E.M. Sholokhov

Luminescence and selective heat radiation of Yb2O3 upon the resonant and thermal laser excitation

Yb2O3 polycrystals with a size of up to 10 mm are synthesized using the sintering and melting of the ultrapure Yb2O3 powders by the CO2-laser radiation with the power PL ≤ 100 W at the wavelength λ = 10.6 μm at the melting point Tm = 2703 K, forming due to surface tension in melt, and crystallization in air. The analysis of the polycrystal microstructure using the methods of optical and electron microscopy and X- ray diffractometry shows that perfect oxide crystallites are formed in the course of crystallization after melting-through. The transformation of the luminescence and selective heat radiation (SHR) spectra of the Yb2O3 polycrystals is studied under the resonant excitation at λ ≈ 975 nm using a laser diode and the laser heating at the wavelength λ = 10.6 μm. When the resonant excitation power of the Yb3+ ions increases from 0.15 to 4.5 W, the Stokes luminescence of the Yb2O3 polycrystals is sequentially transformed into SHR and the thermal radiation of the crystal lattice. The transformation of the emission spectra of the Yb2O3 polycrystals with an increase in the laser heating intensity by about four orders of magnitude can be represented as the low-temperature heat radiation, spectral burst of the thermodynamically nonequilibrium SHR of the Yb3+ ions, and the high-temperature radiation of the crystal lattice. The temperature dependence of the luminescence spectra and SHR of the Yb2O3 polycrystals on the intensity of the laser and laser-thermal excitation and the concentration quenching of the Yb3+ luminescence in oxides indicate the key role of the interaction of the f-electron shell of the Yb3+ ions with the natural oscillations of the crystal lattice in the processes of the multiphonon excitation and nonradiative (multiphonon) and radiative (vibronic) relaxation.

Yb-laser based on the LMA active fiber and multimode Bragg grating

An ytterbium-fiber laser based on an active fiber with an increased core diameter and a multimode grating is created and studied. It is demonstrated that the effective reflection of the Bragg grating amounts to 96%. The laser line width is about 3 nm, which indicates the propagation of a superposition of modes in the multimode fiber. The repetitively pulsed regime of lasing, which can be related to the mode beating, is demonstrated

All-fiber Q-switched Er:Tm laser

Q-switched fiber lasers have found a number of application such as medical treatment, micromachining, diagnostics and others. All-fiber pulsed lasers are of special interest since exhibiting all advantages of fiber lasers, namely compactness, absence of mechanical elements, long time stability etc. One of the promising ways to build such pulsed laser scheme consists in the application of the fiber saturable absorber. Thus, in [1] it was demonstrated that a placing of Sm-doped fiber in the cavity of Yb-dop fiber laser allows one to get the pulse generation with energy of approximately 20 µJ and time duration of 650nx. Another approach was applied in [2], where Bi-doped fiber played a role of the saturated absorber. It is known that Tm-ions exhibit absorption band near 1.58 µm where Er-doped fiber lasers operate. In [3] this face was used to create Q-switched Er-laser with Tm-doped fiber as a saturable absorber. However laser scheme exploited the core pumped configuration, as result the pulse energy was as small as 9 µJ. Pulse duration was measured as 420 ns. To increase the pulse heavily Tm-doped fiber with a high concentration of the ion pairs. In this case the relaxation time is determined by the ions interaction within the pair.

Concentration effects in Ho-doped fiber lasers

Ho-doped fiber lasers exhibit oscillations at the wavelengths above 2 µm [1,2] and can be interesting for medical applications due to the high absorption of tissue and laser location due to the high atmosphere transparency in a range of 2.1–2.2 µm. In this spectral range there is a sharp growth of optical loss in silica glasses due to an influence of the molecular vibration lasers based on silica based fiber. Therefore to achieve the high lasing efficiency a length of the cavity should be not longer than several meters. It means that Ho-ions concentration should be high enough to provide an efficient oscillation using a short active fiber piece. On the other hand an increase of the active ions concentration can lead to the ions clustering. This effect can decrease the lasing efficiency and cause the laser self-pulsing due to the fast relaxation of the ions in clusters to the ground level. In this paper we study the lasing efficiency and dynamic properties of Ho-doped lasers based on the active fibers with different Ho-ions concentration.

All-fiber Q-switched Yb:Ho laser

Q-switched fiber lasers have found a number of applications such as medical treatment, micromachining, diagnostics and others. All-fiber pulsed lasers are of special interest since exhibiting all advantages of fiber lasers, namely compactness, absence of mechanical elements, long time stability etc. One of the promising ways to build such pulsed laser scheme consists in the application of a fiber saturable absorber. Thus, in [1] it was demonstrated that a placing of Sm-doped fiber in the cavity of Yb-doped fiber laser allows one to get the pulse generation with energy of approximately 20 µJ and time duration of 650 ns. Another approach was applied in [2], where Bi-doped fiber played a role of the saturated absorber. Bi active centers have the long lifetime, ∼ 1 ms. To solve this problem Bi-doped fiber was placed in a separate cavity to achieve a laser action at its wavelength and consequently to decrease the lifetime of the active centers in the excited state. That allowed authors to demonstrate the stable pulsed lasing with the pulse energy up to 0.1 mJ and the pulse duration in a range of approximately 1–2 µs. In this paper we suggest a new laser configuration with Ho-doped fiber as a saturable absorber.