Experimental and theoretical investigation of polygon-based swept-source with continuous adjustable free spectral range

In this paper, a method that enables adjustment of the free spectral range (FSR) of a swept-source is presented by introducing a right-angled prism inside a polygon-based filter. Firstly, the feasibility of this method is analyzed in theory. When adopting the prism at two different cases, the theoretical prediction value of FSR changes differently. In Case 1, when the beam incidents from the hypotenuse surface of the prism, a broader FSR is obtained. In Case 2, when the beam incidents from the right-angled surface of the prism, a narrower FSR is achieved. Then, to verify these two different changes, the filters deployed in these two cases, as well as a standard reference filter, are tested and compared on a seed swept-source. There is no prism in the reference configuration which has the FSR of 187.9 nm. Compared with that, in Case 1 and Case 2, the measured FSR increases to be 234.4 nm and decreases to be 156.6 nm, respectively. The experimental results are not only consistent with the predicted changes, but also show the ability of fine and continuous adjustment of the FSR. This method is promising to improve the performance of swept-sources, as well as the imaging systems which utilize swept-sources as the light sources.