Microchip Yb:CaLnAlO4 lasers with up to 91% slope efficiency

In-band pumped Kerr-lens mode-locked Tm,Ho-codoped calcium aluminate laser

We report on a Kerr-lens mode-locked Tm,Ho-codoped calcium aluminate laser with in-band pumping of the Tm ions by a spatially single-mode 1678 nm Raman fiber laser. The structurally disordered CaGdAlO4 host crystal is also codoped also with the passive Lu ion for additional inhomogeneous line broadening. The Tm,Ho,Lu:CaGdAlO4 laser generates soliton pulses as short as 79 fs at a central wavelength of 2073.6 nm via soft-aperture Kerr-lens mode-locking. The corresponding average output power amounts to 91 mW at a pulse repetition rate of ∼86 MHz. The average output power can be scaled to 842 mW at the expense of slightly longer pulses of 155 fs at 2045.9 nm, which corresponds to a peak power of ∼58 kW. To the best of our knowledge, this represents the first demonstration of an in-band pumped Kerr-lens mode-locked Tm,Ho solid-state laser at ∼2 µm.

Growth, spectroscopy and SESAM mode-locking of a "mixed" Yb:Ca(Gd,Y)AlO4 disordered crystal

We present the growth, spectroscopy, continuous-wave (CW) and passively mode-locked (ML) operation of a novel "mixed" tetragonal calcium rare-earth aluminate crystal, Yb3+:Ca(Gd,Y)AlO4. The absorption, stimulated-emission, and gain cross-sections are derived for π and σ polarizations. The laser performance of a c-cut Yb:Ca(Gd,Y)AlO4 crystal is studied using a spatially single-mode, 976-nm fiber-coupled laser diode as a pump source. A maximum output power of 347 mW is obtained in the CW regime with a slope efficiency of 48.9%. The emission wavelength is continuously tunable across 90 nm (1010 - 1100 nm) using a quartz-based Lyot filter. With a commercial SEmiconductor Saturable Absorber Mirror to initiate and maintain ML operation, soliton pulses as short as 35 fs are generated at 1059.8 nm with an average output power of 51 mW at ∼65.95 MHz. The average output power can be scaled to 105 mW for slightly longer pulses of 42 fs at 1063.5 nm.

Semiconductor saturable absorber mirror mode-locked Yb:YAP laser

We report on sub-30 fs pulse generation from a semiconductor saturable absorber mirror mode-locked Yb:YAP laser. Pumping by a spatially single-mode Yb fiber laser at 979 nm, soliton pulses as short as 29 fs were generated at 1091 nm with an average output power of 156 mW and a pulse repetition rate of 85.1 MHz. The maximum output power of the mode-locked Yb:YAP laser amounted to 320 mW for slightly longer pulses (32 fs) at an incident pump power of 1.52 W, corresponding to a peak power of 103 kW and an optical efficiency of 20.5%. To the best of our knowledge, this result represents the shortest pulses ever achieved from any solid-state Yb laser mode-locked by a slow, i.e., physical saturable absorber.

Advances of Yb:CALGO Laser Crystals

Yb:CaGdAlO4, or Yb:CALGO, a new laser crystal, has been attracting increasing attention recently in a myriad of laser technologies. This crystal features salient thermal, spectroscopic and mechanical properties, which enable highly efficient and safe generation of continuous-wave radiations and ultrafast pulses with ever short durations. More specifically, its remarkable thermal-optic property and its high conversion efficiency allow high-power operation. Its high nonlinear coefficient facilitates study of optimized mode locking lasers. Besides, its ultrabroad and flat-top emission band benefits the generation of complex structured light with outstanding tunability. In this paper, we review the recent advances in the study of Yb:CALGO, covering its physical properties as well as its growing applications in various fields and prospect for future development.

Yb:GdScO3 crystal for efficient ultrashort pulse lasers

We present, to the best of our knowledge, the first demonstration of thermal, optical, and laser properties of Yb:GdScO₃ for potentially efficient ultrashort pulse lasers. The stimulated emission cross section at 1025 nm (E//c) is 0.46×10^-20cm² with the emission band width of 85 nm, even broader than the well-known Yb:CaGdAlO₄. It has quite a high thermal conductivity of 5.54W/(m⋅K) at 50°C, comparable with Yb:YAG. In the continuous-wave regime, the maximum output power of 13.45 W at 1063.9 nm was generated with the optical-to-optical efficiency of 63.3%. These results suggest that the Yb:GdScO₃ crystal is a promising candidate for ultrashort pulse lasers.

