JP2020004802A - Laser driver, light source, and image projection device - Google Patents

Abstract

To reduce the size of a laser driver.SOLUTION: A laser driver includes a code generation unit 16 that generates a luminance code for driving a semiconductor laser 18, a drive circuit 12 that generates a drive current for driving the semiconductor laser on the basis of the luminance code, and a current adjusting unit 14 that generates a correction current for correcting the drive current such that the optical output intensity of the semiconductor laser linearly changes in accordance with the change in the luminance code. The current adjustment unit generates the correction current for correcting the drive current when the luminance code is equal to or less than a first threshold.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laser driver, a light source, and an image projection device, for example, a laser driver that outputs a drive current to a semiconductor laser, a light source, and an image projection device.

  2. Description of the Related Art An image projection apparatus that projects an image directly on a retina by scanning a laser beam is known (for example, Patent Document 1). A semiconductor laser is used as a light source of such an image projection device. It is known that a drive current for driving a semiconductor laser is corrected when an IL (Injection current-Light output) characteristic of the semiconductor laser is nonlinear (for example, Patent Document 2).

International Publication No. 2014/192479 JP 2013-226746 A

  In an image projection device as disclosed in Patent Document 1, the intensity of laser light may be weakened in order to irradiate the retina with laser light. In such a case, if a semiconductor laser is used as the light source, a non-linear region of the IL characteristic is used. However, when the drive current is corrected as in Patent Literature 2, the control of the drive current becomes complicated, and the circuit scale of the laser driver increases.

  The present invention has been made in view of the above problems, and has as its object to reduce the size of a laser driver.

  The present invention provides a code generation unit that generates a luminance code for driving a semiconductor laser, a driving circuit that generates a drive current for driving the semiconductor laser based on the luminance code, A current adjustment unit that generates a correction current that corrects the drive current so that the optical output intensity of the semiconductor laser changes linearly. In the following, the laser driver generates a correction current for correcting the drive current.

  In the above configuration, the current adjustment unit may not generate the correction current when the luminance code is larger than the first threshold.

  In the above configuration, the current adjustment unit may be configured to subtract a correction current corresponding to the luminance code from the drive current when the luminance code is equal to or less than the first threshold.

  In the above configuration, the current adjustment unit includes a plurality of current generation circuits each of which generates a correction current, and when the luminance code is equal to or less than the first threshold, each of the corrections output by the plurality of current generation circuits. The current may be combined and subtracted from the drive current.

  In the above configuration, when the luminance code is equal to or less than the first threshold, the current adjustment unit may include a plurality of current generation circuits in a range of consecutive luminance codes among the luminance codes that are equal to or less than the first threshold. A control unit may be provided that causes only one current generation circuit to generate the correction current and causes another current generation circuit to output the maximum current or 0 among the correction currents based on the luminance code.

  In the above configuration, a configuration may be provided that includes a current cutoff unit that cuts off the drive current when the luminance code is equal to or smaller than a second threshold smaller than the first threshold.

  The present invention is a light source including the above laser driver, and a semiconductor laser driven by the laser driver to emit laser light.

  The present invention is an image projection device comprising: the light source; a scanning unit that scans the laser light emitted by the light source; and an image projection unit that projects the scanned laser light as an image onto a retina.

  In the above configuration, the light source includes a plurality of the laser drivers and the plurality of semiconductor lasers, respectively, and a current adjustment unit in each of the plurality of laser drivers includes a plurality of semiconductor lasers when the luminance code is equal to or less than the first threshold. The correction current may be generated such that the light output intensity of the corresponding semiconductor laser among the lasers with respect to the luminance code has linearity.

  In the above configuration, the light source includes three types of the laser driver and the semiconductor laser, the three types of semiconductor lasers generate red, green, and blue laser beams, respectively, and the code generation unit generates a luminance code as a luminance code. , Respectively, for generating red, green and blue color codes.

  According to the present invention, an object is to reduce the size of a laser driver.

