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Abstract
This paper presents the first demonstration of InGaN multiple quantum well (MQW) based micro-photodetectors (µPD) used as the optical receiver in orthogonal frequency-division multiplexing (OFDM) modulated visible communication system (VLC). The 80-µm diameter µPD exhibits a wavelength-selective responsivity in the near-UV to violet regime (374 nm - 408 nm) under a low reverse bias of −3 V. The modulation scheme of 16-quadrature amplitude modulation (16-QAM) OFDM enables the use of frequency response beyond −3 dB cutoff bandwidth of µPD. A record high data rate of 3.2 Gigabit per second (Gpbs) was achieved as a result, which provides the proof-of-concept verification of a viable high speed VLC link.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Visible light communication (VLC) has attracted increasing research attention in recent years due to the growing bandwidth demands and data security for high-speed mobile internet, smart traffic, and Internet of Things (IoT) [1–3]. The utilization of white light-emitting diodes (LEDs) as transmitter has been developed for lighting and data communication dual-function lamps [4, 5]. LED-based VLC links, or “Light Fidelity” (Li-Fi) links, at various transmission distances and data rates have been demonstrated using different modulation techniques [6–8]. Owing to the relatively small modulation bandwidth of visible LEDs, the GaN-based superluminescent diodes (SLDs) and laser diodes (LDs) have lately been used as transmitters in VLC links [9–12] due to their significantly higher −3 dB cutoff bandwidth [13–15].
Compared to the advances in high-speed transmitters of VLC systems, the development of optical receiver is still lagging behind. In infrared regime, InGaAsP and InGaAs MQW-based PDs have shown > 20 Gigabit-per-second (Gbps) data rate for optical telecommunication systems [16, 17]. But such PDs are not suitable as signal receiver in UV and visible color regime due to the bandgap limitation. The silicon based PDs are typically used in VLC studies due to its low cost and wide wavelength coverage. The bandwidths of silicon based PDs vary from tens of MHz to GHz range depending on the design, and the speed of GaN laser enabled VLC links can be increased from 2.5 Gbps to 9 Gbps by employing different modulation schemes [1-2]. The further improvement of the data rate is limited by the absence of high speed receiver from silicon based PDs. Moreover, silicon based PD cannot adapt to harsh environment in space or salty seawater. On the other hand, III-nitride based PD can be tailored for wavelength-selective response to improve received optical power and signal-to-noise ratio. It is the optimal material choice for high speed VLC receiver in various environments. However, the previous III-nitride based PDs studies have focused on the typical characteristics of responsivity and pulse measurements as stand-alone devices using metal-semiconductor-metal (MSM) [18–23], p-i-n [24–27], and multiple quantum-wells (MQW) [28–32] structures. Their performance as optical receiver in VLC links has yet to be investigated.
Various modulation techniques have been investigated in VLC links, such as on-off keying (OOK) and orthogonal frequency-division multiplexing (OFDM) [10, 33–38]. OFDM has attracted attentions due to its robustness against inter-symbol interference (ISI) and frequency selective fading. It is known to be able to increase data rate of the communication link [36]. By dividing a broad fading channel into multiple narrow subchannels, flatness can be assumed on each subchannel and at the receiver, and a simple one-tap equalization can be performed to efficiently eliminate channel effects and enhance communication capabilities of the device [34–38]. This property makes OFDM highly advantageous when being used in band-limited systems as compared to other FDM techniques. However, OFDM system puts forward a higher requirement of signal to noise ratio (SNR) to ensure the orthogonality of subchannels. Thus, to lower the SNR, the dark current of PD used in OFDM system has to be minimized.
In this paper, InGaN/GaN MQW-based micro-photodetectors (µPDs) are fabricated as the high-speed, wavelength-selective optical receivers for VLC. Firstly, the typical PD characteristics of responsivity and frequency response are measured to assess the feasibility of using the PD for data transmission. Secondly, a VLC link is built with µPDs as receiver and 405nm laser as the transmitter. The OFDM modulation schemes with different quadrature amplitude modulation (QAM) numbers are applied in the VLC link and the data rate will be measured.
2. Responsivity and bandwidth of μPD
The µPDs were fabricated on c-plane InGaN/GaN MQW grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD), followed by standard clean room fabrication process. The schematics of the epitaxial structure and the device layout are shown in Fig. 1. We have previously compared the responsivity and frequency response of similar structures with different indium compositions [39], and selected the current structure for a focused development and investigation. The inset of Fig. 1(a) shows the transmission electron microscope (TEM) image of the active region consisting of 15-pairs of InGaN/GaN QWs, which was capped with 100 nm p-AlGaN electron blocking layer (EBL) and 150 nm p-GaN layer. The transparent contact of 5 nm Ni/ 250 nm ITO was deposited by sputtering followed by rapid thermal annealing at 600 °C for 1 min. A layer of 200 nm SiO2 was deposited for electrical isolation of the contact since the probe-pad was extended out of the p-GaN region to reduce shadowing. 10-nm Ni/ 1-μm Au was evaporated on top of the device as metal contacts. Figure 1(b) shows a microscope image of the μPDs with mesa diameter of 80 μm.
