We present our recent results obtained with single-crystal Fe:CdTe lasers. In particular, different pumping schemes and operation temperatures are investigated. Our high-quality Fe:CdTe single-crystal has been grown using the physical vaportransport technology with simultaneous Fe2+-ions doping. When pumped by 40-ns pulses from a Q-switched Er:YAG laser at 2.94 μm, the Fe:CdTe laser produces up to 2 mJ of output energy at room temperature (RT), with a slope efficiency of 16 %. A spectral tunability in the range of 5.1–6.3 μm is achieved. The lifetime of the upper laser level is measured to be 530 ns at RT. When using a 4.1-μm Fe:ZnSe laser (40-ns pulses) as a pump source instead of Er:YAG, the efficiency of the Fe:CdTe laser is increased to 30 % and we obtain an output energy of 5.8 mJ at RT. Furthermore, a record 2300-nm smooth and continuous wavelength tunability over 4.5–6.8 μm is achieved. With the latter system we demonstrate the first application of the Fe:CdTe laser to intracavity absorption spectroscopy. The experimental spectrum of atmospheric H2O shows good agreement with a calculated spectrum from the HITRAN database. The obtained sensitivity corresponds to an effective absorption path length Leff = 10 m, being limited only by the laser-pulse duration. However, by applying longer pump-pulses, the sensitivity can be enhanced by up to seven orders of magnitude, thus enabling extremely sensitive spectroscopic measurements. Furthermore, an energy up-scaling of the laser output is demonstrated at 77 K. The increased upper-state lifetime of 68 μs facilitates pumping by 250-μs pulses of a 4.1-μm Fe:ZnSe laser, and our Fe:CdTe laser produces a record-output of 0.35 J at 77K, with a slope efficiency of 44 %.
Hollow-core fibers (HCFs) which guide light by an antiresonant reflection from arrays of silica walls have been attracting much interest due to their extraordinary optical properties and potential interdisciplinary applications including highly efficient laser-matter interaction, ultra-short pulse delivery, pulse compression and low-loss mid-infrared transmission. There are several types of HCFs having either a photonic crystal cladding, Kagome lattice or a single cycle of capillaries surrounding the core. In the latter case the antiresonant guidance properties depend strongly on the core size and the shape of the core/cladding boundary.
In this work, we focus on the capabilities of two HCF designs (negative curvature of the core/cladding boundary and nodeless capillary structure) to obtain a nearly single-mode guidance from the visible to the mid-infrared spectral regions.
The first HCF (Sample A) was drawn from the stack comprising a cycle of eight touching capillaries having the wall thickness 1.5 µm which provided a negative curvature of the core/cladding boundary. The fiber was intentionally manufactured with the trapezoidal shape of the capillaries in order to minimize the interaction between the surface modes, trapped amidst the touching trapezoids, and the fundamental mode in a hollow core. The negative curvature of the boundary resulting in the octagonal shape of the core was achieved by putting an excess gas pressure inside the capillaries during the drawing process. The second HCF (Sample B) was produced from the stack comprising a cycle of six non-touching capillaries having the wall thickness 2.5 µm with a view to restrict the abovementioned interaction via breaking the surface modes coupling between the adjacent capillaries. As in the first case, the gas pressure was controlled carefully to keep all capillaries separately from each other. In both samples the core diameter was equal to 50 µm ensuring a relatively large effective mode area.
Taking into account the periodic nature of HCFs transmission windows, we simulated and measured accurately transmission spectra and modal properties of the fibers. The simulations were performed using the finite element analysis. The transmission spectra were measured by passing light from the tungsten halogen lamp through the samples of 35 cm long and registering output signal applying three optical spectrometers covering the wavelength range 600-2500 nm.
We observed a good agreement between the simulation and the experiment. The Sample A has transmission windows at the wavelengths 650, 750, 850-900, 950-1050, 1150-1300, 1450-1700, 2000-2300 nm and the Sample B – at the wavelengths 600, 650-700, 750-800, 850-950, 1000-1100, 1150-1350, 1450-1750, 1900-2400 nm. The mid-infrared window for the Sample B is larger and more pronounced in terms of relative transmission due to the larger wall thickness at the core/cladding boundary. Moreover, the Sample B is predicted to be practically single-mode in the considered spectral region, as the losses of the most competitive higher-order modes are estimated to be much above 1 dB/m. A similar regime for the Sample A is expected only when operating at the long-wavelength limit of the spectral region, due to the increase in the fundamental and higher-order modes refractive index difference.
Fiber Bragg grating (FBG) strain sensors are powerful tools for structural health monitoring applications. However, FBG sensor fabrication and packaging processes can lead to a non-linear behavior, that affects the accuracy of the strain measurements. Here we present a novel nondestructive calibration technique for FBG strain sensors that use a mechanical nanomotion transducer. A customized calibration setup was designed based on dovetail-type slideways that were mechanized using a stepping motor. The performance of the FBG strain sensor was investigated through analysis of experimental data, and the calibration curves for the FBG strain sensor are presented.
Recently similariton (or self-similar pulse) fiber lasers have attracted great attention due to their capabilities of highenergy
pulse generation that could find different applications in science and industry. Moreover it is very important to
reach stable pulse generation for the application as a frequency divider in optical frequency standard. Hybrid modelocking
mechanism was used for obtaining stable similariton generation at 38 MHz pulse repetition frequency. It
involves two types of mode-locking mechanisms in the cavity - saturation of carbon nanostructures absorber (recovery
time Trt ~ 500 fs) and nonlinear polarization evolution based on the nonlinear Kerr-effect (Trt ~ 10 fs). It was shown that
total intracavity dispersion should be slightly positive for generating stable similaritons with duration of less than 90 fs
and spectral bandwidth of more than 50 nm at 11.2 mW output average power that could be further applied in an all-fiber
MOPA setup.
Our research deals with studying picosecond pulse propagation from ytterbium laser through the large core multimode
microstructured fibers. Fibers with core sizes from 8 μm to 11 μm were investigated. Pumping in the normal dispersion
regime was made. Supercontinuum (SC) spectra were obtained in all the fiber samples. The main mechanism,
particularly four-wave mixing, responsible for SC generation was investigated theoretically.
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