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High-speed underwater optical wireless communication (UOWC) was achieved using an 80 μm blue-emitting GaN-based micro-LED. The micro-LED has a peak emission wavelength of ~440 nm and an underwater power attenuation of 1 dB/m in tap water. The −3 dB electrical-to-optical modulation bandwidth of the packaged micro-LED increases with increasing current and saturates at ~160 MHz. At an underwater distance of 0.6 m, 800 Mb/s data rate was achieved with a bit error rate (BER) of 1.3 × 10−3, below the forward error correction (FEC) criteria. And we obtained 100 Mb/s data communication speed with a received light output power of −40 dBm and a BER of 1.9 × 10−3, suggesting that UOWC with extended distance can be achieved. Through reflecting the light emission beam by mirrors within a water tank, we experimentally demonstrated a 200 Mb/s data rate with a BER of 3.0 × 10−6 at an underwater distance of 5.4 m.
© 2017 Optical Society of America
1. Introduction
Oceanography investigation, offshore oil exploration and sea floor monitoring require high-speed data communication, especially for real-time video communication. The fiber optic and copper cable can offer high data communication speed, but lack the convenience of wireless links. Acoustic communication, the popular underwater wireless communication technology, suffers from low bandwidth, high latency, multipath propagation, and Doppler spread, only offering data rate of tens of kb/s for km distance communication. Radio frequency (RF) is strongly attenuated by the sea water with increasing frequencies and thus only supports short communication distance less than 10 meters. Optical wireless communication has high bandwidth but is strongly affected by the water absorption and suspended particles scattering. However, the water attenuation is relatively small in the blue-green spectrum region, thus making blue or green-emitting GaN-based LEDs or lasers suitable optical data sources for underwater optical wireless communication (UOWC) [1–7]. The external quantum efficiencies of blue GaN-based LEDs have been significantly improved during the past two decades and widely used in general lighting [8], which enables the recent development of visible light communication, also known as LiFi [9–11]. The blue LED-based free space communication has achieved a data rate of up to 5 Gb/s for a single micro-LED at a distance of 0.75 m [11], but the underwater communication data rate using LEDs is still low [2,3]. Recently, Xu et al. reported a data rate of 161.1 Mb/s at a distance of 2 m using a blue LED [3]. Comparatively, using a blue laser, high data rate of up to 4.8 Gb/s has been achieved at a distance of 5.4 m [4], which may be attributed to the collimated laser beam and low power attenuation for potentially long distance communication and high modulation bandwidth for high speed communication. However, the cost of lasers is higher than LEDs, and lasers may cause safety issues such as damaging human eyes. The −3dB electrical-to-optical modulation bandwidth of commercial LEDs for general lighting is usually limited to ~20 MHz, and the LED beam follows non-collimated Lambertian radiation pattern [9–11]. Therefore, through improving the LED modulation bandwidth and optimizing the optics, the data communication rate and distance using an LED can be significantly improved. It was reported that micro-LEDs with pixel sizes of tens of μms had high modulation bandwidth of up to several hundreds of MHz [10,11], but their application in the UOWC has not been reported.
In this work, we used a packaged 80 μm × 80 μm blue-emitting micro-LED with a maximum modulation bandwidth of ~160 MHz. At a 0.6 m distance, an UOWC data rate of 800 Mb/s using on-off-keying (OOK) modulation was achieved with a bit error rate (BER) of 1.3 × 10−3, below the forward error correction (FEC) criteria. Further work demonstrated a data rate of 100 Mb/s at a received light output power of −40 dBm and a BER of 1.9 × 10−3, suggesting that UOWC with extended distance can be achieved. Using mirrors to reflect light beam of the micro-LED within a water tank, data communication distance of 5.4 m at a data rate of 200 Mb/s and a BER of 3.0 × 10−6 has been experimentally demonstrated. The OOK modulation in this work has already offered a higher speed and a longer distance than previously reported UOWC work based on LEDs [2,3], and we expect that a much higher data rate can be obtained using orthogonal frequency division multiplexing (OFDM) modulation scheme [12]. In addition, the micro-LED fabrication technology is compatible with conventional LED fabrication processes [10,11] and the conventional LED fabrication cost has been greatly reduced due to its wide application in solid-state lighting, so the micro-LED cost can be much lower than the laser cost. Compared with previous papers using blue, green or red lasers for UOWC [4,5,7,13,14], the micro-LED bandwidth is lower and it is more difficult to collimate the light beam of micro-LEDs. However, the micro-LEDs may offer potential advantages of using a micro-LED array include the applications in both underwater micro-display and communication [15], and parallel data communication using multiple micro-LED pixels to further increase the data rate [16].
