Patent Application
20070013765
The invention relates generally to the field of Vertical Cavity Surface Emitting Lasers (VCSELs) or microcavity lasers, and in particular to organic microcavity lasers or organic VCSELS. More specifically, the invention relates to the various flexible arrays of organic laser cavities used as printing engines.
Laser printers rely on the same technology used first in photocopying machines. This process is known as electro photography and was invented in 1938 and developed by Xerox and Eastman Kodak in the later 1980s. Prior art laser printer 3 rely on a laser beam 4 and scanner assembly 5 to form a latent image on a photo-conductor 11, wrapped around a drum, bit by bit. The scanning process, as illustrated in
The core component of this system is the photo-conductor 11 or photoreceptor, typically a revolving drum or cylinder. This drum assembly is made out of highly photoconductive material that is discharged by light photons.
Initially, the drum 11 is given a total positive charge by the charge corona wire 12, a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.) As the drum 11 revolves, the printer shines a tiny laser beam 4 across the surface to discharge certain points. In this way, the laser 6 “draws” the letters and images to be printed as a pattern 13 of electrical charges—an electrostatic image. The system can also work with the charges reversed—that is, a positive electrostatic image on a negative background.
After the pattern 13 is set, the printer 3 coats the drum 11 with positively charged toner (not shown)—a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum 11, but not to the positively charged “background”.
With the powder pattern affixed, the drum rolls over a sheet of paper 14, which is moving along a belt 15 below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire 16 (charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper 14 can pull the toner powder away. Since it is moving at the same speed as the drum 11, the paper 14 picks up the image pattern 18 exactly. To keep the paper from clinging to the drum 11, the paper 14 is discharged by the detach corona wire 17 immediately after picking up the toner. Material other than paper such as plastic etc. can be printed using this device.
Finally, the printer 3 passes the paper 14 through the fuser 19, a pair of heated rollers. As the paper 14 passes through these rollers, the loose toner powder melts, fusing with the fibers in the paper. The fuser rolls the paper to the output, and you have your finished page.
After depositing toner on the paper, the surface of the drum 11 passes the discharge lamp 20. This bright light exposes the entire photoreceptor surface, erasing the electrical image. The drum surface 11 then passes the charge corona wire 12, which reapplies the positive charge.
The most expensive part of the printer described above is the write laser and associated optics, which need to be precision ground and extremely accurate. This is generally the limiting factor in output resolution. There are laser printers capable of 2400 dpi and over, but most are 600 dpi.
LED arrays provide an alternative to lasers as the writing source. While LED arrays are somewhat simpler in design and do not need the rotating mirror, the arrays are expensive to assemble and difficult to align with the photoconductor to achieve the registration necessary for printing. In addition the light from the LED arrays spreads out a great deal more then the light from lasers requiring the LED arrays to be placed in close proximity to the photoconductor. The spaced requirements in electro-photographic printers around the photoconductors is very limited and any writing light source which can be conveniently spaced away from the photoconductor without requiring a complicated optical path provides a definite advantage.
One solution provided by the present invention to the problem stated above is to replace the laser and expensive reflective optics with an array of organic vertical cavity surface emitting lasers (VCSELs) lasers. The array would be cheaper to produce, give faster output times, greater resolution and can be placed further from the photoconductor.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (Kinoshita et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987). 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 (Choquette et al., Proceedings of the IEEE, Vol. 85, No. 11, November 1997). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (Wilmsen, Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (Ishigure et al., Electronics Letters, 16th Mar. 1995, Vol. 31, No. 6). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans 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 can enjoy 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 are 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. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828, issued Dec. 12, 2000), waveguide, ring micro lasers, and distributed feedback (see also, for instance, Kranzelbinder et al., Rep. Prog. Phys. 63, (2000) 729-762 and Diaz-Garcia et al., U.S. Pat. No. 5,881,083, issued Mar. 9, 1999). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically pumped organic lasers is the small carrier mobility of 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 (Kozlov et al., Journal of Applied Physics, Volume 84, No. 8, Oct. 15, 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 having more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (Tessler et al., Applied Physics Letters, Volume 74, Number 19, May 10, 1999). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (Berggren et al., Letters to Nature, Volume 389, page 466, Oct. 2, 1997), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 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 (Tessler, Advanced Materials, 1998, 10, No. 1, page 64).