Fig. 1 (a) Schematic of InGaN/GaN MQW μPD and TEM image showing 15-pairs of 3-nm InGaN / 13.5-nm GaN (scale bar indicates 20 nm). (b) The optical microscope image of μPD with diameter of 80 μm.
The photoelectrical performance was measured using a Keithley 4200-SCS semiconductor characterization system. The devices were illuminated by monochromatic light from a Xenon lamp (350-700 nm) of which intensity was calibrated to 0.66 mW/cm2. Figure 2(a) shows the current -voltage curve under dark condition and under illumination of 392-nm. A low dark current of 37.4 pA was measured at −3 V. It suggests a low background noise level for later system level measurement. Under the illumination, the µPDs had a photocurrent of 2.1 nA at −3 V. Figure 2(b) plots the responsivity spectrum across wavelength range of 350 nm to 600 nm at −3 V. A peak responsivity of 70.7 mA/W was obtained at λ = 392 nm. The top p-GaN layer would absorb light with wavelength shorter than 380 nm, and MQW active region is transparent to light with wavelength longer than 450 nm. Therefore, the responsivity of μPD shows a wavelength selectivity between 374 nm and 408 nm, with a passband full-width at half-maximum (FWHM) of 34 nm. The characteristics of wavelength-selectiveness in responsivity further improve the signal-to-noise ratio for higher data rate.
Fig. 2 Photoelectrical performance of μPD: (a) the current -voltage curve under dark condition and under illumination of 392-nm light at an intensity of 0.66 mW/cm2. (b) The responsivity spectrum of μPD showing a 374 nm - 408 nm bandpass characteristic. The highest responsivity is 70.7 mA/W at 392 nm.
The frequency response is essential for evaluating the high-speed operation. Agilent E8361C network analyzer was used to measure the frequency response of μPD through a RF-prober. The sinusoidal signal was generated by the network analyzer, then used to modulate a 405-nm laser diode and eventually detected by the µPD. The frequency response at −3 V was plotted in Fig. 3(a). The insert shows the zoom-in view of the frequency response in logarithmic scale for the range of 10 to 100MHz. The dip at 20 MHz due to the impedance of the device was excluded in the following OFDM study. The absolute impedance of the μPD reduced from 260 Ω to 31 Ω when frequency increased from 50 MHz to 1 GHz. Because the bandwidth of our devices are not limited by the RC delay, the changes of impedance won’t affect the later data rate measurements. However, for VLC application, the impedance matching would be an important aspect in designing the driver circuit of the receiver. The −3 dB cutoff bandwidth (f-3dB) was measured to be 71.5 MHz. Although this bandwidth was not high, μPD showed extensively flat frequency response until 830 MHz which was limited by RC delay of the device which could be described as
where R is the load resistance of 50 Ω. C is the capacitance of μPD that was measured to be 4 pF as shown in Fig. 3(b).Fig. 3 (a) The frequency response of the μPD at a bias voltage of −3 V. Inset: zoom-in view of the frequency response in log scale. (b) The capacitance-voltage measurements of a μPD. (c) −3 dB bandwidth measured at different bias voltage of μPD and mean value is indicated by the straight line in each bias group.
The reason to choose reverse DC bias of 3 V was due to the dark current and the bandwidth. As shown in Fig. 2(a), when the reverse bias increased, the leakage current, i.e. dark current and thus the noise increased. Furthermore, the −3 dB bandwidth did not show significant and consistent change with different bias as compared in Fig. 3(c). −3 V exhibited relative larger bandwidth compared to other bias voltage. Thus, −3 V DC bias was applied to the μPDs in the following data rate measurement.
3. Data rate of OFDM modulated VLC system
We utilized the µPD as an optical receiver in the OFDM VLC system as illustrated in Fig. 4. The OFDM VLC system consisted of three parts: the transmitter, the receiver and the signal processing unit.
At the transmitter end, a series of pseudo-random binary sequence (PRBS) were generated. After M-ary quadrature amplitude modulation, the signal went through a serial to parallel (S/P) conversion and an inverse fast Fourier transform (IFFT) of length 4096 was performed to convert the signal into time-domain OFDM symbols. To efficiently eliminate the ISI, a cyclic prefix (CP) of 1 point was added in front of each OFDM symbol, and a parallel to serial (P/S) conversion was performed before sending the signal to the arbitrary wave generator (AWG, Tektronix AWG 70002A) to perform digital to analog (D/A) conversion. To battle the intrinsic high peak to average power ratio (PAPR) of the OFDM system, a clipping ratio of 2.6δ was applied, where δ is the standard variance of the signal. This value was meticulously selected after exhaustive experiments to ensure a maximum SNR at the receiver. After that, the signal went through two attenuators (−3 dB and −6 dB) and a 26 dB amplifier (Tektronix, PSPL5865) to modulate 8.8-mW, 405-nm laser diode (Thorlabs, LP405–SF10) with peak-to-peak voltage Vpp of 2V. The laser light was directed to and focused on μPD using customized testing system with fiber and objective lens. The distance between LD and μPD is 50 cm in fiber and 10 cm in free space with the laser beam size of about 100 μm in diameter. Although the transmission distance is limited in our current testing setup, the PD developed is expected to work with substantial transmission distance in VLC links. This can be achieved by properly packaging a ball lens aligned with the PD in future VLC receiver system.
