REMOTE SENSING: Frequency-resolved lidar improves range and vibration measures

Scientists at the National Institute of Standards and Technology (NIST; Boulder, CO) have used a broadband femtosecond fiber laser to make frequency-resolved coherent lidar measurements over a fiber length of 1 km of target displacements as small as 60 mm.¹ The NIST system uses a free-running IR femtosecond laser with a 50-MHz repetition rate emitting a frequency comb with individual linewidths of less than 4 kHz.

The frequency comb serves as the light source and as a precise ruler for measuring the reflected heterodyne signal, which is spectrally resolved into N channels using an arrayed waveguide grating. The data can be processed using software developed at NIST, either incoherently to produce a vibration profile or coherently to produce a range image. In either case, this system has N-times superior performance to a single-channel conventional coherent lidar. To resolve problems related to signal noise and dispersion, measurements are averaged across the channels, effectively multiplying the precision of the result by the number of channels.

The system was used to determine the distance to, and vibration of, a rotating disk under a variety of conditions. In one case, the reflected light was transmitted through 1 km of optical fiber wrapped around a spool and the system measured a 60-mm displacement across the disk surface at the 1-km range. Conventional laser-ranging systems would fail to produce usable data on a target at a 1-km range (false-color image, top). By contrast, the NIST system obtains a clear image of the relative changes in the range to the target with time. The rise and fall of the plot (bottom) represents minute changes in range from the detector to the surface of a rotating wobbling disk.

REFERENCE

1. 1. W.C. Swann, N.R. Newbury, Optics Lett., in press.