A driver module structure includes a flexible circuit board provided with a wiring pattern, a semiconductor device mounted on the flexible circuit board, and an electrically conductive heat-radiating member joined to the semiconductor device. The back mirror has a effB approximately equal to Tune, where effB is an effective coupling constant and Tune is the maximum amount of propagation loss anticipated for an amount of peak tuning required, and a length of the back mirror is made to produce greater than approximately 80% reflectivity. These systems may further include a beam splitting device, such as an optical wedge, to provide two sample beams that intersect to provide a fringe pattern.