Television – Video display – Projection device
Reexamination Certificate
1999-07-27
2002-11-19
Lee, Michael (Department: 2614)
Television
Video display
Projection device
C348S754000, C353S031000
Reexamination Certificate
active
06483556
ABSTRACT:
BACKGROUND OF THE INVENTION
A pulsed laser video imaging system and method is set forth in U.S. Pat. Nos. 4,720,747 and 4,851,918, issued on Jan. 19, 1988, and Jul. 25, 1989, respectively, both hereby incorporated by reference. These patents describe a video imaging system responsive to input signals representing a video image and employ one or more pulsed lasers, such as metal vapor lasers, to provide one or more monochromatic light sources.
SUMMARY OF THE INVENTION
The invention relates to a laser video display system and method. In particular, the invention relates to a full color, laser video image system wherein the pulsed laser light source comprises a green (G) laser with controlled multimode operation to minimize speckle, a tunable blue (B) laser and a red (R) laser with sum of frequency mix.
It is desirable to provide a new and improved pulsed laser projection system and method with one or more of the following improvements. The list of improvements include:
1. all solid state green laser design with controlled multimode operation to minimize speckle;
2. all solid state tunable blue laser design;
3. all solid state red laser design with sum of frequency mix;
4. diode laser pumped, all solid state, monochromatic red, green and blue light source with color space conversion;
5. uniform intensity line generation by a cylindrical asphere lens set;
6. two channel image output with achromatic image inversion;
7. remote image delivery by coherent fiber ribbon;
8. horizontal video line time adjustment (compression and expansion);
9. nonlinear control of vertical scanner (i.e., tangential correction); and
10. horizontal pixel time correction with sub-clock timing.
The various components of the improved laser video image system and method, and the differences and improvements from prior art systems and methods are set forth in detail below.
(1) All Solid State Green Laser Design with Controlled Multimode Operation to Minimize Speckle
The original invention described in U.S. Pat. No. 4,720,747 specifies the use of metal vapor lasers as light sources. U.S. Pat. No. 3,818,129, issued Jun. 18, 1974, to Yamamoto, incorporates use of a cw lamp pumped, repetitively Q-switched, frequency doubled Neodymium:Yttrium Alminum Garnet (Nd:YAG) laser (wavelength=532 nm) to be a light source for green, and it cites the short pulse duration and high average power as the primary reasons for the choice of the above-mentioned laser. However, U.S. Pat. No. 3,818,129 fails to mention another important factor affecting image quality, which is output beam quality from the above-mentioned laser.
Generally speaking, the output beam quality from the cw lamp pumped, repetitively Q-switched, frequency doubled, Nd:YAG laser with high output power tends to have a high multimode structure in transverse direction, which results in high beam divergence; therefore producing undesirable image blur at a screen. The above-mentioned laser can be constructed to produce near diffraction limited, single mode output (TEM
oo
mode) which minimizes the beam divergence; however, TEM
oo
mode output does maximize laser speckle effect, which is not desirable.
Then what is needed is the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser cavity design which produces controlled multimode output in transverse direction that minimizes the laser speckle and still produces reasonably crisp images on the screen surface because of optically manageable beam divergence. M
2
is a measurable quantity which characterizes output beam spot size at far field and its divergence. When M
2
is equal to one, the output beam is called diffraction limited beam or TEM
oo
mode, whereas when M
2
is large (i.e., ~100), then the output beam is said to have a high multimode structure. The acceptable range of M
2
for the laser video display discussed in this invention is semi-empirically determined to be between 10 and 20. To achieve the acceptable range of M
2
=10-20, the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser has an intra-cavity aperture to strip excess modes, and the frequency doubling process is achieved by a Type II LBO (lithium triborate) or KTP (potassium titanyl phosphate) placed within the laser cavity. The schematic drawing of this green laser cavity is shown in FIG.
3
.
(2) All Solid State Tunable Blue Laser
Blue light is produced by a frequency doubled Ti:Sapphire (Ti:Al
2
O
3
) laser, which is longitudinally pumped by the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser (wavelength=532 nm). The Ti:Al
2
O
3
laser has broad range of near infrared emission; thus, it can be tuned to a specific wavelength by a set of birefringent plates, and for this particular application, the IR emission is tuned at 900 nm. The frequency doubling process is achieved by placing a Type I LBO or BBO (beta-barium borate) within the Ti:Al
2
O
3
laser cavity (i.e., intra-cavity frequency doubling), which results in emission of blue light at 450 nm. Finally, the range of blue emission from this frequency doubled Ti:Al
2
O
3
laser can be tunable by adjusting the angle of the birefringent plates. The schematic drawing of the frequency doubled Ti:Al
2
O
3
laser is shown in FIG.
4
.
(3) All Solid State Red Laser with Sum of Frequency Mix
A cw lamp pumped, repetitively Q-switched Nd:YAG laser produces primary laser radiation of 1064 nm. This 1064 nm radiation is used to pump the potassium titanyl arsenate (KTA) based intracavity optical parametric oscillation (OPO) and sum of frequency mix (SFM) mechanism to produce red light in wavelength between 626 nm and 629 nm. When KTA crystal is pumped by 1064 nm, it has been demonstrated to produce the signal (1520 nm~1540 nm) and the idler (~3540 nm) waves, and unlike KTP, the KTA does not exhibit reabsorption of the idler wavelength (~3540 nm); thus, relatively high conversion efficiency is expected from KTA based OPO once the pump beam exceeds OPO threshold. A separate Type III KTP will be used to achieve the sum of frequency mix process, and it has a phase match angle of 77° for SFM process between the wavelength of 1520 nm~1540 nm and 1064 nm, producing the resultant red wavelength of between 626 nm and 629 nm. Similarly, Type II KTA or Type I LBO can be used to achieve sum of frequency mix (SFM) between 1520~1540 nm and 1064 nm to produce the desired red wavelength of 626~629 nm, instead of Type III KTP discussed above.
The lasing mechanism to generate 1064 nm radiation, and KTA based OPO, and subsequent KTP based SFM process can be placed in the same cavity structure (intra-cavity design) or the two can be separated, depending on peak power of 1064 nm radiation. The schematic drawing of the former cavity design is shown in FIG.
5
.
(4) Diode Laser Pumped, All Solid State, Monochromatic Red, Green and Blue Light Source with Color Space Conversion
The all solid state red, green and blue laser designs discussed previously are based on cw lamp pump mechanism to produce primary laser radiation of 1064 nm from Nd:YAG crystal. However, diode laser pumped, all solid state red, green and blue laser light source described in U.S. Provisional Patent Application No. 60/032,269, filed Nov. 29, 1996 (Title: “Monochromatic R,G,B Laser Light Source and Display Systems by Masayuki Karakawa), will also be used as an alternative light source to produce three primary colors. This diode laser pumped, all solid state red (wavelength: 626~629 nm), green (wavelength: 532 or 523.5 nm) and blue (wavelength: 450 or 447 nm) laser light source also incorporates digital color space conversion electronics circuit and produces a very short pulse at a high repetition rate.
(5) Uniform Intensity Line Generation by a Cylindrical Asphere Lens Set
In the laser video projection system described in U.S. Pat. Nos. 4,720,747 and 4,851,918, each pulse of laser light having Gaussian intensity distribution, from one or more laser source must be converted to a line by an optical set up, and enters into an acousto-optic cell which acts as a spatial light modulator
Karakawa Masayuki
Martinsen Robert J.
McDowell Stephen R.
Corporation for Laser Optics Research
Lee Michael
Ropes & Gray
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