EXALOS EVENT PAPER (JULY 2021)

Ultracompact RGB laser diode module for near-to-eye displays

The following excerpts are taken from the paper: Ultracompact RGB laser diode module for near-to-eye displays (July 2021).

Published in Proceedings Volume 11788, Digital Optical Technologies 2021; 117880Q (2021)
Event: SPIE Digital Optical Technologies, 2021, Online Only
DOI: 10.1117/12.2594243

ABSTRACT

EXALOS demonstrate a miniaturized, full-color RGB light source module for near-to-eye display systems, incorporating three semiconductor laser diodes (LDs) that are integrated on a free-space, micro-optical bench together with collimation optics and wavelength filters.

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.

1. INTRODUCTION

Smart Glasses for Augmented Reality (AR) applications need to be lightweight, compact, stylish and comfortable to wear in order to be accepted as accessories by a wider audience in the consumer market. While a number of display architectures are being explored, such as 2D micro-displays based on μLEDs or μOLEDs, digital mirror displays (DMDs) or liquid crystal on silicon (LCOS) devices, there is general agreement that designs based on laser beam scanning (LBS) with micro electro-mechanical mirrors (MEMS) offer the most compact solution with highest wall-plug efficiency.

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 [1]. 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 [2] 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 [3] 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 [4] 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 [5] 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 [6] 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
EXALOS
[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 [5], which, however, also includes two MEMS scanners, and at least half the size of modules based on PIC devices [8].

2. OPTO-MECHANICAL DESIGN AND AUTOMATED ASSEMBLY

Fig. 1 shows a photograph of realized prototype modules of this RGB LD engine, with one module being open (left-hand side) and one module being closed (right-hand side). The lid encapsulates the base and provides an optical window with a clear aperture of 1.0 mm through which the free-space collimated beams exit. The base and the sidewalls have a thickness of 0.6 mm and 0.4 mm, respectively, resulting in an optical bench size of 3.5 mm x 2.2 mm for placing the collimation lenses and the optical combiner filters. The aspheric collimation lenses have a width of 0.6 mm, a height of 1.0 mm and a thickness of 0.7 mm, while the optical filters have a width of 1.2 mm, a height of 1.5 mm and a thickness of 0.4 mm. The collimated output of the blue LD is currently steered with a deflecting 90-degree prism having a leg size of 1.0 mm. The RGB light source engine has ceramic terminal lines with a length of 1.0 mm and a total width of 3.0 mm. The weight of the entire engine, including the lid, is 0.294 g while it is 0.096 g for the base only.

Fig. 1 Photo of open and closed RGB LD module on a gel pack with a raster size of 2.9 mm.

The three LD chips are gold-tin soldered onto a common ceramic substrate that incorporates separate anode and cathode signal lines for each of the three LDs. A next-generation ceramic substrate may also include an additional temperature sensor on the same footprint in order to allow for calibration of temperature-related changes in output power or wavelength. The LD chips have a length of 600 μm and a width of 400 μm and are mounted p-side up onto the ceramic substrate with a lateral distance of 1.1 mm. Fig. 2 shows a 3D CAD model of the light source module as well as the RGB LD engine in operation.
Fig. 2 3D CAD model of RGB LD module with half-transparent lid (left-hand side), top view of RGB LD module in operation (right-hand side).
During the active alignment, the optical light output from the LD chips can be monitored using beam profiler cameras, and the distance of the collimation lenses to the LD chips can be varied to adjust the beam diameter or the divergence of the collimated beams in order to achieve targeted beam properties at a specified reference distance, which was set to 50 mm for these prototype modules. During the alignment of the optical filters, the beam position and the overlap of the centroids have been adjusted to achieve a radial offset of less than 50 μm.

3. RGB LD MODULE PERFORMANCE

The RGB laser diodes have been designed and developed by EXALOS. The red LDs are based on AlGaInP-GaAs semiconductor compounds and emit light at an emission wavelength around 640-645 nm. They have a lasing threshold at 25°C of 17 mA and a slope efficiency from the module of 0.83 W/A. The blue and green LDs are based on InGaN-GaN compounds that EXALOS has been developing for the past 15 years for realizing edge-emitting superluminescent diodes (SLEDs), LDs or similar light sources. The green LDs used for these light modules have a lasing threshold of 27 mA, achieve a slope efficiency of 0.27 W/A and emit light at a wavelength of 515-525 nm. The blue LDs have a lasing threshold of 14 mA, a slope efficiency of 0.43 W/A and emit light at a wavelength of 450-460 nm. The typical spectra and LIV curves of these LDs are shown in Fig. 3. Similar to the GaN-based blue and green LDs, the red GaAs-based LDs are highly polarized in horizontal (TE) direction with a PER of around 20 dB.
Fig. 3 Optical spectra and output power performance (LIV curve) of individual RGB LDs at 25°C

 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 [9].

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.

Fig. 4 RGB beam diameters (50% or FWHM) as a function of propagation distance
Fig. 5 RGB beam diameters (1/e2) as a function of propagation distance
As mentioned before, by adjusting the micro-optical components and beam-collimation optics, the beam divergence can be modified and optimized for other propagation distances. That means that the divergence angles can be further equalized and the ellipticity or circularity can be further improved by more developments on the build process and on the automated alignment routines for the collimation optics.

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.

Fig. 7 Beam overlap at 50% intensity for red, green, blue beam at reference distance of 50 mm. The small dots represent
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 right-hand side of Fig. 7 shows the horizontal and vertical beam offset of the blue and red beam as a function of the propagation distance. The vertical offsets are around 5 μm for both beams at the reference distance of 50 mm and are, otherwise, only slowly increasing with distance.

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.

Table 2 Characteristics of collimated RGB beams at reference distance of 50 mm
Table 3 summarizes the electro-optical properties of the first RGB LD module. As mentioned earlier, the low-threshold LD devices can deliver optical power levels of 10 mW or even more, but they are intended for low-power operation with power levels of 2-5 mW. At a reference output power of 5 mW per color, the total power consumption for cw operation will be 440 mW, with 63% being consumed by the green LD and 26% being consumed by the blue LD. Naturally, the largest reduction of the electrical power consumption of the RGB LD module will be achieved by reducing the threshold currents of the blue and especially of the green LD, also because the forward voltages of these LDs are two to three times higher compared to the red LD.
Table 3 Measured characteristics of first RGB LD light source prototype module

4. SUMMARY

We have demonstrated here the smallest and most lightweight RGB LD engine with a collimated beam output, realized with micro-optical free-space components that are automatically aligned by a high-precision assembly machine. The ultra-compact package has a footprint of 4.4 mm x 4.15 mm with a height of 2.9 mm (corresponding to a volume of only 0.053 cm3) and an optical output window for the collimated and collinearly aligned RGB beams. The collimated beams were optimized for a reference distance of 50 mm and have a 1/e2 beam diameter of 250 to 650 μm with low residual divergence values around 0.5°. At a reference output power of 5 mW per color, the RGB LD module has a power consumption of 440 mW.
Source/Citation
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

 

Full paper available on:

www.spiedigitallibrary.org

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