beneficial to include curved refocusing mirrors into an fabry perot interferometer cavity as seen in fig4 b the curved mirrors are aligned in such a way that beam size is reproduced exactly every round trip time in the cavity configurations in fig4 a and 4b require external separation between the incident reflection path and the combined beams this can be done for example using a standard polarization beam splitter and a suitable waveplate or a faraday rotator the configuration shown in fig4 c has the advantage that all the incident beams are separate from each other and most importantly from the combined beam to combine more than two optical beams the resonant cavity beam combiners and pulse stackers need to be cascaded together two example arrangements are shown in fig5 a and 5b in fig5 a fabry perot interferometer cavities may be arranged in a parallel cascading manner that is optical beams are paired together e g 1 and 2 and each pair of optical beams are combined together concurrently using a series of cavities combined optical beams are likewise paired together e g 51 and 52 and combined with each other using another series of cavities this arrangement is repeated until two optical beams are combined to form a single optical beam 53 fabry perot interferometer cavities may also be arranged in a sequential cascading manner as seen in fig5 b in this arrangement two optical beams e g 1 and 2 are first combined together using one cavity to a form a first combined beam the combined beam is then combined with a third beam i e 3 using another cavity.