Coherent Beam Combining (CBC) is one of the novel methods aiming to increase laser output power and intensity, especially in cases where applications require both high power and high beam quality. CBC offers a way of exceeding limitations of a single-fiber laser source, allowing for excellent scalability and high efficiency operation. Simulating and optimizing the intensity of the far-field is crucial when designing a CBC system. This paper focuses on a way of approaching numerical solution of the electric field intensity along the Rayleigh range of multiple Gaussian beams coherently combined over large distances. It aims to circumvent the restrictions of computational capacity faced by most numerical methods when solving for the optical field propagation over large geometries by combining ray and wave optics approach. Output intensity fields for coherent combination of 6, 12, and 20 channels operating at a wavelength of 1550 nm are presented, using a Cassegrain type telescope as beam combiner. Influence of design parameters and near field arrangement is examined and results are compared with previously reported experimental values.
Advanced resonator designs for Ho3+:YAG lasers and ZGP OPOs are presented. A segmented Ho3+:YAG crystal for power scaling was investigated. An improved power performance could not be shown up to 60W of output power, which is attributed to the low overall crystal temperature. Exploiting the Porro prism resonator concept for the first time, a 200-times increased alignment tolerance compared to a corresponding mirror resonator was achieved. The performance of a ZGP OPO could be improved by a negative lens in the resonator, which significantly enhances the beam quality.
We report on an all-fiber Master Oscillator Power Amplifier (MOPA) laser at a signal wavelength of 2048 nm, designed for pumping an Optical Parametric Oscillator (OPO). This setup offers the benefit of miniaturization while being rugged and thereby suite the demands for mobile counter measures. 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. Thus, 50 W average power at a wavelength of 2048 nm is generated and applied to pump a ZGP OPO. For a ZGP OPO pump power of 20 W, mid-IR output power of 11.1 W is achieved. At this maximum output power, a beam quality M2 of 2.2 is measured for signal and idler. To our knowledge this is the highest mid-IR brightness achieved by a fiber laser pumped ZGP OPO.
Solid-state lasers are pivotal in a wide range of defense applications. In this study, we present a comprehensive simulation model for advancing solid-state laser capabilities, focusing on Ho3+:YAG for SWIR generation. This versatile model integrates different optical components, including mirrors and laser crystals, allowing flexible resonator and free-space modeling. It employs a multi-physics approach utilizing a beam propagation method for simulating laser fields and 3D temperature and stress distributions based on finite difference methods. The combined model allows for the optimization of solid-state laser systems for power output, efficiency, and beam quality.
The demand for high-power lasers in the 2 μm region is growing across various applications. Thulium-Doped Fiber Lasers (TDFL) lead with 50-60 % slope efficiencies and over 1 kW of output power. Thulium:Holmium-co-Doped Fiber Lasers (THDFL) show lower slope efficiencies but operate beyond 2.1 μm, beneficial for atmospheric applications. The latter laser type being so-far less explored. We compare TDFL and THDFL power scaling in a Master-Oscillator Power-Amplifier (MOPA) setup, assessing thermal constraints and output power limits for distinct fiber geometries. This study provides a direct comparison and important background on laser design to meet specific power requirements.
Erbium:ytterbium-codoped fibre lasers (EYDFL) are used in many industrial and defence applications such as free-space communication, range finding and LIDAR. These applications benefit from a low atmospheric absorption and improved eye-safety. They are supported by widely available matured fibre components that simplify the realization of the EYDFL, thus reducing costs and increasing reliability. In many applications, the laser is targeted for an outdoor operation. Therefore, a monolithic setup has to be used with an inherent compactness and reliability. Furthermore, pulsed laser operation is needed for certain applications. This investigation shows an all-fibre pulsed EYDFL suitable for operation in harsh environment.
