Patent Publication Number: US-2006002270-A1

Title: System for applying combined laser light with extended output-power range

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method and a system for irradiation with combined laser light, which is generated by combining laser beams emitted from a plurality of laser-light sources.  
      2. Description of the Related Art  
      Conventionally, combined-laser-light irradiation systems for recording image information in a recording medium made of a photosensitive material or the like with combined laser light are known, where the combined laser light is generated by combining laser beams emitted from a plurality of laser-light sources. In particular, in some combined-laser-light irradiation systems, as disclosed in Japanese Unexamined Patent Publication No. 2000-190563, a plurality of semiconductor lasers having identical characteristics are used, and the optical output power of the combined laser light is controlled by equally increasing or decreasing the driving currents supplied to the plurality of semiconductor lasers so that the measured value of the optical output power of the combined laser light generated by combining laser beams emitted from the plurality of semiconductor lasers is equalized with a target value of the optical output power. At this time, the range within which the optical output power of the combined laser light is controlled is determined on the basis of the range of the optical output power of each semiconductor laser. That is, the range within which the optical output power of the combined laser light is controlled is the range in which the optical output power of the combined laser light varies when each semiconductor laser is driven with the driving current in the range from the oscillation threshold current to the maximum rated current value.  
      On the other hand, the recording mediums in which image information can be recorded by irradiation with the above combined laser light have various photosensitivities. In order to record image information in the recording mediums having various photosensitivities, it is necessary to control the optical output power of the combined laser light according to the photosensitivities of the respective recording mediums.  
      However, if a construction for generating the combined laser light is configured in such a manner that the maximum value of the optical output power of the combined laser light is a great value which is appropriate for a recording medium having a low photosensitivity, in some case, the lower limit of the range within which the optical output power of the combined laser light can be controlled in the system may be greater than another value of the optical output power which is appropriate for a recording medium having a very high photosensitivity. In this case, even when each semiconductor laser is driven with the oscillation threshold current, the optical output power of the combined laser light exceeds the value appropriate for the recording medium having the very high photosensitivity. If a recording medium is irradiated with combined laser light having an inappropriate optical output power, the quality of image information recorded in the recording medium deteriorates.  
      Further, if each semiconductor laser is driven with the driving current lower than the oscillation threshold current (i.e., each semiconductor laser is driven so as to output light by spontaneous emission) in order to lower the optical output power of the combined laser light, the wavelength range of the combined laser light is broadened, so that the recording medium is irradiated with light having wavelengths out of a predetermined wavelength range. In addition, when the driving current is lower than the oscillation threshold current, the optical output power of the combined laser light rapidly varies with the driving current, and therefore control of the optical output power of the combined laser light is difficult. Consequently, it is not practical to drive the semiconductor lasers with the driving current lower than the oscillation threshold current.  
     SUMMARY OF THE INVENTION  
      The present invention has been developed in view of the above circumstances.  
      The object of the present invention is to provide a method and a system for irradiation of a recording medium with combined laser light, the optical output power of which can be controlled within an extended range.  
      In order to accomplish the above object, the first aspect of the present invention is provided. According to the first aspect of the present invention, there is provided a method for irradiating a recording medium with combined laser light generated by combining laser beams emitted from a plurality of laser-light sources each having an oscillation threshold current and a maximum rated current. The method comprises the steps of: (a) selecting a first fraction of the plurality of laser-light sources when a target value of the optical output power of the combined laser light which is determined in correspondence with the photosensitivity of the recording medium is smaller than a predetermined reference value; and (b) driving each of the first fraction of the plurality of laser-light sources with a driving current within a range from the oscillation threshold current to the maximum rated current, and stopping a second fraction of the plurality of laser-light sources which are not selected in step (a), so that the optical output power of the combined laser light is equalized with the target value.  
