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Dynamic shaping of the basic intensity profile of adaptive laser headlights based on resonant MEMS scanning mirrors

Laser-based headlights offer the advantages of high luminance and compact size. Combining laser headlights with MEMS scanning mirror technology will be the key to enable new adaptive illumination functionality with high pixel resolution. This paper reports on applying a resonant biaxial MEMS scanning mirror in a laser phosphor illumination system. A new concept is presented that allows to dynamically shaping the basic intensity distribution of an adaptive laser-phosphor headlight.

1. Introduction

By introduction of laser-phosphor conversion technology in automotive headlights BMW and Audi were able to demonstrate higher luminance and a further range of illumination compared to conventional headlights [1], [2]. The small form factor of laser-based headlights is essential because it offers new degrees of freedom for the design. Besides all technical aspects this is an important factor that will drive developments of next generation automotive headlights. In parallel to laser-based systems important progress was made by Audi's introduction of Matrix-LED-based systems for demonstration of an adaptive glare-free headlight [3]. As a logical consequence it is desirable to combine the advantages of laser-based headlights with the advantages of adaptive glare-free systems in order to demonstrate a high resolution adaptive headlight of high luminance, long range but still compact size. Adaptive laser-based systems with high pixel counts basically can be realized by usage of spatial light modulators such as LCOS, LCD or DLP. However, since those systems work in a subtractive mode by spatially modulating the conversion of laser light into heat the efficiency of a spatial light modulator based approach is critical. Therefore, in this paper a MEMS mirror-based concept is proposed that aims at using almost 100 percent of the available laser light for pattern projection on the phosphor.

The basic setup of a laser phosphor headlight applying a biaxial MEMS scanning mirror is shown in Figure 1. After collimation the laser beam is scanned by a biaxial MEMS scanning mirror across the phosphor. Part of the laser light is only scattered by the phosphor layer while another part is converted into yellow light. By colour mixing of scattered light and converted light white light is generated which is collected and projected by corresponding optics.

Fraunhofer ISIT has used a setup like that for projection of high resolution white light images with up to 1024 x 512 pixels ( Figure 2).

In that projection system a FPGA based control electronics processes and evaluates the capacitive position feedback signals of the 2 resonant scanning axes of the MEMS mirror to synchronize the excitation of the blue laser depending on the image content to be displayed [4].

2. MEMS mirror requirements

The MEMS mirror needs to scan the laser beam at high speed vertically and horizontally across the phosphor. The required minimum scan speed is given by the line resolution of the image content to be displayed and the required number of projected frames per second. Assuming a frame rate of 200 Hz and an image resolution of 100 lines leads to a line scan frequency of 20 kHz. This translates into an oscillation frequency requirement of the MEMS mirror of 10 kHz since 2 lines are being projected at each complete mirror oscillation. A biaxial MEMS scanning mirror with electrostatic comb drives is shown in Figure 3.

For application in a laser phosphor headlight the MEMS mirror will have to be coated by dedicated dielectric multilayers in order to exhibit the required high reflectivity and to minimize heating by absorption. Experiments have shown that standard aluminium coatings can degrade already at intensity levels of 1 watt / mm2. Applying appropriate dielectric multilayer coatings MEMS mirrors can withstand very high laser power levels. Fraunhofer ISIT successfully developed and demonstrated MEMS scanning mirrors for laser cutting processes that can biaxially scan cw-laser beams at power levels as high as 4.5 kW [5]. An example of a MEMS mirror for laser material processing with dielectric coating with

