High-power Laser Operation Research Articles

1.7 W holmium-doped fluoroindate fiber laser at 3920 nm.

A monolithic fiber laser emitting 1.7 W at 3920 nm is experimentally demonstrated in a Ho3+:InF3 fiber. The cavity comprises a pair of highly reflective fiber Bragg gratings written in the active fiber with the femtosecond phase-mask scanning technique and is spliced to the pump diode with a robust silica-to-fluoride fiber splice. This work is an important step toward high-power all-fiber laser operating in the vicinity of 4 µm.

Polarized spectral properties and 2.3 µm laser performance of the Tm:YVO4 crystal

The polarized spectral properties and ∼2.3 µm high-power continuous-wave laser operation of Tm3+-doped yttrium orthovanadate crystal (Tm:YVO4) are reported. For the 3H4 → 3H5 transition, the stimulated-emission cross-section σSE is 1.01 × 10−20 cm2 at 2276 nm corresponding to a large emission bandwidth of 52 nm (for π-polarization). Pumped by a 794 nm laser diode, the 1.5 at.% Tm:YVO4 laser delivered 5.52 W at 2.29 µm with a slope efficiency of 19.9%, a laser threshold of 8.70 W, and a linear laser polarization (π). The Tm laser operated on the cascade scheme (on the 3H4 → 3H5 and 3F4 → 3H6 transitions) which was mainly responsible for the observed high laser slope efficiency. We also report on the first passively Q-switched Tm:YVO4 laser at 2.3 µm by employing porous nano-grained cuprous selenide (PNG-Cu2Se) as a saturable absorber. The shortest pulse duration and the highest single pulse energy amounted to 706 ns and 3.65 µJ, respectively, corresponding to a pulse repetition rate of 62.8 kHz.

Efficient high-power 2.3 μm continuous-wave laser operation of diffusion-bonded composite YVO4/Tm:GdVO4 crystals

Abstract We reported on an efficient high-power continuous-wave laser operation on the 3H4 → 3H5 transition of Tm3+ ions in a diffusion-bonded composite YVO4/Tm:GdVO4 crystal. Pumped by a laser diode at 794 nm, a maximum output power of 7.5 W was obtained from a YVO4/Tm:GdVO4 laser at 2.29 μm, corresponding to a slope efficiency of 40.3% and exceeding the Stokes limit. To the best of our knowledge, this result represents the maximum power ever achieved from a Tm laser at 2.3 μm.

Assessing low laser damage densities of fused silica optics by means of a functional raster scan protocol.

The knowledge of the laser damage resistance of fused silica optics for their use in high-power lasers is of primary importance for the operation and maintenance of these facilities. Among the control procedures developed, one of the most relevant to date is the raster scan procedure [Lamaignère etal., Rev. Sci. Instrum. 78, 103105 (2007)]. This procedure is used to determine the damage density of optical components as a function of fluence. To date, this procedure takes into account all triggered damage sites, regardless of their size and/or morphology. We have added a step to this procedure, which consists in irradiating again all the damage sites with a series of shots to ascertain their ability to grow. This allows us to estimate the densities of growing damage sites, which are most critical for the safe operation of lasers. This pragmatic approach can be considered a functional test procedure. By applying this procedure to large optical areas, we were then able to measure extremely low damage densities (∼10-4 damage cm-2) over a wide range of fluences [0-18Jcm-2], corresponding to fluences to which the optics are irradiated during the operation of high-power lasers. We can then determine the damage law of a given set of optical components. This reference law makes it possible, on the one hand, to predict the lifetime of the optics. On the other hand, any deviation can then be analyzed with regard to this reference law. Thanks to the determination of confidence intervals, this functional procedure can also be used to delimit the reference law by upper and lower bounds.

Spectral Failsafe System of High-Power Laser Using Dual Fiber Bragg Gratings.

Phase-modulated (PM) spectral failsafe systems are necessary to promptly terminate amplification processes following accidental seeding of a high-power laser chain with a non-PM pulse to prevent optical damage. In this work, we present a reliable spectral failsafe system that can indicate the presence or absence of sufficient PM light. This requirement is met by combining dual temperature-sensitive fiber Bragg gratings detection with high-speed RF amplitude comparisons. The failsafe trigger signal is generated when the spectral power at the peak sideband exceeds that at the center. The spectral failsafe system has the ability to distinguish between adequate and inadequate PM pulses, and it exhibits significant robustness in pulse width, TEC temperature drift, and DFB wavelength drift in experiments, making it valuable for safe high-power laser operations and providing a useful reference for other detection system designs.