      In order to accomplish the aforementioned object, the second aspect of the present invention is provided. According to the second aspect of the present invention, there is provided a system for irradiating a recording medium with combined laser light. The system comprises: a plurality of laser-light sources which emit a plurality of laser beams, and each of which has an oscillation threshold current and a maximum rated current; a combining unit which combines the plurality of laser beams so as to generate the combined laser light; an irradiation unit which irradiates the recording medium with the combined laser light; a target-value reception unit which receives a target value of the optical output power of the combined laser light which is determined in correspondence with the photosensitivity of the recording medium; and an optical-output-power control unit which equalizes the optical output power with the target value with the target value by selecting a first fraction of the plurality of laser-light sources, driving each of the first fraction of the plurality of laser-light sources with a driving current within a range from the oscillation threshold current to the maximum rated current, and stopping a second fraction of the plurality of laser-light sources which are not selected, when the target value of the optical output power of the combined laser light is smaller than a predetermined reference value.  
      The oscillation threshold current is the minimum driving current necessary for making each laser-light source output light by stimulated emission.  
      The reference value is predetermined to be the value of optical output power of the combined laser light which is obtained when all of the plurality of laser-light sources are driven with their oscillation threshold currents, or another value which is near to and greater than the value of the optical output power obtained as above.  
      The stopping of the second fraction of the plurality of laser-light sources means to make the second fraction of the plurality of laser-light sources emit no light. However, even in the case where the second fraction of the plurality of laser-light sources are driven with a driving current below the oscillation threshold current, and light by spontaneous emission can be emitted from the second fraction of the plurality of laser-light sources, it is possible to deem that the second fraction of the plurality of laser-light sources are stopped, as long as the light by spontaneous emission does not affect the irradiation of the recording medium.  
      The plurality of laser-light sources may be of any type, and for example, semiconductor lasers, solid-state lasers, or gas lasers.  
      In addition, the plurality of laser-light sources may have identical characteristics. Alternatively, a fraction of the plurality of laser-light sources may have different characteristics from the other of the plurality of laser-light sources.  
      According to the first and second aspects of the present invention, it is possible to decrease the minimum possible value (lower limit) of the optical output power of the combined laser light without decreasing the maximum possible value (upper limit) of the optical output power of the combined laser light. That is, it is possible to extend the range within which the optical output power of the combined laser light can be controlled. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram schematically illustrating a construction of a combined-laser-light irradiation system according to an embodiment of the present invention.  
       FIG. 2  is a graph indicating an example of a relationship between driving current and optical output power of each semiconductor laser.  
       FIG. 3  is a graph indicating examples of relationships between driving current and optical output power of combined laser light.  
       FIG. 4  is a flow diagram indicating a sequence of operations performed when a recording medium is irradiated with the combined laser light.  
       FIG. 5  is a graph indicating a relationship between the relative driving current and the optical output power of combined laser light.  
       FIG. 6  is a graph provided for explaining a way of obtaining the relative driving current by linear interpolation on the basis of data obtained from a data table. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      An embodiment of the present invention is explained in detail below with reference to drawings.  
       FIG. 1  is a diagram schematically illustrating a construction of a combined-laser-light irradiation system according to an embodiment of the present invention. As illustrated in  FIG. 1 , the combined-laser-light irradiation system  100  comprises a plurality (k) of semiconductor lasers  10   a ,  10   b ,  10   c ,  10   d , . . . (which may be hereinafter referred to as the semiconductor lasers  10 ), a combining unit  15 , and an irradiation unit  20 . The combining unit  15  combines laser beams emitted from the plurality of semiconductor lasers  10  so as to generate combined laser light Le. The irradiation unit  20  irradiates a recording medium  90  with the combined laser light Le, where the recording medium  90  is made of a photosensitive material.  
      The combined-laser-light irradiation system  100  further comprises a target-value receiving unit  25  and an optical-output-power control unit  30 . A target value of the optical output power of the combined laser light Le is inputted through the target-value input unit  25 . The optical-output-power control unit  30  selects a first fraction  10   a ,  10   b , . . . of the plurality of semiconductor lasers  10  (which may be hereinafter referred to as the semiconductor lasers  10 E), drives each of the selected semiconductor lasers  10 E with a driving current within a range from the oscillation threshold current to the maximum rated current, and stops a second fraction  10   c ,  10   d , . . . of the plurality of semiconductor lasers  10  (which are not selected, and may be hereinafter referred to as the semiconductor lasers  10 F), so that the optical output power of the combined laser light Le is equalized with the target value, when the target value of the optical output power of the combined laser light Le (which is inputted through the target-value receiving unit  25 ) is smaller than a predetermined reference value.  
