Various applications in the medical, defence and industrial fields exist for thulium-doped fibre lasers (TDFL) emitting in the 2 µm spectral region. All-fibre laser architectures represent optimized designs especially for applications that require high reliability in harsh environments. These architectures can be further improved by reducing the amount of fibre components and therefore reducing the failure probability. We investigate mode field adaption techniques between an active and passive fibre by changing the refractive index profiles of both fibres. The findings of this investigation are used to optimize a core-pumped TDFL with up to 75% slope efficiency.
Exceeding the multi-kW power level with thulium-doped fiber lasers has not been achieved using a single thulium-doped fiber laser. One solution to overcome this limit is the coherent beam combination. We focus on an active phase control with tiled aperture configuration. The setup consists in an amplified seed laser split in three channels. These channels are controlled in phase and amplified again before being launched free space and combined. A SPGD algorithm controls the channel’s phase to provide combination. Rise time below 0.5 ms were achieved with a residual amplitude noise lower than λ/30.
We report on the scaling of a polarization-maintaining MOPA at a signal wavelength of 2048 nm, designed for pumping an optical parametric oscillator (OPO). By utilizing the MOPA structure to design suitable OPO pump pulses the overall mid-IR conversion efficiency is enhanced enabling the scaling of the mid-IR average power. 60 W of average power is achieved and applied to pump different ZGP OPOs. The resonator designs are investigated and compared regarding scalability and beam quality.
In this work we propose a simulation tool to analyze the case of conduction-driven thermal blooming and compare the results with measurements at the 2055 nm absorption line of CO2. Using a split-step beam propagation method and incorporating the spatial refractive index change related to the absorption-driven radial temperature gradient resulting from conduction, the effect of beam distortion can be described for arbitrary wavelengths and various atmospheric conditions. The model is benchmarked by experimental investigations using a tunable 100-W thulium fiber laser.
We present our latest results in power scaling of Midwave-Infrared (MWIR) Optical Parametric Oscillators (OPOs) based on a Zinc Germanium Phosphide (ZGP) crystal, utilizing a single oscillator fiber laser as pump source. To obtain a compact and complexity-reduced pump source emitting at ≥ 2.09 μm, a Q-switched Tm3+:Ho3+- codoped fiber laser was developed. Based on this pump source at an emission wavelength of 2.1 μm, we achieved an MWIR output power of 12.2W with pulse energies of up to 270 μJ and a conversion efficiency exceeding 43 %. This result exceeds the published power records of ZGP-based OPOs pumped by 2 μm Q-switched fiber lasers by 50 % and sets a new benchmark for average power scaling and pulse energy of Q-switched pump sources.
An actively Q-switched diode-pumped Tm3+-doped fiber laser (TDFL) operating at 2050 nm is reported based on a flexible Photonic Crystal Fiber (PCF) with a core diamter of 50 μm. Using a fiber length of 3 m, the TDFL delivers gaussian shaped pulses with a maximum pulse energy of 1.5 mJ, corresponding to a peak power of 16 kW and a pulse width of 88 ns. The measured output spectrum shows a single peak at 2050 nm with a 3-dB-linewidth of 100 pm and 10-dB-linewidth of 270 pm. For a longer fiber length of 7 m, the effective gain is redshifted by reabsorbtion, increasing the achievable pulse energy up to 1.9 mJ. The average output power of the pulsed TDFL can be scaled to more than 100 W with a slope efficiency of 46 %. In all configurations the TDFL delivers nearly diffraction limited beam quality (M2 ⪅1.3).
Thulium-Doped Fiber Lasers (TDFL) emitting at 2 μm wavelength are used in various applications such as imaging, telecommunication and optical countermeasures. Many of these applications require highly integrated and passively cooled lasers with low SWaP (size, weight, and power) architecture that can work in harsh environment at different temperatures. We investigated the temperature dependence of a multi-watt TDFL with a low SWaP architecture for temperatures ranging from 253 K till 573 K. Cladding-pumping with 793 nm diode lasers is used for high-power TDFLs to take advantage of the cross-relaxation effect to double the quantum efficiency. However, since the 3 H4 absorption band is relatively narrow with a 16 nm FWHM compared to the diode wavelength shift of 0.3 nm/K, these diode lasers have to be wavelength or temperature stabilized using volume Bragg gratings or Peltier elements. Both approaches either limit the applicable temperature range1 or decrease the overall efficiency. In contrast in-band core-pumping directly into the 3 F4 level offers a broad absorption band ranging from 1550 nm till 1720 nm and is therefore preferred for low SWaP TDFLs. We investigated therefore a low SWaP TDFL that is core-pumped by an in-house built erbium:ytterbium-codoped fiber laser (EYDFL) with pump wavelength of 1567 nm.
We present our latest results in power scaling of thulium-doped fiber lasers in the 2 μm region based on coherent beam combination with tiled aperture technique. The investigation of a high-power laser system based on coherent beam combination was divided into three individual experiments. First a MOPA architecture was studied with focus on power scaling to kW level with a broad linewidth. Second another MOPA setup was developed to match the requirements for coherent beam combination. Lastly, the combination of milli-watt level channels was investigated using a SPGD algorithm. The performance of these systems will be presented.
We investigated the temperature dependence of a multi-watt thulium-doped fiber laser. For high-power laser operation, thulium-doped fiber lasers are often pumped in the cladding by diode lasers operating at 793 nm to take advantage of the cross-relaxation effect. However, these diode lasers have to be temperature stabilized since the 3H4 absorption band in thulium-doped fibers is narrow and, therefore, not suitable for passively cooled setups. In contrast in-band pumping into 3F4 is an alternative, benefiting from a broad absorption band. The investigated thulium-doped fiber laser is core-pumped by an in-house built erbium:ytterbium-codoped fiber laser. In order to keep the surrounding temperature defined, the thulium-doped fiber was integrated into a metal plate with grooves and embedded in a thermal interface material. In addition, the metal plate was mounted on Peltier elements to control its temperature. During the experiment, the temperature of the metal plate was changed between -20°C and 80°C while the output power, slope efficiency and electrooptical efficiency of the thulium-doped fiber laser were measured. The performance of the laser versus temperature is reported and show minor dependence over a broad temperature range.
A polarization-maintaining (PM) pulsed three-stage master oscillator power amplifier (MOPA) emitting at 2047 nm is reported, generating 19.8W of output power (396 μJ pulse energy) for a 50 ns pulse width at a repetition rate of 50 kHz. The output signal is linearly polarized and a diffraction limited beam quality is achieved. This MOPA laser is used to pump a doubly resonant ZnGeP2 (ZGP) optical parametric oscillator (OPO) in a linear cavity. A mid-IR output power of 8.1W, accordingly 162 μJ of pulse energy, and a conversion efficiency of 44 % are obtained in the 3-5 μm band.
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