Laser communications are more efficient than microwave links

Sending information by radiation Information is coded into a transmitted signal by modulating it. That's a fancy way of saying we change one of its properties in a way that can be measured at a receiver. This could be as simple as varying its brightness, like flashing a light. It could be just like Morse code, but the actual codes used today are more sophisticated than just the letters and numbers defined for Morse. A transmission link that doesn't use a wire or other physical medium operates by sending radio waves, microwaves, light, or any other form of radiation from a transmitter to a receiver. Transmitted radiation is a continuous stream of waves, and so has a wavelength (the distance between successive waves) and a frequency (the number of waves to pass a given point in one second). Electromagnetic radiation in a vacuum always travels at the speed of light. Microwaves have a wavelength of a few centimetres, while light has a wavelength of less than a micrometre, thousands of times smaller. Since the wavelength is so much smaller and they travel at the same speed, many more waves of light pass a given point per second than of microwaves (that is, light has a much higher frequency).

Sending information fast Speed of communication links is measured in 'bits per second' (bps). A 'bit' is the smallest unit of information - like the dot or dash in Morse. To send information as fast as possible, the transmitted radiation must be modulated as fast as possible. But radiation can't be modulated faster than its own frequency - you can't start and stop a stream of waves in a space shorter than the length of a wave! So thousands of times more information can be sent in the same time using light instead of microwaves.

Sending information cheap To successfully transmit one bit of information, enough of the transmitted power must be received to be able to identify the bit. If the coding scheme used were Morse, that means we've got to receive enough power to tell the difference between a dot and a dash. If lots of bits of information are being transmitted, it will take lots of power to carry it. For a spacecraft orbiting Mars, power is very limited and it makes sense to talk about the 'cost per bit' of information transmitted. When radiation is transmitted, it spreads out through space and not all of it goes to the receiver. The more tightly the radiation can be focussed towards the receiver, the less is wasted by being sent off in other directions. There is a physical limit to how tightly radiation can be focussed. It depends on the wavelength (big waves can't be focussed as tightly as little ones) and on the size of the transmitter (a big transmitter can focus radiation more tightly than a small one). So to focus as much as possible of the transmitted power onto the receiver, we want a big transmitter and small wavelengths. A laser transmitter could readily be imagined to measure a few metres in diameter. To match that focussing ability, a microwave transmitter would need to be a few kilometres across. If a 2-metre diameter microwave transmitter were to send data from Mars to Earth, the best focus it could achieve would spread the energy over a spot hundreds of thousands of kilometres in size. A 2-metre laser transmitter could focus its energy into a spot only a few hundred kilometres across. With energy focussed this much better, a million times more power gets to the receiver. The transmitter doesn't need to use so much power to be detected, and the 'cost per bit' of running the communications link plummets.

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Originally created by Andrew McGrath on August 5, 2002