      The combining unit  15  comprises a condensing lens  16  and an optical fiber  17 . The condensing lens  16  converges the laser beams emitted from the semiconductor lasers  10  into a point. The laser beams converged by the condensing lens  16  enter the optical fiber  17  and are combined in the optical fiber  17  to generate the combined laser light Le, which is then outputted from the optical fiber  17 .  
      The irradiation unit  20  comprises a collimator lens  21 , a DMD (digital micromirror device)  22 , a DMD controller  23 , and an image-forming lens  24 . The collimator lens  21  collimates the combined laser light Le outputted from the optical fiber  17 . The DMD  22  reflects the combined laser light Le collimated by the collimator lens  21  so as to spatially modulate the collimated combined laser light Le in accordance with image information (reflection pattern) as explained later. The DMD controller  23  controls the DMD  22 . The image-forming lens  24  forms an image of the spatially modulated light on the recording medium  90 , which is placed on a carrier table  50  provided in the combined-laser-light irradiation system  100 .  
      The DMD  22  is a spatial light-modulation device in which a plurality of micromirrors are arrayed in columns and rows (e.g., 1,024 columns and 756 rows), where each of plurality of micromirrors corresponds to a pixel, and can be individually controlled to change the orientation of the reflection surface. Therefore, a plurality of portions of laser light injected into the DMD  22  are respectively reflected by the plurality of micromirrors, so that the laser light injected into the DMD  22  is spatially modulated.  
      The target-value receiving unit  25  comprises a reading unit  26  and a storage unit  27 . The reading unit  26  reads a bar code  91  being indicated on the recording medium  90  and representing the target value, and the storage unit  27  stores the target value represented by the bar code  91 .  
      The optical-output-power control unit  30  comprises a laser controller  31  and drivers  32   a ,  32   b , . . . (which may be hereinafter referred to as the drivers  32 ). The laser controller  31  receives the target value from the storage unit  27  in the target-value receiving unit  25 , and selects one or more semiconductor lasers to be driven from among the plurality of semiconductor lasers  10 , determines the values of the driving currents of the one or more semiconductor lasers, and stops driving of the other semiconductor lasers. The drivers  32  drives the one or more semiconductor lasers selected by the laser controller  31  with the driving current determined and controlled by the laser controller  31 .  
      Specifically, the laser controller  31  stores in advance basic data indicating a first relationship between the driving current of each semiconductor laser and optical output power of laser light emitted from each semiconductor laser. An example of the first relationship is indicated in the graph of  FIG. 2 . In this example, it is assumed that the plurality of semiconductor lasers  10  have identical characteristics, i.e., all the semiconductor lasers  10  have the relationship of  FIG. 2 . As indicated in  FIG. 2 , light by spontaneous emission is outputted from each semiconductor laser when the driving current is below the oscillation threshold current Th, and light by stimulated emission is outputted from each semiconductor laser when the driving current is in the range from the oscillation threshold current Th to the maximum rated current Tmax.  
      In addition, the laser controller  31  stores in advance reference data which is obtained on the basis of the above first relationships between the driving current and the optical output power of each semiconductor laser. The reference data indicates a second relationship between the values of the driving current common to each semiconductor laser and the optical output power of the combined laser light in each of a plurality of cases where all or a fraction of the plurality of semiconductor lasers  10  are selected and driven.  
