We present the first light source module that is realized with RGB superluminescent LEDs in a compact 14-pin butterfly housing for speckle-free display applications. The module provides a free-space output with collimated RGB beams that are colinearly aligned having 10 mW output power per color.
Superluminescent light emitting diodes (SLEDs) have beam-like optical output similar to laser diodes (LDs) while offering a broader emission wavelength spectrum. They represent, therefore, an interesting alternative to conventional LDs for applications where a short coherence length or low speckle noise are required. Visible SLEDs emitting in the red, blue, and green are ideal candidates for the manufacturing of speckle-free light sources in portable or wearable compact projection systems. In this paper, we review the current status of EXALOS’ GaN-based SLED technology in the violet-blue spectral range and report on our recent progress in terms of performance for devices with 440-460 nm emission. Furthermore, we discuss the challenges in achieving light output at even longer wavelengths. As a matter of fact, lower refractive index contrast between the waveguiding and cladding layers, decreased p-type doping efficiency when growing at low temperatures, low crystal quality and thermal stability of the active region have to be addressed and solved in order to achieve green emission. The epitaxial structures were grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane freestanding GaN substrates. Growth was followed by standard fabrication of SLEDs with a ridge waveguide design. A record CW output power of 150 mW (at an operating current of 330 mA) and a wall-plug efficiency (WPE) of 8% have been obtained at an emission wavelength >440 nm.
We report on the reliability of GaN-based super-luminescent light emitting diodes (SLEDs) emitting at a wavelength of 405 nm. We show that the Mg doping level in the p-type layers has an impact on both the device electro-optical characteristics and their reliability. Optimized doping levels allow decreasing the operating voltage on single-mode devices from more than 6 V to less than 5 V for an injection current of 100 mA. Furthermore, maximum output powers as high as 350 mW (for an injection current of 500 mA) have been achieved in continuous-wave operation (CW) at room temperature. Modules with standard and optimized p-type layers were finally tested in terms of lifetime, at a constant output power of 10 mW, in CW operation and at a case temperature of 25 °C. The modules with non-optimized p-type doping showed a fast and remarkable increase in the drive current during the first hundreds of hours together with an increase of the device series resistance. No degradation of the electrical characteristics was observed over 2000 h on devices with optimized p-type layers. The estimated lifetime for those devices was longer than 5000 h.
Since pico-projectors were starting to become the next electronic "must-have" gadget, the experts were discussing which
light-source technology seems to be the best for the existing three major projection approaches for the optical scanning
module such as digital light processing, liquid crystal on silica and laser beam steering. Both so-far used light source
technologies have distinct advantages and disadvantages. Though laser-based pico-projectors are focus-free and deliver a
wider color gamut, their major disadvantages are speckle noise, cost and safety issues. In contrast, projectors based on
cheaper Light Emitting Diodes (LEDs) as light source are criticized for a lack of brightness and for having limited focus.
Superluminescent Light Emitting Diodes (SLEDs) are temporally incoherent and spatially coherent light sources
merging in one technology the advantages of both Laser Diodes (LDs) and LEDs. With almost no visible speckle noise,
focus-free operation and potentially the same color gamut than LDs, SLEDs could potentially answer the question which
light source to use in future projector applications. In this quest for the best light source, we realized visible SLEDs
emitting both in the red and blue spectral region. While the technology required for the realization of red emitters is
already well established, III-nitride compounds required for blue emission have experienced a major development only
in relatively recent times and the technology is still under development. The present paper is a review of the status of
development reached for the blue superluminescent diodes based on the GaN material system.
GaN/AlN multiple quantum wells (MQWs), designed for intersubband (ISB) absorption in the telecommunication
range, are grown by molecular beam epitaxy. We demonstrate that the use of both AlN template and optimized growth
temperature allows to reach ISB transition energy in the telecom range, i.e. above 0.8 eV (λ = 1.55 μm). Absorption
spectra exhibit narrow linewidth (< 50 meV) with a relative energy broadening of 8%. An electro-optical modulator
based on electron tunnelling in coupled QWs is then fabricated. A modulation bandwidth of 2 GHz at -3 dB cut off
frequency is achieved for 15x15 μm2 mesas. We show that the modulation rate is limited by the device geometry rather
than by the material quality, which makes this technology a good candidate for THz regime.
The authors report on room temperature (RT) lasing action in two different types of nitride-based microcavities (MCs):
vertical cavity surface emitting lasers (VCSELs) and polariton lasers which operate in the weak and in the strong
coupling regime, respectively. Following a brief description of these two operating regimes, an analysis of lasing action
at RT is reported for a crack-free planar VCSEL structure based on a bottom lattice-matched AlInN/GaN distributed
Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding
only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL.
Processing issues of such planar VCSEL structures and electroluminescence characteristics of processed devices are also
reported. Then, an alternative approach relying on the realization of coherent GaN-MC light sources based on the
spontaneous decay of a macroscopic polariton population, the so-called polariton laser, is described. It is shown that this
kind of devices could work at RT with a potentially much lower threshold current density than VCSELs as it does not
necessitate reaching population inversion conditions. As for VCSELs, we demonstrate laser-like properties above
threshold, i.e. an intense polarized emission and a strong spectral narrowing. Differences of polariton lasers with
conventional lasers are also highlighted.