In this paper mode instability in a 500 W ytterbium-doped fiber laser is experimentally examined by changing the pumping wavelength, spectral bandwidth of signal light, active fiber temperature and coiling radius. The magnitude of power transfer from the fundamental mode to the higher order mode due to mode instability is measured as a criterion for its incident. The experiments show that the coiling radius of the first few tens of centimeters of the active fiber plays a significant role in controlling mode instability, and shifting the pumping wavelength from 976 to 973 nm can mitigate mode instability.

Modal instability (MI) and stimulated Raman scattering (SRS) are the main obstacles in the power scaling of fiber lasers and amplifiers. In the power scaling of a high-power ytterbium (Yb)-doped master oscillator power amplifier system, a new type of MI has occurred. Experimentally, it is shown that just at the onset of the SRS effect, MI takes place, and the degradation of the beam quality is observed. By the spectra and beam quality measurements, it is revealed that this type of MI can be mitigated firmly by suppressing the SRS effect in high-power Yb-doped fiber amplifiers.

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To achieve a 3.02 kW Yb-doped fiber laser oscillator, the behavior of transverse mode instability (TMI) is experimentally studied in different pumping configurations; co, hybrid, counter, and bidirectional. A comparative analysis showed that population inversion saturation has a substantial impact on TMI threshold enhancement in high power fiber oscillators. Monitoring the dynamic power exchange of fundamental mode and higher-order mode of laser output beam indicates that in a hybrid pumping scheme, simultaneous pumping with two different wavelengths enhances the TMI threshold to a great stand. Moreover, injecting a few watts of pumping light in the counter direction mitigates the TMI caused by pumping in the co-direction. Calculation of population inversion in different pumping configurations using simulation shows that higher population inversion saturation leads to increasing the TMI threshold.

In this paper we present a novel method to reliably strip the unwanted cladding light in high-power fiber lasers. Soft metals are utilized to fabricate a high-power cladding light stripper (CLS). The capability of indium (In), aluminum (Al), tin (Sn), and gold (Au) in extracting unwanted cladding light is examined. The experiments show that these metals have the right features for stripping the unwanted light out of the cladding. We also find that the metal-cladding contact area is of great importance because it determines the attenuation and the thermal load on the CLS. These metals are examined in different forms to optimize the contact area to have the highest possible attenuation and avoid localized heating. The results show that sheets of indium are very effective in stripping unwanted cladding light.

To enhance the transverse mode instability (TMI) threshold of a fiber oscillator, a novel configuration is presented. In this configuration the oscillator cavity length is considerably reduced and the remaining active fiber is released out of the cavity to absorb the rest of the pump power and amplify the output signal of the cavity. In fiber oscillators, the index gratings generated by both forward and backward propagating signals can interact with light propagating in the backward and forward directions and degrade the output beam quality. In the proposed modified configuration, due to lower intra-cavity forward and backward signal power the index grating is smoothed and the TMI threshold is increased. Experimental results indicate that this modified configuration has a higher TMI threshold than a conventional fiber oscillator. Finally a higher TMI threshold is achieved in the bidirectional pumping scheme of the modified configuration.

Transverse mode instability (TMI) is experimentally investigated in a fiber oscillator and a fiber amplifier. For a reasonable comparison of TMI in these two configurations, the same optical components and design parameters are applied to both. Our experimental results show that the TMI power threshold in a fiber oscillator is lower than in a corresponding fiber amplifier. By using simulation software, a fiber oscillator and an amplifier are designed with similar characteristics, to provide identical conditions for all effective parameters on TMI in both of them. Since the signal propagation in fiber oscillators is different from that of single-pass fiber amplifiers, and also since both forward and backward propagating signals in fiber oscillators can generate thermo-optic index gratings, the observed lower TMI threshold in the fiber oscillator is due to its different interaction of light with index gratings.

The electron transport properties of ultra-scaled phase change material Ge2Sb2Te5 (GST) are investigated in a subthreshold bias range. We used ab-initio molecular dynamics (AIMD) and non-equilibrium Green’s function (NEGF) transport formalism based on density functional theory (DFT). We calculate the conductance and current-voltage (I-V) curve of both crystalline (c-GST) and amorphous GST (a-GST). Our purely ab-initio simulations show that the conduction mechanism of ultra-scaled a-GST is different from that of c-GST. The current-voltage (I-V) curve of a-GST shows linear and exponential behavior. Both the bias induced variation of the transmission coefficients and the enlarging of bias window is responsible for the exponential shape of the I-V curve for a-GST. Whereas the linear part of the I-V curve is a consequence of the bias window enlarging. Moreover, it is revealed that the electron transport properties of ultra-scaled c-GST are dominated by metal-induced gap states (MIGS). The measured ON/OFF ratio and I-V curves are in good agreement with the similar experimental results. The findings of this paper would be useful in designing the ultra-scaled PCM devices based on GST and the designers should consider the difference in conduction mechanism of a-GST and c-GST as a potential reason for the different behavior of their I-V and conductance curve.