Ocean that covers more than 70% of Earth’s surface, is rich in biological resources, petroleum and other mineral resources. Ocean exploration has earned more and more attention since it is very important in science and economics. Although the underwater acoustic communication has been well-established, it suffers from low transmission rate, long transmission delay spread, low confidentiality and larger power consumption because of the strong absorption and scattering of sea water, and simultaneously, an ultrasonic wave could hurt marine life such as dolphins, etc. In the 1980s, it was found that there was a low attenuation “window” of seawater in the blue/green region of visible spectrum and therefore, the underwater wireless optical communication has become a research hot-spot since it can provide a higher data rate, a wider transmission bandwidth and a smaller transmission delay spread, and a low power consumption as well.
Wireless Blue/Green Laser Transmitter The optical transmitter is composed of a blue/green laser module, a driver module, a modulation module, and an optical collimating lens as an optical antenna, where the laser possesses a wavelength of 520nm, a nominal power of 10mW and a bandwidth of 100MHz. In order to design a laser driver and modulation circuit together one IC, ADN2848, is selected, which converts the an input differential signal into an corresponding current signal to directly modulate the laser diode by the Imod pin, while it generates a 36mA bias current to provide a DC bias for the laser diode. An initial output of this IC is determined by values of external resistances from pins of RSET and ERSET to ground.
The optical receiver is constituted of a photodiode, a preamplifier, a limiting amplifier and a CDR circuit. A wireless laser signal received via an optical antenna is converted to an electrical signal by a photodiode (PD), and then amplified twice by a pre-amplifier and a limiting amplifier to recover the high-speed digital signal. Since the Ethernet conversion card developed by us includes a recovery circuit in our real-time service transmission experiment, there is not a recovery circuit here.
A. Test of Bi-directional Communication System in Clear Water
The block diagram of bi-directional communication system that consists of our developing bi-directional transceiver modules and a water tank injected by some height of clear water.
Block diagram of bi-directional communication system
The variation of received optical power for our communication signal going through six different types of seawater. It can be seen that the attenuation of an optical signal through underwater channels becomes larger and its scattering is getting stronger with the increase of salinity and turbidity of seawater. They basically agree with our channel simulation results. The BER curve of received signals of 100Mbps through six different types of seawater. It can be seen that their BERs are all lower than 10-10 whether the green signals goes through a deep sea water, a coastal seawater or a turbid seawater, and simultaneously the BERs gradually increase with augmenting of a turbidity for seawater. For the most turbid seawater, the BER of optical signals has reached 10-1, which is unable to communicate.
In addition, the variation of BERs with changing of bitrates from 100Mbps to 150Mbps is experimentally measured through six different types of seawater channels when a 2151 PRBS generated by a BERT is employed. It can be seen that the BER increases with rising of seawater turbidity at the same data rate and, in the same type of seawater, the BER increases when the data rate goes up.
In this work, two underwater wireless optical transceivers with a green wavelength and high bit-rate are developed, and a low-cost, full-duplex underwater wireless optical communication system supporting data rates over 100Mbps is established. The feasibility of the system is demonstrated and verified experimentally. Moreover, the impact of different types of seawater on the performance of wireless optical channel is experimentally investigated.