A nanosecond-pulsed Tm-doped fiber laser (TDFL) operating at 2050 nm is reported based on a flexible photonic crystal fiber with a core diameter of 50 μm. The TDFL delivers gaussian-shaped pulses with a maximum pulse energy of 1.9 mJ, corresponding to a peak power of 15.4 kW and a pulse width of 116 ns. The average output power of the pulsed TDFL is 114 W at a pulse repetition rate of 60 kHz. The measured output spectrum shows a single peak at 2050 nm with a 3-dB-linewidth of 130 pm and a 10-dB-linewidth of 370 pm. The TDFL serves as an excellent pump source for mid-IR generation through nonlinear-frequency conversion in optical parametric oscillators based on ZnGeP2.
We report our latest results in nonlinear mid-infrared frequency conversion using a fiber-based single-oscillator pumping a ZGP-based OPO. To minimize absorption in the ZGP crystal a simplified Q-switched Tm3+:Ho3+-codoped fiber laser was realized, emitting at 2.1 μm. Based on this pump source, mid-infrared output powers up to 12.2 W were achieved with pulse energies up to 271 μJ and conversion efficiencies of 43.4%. Representing an improvement of about 50% over the state-of-the-art.
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.
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.
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.
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 a Ho3+:YAG laser source and use it to pump a linear ZGP OPO with a novel design intended to improve the mode matching properties of the resonator. Beam quality measurements are used to evaluate the performance of the novel design in comparison with a conventional linear resonator. Operated at 25 kHz repetition rate, the Ho3+:YAG laser delivers 2.2 mJ, 20 ns Q-switched pulses. This results in a pulse peak power of 108 kW while the average output power is 58W. In the optimal ZGP OPO configuration, 14.1W of signal and idler output power are achieved with a conversion efficiency of 49.8 % with respect to the absorbed pump power. A clearly improved beam quality of 2.1 and 3.3 (2.4 and 3.5) in the x- and y-axis of the signal (idler) beam compared to the conventional linear resonator is shown.
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 an advanced field propagation model for end-pumped Ho3+:YAG laser resonators, enabling highly accurate predictions of their performance and spectral properties. The model incorporates two key improvements to our previously published model: simulation of pulsed operation through active Q-switching and spectral resolution for pump and laser fields. Pulsed simulation is achieved by solving the crystal rate equations with a time resolution, capturing dynamic behavior. Spectrally resolved rate equations and cross-sections enable comprehensive analysis of spectral properties. This advanced model provides unprecedented accuracy in simulating Q-switched Ho3+:YAG laser resonators, allowing the design and optimization of high-precision laser systems.
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 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 report on laser resonators with a segmented and a homogeneously doped Ho3+:YAG crystal delivering over 60 W of output power with near-diffraction-limited beam quality. The resonators with both crystals exhibit high slope efficiencies around 67% and maximum pulse energies of 1.14 mJ and 1.04 mJ are measured for the homogeneously doped and segmented crystal, respectively, at a repetition rate of 50 kHz. Q-switched pulses with a pulse peak power of 108 kW are generated with the homogeneous crystal at a repetition rate of 25 kHz. In a slight redesign of the cavity, 1:24 mJ, 33 ns pulses with a pulse peak power of 38kW are measured.
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.
We have set up a Gated Viewing (GV) system operating at a laser wavelength of 2.09 μm in the short-wave infrared (SWIR) spectral range to experimentally assess the potential of such a system for security and military applications like long-range target identification and intelligence, surveillance and reconnaissance (ISR) in low visibility conditions. In particular, we compare this system with GV systems operating at the widely used SWIR wavelength of 1.57 μm. Our focus is on examining physical effects such as laser reflection and speckles at the target surface as well as atmospheric impacts like transmission and turbulence. Finally, estimates of system ranges are made. The gated viewing camera is based on an array of 640 × 512 mercury cadmium telluride (MCT) avalanche photodiodes (APD) with a pitch of 15 μm. The cut-on and cut-off wavelengths are 0.9 μm and 2.55 μm, respectively, providing sensitivity in the extended SWIR (eSWIR) spectral range. This allows to capture both laser wavelengths 1.57 μm and 2.09 μm with the same GV camera. The camera is equipped with an aspherical F/3 lens with a focal length of 600 mm, resulting in a field-of-view (FOV) of 0.92° × 0.73°. The 1.57 μm laser is based on a commercial flashlamp-pumped Nd:YAG laser combined with an optical parametric oscillator (OPO) with a maximal pulse energy of 65 mJ at 20 Hz pulse repetition frequency (PRF) and a pulse width of τ = 11 ns. The 2.09 μm laser is an in-house developed solution with approximately 20 mJ at 20 Hz PRF and τ = 12 ns.
