Frequency tunable resonant scanner Number:6,803,561 from the United States Patent and Trademark Office (PTO) owispatent A display apparatus includes a scanning assembly that scans about two or more axes, typically in a raster pattern. A plurality of light sources emit light from spaced apart locations toward the scanning assembly such that the scanning assembly simultaneously scans more than one of the beams. The scanning assembly is a resonant scanning assembly with a variable resonant frequency. The resonant frequency of the scanning assembly can be actively controlled by controlling partial pressure of fluids in a package containing the scanning assembly. In one embodiment, the increased partial pressure increases the mass of a scanning mirror, thereby changing the resonant frequency. In another embodiment, a gas absorbing material is coupled to a support arm that carries a scanning mirror. As the gas absorbing material absorbs gas, its physical properties change, thereby shifting the resonant frequency of the scanning assembly. Monitoring the resonant frequency relative to a desired frequency provides in error signal that can be used to frequency lock the resonant scanning assembly to an input signal.<H Frequency, resonant, Frequency, tunab, 6803561.html, Patent, pto, 6803561

Title: Frequency tunable resonant scanner

Abstract: A display apparatus includes a scanning assembly that scans about two or more axes, typically in a raster pattern. A plurality of light sources emit light from spaced apart locations toward the scanning assembly such that the scanning assembly simultaneously scans more than one of the beams. The scanning assembly is a resonant scanning assembly with a variable resonant frequency. The resonant frequency of the scanning assembly can be actively controlled by controlling partial pressure of fluids in a package containing the scanning assembly. In one embodiment, the increased partial pressure increases the mass of a scanning mirror, thereby changing the resonant frequency. In another embodiment, a gas absorbing material is coupled to a support arm that carries a scanning mirror. As the gas absorbing material absorbs gas, its physical properties change, thereby shifting the resonant frequency of the scanning assembly. Monitoring the resonant frequency relative to a desired frequency provides in error signal that can be used to frequency lock the resonant scanning assembly to an input signal.

Patent Number: 6,803,561 Issued on 10/12/2004 to Dunfield

Inventors: Dunfield; John C. (Woodinville, WA)
Assignee: Microvision, Inc. (Bothell, WA)

Appl. No.: 10/318,457

Filed: December 13, 2002

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
947642	Sep., 2001	6525310
370791	Aug., 1999	6331909

Current U.S. Class: 250/235 ; 250/216

Field of Search: 250/234,235,216,221 359/197-203,207,213

References Cited [Referenced By]

U.S. Patent Documents


4429564	February 1984	Ikeda et al.
4782334	November 1988	Meaney
4874215	October 1989	Montagu
5165279	November 1992	Norling et al.
5367878	November 1994	Muntz et al.
5451425	September 1995	Vig
5467104	November 1995	Furness et al.
5488862	February 1996	Neukermans et al.
5557444	September 1996	Melville et al.
5629790	May 1997	Neukermans et al.
5640133	June 1997	MacDonald et al.
5645735	July 1997	Bennin et al.
5648618	July 1997	Neukermans et al.
5701132	December 1997	Kollin et al.
5841553	November 1998	Neukermans et al.
5969465	October 1999	Neukermans et al.
6044705	April 2000	Neukermans et al.
6064779	May 2000	Neukermans et al.
6122394	September 2000	Neukermans et al.
6245590	June 2001	Wine et al.
6512622	January 2003	Wine et al.

Other References

Nagle, Schiffman & Gutierrez-Osuna, "The How and Why of Electronic Noses," IEES Spectrum, Sep., 1998..

Primary Examiner: Le; Que T.

Parent Case Text

This application is a division of application Ser. No. 09/947,642 filed Sep. 6, 2001 now U.S. Pat. No. 6,525,310, which is a continuation of application Ser. No. 09/370,791, filed Aug. 5 1999 now U.S. Pat. No. 6,331,909.

Claims

What is claimed is:

1. A method of controlling a scanning motion of a MEMs device, comprising the steps of: activating the MEMs device for periodic motion of a portion of the MEMs device relative to a reference point, the portion having a center of mass offset from the reference point by a selected distance; monitoring the periodic motion of the MEMs device; responsive to the monitored periodic motion of the MEMs device, identifying a deviation of the periodic motion from a desired periodic motion; generating an error signal in response to the identified deviation; and responsive to the error signal, changing the selected distance.

2. The method of claim 1 wherein the step of activating the MEMs device for periodic motion of a portion of the MEMs device includes applying an electrical driving signal to the MEMs device.

3. The method of claim 1 wherein the step of changing the selected distance includes adding mass to or removing mass from the portion of the MEMs device.

4. The method of claim 1 wherein the MEMs device includes a torsional member that supports the portion of the MEMs device and wherein the step of monitoring the periodic motion of the MEMs device includes monitoring torsional stress in the torsional member.

5. The method of claim 1 wherein the step of monitoring the periodic motion of the MEMs device includes optically detecting movement or position of the portion.

6. The method of claim 1 further comprising the steps of: receiving an input scanning signal; and determining from the received input scanning signal the desired periodic motion.

Description

TECHNICAL FIELD

The present invention relates to resonant microelectromechanical devices and, more particularly, to control of frequency in such devices.

BACKGROUND OF THE INVENTION

A variety of techniques are available for providing visual displays of graphical or video images to a user. In many applications cathode ray tube type displays (CRTs), such as televisions and computer monitors produce images for viewing. Such devices suffer from several limitations. For example, CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.

Matrix addressable displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical matrix addressable displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user's field of view. Such displays have been reduced in size, at the cost of increasingly difficult processing and limited resolution or brightness. Also, improving resolution of such displays typically requires a significant increase in complexity.

