Patent Application
20180329270
The present application claims benefit of priority of German Application No, 10 2017 004 574.7 for a application titled “Laserscanner basierend auf elektrisch schaltende Schichten” filed on the 12th of May 2017.
The present disclosure relates to a new concept of deflecting—even high intensity—laser beams used for 1d, 2d and 3d laser scanning applications. The system can be used as a substitute for a 2d galvanometer scanner, that deflects a laser beam over a twodimensional area. Such a system has a broad range of use, that covers more or less every possible application of conventional galvanometer based laser scanners.
Systems that will benefit from the disclosure encompass 3D Laserscanners, 3D Printers, Rapid Prototyping Systems, Laser engraving systems, opthalmological laser systems, confocal microscopes. Laser printers, Laser shows, Laser systems for laserprocessing of ceramics and other materials—generally systems for planar (2d) laserprocessing. The system might also be used in barcode scanners, and various scanners for 2d area and 3d space measurement purposes.
Current laser scanner systems are primarily based on galvanometer setups, that entertain two mirrors tilted towards each other, so that the superposition of the movement of the one with the movement of the other deflects a laser beam to an intended xy coordinate in a spatial angle, that is covered by the movement of the two mirrors. The galvo produces a torque, that makes the mirror tilt back and forth covering essentially a certain range of deflection angles, to which an incoming beam is deflected by reflection of the mirror. The basic deflection mechanism is based on mechanical movements of inertial masses. The disadvantages are obvious. Mechanical movements are not very fast compared to intended GHz jumps of the beam to new xy values, that can be freely chosen in the solid angle that is covered by the scanner. Mechanical systems also are inertial in their movement and their coverage of the deflection range. To change the direction of the movement a vibrating torsional system needs to pass by the reversal point, which obviously leads to delay times, peculiar movement patterns at the borders of the deflection range and dead times, that have to be taken into account in order to produce an adequate xy pattern. The movement of galvanometer mirrors is a ballistic movement, that determines the timing of the control loop of the galvosystem to produce the patterns that are intended. Inaccuracies in x and y have to be taken into account because of heat related changes in the mechanical setup. Even piezobased systems cannot cover the wanted range of deflection angles or the intended speed of operation.
A second class of systems is based on electrooptical materials that are longitudinally traversed to produce the wanted deflection. They suffer from problems related to the amount of energy/intensity that is absorbed by the used materials, which severly limits maximum power of the laser beam that can be deflected and the quality of the deflected laser beam. Even other systems, ones that are based on prisms or on acoustooptical materials, suffer from the same disadvantages.
Accordingly a primary object of the present disclosure is to provide an improved mechanism to deflect a laser beam without using moving mechanical components and without using optical elements that hamper usability for high intensity laser beams due to their high optical depth. A further objective is an increase of the patterning speed and complete freedom to jump with the beam from one xy deflection direction to a totally different xy direction from one time step to the next.
Central to the disclosure is the use of elements that due to their optical switching characterstics can be either made highly transparent or highly reflective. These are used to deflect the laser beam and replace the function of the galvanometer mirrors used in galvo scanners. A means has to be to employed to arrange such elements on the path of the laser beam and to orient them in a tilted fashion with a fixed, precisely defined tilting angle going from one element to the next, to deflect the beam in discrete angular steps, so that the discrete tilt angles (or steps) of a deflected beam coming out of a first cascade of optical switching films/layers accurately cover the wanted total deflection range that is used by subsequent cascades that cover an ever smaller element to element tilt and therefore a smaller deflection range and therefore add smaller further tilts and finer granularity to the discrete angle steps.
The cascade might be called “deflection element”. By switching “on” one element and switching “off” all the others in a “deflection element” a predetermined deflection is chosen that the incoming laser beam is made subject to. A beam can for example be deflected to an angle 0,w1,2*w1,3*w1 and so on, up to an angle (n−1)*w1 for the last element in a “deflection element” that includes n switching films. These essentially are planar comparable to the galvo mirror—but might also be nonplanar, depending on the selection of the switching film/mirror mechanism. Using two or three such “deflection elements” in a cascade makes it possible to deflect the laser beam in ever smaller further deflection steps. It is important to note that the entity “deflection element” may differ in various ways from one deflection angle to the next. These “deflection elements” in a cascade might even not be two or three distinct mechanical compartments. The sole purpose of these “deflection elements” is to describe the way the laser beam is deflected to ever finer discrete deflection angles by cascading setups of sequences of switching films with fixed tilt from one switching film to the next in the sequence.
