Yb-doped femtosecond lasers and their frequency doubling

Laser pulses, Ultrashort;Femtosecond lasers;Ytterbium

Sarmani, Abdul Rahman ; Sibbett, Wilson ; Sibbett, Wilson

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Laser pulses, Ultrashort;    Femtosecond lasers;    Ytterbium;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/781/Abdul%20Rahman%20Sarmani%20PhD%20thesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Ultralow threshold, compact and highly efficient femtosecond lasers based on Yb³⁺-dopedpotassium yttrium tungstate (Yb:KYW) and Yb³⁺-doped vanadium yttrium oxide(Yb:YVO 4 ) have been demonstrated within this PhD-research project. For a continuouswave unmode-locked Yb:KYW laser a threshold as low as 101 mW was obtained with aslope efficiency of 74%. By employing a single prism for dispersion control, the laser wastunable between 1012 nm to 1069 nm. When operated in the mode-locked regime, thislaser produced transform-limited pulses having durations of 210 fs at a central wavelengthof 1044 nm. Stable mode locking was observed for an optimised incident pulse fluence onthe SESAM between 140 μJ/cm²to 160 μJ/cm²which was 2-3 times higher than thedesigned energy pulse fluence of the SESAM (70 μJ/cm²).The employment of several combinations of chirped mirror designs for control ofintracavity group velocity dispersion led to excellent results. The threshold for modelocking was satisfied for a pump power of 255 mW where the slope efficiency wasmeasured to be 62%. This is the most efficient SESAM-assisted femtosecond laser yetreported and the highest optical-to-optical efficiency of 37% is exceptional. Transform-limited pulses with durations as short as 90 fs were produced in a spectral region centredon 1052 nm. The success of this research thus represents a good foundation on which todesign and build more compact configurations that will incorporate just one chirped mirrorfor dispersion compensation.A relatively high nonlinear refractive index, n₂ , of 15 x 10⁻¹⁶ cm²/W was measuredin Yb:YVO₄ and this affords particular potential for this candidate material in Kerr-lensmode locking. In fact, for operation in the femtosecond domain, the threshold power was190 mW with a slope efficiency of 26% and near-transform-limited pulses as short as 61fs were generated at a centre wavelength of 1050 nm. The main objectives in developingthis type of laser relate to a demonstration of high peak power operation in thin disc laserconfigurations.The deployment of a diode-pumped Yb:KYW femtosecond laser as a pump sourcefor frequency doubling in a periodically-poled LiTaO₃ crystal was realised. The maximumrealized output power of 150 mW corresponded to an impressive second harmonicconversion efficiency of 43%. 225-fs duration green pulses (centred at 525 nm) weregenerated under the condition of strong focusing in the nonlinear crystal.