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (McGehee et al., Applied Physics Letters, Volume 72, Number 13, Mar. 30, 1998) or organic (Berggren et al., U.S. Pat. No. 5,881,089, issued Mar. 9, 1999). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, 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 based on a waveguide laser design (Berggren et al., Letters to Nature Volume 389, Oct. 2, 1997). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to avail of optically pumping by incoherent sources. Additionally, in order to lower the lasing threshold it is necessary to choose a laser structure that minimizes the gain volume; 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 with a variety of readily available, incoherent light sources, such as LEDs.
One of the advantages of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. 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. Thus, there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers.
In accordance with one aspect of said present invention there is provided a printing device, comprising:
a photoconductor for receiving a charge;
a plurality of organic vertical cavity surface emitting lasers for producing a charged image pattern on said photoconductor;
a toner application mechanism for applying a toner onto said photoconductor for creating a toner image pattern in accordance with said charged image pattern; and
a transfer mechanism for transferring said toner image pattern onto a media.
In accordance with anther aspect of said present invention there is provided a method for printing an image onto a media, comprising said steps of:
producing a charged image pattern on a photoconductor using said plurality of organic vertical cavity surface emitting lasers;
applying a toner onto said photoconductor for creating a toner image pattern in accordance with said charged image pattern; and
transferring said toner image pattern onto a media.
In accordance with yet another aspect of said present invention there is provided a method for writing an image onto a media, comprising said steps of:
providing a media on which an image is to be created; and
creating said image using a plurality of organic vertical cavity surface emitting lasers.
In accordance with still another aspect of said present invention there is provided a flexible writing head for writing onto photoconductor, comprising:
a flexible substrate having a plurality of organic vertical cavity surface emitting lasers arranged in pattern for producing images on said media.
In accordance with another aspect of the present invention there is provided a printer for printing onto a media, comprising:
a photoconductor for receiving a charge;
a flexible substrate having a plurality of organic vertical cavity surface emitting lasers provided in an arrangement for producing a charged image pattern on said photoconductor;
a toner application mechanism for applying a toner onto said photoconductor for creating a toner image pattern in accordance with said charged image pattern; and
a transfer mechanism for transferring said toner image pattern onto a media.
The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:
In a typical prior art electro-photographic printer using the laser printer the most expensive parts are the write laser and its associated optics. This is also generally the limiting factor in output resolution. This is also true in the case where the LED array is used as the printer because of the LED array's complicated assembly and alignment process.
Instead of using the laser and expensive reflective optics or the LED array and it's complicated assembly it is advantageous to replace these two components with an array of organic lasers. Organic based lasers can be fabricated over large areas and grown on a variety of substrates such as glass, Silica and most importantly flexible plastics. Organic lasers can be available in a broad range of wavelengths allowing optimization with photoconductive material. Print heads made from organic laser arrays will be cheaper to produce with faster output times and higher resolution.
In the present invention, the terminology describing vertical cavity organic laser devices (VCSELs) may be used interchangeably in a short hand fashion as “organic laser cavity devices.” Organic laser cavity structures are fabricated as large area structures and optically pumped with light emitting diodes (LEDs).