One approach to overcoming many limitations of conventional displays is a scanned beam display, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. As shown diagrammatically in FIG. 1, in one embodiment of a scanned beam display 40, a scanning source 42 outputs a scanned beam of light that is coupled to a viewer's eye 44 by a beam combiner 46. In some scanned displays, the scanning source 42 includes a scanner, such as scanning mirror or acousto-optic scanner, that scans a modulated light beam onto a viewer's retina. In other embodiments, the scanning source may include one or more light emitters that are rotated through an angular sweep.

The scanned light enters the eye 44 through the viewer's pupil 48 and is imaged onto the retina 59 by the cornea. In response to the scanned light the viewer perceives an image. In another embodiment, the scanned source 42 scans the modulated light beam onto a screen that the viewer observes. One example of such a scanner suitable for either type of display is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference.

In another embodiment, a micro-electromechanical (MEMs) device operates as the scanner. The MEMs scanner may be uniaxial or biaxial. A number of MEMs scanners are known. For example, one such scanner is described in U.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINED TORSIONAL SCANNER.

Where such MEMs scanners are resonant devices, it may be difficult to synchronize the scanning frequency of the MEMs device to a desired frequency. For example, where the MEMs scanner scans a beam that is modulated according to the horizontal synchronization component of a video signal, incoming data is modulated at a line rate that may or may not match the resonant frequency of the MEMs scanner. Similarly, where the scanner is used as an input device, decoding electronics may utilize a line rate that differs from the resonant frequency of the MEMs scanner.

Returning to display applications, sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user's field of view and presents an image that occupies a region 43 of the user's field of view 45, as shown in FIG. 2A. The user can thus see both a displayed virtual image 47 and background information 49. If the background light is occluded, the viewer perceives only the virtual image 47, as shown in FIG. 2B.

One difficulty that may arise with such displays is raster pinch, as will now be explained with reference to FIGS. 3-5. As shown diagrammatically in FIG. 3, the scanning source 42 includes an optical source 50 that emits a beam 52 of modulated light. In this embodiment, the optical source 50 is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). A lens 53 gathers and focuses the beam 52 so that the beam 52 strikes a turning mirror 54 and is directed toward a horizontal scanner 56. The horizontal scanner 56 is a mechanically resonant scanner that scans the beam 52 periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner 58 that scans periodically to sweep the horizontally scanned beam vertically. For each angle of the beam 52 from the scanners 58, an exit pupil expander 62 converts the beam 52 into a set of beams 63. Eye coupling optics 60 collect the beams 63 and form a set of exit pupils 65. The exit pupils 65 together act as an expanded exit pupil for viewing by a viewer's eye 64. One such expander is described in U.S. Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference. One skilled in the art will recognize that, for differing applications, the exit pupil expander 62 may be omitted, may be replaced or supplemented by an eye tracking system, or may have a variety of structures, including diffractive or refractive designs. For example, the exit pupil expander 62 may be a planar or curved structure and may create any number or pattern of output beams in a variety of patterns. Also, although only three exit pupils are shown in FIG. 3, the number of pupils may be almost any number. For example, in some applications a 15 by 15 array may be suitable.

Returning to the description of scanning, as the beam scans through each successive location in the beam expander 62, the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display 40 can produce the desired image.

Simplified versions of the respective waveforms of the vertical and horizontal scanners are shown in FIG. 4. In the plane 66 (FIG. 3), the beam traces the pattern 68 shown in FIG. 5. Though FIG. 5 shows only eleven lines of image, one skilled in the art will recognize that the number of lines in an actual display will typically be much larger than eleven. As can be seen by comparing the actual scan pattern 68 to a desired raster scan pattern 69, the actual scanned beam 68 is "pinched" at the outer edges of the beam expander 62. That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because the image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern 69, the displaced pixel locations can cause image distortion.

For a given refresh rate and a given wavelength, the number of pixels per line is determined in the structure of FIG. 3 by the mirror scan angle .theta. and mirror dimension D perpendicular to the axis of rotation. For high resolution, it is therefor desirable to have a large scan angle .theta. and a large mirror. However, larger mirrors and scan angles typically correspond to lower resonant frequencies. A lower resonant frequency provides fewer lines of display for a given period. Consequently, a large mirror and larger scan angle may produce unacceptable refresh rates.

SUMMARY OF THE INVENTION

A display includes a primary scanning mechanism that simultaneously scans a plurality of beams of light both horizontally and vertically along substantially continuous scan paths where each beam defines a discrete "tile" of an image. In the preferred embodiment, the scanning mechanism includes a mirror that pivots to sweep the beams horizontally.

Optical sources are aligned to provide the beams of light to the scanning mechanism from respective input angles. The input angles are selected such that the scanning mechanism sweeps each beam of light across a respective distinct region of an image field. Because the respective regions are substantially non-overlapping, each beam of light generates a substantially spatially distinct region of the image. The respective regions are immediately adjacent or may overlap slightly, so that the spatially distinct regions are "tiled" to form a contiguous image. Because movement of the mirror produces movement of all of the beams, the display produces each of the spatially separate regions simultaneously. As described above, the scan angle .theta. and the mirror dimensions determine the number of pixels drawn for each beam. The total number of pixels in a line can thus substantially equal the number of pixels for each beam multiplied by the number of beams.

In one embodiment, the scanning mechanism scans in a generally raster pattern with a horizontal component and a vertical component. A mechanically resonant scanner produces the horizontal component by scanning the beam sinusoidally. A non-resonant or semi-resonant scanner typically scans the beam vertically with a substantially constant angular speed.

In one embodiment, the scanning mechanism includes a biaxial microelectromechanical (MEMs) scanner. The biaxial scanner uses a single mirror to provide both horizontal and vertical movement of each of the beams. In one embodiment, the display includes a buffer that stores data and outputs the stored data to each of the optical sources. A correction multiplier provides correction data that adjusts the drive signals to the optical sources in response to the stored data. The adjusted drive signals compensate for variations in output intensity caused by pattern dependent heating.