Spectroscopy and high-power laser operation of a monoclinic Yb3+:MgWO4 crystal

Monoclinic (wolframite-type) monotungstate crystals are promising for rare-earth doping. We report polarized room- and low-temperature spectroscopy and efficient high-power laser operation of such a ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb³⁺:MgWO₄ crystal featuring high stimulated emission cross section (${\sigma _{\rm SE}}\; = \;{6}.{2}\; \times \;{{10}^{ - 20}}\;{{\rm cm}^2}$σ_SE=6.2×10^-20cm² at 1056.7 nm for light polarization ${\rm E}\;||\;{N_m}$E||N_m), large Stark splitting of the ground state (${765}\;{{\rm cm}^{ - 1}}$765cm^-1), large gain bandwidth (26.1 nm for ${\rm E}\;||\;{N_g}$E||N_g), and strong Raman response (most intense mode at ${916}\;{{\rm cm}^{ - 1}}$916cm^-1). A diode-pumped ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb³⁺:MgWO₄ laser generated 18.2 W at ${\sim}{1056}\;{\rm nm}$∼1056nm with a slope efficiency of ${\sim}{89}\% $∼89% and a linearly polarized laser output.

Wavelength shift with a diode-pumped continuous-wave Yb:CALGO laser

The dependence of an emission wavelength on the crystal temperature was first investigated for a diode-pumped continuous-wave ${{\rm Yb}^{3 + }}$Yb³⁺-doped ${{\rm CaGdAlO}_4}$CaGdAlO₄ (Yb:CALGO) laser. A maximum output power of 11 W was obtained, corresponding to a slope efficiency of 19.8%. The output wavelength varied from 1051.10 nm to 1054.72 nm with increased absorbed pump power. This wavelength shift is attributed to a change in the crystal temperature. This observation is, to our best knowledge, an original conclusion about this phenomenon in a Yb:CALGO laser. We use a temperature-dependent model to explain the emission wavelength shifts that can be generalized to any such quasi-three-level materials.

Ultrafast laser inscribed waveguide lasers in Tm:CALGO with depressed-index cladding

Depressed-index buried and surface channel waveguides (type III) are produced in a bulk 3.5 at.% Tm³⁺:CALGO crystal by femtosecond direct-laser-writing at kHz repetition rate. The waveguides are characterized by confocal microscopy and µ-Raman spectroscopy. Under in-band-pumping at 1679 nm (³H₆ → ³F₄ transition) by a Raman fiber laser, the buried channel waveguide laser with a circular cladding (diameter: 60 µm) generated a continuous-wave output power of 0.81 W at 1866-1947 nm with a slope efficiency of 71.2% (versus the absorbed pump power) and showed a laser threshold of 200 mW. The waveguide propagation losses were as low as 0.3 ± 0.2 dB/cm. The laser performance under in-band pumping was superior compared pumping at ∼800 nm (³H₆ → ³H₄ transition), i.e., the convetional pump wavelength. Vibronic laser emission from the WG laser above 2 µm is also achieved. The low-loss behavior, the broadband emission properties and good power scaling capabilities indicate the suitability of Tm³⁺:CALGO waveguides for mode-locked laser operation at ∼2 µm.

Diode-pumped 45 fs Yb:CALGO laser oscillator with 1.7 MW of peak power

A high-power sub-50 fs diode-pumped Yb:CALGO laser oscillator is demonstrated. The peak power achieved for 45 fs pulses directly from the oscillator was 1.7 MW. To the best of our knowledge, this is the highest peak power in the sub-50 fs pulse regime that has ever been produced directly from any diode-pumped Yb-ion-doped laser oscillator to date. The shortest generated pulses were 38 fs long with 187 kW of peak power.

Wavelength-tunable Hermite-Gaussian modes and an orbital-angular-momentum-tunable vortex beam in a dual-off-axis pumped Yb:CALGO laser

A dual-off-axis pumping scheme is presented to generate wavelength-tunable high-order Hermite-Gaussian (HG) modes in Yb:CaGdAlO₄ lasers. The mode and wavelength can be actively controlled by the off-axis displacements and pump power. The purities of the output HG modes are quantified by intensity distributions and the measured M² values. The highest order reaches m=15 for stable HG_m,0 mode, and wavelength-tunable width is about 10 nm. Moreover, through externally converting the HG_m,0 modes, the vortex beams carrying orbital angular momentum (OAM) with a large OAM-tunable range from ±1ℏ to ±15ℏ are produced. This work is effective for largely scaling the spectral and OAM tunable ranges of optical vortex beams.

Crystal growth and properties of the disordered crystal Yb:SrLaAlO4: a promising candidate for high-power ultrashort pulse lasers

A high quality Yb³⁺-doped strontium lanthanum aluminate crystal, Yb:SrLaAlO₄ is grown by the Czochralski method.