We demonstrate passive mode-locking of a Raman fiber laser using a nanotube-based saturable absorber coupled to a net normal dispersion cavity. This generates highly chirped 500 ps pulses. These are then compressed down to 2 ps, with 1:4 kW peak power, making it a simple wavelength-versatile source for various applications. In telecommunications, Raman-based amplification allows operation beyond the spectral limits of rare-earth devices [3]. Consequently, similar techniques can be applied to ultrafast fiber lasers. The most attractive feature of Raman-based amplification in silica fiber is that gain is available at any wavelength across the transparency window of the medium (300-2300 nm), given a suitable pump source [3]. With advances in high-power fiber laser pump technology and in cascaded Raman fiber lasers, efficient pump systems are now available throughout this entire band.There have been a number of reports utilizing Raman gain in ultrafast sources [4][5][6][7][8]. However, to date, none of these systems has reached a level of performance comparable with state-of-the-art rare-earth-based lasers. In Ref. [4], dissipative four-wave mixing was used for mode-locking, generating a pulsed laser with a very high repetition rate. While this is useful for some applications, the high repetition rate limits the delivered peak power. Nonlinear loop mirrors [5,6] and nonlinear polarization evolution [8] have also been used to provide saturable absorption, but such systems suffer from instabilities due to fluctuations in ambient temperature, and often exhibit poor self-starting performance [5,6,8].Recently, a Raman mode-locked laser using a semiconductor saturable absorber mirror (SESAM) was reported [7]. While the use of a SESAM improves self-starting and robustness against environmental perturbations, there is limited spectral operation from a single device. In addition, the fabrication cost of SESAMs at nonstandard wavelengths is high. Availability of a broadband saturable absorber (SA) to achieve mode-locking at any desired wavelength across the transmission window of silica is an essential prerequisite to fully exploit the flexibility of Raman amplification in ultrafast sources across the visible and near-IR. Recent interest in the application of nano materials, in particular carbon nanotubes (CNTs) [9][10][11][12] and graphene [10,[13][14][15], as SAs in mode-locked lasers have moved the field a step closer to a fully universal device [9][10][11][12][13][14][15]. For example, a single CNT-based SA was used to mode-lock fiber lasers, offering wide wavelength-tunable pulses [11] and broad spectral coverage from 1 to 2 μm [9].Here, we report a passively mode-locked laser combining both Raman gain and a CNT-based SA in a normally dispersive cavity, showing the potential of this flexible approach. Mode-locked lasers based on rare-earth media and operating with a net normal dispersion map, generating so-called dissipative solitons [12,16,17], were suggested as a means of overcoming the limits on pulse energy imposed b...

We demonstrate passive mode-locking of a Raman fiber laser at 1.12 µm, using a nanotube-based saturable absorber. A regular train of pulses, with a duration of 236 ps at the fundamental repetition frequency of the cavity, are generated by the all-normal dispersion oscillator. Importantly, this simple system is pumped with a continuous wave Yb fiber laser, removing the need for complex synchronous pumping schemes, where pulse-shaping depends on the action of the saturable absorber and a balance of dissipative processes. These results illustrate the flexibility of combining Raman amplification with a nanotube-based absorber for wavelength versatile pulsed sources.

No abstract

This paper proposes a new design methodology for discrete multi-pumped Raman amplifier. In a multi-objective optimization scenario, in a first step the whole solution-space is inspected by a CW analytical formulation. Then, the most promising solutions are fully investigated by a rigorous numerical treatment and the Raman amplification performance is thus determined by the combination of analytical and numerical approaches. As an application of our methodology we designed an photonic crystal fiber Raman amplifier configuration which provides low ripple, high gain, clear eye opening and a low power penalty. The amplifier configuration also enables to fully compensate the dispersion introduced by a 70-km singlemode fiber in a 10 Gbit/s system. We have successfully obtained a configuration with 8.5 dB average gain over the C-band and 0.71 dB ripple with almost zero eye-penalty using only two pump lasers with relatively low pump power.

No abstract