Carrier Envelope Phase Stabilisation

A mode locked laser pulse train can be described by the optical oscillations of each pulse known as the carrier, and the envelope that surrounds it. The position of the optical maxima relative to the envelope, also referred to as carrier-envelope phase (CEP), changes from pulse to pulse at a rate given by the carrier-envelope offset (CEO) frequency due to dispersion and nonlinearities in the system. For applications in high-electric field and attosecond phyics, the CEP is vitally important as the peak electric field will vary along with the CEP. As mode locked systems produce shorter pulse durations, the relative importance of the phase becomes higher as there are fewer oscillations under each envelope. For precision metrology and spectroscopy applications employing the frequency comb nature of modelocked lasers, precise control of the CEO frequency is key as it determines the experimental accuracy.

The CEO frequency and the CEP can be controlled to produce an astonishingly well-defined light source tailored to the research needs. One way to achieve this is using an f-to-2f interferometer and a feedback loop that controls the CEO frequency via the pump power supplied to the ultrafast oscillator. Conventionally, the loop controls an acousto optic modulator (AOM) to vary the output from a constant power pump beam before it enters the oscillator. Provided the speed of phase offset variation is slower than the speed at which the AOM can respond, the phase offset can be held constant.

A more recent advancement in this model uses the signal from the interferometer to directly control the pump laser output, circumventing the AOM. This uses the same principle of pump power control to stabilise the phase offset, however the speed and depth of that control can be far higher than with the AOM.

It is also important to consider the noise of the pump laser for CEP systems with a drive in the pump laser manufacture to keep reducing noise. The lower the better is a simple rule.

finesse pure CEP - Ultra low noise and direct pump modulation capability

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