n recent years, underwater wireless optical
communication (UWOC) has attracted much attention
as it can provide a high transmission rate [1]-[6].
Compared to radio-frequency (RF) and acoustic
communications, UWOC has a much higher
transmission bandwidth, and thus providing much
higher transmission rate. Many applications of UWOC
systems have been proposed for environmental
monitoring, underwater oil pipe investigation, and
offshore exploration. With the rapid progress of UWOC
systems, the increasing requirements raise the needs
for long-range and high-speed underwater links. For an
effective implementation of UWOC systems, long
underwater link and high underwater transmission rate
are the key concerns of system designers. Blue light,
around the 405 nm band, is hardly absorbed in the
water. An UWOC system employing 405 nm blue light
laser diode (LD) is therefore expected to provide a long
underwater link. Two-stage injection-locked technique
has been used in a two-way passive optical network [7]
and a 10 m/25 Gbps light-based WiFi (LiFi)
transmission system [8] to improve the transmission
performance. However, it has not been used as a
system performance improvement scheme in an
UWOC system. Two-stage injection-locked technique,
which can greatly enhance the frequency response of
LD, is therefore expected to provide a high
transmission rate in UWOC systems. In this paper, an
8 m/9.6 Gbps UWOC system based on a two-stage
injection-locked 405 nm blue light LD transmitter with
16-quadrature amplitude modulation (QAM)-orthogonal
frequency-division multiplexing (OFDM) modulating
signal is proposed and experimentally demonstrated.
OFDM is an approach of digital modulation in which a
signal is divided into several narrowband channels on
multiple carriers. It is an effective technology which has
very high spectrum efficiency and robust dispersion
tolerance [9]. As far as we know, it is the first one that
employs a two-stage injection-locked 405 nm blue light
LD transmitter in an UWOC system. Over an 8-m
underwater link, acceptable bit error rate (BER)
performance and constellation map are achieved in the
proposed UWOC systems. Previous study has
demonstrated a 7 m/2.3 Gbps UWOC system using onoff
keying non-return-to-zero modulation scheme [2].
However, the transmission rate of 2.3 Gbps is much
less than 9.6 Gbps of our proposed UWOC systems.
And further, a 5.4 m/4.8 Gbps UWOC system has been
illustrated formerly [1]. Nevertheless, the transmission
distance of 5.4 m and the transmission rate of 4.8
Gbps are less than 8 m and 9.6 Gbps of our proposed
UWOC systems. Such proposed two-stage injectionlocked
405 nm blue light LD transmitter-based UWOC
system is shown to be a potential one to present its
feasibility for long-range and high-speed underwater
communications.