In this work, we demonstrate an ultra-narrow linewidth self-injection locked distributed feedback (DFB) diode laser emitting at 452 nm, achieving an intrinsic linewidth of 170 Hz with a fiber output power of 11 mW. The linewidth reduction of the DFB laser is obtained thanks to the coupling with an external cavity based on a fiber Bragg grating (FBG). Experimental results demonstrate excellent agreement with our theoretical modeling of the self-injection dynamics. Furthermore, the tuning capabilities of the system are characterized, with tunability achieved via current modulation, yielding a tuning efficiency of 300 MHz/mA over a continuous mode-hop-free range of 600 MHz. This architecture offers a robust pathway toward an integration within a compact package. Ultimately, such compact, stable, and frequency-tunable visible light sources are key for integrated optical atomic clocks and underwater lidar application.

SLEDs are attractive light sources for various applications in the visible and near-infrared spectral region. Some applications require operation at ultra-high ambient temperatures of up to 150° C. We show that the electro-optical device performance at such challenging conditions is limited by the thermal degradation of the material gain and by the non-radiative Auger recombination processes that are strongly increasing with temperature. Still, with proper design choices, several milliwatts of optical output power can be generated at ultra-high chip temperatures.

High-resolution Optical Coherence Tomography (OCT) requires broadband, spatially coherent light sources. Today, the source of choice for visible light OCT, the supercontinuum (SC), is bulky, expensive, and prone to excess noise. Here we demonstrate high-resolution visible light OCT of the human retina with a combined superluminescent diode (SLD) source. The source is longer in wavelength than the high blue light hazard range but shorter in wavelength than the high photopic efficiency range, ensuring subject safety and comfort. We report an axial resolution of 3.2 micrometers in retina, comparable to high-resolution near-infrared OCT systems. We find that Bruch’s membrane is well-delineated in subjects without ocular pathology, even though the axial resolution is ~3x coarser than SC visible light OCT systems. Clinical imaging of pathological findings in intermediate age-related macular degeneration is shown. We further demonstrate that, within the cyan-green wavelength range of the SLD, optical density spectra resemble those of macular pigments. Overall, while the combined SLD approach does not achieve the micrometer-scale resolution of the SC, it potentially reduces the cost and complexity of visible light OCT while providing novel disease-relevant biomarkers in human retina.

EXALOS recently demonstrated superluminescent light-emitting diodes (SLEDs) exhibiting increased optical modal confinement by implementing an InAlN-based n-type cladding in the epitaxial structure. In this work, we leverage the resulting larger modal gains, together with optimized growth conditions for the active region, to demonstrate SLEDs with emission wavelengths up to 525 nm. In addition, we present laser diodes (LDs) obtained on the same epitaxial wafer, achieving lasing action at 535 nm, enabling us to highlight the challenges related to the realization of pure-green SLEDs.

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 microoptical 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 481 nm to 519 nm, a 3dB bandwidth of 32 nm and a coherence length of 4.5 microns in air.

We have developed a new generation of inertial measurement units (IMUs) that are based on 3-axis closed-loop fiber optic gyroscopes (FOGs) operated by a highly integrated SLED transceiver optical module, thereby eliminating several discrete optical components like depolarizers, fiber couplers or fiber-pigtailed photodiodes. Removing many fiber components lowers system costs but also reduces the number of splices of polarization-maintaining (PM) fibers, which dramatically simplifies the IMU's manufacturing processes and improves the system performance. The optical transceiver module is realized as a Butterfly (BTF) module having three PM output fibers with an integrated 1550nm superluminescent diode (SLED), a 1:3 beam divider, a 3-channel optical circulator and three integrated receiver photodiodes.

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.

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.

We report on the progress of our efforts to apply silicon nitride photonic integrated circuits (PIC) to the miniaturization of optical coherence tomography (OCT) with the goal of facilitating its widespread use in ophthalmology at the point of care. In particular, we highlight the design and optical characterization of photonic building blocks allowing the realization of a silicon nitride PIC-based multi-channel swept-source OCT system in the 1060 nm wavelength region. Apart from waveguide structures, these building blocks include 3D-printed microlenses on the PIC end facets for efficient light coupling to and from the PIC.