2. Experiments
A blue GaN-based micro-LED array fabricated on a sapphire substrate was used for UOWC. The micro-LED structure and fabrication processes are similar to those reported in our previous work [15,17]. The LED structure consists of an n-GaN layer, an InGaN/GaN multiple quantum well (MQW), an AlGaN electron blocking layer, and a p-GaN layer, grown by MOCVD. Ni/Au (10nm/25nm) metal layers were deposited on top of the p-GaN, after which dry etching was used to etch away the Ni/Au layers and the GaN layers down to the n-GaN layer. Rapid thermal annealing at 500 °C was employed to form the p-contact. SiO2 was deposited as the isolation layer and then apertures were formed on the micro-LED mesas and in the n-contact areas. Ti/Au(50nm/200nm) metal layers were deposited as the p- and n-pads to individually address each pixel. The micro-LED array was then bonded onto a PCB with light extracted from the sapphire side. The micro-LED array has different sizes from 20 μm × 20 μm to 80 μm × 80 μm. Previous work has shown that the micro-LED with smaller size has a higher modulation bandwidth but a smaller absolute light output power [10]. In this work, we used an 80 μm micro-LED for UOWC, as both high modulation bandwidth and high light output power are important for high-speed communication [11,12,15]. In addition, the micro-LED package has limited the usable micro-LED bandwidth due to impedance mismatch which will be discussed later. The current versus voltage (I-V) characteristics were measured by a current source under DC. After collimating the light emission of the micro-LED by a nonspherical lens, the light transmitted through a water tank and its characteristics of light output power versus current (P-I) was measured at a distance of 1 m from the micro-LED. The electroluminescence (EL) spectra were collected using an Ocean Optics USB4000 Spectrometer. The −3dB electrical-to-optical modulation bandwidth was measured by driving the micro-LED with a DC bias combined with a small-signal modulation from an Agilent N5225A network analyzer (10 MHz-50 GHz). The light output was collected by a 1.4 GHz Femto photodetector and the frequency response was measured by the network analyzer.
Figure 1(a) shows the schematic setup of UOWC. The micro-LED was driven by combining a DC current source and the signal through a bias tee. The signal was generated by a pulse pattern generator module (0.1-14 Gb/s) and BER was analyzed by an error detector module (0.1-14 Gb/s) from an Anritsu MP1800 signal quality analyzer. Pulse pattern generator generated pseudo-random binary sequences (PRBS) with a standard pattern length of 27-1 bits and a peak-to-peak voltage swing of 2 V. The Agilent N5225A network analyzer generated a clock signal for the MP1800 as our MP1800 does not have an internal clock module. The light emission of the micro-LED was collimated and transmitted through a water tank with tap water inside. High-reflectivity mirrors were used to reflect the light within the water tank for UOWC with extended distance. By increasing the reflection times of the light beam inside the water, the distance that the light propagates in the water can be extended. The distance between the micro-LED and the receiver was 1 m and the length of the water tank was 0.6 m. On the receiver side, the light emission was focused onto the high-speed 1.4 GHz Femto PIN photodetector with 0.4 mm photosensitive diameter or a high-sensitivity 100 MHz Hamamatsu APD with 1.0 mm photosensitive diameter. The received optical signal was converted to electrical signal by the PIN photodetector or the APD, and further sent to the error detector to test the BER. The high-speed photodetector was employed to test the highest achievable data communication speed. The high-sensitivity APD was used to test the UOWC with extended distance and relatively lower speed. The eye diagrams were captured by a 14 GHz Agilent 86100A oscilloscope. Figures 1(b)-1(d) show the images of the packaged micro-LED, communication link and the PIN receiver. In Fig. 1(d), the collimated light transmission path in the water can be seen, indicating the attenuation of light output power by strong scattering of water molecules.
Fig. 1 (a) Schematic setup of UOWC (n represents the reflection times of the light beam); images of (b) the packaged micro-LED, (c) the communication link and (d) the PIN receiver.
