Gold nanoparticles saturable absorber for ultrashort pulse generation in fibre lasers
The advantages of ultrashort pulse lasers have triggered a technological tsunami in the laser field, putting pressure on researchers to discover the simplest fabrication method and thus effectively improve the saturable absorber (SA) preparation technique. The optical property of saturable absorp...
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| Main Author: | |
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| Format: | Thesis |
| Language: | English |
| Published: |
2021
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/103982/1/NOOR%20ZIRWATUL%20AHLAM%20-%20IR.pdf |
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| Summary: | The advantages of ultrashort pulse lasers have triggered a technological
tsunami in the laser field, putting pressure on researchers to discover the
simplest fabrication method and thus effectively improve the saturable absorber
(SA) preparation technique. The optical property of saturable absorption is
directly associated to the embedded materials in SAs themselves. Thus,
manipulation of the embedded materials may improve the overall performance
of the SA. Metallic nanostructures have been known for their optical properties
due to the effect of surface plasmon resonance. Among the metallic
nanostructures, gold nanoparticles (Au-NPs) are widely investigated due to its
saturable and reverse saturable absorption properties. These properties can be
tailored to cater for various applications by manipulating its size and shape.
There are three important aspects in this research work which include the
synthesis of Au-NPs in tetrahydrofuran by pulsed laser ablation, validation of
the ablated Au-NPs as a SA in generating mode-locked pulses and
investigation on the effect of Au-NP size towards optical pulse profiles. The
proposed synthesis of Au-NPs in tetrahydrofuran with stirring condition
produced a good size distribution of spherical Au-NPs ranging from 6.0 to 11.5
nm. The size reduction was influenced by the ablation time increment from 7 to
30 minutes. The effect of stirring was also confirmed by comparing the ablated
material size without stirring. However, the biggest challenge of using this
method was the low yield of ablated Au-NPs. In order to study the effect of Au-
NP size, commercially available Au-NPs of varied sizes were purchased; 10,
20, 40 60 and 80 nm. These two batches would be prepared as fillers inside
polydimethylsiloxane (PDMS) polymer matrix. To fabricate a mode-locker that
can support evanescent wave propagation, a tapered fibre was selected as the
preferred waveguide. The embodiment of Au-NPs with PDMS on the tapered
fibre was deposited using a spin coating technique. The longitudinal
encapsulation of Au-NP/PDMS enables interaction between evanescent wave
and matter (surrounding medium). The fabricated Au-NP-based SA was characterized using a twin balance photo-detection method to obtain its
saturation fluence, non-saturable loss and modulation depth. The functionality
of the fabricated Au-NP-based SA was proven by incorporating it in a ring
cavity erbium-doped fibre laser as a result of optical pulse generation. The
same laser cavity was used throughout the research work to minimize
uncertainties of loss and dispersion. Based on the experimental findings, both
batches of Au-NPs were proven to be able to generate ultrashort pulse with a
pulse duration of less than 1 picosecond. This marks the most significant
finding of the research work. For the ablated Au-NPs, the average size of 7.8
nm was successfully tested to generate mode-locked pulse at 1554.5 nm with
duration in the range of 916 – 994 fs. Even though that the modulation depth of
the fabricated SA was only 0.4%, a stable pulse was produced. For the
commercially available Au-NPs, mode-locked pulse was attained for all sizes to
verify the finding from the previous experiment (ablated Au-NPs). The lasing
performance was evaluated by comparing SA characteristics and pulse
qualities among the sizes. The optimum pulse performance was realized when
the SA was fabricated with 20 and 40 nm Au-NP size. For the former, the timebandwidth
product of 0.34 was demonstrated which was the closest to its
bandwidth-limited pulse. For the former, the fabricated SA exhibited 4.0%
modulation depth and average pulse duration of 886.7 fs. From the
experimental findings, larger Au-NP size of 60 and 80 nm had the tendency to
scatter more lights due to its larger cross section. Therefore, the highest
transmission loss of 8.56 dB was obtained for 80 nm Au-NP size and the pulse
quality deteriorated to 1062.3 fs. The research work has demonstrated the
functionality of Au-NPs as a saturable absorption material to generate
ultrashort pulses. In addition, the size of nanomaterials has influenced on the
characteristics of saturable absorbers that shapes the quality of laser pulse. |
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