Micro projector

Abstract

Microprojector (10) with a laser light source (100) and with at least one movable micromirror (200) for deflecting a laser beam, wherein the laser light source (100) has at least one first laser (110, 120, 130, 240) and at least one laser driver circuit (150), and the microprojector (10) is designed to generate an image by at least one micromirror (200) performing periodic tilting movements which lead to a deflection of the laser beam in two different directions, and at the same time the laser driver circuit (150) is designed to control the output power of the laser light source (100) in continuous wave operation synchronously with the periodic mirror movement and in accordance with the image content to be displayed, characterized in that the laser driver circuit (150) is configured to operate the first laser (110, 120, 130, 140) in pulsed operation with a power that is greater than the maximum possible power in continuous wave operation.

Description

State of the art

[0001] The invention is based on a microprojector with a laser light source and with at least one movable micromirror for deflecting a laser beam, wherein the laser light source has at least one first laser and at least one laser driver circuit.

[0002] In recent years, miniature projectors have become increasingly popular. Such projectors can be based on a variety of imaging technologies. A widely used design uses surface light modulators based on liquid crystals (Liquid Crystal on Silicon, abbreviated LCoS) or micromirror arrays (Digital Mirror Device, abbreviated DMD) as imagers. In addition, miniature projectors have also been developed in which an image is generated via the time-varying deflection of a collimated laser beam. The deflection unit of such a miniaturized projector is typically a MEMS mirror. The image is formed in a similar way to that used with cathode ray tubes. The trajectory of the light spot on a projection surface, which is generated by the time-varying deflection of the laser beam, typically describes a line pattern.Image content can then be generated by temporally modulating the radiation intensity. Miniaturized projectors based on micromirrors are already commercially available. A general trend toward increasing miniaturization can be observed with the goal of integrating a projector module into smartphones. Projector modules based on area light modulators have already found their way into commercially available smartphones. Miniaturized projectors are particularly suitable for presentations. Laser pointers are another highly useful tool in this context.

[0003] Laser pointers typically produce a light spot with a diameter of a few millimeters. The intensity of the spot is usually much greater than the intensity of an image projected in the background. Such intensities are typically not achievable locally with the actual projector. Therefore, without external accessories, only simple pointing aids, such as the mouse pointer commonly used in GUls, can be implemented. Therefore, the use of external accessories such as laser pointers has been common practice to provide pointing aids.

[0004] The documents WO 2006 / 105 249 A2, US 2003 / 0 030 756 A1 and US 2008 / 0 175 284 A1 describe projectors which partially have features of the microprojector according to claim 1. Disclosure of the inventionAdvantages of the invention

[0005] The invention is based on a microprojector with a laser light source and with at least one movable micromirror for deflecting a laser beam, wherein the laser light source has at least a first laser and at least one laser driver circuit. The core of the invention is that the laser driver circuit is configured to operate the first laser in pulsed mode with a power that is greater than the maximum possible power in continuous wave mode. Advantageously, this allows a portion of an image projected by the microprojector to be highlighted or marked by increased light intensity. Advantageously, the increased light intensity can be reproduced in a separate marking symbol in addition to the image content, such as a light point or spot, comparable to the light spot of a laser pointer. Advantageously, however, an existing image section can also be reproduced with increased intensity.

[0006] An advantageous embodiment of the invention provides that the laser light source has at least a second laser, and the laser driver circuit is configured to operate the second laser in continuous wave mode. Advantageously, a first laser is provided in pulsed mode for the bright marking of image portions, while one or more second lasers in continuous wave mode write the actual image. Advantageously, all beam paths for laser beams are aligned such that, after reflection from all micromirrors, the beams are either directed into the same solid angle at all times or can at least be directed into a common solid angle range by applying suitable, different mirror deflections in chronological order.

[0007] Another advantageous embodiment of the invention provides that the laser driver circuit is configured to operate the first laser in both pulsed and continuous-wave mode. Advantageously, one or more first lasers write the image in continuous-wave mode and implement the marking function in pulsed mode. This advantageously allows the marking function to be implemented compactly and cost-effectively without additional lasers.

[0008] Another advantage is that the laser driver circuit is configured for pulsed operation with a duty cycle of <= 0.01. This significantly reduces the heat generated by the laser during pulsed operation, enabling very high-power pulsed operation.

