Single LED-based 46-m underwater wireless optical communication enabled by a multi-pixel photon counter with digital output

Abstract

Presently, most of the long-reach underwater wireless optical communication (UWOC) systems employ laser diodes rather than light-emitting diodes (LEDs) as the transmitters. Due to lasers’ smaller divergence angles, those UWOC systems could realize longer communication links. Although LED-based long-reach UWOC has distinguished advantages, it suffers from very low received optical power. Fortunately, highly sensitive multi-pixel photon counter (MPPC) and energy efficient pulse position modulation (PPM) pave a way towards LED-based long-reach UWOC. In this paper, we investigated the underlying working principle of an MPPC and proposed a UWOC system based on single LED as well as a lens-free MPPC with digital output. The relationship between the MPPC p.e. threshold and its received optical power was theoretically studied and further proved by experimental results. Single 2.3-MHz, 3-W blue LED was used as the transmitter to generate 8-PPM to 64-PPM signals with a 5-MHz slot frequency. After a 46-m underwater transmission, the measured BERs were all below the forward error correction (FEC) limit, which were achieved with less than 100 incident photons during each pulse slot.

Introduction

At present, underwater acoustic communication (UAC) remains the dominant approach for long-reach underwater communication, due to low attenuation of acoustic wave in water. However, UAC suffers from large transmission delay, narrow bandwidth and low data rate even at short distance [1], [2], [3]. Besides, acoustic equipment can hardly be installed in small underwater platforms, such as a small autonomous underwater vehicle (AUV), due to the large size. On the contrary, underwater wireless optical communication (UWOC), normally based on green or blue light with relatively low attenuation in water, has drawn significant attention in recent years due to its low latency, high speed and high security. In addition, UWOC devices with small size, low power consumption and portability enable them to be carried by individual carriers easily, such as divers or AUVs. It has been proven by C. Wang et al. that in clean ocean, green light could achieve a communication distance of over a hundred meters [2].

For a UWOC system, laser diode (LD) and light-emitting diode (LED) have been considered as two major and mature light sources at the transmitting side, both of which are very small in size and light in weight. LD is characterized by highly coherent and directional output light with high optical power. Most LDs require temperature and power control. Many UWOC systems based on LDs had achieved long transmission distance or high data rate. For example, Liu et al. [3] achieved a 34.5-m, 2.7-Gbps UWOC in tap water based on on-off keying (OOK) modulation and a 520-nm laser in 2017; Y. Chen et al. [4] achieved a 5.5-Gbps data rate over a 26-m air-water channel with a 520-nm laser and orthogonal frequency division multiplexing (OFDM) in the same year; S. Hu et al. [5], in 2018, achieved a UWOC with 120-m transmission distance in Jerlov II water by using a pulse laser with a peak power of 10⁵ W, a photomultiplier tube (PMT)-based photon counter, and Reed–Solomon (RS) and low-density parity check (LDPC)-coded pulse position modulation (PPM). On the other hand, LEDs, which emit incoherent light and have big divergence angles, have a relaxed requirement on temperature control. Most LEDs are cheap, robust and convenient to use. Yet, most LED-based UWOCs are short in transmission distance [6], [7], [8], among which the longest transmission distance was achieved by M. Doniec et al. [9]. They realized a 50-m long bidirectional UWOC in an Olympic size pool with a maximum data rate of 2.28 Mbps based on discrete pulse interval modulation (DPIM) and AquaOptical II modem in 2010. The modem containing 18 LEDs with 10-W total output power ensured the communication link over such a long distance with a relieved requirement for link alignment. Others like I. Alsolami et al., provided an M/N-PPM scheme to improve information rate over photon-counting channels [10]. G. Zhang et al. theoretically discussed the minimum average incident light power required for a digital optical communication [11].

Underwater wireless optical signals suffer from severe attenuation and scattering, leading to more decision errors due to degraded signal to noise ratio (SNR) when the transmission distance is extended. This is particularly evident for some advanced modulation formats with multiple signal levels. To ensure sufficient signal SNR in an LED-based long-reach UWOC requiring moderate data rate, the modulation format with fewer levels and higher energy efficiency should be taken as preference. PPM, which denotes m-bit signal with a pulse in $L=2^m$ slots, has the advantages of higher energy efficiency and lower slot error rate (SER) [12]. With its simplicity in implementation, PPM is a preferred modulation format for long-reach UWOC systems [13].

In addition to the suitable modulation format, a sensitive detector with huge gain is also required in the system. Presently, most sensitive detectors used in UWOC systems are avalanche photon diodes (APDs) or PMTs, both of which feature fast response and high gain. A PMT, which amplifies signal by multiple reflections of photoelectrons between dynodes, is poor in mechanical robustness and requires a high operation voltage up to kilovolts, bringing difficulties in practical implementation. An APD can achieve a large gain via carrier avalanche multiplication in a PN junction under a proper reversed voltage. However, to detect signals of only several photons per symbol, a normal APD will meet its bottleneck of sensitivity. Fortunately, a Geiger-mode APD could further increase its gain and sensitivity through stronger carrier multiplication under a reversed voltage higher than the breakdown voltage [13]. In this way, the APD is sensitive enough for photon-scarce detection or even photon counting, and such an APD is called a single photon avalanche detector (SPAD). Once an SPAD detects one or multiple photons during a dead time, it will output a counting pulse, i.e. one count. SPADs have been widely used in optical communications [14], [15], [16] and quantum communications [17], [18].

In our previous work [13], a 46-m long-reach UWOC system was demonstrated via using a blue-light laser, PPM signals and a multi-pixel photon counter (MPPC) with analog output. The system could still work well even when the laser was operated under a spontaneous emission state, and then we naturally predicted that the MPPC may potentially enable a long-reach UWOC system even using low-cost LEDs as the transmitter. Following this work, we did replace the blue-light laser with a compact 3-W LED already available in the laboratory, but the 46-m UWOC link could not work well until we further replaced the original analog-mode MPPC with another digital-mode MPPC possessing decision threshold control.

In this work, we investigated the underlying working principle of an MPPC and proposed a UWOC system based on a cheap, commercially available LED and a lens-free MPPC with digital output. Its feasibility was deduced theoretically, and then demonstrated experimentally through a 46-m underwater channel. By carefully adjusting the decision threshold of the digital-mode MPPC, non-signal counts, such as that arising from background noise and dark count, can be effectively suppressed. PPM signals were used in the experiment to acquire high SNR, low SERs and bit error rates (BERs) as in [13], but the transmitter and receiver, key components of a UWOC system, are different from that used in [13]. The experimental results turned out that BERs were all below the forward error correction (FEC) limit, which were achieved with less than 100 incident photons during each pulse slot. The proof-of-concept single LED-based transmission was currently demonstrated via an underwater channel emulated by a 46-m white Polyvinyl Chloride (PVC) tube. However, we envision that for practical implementation the proposed scheme can be enhanced by using an LED array [9], to further extend the transmission distance and relax the requirement on link alignment [19].