Optical parametric oscillators and amplifiers for ultrashort pulses, but also mode-locked lasers, can be synchronously pumped. This means that the pump light is not delivered continuously, but in the form of ultrashort pulses, and those are synchronized with the pulse(s) circulating in the device.
Synchronous Pumping of Ultrafast OPOs
For optical parametric oscillators (OPOs), synchronous pumping is the only option for generating ultrashort pulses, because a parametric device is not able to store the pump energy for use at later times. Synchronization usually requires a fairly precise match of the pump pulse repetition rate and the round-trip frequency of the OPO resonator – particularly for femtosecond pulses. For deviations from perfect synchronism, a timing deviation between pump and signal pulses occurs, which in the steady state must be compensated in the nonlinear interaction. As a result, the output pulse energy may drop, or the OPO may stop working altogether. It is also possible that the output becomes very unstable.
In some cases, the signal wavelength of the OPO will automatically adjust so that the resonator round-trip frequency matches the pump repetition rate. This means that the OPO output wavelength can be tuned by adjusting the resonator length, because it is not determined solely by a phase-matching condition. Such a wavelength shift may also be used in a feedback system for stabilizing the resonator length based on detected changes of the output wavelength.
All the effects of deviations from synchronism can be well studied with numerical modeling. Due to the complexity of the nonlinear interaction, analytical models usually have a limited usefulness for such studies, while numerical models can take into account far more details.
In contrast to ultrafast OPOs, mode-locked lasers generally have a continuous-wave pump source. The laser gain medium then stores the pump energy in periods between the pulses. Synchronous pumping of mode-locked lasers is used in rare cases where, e.g., the upper-state lifetime is not much longer than the inverse pulse repetition rate of the laser. This is sometimes the case for dye lasers or for semiconductor lasers (e.g. VECSELs), particularly in situations with small pulse repetition rate. Another aspect is that self-starting mode locking may in some cases be achieved by synchronous pumping alone, because that can provide a modulation through cross-phase modulation.
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