[0009] An advantageous embodiment of the invention provides that the laser driver circuit is configured to operate the first laser in pulsed mode at a power that is >= 10 times greater than the maximum possible power in continuous-wave mode. This advantageously achieves a significant local increase in the brightness of the laser light compared to the intensity in continuous-wave mode, enabling a striking marking function.

[0010] According to the invention, the microprojector is designed to generate an image by at least one mirror performing periodic tilting movements which lead to a deflection of the laser beam in two different directions, and at the same time the laser driver circuit controls the output power of the laser source in continuous wave operation synchronously with the periodic mirror movement and in accordance with the image content to be displayed.

[0011] An advantageous embodiment of the invention provides that the laser driver circuit is configured to generate a pulse or a chain of pulses in pulsed operation synchronously with the periodic mirror movement of the micromirror, so that the generation of a current pulse or a chain of current pulses occurs periodically with the construction of a complete image to be projected.

[0012] An advantageous embodiment of the invention provides that the laser driver circuit is designed to generate a pulse or a pulse chain in pulse mode at an adjustable time of use relative to the start of the periodic mirror movement which takes place to build up a complete image.

[0013] The invention provides the functionality of a laser pointer in a micromirror-based projection module. In the described invention, this can be achieved with the simultaneous display of any image content using the same projection module. This eliminates the need for the user to carry a laser pointer as an accessory. The solution does not necessarily rely on an additional beam source in the projector, but rather, in the simplest case, utilizes the existing laser sources of a micromirror projector. Furthermore, the present integrated solution also eliminates the need for an additional micromirror, which would allow any additional beam source intended to serve as a laser pointer to be moved quasi-statically across the image area.

[0014] In a typical application, a user would hold a device incorporating the projection module during a presentation. If the device (e.g., a mobile phone with a built-in micromirror projector) has a touch display, the user could simultaneously follow the projected presentation on the display and, using a touch gesture, move the position of the laser light spot within the projected image area to highlight presentation content. In contrast to an external laser pointer, there is the advantage that the spectral color of the highlighted light spot in the image can be freely adjusted using the color mixing of the existing RGB lasers. A laser pointer as an external accessory, on the other hand, is limited to one color due to the usual use of a single beam source.

[0015] The functionality of a laser pointer is achieved by operating one (or more) laser diodes at high output power on a very short timescale (< 1 µs) during line build-up. The time of switch-on is synchronized with the image build-up, so that the resulting light spot appears stationary. This principle takes advantage of the fact that on the aforementioned timescales (< 1 µs), safe operation of the laser diode is possible at very high currents. These currents can, for example, be a factor of ten higher than the current specified for continuous wave operation. A bright spot is then created on the projection surface, which stands out clearly from the brightness with which the image is displayed. This creates the impression of a laser pointer, which can highlight individual image elements. drawing Fig. 1 shows a schematic image representation of a microprojector according to the invention and an associated laser current over time. Fig. 2 shows a block diagram of a microprojector according to the invention in a first embodiment. Fig. 3 shows a block diagram of a microprojector according to the invention in a second embodiment. Description of implementation examples

[0016] Fig. 1 shows a schematic image representation of a microprojector according to the invention and an associated laser current over time.

[0017] Fig. Figure 1a shows an image content 1000 to be displayed with a micromirror projector according to the invention, in which an element is highlighted in the manner of a laser pointer (e.g., a bar chart). For this purpose, a light spot or light point 1 is generated, which has a significantly higher brightness than the rest of the image.

[0018] Fig. Figure 1b schematically shows a projected image 2000 and is intended to illustrate that the image is generated sequentially; here, using a progressive line scan method. The image area is scanned by a laser beam along a trajectory 2.

[0019] Fig. Figure 1c shows the temporal course of the current modulation for two exemplary lines of trajectory 2, which contain the light spot 1 to be highlighted. While the current I through the laser diode for displaying the image content is within the range of the specification for continuous wave operation (i.e., typically in the order of 100 mA for miniaturized projectors), according to the invention, a pulsed current (duration in the sub-microsecond range) with significantly higher currents (up to the range of several amperes) is used to generate the light spot 1.

[0020] In a first embodiment of the microprojector according to the invention, one or more of the laser diodes of the existing RGB module are energized, as shown in Fig. 2 is shown.

[0021] In a second embodiment of the microprojector according to the invention, an additionally integrated pulse laser diode is alternatively energized, as in Fig. 3 is shown.

