EXALOS CONFERENCE PAPER (JUNE 2019)
A Compact Red-Green-Blue Superluminescent Diode Module:
A Novel Light Source for AR Micro-Displays
The following excerpts are taken from the a paper for Conference: Digital Optical Technologies II in June 2019.
Published in Proceedings Volume 11062, Digital Optical Technologies 2019; 110620F (2019)
Event: SPIE Digital Optical Technologies, 2019, Munich, Germany
There have been major research and development activities over the past years on head-up displays (HUDs) or mixed-reality (MR) and augmented-reality (AR) display architectures, including systems based on digital mirror displays (DMDs), liquid crystal on silicon (LCOS) devices, micro electro-mechanical mirror (MEMS) beam-steering engines or holographic spatial modulators. Many of such systems are using laser diodes (LDs) because a spatially-coherent light source is required for providing a collimated beam with high brightness, for example for flying-spot architectures or when coupling to devices having a single-mode optical waveguide. However, LDs are known to generate speckle noise due to their high temporal coherence. On the other hand, LED-based light sources do not suffer from speckle due to the broad emission spectrum but are not efficient when the light output shall be collimated. Furthermore, holographic display engines based on LCOS or other waveguide devices typically support only one polarization direction, which means that half of the unpolarized LED output power is being lost.
Superluminescent light emitting diodes (SLEDs) are edge-emitting semiconductor devices sharing a ridge-waveguide architecture similar to LDs, therefore providing efficient light output with high spatial coherence. Different from LDs, optical feedback is suppressed by tilting the waveguide, applying anti-reflection coatings at the facets and other chip design optimizations. As a result, these SLED devices do not emit coherent, stimulated emission like LDs but amplified spontaneous emission (ASE) that is broadband and, therefore, temporally incoherent. For example, SLEDs in the visible range have a typical spectral full width at half maximum (FWHM) of 4-10 nm, corresponding to a coherence length of only 20-30 μm. Furthermore, the light output of SLEDs has a high polarization extinction ratio (PER) that is usually in the range of 10-20 dB.
For the above-mentioned reasons, SLEDs are ideal light sources for many display architectures, providing high-quality images with good sharpness and minimum speckle contrast . SLEDs in the red-wavelength region were introduced more than 20 years ago, SLEDs with blue wavelengths were presented ten years ago while SLEDs with green emission have been demonstrated only recently . In this work, we present, to our knowledge, the first light source module that is built with RGB SLEDs in a compact 14-pin butterfly housing, providing 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 beam diameters in the vertical and horizontal direction as well as the beam divergence angles can be modified by adjusting the micro-optical components depending on the particular target requirements. For this particular module presented here, the beam collimation has been optimized for a reference distance of 100 mm. The free-space output power from the RGB module is 10 mW per color, corresponding to a total luminous flux of 5 lm.
2. RED, GREEN AND BLUE SLED PERFORMANCE
Red LDs and SLEDs with an emission wavelength in the range of 630-650 nm are based on AlGaInP semiconductor compounds. Realizing highly efficient devices in this wavelength range is challenging due to the small conduction band offset, resulting in shallow quantum wells in the active region and, thereby, a higher degree of carrier leakage and reduced efficiency . 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 [#ref1].
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 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 [3, 5]. 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.
Fig. 1. Typical output power performance (LIV curve) and ASE spectra of individual RGB SLEDs
3. RGB SLED MODULE REALIZATION
The EXALOS RGB optical module is assembled in a standard 14-pin butterfly package (housing 20.8 mm x 12.7 mm) featuring a hermetically sealed sapphire optical window to allow for free-space transmission of the collimated output beam.
A ceramic plate, which acts as an optical bench, is mounted on top of a high-performance thermo-electric cooler that is soldered to the bottom of the optical package. Each SLED chip is soldered onto an individual ceramic submount, the latter being mounted onto the optical bench. A mechanical CAD drawing is shown in Fig. 3 and a photograph of an open module is shown in Fig. 4.
Fig. 3. 3D Solidworks drawing of 14-pin butterfly RGB SLED
Fig. 4. Photograph of 14-pin butterfly RGB SLED module in operation
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 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.
4. ELECTRO-OPTICAL MODULE PERFORMANCE (EXCERPT)
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%.
Fig. 5. LI characteristics of SLEDs in the RGB module
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%.
|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
The electrical requirements for 10 mW of output power per color (total 5 lm) are shown in Table 1. It is clear that ~80% of the power consumption is currently used by the green SLED. As already mentioned, these green SLEDs are earlyprototype devices with a large potential for performance improvement in terms of drive current and forward voltage (see section 6 for more details). Consequently, we are expecting to reduce the electrical power requirements by more than 50% over the coming months.
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.
5. COLLIMATED BEAM PERFORMANCE (EXCERPT)
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.
Fig. 6 shows the diameters for the RGB beams as a function of distance from the module.
Fig. 6. RGB FWHM beam diameters as a function of propagation distance
Fig. 7. Measured RGB beam profiles from module at 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. As shown in this figure, the red beam has a slightly distorted beam profile with a wider beam width in vertical direction compared to the horizontal direction. The root cause for that is the larger vertical numerical aperture (NA) and, therefore, the larger vertical far field (FF) angles of the red SLED that is not well matched to the acceptance angle of the currently used collimation lens.
Future optimization of the vertical confinement of the red SLED could reduce the vertical FF angles and, thereby, result in a similar beam profile as obtained for the green or blue SLED. Another option might be to choose a collimation lens with a larger NA and larger aperture, which would reduce the beam distortions.
We demonstrate, to our knowledge, the first integrated RGB SLED module in a compact 14-pin butterfly package.
SLEDs at 452 nm, 505 nm and 635 nm are mounted on a temperature-stabilized platform inside this module to provide stable output power and stable wavelength performance. 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. The output power is 10 mW per color, which corresponds to a luminous flux of 5 lm.
We also present a new generation of green SLEDs operating at 517 nm, which is the longest green wavelength reported for SLED devices.
This integrated module of RGB Superluminescent LEDs represents a new platform of micro-optical light source assemblies that enable the advancement of micro-display architectures for augmented-reality (AR) systems and head-up displays (HUDs) by significantly reducing speckle noise compared to LD-based light sources.
A compact red-green-blue superluminescent diode module: A novel light source for AR microdisplays
N. Primerov, J. Dahdah, S. Gloor, T. von Niederhäusern, N. Matuschek, A. Castiglia, M. Malinverni, C. Mounir, M. Rossetti, M. Duelk, C. Velez
EXALOS AG, Wagistrasse 21, CH-8952 Schlieren, Switzerland
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