We present a crossed-Porro prism resonator with a Ho3+:YAG crystal and investigate it with a focus on the alignment stability. Furthermore, we show a single-Porro-ended resonator optimized for Q-switched operation. Both resonators are compared to corresponding mirror resonators. In the crossed-Porro prism resonator, a maximum output power of 30.7 W is reached with a high slope efficiency of 67.4 %. By tilting each of the prism axes one by one and measuring the entailed drop in output power, the alignment sensitivity is determined. In comparison to a corresponding mirror resonator, it is improved by a factor of up to 200. With this design, 170 ns Q-switched pulses with an energy of 0.51 mJ are generated at a repetition rate of 50 kHz. In the single-Porroended resonator significantly shorter pulses with a duration of 55 ns and a maximum pulse energy of 0.8 mJ were achieved.
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.
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.
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 on an actively Q-switched high-pulse-energy Ho3+:YAG laser in-band pumped by a Tm3+-doped fiber laser, both operated at room temperature. The Ho3+:YAG active medium inside a plane-plane cavity is pumped using a commercial Tm3+-doped fiber laser at 1908 nm from one side. In continuous operation a maximum power of 20.1 W with a slope efficiency of 45.1%, central wavelength of 2090 nm and a beam quality factor M2 below 1.5 were achieved. Q-switched operation was achieved using a Brewster-cut acousto-optic modulator (AOM) based on crystalline Quartz. During Q-switching the incident power was kept stable at 47 W to obtain an M2 of 1.3 and a stable thermal lens inside the laser crystal. With the variation of the repetition frequency a lower limit of the quasi-continuous pulsed regime was investigated and measured to be approximately 3 kHz. The maximum pulse energy in Q-switching operation was achieved with a repetition frequency of 700 Hz leading to an energy of 15 mJ at 12.1 ns pulse width, corresponding to a peak power of 1.2 MW. The laser showed no sign for a loss of performance during many hours of testing. Using this laser as a pump source for a double resonant OPO, a maximum mid-infrared output power of 6.3 W could be achieved at a repetition frequency of 2 kHz, accompanied by a low threshold power of 1.6 W and a slope efficiency of 49.2%.
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.
A bidirectional 793 nm diode-pumped actively Q-switched Tm3+, Ho3+-codoped silica polarization maintaining (PM) double-clad (DC) fiber laser is reported. An average output power of 55 W with 100 ns pulse width, 200 kHz repetition rate and 2.09 μm wavelength is obtained with this fiber laser. To the best of our knowledge, this is the highest ever demonstrated average output power emitted from a Q-switched single oscillator 2 μm fiber laser. Thanks to end-caps fusion-spliced on both fiber tips, this fiber laser source is presenting good beam quality factors (M² < 1.7) and no thermal-induced damaging is observed at 55 W average output power. Further power scaling of the laser is only pumppower- limited in the range of the total available pump power in this setup (180 W). Direct OPO ZnGeP2 (ZGP) pumping with this fiber laser has been performed for optical nonlinear conversion in the mid-IR regime. More than 8 W of average output power has been generated in the 3-5 μm band.
