Q-switched and mode-locked pulse generation with organic dyes based saturable absorber / Rawan M S Soboh

Rawan , M S Soboh (2021) Q-switched and mode-locked pulse generation with organic dyes based saturable absorber / Rawan M S Soboh. PhD thesis, Universiti Malaya.

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Abstract

The manifold applications of pulsed fiber lasers are an evidence of the importance of laser technology in science and engineering. Saturable absorbers (SA) play a key role in the generation of pulsed laser, whether passive Q-switched or passive mode-locked, without using the modulators. In this thesis, two saturable absorbers (SAs) based on organic materials were fabricated, characterized, and successfully incorporated into the erbium-doped fiber laser (EDFL) and the Ytterbium-doped fiber laser (YDFL) cavities to produce both Q-switched and mode-locked pulsed lasers. Zinc Phthalocyanine (ZnPc) SAs were constructed in three different ways; by embedding ZnPC in the polyvinyl alcohol (PVA) film to obtain embedded ZnPc:PVA, by casting ZnPC onto the PVA film to get casted ZnPc/PVA, and by spin-coating ZnPc onto the film to produce spin-coated ZnPc:PVA. The three samples of the ZnPc-SA were inserted one by one between the two fiber ferrules to form a fiber compatible Q-switch, and were placed in an EDFL cavity. Stable Q-switched pulse trains operating at the central wavelengths of 1561.4 nm, 1560.4 nm, and 1559.5 nm were produced by the SAs formed from embedded ZnPc:PVA, casted ZnPc/PVA, and spin-coated ZnPc/PVA thin films, respectively. The maximum achieved pulse energies for these Q-switched laser pulses were 71 nJ, 32 nJ, and 50 nJ. For Q-switched operation in the EDFL cavity, the shortest pulse width was 3.6 μs and the maximum repetition rate was 48 kHz, obtained with the use of the embedded ZnPc:PVA SA. By integrating the fabricated ZnPc:PVA SA inside the ring cavity, soliton EDFL mode-locked pulses with a pulse width of 1.83 ps, pulse energy of 2.1 nJ, and a peak power of 1.15 kW were also obtained. When the ZnPc:PVA SA was placed in a YDFL cavity, Q-switched pulses were generated with the minimum pulse width of 2.2 μs, maximum repetition rate of 63 kHz, pulse energy of 91 nJ, and an average output power of 5.7 mW. In addition, successful generation of mode locked YDFL pulses using the ZnPc/PVA SA, at a fixed repetition rate of 3.3 MHz with a picosecond pulse width, were also demonstrated. The highest pulse energy for mode locked YDFL by using ZnPc/PVA SA was 1.36 nJ. Finally, an SA made of a lawsone film was successfully prepared and tested in generating a passive Q-switched EDFL, realizing the highest pulse energy of 53.7 nJ and the shortest pulse width of 1.7 μs. The results indicate that both ZnPc and Lawsone thin films have great potential as alternative SA materials for Q-switched and mode-locked pulse generation.

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