Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm<sup>2</sup> near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M<sup>2</sup> ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.
In this study, we propose a widely tunable in the 1.6-2.65 μm range femtosecond fiber laser source, generating high-quality sech-shaped pulses with the duration of order 100 fs. Experimental setup contains hybrid all-fiber Er/Tm pump laser generating 150 fs pulses of 2 nJ in Erbium (1.56 μm) channel and 125 fs pulses of 4 nJ in Thulium (2 μm) channel respectively. This laser source was coupled to a 50 cm piece of suspended-core microstructured TeO<sub>2</sub>-WO<sub>3</sub>- La<sub>2</sub>O<sub>3</sub> glass fiber with launching efficiency of about 10%. We have observed Raman self-frequency shifting solitons in this fiber with maximum red shift of 2.25 μm for Erbium channel and 2.65 μm for Thulium channel. By varying energy of pump pulses, solitons can be tuned in broadband spectral region. We have made theoretical studies of nonlinear pulse dynamics in the tellurite fiber with carefully measured and calculated parameters. Numerical simulation is in a very good agreement with the experiment
We proposed and investigated a novel tapered ytterbium-doped fiber design. The tapered fiber has length of 2.1 m, single-mode thin end and thick end with core/clad diameter of about 50/430 µm. Master-oscillator power-amplifier monolithic fiber scheme have been developed using this fiber and 5 ps duration, 28 nm spectral width and 0.5 MW peak power pulses was achieved at the output. FROG measurements reveal linear chirp that allow one to expect decompression of these pulses down to sub-100 fs duration.
The use of double-clad fibers for short pulses amplification requires high active ions concentration in order to keep the active fiber length short. In the case of Er-doped fibers an increase of concentration leads to a significant drop of efficiency due to Er ions clustering. We have demonstrated through numerical simulation that efficiency of amplifiers based on double-clad P<sub>2</sub>O<sub>5</sub>-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> (PAS) Er-doped fibers decreases slower with Er-concentration growth if compared with standard Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> fibers. In this paper, we present single-mode large-mode-area heavily Er-doped double-clad fiber based on PAS glass matrix for short pulses amplification. The developed PAS fiber has a 36 μm singlemode core and a small signal cladding absorption of 3 dB/m at 980 nm leading to an optimal fiber length in range of 5-8 m depending on the central wavelength. At first, an all-fiber nanosecond MOPA at 1560 nm was built using our PAS fiber as the final amplifier. We obtained 28 W of average output power (efficiency of 25 % with respect to the launched pump power at 976) limited by amplified spontaneous emission. Pulse energy of 1.5 mJ was achieved at pump power level of ~120 W. We believe that it is the first demonstration of mJ-energy level single-mode nanosecond fiber system. Then, direct amplification of 100-fs source was performed using this fiber. We obtained 12 nJ pulse energy and 100 kW of peak power from the fiber which is close to the record value for Er-doped fiber amplifiers.
The possibility to build up an optical source of femtosecond pulses that are smoothly tuned in the telecommunication range using a dispersion-decreasing fiber is demonstrated. The smooth tuning is based on the Raman frequency conversion of ultrashort pulses, which can be effectively tuned due to the compression mechanism for maintaining of a relatively high pulse intensity in the medium with a monotonically decreasing anomalous dispersion. The generation of a 90-fs soliton pulse whose wavelength is smoothly tuned in the wavelength range 1.55 - 1.85 μm is experimentally demonstrated.
We put forward a scheme of the efficient terahertz and mid/far-infrared pulsed generation via an intracavity difference-frequency mixing of lasing fields due to the resonant intersubband and non-resonant bulk nonlinearities in the mode-locked dual-wavelength heterolasers.