WO2006007866A1

WO2006007866A1 - Method and device to generate a dot by depositing a spot on a medium using a vcsel light source

Info

Publication number: WO2006007866A1
Application number: PCT/EP2004/008060
Authority: WO
Inventors: Bart Wattyn; Michael Peeters; Guy Verschaffelt; Hugo Thienpont
Original assignee: Strobbe Graphics Nv
Priority date: 2004-07-16
Filing date: 2004-07-16
Publication date: 2006-01-26

Abstract

Method and device to generate a dot (67) on a medium (23) by depositing a spot (22) on a medium (23) using a VCSEL light source (10), whereby a light beam (11) is generated using a VCSEL light source (10), the energy distribution of the generated light beam (11) is reshaped using a wave front engineering element (20) into a light beam (21) with a reshaped energy distribution to deposit a spot (22) on the medium (23).

Description

Method and device to generate a dot by depositing a spot on a medium using a VCSEL light source.

The invention relates to a method to generate a dot on a medium by depositing a spot on a medium using a VCSEL light source. The invention also relates to a device to generate a dot on a medium by depositing a spot on a medium comprising a VCSEL light source.

In the following description, the term spot means spatially localised energy distribution, the term medium means energy sensitive medium and the term dot means a deposited spot on a medium i.e. the end result of the interaction between the energy deposited and the medium.

It is known one can spots on a medium using edge- emitting laser diodes that generate a laser beam. In order to generate a dot on a medium, a minimum energy has to be deposited on the medium. The laser beam of edge-emitting laser diodes has a large angle of divergence. However, it is straightforward to focus such a laser beam using an optical lens system. A combination of such edge-emitting laser diodes and such an optical lens system is rather expensive. Unfortunately, due to the high power density at the output aperture of such edge-emitting laser diodes, the lifetime of such edge-emitting laser diodes is limited.

To generate a dot on a medium one can use a vertical cavity surface emitting laser diode, also known as a VCSEL or VCSEL light source. A VCSEL is made of semiconductor material. A VCSEL emits a light beam perpendicular to the substrate whereon it has been grown. A small-sized VCSEL has a diameter of approximately five micrometer or less, a medium-sized VCSEL has a diameter of approximately twenty micrometer, while a large-sized VCSEL has a diameter of approximately fifty micrometer or more. The power density at the output aperture of a VCSEL is such that the lifetime of a VCSEL is longer compared to the lifetime of edge-emitting laser diodes.

A small-sized VCSEL usually operates in the single fundamental transverse mode, when both the near-field and the far-field are Gaussian. For this reason, a small-sized VCSEL can be called a single mode VCSEL. Such a VCSEL is suited to image a spot on a medium. However the power that can be provided by such a small-sized VCSEL is limited, such that a dot can only be generated on a high sensitivity medium. Even if several light beams of small-sized VCSEL light sources could be focused together with an optical system to form one spot, the total power would still be not sufficient to generate a dot on a low sensitivity medium.

In order to provide more power, a medium-sized or a large-sized VCSEL has to be used. Such medium-sized and large-sized VCSEL light sources do not operate in a single mode, and are called multi-mode. The far- field and near-field patterns of multi-mode VCSEL light sources are not Gaussian, and therefore such a VCSEL is less suited to be used to image a spot on a medium. A light beam out of such a VCSEL can be imaged with an optical lens system to a non-uniform spot, for example a spot consisting of several random small spots .

A VCSEL will emit light when a current is sent through the VCSEL. In use a VCSEL will be activated during certain time intervals and will be not activated during the other time intervals. It is known that the temperature of the VCSEL will rise to a certain level when the VCSEL is activated. It is also known that the power of the emitted light beam will eventually decrease if the VCSEL temperature exceeds a certain limit. For this reason the maximal current that can be fed to the VCSEL is limited. The so-called nominal VCSEL current is the current that can be fed continuously to the VCSEL whereby the temperature of the VCSEL will not exceed a certain value or whereby the power of the generated light beam will not substantially decrease in time.

The object of the invention is a method and a device to enable the generation of dots on a medium, more particularly on a rather low sensitivity medium, fey using one or more VCSEL light sources.

