Three-dimensional imaging is a topic in the field of optical sensing with ever-increasing interest and with enormous achievements during the past years. Most of the techniques are based on the measurement of distances combined with a scanning in the transversal direction. Two well-known techniques are optical frequency-modulated continuous-wave (FMCW) reflectometry and swept-source optical coherence tomography. One major challenge is to find a balance between resolution and maximum measurement distance. The paper of DiLazaro and Nehmetallah presents a very interesting concept to create a setup enabling long-distance together with high-resolution measurements. The basis of their concept is a multi-wavelength laser module that is designed for tunable transmitters in telecommunication applications. The authors developed a method to use this element together with auxiliary Mach-Zehnder interferometers and a hydrogen cyanide gas cell as a compact tunable laser source that is linearly tunable over a wide wavelength range in a very short time. Especially the correct stitching of the individual lasers is remarkable. The axial point spread function is determined taking into account also the group velocity and the dispersion slope of the used optical fiber. Furthermore, the scan distortion and tilt are also discussed.

The combination of high-performance components and sophisticated signal processing results in an FMCW tomography setup for long-distance measurements at high resolution. Several impressive examples range from surface profilometry over remote 2D tomography to large-volume 3D imaging. Especially the tomographic results from foam materials showing the individual cells of the foam and the measurement of a laminated embossed medallion, where the surface of the transparent laminate is clearly visible, must be highlighted here. Animations supplement the results from the large-volume measurements.

Despite the fact that the paper gives a detailed insight in the high-level research of the authors, they added also a very nice overview on the state of the art in their field. The paper is also a good starting point for those who just entered this field of optical distance measurement, three-dimensional imaging, and tomography.

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