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.
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.
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).
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 present a study of perforation time for a polymer material. Two different colors of the same polymer were investigated: natural and black. The study compares two different fiber laser wavelengths: 1 μm and 2 μm. The beam diameter on the polymer material was kept the same to provide a fair comparison between wavelengths. The irradiance was varied between 0.1 and 0.5 kW/cm2. Over the studied cases the perforation time was found to be shorter for the 2 μm fiber laser.
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.
We present the performance of a thulium:holmium-codoped fiber amplifier operating at 2050 and 2090 nm signal wavelength. This amplifier is built with polarization-maintaining fibers and is clad-pumped at 793 nm. We demonstrate more than a watt of signal power for both signal wavelengths. We compare the performance of this amplifier with a previously developed holmium-doped fiber amplifier operating at the same signal wavelengths. The amplifiers are compared in view of their optical-to-optical efficiency, optical bandwidth, gain, and noise figure.
We report on current advances in polarization-maintaining (PM) Thulium (Tm3+):Holmium (Ho3+)-codoped triple-clad fiber (THTF) laser. First fundamental studies were performed in a continuous-wave (CW) regime. A fiber laser with a 7 m active fiber delivered high output powers of up to 180 W for an emission wavelength centered at ~2050 nm. In addition, a setup with a 5 m active fiber was investigated with a slope efficiency of 41.6 % and a close to diffraction limited beam propagation with an M2x,y < 1:1. Operating in Q-switched regime at a repetition rate of 63 kHz, the pulses of the THTF laser had a pulse width of 45.6 ns and a pulse energy of 760 μJ, resulting in a peak power of 15.7 kW with an average output power of 48 W. With an M2x,y < 1:2 and a FWHM of 290 pm for the emission spectrum (compared to 54 pm in CW) the fiber laser shows a good basis for efficient frequency conversion.
A diode-pumped actively Q-switched Tm3+-doped fiber laser is reported generating pulse energies of 800 μJ, pulse widths of 43 ns and peak powers of 17.5 kW. By using the single-oscillator as a pump source for nonlinear frequency conversion, mid-IR pulse energies of 230 μJ are extracted from a ZnGeP2 (ZGP) optical parametric oscillator (OPO).
We report the design and performance of Holmium-doped fiber amplifiers (HDFAs) with novel alternative in-band pump wavelengths in the 1720—2000 nm spectral region. We demonstrate through simulations that pump wavelengths of 1840—1860 nm can yield significantly improved output power (3—6 dB), gain (8—10 dB) , and optical-optical conversion efficiency compared to the previous technical and industry standard pump wavelength of 1940 nm. Experimental results fully confirm our simulations.
We report on the development of a pulsed MOPA laser as a pump source for nonlinear fibers, for supercontinuum generation, or optical parametric generation in the mid-IR. The master oscillator is a directly modulated semiconductor laser, while the power amplifier is a two-stage polarization-maintaining fiber amplifier. Such a laser setup allows a flexible output pulse. We investigate the amplification of pulses from 5 to 50 ns long with pulse frequencies from 10 kHz to 1 MHz at a signal wavelength of 2050 or 2090 nm with the goal of kW peak power level for a rectangular output pulse shape.
Efficient high-power 2 μm fiber laser sources play a growing role in direct pumping of nonlinear crystals for frequency conversion into to mid-IR. There is an ongoing progress in fiber development and cavity improvement achieving outstanding laser performance for an efficient optical parametric generation. In this paper, we report on the investigation and characterization of a polarization-maintaining (PM) Thulium-Holmium-codoped triple-clad fiber (THTF). First fundamental studies were performed in a continuous-wave (CW) regime and showed highly promising results as a high power pump source for frequency conversion into the mid-IR. The paper focuses on first fundamental studies and the comparison of laser setups based on a 4.1 m and a 7 m active fiber length. Using the 7 m fiber, the THTF laser delivered an output power of 181 W. The laser had a degree of polarization of 98 %, a slope efficiency of 34.1 %, an optical-to-optical efficiency of 30% and a linewidth of 250 pm centered at 2050 nm. The laser output performance is compared with previous data of a THTF laser with a 4.1 m long fiber.1
We report on the first polarization-maintaining (PM) Thulium (Tm3+):Holmium (Ho3+)-codoped triple-clad fiber (THTF) laser. First fundamental studies were done in a CW regime and showed highly promising results as a high power pump source for frequency conversion in ZGP crystals. For an unpolarized output, the fiber laser delivered up to 145 W. Switching to a polarized operation, up to 140 W of optical output power with a slope efficiency of 36.3 %, an optical-to-optical efficiency of 32 %, and a beam propagation factor of M2 x,y < 1:9 was obtained with a degree of polarization > 99:8 %. The laser output wavelength was tunable from 2022 nm to 2068 nm. Operating in Q-switched regime at a repetition rate of 140 kHz, the pulses of the THTF laser had a pulse width of 96 ns, a pulse energy of 275 µJ, and a peak power of 2.84 kW. The emission spectrum was centered at ~2050 nm with a linewidth of 60 pm.
New developments in LIDAR and atmospheric sensing experiments highlight the need for studies of the optical bandwidth and wavelength dependence of multi-watt, large bandwidth, high dynamic range polarization-maintaining optical amplifiers in the 2—2.1 μm band. Recently we have demonstrated a hybrid single clad-double clad Tm-doped fiber amplifier with greater than 20 W output and a dynamic range of <20 dB in the 2 μm band, and a <25W output PM hybrid HDFA/TDFA with a dynamic range of 34 dB. Both demonstrations were carried out at a single input wavelength of 2051 nm. In this paper we extend our experimental studies to the signal wavelength dependence of a PM hybrid HDFA/TDFA with a single clad Hodoped preamplifier [4-7] and a double clad Tm-doped power amplifier. We have studied the performance of the amplifier from 2004 to 2108 nm, and in this paper, we report first experimental results for this wavelength region. We find that our hybrid Ho-Tm-doped design provides a PM fiber amplifier with a combination of large output optical signal-to-noise ratio, broad operating bandwidth, and high Pout of 28.5 W at λs = 2069 nm.