A compact optical sensor for risk assessment from high-power laser scattering

The outdoor operation of multi-kilowatt high-power lasers involves the potential hazard of exposing the human eye and skin to scattered radiation. In present laser safety standards and regulations, the effects of atmospheric off-axis scattering and the dynamic interaction between the laser beam and the target object are not adequately represented. Therefore, experimental studies are dedicated to providing quantitative information on scattered radiation for comprehensive laser safety evaluations. With this end in view, a mobile detection system has been developed that relies on the technical specifications (i.e. entrance aperture and field-of-view) of laser safety regulations. The mobile detection system is deployed at the German Aerospace Center (DLR) laser test range at Lampoldshausen. Atmospheric scattering experiments are performed by placing the calibrated detection system under specified scattering angles relative to the direction of the laser beam and determining the scattered radiation, i.e. absolute values for scattered laser power. In addition, the detection system is used for target scattering experiments. In these experiments, the sensor’s versatility is demonstrated by measuring the specularly and diffusely scattered laser radiation from metallic target objects. The dependence of the scattering power as a function of the scattering angle, distance, and laser output power is investigated during laser-matter interaction to demonstrate the capabilities of the system.

Direct generation of 5 W all-fiber red laser at 635 nm

We propose and experimentally demonstrate a compact high-power all-fiber Pr3+-doped red laser at 635 nm using highly-resistant dielectric-coated fiber mirror. The fiber laser consists of a 443 nm pump laser diode (LD), a double-clad Pr3+-doped fluoride fiber and a 443/635 nm dichroic fiber mirror. The 635 nm laser is directly generated by down-conversion gain of the Pr3+-doped fiber pumped by the 443 nm LD. The homemade dichroic fiber mirror is specifically designed for anti-reflection at 443 nm pump wavelength and high-reflection at 635 nm laser wavelength, and shows a high damaged density of >15 MW/cm2, which is very favorable to high-power visible laser operation. The red all-fiber laser directly generates a highest average power of ∼5 W at 635 nm which is more than twofold higher than previously reported, and the slope efficiency is as high as 25.7%. Different from the traditional visible laser by frequency doubling, this work offers a promising way for miniaturized, high-power fiber lasers in visible wavelength region.

Growth, spectroscopy and high-power laser operation of Er:YAP crystal with different Er3+ concentrations

Er3+-doped YAP crystals with concentrations of 5 at%, 10 at%, 15 at% and 20 at% were grown successfully by Czochralski method. X-ray rocking curves suggest high crystalline quality. Lifetime ratio of 4I13/2 and 4I11/2 levels decreases obviously with the increase of Er3+ concentrations, which is beneficial to realize population inversion and decrease the laser threshold. Spectroscopy and laser show the 15 at% Er:YAP crystal has strongest mid-infrared fluorescence intensity and best laser performance by 968-nm LD side-pumping. Maximum average powers of 23.52 and 11.08 W are achieved at 250 and 1000 Hz, corresponding to the slope efficiency of 12.0% and 6.2%, respectively. Therefore, the 15 at% Er:YAP crystal is a promising candidate for the high-power and high-frequency mid-infrared laser.

High-power continuous-wave Tm3+:Ho3+-codoped fiber laser operation from 2.1 µm to 2.2 µm

A Tm3+:Ho3+-codoped free-space single-oscillator fiber laser is under investigation with special focus on the power scalability of emission wavelengths from 2.1 µm to 2.2 µm. Using a tunable diffraction grating, a 200-nm tunable laser source is built. Laser output powers above 10 W are delivered from 1990 nm up to 2190 nm, demonstrating the range for stable high-power laser operation. By replacing the diffraction grating by a highly reflective, narrow linewidth volume Bragg grating, power scaling is performed at 2.1 µm and is even enabled at a wavelength of 2.2 µm. Using a volume Bragg grating (VBG) at 2.1 µm, a slope efficiency of 49% is measured with an output power of 262 W. Using another VBG with a center wavelength of 2.2 µm, the fiber laser delivers a record power of 77 W with a slope efficiency of 29%.

High-power and highly-efficient laser operation of Tm3+:Ho3+-codoped silica fiber lasers emitting at 2.1 μm and 2.2 μm

High-power and highly-efficient laser operation of Tm³⁺:Ho³⁺-codoped silica fiber lasers emitting at 2.1 μm and 2.2 μm

β-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters.

