For maximum fibre-coupled power, high power broad area diode lasers must operate with small lateral far field angles at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. In order to experimentally determine the origin of the low beam quality, spectrally resolved near and far field measurements were performed for a diode laser with 50 µm stripe width. Within the range measured (CW optical output powers to 1.5 W) the laser is shown to operate in just six stable lateral modes, with spatially periodic profiles. Comparisons of the measured profiles with the results of two-dimensional modal simulation demonstrate that current-induced thermal lensing dominates the lateral waveguiding, in spite of the presence of both strong built-in index guiding and gain guiding. No evidence is seen for filamentation. Building on the diagnosis, proposals are presented for improvements to beam quality.

https://iopscience.iop.org/article/10.1088/0268-1242/27/4/045001/pdf

Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers

When active regions that use low refractive index quantum barriers (LIQB) are combined with super large optical cavity (SLOC) designs in GaAs-based diode lasers, high-power operation with extremely narrow vertical far-fields is observed. However, LIQB designs are found to have lower slope efficiency and increased operation voltage. Comparison of experiment and finite element device simulation shows that this is due to hole accumulation at the edge of the active region. Example devices using an 8.6-μm thick SLOC deliver 30 W at 1065 nm with vertical divergence of 15.6° (95% power).

Combination of Low-Index Quantum Barrier and Super Large Optical Cavity Designs for Ultranarrow Vertical Far-Fields From High-Power Broad-Area Lasers

Diode lasers generating optical pulses with high peak power and lengths in the nanosecond range are key components of systems for free-space communication, metrology, material processing, spectroscopy, and light detection and ranging (LiDAR) as needed for object detection and autonomous driving. Automotive LiDAR systems demand additionally a good beam quality and low wavelength shift with temperature due to the wide operating temperature span. We present here internally wavelength stabilized lasers emitting ns optical pulses from an emission aperture between 30 μm and 100 μm with peak powers of tens of Watts at wavelengths around 905 nm. The vertical structure based on AlGaAs (confinement and cladding layers) and InGaAs (active quantum well) is especially optimized for pulsed operation with respect to the implementation of a surface Bragg grating with a high reflectivity. The fabricated 6 mm long distributed Bragg reflector (DBR) broad area (BA) lasers are electrically driven by an in-house developed high-speed unit generating 3 to 10 ns long nearly rectangular shaped current pulses with amplitudes of up to 250 A. Such lasers emit optical pulses with a peak power of more than 30 W at ~95 A pulse current up to a temperature of 85°C with a wavelength shift as low as 65 pm/K and a lateral beam propagation factor less than 10. The influence of the lateral aperture width and the pulse length on the beam quality will be shown. A monolithic integration of 3 DBR BA lasers on a single chip whose emission can be combined into a single beam raises the output power to more than 100 W.

Wavelength-Stabilized High-Pulse-Power Laser Diodes for Automotive LiDAR

Semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with a broadened waveguide and emitting in the wavelength range 1.0 – 1.1 μm are studied. It is found that the intensity of spontaneous emission from the active region increases with increasing pump current above the lasing threshold and that this is caused by a growth in the concentration of charge carriers in the active region due to the modal gain enhancement needed to compensate for the growing internal optical loss at high pulsed pump currents. It is shown that the increase in the internal optical loss with increasing pulsed pump current is one of the main reasons for saturation of the light – current characteristics of high-power semiconductor lasers.

Saturation of light – current characteristics of high-power lasers (λ = 1.0 – 1.1 mm) in pulsed regime

A root-cause analysis of thermal power saturation in broad area diode lasers is presented. Thermal power saturation limits in largest parts the optical power in continuous wave (CW) driven diode lasers, as junction temperature increases with bias. Accordingly, gain decreases and, hence, the carrier density in the quantum well increases (nonpinning) to compensate the low gain and the additional photon loss. A systematic variation of epitaxial designs was used to experimentally clarify the impact of modal gain on the temperature dependence of differential internal efficiency and optical loss. To account for this effect in simulation, a rate equation model was applied to determine temperature dependent slope efficiency. Longitudinal spatial hole burning (LSHB) causes an asymmetry of the carrier density profile, leading to high nonlinear carrier loss at the back facet, but also the average carrier density increases with temperature, enhancing the impact of LSHB. This enhancement effect leads to rapid degradation of internal differential efficiency with temperature and, hence, early power saturation. The extreme triple asymmetric vertical design is introduced, that enables devices to be realized that show minimized degradation of internal differential efficiency with temperature and therefore increased performance in CW operation.

Impact of Carrier Nonpinning Effect on Thermal Power Saturation in GaAs-Based High Power Diode Lasers

Many physical effects can potentially limit the peak achievable output power of single emitter broad area diode lasers under high current, pulse-pumped operation conditions. Although previous studies have shown reliable operation to high pump levels (240 A, 300 ns, and 1 kHz), power was found to saturate. We present here results of a systematic study to unambiguously determine the sources of this power saturation. A combination of detailed measurements and finite element device simulation were used for the diagnosis. We find that the measured power saturation is dominated by electron leakage caused by band bending at high bias due to the low mobility of the p-type waveguide. However, the power saturation is only fully reproduced when longitudinal spatial hole-burning is included. Higher powers are expected to be achieved if higher energy barriers and lower confinement factors are used to mitigate leakage and longitudinal hole-burning, respectively.

Root-Cause Analysis of Peak Power Saturation in Pulse-Pumped 1100 nm Broad Area Single Emitter Diode Lasers

High power, high beam quality and narrow, stable spectra are achieved simultaneously using a truncated-tapered optical amplifier in a master-oscillator power amplifier-system. We compare the influence of lateral geometric design on amplifier performance, by using devices with super large optical waveguide (4.8 μm) and relative low confinement factor (Γ = 1%) . We find that the use of an amplifier with a larger active region results in both high power and high beam quality. An abandonment of cavity spoiling grooves leads to strongly improved beam characteristics.

17-W Near-Diffraction-Limited 970-nm Output From a Tapered Semiconductor Optical Amplifier

As an important part of power development planning, power load forecasting is an important prerequisite for power generation, transmission and reasonable distribution of electric energy, and improving the accuracy of load forecasting is an important means to guarantee the stable operation of the power grid and improve the economic benefits of the power system. Aiming at short-term power load forecasting, this paper used the Autoregressive Integrated Moving Average (ARIMA) model to forecast the power load time series data, and combined the climate data, the main influencing factor of power load, to optimize the ARIMA residual by Long Short Term Memory (LSTM) model. A combined forecasting model based on ARIMA-LSTM was proposed by fully combining the advantages of both models. Taking the power load in Europe as the prediction object, the results showed that the root mean square error (RMSE) of the combined model was only 0.7835 MW, and the mean absolute percentage error (MAPE) was only 0.06 %. Compared with the LSTM model and ARIMA model, RMSE had been reduced by 91.18 % and 98.48 %, respectively. The combined model has improved the prediction accuracy and had specific practical application value.

Xiaozhuo Wang

Papers

Root-Cause Analysis of Peak Power Saturation in Pulse-Pumped 1100 nm Broad Area Single Emitter Diode Lasers

17-W Near-Diffraction-Limited 970-nm Output From a Tapered Semiconductor Optical Amplifier

Research on short-term power load forecasting based on ARIMA-LSTM model

Research on thermal conductivity and electrical insulation properties of epoxy composites with fluorescent grafted zero-dimensional Al2O3