Simultaneous wavenumber measurement and coherence detection using temporal phase unwrapping

Abstract

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.

Correction of Scanning Steps to Improve Accuracy in Interferometric Profilometer

“…In present time a lot of attention is paid to improvement of interference measurement accuracy [1][2][3][4]. The presence of noise in interferograms and non-linearity of the scanning system leads to distortion of the correlogram functions and error in the measurement results.…”

Improvement of the depth resolution in depth-resolved wavenumber-scanning interferometry using multiple uncorrelated wavenumber bands

“…Wide-field depth-resolved wavenumber-scanning interferometry (DRWSI) or swept-source optical coherence tomography (SSOCT) is a noninvasive 3D imaging technique, using wavenumber-scanning technology to detect the 3D structure and deformation field, etc., with the advantages of a large depthresolved range and stationary components [1][2][3][4][5]. The technology can be traced back to digital phaseshifting interferometry using a directly frequencymodulated diode laser in the 1980s, when phase shifting was found by altering the current to tune its wavelength [6].…”

“…The advantage is that this range of wavelength scanning makes the DRWSI have the finest depth resolution, at the expense of a relatively complex bulky optical system. Ruiz and Huntley et al provided a solution to employ a wide-field CCD camera and an external-cavity diode laser or a Ti:sapphire laser as the laser sources [2][3][4][5] in the DRWSI. The advantage is that it makes full use of the mature technology so as to improve its performance Due to its low cost, compact size, and multiple wavelength bands, the single-longitude-mode FabryPerot diode laser is a very promising application of DRWSI.…”

“…The only obstacle to prevent its further development is mode hopping in the output of these laser sources, which breaks the whole range of the wavelength or wavenumber scanning into uncorrelated narrow bands, and results in a poor depth resolution in the DRWSI. In 2012, Davila et al described a method to record the fringe patterns of multiple optical wedges to measure the wavenumbers on time with high resolution and immunity to the ambiguities caused by large wavenumber jumps [4,5]. Furthermore, in order to overcome the nonlinearity or the mode hops of the laser, the series of unequal-space sampling is linearly fitted for the Fourier transform (FT).…”

Comparison study of algorithms and accuracy in the wavelength scanning interferometry

“…A scanning range up to 100 nm was achieved by using either a solid state laser, such as Ti:sapphire [9], or an acousto-optic tunable filter (AOTF) coupled to a halogen light source [10]. Further enhancement in the measurement resolution made clear that the selection of a suitable algorithm could achieve nanometer accuracy without the need to combine WSI with other interferometry techniques.…”