3. Results
The I-V and P-I characteristics are shown in Fig. 2(a). The series resistance Rseries = dV/dI at operation currents from 20 mA to 90 mA keeps almost constant and was fitted to be ~20 Ω. The small signal modulation voltage induced variations of current and further light output power. From the I-V and P-I characteristics, we can obtain light output power versus voltage (P-V) characteristics and then good linear modulation characteristics for the micro-LED device. To accurately determine the water attenuation of light output power, we measured the light output power with a fixed distance of 1 m from the micro-LED in free space (without tank and water), with tank, and with water in the tank. The light output attenuations by the tank and the 0.6 m water were 17% and 13%, respectively. Water attenuation ~1 dB/m can be extracted from Fig. 2(a), which is much lower than the RF technique but is higher than reported value 0.39 dB/m of the open ocean [1]. In our future work, different water types, e.g. coast ocean and turbid harbor [1], may be demonstrated for the UOWC. The higher attenuation coefficient in our work may be caused by the large diameter of the collimated light emission beam (compared with laser) as shown in Fig. 1(d). For UOWC with longer distance, the collimation of the LED light emission by optimizing optics is quite important, as most light emission of the LEDs distributed within a large angle of ~120° [18]. In Fig. 2(b), the EL peaks show blue shift from ~446 nm at 0.8 mA to ~439 nm at 61 mA and then keep constant until 88 mA. The blue shift was caused by carrier screening effect and band filling effect, and self-heating affected the EL spectra of the micro-LED at higher currents [19]. The high junction temperature induced by self-heating can degrade the micro-LED’s aging and reliability characteristics significantly [20], as well as the modulation bandwidth [21], so moderate driving current should be chosen to achieve both high light output power and good device reliability.
Fig. 2 (a) The I-V and P-I characteristics; (b) the EL spectra of the micro-LED under different injection currents.
Figure 3(a) shows the normalized electrical-to-optical frequency response of the micro-LED from 1 mA to 88 mA, and the characteristics of extracted −3dB modulation bandwidth versus current are summarized in Fig. 3(b). With higher currents, the bandwidth increases to ~160 MHz at 61 mA and then keeps constant. Previous studies have demonstrated that the micro-LEDs showed a much higher bandwidth using a high-speed probe, but after device bonding the maximum bandwidth was limited by a CMOS driver [10]. We consider that in this work the impedance mismatch between the micro-LED and package may be the dominant factor for the bandwidth saturation. To improve the bandwidth of the packaged micro-LED system, the micro-LED impedance needs to be clarified. Our previous work found that the micro-LED showed negative capacitance at a normal micro-LED operation voltage, e.g. 5 V, the effect of which on the modulation bandwidth and the impedance matching still needs further study [22].
Fig. 3 (a) Frequency response of the micro-LED at injection currents from 1 mA to 88 mA, and (b) the extracted −3dB modulation bandwidth versus current characteristics.
In Fig. 4(a), the BER at different driving currents was tested from 16 mA to 88 mA. A higher data communication speed can be achieved at a higher driving current, due to the increase of modulation bandwidth and light output power. From 61 mA to 88 mA, a slightly higher speed than 800 Mb/s was achieved, as the bandwidth already saturated at 61 mA. At 88 mA, a data rate of 800 Mb/s was obtained with a BER 1.3 × 10−3 (below FEC 3.8 × 10−3). With 7% overhead of FEC, the data rate of the 0.6 m UOWC was ~750 Mb/s [23]. To the best of our knowledge, this is the highest UOWC data rate based on LEDs [1–3]. In our future study, we expect that using an OFDM modulation scheme the data rate can be further improved, which also allows different modulation schemes for UOWC to be compared with each other [1,3,12].
Fig. 4 (a) The BER at different driving currents; (b) the BER at different received light output powers (driving current: 61 mA) with a PIN or APD (inset figure) as the receiver.
For UOWC with extended distance, at a fixed driving current the received light output is the main limiting factor due to the strong underwater power attenuation. We used a driving current of 61 mA to characterize the BER versus received light output power. The micro-LED was not driven at the maximum sustainable current, as a higher current, e.g. 88 mA, can offer a slightly better BER but the micro-LED reliability may be degraded [20]. The received light output power was adjusted by a neutral density filter between the lens for the micro-LED and the water tank. In Fig. 4(b) and the inset figure, we achieved 100 Mb/s data rate at received light output powers of −11 dBm (~80 μW) and −40 dBm (~0.1 μW) using PIN and APD receivers, respectively. Assuming the obtained 1 dB/m attenuation from Fig. 2(a) also applies to the UOWC with extended distance, the system provides potential to achieve 100 Mb/s data communication speed at distances of ~11 m with a PIN receiver and ~40 m with an APD receiver. However, the estimated distance can be strongly affected by the beam spreading, receiver diameter, turbulence, and different water types [1].
