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
1. Introduction and motivation
Optical coherence tomography (OCT) has been established as powerful tool for cross-sectional imaging of microstructures in biological systems, with examples ranging from scans of the retina [1,2] and cornea [3,4] to investigations of the coronary arterial wall  or imaging of human skin [6,7] or teeth [8,9]. Recently, the great potential of OCT for material investigations as contact-free, non-destructive method with resolution in the ten-micrometer range has been demonstrated. However, only few applications, when compared with OCT in the biomedical field, have been reported up to now [10–14].
OCT employs low-coherence interferometry to obtain depth-resolved reflectivity images of the sample. Scattering and reflecting structures, especially surfaces and interfaces are visualized. Using broadband light sources in the near infrared, the penetration depth can reach some millimeters for translucent and turbid materials (e.g., polymer materials ). The axial (depth-) resolution is mainly determined by the coherence length of the light sources. Commonly, superluminescence diodes (SLDs) are used, which are capable of an axial resolution of about 10–20 µm. For details on the principles of OCT see, e.g., refs.  and .
OCT for material analysis poses high demands on the setup: the materials investigated are mainly plastics, polymers and compounds with feature sizes (e.g., diameter of glass fibers, thickness of layers, size of inclusions) often in the range of some few microns. Therefore, the depth resolution obtained with SLDs is too low for these applications. Only ultra-high resolution OCT with sophisticated broadband light sources provides axial resolutions down to 1 µm . In general, cross-sectional scans (B-scans) are obtained by OCT. Nevertheless, for many applications transversal scans with the imaging plane parallel to the surface (en-face scans) are advantageous, e.g., when the lateral distribution of inclusions should be determined. Recently, en-face scanning OCT techniques have been introduced, however with depth resolutions around 15 µm, using light emitting or superluminescence diodes (LED, SLD) as light source [10,11,18,19]. High-resolution en-face OCT has already been realized with full-field OCT setups, e.g. presented by Dubois et al.  and De Martino et al. , but only for lateral scan regions limited to areas as small as 300×300 µm2. Finally, the time required for a single OCT scan is a decisive factor. Especially when OCT is used for inline quality control, only setups capable of high-speed measurements can be applied.
In this work, we combine transversal with ultra-high resolution OCT for material investigation. Using the basic transversal OCT concept of Hitzenberger et al.  and refined by Pircher et al.  for polarization-sensitive OCT, we are able to obtain large-area en-face scans within short measurement times. For the first time, to the best of our knowledge, a Ti:sapphire femto-second (fs-)laser is used in combination with acousto-optic modulators (AOMs) for heterodyne signal generation. So far, the influence of AOMs on the broadband spectrum has not been investigated for OCT imaging, but for high-resolution applications it is essential that the full width of the spectrum is transmitted unaltered through the AOMs. By measurement of different samples we demonstrate, on the one hand, the benefits of OCT for material investigations and, on the other hand, the striking advantage of en-face scans for selected applications.
2. Experimental setup
The experimental setup (see Fig. 1) is based on a Mach-Zehnder interferometer, where sample and reference beam are guided in separate arms . In the reference arm, two polarization insensitive acousto-optic modulators (dense flint glass) shift the light frequency of the first order diffracted beams by 40 MHz and -38 MHz, respectively, resulting in a total frequency shift of 2 MHz . The first order diffracted beam from AOM 1 serves as input for the second AOM and is selected by an iris aperture. Interference with the sample beam results in beat and in a carrier frequency of the signal of 2 MHz, allowing high data acquisition rates by using a very fast lock-in amplifier (DSP lock-in 7280 from Signal Recovery, frequency range up to 2 MHz) and a PC with a data acquisition board.
In the sample arm, the laser beam is scanned over the sample by an xy-galvano scanner unit (Laser2000) which consists of a pair of scanning mirrors with a maximum scanning frequency of around 500 Hz. Both scanning mirrors are driven by sawtooth voltages, realizing fast scanning along the x- and slow scanning along the y-direction. In combination with the focusing lens in front of the sample with a focal length of 30 mm, large en-face scan areas can be acquired. The system can be switched instantly from cross sectional (only the x-galvano mirror in combination with the motorized stage of the reference mirror are driven) to transversal (en-face) scanning where x- and y-mirror are scanned simultaneously with the reference mirror kept fixed at a defined imaging depth. For a high lateral resolution a small focal spot size on the sample is required. Therefore, the beam originally having 1 mm diameter is expanded to a diameter of about 6 mm by a telescope before focusing on the sample, resulting in a numerical aperture of 0.1 and a diffraction limited focal spot size of 5 µm (1/e2 decay of the intensity) for the 30 mm focusing lens.