In order to reach that object the method according to the invention comprises generating a light beam using a VCSEL light source, reshaping the energy distribution of the generated light beam using a wave front engineering element into a light beam with a reshaped energy distribution to deposit a tailored spot on the medium. The dot is the result of the interaction between the energy spot and the medium. In a preferred embodiment, the method comprises depositing a spot on a medium originating from at least one high power multi mode VCSEL light source.

The method according to the invention allows generating a dot on a medium. If there is a weak interaction between a laser light beam and a particular medium, a lot of energy must be provided on the medium in order to generate a dot on the medium. To provide this energy one or more VCSELs can be used, preferably one or more medium-sized or large-sized VCSELs. A wave front engineering element, abbreviated as WFEE is used to reshape and/or combine the emitted energy distribution, more particularly to reshape a non-Gaussian near-field pattern of a light beam out of a multi-mode VCSEL into a more suitable near-field pattern. Such a reshaped light beam is better suited to deposit a spot on a medium, more particularly can have a tailored near-field pattern that is for example substantially Gaussian, top-hat or super Gaussian. This tailored spot now should contain enough power to generate a dot. Due to the WFEE a VCSEL does not need to generate a light beam with a substantially Gaussian near-field pattern any more in order to be suitable to deposit spots on a medium, such that a middle-sized or a large-sized VCSEL light source can be used to this end. Furthermore, one is not limited to using only one VCSEL light source to generate a light beam. However, in case several VCSELs are used to generate a light beam, the total output light beam generated by these several VCSEL light sources can also be reshaped using one or more corresponding WFEEs to provide at least one light beam having a tailored near-field pattern suitable to deposit spots on the medium. In order to reach the object of the invention a further method according to the invention, that can be combined or not with the former method according to the invention comprises feeding short time current pulses to the VCSEL light source, where the short time current pulses have an amplitude that is substantially higher than the nominal VCSEL current in order to generate short high power light beams, more particularly light beam pulses that are suited to generate a dot on a medium. In a preferred embodiment, the method comprises reshaping the short high power light beam pulses using a wave front engineering element into reshaped short high power light beam pulses that are suited to generate a dot on a medium.

The method according to the invention allows generating a light beam pulse providing a high power during a short time interval, this is a power that is substantially higher than the nominal power of a light beam out of the VCSEL. This high power light beam pulse offers the advantage that it can provide a sufficiently high energy to the medium in a short time interval. Such a high power light beam pulse from a VCSEL offers the further advantage that the near-field and far-field pattern of such a light beam pulse is much more Gaussian than a nominal light beam pulse from said VCSEL using the nominal current fed to the VCSEL. Such a high power light beam pulse can be reshaped by a wave front engineering element according to the invention to be suited to deposit a spot on a medium or can possibly directly be used to deposit a spot on a medium.

According to a preferred embodiment, the time interval for activating the VCSEL in order to generate a light beam pulse is substantially shorter than the time interval where the VCSEL is not activated. During the activation, the current fed to the VCSEL is substantially higher than the nominal VCSEL current. Due to this, the actual peak output power of the light beam out of the VCSEL is substantially higher than the nominal output power of the VCSEL, because the temperature increase due to such a short pulse is small. A high output power is needed to generate a dot on a rather low sensitivity medium.

According to a preferred embodiment, when two spots are deposited one after the other, the time interval during which the VCSEL is not activated is chosen such that the temperature of the VCSEL at the end of this time interval is rather low and approximately constant. During the activation of the VCSEL the temperature inside the VCSEL will rise and during the time interval when the VCSEL is not activated this temperature will decrease. Because the temperature inside the VCSEL at the beginning of each activation of the VCSEL is always the rather low and approximately constant temperature, there will be no substantial output power variation from pulse to pulse. For example, when two spots are deposited one after the other, the time interval for activating the VCSEL is approximately one fifth of the time interval where the VCSEL is not activated and the current fed to the VCSEL is approximately three times the nominal VCSEL current.