Figures 5(a) and 5(b) show the eye diagrams at 400 Mb/s and 700 Mb/s using a PIN photodetector. The micro-LED was driven at a DC 88 mA and the received light output power at the receiver is 1.1 mW. Open eye diagram can be seen in Fig. 5(a), indicating error free data communication. The eye diagram in Fig. 5(b) is almost closed which corresponds to a BER of 4.8 × 10−7. Using an APD with higher sensitivity and a driving DC 61 mA, Figs. 5(c) and 5(d) demonstrate the eye diagrams with received light output of ~80 μW and ~0.1 μW. Similarly, open eye diagram in Fig. 5(c) suggests an error-free data communication, and the BER for the eye diagram in Fig. 5(d) increases to 1.9 × 10−3.
Fig. 5 Eye diagrams at data rates of (a) 400 Mb/s and (b)700 Mb/s using a PIN photodetector with received light output power of 1.1 mW. Eye diagrams with received light output powers of (c) ~80 μW and (d) ~0.1 μW at 100 Mb/s using a high-sensitivity APD. The UOWC distance is 0.6 m, and the received light output power was adjusted by a neutral density filter.
To experimentally evaluate the maximum UOWC distance in our setup, we installed high-reflectivity mirrors on the sidewalls of the water tank, as illustrated in Fig. 1(a). The APD was used to test the communication speed. As the −3 dB bandwidth of the APD is 100 MHz, we only show the BER up to 300 Mb/s in Table 1. At the data rate of 300 Mb/s, error-free UOWC was achieved at a distance of 0.6 m; the BER increased at 3.0 m; at 5.4 m, the signal synchronization was not obtained to build a communication link. With a longer distance, the light output power decreased at the receiver, so at 300 Mb/s the BER increased with longer distance. In addition, it was more difficult to collimate the light beam with increased reflection times of the light beam. High data communication speed of 200 Mb/s can be achieved at a distance of 5.4 m with a BER of 3.0 × 10−6, which is better than the recent reported data communication speed of 161 Mb/s at a distance of 2 m using OFDM modulation [3]. Further adding mirrors to extend the UOWC length to 7.8 m in our experiment caused large attenuation of light output power due to light beam spreading, and thus we could not obtain acceptable BER at a data rate of 100 Mb/s at such distance. However, we expect that a longer distance for UOWC applications may be achievable in our future work after optimizing the optical antennas, i.e. optics techniques for collimating the micro-LED light beam for the transmitter and focusing the beam for the receiver.
Table 1. Experimental demonstration of UOWC using a micro-LED at distances of 0.6 m, 3.0 m, and 5.4 m.
4. Summary
We used a high bandwidth micro-LED with a size of 80 μm and a peak mission wavelength of ~440 nm to achieve high-speed UOWC. The underwater attenuation was estimated to be 1 dB/m after collimating the micro-LED light emission beam. Due to the impedance mismatch, the maximum modulation bandwidth of the packaged micro-LED was limited to be ~160 MHz. A record data rate of 800 Mb/s at an underwater distance of 0.6 m and a BER of 1.3 × 10−3 was achieved for UOWC based on micro-LEDs. In addition, we experimentally demonstrated that 100 Mb/s data rate was achievable with a received light output of −40 dBm and a BER of 1.9 × 10−3, which indicated that UOWC with extended distance can be achieved. Through adding reflection mirrors, the underwater distance was extended to 5.4 m and a data rate of 200 Mb/s was experimentally demonstrated with a BER of 3.0 × 10−6. Our work implies that micro-LED arrays offer significant potential for high-speed UOWC. We expect that the data rate can be increased and the communication distance can be further extended through improving the bandwidth of the packaged micro-LED system, optimizing collimation optics, using multiple micro-LEDs for parallel data communication, and employing OFDM modulation scheme.
Funding
National Natural Science Foundation of China No. 61571135; Start-up Research Funding JJH1232106 from Fudan University; State Key Laboratory of Luminescence and Applications funding SKLA201619.
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