1. INTRODUCTION

Visible light communication (VLC) is an optical wireless communication technology whereby data is transmitted using visible light with wavelengths between 375 and 780 nm [1,2]. The benefits of the plentiful spectrum (400–800 THz) have attracted increasing interest in VLC research worldwide [1–5]. VLC can be considered as an essential component of next-generation 6G wireless networks [6]. VLC technology is characterized by electromagnetic immunity and high security and is license-free [7,8]. These characteristics are ideal for certain settings, including stadiums, hospitals, airplanes, and laboratories [7,9]. Therefore, VLC plays an important role in local networks where it is used to address the foreseeable limited radio frequency (RF) spectrum resources [10]. Due to its unique light penetration characteristics underwater, VLC not only facilitates underwater communication but also integrates seamlessly with wireless communication systems in space, enabling a unified communication network across both aquatic and aerial environments. To date, light-emitting diode (LED)-based VLC systems have demonstrated significant potential and benefits in several applications [11–15]. For instance, a record 7.91 Gbps data transmission rate based on a micro-LED was accomplished [16]. However, the rapid increase in the requirement for high data transmission rates has resulted in a greater demand for VLC systems [17]. Because the carrier lifetime of an LED is of the order of nanoseconds, the bandwidth of the LED always limits the data rate of an LED-based VLC system [18,19]. Compared with LED, the modulation response of a laser diode (LD) is controlled by the photon lifetime, which is at the picosecond level. Consequently, the LD exhibits a higher modulation bandwidth, which is more suitable for high-speed VLC [20–23]. Moreover, the LD exhibits improved optical directionality and optical spectrum purity, both of which help to extend the transmission distance in VLC systems.

High-speed gallium nitride (GaN)-based blue LDs are key devices for VLC. A GaN-based LD was developed after Nakamura demonstrated the first violet laser in 1996 [24]. With the development of the GaN growth technique and improvement of the p-doping level, GaN-based LDs have demonstrated suitability for applications including optical storage [25], bio-medical treatment [26], and high brightness lighting [27,28]. However, the design and fabrication of high-speed LDs for VLC are yet to be thoroughly investigated, and the modulation bandwidth for visible LDs is still far below that of InP-based and GaAs-based NIR LDs [29,30]. The progress of research into high-speed GaN-based LDs is presented in Table 1. The modulation bandwidth of the c-plane LD was still limited to 3 GHz, which did not satisfy the fast-growing demands for higher communication data rates. Low slope efficiency and high threshold current density remain issues that are to be resolved. Although an LD grown on a semipolar GaN substrate has a modulation bandwidth of over 5 GHz [32], problems such as costly substrate and immature material growth process remain. Therefore, it is critical to design and fabricate high-speed GaN-based blue LDs on c-plane substrates.

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

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Various approaches can be used to achieve high-speed LDs. Both active region and device structure design are critical for the improvement of modulation bandwidth in GaN-based LDs. For example, adopting quantum-dots (QDs) can enhance the electron–photon interactions as well as the coupling between the active region and the optical field, resulting in a reduced threshold current and increased differential gain [35,36]. Those features contribute to a higher resonance frequency. However, the low conversion efficiency and limited output optical power at high current densities hinder the application of GaN-based QD lasers [37]. The introduction of a Bragg grating along the ridge waveguide to develop a single-mode GaN-based distributed-feedback (DFB) LD helps improve mode stability and modulation response [38]. However, the complicated fabrication process hinders the realization of high-performance DFB gratings at blue color regime. Adopting semipolar GaN quantum wells (QWs) can effectively suppress the quantum confinement Stark effect (QCSE) and improve the recombination efficiency, but the growth of such high-quality epitaxial layers on c-plane substrates remains challenging [39]. Since the miniaturization of transistors and LEDs has yielded a significant improvement in device performance, the study of small form-factor GaN-based LDs has become another promising approach toward high-speed operation. Studies reveal that the structure of the ridge width and cavity length affect the dynamic properties and high-speed performance of NIR LDs [40]. Therefore, the dimensional scaling down in GaN-based LDs becomes an important topic to investigate.