A series of β-TCBD (1,1,4,4-tetracyano-buta-1,3-diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H2-PE2, and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H2-PE2. The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of -0.06 to -0.10 V are the consequence of the TCBD moiety present at the β-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H2-PE2) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the M-TCBD series. The single-beam femtosecond Z-scan measurements were performed to elucidate the third-order nonlinear optical properties, and the temporal response of these porphyrin molecules was investigated using optical pump-probe spectroscopy to study the excited state absorption dynamics. Z-scan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (β) and the imaginary part of third-order nonlinear optical susceptibility (χ(3)) were obtained from the open aperture experiment, whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n2) and the real part of χ(3). Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90-2.33 × 1015 W/m2) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced Damage Thresholds.

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca3(TeO3)2(MO4) (M = Mo, W) were grown from TeO2-MO3 flux by high-temperature solution methods. Ca3(TeO3)2(MoO4) and Ca3(TeO3)2(WO4) are isostructural, which feature novel structures consisting of asymmetric MO4 tetrahedra and TeO3 trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 μm, and superhigh laser damage thresholds (1.63 GW/cm2 and 1.50 GW/cm2), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS2, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca3(TeO3)2(MO4) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca3(TeO3)2(MO4) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Mode-Field Matched Pump-Signal Combiner for High Power Fiber Laser in Advanced Manufacturing

Stable output and beam profile at high power laser operation is a prime requirement for precision high-end manufacturing. The stability of laser charateristics during power scaling in a single-oscillator is limited by several unavoidable non-linear effects. Hence, kW-level power scaling through multiple amplification stages or using signal combination are feasible solutions. In a master oscillator power amplifier (MOPA) architecture, pump-signal combiner with high pump coupling efficiency and low loss signal transmission maintaining seed beam quality during coupling from a single mode fiber into a large mode area fiber, is an important component. In this respect, an effective method of fabricating mode-field matched pump-signal combiner has been presented employing a section of intermediate fiber, having adiabatically tapered core and uniform cladding, along with a conventional signal fiber (termed hybrid signal fiber) inside the input fiber bundle to obtain effective mode-field matching of signal light in between the input fiber bundle and the output fiber. Additionally a taper-less signal combiner design has been proposed for scaling laser power through incoherent combination. The reduced-clad signal fiber with low loss core guidance supports bundling of the input fibers inside an economic low-OH silica tube to obtain good transmission efficiency and beam profile.

Internal optical loss and internal quantum efficiency of a high-power GaAs laser operating in the CW mode

We continue the comparative studies of high-power AlGaAs/InGaAs/GaAs semiconductor lasers with different waveguide designs. In this work, measurements were carried out for a continuous-wave mode of operation: light–current and current–voltage characteristics, threshold characteristics and their temperature dependences and crystal temperature depending on the pump current. The data obtained made it possible to reconstruct the dependences of internal optical losses and internal quantum efficiency using calculations. It was shown that the main factor affecting the power characteristics in the continuous mode of operation is the temperature stability of the laser and its relationship with internal optical losses and internal quantum efficiency. Thus, lasers based on a heterostructure with a doped GaAs waveguide with characteristic temperatures T 0 and T 1 of 120 K and 170 K, respectively, demonstrated an increase in internal optical losses to 2 cm−1 and a drop in the quantum efficiency to 90%, which led to saturation of the output power by high pump currents. Lasers based on the heterostructure with an AlGaAs waveguide, which are characterized by higher temperature stability (T 0 = 161 K and T 1 = 280 K), had lower internal optical losses and a higher quantum efficiency. This made it possible to obtain optical power 1 W higher than that for lasers with GaAs waveguides.

Graphene-chitin bio-composite polymer based mode locker at 2 micron region

In the field of pulsed fiber laser, graphene is a well-known two-dimensional (2D) material for its excellent optical properties. An alternative approach to the existing method, graphene based filament originally intended for 3-dimensional (3D) printing was used as starting material. Coupled with a newly introduced chitin nanofiber as the host polymer, it was demonstrated and reported as passive mode locker at 2-micron region. The conventional soliton operated at operating wavelength of 1982.7 nm with repetition rate of 11.35 MHz. The produced average output power, pulse width, time-bandwidth product (TBP) and signal to noise ratio (SNR) was 76.83 µW, 1.88 ps (HAC200), 0.416 and 43 dB, respectively. When the pulse was amplified with 5.4 dB of Thulium doped fiber amplifier (TDFA), the average output power increased to 3.43 mW and produced a broad operating wavelength around the 2-micron region. At the same repetition rate of 11.35 MHz, the measured pulse width, SNR, pulse energy and peak power of 7.033 ps (pulseCheck 150), 42.0 dB, 0.30 nJ, and 42.98 W, are obtained, respectively. High power laser operation in this region can find applications in medical field and sensors technology.

