Optical Power Efficiency Research Articles

Enhancing optical output power efficiency in nitride‐based green micro light‐emitting diodes by sidewall ion implantation

AbstractThough micro‐LED (light‐emitting diode) displays are considered the emerging display technology, the micron‐scale LED chip size encounters severe efficiency degradation that may impact the power budget of the displays. This work proposes an ion implantation method to deliberately create a high‐resistivity sidewall in the InGaN/GaN green LED. Our study demonstrates that ion implantation suppresses reverse leakage current due to the mitigation of sidewall defects. For an LED mesa size of 10 × 10 μm2, optical output power density is improved by 36.2% compared to the device without implantation. Compared to a larger 100 × 100 μm2 device without implantation, we achieve only 21.3% degradation of output power density under 10 A/cm2 injection for a 10 × 10 μm2 LED with implantation. In addition, the ion implantation method can lower the wavelength shift by reducing light emission from the region near the sidewall (where the amount of Indium [In] clustering differs from the mesa region). The results show promise in addressing the efficiency challenges of micro‐LED displays by selective ion implantation.

Dual polarization for efficient III-nitride-based deep ultraviolet micro-LEDs

The deep ultraviolet (DUV) micro-light emitting diode (μLED) has serious electron leakage and low hole injection efficiency. Meanwhile, with the decrease in the size of the LED chip, the plasma-assisted dry etching process will cause damage to the side wall of the mesa, which will form a carrier leakage channel and produce non-radiative recombination. All of these will reduce the photoelectric performance of μLED. To this end, this study introduces polarized bulk charges into the hole supply layer (p-HSL) and the electron supply layer (n-ESL) respectively (dual-polarized structure) of the DUV μLED at an emission wavelength of 279 nm to enhance the binding of carriers and increase the injection efficiency of carriers. This is because the polarization-induced bulk charge can shield the polarized sheet charge on the interface and reduce the polarization electric field. The reduced polarization electric field in p-HSL can increase the effective barrier height of the conduction band in the p-type region and reduce the effective barrier height of the valence band. The decrease in the polarized electric field of n-HSL can reduce the thermal velocity of electrons, thereby enhancing the electron injection efficiency, reducing the Shockley–Read–Hall (SRH) recombination, and increasing the effective barrier height of the valence band. The study results indicate that the electron concentration and hole concentration of a μLED with dual polarization were increased by 77.93% and 93.6%, respectively. The optical power and maximum external quantum efficiency of μLED reached 31.04 W/cm2 and 2.91% respectively, and the efficiency droop is only 2.06% at 120 A/cm2. These results provide a new approach to solving the problem of insufficient carrier injection and SRH recombination in high-performance DUV μLEDs.

Investigation of GaN-Based Micro-LEDs with Effective Tetramethylammonium Hydroxide Treatment

This study investigates the effects of TMAH treatment on 5 μm-sized GaN-based Micro LEDs. Compared with untreated GaN Micro LEDs, the optical output power and external quantum efficiency of TMAH treated Micro LEDs are significantly improved. These results can be attributed to the formation of microstructures on the sidewall of Micro LEDs through the TMAH treatment and the effective light reflection is therefore constructed. This research not only improves the characteristics of LEDs, but also paves the way for green and advanced optoelectronic devices.

Optical circuit compactification for ultracold atoms.

We develop a modular and compactified optical circuit for the generation of optical beams for cooling, imaging, and controlling ultracold atoms. One of the simplifications that is made in our circuit is to admix the repumping beams to each other optical beams in its dedicated single-mode fiber. We implement our design, characterize the output, and show that the optical power efficiency of the circuit is in the region of 97%, and after fiber coupling, the efficiencies are in the range of 62-85%. Given its compact design and controllable optical sources, this setup should be adaptable to a variety of quantum experiments based on ultracold gases.