As light source for ultra-high resolution imaging, we use a femto-second Ti:sapphire mode-locked laser (Integral OCT from Femtolasers) with attached single mode fiber and an averaged output power of 50 mW ex-fiber at a pulse rate of 85 MHz. The measured spectral full-width-at-half-maximum (FWHM) of 124 nm at a center wavelength of 800 nm results in a theoretical coherence length  and axial resolution of 2.3 µm in air and correspondingly, of 1.6 µm in material (assuming a refractive index of 1.4 which is a typical value for plastics and polymers). Balanced detection (balanced photoreceiver from Femto) is applied to obtain a reduction of laser and excess noise, thus providing an enhanced signal to noise ratio. In our configuration, up to 5 mW of power is delivered to the sample. An adjustable neutral density filter was placed in the sample arm to avoid occasionally occurring saturation of the detectors by the sample signal. The reference beam is permanently attenuated with a 1.5 neutral density filter to reach shot noise limited detection.
Ultra-high resolution OCT requires an exact adjustment of the dispersion between reference and sample arms to achieve a narrow coherence peak. We use a BK7 prism pair in the reference arm to compensate second order dispersion due to the three additional lenses in the sample arm. For compensation of the AOMs, the same dense flint glass blocks as used inside the AOMs are placed in the sample arm.
Finally, it should be underlined that our chosen transversal configuration allows an easy implementation of dynamic sample focusing. For one cross-sectional scan, the reference mirror is moved only once (backwards), for an en-face scan the position is even fixed. Therefore, we are able to achieve dynamic focusing by placing the sample holder on an additional translation stage which can be moved independently and simultaneously with the reference mirror to match the depth of the focal point with the coherence gate.
3.1 Characteristics of the transversal OCT setup
Diffraction by the first AOM causes an angular spectral spreading of the reference beam, and after passing the second AOM, the beam is parallely re-collimated but remains laterally broadened with a continuous distribution of the different wavelengths across the beam diameter. Considering these effects, two questions arise: (i) is the full spectrum transmitted through the AOMs, and (ii) is a complete coupling into a single mode (SM) fiber (leading to the detector) possible. We measured the spectrum of the femto-second Ti:sapphire laser ex-fiber and in the reference arm after passing both AOMs and after coupling into the SM fibers of the two detectors, using a CCD spectrometer (AvaSpec 2000, Avantes). Comparison of the spectra shows that the Gaussian shape is nearly sustained and that the FWHM is not degraded (see Fig. 2), but that the full spectral width is passed through the AOMs and through the coupling lens-SM fiber system to the detector. The envelope function of the interferogram (insert in Fig. 2) obtained directly from the lock-in amplifier with a mirror as sample shows a FWHM of 2.95 µm (in air) for full second order dispersion compensation. The deviation from the theoretical value of 2.3 µm is caused by the limited performance of the standard 30 mm achromatic lenses used in the sample arm (one in the telescope and one for focusing on the sample). Measurement of a resolution target proves a lateral resolution better than 4 µm (smallest feature size of our target) at the maximum scan rate of about 500 Hz of our galvano scanner (corresponding to a time of 2 ms per scan line).
The imaging speed of the system is mainly restricted by the maximum scanning frequency. An en-face image with, e.g., 1000×1000 pixels and a scanning frequency of 500 Hz of the fast (x-) galvano mirror (2 ms per scan line and 2 µs per data point, 2 µs integration time at the lock-in amplifier, sampling rate of 0.5 mega samples per second) results in 2 seconds measurement time per en-face scan (0.5 Hz scanning frequency of the slow (y-) mirror). The galvano scanner and the optical configuration of the sample focusing unit were demonstrated to be capable of scanning areas of more than 6×6 mm2 at the 500 Hz scanning frequency. However, the non-linear response of the galvano scanner to the driving sawtooth voltage leads to already noticeable image distortions at the border regions of the acquired image, which could be in principle compensated by the acquisition software and will be dealt with in future refinements of the set-up. At a reduced x-scan rate of 100 Hz, the galvano mirror follows the driving voltage linearly. Therefore, most images presented in the next section were generated at an x-mirror frequency of 100 Hz with a measured system sensitivity of 100 dB for scan regions as large as 3×3 mm2. These images cover scan areas 100 times larger as compared to the en-face images acquired with, e.g., full-field OCT systems , obtained in comparable measurement times.
