Niels Meiser

Abstract

The subject of this thesis is the investigation of novel technologies for mode-locking of diode-pumped, solid-state lasers. Novel saturable absorbers are used: quantum dots (QDs) and carbon nanotubes (CNTs), which both are low-dimensional nanoformations.

In addition, mode-locking by cascaded nonlinearities is explored. Absorber structures containing self-assembled InGaAs QDs are characterised in detail by pump-probe experiments, time-resolved photoluminescence spectroscopy, and measurement of the nonlinear reflectivity. The samples show sub-picosecond relaxation times of the reflectivity, modulation depths between 0.18 % and 2.9 %, as well as low saturation fluences on the order of 1–10 μJ/cm². The structures’ design parameters are related to their transient and nonlinear performance.

The characterised QD saturable absorbers are then used for mode-locking of diode-pumped, solid-state lasers, delivering picosecond pulses with optical spectra in the region of 1020–1040 nm. In particular, a QD absorber with a saturation fluence of 4 μJ/cm² and a relaxation time <200 fs is successfully employed for fundamental mode-locking of an Yb:KYW laser at a repetition rate of 1 GHz. This laser emits pulses with a duration of 1.7 ps at an output power of 339 mW. Apart from this, an Yb-thin-disc laser is demonstrated, emitting pulses with a duration of 1.6 ps at an output power of 13 W, thereby showing, that the absorber withstands fluences of up to 2.4 mJ/cm² without being damaged.

An absorber with a linear loss of only 1 % is obtained by embedding CNTs in a thin plastic film, coated onto a glass substrate. Using this absorber, mode-locking of an optically-pumped semiconductor disc-laser is achieved. The laser emits pulses with a duration of 1.12 ps at a repetition rate of 613 MHz and with an average output power of 136 mW.

For cascaded mode-locking, a periodically-poled KTP crystal is placed inside a laser cavity and the two second-order nonlinearities from second-harmonic generation and back-conversion are used to emulate a third-order nonlinearity with an effective nonlinear refractive index of 2.33 · 10−17 m²/W. For precise control of the nonlinearity, the laser’s spectrum is fixed to a wavelength of 1029.1 nm by a volume Bragg grating. The laser emits pulses with a duration of 16 ps at a repetition rate of 210 MHz and with an output power of 690 mW.