With pulse durations approaching the single cycle limit, the phase relationship between the carrier and its envelope becomes increasingly important. Controlling the carrier envelope phase (CEP) enables the exploration of a whole new area of extreme nonlinear physics where the relationship between the phase of the carrier wave and its envelope becomes crucial to the observed phenomena, e.g. in attosecond physics, where the front end of the machines generating isolated attosecond light pulses traditionally are based on CEP-stabilized oscillators. Another field which has opened up is the exploration of CEP dependent effects in electron emission spectroscopy, or more generally in the field of nano optics, plasmonics and femtosecond electron point sources.
Stabilizing the CEP of a laser has relied on optical self-referencing (using different methods) to generate a signal sensitive to the CEP, to feedback into the oscillator to control the difference in group and phase velocity which causes the phase slippage.
A common implementation of the self-referencing is the use of an f-to-2f interferometer, which relies on being able to interfere two parts of the spectrum from the laser beam, where one part is twice the frequency of the other.
In venteon’s octave spanning laser, the unique venteon CEP5, the most direct and natural stabilization scheme possible today is employed, using the natural bandwidth output as a self-referencing supply for the f-to-2f interferometer and eliminating the need for additional spectral broadening techniques.
In the venteon CEP5, only the extreme spectral wings are used for self-referencing, using less than 10% of the original laser output power resulting in very high efficiency and sub-6 fs pulses of maximum power available for the application. With optional modifications to the interferometer, a CEP offset frequency of zero is also possible, resulting in a train of pulses with identical CE phase (versus every fourth in standard configuration) with a maximum electrical field.
For applications needing a more contained not quite octave spanning spectrum, the more conventional approach using a PCF fibre for slight additional broadening is available in the venteon power with CEP stabilization.
Both implementations benefit from the advancement of the feedback system that inputs the f-to-2f interferometer signal directly into the pump laser using CEPLoQ™ technology. This is achieved by directly controlling a ±1% power modulation of the pump laser, covering a range of DC to 1 MHz, with better than 90 degrees phase behaviour up to 700 kHz, leading to a larger locking bandwidth than traditional methods.
The parametric amplification process used in the venteon OPCPA preserves the CEP and hence the amplified pulses are also CEP stable. Using sophisticated engineering and stretching scheme, the extremely low CEP noise is best preserved in the amplifier. A slow loop drift stabilization of the amplifier is available for long-term CEP stable operation.