Properties of Cr:LiSrAIF(6) crystals for laser operation

Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2 cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35 W) limited the cw powers we could achieve experimentally to 2.25 W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10 W level.

Site-splitting inhibition and near-infrared luminescence properties of Cr3+ activated magnetoplumbite SrAl12O19 modified by La and Mg

Near-infrared phosphor-converted light-emitting diodes (pc-LEDs) are attractive in application like in vivo imaging and nondestructive examination in food industry. Plenty of aluminium oxides with high melting point and rigidity have been researched towards high photoluminescence quantum yield and thermal stability. In this work, a series of Cr³⁺ activated La, Mg modified SrAl₁₂O₁₉ phosphors, i.e. Sr_0.7La_0.3Mg_0.3Al_11.7O₁₉, are prepared via a solid-state reaction route. The site splitting and structural evolution towards relative thermodynamic stability in the modified host are discussed, Cr³⁺ ions with multisite occupation in the host show intense zero-phonon-line (ZPL) emission and tunable broadband emission. The optimized phosphor exhibits excellent luminescence thermal stability, with 98% residual integrity intensity of that at room temperature at 473 K. The results provide a reference for developing novel NIR-emitting magnetoplumbite phosphor with high efficiency and excellent thermal quenching resistance.

LED-pumped Cr:LiSAF laser system operating at 100 Hz based on a multipass amplifier

The LED-pumping technology is used for the first time, to the best of our knowledge, to develop a complete master oscillator power amplifier (MOPA) system including a multipass amplifier. A pumping head using an original slab architecture is developed integrating a Cr:LiSAF slab pumped by 2112 blue LEDs via a Ce:YAG luminescent concentrator. The slab configuration enables the reaching of a large number of passes-up to 22-together with access to efficient cooling, allowing for a repetition rate scale up. For 22 passes, the amplifier delivers pulses with energy up to 2.4 mJ at 10-Hz repetition rate with a gain of 4.36 at 825 nm. A complete study of the MOPA is described, concluding in nearly constant performances versus the repetition rate, up to 100 Hz.

Mode-locked Cr:LiSAF laser far off the gain peak: tunable sub-200-fs pulses near 1 µm

We report, to the best of our knowledge, the first mode-locking results of a Cr:LiSAF laser near the 1 µm region. The system is pumped only by a single 1.1 W high-brightness tapered diode laser at 675 nm. A semiconductor saturable absorber mirror (SESAM) with a modulation depth of 1.5% and non-saturable losses below 0.5% was used for mode-locking. Once mode-locked, the Cr:LiSAF laser produced almost-transform-limited sub-200-fs pulses with up to 12.5 mW of average power at a repetition rate of 150 MHz. Using an intracavity birefringent filter, the central wavelength of the pulses could be smoothly tuned in the 1000-1020 nm range. Via careful dispersion optimization, pulse widths could be reduced down to the 110-fs level. The performance in this initial study was limited by the design parameters of the SESAM used, especially its passive losses and could be improved with an optimized SESAM design.

DFT calculation of 229thorium-doped magnesium fluoride for nuclear laser spectroscopy

The ²²⁹thorium nucleus has an extremely low-energy isomeric state that could be manipulated with light in the vacuum ultraviolet (VUV) range. Recent measurements based on internal conversion electrons place the isomer energy at 8.28(17) eV (Seiferle B et al 2019), within the transmission window of large-band-gap materials, such as fluoride single crystals. Doping ²²⁹Th into VUV-transparent materials realizes a spectroscopy target with a high nuclei density and might form the basis of a solid-state nuclear clock. This paper presents a theoretical study of the optical properties of a thorium-doped MgF₂ crystal. Using the Vienna Ab-initio Simulation Package, we perform density functional theory calculations of the electronic and optical properties of Th:MgF₂. We determine whether thorium will be accepted as a dopant and identify the charge compensation mechanism and geometry. The simulations indicate, that the band gap of Th-doped MgF₂ will be significantly reduced compared to undoped MgF₂, below the expected ²²⁹Th isomer energy. This result is in striking contrast to a similar study performed for Th-doped CaF₂ (Dessovic P et al 2014 J. Phys. Condens. Matter 26 105402).