reflectivity exceeding 99.9% is shown in Figure 4. However, dielectric coatings exhibit high mechanical stress that may lead to unwanted curvature of the MEMS mirror. To compensate for this it is necessary to increase the MEMS mirror thickness. By way of example the MEMS mirror of Figure 4 exhibits a mirror thickness of 0.7 mm in contrast to a standard MEMS mirror thickness of less than 0.1 mm. An increased thickness on the other hand increases the mass moment of inertia. As a consequence the mass moments of inertia of the two MEMS mirrors of Figure 3 and Figure 4 differ by 6 orders of magnitude! To drive and actuate MEMS mirrors of such high mass moment of inertia Fraunhofer ISIT applies hermetic wafer level vacuum packaging in combination with different actuator concepts (internal electrostatic drive or external drive). For optimum exploitation of the beneficial reduction of damping achieved by vacuum packaging the biaxial MEMS mirrors are operated at their resonant frequencies in both axes. In this case maximum optical scan angles are achieved. This kind of operation is advantageous because it allows designing and fabricating a very stiff and robust MEMS mirror that is not sensitive to shock and vibration as it was for a MEMS mirror that has to operate in quasistatic (non-resonant) mode [6].

3. Laser intensity profile

The bi-resonant operation of a biaxial MEMS scanner leads to a spatial intensity distribution of the scanned laser projected pattern that is at first glance disadvantageous for an automotive headlight due to the fact that the resonant MEMS mirror performs harmonic oscillation with sine-shaped velocity characteristic. Such characteristic causes the laser beam to be scanned at maximum speed in the centre part of the projected pattern whereas the velocity gets zero at the edges of the projected pattern. As a consequence the laser intensity is high at the boundaries of the projected pattern and minimum in the centre (see Figure 5). For an adaptive automotive headlight however, the inverse basic intensity distribution is preferred. Maximum intensity should arise in the centre whereas intensity should decrease towards the boundaries of the basic intensity distribution.

4. Dynamic shaping of the basic intensity profile

The disadvantageous basic intensity distribution described before can be avoided by a sufficiently fast modulation of the oscillation amplitude of the resonant scanning mirror [7]. That effect can be explained as follows: If the size of the projected pattern is rapidly changed then the bright boundaries of the sequentially projected patterns do not coincide any longer. If a small projected scan pattern (corresponding to a low oscillation amplitude) is projected more frequently than the largest scan pattern (corresponding to maximum oscillation amplitude) then the probability of presence of the laser beam increases at the central parts of the covered projection area and decreases at the outer parts of the covered projection area. As a consequence the average laser intensity in the centre increases whereas the average intensity at the boundaries decreases. Figure 6 shows the experimental verification. The oscillation amplitude of the vertical scan axis of the 2D-MEMS scanning mirror was modulated at a frequency of 200 Hz ( Figure 7). The resonant frequency of that axis is around 15 kHz. The fast modulation of the oscillation amplitude results in an intensity redistribution with a maximum intensity in the centre whereas the intensity is minimum at the upper and lower boundaries of the total projection area ( Figure 6).

The modulation of the mechanical oscillation amplitude can be achieved by periodic modulation of the drive signal. This can be phase modulation of the drive signal, amplitude modulation, frequency modulation, pulse width modulation or combinations of those methods.

The basic intensity distribution that is achieved based on this new concept depends also on the depth of amplitude modulation and thus in principle would allow to be changed dynamically depending on the traffic situation. That is shown in Figure 8. The laser projection by a biaxial MEMS mirror with two fast scan axes and resonant frequencies around 15 kHz was simulated for amplitude modulation of the oscillation of the vertical axis. That configuration with two fast axes corresponds to the MEMS mirror which was used for the experiments shown by Figures 5 and 6. If the modulation depth is zero then the original disadvantageous basic intensity distribution is achieved. Increasing the value of modulation depth from 0% to values of 20%, 50%, 80% and 100% results in the different patterns depicted in Figure 8. These results show that the modulation depth directly determines the width of the bright horizontal bar in the projected basic intensity pattern. Thus, modulation depth could be used as a parameter to switch between different basic intensity distributions of a headlight for adaptation of lighting. It should be mentioned and emphasized at this point that the amplitude modulation is only to influence the basic intensity profile. Needless to say that by additional modulation of the laser source it is possible to overlay and project any kind of pixel pattern, e.g. for glare-free adaptation or other functionalities.