FIGS. 1A and 1B are block diagrams of a light source in which the laser driver according to the first embodiment is used. FIGS. 2A and 2B are diagrams illustrating IL characteristics of the semiconductor laser according to the first embodiment. FIG. 3 is a flowchart illustrating a process of the current adjustment unit according to the first embodiment. FIG. 4A illustrates a relationship between the color code C and the current Id in the first embodiment, and FIG. 4B illustrates a relationship between the current Id and the color code C and the optical output power P in the first embodiment. FIG. FIG. 5 is a diagram illustrating the relationship between the drive current Iop, the current Id, the color code C, and the optical output power P in the first embodiment. FIG. 6 is a block diagram illustrating an example of a current adjustment unit according to the first embodiment. FIG. 7 is a diagram illustrating the correction current Is for the color code C in the first embodiment. FIG. 8 is a flowchart illustrating a process executed by the control circuit in the first embodiment. FIG. 9 is a block diagram of a light source in which the laser driver according to the first modification of the first embodiment is used. FIG. 10 is a flowchart illustrating a process of the current interrupting unit and the current adjusting unit according to the first modification of the first embodiment. FIG. 11 is a diagram illustrating a relationship between the drive current Iop, the current Id, the color code C, and the optical output power P in the first modification of the first embodiment. FIG. 12 is a block diagram of the image projection apparatus according to the second embodiment. FIGS. 13A to 13C are diagrams illustrating the optical output power P of the semiconductor laser with respect to the color code C according to the second embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIGS. 1A and 1B are block diagrams of a light source in which the laser driver according to the first embodiment is used. As shown in FIG. 1A, the light source 30 includes a plurality of semiconductor lasers 18a to 18c, a plurality of laser drivers 10a to 10c, and a code generation unit 16. The plurality of semiconductor lasers 18a to 18c have an anode connected to a power supply and a cathode connected to the laser drivers 10a to 10c. The laser drivers 10a to 10c supply the drive current Iop to the semiconductor lasers 18a to 18c, respectively. The code generator 16 generates a luminance code based on luminance (level) data of the image. Here, the plurality of semiconductor lasers 18a, 18b and 18c included in the light source 30 are three-color semiconductor lasers of red (R), green (G) and blue (B), respectively. For the laser drivers 10a, 10b, and 10c that drive the semiconductor lasers 18a, 18b, and 18c, respectively, the code generator 16 generates color codes CR, CG, and CB that are luminance codes corresponding to the luminance levels of the respective colors. Output to each of the laser drivers 10a, 10b and 10c. The color codes CR, CG and CB correspond to the RGB of the pixels in the image data.

  The laser drivers 10a to 10c supply driving currents IopR, IopG and IopB corresponding to the color codes CR, CG and CB to the respective semiconductor lasers 18a to 18c. The semiconductor lasers 18a to 18c emit red laser light (wavelength: about 610 nm to 660 nm), green laser light (wavelength: about 515 nm to 540 nm), and blue laser light (wavelength: about 515 nm to 540 nm) having optical output powers corresponding to the drive currents IopR, IopG, and IopB, respectively. (Wavelength: about 440 nm to 480 nm).

  FIG. 1B illustrates one of the three semiconductor lasers 18 a to 18 c and the three laser drivers 10 a to 10 c as the semiconductor laser 18 and the laser driver 10. The laser driver 10 includes a drive circuit 12, a current adjustment unit 14, and a code generation unit 16. The code generator 16 may be provided for a plurality of laser drivers 10a to 10c as shown in FIG. 1A, or may be provided in the laser driver 10 as shown in FIG. Is also good. The drive circuit 12 generates the current Id based on the corresponding color code C among the color codes CR, CG, and CB generated by the code generation unit 16. The direction of the current Id is the forward direction of the diode of the semiconductor laser 18. The current adjusting unit 14 generates a correction current Is for the semiconductor laser 18 based on the color code C. The currents Id and Is are in opposite directions. Currents Id and Is are combined at node N. Thus, the current Id-Is is supplied to the semiconductor laser 18 as the drive current Iop.

[IL Characteristics of Semiconductor Laser]
The IL characteristics of the semiconductor laser 18 will be described. FIGS. 2A and 2B are diagrams illustrating IL characteristics of the semiconductor laser according to the first embodiment. FIG. 2B is an enlarged view of a range A in FIG. The horizontal axis is the forward drive current Iop supplied to the semiconductor laser 18, the current Id output by the drive circuit 12, and the color code C, and the vertical axis is the optical output power of the semiconductor laser 18. The IL curve 60 is shown by a thick solid line.

