EXALOS EVENT PAPER (JULY 2021)
Ultracompact RGB laser diode module for near-to-eye displays
Published in Proceedings Volume 11788, Digital Optical Technologies 2021; 117880Q (2021)
Event: SPIE Digital Optical Technologies, 2021, Online Only
The ultra-compact package has a footprint of 4.4 mm x 4.15 mm with a height of 2.9 mm (0.053 cm3) and an optical output window for the collimated and collinearly aligned RGB beams. The light source module delivers up to 10 mW per color at low power dissipation values of 640 mW and provides low-divergent output beams having a high circularity and a diameter of 250-650 μm at a reference distance of 50 mm.
Here, the optical architecture of the smart glasses might be based on embedded optical waveguide combiners with refractive, reflective, diffractive or holographic coupling elements for light input and output . Typically, these waveguides are optically inefficient and require output power levels of 20-80 mW per color from the light source engine.
The other prominent optical architecture for LBS-based smart glasses is free-space where light is reflected off the surface of the glasses and a visual image is generated by scanning low-power laser light directly onto the retina, typically requiring only 2-5 mW per color from the light source engine.
Table 1: Comparison of various RGB laser diodes modules (n/s = not specified)
|Group||Dimensions||Volume||Weight||Optical Output Power||Optical Design|
|Trilite ||9.0mm x 8.0mm x 6.2 mm||0.446 cm3||3.1 g||n/s||Separate FAC and SAC, no collinear alignment|
|Sumitomo ||13.0mm x 11.0mm x 5.7mm||0.815 cm3||n/s||80+55+50 mW||Individual collimation lenses & free-space beam combiner, collinear alignment|
|OSRAM ||7.0mm x 4.6mm x 1.2mm||0.039 cm3||n/s||100+50+80 mW||No collimation or beam align-ment, only prism deflectors|
|ST / OSRAM ||11.0mm x 10.0mm x 6.0mm||0.660 cm3||2.5 g||100+50+80 mW||Individual collimation lenses & free-space beam combiner, collinear alignment, two MEMS scanners, monitor photodiode|
|Seiren KST ||11.0mm x 4.8mm x 3.0mm||0.158 cm3||0.7 g||n/s||PIC device for WDM and collinear beam combining, external collimation lens|
|TDK / NTT [7,8]||6.7mm x 5.5mm x 2.7mm||0.100 cm3||0.4 g||5+5+5 mW||PIC device for WDM and collinear beam combining, external collimation lens|
[this work here]
|4.4mm x 4.2mm x 2.9mm||0.053 cm3||0.29 g||10+10+10 mW||Individual collimation lenses & free-space beam combiner, collinear alignment|
Earlier light source engines with red, green, blue (RGB) edge-emitting laser diodes (LDs) were based on individual TO cans, which resulted in rather large footprints that are difficult to integrate into compact AR smart glasses. Newer light sources are based on individual RGB LD chips that are assembled in combination with photonic integrated circuit (PIC) devices or with free-space micro-optical components for the beam collimation and alignment. Table 1 provides an overview of various RGB LD engines that have been reported to date, including this work. Even though a direct comparison of those light source modules may not be appropriate since they are based on different technologies and probably tailored to slightly different requirements, we are demonstrating here the smallest RGB LD engine with a collimated beam output, realized with micro-optical free-space components. This engine is over ten times smaller than a comparable module , which, however, also includes two MEMS scanners, and at least half the size of modules based on PIC devices .
2. OPTO-MECHANICAL DESIGN AND AUTOMATED ASSEMBLY
Fig. 1 Photo of open and closed RGB LD module on a gel pack with a raster size of 2.9 mm.
3. RGB LD MODULE PERFORMANCE
These LDs have been optimized for operation at low output power levels of up to 10 mW using EXALOS-patented design concepts. Recently, significant improvements in lowering the lasing threshold currents for blue and green LDs have been achieved by EXALOS with lasing threshold currents around 5 mA for blue LDs and around 15 mA for green LDs, thereby further reducing the electrical power dissipation of these light modules .
