Thesis Summary

This thesis has reported the development of monolithic waveguide lasers around 1 µm based on Yb:tungstate crystals. These crystals were chosen due to their excellent spectroscopic properties which include [1]:

- high absorption and emission cross-sections, suitable for short crystal lengths

- strong absorption at 980 nm, making the crystals suitable for diode pumping

- simple two-level electronic structure of the ytterbium ion which reduces detrimental processes such as excited state absorption, up-conversion, cross relaxation and concentration quenching

- a small Stokes shift resulting in high laser slope efficiencies

- broad emission bandwidths suitable for ultrashort pulse generation.

As a precursor to the waveguide theme a novel QD-SESAM was investigated in a bulk cavity configuration for its suitability as a mode-locking element in Yb:tungstate lasers. It was found to be a highly suitable device, as pulses as short as 114 fs at 0.5 W were demonstrated in an Yb:KYW cavity, and a higher average output power of 1.15 W was observed with 200 fs pulses. The excellent performance was attributed to low non-saturable losses of 0.2 % and low saturation fluence of 25 µJcm-2, together with very fast relaxation times, surpassing the performance of QW-SESAMs [2]. This highlighted the advantages of using QD materials as saturable absorbers and led onto the inscription of channel waveguides in a QD-doped glass using ULI [3]. The losses at 1.5 µm were determined to be 2.9 dBcm-1, and the absorption in the waveguide region was found to be reduced compared to the bulk, indicative of modification to the QDs within the written volume. The exact nature of this modification requires further investigation. Nevertheless this has been an important first step towards the

creation of waveguide saturable absorbers for future integration with channel waveguide lasers to form fully integrated ultrafast waveguide-based devices.

A planar waveguide with an active Yb:KYW guiding layer was grown by LPE with losses of less than 0.1 dBcm-1. This waveguide was the first demonstration of lasing from Yb:KYW in a monolithic cavity configuration [4]. Using surface tension provided by fluorinert liquid, a mirror and output coupler were held against the crystal end-facets. Using a 4.1 % output coupler a maximum output power of 148 mW was observed with 62 % slope efficiency at 1039 nm. A higher output power of 190 mW with a 76 % slope efficiency was obtained with a bulk 10% output coupler, held to the crystal facet using a mirror mount. These output powers and slope efficiencies are comparable with results from more typical z-fold cavity configurations using bulk Yb:KYW crystals, but from devices with a much smaller footprint. With 1 % output coupling a threshold of 40 mW absorbed pump power was observed. This is the lowest threshold reported in an Yb:KYW laser. This result shows the advantages of waveguide lasers over their bulk counterparts, offering comparable output powers and slope efficiencies but with the additional advantages of compact geometries and reduced lasing thresholds.

Replacing the output coupler with an OC-SESAM, Q-switching was demonstrated. Pulse durations were as short as 170 ns with up to 33 mW average output powers at repetition rates exceeding 700 kHz. The maximum pulse energy was found to be 44 nJ. The advantage of partially unstable cavities in offering optimal output coupling was then demonstrated as the average output power from the same Q-switched laser increased to over 90 mW in such a geometry. The repetition rate and pulse duration of this laser remained similar to the stable cavity performance, but the increased average power led to a pulse energy of 140 nJ. Although unstable cavities are known to be able to improve beam quality from a laser, in this case the beam performance along the unguided direction remained poor. This was attributed to the low magnification in a plane-plane cavity which does not lead to transverse mode discrimination and therefore does not produce a good far-field beam pattern. In a monolithic cavity the best option for high beam quality is to use a channel waveguide, and this was the next topic which was investigated.

Channel waveguides were fabricated in Yb:KGdW and Yb:KYW using ULI [5]. This was the first example of ULI waveguides fabricated in Yb:KGdW. These channel waveguides were also used to build monolithic lasers, where in Yb:KGdW four waveguides were found to lase and in Yb:KYW sixteen lasing structures were identified. Interestingly, guiding regions were found to be polarisation dependent. In Yb:KGdW, E||b gave rise to guiding in left, central and right hand regions where optimal guiding was found in structures written with 369 nJ – 389 nJ of pulse energy and a 20 µm scan separation. Lasing powers were as high as 11.2 mW at 1036 nm and propagation losses were found to be 2.9 dBcm-1. With E||a guiding was observed in upper and lower regions, where the optimal structure was written at 408 nJ pulse energy and a 20 µm scan separation, which had produced a cracked region between the two filaments. The best lasing performance was obtained with this structure, where 18.6 mW was achieved with a 9.3% slope efficiency using a 5 % output coupler. This waveguide lased at 1023 nm and had losses of ~2 dBcm-1. The M 2 of this laser was determined to be 1.5 and 1.2 along the a and b axes respectively. The waveguide numerical aperture was used to estimate the refractive index change in the waveguides to be 0.8×10-3 in the E||a axis and 1.3×10-3 in the E||b axis.

In Yb:KYW E||a only gave poor guiding results, but E||b gave guiding in left, central and right hand regions. Central and left hand regions were consistently found to lase in structures written with a 20 µm scan separation and with pulse energies between 355 nJ and 432 nJ, where pulse energies between 392 nJ and 411 nJ gave optimal results. Output powers were low with 8.2 mW being the maximum demonstrated using a 5 % output coupler, and the losses were found to be greater than 3.7 dBcm-1. The output powers and slope efficiencies were very poor when compared to those reported in the LPE planar waveguide. This was due to the large losses of the channel waveguides. However, the excellent beam quality shows the advantages channel waveguides can offer. By reducing the propagation losses of the channel waveguides excellent power, slope efficiency and beam quality should be achievable. In this work the writing polarisation, pulse energy and scan separation were varied to optimise the waveguide quality. By optimising other writing parameters it is anticipated that losses can be reduced even further.

Finally micro-spectroscopy measurements were performed on the channel waveguides in order to identify some of the structural changes that had resulted in the guiding behaviour. Micro-luminescence and micro-Raman maps of both lasing and non-lasing structures were taken. It was found that in all structures the intensity of both the luminescence and Raman peaks dropped at the written filament, suggesting increased damage and disorder in these regions, in agreement with the crystal damage which could be observed in end-facet images. This resulted in a lower material density in the filaments with reduced refractive index, forming an effective boundary for the waveguides. Higher writing pulse energies led to greater drops in intensity in the filaments, signifying increased damage, which was expected with greater writing energy. The most significant observations were changes to the energy shifts seen in several of the Raman lines in guiding regions. In Yb:KGdW the higher energy shift in the 898 cm-1 and 682 cm-1 lines were attributed to a densification of the WO2W

bridge, which increased the refractive index in these regions and explained the observed E||a guiding regions. In the 754 cm-1 and 764 cm-1 Raman lines a lower energy shift is characteristic of densification of the WO2W bridge in the lattice, which

again resulted in an increased refractive index in these regions, which supported guiding in both Yb:KGdW and Yb:KYW along the crystallographic b axis. This technique has therefore been shown to be able to identify not only guiding regions, but also the nature of the crystal modification which has resulted in the waveguides.

This thesis has therefore presented a variety of work which has resulted in the demonstration of monolithic waveguide lasers in Yb:tungstates, and also a method for characterising the fabricated waveguides. Excellent output power and slope efficiencies were demonstrated in LPE-grown planar waveguides, and excellent beam qualities were demonstrated in channel waveguide fabricated by ULI. QD-SESAMs were also characterised to be highly suitable mode-locking elements in Yb:tungstate cavities. Future work will of course build on this to combine the excellent performance from each result into a single device.