At the receiver end, µPD was connected to a bias-T (Tektronix, PSPL 5545) under DC bias of −3V. The AC signal from µPD was sent to a real-time oscilloscope (RTO, Tektronix DPO 72004C) through a 26 dB amplifier (Tektronix, PSPL5865), and then downloaded to a personal computer (PC) to perform offline analysis using MATLAB. Through resampling, window synchronization, CP removal, fast Fourier transform (FFT), channel estimation and QAM demodulation, the received bits were compared bit-wise to calculate the bit error rate (BER).
The OFDM signal was generated using a MATLAB program. Each subchannel occupied a bandwidth of 0.97 MHz calculated from the Eq. (2)
where fs is the sampling rate of the AWG fixing at 4 GSamples/s. N is the size for the FFT and IFFT, chosen to be 4096. This bandwidth of less than 1 MHz was narrow enough to ensure flatness on each subchannel, so that simple one-tap equalization could be applied at the receiver, while the computational complexity was still within a reasonable range.When the OFDM signal was generated, the first 9 subchannels were set unused to avoid low-frequency interferences from the environment. A pilot tone was inserted every 16 subcarriers to assist the channel estimation process. The transmission speed was calculated as:
where Nsubcarrier is the number of valid data subcarriers, M is the order of QAM modulation, NIFFT is number of the IFFT size of 4096, and CP is the length of the cyclic prefix equal to 1.In the communication process, the BER is an important factor to evaluate the communication quality, and it should be lower than the FEC limit of 3.8 x 10−3. Two approaches to promoting high speed in OFDM modulation scheme are 1) to use a higher QAM order and modulate relatively fewer subcarriers and 2) to use a lower QAM order and modulate relatively more subcarriers. The first approach requires a higher subcarrier SNR, while the second one increases computational complexity.
We have optimized the modulation scheme, including adjusting the combination of QAM orders and modulated subcarrier numbers to extend the data rate in the VLC link. 4-QAM, 8-QAM and 16-QAM modulation were used in the optimization as indicated in Fig. 5. 16-QAM with subcarrier number 10 to 880 has the highest transmission speed with a BER of 3.7 x 10−3. Thus, the 16-QAM modulation could push the frequency response up to 853 MHz. With 816 valid data subcarriers, a sampling rate of 4 x 109, IFFT size of 4096, and a QAM order of 16, a corresponding data rate of 3.2 Gbps was achieved. If a fixed FEC overhead of 7% is taken into consideration, an error-free transmission of 2.96 Gbps can be achieved.
Fig. 5 BER vs. data rate of the VLC link with different OFDM QAM order. Insets: the corresponding constellation diagrams at 4-QAM, 8-QAM, and 16-QAM.
A summary of the key parameters achieved in this study are listed in the Table 1. Our work features the record high data rate of VLC link using InGaN μPDs as receiver, which is the first demonstration of such system. In addition, compared with the prior reported results, our device was operated at lower bias voltage, and the carrier lifetime was shorter for high speed modulation. The InGaN based active region can be further optimized to reduce the carrier lifetime for fast speed. The device configuration of µPD array and avalanche photodetector can also be explored to improve the bandwidth of InGaN based PDs. In addition to the device optimization, the OFDM-based VLC-link scheme can be further improved by using pre-equalization [36, 37] or bit/power loading techniques [38]. Nevertheless, this study demonstrated the great potential of InGaN/GaN based μPDs to facilitate Gbps data transfer rate. They can be used as the basic building blocks to enable wide range communication coverage beyond point-to-point communication link.
Table 1. Recent III-nitride based PDs with different configurations.
4. Conclusions
In summary, a record-high data rate of 3.2 Gbps was achieved by using InGaN/GaN MQW µPD as a receiver in OFDM modulated communication link. Owing to the wavelength selective in UV and visible region (374 nm - 408 nm) and the low reverse bias of −3 V, the background noise of the µPD was minimized. The OFDM modulation enabled the use of an extended frequency response beyond the −3 dB cutoff frequency of 71.5 MHz. The highest data rate with BER below FEC limit demonstrated the great potential of III-nitride based µPDs used in VLC link as optical signal receiver.
Funding
The authors gratefully acknowledge the financial support from King Abdulaziz City for Science and Technology (KACST), Grant No. KACST TIC R2-FP- 008, King Abdullah University of Science and Technology (KAUST) baseline funding, BAS/1/1614-01-01 and BAS/1/1657-01-01, and KAUST-KFUPM Special Initiative, Grant No. REP/1/2878.
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