In another embodiment, an imager acquires images in tiles by utilizing two separate detector and optical source pairs. One embodiment of the imager includes LEDs or lasers as the optical sources, where each of the optical sources is at a respective wavelength. The scanning assembly simultaneously directs light from each of the optical sources to respective regions of an image field. For each location in the image field, each of the detectors selectively detects light at the wavelength, polarization, or other characteristic of its corresponding source, according to the reflectivity of the respective location. The detectors output electrical signals to decoding electronics that store data representative of the image field.

In one embodiment, the imager includes a plurality of detector/optical source pairs at each of red, green, and blue wavelength bands. Each pair operates at a respective wavelength within its band. For example, a first of the red pairs operates at a first red wavelength and a second of the red pairs operates at a second red wavelength different from the first.

In one embodiment, a pair of optical sources alternately feed a single scanner from different angles. During forward sweeps of the scanner, a first of the sources emits light modulated according to one half of a line. During the return sweep, the second source emits light modulated according to the second half of the line. Because the second sweep is in the opposite direction from the first, data corresponding to the second half of the line is reversed before being applied to the second source so that light from the second source is modulated to write the second half of the line in reverse.

In one embodiment of the alternate feeding approach, a single light emitter feeds an input fiber that is selectively coupled to one of two separate fibers by an optical switch. During forward sweeps, the optical switch couples the input fiber to a first of the separate fibers so that the first separate fiber forms the first optical source. During reverse sweep, the optical switch feeds the second separate fiber so that the second separate fiber forms the second source. This embodiment thus allows a single light emitter to provide light for both optical sources.

The alternate feeding approach can be expanded to write more than just two tiles. In one approach, the input fiber is coupled to four fibers by a set of optical switches, where each fiber feeds the scanning assembly from a respective angle. The switches are activated according to the direction of the sweep and according to the tracked location of the user's vision. For example, when the user looks at the top half of the image, a first fiber, aligned to produce an image in the upper left tile feeds the scanner during the forward sweeps. A second fiber, aligned to produce an upper right tile feeds the scanner during reverse sweeps. When the user looks at the lower half of the image, a third fiber, aligned to produce the lower left tile, feeds scanner during forward sweeps. A fourth fiber, aligned to produce the lower right tile, feeds the scanner during reverse sweeps.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of a display aligned to a viewer's eye.

FIG. 2A is a combined image perceived by a user resulting from the combination of light from an image source and light from a background.

FIG. 2B is an image perceived by a user from the display of FIG. 1 where the background light is occluded.

FIG. 3 is a diagrammatic representation of a scanner and a user's eye showing bi-directional scanning of a beam and coupling to the viewer's eye.

FIG. 4 is a signal-timing diagram of a scan pattern scanner in the scanning assembly of FIG. 3.

FIG. 5 is a signal position diagram showing the path followed by the scanned beam in response to the signals of FIG. 4, as compared to a desired raster scan path.

FIG. 6 is a diagrammatic representation of a display according to the one embodiment invention including dual light beams.

FIG. 7 is an isometric view of a head-mounted scanner including a tether.

FIG. 8 is a diagrammatic representation of a scanning assembly within the scanning display of FIG. 6, including a correction mirror.

FIG. 9 is an isometric view of a horizontal scanner and a vertical scanner suitable for use in the scanning assembly of FIG. 8.

FIG. 10 is a diagrammatic representation of scanning with two input beams, showing slightly overlapped tiles.

FIG. 11 is a top plan view of a biaxial scanner showing four feeds at spatially separated locations.

FIG. 12 is a diagrammatic representation of four tiles produces by the four feed scanner of FIG. 11.

FIG. 13 is a schematic of a system for driving the four separate feeds of FIG. 11, including four separate buffers.

FIG. 14 is a signal-timing diagram comparing a ramp signal with a desired signal for driving the vertical scanner.

FIG. 15 is a signal timing diagram showing positioning error and correction for the vertical scanning position.

FIG. 16 is a side cross sectional view of a piezoelectric correction scanner.

FIG. 17A is a top plan view of a microelectromechanical (MEMs) correction scanner.

FIG. 17B is a side cross-sectional view of the MEMs correction scanner of FIG. 17A showing capacitive plates and their alignment to the scanning mirror.

FIG. 18 shows corrected scan position using a sinusoidally driven scanner through 90% of the overall scan.

FIG. 19 shows an alternative embodiment of a reduced error scanner where scan correction is realized by adding a vertical component to the horizontal mirror.

FIG. 20 is a position diagram showing the scan path of a beam deflected by the scanner of FIG. 19.

FIG. 21 is a diagrammatic view of a scanning system, including a biaxial microelectromechanical (MEMs) scanner and a MEMs correction scanner.

FIG. 22 is a diagrammatic view of a correction scanner that shifts an input beam by shifting the position or angle of the input fiber.

FIG. 23 is a diagrammatic view of a correction scanner that includes an electro-optic crystal that shifts the input beam in response to an electrical signal.

FIG. 24 is a diagrammatic view of an imager that acquires external light from a target object.

FIG. 25 is a diagrammatic view of an alternative embodiment of the imager of FIG. 24 that also projects a visible image.

FIG. 26 is a signal timing diagram showing deviation of a sinusoidal scan position versus time from the position of a linear scan.

FIG. 27 is a diagram showing diagrammatically how a linear set of counts can map to scan position for a sinusoidally scan.

FIG. 28 is a system block diagram showing handling of data to store data in a memory matrix while compensating for nonlinear scan speed of the resonant mirror.

FIG. 29 is a block diagram of a first system for generating an output clock to retrieve data from a memory matrix while compensating for nonlinear scan speed of the resonant mirror.

FIG. 30 is a block diagram of an alternative embodiment of the apparatus of FIG. 29 including pre-distortion.

FIG. 31 is a detail block diagram of a clock generation portion of the block diagram of FIG. 29.