For many applications in material characterization, conventional cross-sectional OCT can provide important information on the quality of samples and their structure, like, e.g., the thickness, the number or the sequence of layers: as an example, we have measured commercial polyester and polyethylene composite foils as used in the food packaging industry. In detail, we investigated the welded seam between two of these foils (Fig. 3). In the OCT images, darker gray scale values correspond to higher signal values where more light was returned from the sample. Cross-sectional OCT reveals the complex composition of the two different foils, with the upper foil consisting of eight and the lower foil of four individual sheets. Different gray shadings give an indication of the different materials used. The complex structure of these foils enhances the structural strength (i.e., longitudinal and transversal tensile strength) and reduces their gas permeability (diffusion barriers for oxygen, carbon dioxide, nitrogen and water vapor), crucial for modified atmosphere packaging . Therefore it is of special interest, if the nominal sheet thicknesses (individual layers and total foil) are accurate within the specified tolerances and if damages occur during the subsequent steps of processing. In the example of Fig. 3, a thermal damage of the lower foil was detected with OCT on the left side of the welded region, where the multilayer structure was thinned and destroyed (molten), thus deteriorating the performance as diffusion barrier.
For other applications, often information within a layer is of interest, which cannot be obtained easily from cross-sectional OCT images. Of current interest for many products is an engineering of the surface for refinement purposes. As an example, protective coatings of laminate floor panels are provided with ceramic particles to obtain a better resistance towards abrasion. In this application, a homogenous lateral distribution of the particles is of crucial importance for the wear resistance and also for the optical appearance of the product to the eye (e.g., agglomerations of particles may deteriorate the appearance of the underlying decorative layer). By conventional cross-sectional OCT (Fig. 4(a), taken at an increased x-scan rate of 500 Hz), the particles within the resin layer are clearly resolved as dark spots and grey-shaded areas surrounded by the transparent lacquer matrix. The bright vertical lines are due to shadowing effects of opaque particles. However, information about the lateral distribution at a certain depth is difficult to obtain. When we perform en-face measurements (Fig. 4(b), taken also at 500 Hz x-scan rate), the lateral distribution of the particles at a defined depth is revealed within one single scan, without the need to reconstruct the same lateral information from whole three-dimensional sets acquired by thousands of subsequent cross-sectional scans.
Another promising application for transversal OCT is the investigation of soft cellular materials. Since long, foams and porous materials have found widespread applications as thermal insulators or absorbers of acoustic energy. Most recently, internally charged cellular polymers have emerged as a new class of piezoelectric materials with piezoelectric coefficients comparable or surpassing those of ferroelectric materials . The shape, size and the distribution of the voids are decisive for the mechanical and piezoelectrical properties of these materials. The anisotropy in the elastic stiffness and piezoelectric tensor elements is, for example, correlated with the three-dimensional structure of the voids . Future applications range from totally flat and flexible loudspeakers (“talking advertisement poster”) to large area pressure sensitive sensors . So far, the geometrical parameters of the cells could only be determined by a tedious destructive sectioning procedure and subsequent scanning electron microscopy analysis, yielding only cross-sectional information on the void size . We have performed OCT measurements on a closed-cell polyolefin foam sample to demonstrate the potential for fast and non-destructive characterization of these voids. The cross-sectional OCT image (Fig. 5(a)) shows a penetration depth of around four layers of voids. By performing en-face OCT scans, we can image the shape and size and the distribution of the cells within the layer (Fig. 5 (b)). Information about the wall thickness is obtained and, in that case, the occurrence of double wall structures is observed. When we record en-face scans at different depths, the development of single structures with increasing depth can be tracked (movie in Fig. 5(b)). En-face scans are, on the one hand important for modeling the mechanical and piezoelectric properties of cellular polymers, and on the other hand also for the optimization of the void forming process and can in the future be used, for example, for an in-line control of the foaming process.