According to an embodiment, the method further comprises arranging a number of VCSEL light sources in at least one array to be able to deposit one or several spots on the medium, more particularly one or several spots on the medium at a time. This allows the generation of one or more dots at a time. According to a further embodiment, the method further comprises moving relative to each other the medium on the one hand, and the at least one VCSEL with a corresponding at least one WFEE on the other hand. In this way it is possible to form an image of several dots on the medium.

According to an embodiment, the method comprises depositing a spot on a flat medium. According to another embodiment, the method comprises depositing a spot on a cylindrical medium.

In order to reach the object of the invention the device according to the invention comprises at least one VCSEL light source to generate a light beam and a wave front engineering element to reshape the energy distribution of the generated light beam into a light beam with a reshaped energy distribution to deposit a tailored spot on the medium. According to the invention, the device comprises a feeding unit for feeding short time current pulses to the at least one VCSEL light source to generate high power short time light beams, more particularly light beam pulses, where the short time current pulses have an amplitude that is substantially higher than the nominal VCSEL current.

According to an embodiment, the device comprises a number of VCSEL light sources for generating light beams and a number of wave front engineering elements to reshape the energy distribution in the generated light beams of the corresponding VCSELs. Due to this, it is possible to deposit several spots on a medium at the same time. The VCSEL light sources are preferably arranged such that the distance between the VCSEL light sources is larger than the distance between the spots to be deposited on the medium.

According to an embodiment, a number of VCSEL light sources and the corresponding wave front engineering elements are arranged to deposit a single spot originating from light beams of a number of VCSEL light sources, more particularly depositing the reshaped total energy output originating from several VCSELs into a single spot. This allows to achieve the necessary energy deposition on a medium, in other words to provide a sufficient power to generate a dot on the medium. Such a combination of a WFEE and a number of VCSELs can be repeated a number of times in order to deposit several spots on the medium at the same time.

According to an embodiment, the wave front engineering elements are arranged in at least one array and are held in a holding element. Preferably, a number of identical wave front engineering elements are held in a holding element. According to a further embodiment, the device further comprises at least one drive system for moving relative to each other the medium and at least one VCSEL with a corresponding WFEE. In this way it is possible to sequentially form an image of several spots on the medium.

According to an embodiment, the wave front engineering element comprises a diffractive optical element, which is a microstructure, which can reshape the energy distribution of a light beam into a different energy distribution.

The characteristics and further advantages of the invention will be further explained on the basis of non-restricting exemplifying embodiments represented in the attached drawings and in the following detailed description. In this description reference is made to the following drawings in which:

Figure 1 represents schematically a device according to the invention;

Figure 2 represents a device according to the invention with several VCSEL light sources;

Figure 3 represents a near-field pattern of a light beam generated by a multi mode VCSEL light source;

Figure 4 represents a so-called super Gaussian near-field pattern of a light beam; Figure 5 represents a so-called top-hat near-field pattern of a light beam;

Figure 6 represents a substantially Gaussian near- field pattern of a light beam; Figure 7 represents schematically a cross section through a wave front engineering element; Figure 8 represents a device according to the invention that is similar to the device according to figure 2;

Figures 9 and 10 each represent a possible image containing many dots generated with a device according to the invention/ Figure 11 and 12 represent a variant of the device according to figure 8;

Figure 13 represents a possible curve for the current fed to the VCSEL according to the time; Figure 14 represents a temperature curve of the VCSEL according to the time if a current as in figure 13 is fed to the VCSEL;

Figure 15 represents a variant of figure 13.

The device according to the invention represented in figure 1 comprises a VCSEL light source 10 for generating a light beam 11 and a wave front engineering element 20 for reshaping the energy distribution of the light beam 11 into a reshaped light beam 21 that is transmitted to the medium 23 in order to deposit a spot 22 on a medium 23. In this embodiment the medium 23 consists of an energy sensitive flat plate, more particularly a plate that is sensitive to heat. The absorbed power in the spot will be converted into a dot. The device further comprises a feeding unit 30 for feeding a current to the VCSEL light source 10 via electrical conductors 31 and 32. Preferably the feeding unit 30 can generate short time current pulses to the VCSEL light source 10 such that due to these short time current pulses the VCSEL light source 10 generates pulsed light beams 11, also called short light beam pulses. Using a device according to figure 1, it is possible to generate high power light beam pulses and to deposit one spot 22 at a time on the medium 23, thereby generating dots. In the embodiment of figure 2, the device according to the invention comprises a number of VCSEL light sources 10, each generating a corresponding light beam. The VCSEL light sources 10 are arranged in an array 15 and can independently be fed with current from a feeding unit 30. The device further comprises an equal number of wave front engineering elements 20 for reshaping the energy distribution of the generated light beams of the corresponding VCSEL light sources 10 into reshaped light beams 21 that are transmitted to deposit a spot 22 on a flat medium 23. The wave front engineering elements 20 are arranged next to one another and are held in a holding element 24. The holding element 24 also supports the VCSEL light sources 10. Such a device can deposit one or several spots at a time on the medium 23, more particularly a row of spots 22 at a time.

