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Abstract
We propose and implement a high-bandwidth white-light visible light communication (VLC) system accomplishing data rate of 2.805 Gbit/s utilizing a semipolar blue micro-LED. The system uses an InGaN/GaN semipolar (20-21) blue micro-LED to excite yellow phosphor film for high-speed VLC. The packaged 30 μm 2 × 4 blue micro-LED array has an electrical-to-optical (EO) bandwidth of 1042.5 MHz and a peak wavelength of 447 nm. The EO bandwidth of the white-light VLC system is 849 MHz. Bit error rate (BER) of 2.709 × 10−3 meeting the pre-forward error correction (FEC) threshold is accomplished by employing a bit and power loaded orthogonal frequency division multiplexing (OFDM) signal. The proposed white-light VLC system employs simple and inexpensive yellow phosphor film for white-light conversion, complex color conversion material is not needed. Besides, no optical blue filter is employed in the white-light VLC system. The fabrication of the InGaN/GaN semipolar (20-21) blue micro-LED is discussed, and its characteristics are also evaluated.
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1. Introduction
Phosphor based white-light light-emitting diodes (LED) are currently the most low-cost and popular lighting devices. It has the characteristics of high energy conversion efficiency and long lifetime. Visible light communication (VLC) technology allows the LED-based lighting infrastructure to provide the possibility of lighting and data communication simultaneously. In recent years, VLC systems utilizing white-light LEDs have been demonstrated [1–6]. The electrical-to-optical (EO) bandwidth of traditional white-light LED is only a few MHz [1]. Researchers have proposed many methods to increase the speed. In 2009, Minh et al. revealed a 100 Mbit/s on-off-keying (OOK) transmission utilizing a simple first-order analogue equalizer to equalize a 50 MHz bandwidth phosphor white-light LED [1]. In 2014, Yeh et al. demonstrated a real-time 1.5 m video transmission using ∼ 1 MHz bandwidth phosphor white-light LED [3]. In the same year, Li et al. illustrated a 340 Mbit/s OOK link employing a post-equalization circuit to extend the white-light LED bandwidth to 151 MHz [4]. In 2015, Huang et al. achieved a 2.0 Gbit/s orthogonal frequency division multiplexing (OFDM) white-light LED transmission utilizing pre-equalization and differential output receiver [5].
With the advancement of display technologies, micro-LED is one of the fast growing technologies in the world [7–9]. Micro-LED is defined with chip size < 100 μm. The smaller chip size reduce the junction capacitance, and it is very suitable for high-speed VLC systems. In 2014, Chun et al. illustrated a high speed 1.68 Gbit/s VLC system using a blue micro-LED and a yellow fluorescent polymer [10]. In 2018, Huang, et al. demonstrated a blue micro-LED array with yellow fluorescent material achieved 127.3 MHz EO bandwidth [11]. Moreover, micro-LED combined with quantum dots is considered as a promising color conversion technique in VLC. In 2018, Mei et al. showed a 300 Mbit/s OOK VLC link utilizing a blue micro-LED and yellow-emitting perovskite quantum dots [12]. In 2019, Cao et al. demonstrated a 675 Mbit/s OOK VLC link utilizing blue micro-LED array and CdSe/ZnS semiconductor quantum dots achieving 637.6 MHz bandwidth [13]. In 2021, Wang et al. accomplished a 1.7 Gbit/s OOK VLC system using blue micro-LED with perovskite nanocrystal-polymethyl methacrylate (PNC-PMMA) films [14].
In addition, many researchers have increased the transmission speed of LEDs by improving the manufacturing methods. In 2020, Khoury et al. illustrated polarized monolithic white semipolar (20–21) InGaN micro-LED can exhibit a 660 MHz EO bandwidth [15]. In the same year, Wan et al. fabricated a phosphor-free single chip self-assembled InGaN quantum dots white LED with modulation bandwidth of 150 MHz [16]. In 2021, the same group fabricated nanohole array structured GaN-based white LEDs, increasing the transmission speed to 2.21 Gbit/s [17]. Table 1 summaries of the recent achievements of different high-speed white-light micro-LED VLC systems, illustrating different white-light conversion schemes, modulation formats, achieved data rate, etc.
