Carrier-envelope phase control for the advancement of attosecond pulse generation

When the optical pulses emitted by a laser become so short in time
that they encompass only a few cycles of the carrier wave, the
phase between carrier and envelope becomes a crucial parameter. The
ability to control this carrier-envelope phase (CEP) is elemental
to experiments probing the fastest processes in the microcosm,
occurring on the time-scale of attoseconds. More than a decade into
the attosecond era, the limitations of the established CEP
stabilisation technique have begun to curtail experimental
progress. First, increasingly complex experiments require many
hours of uninterrupted operation at the same waveform. Second, the
pulses used in experiments are approaching the single-cycle
boundary, calling for ever-decreasing CEP noise. With the
conventional stabilisation technique, already these two
requirements cannot be fulfilled simultaneously. Ultimately, the
low efficiency of the underlying nonlinear processes can only be
compensated by driver lasers at a higher repetition rate than
available at present. In order to advance attosecond pulse
generation, novel approaches to CEP control thus face a threefold
challenge that outlines this thesis: To simultaneously provide low
CEP noise and long-term operation to present-day few-cycle lasers
and amplifiers, and to investigate CEP control capability in high
average power sources that are currently under development. This
thesis describes the adaptation of cavity-external CEP
stabilisation for use with few-cycle pulses. The intrinsic
limitations of the conventional feed-back technique are lifted. A
laser oscillator is demonstrated to maintain record-low CEP noise
for tens of hours of operation free from phase discontinuities. In
addition, a modification of the technique is presented that further
enhances the applicability to amplified systems. Two routes are
investigated to achieve CEP control in system architectures that
represent potential megahertz repetition rate driver sources. In
combination with temporal pulse compression, a thin-disk laser is
shown to yield few-cycle pulses. Experiments are presented that
provide the groundwork towards the first CEP-stabilised thin-disk
oscillator. The second approach targets the seed oscillator of a
fibre chirped-pulse amplifier. The CEP noise properties of
different amplification regimes are examined. Intensity enhancement
of the output pulses in a passive resonator is shown to benefit
greatly even from a coarse lock of the CEP slip rate. For few-cycle
pulse energy to reach the millijoule level and above, amplification
and temporal compression will remain indispensable in the
foreseeable future. Maintaining CEP stability across such stages is
crucial, irrespective of the technology employed. Cavity-external
CEP control is demonstrated to enable more than 24 hours of
constant-CEP operation in chirped-pulse amplifiers. Furthermore, a
novel actuator is introduced that, in conjunction with a fast means
of measuring the CEP, is able to provide phase correction of the
amplified waveform up to several kilohertz bandwidth. The result is
a train of millijoule-level pulses with residual CEP noise
comparable to that of state-of-the-art nanojoule oscillators.
Eventually, an experiment is presented to examine the influence of
different types of hollow-core fibre-based temporal compression on
the CEP. The findings shed new light on the origin of adverse
effects introduced by this technique, and point out ways towards
effective compensation.