High-power, frequency-doubled Nd:GdVO4 laser for use in lithium cold atom experiments

We report on an 888 nm-pumped Nd:GdVO₄ ring laser operational over a wavelength range from 1340.3 nm to 1342.1 nm with a maximum output power of 7.4 W at 1341.2 nm and a beam quality parameter M²<1.1. To our knowledge this is the highest single-longitudinal-mode power obtained with a Nd:GdVO₄ crystal laser. We use a commercial frequency-doubling cavity to achieve 1.2 W at 671.0 nm and 4.0 W at 670.6 nm for use in lithium cold atom experiments. Respectively, these wavelengths are approximately resonant with and 250 GHz blue-detuned from the lithium D-lines. Thus, this source provides ample power for laser cooling of lithium atoms while also offering substantial power for experiments requiring light 10's to 100's of GHz blue-detuned from the primary lithium transitions.

Power scaling potential of continuous-wave Cr:LiSAF and Cr:LiCAF lasers in thin-disk geometry

Continuous-wave (cw) output power from Cr:Colquiriite lasers is currently limited to the 3 W level due to weak thermal and mechanical properties of the Colquiriite host that hinders power scaling efforts. Thin-disk (TD) geometry, which has enabled record power levels in Yb:YAG gain media, has not yet been thoroughly studied for Cr:Colquiriites. In this work, we have numerically investigated the cw lasing potential of TD Cr:LiSAF and Cr:LiCAF lasers in detail. Our analysis has shown that, due to their low thermal conductivity and high susceptibility to temperature quenching of fluorescence lifetime, TD lasers based on Cr:LiSAF will require usage of ultrathin (100 μm thick) crystals with large diameters (15-25 mm), which will be difficult to prepare/handle due to its glass-like thermomechanical strength. Assuming such TD crystals could be produced, we estimate that cw powers above 15 W might be feasible from Cr:LiSAF thin-disk lasers (TDLs) pumped by a 100 W red diode. As an alternative, usage of Cr:LiCAF, which is the strongest member of Cr:Colquiriites in terms of thermomechanical properties, enables tighter focusing of a pump beam on the TD, which in turn facilitates adoption of smaller-diameter samples (10 mm). We have estimated that, when the thermally stronger c axis is used for heat extraction and the a axis is used for lasing, cw powers exceeding 30 W could be achieved from Cr:LiCAF TDLs at an incident pump power of 100 W. On the other hand, our model shows that the small signal gain that could be achieved via cw pumping of Cr:Colquiriite TD elements is below 0.5%. Hence, the aforementioned performance requires usage of high-quality factor cavities with minimal intracavity losses.

The generation of amplified spontaneous emission in high-power CPA laser systems

An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high-intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high-intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high-contrast front-end design are derived regarding the necessary contrast improvement and the amplified "clean" output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high-intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.

2.1-watts intracavity-frequency-doubled all-solid-state light source at 671 nm for laser cooling of lithium

We present an all-solid-state laser source emitting up to 2.1 W of single-frequency light at 671 nm developed for laser cooling of lithium atoms. It is based on a diode-pumped, neodymium-doped orthovanadate (Nd:YVO(4)) ring laser operating at 1342 nm. Optimization of the thermal management in the gain medium results in a maximum multi-frequency output power of 2.5 W at the fundamental wavelength. We develop a simple theory for the efficient implementation of intracavity second harmonic generation, and its application to our system allows us to obtain nonlinear conversion efficiencies of up to 88%. Single-mode operation and tuning is established by adding an etalon to the resonator. The second-harmonic wavelength can be tuned over 0.5 nm, and mode-hop-free scanning over more than 6 GHz is demonstrated, corresponding to around ten times the laser cavity free spectral range. The output frequency can be locked with respect to the lithium D-line transitions for atomic physics applications. Furthermore, we observe parametric Kerr-lens mode-locking when detuning the phase-matching temperature sufficiently far from the optimum value.

