Graphite Saturable Absorber for Q-Switched Fiber Laser

This paper reported a successful demonstration on Q-switched fiber laser by using graphite as saturable absorber (SA). The graphite is deposited on the fiber ferrule through a simple mechanical exfoliation method. The modulation depth of the graphite SA is 19.2% with a saturation intensity of 85 MW/cm². The maximum achievable pulse repetition rates and pulse width are 42.41 kHz and 3.40 μs respectively. Meanwhile, its optical signal-to-noise ratio is about 50.81 dB. The Q-switched pulses have the maximum pulse energy of 5.84 nJ. These outcomes demonstrated that a stable output of passively Q-switched fiber laser is produced and can be applied for various optical fiber applications.

Direct-Gap Photoluminescence from a Si-Ge Multilayer Super Unit Cell Grown on Si0.4Ge0.6

Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. One way to overcome this limitation is through band gap engineering. In this regard, M. d’Avezac et al. [Phys. Rev. Lett., 108, 027401 (2012)] predicted that a strained SiGe2Si2Ge2SiGen super unit cell on Si0.4Ge0.6 would have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber applications. Here we report on the epitaxial growth of such a structure and its optical properties, for which purpose two similar samples were prepared by molecular beam epitaxy and solid phase epitaxy. Photoluminescence (PL) spectra were obtained at low temperatures (6–25 K) with excitation at wavelengths of 405 and 458 nm, selected to emphasize the light emission from the sample superstructure. A strong low-energy PL quadruplet is seen, with peaks near 727, 758, 792 and 822 meV at 6 K, together with a much weaker peak at 871 MeV. The ratio of intensities of the strong and weak peaks is the same in both samples. The weak peak at 871 meV is assigned to the dipole-allowed direct-gap transition associated with the super unit cell. The four strong peaks are attributed to dislocation related emission lines of the thick relaxed Si0.4Ge0.6 transition layer on Si.