Gunay Yurtsever

Photonic integrated Mach-Zehnder interferometer with an on-chip reference arm for optical coherence tomography

Optical coherence tomography (OCT) is a noninvasive, three-dimensional imaging modality with several medical and industrial applications. Integrated photonics has the potential to enable mass production of OCT devices to significantly reduce size and cost, which can increase its use in established fields as well as enable new applications. Using silicon nitride (Si3N4) and silicon dioxide (SiO2) waveguides, we fabricated an integrated interferometer for spectrometer-based OCT. The integrated photonic circuit consists of four splitters and a 190 mm long reference arm with a foot-print of only 10 × 33 mm(2). It is used as the core of a spectral domain OCT system consisting of a superluminescent diode centered at 1320 nm with 100 nm bandwidth, a spectrometer with 1024 channels, and an x-y scanner. The sensitivity of the system was measured at 0.25 mm depth to be 65 dB with 0.1 mW on the sample. Using the system, we imaged human skin in vivo. With further optimization in design and fabrication technology, Si3N4/SiO2 waveguides have a potential to serve as a platform for passive photonic integrated circuits for OCT.

Ultra-compact silicon photonic integrated interferometer for swept-source optical coherence tomography

We demonstrate an ultra-compact silicon integrated photonic interferometer for swept-source optical coherence tomography (SS-OCT). The footprint of the integrated interferometer is only 0.75×5  mm2. The design consists of three 2×2 splitters, a 13 cm physical length (50.4 cm optical length) reference arm, and grating couplers. The photonic integrated circuit was used as the interferometer of an SS-OCT system. The sensitivity of the system was measured to be -62  dB with 115 μW power delivered to the sample. Using the system, we demonstrate cross-sectional OCT imaging of a layered tissue phantom. We also discuss potential improvements in passive silicon photonic integrated circuit design and integration with active components.

Compact grating couplers on silicon-on-insulator with reduced backreflection

The backreflection in commonly used grating couplers on silicon-on-insulator (SOI) is not negligible for many applications. This reflection is dramatically reduced in our improved compact grating coupler design, which directs the reflection away from the input waveguide. Realized devices on SOI show that the reflection can be reduced down to -50 dB without an apparent transmission penalty.

Integrated photonic circuit in silicon on insulator for fourier domain optical coherence tomography

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.

Heterodyne laser Doppler vibrometers integrated on silicon-on-insulator based on serrodyne thermo-optic frequency shifters

Miniaturized laser Doppler vibrometers (LDVs) have many advantages over conventional bulk LDVs. In this paper, the realization of a miniaturized heterodyne LDV integrated on silicon-on-insulator substrate is reported. The optical frequency shifters in these on-chip LDVs employ a serrodyne technique, and they generate a frequency shift at 2 kHz. Vibrations of a mirror for the frequency range between 1.1 and 123 Hz and the velocity range between 0.8 and 400 μm/s are measured by both an on-chip LDV and a commercial LDV. The measurement results agree well. A compensation method for the influence of on-chip spurious reflections is also demonstrated.

Rapid pulse shaping with homodyne detection for measuring nonlinear optical signals

We have designed a common-mode interferometric acousto-optic pulse shaper that is capable of shaping individual pulses differently from a mode-locked laser. The design enables the measurement of weak nonlinear optical signals such as two-photon absorption and self-phase modulation at megahertz rates. The experimental apparatus incorporates homodyne detection as a means of resolving the phase of the detected signals. The fast data acquisition rate and the ability to perform measurements in scattering media make this experimental apparatus amenable to imaging applications analogous to measurements of two-photon fluorescence using a mode-locked laser.

Low dispersion integrated Michelson interferometer on silicon on insulator for optical coherence tomography

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.

Two-Photon Absorption of Oxyhemoglobin and Methemoglobin for Microscopic Imaging

We measured two-photon absorption of oxyhemoglobin and methemoglobin with modest laser powers. In combination with our previous work on melanin tissue imaging, this shows two-photon absorption imaging of hemoglobin state is feasible.

Miniaturized laser Doppler vibrometers integrated on silicon-on-insulator with thermo-optic serrodyne optical frequency shifter

A miniaturized heterodyne laser Doppler vibrometer integrated on silicon-on-insulator is reported and compared with a commercial LDV. The optical frequency shift was realized by applying the serrodyne method on a thermo-optic phase modulator.

Compact demultiplexer using cascaded planar concave grating and ring resonators on SOI

We present a compact demultiplexer based on cascaded planar concave grating and ring resonators. The demultiplexer has 28 channels, with 1 nm channel spacing. The device was fabricated on a silicon-on-insulator (SOI) platform using CMOS compatible deep UV lithography. The size of the structure is 1.5 times 3 mm. Insertion loss is 15 dB, channel to channel variation is 5 dB.