We demonstrate the first broadband light source based on spectral combination of four superluminescent diodes (SLEDs) in the cyan-to-green wavelength range, suitable for high-resolution, visible optical coherence tomography (OCT). Two integrated combined-SLED sources, each comprising two wavelength-shifted green SLEDs, are realized through micro-optical module integration. Each of those two combined-SLED sources is delivering a highly polarized output spectrum at a polarization-maintaining (PM) fiber. The output of the two PM fibers is then spectrally combined with a free-space, micro-optical combiner module to a common, single-mode fiber output with a broadband output spectrum having a 10dB wavelength range from 481nm to 519nm, a 3dB bandwidth of 32nm and a coherence length of 4.5 microns in air.
We introduce an enhanced version of a full-color, RGB hybrid LD-SLED light source module for near-to-eye display systems, predominantly tailored for laser beam scanning (LBS) architectures. This light source module integrates blue and green semiconductor laser diodes (LD) emitting at wavelengths of 455 nm and 520 nm, respectively, along with a red superluminescent diode (SLED) operating at 638 nm. Besides the RGB emitter devices, this micro-RGB module includes collimation optics, wavelength-combining filters, and a prism pair to achieve circular output beams, all packaged with an innovative micro-optical, free-space bench architecture. With a compact footprint of 5.5 mm x 8.6 mm, this module produces collimated, circular, and collinearly aligned RGB beams with minimal divergence and large diameters of 1.0-1.3 mm at the module output. This third generation of an micro-RGB light source module delivers up to 50 mW of optical power per color at a total power dissipation of 1.2 W.
Laser beam scanners are of strong interest, as they offer a compact and low-power consumption solution for head-worn AR-displays and, more in general, for efficient and focus-free projection displays. In this framework, EXALOS has been leading the research and development of active light sources, focusing on optimizing red, green, and blue edge-emitting devices, including superluminescent diodes (SLEDs) and laser diodes (LDs). This work provides an overview of the performance achieved by single-emitter SLEDs and low-threshold LDs based on GaN, as well as AlGaInP, III-V semiconductors. Furthermore, we report on the development of emitter arrays. In particular, narrow-pitch devices with an emitter-to-emitter spacing of 10 μm and also a novel device design featuring both anodes and cathodes on the top chip surface are reported. The individual cathodes are fully independent and electrically insulated, ensuring an emitter-to-emitter resistance close to 1MOhm. Compared to conventional arrays with common cathode, the new architecture allows for integration with industry-standard current-sink drivers for efficient multi-LD modulation.
An n-type InAlN cladding design based on multiple GaN/InAlN pairs is successfully implemented in edge-emitting laser diodes (LDs) and superluminescent light emitting diodes (SLEDs) emitting in the blue and green spectral range. Thanks to the stronger refractive index contrast with respect to waveguiding layers enabled by this approach, larger optical confinement factors are obtained. The resulting larger modal gains translate into remarkable performance improvements for LD and SLEDs with respect to conventional AlGaN based claddings. LDs with threshold currents as low as 3 mA in the blue and 12 mA in the green spectral range are demonstrated. Similarly, an operating current decrease of >100 mA is reported for state-of-the-art green SLEDs.
We present, to the best of our knowledge, the first compact, full-color, hybrid RGB LD-SLED light source module designed for near-to-eye display systems. This module integrates a blue and green semiconductor laser diode (LD) at a wavelength of 453 nm and 520 nm, respectively, and a red superluminescent diode (SLED) at 639 nm in combination with a novel micro-optical, free-space architecture. The light source module includes circularizing optics, wavelengthcombining filters, and a single aspheric collimation lens. The light source module has a compact footprint of 7.7 mm x 10.8 mm and generates collimated, circular and collinearly aligned RGB beams with low divergence and large diameters in the range of 1.7 mm to 2.2 mm at the optical output. The current generation of this light source module delivers up to 15 mW of optical power per color, with a total power dissipation value of only 430 mW.
We demonstrate a miniaturized, full-color RGB light source module for near-to-eye display systems, incorporating three semiconductor laser diodes (LDs) that are integrated on a free-space, micro-optical bench together with collimation optics and wavelength filters. The ultra-compact package has a footprint of 4.4 mm x 4.15 mm with a height of 2.9 mm (0.053 cm3) and an optical output window for the collimated and collinearly aligned RGB beams. The light source module delivers up to 10 mW per color at low power dissipation values of 640 mW and provides low-divergent output beams having a high circularity and a diameter of 250-650 μm at a reference distance of 50 mm.
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.
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