The generation of mid-infrared (mid-IR) radiation, ranging from 2 - 5 μm, is getting much attention in recent years thanks to many applications it can be used for, e.g. in free space optical communication, range finding, counter measures and remote chemical sensing systems. It also plays an increasing role in medicine, for instance in optical tissue ablation or optical coherence tomography, owing to the high water absorption in that wavelength range. In this research study, a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fiber is pumped by a Q-switched mode-locked (QML) thulium (Tm3+)- doped double-clad silica fiber laser, emitting at around 2 μm, to generate mid-IR supercontinuum (SC). Further spectral broadening of this SC radiation is achieved by coupling it into a chalcogenide arsenide-selenide (AsSe) photonic crystal fiber (PCF). An output power of 24 W at 2 μm has been achieved in QML operation for the Tm3+-doped fiber laser. The SC output power from the ZBLAN fiber has been 7.8 W with a spectrum extending to approximately 4.1 μm. For further wavelength broadening experiments, a long-wave-pass filter with a 3 dB edge around 3.6 μm has been implemented between the ZBLAN and the AsSe fiber to cut out the residual pump light at 2 μm and the radiation between 2 μm and 3.5 μm. The pump power was approximately 120 mW with a spectrum from 3.5 μm to 3.9 μm. First proof of principal experiments has been performed with 20 mW of averaged output power and a spectrum extending to 4.9 μm. The coupling efficiency of the SC radiation from the ZBLAN fiber into the AsSe fiber has been around 30%.
6.5 W of average power have been generated by a mid-infrared ZnGeP2 (ZGP) optical parametric oscillator (OPO)
pumped directly by a Q-switched Tm3+-doped single-oscillator fiber laser. The Tm3+-fiber pump laser based on a silica
polarization-maintaining (PM) double-clad fiber provided average powers of up to 23 W at pulse widths of 65 ns at
40 kHz repetition rate. The ZnGeP2 OPO produces 45 ns mid-IR pulses. The OPO slope efficiency was 40% and the
optical-to-optical conversion efficiency 32%.
The generation of mid-infrared (mid-IR) supercontinuum (SC) radiation, ranging from 2 - 5 μm, is subject of intense research due to its wide range of applications. A very popular host media for mid-IR SC generation are soft glass fibers owing to their low-loss transmission in the mid-IR wavelength regime, particularly fluoride fibers are very attractive for high-power operation. In this research study, a diode-pumped Q-switched mode-locked (QML) thulium (Tm3+)-doped double-clad silica fiber laser is used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-IR SC generation. The QML regime of the fiber laser is actively generated by two acousto-optic modulators. The Tm3+-fiber laser provided up to 23.5 W (26 W) of average output power in QML (continuous wave) regime with a slope efficiency of 36 % (32 %). The measured beam quality has been close to the diffraction-limit in QML regime. The system delivered mode-locked pulses with a duration of 7.5 ps, measured with a commercial autocorrelator system, at a repetition rate of 46 MHz. The Q-switched envelopes had a width between 50 and 150 ns depending on the output power level and the adjustable repetition rate. Mid-IR SC with an average output power in all spectral bands of 4.5 W have been achieved with more than 3 W/ 1.7 W/ 1 W/ 0.36 W after a long-wave-pass filter with a 3 dB-edge at 2.15 μm/ 2.65 μm/ 3.1 μm/ 3.5 μm.
This paper describes new laser sources and non linear conversion setups for 2 μm and mid-IR generation based
on fiber technologies developed at ISL. Especially for jamming heat-seeking missiles, these novel designs allow
to propose future compact, efficient and integrable laser systems. The specialty of the ISL technology lies in the
use of single 2 μm fiber laser oscillators, which deliver the full output peak power to pump optical parametric
oscillators or nonlinear fibers. No multi-stage amplifiers at 2 μm or 1.55 μm are necessary to efficiently pump
non linear converters to obtained useful energies in the mid-infrared spectral range. This technology leads to
efficient, simple and promising setups to be implemented in flying platforms. The best results achieved in continuous-wave (CW), Q-switched (QS) and mode-locked (ML) regimes with fiber lasers based on Tm3+-doped and Tm3+,Ho3+-codoped fibers are presented. Up to 70 W of average power was achieved around 2 μm with a Tm3+-doped fiber in CW regime. In ML regimes, at a repetition rate of 66 MHz, 50 W of average power was reached. In QS regime, up to 32 W of average power was generated around 2 μm with a polarization maintaining Tm3+-doped fiber at a repetition rate of 40 kHz. With a Tm3+,Ho3+-codoped fiber, up to 25 W of average power was obtained around 2070 nm in Q-switched regime. For example at 50 kHz, the pulse duration was around 50 ns at the maximum output power. The M2 was estimated to be less than 1.2.