[0022] The current pulse is preferably synchronized with the image content and thus also with the mirror movement of a deflectable micromirror of the microprojector in such a way that the light spot 1 remains stationary at a freely selectable point. Fig. 1c, this is due to the fixed time offset τ _onset relative to the reversal point 20 of the mirror movement of a deflectable micromirror of the microprojector.

[0023] Alternatively, the light spot can also be arranged in a movable manner by adjusting the time offset τ_offset is changed. For example, light point 1 can move in a circle around a detail of the image content, or even underline it.

[0024] Fig. Figure 2 shows a block diagram of a microprojector according to the invention in a first exemplary embodiment. It shows a microprojector 10 with a laser light source 100 and a movable micromirror 200 for deflecting a laser beam. The laser light source 100 has three first lasers 110, 120, 130, a beam combiner 170, and a laser driver circuit 150. Light generation typically takes place in a laser diode, since this allows direct conversion of the supplied electrical power into light radiation. Laser diodes achieve a high degree of compactness and efficiency. Another advantage for use in laser projectors is that high-frequency modulation of the radiation intensity is possible. In some cases, optically pumped semiconductor or solid-state lasers with subsequent frequency doubling are also used.Here, the laser driver circuit, which is part of the invention, would then be responsible for supplying current to the laser diodes that serve as the pump light source.

[0025] According to the invention, the laser driver circuit 150 is configured to operate one or more of the lasers 110, 120, 130 in pulsed mode with a power that is greater than the maximum possible power in continuous-wave operation. For this purpose, the laser driver circuit 150 comprises an ASIC / FPGA 152 and a laser driver IC 154. Image data 1010 is supplied to the ASIC / FPGA 152, from which sequential pixel information for image generation 1110 is generated and supplied to the laser driver IC 154. The ASIC / FPGA 152 also generates a micromirror control signal 1512, which is supplied to a mirror drive controller 1200. The mirror drive controller 1200 generates a micromirror drive signal 1220, which is supplied to the micromirror 200 and causes the mirror to deflect in two directions.Alternatively, several mirrors can be arranged one after the other in an optical beam path, each of which can be deflected in only one direction, for example. The optical beam paths of the first three lasers 110, 120, 130 are combined into a single beam path in a beam combiner 170, so that the laser beams emitted by the three lasers together impinge on the micromirror 200. The deflection of the micromirror 200 in two directions causes the projected image 2000 to be written along the trajectory 2 by the first three lasers 110, 120, 130.

[0026] The ASIC or FPGA 152 also generates a TTL trigger pulse 1120, which is synchronized with the signal for line-by-line image generation 1110 according to the input data in the form of pixel coordinates 1020 for the center of light spot 1. This trigger pulse 1120 causes a pulse stage in the laser driver IC 154 to generate a short current pulse. The pulse stage can also be implemented externally in a discrete or integrated circuit. The current pulse is superimposed on the current for continuous wave operation (cw current), with which an image is generated via the existing laser diodes of the RGB module. The resulting combined cw / pulse laser current 160 is fed to the first lasers 110, 120, 130. In this exemplary embodiment, the color of light spot 1 is, in principle, freely adjustable. For this purpose, it can be provided that the laser driver receives color information from the ASIC in addition to the trigger pulse.This is done, for example, by writing to a corresponding register via an SPI bus. Light spot 1 functions as a laser pointer. The invention thus creates, in a sense, a laser projection system with an integrated laser pointer function.

[0027] Fig. Figure 3 shows a block diagram of a microprojector according to the invention in a second exemplary embodiment. In contrast to the first exemplary embodiment, three second lasers 210, 220, 230 are used for the RGB module. A cw laser current 250 is supplied to these lasers by the laser driver IC 154, which is intended to enable continuous-wave operation of the second lasers. The trigger pulse 1120 causes a pulse stage in the laser driver IC 154 to generate a short current pulse 260, which is separately supplied to a first laser 240, which is intended to enable pulsed operation. In the example described, this is a pulsed laser diode used specifically for the laser pointer function.

[0028] The invention utilizes, among other things, the fact that when writing an image sequentially, only a relatively short period of time is available for each pixel and, due to the inertia of the viewer's eye, this is also only necessary.

