Superluminescent Diode Research Articles

Enhanced ultraviolet electroluminescence from ZnO nanoparticles via decoration of partially oxidized Al layer.

We construct the ZnO-based superluminescent light-emitting diodes (SLEDs) by spin-coating ZnO nano-particles onto p-GaN/sapphire substrate. By inserting another thin Al layer to form an n-ZnO/Al/n-ZnO/p-GaN sandwich structured SLD, the intensities of the photoluminescence and electroluminescence were greatly enhanced, which can be attributed to the surface plasmon resonance of this Al layer. The tendency of the intensities of the entire electroluminescence spectra shows a super-linearly behavior with increasing the forward bias. Besides, the spectral bandwidth is narrowed down enormously owing to the achievement of the SLD. Furthermore, the interfacial emissions between ZnO/GaN are effectively suppressed by partially oxidizing the Al layer.

Wide and tunable spectral asymmetry between narrow and wide facet outputs in a tapered quantum-dot superluminescent diode.

A wide spectral asymmetry between the front and rear facets of a tapered chirped quantum dot multi-section superluminescent diode is reported. The spectral asymmetry between the two facet outputs was found to be tunable and highly dependent on the bias asymmetry between the two contact sections, with a spectral mismatch of up to 14 nm. Numerical simulations confirmed a relationship between this spectral asymmetry and the non-uniform filling of the quantum dots' confined states when different current densities are applied to the device electrodes. The results from this investigation open up an additional degree of freedom for multi-section superluminescent diodes, which could pave the way for optical bandwidth engineering via multiplexing the spectral output from both facets, using only a single device.

Demonstration of a low-complexity memory-polynomial-aided neural network equalizer for CAP visible-light communication with superluminescent diode

Visible-light communication (VLC) stands as a promising component of the future communication network by providing high-capacity, low-latency, and high-security wireless communication. Superluminescent diode (SLD) is proposed as a new light emitter in the VLC system due to its properties of droop-free emission, high optical power density, and low speckle-noise. In this paper, we analyze a VLC system based on SLD, demonstrating effective implementation of carrierless amplitude and phase modulation (CAP). We create a low-complexity memory-polynomial-aided neural network (MPANN) to replace the traditional finite impulse response (FIR) post-equalization filters of CAP, leading to significant mitigation of the linear and nonlinear distortion of the VLC channel. The MPANN shows a gain in Q factor of up to 2.7 dB higher than other equalizers, and more than four times lower complexity than a standard deep neural network (DNN), hence, the proposed MPANN opens a pathway for the next generation of robust and efficient neural network equalizers in VLC. We experimentally demonstrate a proof-of-concept 2.95-Gbit/s transmission using MPANN-aided CAP with 16-quadrature amplitude modulation (16-QAM) through a 30-cm channel based on the 442-nm blue SLD emitter.

Broadened Bandwidth Amplified Spontaneous Emission from Blue GaN-Based Short-Cavity Superluminescent Light-Emitting Diodes

We report broad bandwidth blue superluminescent light-emitting diodes (SLEDs) based on a short-cavity active region. The dependencies of amplified spontaneous emission (ASE) output power and gain bandwidth on cavity length were investigated in devices whose gain medium consists of a ridge waveguide with embedded InGaN/GaN quantum wells sandwiched between one etched facet coated with a high reflectivity distributed Bragg mirror and one cleaved facet with an anti-reflection coating. 250 μm-long blue SLEDs exhibit a spectral bandwidth up to 7.5 nm at 1.72 mW output power at a wavelength of 427 nm. As cavity length decreases, the bandwidth gradually broadens up to 15 nm for the shortest, 40 μm-long, SLED devices. ASE is confirmed by current-dependent electroluminescence spectra and polarization-dependent emission intensity measurements. The optical features of those short-cavity devices could be helpful for designing broad bandwidth SLEDs aiming for various applications such as optical coherence tomography, next generation displays, on-chip biosensing and imaging.