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.

We report on $300~\mu \text{m}$ short cavity single transverse mode LDs employing $n$ -type InAlN cladding layers. Threshold currents in the blue and green spectral range of 3.3 and 12.0 mA are demonstrated, respectively. This is accompanied by low electrical power consumption for optical powers in the mW range. Compared to conventional AlGaN-based devices having nominally identical active regions, an increase in modal gain is demonstrated, responsible for a ≈25% reduction in threshold currents. Fast-axis far field divergence angle measurements indicate that the modal gain improvement can be ascribed to a higher optical confinement factor when using InAlN.

A novel, 14-pin Butterfly (BTF) light source module, incorporating a broadband Superluminescent Light-Emitting Diode (SLED) at 1550 nm, has been developed for inertial measurement unit (IMU) applications. The compact BTF module integrates micro-optical lenses, a flatpack optical isolator, a monolithic 1:3 beam divider and other components on a temperature-controlled, free-space optical bench platform and is delivering, at an SLED current of 300 mA, more than 3.5 mW of output power at each of the three polarization-maintaining (PM) output fibers with a polarization extinction ratio (PER) larger than 18 dB.

We present the first AIInGaN-based Superluminescent Light Emitting Diodes (SLEDs) featuring an InAIN cladding with an emission wavelength above 51 Onm and state-of-the-art performance. While conventional edge-emitting devices employ non-lattice-matched AIGaN cladding layers (with Al composition of 3-8%), InAIN with an 18% indium composition is lattice-matched to GaN and has a 4-times larger refractive index contrast to waveguiding layers if compared to Alo.o6Gao.94N. These properties make InAIN an ideal cladding candidate for Laser Diodes (LDs) and SLEDs. The increased refractive index contrast is crucial in the green spectral range, where nitride-based devices suffer from strongly reduced modal gain and substrate optical leakage, leading to lower efficiency and far-field emission profiles of lower quality.

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, to our knowledge for the first time, an integrated light source module that emits broadband amplified spontaneous emission (ASE) light from two superluminescent diodes (SLEDs) through a single-mode fiber output simultaneously on two orthogonal polarizations. The dual-polarization SLED source is realized on a free-space, temperature-stabilized, micro-optical bench and integrated in a 14-pin butterfly module. The broadband light output from the single-mode fiber has twice the output power of a single-SLED source and achieves a polarization extinction ratio (PER) of 0 dB when the relative power levels of both SLED sources are carefully balanced.

An optical transceiver module, based on a broadband Superluminescent Light-Emitting Diode (SLED) at 1550 nm and an integrated receiver photodiode, has been developed. The compact 14-pin butterfly (BTF) module integrates micro-optical lenses, a flatpack optical isolator, a 50:50 beam splitter and other components on a temperature-controlled, free-space optical bench platform and is delivering, at an SLED current of 250 mA, around 5 mW of output power at a polarization-maintaining (PM) output fiber with a polarization extinction ratio (PER) larger than 20 dB over a temperature operating range from $- 40 ^{\circ}\mathrm{C}$ to $+ 85 ^{\circ}\mathrm{C}$.

We demonstrate state-of-the-art superluminescent light emitting diodes emitting in the green spectral range with InAlN n-type claddings. The implementation of an InAlN optical blocking layer for suppressing modal substrate leakage leading to high farfield quality is first reported. An optimized structure by optical mode simulations featuring a bottom n-type InAlN cladding layer is then presented. Compared to conventional AlGaN cladding based devices, the electro-optical performance, optical confinement, and modal substrate leakage are greatly improved. Performance improvements in green laser diode devices are also presented.

We report on a swept-source OCT system based on a photonic-electronic integrated circuit. It enables a parallelization of data acquisition resulting in an effective A-scan rate of 4x100 kHz at a central wavelength of 840 nm. The monolithic co-integration of photonic elements forming the multiplexed interferometers and the system electronics on one chip allows a very compact OCT engine in a photonic package. Integrated in an ophthalmic system, the maximum sensitivity was estimated to be 91 dB with an optical power of 4x520 µW at the model eye. An eye phantom was imaged at 400 kHz showing its layered structure.