All LDs show a nearly Gaussian far-field (FF) distribution along the horizontal direction with similar FWHM far field angles of 9-10° for all three colors, demonstrating lateral single-mode operation. In the vertical direction, the FF distributions are typically a bit different for GaAs-based red LDs and GaN-based blue and green LDs, with FWHM farfield angles of 27° for red, 20° for green and 22° for blue. While the horizontal FF distribution is mainly governed by the etch depth and the width of the ridge waveguide, the vertical FF distribution is determined by the epitaxial layer structure, namely by the refractive index contrast of the waveguide and, thus, by the mode confinement. Normally, GaN based epitaxial structures feature a lower index contrast and hence a smaller confinement factor, which translates into a larger vertical near field and, consequently, into a narrower far field distribution in vertical direction.
The collimation efficiency with the micro-optical lenses is nearly 100% when the LDs are driven above the lasing threshold as hardly any residual spontaneous emission or substrate leakage modes are emitted from the LD chips once they are operating in the stimulated-emission mode. The high-performance dielectric edge filters have low insertion losses of less than 2%, which means that the slope efficiency values of the RGB LD module is very similar to the one of the free-space LD chips.
Fig. 4 and Fig. 5 show the diameters for the RGB beams as a function of distance from the module, either at the 50% relative intensity level (Fig. 4) or at the 1/e2 or 13.5% relative intensity level (Fig. 5). The first RGB LD prototype modules were built for a reference distance of d=50 mm, which means that the beam shapes and beam overlap were optimized at this distance from the RGB module.
Furthermore, the collimated beam of the green LD was used as a reference beam as it has the strongest contribution to the visual perception of a white-light beam. This beam has, at a distance d=50 mm, almost perfect circularity or ellipticity of 0.97 as the 1/e2 beam diameters in the horizontal direction (250 μm) and in the vertical direction (245 μm) are nearly identical. The collimated blue beam has 1/e2 beam diameters of 425 μm and 325 μm, respectively, and thus achieves a circularity higher than 80%. Given the similar horizontal and vertical far-field angles of the green and blue LD, a better matching to the beam parameters of the green beam should be achievable for the blue beam, though.
For the red beam, the 1/e2 horizontal beam diameter is 440 μm, which is similar to what has been achieved for the blue beam. However, in the vertical direction, the 1/e2 beam diameter is 650 μm, which is significantly larger than the vertical beam size of the blue and green beam. This is mostly related to the larger vertical divergence of the red LD chip, as mentioned earlier, which is something that EXALOS will correct shortly with a new generation of red LD (and SLED) chips that
have similar vertical divergence values as the blue devices.
Besides beam divergence and ellipticity, another important parameter is the beam overlap. Here again, the collimated green beam was taken as a reference and the collimated red and blue beams were aligned relative to it by adjusting the dichroic edge filters. The left-hand side of Fig. 7 shows the beam profiles at 50% relative intensity and their overlap at the reference distance of 50 mm. In line with the discussions above, the green beam has the best circularity while the red beam has a larger ellipticity in the vertical direction. The three dots indicate the center points of the three beams with the green dot being at the origin.
the center points of the beams (left). Horizontal and vertical beam offset of red and blue beam, relative to green beam,
as function of the propagation distance (right).
The horizontal offset for the blue beam remains more or less constant at 20 μm, which indicates a small deviation in the lateral position of the deflecting prism for the blue beam. The horizontal offset for the red beam has an acceptable small value of 17 μm at the reference distance of 50 mm, but shows a strong change with propagation distance, which indicates a deviation in the angular position of the last dichroic edge filter in the beam path. Those alignment errors can be minimized with further process development on the automated placement of the micro-optical components. Table 2 summarizes the beam properties of the first RGB LD module at the reference distance of d=50 mm.
N. Primerov, J. Ojeda, S. Gloor, N. Matuschek, M. Rossetti, A. Castiglia, M. Malinverni, M. Duelk, and C. Velez “Ultracompact RGB laser diode module for near-to-eye displays”, Proc. SPIE 11788, Digital Optical Technologies 2021, 117880Q (20 June 2021); https://doi.org/10.1117/12.2594243
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