Underwater laser communication is a technique that employs laser propagation under water to transmit data from one point to another. The technology is useful where physical connections through fibre optic cables are impractical due to high costs or other considerations.

Free Space Optics (FSO) technology is based on connectivity between FSO-based optical wireless terminals, each consisting of an optical transceiver to provide full duplex (bi-directional) communication functionality. Each optical wireless terminal uses an optical source (laser LED) and a telescope that transmits light through the atmosphere to the receiver. The receiver consists of an optical system and an opto-electronic front end that, in turn, comprises a pin photodiode or avalanche photodiodes, which convert the optical input into an electrical output.

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FSO technology for high-speed and secure data transmission was originally developed by the US military and NASA. It has been used for more than three decades in various forms to provide fast communication links in remote locations. Over the years, FSO technology has gained acceptance in the telecommunication industry, particularly in the enterprise campus networking environment. Due to the enormous bandwidth capabilities of FSO transmission and the worldwide unlicensed nature of the transmission spectrum, FSO technology has enormous potential for short-distance wireless connections.

Apart from the obvious advantages of the high rates of laser communication underwater, such transmission is also inexpensive, immune to jamming and has a low signal-to-noise ratio. However, line-of-sight is a major disadvantage. The basic concept of laser communication between two computers, both in free space and under water, is more or less the same. However, factors that impact performance underwater, like water salinity and turbidity, have to be taken into consideration.

In this article, design of the actual concept of underwater communication is proposed to be configured around green laser module that is intensity modulated by the intelligence signal to be transmitted. On the receiver side, a suitable photo-sensor-based circuit is used to demodulate the signal. On the transmitter and receiver side, the computer interface is through RS-232. The data file to be transmitted is converted into a serial bit stream, and then used to modulate the laser through the RS-232 serial interface. On the receiver side, the laser beam is detected and the serial bit stream is recovered. The bit stream is converted back to the desired format in the PC. The latter part of the article covers the advantages of laser communication over optical communication under water, and also discusses the emerging concepts and their implementation.

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Operational methodology
The following section covers the operational principle of underwater laser communication links. The proposed communication setup comprises five major elements—the source of the data to be transmitted (in this case, the PC), the transmitter, communication channel, receiver and the data receiving terminal (PC).

Fig. 1: The transmitter block schematic
Fig. 1: The transmitter block schematic
Fig. 2: The receiver block schematic
Fig. 2: The receiver block schematic

The data file in the PC, which is to be transmitted to another remote PC, is first converted into a serial bit stream in RS/EIA232 format with the help of hyper terminal software that comes with MS Windows and is available on the PC.

Fig. 3: Laser communication for naval applications
Fig. 3: Laser communication for naval applications

In the next step, this serial bit stream (representing data) is converted into TTL compatible pulses with the help of integrated circuit (IC) MAX232. This is a dual EIA232 driver/receiver. It has two on-chip independent EIA232-to-TTL/CMOS drivers and two on-chip independent TTL/CMOS-to-EIA232 receivers. On the transmitter side, one of the receivers has been used to convert the EIA232 bit stream into equivalent 5-volt TTL/CMOS-compatible pulses.

The TTL-compatible output pulses are then fed to a non-inverting adder circuit configured around the low- noise op-amp LM324. The other input to the adder is a DC voltage, which can be adjusted from approximately 0.5 volt to 1.7 volts. The magnitude of TTL pulses is also adjustable. This is required to get the desired magnitude of the drive current through the laser diode. This circuit is part of the overall laser diode driver circuit.

The output of this circuit feeds a constant current source driver circuit configured around the op-amp LM324 and an npn Darlington transistor. The magnitude of constant current is given by the voltage appearing at terminal 3 of this op-amp and the value of resistance RE.

The laser source used is a diode pumped solid state laser operating at 532 nm (green) with an output power of 10 mW. Green laser is used for underwater laser communication links, as it has the least attenuation in water. The proposed high-power green laser, which combines high power and high brightness with low signal noise at high repetition rates, has a great potential.

The current drive to the laser varies between two values corresponding to the low and high levels of the TTL pulse stream. As a result, the laser beam gets intensity modulated, i.e., its intensity or power gets modulated. It is a type of pulse amplitude modulation. The modulated laser beam is transmitted towards the receiver. The laser communication needs line-of-sight conditions, i.e., the transmitter and receiver should be in line-of-sight.

