Q-switching performance

Figure 5.33 shows the output power characteristics of both the Q-switched TBFL and TDFL against the input pump power. There is no Q-switching pulse observed below the threshold pump power for both lasers. The threshold pump power of the Q-switched TBFL is obtained at 106.6 mW, which is much lower than that of the conventional TDFL. Stable Q-switching pulse can only be generated within a pump power range from 187.3 to 194.2 mW for the TDFL. At the threshold pump power, the output power of TBFL and TDFL are obtained at 0.37 mW and 0.40 mW, respectively.

This shows that the efficiency of the TBFL is significantly higher than the conventional TDFL. The improved threshold and efficiency characteristics are due to the use of highly concentrated TBF as the gain medium, which reduces the cavity length of the laser. In addition, the presence of active bismuth ions in the gain medium improves the population inversion via the energy transfer process. However, the efficiency of both

TBFL and TDFL are comparatively low due to the insertion loss of the WDM, the output coupler and a saturable absorber.

Figure 5.33: The lasing characteristic of the Q-switched laser with two different gain media.

The output spectra of both Q-switched lasers were also monitored by an OSA, whose resolution is limited at 0.05 nm. Figure 5.34 shows the optical spectra of the TBFL and TDFL at the pump power threshold of 106.6 mW and 187.3 mW respectively. The TBFL operates at the center wavelength of 1857.8 nm, which is shorter than TDFL operating at 1892.4 nm due to the energy transfer from active bismuth, which provides a higher gain at shorter wavelength. The FWHM of the output spectra are obtained at ~0.18 nm and ~10.9 nm for TBFL and TDFL, respectively. The absence of spectral broadening in the Q-switched TBFL is due to the gain medium length used which is just around 0.4 m and thus prevents the non-linear effect to take place in the cavity. It is worth noting that the operating wavelength shifts to a shorter wavelength with the incorporation of the MWCNT-SA for both experiments due to the

insertion loss of the SA as compared to CW laser’s operating wavelength. As the cavity loss increases, the oscillating laser shifts toward the peak absorption wavelength of the gain medium as explained earlier.

Figure 5.34: The output spectra of the Q-switched TBFL and TDFL at the threshold pump power.

When the SA is inserted into the ring cavity, a stable and self-starting Q-switching operation is obtained within a pump power of 106.6 - 160.0 mW and 187.3 - 194.2 mW for TBFL and TDFL, respectively. Figure 5.35 shows the typical oscilloscope traces of those Q-switched pulse trains, which were measured at its threshold pump power by an oscilloscope via photo-detector. As shown in the figure, the pulse to pulse durations for the TBFL and TDFL are measured at 77.9 and 263.2 µs, which correspond to the repetition rates of 12.84 kHz and 3.8 kHz, respectively. The TBFL has a relatively higher repetition rate due to the gain medium length used which is reasonably shorter. The corresponding pulse widths are 9.6 µs and 22.1 µs for the

TBFL and TDFL, respectively. At the threshold pump powers of 106.6 and 187.3 mW, the average output power of the Q-switched TBFL and TDFL are 0.37 mW and 0.4 mW, respectively.

Figure 5.35: Q-switching pulse train observed from an oscilloscope for TBFL and TDFL.

Figure 5.36 shows how repetition rate and pulse width for both Q-switched lasers are related to the pump power. The pulse repetition rate can be seen to increase almost linearly with pump power, while the pulse width decreases in the same fashion for both lasers. The pulse repetition rate of the Q-switched TBFL can be widely tuned from 12.84 to 29.48 kHz by varying the pump power from 106.6 to 160.0 mW. This agrees well with the passive Q-switching theory with a saturable absorber (Spühler et al., 1999). Meanwhile, its pulse width reduces from 9.6 to 6.1 µs. The tuning range of the repetition rate for TDFL is smaller than the one for TBFL due to the limited available pump power range. The lowest pulse width of the TDFL is 18.3 µs, which is

obtained at the maximum pump power of 194.2 mW. The repetition rate / pulse width of the TDFL is expected to increase / drop further by increasing the pump power, as long as the damage threshold of the SA is not exceeded. Compared to the TDFL, the pulse width of the Q-switched TBFL is significantly shorter due to the use of a higher doped fiber, which shortens the cavity length of the fiber laser.

Figure 5.36: Repetition rate and pulse width as a function of pump power.

Figure 5.37 shows the average output power and pulse energy characteristics for both Q-switched lasers against the injected pump power. It is found that the output power and pulse energy increase with the pump power for both lasers. For instance, the output power of the TBFL increases from 0.37 to 1.82 mW and the pulse energy also increases from 28.8 to 61.7 nJ as the pump power is varied from 106.6 mW to 160.0 mW. However, the highest pulse energy of 126.1 nJ is obtained by the TDFL at the maximum pump power of 194.2 mW. This is attributed to the repetition rate of the Q-switched TDFL, which is significantly lower than that of the TBFL. The new fabricated

TBF exhibits a better Q-switching laser performance compared to the conventional TDF. This is most probably due to the use of TBF, which has a suitable lifetime for enhancing Q-switching operation (Keller, 2003).

Figure 5.37: Average output power and pulse energy as a function of pump power.