We demonstrate an optically tunable graphene saturable absorber to manipulate the laser operation in pulsed fiber laser system. Owing to the strongly enhanced evanescent field interaction with monolayer graphene, we could realize an efficient control of modulation depth in the graphene saturable absorber by optical means through cross absorption modulation method. By integrating the tunable graphene saturable absorber into the fiber laser system, we could switch the laser operation from Q-switching through Q-switched mode-locking to continuous wave mode-locking by adjusting only the optical power of the control beam. In addition, we realized a hybrid Q-switching of fiber laser by periodical modulation of the absorption of the graphene saturable absorber, where we observed that the repetition rate of the Q-switched laser could be continuously tuned according to the modulation frequency of the applied external signal.
The
1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits
different photoluminescence (PL) and aggregation-caused quenching
properties due to its crystal polymorphism, which depends on the solvent
evaporation process in tetrahydrofuran solution. In the slow drying
process, molecules aggregated into an energetically more stable form
(time-dependent density functional theory calculation), of which the
PL peak maximum was 453 nm, corresponding to blue emission at the
365 nm excitation. However, the fast evaporation process induces an
energetically less stable form, with a PL peak maximum of 508 nm,
corresponding to green emission. The main difference between the two
crystal structures is the alkyl conformation, as confirmed by X-ray
single-crystal analysis. Due to the different alkyl conformations,
NI groups aggregated into more obliquely aligned structures that emit
blue PL, which plays a role in weakening the π–π
interactions between molecules relative to green PL crystals. We found
that the conformational stable molecular stacking induced instability
in the electronic energy levels of the blue crystal compared to the
green crystal.
We demonstrated a 1.1-µm band extended wideband wavelength-swept laser (WSL) that combined two semiconductor optical amplifiers (SOAs) based on a polygonal scanning wavelength filter. The center wavelengths of the two SOAs were 1020 nm and 1140 nm, respectively. Two SOAs were connected in parallel in the form of a Mach-Zehnder interferometer. At a scanning speed of 1.8 kHz, the 10-dB bandwidth of the spectral output and the average power were approximately 228 nm and 16.88 mW, respectively. Owing to the nonlinear effect of the SOA, a decrease was observed in the bandwidth according to the scanning speed. Moreover, the intensity of the WSL decreased because the oscillation time was smaller than the buildup time. In addition, a cholesteric liquid crystal (CLC) cell was fabricated as an application of WSL, and the dynamic change of the first-order reflection of the CLC cell in the 1-µm band was observed using the WSL. The pitch jumps of the reflection band occurred according to the electric field applied to the CLC cell, and instantaneous changes were observed.
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