Angular dependence of the thermal-lens effect on LiSrAlF6 and LiSrGaF6 single crystals

We apply thermal-lens (TL) spectrometry to measure the angular dependence of the TL effect on colquiriite single crystals. The experiments were performed with LiSrAlF(6) and LiSrGaF(6) using a two-beam mode-mismatched configuration. The results show that it is possible to minimize the TL effect by selecting the appropriate crystal orientation. Our data also show that the anisotropy of the linear thermal expansion coefficient drives the amplitude of the TL effect, including the inversion from focusing to defocusing as the crystal orientation angle tends to the c-axis direction. The results may be useful for those working to develop a high-power laser using LiSrAlF(6)(:Cr) and LiSrGaF(6)(:Cr) single crystals, allowing for optimization of the designed laser cavity.

30 W Cr:LiSrAlF 6 flashlamp-pumped pulsed laser

We report the performance of a flashlamp-pumped Cr:LiSrAlF(6) (Cr:LiSAF) laser developed and built by us. The pumping cavity incorporates filters that select the flashlamps' emission spectrum to match the absorption bands of the gain medium, allowing control of the amount of nonradiactive decay heat contribution of the optical cycle, minimizing thermal effects on the laser operation. The laser generated 2 J pulses at 15 Hz, resulting in 30 W of average power, the highest power extracted from a Cr:LiSAF rod laser to our knowledge. We were able to conclude that the laser efficiency is affected by resonator configuration changes due to thermal lens effects, and not to thermal quenching of the Cr:LiSAF luminescence.

Development of a flashlamp-pumped Cr:LiSAF laser operating at 30 Hz

Cr3+:LiSrAlF6 crystals are an interesting laser medium because of their spectroscopic characteristics: They present a broad emission band in the near infrared and can be pumped either by a flashlamp or by diodes. Up to now, their limitation has been mostly due to their poor thermal properties that limit the laser performance either in the repetition rate in a pulsed system or output power in cw systems. We have designed and constructed a flashlamp-pumped laser using a standard rod pumping cavity that avoids most of the heat generated in the pumping process and allows operation at a fairly high repetition rate of 30 Hz with a high average power of 20 W in a conservative operation mode.

Fluorescence quantum efficiency measurements in the presence of Auger upconversion by the thermal lens method

Mode-mismatched thermal lens (TL) measurements in Cr3+-doped fluoride crystals (LiSrAIF6 and LiSrGaF6) are reported. A nonlinear increase of the TL signal, and decrease of quantum efficiency, with increasing excitation power was observed and attributed to energy-transfer upconversion (ETU). Assuming an upconversion rate that is proportional to the excited-state population, Wup = gammaN(e), the theoretical model developed fits the experimental data well. The ETU parameters (gamma) were determined with unprecedented accuracy for low Cr concentrations. Thermo-optical parameters (K, ds/dT) were also obtained.

Chromium-doped LiCAF laser passively Q switched with a V3+:YAG crystal

A tunable, lamp-pumped Cr3+:LiCaAlF6 laser that operates in the 760-800-nm wavelength range is demonstrated. As a passive Q switch the V3+:YAG crystal, characterized by wide saturable absorption in the 760-900-nm wavelength range, was applied. The V3+:YAG parameters were determined by spectroscopic and saturable transmission measurements. The 30-mJ output energy at 2-Hz repetition rate in the free-running regime was obtained in a cavity reinforced by a diaphragm near the fundamental mode. The dispersive prism inserted into the cavity enabled tuning in the 760-800-nm wavelength range. In the passive Q-switching regime we achieved greater than 1-mJ energy with 50-ns pulse duration in a 35-cm-long cavity. A numerical model that takes into account the short recovery time of V3+:YAG (approximately 5 ns) and excited-state absorption was used to analyze such a laser.