MIMO Channel Measurement in Heterogeneous Networks

Cutting-edge technologies focusing on optical efficiency, data rates, and transmission power efficiency are propelling the evolution of multiple-input-multiple-output (MIMO) systems. These innovations facilitate higher aggregation rates, particularly beneficial in large MIMO generation environments. However, the fixed nature of MIMO results in a smaller channel shape in the spatial domain, limiting dynamic adoption in various 2D channels. Despite this constraint, the distinctive channel characteristics of MIMO present opportunities for significant improvements in 5G network capacity. This study explores and compares the channel nature of MIMO, shedding light on its potential to optimize network performance in heterogeneous environments, marking a noteworthy stride in the pursuit of enhanced wireless communication technologies.

A CA and ML approach for M-MIMO optical non-orthogonal multiple access power efficiency

Abstract The non-orthogonal multiple access (NOMA) multiple access approach can be used in future wireless communication systems to support massive connections and increase spectrum efficiency. Because user signal intensities and interference levels vary, precise channel assessment is essential in NOMA. Optimal power allocation and decoding order are made possible by precise algorithms, increasing system effectiveness and performance. However, NOMA can be adversely impacted by high peak-to-average power ratio (PAPR) values, leading to worsened system performance and more complex power amplifiers. In order to solve this issue, this paper recommends PAPR reduction in NOMA using companding methods for 512, 256, and 64 sub-carriers. Nonlinear companding techniques, such as MA and A-law companding, can efficiently reduce the high peak power of NOMA signals while reducing distortion and enhancing overall system dependability. The effectiveness of the proposed companding methods is evaluated using simulations, and the results demonstrate a significant decrease in PAPR, ensuring higher bit error rate (BER) effectiveness and transmission resilience in NOMA-based communication systems. The proposed approach is compared to the traditional Ml (C- Ml) and A-Law (C- A-Law).

Inverse design of omnidirectional coherent absorbers for optical power beaming applications.

An efficient photovoltaic power converter is a critical element in laser power beaming systems for maximizing the end-to-end power transfer efficiency while minimizing beam reflections from the receiver for safety considerations. We designed a multilayer absorber that can efficiently trap monochromatic light from broad incident angles. The proposed design is built on the concept of a one-way coherent absorber with inverse-designed aperiodic multilayer front- and back-reflectors that enable maximal optical absorption in a thin-film photovoltaic material for broad angles. We argue that the broad bandwidth is achieved through an optimization search process that automatically engineers the modal content of the cavity to create multiple overlapping resonant modes at the desired angle or frequency range. A realistic design is provided based on GaAs thin films with inverse-designed multilayer binary AlAs/AlGaAs mirrors. The proposed device can pave the way for efficient optical power beaming systems.

Optical vortex convolution generator and quasi-Talbot effect.

In this Letter, a simple optical vortex convolution generator is proposed where a microlens array (MLA) is utilized as an optical convolution device, and a focusing lens (FL) is employed to obtain the far field, which can convert a single optical vortex into a vortex array. Further, the optical field distribution on the focal plane of the FL is theoretically analyzed and experimentally verified using three MLAs of different sizes. Moreover, in the experiments, behind the FL, the self-imaging Talbot effect of the vortex array is also observed. Meanwhile, the generation of the high-order vortex array is also investigated. This method, with a simple structure and high optical power efficiency, can generate high spatial frequency vortex arrays using devices with low spatial frequency and has excellent application prospects in the field of optical tweezers, optical communication, optical processing, etc.

VCSEL quick fabrication of 894.6 nm wavelength epi‐material for miniature atomic clock applications

A vertical cavity surface emitting laser (VCSEL) quick fabrication (VQF) process is applied to epitaxial materials designed for miniature atomic clock applications (MACs). The process is used to assess material quality and uniformity of a full 100 mm (4-inch) wafer against the stringent target specification of VCSELs for MACs. Target specifications in optical power (>0.6 mW) and differential efficiencies (<0.5 W/A) are achieved over large portions of a wafer; however, the variation in the oxide aperture diameter is shown to limit the yield. The emission of the fundamental mode at 894.6 nm at 70 ℃ $^{\circ}\mathrm{C}$ is met over a significant area of the wafer for ∼4 μ $\mu $ m aperture multi-mode devices. The consideration of the on-wafer variation reveals that further optimisation is required to increase the device yield to levels required for volume manufacture.

