Fig. 1. Design and simulation of the high-speed GaN mini-LD (with a ridge width of 1.8 μm and cavity length 500 μm). (a) Simulated dynamic parameters dfR/d(I−Ith)1/2 curve slope efficiency (red) and optical confinement factor (blue) of the devices with various quantum barrier thickness and 2 nm quantum well. (b) Histogram of simulated differential gain (red) and −3 dB bandwidth for an injection current of 140 mA, various quantum well thicknesses, and a 5 nm quantum barrier. (c) Simulated light field profile of quasi-TE fundamental mode in LD with 3 nm/5 nm active region. A ridge of 1.8 μm width is visualized in the white frame. Γ is the optical confinement factor. (d) Simulated frequency response under injection currents ranging from 110 to 140 mA of a 3 nm/5 nm active region structure. The dashed line in the figure is the −3  dB line related to the measurement start point (100 MHz). Damping behavior at the low frequency marked by the blue dashed circle is related to the RC (resistance-capacitance) roll-off.

Fig. 2. Macroscopic and microscopic structures of the laser. (a) 3D illustration of the fabricated laser. The annotation on the right shows the epitaxial layer structure of the active region from top to bottom. (b) Far-field emission pattern of the laser. (c) Optical microscopy image of the fabricated laser. The n- and p-electrodes are marked in the picture. (d) Scanning electron microscope (SEM) image of cross-sectional view. The ridge waveguide width is ∼1.8  μm. (e) STEM image of the active region. From top to bottom are the p-cladding layer, electron blocking layer (EBL), upper waveguide, and MQWs. (f) Indium mapping of the active region. The two high-brightness lines (marked with the yellow triangle) are quantum wells separated by a quantum barrier. (g) Aluminum mapping of active region. The line with high brightness (marked with the yellow triangle) is the EBL (electron blocking layer).

Fig. 3. (a) Light–current–voltage (L–I–V) characteristic of the laser under the condition of continuous wave (CW) injection. (b) Spectra of the mini-LD for injection currents ranging from 60 to 140 mA at room temperature.

Fig. 4. (a) Measured frequency response of the fabricated mini-LD for injection currents ranging from 100 to 140 mA. (b) S11 response for injection currents ranging from 100 to 140 mA. (c) Extracted intrinsic S21 response for injection currents ranging from 100 to 140 mA. The −3 and −10  dB are labelled in the figure using dashed lines. (d) Extracted intrinsic modulation bandwidth (−3 and −10  dB) versus injection currents ranging from 70 to 140 mA. (e) Relationship between resonance frequency (fR) and (I−Ith)1/2, where I is the injection current and Ith is the threshold current. The line in this figure is the fitting curve. (f) Relationship between square of resonance frequency (fR) and the damping factor. The line in this figure is the fitting curve. K is the slope of the curve, and γ0 is the intercept on the Y axis.

Fig. 5. (a) Transmitter in the system using the discrete multitone (DMT) bit-power loading modulation method. The digital signal is processed through bit-power loading, DMT modulation, and digital pre-equalization. AWG is the arbitrary waveform generator. EA is the electronic amplifier. The generated signal is combined with the direct current (D.C.) of the bias and then transmitted to the mini-LD. (b) Experimental setup of the high-bandwidth mini-LD-based high-speed VLC system with DMT bit-power loading modulation. The mini-LD is equipped with a heat sink with the TEC to ensure constant temperature (20°C) and the transmitted signal is collected by the PD. (c) The receiver in the system which contains the post-processing of the signal. PD is the photodetector, OSC is the oscilloscope, LMS Volterra is the least mean square Volterra nonlinear filter, and ZF Equ. is the zero-forcing equalizer. After the post-processing, we obtained the transmitted signal.

Fig. 6. Experimental results of the VLC system using fabricated mini-LD. (a) Bit-power loading scheme of each subcarrier according to measured channel SNR. The effective carrier number is 504. (b) Received constellation diagrams of the system, including 128 QAM, 64 QAM, 32 QAM, 16 QAM, 8 QAM, 4 QAM, and 2 QAM. (c) Partial T/R symbol of the mini-LD-based VLC data links. The tested BER is 0.0030.

Table 1. Summary of the Key Parameters for High-Speed Blue GaN LDs^a