For example the use of a “deflection element” that covers an angle w of deflection angles having 12 switching films and the subsequent use of a deflection element that covers the angle w/12, with the same number of switching elements and the use of a last “deflection element” with a covered range of angle of w/(12*12), that again includes 12 switching elements makes it possible to hit 12*12*1.2=1728 discrete deflection angles in the angle coverage of w.
It is obvious that the switching films have to be perfectly aligned in their tilting angles, so that even in the case of the smallest tilt angles of the last deflection elements the whole deflection angle range is covered without gaps, without overlapping and equidistantly from one deflection angle step to the next. These constructive constraints are a prerequisite for a high spatial fidelity xy deflection of the laser beam. Due to high metrological precision that is possible in MEMS systems the necessary technology exists to make such a setup possible.
A possible embodiment of the optical switching elements might be based on metallic switching films. An other embodiment might be based on possibly stacked (possibly amorphous) dielectric films, with minor mismatches in the dielectric constants of one sequence of films in the stack compared to the other—a complementary sequence of films. In both cases it is necessary to maintain high planarity of the films, low overall surface roughness better than λ/10, high damage threshold of the films (for Lasers with kW output power). Sputtering or CVD processes are state of the art means to produce films of the wanted quality. A further aspect is the possibly huge area of the planar thin film, that is necessarily going to be implemented.
Metallic switching films are known to switch from transparrent to metallic in fractions of a ns (see reference [2000] Stefanovich et al.). The tests that are covered in the reference are based on the usage of a MOM Sandwich. The film, that might actually be used in a likely embodiment would preferably be very thin. For a full reflection a VO2 film of 14 nm thickness actually is enough. Such a material in its transparent state would not absorb intensity due to the very small optical depth (Beer Lambert law, very small thickness of the medium). If a MOM sandwich is going to be used for the switching mechanism, the metallic film must be transparent, again as thin as possible, especially at the areas where the laser beam shines through. Possible setups include vertical sandwiches and horizontal sandwiches, and electrode layers that omit the path of the laser beam. For electrochromic switching even of bigger areas the possible switching setup has been proven viable.
The article of Stefanovich proves switching times of 1 ns for a switching voltage of 8V The profound meaning of that information is, that it is possible to scan a field of 4096 times 4096 scanning points with a “frame rate” of 60 Hz. A whole new world of generative laser processes becomes possible. Laser TV with very high frame rates becomes possible, for example 4K Laser TV with a frame rate of 120 Hz can be produced (1 GHz switching frequency of the optical switches).
Materials for dielelectric switching films would include chalcogenide films, that have been proven to switch in the ns range (see reference [2013] Myoung-Jae Lee et al.). There is a broad class of chalcogenide materials that can be used to procure a material for the purpose of fastly switching the optical properties to control a slight mismatch of dielectric properties for two complementary films for highly reflective dielectric stacks or multilayer structures. Filters with a reflectivity of 99.999% are possible—and this not only for one preferential wavelength, as in a case of a laser beam with fixed wave length. Such mirrors might also be used for adaptive optics for astronomical instruments. The switching times of ins are way beyond state of the art adaptive optics systems.
Other more speculative embodiments might include phase changes in a fluid medium, that rapidly build up a highly reflective membrane on being electrically switched with selectable electrode lines, making it possible to choose the tilt angle with firing up a selection of possible electrodes, that are all located in the same medium, hereby drastically removing a huge number of switching films that potentially have to be traversed. Again, the rapid buildup of the reflective area is the key to the solution of the problem.
The embodiment of the complete scanner would include cascades of “deflection elements”.
Actually there would be a cascade of three “deflection elements” for x followed by a cascade of three “deflection elements” for y—for the orthogonal tilt. A last deflection element might be used to balance the mismatch of traversed optical depths. The exiting laser beam, that has been trimmed to the same intensity for every combination of x and y may pass a last element to correct the wavefront distortion.
