We show for the first time that multi-ten Watt operation of an Alexandrite laser can be achieved with direct red diode-pumping and with high efficiency. An investigation of diode end-pumped Alexandrite rod lasers demonstrates continuous-wave output power in excess of 26W, more than an order of magnitude higher than previous diode end-pumping systems, and slope efficiency 49%, the highest reported for a diode-pumped Alexandrite laser. Wavelength tuning from 730 to 792nm is demonstrated using self-seeding feedback from an external grating. Q-switched laser operation based on polarization-switching to a lower gain axis of Alexandrite has produced ~mJ-pulse energy at 1kHz pulse rate in fundamental TEM00 mode.

High efficiency >26 W diode end-pumped Alexandrite laser

Measurements of two-photon fluorescence excitation (TPE) spectra are presented for 11 common molecular fluorophores in the excitation wavelength range 690 nm < λ < 1050 nm. Results of excitation by ∼100-fs pulses of a mode-locked Ti:sapphire laser are corroborated by single-mode cw Ti:sapphire excitation data in the range 710 nm < λ < 840 nm. Absolute values of the TPE cross section for Rhodamine B and Fluorescein are obtained by comparison with one-photon-excited fluorescence, assuming equal emission quantum efficiencies. TPE action cross sections for the other nine fluorophores are also determined. No differences between one-photon- and two-photon-excited fluorescence emission spectra are found. TPE emission spectra are independent of excitation wavelength. With both pulsed and cw excitation the fluorescence emission intensities are strictly proportional to the square of the excitation intensity to within ±4% for excitation intensities sufficiently below excited-state saturation.

Measurement of two-photon excitation cross sections of molecular fluorophores with data from 690 to 1050 nm

https://iopscience.iop.org/article/10.1088/1612-202X/aa857f/pdf

High power continuous-wave dual-wavelength alexandrite laser

Cr: Colquiriite Lasers: Current Status and Challenges for Further Progress

In this study, we numerically and experimentally investigate application of birefringent filters (BRFs) as frequency selective elements in multicolor lasers. A BRF plate made out of crystalline quartz with an arbitrarily oriented optical axis has been explored. Simulation results have shown that compared to regular BRFs where the optical axis lies in the plane of the plates surface, a BRF with an optical axis pointing out of its surface enables design flexibility in filter parameters, providing access to a wider set of free spectral range and bandwidth values. As a result, multicolor operation could be obtained in many wavelength pairs using a single BRF plate. In the experiments a 3-mm thick quartz BRF with an optical axis 45° to the surface plane has been used. With Cr:Nd:GSGG as a laser medium two-color and three-color cw laser operation has been demonstrated in 11 and 3 different transition combinations, respectively. Moreover, two-color laser operation has been demonstrated in 10 different wavelength pairs in Cr:LiSAF. To our knowledge, this study is the first detailed investigation and experimental demonstration of BRFs with tilted optical axis for multicolor operation of solid-state lasers. Compared to other methods, BRFs enable a rich selection of transition pairs and also the ratio of the power in each line could be regulated by fine adjustment of the rotation angle. Implementation of tilted-axis BRFs should boost development of efficient and low-cost multicolor lasers in other gain media as well.

Multicolor lasers using birefringent filters: experimental demonstration with Cr:Nd:GSGG and Cr:LiSAF

Low-cost multi-mode diode pumped Tm:YLF laser: Multi-color & Q-switching operations

We present low-cost and highly efficient continuous-wave Alexandrite lasers pumped by recently developed high-brightness diodes. A 19-mW lasing threshold, 38% slope efficiency, 200 mW output power, and a broad tuning range (724-816 nm) were realized.

Efficient and Low-Threshold Alexandrite Lasers Pumped by High-Brightness Diodes

We report a continuous-wave operation of Tm:YAG and Tm:LuAG lasers pumped with a low-cost, multimode AlGaAs laser diode. First, the lifetime and the absorbance behavior of 5 mm, 6% Tm -doped YAG and LuAG crystals were thoroughly investigated. A low-cost multimode 3W laser diode at 781 nm was then used as a pump source for the Tm -doped laser systems. Using three different output couplers, up to 636 mW of output power was obtained from Tm:YAG laser, with a slope efficiency of 29% at 2017 nm. The maximum output power was 637 mW in the Tm:LuAG laser, with a slope efficiency of 28% at 2023 nm. The lasing performances showed a decreasing slope efficiency with an increasing level of output coupling, which leads to a high upconversion. Furthermore, using a birefringent filter in the resonators, the laser outputs were tuned from 1942 to 2086 nm in the Tm:YAG resonator and from 1931 to 2107 nm in the Tm:LuAG case.