[0029] In a laser projector with micromirrors as the deflection unit, the mirror deflection in one direction is typically repeated in the line scan process at a frequency of approximately 60 Hz, while the deflection frequency in the other direction is, for example, 21 kHz. In this case, an image with approximately 700 lines is generated 60 times per second if the angular movement with the lower frequency describes an ideal sawtooth shape. If the overall image information is to be displayed with a resolution of 1280 x 720 pixels (HD resolution), the beam sources must be modulated with a total clock rate of approximately 55 MHz. For the sake of simplicity, the possibility of generating subpixels is not discussed here. Blanking intervals at reversal points of the mirror movement are also not taken into account. A single pixel corresponds to trajectory 2, which light spot 1 describes within approximately 20 ns.

[0030] The dot that a laser pointer would create on the projection surface would typically have an extension corresponding to a few pixels of the displayed image. Applied to the trajectory of the laser beam, for example, five pixels would correspond to a time interval of approximately 100 ns according to the above estimate. Such a time interval is typical for pulsed operation within the meaning of the present invention.

[0031] Within a current pulse with a duration in the submicrosecond range, semiconductor lasers, such as those used as beam sources in projectors, typically do not heat up the component significantly. This does not change even if the pulse is repeated, provided the repetition rate remains in the kHz range (the duty cycle should ideally remain below 1%). Semiconductor lasers for miniaturized projectors enable output powers in continuous-wave operation that are typically in the range of 50-100 mW. For the purposes of the present invention, the term "continuous-wave operation" represents all intensity modulations and switching times used in the prior art to display a planarly projected image content of a laser projector.Continuous-wave operation in the true sense of the word would ultimately only result in the display of a monochrome and almost homogeneous image in a laser projector. Higher power levels, however, are generally not possible in continuous-wave operation due to the component's self-heating. However, since no significant self-heating occurs in pulsed operation, the laser diode can be operated with significantly higher currents within a pulse, so that the output power in the pulse can also assume significantly higher values ​​(up to the watt range). However, it is also important to ensure that irreversible facet damage to the light exit surfaces of the semiconductor laser, also known as "catastrophic optical damage" (COD) due to excessive power densities, is avoided.

[0032] The projector described in this invention now includes, in one embodiment, a laser driver that allows the superimposition of short current pulses with durations in the sub-microsecond range onto the modulated laser current, as required to generate any desired image content. Fig. 1 shows the level of the laser current plotted as a function of time. The exemplary time interval considered here corresponds to the time required to display two lines. To display the pixels of the image content, the laser source is operated within a line according to the desired brightness per pixel at output powers typically between 0 - 100 mW. According to the state of the art, the laser sources can be completely switched off at the beginning and end of the line to prevent unacceptably high brightnesses from occurring at the trajectory reversal point. At a selectable point within the line interval, the current is increased many times over for a period in the sub-microsecond range. In the next line, the pulse is then repeated at a time determined, among other things, by the line frequency. This synchronization with the mirror movement is achieved.After a few lines have been generated, the laser pulse generation for the remaining lines to be displayed is paused. Applied to the trajectory that the light spot follows on a projection surface during the displayed period, a contiguous area of ​​a few pixels in size is created, within which the brightness is significantly increased compared to the rest of the image content. This creates a spot comparable to that of a laser pointer. If the pulse is repeated across all lines at fixed times determined by the line frequency, even a vertical line can be displayed that stands out clearly from the background.

[0033] By varying the temporal distance between the pulse and the end of the line, the spatial position of the dot can be changed in one direction (typically horizontally). However, selecting the lines in which pulse overlay occurs allows the dot to be shifted in the other direction (vertically).

[0034] Up to this point, the embodiment of the invention has been described in accordance with Fig. 2, which utilizes existing laser sources. This is a preferred option for a cost-effective projector module. However, the laser diodes used would have to be suitable for both continuous wave (CW) and pulsed wave operation, i.e., operation with short and, compared to continuous wave (CW) pulses, high current pulses, and approved for both operating modes.

[0035] Alternatively, in the embodiment of the invention according to Fig. 3 To implement an integrated laser pointer function, however, an additional laser source can also be provided in the projector, the beam of which is guided coaxially to the combined beam of the existing RGB laser module. This could, for example, be a 240-degree pulsed laser diode with emission in the visible light spectrum (e.g., at a wavelength of 670 nm), which has been specifically optimized for pulsed operation, i.e., for pulse lengths in the sub-microsecond range and repetition rates up to the kHz range, and which is therefore insensitive to irreversible facet damage and self-heating. Broad-area lasers, for example, can also be used here, in which the power density at the exit facets is significantly reduced compared to laser diodes with a single-mode and therefore narrower beam profile.This solution would still have the advantage that the existing mirrors could be used to deflect the beam and thus to move the laser pointer within the image area.

