 10, further comprising: dividing the focus error signal by the sum of the reflected optical energy captured by the first focus sensing structure and the optical energy captured by the second focus sensing structure to produce a normalized focus error signal; and feeding the normalized focus error signal to the control system to adjust the at least one optical system component, wherein the sign of the normalized focus error signal determines the direction the at least one optical system component is adjusted and wherein the normalized focus error signal is used in place of the focus error signal.

12. The method of claim 10, wherein the One or more structures, the first focus sensing structure and the second focus sensing structure are optical waveguides contained in an integrated optics chip.

13. The method of claim 12, wherein an edge of the integrated optics chip is cut so that the edge of the integrated optics chip corresponds to the front ends of the one or more structures, the front end of the first focus sensing structure and the front end of the second focus sensing structure.

14. The system of claim 12, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more structures, the front end of the first focus sensing structure and the front end of the second focus sensing structure terminate within the cladding material.

15. The method of claim 12, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more structures and the front end of the second focus sensing structure terminate within the cladding material and further wherein the front end of the first focus sensing structure terminates at the edge of the integrated optics chip.

16. The method of claim 10, wherein the one or more structures, the first focus sensing structure and the second focus sensing structure are fiber optics.

17. An optical waveguide configuration for focus sensing, comprising: one or more read/write waveguides, wherein the one or more read/write waveguides are for reading information from a surface, writing information to the surface, or both reading information from the surface and writing information to the surface, wherein each of the one or more read/write waveguides has a front end, wherein the front ends of the one or more read/write waveguides are coplanar, wherein optical energy travelling through the one or more read write waveguides passes through a lens and is at least partially reflected off a surface and back through the lens, and wherein the at least partially reflected optical energy is collected by the one or more read/write waveguides; a first focus sensing waveguide having a front end, wherein the front end of the first focus sensing waveguide terminates beyond the front ends of the one or more coplanar read/write waveguides by a first distance, wherein optical energy travelling through the first focus sensing waveguide passes through the lens and is at least partially reflected off the surface and back through the lens, and wherein the at least partially reflected optical energy is collected by the first focus sensing waveguide; a second focus sensing waveguide, wherein the front end of the second focus sensing waveguide terminates before the front ends of the one or more coplanar read/write waveguides by a second distance, wherein the first distance is substantially equal to the second distance, wherein optical energy travelling through the second focus error waveguide passes through the lens and is at least partially reflected off the surface and back through the lens, wherein the at least partially reflected optical energy is collected by the second focus sensing waveguide, and wherein the difference between the reflected optical energy collected by the first focus sensing waveguide and the reflected optical energy collected by the second focus sensing waveguide produces a focus error signal that is used to make focus adjustments in an optical system.

18. The configuration of claim 17, wherein the one or more read/write waveguides, the first focus sensing waveguide and the second focus sensing waveguide are integrated into an optics chip.

19. The configuration of claim 17, wherein the front ends of the one or more read/write waveguides, the front end of the first focus sensing waveguide, and the front end of the second focus sensing waveguide correspond to an edge of the integrated optics chip.

20. The system of claim 17, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more read/write waveguides, the front end of the first focus sensing waveguide and the front end of the second focus sensing waveguide terminate within the cladding material.

21. The configuration of claim 17, wherein the integrated optics chip comprises cladding material and wherein the front ends of the one or more read/write waveguides and the front end of the second focus sensing waveguide terminate within the cladding material and further wherein the front end of the first focus sensing waveguide terminates at the edge of the integrated optics chip.