29 Jun 2009
A laser ranging system targets future formation-flying satellite missions.
Researchers at the National Institute of Standards and Technology (NIST), US, have built a laser ranging system that can pinpoint multiple objects with nanometre precision over distances up to 100 km. The LIDAR (light detection and ranging) system could have applications from precision manufacturing on Earth to maintaining networks of satellites in perfect formation (Nature Photonics DOI: 10.1038/NPHOTON.2009.94).
"What is particularly exciting is that this system is more than the sum of its parts," NIST's Ian Coddington, told optics.org. "By combining interferometric and time of flight measurements we are able to perform absolute distance measurements with nanometre-scale resolution."
The key to the NIST device is the use of two coherent broadband fibre-laser frequency combs. Frequency combs output a series of stable short pulses that also contain a highly coherent carrier that extends across the pulse train. This means a frequency comb can be employed to simultaneously make an interferometric measurement as well as a time-of-flight measurement.
In the set-up, two phase-locked frequency combs are used in a coherent linear optical sampling configuration, also known as a multi-heterodyne. What this means is that one frequency comb measures the distance path, while the other is used to read out the distance information encoded in the light of the first comb.
Pulses from one frequency comb are launched out of the fibre and directed towards two glass plates (a target and a reference). The plates reflect 4% of the pulse back down the fibre, effectively creating two new pulses. The time separation between the two pulses gives the distance between the target and reference plates.
A second frequency comb is tightly phase-locked with the first, but has a slightly different repetition rate. Due to the different delay between consecutive pulses when the sources interfere, the second frequency comb samples a slightly different part of the light from the electric field of the first comb.
"The steps, or time resolution, can be quite small (500 fs in our case), which allows us to get such a precise time-of-flight measurement," explained Coddington. "What's more, optical sampling allows us to avoid detector bandwidth issues that often limit time-of-flight systems as well as allowing us to see the carrier phase needed for the interferometric measurement."
Coddington and colleagues next intend to decrease the time required to obtain nanometre-level measurements. "Currently we require 60 ms but we believe we should be able to get this down to 0.2 ms," he concluded. "This is fast enough to see a lot of acoustic noise and it would be nice to demonstrate."