The Application of High Efficiency and Low Noise Switching Power Supply in Fiber Optic Gyroscope

For high-precision fiber optic gyro systems, the efficiency and noise in the switching power source limits its application and also affects the system's light source and signal processing circuit, resulting in output errors. The interference mechanism of switching power supply ripple noise on super luminescent diode (SLD) light source and demodulation module in fiber optic gyro system is analyzed. A high-efficiency and low-noise switching power supply based on synchronous rectification is proposed. The power supply uses RC circuit to realize adjustable switching frequency. The push-pull isolation topology, LC filter and LDO regulator circuit are used to achieve constant voltage output with peak-to-peak ripple noise of less than 1mV. The conversion efficiency of the switching power supply is improved by 10%, which provides a high-quality power source for the high-precision fiber optic gyroscope. In the meanwhile, the switching power source for FOG is 541cm in size, which is beneficial to the miniaturization of inertial navigation system.

On the Feasibility of Absolute Distance Measurement by Using Optical Feedback Into a Superluminescent Diode Cavity

The development of accurate absolute distance interferometers is a long-term research goal. The key impact of this article is the demonstration of an absolute distance interferometer based on the optical feedback (OF) into a superluminescent diode cavity. Due to the OF, we obtained huge interference fringes even with diffusive targets and without using high-sensitivity and low-noise detectors but directly exploiting the monitor photodiode enclosed in the same package of the optical source. The low coherence of the optical source makes the system robust to any stray light along the optical path and thus suitable to work in biological media and dusty industrial environments. The proposed signal processing exploits a zero-crossing approach to determine the absolute target position. Preliminary results show a 95% expanded uncertainty to measuring interval ratio of about 60 ppm over a measuring interval of 50 mm.

High-power single mode GaSb-based 2 μm superluminescent diode with double-pass gain

We report a broadband superluminescent diode operating around a 2 μm wavelength, optimized for high-power broadband operation. The high power operation is achieved by using a GaInSb/AlGaAsSb heterostructure positioned in a ridge waveguide with a J-shaped layout to form a double-pass geometry. To avoid lasing at high current while enabling high gain, a cavity suppression element is used. This combination allows demonstration of an output power as high as 120 mW for continuous-wave (CW) operation at room temperature, with a spectral full width at half maximum of about 43 nm. The maximum power spectral density was measured to be 1.8 mW/nm, which is about a fourfold increase compared to the state-of-the-art results for this wavelength range. To avoid heating, the diode was also driven with low duty-cycle current pulses; in this case, a peak power of more than 300 mW was achieved without any sign of roll-over (power was limited by the current injected). For CW operation, the central emission wavelength could be tuned by current injection between 1900 nm at 200 mA and 2027 nm at 2000 mA. Devices produce a Gaussian output beam that is suitable for coupling to single mode waveguides.

GaAs-Based InPBi Quantum Dots for High Efficiency Super-Luminescence Diodes.

InPBi exhibits broad and strong photoluminescence at room temperature, and is a potential candidate for fabricating super-luminescence diodes applied in optical coherence tomography. In this paper, the strained InPBi quantum dot (QD) embedded in the AlGaAs barrier on a GaAs platform is proposed to enhance the light emission efficiency and further broaden the photoluminescence spectrum. The finite element method is used to calculate the strain distribution, band alignment and confined levels of InPBi QDs. The carrier recombinations between the ground states and the deep levels are systematically investigated. A high Bi content and a flat QD shape are found preferable for fabricating super-luminescence diodes with high efficiency and a broad emission spectrum.

Tilt metrology on rough dielectric surfaces using low coherence scanning interferometry.

In this investigation, we propose a technique to obtain not only the dimensional surface profile but also tilt information of the rough dielectric surface having a few microns root-mean-square roughness. This technique is based on low coherence scanning interferometry (LCSI) using a compound light source by combining a superluminescent light-emitting diode with ytterbium-doped fiber amplifier. Tilt angle and direction of the measured surface is extracted by the principal component analysis (PCA) from the measurement surface data and the centroid peak detection algorithm. To verify the performance of the proposed tilt measurement method, standard angle gauge block and certified step height sample were used as specimens. LCSI tilt measurement was about 3 times superior to the conventional auto-collimator in terms of the measurement precision in the practical camera module manufacturing process of smartphones. The proposed method was also discussed the dynamic tilt evaluation for the moving object.

What limits the efficiency of GaN-based superluminescent light-emitting diodes (SLEDs)?

