Laser based electronic imaging systems have been developed for use both in projection display, and even more extensively, for printing applications. In particular, laser projection display systems have been developed with several basic architectures, which include vector scanning, raster scanning, one-dimensional (1-D) scanning, and two-dimensional (2-D) area imaging systems. The development of laser projection systems, which are typically intended to be multi-color, have been generally limited by the minimal availability of useful visible wavelength lasers. On the other hand, laser based printing systems have been extensively developed using all of these same architectures, with the possible exception of vector scanning. As a result, many of the laser beam shaping and laser beam modulation techniques applicable to laser projection have been previously developed in great detail and variation by the efforts directed to laser printing. Notably, however, most of the laser printing systems described in the prior art are monochrome, and utilize infrared lasers, rather than the multiple visible spectrum lasers desired for laser projection.
In a typical laser printer, radiation from a laser is shaped, and imaged onto a film plane to produce the desired spot size. The spot, called a pixel, forms the smallest image element of the image. The laser radiation is modulated to create the correct density of each spot, pixel by pixel. The laser spot is scanned in the line direction, and the media is moved in the page scan direction to create a two-dimensional image. In a printer system with a continuous wave (CW) gas or solid state laser, an external modulator, such as an acousto-optical device, is often used to input the image data into the optical beam. Whereas, in systems with semi-conductor diode lasers, the laser radiation is typically modulated directly by varying the current input to the laser. For printers using high sensitivity media such as a silver halide film, high printing throughput is obtained by scanning the laser beam in the line direction with a polygonal mirror or a galvanometer. These printers are called “flying spot” printers.
By comparison, when the print media has a low optical sensitivity (such as most thermal media), the typical laser printer employs high power laser sources and slow line and page scan speeds to meet the high exposure requirements. One way to achieve this type of scan is to configure the printer like a “lathe,” where the page scan is obtained by rotating a drum which holds the film, and line scan, by translating the laser in a direction parallel to the axis of rotation of the drum. To achieve this high optical power throughput, in a small package, with a relatively low cost, the technology has adapted to provide multiple writing spots directed to the target plane.
Multi-spot printers have been configured in systems using a single laser as the light source, where the light illuminates a linear spatial light modulator array, which is in turn imaged to the target plane. Exemplary systems are described in several prior art patents, including U.S. Pat. Nos. 4,389,659 by Sprague, 4,591,260 by Yip, and 4,960,320 by Tanuira. However, the high power single beam lasers are typically too large and expensive to use in many printing applications. Moreover, such systems are sensitive to the potential failure of the laser source.
In another approach, a monolithic array of laser sources is imaged directly onto a light sensitive media to produce multiple spots. The power to each element of the laser array is individually modulated to obtain pixel densities. Such a system, as described by U.S. Pat. No. 4,804,975, potentially has a low cost and high light efficiency. On the other hand, this type of system is susceptible to emitter failure, and the consequent introduction of a pattern error. It can also be difficult to properly modulate the diodes, due both to the high current inputs needed by the diodes and the sensitivity to thermal and electrical crosstalk effects between laser emitters.
As a hybrid approach, linear diode laser arrays are used as light sources without direct addressing, and the laser light from the multitude of emitters is subsequently combined to flood illuminate a linear spatial light modulator array. In many such systems, the lasing emitters provide single mode Gaussian light emission in the cross array direction, and spatially multi-mode emission in the array direction. A typical emitter might be ˜100 μm in length in the array direction, and only ˜3 μm wide in the cross array direction. The addressed pixels of the modulator array break up the light into image elements, and each pixel of the modulator is subsequently imaged onto the media plane to form the desired array of printing spots. Printing systems employing this approach are described by prior art patents U.S. Pat. No. 4,786,918 by Thornton et al., U.S. Pat. Nos. 5,517,359 by Gelbart, and 5,521,748 by Sarraf. A variety of linear spatial light modulators are appropriate for use in such systems, including the “TIR” modulator of U.S. Pat. No. 4,281,904 by Sprague, the grating light valve (GLV) modulator of U.S. Pat. No. 5,311,360 by Bloom et al., the electro-optic grating modulator of U.S. Pat. No. 6,084,626 by Ramanujan et al., and the conformal grating modulator of U.S. Pat. No. 6,307,663 by Kowarz. Certainly numerous other modulator array technologies have been developed, including most prominently the digital mirror device (DMD) and liquid crystal displays (LCDs), but these devices are less optimal as linear array modulators which experience the high incident power levels needed in many printing and display applications.
In such systems, it is important that the illumination provided to the modulator plane be as uniform as possible. To begin with, if the emitted light is spatially and temporally coherent from one emitter to the next, the overlapped illumination at the modulator can suffer variation from interference fringes. Even with laser array consisting of long 1-D multimode emitters, laser filamentation, residual coherence, and non-uniform gain profiles can cause significant macro- and micro- non-uniformities in the array direction light emission profiles, which can result in the modulator illumination being significantly non-uniform. These issues have been addressed by a variety of methods.
