hand, the pulse energy fluctuates with the pump power increment. The highest pulse energy of 54.1 nJ is achieved at the pump power of 650 mW. The Q-switched fiber laser shows a stable operation without significant degradation of the deposited MWCNTs with such output pulse energy level.

Figure 5.41: Average output power and pulse energy versus pump power.

Table 5.1: Q-switched performance of the GSA and MWCNTs

Types of SA

Gain media

Pumping excitation (nm) / length

of fiber (m)

Input range (mW)

Highest average output (mW)

Lowest pulse width (µs)

Highest Repetition rate (kHz)

Highest Pulse energy

(nJ)

Graphene TDF

800 / 2 186 – 207 1.01 19.3 13.1 77.2

1552 / 4 513 – 539 1.01 8.17 24.16 90.64

MWCNTs

TDF

800 / 2 187.3 –

194.2 0.58 18.3 4.6 126.1

1552 / 5 302.2 –

382.1 2.2 21.7 46.1 103.4

TBF

800 / 0.4 106.6 – 160 1.82 6.1 29.48 61.7 1552 / 1.5 500 – 800 3.35 4 61.99 54.1

The Q-switch TDFL was successfully demonstrated using a 2 m long TDF, which is pumped by an 800 nm laser diode and a GSA. At the maximum pump power of 207 mW, the laser produces the highest repetition rate of 13.1 kHz and the highest pulse energy of 77.2 nJ. The pulse width trend follows a V-shaped curve due to the effect of heat transfer to the GSA, which was initiated by the non-radiative emission from 3H4 to 3F4 level of thulium. The curved is eliminated by using 1552 nm (3H6 to 3F4) pumped wavelength to excite the thulium ions. The attainable pulse energy can be increased to 90.64 nJ. We also demonstrated a Q-switched TBFL using a MWCNTs based SA in conjunction with 800 nm pumping. The performance of the TBFL is also compared to that of a TDFL. The TDFL generates an optical pulse train with a repetition rate from 3.8 to 4.6 kHz with a pulse width of 22.1 to 18.3 µs when the pump power is tuned from 187.3 to 194.2 mW. Meanwhile, the pulse train of the Q-switched TBFL has a repetition rate ranging from 12.84 to 29.48 kHz, pulse width of 9.6 to 6.1 µs with the pump power varied from 106.6 to 160 mW. The Q-switched laser is generated at the lowest threshold pump power of 106.6 mW. The Q-switched TBFL is also

demonstrated using 1552 nm pumping. It operates within a wide pump power range of 300 mW. The repetition rate increases from 22.52 to 61.99 kHz while the pulse width reduces from 5.6 to 4.0 µs by increasing the pump power from 500 to 800 mW. The proposed laser is expected to have various practical applications in fiber communications and sensors.

CHAPTER 6 6

CONCLUSIONS AND FUTURE WORKS

6.1 Conclusion

This research work is devoted to the construction of fiber lasers that emit longer wavelength beams (in the spectral region of around 1.9 µm) utilizing thulium doped and co-doped fiber laser for the improvement of lasing efficiency and threshold pump power. The approach taken involves employing two different co-doping elements which are Ytterbium-Thulium doped fiber (YTDF) and Thulium-Bismuth doped fiber (TBF) as the gain media. Both fibers are newly fabricated and their spectroscopic properties as well as energy transfer processes have been investigated. In the Q-switched laser, new implementation of saturable absorbers which are graphene and multi-walled carbon nanotubes (MWCNTs) have been proposed using commercial Thulium-doped fiber (TDF) and TBF as the gain media. The proposed pulse laser performances are comparable to the reported published works.

A spectroscopic study of YTDF exhibit possible degradation of the fiber laser performance in 2 µm region due to the upconversion (UC) processes which contribute to the effect of excited state absorption (ESA). A lasing action was successfully obtained using two YTDF samples with different Yb:Tm concentration (LTY6 and LTY8) based on a cladding pumping technique through the transition of thulium ions from 3F4 to 3H6 with the assistance of ytterbium to thulium ion energy transfer. With a ring configuration, the laser is more efficient when coupled with a 905 nm pumping source compared to the 931 nm pumping source. In the Fabry–Perot cavity with two FBGs configuration, the YTDF laser (YTDFL) operates at 1901.6 nm with an efficiency

ytterbium to thulium concentration ratio of 4.0:1 contribute to more efficient energy transfer between the sensitizer and acceptor ions in YTDF, which in turn lowers the threshold of the laser. Higher NA and smaller core radius also contribute to the higher lasing efficiency. The use of multimode pump with slightly lower wavelength than 931 nm is shown to improve both the laser’s threshold and efficiency of the YTDFL.

Finally, a series of dual-pumping schemes are proposed to improve the lasing efficiency of YTDFL based on linear cavity configuration. However, the proposed YTDFL is experimentally less efficient than other reported works due to three possible reasons.

