Ajustes y pruebas de regresión

Two pump wavelengths were used to investigate the DFB laser performance. As mentioned in Section 6.1, the quantum efficiency limit for 1119 nm and 1836 nm, at the lasing wavelength, is ~52% and ~86%, respectively.

Figure 6.6:Experimental set-up of the5I6pump band of the Ho3+-doped DFB fibre laser.

Figure 6.6 shows the experimental set-up for the 5I6 pump band of the Ho3+-doped DFB fibre laser. The pump source was an Yb3+-doped fibre laser operated at 1119 nm. The Yb3+-doped fibre laser basically consisted of a spool of GTWave fibre, a high reflectivity grating and an output coupler (OC) grating at 1119 nm. The GTWave fibre assembly consists of an Yb3+-doped fibre and two un-doped silica pump fibres wrapped around the doped fibre. The GTWave fibre is 1 km long and is pumped by a pigtailed 977 nm multimode laser diode which was spliced to the pump fibre. As the pump light propagates along the pump fibre, its evanescent field is coupled to the Yb3+-doped fibre where it is absorbed. The operating wavelength of the Yb3+-doped fibre laser was determined by the Bragg gratings at 1119 nm which coincides with the optimal performance of the Yb3+-doped laser. The output power of the laser, measured at the end of the OC grating, was ~230 mW with an input power from the pump diode of 3W. The DFB fibre laser was configured in the forward pumping scheme in which a WDM coupler is eliminated. One end of the DFB laser, the end further away from the phase shift, was spliced to the output end of the Yb3+- doped fibre laser. The output end of the DFB laser, the end closer to the phase shift, was measured with the automatic monochromator.

The DFB laser was pumped with 230 mW and the power at the output end of the laser, measured with the thermal power meter, was ~120 mW. The measured power might consist of the signal and unabsorbed power, for this reason, the monochromator was used to investigate the spectrum at the output end of the DFB laser. Figure 6.7 shows the spectrum at the output end of the DFB laser as a function

HR Grating @1119 nm OC Grating @1119 nm Automatic Mono- chromator/ Thermal Power Meter 977 nm MM Laser Diode Module GTWave Fibre Spool Ho3+_-doped DFB

of wavelength that was scanned by the monochromator. From the spectrum, a peak was observed at 2123 nm. This peak wavelength was far from the designed operating wavelength of the laser, which is at 2140 nm. So, in order to further investigate the possibility of this being due to the pump source itself, the DFB laser was broken at the splice point and a 50 cm piece of Ho3+-doped fibre was spliced at the OC grating end. A peak at 2123 nm was still observed with a decreasing in amplitude. Obviously, this cannot be due to the pump source as the amplitude should be the same for both cases. The other possibility was lasing actually occurred at the maximum emission peak of Ho3+as the threshold power of the cavity could be lower than the DFB cavity. As observed in Section 4.2, the DFB laser with κL~5required a much higher threshold power than κL~10. This could be the reason as the κL of the Ho3+DFB was low, i.e. ~4, and the threshold power is even higher in order for lasing to occur. 2000 2050 2100 2150 2200

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Figure 6.7:Spectrum with and without DFB fibre laser at 230 mW pump power.

From the measured unabsorbed power ~120 mW, the absorption of the fibre at 1119 nm was ~0.27 dB/cm, which is about the same as the absorption loss measured earlier. The absorption of this fibre is low and the pump power available to reach the

the 1670 nm cut-off wavelength of the fibre, higher order modes could therefore be presented and hence power competition between the modes would occur. The following experimental set-up was used to pump the DFB laser above the cut-off wavelength of the fibre. Using the 1836 nm Tm3+DFB MOPA laser as pump source, the V-number at this pump wavelength was 2.19 so there was only one mode present. In addition, it is an in-band pumping scheme at 1836 nm, in which the theoretical quantum efficiency limit is ~86%. With this pump wavelength, the Ho3+ ions are excited to the upper laser level 5I7 directly, so the photon conversion efficiency for 2.1 μm emission will be higher than with the other pumping schemes. Figure 6.8 shows the experimental set-up of the5I7pump band of the Ho3+-doped DFB laser.

Figure 6.8:Experimental set-up for the5I7pump band of Ho3+-doped DFB laser.

The same experimental set-up for the Tm3+ DFB MOPA laser as in Section 6.2.3, used for obtaining the emission of Ho3+, was used to pump the Ho3+-doped DFB laser. The amplifier fibre length of 1.295 m was used, instead of 1.75 m. The maximum MOPA output power of ~110 mW was obtained with 960 mW pump power. The end further away from the phase shift was spliced to the MOPA laser output and the power after the Ho3+-doped DFB laser was ~ 45 mW, measured with the power meter. Again, the monochromator was used to investigate the output characteristics of the laser. Figure 6.9 shows the output characteristics of the DFB laser at a pump power of 110 mW and also 65 mW. For 65 mW pump power, the power at the output of the DFB laser was 27 mW. Results show that the ASE of the Ho3+-doped fibre was increased as the pump power increased and the DFB laser did not lase with these pump power levels. The pump absorption at this wavelength was ~0.2 dB/cm. E -TEK Isolator Automatic Mono- chromator/ Thermal Power Meter Tunics Wavelength Tunable Laser Diode Source 1565 nm 1W SPI High Power Amplifier Tm 3+_DFB Laser Ho3+-doped Fibre Tm3+_Co-doped Fibre

Pump wavelengths at 1119 nm and 1836 nm were used to pump the DFB laser, neither wavelength showed that lasing occurred. This could be due to the quality, Q, of the cavity that is reduced as compared with the Q-value of a grating strength of ~10. Consequently, the loss in the cavity has increased as a result of decreasing the Q of the cavity. Furthermore, the gain in the fibre could be low and hence, it is insufficient to overcome the losses. The estimated loss introduced by the reflective gratings was ~0.11 dB. The other problem could be due to the low pump absorption in a 12-cm long laser cavity; to resolve this, a distributed Bragg reflector (DBR) was constructed in the following section.

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Figure 6.9:The output spectrum of the Ho3+-doped DFB laser pumped at5I7band.