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En-face scanning optical coherence tomography with ultra-high resolution for material investigation

Karin Wiesauer, Michael Pircher, Erich Götzinger, Siegfried Bauer, Rainer Engelke, Gisela Ahrens, Gabi Grützner, Christoph K. Hitzenberger, and David Stifter

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Abstract

Optical coherence tomography (OCT) is an emerging technique for cross-sectional imaging, originally developed for biological structures. When OCT is employed for material investigation, high-resolution and short measurement times are required, and for many applications, only transversal (en-face) scans yield substantial information which cannot be obtained from cross-sectional images oriented perpendicularly to the sample surface alone. In this work, we combine transversal with ultra-high resolution OCT: a broadband femto-second laser is used as a light source in combination with acousto-optic modulators for heterodyne signal generation and detection. With our setup we are able to scan areas as large as 3×3 mm2 with a sensitivity of 100 dB, representing areas 100 times larger compared to other high-resolution en-face OCT systems (full field). We demonstrate the benefits of en-face scanning for different applications in materials investigation.

©2005 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. Schematic OCT setup based on a Mach-Zehnder interferometer. The light source is a femtosecond (fs-) Ti:sapphire broadband laser. In the reference arm, two acousto-optic modulators (AOMs) are placed. The laser-beam is scanned over the sample by an xy-galvanometer scanner. Abbreviations: SMF-single mode fiber; NPBS-non-polarizing beamsplitter; DAQ-data acquisition.

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Fig. 2.
Fig. 2. Normalized spectra (vertically shifted) of the Ti:sapphire fs-laser directly ex-fiber from the laser (green), in the reference arm after passing the AOMs and after coupling into the single mode fibers of the two detectors of the balanced receiver (blue). The shape and width of the original spectrum is nearly sustained. Insert: Demodulated interferogram (from a single depth- or A-scan) directly from the lock-in amplifier with a mirror as sample. For full second order dispersion compensation, a FWHM and axial resolution of 2.95 µm (in air) is obtained.

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Fig. 3.
Fig. 3. Cross-sectional OCT image (composed of three individual scans) of two welded polyester and polyethylene composite foils as used in the food packaging industry. Clearly, the multi-layered structure of the foils is revealed, with eight layers for the upper and four layers for the lower foil with a thermally damaged region on the left side of the image (arrow).

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Fig. 4.
Fig. 4. (a) Cross-sectional and (b) 3×3 mm2 en-face scan of a laminate floor panel with ceramic particles within the resin layer, recorded at an increased x-scanning frequency of 500 Hz. From the en-face scan, the lateral distribution of the particles at a depth indicated by the dashed line in (a) can be determined.

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Fig. 5.
Fig. 5. (a) Cross-sectional scan and (b) (0.9 MB) movie of 3×3 mm2 en-face scans of a polyolefin foam specimen recorded at different depths in steps of 40 µm, starting close to the surface (larger thickness of the cell walls). Not only the evolution of single cells with increasing depth can be tracked, but also the three-dimensional structure of the voids can be obtained.

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Fig. 6.
Fig. 6. (a) Cross-sectional scan and schematic drawing of a mould for a micro mechanical wheel in a 1.3 mm thick photoresist layer on a gold coated wafer; (b)–(d) 3×3 mm2 en-face scans of the structure. In (a), only the surfaces of the bare resist and the wafer, and the resist-wafer interface can be distinguished. In (b)–(d), the full geometric information of the structure at these levels is obtained. In (b) the resist surface is imaged, (c) and (d) were recorded at depth positions of the optical path length corresponding to the bare wafer surface and the resist-wafer interface (as shown in (a)), respectively. Design of the wheel structures: Micromotion GmbH, Mainz (Germany).

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