Patent Application
20070070309
1. Field of the Invention
The present invention generally relates to projecting a two-dimensional image in color by employing an electro-absorption green laser system during such image projection in order to achieve low power consumption, high resolution and miniature compact size and weight.
2. Description of the Related Art
It is generally known to project a two-dimensional color image on a screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a laser beam derived from red, blue and green laser systems over a raster pattern. The red and blue laser systems include solid-state, semiconductor lasers which are readily directly modulated and pulsed at frequencies on the order of 100 MHz. However, the currently available green solid-state lasers cannot be pulsed at such high frequencies. As a result, the green laser system includes an infrared diode-pumped YAG crystal laser whose output beam has a wavelength on the order of 1060 nm, and a non-linear frequency doubling crystal, preferably included in the laser cavity, to cause a green laser beam having a wavelength on the order of 530 nm to be emitted. An external, acousto-optical modulator is employed to pulse the emitted green beam.
Although generally satisfactory for its intended purpose, the known frequency-doubled, diode-pumped, solid-state, externally-modulated, green laser system accounts for approximately half the size, weight, cost and electrical power consumption of the arrangement for projecting the color image, thereby rendering such known image projection arrangements impractical for use in miniature, hand-held, battery-operated applications where physical size, weight, cost and power consumption must be kept to a minimum.
Accordingly, it is a general object of this invention to minimize power consumption, physical size, weight and cost of a color image projection arrangement.
Another object of this invention is to provide an alternative green laser system for use in the color image projection arrangement.
An additional object is to provide a miniature, compact, lightweight, energy-efficient, and portable color image projection arrangement useful in many instruments of different form factors, especially handheld instruments.
In keeping with these objects and others which will become apparent hereinafter, one feature of this invention resides, briefly stated, in an image projection arrangement for projecting a two-dimensional, color image. The arrangement includes a support; a plurality of red, blue and green lasers for respectively emitting red, blue and green laser beams; a scanner for sweeping a pattern of scan lines in space at a working distance from the support, each scan line having a number of pixels; and a controller for causing selected pixels to be illuminated, and rendered visible, by the laser beams to produce the color image.
In the preferred embodiment, an optical assembly is provided on the support between the lasers and the scanner, for optically focusing and collinearly arranging the laser beams to form a composite beam directed to the scanner. The scanner includes a pair of oscillatable scan mirrors for sweeping the composite beam along generally mutually orthogonal directions at different scan rates and at different scan angles. At least one of the scan rates exceeds audible frequencies, for example, over 18 kHz, to reduce noise. At least one of the scan mirrors is driven by an inertial drive to minimize power consumption. The image resolution preferably exceeds one-fourth of VGA quality, but typically equals or exceeds VGA quality. The support, lasers, scanner, controller and optical assembly preferably occupy a volume of less than thirty cubic centimeters.
The arrangement is interchangeably mountable in housings of different form factors, including, but not limited to, a pen-shaped, gun-shaped or flashlight-shaped instrument, a personal digital assistant, a pendant, a watch, a computer, and, in short, any shape due to its compact and miniature size. The projected image can be used for advertising or signage purposes, or for a television or computer monitor screen, and, in short, for any purpose desiring something to be displayed.
In accordance with this invention, the green laser includes an edge-emitting infrared laser diode for emitting an infrared beam having a wavelength on the order of 1060 nm, an electro-absorption modulator for modulating the infrared beam, and a second harmonic generator for converting the modulated, infrared beam to a green laser beam having a wavelength on the order of 530 nm. The infrared diode is preferably a distributed feedback laser diode which is fabricated on a common semiconductor chip with the modulator. The infrared diode could also be a three section, distributed Bragg reflector. The second harmonic generator is preferably a periodically poled waveguide.
This green laser is energy efficient and consumes much less power than other green laser systems. This green laser is likewise lighter in weight and smaller in size than other green laser systems. This green laser allows the image projector to be more compact and to be used in many more applications, especially in hand-held instruments.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Reference numeral 10 in
As shown in
The parallelepiped shape of the instrument 10 represents just one form factor of a housing in which the arrangement 20 may be implemented. The instrument can be shaped as a pen, a cellular telephone, a clamshell or a wristwatch, as, for example, shown in U.S. Pat. No. 6,832,724, assigned to the same assignee as the instant application, and incorporated herein by reference thereto.