A schematic of a vertical cavity organic laser device 25 is shown in
The preferred material for the organic active region 40 is a small-molecular weight organic host-dopant combination typically deposited by high-vacuum thermal evaporation. These host-dopant combinations are advantageous since they result in very small unpumped scattering/absorption losses for the gain media. It is preferred that the organic molecules be of small molecular weight since vacuum deposited materials can be deposited more uniformly than spin-coated polymeric materials. It is also preferred that the host materials used in the present invention are selected such that they have sufficient absorption of the pump beam 60 and are able to transfer a large percentage of their excitation energy to a dopant material via Förster energy transfer. Those skilled in the art are familiar with the concept of Förster energy transfer, which involves a radiationless transfer of energy between the host and dopant molecules. An example of a useful host-dopant combination for red-emitting lasers is aluminum tris(8-hydroxyquinoline) (Alq) as the host and [4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran] (DCJTB) as the dopant (at a volume fraction of 1%). Other host-dopant combinations can be used for other wavelength emissions. For example, in the green a useful combination is Alq as the host and [10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[1]Benzopyrano[6,7,8-ij]quinolizin-11-one] (C545T) as the dopant (at a volume fraction of 0.5%). Other organic gain region materials can be polymeric substances, e.g., polyphenylenevinylene derivatives, dialkoxy-polyphenylenevinylenes, poly-para-phenylene derivatives, and polyfluorene derivatives, as taught by Wolk et al. in commonly assigned U.S. Pat. No. 6,194,119 B1, issued Feb. 27, 2001, and referenced herein. It is the purpose of the organic active region 40 to receive transmitted pump beam light 60 and emit laser light.
The bottom and top dielectric stacks 30 and 50, respectively, are preferably deposited by conventional electron-beam deposition and can comprise alternating high index and low index dielectric materials, such as, TiO2 and SiO2, respectively. Other materials, such as Ta2O5 for the high index layers, could be used. The bottom dielectric stack 30 is deposited at a temperature of approximately 240° C. During the top dielectric stack 50 deposition process, the temperature is maintained at around 70° C. to avoid melting the organic active materials. In an alternative embodiment of the present invention, the top dielectric stack is replaced by the deposition of a reflective metal mirror layer. Typical metals are silver or aluminum, which have reflectivities in excess of 90%. In this alternative embodiment, both the pump beam 60 and the laser emission 70 would proceed through the substrate 28. Both the bottom dielectric stack 30 and the top dielectric stack 50 are reflective to laser light over a predetermined range of wavelengths, in accordance with the desired emission wavelength of the laser cavity 25.
The use of a vertical microcavity laser with very high finesse allows a lasing transition at a very low threshold (below 0.1 W/cm2 power density). This low threshold enables incoherent optical sources to be used for the pumping instead of the focused output of laser diodes, which is conventionally used in other laser systems. An example of a pump source is a UV LED, or an array of UV LEDs, e.g. from Cree (specifically, the XBRIGHT® 900 UltraViolet Power Chip® LEDs). These sources emit light centered near 405 nm wavelength and are known to produce power densities on the order of 20 W/cm2 in chip form. Thus, even taking into account limitations in utilization efficiency due to device packaging and the extended angular emission profile of the LEDs, the LED brightness is sufficient to pump the laser cavity at a level many times above the lasing threshold.
Organic lasers open up a more viable route to output that spans the visible spectrum. Organic based gain materials have the properties of low un-pumped scattering/absorption losses and high quantum efficiencies. VCSEL based organic laser cavities can be optically pumped using an incoherent light source such as light emitting diodes (LED) with lasing power thresholds below 5W/centimetersquared.
One advantage of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity. Lasers based on amorphous gain materials can be fabricated over large areas without regard to producing large regions of a single crystalline material and can be scaled to arbitrary size resulting in greater power output. Because of the amorphous nature, organic based lasers can be grown on a variety of substrates: thus, materials such as glass, flexible plastics and Si are possible supports for these devices.
The efficiency of the laser is improved further using an active region design as depicted in
An organic laser cavity structure is a predetermined arrangement of a plurality of organic laser cavity devices 200.
