Feature Of The Week 6/3/12: Towards OCT On a Chip - A Hard Goal but One that Could Eventually Revolutionize OCT Systems and Applications
Miniaturization of Optical Coherence Tomography (OCT) systems is one of the most exciting and promising areas for instrumentation research and offers tremendous commercial potential. While there has been progress over the past decade in reducing the size and cost of OCT systems, the future of truly compact and low-cost OCT systems, such as a one chip solution, has not been realized. The potential of the core of an OCT system (source, interferometer, detector, TIA, frontend DSP, etc) on a chip is compelling. In the future it seems quite promising that advances in active and passive photonic integrated circuits along with advances in ASICs (with integrated ADCs, DACs, controll electronics, and DSP) are on a very promising course to offer revolutionary advances for OCT systems. Such advances are allready happening in other fields such as optical telecommunications. In fact the Arrayed-Waveguide Grating (the subject of the article below) was invented and initially used in WDM Fiber Optical Communications Systems. Dramatically lower size and cost could enable many new applications and improve utility in existing ones. Several research groups and a few commercial startups (e.g. MedLumics, Tornado) around the world are actively pursuing this goal. One such research group that has been very active in this area is the University of Twente. Below is a summary of some of their recent work.
- Eric Swanson
Optical low-coherence reflectometry (OLCR) is a one-dimensional optical ranging technique for measuring the position of reflective surfaces in a sample. It was first developed for characterization of fiber-based waveguide devices; in the following years the transverse scanning capability enabled cross-sectional imaging, i.e., optical coherence tomography (OCT). The size and cost of OLCR and OCT systems can be decreased significantly by the use of integrated optics. In our previous work we successfully demonstrated cross-sectional imaging of a multilayered phantom by use of an arrayed-waveguide-grating (AWG) spectrometer in a spectral-domain (SD) OCT system. There are still some possibilities to improve the performance of the existing AWG spectrometers. The first improvement is enhancing the depth range by omitting the output channels (or removing them from a conventional AWG by dicing). In this way, the wavelength discretization will be determined by the number of pixels on the camera, which can be much larger than the number of output channels. The other possibility is eliminating the polarization dependency of AWG spectrometers by using non-birefringent waveguides in the arrayed waveguides. In this way the periodic signal fading in the sensitivity roll-off will be eliminated and the cost and size of integrated OLCR and OCT systems will reduce significantly by eliminating the components for polarization control.
In this work, we discuss the impact of discrete output channels and polarization dependency of an AWG on SD-OLCR performance. The AWG is designed in silicon-oxynitride (SiON) for operation around 800 nm wavelength, with an free spectral range of 19.4 nm and a wavelength resolution of 0.16 nm. As SiON is transparent over a broad wavelength range that covers all the frequently used OCT wavelength bands at 800, 1000, and 1300 nm, AWGs can be fabricated for all these wavelength ranges in the same material system. Single-mode SiON channel waveguides of 1.5 µm width and 0.8 µm height are fabricated with core and cladding refractive indices of 1.5 and 1.454 at λc= 800 nm, respectively. The size of the fabricated device is 2.6 cm x 2.1 cm. By removing the output waveguides of the AWG, the depth range is enhanced from 1 mm 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 non-birefringent AWG centered at 1300 nm was designed with spectral resolution of 0.4 nm (results in a maximum depth range of 1 mm), free spectral range of 20 nm, and an overall device size of 1.2 cm x 1.0 cm is demonstrated. Single-mode SiON channel waveguides of 2.2 µm width and 1 µm height are fabricated with core and cladding refractive indices of 1.535 and 1.4486 at λc= 1300 nm, respectively. No polarization dependent shift for the center and outer channels are measured. Such AWG spectrometers would eliminate the need for polarization control with its associated noise, size, and cost penalties in OLCR and, hence, OCT systems.
For more information see recent Article. Courtesy Imran Akca from the University of Twente.