The emission from QS fiber lasers was used to directly pump OP-GaAs and ZGP OPOs. For example, in band II,
up to 6.5 W of averaged power was recently obtained from a ZGP OPO pumped by a Tm3+-doped fiber laser. At
40 kHz repetition rate, the pulse duration was around 65 ns at the maximum output power. For 3 W of averaged
output power, the M2 of the signal beam was estimated to be less than 2.1 and less than 2.4 for the idler beam.
Using a mode-locked Tm3+-doped fiber laser to pump a ZBLAN fiber at an injection efficiency of ~60%, an
overall supercontinuum power of up to 2.2 W from a pump power of 5.4 W was achieved. The power above 2650 nm was 0.7 W.
CdSiP2 (CSP) is a very promising nonlinear crystal for the mid-infrared spectral range with a nonlinear coefficient slightly larger than that of ZnGeP2 (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage threshold of uncoated CSP exists. In this work, we compare the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09 μm (1 kHz, 21 ns) and 1.064 μm (100 Hz, 8 ns).
CdSiP2 (CSP) is a very promising nonlinear crystal for the mid-IR spectral range with a nonlinear coefficient slightly larger than that of ZnGeP2 (ZGP). In contrast to ZGP, CSP is phase-matchable and can be employed in 1.064-μm pumped optical parametric oscillators (OPOs) without two-photon absorption. Although low damage resistivity has been reported in such initial OPO tests of CSP, no reliable data on the damage-threshold of uncoated CSP exists. We compare in this work the damage resistivity of uncoated CSP with ZGP at two wavelengths, 2.09 μm (1 kHz, 21 ns) and 1.064 μm (100 Hz, 8 ns).
The paper describes two laser prototypes devoted to the jamming or the damaging of heat-seeking missiles for use in field trials. The semi-ruggedized compact jamming prototype is based either on an OP-GaAs or a ZnGeP2 (ZGP) OPO directly pumped by a 2.09 μm Q-switched Ho3+:YAG laser with up to 20 W of average power around 2.1 μm and an M2 of less than 1.1. For jamming in band II, up to 3.5 W of average power were obtained and repetition rates from 20 kHz to 100 kHz were achieved. For 3.5 W of averaged output power, the M2 of the signal and idler beams were estimated to be less than 1.2. The destruction laser consists of a Ho3+:LLF MOPA laser system which is used to pump a ZGP OPO. The maximum pulse energy of the Ho3+:LLF MOPA was 82 mJ at a repetition rate of 100 Hz. The pump beam quality was measured to M2x = 1.01 and M2y = 1.03 at a wavelength of 2053 nm. The total 3-5 μm energy obtained for destruction was 23.4 mJ, corresponding to an optical-to-optical conversion efficiency of 51 %. The M2 values of the signal were M2x = 1.81 and M2y = 1.98. The M2 values of the corresponding idler beam were M2x = 1.91 and M2y = 1.94, respectively. ISL is also currently working on new laser sources and non linear conversion setups for proposing new versions that should be more compact, more efficient and more integrable.
