Adaptive delay lines for absolute distance measurements in high-speed long-range frequency scanning interferometry

Christos A Pallikarakis,Pablo D Ruiz,Jonathan M Huntley

doi:10.1364/osac.404581

Abstract

The application of frequency scanning interferometry to long-range (∼10 m) high-speed (upwards of 105 coordinates s−1) absolute distance measurement is currently impractical at reasonable cost due to the extremely high modulation frequencies (typically 100 GHz or more). A solution is proposed here based on an adaptive delay line architecture, in which the reference beam passes through a series of N switchable delay lines, with exponentially-growing delays. The benefits include a reduction by a factor of 2 N in the required signal sampling rate, in the size of dataset to be processed, and in minimum allowable source coherence length, thus paving the way for the use of fast sweeping sources such as vertical-cavity surface-emitting lasers (VCSELs) and Fourier-domain mode-locked (FDML) lasers for long-range lidars. The validity of the principle has been demonstrated experimentally by means of a three-switch prototype.

Highlights

Optical techniques for absolute distance measurement have been developed in many branches of science and engineering
Those based on a tunable laser source find applications over length scales from the sub-mm in the form of swept source Optical Coherence Tomography (OCT), to tens of m or more with Frequency Scanning Interferometry
When all three half wave plates (HWP) are in this orientation, implementing the 000 bit configuration, the reference beam follows the blue path and the optical path difference (OPD) for the interferometer is the difference in optical path between the red and blue lines

Summary

Introduction

Optical techniques for absolute distance measurement have been developed in many branches of science and engineering. The resulting 1024 steps defined by the bit pattern B = bN−1bN−2 · · · b1b0 through which the imbalance between the reference and the object arms can be adjusted, cover the range 0 to ∼ 10 m whilst the maximum required sampling frequency is reduced by 1024×, to that for a Λ of 10 mm.

Results

Conclusion