As an example, U.S. Pat. No. 4,786,918 provides a laser diode array in which alternating single mode laser emitters are offset in one of two rows, so that the emitters are uncoupled and mutually incoherent. The emitted light subsequently overlaps in the far field, without any assistance from light homogenizing optics, to provide a substantially Gaussian light profile without interference.
In contrast, prior art U.S. Pat. Nos. 5,517,359 and 5,521,748 both utilize linear laser diode array consisting of broad area emitters. These high power laser arrays used in these systems typically emit 20-30 Watts of near infrared light, at wavelengths in the 810-950 nm range, with emission bandwidths of 3-4 nm. In both of these systems, the laser emitters are imaged directly, in an overlapping fashion, with the assistance of a lenslet array, onto the modulator array at a high magnification. As the array direction light emission profile for each of these emitters suffers a light fall off at the ends of the emitters, the system of U.S. Pat. No. 5,517,359 provides a mirror system to partially compensate for these problems, by substantially removing the macro-nonuniformities, but at the cost of some reduced brightness due to the increased angular spread of the illumination to the modulator. The method of U.S. Pat. No. 5,517,359 also only works well when the light profile across the emitting elements already has large areas that are substantially uniform.
A variety of systems have been disclosed for improving the illumination uniformity provided to the spatial light modulator array from the laser array. In particular, U.S. Pat. No. 5,923,475 by Kurtz et al. describes systems where a fly's eye integrator is used to homogenize the array direction illumination incident to the modulator array. Similarly, U.S. Pat. No. 6,137,631 by Moulin utilizes an integrating bar to homogenize the light.
As these laser diode arrays also typically suffer from “laser smile”, which is a cross array deviation of the emitter location from co-linearity (typical total deviation is 10 μm or less), cross array optics have been developed to correct for this problem. A variety of smile correction methods are described in prior art patents U.S. Pat. No. 5,854,651 by Kessler et al., U.S. Pat. No. 5,861,992 by Gelbart, and U.S. Pat. No. 6,166,759 by Blanding. Laser diode array bars have also been stacked in the cross array direction, with the goal of increasing the incident light available to the target plane. Exemplary laser beam shaping optics designed for stacked laser arrays are described in prior art patents U.S. Pat. No. 6,215,598 by Hwu and U.S. Pat. No. 6,240,116 by Lang et al.
Numerous color laser printers, with color lasers or infrared lasers and false color media, have been developed. In general, the most thoroughly developed architecture for color laser printing utilizes co-aligned beams in a flying spot printer. Exemplary prior art patents include U.S. Pat. No. 4,728,965 to Kessler et al. and U.S. Pat. No. 4,982,206, also to Kessler et al.
However, the visible color laser systems used both in display and printing applications have under utilized this very effective architecture of having a laser diode array flood illuminate a spatial light modulator array, with or without intervening light uniformizing optics. The system described in U.S. Pat. No. 5,982,553 by Bloom et al. utilizes solid state lasers (red, green, and blue) to illuminate a spatial light modulator array, which is turn imaged and scanned across a screen. As with the comparable laser printing systems, U.S. Pat. No. 5,982,553 system relies on a single laser source (for each color), and is thus sensitive to the failure of that laser source.
In the prior art patents U.S. Pat. No. 5,614,961 by Gibeau et al. and U.S. Pat. No. 5,990,983 by Hargis et al., color laser arrays are directly modulated and scanned across the screen. Thus, these systems do not utilize a system architecture which flood illuminates a spatial light modulator array, and thus the systems lack laser redundancy, and they too are sensitive to laser emitter failure. Additionally, the color laser arrays described by U.S. Pat. Nos. 5,614,961 and 5,990,983 are costly and difficult to fabricate. Because the laser arrays rely on inorganic semiconductor or solid-state laser media which do not emit light directly in the blue (440-470 nm) and green (520-550 nm) regions of the spectrum, nonlinear optics are required to frequency double the light emission to the desired wavelengths. Reliable lasers based on the nitride system that emit sufficient power directly in the blue and green spectral regions do not appear to be available in the near future. For the present, the nonlinear optics increase the cost and complexity of the laser arrays, and also reduce the efficiency of the laser system. Furthermore, the requirement to directly modulate the laser arrays in U.S. Pat. Nos. 5,614,961 and 5,990,983 necessitates the inclusion of an external modulating element to each emitter in the laser arrays to avoid chirp in semiconductor laser systems or limits due to the long upper state lifetime in solid-state laser systems.
Therefore, it can be seen that a laser projection display system using the optical system architecture combining a laser diode array with a flood illuminated spatial light modulator array would be advantaged. Moreover, it can be seen that improved, robust, low cost, color laser diode arrays would be advantaged over the existing color laser arrays, and would in turn further advantage this same optical system architecture.