Unstable Cr:LiSAF laser resonator with a variable reflectivity output coupler

The performance of a flash-lamp-pumped Cr:LiSAF unstable laser resonator utilizing a fourth-order super-Gaussian variable reflectivity mirror as an output coupler is described. The super-Gaussian mirror results in a smooth, flattop transverse beam profile in the near field that is advantageous for nonlinear frequency-conversion applications. Long-pulse and Q-switched operation of the Cr:LiSAF unstable laser resonator are described and compared with stable resonator operation. We obtained tunable ultraviolet radiation extending from 267 to 290 nm by frequency mixing theQ-switched Cr:LiSAF laser output with lithium triborate and beta-barium borate nonlinear crystals.

Broadly tunable femtosecond Cr:LiSAF laser

We demonstrate wavelength tunability of femtosecond pulses over a range of more than 50 nm from a diode-pumped Cr:LiSAF laser. The mode-locking mechanism can be explained by soliton mode locking that is started and stabilized by a broadband semiconductor saturable-absorber mirror designed for low loss over the full reflection bandwidth of the underlying Bragg mirrors, which ultimately limits tunability. We obtain 45-fs pulses at 105-mW average output power and a maximum mode-locked average output power of 125 mW with 60-fs pulses.

1.1-W cw Cr:LiSAF laser pumped by a 1-cm diode array

We demonstrate 1.1-W cw output power from a diode-laser array-pumped Cr:LiSAF laser based on a concept that allows for pumping low-gain solid-state lasers at reduced temperature rise. We discuss scaling to higher powers as a function of diode power and define a figure of merit for evaluating given diode lasers as pump sources for low-gain solid-state lasers.

Small-signal gain investigations for a continuous-wave diode-pumped Q-switched Cr:LiSAF laser

We measured small-signal gain in a cw diode-pumped Cr:LiSAF and showed that upconversion and thermal quenching of fluorescence strongly limit small-signal gain. Then we optimized the gain in a Cr:LiSAF laser pumped by two 400-mW red diodes. In Q-switched operation, this laser produced tunable nanosecond pulses between 800 and 900 nm. At 850 nm, we obtained 230-ns pulses with an energy of 6.5 microJ at a repetition rate of 10 kHz.

Theory and optimization of lens ducts

Lens ducts are simple optical devices that have found application in the coupling of pump radiation from extended two-dimensional semiconductor laser diode arrays into solid-state laser gain media. The operation of these devices relies on the combined effects of lensing at their curved input surface and channeling by total internal reflection off their canted planar sides, to contain and couple semiconductor diode laser light efficiently to the input face of a solid-state laser crystal or glass. The lens duct provides a robust method for amplifying the irradiance of laser diode array pump sources and has made possible a scalable diode end-pumping architecture that offers the opportunity to expand significantly the number of ions and transitions that can be practically engaged in diode-pumped solid-state laser systems. An analytic model that describes the transfer efficiency of lens ducts and aids in the optimization of their design is presented.

Single-mode continuous-wave Cr(3+):LiSAF ring laser pumped by an injection-locked 670-nm broad-area diode laser

We report on an efficient continuous-wave Cr(3+):LiSrAlF(6) ring laser pumped by the near-diffraction-limited output of an injection-locked 665-nm broad-area diode laser. With an absorbed pump power of 120 mW the ring laser generates 22 mW of single-mode 855-nm radiation with a spectral linewidth of less than 10 MHz. Unidirectional operation and longitudinal-mode control of this ring laser is achieved by injection of 855-nm diode-laser radiation into the ring cavity. Optical feedback of ring-laser radiation into the diode laser narrows the diode laser's linewidth and locks its frequency to a resonance of the ring resonator.

All-solid-state continuous-wave tunable blue-light source by intracavity doubling of a diode-pumped Cr:LiSAF laser

We describe a diode-pumped cw Cr(3+):LiSrAlF(6) laser that produces 190-mW cw output at 860 nm. By frequency doubling in a KNbO(3) crystal we generate 13 mW of second-harmonic light tunable from 427 to 443 nm.