Fabry-Pérot resonant avalanche-mode silicon LEDs for tunable narrow-band emission.

We report on the effect of Fabry-Pérot (FP) resonance on hot-carrier electroluminescence (EL) spectra and the optical power efficiencies of silicon (Si) avalanche-mode (AM) LEDs in the wavelength range from 500 nm to 950 nm. The LEDs, fabricated in a silicon-on-insulator photonics technology, consist of symmetric p-n junctions placed within a 0.21 µm thick Si micro-ring of varying width and radius. We show that the peak wavelength in the EL-spectra can be tuned within a range of 100 nm by varying the ring width from 0.16 µm to 0.30 µm, which is explained by FP resonance. The measured EL-spectra features relatively narrow bands (with a spectral width of ∼50 nm) with high intensities compared to conventional Si AMLEDs. By varying the ring radius and using a high doping level, we obtain a record high optical power efficiency of 3.2×10-5. Our work is a breakthrough in engineering the EL spectrum of Si, foreseen to benefit the performance of Si-integrated optical interconnects and sensors.

Constant-Envelope OFDM for Power-Efficient and Nonlinearity-Tolerant Heterodyne MMW-RoF System With Envelope Detection

The radio over fiber (RoF) system using optical heterodyne (OH) for broadband up-conversion and envelope detection (ED) for phase-noise-insensitive down-conversion stands out as a simple, robust, and cost-effective solution for exploiting the abundant bandwidth resource at the millimeter-wave (MMW) band. In this paper, we dedicate efforts to experimentally investigate such an MMW-RoF system based on the constant-envelope orthogonal frequency division multiplexing (OFDM) (CE-OFDM) technique. A critical benefit of employing CE-OFDM is its excellent tolerance to the nonlinear distortions, which are essential for the wide acceptance of analog RoF systems. Also, thanks to the analog angle modulation transforming the OFDM signal to a constant envelope signal, the carrier-suppressed double-sideband (CS-DSB) modulation can be implemented for encoding the CE-OFDM signal onto the optical carrier, leading to significant improvements in power efficiencies of both fiber and radiofrequency (RF) links. In experiments, a 60-GHz OH-ED MMW-RoF system is established, of which the distances of fiber and MMW wireless transmissions are 10 km and 1.2 m respectively. Under the case of 2-GHz 64-quadrature amplitude modulation (QAM) OFDM baseband signal, the achieved experimental results show that the proposed OH-ED MMW-RoF system outperforms its counterpart using the conventional OFDM in terms of nonlinearity insensitivity, RF and optical power efficiency enhancements (indicated by over 11.5-dB and 3.2-dB reductions in the lowest transmit RF and receive optical powers for satisfying the 8% error-vector-magnitude (EVM) limit). In addition, when the QAM order of the 2.5-GHz OFDM baseband signal reaches 256, the CS-DSB CE-OFDM scheme enables an acceptable EVM performance of 4.43%.

Lower threshold current density of GaN-based blue laser diodes by suppressing the nonradiative recombination in a multiple quantum well.

The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm2, and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.

