Actively Mode Locked Raman Fiber Laser with Multimode LD Pumping

We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 > 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime

Fiber Lasers Based on Dynamic Population Gratings in Rare-Earth-Doped Optical Fibers

Long dynamic population gratings (DPGs) formed in rare-earth-doped fibers have unique spectral characteristics compared to other types of fiber gratings, making them suitable for controlling the spectral composition of lasers. Depending on the type, length, and position of the DPGs in the cavities of lasers, they can be used for various purposes, ranging from the stabilization of single-frequency radiation to regular wavelength self-sweeping (WLSS) operation. Lasers based on DPGs are sources of narrow-band radiation with a fixed or sweeping generation spectrum. One of the main advantages of such lasers is the simplicity of their design, since they do not require special spectral elements or drivers for spectrum control. In this paper, we review the research progress on fiber lasers based on DPGs. The basic working principles of different types of DPGs will be introduced in the theoretical section. The operation of lasers based on absorption and gain DPGs and their practical applications will be discussed and summarized in experimental section. Finally, the main challenges for the development of such lasers will be presented

Mechanism of brightness enhancement in multimode LD-pumped graded-index fiber Raman lasers

A theory describing experimental beam profiles of multimode fiber Raman lasers is developed: random mode coupling, Kerr self-cleaning and spatial filtering play a decisive role in shaping Stokes beams, whereas depleted pump beams remain insensitive

Fiber lasers with regular and random distributed feedback

We review our recent results on fiber lasers with distributed feedback based on π-shifted or random fiber Bragg gratings, and random index structures inscribed by femtosecond pulses in singlemode or multicore/multimode fibers

Spatio-spectral beam control in multimode diode-pumped Raman fibre lasers via intracavity filtering and Kerr cleaning

Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them