The operation of the above-mentioned devices according to the invention can be derived easily from the following explanation. The VCSEL light source 10 is fed with a current from the feeding unit 30 in order to generate a light beam 11. The light beam 11 is transmitted to the wave front engineering element 20 and when passing through the WFEE 20 the energy of light beam 10 is reshaped into a light beam 21. The reshaped light beam 21 is transmitted to the medium 23 and deposited in a spot 22. The energy in the spot -2-2- 22 is taken up by the energy sensitive medium 23. Due to the fact that the energy is taken up, a dot is formed on the medium 23. It is preferred to use a low energy sensitive medium 23, this means a medium that has to take up a lot of energy in order that a dot can be formed via a spot 22. A high sensitivity medium has to be treated by chemicals, for example acids which are more unfavourable for the environment and should be avoided. A more environmentally friendly rather low sensitivity medium, however, needs a more powerful light beam 21 in order to generate a dot on this medium 23.

The operation of the WFEE 20 is as follows. If a so- called multi mode large-sized VCSEL light source 10 is used, a light beam 11 with a near-field pattern 35 as shown in figure 3 can be generated. Such a near-field pattern contains several tops or peaks 33 and also contains depths 34. When this light beam 11 passes the WFEE 20, each part of the light beam 11 is reshaped due to at least the interaction with the upper surface of the WFEE 20 and/or the lower surface of the WFEE 20. Due to the shape of the upper surface and/or the shape of the lower surface the relative phase of different parts of the light beam will be changed, resulting in a reshaped energy distribution of the light beam 21 with respect to the light beam 11 and in the spot 22 after propagation to the medium 23. The change in relative phase between different parts of the light beam can be calculated in advance and the wave front engineering element can be designed to optimijs-s-e the efficiency of the energy redistribution to obtain an energy distribution of the light beam 21 to deposit a spot 22 that is favourable to deposit a spot 22 on a medium 23.

Using a wave front engineering element 20 according to the invention a reshaped light beam 21 with a near- field pattern 36 in the spot 22 at the height of the medium 23 can be obtained that is super Gaussian as shown in figure 4, such a pattern 37 that is top-hat as shown in figure 5 or such a pattern 38 that is approximately Gaussian as shown in figure 6. These patterns are all suitable to concentrate the energy in a spot that will generate a dot on the medium 23. The WFEE 20 thus allows reshaping an energy distribution in the light beam 11 that is less suitable to deposit a spot on a medium into a reshaped light beam 21 with an energy distribution that is more suitable to deposit a spot on a medium in order to generate a dot on the medium.

To reshape the energy distribution of a light beam 11 into a light beam 21, the wave front engineering element 20 as shown in figure 7 comprises a diffractive optical element 60 that comprises a microstructure made along the upper surface 63 of the diffractive optical element 60 and a smooth lower surface 66. In this example the upper surface 63 comprises a microstructure with grooves 62 and structural elements 64 having side walls 65 near the grooves 62. In this example, each structural element 64 and each groove 62 have dimensions of a fraction of a micrometer or dimensions of one or a few micrometers. The WFEE 20 as shown in figure 7 has dimensions in the same order of magnitude as the light beam 11 that has to be reshaped. Such a diffractive optical element 60 is suitable to reshape a laser light beam out of a VCSEL light source.

