As the demands for higher data rates, lower latency, and ubiquitous connectivity continue to escalate, researchers and industries are intensifying their efforts to develop next-generation communication technologies capable of meeting these evolving requirements. Visible light communication (VLC) has emerged as a promising candidate for future wireless networks, and its unique blend of advantages ideally suited for the demands of 6 G [1,2]. By harnessing the vast bandwidth of the visible light spectrum, VLC facilitates high-speed data transmission while alleviating spectrum congestion challenges encountered in traditional radio frequency systems [3]. Furthermore, the compatibility of VLC with existing lighting infrastructure presents opportunities for energy-efficient dual-functionality deployments, enhancing sustainability and reducing operational costs [4].

Light emitting diodes (LEDs) are commonly used as light sources for daily illumination, emitting light that is relatively diffuse. Therefore, LED-based VLC holds the promise of supporting large-scale data transmission and coverage. Since the LED light sources could possess a certain degree of divergence angle, attention has been directed toward addressing receiver angles. In 2016, Manousiadis et al. designed a fluorescent antenna with a field of view (FOV) semi-angle of 60° [5]. Utilizing this fluorescent concentrator and a blue micro-LED, Quintana et al. achieved a data link of 300 Mbps at 2 m within 45° [6]. Nabavi et al. prototyped a VLC receiver with a multi-photodetector array realizing a 360° FOV in a white phosphorous LED VLC system [7]. LEDs offer wide coverage while the transmitting rate and distance are limited.

In contrast, laser diodes (LDs) present distinct advantages over LEDs in various lighting and communication scenarios, including high luminous flux, compact size, and large modulation bandwidth [8]. BMW has developed laser-based headlight to support long distance illumination [9]. Consequently, visible laser light communication (VLLC) systems have attracted significant scholarly attention as researchers seek to leverage the unique capabilities of LDs for higher data rates and facilitating substantial enhancements in both power efficiency and transmission distances.

Research into wide-FOV VLLC has recently gained momentum over the past few years. Similar to LED-based systems, enlarging the receiving aperture can effectively expand the transmission FOV. Yu et al. proposed a sandwich structure light-trapping fluorescence antenna to enhance the FOV and signal-to-noise ratio (SNR) of a vehicle-to-vehicle VLC system [10]. Additionally, Alkhazragi et al. employed fused fiber-optic tapers (FFOTs), which consist of hundreds of thousands of tapered optical fibers, enabling Gbps data transmission within a ±30° acceptance angle [11]. By utilizing a fisheye lens with a ± 90° FOV at the receiver, along with a mobile scanning device (MSD) to track variations in imaging position, Hua et al. realized a data rate of 400 Mbps in a 7-m tap water channel [12]. In our previous work, we have also conducted research into wide-FOV VLLC. We employed a fish-eye lens and achieved over 3 Gbps transmitting data rate in 34° coverage [13]. By substituting a silicon photomultiplier (SiPM) at the receiver end, the coverage of the VLC system can be further expanded to 180°, with over 500 Mbps data transmitting rate [14].

Simultaneously, dealing with the small divergent angle of LD is also a crucial challenge to overcome, and expanding the radiation area of LD is a solution. Chi et al. demonstrated a white light phosphorous diffuser to diverge the beam spot, and the system can transmit a 5.2 Gbps orthogonal frequency-division multiplexing (OFDM) signal over a 0.6 m free-space link [15]. The structure of combining different optical diffusers and LDs in both air and water channels was verified to ensure a Gigabit-bit level high data rate while obtaining ±12° coverage [16]. In the listed works, most wide-FOV receiving solutions are structurally complex and fail to meet the requirements of mobile applications. The transmitting side does not fully leverage the advantages of lasers in long distance and high capacity. Therefore, there is a fundamental need for enhancements in high-performance light sources to meet the practical application requirements of future VLC in mobile scenarios integrated with lighting.

In this work, we designed a laser-based white light transmitter with wide coverage, high luminance, and stability. It utilizes a high-power blue laser diode to excite yellow phosphor, thereby generating white light. This emission structure can be employed in VLC systems. We examine both the illumination parameters and communication performance of such systems. The FOV of this system is approximately 40°, and at a distance of 5 m, the maximum communication rate can exceed 2.5 Gbps. This research not only demonstrates the feasibility of utilizing white light emitter based on laser for VLC applications and solid-state lighting but also underscores their potential to significantly advance the development of the next generation optical wireless communication technology.

