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.
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 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 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.
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, 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.
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.
The miniaturization of optical coherence tomography (OCT) systems could open up potential new markets, such as point-of-care application, home OCT to regularly monitor disease and treatment progress, and in low-resource settings. Photonic integrated circuits (PIC) are considered an attractive approach to miniaturize OCT. We present our recent achievements in in vivo retinal imaging with a PIC-based Mach-Zehnder interferometer integrated in a state-of-the-art ophthalmic OCT system. The system achieves 94 dB at 750 µW on the sample, running at 50 kHz. Preliminary results of a fully packaged 4-channel opto-electronic OCT engine further demonstrate the potential of PIC-based OCT.
Silicon nitride waveguide based photonic integrated circuits (PICs) are intensively investigated for a wide range of sensing applications in the visible to sub 1-µm near-infrared spectral region. The monolithic co-integration of silicon photodiodes and read-out electronics offers additional benefits in terms of performance and miniaturization. We discuss challenging aspects related to the efficient coupling and routing of light to, from, and within PICs and present interfacing photonic building blocks offering potential solutions. We demonstrate the suitability of these interfacing building blocks by using them for the realization of a PIC-based multi-channel optical coherence tomography concept at 840 nm.
We demonstrate a 1300-nm ultra-broadband and compact light source module with 180 nm FWHM optical bandwidth and 22 mW of output power, realized with four superluminescent diodes (SLEDs) that are integrated on a temperature-stabilized, free-space, micro-optical bench in a standard 14-pin Butterfly package. The light output of four SLED chips at 1220 nm, 1270 nm, 1310 nm and 1360 nm is collimated by micro-optical collimation lenses, spectrally combined through free-space dielectric edge filters and focused into a SMF-28 single-mode fiber. The combined broadband spectrum corresponds to a coherence length of 5.4 μm in air, suitable for ultra-high-resolution OCT systems at 1300 nm.
We demonstrate the first fiber-coupled, broadband master oscillator power amplifier (MOPA) module where amplified spontaneous emission (ASE) light from an 840-nm superluminescent diode (SLED) is amplified by a low-confinement,
broadband 840-nm semiconductor optical amplifier (SOA), generating power levels of more than 60 mW in single-mode fibers. The SLED and SOA are integrated, without an optical isolator in between, in a compact 14-pin Butterfly module on a temperature-stabilized optical bench. The highly-polarized ASE output has a polarization extinction ratio (PER) of more than 30 dB and a 10-dB bandwidth of more than 50 nm, resulting in a coherence length of 10 microns in air.
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.
A novel lipid sensitive OCT setup is presented, using light source with a central wavelength of 1280 nm and a spectral tuning range of 201 nm. A cholesterol plaque located in the aorta was imaged post mortem. Different spectral bands were chosen in post processing. Analyzing the signal attenuation of the different spectral bands enables us to see a clear difference between the lipid absorption in the plaque between the bands. In the normal tissue no clear separation of the signal attenuation can be found. This enables us to visualize the plaque on a three dimensional level.
We demonstrate a novel light source for multi-modality imaging where three superluminescent diodes (SLEDs) are integrated on a free-space, temperature-stabilized, micro-optical bench in a standard 14-pin butterfly package. The light output of the three SLED chips is collimated by individual micro-optical collimation lenses and then spectrally and spatially combined in free space through dielectric edge filters before being coupled into a common HI-780 single-mode output fiber. The two SLEDs at 840 nm and 880 nm deliver a combined broadband spectrum with 120 nm bandwidth and 8.5 mW of output power for ultra-high-resolution optical coherence tomography (OCT) imaging with an axial resolution of ~4 microns in air. The third SLED at 750 nm is suitable for implementing eye tracking based on a scanning laser ophthalmoscope (SLO). Because of the free-space architecture, the polarization of all SLEDs is intrinsically aligned such that the UHR-OCT broadband source has a high PER of 20 dB across the entire spectrum.
We demonstrate, to our knowledge for the first time, an integrated broadband master oscillator power amplifier (MOPA) module where amplified spontaneous emission (ASE) light from an 840-nm superluminescent diode (SLED) is amplified by a low-confinement, broadband 840-nm semiconductor optical amplifier (SOA) in order to generate power levels of more than 100 mW in free space. The SLED-SOA MOPA architecture is realized on a free-space, temperaturestabilized, micro-optical bench and integrated in a 14-pin butterfly module with an optical window output. The highlypolarized ASE light from the SLED is intrinsically aligned to the polarization of the SOA, thereby providing high-power amplified ASE light with an extinction ratio of more than 20 dB and with an optical bandwidth of more than 35 nm FWHM. Optimization of the ASE input signal and of the booster SOA design may either increase the optical bandwidth of the amplified ASE output signal or may also increase the output power levels to a regime of 200-300 mW.
