Patent Application
20090029292
The invention relates to doped fiber amplification (DFA) of a plurality of laser sources composed of different wavelength laser diodes to produce printing blocks more efficiently.
In current printing technology the final image is conveyed to the substrate by transferring ink from a printing block to the imaged surface. The printing block comprises light sensitive material wherein the image is formed by light exposure of selective areas with laser source. The light exposure can be made through a patterned mask (film), however, most often this is achieved by directly controlling the exposing light beam commonly in a process referred to as computer to plate (CtP); in a CtP system a laser beam is scanned over the surface (plate), and the intensity of the laser is modulated according to the data generated by a computer.
One family of plates used in direct laser imaging is known as flexographic (flexo) printing plates made of a flexible material, such as polymer or rubber, so that it can be attached to a roller or cylinder for ink application. Ink transfer occurs when raised images on the plate come into contact with the substrate during the printing process. For direct laser image setting, the flexo plate consist of upper level light sensitive material that acts as photo resist mask for a lower layer photopolymer material. In a first step, the upper level is imaged (ablated) to a desired pattern. In a subsequent step the flexo plate is exposed with ultraviolet (UV) light. The photopolymer cures material beneath the non removed areas of the mask, while under the removed areas the photopolymer stays in its uncured form and washes out in subsequent developing step. For mechanical support these two layers rest on a third substrate layer made of flexible polymer.
An alternative method for producing high resolution flexo plates where the UV exposure and subsequent developing steps are eliminated, is by direct engraving into the polymer. Imaging of the plate is achieved by selective removal of the material by laser ablation. The laser is applied one or more times on the surface of the plate until a 3D feature with required depth is formed. The depth of ablation depends on material properties such as ability of the material to absorb laser energy, the absorption depth, and other properties of the material. For a given material, there is a minimum required energy density for the laser beam for the ablation to occur, above this threshold the productivity of the plate setters will depend on the power of the laser.
This requirement of high power from a laser source is often contradictory to the requirement of high resolution imaging. For the laser to provide an adequate imaging resolution the laser spot must have the ability to be focused to a particular spot size. However, due to diffraction effects there is a fundamental minimum to the size the laser spot can be focused, which depends on the wavelength of the laser source and the angular divergence of the laser beam.
A laser spot focused to a particular spot size with minimum angular divergence set by the theoretical limit is said to be diffraction limited. A quantitative measure to how much the laser beam exceeds this theoretical limit is provided by the so called the M2 model. Characterization of laser beams by “M2 model” is discussed by (Thomas Johnston and Michael Sanset in “Handbook of Optical and Laser Scanning”, Ed. G. F. Marshal). Essentially, a spot produced by laser source and ideal lens used to focus the laser beam is given by the following formula:
Where λ is wavelength of the laser source, f is the focal length of the imaging lens, and D is the input beam diameter at the lens. M2 can be thought as the number of times the beam divergence exceeds the diffraction limit. For diffraction limited beam M2=1. CO2 laser which emit light at wavelength, λ=10.6 λm is an example of popular laser system that is limited because of a long wavelength. On the other hand, high power laser diodes are usually designed to emit at wavelength below 1 λm, but powerful emission can only be realized with multimode laser cavity with considerable divergence, that is, M2 many times larger than unity.
For these reasons, and with the advent of fiber lasers technology, fiber lasers, based on ytterbium (Yb) doped glass fiber, emerge as a technology of choice for high power laser application, providing high beam quality: M2 close to unity, and wavelength of the order λ≈1.1 μm. Fiber lasers capable of emitting many hundreds of watts are commercially available from various companies, such as for example IPG Photonics. A review on fiber lasers is given by “Rare Earth Doped Fiber Lasers and Amplifiers”, Second Edition, Ed. Michel J. F. Digonnet.
