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A pulsed regime of short-cavity, heavily erbium-doped fiber lasers is of high interest for its possible applications in telecommunications and sensorics. Here, we demonstrate these lasers in two configurations, distributed feedback laser and compare it with a classic Fabry-Perot type laser. We have managed to create lasers that function stably with cavities as small as 50 mm. Pulse properties such as amplitude, frequency and duration, are in a good agreement with our theoretical analysis, which takes into account spontaneous emission. We report the observation of the thermal switching effect, which consists of the pulsing regime changing to CW upon cooling the laser cavities down to the liquid nitrogen temperature. We theoretically show that this effect may be explained by weakening of the up-conversion process responsible for the pulsed regime. The slowing of the up-conversion processes is due to the energy mismatch in this process, which is overcome by interaction with phonons. At low temperatures, the number of phonons decreases and pulsing switches off.Fiber lasers based on erbium-doped silica glass are actively used in modern fiber optics. [1][2][3][4][5] Erbium lasers operating at wavelengths near 1.5 μm are widely used in telecommunications, 6-10 optical sensing, 11-13 and radiophotonics. 14 ASSOCIATED CONTENT

In this paper, we demonstrated a novel, all-fiber highly sensitive bend sensor based on a four-core fiber rod with a diameter of 2.1 mm. We observed a high resolution of the sensor at a level of 3.6 × 10−3 m−1. Such a sensor design can be used in harsh environments due to the relatively small size and all-fiber configuration, containing no adhesive, nor welded joints.

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Photoluminescence spectra and decay kinetics of bismuth inclusions in silica optical fibres containing fluorine additive in the core glass are studied in the vicinity of a wavelength of 1420 nm at temperatures of 80-900 K under a continuous wave (CW) and a pulsed diode laser pump at a wavelength of 808 nm. At high fluorine concentration and low temperatures, luminescence decay kinetics becomes essentially bi-exponential, typical lifetimes being 720 and 1200 µs. Hydrogen and deuterium loading at pressures of up to 125 bar leads to a decrease of the steady-state luminescence intensity and lifetime. We attribute this to the appearance of an energy transfer bridge from bismuth clusters to vibrational degrees of freedom of diatomic molecules. It is found that in the presence of H(2) or D(2) molecules experiencing random walking in silica, luminescence decay kinetics stop following a single exponential function even in fluorine-free silica-core fibre, deviation from the single exponent being greater at higher temperatures. The induced quenching rate increases with the increase of temperature as well and is greater for H(2) molecules. All conditions being equal, the equilibrium concentration of hydrogen molecules is greater in heavily fluorinated silica. At temperatures below ~250 K, the presence of dissolved molecules has no effect, which speaks for the primary importance of having rotational degrees of freedom of migrating interstitial diatomic molecules in an excited state for effective quenching of bismuth electronic excitations. It is found that the influence of dissolved deuterium is weaker than that of hydrogen. We attribute this feature to a greater angular momentum of the D(2) molecule and correspondingly smaller energy of the molecule's rotational quantum. The results of the experiments show that bismuth clusters mainly located in voids of the silica network, rather than bismuth point defects, are responsible for near-infrared luminescence.

Photoluminescence spectra and kinetics of the bismuth doped silicon dioxide are studied in the wavelength interval 0.7-1.6 μm in the absence and in the presence of aluminum co doping. It is demon strated that a broad spectral band that is centered at 1420 nm and exhibits a decay time of 600 μs dominates in the long wavelength part of the spectral interval regardless of the presence of aluminum in the directly deposited unfused bismuth doped silicon dioxide. The melting of the plasma deposited aluminosilicate material leads to the shift of the center of the band to a wavelength of 1100 nm, whereas the melting of the aluminum free material weakly affects the spectrum. It is demonstrated that an increase in the temperature to 700 K causes an increase in the intensity of the band centered at a wavelength of 1420 nm, which is most clearly seen under the excitation at a wavelength of 808 nm. An increase in the intensity is accompanied by an increase in the lifetime and a sharp decrease in the intensity of the band peaked at a wavelength of 825 nm. The spectral features and kinetics of photoluminescence are interpreted in the framework of the three level scheme of electronic transitions for two types of bismuth defects that simultaneously exist in the aluminosil icate materials and exhibit close energies of the stationary electronic states.

Al, P, Ge additives to silica separately and/or in combination are used to shape refractive index profiles of Bi-doped-silica-core fibres. Loss spectra and luminescent properties of the fibres, critical for light amplification at 1.1-1.5 microns wavelengths are discussed.