  As shown in FIG. 2A, when the drive current Iop is 0 A, the optical output power P is almost 0 W. Even if the drive current Iop increases from 0, the optical output power P is almost 0. When the drive current Iop exceeds a certain current, the optical output power P starts to increase. Thereafter, the optical output power P increases with respect to the drive current Iop. An approximate straight line 62 between the drive current Iop and the optical output power P is indicated by a broken line. The drive current Iop at which the approximate straight line 62 intersects the axis of the optical output power P = 0 is the threshold current Ith. As shown in FIG. 2B, when the drive current Iop is equal to or less than the current value Ith1, the optical output power P of the IL curve 60 becomes larger than the approximate straight line 62. As described above, when the drive current Iop is equal to or smaller than the current value Ith1, the nonlinearity of the IL characteristic becomes large.

  When the light source 30 is used for generating an image, the color code C and the optical output power P are required to be substantially proportional. When the semiconductor laser 18 is used in a range where the optical output power is large, if the drive circuit 12 is set to output the current Id linearly with respect to the color code C, the color code C and the optical output power P are almost proportional. The drive circuit 12 that linearly sets the current Id for the color code C has a simple circuit configuration and can be downsized.

  When the semiconductor laser 18 is used in a range where the optical output power is small, the influence of a nonlinear range of IL characteristics (a range of current value Ith1 or less) becomes large. Therefore, the relationship between the color code C and the drive current Iop is stored in a table, and when the color code C is input to the drive circuit 12, the drive circuit 12 acquires data corresponding to the drive current Iop from the table and outputs the data. It is possible to do. However, for example, when the color code C is 10 bits, a memory having a large storage capacity is required to store a table of the color code C for 1024 gradations and the driving current Iop. It is also conceivable that the drive circuit 12 calculates the drive current Iop from the color code C using a correction formula or the like. However, the circuit scale of the drive circuit 12 increases.

  Therefore, in the first embodiment, the current Id output from the drive circuit 12 at the time of the color code C0 is substantially set to the threshold current Ith. In the case of the color code Cmax, the current Id output from the drive circuit 12 is defined as a current Imax at which the optical output power P becomes Pmax. The color code C corresponding to the current value Ith1 where the IL curve 60 deviates from the approximate straight line 62 is referred to as a code Cth1. The drive circuit 12 generates a linear current Id for the color code C. The current adjusting unit 14 generates the correction current Is when the color code C is equal to or less than the code Cth1, and does not generate the correction current Is when the color code C is larger than the code Cth (for example, the correction current Is is set to 0A).

[Control of current adjustment unit]
Control of the current adjustment unit 14 will be described. FIG. 3 is a flowchart illustrating a process of the current adjustment unit according to the first embodiment. As shown in FIG. 3, the current adjustment unit 14 acquires the color code C from the code generation unit 16 (Step S10). The current adjusting unit 14 determines whether the color code C is equal to or less than the predetermined code Cth1 (Step S12). When the determination is Yes, the current adjustment unit 14 performs the current adjustment (Step S14). That is, the current adjusting unit 14 generates the correction current Is in the direction opposite to the current Id. If No in step S12, the current adjustment unit 14 does not perform current adjustment (step S16). That is, the current adjusting unit 14 does not generate the correction current Is (for example, Is is set to 0 A). Thereafter, the process returns to step S10 to acquire the next color code C.

  The control of the drive circuit 12 and the current adjustment unit 14 will be specifically described.

[Description of Current Generated by Drive Circuit]
FIG. 4A is a diagram illustrating a relationship between the color code C and the current Id in the first embodiment. The dots are points indicating the relationship between the color code C and the current Id output by the drive circuit 12, and the broken line 65 is a line connecting the dots. As shown in FIG. 4A, the color code C and the current Id are in a proportional relationship. For example, the color codes C0 to C6 respectively correspond to the currents Id0 to Id6, and the color code Cmax is the maximum value of the color code C and corresponds to the current Imax. When the color code C is 10 bits, the color code C0 is 0 and the color code Cmax is 1023.