Current developments in LIDAR and atmospheric sensing experiments highlight the need for multi-watt, large bandwidth, high dynamic range polarization-maintaining optical amplifiers in the eye safe 1.9—2.15 μm band. So far, as an illustration of the previous state of the art for high power devices, multi-watt Tm-doped fiber amplifiers (TDFAs) have been demonstrated by Goodno et al. with an output power of 608 W at a signal wavelength of 2040 nm. As for Ho-doped fiber amplifiers (HDFAs) Hemming et al. have reported output powers of 265 W at 2110 nm. For this HDFA, a double clad Ho-doped fiber pumped by high power fiber lasers made the configuration complex and yielded an optical slope efficiency of 41%. Both of these achievements were with standard (non-polarization-maintaining) fiber.
We present a kW level pulsed laser based on a master oscillator power amplifier (MOPA) configuration. The directly modulated single frequency laser at 1952 nm was pulsed in the nanosecond regime with a repetition rate frequency from 10 kHz to 2 MHz. The MOPA topology was based on a two stage amplifier using single clad Thulium-doped fiber: it consisted of a pre-amplifier stage followed by a booster stage. We investigated the performance of this pulsed laser for two different TDFs with different saturating energies in the booster stage. The direct modulation allowed us to demonstrate more than 1 kW of output peak power over pulse repetition frequencies from 10 kHz to 500 kHz. For a pulse duration of 21 ns, we measured output energy of 13 μJ and 29 μJ for booster fiber saturating energies of 15 μJ and 30 μJ, respectively.
We report the experimental performance and simulation of a multiwatt two-stage TDFA using an L-band (1567 nm) shared pump source. We focus on the behavior of the amplifier for the parameters of output power Pout, gain G, noise figure NF, signal wavelength λs, and dynamic range. We measure the spectral performance of the TDFA for three specific wavelengths (λs= 1909, 1952, and 2004 nm) chosen to cover the low-, mid-, and upper-wavelength operating regions of the wideband amplifier. We also compare the performance of the two-stage shared pump TDFA with a one stage shared pump amplifier. Experimental results are in good agreement with simulation.
We report the performance of a two stage single clad (SC) Thulium-doped fiber amplifier (TDFA), delivering an output power of 5 W at 1952 nm without stimulated Brillouin scattering (SBS) for a single-frequency input signal. A slope efficiency greater than 60 %, a signal gain greater than 60 dB and an input dynamic range > 30 dB are achieved. The amplifier topology was optimized with a modelization tool of the SC TDFA performance: experimental results and simulations are in good agreement.
We report the design, experimental performance, and simulation of a single stage, co- and counter-pumped Tmdoped fiber amplifier (TDFA) in the 2 μm signal wavelength band with an optimized 1567 nm shared pump source. We investigate the dependence of output power, gain, and efficiency on pump coupling ratio and signal wavelength. Small signal gains of >50 dB, an output power of 2 W, and small signal noise figures of <3.5 dB are demonstrated. Simulations of TDFA performance agree well with the experimental data. We also discuss performance tradeoffs with respect to amplifier topology for this simple and efficient TDFA.
A simple engineering design is important for achieving high Thulium-doped amplifier (TDFA) performance such as good power conversion, low noise figure (NF), scalable output power, high gain, and stable operation over a large dynamic range. In this paper we report the design, performance, and simulation of two stage high-power 1952 nm hybrid single and double clad TDFAs. The first stage of our hybrid amplifier is a single clad design, and the second stage is a double clad design. We demonstrate TDFAs with an output power greater than 20 W with single-frequency narrow linewidth (i.e. MHz) input signals at both 1952 and 2004 nm. An optical 10 dB bandwidth of 80 nm is derived from the ASE spectrum. The power stage is constructed with 10 μm core active fibers showing a maximum optical slope efficiency greater than 50 %. The experimental results lead to a 1 dB agreement with our simulation tool developed for single clad and double clad TDFAs. Overall this hybrid amplifier offers versatile features with the potential of much higher output power.
A careful comparison of experiment and theory is important both for basic research and systematic engineering design of Thulium fiber amplifiers operating in the 2 μm region for applications such as LIDAR or spectroscopy (e.g. CO2 atmospheric absorption at 2051.4 nm). In this paper we report the design and performance of a multistage high-power PM Tm-doped fiber amplifier, cladding pumped at 793 nm. The design is the result of a careful comparison of numerical simulation, based on a three level model including ion-ion interactions, and experiment. Our simulation model is based on precise measurements of the cross sections and other parameters for both 6 and 10 μm core diameter fibers. Good agreement for several single and multistage amplifier topologies and operating conditions will be presented. Origins of the difference between theory and experiment are discussed, with emphasis on the accuracy of the cross sections and the cross relaxation parameters. Finally based on our simulation tool, we will demonstrate a design with an output power greater than 10 W for a multistage amplifier with a single-frequency signal at 2050 nm. The power stage was constructed with a 6 μm active fiber showing a 64 % optical slope efficiency. The output power is found to be within 5 % of the simulated results and is limited only by the available launched pump power of ~24 W. No stimulated Brillouin scattering is observed at the highest output power level for an active fiber well thermalized.
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