Feature Of The Week 2/2/14: Tsinghua University.Demonstrates Linear-in-Wavenumber Swept Laser with an Acousto-Optic Deflector for OCT
Compared to the conventional time domain OCT, the frequency domain (FD) detection techniques enable a dramatic increase in imaging speed. Spectrometer-based FD-OCT (SD-OCT) systems are already widely used, especially for ophthalmic application in the 800 nm wavelength range. Alternatively, the application of FD-OCT systems based on a rapidly swept, narrowband light source, i.e. swept source OCT (SS-OCT) offers the additional advantages of dual balance detection, potentially longer ranging depth and higher imaging speeds.
However, a general disadvantage of most FD-OCT systems is the requirement to resample and recalibrate the detected OCT interference fringe prior to fast Fourier transformation, in order to provide data evenly sampled in optical frequency. Although a SS-OCT system based on a light source with a polygonal scanner exhibits small nonlinearity in wavelength over time, evenly resampling in wavenumber is necessary due to the wavenumber-to-wavelength relation k=2π/λ , especially when the tuning range is large. Swept sources based on resonant galvanometers, MEMS scanners, or typical Fabry–Perot filters have even more pronounced nonlinear swept characteristics. The recalibration in SS-OCT can be performed by clocking the analog to digital converter with an electronic signal generated by a second interferometer, accounting for the non-linear sweep operation. However, it requires complex electronic hardware and usually isn’t easily adjustable to different operation frequencies of the source. Although increasing computing power, driven in part by GPUs, has made the task of numerically resampling the signal into k-space a less burdensome task, the numerical process, which may also introduce extra noise, takes time and thus reduces the imaging rate. Therefore a swept laser that can itself scan or tune its wavenumber linearly over time is in high demand for high speed SS-OCT imaging.
Professor Ping Xue, PhD Student Tiancheng Huo and their colleagues from Tsinghua University in China, have made important progress in the new design of swept laser. They have demonstrated a novel high speed linear-in-wavenumber (k-linear) swept laser source based on an acousto-optic deflector (AOD). The AOD-based optical filter includes an acousto-optic deflector and a reflection grating. The laser may scan the wavenumber k linearly over time due to its appropriate configuration. Because of avoiding data resampling and recalibration that are generally required in conventional SS-OCT, this linear-in-wavenumber swept laser is very favorable and promising for fast imaging.
The researchers have achieved a good k-linear swept laser with Pearson’s r correlation coefficients of 0.99997. The laser has a tuning range of 50 nm, 3 dB swept range of 42 nm (FWHM), output power of 2.56 mW, 6 dB sensitivity roll-off depth of ~1 mm, and central wavelength of 1064 nm at a scanning rate of ∼20 kHz. Owing to the fast response time of the acousto-optic crystal, scanning rate as high as ∼400 kHz is also achieved for this laser with the tuning range of 49 nm, swept linearity of 0.99990, output power of 2.30 mW, and 6 dB sensitivity roll-off depth of ~0.6 mm. It has proved that the sweep rate of ≥ 2 MHz is also feasible for this laser. 2D imaging with this k-linear swept laser was also demonstrated to verify that neither recalibrating nor interpolation is needed to obtain the standard OCT image.
For more information see recent Article. Courtesy Ping Xue from Tsinghua University.