The way in which the light beam 11 will be reshaped is mainly determined by the profile of the sidewalls 65 and by the depth and the width of the grooves 62. A light beam with a given energy distribution can be easily reshaped into a light beam with an almost totally different energy distribution. The depth of the grooves 62, the width of the grooves 62 and the profile of the side walls 65 can be calculated using optical formula or simulated using optical models. Hereby one should take account of the near-field pattern of the light beam 11 transmitted to the diffractive optical element 60, the near-field pattern of the expected reshaped light beam 21 and the properties of the material of the diffractive optical element 60. As an example, this material can be formed by a kind of glass. The diffraction theory is also the theory behind holography, such that diffractive optical elements, abbreviated as DOE, are of the same component family as holograms or holographic optical elements .

In the example of figure 7, the optical diffractive element 60 comprises a smooth lower surface 66.

According to a variant not shown, the lower surface 66 of the diffractive optical element 60 is also provided with a microstructure with grooves 62 and structural elements 64 having side walls 65 near the grooves 62, similar to the microstructure of the upper surface 63.

In the embodiment of figure 2 and 8, the medium 23 can have a width in the width direction X of 1000 millimeter and a length in the length direction Y of 800 millimeter, while the array 15 of VCSEL light sources 10 extend along the length direction of the medium 23. The array 15 of VCSEL light sources 10 has a length of for example 400 millimeter. If VCSEL light sources 10 with a diameter of fifty micrometer are used, in an example 4000 VCSEL light sources will be arranged in an array 15. This means that for each hundred micrometer, one VCSEL light source 10 is arranged. Normally a VCSEL light source 10 with a diameter of fifty micrometer will be able to deposit a spot 22, after reshaping its generated light beam by a WFEE 20, in . which the main part of the power is contained inside a diameter of approximately ten micrometer. According to a variant, the array 15 of VCSEL light sources can have a length of 800 millimeter and can contain 8000 VCSEL light sources.

In order to form an image consisting of individual dots 67 using a method according to the invention, in other words to achieve a specific energy deposition pattern on the medium 23, the holding element 24 with the VCSEL light sources 10 and the corresponding wave front engineering elements 20 have to move with respect to the medium 23. As is shown in figure 8, the device further comprises a drive system 40 for moving the VCSEL light sources 10 and the corresponding wave front engineering element 20 relative to the width direction of the medium 23, more particularly to move them in steps according to the width direction of the medium. In the schematic example shown, the drive system 40 comprises two drive motors 41 that are driven in synchronism and that each drive a screw 42, whereby the screws 42 are guided in a housings 43 and work together with corresponding screw thread provided in the holding element 24 to move the holding element 24 along the width direction. According to a variant, the drive system 40 can comprise other means to move the holding element 24.

In order to form a practically continuous row of dots along the width direction of the medium 23 using a device according to figure 8, it is preferable to be able to deposit a spot 22 each ten micrometer along the width direction. This results in that the holding element 24 has to be able to move relative to the medium 23 along the width direction in steps of ten micrometer. After each step in the width direction a row of spots 22 in the length direction can be deposited. In practice about 1600 rows of spots per second can be produced. In this example, whereby the VCSEL light sources 10 are arranged at hundred micrometer one from the other, spots of ten micrometer at a mutual distance of hundred micrometer along the length direction of the medium 23 can be deposited on the medium 23. The distance of hundred micrometers between such two spots 22 is substantially larger than the diameter of ten micrometer of one spot 22. An image of dots 67 that can be obtained in this way is schematically shown in figure 9.

In order to form a practically continuous row of dots along the length direction of the medium 23, it is preferable to be able to deposit a spot 22 each ten micrometer along the length direction. This results in that the holding element 24 also has to be able to move relative to the medium 23 along the length direction in steps of ten micrometer. To this end, the device according to figure 8 further comprises a drive system 45 for moving the VCSEL light sources 10 and the corresponding wave front engineering element 20 relative to the length direction of the medium 23, more particularly to move them in steps according to the length direction of the medium 23. In the schematic example shown, the drive system 45 comprises two drive motors 46 that are driven in synchronism and that each drive a screw 47, whereby the screws 47 support the housing 43 to move the this housing 43 and the holding element 24 along the length direction with respect to housings 44 that are arranged fixedly.