[0036] To implement the laser pointer function, a customized laser driver 154 is required. This driver should contain a special driver stage suitable for generating short current pulses with durations in the sub-µs range. The current I can reach values ​​of up to several amperes in the pulse. Suitable implementations are known in the prior art, for example, in which a capacitance across the laser diode is discharged via a MOSFET or an avalanche transistor when a trigger signal is applied. In the present invention, the current pulse of this driver stage is either fed directly to a pulsed laser diode 240 specifically integrated for the laser pointer function, or is superimposed with the current modulated according to the image content and then fed to the laser diodes of the existing RGB module. Alternatively, driver ICs with multiple integrated driver stages can also be used.A laser diode can then be operated with one driver stage in continuous wave mode, while when a control signal is applied, further driver stages are switched on, which provide the higher currents required in pulsed operation for the required period of < 1 µs.

[0037] The laser driver can be synchronized with the ASIC or FPGA responsible for image generation and mirror control in various ways. For example, the ASIC / FPGA can send a TTL signal to a corresponding input of the laser driver at the time corresponding to the selected position of the laser pointer within the image during line-by-line image generation, as shown in the block diagram in Fig.2. Alternatively, the signal can also be transmitted according to other standards; for example, the control signal can also be transmitted according to the LVDS standard. A rising signal edge is then used by the laser driver to generate a short current pulse in the sub-microsecond range. This current pulse is then superimposed on the cw current of the existing laser sources or fed to an additional pulsed laser diode with visible emission. Alternatively, communication via the SPI bus is also conceivable. The ASIC / FPGA would assume the role of the bus master, while the laser driver would be assigned the role of the slave. Communication with the laser driver IC would then have to be established via a corresponding slave select level (zero).At the specified time (see above), a register in the laser driver IC would then be written via the MOSI line with a value to be defined, signaling the laser driver to generate a current pulse immediately. Any delays in command processing by the slave would have to be taken into account by the master to ensure the laser pointer spot is actually generated at the desired location in the image.

[0038] In the above explanations, the realization of a bright light spot in the image was described in relation to a progressive line scan method. However, the laser pointer function could also be implemented using other image generation methods. One example is the so-called Lissajous scan. However, this also requires appropriate synchronization of the laser pulse with the mirror movement.

[0039] It is also conceivable to display a marking object such as a mouse pointer, an underline, or another structured object with increased brightness instead of a simple bright point of light. It is also conceivable to project parts of the projector image itself with increased brightness, thus highlighting and highlighting them against the surroundings. These could be letters or entire words of a text, for example, or even parts of an image. The only prerequisite for this is that the extent of the object or area of ​​increased brightness is small compared to the extent of the entire projector image, such that pulsed operation is still possible without excessively heating the laser.

Claims

[1] Microprojector (10) with a laser light source (100) and with at least one movable micromirror (200) for deflecting a laser beam, wherein the laser light source (100) has at least one first laser (110, 120, 130, 240) and at least one laser driver circuit (150), and the microprojector (10) is designed to generate an image by at least one micromirror (200) performing periodic tilting movements which lead to a deflection of the laser beam in two different directions, and at the same time the laser driver circuit (150) is designed to control the output power of the laser light source (100) in continuous wave operation synchronously with the periodic mirror movement and in accordance with the image content to be displayed, characterized by , that the laser driver circuit (150) is configured to operate the first laser (110, 120, 130, 140) in pulsed operation with a power which is greater than the maximum possible power in continuous wave operation. [2] Microprojector according to claim 1, characterized by that the laser light source has at least a second laser (210, 220, 230) and the laser driver circuit (150) is configured to operate the second laser in continuous wave operation. [3] Microprojector according to claim 1, characterized by that the laser driver circuit (150) is configured to operate the first laser (110, 120, 130) both in pulsed mode and in continuous wave mode. [4] Microprojector according to one of the preceding claims, characterized by that the laser driver circuit (150) is configured for pulse operation with a duty cycle of 0.01 or less than 0.

01. [5] Microprojector according to one of the preceding claims, characterized by that the laser driver circuit (150) is designed to operate the first laser (110, 120, 130, 240) in pulsed operation with a power which is at least a factor of 10 greater than the maximum possible power in continuous wave operation.

Images