Feature Of The Week 11/9/14: Dynamics of a Short Cavity Swept Source OCT Laser
Great advances in OCT speed and sensitivity have been possible due to the introduction of fast frequency swept laser sources. One of the most commonly used swept source devices is a short (~5 cm) cavity Fabry-Perot laser. We present both experimental characterisation and numerical modelling of such an OCT swept source and also analyse the underlying physics to better understand how such sources may be optimised.
For the experimental characterisation we use a high-speed digital oscilloscope for real time analysis of the fast temporal intensity and frequency evolution. For the theoretical analysis we use a set of delay differential equations describing the electric field envelope and saturable gain. We show that numerical simulations agree extremely well with the experiments. We describe various lasing regimes (mode hopping, sliding frequency mode locking and chaos), explain their origin and use the model to provide guidelines for further laser OCT performance optimisation.
It is known that fast sweeping lasers exhibit an asymmetry in their performance depending on the sweep direction. The swept source used in this work is no different. The asymmetry is a consequence of the non-zero phase amplitude coupling obtained with semiconductor gain material and therefore the swept source generates different lasing regimes during forward and backward sweeps. During the forward sweep the laser exhibits quasi-periodic pulsations, characteristic of mode locking. Unlike classical mode-locking however, the central frequency of each pulse changes following the filter central frequency. Thus it is a sliding frequency mode locking. It appears at sweep speeds exceeding some threshold value (for example, for a 5 cm cavity length the sweep speed has to exceed ~0.5 GHz/ns ). The origin of the phenomenon is nonlinear mixing of the laser cavity modes lying within the filter bandwidth. It is this regime that is desirable for OCT application. For the backward sweep direction the laser generates a chaotic pulse train and this part of the sweep is less desirable for imaging.
We use the model to guide device optimisation and consider two key parameters of interest for OCT imaging: sweep speed and instantaneous linewidth. We conclude that wider filters can be used in order to achieve higher sweep speeds. However, wider filters lead to increased instantaneous linewidth, therefore leading to a need to compromise between the desired maximum sweep speed and the desired imaging depth.
For more information see recent Article. Courtesy Bryan Kelleher from Cork Institute of Technology.