Phase locking of optical sources and impact on system

Example 1 - Phase difference between lasers

Two free-running lasers, each with linewidth 250 kHz, are used as the signal laser and the LO in a coherent optical transmission system. What is the rms increase in the phase difference between the laser over a time interval of 1 µs? 

Example 2 - OPLL with delay

A prototype first-order OPLL is constructed from optical components with fibre pigtails.  The total length of fibre in the loop is 2 m.  What is the maximum loop bandwidth if the propagation delay  through the fibre is the dominant delay?