  FIG. 4B is a diagram illustrating a relationship between the current Id and the color code C and the optical output power P in the first embodiment. The dot is a point indicating the target optical output power P in each color code C (that is, the current Id output from the drive circuit 12). The broken line 64 is a line connecting the dots. As shown in FIG. 4B, when the color code C is C0 to C6, the currents Id are Id0 to Id6, respectively. The intervals ΔId of the currents Id between adjacent color codes C are substantially equal. The target optical output powers P for the color codes C0 to C6 are P1 to P6, respectively. The optical output power P0 is, for example, 0 W. The intervals ΔP between the optical output powers corresponding to the adjacent color codes C are substantially equal.

  In the first embodiment, as shown in FIGS. 4A and 4B, the relationship between the color code C and the current Id output from the drive circuit 12 is made a linear relationship (that is, a linear relationship). When the color code C and the current Id have a linear relationship, the circuit size of the drive circuit 12 can be reduced.

[Description of current generated by current adjusting unit]
FIG. 5 is a diagram illustrating the relationship between the drive current Iop, the current Id, the color code C, and the optical output power P in the first embodiment. In step S14 of FIG. 3, the correction current Is generated by the current adjustment unit 14 is indicated by an arrow. As shown in FIG. 5, for example, when the color code is C1, the drive circuit 12 outputs a current Id1 (see FIG. 4B). At this time, the target of the optical output power P is P1, but the actual optical output power P is the optical output power (Pa) at the point A on the IL curve 60, which is larger than the target P1. Therefore, the current adjusting unit 14 generates the correction current Is1. When the correction current Is1 is combined with Id1, the drive current Iop becomes I1 = Id1-Is1. Accordingly, the light output power P at the time of the color code C1 becomes substantially the target P1, that is, the light output power P1 at the position of the point B on the broken line 64, and the light output power linear with respect to the color code C. The semiconductor laser 18 can be turned on by power.

  Similarly, for the color codes C2 to C6, the current adjustment unit 14 generates correction currents Is2 to Is6, respectively. As a result, the drive currents I2 to I6 become Id2-Is2 to Id6-Is6, respectively, and the optical output power P becomes the target P2 to P6, respectively. That is, by performing such a current correction, the light output power P changes from P2 to P6 with respect to the color codes C2 to C6. Output power P can be obtained. In other words, by changing the color code C and performing current correction according to the change, the optical output power P can be changed linearly. In the case of the color code C0, if the target optical output power P0 has a finite value, the current adjusting unit 14 generates the correction current Is0, as in the case of the color codes C1 to C6. When the target optical output power P0 is 0 W, the current adjustment unit 14 may set the current Id of the drive circuit 12 to 0A.

[Example of current adjustment unit]
FIG. 6 is a block diagram illustrating an example of a current adjustment unit according to the first embodiment. As shown in FIG. 6, the current adjusting unit 14 includes a plurality of current generating circuits 20a to 20e, a control circuit 22, and a memory 24. The current generation circuits 20a to 20e generate correction currents Isa to Ise, respectively. The current obtained by combining the correction current Isa and the Is is the correction current Is. The control circuit 22 causes the current generation circuits 20a to 20e to generate the correction currents Isa to Ise based on the color code C. The memory 24 stores parameters for generating Is from the correction current Isa based on the color code C.

  FIG. 7 is a diagram illustrating the correction current Is for the color code C in the first embodiment. As shown in FIG. 7, the current adjustment unit 14 generates a correction current Is corresponding to the color code C in step S14 of FIG. The correction currents Is1 to Is6 for the color code C in FIG. 5 are assumed to be ideal correction currents Is and are represented by a dotted curve 68 in FIG. In the first embodiment, the color codes C for which the correction current Is is 0 and Ia1 to Ia5 are Ca0 to Ca5, respectively. The dotted curve 68 is approximated by five straight lines 66. The correction current Is between 0 and Ia1 corresponds to the correction current Isa generated by the current generation circuit 20a. The correction current Is between Ia1 and Ia2 corresponds to the correction current Isb generated by the current generation circuit 20b. Hereinafter, similarly, the correction current Is between Ia2 and Ia3, between Ia3 and Ia4, and between Ia4 and Ia5 respectively correspond to the correction currents Isc to Ise generated by the current generation circuits 20c to 20e. The maximum current value generated by each of the current generation circuits 20a to 20e is defined as Ism. In this example, the maximum current values Ism of the correction currents Isa to Ise are substantially the same. The maximum current values of the correction currents Isa to Ise may be different from each other.