In this study, we design and fabricate a blue GaN mini-LD with reduced cavity length from millimeter level to sub-millimeter (500 μm) and a narrow ridge width of 1.8 μm. The design principles of GaN mini-LD epi-structures concerning the modulation response are also investigated. The use of multilayer QW/quantum barriers (QBs) can improve the relaxation resonance frequency by increasing differential gain yet increasing the threshold current [32]. Our mini-LD features a double-QW active region with optimized QW/QB thickness. The total effective active region of the fabricated mini-LD is less than $0.001{\mathrm{mm}}^2$. The threshold current and the corresponding current density are 31 mA and $3.4\mathrm{kA}/{\mathrm{cm}}^2$, respectively. The slope efficiency of the single-mode mini-LD reaches 1.02 W/A. The $-3\mathrm{dB}$ modulation bandwidth is measured to be 5.9 GHz. Adopting this mini-LD in the VLC system allowed us to obtain a data transmission rate exceeding 20 Gbps. To the best of our knowledge, this is the highest data rate reported for a blue LD-based VLC system.

2. DEVICE DESIGN AND FABRICATION

To design a high-speed GaN-based LD, a dynamic response model is established to analyze the effect of the active region on the bandwidth. The transfer function can be extracted from the carrier-photon rate formula [41], which is expressed as [42]

Here, $f_R$ is the relaxation resonance frequency, $\gamma =K{f}_R^2+{\gamma }_0$ is the damping coefficient, and $K$ is a constant whose value indicates the damping effect. Parameter $f_R$ mainly depends on the differential gain, optical field volume, and injection efficiency at a given injection current level.

The optical and electronic properties of the LDs with different quantum well/barrier thicknesses were simulated using PICS3D. In the simulation, the spontaneous emission coefficient is set as $1\times {10}^{-4}$, and the screening coefficient is 0.35. The additional nonlinear gain suppression is not considered. Figure 1(a) shows LD with the 5 nm barrier is featured with the highest $f_R$ response and low ${\gamma }_0$ level when the quantum well thickness is 2 nm. Meanwhile, low-threshold currents appeared in structures with smaller barrier thicknesses, leading to a higher $f_R$. Figure 1(b) shows that the device with a 3 nm/5 nm active region exhibited a higher differential gain and $-3\mathrm{dB}$ bandwidth. Note that the comparison at a relatively low current is fair, avoiding the self-heating and saturation of $f_R$. Figure 1(c) suggests that the LD has an optical confinement factor of 0.0152. The simulated $-3\mathrm{dB}$ bandwidth can reach $\sim 6\mathrm{GHz}$ at 140 mA, as shown in Fig. 1(d).

 

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 $\mathrm{d}{f}_R/\mathrm{d}{(I-I_{\mathrm{th}})}^{1/2}$ 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. $\mathrm{\Gamma }$ 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 $-3\mathrm{dB}$ 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.

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Based on the simulation results, we design and fabricate the III-nitride mini-LDs. The devices were grown on a c-plane (0001) GaN substrate using the metal-organic chemical vapor deposition (MOCVD) technique. The epitaxial structure consists of a 3 μm Si-doped n-GaN contact layer ($[\mathrm{Si}]=2\times {10}^{18}{\mathrm{cm}}^{-3}$), a 1000 nm $\mathrm{n}\text{-}{\mathrm{Al}}_{0.07}{\mathrm{Ga}}_{0.93}\mathrm{N}$ cladding layer ($[\mathrm{Si}]=3\times {10}^{18}{\mathrm{cm}}^{-3}$), and a 150 nm $\mathrm{n}\text{-}{\mathrm{In}}_{0.03}{\mathrm{Ga}}_{0.97}\mathrm{N}$ waveguide layer ($[\mathrm{Si}]=2\times {10}^{18}{\mathrm{cm}}^{-3}$). The active region consists of two 3 nm ${\mathrm{In}}_{0.21}{\mathrm{Ga}}_{0.79}\mathrm{N}$ QWs separated by three 5 nm GaN quantum barriers, followed by a $\mathrm{p}\text{-}{\mathrm{In}}_{0.03}{\mathrm{Ga}}_{0.97}\mathrm{N}$ waveguide layer. Then, an 8 nm ${\mathrm{Al}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$ Mg-doped electron blocking layer (EBL) is grown on the topside ($[\mathrm{Mg}]=5\times {10}^{18}{\mathrm{cm}}^{-3}$) followed by a 300 nm $\mathrm{p}\text{-}{\mathrm{Al}}_{0.07}{\mathrm{Ga}}_{0.93}\mathrm{N}$ cladding layer. This is then followed by a 100 nm highly doped ${\mathrm{p}}^{+}\text{-}\mathrm{GaN}$ contact layer with a doping concentration of $1\times {10}^{19}{\mathrm{cm}}^{-3}$ optimized to reduce the series resistance ($[\mathrm{Mg}]=1\times {10}^{19}{\mathrm{cm}}^{-3}$). Transmission electron microscopy (TEM) images of the active region and energy-dispersive X-ray spectroscopy (EDS) mapping of the active region, including In and Al, are shown in Figs. 2(f) and 2(g), respectively. The epitaxial layer was etched into a ridge waveguide structure using lithography and an inductively coupled plasma (ICP) process. To achieve passivation and electrical isolation, 200 nm ${\mathrm{SiO}}_2$ was sputtered on the wafer. The epitaxial layers between the n-contact and p-contact layers were covered by the ${\mathrm{SiO}}_2$ layer from bottom to top. A self-alignment process was adopted to selectively remove the ${\mathrm{SiO}}_2$ layer from the ridge. Pd/Ti/Pt/Ti/Au and Ti/Pt/Ti/Au were deposited as the p- and n-GaN metal electrodes, respectively. The LD resonance cavity was formed by mechanical cleavage. Facet coatings with pairs of ${\mathrm{SiO}}_2/{\mathrm{TiO}}_2$ were deposited on the front and rear facets to achieve reflectivities of 70% and 99%, respectively.