5. New options by circle scanning mirrors

The scanned laser projected pattern does not necessarily have to be rectangular for application in an automotive headlight. However, a rectangular projection pattern is always achieved if the two axes of the MEMS mirrors are being driven at different frequencies. An alternative to that rectangular scan pattern could be a circular or elliptical scan pattern which can only be achieved if the oscillation frequencies of the two axes are identical. Fraunhofer ISIT has developed several types of such circle scanning mirrors with mirror diameters of 2 mm, 7 mm and 20 mm. For all investigations presented in this paper only the 7 mm MEMS mirror shown in Figure 9 was used.

The mirror plate of this MEMS mirror is suspended by three identical bending beams. As all other MEMS mirrors shown before also this mirror is hermetically sealed and vacuum packaged already on wafer level. The vacuum cavity minimizes damping losses which leads to an advantageous high Q-factor of some 10,000s. This biaxial MEMS mirror exhibits two almost identical eigenfrequencies around 1.5 kHz that provide the capability to project lines, ellipses, circles and more complex trajectories. The circle scan capability is shown in Figure 10.

As the examplary experiments depicted in Figure 11 show amplitude modulation of a circle scanning mirror allows for new options to realize different basic intensity distributions that are dynamically switchable for adaptation purpose. Spiral scanning for example could be the appropriate way to shift the maximum of the basic white light intensity distribution to any target radius within the circular projection area.

6. Experiments with high laser power

By cooperation of Fraunhofer ISIT with Laser Display Technology GmbH (LDT) and with the colleagues from Fraunhofer IKTS basic experiments were performed to investigate the general feasibility of an adaptive MEMS mirror-based laser phosphor headlight. For that purpose a fibre coupled high power blue laser from LDT was set up together with the already described 7mm-tripod-MEMS mirror of Fraunhofer ISIT and with a ceramic phosphor from Fraunhofer IKTS. The overall test setup is shown in Figure 12 and Figure 13. A principal advantage of using a fibre coupled laser source for any future headlight application is the option to spatially separate the heat generating laser diodes from the headlight module in the front of a car where active cooling can become a challenge because of the

required compactness of the module. In the laboratory setup two lenses were placed behind the fibre exit for beam expanding and collimation. The lenses were adjusted to achieve a beam diameter on the MEMS mirror of about 5 mm. The MEMS mirror was excited to produce an elliptical trajectory of the reflected laser beam on the transmissive phosphor from IKTS. About 15 cm behind the phosphor a projection lens was placed to collect and project part of the generated white light to a projection screen ( Figure 14).

The experiments demonstrated that laser power as high as 12 watts could be applied to the MEMS mirror and to the phosphor without damage. Further 

increase of laser power then caused degradation of the mirror's reflectivity. This can be explained by the fact that the MEMS mirror was coated only by aluminium. A tailored dielectric coating would enable to scan and project a laser beam of several times higher laser power. Besides transmissive ceramic phosphors also a large reflective ceramic phosphor with a diameter of 4 inch developed by Fraunhofer IKTS was successfully tested in reflection geometry in the same setup ( Figure 15).

7. Conclusion

State of the art MEMS scanning mirror technology was combined with advanced new phosphor technology and fibre coupled high power blue lasers to demonstrate the principle functionality of an adaptive high power white light source. A new concept has been presented that includes fast amplitude modulation of the bi-resonant MEMS scanning mirror in order to adapt and advantageously reshape the basic intensity distribution of the scanned laser projected pattern. Based on that approach the former disadvantageous intensity maximum at the boundaries of the projected area could be shifted to the centre.

Future work will have to focus on design and fabrication of a much faster circle scanning mirror with dielectric multilayer coating to allow for even higher laser power levels.

8. Acknowledgement

The authors want to thank all contributing and supporting colleagues at Fraunhofer IKTS. Furthermore we thank all contributing and supporting colleagues at LDT Laser Display Technology GmbH and the colleagues at Fraunhofer ISIT.