Fiber lasers emitting in the 2 μm wavelength range doped with thulium ions can be used as highly efficient pump
sources for nonlinear converters to generate mid-infrared radiation. For spectroscopic purposes, illumination and
countermeasures, a broad mid-infrared emission spectrum is advantageous. This can be reached by supercontinuum
generation in fibers, e.g. fluoride fibers, which up to now has, however, only been presented with either low
average power, complex Raman-shifted 1.55 μm pump sources or multi-stage amplifier pump schemes. Here we
present recent results of a new actively-mode-locked single-oscillator scheme that can provide the high-repetition
rate sub-ns pump pulses needed for pumping supercontinuum generators. A thulium-doped silica fiber laser is
presented that provides > 11 W of average power CW-mode-locked pulses at 38 MHz repetition rate at ~ 38 ps
pulse width. Upgrading the setup to allow Q-switched mode-locked operation yields mode-locked 40 MHz pulses
arranged in 60 kHz bunched Q-switch envelopes and thus increases further the available peak power. In this
Q-switched mode-locked regime over 5 W of average power has been achieved.
Due to a wide transparency range (0.9-17 μm), a low absorption loss (~ 0.01 cm-1), and a laser damage threshold
comparable to ZGP crystals (~ 2 J/cm2), combined with excellent nonlinear, thermal and mechanical properties,
quasi-phase-matched orientation-patterned gallium arsenide (OP-GaAs) crystals are well adapted for efficient
mid-infrared optical parametric oscillators (OPOs).
The paper discusses the best results obtained, to our knowledge, with an OP-GaAs OPO pumped by a Qswitched
2.09 μm Ho3+:YAG laser. The compact (33 × 48 cm) high-repetition rate source developed allows to
achieve 4.0 W of average output power in the 3-5 μm range at 40 kHz repetition rate with a 45 % slope
efficiency and a very good beam quality (M2 < 1.8). 6.4 W were obtained at 70 kHz with a 51 % slope
efficiency, and 7.7 W at 100 kHz with a 46 % slope efficiency. At 40 kHz and 70 kHz, an optical damage
occurred at a fluence of 1.9 J/cm2 and 1.5 J/cm2 respectively. The power is limited by the OP-GaAs crystal
thickness and is expected to be scaled in thicker samples recently fabricated.
Improvement in hybrid vapour phase epitaxy growing techniques of quasi-phase-matched orientation-patterned GaAs
(OP-GaAs) allows larger sample thickness and permits efficient operation as a mid-infrared optical parametric oscillator
at Watt-level average output powers [1-3]. Especially its low absorption loss (- 0.01 cm-1), its laser damage threshold
comparable to ZGP (- 2 J/cm2) combined with a large nonlinear coefficient, a good thermal conductivity, excellent
mechanical properties, and a wide transparency range (0.9-17 μm) are suitable properties for efficient non-critical phase
matched OPOs. As there is no natural birefringence in GaAs, phase matching is independent of polarization and
propagation direction, offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized
or poorly polarized sources like simple fiber lasers have been efficiently used as pump sources [4-5].
The paper discuss the best OP-GaAs OPO results achieved, to our knowledge, using a Q-switched 2.09 μm Ho:YAG
laser as pump source as well as results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched
Tm,Ho:silica fiber laser. With a 2.09 μm Q-switched Ho:YAG fiber laser pump source up to 2.9 W of average output
power was achieved at 20 kHz repetition rate, 3.9 W at 40 kHz and 4.9 W at 50 kHz. With a 2.09 μm Q-switched
Tm3+,Ho3+:silica fiber laser pump source, up to 2.2 W of average output power was achieved at 40 kHz repetition rate,
1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.
For some medical fields in laser surgery and as a pump source for nonlinear materials to generate mid-IR
radiation, e.g. for countermeasure applications, it is very useful to have a solid-state laser with high pulse energy
at 2 μm. The rare earth ion Thulium offers a cross relaxation and can thus be directly diode pumped with
common laser diodes around 800 nm for an efficient pumping. However, it was not considered for high pulse
energy operation due to the high saturation fluence of around 62 J/cm2 at 2 μm. A limiting factor has always
been the damage threshold of the optical elements inside the cavity. One of the reasons is the strong thermal
lens of YAG, which affects a change of the beam radius inside the resonator and additionally degrades the beam
quality with increasing pump power. Using a new pump geometry of the Tm3+:YAG laser system, it is now
possible to reach pulse energies > 13 mJ at a diffraction limited beam quality of M2 < 1.1. The Q-switched
Tm3+:YAG laser system uses an AOM operating at 100 Hz and will be described in detail. Due to the high pulse
energy and very good beam quality, this laser is very interesting for nonlinear parametric frequency conversion.
