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.
In this work we present a study on teh Super Luminescent LIght Emitting Diodes (SLEDs) performance under high doses of gamma radiation. We investigate GaAs SLEDs with emission wavelengths around 830 nm. The devices were exposed to ionising radiation at a dose rate of about 4.7 Gy/s, up to a cumulated dose of 10.1 MGy in the CMF facility of the Belgian nuclear research centre SCK•CEN. We measured the device characteristics before adn after irradiation. We show that the SLED performance is only marginally affected.
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.
We show a broad range of swept source performances based on a highly-flexible external cavity laser architecture.
Specifically, we demonstrate a 40-kHz 1300-nm swept source with 10 mm coherence length realized in a compact
butterfly package. Fast wavelength sweeping is achieved through a 1D 20-kHz MEMS mirror in combination with an
advanced diffraction grating. The MEMS mirror is a resonant electrostatic mirror that performs harmonic oscillation only
within a narrow frequency range, resulting in low-jitter and long-term phase-stable sinusoidal bidirectional sweep
operation with an A-scan rate of 40 kHz. The source achieves a coherence length of 10 mm for both the up- and downsweep
and an OCT sensitivity of 105 dB.
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.
We have developed ultra-broadband Super-Luminescent Emitting Diodes (SLEDs) at 840 nm with a 3-dB bandwidth of
45-75 nm. The SLEDs show high robustness against back-reflections of up to 50% with little change in coherence
length, sidelobe suppression ratio and secondary peak suppression over a wide range of back-reflections. First long-term
measurements do not show any signs of device degradation. Hence, these SLEDs can be employed in OCT systems
without costly broadband optical isolators.
We have fabricated superluminescent light-emitting devices in the 840nm wavelength range with flat top spectral shape.
The novel design allows more than 50nm bandwidth and up to 34mW of optical power at the chip facet. Moreover, the
3dB bandwidth changes by less than 2nm within a driving current range between 120mA and 200mA. This corresponds
to a power level change between 17mW and 34mW, without considerable shape changes, which is one of the main
concerns for many applications. The stability of the spectral bandwidth is also reflected by the central wavelength that
changes by less than 1nm in the same range of currents. The device shows great stability of the optical far field with
respect to the driving current, allowing stable coupling of the emitted beam in optical fibers. We have also measured the
coherence function of this device using an interferometric spectrum analyzer. Results show good side-lobes suppression
ratio of more than 10dB, which remains almost unchanged over the whole range of driving currents.
A Superluminescent Light Emitting Diode (SLED) is an ideal optical broadband source for applications like Optical Coherence Tomography (OCT) and other fiber optic based imaging techniques. High optical output power and large optical bandwidth are key features for these devices. The short coherence length related to this large bandwidth allows the realization of OCT systems with higher sensitivity.
Semiconductor devices based on quantum dots (QD) are ideally suited as the active material for SLEDs since the size dispersion typical of self-assembled growth naturally produces a large inhomogeneous broadening. The large spacing between different energy levels can lead to improved thermal stability as well.
In this paper we report, ridge-waveguide devices based on five stacks of self-assembled InAs/GaAs QDs. SLED devices with output powers up to 1.5 mW emitting around 1300 nm have been realized. Spectral analysis at 20°C shows a 121 nm FWHM. Temperature characteristics in the range 10-80°C are also reported.
A Superluminescent Light Emitting Diode (SLED) is an ideal optical broadband source for applications like Fiber Optic Gyroscopes and other fiber optic based sensors used in navigation systems. High optical output power and large optical bandwidth are key features for these devices. The short coherence length related to this large bandwidth allows the realization of sensors with improved sensitivity.
Semiconductor devices based on quantum dots (QD) are ideally suited as the active material for SLEDs since the size dispersion typical of self-assembled growth naturally produces a large inhomogeneous broadening. The large spacing between different energy levels can lead to improved thermal stability as well. In this paper we report, ridge-waveguide devices based on five stacks of self-assembled InAs/GaAs QDs. SLED devices with output powers up to 1.5 mW emitting around 1300 nm have been realized. Spectral analysis at 20°C shows a 121 nm FWHM. Temperature characteristics in the range 10-80°C are also reported.