Lantian Chang

Polarization-Independent Enhanced-Resolution Arrayed-Waveguide Grating Used in Spectral-Domain Optical Low-Coherence Reflectometry

The performance of an arrayed-waveguide grating (AWG) as an integrated spectrometer in spectral-domain optical low-coherence reflectometry (SD-OLCR) is significantly improved. By removing the output waveguides of the AWG, the depth range is enhanced from 1 to 3.3 mm at 800 nm and 4.6 mm at 1300 nm. Periodic signal fading, that was previously observed in the sensitivity roll-off curve in depth ranging measurements, is shown to be evoked by beat-frequency generation between the two polarizations of partially polarized signal light in a birefringent AWG. By carefully controlling the polarization state-of-light, the signal fading is eliminated. As a permanent solution to this problem, a polarization-independent AWG is demonstrated, which can reduce the size and cost of OCLR and optical coherence tomography systems further by eliminating the components for polarization control.

Waveguide-coupled micro-ball lens array suitable for mass fabrication.

We demonstrate a fabrication procedure for the direct integration of micro-ball lenses on planar integrated optical channel waveguide chips with the aim to reduce the divergence of light that arises from the waveguide in both horizontal and vertical directions. Fabrication of the lenses is based on photoresist reflow which is a procedure that allows for the use of photolithography for careful alignment of the lenses with respect to the waveguides and enables mass production. We present in detail the design and fabrication procedures. Optical characterization of the fabricated micro-ball lenses demonstrates a good performance in terms of beam-size reduction and beam shape. The beam half divergence angle of 1544 nm light is reduced from 12.4 ° to 1.85 °.

Chip based common-path optical coherence tomography system with an on-chip microlens and multi-reference suppression algorithm

We demonstrate an integrated optical probe including an on-chip microlens for a common-path swept-source optical coherence tomography system. This common-path design uses the end facet of the silicon oxynitride waveguide as the reference plane, thus eliminating the need of a space-consuming and dispersive on-chip loop reference arm, thereby obviating the need for dispersion compensation. The on-chip micro-ball lens eliminates the need of external optical elements for coupling the light between the chip and the sample. The use of this lens leads to a signal enhancement up to 37 dB compared to the chip without a lens. The light source, the common-path arm and the detector are connected by a symmetric Y junction having a wavelength independent splitting ratio (50/50) over a much larger bandwidth than can be obtained with a directional coupler. The signal-to-noise ratio of the system was measured to be 71 dB with 2.6 mW of power on a mirror sample at a distance of 0.3 mm from the waveguide end facet. Cross-sectional OCT images of a layered optical phantom sample are demonstrated with our system. A method, based on an extended Fourier-domain OCT model, for suppressing ghost images caused by additional parasitic reference planes is experimentally demonstrated.

Non-traditional whispering gallery modes inside microspheres visualized with Fourier analysis

Non-traditional whispering gallery modes are studied in a glass microsphere. Geometrical ray tracing is used to explain and calculate these modes. Thermal emission and Raman scattering are used as an internal light source to excite these modes inside the glass microsphere. The thermal and Raman emission spectra are modified due to the existence of these modes. Fourier analysis is then used to distinguish the individual modes. The understanding of these non-traditional WGM may lead to alternative design strategies for sensor applications or laser cavity configurations.

Chip Based Common-path Swept-source Optical Coherence Tomography Device

Optical coherence tomography (OCT) is an optical imaging technique which provides three-dimensional images with micrometer-resolution. OCT has been extensively used for disease diagnostics, treatment planning, and surgical guidance. Currently, most of the OCT systems are based on discrete free-space optical components and optical fibers. These discrete components keep these instruments costly and bulky. The development in integrated optical circuit technology provides the opportunity to develop miniaturized, stable and maintenance-free OCT systems. The goal of this study is developing an OCT chip with external light source and detector. The developed chip is intended to be used in a hand-held OCT probe for skin applications. In this study, a literature survey summarized the achievements and limitations of current chip-based OCT systems. Our common-path design uses the end facet of the waveguide as the reference plane, thus eliminating the need for a space-consuming and dispersive on-chip loop reference arm, thereby reducing the chip size and obviating the need for dispersion compensation. Our on-chip micro-ball lens eliminates the need of external optical elements for coupling the light between the chip and the sample, thereby further reducing the size and the complexity of the system. The signal to noise ratio (SNR) of our system was measured to be 71 dB with 2.6 mW of power on a mirror sample at a distance of 0.3 mm from the waveguide end facet. At this moment, the SNR of our chip system is slightly lower (11 dB lower) than a fiber-based system under the same experimental condition. The suggestions for improving the SNR of our chip system were discussed. Multiple ghost images caused by additional reference planes (originating from the lens surface) could be largely suppressed using a deconvolution scheme. Finally, we presented the design and fabrication results of a common-path parallel SS-OCT (PSS-OCT) chip followed by suggestions for characterization measurements and for possible improvements in the parallel chip design. We believe that by integrating a micro-ball lens onto the chip and using a common-path configuration we have moved a significant step forward in the development of on-chip SS-OCT systems.

Integrated polymer microlenses for two-dimensional collimation of light from single-mode optical waveguides

In this work, we demonstrate an on-chip reflowed polymer microlens design to enable light collimation in both, horizontal and vertical directions, and the experimental results show reduction of the divergence angle by a factor of 25.

Advanced integrated spectrometer designs for miniaturized optical coherence tomography systems

Optical coherence tomography (OCT) has enabled clinical applications that revolutionized in vivo medical diagnostics. Nevertheless, its current limitations owing to cost, size, complexity, and the need for accurate alignment must be overcome by radically novel approaches. Exploiting integrated optics, the central components of a spectral-domain OCT (SD-OCT) system can be integrated on a chip. Arrayed-waveguide grating (AWG) spectrometers with their high spectral resolution and compactness are excellent candidates for on-chip SD-OCT systems. However, specific design-related issues of AWG spectrometers limit the performance of on-chip SD-OCT systems. Here we present advanced AWG designs which could overcome the limitations arising from free spectral range, polarization dependency, and curved focal plane of the AWG spectrometers. Using these advanced AWG designs in an SD-OCT system can provide not only better overall performance but also some unique aspects that a commercial system does not have. Additionally, a partially integrated OCT system comprising an AWG spectrometer and an integrated beam splitter, as well as the in vivo imaging using this system are demonstrated.