RGB SLED/LD MODULES
A novel light source for Augmented Reality Micro-Displays.
RGB SLED/LD MODULES
A novel light source for Augmented Reality Micro-Displays.
SLED MODULES
EXALOS presents the first light source module that is built with RGB SLEDs in a compact 14-pin butterfly housing, including the first green SLED with a wavelength of 510 nm. The module provides a free-space output of collimated RGB light beams that are colinearly aligned in beam shape and polarization and that have a high circularity and low divergence.
The light output of these SLED devices is collimated by micro-optical lenses and then spectrally combined and colinearly aligned using dielectric edge filters. The free-space beam output has good circularity and similar beam shapes with low divergence for all three colors.
RGB Laser Modules
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“Speckle-free” Lasers for Head-Mounted and Head-Up Displays
The light output of each SLED chip is collimated by a micro-optical lens (1.0 mm x 1.0 mm) and then spectrally combined and colinearly aligned using dielectric edge filters that transmit longer wavelengths and reflect shorter wavelengths.
Other beam-deflecting optics inside the package ensure that the combined RGB free-space beam exits the package through an optical window with a well-defined beam shape (circularity >75%).
The collimation lenses and other beam-shaping optics were aligned to provide circular beams and good collinearity among the three colors at a reference distance of 100 mm. By changing the alignment of the micro-optical components, the beam properties can also be optimized for other distances.
The optical bench also hosts a monitor photodiode (MPD) and an NTC temperature sensor such that the SLEDs are operated on a temperaturestabilized optical platform. The micro-optical components are actively aligned with beam profilers by an automated assembly robot that performs automatic alignment with sub-micron precision and fixation of such components using
UV-curable epoxy glues.
EXALOS VISIIIBLES RGB SLED Features
- Next Generation of GaAS/GaAs-based design
- Low speckle, broadband output
- Enabling sharp images
- High directionality, low etendue beam
- Diffraction-limited (single spatial mode)
- Polarized output
- Energy efficient
- High damage threshold
- Perfect for compact size applications, free-space
or fiber coupled architectures
The VISIIIBLES RGB module is designed for the development of:
- Holographic Displays
- Near-to-eye Displays for AR/VR/MR, e.g. Smart Glasses
- Color-sequential LCOS, DLP, SLM and Scanning MEMS Mirrors
- Micro Displays
- Military & Industrial HUDs
- Pico Projectors
- Machine Vision
- Metrology
- Microscopy
The following measurements are taken with an early prototype and were published in a conference paper: Digital Optical Technologies II in June 2019 [ 1 ].
RED: While SLEDs at 650 nm or longer wavelengths have been commercially available for several years, an efficient SLED at 630-640 nm was introduced only at the end of 2017 by EXALOS. These SLEDs emit a Gaussian optical spectrum with a FWHM of 5.5 nm at a center wavelength of 635 nm, as shown in Fig. 1, with a PER of ~16 dB. They have an ASE threshold current of around 35 mA with a high slope efficiency of 0.50 W/A and reach a free-space output power of 10 mW at a current of less than 60 mA.
Blue and Green semiconductor devices are based on III-nitride compounds. GaN-based SLEDs at 405 nm (violet) or 450 nm (blue) have been commercially available for the past years but green SLEDs at wavelengths above 500 nm have been only introduced in early 2018 by EXALOS and are still under active development.
While blue SLEDs have a good efficiency with an ASE threshold current of around 125 mA and a slope of 0.20 W/A (see Fig. 1), realizing green SLEDs has been challenging as the optical confinement and hence the modal gain is rapidly decreasing when moving to longer emission wavelengths.
The generation of green SLEDs used here operate, as shown in Fig. 1, at a wavelength of 505 nm and have a high ASE threshold current of 475 mA with a low slope efficiency of 0.05 W/A.
When looking at the electrical power requirements for the RGB chip set, the higher forward voltages of typically 5-6V for blue and green emitters, compared to 2-3V for red emitters, has to be considered.
Therefore, the electro-optic efficiency of the first RGB SLED module is currently dominated by the performance of the green SLED. Similar to the red SLEDs, the GaN-based blue and green SLEDs are highly polarized in horizontal (TE) direction with a PER >10 dB. The 3-dB spectral width is 4-5 nm for the blue and 8-10 nm for the green SLED.
Far field distribution along horizontal and vertical direction for red, green and blue SLED
Fig. 1 The curves show measurements of the far field (FF) intensity distribution for three SLEDs along the vertical and horizontal direction. All SLEDs show a nearly Gaussian FF distribution along the horizontal direction with similar FWHM far field angles of 13° for red, 13° for green and 12° for blue, demonstrating lateral single-mode operation.
In the vertical direction, the FF distributions are quite a bit different for GaAs-based red SLEDs and GaN-based blue and green SLED devices, with FWHM far field angles of 34° for red, 17° for green and 24° 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.
