4. Proyecto AGORA: Programa de prácticas transnacional para estudiantes de
4.3. Desarrollo: fases de AGORA
An indium gallium arsenide diode laser system was used to pump the YbAG film at a wavelength of 940 nm with a potential total power of 50 W. Two lenses were used to focus the pump light into a small volume of the film, forming a gain region. Two types of laser cavity were formed around the gain region to see if the film would lase. Both cavities are simple for testing purposes but the pump radiation can be used more efficiently by passing it through the disk several times as has been reported previously elsewhere [35]. The experimental arrangement used for pumping and optical analysis of the YbAG film is shown below in figure 4.8.1, and the two types of cavities attempted are shown in figure 4.8.2 and figure 4.8.3.
chopper chopper
control
B.
detector amplifier oscilloscope
fiber optic
coupler opticalfiber
optical spectrum analyser A. pump filter A or B laser cavity diode laser lenses
Figure 4.8.1: Experimental arrangement for pumping and optical analysis of the YbAG film.
to heatsink
water chiller heatsink
thin-disk or film cavity mirror resonant path pump light unused pump light mirror HR @ laser HT @ pump laser output
Figure 4.8.2: Coupled-cavity for a thin-disk laser.
The coupled-cavity arrangement utilises the occurrence of etalon cavities between the interfaces of the crystal. An external cavity is made using a flat mirror that transmits the pump wavelength but reflects the laser wavelength, and a curved mirror that reflects both the pump and laser wavelengths, but allows some of the laser wavelength to be transmitted as output. An advantage of such a cavity is that the pump light is passed through the device twice, but alignment is critical for it to overlap its original path through the crystal, and parallelism of the crystal interfaces is also critical for the etalon cavities to exist.
HeNe alignment beam to heatsink water chiller heatsink thin-disk or film cavity mirror resonant path pump
light pump lightunused
Brewster angle
laser output
The Brewster-angle-cavity takes advantage of the fact that light polarised parallel to the plane of incidence is transmitted without any Fresnel reflection at the Brewster angle. The cavity is formed by two curved mirrors that must be set very precisely for lasing to be successful. A disadvantage of this cavity arrangement is that as the device thickness is scaled downwards, it becomes increasingly difficult to align the cavity mirrors. With smaller thicknesses, the pump light must be focussed more tightly for it to still be absorbed efficiently, and the volume of the gain region is therefore reduced. For lasing to occur, the resonant path between the two cavity mirrors must be aligned with the gain region and this task is much more difficult as the volume of the gain region is made smaller.
4.8.2 Absorption spectra and film losses
The absorption spectrum of the YbAG film was measured using a spectrophotometer and a polished blank YAG substrate was used as a reference. This allowed the absorption coefficient and an upper limit for the film transmission loss to be estimated.
4.8.3 Threshold power for lasing
The experimental arrangement shown in figure 4.8.1:B was used to detect the occurrence of lasing from the YbAG film. HeNe laser alignment beams were used to try and make the pumped volume and resonant path between the cavity mirrors coincide as closely as possible, before further adjustments were made to the alignment of the cavity mirrors.
4.9 Conclusions
● The technique of PLD has been described and some of the challenges involved in the growth of thick films have been discussed in detail. An improved substrate holder design allowed substrates to be heated with a homogeneous temperature distribution, and deposition using multiple growth runs and target reconditioning was found to be a route to thick film growth without the occurrence of particulates increasing significantly.
● The techniques used for the characterisation and preparation of films have been described and a limit was found for the thickness of waveguiding films, due to cracking at the polishing stage of preparation. The use of 1 mm thick substrates allowed thicker films to be polished without the substrate cracking, but difficulties were still experienced with films cracking.
● The experimental setups for waveguide and thin-disk laser cavities, and the techniques used for their qualitative analysis have been described. Some theory necessary for the derivation of properties such as propagation loss and launch efficiency has been discussed.
Chapter 5
Nd:GGG Film Results
5.1 Introduction
All of the results obtained from Nd:GGG films are presented in this chapter. Firstly, there is an overview of the results from basic material analysis that are the same or similar for each film. The lasing results from a 40 μm thick Nd:GGG film deposited in a single growth run and pumped with a Ti:sapphire laser are presented and discussed, along with the results from a 50 μm thick Nd:GGG film deposited using multiple growth runs and pumped with a Ti:sapphire laser, and subsequently with a laser diode array. Finally, the results obtained from self-imaging in a Nd:GGG film are presented and conclusions are drawn from all of the results.