Table 1. Summary of recent achievements of different high-speed white-light micro-LED VLC systems
In this work, we propose and implement a high-bandwidth white-light VLC system accomplishing data rate of 2.805 Gbit/s utilizing semipolar blue micro-LED. The system uses InGaN/GaN semipolar (20-21) blue micro-LED to excite yellow phosphor to produce white-light. The bandwidth improvement is attributed by the use of semi-polar (20-21) micro-LED. As discussed in [18], typical GaN LEDs are usually grown on (0001) polar c-plane sapphire substrates, which can result in a high quantum-confined Stark effect (QCSE) leading to reduced efficiency. Besides, as discussed in [19], the semi-polar (20-21) orientation can opposite the built-in junction field, allowing the band profile in the quantum wells to be similar to a flat-band condition. This increases the electron overlap, improving the radiative efficiency and improving the droop characteristics. Previous semi-polar devices also show improvement performances [20,21]. The proposed white-light VLC system employs simple and inexpensive yellow phosphor film for white-light conversion, complex color conversion material is not needed. The packaged 30 μm 2 × 4 blue micro-LED array has an EO bandwidth of 1042.5 MHz and a wavelength of 447 nm. The EO bandwidth of the white-light VLC system is 849 MHz. Bit error rate (BER) of 2.709 × 10−3 meeting the pre-forward error correction (FEC) threshold is accomplished by employing bit and power loaded OFDM signal. Besides, no optical blue filter is employed in the white-light VLC system. Furthermore, the fabrication of the InGaN/GaN semipolar (20-21) blue micro-LED is discussed, and its characteristics are also evaluated. It is also worth to note that white-light VLC system can also be achieved using UV/blue LD as pump sources. For examples, a 1 Gbit/s white-light VLC system was realized by a 410 nm UV LD with red-green-blue (RGB) emitting phosphors [22]; and a 6.915 Gbit/s white-light VLC was achieved by a 460 nm blue LD with yellow phosphor [23]. Although the UV/blue LD based VLC systems can provide high modulation bandwidth, the LED based system could offer simpler driving and temperature stabilizing circuits at the Tx. Besides, LED could provide less thermal damage to the color conversion materials [14].
2. Device architecture and experimental demonstration
Figure 1(a) presents the architecture of our semipolar blue micro-LED. The (20-21) oriented semipolar GaN layer is grown on a (22-43) oriented patterned sapphire substrate (PSS) through metal organic chemical vapor deposition (MOCVD). The (22−43) PSS is produced by reactive ion etching (RIE). Ge-doping GaN is employed at the starting epitaxy to reduce the stacking fault. Then, a bulk un-doped GaN layer is deposited onto the Ge-doped GaN layer. Chemical mechanical planarization (CMP) is utilized for the planarization of the device. As shown in Fig. 1(a), the active region of the micro-LED consists of an n-GaN layer, followed by the InGaN/GaN single quantum well (SQW), and a p-type GaN layer. Then, an indium tin oxide (ITO) layer is deposited on top of the p-GaN layer. Ti/Al/Ti/Au is deposited with thickness of 20/125/45/75 nm as electrodes. Atomic layer deposition (ALD) of Al2O3/SiO2 is used for the passivation layer of device. Ti/Al/Au is deposited with thickness of 20/250/300 nm as metal pads. Figures 1(b) and 1(c) show the photographs of our fabricated blue micro-LED array without and with the bias current (@ 20 mA) respectively. It consists of 2 × 4 micro-LEDs having a diameter of 30 μm in each micro-LED. The micro-LED is mounted in 50 mm × 74 mm SMD (surface mount device) package, as shown in Fig. 1(d). After this, a yellow phosphor film (Intematix Corporation NYAG4255) is attached to the device to produce white-light as illustrated in Fig. 1(e).
Fig. 1. (a) Proposed architecture of the semipolar blue micro-LED. Photographs of the 2 × 4 blue micro-LED array (b) without current, (c) with current, (d) on a SMD package. (e) Illustration of a yellow phosphor film attached to the device.
The micro-LED is fixed in a SMD package and yellow phosphor film is covered on the package. The package is soldered on a printed circuit board (PCB). SMA connector on the PCB is used to connect the equipments. Figures 2 (a) to 2(c) present the photographs of the micro-LED array with the yellow phosphor film at OFF, ON and ON with lens. The yellow phosphor has an emission peak wavelength at 545 nm. The polydimethylsiloxane (PDMS) consisted of A and B elastomer kit is purchased from Sil-More Industrial Ltd. The film fabrication begin with the preparation of PDMS, which consists of a weight ratio 10:1 of A and B elastomer kits. Then, the phosphor is added to the prepared PDMS with a mixing ratio of 1:15, and manually mixed until the solution is uniform. To solidify the compound solution, it is poured into a container and baked at 50°C. After baking for 3 hours, the solution solidify into a yellow translucent film with a thickness of 0.5 mm ∼ 0.6 mm. The fully dried film is cut to an appropriate size (50 mm × 50 mm) for the SMD package.
