Wavelength scanning interferometry and swept-source optical coherence tomography require accurate measurement of time-varying laser wavenumber changes. We describe here a method based on recording interferograms of multiple wedges to provide simultaneously high wavenumber resolution and immunity to the ambiguities caused by large wavenumber jumps. All the data required to compute a wavenumber shift are provided in a single image, thereby allowing dynamic wavenumber monitoring. In addition, loss of coherence of the laser light is detected automatically. The paper gives details of the analysis algorithms that are based on phase detection by a two-dimensional Fourier transform method followed by temporal phase unwrapping and correction for optical dispersion in the wedges. A simple but robust method to determine the wedge thicknesses, which allows the use of low-cost optical components, is also described. The method is illustrated with experimental data from a Ti:sapphire tunable laser, including independent wavenumber measurements with a commercial wavemeter. A root mean square (rms) difference in measured wavenumber shift between the two of ~4 m⁻¹ has been achieved, equivalent to an rms wavelength shift error of ~0.4 pm.
Range (i.e., absolute distance), displacement, and velocity of a moving target have been measured with a frequency scanning interferometer that incorporates a
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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
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have been demonstrated over a depth measurement range of 300 mm.
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
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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.
A chip-scale solid-state wavelength measuring device based on a silicon photonics platform is presented. It has no moving parts and allows single-shot wavelength measurement with high precision over a nominal bandwidth of 40 nm in the Oband. The wavemeter design is based on multimode interferometer (MMI) couplers and a multi-band Mach-Zehnder interferometer (MZI) structure with exponentially increasing optical path differences and in-phase quadrature detection. The design of the MMI couplers is supported by simulations using the Finite-Difference Time-Domain (FDTD) method. The design, experimental evaluation, and calibration of the device are discussed. Observed performance indicates a spectral support of 38.069 nm (i.e., frequency bandwidth 6.608 THz), with a resolution of 8.3 pm (1σ), corresponding to 1 part in 4,587. This wavelength meter approach has emerged from a need in absolute distance measurements using frequency scanning interferometry, where knowledge of the instantaneous wavelength of a tunable laser is required to relate signal frequency with target range. We also present an adaptive delay line on a chip, demonstrate its use for range measurements, and suggest how the wavelength meter could evolve for real-time applications.
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