 

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 $\sim 1.8\mathrm{\mu m}$. (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).

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3. EXPERIMENTAL RESULTS AND DISCUSSIONS

The light–current–voltage (L–I–V) characteristics of the fabricated mini-LDs are shown in Fig. 3(a). The threshold current density of the LD was $3.4\mathrm{kA}/{\mathrm{cm}}^2$ and the slope efficiency was 1.02 W/A. Continuous (CW) measurements were performed using a Keithley 2450 source measure unit (SMU) and a Newport 2936-R with a calibrated 818-SL Si detector as the power meter. Compared to other reports on GaN laser diodes for VLC, our mini-LD shows a lower threshold current density and higher slope efficiency [17,31,34,43]. The electrical luminescence spectra of the blue LD were collected using a high-resolution optical spectrum analyzer (Advantest Q8347), and the results are shown in Fig. 3(b). The measurement was performed at room temperature with a pulsed injection current (PL202 sourcemeter). The pulse width is 200 μs and the duty cycle is set at 1% to eliminate the effects of self-heating. The peak wavelength was 451 nm for injection currents in the range of 60–140 mA. The peak wavelength of the chip remained constant during the testing and a single longitudinal mode was observed from the fabricated mini-LD.

 

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.

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The small-signal modulation characteristics of mini-LDs were investigated experimentally. Prior to the ${\mathit{S}}_{21}$ test, we adopted the short-load-open-through (SLOT) method using an MPI AC2 calibration board to calibrate the entire experimental setup to the tips of the GS probe (MPI TITAN-T26-GS150). Sinusoidal small-signal modulation was performed using an Agilent N5230C PNA-L network analyzer. A 10 GHz photodetector (PD, Newport 818-BB-45A) was used as the receiver.

The measured small-signal modulation response of the entire system $S_{21}^M$ can be expressed as

Here, $S_{21}^L$ and $R$ refer to the electrical-optical and optical-electrical modulation responses of the laser and PD, respectively. Since the $-3\mathrm{dB}$ modulation bandwidth of the PD was 10 GHz and above, we considered $R$ to be constant. Therefore, $S_{21}^M$ revealed the modulation characteristics of the laser when $S_{21}^M<10\mathrm{GHz}$. The $S_{21}^L$ for different injection currents are shown in Fig. 4(a). To extract the intrinsic ${\mathit{S}}_{21}$ response from the original response, we established an equivalent circuit model of laser diodes as follows:

$\eta$ is the current modulation response coefficient of the intrinsic laser, $Z_{\mathit{r}}$ is the specific impedance, and $S_{22}^P$ and $S_{21}^P$ are the scattering parameters of parasitic part in cascaded networks. The component values of the circuit can be obtained from the ${{\mathit{S}}_{11}}_{\mathrm{}}^{}$ results, as shown in Fig. 4(b). The intrinsic modulation characteristics of the LD through the de-embedding of the parasitic network response are shown in Fig. 4(c). The results showed that the $-3\mathrm{dB}$ bandwidth of the laser chips increased with the injection current from 100 to 140 mA. Further increasing the injection current led to a significant degradation in the modulation characteristics, which was caused by a strong nonlinear gain with a self-heating effect. The highest modulation bandwidth achieved was 5.90 GHz. To the best of our knowledge, this is the highest $-3\mathrm{dB}$ modulation bandwidth for c-GaN laser diodes. The plot of extracted $-3$ and $-10\mathrm{dB}$ modulation bandwidth versus injection current is shown in Fig. 4(d). The results revealed that the nonlinear effect was not significant at 140 mA.