Applications of such one-dimensional organic laser cavity structures 221 and two-dimensional organic laser cavity structures 222 include line and area photo-activated printing processes, line and area emissive displays, and the like. The regular repetition of the light emitting organic laser cavity devices 200 as a consequence of the fabrication process produces an exposure device for printing and display applications. The spacing of the organic laser cavity devices 200 in such structures is dictated by the resolution requirements of the application. For example, in a printer application, the organic laser cavity devices 200 may be circular with diameters of approximately 20 to 50 micrometer, while the spacing between such organic laser cavity devices 200 (the inter-pixel regions 210) may be of comparable distances. Although not depicted, an arrangement whereby the diameter of the organic laser cavity devices 200 varies within the array is also considered an embodiment of the present invention.
Referring again to
Flexible organic laser cavity structures 228 can be produced, because of the relaxed substrate requirements for organic laser cavities as previously mentioned. Such flexible organic laser cavity structures 228 offer many advantages in that the structure can be lightweight and made to conform to a variety of non-planar surfaces. Additionally, the spatial relationship between organic laser cavity devices 200 may be affected by producing such devices on a flexible substrate. In this way the spatial relationship among the plurality of organic laser cavity devices changes with respect to each other. Stretching a flexible substrate may be used to alter the degree of coherence among organic laser cavity devices 200. It is to be understood that any of the organic laser cavity structures features (multiwavelength, control of coherence among elements, etc.) can be realized in combination with flexible organic laser cavity structures 228.
Organic lasers as previously described open up a more viable route to output that spans the visible spectrum. Organic based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. VCSEL based organic laser cavities can be optically pumped using an incoherent light source such as light emitting diodes (LED) with lasing power thresholds below 5W/centimeter squared.
One advantage of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity. Lasers based on amorphous gain materials can be fabricated over large areas without regard to producing large regions of a single crystalline material and can be scaled to arbitrary size resulting in greater power output. Because of the amorphous nature, organic based lasers can be grown on a variety of substrates: thus, materials such as glass, flexible plastics and Si are possible supports for these devices.
A laser printer 250 comprising an organic laser printer array 300 made in accordance with the present invention is illustrated in
Embodiments of two other light sources 229 are shown in
The following equations apply to the organic laser cavity array structure 300 in the laser printer 250:
For a typical laser printer the photoconductive roller needs 1 erg/cm2 to discharge the localized area.
1 erg/cm2=0.01 erg/mm2=1e−9J/mm2
For 2400 dpi printing
Dot size=(25.4/2400)(25.4/2400)=1.2e−4 mm2
Power per dot=energy per dot*area of dot=1.2e−13 J
An embodiment of the proposed arrays would have lasers 10 microns between centers with a diameter of 5 microns per display
Laser area=1/4*pi*d*d=¼*pi*(5e−6)(5e−6)=1.9e−5 mm2
Lasers that make up the area as in EK application number have been found to have laser output/area of 8e−5 W/mm2
The 5 micron lasers described would have laser power=laser power per area*area 8e−5*1.9e−5=1.6e−9 W
Time to process each dot=power per dot/5 micron power=1.2e−13 J/1.6e−9 W=7.5e−5 s or 75 microseconds
Since there is a row of lasers the time to print a page is the number of dots up the page times the time per dot. Dots per page length=26400(2400*11)
TIME PER PAGE˜2 seconds.
By increasing the output of the laser or decreasing the energy needed to discharge the photoconductor printing time could be reduced. Using Organic VCSELs allows the selection of an output wavelength that will require the least amount of energy for a particular photoconductor. Additionally the power incident on the photoreceptor drum could be increased by an array configuration as seen in
An additional method to reduce printing time is illustrated in
In another embodiment shown in
Electro-photographic printers usually use 780 nm wavelength of light and longer extending into the infrared range. Because of the limitations of the light sources used in electro-photographic printers, photoconductors have been designed to match the light sources. There is a definite advantage inherent in a light source that can be tuned to a variety of wavelengths of visible light such as can be done by the organic VCSELs.
The ability of organic VCSELs to be tuned to specific wavelengths in the range of 430 nanometers to 800 nanometers provides the opportunity to design the organic laser to better match the absorption spectrum of the photoconductor. The matching provides the opportunity to balance the appropriate electron penetration depth. Both of these lead to gains in printing efficiency.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
