A group of researchers from Singapore, led by Dingyuan Tang are using a Graphene-Polymer composite as a laser locker. A laser locker is used to oscillate laser modes in phase, giving rise to short and very intense light pulses. Graphene can be used as a mode locker because it is a ‘saturable absorber’, that is, its light absorbance decreases rapidly with increasing light intensity — an essential property for passive mode lockers.
The team approached this application by introducing a graphene–polymer composite film into an erbium-doped fiber laser. “Encapsulating graphene in the polymer host passivate the graphene from ambient oxygen, which prevents photooxidation and optical bleaching by high-powered lasers,” says Tang.
The researchers observed efficient mode locking with pulses as short as 700 fs at a wavelength of 1590 nm with a pulse energy of 3 nJ. These characteristics surpass those achieved by the same laser using other types of mode lockers such as carbon nanotubes and semiconductor saturable absorber mirrors. Furthermore, careful examination of the pulse shape revealed that the pulse was composed of one soliton — a nonlinear localized wave. This is a major improvement compared with standard mode lockers, which usually give rise to multiple solitons.
The results represent a real advance in photonics applications of graphene. “Graphene–polymer composite is so far the only mode locker that can be potentially employed for different wavelengths of lasers, ranging from visible to infrared, due to graphene’s super-broadband saturable absorption,” says Tang.