FIG. 32 is a representation of a data structure showing data predistorted to compensate for vertical optical distortion.

FIG. 33 is a top plan view of a MEMs scanner including structures for electronically controlling the center of mass of each mirror half.

FIG. 34 is a top plan view of the MEMs scanner of FIG. 32 showing flexing of protrusions in response to an applied voltage.

FIG. 35 is a top plan view of a MEMs scanner including comb structures for laterally shifting the center of mass of each mirror half.

FIG. 36 is a side cross sectional view of a packaged scanner including electrically controlled outgassing nodules.

FIG. 37 is a top plan view of a MEMs mirror including selectively removable tabs for frequency tuning.

FIG. 38 is a diagrammatic view of a four source display showing overlap of scanning fields with optical sources.

FIG. 39 is a diagrammatic view of a four source display with small turning mirrors and offset optical sources.

FIG. 40 is a diagrammatic view of the display of FIG. 39 showing beam paths with the small turning mirrors and a common curved mirror.

FIG. 41 is a diagrammatic view of a single emitter display including switched optical fibers each feeding a separate tile.

FIG. 42 is a diagrammatic view of a display including four separate fibers feeding a scanner through a set of optical switches in response to a detected gaze direction to produce four separate tiles.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 6, a scanned beam display 70 according to one embodiment of the invention is positioned for viewing by a viewer's eye 72. While the display 70 is presented herein is scanning light into the eye 72, the structures and concepts described herein can also be applied to other types of displays, such as projection displays that include viewing screens.

The display 70 includes four principal portions, each of which will be described in greater detail below. First, control electronics 74 provide electrical signals that control operation of the display 70 in response to an image signal V.sub.IM from an image source 76, such as a computer, television receiver, videocassette player, DVD player, remote sensor, or similar device.

The second portion of the display 70 is a light source 78 that outputs modulated light beams 80, each having a modulation corresponding to information in the image signal V.sub.IM. The light source 78 may utilize coherent light emitters, such as laser diodes or microlasers, or may use non-coherent sources such as light emitting diodes. Also, the light source 78 may include directly modulated light emitters such as the light emitting diodes (LEDs) or may include continuous light emitters indirectly modulated by external modulators, such as acousto-optic modulators.

The third portion of the display 70 is a scanning assembly 82 that scans the modulated beams 80 through two-dimensional scanning patterns, such as raster patterns. The scanning assembly 82 preferably includes a periodically scanning mirror or mirrors as will be described in greater detail below with reference to FIGS. 3-4, 8, 11, 19-22.

Lenses 84, 86 positioned on opposite sides of the scanning assembly 82 act as imaging optics that form the fourth portion of the display 70. The lenses 86 are cylindrical graded index (GRIN) lenses that gather and shape light from the light source 78. Where the light source 78 includes optical fibers that feed the lenses 86, the lenses 86 may be bonded to or integral to the fibers. Alternatively, other types of lenses, such as doublets or triplets, may form the lenses 86. Also, other types of optical elements such as diffractive elements may be used to shape and guide the light. Regardless of the type of element, the overall optical train may incorporate polarization sensitive materials, chromatic correction, or any other optical technique for controlling the shape, phase or other characteristics of the light.

The lens 84 is formed from a curved, partially transmissive mirror that shapes and focuses the scanned beams 80 approximately for viewing by the eye 72. After leaving the lens 84, the scanned beams 80 enter the eye 72 through a pupil 90 and strike the retina 92. As each beam of scanned modulated light strikes the retina 92, the viewer perceives a respective portion of the image as will be described below.

Because the lens 84 is partially transmissive, the lens 84 combines the light from the scanning assembly 82 with the light received from a background 89 to produce a combined input to the viewer's eye 72. Although the background 89 is presented herein as a "real-world" background, the background light may be occluded or may be produced by another light source of the same or different type. One skilled in the art will recognize that a variety of other optical elements may replace or supplement the lenses 84, 86. For example, diffractive elements such as Fresnel lenses may replace either or both of the lenses 84, 86. Additionally, a beamsplitter and lens may replace the partially transmissive mirror structure of the lens 84. Moreover, various other optical elements, such as polarizers, color filters, exit pupil expanders, chromatic correction elements, eye-tracking elements, and background masks may also be incorporated for certain applications.

Although the elements of FIG. 6 are presented diagrammatically, one skilled in the art will recognize that the components are typically sized and configured for the desired application. For example, where the display 70 is intended as a mobile personal display the components are sized and configured for mounting to a helmet or similar frame as a head-mounted display 70, as shown in FIG. 7. In this embodiment, a first portion 171 of the display 70 is mounted to a head-borne frame 174 and a second portion 176 is carried separately, for example in a hip belt. The portions 174, 176 are linked by a fiber optic and electronic tether 178 that carries optical and electronic signals from the second portion to the first portion. An example of a fiber-coupled scanner display is found in U.S. Pat. No. 5,596,339 of Furness et al., entitled VIRTUAL RETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporated herein by reference.

An exemplary embodiment of the scanning assembly 82 will be described next with reference to FIG. 8. The scanning assembly 82 includes several components that correspond to the scanning source 42 of FIG. 3, where components common to the scanning assembly 82 and scanning source 42 are numbered the same. Additionally, only central rays 55 are presented for the beams 52 for clarity of presentation.

In this embodiment, a pair of fibers 50 emit light from the light sources 78 (not shown) and the lens 84 is represented as a common refractive lens rather than as a partially transmissive mirror. Unlike the scanning source 42 of FIG. 3, the scanning assembly 82 includes an active correction mirror 100 that can pivot to scan the light beam 80 along the vertical axis. As will be explained below, the correction mirror 100 produces a varying corrective shift along the vertical axis during each sweep (forward or reverse) of the horizontal scanner 56. The corrective shift offsets vertical movement of the beams 80 caused by the vertical scanner 58 to reduce the overall deviation of the scanning pattern from the desired pattern shown in broken lines in FIG. 5.