  FIG. 8 is a flowchart illustrating a process executed by the control circuit 22 in the first embodiment, and corresponds to the process of step S14 in FIG. As shown in FIG. 8, the control circuit 22 determines whether the color code C is smaller than Ca1 (Step S30e). If Yes, the control circuit 22 sets Isd to a constant Ism from the correction current Isa (Step S32e). The control circuit 22 sets the correction current Ise to a current value linearly corresponding to the color code C (step S36e). Then, the process ends.

  If No in step S30e, the control circuit 22 determines whether the color code C is smaller than Ca2 (step S30d). At Yes, the control circuit 22 sets Isc to Ism from the correction current Isa (Step S32d), sets the correction current Ise to 0A (Step S34d), and linearly corresponds the correction current Isd to the color code C. The current value is set (step S36d). Then, the process ends.

  If No in step S30d, the control circuit 22 determines whether the color code C is smaller than Ca3 (step S30c). At Yes, the control circuit 22 sets the correction currents Isa and Isb to Ism (step S32c), sets the correction currents Isd and Ise to 0A (step S34c), and linearly converts the correction current Isc to the color code C. The corresponding current value is set (step S36c). Then, the process ends.

  If No in step S30c, the control circuit 22 determines whether the color code C is smaller than Ca4 (step S30b). If Yes, the control circuit 22 sets the correction current Isa to Ism (step S32b), sets Is to 0 A from the correction current Isc (step S34b), and linearly corresponds the correction current Isb to the color code C. The current value is set (step S36b). Then, the process ends.

  If No in step S30b, the control circuit 22 sets Is to 0 A from the correction current Isb (step S34a), and sets the correction current Isa to a current value linearly corresponding to the color code C (step S36a). Then, the process ends.

  The control circuit 22 controls the correction currents Isa to Ise generated by the current generation circuits 20a to 20e to Ism or 0 based on the color code C and controls the correction currents Isa to Ise to be linear with respect to the color code C. Just do it. When the color code C and the generated correction currents Isa to Ise have a linear relationship, the circuit size of the control circuit 22 can be reduced. Further, the number of parameters to be stored can be reduced. Further, in the case where control is performed to set Is from Isa or 0 A from the correction current Isa based on the color code C, the circuit scale of the control circuit 22 can be reduced. Further, the number of parameters to be stored can be reduced. Therefore, the circuit scale of the current adjustment unit 14 can be reduced. The control circuit 22 may be realized using a dedicated logic circuit, or may be realized by a processor cooperating with a program.

[Modification 1 of Embodiment 1]
FIG. 9 is a block diagram of a light source in which the laser driver according to the first modification of the first embodiment is used. As shown in FIG. 9, the laser driver 10 includes a current cutoff unit 26. The current cutoff unit 26 may be realized using a dedicated logic circuit, or may be realized by a processor cooperating with a program. Other configurations are the same as those in FIG.

  FIG. 10 is a flowchart illustrating a process of the current interrupting unit and the current adjusting unit according to the first modification of the first embodiment. As shown in FIG. 10, after step S10, the current interrupting unit 26 determines whether the color code C is equal to or smaller than a predetermined code Cth2 smaller than Cth1 (step S18). If Yes, the current cutoff unit 26 causes the drive circuit 12 and the current adjustment unit 14 to cut off the drive current Iop (step S20). For example, the current Id and the correction current Is are set to 0A. Then, the process ends. If No in step S18, the process proceeds to step S12. The other flow is the same as that in FIG. 3 and the description is omitted.

  FIG. 11 is a diagram illustrating a relationship between the drive current Iop, the current Id, the color code C, and the optical output power P in the first modification of the first embodiment. As shown in FIG. 11, Cth2 smaller than Cth1 is assumed to be C2, for example. The current Id at this time is Ith2. When the color code C ranges from C0 to C2, the current adjustment unit 14 sets the drive current Iop to 0A. For example, the current adjusting unit 14 sets the currents Id and Is to 0A. Thus, the light output power P becomes 0 W when the color codes are C0 to C2. The other parts are the same as those in FIG.