      Two examples of the above second relationships (reference data) are indicated in  FIG. 3 , which is a graph indicating the examples of the second relationships between driving current and optical output power of combined laser light. In  FIG. 3 , the curve Ro indicates a relationship (output characteristic) in the case where all of the semiconductor lasers  10  are driven, and the curve Re indicates a relationship (output characteristic) in the case where only two (the semiconductor lasers  10   a  and  10   b ) of the semiconductor lasers  10  are driven. In addition, light by spontaneous emission is emitted from each semiconductor laser when the driving current I of each semiconductor laser is below the oscillation threshold current Th, and light by stimulated emission is emitted from each semiconductor laser when the driving current I of each semiconductor laser is equal to or greater than the oscillation threshold current Th, and smaller the maximum rated current Tmax. The combined-laser-light irradiation system  100  uses only the light by stimulated emission for recording in the recording medium  90 .  
      The aforementioned reference value is predetermined in the range from the value Q 1  to the value P 2 , which is indicated by S in  FIG. 3 , and and stored in the laser controller  31 . The value Q 1  is a value of the optical output power of the combined laser light which is obtained when all of the semiconductor lasers  10  are driven with the oscillation threshold current Th, and the value P 2  is a value of the optical output power of the combined laser light which is obtained when only a fraction of the semiconductor lasers  10  (e.g., only the two semiconductor lasers  10 E) are driven with the maximum rated current Tmax.  
      As indicated in  FIG. 3 , the optical output power of the combined laser light which is obtained when all of the semiconductor lasers  10  are driven with a driving current in the range from the oscillation threshold current Th to the maximum rated current Tmax is in the range from the value Q 1  to the value Q 2 , and the optical output power of the combined laser light which is obtained when only a fraction of the semiconductor lasers  10  (e.g., only the two semiconductor lasers  10 E) are driven with a driving current in the range from the oscillation threshold current Th to the maximum rated current Tmax is in the range from the value P 1  to the value P 2 . In particular, when the target value (indicated by S 1  in  FIG. 3 ) of the optical output power of the combined laser light Le is smaller than the aforementioned reference value, only the fraction of the semiconductor lasers  10  (e.g., only the two semiconductor lasers  10 E) are driven, the other semiconductor lasers  10 F are stopped, and the optical output power of the combined laser light is controlled within a range from the value P 1  to a value smaller than the reference value S 1 . On the other hand, when the target value (indicated by S 1  in  FIG. 3 ) of the optical output power of the combined laser light Le is equal to or greater than the reference value, all of the semiconductor lasers  10  are driven, and the optical output power of the combined laser light is controlled within the range from the reference value S to the value Q 2 . Thus, it is possible to realize the optical output power of the combined laser light which is below the range of the optical output power which is obtained when all of the semiconductor lasers  10  are driven.  
      Hereinbelow, operations performed by the combined-laser-light irradiation system  100  are explained.  
      First, operations for recording image information in a recording medium  90   a  having a low photosensitivity are explained below.  
      When the recording medium  90   a  is placed on the carrier table  50  in the combined-laser-light irradiation system  100 , the reading unit  26  reads the bar code  91   a  indicated on the recording medium  90   a , and a target value Ma of the optical output power represented by the bar code  91   a  is stored in the storage unit  27 . Thereafter, the laser controller  31  in the optical-output-power control unit  30  reads out the target value Ma from the storage unit  27 , and compares the target value Ma with the reference value S 1 . In this case, the target value Ma is equal to or greater than the reference value S 1  (Ma≧S 1 ). Therefore, the laser controller  31  controls the drivers  32  to drive each of the semiconductor lasers  10  with a driving current in the range from the oscillation threshold current to the maximum rated current, and equalize the optical output power of the combined laser light Le with the target value Ma.  
      In order to drive each of the semiconductor lasers  10  with a driving current in the range from the oscillation threshold current to the maximum rated current, and equalize the optical output power of the combined laser light Le with the target value Ma, the laser controller  31  refers to portions of the reference data (output characteristic) corresponding to the curve Ro in  FIG. 3 , determines a driving current T 1  for each semiconductor laser corresponding to the target value Ma of the optical output power, and controls the respective drivers  32   a ,  32   b , . . . to drive the semiconductor lasers  10   a ,  10   b , . . . with the driving current T 1 . Thus, each of the semiconductor lasers  10  outputs a laser beam having an optical output power of Ma/k (i.e., the target value Ma divided by the number k of the semiconductor lasers  10 ), so that the optical output power of the combined laser light Le generated by the combining unit  15  becomes Ma.  
