Style Guides and Templates

0 downloads 0 Views 378KB Size Report
Apr 24, 2015 - MEMS scanning mirrors for high power laser display and lighting applications ... Applying high reflective dielectric coating enables these MEMS.
The 4th Laser Display and Lighting Conference (LDC’15), Yokohama, Japan, Apr. 22 - Apr. 24, 2015

MEMS scanning mirrors for high power laser display and lighting applications F. Senger, T. v. Wantoch, C. Mallas, J. Janes, U. Hofmann, B. Wagner, W. Benecke Fraunhofer ISIT, Fraunhoferstrasse 1, D-25524 Itzehoe, Germany, phone 49 4821 17-1495 fax 49 4821 17-4250, [email protected]

Abstract: This paper reports about a new development of MEMS scanning mirrors specifically designed for high power laser displays, high power lighting applications and for laser material processing. Applying high reflective dielectric coating enables these MEMS scanning mirrors to comply with laser power loads exceeding 500 watts.

1. Introduction MEMS mirror technology predominantly has been used for scanning of low power laser beams not exceeding a few hundred milliwatts [1]. Emerging automotive lighting and display applications however will have to exhibit high brightness and therefore will need to implement lasers of much higher power levels. Furthermore, specially tailored scanning solutions are demanded for applications based on laser material processing. These devices have to be capable of scanning laser power loads exceeding 500 W. In this work we present the development of new MEMS scanning mirrors specifically designed to enable highspeed scanning of high power laser beams. 2. Two MEMS scanning mirror concepts The wide range of laser power which extends from a few watts for display and lighting applications up to kilowatts for laser material processing requires two different MEMS mirror concepts. Both concepts apply electrostatic actuation in combination with hermetic wafer level vacuum packaging technology. 2.1 Concept 1: Laser power loads of less than 20 watts In automotive laser headlights blue laser light excites a yellow phosphor layer which then emits white light of high brightness. While currently available systems are static, future laser headlights can offer dynamic lighting functions by combining the laser phosphor conversion technology with MEMS scanning mirror technology.

Fig. 1: New quadpod- scanner with 1 mm mirror diameter, a robust suspension and two orthogonal scan axes with resonance frequencies of about 15 kHz.

© 2015 The Japan Society of Applied Physics

One important aspect in the design of a MEMS mirror for such applications is to consider the vibration spectrum in a car which extends well above 2 kHz. We propose a scanning solution based on two fast scan axes, which differ in frequency only by the image repetition frequency. As a result of the two fast axes the design is robust against shock and vibration. Furthermore, this scanning concept is advantageous with respect to avoidance of flicker artifacts [2]. A possible realization of this scanner concept is demonstrated in Fig. 1. The gimbal-less quadpod MEMS mirror is actuated by electrostatic comb-drives and suspended by four almost identical springs. The four springs allow for advantageous heat conduction to the surrounding chip frame. Absorbed laser energy always affects both scan axes simultaneously without delay. According to the two high scan frequencies thermally introduced frequency shifts can be compensated very quickly by implementing two phase locked loops for frequency and phase control. In contrast to conventional raster scanning systems a MEMS mirror with two fast scan axes additionally allows to spatially redistribute the projected laser intensity without modulating the laser source. This feature may become of significant importance for fast situation dependent dynamic control in future laser headlight systems. The capability to redistribute the laser intensity pattern is achieved by applying a fast modulation of the MEMS mirror’s oscillation amplitude. An exemplary intensity redistribution is shown in Fig. 2. It has been realized based on a quadpod MEMS mirror with two scan frequencies around 15 kHz.

Fig. 2: Advantageous redistribution of projected laser intensity pattern can be achieved by fast modulation of the MEMS mirror’s oscillation amplitude (lower image). Without this amplitude modulation a disadvantageous intensity distribution results in high intensity at the outer edges of the scan pattern (upper image).

The 4th Laser Display and Lighting Conference (LDC’15), Yokohama, Japan, Apr. 22 - Apr. 24, 2015

A laser phosphor display based on a biaxial vacuum packaged MEMS scanning mirror with integrated capacitive sensors for synchronizing laser modulation and controlling mirror actuation is shown in Fig. 3. It generates 1024 x 512 pixels, although light scattering partially lowers the achievable optical resolution.

Fig. 5: Laser marking experiment applying a resonant 7 mm MEMS scanner in combination with a linear scanning galvanometer scanner. A ns-pulsed laser (𝜆 = 1064nm, average power = 10 W) was used.

Biaxial MEMS mirrors with aperture sizes up to 20 mm have also been developed for use in laser cutting and laser welding applications (Fig. 6). These vacuum packaged mirrors were successfully exposed to cw-laser power loads up to 600 watts without damage [3].

Fig. 3: MEMS scanner based laser phosphor display.

2.2 Concept 2: Laser power up to several hundred watts For applications that require much higher laser power loads such as for example used in laser material processing (e.g. laser marking, laser welding, laser cutting) mirror diameters between 4 and 20 mm have been realized. In order to minimize the absorbed laser energy, an optimized high reflective dielectric coating with a reflectivity higher than 99.9 % is necessary. As this multilayer coating introduces high residual stress, a MEMS mirror of standard thickness (80 µm) exhibits considerable deformation. To maintain a flat mirror, the thickness of this kind of MEMS scanner is about 10 times higher. All mirror structures possess full wafer thickness. Fig. 4 shows a 7 mm single axis MEMS scanner designed as an amplitude magnifying double resonator.

Fig. 6: Vacuum packaged biaxial 20 mm MEMS scanning mirror for laser cutting with resonant frequencies around 1 kHz.

3. Conclusion In this contribution we demonstrate that MEMS mirror technology can be extended and adapted for use with high power lasers of up to several hundred watts. The two presented MEMS scanner concepts enable new promising applications based on high power laser scanning for automotive and general lighting and display solutions as well as for laser material processing. Additionally, biaxial resonant MEMS mirrors with two fast scan axes enable dynamic intensity redistribution which is expected to become important for future automotive laser phosphor headlights. References

Fig. 4: Vacuum encapsulated full wafer thick 7 mm MEMS scanner with integrated getter pill for vacuum stabilization.

This large MEMS mirror achieves a high optical scan angle of 35 degrees at a scan frequency of 3.2 kHz. Fig. 5 demonstrates its use in a laser marking experiment performed by our partner Fraunhofer IWS in Dresden. Measurements of the laser damage threshold of the dielectric coating provided a value of 1.76 J /cm2 corresponding to a pulse power density of 176 MW / cm2.

© 2015 The Japan Society of Applied Physics

[1] Holmström et al., “MEMS Laser Scanners: A Review,” J. Microelectromech. Syst. 23 (2), 259-275 (2014). [2] Hofmann et al., "Wafer-level vacuum-packaged two-axis MEMS scanning mirror for pico-projector application," in Proc. SPIE 8977, San Francisco , 2014. [3] Senger et al., “Centimeter-Scale MEMS Scanning Mirrors for High Power Laser Application,” in Proc. SPIE 9375, San Francisco , 2015.