One new laser technology that could be particularly advantaged for providing visible wavelength laser arrays, which could be useful both for projection and display, is the organic vertical cavity laser.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) are more commonly known than are the newer, organically based lasers. Inorganic VCSELs have been developed since the mid-80's (“Circular Buried Heterostructure (CBH) GaAl As/GaAs Surface Emitting Lasers” by K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, pp. 882-888 (1987)), and they have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years. With the success of these near-infrared lasers, attention in recent years has turned to using inorganic material systems to produce VCSELs emitting in the visible wavelength range, but despite significant efforts worldwide, much work remains to create viable inorganic laser diodes spanning the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials may have a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers should be relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip.
Given this potential, interest in making organic-based solid-state lasers is increasing. In the efforts to date, the laser gain material has been either polymeric or small molecule, with these materials utilized in a variety of resonant cavity structures. The exemplary cavity structures used have included micro-cavity structures (U.S. Pat. No. 6,160,828 by Kozlov et al.), waveguide structures, ring micro-lasers, and distributed feedback structures (U.S. Pat. No. 5,881,083 by Diaz-Garcia et al.). Notably, these new devices have all used a laser pump source to excite the organic laser cavities. Electrical pumping is generally preferred, as the laser cavities are more compact and easier to modulate.
A main barrier to achieving electrically-pumped organic lasers is the small carrier mobility of the organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (“Study of lasing action based on Förster energy transfer in optically pumped organic semiconductor thin films” by V. G. Kozlov et al., J. Appl. Phys. 84, pp. 4096-4106 (1998)). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude more charge carriers than singlet excitons. One consequence of this is that charge-induced (polaron) absorption can become a significant loss mechanism. Assuming laser devices have a 5% internal quantum efficiency, while using the lowest reported lasing threshold to date of ˜00 W/cm2 (“Light amplification in organic thin films using cascade energy transfer” by M. Berggren et al., Nature 389, pp. 466-469 (1997)), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of only 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (“High Peak Brightness Polymer Light-Emitting Diodes” by N. Tessler, Adv. Mater. 19, pp. 64-69 (1998)).
One way to avoid some of the problems affecting electrical pumping of organic laser devices is to use crystalline organic material instead of amorphous organic material as the lasing media. For example, an organic laser, comprising a thick layer single crystal tetracene gain material and a Fabry-Perot resonator, has demonstrated room temperature laser threshold current densities of approximately 1500 A/cm2.
However, it would be preferable to fabricate organic-based lasers with amorphous layers instead of crystalline layers (either inorganic or organic materials), as the manufacturing costs are significantly reduced. Furthermore, amorphous organic lasers can more readily be fabricated over large areas, as compared to producing large regions of single crystalline material. Additionally, because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and Si are possible supports for these devices. In combination, the amorphous organic laser has the potential to be scalable to arbitrary size (resulting in greater output powers) and arbitrary shape.
Optical pumping of amorphous organic lasers provides the significant advantage that the lasing structure is no longer impacted by the problems experienced by electrical pumping. The organic lasers can be pumped not only by exterior laser sources, but also incoherent light sources, such as light emitting diodes (LEDs) and lamps. For example, the combinations of using an organic DFB laser with inorganic LEDs (“Semiconducting polymer distributed feedback lasers” by M. D. McGehee et al. Appl. Phys. Lett. 72, pp. 1536-1538 (1998)) or organic waveguide lasers with organic LEDs (U.S. Pat. No. 5,881,089 by Berggren et al.) have been described. Optical pumping of organic laser systems is enabled by the fact that scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength are greatly reduced, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically-pumped threshold for organic lasers to date is 100 W/cm2, in a device using a waveguide laser design (“Light amplification in organic thin films using cascade energy transfer” by M. Berggren et al., Nature 389, pp. 466-469 (1997)). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, a different device architecture is required to achieve optical pumping with incoherent sources, particularly with LEDs. In order to lower the lasing threshold additionally, it is necessary to choose a laser structure that minimizes the gain volume; and a VCSEL-based microcavity laser satisfies this criterion. Using VCSEL-based organic laser cavities should enable optically-pumped power density thresholds below 5 W/cm2. As a result practical organic laser devices can be driven by optically pumping then with a variety of readily available, incoherent light sources, such as LEDs.
There are a few disadvantages to organic-based gain media, but with careful laser system design these can be overcome. Organic materials can suffer from low optical and thermal damage thresholds. Devices will have a limited pump power density in order to preclude irreversible damage to the device. Organic materials additionally are sensitive to a variety of environmental factors like oxygen and water vapor; efforts to reduce sensitivity to these variables typically result in increased device lifetime.
In general, the field of organic lasers has not been fully developed. Moreover, the favorable laser architecture of amorphous organic materials, vertical micro-cavity structures, and optical pumping with either coherent or incoherent light sources, has likewise not been fully developed. In particular, the extension of the optically pumped organic vertical cavity laser into configurations favorable for various systems applications has not occurred. As organic lasers, can be fabricated by high-vacuum thermal evaporation methods, using masks and photo-resists for patterning, a wide variety of laser structures, including laser array structures can be created. It may also be possible to fabricate organic lasers in part by utilizing printing methods (as is done with organic LEDs), such as ink jet or laser thermal deposition. As a result, the organic laser structures can be optimized in new and unique ways to match the specific intended applications, such as printing and display.