| 摘要 |
<p>First and second coherent beams illuminate a common area on an exposure station (17). A phase detector, comprising left and right photodiodes (26), senses the relative phase between the first and second beams (31 and 32) to provide a control signal. There is at least one phase shifter (19) in the path of at least one of the coherent beams (31 and 32). The control signal is coupled to the phase shifter (19) to adjust the phase imparted thereby so that the relative phase between the first and second coherent beams (31 and 32) is substantially constant.</p> |