A DFB Laser With Integrated Passive Region Suitable for PAM-4 Modulation Signal

Numerous studies have shown that the use of PAM-4 modulation format for DFB lasers with integrated passive regions (DFB IPE lasers) can greatly improve information transmission capabilities. In this paper, it is found by theoretical analysis that a passive waveguide can reduce the photon concentration and injection current in the cavity under the same output optical power. The DFB IPE laser has large saturated optical power and slope efficiency, which reduces the distortion of the electro-optical conversion of the PAM-4 signal. A DFB IPE laser was fabricated with a total cavity length and passive waveguide length of 130 um and 40 um, respectively. The saturation output power, slope efficiency, and modulation bandwidth of this laser are 23.5 mW (61 mA), 0.41 W/A, 22 GHz, respectively. The relative intensity noise is also below −145 dB/Hz. Besides, we have verified the high temperature working capability of this laser in a 40 Gbps PAM-4 signal back-to-back (BTB) transmission system. When the signal peak-to-peak voltage is 750 mV, a clear eye diagram with an eye height of 196 mV is obtained at 343 K.

High-performance diode-end-pumped Nd:YLF laser operating at 1314 nm.

A stable, efficient, and powerful 1314 nm Nd:YLF laser inband-pumped by a wavelength-locked narrowband 880 nm laser diode is demonstrated. The influence of mode-to-pump ratio on the performance of the diode-end-pumped Nd:YLF laser has been systematically investigated by taking into account the thermal effect and the energy transfer upconversion effect. For the optimum mode-to-pump ratio of 0.84, the maximum continuous wave output power of 21.9 W was extracted under the pump power of 70 W, which corresponded to the optical power efficiency of 31.3% and the beam quality of M2 ≈ 1.6. The resultant output power stability was determined to be 0.059% (RMS) within 1 h. In addition, by increasing the mode-to-pump ratio to 1.0, the near-diffraction-limited beam (M2 ≈ 1.3) was achieved with the output power of 17.0 W and the optical power efficiency of 24.3%.

9 Tb/s Transmission Using 29 mW Optical Pump Power Per EDFA With 1.24 Tb/s/W Optical Power Efficiency Over 15,050 km

We transmit 143x32.6 GBd 8D-QPSK signal over 15,050 km with EDFAs consuming 29 mW of 980 nm-pump power. We experimentally achieve record optical power efficiency from 6.5 to 1.24 Tb&#x002F;s&#x002F;W with the corresponding optimum spectral efficiency from 2.4 to 2.0 b&#x002F;s&#x002F;Hz for transmission distances ranging from 6,020 to 15,050 km respectively. These results are enabled by: precisely tuned information spectral efficiency and EDFA output power based on our power efficiency model to maximize optical power efficiency; the use of coded modulation format with &lt;1.6 dB gap to Shannon limit and variable overhead FEC; the use of ultra-low loss fiber; more power efficient gain equalization with a single gain flattening filter per two EDFAs; and low back-to-back implementation penalty. We also confirmed the validity of our power efficiency model comparing the designed and measured OSNR evolution, achieved optical power efficiency and the corresponding spectral efficiency.

Optimization of Power Efficient Spatial Division Multiplexed Submarine Cables Using Adaptive Transponders and Machine Learning

Historically, undersea systems capacity increases were primarily based on maximizing the total throughput per fiber. Whilst previously designed to operate at optimum launch power of the fiber, subsea systems are now designed for higher optical power efficiency. Next generation of spatial division multiplexed (SDM) cables require operation at lower signal power enabling repeater pump farming and providing higher reliability and efficiency. To adapt to these power constraints, the benefits of adaptive transponders and the application of machine learning to submarine line design are investigated.

Polarization Modulation at Last Quantum Barrier for High Efficiency AlGaN-Based UV LED

The performance of AlGaN-based light-emitting diodes (LEDs) emitting at UVA–UVC regions can be severely compromised due to the polarization difference (ΔP) between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this work, the different situations of the bandgap difference (ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) and ΔP of InAlN/AlGaN and AlGaN/AlGaN heterojunctions fully strained on GaN and AlN substrates are discussed. It shows that the InAlN/AlGaN heterojunctions could produce positive or negative sheet charges at the heterointerface under ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> >0, which could not be realized by the conventional AlGaN/AlGaN heterojunctions. To demonstrate and utilize the feature, the polarization-modulated InAlN LQBs with 0.14–0.16 indium compositions of 320 nm UVB LEDs are designed and investigated. It is observed that the InAlN LQBs could replace the conventional AlGaN LQB to improve electron confinement and hole injection by affecting effective barrier heights. By modulating the LQB/EBL polarization using InAlN, the proposed UV LED has a 32% enhancement in internal quantum efficiency and lower efficiency droop (from 16.9% to 0.7%) compared with the conventional one without modulation. The operation voltage at the same current also significantly decreases. The improvement of optical output power and wall plug efficiency at 60 mA in proposed structures are near 90% and 100%, respectively. This study provides a novel and highly effective methodology for development of high efficiency UV LEDs.

