Superluminescent LEDs Target Displays
Superluminescent LEDs with record-breaking output powers and efficiencies underscore the promise of these devices for head-up displays and pico-projectors by Marco Malinverni from Exalos
One of the hottest research topics right now is the development of energy-efficient display technologies for augmented-reality head-up displays and pico-projectors.
Today, these technologies tend to use laser diodes or LEDs for their light sources "“ but neither is ideal. LEDs are held back by their Lambertian emission profile that translates into low directionality, which hampers coupling to optical elements, such as fibres and waveguides. Laser diodes are better in this regard, because they emit a narrow beam of light that enables simple, efficient coupling to optical elements. What's more, they have a reduced output dimension, so are much closer to an ideal point-like source, aiding optical system design. However, their monochromatic spectral emission, arising from the Fabry-Pérot cavity that is required for optical feedback, results in a high degree of temporal coherence. This is undesirable: when the laser is used to form an image, it is plagued by an interference pattern, commonly referred to as speckle. This is a particularly troublesome for augmented-reality applications, where high-fidelity images are critical. Despeckling solutions have been developed to minimize interference. But success is limited, and the hardware that is required is often too bulky or expensive to incorporate into consumer electronic devices.
What's needed is a source that combines the best attributes of the laser and the LED. And the good news is that this device already exists, in the form of a superluminescent LED (SLED). It provides a directional light source with a broadband emission spectrum that yields a low degree of speckle noise and a high image quality (see Figure 1).
We are pioneering SLEDs at Exalos, Switzerland. One of our milestones came in 2009, when after three years of development, we introduced the industry's first blue-violet (420 nm) SLED in collaboration with the Ecole Polytechnique Fédérale de Lausanne. Fast-forward to today and we are still the only manufacturer of violet and blue SLEDs. That's not our only product, however "“ we are also the leading supplier of a wide range of infrared SLED devices, having shipped more than 300,000 devices for use in a diverse range of applications, from current sensing to fibre-optic gyroscopes and optical coherence tomography.
Recently, we have focused our efforts on increasing the emission wavelength of our SLEDs. If blue and green devices, made from the III-nitride material system, can be paired with red SLEDs based upon more well-characterized GaAs structures, then this triumvirate can be used to form a full-colour display. There is much work to do, though, as the performance of blue SLEDs can still be improved, while those in the green are still in their infancy.
The SLED design
All our SLEDs share the edge-emitting, ridge-waveguide architecture of the laser diode. However, to prevent optical feedback and lasing, cavity effects are limited by increasing mirror losses at the output facet. There are several options for realizing this: tilting the output facet by several degrees, applying an anti-reflection coating to this facet, or adopting a combination of the two. By inhibiting the laser action, a directional, broadband emission beam is realized through the stimulated emission of carriers. The spectral bandwidth is far broader than that of a laser, and governed by the gain characteristics of the semiconductor material.