LI characteristics of SLEDs in the RGB module
Fig. 2: The collimation efficiency of the micro-optical lenses is around 90%, which means that 10% of the ex-facet output power performance shown in Fig. 1 is being lost at the first lens. The dielectric edge filters and the deflection optics have additional insertion losses that are in the range of 5-10%.
Furthermore, residual spontaneous emission (SE) that is exiting the front facet of the SLED chips is not being collimated and is therefore not propagating to the optical output window of the module. This results in steepening of the LI curves near the ASE threshold current and in better suppression of the SE background below the ASE threshold, as demonstrated by comparing the LI curves of Fig. 1 and Fig. 2, especially for the blue SLED.
To reach an output power of 10 mW per color from the integrated RGB module, electrical drive currents of 60 mA (red), 750 mA (green) and 200 mA (blue) are needed with the current generation of RGB SLED chips, as shown in Table 1. For the green SLED, this value is for pulsed operation with a duty cycle of 25%.
Operating conditions for 10mW per color (total = 7.5 lm):
| Current | Voltage | Power Consumption | |
| Red (635 nm) | 60 mA | 2.30 V | 0.14 W |
| Green (505 nm) | 750 mA | 6.37 V | 4.78 W |
| Blue (452 nm) | 200 mA | 5.83 V | 1.17 W |
Table 1: Electrical characteristic of first RGB SLED prototype module (pulsed operation) for 10 mW output each
~80% of the power consumption is used by the early prototype devices of the green SLED. The design provides a large potential for performance improvement in terms of drive current and forward voltage and later prototypes reduce the electrical power requirements by more than 50%.
Thermo-electric cooling
The 14-pin butterfly module has a thermo-electric cooler (TEC) that provides operation with stable output power values and stable output wavelengths over a wider range of ambient temperatures.
Given the elevated power consumption of the current RGB chip set, the power dissipation of the TEC is also increased and reaches a few Watts at higher heat sink temperatures. For the same reason, the maximum ambient temperature for this cooled butterfly module is currently limited to around 35°C (for cw operation). Improvements in those aspects are expected with upcoming generations of green SLEDs.
The following measurements are taken with an early prototype and were published in a conference paper: Digital Optical Technologies II in June 2019 [ 1 ].
Measured RGB beam profiles at a reference distance of 100 mm
The plots show the 2D intensity for each color, together with the intensity profiles in horizontal (H) and vertical (V) direction at reference distance of 100 mm.
RGB FWHM beam diameters as a function of propagation distance
The measurement shows the diameters for the RGB beams as a function of distance from the module. At a reference distance of 100 mm, the blue beam has a horizontal FWHM beam size of 277 μm and a vertical FWHM beam size of 234 μm. At the same distance, the green beam has slightly larger dimensions with a horizontal FWHM of 311 μm and a vertical FWHM of 421 μm, while the red beam has a horizontal FWHM of 437 μm and a vertical FWHM of 578 μm.
Divergence of red, green and blue beams along horizontal and vertical directions as a function of distance
The residual divergence remains below 0.5° for all three beams over a large propagation distance. At short distances, the blue beam has a negative divergence in vertical direction, which means that the beam is convergent. By adjusting the micro-optical components and beam collimation optics, the beam divergence can be modified and optimized for other propagation distances.
A new alignment and build process (active feedback loops based on beam profiler cameras) provides better agreement for RGB beams and for horizontal/vertical beam shapes.
Circularity for red, green and blue beams versus distance
The measurement shows the beam circularity for each color as a function of distance. The circularity is defined here as the ratio of the smaller and larger beam diameter in horizontal and vertical direction at 50% relative intensity. Hence, a perfectly circular beam would reach the maximum circularity value of one or 100%. This is quite well achieved for the blue beam at larger distances where the beam diameters in horizontal and vertical direction are similar and hence the circularity is close to unity. For the green beam, the circularity remains pretty constant at 75% over a wide range of distances, while again the red beam shows the strongest changes. At small distances up to 40 mm, the circularity is below 70%, which is a consequence of beam diffraction along the fast axis with very small beam diameters in vertical direction. However, at the reference distance of 100 mm, the beam circularities for all three colors are above the target value of 70%.
[ 1 ] A Compact Red-Green-Blue Superluminescent Diode Module: A Novel Light Source for AR Micro-Displays
Published in Proceedings Volume 11062, Digital Optical Technologies 2019; 110620F (2019)
Event: SPIE Digital Optical Technologies, 2019, Munich, Germany
DOI: 10.1117/12.2527626
More from our Blog
A Compact Red-Green-Blue SLED Module:
A Novel Light Source for AR Micro-Displays
EXALOS CONFERENCE PAPER (JUNE 2019)
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