 

Fig. 4. (a) Measured frequency response of the fabricated mini-LD for injection currents ranging from 100 to 140 mA. (b) ${\mathit{S}}_{11}$ response for injection currents ranging from 100 to 140 mA. (c) Extracted intrinsic ${\mathit{S}}_{21}$ response for injection currents ranging from 100 to 140 mA. The $-3$ and $-10\mathrm{dB}$ are labelled in the figure using dashed lines. (d) Extracted intrinsic modulation bandwidth ($-3$ and $-10\mathrm{dB}$) versus injection currents ranging from 70 to 140 mA. (e) Relationship between resonance frequency ($f_R$) and ${(I-I_{\mathrm{th}})}^{1/2}$, where $I$ is the injection current and $I_{\mathrm{th}}$ is the threshold current. The line in this figure is the fitting curve. (f) Relationship between square of resonance frequency ($f_R$) and the damping factor. The line in this figure is the fitting curve. $K$ is the slope of the curve, and ${\gamma }_0$ is the intercept on the $Y$ axis.

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We further explore the relationship between the resonance frequency and the injection current:

To characterize the differential gain of the laser, we investigated the relationship between the resonance frequency and ${(I-I_{\mathrm{th}})}^{1/2}$. In Eq. (4), $f_R$ is the resonance frequency, $I$ is the injection current, $I_{\mathrm{th}}$ is the threshold current, $\mathrm{\Gamma }$ is the confinement factor, ${\upsilon }_g$ is the group velocity, $\frac{\mathrm{d}g}{\mathrm{d}n}$ is the differential gain, $q$ is the elemental charge, $V$ is the volume of the active region, and ${\eta }_i$ is the injection efficiency. The square root of the driving current above the threshold is linearly proportional to the resonance frequency ($f_R$). For the fabricated LD, the $\mathrm{d}{f}_R/\mathrm{d}{(I-I_{\mathrm{th}})}^{1/2}$ is obtained as $0.32\mathrm{GHz}/{\mathrm{mA}}^{1/2}$ as seen in Fig. 4(e), which was relatively high and close to the simulation results. The extracted damping factors of the LD are shown in Fig. 4(f).

The relationship between the damping factor and the resonance frequency can be expressed as

To characterize the communication performance of the LD, an experimental VLC system was established, as shown in Fig. 5. Figure 5(a) shows a schematic of the transmitter end, where discrete multitone (DMT) modulation with a bit-power loading scheme is employed [45,46]. The generated modulated signal [$x(t)$] can be expressed as

 

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.

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The transmitted signal is generated using an arbitrary waveform generator. The signal is then amplified by an electronic amplifier (EA) and sent to the bias tee, where direct current is coupled with the amplified signal. Finally, the merged signal is directly imposed on the laser. Figure 5(b) shows an image of the communication test system. The laser chip is fixed to a heat sink equipped with a thermoelectric cooler (TEC). Light is collected through the objective and delivered to the PD. The lower-right corner of Fig. 5(c) shows the system receiver. The transmitted light signal is converted to an electrical signal by the PD and transmitted to an oscilloscope (OSC). The bit and power loading signals received from the oscilloscope are processed by post-equalization and demodulation of the DMT model. A 4 GHz bandwidth is used to realize a high data rate of 20.06 Gbps. The measured SNR and allocated bits in each subcarrier channel are shown in Fig. 6(a). The obtained constellation diagram in Fig. 6(b) shows a clear profile. The transmitted and received partial symbols exhibit a high degree of overlap. The measured bit error ratio (BER) is 0.0030, which satisfies the threshold for forward error correction (FEC) coding (0.0038), as illustrated in Fig. 6(c).

 

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.

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4. CONCLUSIONS

In this study, we demonstrated a c-plane GaN-based blue mini-LD with a high $-3\mathrm{dB}$ modulation bandwidth of 5.9 GHz. The fabricated laser chip is characterized by a relatively low threshold current density of $3.4\mathrm{kA}/{\mathrm{cm}}^2$ at room temperature and a relatively high slope efficiency of 1.02 W/A. The device shows a K factor of 0.395 ns and ${\gamma }_0$ of 0.20 GHz. Using this mini-LD as the transmitter, we developed a blue-laser-based VLC system that demonstrated a record data rate of 20.06 Gbps. The system currently utilizes only 4 GHz of the mini-LD’s bandwidth owing to the hardware limitations of our testing system. Exploring avenues for achieving higher data rates remains an area for further work. Nonetheless, our work reveals that this high-performance GaN blue mini-LD has great potential for applications in next-generation 6G wireless optical communication systems as well as photonic integrated chips in the visible color regime.