D. Castro Alves, M. Abreu, A. Cabral, J. M. Rebordão

Abstract
We present an absolute distance measurement concept based on pulse frequency repetition (PRF) sweep with a mode-locked laser diode. This technique requires a Michelson interferometer, a mode-locked laser source and a cross-correlation detector. The role of the cross-correlator is to analyze the state of superposition of a pair of pulses, one travelling over the reference arm of the interferometer, the other, corresponding to the measuring arm, travelling from the apparatus to the target and reflected back. Using two distinct laser pulse repetition frequency and knowing the cross-correlation peak position at reference arm of the interferometer for each frequency, it’s possible to obtain the length of the measuring arm, i.e., a distance measuring. The main techniques for performing pulse correlation are based in 2nd order effects over second harmonic crystal (SHG) or a two-photon absorption (TPA) process in a semiconductor junction. Both SHG and TPA methods require reasonably high pulse energies to achieve fair signal to noise ratios. The use Degree of Polarization (DOP) technique for cross-correlation detection allows the use of optical powers as low as -60 dBm. Such high sensitivity can be very convenient for low energy pulse sources such is the case of high frequency mode-locked laser diodes. The DOP technique can however limit the maximum measurement range, constrained by the loss of coherence of the laser source for longer distances. In this paper we present the first results of this measurement concept, based in the DOP correlator, discussing the main limitations of this technique for long distance measurements. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Keywords
Mode-locked: Semiconductor: laser – Metrology – Pulse: Repetition: Frequency – Long: Absolute: Distance: Measurement

8th Iberoamerican Optics Meeting and 11th Latin American Meeting on Optics, Lasers, and Applications
M. F. P. C. M. Costa

SPIE
Volume 8785
2013 November

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