Gallium arsenide combines a large nonlinear coefficient, a good thermal conductivity, excellent mechanical properties,
and a wide transparency range (0.9-17μm). Improvement in hybrid vapour phase epitaxy growing techniques of quasiphase-
matched orientation-patterned GaAs (OP-GaAs) allows larger sample thickness and permits efficient operation as
a mid-infrared optical parametric oscillator at Watt-level average output powers.
Especially its low absorption loss (~; 0.01 cm-1), its laser damage threshold comparable to ZGP (~ 2 J/cm2) are suitable
properties for efficient non-critical phase matched OPOs.
As there is no natural birefringence in GaAs, phase matching is independent of polarization and propagation direction,
offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized or poorly polarized
sources like simple fiber lasers have been efficiently used as pump sources.
The paper will discuss recent results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched
Tm,Ho:silica fiber laser and a study on polarization effects using a Q-switched 2.09 μm Ho:YAG laser as the pump.
With a 2.09 μm Q-switched Tm,Ho:silica fiber laser pump source, up to 2.2 W of average output power was achieved at
40 kHz repetition rate, 1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.
Nonlinear optical materials play a key role in the development of coherent sources of radiation as they permit the
frequency conversion of mature solid-state lasers into spectral ranges where lasers do not exist or perform poorly. The
availability of efficient quasi-phasematched infrared materials is thus considered as important for the development of
several optronics applications.
This paper will review the recent progresses achieved with thick Orientation-Patterned GaAs structures. We will present
results obtained in growing a 500 μm thick layer on 2 cm long structures with low optical losses (less than 0.02 cm-1).
This loss coefficient is low enough to allow the operation of a highly efficient GaAs OPO in the Mid-IR range.
Nonlinear optical materials play a key role in the development of coherent sources of radiation as they permit the
frequency conversion of mature solid-state lasers into spectral ranges where lasers do not exist or perform poorly. The
availability of efficient quasi-phasematched infrared materials is thus considered as important for the development of
several defense optronics applications.
This paper will review the recent progresses we achieved with thick Orientation Patterned-GaAs structures. We will
present results obtained in growing thick-layer (500 µm) on 2 cm long structures with very low optical losses (less than
0.02 cm-1). This loss coefficient is low enough to allow the realization of a high power OPO in the MIR band.
Efficient generation with good beam quality and high output power in the 3-5 μm wavelength range is desired and a 2 μm-pumped Optical Parametric Oscillator is a promising design for this purpose.
The low quantum defect of a resonant laser pumping of the Ho 5I7 manifold using 1.9 μm radiation leads to an efficient 2.09 μm laser. Due to a long fluorescence lifetime and a good energy storage capability, the system is well-suited for either increased average power at high repetition rates (more than tens of kHz) or high energy output at low repetition rates (several tens of Hz).
For both repetition rate ranges, we propose to evaluate critical components of the system such as the Tm pump laser source. Performances of a Ho:YAG laser pumped either by a 50 W Tm:fiber laser or by a 20 W Tm:YLF laser are compared. Efficiencies of the modulation devices are reported for an AOM and a RTP Q-switch cell.
Q-switched and diode-pumped 2 μm solid state lasers are becoming of increasing interest for efficient pumping of mid-infrared emitting optical parametric oscillators (OPOs). In particular, Thulium and Holmium rare earths seem to be most suited for systems with high efficiency due to their long upper state lifetime. In addition, the Ho:YAG emission around 2.1 μm is not in the strong absorption spectral band of water and it is a suitable wavelength for non linear crystal pumping, such as ZnGeP2. Several works on Ho:YAG laser end-pumped by diode-pumped Tm:YLF laser have demonstrated high power operations1. To simplify the set-ups, experiments with Tm-Ho intracavity lasers have been done; they demonstrated a 36.5% slope efficiency2. Unfortunately these set-ups3 did not allow Q-switched operations and the thermal lens in the rods led to relatively poor beam quality (M2~5-6).