While a single fiber laser is capable of providing ample output power, there is a practical limitation to the power of a single laser beam that can be useful in a CtP system. Conventionally, in a CtP, the plate is clamped to a rotating drum and the laser beam is scanned over the plate in an axis parallel to axis of rotation. The productivity can be expressed as the total area of plate material processed per unit time. Therefore to take advantage of increased power form a laser source to increase productivity of the platesetter, the drum needs to be rotated fast. Similarly, if a linear scanning method is used, known as “flatbed or “capstan”, the plate is scanned relative to the laser beam linearly the linear velocity needs to be increased accordingly.
Usually however, there are mechanical constrains which makes higher scanning speeds increasingly complicated. Therefore, the common approach for increased productivity is to use a plurality of fiber laser beams, positioned into a contiguous array, each beam is modulated independently for simultaneous imaging of the plate. Such approach is commonly used with plurality of laser diodes sources, wherein each diode is modulated by direct modulation of the driving current. However, the laser diode fiber laser is limited to direct current modulation to moderate frequency range, usually less than 100 KHz which is less than required for high speed imaging.
To overcome this shortcoming of fiber laser, the standard approach was to use acousto optic modulator (AOM) to modulate the beam. For plurality of beams, a number of AOMs are utilized, and each beam was provided with a separate AOM. As an alternative, acousto optic deflector (AOD), driven by multiple RF voltage frequencies, was provided with each RF frequency amplitude modulated corresponding for a particular beam channel. U.S. Pat. No. 6,822,669 (Fischer et al.) describes such arrangement of plurality of fiber lasers in conjugation with number of AOM, or single AOM driven by several RF voltages.
While AOM can provide the means for high rate of modulation, there are significant disadvantages associated with it, in addition to increased complexity of adding additional components and costs to the imaging head. Since the AOM is not perfectly transparent the light transmitted through the modulator is attenuated.
Another disadvantage associated with AOM is related to its reliability. When light beam is amplitude modulated by the AOM, the rise time of the modulation is proportional to the beam diameter passing through the modulator. If the AOM is operated at high modulation rates the laser beam diameter incident on the AOM must be small, and therefore the standard approach has been to focus the beam at the input aperture of the modulator. This results in significant increase in the optical power density of the beam and can lead to damage of the AOM. Even slight excess of power density can damage the crystal inside AOM, leading to optical absorption of higher portions of the laser beam and inevitable failure of the device. Yet, another disadvantage of AOM is associated with the RF voltage driver. As the electric to acoustic energy conversion is not efficient, the heat produced by the dissipated electrical energy within the RF driver needs to be removed from the system, often by water cooling, which adds to complexity and impairs the reliability of the CtP. It is the objective of this invention to provide a method for constructing a plurality of laser beams for CtP utilizing fiber laser technology which circumvents the use of acousto optics devices.
In a fiber laser the gain medium for laser action is a fiber doped with ions such as ytterbium (Yb3+), erbium (Er3+), neodymium (Nd3+), or other rare-earth metals, that is pumped with one or more laser diode. For laser action to take place a cavity is formed by introducing a type of resonant reflector into the fiber, which can be a mirror, a fiber ring, fiber optic couplers, or other arrangements described in the literature. If no resonant reflectors are introduced into the gain medium, the doped and pumped fiber can serve as a light amplifier to low power laser light which is launched into it. The arrangement is then generally referred to as ‘doped fiber amplifier’ (DFA). The action of DFA gain medium is not limited to amplification of a single low power laser light. Several laser inputs can be amplified simultaneously by the same DFA provided that the launched wavelengths of laser light lies within the optical spectral bandwidth of the DFA. Furthermore, the DFA can be cascaded in several stages to provide several stages of amplification.
DFA applications are extensively used in optical fiber communication, particularly in wavelength-division multiplexing (WDM) where predominantly erbium (Er3+) doped fiber amplifiers (EDFA) are used to amplify optical signals within several channels, each channel of different wavelength that propagate in optical fibers.