The design of the wide-angle white light transmitter is illustrated in Fig. 1. We utilize a high-power blue laser array, consisting of 14 InGaN/GaN quantum well-based laser diodes, on a heatsink to address thermal management issues. Above the laser array, an engineered diffuser is placed to scatter multiple laser beams into a uniform wide-angle light source, thus reducing the laser safety associated concerns. The phosphor, inclined at an angle of 45°, allows blue light to pass through and emit white light.

 

Fig. 1. Wide-coverage laser-based white light source structure design.

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Three lenses, each with a diameter of 25 mm, are positioned respectively to the left, above, and right of the phosphor. The lenses on the left and above adjust the direction of the light beams, directing those that pass directly through the phosphor, and those that are reflected to the left redirect toward the right. The lens on the right, meanwhile, functions to further homogenize the light source and broaden the emission angle. All components are enclosed within a black box with an emission window on the right, forming our transmitter structure.

Figure 2 illustrates the experimental setup employed in the study. The transmission signal is generated offline via MATLAB coding and then sent to an arbitrary waveform generator (AWG, Keysight M8190A) to produce the electrical signal. This waveform is then amplified by an electric amplifier (Mini-Circuits ZHL-5W-1+) and subsequently coupled with a direct current signal through a bias-tee (Mini-Circuits ZFBT-282-1.5A+), driving the laser diode for emission. Following transmission through free-space over a certain distance, the light signal reaches the receiving end. It should be acknowledged that the effective operating frequency range of the amplifier is specified as 5–500 MHz, which may impose certain limitations on the system’s performance to some extent.

 

Fig. 2. Experimental setup includes signal processing, communication flow, and photos of the experimental equipment: (i) white light transmitter; (ii) emission pattern from the transmitter; (iii) receiving end of the system; (iv) VLC system in a lab testbed.

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At the receiving end, the light signal is first focused through a lens and then filtered to extract the blue light using a filter (LBTEK MBF10-450-25). The filtered light is then received by a silicon-based avalanche photodetector (APD), which converts the received optical signal into an electrical signal. This electrical signal is sampled by an oscilloscope (Keysight MSO9404A). The sampled data is then sent back to a computer for offline digital signal processing (DSP) and bit error rate (BER) calculation.

Discrete multi-tone (DMT) bit-loading modulation is used to achieve high spectral efficiency. In this modulation scheme, the entire bandwidth is divided into 256 subcarriers, and the SNR is computed for each subcarrier. Based on varying SNR levels, each subcarrier is allocated an appropriate Quadrature Amplitude Modulation (QAM) order, so that a high data transmission rate can be achieved.

Figure 3(a) presents the electroluminescence (EL) spectrum and chromaticity coordinates of the white light based on a laser. The light emission process involves a blue laser diode with a central wavelength of around 450 nm, which undergoes color conversion through yellow phosphor glass, resulting in the emission of white light.

 

Fig. 3. (a) Spectrum and chromaticity coordinates of the laser-based white light we employ in this work. (b) Variation of the correlated color temperature (CCT) and color rendering index (CRI) with varying injection currents.

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Given the diverse operational scenarios encountered with high-power lasers, we conducted an examination of the correlated color temperature (CCT) and color rendering index (CRI) stability across different injection current levels. Measurements were taken when the operation current is above the threshold of the laser, varying the current from 700 to 1500 mA. As illustrated in Fig. 3(b), the numerical values of both parameters exhibited remarkable stability over this current range.

Specifically, the CCT remained consistent at approximately 8300 K, indicative of a cool white light appearance, and CRI around 65 across the tested current range. Both fluctuations are within 5%.

Then, we conducted FOV testing on the system to assess its angular coverage capabilities. The testing methodology is depicted in Fig. 4(a). Firstly, we established a standardized measurement distance of 1 m between the transmitter and receiver. The light source was facing the receiver, and the receiver was fixed in place to maintain consistency throughout the testing process. While maintaining the position of the transmitter fixed, we systematically rotated its orientation. This rotation allowed us to explore different angular positions relative to the receiver. The angle formed between the direction of the light source and the line connecting the transmitter and receiver served as the measurement position’s angle. We define the clockwise direction as positive and the counterclockwise direction as negative.

 

Fig. 4. (a) Schematic diagram of the field-of-view testing method. (b) Testing results of illuminance at 1, 2, 3, and 5 m.

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To further examine the system’s FOV characteristics, we varied the distance between the transmitter and receiver. Measurements were conducted at distances of 2, 3, and 5 m, following the same testing procedure outlined above. Varying both the angular orientation and distance between the transmitter and receiver, we were able to comprehensively assess the system’s FOV across different spatial configurations.