Further to achieve this, it might be necessary to widen the solid angle that the deflected beam is covering with a setup of lenses, after it left the scanner. Hereby it might be possible to handle only a small solid angle with the new scanner setup that afterwards is properly widened.
The so called “deflection elements” might have a film for shuttering purposes in front of the sequence of the switching films. In case of using ultra short pulses, laser synchronization might be the way to go to screen off the transient time period switching from one xy to the next. For cw Lasers the use of shutter films might do the trick of screening off the period between two switching states, but due to the high intensities it might be necessary to ward off the beam to a heat sink by proper total reflection.
A huge adaptive optics system, a planar CCD like dielectric mirror setup might be used to deflect the beam a last time to correct the wavefront distortion and to make the beam usable for focusing applications. The beam might have crossed 2*n*6 thin films (2*n for every “deflection element”). The distortion might be substantial (and might be measured with a Shack Hartmann wavefront sensor for example).
A means or method has to be put in place to sense the distortion for every combination of x and y in the deflection range, to have precomputed adaptive setups to cover all possible cases of deflection directions, of which every single one has its unique distortion characteristics. Under normal operation conditions those measurements would be fed into the correction element synchronously with the deflection angle xy that have been employed, to ensure a high quality beam for every deflection angle that can be attended to with the scanner system.
As a matter of fact the bandwidth and the covered area of state of the art adaptive optics is a problem. The systems in use today are limited by a bandwidth of 10 kHz.
A possible solution would include the use of the same switching film mechanisms that are employed to build up the scanner—with one significant distinction. This time the reflective film would be an xy resolved matrix that can be switched on and off by two applied voltages at precisely one point. Having a number m of such films—one stacked on the other—it would be possible to selectively set the optical path, that has to be traversed on a “per pixel” basis, again with speeds in the GHz range. The “stacks” might be built with metallic or dielectric xy matrices. Switching films would be combined with spacer/transparent pieces of fixed length (again in cascaded setups with ever shorter spacer lengths), to tackle the discrepancy of range of lengths that has to be covered on the one side with a finely grained discreteness of steps of optical depth that can be additively achieved on the other.
Similarly it would be possible to produce films with a fixed deflection angle, arranged as a xy resolved matrix, that can be switched by two applied voltages at precisely one point. By using a number m of such films, it would be possible to selectively set the deflection angle for the beam on a “per pixel basis”, thereby making it possible to recover the wavefront to the directional signature of the undistorted beam. There might be configurations of adjacent switched pixels in the xy matrices of switching films, that would hinder the reflected beam from a pixel to leave the wavefront correction element due to a masking of a protruding adjacent pixel of a “higher” xy matrix. Therefore it might be necessary to put effort into the choice o possible deflection angles of the switching films. The choice would be in favor of smaller deflections and of deflection angles, that minimize the number of different maskings that have to be taken care of. As a possible solution to the masking configurations it would be possible to work out pixels that have various active fields, that can be individually switched on and off, thereby, switching off a masking area, either on the reflecting pixel or on the protruding pixel that is hit by the reflected beam from a pixel, that lies in a deeper stack. This would be a guarantee that the incoming intensity is totally reflected and not absorbed from bouncing multiple reflections in the stack of switching xy matrices.
It might be necessary to use a row/cascade of two or three said such stacks for phase correction and a row/cascade of two, or three said such stacks for deflection angle correction, with only a fraction of possible deflection range, but higher angular resolution or depth resolution for each subsequent stack in a row/cascade. Hereby it would be possible to finely deflect individual parts of the wavefront and finely correct the phase and angle of deflection of the wavefront on a “per pixel basis”.
The whole adaptive wavefront correction solution would be quite similar to cascading “deflection elements” to produce the scanner. It would be a quantum leap in adaptive optics technology (GHz adaptive speed/frequency) making obsolete the state of the art systems (for example deformable mirrors, MEMS systems) that are in use today, with a broad scope of uses in astronomy, 3D imaging, vision, and biomedical applications.
In order to more fully understand the objects, the following detailed description of the illustrative embodiments should be read in conjunction with the accompanying drawings, wherein:
By using elements as depicted in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 004 574.7 | May 2017 | DE | national |