Before describing the effects of the correction mirror 100 and the relative timing of the various signals, exemplary embodiments of mechanically resonant scanner 200, 220 suitable for use as the horizontal scanner 56 and vertical scanner 58 will be described with reference to FIG. 9.

The principal scanning component of the horizontal scanner 200 is a moving mirror 202 mounted to a spring plate 204. The dimensions of the mirror 202 and spring plate 204 and the material properties of the spring plate 204 have a high Q with a natural oscillatory ("resonant") frequency on the order of 1-100 kHz, where the selected resonant frequency depends upon the application. For VGA quality output with a 60 Hz refresh rate and no interlacing, the resonant frequency is preferably about 15-20 kHz. As will be described below, the selected resonant frequency or the achievable resolution may be changed through the use of a plurality of feeds.

A ferromagnetic material mounted with the mirror 202 is driven by a pair of electromagnetic coils 206, 208 to provide motive force to mirror 202, thereby initiating and sustaining oscillation. The ferromagnetic material is preferably integral to the spring plate 204 and body of the mirror 202. Drive electronics 218 provide electrical signals to activate the coils 206, 208, as described above. Responsive to the electrical signals, the coils 206, 208 produce periodic electromagnetic fields that apply force to the ferromagnetic material, thereby causing oscillation of the mirror 202. If the frequency and phase of the electric signals are properly synchronized with the movement of the mirror 202, the mirror 202 oscillates at its resonant frequency with little power consumption.

The vertical scanner 220 is structured very similarly to the resonant scanner 200. Like the resonant scanner 201, the vertical scanner 220 includes a mirror 222 driven by a pair of coils 224, 226 in response to electrical signals from the drive electronics 218. However, because the rate of oscillation is much lower for vertical scanning, the vertical scanner 220 is typically not resonant. The mirror 222 receives light from the horizontal scanner 201 and produces vertical deflection at about 30-100 Hz. Advantageously, the lower frequency allows the mirror 222 to be significantly larger than the mirror 202, thereby reducing constraints on the positioning of the vertical scanner 220. The details of virtual retinal displays and mechanical resonant scanning are described in greater detail in U.S. Pat. No. 5,467,104, of Furness III, et al., entitled VIRTUAL RETINAL DISPLAY which is incorporated herein by reference.

One skilled in the art will recognize a variety of other structures that may scan a light beam through a generally raster pattern. For example, spinning polygons or galvanometric scanners may form either or both of the scanners 56, 58 in some applications.

In another embodiment, a bi-axial microelectromechanical (MEMs) scanner may provide the primary scanning. Such scanners are described in U.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINED TORSIONAL SCANNER, which is incorporated herein by reference. Like the scanning system described above, the horizontal components of the MEMs scanners are typically defined by mechanical resonances of their respective structures, as will be described in greater detail below with reference to FIGS. 17A-B and 21. Like the two scanner system described above with reference to FIGS. 3 and 8, such biaxial scanners may suffer similar raster pinch problems due to movement along the slower scan axis during sweeps along the faster scan axis. Other scanning approaches may also apply. For example, acousto-optic scanners, electrooptic scanners, spinning polygons, or some combination of scanning approaches can provide the scanning function. Some of these approaches may not require pinch correction.

Returning to FIGS. 6, 8 and 9, the fibers 50 output light beams 80 that are modulated according to the image signal from the drive electronics 218. At the same time, the drive electronics 218 activate the coils 206, 208, 224, 226 to oscillate the mirrors 202, 222. The modulated beams of light strike the oscillating horizontal mirror 202 (of the horizontal scanner 56), and are deflected horizontally by an angle corresponding to the instantaneous angle of the mirror 202. The deflected beams then strike the vertical mirror 222 (of the vertical scanner 58) and are deflected at a vertical angle corresponding to the instantaneous angle of the vertical mirror 222. After expansion by the beam expander 62, the beams 52 pass through the lens 84 to the eye. As will also be described below, the modulation of the optical beams is synchronized with the horizontal and vertical scans so that, at each position of the mirrors, the beam color and intensity correspond to a desired virtual image. Each beam therefore "draws" a portion of the virtual image directly upon the user's retina.

One skilled in the art will recognize that several components of the scanning assembly 82 have been omitted from the FIG. 9 for clarity of presentation. For example, the horizontal and vertical scanners 200, 220 are typically mounted to a frame. Additionally, lenses and other optical components for gathering, shaping, turning, focusing, or collimating the beams 80 have been omitted. Also, no relay optics are shown between the scanners 200, 220, although these may be desirable in some embodiments. Moreover, the scanner 200 typically includes one or more turning mirrors that direct the beam such that the beam strikes each of the mirrors a plurality of times to increase the angular range of scanning. Further, in some embodiments, the scanners 200, 220 are oriented such that the beam can strike the scanning mirrors a plurality of times without a turning mirror.

Turning to FIGS. 10 and 11, the effect of the plurality of beams 80 will now be described. As is visible in FIG. 10, two fibers 50 emit respective light beams 80. The GRIN lenses 86 gather and focus the beams 80 such that the beams 80 become converging beams 80A, 80B that strike a common scanning mirror 1090.

For clarity of presentation, the embodiment of FIG. 10 eliminates the mirror 84, as is desirable in some applications. Also, the embodiment of FIG. 10 includes a single mirror 1090 that scans biaxially instead of the dual mirror structure of FIG. 9. Such a biaxial structure is described in greater detail below with reference to FIGS. 11, 17A-B and 21. One skilled in the art will recognize that a dual mirror system may also be used, though such a system would typically involve a more complex set of ray traces and more complex compensation for differing optical path lengths.

