  We present an experimental realization of the Optical Frequency Locked Loop
(OFLL) applied to long-term frequency difference stabilization of broad-line
DFB lasers. The presented design, based on an integrated phase-frequency
detector chip, is digitally tunable in real-time, robust against environmental
perturbations and compatible with commercially available laser current control
modules. We present a simple model and a quick method to optimize the loop for
given hardware using only simple measurements in time domain and approximate
laser linewidth. We demonstrate frequency stabilization for offsets
encompassing entire 4-15 GHz capture range. We achieve < 0.5 Hz long-term
stability of the beat note frequency for 1000 s averaging time. Step response
of the system as well as phase characteristics closely adhere to the
theoretical model.
