Q-switching and mode-locking are two of the most used approaches to generate optical pulses. Q-switched pulses typically have lower repetition rate, larger pulse energy and longer pulse duration than those generated by mode-locking. Therefore, Q-switched lasers are ideal for applications in which a large pulse energy is desirable and short pulse duration is not necessary, such as industrial material processing and optical sensing.
Saturable absorbers (SAs) are currently the most utilized means to produce Q-switched pulses. They are fabricated from nonlinear optical materials, including dyes, color filter glasses, ion-doped bulk crystals, and semiconductors. All these SAs have drawbacks, such as narrow operation bandwidth, high loss, and expensive fabrication/integration. In particular, these SAs are not suitable for all-fiber Q-switched lasers, which are considered as alternative to the currently widely used bulk solid-state lasers, due to efficient heat dissipation and alignment-free waveguide format offered by the optical fibers. Ion-doped bulk crystals and semiconductor saturable absorber mirrors (SESAMs) are the most common SAs for passively Q-switched fiber lasers. They normally have narrow operation bandwidth (~ few tens of a nanometer), thus are not adequate for broadband tunable pulse generation. They also typically need free-space coupling components, eliminating the alignment-free waveguide feature of fiber lasers.
Graphene is a promising SA, at the centre of an ever growing research effort since the first demonstration of a mode-locked fiber laser in 2009, due to its ultra-broad bandwidth, ultrafast response time, low-cost and easy fabrication/integration. It has since been used successfully to mode-lock various lasers (including fiber, solid-state, waveguide and semiconductor lasers), as well as being integrated into Q-switched lasers.
Wu et al. report a high-energy, wavelength-tunable, all-fiber laser passively Q-switched with a graphene SA. Graphene is prepared by chemical vapor deposition, and then transferred to form a fiber-compatible SA, by sandwiching it between two fiber connectors. Single-layer graphene was selected to facilitate broadband wavelength tunability with low insertion losses (<2.3%). Wavelength tuning was obtained via a Fabry–Pérot tunable fiber filter. The central wavelength can be continuously tuned from ~1531 to ~1547nm. The gain medium is an Er:Yb co-doped double-clad fiber, which offers large gain for high energy pulse generation. Most of the intra-cavity light generated in the gain fiber was coupled outside the cavity for output performance characterization. This also reduced the heat load in graphene, placed after the output coupler. For a 582mW pump power, the maximum average Q-switched output power was 25.6mW. The corresponding single pulse energy was~1μJ, much higher than the previously reported all-fiber graphene Q-switched lasers (e.g.,~40nJ from the first all-fiber tunable laser Q-switched with graphene in the 1522-1555 nm range).
These results underline the great potential of graphene SAs to realize high-energy, low-cost, wavelength-tunable pulsed light sources. Further investigations, e.g., improving optical-to-optical conversion efficiency, may lead to a significant increase of the output energy, wavelength tuneability and stability, to meet the requirements for a range of applications in industrial material processing, spectroscopy, metrology and biomedical diagnostics.
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