We present an integrated silicon Michelson interferometer for OCT fabricated with wafer scale deep UV lithography. Silicon waveguides of the interferometer are designed with GVD less than 50 ps/nm.km. The footprint of the device is 0.5 mm x 3 mm. The effect of sidewall roughness of silicon waveguides has been observed, possible solutions are discussed.
Optical coherence tomography (OCT) is a medical imaging technology capable of producing high-resolution, crosssectional
images through inhomogeneous samples, such as biological tissue. It has been widely adopted in clinical
ophthalmology and a number of other clinical applications are in active research. Other applications of OCT include
material characterization and non-destructive testing. In addition to current uses, OCT has a potential for a much wider
range of applications and further commercialization. One of the reasons for slow penetration of OCT in clinical and
industrial use is probably the cost and the size of the current systems. Current commercial and research OCT systems
are fiber/free space optics based. Although fiber and micro-optical components have made these systems portable,
further significant miniaturization and cost reduction could be achieved through the use of integrated photonic
components. We demonstrate a Michelson interferometer using integrated photonic waveguides on nanophotonic silicon
on insulator platform. The size of the interferometer is 1500 μm x 50 μm. The structure has been tested using a mirror as
a reflector. We can achieve 40 μm axial resolution and 25 dB sensitivity which can be substantially improved.
We develop a new approach in imaging nonfluorescent species with two-color two-photon and excited state absorption microscopy. If one of two synchronized mode-locked pulse trains at different colors is intensity modulated, the modulation transfers to the other pulse train when nonlinear absorption takes places in the medium. We can easily measure 10−6 absorption changes caused by either two-photon absorption or excited-state absorption with a RF lock-in amplifier. Sepia melanin is studied in detail as a model system. Spectroscopy studies on the instantaneous two-photon absorption (TPA) and the relatively long-lived excited-state absorption (ESA) of melanin are carried out in solution, and imaging capability is demonstrated in B16 cells. It is found that sepia melanin exhibits two distinct excited states with different lifetimes (one at 3 ps, one lasting hundreds of nanoseconds) when pumped at 775 nm. Its characteristic TPA/ESA enables us to image its distribution in cell samples with high resolution comparable to two-photon fluorescence microscopy (TPFM). This new technique could potentially provide valuable information in diagnosing melanoma.
We have demonstrated a new method for imaging melanin with two-color excited state absorption microscopy. If one of
two synchronized mode-locked pulse trains at different colors is intensity modulated, the modulation transfers to the
other pulse train when nonlinear absorption takes place in the medium. We can easily measure 10<sup>-6</sup> absorption changes
caused by either instantaneous two-photon absorption or relatively long lived excited state absorption with a RF lock-in
amplifier. Eumelanin and pheomelanin exhibit similar excited state dynamics. However, their difference in excited state
absorption and ground state absorption leads to change in the phase of the transient absorption signal. Scanning
microscopic imaging is performed with B16 cells, melanoma tissue to demonstrate the 3D high resolution imaging
capability. Different melanosome samples are also imaged to illustrate the differences between eumelanin and
pheomelanin signals. These differences could enable us to image their respective distribution in tissue samples and
provide us with valuable information in diagnosing malignant transformation of melanocytes.
Multiphoton excitation fluorescence microscopy has proven to be a powerful method for non-invasive, in vivo, thick tissue imaging with molecular specificity. However, many important endogenous biomolecules do not fluoresce (NAD) or fluoresce with low efficiency (Melanin). In this report femtosecond pulse shaping methods are used to measure two-photon absorption (TPA) directly with very high sensitivity. Combining with the laser scanning microscope, this Two-photon Absorption Microscopy (TPAM) retains the penetration and localization advantages of two-photon fluorescence microscopy and permits direct observation of important endogenous molecular markers (melanin or hemoglobin) which are invisible in multiphoton fluorescence microscopy. We have demonstrated here for the first time that TPAM can successfully and more efficiently image melanoma cells and tissues and provide a good melanin contrast in optical sectioning of the melanoma lesions which are comparable to pathological histology. Combining with the two-photon fluorescence images acquired simultaneously, the distribution patterns of the melanocytes and their intratissue behavior could be studied without cutting the lesions from patients. TPAM will undoubtedly find the applications in the clinical diagnosis and biomedical research.