Ultrafast waveguide lasers

Choudhary, Amol (2014) Ultrafast waveguide lasers. University of Southampton, Physical Sciences and Engineering, Doctoral Thesis, 156pp.

Record type: Thesis (Doctoral)

Abstract

Mode-locked lasers with repetition-rates in excess of 1 GHz have many applications in areas such as optical sampling, non-linear microscopy, and optical frequency metrology. To date there have been very few demonstrations of such high repetition-rate lasers with sub-picosecond operation and high average power. This thesis deals with the realisation of such compact sources using an integrated-optics platform. Waveguides offer certain key advantages, including a low threshold power, high slope efficiency, compatibility with monolithic devices, and a low mode-locking threshold, making them very promising candidates for such devices. Ultrafast multi-GHz waveguide lasers are described in this thesis, which are compact, mass-producible and low-cost making them very exciting candidates for industrial applications.
Mode-locking was demonstrated in an ion-exchanged Yb:phosphate glass waveguide laser with integrated saturable absorber elements. An average output power as high as 80 mW was achieved at a pulse repetition frequency (PRF) of 4.9 GHz, at a wavelength around 1 µm and with pulse durations as short as 740 fs. Using shorter cavity lengths, waveguide lasers with PRFs of 10.4 GHz, 12 GHz and 15.2 GHz were achieved with pulse durations between 757 fs and 824 fs. A Gires Tournois Interferometer (GTI) effect was used to facilitate soliton mode-locking in the waveguides via accurate control of the gap between the waveguide and the output coupling mirror. This is a convenient technique to control the dispersion without introducing any extra elements in the laser cavity.
Two further Yb-doped ultrafast laser hosts, RbTiOPO₄ and KY(WO₄)₂, were investigated for their potential as ultrafast waveguide laser sources, having both been previously demonstrated as good bulk ultrafast systems. Laser action was demonstrated for the first time in an (Yb,Nb):RbTiOPO₄ planar waveguide laser, fabricated by liquid-phase epitaxy. Ion-beam milling was then used to fabricate the first ever single-mode rib waveguides in (Yb,Nb):RbTiOPO₄ fabricated by dry etching techniques but laser action was not possible due to propagation losses of ~3dB/cm. A systematic study of the reactive ion etching of RbTiOPO₄ was then carried out to minimise the surface roughness, in an attempt to reduce the propagation losses. The first ever demonstration of single-mode waveguiding in (Yb,Nb):RbTiOPO₄ fabricated by reactive ion etching was demonstrated but the propagation losses remained high. Using (Yb,Gd,Lu):KY(WO₄)₂ as a gain media, efficient laser action was demonstrated in an "inverted-rib" waveguide laser structure fabricated by ion-beam milling. This laser was found to have a threshold power as low as 13 mW and a maximum slope efficiency of 58% and showed characteristics of a pure 3-level laser by lasing at 981 nm. However, further loss reduction is again required in order for efficient ultrafast operation to be obtained in the future.
Mode-locked waveguide laser operation was extended to the 1.5µm spectral region based on an ion-exchanged Er,Yb:phosphate glass waveguide laser using a novel SESAM based on a quantum dot in well (DWELL) structure. 2.5 ps pulses at a PRF of 4.8 GHz and an average output power of 9 mW were achieved. With a shorter waveguide sample, a PRF of 6.8 GHz with an average output power of 30 mW and pulse duration of 5.4 ps was achieved. The repetition-rate of the laser was finely tuned by controlling the pump power offering an attractive technique for enabling future frequency-comb stabilisation. This is the highest reported repetition-rate from a mode-locked waveguide laser at 1.5 µm and is also the first ever waveguide laser mode-locked by a quantum dot SESAM.
Finally, as an initial step towards further extension to the 2µm spectral region, laser action was demonstrated, for the first time, in an ion-exchanged Tm:glass waveguide laser with a threshold power as low as 44 mW and a maximum slope efficiency of 6.8% around 1.9 µm.
Designs for power-scaling of such sources have also been discussed in this thesis.