Peng Bu

Full-range parallel Fourier-domain optical coherence tomography using sinusoidal phase-modulating interferometry

We demonstrate a full-range parallel Fourier-domain optical coherence tomography (FD-OCT) in which a tomogram free of mirror images as well as DC and autocorrelation terms is obtained in parallel. The phase and amplitude of two-dimensional spectral interferograms are accurately detected by using sinusoidal phase-modulating interferometry and a two-dimensional CCD camera, which allows for the reconstruction of two-dimensional complex spectral interferograms. By line-by-line inverse Fourier transformation of the two-dimensional complex spectral interferogram, a full-range parallel FD-OCT is realized. Tomographic images of two separated glass coverslips obtained with our method are presented as a proof-of-principle experiment.

In situ aberration measurement technique based on principal component analysis of aerial image

We propose a novel in situ aberration measurement technique for lithographic projection lens by use of aerial image based on principal component analysis (AMAI-PCA). The aerial image space, principal component space and Zernike space are introduced to create a transformation model between aerial images and Zernike coefficients. First the aberration-induced aerial images of measurement marks are simulated to form an aerial image space with a statistical Box-Behnken design pattern. The aerial image space is then represented by their principal components based on principal component analysis. The principal component coefficients of the aerial images are finally connected with Zernike coefficients by a regression matrix through regression analysis. Therefore in situ aberration measurement can be achieved based on the regression matrix and the principal component coefficients of the detected aerial images. The measurement performance of the proposed AMAI-PCA technique is demonstrated superior compared to that of the conventional TAMIS technique by using a lithographic simulator tool (Prolith). We also tested the actual performance of AMAI-PCA technique on a prototype wafer exposure tool. The testing results show our proposed technique can rapidly measure the aberrations up to high-order Zernike polynomial term with 1σ repeatability of 0.5 nm to 2.3 nm depending on the aberration type and range.

One-shot parallel complex Fourier-domain optical coherence tomography using a spatial carrier frequency

We propose a novel method of one-shot parallel complex Fourier-domain optical coherence tomography using a spatial carrier frequency for full range imaging. The spatial carrier frequency is introduced into the 2-D spectral interferogram in the lateral direction by using a tilted reference wavefront. This spatial-carrier- contained 2-D spectral interferogram is recorded with one shot of a 2-D CCD camera, and is Fourier-transformed in the lateral direction to obtain a 2-D complex spectral interferogram by a spatial-carrier technique. A full-range tomogram is reconstructed from the 2-D complex spectral interferogram. The principle of this method is confirmed by cross-sectional imaging of a glass slip object. (c) 2008 Society of Photo-Optical Instrumentation Engineers.

Dynamic full-range Fourier-domain optical coherence tomography using sinusoidal phase-modulating interferometry

We propose a technique for dynamic full-range Fourier-domain optical coherence tomography by using sinusoidal phase-modulating interferometry, where both the full-range structural information and depth-resolved dynamic information are obtained. A novel frequency-domain filtering algorithm is proposed to reconstruct a time-dependent complex spectral interferogram from the sinusoidally phase-modulated interferogram detected with a high-rate CCD camera. By taking the amplitude and phase of the inverse Fourier transform of the complex spectral interferogram, a time-dependent full-range cross-sectional image and depth-resolved displacement are obtained. Displacement of a sinusoidally vibrating glass cover slip behind a fixed glass cover slip is measured with subwavelength sensitivity to demonstrate the depth-resolved dynamic imaging capability of our system. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Full-range parallel Fourier-domain optical coherence tomography using a spatial carrier frequency

We propose a method of parallel full-range Fourier-domain optical coherence tomography (FDOCT) that is capable of acquiring an artifacts-free B-scan image by a single shot of a two-dimensional (2D) CCD camera. This method is based on a spatial carrier technique in which a spatial carrier-frequency is instantaneously introduced into the 2D spectral interferogram registered in parallel FDOCT by using a grating-generated line-reference beam. The spatial-carrier-contained 2D spectral interferogram is processed through Fourier transformation to obtain a complex 2D spectral interferogram, from which a full-range B-scan tomogram is reconstructed. The principle of our method is confirmed by imaging a tropical fish eyes anterior chamber and a shrimp telson in vivo. The suppression ratio of the complex conjugate artifact can reach 36 dB.

Full-range Fourier domain Doppler optical coherence tomography based on sinusoidal phase modulation

A novel full-range Fourier domain Doppler optical coherence tomography (full-range FD-DOCT) using sinusoidal phase modulation for B-M scan is proposed. In this sinusoidal B-M scan, zero optical path difference (OPD) position does not move corresponding to lateral scanning points in contrast to linear B-M scan. Since high phase sensitivity arises around the zero OPD position, the proposed full-range FD-DOCT can achieve easily high velocity sensitivity without mirror image around the zero OPD position. Velocity sensitivity dependent on the OPD and the interval of scanning points is examined, and flow velocity detection capability is verified through Doppler imaging of a flow phantom and an in vivo biological sample.

Impact of mask absorber thickness on the focus shift effect in extreme ultraviolet lithography

A focus shift is a common phenomenon in extreme ultraviolet lithography. It depends on the pattern pitch and has a significant effect on the process window. This work proposes an approximate but analytical method to investigate the focus shift effect using a modified thin mask model and the Hopkins formula of imaging. The source of the focus shift is identified, and the impact of the absorber thickness is discussed. The focus shift effect can be reduced and the process window can be improved by choosing the appropriate absorber thickness.

Single-shot parallel full range complex Fourier-domain optical coherence tomography

We present a method of parallel full range complex Fourier-domain optical coherence tomography (FDOCT) that is capable of acquiring an artifacts-free two-dimensional (2-D) cross-sectional image, i.e. a full range B-scan tomogram, by a single shot of 2-D CCD camera. This method is based on a spatial carrier technique, in which the spatial carrier-frequency is instantaneously introduced into the 2-D spectral interferogram registered in parallel FDOCT by using a grating-generated reference beam. The spatial-carrier-contained 2-D spectral interferogram is processed through Fourier transformation to obtain a complex 2-D spectral interferogram. From the 2-D complex spectral interferomgram, a full range B-scan tomogram is reconstructed. The principle of our method is confirmed by imaging an onion sample.

A method for obtaining scattering decomposed images in optical coherence tomography

A method for obtaining scattering decomposed images in optical coherence tomography based on a multiple scattering model is proposed. Images of different scattering components can be obtained with this method. Employing the proposed method, scattering decomposed images of a biological sample are obtained. Then spatial mean filtering for denoising is performed on only the multiple scattering component, and the total signal is reconstructed. This method is useful for observing the images of different scattering components and has the potential to be applied to information extraction or signal processing on these components separately.

Three-dimensional full-range complex Fourier domain optical coherence tomography for in-vivo volumetric imaging of human skin

A three dimensional full-range complex Fourier domain optical coherence tomography (complex FDOCT) system based on sinusoidal phase-modulating method is proposed. With the system, the range of imaging depth is doubled and the sensitivity degradation with the lateral scan distance is avoided. Fourier analysis of B-scan data along lateral scan distance is used for reconstructing the complex spectral interferograms. The B-scan based Fourier method improves the system tolerance of sample movement and makes data processing less time consuming. In vivo volumetric imaging of human skin with the proposed full-range FDOCT system is demonstrated. The mirror image rejection ratio is about 30 dB. The stratum corneum, the epidermis and the upper dermis of skin can be clearly identified in the reconstructed three dimensional FDOCT images.