the biaxial anisotropy of Nd:YLF crystal, the parameters, such as the stimulated-emission cross section, thermo-optical coefficient and thermo-expansion coefficient, along the a and c axes are different, which leads to a quite different performance of the thermal lens effect for a and c directions. The former studies [5-9] on the thermal effect of Nd:YLF crystal revealed that there is an obvious difference between the thermal lens focal length along a and c axes for both π and σ polarizations. According to Ref.[5], the thermal lens focal lengths of π and σ polarizations along a direction are negative and positive, respectively, for example −100 mm and 300 mm at 12 W end pump power under non-lasing condition. However, along the c direction, the thermal lens focal lengths of the two polarizations are both too weak to be measured. It is interesting that there is a so great difference on the thermal lens focal length between the a and c axes. It is explained by the elliptical pump beam waist and the surmise of that the thermal conductivity for the c axis is twice of the a axis in Ref. [5]. And rarely experimental and theoretical discussions about this phenomenon were made in other papers as far as we know.In our experiment, the same phenomenon as that in the referred papers above is observed, however, some differences are also discovered. The weak thermal lens effect for π polarization along c axis is measured, and the contradiction between the experimental results and the theoretical analysis is discussed. It is found that the complementation among the thermo-optical coefficient, the thermal end bulging and the photoelastic effects contributes to the weak and asymmetry thermal lens effect of Nd:YLF crystal. Experimental measurementsThe experimental setup is shown in Fig. 1. The Nd:YLF crystal is end pumped by a diode laser (LD) with central Abstract The thermal lens effect of Nd:YLF crystal for different polarized beams is experimentally and theoretically studied in this paper. In the experiment, the different thermal lens effects of Nd:YLF crystal along a and c axes for π-and σ-polarized probe beams are observed, and the values of the focal lengths are measured. The theoretical analysis is made to explain the extremely weak thermal lens effect along the c axis, as well as that for the σ-polarized beam. And it is corroborated that the complementation among the thermo-optical coefficient, the thermal end bulging and the photoelastic effects contributes to the weak and asymmetry thermal lens effect of Nd:YLF crystal.
A theoretical method was proposed to compensate the burst envelope distortion in a solid-state master-oscillator power-amplifier (MOPA) system operating in burst mode at an intra-burst repetition rate of 40 MHz. Arbitrary envelope shapes were achieved at inter-burst repetition rate of 100 kHz with 40 pulses in the burst, showing excellent agreement with the calculated ones. This is the first demonstration of arbitrary burst envelope without an adaptive feedback loop in a solid-state laser system. The maximum pulse energy of 100 μJ was achieved at inter-burst repetition rate of 40 kHz, with 10 pulses in the burst.
In the past few years, rapid progress has been made in scaling of the amplified spontaneous emission (ASE) output. For a high-power SFS in the 1-μm spectral region [5][6][7][8][9], Schmidt et al. [9] reported a narrow-band SFS with maximum output power of 697 W at a center wavelength of 1,030 nm using a two-stage master oscillator power amplifier (MOPA) configuration. In comparison with SFSs in the 1-μm spectral region, few studies have been reported to date on high-power broadband thulium (Tm)-doped SFSs in the 2-µm spectral region [10][11][12]. In 2008, Shen et al.[13] demonstrated a broadband Tm-doped fiber SFS with a single-ended output power of 11 W using a free-space pump configuration. The slope efficiency was 38 %, and the full width at half maximum (FWHM) bandwidth was 36 nm. Recently, Liu et al. [14] reported a 122-W broadband superfluorescence output using a MOPA configuration, with a wavelength range that spanned from 1,935 to 2,075 nm and a FWHM bandwidth of 25 nm. Obviously, a MOPA scheme can produce a higher ASE output than a single-stage SFS generation scheme. However, the main drawback of the MOPA scheme is that it is rather complex. It requires two kinds of fibers (and their associated pump sources) and one or more Faraday isolators to provide the required degree of attenuation of feedback to both the seed and the amplifier. In contrast, a single-stage SFS generation scheme needs fewer elements and requires less space, so it is both simpler and more compact. In addition, it is not easy to realize an all-fiber SFS for a MOPA scheme because of the destructive effect of the strong backward ASE on the pump source and the other optical components.In this paper, we report a simple approach for scaling of the output power from a Tm-doped SFS using a singlestage configuration in combination with a double anglecleaved facet geometry to reduce the feedback from the fiber-end facets and provide effective suppression of the Abstract In this paper, we report a high-power thulium (Tm)-doped superfluorescent fiber source (SFS) in the 2-μm spectral region. The SFS is based on double anglecleaved facet operation and uses a simple single-stage geometry. The copropagating amplified spontaneous emission (ASE) yields a maximum output of 20.7 W at a center wavelength of 1,960.7 nm, with a full width at half maximum (FWHM) of ~45 nm. The counterpropagating ASE yields a maximum output of 25.2 W at a center wavelength of 1,948.2 nm, with a FWHM of ~50 nm. The maximum combined output of the SFS is as much as 45.9 W, which corresponds to a slope efficiency of 38.9 %. In addition, a model of the ~2 μm SFS in Tm-doped silica fibers pumped at ~790 nm is developed, and the influence of fiber length and end-facet reflectivity on the ASE output performance and the parasitic lasing threshold are studied numerically.
A high repetition rate optical parametric oscillator (OPO) generating an idler laser with a wavelength as long as 4.0 μm at 200 and 400 kHz was demonstrated in this paper. The OPO was pumped by a master oscillator power amplifier structure fiber laser with excellent characteristics. The pump pulse from the fiber laser had a steep leading edge, which was theoretically proved to improve the OPO's performance, compared with the Gaussian pump pulse. A homemade periodically poled magnesium-oxide-doped lithium niobate crystal with a grating period of 29 μm was employed in our experiment. By optimizing the resonator, 2.75 and 1.67 W idler lasers were finally achieved at repetition rates as high as 200 and 400 kHz, respectively, with a wavelength as long as 4.0 μm. The conversion efficiencies were 12.03% and 7.31%, respectively.
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