Fig. 2. Photographs of the blue micro-LED array with yellow phosphors film soldered on PCB at (a) OFF, (b) ON, and (c) ON with lens
We then evaluate different characteristics of the proposed micro-LED. Figure 3(a) shows the experimental optical spectrum of the yellow phosphor based 2 × 4 blue micro-LED array measured by an optical spectrum analyzer (OSA, Ocean Optics USB2000+). When driving at 70 mA, a peak wavelength of 447 nm is observed. Besides, a peak wavelength of the yellow phosphor of 545 nm is also observed. Figure 3(b) shows the CIE 1931 chromaticity diagram of micro-LED array. The correlated color temperature (CCT) of the micro-LED array is 7509 K at driving current of 70 mA. This could be suitable for cool-white applications. The generated white-light has the Color Rendering Index (CRI) of 69, illuminance of 115 lux@16 cm, and the photoluminescence quantum yield (PLQY) of 40%. Figure 3(c) shows the measured normalized frequency response of micro-LED array at driving current of 70 mA, evaluated by a vector network analyzer (VNA, Rohde & Schwarz ZND). The VNA measurement frequency range is from 1 MHz to 1.2 GHz. The 3-dB EO bandwidth of phosphor based white-light micro-LED array is 849 MHz, while the bandwidth of the blue micro-LED array is 1042.5 MHz. The slow response of the yellow phosphor used in color conversion will sacrifice some received powers and bandwidth. Due to the higher electron-hole wave function overlap and shorter carrier lifetime, LEDs grown in non-polar or semi-polar orientations can provide higher modulation response [22–25]. We also measured the yellow phosphor film and the 3-dB bandwidth of the yellow component is 24 MHz. This result is similar to the literatures [4,25].
Fig. 3. (a) Measured optical spectrum at different biases. (b) Measured CIE 1931 chromaticity diagram of micro-LED. (c) Measured normalized frequency response of micro-LED array.
Figure 4(a) shows the experimental architecture of the proposed semipolar micro-LED white-light VLC system. An arbitrary waveform generator (AWG, Tektronix AWG70001) is used to produce the electrical OFDM data to drive the white-light micro-LED array. The OFDM encoder at the transmitter (Tx) side contains input random data serial-to-parallel conversion (S/P), symbol mapping to quadrature-amplitude-modulation (QAM), inverse fast-Fourier-transformed (IFFT), parallel-to-serial conversion (P/S), and cyclic-prefix insertion (CP). In the experiment, a CP size of 1/32 and an FFT size of 512 are used. The digital domain OFDM signal is transformed into analog domain OFDM signal via a digital-to-analog converter (DAC), which is the AWG. The generated OFDM waveform is applied to the white-light micro-LED array via a bias tee (Tektronix PSPL5575A). The white-light VLC signal is received by an avalanche photodiode (APD, Menlo Systems APD210) attached to a 25dB radio-frequency (RF) amplifier (HP 8447D). The APD is silicon based with the detection wavelength range of 400 - 1000 nm. The detector diameter is 0.5 mm and the 3-dB bandwidth is 1 GHz. The free-space transmission distances between the Tx and receiver (Rx), as well as between the two focus lenses are 16 cm and 12 cm respectively, as shown in Fig. 4(b). In this proof-of-concept demonstration, the transmission distance is only 12 cm due to the low optical power emitted by the small emitter area of the micro-LED array. The potential application could be short-range indoor transmissions, similar to the non-contact wireless applications provided by Radio Frequency IDentification (RFID) or Near Field Communication (NFC). The transmission distance could be extended by using lens system. Then, the real-time oscilloscope (RTO, Teledyne LeCroy 816ZI-B) captures the OFDM signal. It is also the analog-to-digital converter (ADC) transforming the received analog domain OFDM signal into digital domain OFDM signal. The OFDM decoder contains CP removal, S/P conversion, FFT, signal equalization (EQ), symbol de-mapping, and P/S conversion. In order to enhance the transmission capacity of the white-light VLC system, bit and power loading are implemented. During the training process, all the OFDM subcarriers utilize 16-QAM formats. Then, based on the receiver signal-to-noise ratio (SNR) of different OFDM subcarriers, the optimum bit loading and power loading are applied to individual OFDM subcarrier. Figure 4(b) shows the photograph of the micro-LED white-light VLC experiment. In order to reduce the size of the Tx, a poly (methyl methacrylate) (PMMA) convex lens is attached in front of the micro-LED. The other convex lens is fixed to the mirror mount in front of the APD as illustrated in Fig. 4(c). It is worth to method that no optical blue filter is employed in the white-light VLC system. Since micro-LED is used in this work, low optical output power is produced by the small micro-LED emitter area. Based on the analysis in [26], the blue filter will greatly reduce the received optical power; hence, it will also significantly reduce the received SNR. Besides, as also suggested in [26], for the digital-signal-processing (DSP) based modulation format, likes the OFDM used here, the signal equalization can be performed by the DSP at Rx, and there is no need to perform optical equalization via the blue filter.
