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Hybrid bulk/fibre MOPA system based on Yb:KYW laser
Solid-state lasers with active media based on single crystals of potassium-yttrium (KY(WO4)2 – KYW) or potassium-gadolinium (KGd(WO4)2 – KGW) tungstate doped with ytterbium ions possess a number of features [1–3] that have been spurring active research in the domain of these lasers. One major attractive property of Yb:KYW/KGW lasers is their ability to be directly and efficiently pumped with standard diode lasers emitting in the range of 975–980 nm. Spectral domain of generation of Yb:KYW/KGW lasers lies around 1010–1080 nm  with a maximum of radiation output power in the vicinity of 1045-nm wavelength. Presence in active Yb:KYW/KGW media of gain bands with typical width of several dozens of nanometres allows implementation in such lasers of femtosecond pulse generation with less than 100 fs pulse duration . When longer pulses are generated their central wavelength can be, correspondingly, tuned in a range of up to ~ 70 nm. Another important feature of Yb:KYW/KGW lasers is the fact that their output wavelengths fall within the gain band of fibre-based ytterbium amplifiers. The highest gain of fibre Yb amplifiers is reached around 1070 nm, however the short-wavelength wing of gain band in these amplifiers covers the spectral range of Yb:KYW/KGW laser generation, thereby allowing the use of fibre Yb amplifiers in combination with solid-state Yb:KYW/KGW master oscillators. Such hybrid bulk/fibre systems for generation of powerful sub-picosecond pulses within the spectral range of Yb:KYW/KGW generation have already been used before and demonstrated sufficiently high efficiency with the added benefit of relatively simple implementation. The average output power of a hybrid system based on Yb:KYW/KGW lasers may reach more than 100 W  while delivering sub-picosecond pulse durations and pulse repetition rates on the level of 100 MHz. Usually, in fibre-based optical amplifiers of such systems a double-clad Yb-doped fibre is used and the pump radiation is fed into the amplifier through a dichroic mirror placed, as a rule, in the path of the beam exiting the amplifier (counter-propagating pump). Guiding the pump radiation into an optical fibre amplifier through discrete optical elements (lens, mirror) obviously complicates the system and does not allow full realisation of possible advantages inherent in optical fibre amplification systems. Moreover, free-space optics creates discontinuities in multi-stage fibre systems, thereby introducing open intervals with bulk optics. The presence of such open intervals considerably weakens “fibre advantages” of such systems.
GTWave technology of fibre optical amplifiers developed recently  makes it possible to guide the pump radiation into the active fibre directly through a standard quarts fibre. Using this technology allows elimination of any discrete optical elements from the fibre amplification train. In the present work, reported for the first time are results of investigation into hybrid bulk-fibre sub-picosecond systems based on a Yb:KYW laser and an ytterbium amplifier implemented with GTWave technology.
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