Femtosecond comb lasers have the ability to precisely measure absolute distances in formation flying space missions, according to a new report from the European Space Agency and National Physics Laboratories (NPL). Femtosecond comb lasers emit light with extremely short pulses, each lasting just a few femtoseconds. (One femtosecond equals one billionth of one millionth of a second). Formation flying missions entail numerous spacecrafts flying from tens to hundreds of metres apart, each controlling their position relative to the others. Such a mission would be able to gather data in a completely different way to a standard spacecraft; a formation effectively would act as one large compound sensor. But accurately measuring and monitoring distance between the spacecraft would be critical.
Femtosecond comb lasers are one way of taking these measurements, as the minute pulse times allow time of flight measurements to be used to determine distance to within a few microns. For example, one mission, the Laser Interferometer Space Antenna (LISA) is being planned to search for gravity waves. This will entail three craft flying approximately 5 million km apart. In this case, knowing the absolute distance between the crafts would not be needed, since extremely small changes in their separation on timescales of 10 seconds to 10,000 seconds could be a sign that gravity waves have been detected.
This and other missions would benefit from the use of femtosecond combs. A number of groups around the world are currently developing other systems using them for distance measuring instruments. Prototype laser systems will have to have uncertainty claims verified by a national standards laboratory, such as NPL. They will also need to meet stringent specifications for size, weight and power consumption, plus go through other tests such as the system’s ability to withstand strong gravitational fields or radiation in order to become fully spaceworthy.