A recent development for the production of micro electromechanical systems (MEMS) is the electrochemical moulding of metallic parts in thick resist layers which have been patterned by photo or X-ray lithography before . High aspect ratio structures (height vs. width) which are not achievable with other methods can be made by these LIGA like techniques (LIGA-lithography, electroplating and moulding). The very high quality and reproducibility require a precise control of the resist structures. We have investigated resist moulds for micromechanical parts (gear wheels) in 1.3 mm thick photoresist on a gold coated wafer substrate. A cross-sectional OCT scan across a wheel mould makes the surfaces and interfaces of the structure oriented parallel to the wafer surface (Fig. 6(a)) clearly visible. The different levels represent the resist surface to air, the bare wafer where the resist has been removed, and the interface between resist and wafer, respectively. The resist-wafer interface shows a different depth compared to the bare wafer surface because of the larger optical path length of the OCT sample beam within the resist layer with a refractive index n=1.6. However, besides the thickness distribution of the resist layer hardly any information about the geometrical structure of the wheel mould itself can be obtained.
In contrast, the en-face scans in Fig. 6(b)-(d) recorded at the three levels clearly show the structure of a wheel. Information about dimensions and distances or defects like deformed teeth can directly be obtained from all three images. In addition, the observed surface and interface patterns are of special interest. The image of the resist surface to air exhibits a wavy appearance, resist residues and other defects (e.g., particles) can be detected at the bare wafer surface, and a peculiar network of bright lines appears at the resist-wafer interface. By comparative profilometry and atomic force microscopy measurements of resist structures removed from the wafer, the surface structures could be linked to surface roughness with the highest elevations reaching up to 100 nm. The bright lines at the interface could be attributed to ridges with an average width of about 50 µm and a mean height of around 300 nm covering the rear surface of the resist. These ridges are detected at the resist-wafer interface “in situ” for the first time. Their nature is not clear up to now and an explanation is highly speculative. Probably, they could be the result of resist shrinking during the photochemical crosslinking reaction, but other causes like underetching or insufficient resist adhesion are also possible. However, information like those from Fig. 6(b) to (d) is of crucial interest for a quick and non-destructive characterization of the high aspect ratio lithographic resist patterns, which is not achievable up to now. Transverse OCT as demonstrated has the potential to accomplish such important data within seconds even for feature sizes in the 100 nanometer range in the vertical dimension. This is of special interest in order to avoid further (expensive) processing which is at present necessary before defaulting structures can be detected.
4. Conclusions and outlook
For the first time, to the best of our knowledge, we have demonstrated that a broadband fs-Ti:sapphire laser can be used in combination with AOMs in an ultra-high resolution transversal OCT setup. Heterodyne detection and the use of a galvano scanner allow to obtain large-area en-face images with scan sizes of several mm2 within few seconds (e.g., 3×3 mm2 with 4 µm lateral resolution and 100 dB sensitivity). The imaging speed of our OCT system is mainly restricted by the maximum scanning frequency of the galvano-scanner. This limitation can be overcome by the use of faster resonant scanners with scanning frequencies up to some kHz. Our setup was demonstrated to be capable of scanning areas of more than 6×6 mm2 at the 500 Hz scanning frequency, however with decreasing sensitivity and image distortion at the borders of the image, which could be improved by taking into account the non-linear response of the galvano scanner (software compensation) and by a more sophisticated design of the beam-focusing optics placed between scanner and sample.
As to the applications outside the biomedical field, we have demonstrated for different examples, that high-resolution en-face OCT provides information which can hardly be obtained by conventional cross-sectional imaging. The large scan areas allow a statistical evaluation of lateral distributions within a layer, and in-plane structures even of covered interfaces within microstructures can be imaged. Thus, en-face scanning OCT is a promising tool for many applications in material analysis and characterization: although ultra-high resolution transversal OCT is still far away from being applied as a standard technique in industrial inspection (e.g. as a 100% quality control device), it might prove to be a valuable tool (1) for statistical random sampling in production (e.g., in incoming good inspection to check required specifications), (2) for industrial process and product development and (3) for material research, as a fast and non-destructive characterization technique.
This work has been supported by the Austrian Science Fund FWF (Projects: P16585-N08 and P16776-N02) and the European Commission (FP6 CRAFT Project: COOP-CT-2003-507825).
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