Diode End-Pumped 1.37 W Er:YSGG Continuous-Wave Laser Around 2.8-μm

We demonstrate the laser performance of a LD end-pumped Er:YSGG crystals with different doping concentrations comparatively. The experimental results show that 30 at. % Er:YSGG crystal with dimensions of 2 mm ×2 mm ×8 mm has the best laser output. In continuous-wave (CW) mode and pulse mode, the maximum output powers of 1.37 and 1.20 W are achieved, corresponding to slope efficiencies of 23.62% and 22.91%, respectively. To the best of our knowledge, compared with other Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped garnet laser crystals, we obtained a larger power and higher efficiency CW laser simultaneously for LD end-pumped Er:YSGG crystal. The beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in x and y directions are determined to be 1.64/1.65, suggesting the Er:YSGG laser possesses a higher beam quality. All these results indicate that Er:YSGG is a promising mid-infrared gain medium for high efficiency and high power laser operation.

The investigations of Dy:YAG and Dy,Tb:YAG as potentially efficient GaN blue LD pumped solid state yellow laser crystals

With the fast development of GaN blue laser diode, Dy3+ doped laser crystals are receiving renewed interests due to their potential yellow laser emission under GaN laser diode pumping. In this study, Dy:YAG and Dy,Tb:YAG with high quality have been grown successfully with Czochralski method. Their spectroscopic properties as well as the energy transfer mechanism between Dy3+ and Tb3+ were discussed in details. The 583 nm emission cross-section of Dy:YAG crystal is improved by co-doping of Tb3+, which indicates the efficient quasi-resonant energy transfer between the Dy3+-6H13/2 state and Tb3+-7F4 state is realized. The thermal properties of Dy:YAG, including specific heat and thermal conductivity, were investigated. The high thermal conductivity suggests the Dy:YAG crystal should be very suitable for high-power laser operation. In addition, considering the high phonon energy of Dy:YAG and Dy,Tb:YAG crystals, the depletion of population on the lower laser level (6H13/2) of Dy3+ could be accelerated. The results suggesting that both Dy:YAG and Dy,Tb:YAG could be highly appraised as excellent competitors for all solid state yellow lasers.

Photonic bandgap engineering using second-order supersymmetry

First-order supersymmetry (SUSY) adapted from quantum physics to optics manipulates the transverse refractive index of guided-wave structures using a nodeless ground state to obtain intended modal content. Second-order SUSY can be implemented using excited states as a seed function, even with the presence of nodes. We apply second-order SUSY to the coupled-mode equations by recasting them as the Dirac equation. This enables the engineering of non-uniform surface corrugation of waveguide gratings and coupling potential, which encapsulates the Bragg interaction between counterpropagating modes. We show that the added bound states appear as transmission resonances inside the bandgap of the finite grating. The probability density of each state provides the longitudinal modal energy distribution in the waveguide grating. The smooth modal energy distribution of the states obtained by SUSY can mitigate longitudinal spatial hole burning in high power laser operation. We demonstrate that degenerate second-order SUSY allows the insertion of two states, which can coalesce into Friedrich-Wintgen type bound states in the continuum (BIC) for one-dimensional grating. We show that the eigenfunctions of BIC states are doubly degenerate with opposite parity, and the corresponding transmission resonances have phase changes of 2π across these states. One-dimensional BIC states can find application as robust high-speed all-optical temporal integrators by lifting restrictions on the length of various sections in the phase-shifted grating.

Automation of Target Delivery and Diagnostic Systems for High Repetition Rate Laser-Plasma Acceleration

Fast solid target delivery and plasma-ion detection systems have been designed and developed to be used in high intensity laser-matter interaction experiments. We report on recent progress in the development and testing of automated systems to refresh solid targets at a high repetition rate during high peak power laser operation (&gt;1 Hz), along with ion diagnostics and corresponding data collection and real-time analysis methods implemented for future use in a plasma-based ion acceleration beamline for multidisciplinary user applications.

Holographic reflective Bragg gratings of gain and their role in the operation of high-power pulsed lasers

The influence of reflective Bragg gratings that are formed in an active medium due to the spatial periodic modulation of gain on the spectral properties of lasing are investigated by an example of a pulsed Nd : YAG laser. It is shown that, at a certain choice of the formation regime of reflective gratings of gain, the lasing bandwidth can be reduced by a factor of 3 – 5, with a corresponding increase in the pulse energy and significant improvement of spatial beam quality.