Fig. 4. (a) Experiment of the proposed VLC system. IFFT: inverse fast-Fourier-transformed (IFFT); DAC: digital-to-analog converter, APD: avalanche photodiode; AWG: arbitrary waveform generator; RTO: real-time oscilloscope; ADC: analog-to-digital converter. Photographs of (b) the experiment and (c) APD based Rx.
3. Results
Figures 5(a) and 5(b) show the measured SNR, bit and power loaded white-light OFDM signal respectively. The average SNR of all the 220 OFDM subcarriers is 12.78 dB. Bit-loading order of 4 can be realized in the lower frequency OFDM subcarriers, achieving 16-QAM format. Bit-loading orders of 3 or 2 can be realized in the higher frequency OFDM subcarriers, corresponding to 8-QAM format or quadrature phase shift keying (QPSK) format respectively. The typical 16-QAM, 8-QAM and QPSK constellation diagrams are shown in the inset of Fig. 5(b).
Fig. 5. Measured (a) SNR, bit-loaded, and (b) power-loaded white-light OFDM signal. Insets: 16-QAM, 8-QAM and QPSK constellation diagrams.
Figure 6(a) shows the measured optical output powers and voltage-current curve of the micro-LED under different bias currents. The output optical power is measured by an optical power meter (Thorlabs PM100D), and the voltage-current curve is measured by a source measurement unit (Keithley 2401). The output optical power is 0.8 mW at driving current of 100 mA. The optical output power gradually saturates from 80 mA to 100 mA. Furthermore, the voltage increases linearly from 40 mA to 100 mA. Figure 6(b) shows the BER curve of white-light VLC system at different received optical powers. The data rate of 2.805 Gbit/s is achieved at the received optical power of 0.677 mW at BER of 2.709 × 10−3, which is lower than the 7% pre-FEC threshold (BER = 3.8 × 10−3). As also shown in Fig. 6(b), no error floor is observed in the BER curve.
Fig. 6. (a) Output power-current-voltage characteristics for micro-LED array. (b) Measured BER curve of the white-light VLC system at different received optical powers.
4. Conclusion
The traditional LED grown on polar c-plane GaN suffers from significant spontaneous and piezoelectric polarization, resulting in QCSE that greatly reduces the luminous efficiency and speed of LEDs. However, the semipolar structure can mitigate this issue. Here, we proposed and implemented a high-bandwidth white-light VLC system accomplishing data rate of 2.805 Gbit/s utilizing semipolar blue micro-LED. The VLC system utilized InGaN/GaN semipolar (20-21) blue micro-LED to excite yellow phosphor to produce white-light. The proposed white-light VLC system employed simple and inexpensive yellow phosphor film for white-light conversion, complex color conversion material was not needed. When the white-light micro-LED was driven at 70 mA, peak wavelengths of 447 nm and 545 nm due to the blue and yellow components were observed respectively. The CCT of the micro-LED array was 7509K at 70 mA driving current. The 3-dB EO bandwidth of phosphor based white-light micro-LED array is 849 MHz, while the bandwidth of the blue micro-LED array is 1042.5 MHz. BER of 2.709 × 10−3 meeting the pre-FEC threshold was accomplished by employing bit and power loaded OFDM signal. No error floor was observed in the BER curve. The average SNR of all the 220 OFDM subcarriers was 12.78 dB. In this work, no optical blue filter was employed in the white-light VLC system.
Funding
Ministry of Science and Technology, Taiwan (MOST-109-2221-E-009-155-MY3, MOST-110-2221-E-A49-057-MY3).
Acknowledgments
The authors would like to acknowledge Prof. Jun Han of Yale University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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