Comparison of pumping schemes in Tm 3+ doped tellurite samples

In section 4.3 the performance characteristics of the Tm2.0:TZNG glass element pumped by an SDL system at 1211 nm were presented. The results were published in Optical Materials[9] and it is interesting to compare them with the 793 nm Ti:Sapphire pumping reported in section 3.2 on the same gain media. In both cases the laser performance characteristics were recorded under the same environmental settings, the temperature of the sample was kept at 15 ºC and the same set of mirrors and output couplers were employed. Table 4.1 summarizes the relevant parameters of comparison.

Tab 4.1 The continuous wave laser performance characteristics and some spectroscopic parameters of the Tm2.0:TZNG sample pumped at two different wavelengths.

The output to absorbed slope efficiency recorded with a 4 % OC was of 25.6 % and 22.4 % for the 793 nm and the 1211 nm pumping respectively. Similar laser threshold values are also found in the two cases. The net difference around 3 % between the efficiencies of the two lasers is low and confirms that excitation of the 3H5 level of thulium via 1200 nm radiation for this Tm3+ concentration in this tellurite glass is comparable to 3H4 Ti:Sapphire pumping[10]. The two different values of δ793nm = 2 % and δ1211nm = 3.7 % that were found were probably due to the fact that different spots of the same glass sample may be not homogeneous in quality. It is also interesting to give an explanation of the energy transfer mechanisms that come into play in the two cases.

Fig 4.13. Simple Tm3+ Energy levels diagram. Continuous straight lines highlight the main transitions a) 3H5 pumping and b) 3H4 pumping. The main energy transfer mechanism involved

are excited state absorption ( ESA and UC ) and cross-relaxation ( CR ). Dashed lines indicate phonon assisted transitions, nonradiative energy transfer.

Parameters strongly connected to the host-dopant combination and common to all types of excitations are the lifetimes of the energy levels and branching rations of the radiative transitions. In section 2.5.1 the luminescence lifetimes of the 3F4 and of the 3H4 were measured as 1.3 ms and 319 µs respectively. The two levels were highlighted accordingly to their lifetimes in Fig 4.13. The radiative branching ratios of the 3H4 level on similar Tellurite based glass[11] shows that almost 90 % of the radiating ions excited in 3H4 will radiatively relax to ground, ~8 % would relax to 3F4 and ~2 % would relax to 3

H5. As it was introduced in chapter 2, many parameters affect the performances of the 3

F4 → 3H6 laser transition and in this discussion a qualitative indication of the various radiative and nonradiative effects in the case of the two pump regimes is presented.

In the case of 3H5 pumping of the Tm3+:TZNG glass sample a blue upconversion signal was produced and its intensity appeared weaker during lasing and vice versa. This showed that the well known sequential excited state absorption ESA of three pump photons promoted ions to the 1G4 at 480 nm, Fig. 4.13a, and that it was dependent on the population of level 3F4. Out of all the ions excited to 3H5 the vast majority of them would quickly relax to the lasing level in multi-phonon aided transition, some of them would upconvert to 3H4 and ultimately to 1G4 and a very tiny part promoted in 3H4

would undergo cross-relaxation CR and populate the lasing level. Under this excitation the laser emission is less dependent on the cross-relaxation CR and therefore less dependent on the dopant concentration, the limiting factors are the ESA efficiency, the concentration quenching and the lifetime quenching described in chapter 2.

In the case of 3H4 pumping of the glass the blue upconversion was inversely proportional to the laser output power and also a red radiation scattered from the pump could be seen. The energy step from 3F4 to 1G4 is 15280 cm-1 in this glass and corresponds to a 654 nm photon wavelength, while the energy step from 3H5 to 1G4 is 12580 cm-1 and corresponds to a 794 nm photon wavelength therefore it is thought that the blue upconversion to 1G4 is taking place from the 3H5 level via an upconversion mechanism UC. The population in 3H5 would then be supplied either by multi-phonon relaxation from 3H4 or phonon assisted transitions[12] from 3F4 as shown in Fig. 4.13b. In steady state conditions most of the ions pumped in 3H4 would nonradiatively relax to the lasing level 3F4 or cross-relax to populate 3F4 once again. A small part would upconvert to 1G4 and or emit to the ground level 3H6. Having the correct dopant concentration is paramount in 3H4 pumping as the CR plays the major role in the overall efficiency of the laser. Since the CR is directly proportional to the concentration[13] a tradeoff needs to be found between the CR and the limiting factors such as upconversion, concentration quenching and lifetime quenching.

It would be important to define which of the two pumping schemes would perform best and this can only be done by extensive spectroscopy and laser characterisation of samples with different combination of hosts (changing the phonon energy ħω) and varying dopant concentration. The relative upconversion signal strengths would need to be quantified and that would require the design of an excitation setup where the pump wavelengths can be easily swapped and where the upconverted luminescence can be measured while the 2 µm laser is running. The upconversion signals in the Tm3+:TZNG sample with both pump wavelengths were very weak for many reasons. The short lifetime of 3H4 compared to the tens of milliseconds of lower doped samples would not favor the upconversion and in the case of the 3H5 excitation the 1211 nm pump was almost completely off the excited absorption ( ESA ) spectra as measured by Jackson et al.[14].