U.S. Pat. No. 6,212,310 (Waarts et al.) describes coupling a plurality of laser sources into a single fiber waveguide. The signal amplification in the single waveguide is achieved via doped fiber amplification means.
EP Patent No. 0846562 (Tamaki) describes an image recording apparatus and method, utilizing doped fiber amplification means. The DFA is used in conjunction with amplification of a single laser source, the amplified laser source is then applied for purposes of engraving a printing block.
Briefly, according to one aspect of the present invention an apparatus for direct engraving comprises: a plurality of laser diode emitting at different wavelengths; a multiplexer for collecting the plurality of laser sources into a single laser beam; a rare earth doped fiber amplifier to amplify the single laser beam to form an amplified single laser beam; a demultiplexer to split the single laser beam into a plurality of amplified laser sources; and an imaging means to apply the plurality of amplified laser sources for imaging a printing plate.
A method for producing a printing block with a multiple laser sources composed from different wavelength laser diodes 10, as is depicted in
With reference made to
Most commonly optical multiplexers are based on dispersive components, such as diffraction gratings or prisms, but can be realized on principles of interferometery.
In
In
It is understood that the path of the light beam can be retraced as propagating in reverse direction, then the device operates as a multiplexer, combining the light of different ports 25 into a single port 23. Optical multiplexer are discussed in “Fundamentals of Optical Waveguides” by Katsunari Okamoto, Academic Press Inc.
The laser sources 10, at different wavelength are fiber coupled semiconductor diode lasers. Since the beams of laser diodes are amplified by the DFA, and because optical amplification occurs in a finite range of optical frequencies called the gain bandwidth, the wavelength of the laser sources must be positioned within the operational wavelength range of the DFA. For doping with erbium ions the useful range for amplification is 1535 nm to 1565 nm and can be extended to 1610 nm. When doping is with ytterbium ions the applicable wavelength is 1030 to 1100 nm.
It is important that spectral overlapping is to be avoided between the input laser sources; width for the individual laser sources needs to be narrower than the wavelength separation between individual sources. This is described in
Since it is desirable to operate with high beam quality, i.e., M2 close to unity, the preferred DFA is a single mode fiber. The laser sources therefore are preferably single mode laser with single mode fiber output. Because the laser diodes are amplified high power is not required and therefore single mode operation does not introduce a constraint on output power requirement. Furthermore the great advantage of laser diodes is the ability of internal intensity modulation, by modulating the drive current of the laser diode. Single mode laser diodes are better suited for internal modulation at high rates. Single mode laser diodes at wavelength suitable for ytterbium DFA, or erbium DFA are available, for examples available by Lumics—GmbH http://www.lumics.com/.
The wavelength division de-multiplexer channels are chosen according to the wavelength of the laser sources, and the number of ports determined by the number of laser sources. The DFA, can be ytterbium doped fiber amplifier (YDFA), which is suitable for amplification in the wavelength range of 1050-1100 nm as shown in
As can be inferred from their curve in
It is evident from
In preferred arrangement the proposed method of the imaged plate is insensitive to wavelength over the useful range of the fiber amplifier used, and the spectral bandwidth used. As an example, if ytterbium ion based DFA is used, the useful spectral range for amplification 1030 to 1100 nm. In a possible arrangement the CtP consists of 8 beams with wavelength spacing of 5 nm between channels with the first channel centered at 1070 nm. Since the spacing is 5 nm, the second at 1075 nm, and so on, the last eighth channel 8th at 1105 nm. This is described in
The method of this invention offers the advantage of deploying a modular approach, allowing of cascading several amplifier stages, amplifying the output power to the required level.
Another benefit of using a amplification stage rather than discrete powerful laser is that it is much simpler to control the modulation of lower power of individual laser diodes than that of a powerful laser source, moreover, since internal current modulation of laser diodes is straightforward.
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 scope of the invention.