Gallium-nitride-based SLEDs are attractive light sources for augmented reality displays and other applications. However, the electrical-to-optical power conversion efficiency (PCE) of SLEDs is still far below the record-high values reported for LEDs. Utilizing advanced numerical device simulation, this paper investigates the internal physical processes that cause the low PCE of SLEDs. The poor hole conductivity strongly reduces the electrical efficiency, similar to laser diodes. However, in contrast to laser diodes, the rising carrier density in the active layers is identified as main reason for enhanced Auger recombination that severely limits the internal quantum efficiency. Design improvement options are demonstrated.

Effect of an incoherent pump on two-mode entanglement in optical parametric generation

Pumping a nonlinear crystal by an intense radiation results in the optical parametric generation of photons in two modes (the signal and the idler). The quantized electromagnetic field in these modes is described by a continuous-variable quantum state, which is entangled if the pump is a coherent state produced by a laser. The signal and the idler modes remain populated by photons even if the pump becomes incoherent (dephased by a medium, superposed with a thermal state, or produced by an alternative source such as the superluminescent diode). However, the incoherent pump does effect the entanglement and purity of the signal and the idler modes, which is of vital importance for quantum information applications and interferometry. Here we develop an approach to infer the signal-idler entanglement and purity for a general quantum incoherent pump with the given Glauber-Sudarshan function. We show that the signal-idler entanglement is extremely sensitive to the phase distribution of the pump and illustrate our findings by physically relevant examples of the incoherent pump: the noisy coherent state, slightly dephased and phase-averaged coherent states, the thermal state, and states modulated by the Kerr medium. The effect of an incoherent pump on the combined quadratures is discussed as well.

Review—Review on Optimization and Current Status of (Al,In)GaN Superluminescent Diodes

Superluminescent diodes represent a device type which can fill the gap between light emitting diodes (LEDs) and laser diodes. The light generation based on Amplified Spontaneous Emission promotes high brilliance of the light source (better than in case of LEDs), and by this efficient light harnessing e.g. coupling to optical fibers. At the same time the low time coherence of the light reduces the interference effects, unfavorable in applications such as Optical Coherence Tomography or in direct image projection. This article reviews the development process and the current status of nitride superluminescent diodes, from their first introduction in 2009 until present.

Group-III-Nitride Superluminescent Diodes for Solid-State Lighting and High-Speed Visible Light Communications

Group-III-nitride superluminescent diodes (SLDs) are emerging as light sources for white lighting and visible light communications (VLC) owing to their droop-free, low speckle noise, and large modulation bandwidth properties. In this paper, we discuss the development of GaN-based visible SLDs, and analyze their electro-optical properties by studying the optical power-bandwidth products and injection current densities. The significant progress in blue SLDs and their applications for white light VLC is highlighted. A blue SLD, with an optical power of >100 mW and large PBP of 536 mW·nm, is utilized to generate white light, resulting in a high color rendering index (CRI) of 88.2. In a modulation experiment designed for an SLD-based VLC system, an on-off keying scheme exhibits a 1.2 Gbps data rate, with a bit error rate of 1.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which satisfies the forward error correction criteria. A high data rate of 3.4 Gbps is achieved using the same SLD transmitter, by applying the 16-quadrature-amplitude-modulation (16-QAM) discrete multitone modulation scheme for high-speed white light communication. The results reported here unequivocally point to the significant performance and versatility that GaN-based SLDs could offer for beyond-5G implementation, where white lighting and high spectral efficiency VLC systems can be simultaneously implemented.

Energy Efficiency Analysis of GaN-Based Blue Light Emitters

GaN-based light sources are in high demand for lighting, displays, medical equipment and other applications. InGaN/GaN blue light-emitting diodes (LEDs) reach an electrical-to-optical power conversion efficiency of more than 80% but less than 10% are reported for blue superluminescent light-emitting diodes (SLEDs) and less than 50% for blue laser diodes (LDs). We here analyze the physical mechanisms behind this surprising discrepancy in peak energy efficiency of GaN-based light emitters. Our study reveals that the Mg-doping of group-III-nitride layers, which was pioneered by Isamu Akasaki and collaborators, plays a key role in understanding this efficiency difference.

High power surface emitting InGaN superluminescent light-emitting diodes

A high power InGaN superluminescent light-emitting diode emitting normal to the substrate is demonstrated. The device uses a structure in which a monolithically integrated turning mirror reflects the light at both ends of the in-plane waveguide to direct amplified spontaneous emission downward through the transparent GaN substrate. Record optical peak powers of &amp;gt;2 W (both outputs) are reported under pulsed operation at 1% duty cycle. A broad, smooth emission spectrum with a FWHM of 6 nm centered at 416 nm is measured at peak output and ascribed to very low feedback associated with the turning mirror and antireflection coating.