The first reason is the size of the fabricated fiber core diameter, which is approximately two times larger compared to that of the FBG fiber (around 7–8 micron). When both fibers are spliced together, a higher splicing loss of around 1 dB is generated as a large portion of the pump power leaks out. The second reason is due to the poor thermal management of the YTDF. The YTDF cannot operate at pump powers higher than 3 W due to the fiber damage at the splicing point. The burnt fiber problem has been solved using a water bath, however the output power is still low. The third reason is due to the high possibility of multi-step energy transfer which leads to UC and blue emission.

In comparison with YTDF, the enhancement of lasing performance has been identified in TBF using three TBF samples with different thulium and active bismuth concentration (TB1, TB2, and TB3). The incorporation of active bismuth ions in the gain medium helps increase the 3F4 population and thus improves the efficiency of the laser through energy transfer processes without any degradation from UC. The experimental results reveal that TB2 with a comparatively short length (0.4 m) of fiber can achieve higher efficiency as compared to the conventional TDF. This is attributed to the incorporation of Bi ions in the gain medium which help to increase the 3F4

population through energy transfer processes. TB2 which has the highest amount of active bismuth and thulium concentrations exhibits the highest lasing efficiency. An

efficient TBFL with dual pumping at 800 nm and 1552 nm wavelength was also demonstrated. Higher efficiency than single pumping method is attributed to the use of an additional 1552 nm pump to complement the 800 nm pumping. Apart from that, the energy transfer processes can be optimized by modifying the dopants compositions thus increase the efficiency of stepwise energy transfer. By pumping the TBF with an 800 nm pumping excitation, 2 energy transfer processes may occur which are cross relaxation and energy transfer from active bismuth to Tm3+ ions. Due to the efficient energy transfer, a broadband Amplified Spontaneous Emission (ASE) from TBF was proposed. Finally, an all-fiber dual wavelength TBFL operating in the 1900 nm region is demonstrated using a single FBG in a ring configuration. A dual wavelength laser output is obtained as the polarization controller (PC) orientation is adjusted to balance the loss between the two wavelengths. By controlling the intracavity polarization, the dual wavelength laser can also be switched to operate in the single-wavelength.

In the final part of this research work, an all-fiber 1.9 µm Q-switched laser has been successfully produced using commercial TDF and TBF as the gain media in a ring cavity configuration. Reliable self-starting Q-switched based on graphene saturable absorber (GSA) and multi-walled carbon nanotube saturable absorber (MWCNT-SA) were observed. Both of the GSA and MWCNT-SA were fabricated in-house. The graphene flakes were fabricated using electrochemical exfoliation technique whereas MWCNT thin-film was used as received without extra purification process. Both of the SAs were fabricated by cutting a small part of the prepared film and sandwiching it between two FC/PC fiber connectors, after depositing an index-matching gel onto the fiber ends. By using the GSA, a Q-switched fiber laser has been demonstrated using a TDF pumped by an 800 nm pump wavelength and the experiment has been repeated using a 1552 nm pump wavelength. The V-shaped curve of the pulse duration is observed when an 800 nm pump is used which is attributed to the contribution of heat

transferred to the GSA initiated by the non-radiative emission from 3H4 to 3F4 level of thulium. This curve was not observed when the 1552 nm pump source was used to excite the Tm3+ ions due to the absence of non-radiative decay and lattice vibration in the system. Unlike the GSA, the MWCNT-SA successfully generated the Q-switching for both the TDF and TBF-based gain media. The Q-switched performance comparison between TDF and TBF has been made. It is found that the best Q-switched laser performance is exhibited by a TBF pumped by a 1552 nm laser in conjunction with the use of the MWCNTs-SA. It is proven that the pumping of the TDF using a 1552 nm laser excitation enables the possibility to eliminate the accumulated heat in the SA. The Q-switched TBFL has the lowest threshold pump power of 106.6 mW using an 800 nm pumping excitation. With a 1552 nm laser excitation, Q-switched TBFL based MWCNTs pulses have been observed within a wide pump power range of 500 to 800 mW. The corresponding pulse train has a repetition rate ranging from 22.52 to 61.99 kHz with a pulse width of 5.6 to 4.0 µs.

The primary focus on the development of CW and Q-switching fiber laser at 1.9 µm region has been achieved. Even though several results are not as expected, the discussion and analysis of the performances have been carried out. The laser performances on the effect of co-doping ytterbium and bismuth to the thulium ions have been investigated. The experimental results show that TBFL has better performance compared to the commercial TDFL and YTDFL. Although YTDFL has been expected to give better lasing performance, several limiting factors have been identified to be the reason of poor lasing performance. The successful construction of a passively Q-switched fiber laser at the 1.9 µm region using in-house fabricated GSA and MWCNTs-SA facilitates numerous applications in fiber communications and sensor. The findings in this work could be used in the future for the development of fiber based sources targeting new applications.

In document THESIS SUBMITTED IN FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF (halaman 188-195)