  In a region where the light output power P is small, the relationship between the drive current Iop and the light output power P is unstable. Therefore, as in the first modification of the first embodiment, when the color code C is equal to or less than the predetermined code Cth2, the drive current Iop is cut off. Thereby, it is possible to suppress an unintended optical output power.

  According to the first embodiment and its modification, the code generator 16 generates a color code C (luminance code) for driving the semiconductor laser 18. The drive circuit 12 generates a current Id (drive current) for driving the semiconductor laser 18 based on the color code C. The current adjustment unit 14 generates a correction current Is for correcting the current Id such that the optical output power P (optical output intensity) of the semiconductor laser 18 changes linearly according to the change of the color code C. When the color code C is equal to or smaller than the code Cth1 (first threshold), the current adjustment unit 14 generates a correction current Is for correcting the current Id. Accordingly, the drive circuit 12 generates the current Id based on the color code C, so that the drive circuit 12 can be downsized. When the color code C is equal to or smaller than the code Cth1, the current adjusting unit 14 generates a correction current Is for correcting the current Id. This makes it possible to reduce the circuit scale as compared with the case where a non-linear driving current is generated for the color code C over the entire color code C.

  When the color code C is larger than Cth1, the current adjusting unit 14 does not generate the correction current Is. Thereby, the circuit scale can be further reduced.

  If Cth1 is too large, the circuit scale will increase. For this reason, Cth1 is preferably １ ／ or less, and more preferably １ ／ or less, of the maximum value of the color code Cmax of the color code C. If Cth1 is too small, the meaning of using the current adjustment unit 14 becomes thin. Therefore, Cth1 is preferably 1/100 or more of Cmax, more preferably 1/20 or more.

  When the input color code C is equal to or smaller than Cth1, the current adjusting unit 14 subtracts the correction current Is (second current) corresponding to the input color code C from the current Id. Thereby, the drive current Iop can be adjusted.

  As shown in FIGS. 6 to 8, the current adjustment unit 14 includes a plurality of current generation circuits 20a to 20e that respectively generate correction currents Isa to Ise (second current) that linearly correspond to the color code C. I have. When the input color code C is equal to or smaller than Cth1, the current adjustment unit 14 combines the correction current Isa output from the plurality of current generation circuits 20a to 20e with the Ise and subtracts it from the current Id. Thereby, the drive current Iop can be adjusted. The number of the current generation circuits 20a to 20e can be set as appropriate.

  Further, when the input color code C is equal to or less than Cth1, the control circuit 22 generates one of the plurality of current generation circuits 20a to 20e in a continuous code range of the color code C equal to or less than Cth1. A current is generated (for example, Isc) only in the circuit (for example, 22c). The control circuit 22 (control unit) outputs the maximum current value Ism or 0 based on the color code C input to the other current generation circuits (for example, 22a, 22b, 22d, and 22e). Accordingly, since the plurality of current generation circuits 20a to 20e may be controlled linearly with respect to the color code C, the circuit size of the control circuit 22 can be reduced.

  As in the first modification of the first embodiment, the current adjusting unit 14 (current interrupting unit) interrupts the driving current Iop when the input color code C is equal to or smaller than Cth2 (second threshold) smaller than Cth1. This makes it possible to suppress the optical output power from becoming unstable in a region where the optical output power P is small.

  The light source 30 includes the laser driver 10 according to the first embodiment and its modification, and a semiconductor laser 18 driven by the laser driver 10 to emit a laser beam. Thus, linearity between the color code C and the optical output power of the semiconductor laser 18 can be obtained, and the size of the laser driver 10 can be reduced.

  In the first embodiment and its modifications, the correction of the light output power by the configuration of the three-color display based on the color code has been described. However, the light output power may be reduced by using a monochromatic light source or an invisible light source such as an infrared ray (IR). The output power may be corrected.

  FIG. 12 is a block diagram of the image projection apparatus according to the second embodiment. As shown in FIG. 12, the image projection device 100 includes a light source 30, a scanning mirror 32, a reflection mirror 34, a projection mirror 36, a control unit 38, and an image input unit 40.