After such a movement, several spots 22 can again be deposited along the width direction of the medium 23, in other words a continuous row of spots along the width direction X can be deposited next to the previous one. In this way, an image as shown in figure 10 can be obtained. In the above-mentioned example, when such a mutual movement along the length direction Y is carried out several times, several rows of dots according to the width direction can be produced next to each other. In the above example, after the holding element 24 has been moved ten steps one after the other forward in the length direction, the holding element 24 has to be moved backwards over ten steps. After each step in the length direction, the holding element 24 can be moved back or forth along the width direction or the direction perpendicular to the length direction. Due to this, it is possible to form an image on the medium 23 across the whole area of the surface of the medium.

According to a variant not shown, the holding element 24 with the VCSEL light sources 10 and the wave front engineering elements 20 can be arranged fixedly, while the medium 23 will be moveable with respect to the holding element 24 along the width direction and/or along the length direction. According to still another variant, the holding element 24 can be arranged to move along the length direction, while the medium 23 can be arranged to move along the width direction. It is also possible that the holding element 24 can be arranged to move along the width direction, while the medium 23 can be arranged to move along the length direction. This mutual movement allows creating an image made up out on many dots, generated by the deposition of energy spots.

In the embodiment of figure 11 the medium 23 is provided along a cylindrical support and forms a cylindrical medium 23. In this example the cylindrical medium 23 is rotated according to arrow R to allow the formation of an axial row 16 of dots along a peripheral of the medium 23. In order to form another axial row 17 of dots next to the previous one, the holding element 24 with the VCSEL light sources 10 and the corresponding wave front engineering elements 20 can be displaced axially according to arrow D with respect to the medium 23, this means a displacement perpendicular to the peripheral row. After each axial displacement a next row of dots can be formed along a peripheral of the medium 23. In this way due to the several axial rows of relative small dots an image can be formed on the medium 23 that extends over the whole area of the surface of the medium 23. Drive means such as a drive motor can be provided for the rotation of the medium 23 and drive means similar to the ones as shown in figure 8 can be provided for the axial displacement of the holding element 24.

In figure 12 a number of three arrays 15 of VCSEL light sources 10 with corresponding wave front engineering elements 20 are arranged to deposit spots 22 on a medium 23, whereby each spot 22 is originating from light beams generated by three VCSEL light sources 10. In other words three light beams 11 generated by three different VCSEL light sources 10 are combined together to form a single spot 22 on the medium 23. This allows to form a spot 22 with the power transmitted by three light beams 11, more particularly by three corresponding reshaped light beams 21 reshaped by three respective wave front engineering elements 20 or by a common wave front engineering element. In this way a high power can be achieved to form one spot 22. The three VCSEL light sources 10 that form one spot 22 can be controlled independently or can be controlled together. When these three VCSEL light sources 10 are controlled independently, each VCSEL light source 10 can be fed with current via respective conductors . When these three VCSEL light sources are controlled together, each VCSEL light source 10 can be fed with current via conductors that are connected in parallel or via conductors that are connected in series or by a combination of both. This device also allows achieving a row of several spots that are deposited at the same time.

According to the invention it is preferred to feed the VCSEL light sources 10 with short time current pulses. In the example of figure 13, the short time current pulses 50 preferably have an amplitude 51 that is substantially higher than the nominal VCSEL current as indicated by line 52, such that short powerful light beam pulses 11 can be generated. For a VCSEL with a diameter of fifty micrometer, this nominal VCSEL current can be approximately fifty milliampere. Short powerful light beams 11 or light beam pulses are suitable to be used to deposit a spot 22 on a medium 23. Such short time high power light pulses allow to generate a dot on a medium even on a rather low sensitivity medium 23. Due to the short time or fast pulses, all energy deposited on the medium 23 in the spot 22 can be used to generate a dot, because dissipation of energy through heat conduction in the medium 23 is a much slower process.