In the preferred embodiment, the arrangement 20 measures less than about 30 cubic centimeters in volume. This compact, miniature size allows the arrangement 20 to be mounted in housings of many diverse shapes, large or small, portable or stationary, including some having an on-board display 12, a keypad 14, and a window 16 through which the image is projected.
Referring to
Another solid-state, semiconductor laser 28 is mounted on the support and, when energized, emits a diffraction-limited blue laser beam at about 475-505 nanometers. Another biaspheric convex lens 30 and a concave lens 32 are employed to shape the blue beam profile in a manner analogous to lenses 24, 26.
A green laser beam having a wavelength on the order of 530 nanometers is generated not by a semiconductor laser, but instead by a green module 34 having an infrared diode-pumped YAG crystal laser whose output beam at 1060 nanometers. A non-linear frequency doubling crystal is included in the infrared laser cavity between the two laser mirrors. Since the infrared laser power inside the cavity is much larger than the power coupled outside the cavity, the frequency doubler is more efficient in generating the double frequency green light inside the cavity. The output mirror of the laser is reflective to the 1060 nm infrared radiation, and transmissive to the doubled 530 nm green laser beam. Since the correct operation of the solid-state laser and frequency doubler require precise temperature control, a semiconductor device relying on the Peltier effect is used to control the temperature of the green laser module. A thermo-electric cooler can either heat or cool the device depending on the polarity of the applied current. A thermistor is part of the green laser module in order to monitor its temperature. The readout from the thermistor is fed to the controller, which adjusts the control current to the thermo-electric cooler accordingly.
As explained below, the lasers are pulsed in operation at frequencies on the order of 100 MHz. The red and blue semiconductor lasers 22, 28 can be directly pulsed at such high frequencies, but the currently available green solid-state lasers cannot. As a result, the green laser beam exiting the green module 34 is pulsed with an acousto-optical modulator 36 which creates an acoustic standing wave inside a crystal for diffracting the green beam. The modulator 36, however, produces a zero-order, non-diffracted beam 38 and a first-order, pulsed, diffracted beam 40. The beams 38, 40 diverge from each other and, in order to separate them to eliminate the undesirable zero-order beam 38, the beams 38, 40 are routed along a long, folded path having a folding mirror 42. Alternatively, the electro-optical modulator can be used internally to the green laser module to pulse the green laser beam. Other possible ways to modulate the green laser beam include electro-absorption modulation as described below, or a Mach-Zender interferometer.
The beams 38, 40 are routed through positive and negative lenses 44, 46. However, only the diffracted green beam 40 is allowed to impinge upon, and reflect from, the folding mirror 48. The non-diffracted beam 38 is absorbed by an absorber 50, preferably mounted on the mirror 48.
The arrangement includes a pair of dichroic filters 52, 54 arranged to make the green, blue and red beams as collinear as possible before reaching a scanning assembly 60. Filter 52 allows the green beam 40 to pass therethrough, but the blue beam 56 from the blue laser 28 is reflected by the interference effect. Filter 54 allows the green and blue beams 40, 56 to pass therethrough, but the red beam 58 from the red laser 22 is reflected by the interference effect.
The nearly collinear beams 40, 56, 58 are directed to, and reflected off a stationary bounce mirror 62. The scanning assembly 60 includes a first scan mirror 64 oscillatable by an inertial drive 66 (shown in isolation in
The inertial drive 66 is a high-speed, low electrical power-consuming component. Details of the inertial drive can be found in U.S. patent application Ser. No. 10/387,878, filed Mar. 13, 2003, assigned to the same assignee as the instant application, and incorporated herein by reference thereto. The use of the inertial drive reduces power consumption of the scanning assembly 60 to less than one watt and, in the case of projecting a color image, as described below, to less than ten watts.