Also, although the fibers 50 and lenses 84 of FIG. 10 appear positioned in a common plane with the scanning mirror 1090, in many applications, it may be desirable to position the fibers 50 and lenses 84 off-axis, as is visible in FIG. 11. Moreover, where four fiber/lens pairs are used, as in FIG. 11, a beam splitter or other optical elements can allow the fiber/lens pairs to be positioned where they do not block beams 80A-D from other fiber/lens pairs. Alternatively, other approaches, such as small turning mirrors can permit repositioning of the fiber/lens pairs in non-blocking positions with little effect on the image quality. Such approaches are described in greater detail below with reference to FIGS. 11 and 38-40.

After exiting the lens 86, the first beam 80A strikes the scanning mirror 1090 and is reflected toward an image field 1094. The second beam 80B is also reflected by the scanning mirror 1090 toward the image field 1094. As shown by the ray tracing of FIG. 10, the horizontal position of the beams 80A-B in the image field 1094 will be functions of the angular deflection from the horizontal scanner 56 and the position and orientation of the lens 86 and fiber 50.

At the image field 1092, the first beam 80A illuminates a first region 1092 of the image field 1094 and the second beam 80B illuminates a second region 1096 that is substantially non-overlapping with respect to the first region 1092. To allow a smooth transition between the two regions 1092, 1096, the two regions 1092, 1096 overlap slightly in a small overlap region 1098. Thus, although the two regions are substantially distinct, the corresponding image portions may be slightly "blended" at the edges, as will be described below with reference to FIGS. 12 and 13.

While only two beams 80A-B are visible in FIG. 10, more than two fiber/lens pairs can be used and the fiber/lens pairs need not be coplanar. For example, as can be seen in FIG. 11, four separate lenses 86 transmit four separate beams 80A-D from four spatially separated locations toward the mirror 1090. As shown in FIG. 12, the mirror 1090 reflects each of the four beams 80A-D to a respective spatially distinct region 1202A-D of the image field 1094.

Thus, the four beams 80A-D each illuminate four separate "tiles" 1202A-D that together form an entire image. One skilled in the art will recognize that more than four tiles may form the image. For example, adding a third set of fiber/lens pairs could produce a 2-by-3 tile image or a 3-by-2 tile image.

To produce an actual image, the intensity and color content of each of the beams 80A-D is modulated with image information as the mirror 1090 sweeps through a periodic pattern, such as a raster pattern. FIG. 13 shows diagrammatically one embodiment where the beams 80A-D can be modulated in response to an image signal V.sub.IM to produce the four tiles 1202A-D.

The image signal V.sub.IM drives an A/D converter 1302 that produces corresponding data to drive a demultiplexer 1304. In response to the data and a clock signal CK from the controller 74 (FIG. 8), the demultiplexer 1304 produces four output data streams, where each data stream includes data corresponding to a respective image tile 1202A-D. For example, the demultiplexer 1304 outputs data corresponding to the first half of the first line of the image to a first buffer 1306A and the data corresponding to the second half of the first line to a second buffer 1306B. The demultiplexer 1304 then outputs data corresponding to the second line of the image to the second lines of the first two buffers 1306A, B. After the first two buffers 1306A, B contain data representing the upper half of the image, the demultiplexer 1304 then begins filling third and fourth buffers 1306C, D. Once all of the buffers 1306A-D are full, an output clock CKOUT clocks data simultaneously from all of the buffers 1306A-D to respective D/A converters 1308A-D. The D/A converters 1308A-D then drive respective light sources 78 to produce light that is scanned into the respective regions 2102A-D, as described above. The actual timing of the pixel output is controlled by the output clock CKOUT, as described below with reference to FIGS. 28-31.

One skilled in the art will recognize that, although the system of FIG. 13 is described for four separate regions 1201A-D, a larger or smaller number of regions may be used. Also, where some overlap of the regions 1202A-D is desired, common data can be stored in more than one buffer 1202A-D. Because the sets of common data will duplicate some pixels in the overlapping region, the data may be scaled to limit the intensity to the desired level.

One approach to improving image quality that is helpful in "matching" the image portions 1202A-D to each other will now be described with reference to FIGS. 14 and 15. Because the angle of the beams 80A-D is determined by the angles of the vertical and horizontal scanner (for the uniaxial, two scanner system) or the horizontal and vertical angles of the single mirror (for the biaxial scanner), the actual vector angle of the beams 80A-D at any point in time can then be determined by vector addition. In most cases, the desired vertical portions of the scan patterns will be a "stair step" scan pattern, as shown by the broken line in FIG. 14.

If the turning mirror 100 (FIG. 8) is disabled, the pattern traced by the ray will be the same as that described above with respect to FIGS. 3-5. As represented by the solid line in FIG. 14, the actual vertical scan portion of the pattern, shown in solid line, will be an approximate ramp, rather than the desired stair step pattern.

On approach to providing the stair step pattern would be to drive the vertical scanner 58 with the stair step voltage. However, because the vertical mirror is a physical system and the stair step involves discontinuous motion, the vertical mirror will not follow the drive signal exactly. Instead, as the vertical mirror attempts to follow the stair step pattern, the vertical mirror will move at a maximum rate indicated largely by the size and weight of the vertical mirror, the material properties of the mirror support structure, the peak voltage or current of the driving signal, and electrical properties of the driving circuitry. For typical vertical scan mirror size, configuration, scan angle and driving voltage, the vertical scanner 58 is limited to frequencies on the order of 100 to 3000 Hz. The desired scan pattern has frequency components far exceeding this range. Consequently, driving the vertical scanner 58 with a stair step driving signal can produce a vertical scan pattern that deviates significantly from the desired pattern.

