Versatile Swept Source With Adjustable Coherence Length

An electronically controlled optical swept source at 1550 nm using mode locking in a dispersive ring cavity is described. Active mode-locking was achieved by directly modulating the current of a semiconductor optical amplifier (SOA) used as a gain medium. In the static regime, parameters such as linewidth, tuning bandwidth, and contrast were measured, whereas the axial range was determined dynamically. Two types of fiber, dispersion compensation and single mode, are employed in the laser ring cavity. It is demonstrated that the relative lengths of the two types of fiber have little effect on the linewidth, whereas more control on the linewidth is obtained via the frequency of the signal driving the SOA. Linewidths less than 60 pm and over 1 nm were measured in the static regime while driving the SOA at 50-500 MHz. The narrowest linewidths were achieved where the proportion of dispersion compensation fiber in the cavity was 80%-90% of the total length. The optical source is developed to respond to the demands of several optical coherence tomography applications that do not necessarily need long coherence length swept sources.