# In-Band Pumped Thulium-Doped Tellurite Glass Microsphere Laser
## 4 Experimental Results and Discussion
(**c**) A microphotography of single-index fiber produced from the synthesized glass. (**d**,**e**) The images of the experimentally produced tellurite glass microsphere in perpendicular planes from two CCD cameras. The coordinate axes in (**d**,**e**) correspond to the axes in Figure 1.  
images in perpendicular planes. An example of images from CCD cameras is given in Figure 4d,e, where one can also see near-IR light (\"pink belt\" in the equatorial plane) that is explained by very weak (neglected in simulation) energy transfer up-conversion processes in thulium ions (the pump light at 1.55 \(\upmu\)m is invisible to our CCD cameras). Sub 0.1-\(\upmu\)m adjustment of the relative position between the microsphere and the silica taper was implemented with a 3-axis translation stage with precision piezo actuators (Thorlabs MAX312D). The output spectrum of the transmitted pump light was recorded by Telecom OSA (Optical Spectrum Analyzer) Yokogawa AQ6370D (600-1700 nm). To record the spectrum of laser generation beyond the spectral boundary of Telecom OSA, we used a scanning grating monochromator (MS2004i, SOL Instruments Ltd., Minsk, Belarus) and an amplified PbSe detector (PDA20H, Thorlabs) integrated into the computer controlled setup. Figure 5 demonstrates a composite spectrum of the transmitted pump wave (in dB scale) and the generated laser wave (in linear scale). The pump power launched to the taper is about 5 mW. The laser wavelength is about 1.9 \(\upmu\)m, which agrees with the numerical results shown in Figure 3b, where for \(N_{Tm}\) = 5\(\cdot\)10\({}^{19}\) cm\({}^{-3}\) and Q-factor of the order of 10\({}^{6}\), the expected laser wavelength is the same. We estimated the threshold power absorbed in the resonator as 0.5 mW. The slope efficiency estimated by modulating the pump power and measuring the response of the generated laser signal was about 10%, which also agrees with theoretical predictions (see Figure 3c, where for \(N_{Tm}\) = 5\(\cdot\)10\({}^{19}\) cm\({}^{-3}\) and Q factor of 10\({}^{6}\), the calculated value is 11%). Note that measurements of dependencies of the laser output characteristics on the pump power are complicated because changes in the pump power result in changes of the heat dissipated in the microresonator and the corresponding shift of WGM resonance [36], thus changing the pump coupling conditions. Note, despite the fact that lasing in Tm-doped tellurite microspheres was achieved in several works listed in Table 1, an in-band pump was used only in [20]. In [20], at a Tm\({}^{3+}\) concentration of 4.2 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) for a microsphere with a Q factor of 2 \(\times\) 10\({}^{6}\), the central lasing wavelength was 1975 mn, which agrees with the results of our theoretical prediction. Therefore, we believe that our experimental and theoretical work can be useful in the development of relatively simple Tm-doped in-band pumped microlasers. Figure 5: The experimental spectra of the transmitted pump wave (green, left axis, recorded by Telecom OSA in dB scale) and the generated laser wave (red, right axis, obtained using monochromator and PbSe detector in a linear scale).