To reduce this problem, the scanning assembly 82 of FIG. 8 separates the vertical scan function into two parts. The overall vertical scan is then a combination of a large amplitude ramp function at about 60 Hz and a small amplitude correction function at twice the horizontal rate (e.g., about 30 kHz). The vertical scanner 58 can produce the large amplitude ramp function, because the 60 Hz frequency is well below the upper frequency limit of typical scanning mirrors. Correction mirrors 100 replace the turning mirrors 100 and provide the small amplitude corrections. The correction mirrors 100 operate at a much higher frequency than the vertical scanner; however, the overall angular swings of the correction mirrors 100 are very small.

As can be seen from the signal timing diagram of FIG. 15, the correction mirror 100 travels from approximately its maximum negative angle to its maximum positive angle during the time that the horizontal scanner scans from the one edge of the field of view to the opposite edge (i.e. from time t.sub.1 to t.sub.2 in FIG. 15). The overall correction angle, as shown in FIGS. 14 and 15, is defined by the amount of downward travel of the vertical scan mirror during a single horizontal scan. The correction angle will vary for various configurations of the display; however, the correction angle can be calculated easily.

For example, for a display where each image region 1202A-D has 1280 vertical lines and a total mechanical vertical scan angle of 10 degrees, the angular scan range for each line is about 0.008 degrees (10/1280=0.0078125). Assuming the vertical scanner 58 travels this entire distance during the horizontal scan, an error correction to be supplied by the correction mirror 100 is about plus or minus 0.0039 degrees. The angular correction is thus approximately .theta./N, where .theta. is the vertical scan angle and N is the number of horizontal lines. This number may be modified in some embodiments. For example, where the horizontal scanner 56 is a resonant scanner, the correction angle may be slightly different, because the horizontal scanner 56 will use some portion of the scan time to halt and begin travel in the reverse direction, as the scan reaches the edge of the field of view. The correction angle may also be modified to correct for aberrations in optical elements or optical path length differences. Moreover, the frequency of the correction scanner 100 may be reduced by half if data is provided only during one half of the horizontal scanner period ("unidirectional scanning"), although raster pinch is typically not problematic in unidirectional scanning approaches.

As can be seen from the timing diagrams of FIGS. 14 and 15, the correction mirror 100 will translate the beam vertically by about one half of one line width at a frequency of twice that of the horizontal scanner 56. For a typical display at SVGA image quality with bidirectional scanning (i.e., data output on both the forward and reverse sweeps of the horizontal scanner 56), the horizontal scanner 56 will resonate at about 15 kHz. Thus, for a typical display, the correction scanner 100 will pivot by about one-tenth of one degree at about 30 kHz. One skilled in the art will recognize that, as the resolution of the display increases, the scan rate of the horizontal scanner 56 increases. The scan rate of the correction mirror 100 will increase accordingly; but, the pivot angle will decrease. For example, for a display having 2560 lines and an overall scan of 10 degrees, the scan rate of the correction mirror 100 will be about 60 kHz with a pivot angle of about 0.002 degrees. One skilled in the art will recognize that, for higher resolution, the minimum correction mirror size will typically increase where the spot size is diffraction limited.

FIG. 16 shows a piezoelectric scanner 110 suitable for the correction mirror 100 in some embodiments. The scanner 110 is formed from a platform 112 that carries a pair of spaced-apart piezoelectric actuators 114, 116. The correction mirror 100 is a metallized, substantially planar silicon substrate that extends between the actuators 114, 116. The opposite sides of the piezoelectric actuators 114, 116 are conductively coated and coupled to a drive amplifier 120 such that the voltage across the actuators 114, 116 are opposite. As is known, piezoelectric materials deform in the presence of electric fields. Consequently, when the drive amplifier 120 outputs a voltage, the actuators 114, 116 apply forces in opposite directions to the correction mirror 100, thereby causing the correction mirror 100 to pivot. One skilled in the art will recognize that, although the piezoelectric actuators 114, 116 are presented as having a single set of electrodes and a single layer of piezoelectric material, the actuators 114, 116 would typically be formed from several layers. Such structures are used in commercially available piezoelectric devices to produce relatively large deformations.

A simple signal generator circuit 122, such as a conventional ramp generator circuit, provides the driving signal for the drive amplifier 120 in response to the detected position of the horizontal scanner 56. The principal input to the circuit 122 is a sense signal from a sensor coupled to the horizontal scanner 56. The sense signal can be obtained in a variety of approaches. For example, as described in U.S. Pat. No. 5,648,618 to Neukermanns et al., entitled MICROMACHINED HINGE HAVING AN INTEGRAL TORSIONAL SENSOR, which is incorporated herein by reference, torsional movement of a MEMs scanner can produce electrical outputs corresponding to the position of the scanning mirror. Alternatively, the position of the mirror may be obtained by mounting piezoelectric sensors to the scanner, as described in U.S. Pat. No. 5,694,237 to Melville, entitled POSITION DETECTION OF MECHANICAL RESONANT SCANNER MIRROR, which is incorporated herein by reference. In other alternatives, the position of the beam can be determined by optically or electrically monitoring the position of the horizontal or vertical scanning mirrors or by monitoring current induced in the mirror drive coils.

When the sense signal indicates that the horizontal scanner 56 is at the edge of the field of view, the circuit 122 generates a ramp signal that begins at its negative maximum and reaches its zero crossing point when the horizontal scanner reaches the middle of the field of view. The ramp signal then reaches its maximum value when the horizontal scan reaches the opposite edge of the field of view. The ramp signal returns to its negative maximum during the interval when the horizontal scan slows to a halt and begins to return sweep. Because the circuit 122 can use the sense signal as the basic clock signal for the ramp signal, timing of the ramp signal is inherently synchronized to the horizontal position of the scan. However, one skilled in the art will recognize that, for some embodiments, a controlled phase shift of the ramp signal relative to the sense signal may optimize performance. Where the correction mirror 100 is scanned resonantly, as described below with reference to FIG. 18, the ramp signal can be replaced by a sinusoidal signal, that can be obtained simply be frequency doubling, amplifying and phase shifting the sense signal.

