Mode-locked lasers, which generate a train of ultra-short pulses, have become an important tool in precision optical frequency measurement  and are useful for the design of future ultrahigh-speed optical communication systems . This note will discuss how to use FullWAVE™'s custom time envelope feature to create this type of pulse train. Details about generating a custom time envelope can be found in Section 4.B.2 of the FullWAVE manual. Users can access design files on the Customer Support Portal.
The output of a mode-locked laser is a comb of equidistant lines in the frequency domain, separated by a bandwidth of frep. In the time domain, this corresponds to a train of pulses whose temporal separation is given by the cavity round-trip time trt = 1/frep. The offset of the comb, fceo is connected to the pulse-to-pulse phase slip
Δϕ = 2π(fceo/frep). We are going to create a pulse train with fceo = frep/4 , such that every fifth pulse is identical, as shown in Figure 1 .