Pulses oscillating inside the cavity of a mode-locked laser have a frequency spectrum and phase that is fixed to that cavity. The purpose mode-locking mechanism is to lock the phase and intensity of the different resonator modes together across a large spectral range and thus create temporally short pulses. If - for example - the cavity length changes the resonator modes will also move to different frequencies. The pulse will then adjust within several roundtrips to again be mode-locked and resonant inside the cavity. If two pulses are oscillating in such a cavity at the same time and the cavity changes over time one can detect that change by observing a difference in frequency between the two pulses. This difference can be easily detected outside the cavity by interfering both pulses and measuring the beat node they produce. Since both pulses are part of the same cavity many imperfections of the setup cancel out automatically.

To avoid backscattering from one pulse into the other it is however necessary to have the 2 pulses inside the cavity cross in vacuum or air. It is also desirable to avoid energy storage inside the gain medium as a means of the pulse to influence one another. One design that allows this is an optical parametric oscillator (OPO). The pulses inside a singly resonant OPO owe their mode structure and phase the the OPO cavity and their envelope to the pump cavity. In order to provide a stable envelope it is necessary that also the pulses that pump the OPO come from the same cavity. This can be realized in an intracavity pumped OPO setup where the OPO gain crystal is part of the pump cavity. The intracavity pumped setup also allowes us to utilize the much higher intracavity intensities of the pump laser.

An intracavity OPO is a complex system of coupled cavities. To better understand is dynamics we have developed a computer simulation of the complete system.

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