The modulated laser beam falls on the photo diode (type FND100). FND100 is a silicon PIN photo diode. The corresponding current signal generated by the photo diode is converted into an equivalent voltage signal, which is then amplified in a non-inverting amplifier circuit configured around op-amp AD829—a high-speed, high- bandwidth op-amp.

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The amplified signal is then fed to a voltage comparator configured around op-amp comparator LM319, which is a high-speed dual comparator. One of the two comparators available on the chip has been used here. The comparator output is a TTL compatible pulse stream representing data.

This pulse stream is then fed to a TTL buffer of non-inverting type (SN 74LS244). 74LS244 is a hex non-inverting buffer. Three buffers are used in parallel for higher current drive capability. The buffer output feeds the TTL/CMOS input of one of the drivers in the MAX232 converter. The corresponding EIA232 output feeds the PC through the serial port. The hyper terminal software in the PC converts the EIA232 bit stream back to a data file, which is stored in the memory and is available on the desktop.

Laser communication—advantages and applications
Fibre-optic communication can be implemented for a longer distance underwater as compared to a laser system since distortion is much higher underwater than in free space, but underwater laser communication gives us the distinct advantage of flexibility. Fibre-optic communication can only be done where the optical fibre communication setup is available and installed. Two submarines can easily transfer data anywhere in real time using an underwater laser system, just by establishing line-of-sight.

Fig. 4: Satellite-to-submarine communication
Fig. 4: Satellite-to-submarine communication
Fig. 5: An interplanetary TV lin
Fig. 5: An interplanetary TV lin

Laser communication represents a mature, reliable approach for broadband access. Such systems have been engineered to provide robust performance that is highly competitive with other access approaches, offering high capacity, excellent availability of 99.9 per cent, lowest cost per bps, and rapid deployment in less than one hour. The use of an encoder and decoder allows the development of secure communication links. Secure optical communication links are being extensively used in the defence industry to transmit secret information. These systems are compatible with a wide range of applications and markets, and are sufficiently flexible to be easily implemented using a variety of different architectures. Because of these features, market projections indicate healthy growth for optical wireless sales.

Microwaves cannot be used for underwater laser communication as they do not penetrate through water. Very-low-frequency sound waves transmit through water but they cannot enable high rates of data transfer. Blue-green lasers penetrate through water and are used for high-speed communication applications.

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Inter-satellite links employ infrared laser beams for transmitting data from satellite to satellite. Lasers are used, as they do not suffer from attenuation in space and do not have precise pointing requirements.

The application potential of laser communication is expanding at a brisk pace and many new uses are emerging. Laser communication is also being used for satellite-to-satellite links, satellite-to-submarine communication and interplanetary TV links.

Emerging trends in laser communication
Satellites can be used to communicate with many submarines that are submerged in sea water at depths of 100 metres or so. This would eliminate the need for submarines to come to the surface to establish communication, which would reduce their vulnerability. This concept is highlighted in Fig. 4. Satellites in geostationary orbit transmit a large number of narrow blue-green laser beams to create random spots on the ocean, with each beam transmitting encrypted data. A large number of spots are generated, creating empty positions so as not to give away the location of the submarines. Blue-green laser is used for maximal penetration in sea water.

The concept of the interplanetary TV link is shown in Fig. 5. The setup makes use of a satellite orbiting around a planet, with which the link has to be established, and a satellite moving in a geostationary orbit around the earth. The planetary satellite makes use of a low-power laser to transmit signals. The earth-orbiting satellite will have a sensor to receive the optical signal, process it and convert it into microwave signals. The signal is converted from the optical spectrum to the microwave spectrum, as the optical signals do not penetrate clouds and are highly attenuated by rain. The conversion therefore allows the establishment of a non-interruptive link, which enables the monitoring of events happening on different planets on a real-time basis.

Underwater laser communication is an immensely interesting and attractive research topic for scholars and academics. Many advanced countries around the world are trying to successfully implement the technology for military purposes. Successful implementation of this technology for long- distance use is a challenging and uphill task; however, if achieved, it will be a major step forward in the field of communication.

The author is a technical editor trainee with Wiley Publications