A swept source optical coherence tomography (SS-OCT) system with the interferometer engine being a photonic integrated circuit (PIC) has been developed. Furthermore, an Arrayed Waveguide Grating (AWG), representing a grating on a PIC, for spectral domain OCT (SD-OCT) has been integrated in a fiber-based OCT system. With measured sensitivities of ~87 dB (SS-OCT) and ~80 dB (SD-OCT), scattering tissue imaging becomes feasible for OCT-on-chip systems. In this study, we present two OCT-on-chip systems and first results of biological tissue imaging in-vivo and exvivo.
We demonstrate an 840-nm broadband and compact light source with 140 nm FWHM optical bandwidth and 8 mW of output power, realized with three superluminescent diodes (SLEDs) that are, to our knowledge for the first time, integrated on a free-space, micro-optical bench in a standard 14-pin butterfly package. The three SLED chips at 790 nm, 840 nm and 880 nm are mounted on a ceramic baseplate that is temperature-stabilized by a thermo-electric cooler. The light output of the SLEDs is collimated by micro-optical collimation lenses, spectrally combined through free-space dielectric edge filters and focused into a HI-780 single-mode fiber. The combined broadband spectrum corresponds to a coherence length of 2.9 μm in air, suitable for ultra-high-resolution OCT systems.
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.
KEYWORDS: Signal processing, Field programmable gate arrays, Optical coherence tomography, Imaging systems, Visualization, Data processing, Signal to noise ratio, Data acquisition, Optical simulations, Computer simulations
We have developed a Swept-Source Optical Coherence Tomography (Ss-OCT) system with high-speed, real-time signal processing on a commercially available Data-Acquisition (DAQ) board with a Field-Programmable Gate Array (FPGA). The Ss-OCT system simultaneously acquires OCT and k-clock reference signals at 500MS/s. From the k-clock signal of each A-scan we extract a remap vector for the k-space linearization of the OCT signal. The linear but oversampled interpolation is followed by a 2048-point FFT, additional auxiliary computations, and a data transfer to a host computer for real-time, live-streaming of B-scan or volumetric C-scan OCT visualization. We achieve a 100 kHz A-scan rate by parallelization of our hardware algorithms, which run on standard and affordable, commercially available DAQ boards. Our main development tool for signal analysis as well as for hardware synthesis is MATLAB® with add-on toolboxes and 3rd-party tools.
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.
KEYWORDS: Optical coherence tomography, Field programmable gate arrays, Signal processing, Fourier transforms, Computer simulations, Data acquisition, Digital signal processing, Convolution, Algorithm development, Imaging systems
We developed an FPGA-based engine for Fourier-domain OCT that performs real-time signal processing based on Non- Uniform Fast Fourier Transform (NUFFT). The basic NUFFT algorithm is discussed and compared with cubic-spline interpolation regarding efficient re-sampling in k-space with different phase nonlinearities of sinusoidal swept sources. The NUFFT algorithm was adapted for an implementation in an FPGA and its accuracy is analyzed and assessed using simulated numerical data. When implemented, the NUFFT algorithm allows a processing performance at a sampling rate of 100 MS/s. The real-time processing capability was tested with sinusoidal bi-directional swept sources with A-scan rates of 50 kHz.
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 present a measurement method which is capable of measuring the instantaneous coherence length as a
function of the wavelength while the source is working at its full sweep rate. The measurement principle is based
on the dynamic decrease of fringe contrast as a function of the optical path difference. The measurement setup
consists of a free-space Mach-Zehnder interferometer with a variable optical path difference. We present results
for instantaneous coherence lengths in a range from 0 mm to 50 mm with a mean standard deviation of 0.42 mm
at sweep rates of up to 120 kHz.
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.
Current IP/ATM routers use an electrical switching fabric/backplane and either optical or electrical interconnects between the line cards to route data. An electrical backplane and associated connectors limit the number of high speed interconnections due to practical considerations. Scaling these so-called third generation routers to the terabit regime with high port densities faces enormous problems due to the high interconnect and electrical power density. Packet routers with an optical switching fabric are considered as the next generation of IP/ATM Multi-Service routers which enable scalability towards the Terabit and Petabit regime. In this paper, various architectures with electrically active or passive optical switch fabrics will be highlighted and differences in terms of switching speed and scalability will be discussed. Recent results using a passive Array Waveguide Grating (AWG) router and fast wavelength tunable laser are presented in the second half of this paper.
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