  The image projection device 100 is, for example, a glasses-type head-mounted display. The light source 30 is, for example, the light source illustrated in FIGS. 1A and 1B, and is installed on, for example, a vine of spectacles. The projection mirror 36 also functions as, for example, a lens of spectacles. The control unit 38 and the image input unit 40 are provided, for example, on a crane. The control unit 38 and the image input unit 40 may be provided in an external device (for example, a mobile terminal) without being provided in the head mounted display.

  The image input unit 40 receives image data from a camera and / or a recording device (not shown). The control unit 38 controls the light source 30 based on the image data. The control unit 38 may perform processing in cooperation with a program, for example, by a processor. The control unit 38 may be a circuit designed specifically.

  The light source 30 emits, for example, RGB (red, green, and blue) laser lights as the laser light 50. Further, the light source 30 is one light source, and may emit laser light of a single wavelength.

  The scanning mirror 32 is, for example, MEMS (Micro Electro Mechanical Systems), and scans the laser beam 50 two-dimensionally. The scanned laser beam 52 is reflected by the reflection mirror 34 and is irradiated on the projection mirror 36.

  The projection mirror 36 is arranged in front of the user's eyeball 42 and has a positive condensing power. The projection mirror 36 focuses the laser light 52 on or near the crystalline lens 46 and focuses on the vicinity of the retina 44. Thereby, an image is projected on the retina 44.

  The light source 30 is the light source shown in FIG. 1A, and includes three color semiconductor lasers 18a to 18c and laser drivers 10a to 10c for driving the semiconductor lasers 18a to 18c.

  FIGS. 13A to 13C are diagrams illustrating the optical output power P of the semiconductor laser with respect to the color code C according to the second embodiment. FIGS. 13A to 13C correspond to, for example, the semiconductor lasers 18a to 18c. A dashed curve 70 indicates the optical output power P for the color code C when the current adjustment unit 14 of the laser drivers 10a to 10c does not generate the correction current Is, and a solid line 72 indicates when the current adjustment unit 14 generates the correction current Is. Of the optical output power P for the color code C shown in FIG.

  The IL characteristics of the semiconductor lasers 18a to 18c may be different. For example, the slope of the approximate straight line 62 in FIG. 3A and FIG. 3B and the current value Ith1 at which the threshold current Ith and the IL curve 60 become non-linear differ depending on the semiconductor lasers 18a to 18c.

  As shown in FIGS. 13A to 13C, unless the current adjusting unit 14 of the laser drivers 10a to 10c generates the correction current Is, the light output power P with respect to the color code C is changed as indicated by a broken line curve 70. The degree of nonlinearity differs between the semiconductor lasers 18a to 18c. For example, P when the Cth1 and / or C at which the broken line curve 70 becomes nonlinear is 0 differs between the semiconductor lasers 18a to 18c. At this time, color shift is likely to occur in a pixel having a low luminance and a low light output power. For example, in the examples of FIGS. 13A to 13C, the nonlinearity of the semiconductor laser 18c is small. Therefore, when the color code C is around 0, the optical output power of the semiconductor laser 18c is smaller than the optical output power of the semiconductor lasers 18a and 18b. For this reason, color shift is likely to occur in a pixel having a low light output power.

  Therefore, the current adjusting units 14 of the laser drivers 10a to 10c independently generate the correction current Is and correct the current Id, as in the laser driver 10 of the first embodiment and the modification 1 thereof. As a result, the optical output power P of the semiconductor lasers 18a to 18c with respect to the color code C becomes substantially a straight line as shown by a solid line 72 in FIGS. 13A to 13C. Therefore, color shift and the like can be suppressed.

  According to the second embodiment, the image projection device 100 includes the light source 30 according to the first embodiment and its modification. The scanning mirror 32 (scanning unit, scanner) scans the laser light 50 emitted from the light source 30. The projection mirror 36 (image projection unit, projector) projects the scanned laser beam 52 as an image on the retina 44. In such an image projection apparatus 100, the retina 44 is irradiated with the laser light 52. For this reason, from the viewpoint of safety, it is required to reduce the optical output power of the laser light output from the semiconductor laser. Therefore, a region where the IL characteristic is nonlinear is used. Therefore, the laser driver according to the first embodiment and its modification is used. Thereby, the laser driver can be downsized.