The recorded illuminance measurements at various angular positions during the FOV testing are plotted in Fig. 4(b). It is obvious that the illuminance is maximized at an angle of 0°, with a gradual decrease as the angles on either side increase. At a distance of 1 m, the illuminance at the central angle exceeds 4500 lx. At around ±20°, the illuminance is about half of its peak value. At 2 m, the central illuminance approaches 1000 lx, while at 3 m it is 570 lx, meeting the standards for classroom blackboards and precision industrial lighting. At 5 m, the illuminance measures 140 lx.

Modulation bandwidth is a critical parameter in VLC systems. From the measured frequency response of the white light transmitter without an optical filter, the −3 and −10 dB bandwidth is 2.6 and 7.7 MHz, separately. Due to the influence of phosphors, the frequency response of white light can be affected. We compared the normalized S21 response with and without the filter at 1200 mA, as depicted in Fig. 5(a). It is evident that after filtering out the phosphor converted light, there is a notable improvement in the optical source characteristics. The −3 dB bandwidth increases to 42.8 MHz, and the −10 dB bandwidth rises to 534.5 MHz. Based on these results, we observe that incorporating optical filtering at the receiver to mitigate the effects of fluorescence conversion processes can enhance system performance.

 

Fig. 5. (a) Normalized frequency response of the laser-based VLC system, with and without filter in the receiver end. The driving current of the laser transmitter is set at 1200 mA. (b) Measured data rate versus angle of the system at 1, 2, 3, and 5 m. (c) Bit allocation and SNR of each subcarrier measured in the VLC link at 5 m. The data rate is 2.5 Gbps. Insets are the corresponding constellation diagrams of 2-QAM, 4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, and 128-QAM.

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After completing the setup of the experimental system, our initial step was to search for the optimal operating point of the light source, including both the injection current and signal amplitude. This was crucial to determine the best parameter settings for the light source in terms of performance. Through this process, we aimed to optimize the output of the light source to achieve a more efficient and stable operating state. The injection current was varied from 1000 to 1300 mA, while the signal V_PP ranged from 0.5 to 1.1 V. Ultimately, after thorough experimentation, we determined that an injection current of 1200 mA combined with a signal V_PP of 0.6 V provided the optimal performance.

We conducted tests on the variation of the data rate with an angle, using the same methodology as the FOV measurement, and the results are shown in Fig. 5(b). Different colors represent different transmission distances from the transmitter to the receiver, with all results achieving a BER below the 7% forward error correction (FEC) threshold (3.8 × 10⁻³). The selected range for testing, spanning from 1 to 5 m, covers the majority of indoor scenarios. Clearly, at each transmission distance, the peak data rate occurs at the middle angle. The highest data rates at distances of 1, 2, 3, and 5 m are 3.24, 2.99, 2.66, and 2.52 Gbps, respectively. On either side of the peak, as the angle increases, the corresponding decrease in data rate aligns with the illuminance distribution we measured, as depicted in Fig. 4(b). At a transmission distance of 1 meter, the maximum data rate can reach 3.2 Gbps, maintaining above 3 Gbps within 16° and remains above 2.5 Gbps within the range of 36°. Thus, high-speed and stable data transmission is ensured within most of the light scattering range. Similarly, at a transmission distance of 5 m, the maximum data rate also reaches 2.5 Gbps. Within a 40° range, the illuminating area reaches approximately 10 m², and the data rate stays above 1.5 Gbps.

The SNR and bit allocation for each subcarrier at peak data rate at 5 m are illustrated in Fig. 5(c). The red data represents the SNR measured on each subcarrier, while the gray data indicates the number of bits allocated per symbol transmission based on the SNR. The actual utilized bandwidth is 575 MHz, corresponding to the −10 dB bandwidth of the filter depicted in Fig. 5(a). The actual received constellation diagram is inserted in Fig. 5(c), where the constellation points are clearly distinguishable. It can be observed that this wireless optical communication channel can support up to 128-QAM modulation, namely each symbol contains 7 bits, fully leveraging its advantage as a laser light source. The BER for this point is 3.7 × 10⁻³, meeting the FEC threshold.

In this study, we demonstrated a wide-angle laser-based white light transmitter, expanding its field-of-view to 40° for high-speed transmission up to 5 m, achieving wide coverage VLC. At a distance of 1 m and 0°, the illuminance measures approximately 4500 lx, facilitating a transmission speed of 3.2 Gbps. As the angle increases, the transmission speed gradually decreases to 2 Gbps. At 5 m, an illuminance of 140 lx was measured (at 0°), which satisfies the household lighting requirement, and achieve a data rate of 2.5 Gbps. The minimum data transmission rate reaches 1.5 Gbps at 5 m. Our research on wide-angle communication systems addresses a critical challenge in VLC, laying the groundwork for its integration into future optical wireless communication networks.