Control strategy for a monolithically integrated widely tunable laser system on InP for Optical Coherence Tomography

An InP integrated widely tunable laser is investigated for the use as a swept source in optical coherence tomography (OCT) applications. The laser is realized on a generic integration technology platform. It consists of a gain medium and a bandpass filter with 3 cascaded asymmetric Mach-Zehnder interferometers. The additional presence of a balanced Mach-Zehnder modulator as variable out-coupler is instrumental to increase the laser tuning range to 90 nm between 1480 and 1570 nm but can add to additional filtering effects in the laser cavity. In this work, we propose an optimized control strategy for the wavelength calibration of this widely tunable laser source, for a stepwise wavelength scan that is suitable for OCT. The aim is to obtain a wavelength scan with at least 1000 of 10 GHz equally spaced optical frequencies, having uniform power around 100 µW and 1 GHz accuracy. The control strategy is based on the a-priori knowledge of the coarse and the medium filter tuning and on an optimization of the fine filter tuning and the longitudinal cavity mode tuning that can be frequently updated. In this way, the calibration of the laser system can be kept sufficiently accurate and stability of the scan quality can be ensured. With this strategy, 10 GHz spaced optical lasing frequencies are obtained over 30 nm making the calibrated laser suitable as an OCT source