The drive 66 includes a movable frame 74 for supporting the scan mirror 64 by means of a hinge that includes a pair of collinear hinge portions 76, 78 extending along a hinge axis and connected between opposite regions of the scan mirror 64 and opposite regions of the frame. The frame 74 need not surround the scan mirror 64, as shown.
The frame, hinge portions and scan mirror are fabricated of a one-piece, generally planar, silicon substrate which is approximately 150μ thick. The silicon is etched to form omega-shaped slots having upper parallel slot sections, lower parallel slot sections, and U-shaped central slot sections. The scan mirror 64 preferably has an oval shape and is free to move in the slot sections. In the preferred embodiment, the dimensions along the axes of the oval-shaped scan mirror measure 749μ×1600μ. Each hinge portion measure 27μ in width and 1130μ in length. The frame has a rectangular shape measuring 3100μ in width and 4600μ in length.
The inertial drive is mounted on a generally planar, printed circuit board 80 and is operative for directly moving the frame and, by inertia, for indirectly oscillating the scan mirror 64 about the hinge axis. One embodiment of the inertial drive includes a pair of piezoelectric transducers 82, 84 extending perpendicularly of the board 80 and into contact with spaced apart portions of the frame 74 at either side of hinge portion 76. An adhesive may be used to insure a permanent contact between one end of each transducer and each frame portion. The opposite end of each transducer projects out of the rear of the board 80 and is electrically connected by wires 86, 88 to a periodic alternating voltage source (not shown).
In use, the periodic signal applies a periodic drive voltage to each transducer and causes the respective transducer to alternatingly extend and contract in length. When transducer 82 extends, transducer 84 contracts, and vice versa, thereby simultaneously pushing and pulling the spaced apart frame portions and causing the frame to twist about the hinge axis. The drive voltage has a frequency corresponding to the resonant frequency of the scan mirror. The scan mirror is moved from its initial rest position until it also oscillates about the hinge axis at the resonant frequency. In a preferred embodiment, the frame and the scan mirror are about 150μ thick, and the scan mirror has a high Q factor. A movement on the order of 1μ by each transducer can cause oscillation of the scan mirror at scan rates in excess of 20 kHz.
Another pair of piezoelectric transducers 90, 92 extends perpendicularly of the board 80 and into permanent contact with spaced apart portions of the frame 74 at either side of hinge portion 78. Transducers 90, 92 serve as feedback devices to monitor the oscillating movement of the frame and to generate and conduct electrical feedback signals along wires 94, 96 to a feedback control circuit (not shown).
Alternatively, instead of using piezo-electric transducers 90,92 for feedback, magnetic feedback can be used, where a magnet is mounted on the back of the high-speed mirror, and external coil is used to pickup the changing magnetic field generated by the oscillating magnet.
Although light can reflect off an outer surface of the scan mirror, it is desirable to coat the surface of the mirror 64 with a specular coating made of gold, silver, aluminum, or a specially designed highly reflective dielectric coating.
The electromagnetic drive 70 includes a permanent magnet jointly mounted on and behind the second scan mirror 68, and an electromagnetic coil 72 operative for generating a periodic magnetic field in response to receiving a periodic drive signal. The coil 72 is adjacent the magnet so that the periodic field magnetically interacts with the permanent field of the magnet and causes the magnet and, in turn, the second scan mirror 68 to oscillate.
The inertial drive 66 oscillates the scan mirror 64 at a high speed at a scan rate preferably greater than 5 kHz and, more particularly, on the order of 18 kHz or more. This high scan rate is at an inaudible frequency, thereby minimizing noise and vibration. The electromagnetic drive 70 oscillates the scan mirror 68 at a slower scan rate on the order of 40 Hz which is fast enough to allow the image to persist on a human eye retina without excessive flicker.
The faster mirror 64 sweeps a horizontal scan line, and the slower mirror 68 sweeps the horizontal scan line vertically, thereby creating a raster pattern which is a grid or sequence of roughly parallel scan lines from which the image is constructed. Each scan line has a number of pixels. The image resolution is preferably XGA quality of 1024×768 pixels. Over a limited working range, a high-definition television standard, denoted 720 p, 1270×720 pixels, can be displayed. In some applications, a one-half VGA quality of 320×480 pixels, or one-fourth VGA quality of 320×240 pixels, is sufficient. At minimum, a resolution of 160 x 160 pixels is desired.