Intermedial annealing process applied during the growth of quantum wells and its influence on the performance of GaN-based laser diodes.

An intermedial annealing treatment is adopted during epitaxial growth of InGaN/GaN multiple quantum well (MQW) by the metal-organic chemical vapor deposition (MOCVD), which is employed after each GaN cap layer growth is finished. Optical power, threshold current and slope efficiency of GaN-based laser diodes is improved through an appropriate intermedial annealing process. A further investigation about the influence of annealing duration on the luminescence characteristics of light-emitting diodes and the surface topography evolution of single quantum well layers is conducted through the study of electroluminescence, temperature dependent photoluminescence and atomic force microscopy. It is found that the improvement of GaN-based laser diode is attributed to reduction of nonradiative recombination centers in MQW, which is due to a better interface quality between well and barrier layers after an intermedial annealing process.

DFT Spread-Optical Pulse Amplitude Modulation for Visible Light Communication Systems

DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been proposed in visible light communication (VLC) to overcome the limited modulation bandwidth of light emitting diode (LED). Due to the implementation of the inverse fast Fourier transform at the DCO-OFDM transmitter, DCO-OFDM suffers from its high peak-to-average power ratio (PAPR), which restricts its use in some VLC applications, especially where the optical power efficiency is a crucial requirement. That is because the LEDs used in VLC systems have a limited optical power-current linear range. To this end, a novel discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) signal scheme based on the single carrier-interleaved frequency division multiple access (SC-IFDMA) signal is introduced in this paper to address the high PAPR issue of OFDM. DFTS-OPAM is achieved by considering a PAM as an SC-IFDMA data symbol and duplicate the output vector of the fast Fourier transform at the SC-IFDMA transmitter side. Simulation results show that the PAPR of the proposed scheme is 7 dB lower than that of DCO-OFDM. Furthermore, this significant PAPR improvement is experimentally investigated where the practical results show that the proposed scheme can provide more 2.5 dB reduction in the average transmitted power requirement compared to DCO-OFDM and can subsequently increase the maximum achieved distance between the transmitter and the receiver up to 44%.

Contact Geometrical Study for Top Emitting 980 nm InGaAs/GaAsP Vertical-Cavity Surface Emitting Lasers

Geometrical contacts of a double mesa structure with 16 rows ×15 columns arrays of top emitting GaAs based 980 nm vertical cavity surface emitting lasers (VCSELs) are fabricated and characterized. In this paper, 5 strained In0.22Ga0.78As/Ga0.9AsP0.1 quantum wells (QWs) within λ/2 thick cavity have been employed. The top and the bottom epitaxially grown mirrors are based on the linear graded Al0.9Ga0.1As/GaAs distributed Bragg reflectors (DBRs) with 20.5 and 37 periods, respectively. Static parameters including threshold currents, rollover currents, maximum optical output power and wall-plug efficiency are extracted from light out power-current-voltage (LIV) of VCSELs with fixed oxide aperture diameter of ∅~ 6 μm and various mesa2 diameters. In addition, spectral emission for 980 nm VCSELs of oxide aperture between ∅~ 6 and 19 μm and with fixed ∅~ 6 μm and different bias currents are analyzed. The highest optical output power of around 33 dBm is observed at bias current of 0.8 mA for short−reach optical interconnect applications.