Innovative technique for generation of femtosecond high energy pulses in fiber lasers

In this thesis is presented the realization of a Mamyshev oscillator. It is one of the most promising solutions for environmentally stable fiber laser oscillators for ultrashort pulse generation, and this architecture represents an alternative to SESAM to achieve passive modelocking by means of a saturable absorber (SA) effect in fiber lasers. The Mamyshev oscillator is an oscillator composed of two concatenated Mamyshev regenerators usually seeded by an external laser source. Its operating principle is based on self-phase modulation (SPM), which allows spectral broadening of the signal in the fiber, and on two non-overlapping spectral filters, which prevent CW lasing and thus force pulse formation. The Mamyshev oscillator proposed in this work is based on single-mode, polarization-maintaining (PM) fibers and on a low-power (∼ 400 mW) single-mode pump laser diode. To start the Mamyshev oscillator, we injected into the cavity a single pulse emitted by a sub-nanosecond passively Q-switched laser spectrally broadened by the nonlinear four-wave mixing (FWM) effect while propagating in a standard single-mode PM passive fiber. For the spectral filtering, we investigated different solutions based on super-gaussian filters and Gaussian filters. After a careful experimental investigation, we chose a combination of the two filters, obtaining 3.5 nJ pulse energy and a minimum compressed pulse duration of ∼ 170 fs. In order to increase the pulse energy, I also realized a chirped pulse fiber amplifier reaching ∼ 100 nJ of pulse energy with a pulse duration after compression of 200 fs. This was pivotal for the optical parametric generation experiments I performed using the Mamyshev oscillator pulses as pump source for two different parametric crystals: a 42-mm-long MgO:PPLN and a 19-mm-long PPLN. Such long crystals are not prone to group velocity mismatch effects when pumped at 1064 nm for generation of a signal around 1.55 µm, because the dispersion properties of the nonlinear materials are such that the pump and idler present the same group velocity for the wavelengths for which the phase-matching condition occurs. At the output we obtain similar results for both crystals: ∼ 40 nJ of signal and ∼ 20 nJ of idler with a conversion efficiency of 50% and 20% respectively, with signal pulse duration of about ∼ 300 fs close to the Fourier limit, and a wide tuning range. Both the crystals showed very good average output power stability in Gaussian TEM00 beam profiles with beam quality factor M2 ≤ 1.5. The very good nonlinear frequency conversion efficiency, beam quality and reasonable pulse duration preservation are a clear evidence of the effectiveness of the femtosecond fiber oscillator and amplifier based on the Mamyshev architecture I developed.