Changing the speed of optical coherence in free space.

It is typically assumed that the fluctuations associated with a stationary broadband incoherent field propagate in free space at the speed of light in vacuum c. Here we introduce the concept of "coherence group velocity" which, in analogy to the group velocity of coherent pulses, is the speed of the peak of the coherence function. We confirm experimentally that incorporating a judicious spatio-temporal spectral structure into a field allows tuning its coherence group velocity in free space. Utilizing light from a superluminescent diode, we interferometrically measure the group delay encountered by the cross-correlation of a structured field synthesized from this source with the unstructured diode field. By tracking the propagation of this cross-correlation function, we measure coherence group velocities in the range of 12c to -6c.

Emission spectrum of broadband quantum dot superluminescent diodes

We present a microscopic theory of the amplified spontaneous emission of a spectrally broadband quantum dot superluminescent diode with tilted end facets within the quantum white noise limit. From this multimode quantum theory, we have the ability to obtain all orders of temporal correlation functions. In particular, we derive rate equations for the optical power densities, the level occupation of inhomogeneous ensemble of quantum dots within the diode, as well as the emitted optical spectra. As the main result, we find the external power spectrum as a convolution of the intra-diode photon spectrum with a Lorentzian response. Assuming a Gaussian light-matter coupling results in a similar shaped Gaussian output spectrum, which agrees very well with available experimental data.

Superluminescent diodes based on asymmetric double-quantum-well heterostructures

The power and spectral characteristics of near-IR superluminescent diodes (SLDs) based on asymmetric double-quantum-well GaAs/InGaAs heterostructures are studied experimentally. It is shown that, varying the active layer composition and the spatial-single-mode active channel length of these SLDs, it is possible to widely change the achievable output optical power and the spectral width of the symmetric bell-shaped spectrum. The studied SLDs have a lower coherence function pedestal, a weaker dependence of the spectral width on the injection current, and a higher polarisation degree than the widely spread SLDs based on single-quantum-well heterostructures with the same spectral widths.

In vivo subdiffuse scanning laser oximetry of the human retina.

.Scanning laser ophthalmoscopes (SLOs) have the potential to perform high speed, high contrast, functional imaging of the human retina for diagnosis and follow-up of retinal diseases. Commercial SLOs typically use a monochromatic laser source or a superluminescent diode for imaging. Multispectral SLOs using an array of laser sources for spectral imaging have been demonstrated in research settings, with applications mainly aiming at retinal oxygenation measurements. Previous SLO-based oximetry techniques are predominantly based on wavelengths that depend on laser source availability. We describe an SLO system based on a supercontinuum (SC) source and a double-clad fiber using the single-mode core for illumination and the larger inner cladding for quasi-confocal detection to increase throughput and signal-to-noise ratio. A balanced detection scheme was implemented to suppress the relative intensity noise of the SC source. The SLO produced dual wavelength, high-quality images at with a maximum 20 deg imaging field-of-view with any desired combination of wavelengths in the visible spectrum. We demonstrate SLO-based dual-wavelength oximetry in vessels down to in diameter. Reproducibility was demonstrated by performing three different imaging sessions of the same volunteer, 8 min apart. Finally, by performing a wavelength sweep between 485 and 608 nm, we determined, for our SLO geometry, an approximately linear relationship between the effective path length of photons through the blood vessels and the vessel diameter.

1 μJ nanosecond low-coherent laser source with precise temporal shaping and spectral control

A μJ-level low-coherent pulse with dynamic and precise temporal shaping and spectral control is demonstrated. Low-coherent light delivered by a superluminescent diode is precisely shaped through a wave guide amplitude modulator driven by an arbitrary waveform generator. Utilizing a three-stage axial-alternating polarization-maintaining single-mode fiber amplifier, μJ-level low-coherent pulses with 3 ns durations and 20 nm instantaneous bandwidths at a 1 kHz repetition rate are obtained. In addition, a spectral control unit is used to adjust the spectrum shape. As a result, near flat-top, saddle-type, or other required special spectra can be produced. The low temporal coherence characteristics and output stability are characterized experimentally.