With VISIIIBLESGREEN EXALOS provides a high perfomance SLED with low etendue and low speckle noise for the visible green spectral range.
Based on our next-generation Gallium Nitride design the worldwide first green SLED provides a unique wavelength of 510 nm.
EXALOS VISIIIBLESRGB SLED technology offers strong benefits, like high color gamut, high efficiency and effective collimation. Perfect for the next generation of projection technology, e. g. augmented reality displays, illuminating micro- or head-up displays, for holography, metrology or spectroscopy.
Thanks to the low temporal coherence – a unique characteristic of SLEDs – VISIIIBLESGREEN are free of speckle, associated with laser-based projection displays. Moreover: based on the broad spectrum and high spatial coherence, EXALOS SLEDs offer high directionality (or low etendue) and high sharpness in image generation, outperforming existing LED- and laser-based projection systems.
The device comes as TO-9 package and can be used as tunable laser when fit to external cavities. Special form factors and advanced solutions for next generation devices are available on request.
- Next Generation of EXALOS´ GaN-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
VISIIIBLES GREEN are designed for:
- 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
EXALOS Gallium Nitride Epitaxial Structure
EXALOS´ blue and green SLED technology is based on III-nitride compound semiconductors. The active region of these devices is realized with strained InGaN QWs.
The advanced device structures are based on epitaxial layers grown by metal organic chemical vapour deposition on free-standing GaN substrates (see below). They rely on the light emission from multiple InGaN quantum wells (QWs) sandwiched in the middle of a low Indium content InGaN waveguiding layer, positioned in between lower refractive index AlGaN cladding layers. The latter provide optical confinement and are p- and n-doped for electrical injection.
The peak wavelength is dependent on the amount of indium introduced in the InGaN quantum wells present in the active region. The higher the Indium content of the QWs, the longer the emitted wavelength. Blue SLEDs require QWs with moderate indium content (typically of the order of 10 to 15%). Increasing the emission wavelength to 500 nm and beyond, to produce a true-green light source, requires values above 20%.
Based on an innovative epitaxial layer design, EXALOS was able to successfully increase the Indium concentration of the InGaN MQW layers to a stable design.
Challenges for GREEN & BLUE SLEDs
T he development of a market-ready green SLED, especially the high indium content of the QWs, has to overcome great challenges.
The high Indium contents have a negative impact on the device performance caused by the higher lattice mismatch. This leads to a reduction of the critical QW thickness and, thus, to a reduced gain, higher defect formation as well as higher internal electric fields on substrates with polar orientation (c-plane).
Consequently, the modal gain for green SLED structures is rather low and higher injection currents are needed to reach the ASE regime. Additionally, the refractive index difference between active region and surroundingwaveguide layers is rapidly decreasing with longer wavelengths,even over few nanometers!
Finally, the modal gain, which strongly influences the output power of SLED, also decreases rapidly with the higher wavelength.
Growing Indium-rich quantum wells is challenging
Consequently a shorter wavelength design needs to face the following challenges:
- High crystal strain, low uniformity.
- Increasing lattice mismatch to GaN layers.
The reduction of the critical QW thickness and, thus, to a reduced gain, leads to a higher defect formation as well as higher internal electric fields on substrates with polar orientation (c-plane). Consequently, the modal gain for green SLED structures is rather low and higher injection currents are needed to reach the ASE regime.
- Decomposition of In-rich QWs
The risk of decomposition needs to reduce growth temperature of the upper layers. The crystal quality deteriorates and p-doping becomes more challenging.
- Reduced wave function overlap of electrons & holes due to polarization fields (quantum-confined Stark effect, QCSE)
Indium-rich QWs result in a reduced material gain at longer wavelengths and thus a lower efficiency.
Challenges of the low optical confinement
The output power of SLEDs is strongly influenced by modal gain. The main challenge results from the reduced modal gain at increasing wavelength. The refractive index contrast between the waveguide and surrounding waveguide layers is rapidly decreasing, even over few nanometers. The lower optical confinement factor leads to a higher internal loss coefficient caused by increasing free-carrier absorption.
The modal gain also decreases rapidly with the wavelength.
Obviously, red, blue, and green SLEDs operate in completely different regimes with respect to the modal gain. The modal gain for red SLEDs is very high.
For blue SLEDs it is significantly smaller, and for green SLEDs it is another 50% smaller at the same current density compared to blue SLEDs. This explains why the typical operating current is very different for the three RGB colors. 1
A detailed analysis of the modal gain in these structures shows that optimizing the confinement factor and achieving better carrier injection efficiency may lead to significantly lower drive current and therefore a higher device efficiency.
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Superior Image Quality
Low Speckle. High Sharpness. Efficient Beam Collimation.
Modern holographic displays can reconstruct 3D images with full wavefront information and in high quality, free from discontinuous motion parallax, crosstalk and lack of accommodation depth cue.
The key factor for image quality is the used light source. A high degree of coherence is required to realize artifact free, sharp presentation. 1
Consequently, a light source with high spatial coherence and low temporal coherence is ideal for a holographic display in order to obtain high quality images with good sharpness and minimum speckle. sLEDs … are suitable light sources for this purpose. 1
The high spatial coherence typical of SLED emission – which corresponds to a directional light beam output – leads to a reduced complexity of collimation optics required in the optical subsystems. The low temporal coherence – resulting from the large spectral bandwidth – obviates the need for bulky dephasers used to reduce undesirable speckle noise.
SLEDs: The optimal hybrid light source
Benefits for modern display applications, compared to LEDs and Lasers:
|Speckle Noise Generation||Low||Low||High|
|Coupling into Single-Mode Fibers||Poor||Efficient||Efficient|
Comparison of speckle noise in the far-field pattern of a blue LD (a) and SLED (b).
(c) Directional emission from 5.6mm TO-packaged blue SLED.
1 Coherence properties of different light sources and their effect on the image sharpness and speckle of holographic displays (2017).
Yuanbo Deng & Daping Chu. Scientific Reports Volume 7, Article number: 5893 (2017) / DOI:10.1038/s41598-017-06215-x
Published with Open Access under Creative Commons Attribution 4.0 International License
External Link: www.nature.com – Scientific Reports
2 RGB Superluminescent Diodes for AR Micro-Displays
Marco Rossetti, Antonino Castiglia, Marco Malinverni, Christian Mounir, Nicolai Matuschek, Marcus Duelk, Christian Vélez
SID 2018 Digest.
From our Blog
Quality Comparison of potential light sources
for Holographic Projection
The test from Yuanbo Deng & Daping Chu, published in Scientific Reports 7: 5893, compares the Coherence properties of 5 light sources and their effect on the image sharpness and speckle of holographic displays.
VISIIIBLES GREEN 510 nm
Maximum Ratings *
Product number EXS210115-00
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typ. 3dB Bandwidth ( λ )
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(TO-9 module) *
Tcase: 25 °C; P: 5 mW
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