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

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.

Adaptive delay lines implemented on a photonics chip for extended-range, high-speed absolute distance measurement

“…Frequency scanning interferometry (FSI), also known as frequency modulated continuous wave lidar (FMCW), is nowadays an established technique for absolute distance measurements [1][2][3][4]. It can be implemented in single-point, rasterscanning and diverging beam configurations, and incorporated into laser trackers.…”

“…The main advantage of FSI vs. fringe counting (or differential) interferometers is that blocking the beam does not require homing a retroreflector and moving it back to the target position while maintaining line of sight to keep track of the fringe order. Applications range from measuring dimensional changes in the ATLAS detector in the Large Hadron Collider [5], to shape measurement [6,7] and displacement and target velocity measurements [1,2,8,9]. By incorporating gas cells, the range estimate can be made traceable to standards [10].…”

“…Their interference signal has a frequency proportional to 𝛬 and 𝑓 𝑠 , the optical path difference (OPD) between the two waves, and the laser scan repetition rate, respectively. For a linear frequency ramp and a non-dispersive medium 𝑓 = 𝛬 𝑓 𝑠 ∆𝜆/𝜆 𝑐 2 [1,2] with ∆𝜆 the tuning range and 𝜆 𝑐 the centre wavelength. State-of-the-art tunable laser sources such as VCSELs and FDML lasers can produce scan rates of 100s of kHz to several MHz, respectively, combined with ∆λ values exceeding 100 nm at 𝜆 𝑐 = 1.0, 1.3 or 1.5 µm.…”

“…Such systems are currently impractical due to the extremely high cost associated with sampling at these signal frequencies. Adaptive delay lines (ADLs) were recently proposed as a solution to balance the interferometer and therefore reduce sampling rate requirements by a factor of 2 N , where N is the number of switches in the ADL [1,2]. The technique has been successfully demonstrated in the lab using bulk optics and optical fiber configurations, and further reduction in size and cost will increase the breadth of metrology applications that can be addressed.…”

Study of 100 meters laser ranging with a non-optical interferometry

“…The traditional absolute distance measurement method, based on the principle of Michelson interferometer, generally adopts laser interferometric ranging technology, which has been widely used in absolute distance measurement [1], displacement measurement [2], surface topography measurement [3], Microstructure measurement [4], MEMS [5] etc.. However, It has a major problem "phase ambiguity".…”