The vertical movements of the beams 80A-D induced by the correction mirrors 100 offset the movement of the beams 80A-D caused by the vertical scanner 58, so that the beams 80A-D remain stationary along the vertical axis during the horizontal scan. During the time the horizontal scan is out of the field of view, the beams 80A-D travel vertically in response to the correction mirrors 100 to the nominal positions of the next horizontal scan.

As can be seen from the above discussion, the addition of the piezoelectrically driven correction mirrors 100 can reduce the raster pinching significantly with a ramp-type of motion. However, in some applications, it may be undesirable to utilize ramp-type motion. One alternative embodiment of a scanner 130 that can be used for the correction mirror 100 is shown in FIGS. 17A and 17B.

The scanner 130 is a resonant micorelectromechanical (MEMs) scanner, fabricated similarly to the uniaxial embodiment described in the Neukermanns '790 patent. Alternatively, the scanner 130 can be a mechanically resonant scanner very similar to the horizontal scanner 54 of FIG. 9; however, in such a scanner it is preferred that the dimensions and material properties of the plate and mirror be selected to produce resonance at about 30 kHz, which is twice the resonant frequency of the horizontal scanner 200. Further, the materials and mounting are preferably selected so that the scanner 130 has a lower Q than the Q of the horizontal scanner 56. The lower Q allows the scanner 130 to operate over a broader range of frequencies, so that the scanner 130 can be tuned to an integral multiple of the horizontal scan frequency.

The use of the resonant scanner 130 can reduce the complexity of the electrical components for driving the scanner 130 and can improve the scanning efficiency relative to previously described approaches. Resonant scanners tend to have a sinusoidal motion, rather than the desired ramp-type motion described above. However, if the frequency, phase, and amplitude of the sinusoidal motion are selected appropriately, the correction mirror 100 can reduce the pinch error significantly. For example, FIG. 18 shows correction of the raster signal with a sinusoidal motion of the correction mirror where the horizontal field of view encompasses 90 percent of the overall horizontal scan angle. One skilled in the art will recognize that the error in position of the beam can be reduced further if the field of view is a smaller percentage of the overall horizontal scan angle. Moreover, even further reductions in the scan error can be realized by adding a second correction mirror in the beam path, although this is generally undesirable due to the limited improvement versus cost. Another approach to reducing the error is to add one or more higher order harmonics to the scanner drive signal so that the scanning pattern of the resonant correction scanner 130 shifts from a sinusoidal scan closer to a sawtooth wave.

Another alternative embodiment of a reduced error scanner 140 is shown in FIG. 19 where the scan correction is realized by adding a vertical component to a horizontal mirror 141. In this embodiment, the horizontal scanner 140 is a MEMs scanner having an electrostatic drive to pivot the scan mirror. The horizontal scanner 140 includes an array of locations 143 at which small masses 145 may be formed. The masses 145 may be deposited metal or other material that is formed in a conventional manner, such as photolithography. Selected ones of the masses 143 are removed to form an asymmetric distribution about a centerline 147 of the mirror 141. The masses 145 provide a component to scan the correction along the vertical axis by pivoting about an axis orthogonal to its primary axis. As can be seen in FIG. 20, the vertical scan frequency is double the horizontal scan frequency, thereby producing the Lissajous or "bow-tie" overall scan pattern of FIG. 20. The masses 145 may be actively varied (e.g. by laser ablation) to tune the resonant frequency of the vertical component. This embodiment allows correction without an additional mirror, but typically requires matching the resonant frequencies of the vibration and the horizontal scanner.

To maintain matching of the relative resonant frequencies of the horizontal scanner 56 and the correction scanner 100, the resonant frequency of either or both scanners 56, 100 may be tuned actively. Various frequency control techniques are described below with reference to FIGS. 33-36. Where the Q of the scanners 56, 100 are sufficiently low or where the scanners 56, 100 are not resonant, simply varying the driving frequency may shift the scanning frequency sufficiently to maintain synchronization.

As shown in FIG. 21, another embodiment of a scanner 150 according to the invention employs a biaxial scanner 152 as the principal scan component, along with a correction scanner 154. The biaxial scanner 152 is a single mirror device that oscillates about two orthogonal axes. Design, fabrication and operation of such scanners are described for example in the Neukermanns '790 patent, in Asada, et al, Silicon Micromachined Two-Dimensional Galvano Optical Scanner, IEEE Transactions on Magnetics, Vol. 30, No. 6, 4647-4649, November 1994, and in Kiang et al, Micromachined Microscanners for Optical Scanning, SPIE proceedings on Miniaturized Systems with Micro-Optics and Micromachines II, Vol. 3008, February 1997, pp. 82-90 each of which is incorporated herein by reference. The bi-axial scanner 152 includes integral sensors 156 that provide electrical feedback of the mirror position to terminals 158, as is described in the Neukermanns '618 patent.

The correction scanner 154 is preferably a MEMs scanner such as that described above with reference to FIGS. 17A-B, although other types of scanners, such as piezoelectric scanners may also be within the scope of the invention. As described above, the correction mirror 154 can scan sinusoidally to remove a significant portion of the scan error; or, the correction mirror can scan in a ramp pattern for more precise error correction.

Light from the light source 78 strikes the correction mirror 154 and is deflected by a correction angle as described above. The light then strikes the biaxial scanner 152 and is scanned horizontally and vertically to approximate a raster pattern, as described above with reference to FIGS. 3-5.

Another embodiment of a display according to the invention, shown in FIG. 22, eliminates the correction mirror 100 by physically shifting the input beam laterally relative to the input of an optical system 500. In the embodiment of FIG. 22, a piezoelectric driver 502 positioned between a frame 504 and an input fiber 506 receives a drive voltage at a frequency twice that of the horizontal scan frequency. Responsive to the drive voltage, the piezoelectric driver 502 deforms. Bec