High-speed range and velocity measurement using frequency scanning interferometry with adaptive delay lines [Invited]

Abstract

Range (i.e., absolute distance), displacement, and velocity of a moving target have been measured with a frequency scanning interferometer that incorporates a 100 , 000 s c a n s − 1 vertical-cavity surface-emitting laser with 100 nm tuning range. An adaptive delay line in the reference beam, consisting of a chain of switchable exponentially growing optical delays, reduced modulation frequencies to sub-gigahertz levels. Range, displacement, and velocity were determined from the phase of the interference signal; fine alignment and linearization of the scans were achieved from the interferogram of an independent reference interferometer. Sub-nanometer displacement resolution, sub-100-nm range resolution, and velocity resolution of 12 µ m s − 1 have been demonstrated over a depth measurement range of 300 mm.

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.…”

“…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. These characteristics enable high-accuracy long-range distance measurement at rates in the range 10 5 −10 6 s −1 [2], thus offering significant potential for dimensional quality control at rates 2-3 orders of magnitude higher than current commercial FSI-based instruments. However, there are significant challenges that need to be overcome.…”

“…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.…”