      The combined laser light Le obtained as above enters the irradiation unit  20 , propagates through the collimator lens  21 , is spatially modulated by the DMD  22 , and is then applied through the image-forming lens  24  to the recording medium  90   a , which cam be moved with the carrier table  50 . Thus, image information corresponding to the spatial modulation by the DMD  22  is recorded in the recording medium  90   a.    
      Next, operations for recording image information in a recording medium  90   b  having a high photosensitivity are explained below.  
      When the recording medium  90   b  is placed on the carrier table  50  in the combined-laser-light irradiation system  100 , the reading unit  26  reads the bar code  91   b  indicated on the recording medium  90   b , and a target value Mb of the optical output power represented by the bar code  91   b  is stored in the storage unit  27 . Thereafter, the laser controller  31  in the optical-output-power control unit  30  reads out the target value Mb from the storage unit  27 , and compares the target value Mb with the reference value S 1 . In this case, the target value Mb is smaller than the reference value S 1  (Mb&lt;S 1 ). Therefore, the laser controller  31  selects only a fraction of the semiconductor lasers  10  (e.g., two semiconductor lasers  10 E), and controls a fraction of the drivers  32  (e.g., two drivers  32   a  and  32   b ) to drive the selected fraction of the semiconductor lasers  10  (e.g., two semiconductor lasers  10 E) with a driving current in the range from the oscillation threshold current to the maximum rated current, and the other of the drivers  32  to stop the other semiconductor lasers  10 F, so that the optical output power of the combined laser light Le is equalized with the target value Mb.  
      In order to drive each of the selected fraction of the semiconductor lasers  10  (e.g., the two semiconductor lasers  10 E) with a driving current in the range from the oscillation threshold current to the maximum rated current, and equalize the optical output power of the combined laser light Le with the target value Mb, the laser controller  31  refers to portions of the reference data (output characteristic) corresponding to the curve Re in  FIG. 3 , determines a driving current T 2  for each semiconductor laser in the fraction of the semiconductor lasers  10  (e.g., the two semiconductor lasers  10 E) corresponding to the target value Mb of the optical output power, and controls the fraction of the drivers  32  (e.g., the two drivers  32   a  and  32   b ) to drive the fraction of the semiconductor lasers  10  (e.g., the two semiconductor lasers  10 E) with the driving current T 2 . Thus, for example, when the number of the selected semiconductor lasers is two, each of the two semiconductor lasers  10 E outputs a laser beam having an optical output power of Mb/2 (i.e., the target value Mb divided by two), so that the optical output power of the combined laser light Le generated by the combining unit  15  is equalized with the target value Mb.  
      When the combined laser light is generated as explained above, it is possible to record image information in recording mediums having photosensitivities in a wider range, with appropriate optical output power. That is, it is possible to extend the range in which the optical output power of the combined laser light can be controlled.  
      Although each semiconductor laser has an identical characteristic, and an identical relationship between the driving current and the optical output power of laser light in the above example, alternatively, the semiconductor lasers may have different characteristics and different relationships between the driving current and the optical output power of laser light. When the combined laser light is controlled in the manner explained below, it is possible to control the optical output power of the combined laser light regardless of whether or not the characteristics of the semiconductor lasers are identical.  
      Even when the characteristics of the semiconductor lasers are not identical, and the oscillation threshold currents and the maximum rated currents of the semiconductor lasers are different, the optical output power of the combined laser light can be controlled more easily by controlling the relative driving current, which is defined by the formula, 
 
Relative Driving Current=( I−Ith )/( Imax−Ith ), 
 
 where I is the driving current, Ith is the oscillation threshold current, and the Imax is the maximum rated current. The maximum rated current is the driving current which makes each semiconductor laser output a maximum rated amount of light. 