We designed an original intracavity configuration with a dichroic polarizing beamsplitter to decouple Tm:YLF and Ho:YAG cavities. This solution was to improve the beam quality and allow Q-switched operations. 1.9 W of 2.09 μm at the 17.3 W diodes pump level were obtained. The slope efficiency of the diode-pump to the Ho laser output and the optical-to-optical conversion efficiency achieved were respectively ~ 21.4 % and ~ 11 %. As anticipated 4, a shift of Tm:YLF emission was experimentally scaled from 1.908 to 1.953 μm which led to an efficiency decrease for the Tm laser. Unfortunately, in this intracavity geometry, Ho:YAG acted as a saturable absorber. Instead of a cw operation in free running, random Tm:YLF laser pulses of ~ 2.5 μs were observed that each resulted in an Ho pulse (~ 200-250 ns). When the acousto-optic modulator (AOM) worked, the Ho pulses did not follow the Q-switched frequency. In fact Ho emission depends on the Tm:YLF pump energy accumulated between two gates of the AOM.
Some possible ways to optimize the efficiency and to avoid the passive Q-switching behaviour of Ho:YAG are tested and first results are presented in this paper.
Q-switched and diode-pumped 2 μm solid state lasers are becoming of increasing interest for efficient pumping of mid-infrared emitting optical parametric oscillators (OPOs). In particular, Thulium and Holmium rare earths seem to be most suited for systems with high efficiency due to their long upper state lifetime. Several works on Ho:YAG laser end-pumped by diode-pumped Tm:YLF laser have demonstrated high power operations. To simplify the set-ups, experiments with Tm-Ho intracavity lasers have been done; they demonstrated a 36.5% slope efficiency. Unfortunately these set-ups did now allow Q-switched operations and the thermal lens in the rods led to relatively poor beam quality (M2 ~ 5-6). We design an original intracavity configuration with a dichroic polarizing beamsplitter to decouple Tm:YLF and Ho:YAG cavities. This solution improves the beam quality and allows Q-switched operations. We obtained 1.9 W of 2.09 μm at the 17.3 W diodes pump level. The slope efficiency of the diode-pump to the Ho laser output and the optical-to-optical conversion efficiency achieved are respectively ~ 21.4% and ~ 11%. As anticipated, we experimentally scaled a shift of Tm:YLF emission from 1.908 to 1.953 μm that leads to an efficiency decrease for the Tm laser. In this intracavity geometry, Ho:YAG acted as a saturable absorber. Instead of a cw operation in free running, we observed random Tm:YLF laser pulses of ~ 2.5 μs that each resulted in a Ho pulse (~ 200-250 ns). When the acousto-optic modulator (AOM) worked, the Ho pulses did not follow the Q-switched frequency. In fact Ho emission depends on the Tm:YLF pump energy accumulated between two gates of the AOM. Possible ways to optimize the efficiency and avoid the passive Q-switching behaviour of Ho:YAG are suggested.
Up to now, no technique was available to perform inventories and surveys of immersed macroalgae. This article presents a method making it possible to discriminate between macroalgae and sea-floor and to identify the various macroalgae groups. To our knowledge, this is the first EEM of macroalgae ever carried out allowing the identification of the spectral characteristics of each macroalgae group. Fluorescence imaging provides the spatial dimension besides spectral properties. An OPO laser excites a test scene of macroalgae in a sea-water aquarium. Laser-induced fluorescence images from 400 nm to 640 nm displayed by steps of 10 nm are filtered at 680 nm. Their analysis, combined to the water depth parameter, leads to an identification method for macroalgae groups in a false-color image: each of the three macroalgae groups is enhanced by one defined threshold on this false-color image.
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