The roles of the mirrors 64, 68 could be reversed so that mirror 68 is the faster, and mirror 64 is the slower. Mirror 64 can also be designed to sweep the vertical scan line, in which event, mirror 68 would sweep the horizontal scan line. Also, the inertial drive can be used to drive the mirror 68. Indeed, either mirror can be driven by an electromechanical, electrical, mechanical, electrostatic, magnetic, or electromagnetic drive.
The slower mirror is operated in a constant velocity sweep-mode during which time the image is displayed. During the mirror's return, the mirror is swept back into the initial position at its natural frequency, which is significantly higher. During the mirror's return trip, the lasers can be powered down in order to reduce the power consumption of the device.
The image is constructed by selective illumination of the pixels in one or more of the scan lines. As described below in greater detail with reference to
Referring to
The image is created in the raster pattern by modulating or pulsing the lasers on and off at selected times under control of the microprocessor 114 or control circuit by operation of the power controllers 116, 118, 120. The lasers produce visible light and are turned on only when a pixel in the desired image is desired to be seen. The color of each pixel is determined by one or more of the colors of the beams. Any color in the visible light spectrum can be formed by the selective superimposition of one or more of the red, blue, and green lasers. The raster pattern is a grid made of multiple pixels on each line, and of multiple lines. The image is a bit-map of selected pixels. Every letter or number, any graphical design or logo, and even machine-readable bar code symbols, can be formed as a bit-mapped image.
As shown in
Feedback controls are also shown in
The scan mirrors 64, 68 are driven by drivers 168, 170 which are fed analog drive signals from DACs 172, 174 which are, in turn, connected to the microprocessor. Feedback amplifiers 176, 178 detect the position of the scan mirrors 64, 68, and are connected to feedback A/Ds 180, 182 and, in turn, to the microprocessor.
A power management circuit 184 is operative to minimize power while allowing fast on-times, preferably by keeping the green laser on all the time, and by keeping the current of the red and blue lasers just below the lasing threshold.
A laser safety shut down circuit 186 is operative to shut the lasers off if either of the scan mirrors 64, 68 is detected as being out of position.
As previously described, the green module 34 having an infrared diode-pumped YAG crystal laser and a non-linear frequency doubling crystal, as well as the acousto-optical modulator 36, account for approximately half the size, weight, cost and electrical power consumption of the image projection arrangement 20.
The alternative green laser system includes an infrared laser 200 for emitting an infrared beam having a wavelength of about 1060 nm. The laser 200 is preferably a wavelength stabilized, edge-emitting laser diode which, as shown in
An electro-absorption modulator (EAM) 204 is a semiconductor device which allows the intensity of the infrared beam emitted by the laser diode 200 to be controlled via an electrical voltage based on the Franz-Keldysh effect. The EAM 204 includes a modulator waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated infrared beam in order to control the optical transmission thereof. Compared to the AOM 36, the EAM 204 operates with much lower voltages, requires much less power, and operates at very high modulation speeds. A modulation bandwidth of tens of gigahertz can be achieved.
Conveniently, the EAM 204 is integrated with the DFB laser diode on the same chip 202. Although the EAM can be a separate chip, such integration allows better matching of the laser wavelength to the EAM bandgap and eliminates the need for alignment between separate chips. A taper, as depicted in
The modulated beam output by the EAM 204 is coupled into a second harmonic generating (SHG) crystal which can either be a bulk device (such as KTP) or a waveguide 206, as depicted in
In order to maintain stability of the wavelength of the infrared beam, a thermo-electric cooler 208 is employed to maintain the laser 200 at a constant temperature. There may be a requirement to stabilize the temperature of the SHG waveguide 206 as well.
Since the conversion from infrared to green light is proportional to the intensity square of the output power of the infrared laser beam, the modulation performed by the EAM should be calibrated if a linear variation of the green laser output is desired.
It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a color image projection arrangement and method employing an electro-absorption modulated green laser system, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.