 
      The above formula indicates that the relative driving current is zero (0%) when the actual value of the driving current is equal to the oscillation threshold current, and is one (100%) when the actual value of the driving current is equal to the maximum rated current.  
      Hereinbelow, operations for controlling the optical output power of the combined laser light by controlling the relative driving current are explained in detail.  
       FIG. 4  is a flow diagram indicating a sequence of operations for controlling the optical output power of the combined laser light by controlling the relative driving current. In this example, the semiconductor lasers are controlled in such a manner that the relative driving currents of all the semiconductor lasers are identical. In  FIG. 4 , the optical output power of the combined laser light is also referred to the total optical output power.  
      In step  1 , for example, two options for the number of the activated (driven) semiconductor lasers (light-emitting elements) are determined. Specifically, the number m of the activated (driven) semiconductor lasers (light-emitting elements) is determined on the basis of an appropriate exposure (amount of light) for a recording medium having a low photosensitivity, and the number n of the activated (driven) semiconductor lasers (light-emitting elements) is determined on the basis of an appropriate exposure (amount of light) for a recording medium having a high photosensitivity, where m&gt;n.  
      In step  2 , data indicating a relationship between the relative driving current and the total optical output power P for each of the numbers m and n are obtained and stored in the form of a data table.  FIG. 5  is a graph indicating an example of the above relationship between the relative driving current and the optical output power of the combined laser light P. In  FIG. 5 , the curve Rm indicates a relationship between the total optical output power P and the relative driving current of each of activated semiconductor lasers in the case where the number of the activated semiconductor lasers is m, and the curve Rn indicates a relationship between the total optical output power P and the relative driving current of each of activated semiconductor lasers in the case where the number of the activated semiconductor lasers is n. In  FIG. 5 , the total optical output power which is obtained when the relative driving current is one (100%) is the maximum value Pm max in the case where the number of the activated semiconductor lasers is m, and the maximum value Pn max in the case where the number of the activated semiconductor lasers is n.  
      In step  3 , a target value Pt of the optical output power of the combined laser light is set in correspondence with the photosensitivity of the recording medium to which the combined laser light is to be applied.  
      In step  4 , one of the numbers m and n is selected on the basis of the above target value Pt, and the data corresponding to the selected number and indicating the relationship between the relative driving current and the total optical output power P as illustrated in  FIG. 5  is referred to. That is, the data representing the curve Rm (corresponding to the number m) are referred to when the target value Pt is equal to or greater than the maximum value Pn max, and the data representing the curve Rn (corresponding to the number n) are referred to when the target value Pt is smaller than the maximum value Pn max. In this case, the maximum value Pn max is the aforementioned reference value.  
      In step  5 , the data representing the curve Rm is referred to, since, in this example, it is assumed that the the target value Pt is equal to or greater than the maximum value Pn max. Then, data items of the two points U 1  (Pm 1 , Im 1 ) and U 2  (Pm 2 , Im 2 ) on the curve Rm which are nearest to the target value Pt and on both sides of the target value Pt are extracted from the data table.  
       FIG. 6  is a magnified portion of the graph of  FIG. 5 , which is provided for explaining a way of obtaining the relative driving current by linear interpolation on the basis of data obtained from the data table. In step  6 , as illustrated in  FIG. 6 , a straight line L 1  connecting the above two points U 1  (Pm 1 , Im 1 ) and U 2  (Pm 2 , Im 2 ) is obtained, and then the value It of the relative driving current corresponding to the target value Pt is obtained.  
      Although, in the above example, two options for the number of the activated (driven) semiconductor lasers (light-emitting elements) are initially determined in step  1 , it is possible to initially determine more than two options for the number of the activated (driven) semiconductor lasers (light-emitting elements).  
      Further, the data indicating a relationship between the relative driving current and the total optical output power P for each of the numbers m and n may be stored in the form of an approximate straight line instead of a data table.  
      In addition, all of the contents of the Japanese patent application No. 2004-183571 are incorporated into this specification by reference.