Of course these pulsed short time light beams 11 can similarly be reshaped using a wave front engineering element 20 into reshaped light beams 21 to deposit a spot 22 on a medium 23. The near-field and far-field pattern of a high power light beam 11 out of a VCSEL light source 10 fed with short high current pulses is more Gaussian, than the multi mode pattern out of a same VCSEL light source 10 fed with longer pulses at the nominal current. It could be possible to use such a light beam 11 to deposit a spot on a medium, although it is preferred to reshape such a light beam using a WFEE 20 and to use the reshaped light beam to deposit a spot 22 on the medium. For short high power light beams, the efficiency of the WFEE 20 can be higher than for a multi mode energy distribution.

In the example of figure 13, several current pulses 50 are fed to the VCSEL 10 at times Tl, T2, T3, T4, etc. in order to form several spots one after the other. When a current pulse 50 is fed to the VCSEL 10 the temperature of the VCSEL 10 will increase as indicated with the curve 55 in figure 14. Due to the high power current pulse 50 the increase in temperature will be significant. When the current is no longer fed to the VCSEL 10, the temperature of the VCSEL 10 will decrease again. Preferably, the time interval whereby the VCSEL 10 is not activated is chosen such that the temperature of the VCSEL 10 at the end of this time interval is a low and approximately constant temperature TO as indicated by line 56, for example a few degrees higher than the environment temperature.

If two spots 22 have to be deposited one after the other, it is preferable that before depositing the second spot, the temperature of the VCSEL 10 should have decreased to the low temperature TO. To obtain this, the time interval for activating the VCSEL 10 in order to generate a light beam pulse has to be substantially shorter than the time interval where the VCSEL 10 is not activated. In a preferred embodiment, the time interval for activating the VCSEL 10 is approximately one fifth of the time interval where the VCSEL 10 is not activated and the current fed to the VCSEL 10 is approximately three times the nominal VCSEL current. According to a variant, the substantial higher current fed to the VCSEL 10 can, for example, be between two and five times the nominal VCSEL current while the substantial shorter time for activation can, for example, be between one third and one seventh of the time of not activation. In this example the average current fed to the VCSEL will be in the same order and preferably lower than the previous defined nominal VCSEL current.

It is known that an increased temperature of a VCSEL 10 normally can lead to a decreased generation of the power of the light beams out of the VCSEL 10 for a given current fed to the VCSEL 10. It is clear that the VCSEL light source 10 can be cooled to keep their temperature low. It is known that VCSEL light sources 10 have only a small surface that can come into contact with a cooling fluid. A forced water-cooling or similar cooling system may be advantageous for cooling such VCSEL light sources.

Depending on the image to be formed, it is possible that at a given time no spot has to be deposited originating from a particular VCSEL light source 10, in other words no pulse 50 has to be fed to a particular VCSEL light source 10. In the example as shown in figure 15, a spot originating from a particular VCSEL light source 10 has to be deposited at times Tl, T3 and T4 while at time T2 no spot has to be deposited.

According to the invention, it is not excluded to provide a further optical system between the VCSEL light sources and the wave front engineering elements and/or to provide a further optical system between the wave front engineering elements and the medium. Such a further optical system can comprise a lens system that is able to focus a light beam. A wave front engineering element 20 differs from an optical lens system in that the wave front engineering element 20 is able to reshape the energy distribution of a light beam fed to it into a light beam with an energy distribution that is totally different from the light beam fed to it.

The methods according to the invention which may be applied by making use of devices according to the invention, and the devices according to the invention which are designed to be able to apply the methods according to the invention are obviously not restricted to the embodiments described in the examples, but may likewise comprise variants and combinations of these embodiments.

Claims

Claims .

1. Method to generate a dot (67) on a medium (23) by depositing a spot (22) on a medium (23) using a VCSEL light source (10) , characterised in that the method comprises generating a light beam (11) using a VCSEL light source (10) , reshaping the energy distribution of the generated light beam (11) using a wave front engineering element (20) into a light beam (21) with a reshaped energy distribution to deposit a spot (22) on the medium (23) .

2. Method according to claim 1, characterised in that the method comprises depositing a spot (22) on a medium (23) originating from at least one high power multi mode VCSEL light source (10) .

3. Method, more particularly a method according to claim 1 or 2, characterised in that the method comprises feeding short time current pulses (50) to the VCSEL light source (10), where the short time current pulses (50) have an amplitude that is substantially higher than the nominal VCSEL current in order to generate short high power light beams (11), more particularly light beam pulses that are suited to generate a dot (67) on a medium (23) .

