Graphene is at the center of a significant research effort. Near-ballistic transport at room temperature and high mobility make it a potential material for nanoelectronics. Its electronic and mechanical properties are also ideal for micro- and nanomechanical systems, thin-film transistors, and transparent and conductive composites and electrodes. Here we exploit the optoelectronic properties of graphene to realize an ultrafast laser. A graphene-polymer composite is fabricated using wet-chemistry techniques. Pauli blocking following intense illumination results in saturable absorption, independent of wavelength. This is used to passively mode-lock an erbium-doped fiber laser working at 1559 nm, with a 5.24 nm spectral bandwidth and approximately 460 fs pulse duration, paving the way to graphene-based photonics.

We demonstrate a wideband-tunable Q-switched fiber laser exploiting a graphene saturable absorber. We get∼2µs pulses, tunable between 1522 and 1555nm with up to∼40nJ energy. This is a simple and low-cost light source for metrology, environmental sensing and biomedical diagnostics.Q-switching and mode-locking are the two main techniques enabling pulsed lasers [1]. In mode-locking, the random phase relation originating from the interference of cavity modes is fixed, resulting in a single pulse [1], with typical duration ranging from tens ps to sub-10 fs [2], and a repetition rate corresponding to the inverse of the cavity round-trip time [2]. In mode-locking, many aspects, including the dispersive and nonlinear proprieties of the intracavity components, need to be precisely balanced in order to achieve stable operation [1,2]. Q-switching is a modulation of the quality factor, Q, of a laser cavity[1], Q being the ratio between the energy stored in the active medium and that lost per oscillation cycle[1] (thus, the lower the losses, the higher Q). In Q-switching, the active medium is pumped while lasing is initially prevented by a low Q factor[1]. The stored energy is then released in a pulse with duration ranging from µs to ns when lasing is allowed by a high Q factor[1]. The time needed to replenish the extracted energy between two consecutive pulses is related to the lifetime of the gain medium, which is typically∼ms for erbium-doped fibres [1]. Thus the repetition rate of Q-switched lasers is usually low (∼kHz[1]), much smaller than mode-locked lasers [1,2]. On the other hand, Q-switching enables much higher pulse energies and durations than mode-locking[1]. Q-switching has advantages in terms of cost, efficient operation (i.e. input power/output pulse energy) and easy implementation, compared to mode-locking, which needs a careful design of the cavity parameters to achieve a balance of dispersion and nonlinearity [1,2]. Q-switched lasers are ideal for applications where ultrafast pulses (<1ns) are not necessary, or long pulses are advantageous [3,4], such as material processing, environmental sensing, range finding, medicine and long-pulse nonlinear experiments [3][4][5].

Ultrafast fiber lasers with short pulses and broad bandwidth are in great demand for a variety of applications, such as spectroscopy, biomedical diagnosis and optical communications. In particular sub-200fs pulses are required for ultrafast spectroscopy with high temporal resolution. Graphene is an ideal ultra-wide-band saturable absorber. We report the generation of 174fs pulses from a graphene-based fiber laser.

We report an ultrafast laser mode-locked with a graphene saturable absorber. The linear dispersions of the Dirac electrons in graphene enable wideband tunability. We get ~1 ps pulses, tunable between 1525 and 1559 nm, with stable mode-locking, insensitive to environmental perturbations.

We exfoliate graphite in both aqueous and non-aqueous environments through mild sonication followed by centrifugation. The dispersions are enriched with monolayers. We mix them with polymers, followed by slow evaporation to produce optical quality composites. Nonlinear optical measurements show $5% saturable absorption. The composites are then integrated into fiber laser cavities to generate 630 fs pulses at 1.56 mm. This shows the viability of solution phase processing for graphene based photonic devices. 1 Introduction Ultrafast lasers have many applications, ranging from basic research and metrology to telecommunications, medicine, and materials processing. Most employ a mode-locking technique, whereby a nonlinear optical element -called saturable absorber -turns the laser continuous wave into a train of ultrashort pulses. Semiconductor saturable absorber mirrors (SESAMs) currently dominate passive mode-locking [1]. However, these have a narrow tuning range (tens of nanometer), and require complex fabrication and packaging [1]. A simpler and costeffective alternative relies on single wall carbon nanotubes (SWNTs) [2][3][4][5][6][7], where the operating wavelength is defined by the SWNT diameter (i.e., bandgap) [2,4]. Tunability is possible by combining SWNTs with a diameter distribution [5]. However, for a chosen wavelength, the SWNTs not in resonance are not used, and contribute insertion losses, compromising device-performance. Novel nonlinear materials with broadband absorption are therefore required for wideband, tunable operation.The linear dispersion of Dirac electrons in graphene offers the ideal solution: for any excitation there is an electron-hole pair in resonance. Due to the ultrafast carrier dynamics [8][9][10] and large absorption of incident light per layer ($2.3% [11,12]), graphene behaves as a fast saturable absorber over a wide spectral range [2,[13][14][15]. Unlike SESAMs and SWNTs, graphene saturable absorbers do not need band-gap engineering or chirality/diameter control.

We demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94 μm, using a graphene-polymer based saturable absorber. The laser outputs 3.6 ps pulses, with ~0.4 nJ energy and an amplitude fluctuation ~0.5%, at 6.46 MHz. This is a simple, low-cost, stable and convenient laser oscillator for applications where eye-safe and low-photon-energy light sources are required, such as sensing and biomedical diagnostics.

Optical gas sensors play an increasingly important role in many applications. Sensing techniques based on mid-infrared absorption spectroscopy offer excellent stability, selectivity and sensitivity, for numerous possibilities expected for sensors integrated into mobile and wearable devices. Here we review recent progress towards the miniaturization and integration of optical gas sensors, with a focus on low-cost and low-power consumption devices.

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