B.I. Akça

Miniature spectrometer and beam splitter for an optical coherence tomography on a silicon chip.

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, we assemble the central components of a spectral-domain OCT system on a silicon chip. The spectrometer comprises an arrayed-waveguide grating with 136-nm free spectral range and 0.21-nm wavelength resolution. The beam splitter is realized by a non-uniform adiabatic coupler with its 3-dB splitting ratio being nearly constant over 150 nm. With this device whose overall volume is 0.36 cm(3) we demonstrate high-quality in vivo imaging in human skin with 1.4-mm penetration depth, 7.5-µm axial resolution, and a signal-to-noise ratio of 74 dB. Considering the reasonable performance of this early OCT on-a-chip system and the anticipated improvements in this technology, a completely different range of devices and new fields of applications may become feasible.

Integrated approach to laser delivery and confocal signal detection

We present an on-chip arrayed waveguide grating (AWG) sensor based on the confocal arrangement of two AWGs, one acting as focusing illuminator and one as signal collector. The chip can be close to, or in direct contact with, a sample, e.g., biological tissue, without the need of external optics. The collection efficiency of our device can be more than 1 order of magnitude higher than that of a standard AWG, in which light is collected by one input channel. Experimental results on the collection efficiency and volume are presented, together with a demonstration of multiwavelength imaging.

Broad-spectral-range synchronized flat-top arrayed-waveguide grating applied in a 225-channel cascaded spectrometer

Summary form only given. There is a strong need for arrayed waveguide gratings (AWGs) with low-loss and box-like pass band over a broad spectral range. Most of the approaches proposed to implement flat-top AWGs suffer from increased insertion loss and large device size [1,2]. A Mach-Zehnder-interferometer (MZI) synchronized AWG [3] can provide a flattened pass band without introducing intrinsic loss. However, due the wavelength dependency of the directional couplers in the MZI, the achievable bandwidth is limited. We demonstrate a new pass-band flattening method by introducing 3-dB balanced couplers [4] to a MZI-synchronized AWG configuration over a broad spectral range (Fig. 1a). A low-loss cascaded AWG system is demonstrated when using this flat-top AWG as a primary filter. The lengths of the straight coupler sections were calculated as L1 = 149 μm and L2 = 34 μm, and the delay length was found to be ΔL = 0.281 μm, as summarized in Fig. 1b. The simulation and measurement result of the coupler is shown in Figs. 1c and 1d, respectively. The FSR and wavelength resolution of the primary flat-top AWG spectrometer were chosen as 90 nm and 18 nm, respectively, whereas for the secondary AWGs these values were 20.4 nm and 0.4nm, respectively. A 0.5-dB bandwidth of 12 nm and a central excess loss value of 1 dB were measured for the MZI-synchronized AWG. Electrical heaters were placed on both arms of the MZI in order to compensate the fabrication-related performance degradations. The thermal tuning effect on the transmission spectrum of one of the output channels of the MZI-synchronized AWG is shown in Fig. 1e. For the characterization of the cascaded AWG system, several of the outer and central output waveguides of each secondary AWG were measured, as shown in Fig. 1f. A central excess-loss value of 4.5 dB (1 dB from the primary AWG and 3.5 dB from the secondary AWG) and a non-adjacent crosstalk value of 30 dB were obtained. Such low-loss, broad-spectralrange AWGs are very desirable for high-density cascaded multiplexer/demultiplexer systems.

Flat-focal-field integrated spectrometer using a field-flattening lens

Integrated spectrometers are usually of the Rowland mounting type [1]. For applications in which a continuous output spectrum needs to be imaged directly onto a linear detector array, the curved image plane of the Rowland mounting would result in additional losses and aberrations at the outer detector channels. Therefore, a flat-focal-field spectrometer would be highly desirable for such applications. Different methods have been proposed for the design of a flat-focal-field spectrometer [2,3], with limited success. In this work, an alternative way of designing a flat-focal-field arrayed-waveguide grating (AWG) [4] using an integrated field-flattening lens is presented.

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.

Towards a miniaturized optical coherence tomography system

We present experimental results of a spectral-domain optical coherence tomography system that includes an integrated spectrometer. A depth range of 1 mm and axial resolution of 22 μm was measured. A Scotch tape was imaged.