4. Method according to claim 3, characterised in that the method comprises reshaping the light beam pulses using a wave front engineering element (20) into reshaped light beam pulses that are suited to deposit a spot (22) on a medium (23) .

5. Method according to any one of the claims 1 to 4, characterised in that, when two spots (22) are deposited one after the other, the time interval for activating the VCSEL (10) in order to generate a light beam pulse is substantially shorter than the time interval where the VCSEL (10) is not activated, more particularly the time interval for activating the VCSEL (10) is approximately one fifth of the time interval where the VCSEL (10) is not activated.

6. Method according to any one of the claims 1 to 5, characterised in that, when two spots (22) are deposited one after the other, the time interval whereby the VCSEL (10) is not activated is chosen such that the temperature of the VCSEL at the end of the time interval whereby the VCSEL (10) is not activated is rather low and approximately constant.

7. Method according to any one of the claims 1 to, β, characterised in that, when two spots (22) are deposited one after the other, the current fed to the VCSEL (10) is substantially higher than the nominal VCSEL current, more particularly the current fed to the VCSEL (10) is approximately three times the nominal VCSEL current.

8. Method according to any one of the claims 1 to 7, characterised in that the method comprises arranging a number of VCSEL light sources (10) in at least one array (15) to be able to deposit one or several spots (22) on the medium (23), more particularly one or several spots (22) on the medium (23) at a time.

9. Method according to any one of the claims 1 to 8, characterised in that the method comprises moving relative to each other the medium (23) on the one hand, and the at least one VCSEL (10) with a corresponding at least one WFEE (20) on the other hand.

10. Method according to any one of the claims 1 to 9, characterised in that the method comprises depositing a spot (22) on a flat medium (23) or depositing a spot (22) on a cylindrical medium (23) .

11. Device to generate a dot (67) on a medium (23) by depositing a spot (22) on a medium (23) comprising a VCSEL light source (10), characterised in that the device comprises at least one VCSEL light source (10) for generating a light beam (11) and a wave front engineering element (20) for reshaping the energy distribution of the generated light beam (11) into a light beam (21) with a reshaped energy distribution to deposit a spot (22) on the medium (23) .

12. Device, more particularly a device according to claim 11, characterised in that the device comprises a feeding unit (30) for feeding short time current pulses (50) to the at least one VCSEL light source

(10) for generating short time high power light beams

(11) , more particularly light beam pulses, where the short time current pulses (50) have an amplitude that is substantially higher than the nominal VCSEL current.

13. Device according to claim 11 or 12, characterised in that the device comprises a number of VCSEL light sources (10) for generating a light beam (11) and a number of wave front engineering elements (20) for reshaping the energy distribution of the generated light beams (11) of the corresponding VCSEL light sources (10) .

14. Device according to any one of the claims 11 to 13, characterised in that the VCSEL light sources (10) are arranged such that the distance between the VCSEL light sources (10) is larger than the distance between the spots (22) to be deposited on the medium (23) .

15. Device according to any one of the claims 11 to

14, characterised in that a number of VCSEL light sources (10) and the corresponding wave front engineering elements (20) are arranged to deposit a same spot (22) originating from light beams (11) of a number of VCSEL light sources (10) .

15. Device according to any one of the claims 11 to

15, characterised in that the wave front engineering elements (20) are arranged in at least one array (15) and are held in a holding element (24) .

17. Device according to any one of the claims 11 to

16, characterised in that the device comprises at least one drive system (40,45) for moving relative to each other the medium (23) on the one hand, and the at least one VCSEL (10) with a corresponding at least one WFEE (20) on the other hand.

18. Device according to claim 17, characterized in that the medium (23) is arranged fixed, while the at least one VCSEL (10) and the corresponding at least one WFEE (20) are arranged to move with respect to the medium (23) .

19. Device according to any one of the claims 11 to 18, characterised in that a wave front engineering element (20) comprises a diffractive optical element (60) that can reshape the energy distribution of a light beam (11) into a different energy distribution of a reshaped light beam (21) .