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A distributed temperature monitoring inside a cavity of ytterbium DFB and holmium fiber lasers has been demonstrated with a spatial resolution of 1 and 5 mm, respectively, for the first time to the best of our knowledge. For this, we use an optical backscatter reflectometer, which measures intracore temperature, and compare it with the data of an IR thermographic camera, which measures temperature from the surface of a fiber. In the case of holmium fiber laser pumped at a wavelength of 1125 nm with a power of 6 W, the maximum temperature variation along the ∼3-m active fiber reaches ∼60 °C. In the case of ytterbium DFB laser, we observe a strong inhomogeneity of the temperature along the DFB cavity, which leads to a significant decrease in the lasing efficiency. When pumped by a single-mode laser diode at a wavelength of 976 nm with a power of up to 526 mW, the maximum temperature difference reaches 37 °C for the 37-mm DBF cavity.

Holmium (Ho 3+ ) laser systems emitting in the spectral range of 2–2.1 μm are widely used in many medical fields: urology, dermatology, surgery, and etc. This is due to the high absorption of laser radiation by water contained in soft tissues. It is of interest to compare the efficiency of soft tissue ablation using 2 μm lasers operating in continuous wave (CW) and ultrashort pulsed modes. In previous studies the Tm-doped fiber laser [1] , Tm:YLF and Ho:YAG lasers [2] exposure have compared.

Distributed stress and temperature measurements along the active fiber of operating laser is a new dimension of laser systems analysis. Thermal loading of the active fiber can lead to the switching of the laser operation mode [1] , gain spectrum changing, and modes instability appearance. To realize distributed temperature measurements one can use external thermal camera (thermograph) or optical backscatter reflectometer. The first one measures the temperature of the fiber surface, and the second one measures temperature directly in the fiber core. By using optical backscatter reflectometry spatial resolutions down to ~10 microns and temperature resolution of ~0.1 °C are achieved [2] , which allows high-performance analysis of fiber-optic schemes. In [3] distributed temperature measurement in the active medium of a high-power ytterbium-doped fiber laser is demonstrated using this approach. On the other hand, a similar or greater temperature difference can be measured in fibers doped with active ions and pumped with a large quantum defect. Holmium (Ho 3+ ) can become the most attractive ion because it can be pumped at 1.125–1.15 µm, while emitting wavelength is around 2 µm. In this work, we study the temperature distribution in Ho-doped DBR fiber lasers pumped at a wavelength of 1.125 µm. We use LUNA OBR 4600 reflectometer with down to 1 mm spatial resolution [2] and the IR-thermograph IRTIS-2000 ME to measure temperature distribution along a fiber specimen.

An all-fiber scheme of Holmium laser with active Q-switching, implemented by an intracavity Mach–Zehnder interferometer, was realized. Influence of the cavity output mirror reflection coefficient (4 and 20%), modulation voltage and different pump power levels on the pulse duration was investigated. The minimum pulse duration of 0.62 μs was obtained in a scheme with an output Bragg grating reflection coefficient of 20%. Wherein the peak power was 13 W.

We present the numerical simulation and experimental results on generation dynamics of an all-fiber holmium laser with an intracavity modulator based on a Mach–Zehnder interferometer. The experimental data were compared with numerical simulation results in a wide range of parameters. An explanation for the observed increase in the pulse repetition rate from 5.8 to 11.6 kHz, as well as the transition to the chaotic pulsations mode, was given. A numerical variation of the unknown or difficult-to-measure losses in the experiment was carried out. The obtained set of parameters allowed us to implement the correct numerical optimization of an active medium and cavity. As a result, the peak powers of about 9.5 and 13 W were obtained during the experiments, as well as in the numerical simulation.