/pdf/low-cost-multimode-diode-pumped-tm-yag-and-tm-luag-lasers-kvkxoi9u36.pdf

Low-cost multimode diode-pumped Tm:YAG and Tm:LuAG lasers

Eye-safe, diode-pumped all-solid-state lasers working in the 2 μm region are favourable for many applications including remote sensing, medicine, and gas sensing. Tm-doped crystals such as Tm:YAG and Tm:LuAG are among the commonly employed laser gain media for this purpose. These systems are usually pumped by laser diodes operating around 780 nm [1]. The corresponding stokes limit for laser slope efficiency is then about 39%. However, for highly-doped samples, the two-for-one cross-relaxation process enables laser efficiencies exceeding this limit, and a slope efficiency of 59% has been reported from a 785 nm Ti:Sapphire pumped Tm:YAG laser, indicating a cross-relaxation efficiency of around 1.5 (one 780 nm pump photon generating 1.5 laser photons around 2 μm) [2]. As an alternative pumping approach, Stoneman et al. pointed out the possibility to pump Tm-based lasers around 681 nm, due to the much stronger absorption line at this wavelength [3]. Compared to 785 nm pumping, using the stronger 681 nm line warrants utilization of shorter Tm-crystals, which minimizes self-absorption losses [3]. Moreover, 681 nm pumping could also facilitate usage of thinner crystals in thin-disk geometry and/or could decrease the required number of bounces on the thin-disk for sufficient absorption of the pump light [4]. Moreover, pumping at a wavelength with stronger absorption coefficient creates a more densely populated gain medium, which could increase the effectiveness of cross-relaxation process. As a drawback, the stokes limited slope efficiency reduces to around 34% for 681 nm pumping, and decreased absorption lengths might also induce stronger thermal effects. In their early work, Stoneman et al. used a Ti:Sapphire laser operating at 681 nm to pump a Tm:YAG crystal, but the obtained slope efficiency was only around 35%, indicating a weak cross-relaxation process [3].

Efficient Tm:YAG and Tm:LuAG Lasers Pumped by Red Tapered Diodes

Dual-wavelength operation of lasers are attractive for many applications such as coherent terahertz wave generation, ultrahigh pulse repetition rate creation by optical beating, and laser ranging. Several methods could be employed for achieving multicolor laser operation including usage of specially coated cavity optics, employment of dual laser crystals, utilization of coupled cavities, and deployment of Fabry-Perot band-pass filters or volume Bragg gratings. Most of the time, these approaches provide multicolor laser operation at a single wavelength pair, and laser power in each line might fluctuate with experimental conditions (such as crystal temperature and pump power variations). In recent years, usage of intracavity birefringent filters (BRFs) was proposed as a flexible method for the generation of multicolor laser operation [1–4]. Advantages of BRFs in multicolor lasing include: (a) low cost, (b) simple operation, (c) one BRF plate empowering multicolor laser operation in many different wavelength pairs, (d) capability to control laser power in each line, (e) ability to use a rich number of filter parameters from a single BRF plate, (f) effectiveness in continuous-wave, gain-switched, Q-switched and cw mode-locked regimes, and (g) universal usage of the device in any laser that lies within the transmission bandwidth of the BRF plate. In our earlier work, we have employed a 3-mm thick, off-surface optic axis crystalline quartz BRF with an optic axis 45° to the surface of the plate to generate two-color cw laser operation in 11 and 10 different transition combinations in Cr:Nd:GSGG and Cr:LiSAF [3], respectively. In this study [5], we have demonstrated that the same BRF plate could also be used for multicolor laser operation of solid-state lasers around 2 μm. Optimization of the rotation angle of the intracavity inserted BRF plate around different orders (rotation angles) facilitated two-color laser operation in 11, 12 and 8 different wavelength pairs in Tm:YLF, Tm:LuAG and Tm:YAG, respectively [5]. For most of the cases, fine tuning of the BRF rotation angle enabled adjustment of laser power in each line. Figure 1 (a) shows sample optical spectra for Tm:YLF, and Fig. 1 (b) demonstrates details of cavity dynamics for synchronous dual-wavelength operation of Tm:YLF at 1868 & 1991 nm pair. To our knowledge, this is the first report of multicolor laser operation in Tm:LuAG and Tm:YAG, and first results with a BRF in Tm:YLF [6]. Moreover, the demonstrated wide-tunability of two-color laser wavelengths is quite rare in literature.

Widely-Tunable Dual-Wavelength Tm:YLF, Tm:LuAG, and Tm:YAG Lasers

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