  According to the second embodiment, as shown in FIG. 1A, the light source 30 includes a plurality of laser drivers 10a to 10c (laser drivers) and a plurality of semiconductor lasers 18a to 18c. As shown in FIGS. 13A to 13C, when the color code C is equal to or less than the code Cth1, the current adjusting units 14 in the plurality of laser drivers 10a to 10c respectively control the plurality of semiconductor lasers 18a to 18c. The correction current Is is generated so that the optical output power P of the corresponding semiconductor laser with respect to the color code C has linearity. Thereby, color shift and the like can be suppressed.

  The light source 30 includes three laser drivers 10a to 10c and three semiconductor lasers 18a to 18c. The three types of semiconductor lasers 18a to 18c generate red, green, and blue laser beams, respectively. The code generator 16 generates red, green, and blue color codes as luminance codes, respectively. Thereby, color shift and the like can be further suppressed.

  Although the head mounted display has been described as an example of the image projection device 100, the image projection device 100 may be a visual inspection device. The laser driver according to the first embodiment and the modified example thereof may be used other than the image projection device.

  As described above, the embodiments of the present invention have been described in detail. However, the present invention is not limited to the specific embodiments, and various modifications and changes may be made within the scope of the present invention described in the appended claims. Changes are possible.

10, 10a-10c Laser driver 12 Drive circuit 14 Current adjustment unit 16 Code generation unit 18, 18a-18c Semiconductor laser 20a-20e Current generation circuit 22 Control circuit 24 Memory 30 Light source 32 Scanning mirror 34 Reflecting mirror 36 Projection mirror 42 Eyeball 44 Retina 46 lens

Claims (10)

A code generation unit that generates a luminance code for driving the semiconductor laser,
A drive circuit that generates a drive current for driving the semiconductor laser based on the luminance code;
A current adjusting unit that generates a correction current for correcting the drive current, so that the light output intensity of the semiconductor laser linearly changes in accordance with the change in the luminance code.
A laser driver configured to generate a correction current for correcting the drive current when the luminance code is equal to or less than a first threshold.   The laser driver according to claim 1, wherein the current adjustment unit does not generate the correction current when the luminance code is larger than the first threshold.   The laser driver according to claim 1, wherein the current adjustment unit subtracts a correction current corresponding to the luminance code from the driving current when the luminance code is equal to or less than the first threshold.   The current adjustment unit includes a plurality of current generation circuits each generating a correction current, and when the luminance code is equal to or less than the first threshold, synthesizes respective correction currents output from the plurality of current generation circuits. The laser driver according to claim 1, wherein the driving current is subtracted from the driving current.   When the luminance code is equal to or less than the first threshold, the current adjustment unit is configured to generate one of the plurality of current generation circuits in a range of consecutive luminance codes among the luminance codes that are equal to or less than the first threshold. 5. The laser driver according to claim 4, further comprising a control unit that causes only a circuit to generate the correction current and causes another current generation circuit to output a maximum current or 0 among the correction currents based on the luminance code. 6.   6. The laser driver according to claim 1, further comprising a current cutoff unit that cuts off the driving current when the luminance code is equal to or smaller than a second threshold smaller than the first threshold. 7. A laser driver according to any one of claims 1 to 6,
A semiconductor laser driven by the laser driver to emit laser light;
A light source. A light source according to claim 7,
A scanning unit that scans the laser light emitted by the light source,
An image projection unit that projects the scanned laser light as an image onto the retina,
An image projection device comprising: The light source includes a plurality of the laser driver and the semiconductor laser, respectively,
The current adjustment units in the plurality of laser drivers each have a linear light output intensity with respect to a brightness code of a corresponding semiconductor laser among the plurality of semiconductor lasers when the brightness code is equal to or less than the first threshold. 9. The image projection device according to claim 8, wherein the correction current is generated as described above. The light source includes three types of the laser driver and the semiconductor laser, respectively.
The three types of semiconductor lasers generate red, green, and blue laser beams, respectively.
The image projection device according to claim 9, wherein the code generation unit generates red, green, and blue color codes as luminance codes, respectively.

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