Femtosecond pulse generation from a SESAM mode-locked Tm,Ho:SrF2 laser at 2.08 µm.

We report on a passively mode-locked Tm,Ho:SrF2 laser employing a SESAM as saturable absorber (SA), delivering nearly Fourier-transform-limited 246 fs pulses at 2084nm without any additional intra- or extra-cavity dispersion compensation elements. This represents, to the best of our knowledge, the shortest pulses generated from the mode-locked fluoride bulk lasers in the 2-µm spectral range. Such compact femtosecond laser can be a potential seed source for large-sized fluoride bulk amplifier systems with exact gain match, enabling the generation of ultrashort intense pulses around 2 µm.

Sub-60-fs mode-locked Tm,Ho:CaYLuAlO4 laser at 2.05†µm.

Tm,Ho:CaYLuAlO4 (Tm,Ho:CALYLO) crystal has wide emission spectra both for π-polarization and σ-polarization, showing significant potential for the generation of ultrashort pulses. Here, a widely tunable and passively mode-locked laser operation based on Tm,Ho:CALYLO crystal under two polarizations was demonstrated for what we believe to be the first time ever. For π-polarization, a maximum output power of 1.52 W and a tuning range of 255.3 nm were achieved in the continuous wave (CW) regime. In the mode-locked regime, a pulse duration of 68 fs and an average output power of 228 mW were achieved upon GaSb-based semiconductor saturable absorber mirror (SESAM). As for σ-polarization, a broader tuning range of 267.1 nm was realized, leading to the shorter pulse duration of 58 fs at 79.7 MHz repetition rate.

Room-temperature high-power laser emission of erbium-doped fluorite crystals at 2.8†µm.

We report on a high-power continuous-wave (CW) laser at 2.8 µm employing erbium (Er)-doped fluorite crystals as gain materials. With an optimized Er3+ ion concentration, thin "slab" geometry of the sample matching with the tailored pump beam profile and compensated negative thermal lens using a pair of concave mirrors cavity configuration, a highest power of 14.5 W is achieved from a dual-end-pumped Er:CaF2 laser, which, to the best of our knowledge, presents the record power from the room-temperature Er-bulk lasers in the 3-µm spectral range. In addition, 8.05 W output power is obtained from the Er:SrF2 laser with an RMS power stability of 0.35%. This work indicates that Er-doped fluorite crystals with large-scale available fabrication are promising candidates for high-power laser emission at ∼3 µm.

Tunable and mode-locked Tm,Ho:GdScO3 laser.

A novel, to the best of our knowledge, Tm,Ho:GdScO3 crystal grown using the Czochralski method was investigated for its polarized spectroscopic properties and laser performance in both tunable continuous-wave (CW) and mode-locked regimes. The crystal's multisite structure (Gd3+/Sc3+ site) and Tm3+/Ho3+ dopants contributed to spectral broadening, enabling a tunable laser operation from 1914 to 2125 nm (with a broad range of 215 nm). Additionally, a pulse duration of 72 fs was achieved for E || b polarization. These results demonstrate the potential of the Tm,Ho:GdScO3 perovskite crystal as a promising gain material for ultrafast lasers operating around 2 µm.

Wavelength-tunable Tm:Y2O3 ceramic vortex laser with a changeable orbital angular momentum state.

Wavelength-tunable orbital angular momentum (OAM) lasers with controllable topological charges have the potential for serving as light sources for large-capacity optical communication by combining conventional wavelength division multiplexing (WDM) with OAM mode-division multiplexing (OAM-MDM). In this study, we demonstrate a wavelength-tunable Tm-bulk laser that can control OAM states in the 2-µm spectral range. The excitation conditions for different Laguerre-Gaussian (L G 0,l ) modes in a bulk laser cavity are theoretically determined by measuring the spatial propagation dynamics of the annular pump beam. As a proof-of-principle study, we experimentally generate OAM states of |ℏ| and |2ℏ| from a T m:Y 2 O 3 ceramic laser with a tunable emission wavelength using a Lyot filter (LF). The spatial properties of the scalar optical vortices are well conserved during wavelength tuning, indicating the feasibility of our approach for producing wavelength-tunable structured light. These OAM laser sources, which are characterized by their robustness and compactness, have potential applications in various areas such as optical communications, quantum optics, super-resolution microscopes, and more.

Reduction of depolarization loss in a Tm-doped sesquioxide ceramic laser using a ring-shaped pump beam.

We report on a simple method for reduction of the depolarization loss in an end-pumped Tm:Y2O3 ceramic laser by using a near-field ring-shaped pump beam. Initially, we theoretically derive the expression of the depolarization loss in a bulk laser end-pumped with a near-field flat-top-hat or ring-shaped beam, where a significant reduction of depolarization loss in the latter case is presented. Experimental verification is thereafter carried out with a Tm:Y2O3 ceramic laser employing these two different pump configurations. It shows that the experimentally measured depolarization losses are close to the simulated values; the loss in the case of the annular-beam pump is almost 18 times lower than that with a quasi-top-hat beam at a same absorption pump power of 7.4 W. This work, as a proof-of-principle study, indicates that depolarization loss in the end-pumped bulk lasers can be significantly reduced simply by using a ring-shaped pump beam.

Reduction of thermal effects in a 2.7-µm Er:Y2O3 ceramic laser with annular pumping.

Reduction of thermal effects is a challenging aspect for power scaling of 2.7-µm bulk Er-lasers due to the large quantum defect when pumping at ∼ 0.97 µm. Here, we demonstrate that thermal effects in an Er:Y2O3 ceramic laser can be significantly reduced pumping by an annular beam, thus improving the continuous-wave (CW) laser performance in the 3-µm spectral range. The excitation conditions of the TEM00 mode were determined theoretically by taking into account the propagation characteristics of the annular pump beam. For a comparison, the temperature and stress distributions are at first theoretically studied with three different pump configurations. In the experiment, output power of the Er:Y2O3 ceramic laser improved by ∼ 60% by changing the pump beam from coventional quasi-top-hat to a designed annular one. This work, as a proof-of-principle study, indicates the potential of power scaling of the 2.7-µm bulk Er-lasers pumping with an annular beam.

Spectroscopy and efficient laser performances of a Tm,Ho:CaY0.9Lu0.1AlO4 crystal.

We report on the spectral properties and laser performances of a novel, to the best of our knowledge, Tm,Ho:CaY0.9Lu0.1AlO4 (Tm,Ho:CYLA) crystal. The polarized absorption spectra, luminescence spectra, and fluorescence lifetime are systematically investigated, presenting a broad and smooth luminescence band. Furthermore, a maximum continuous wave (CW) laser output power of 0.51 W at 2092 nm is obtained under an absorbed pump power of 2.89 W, corresponding to a slope efficiency of 20.4%. The beam quality factors (M2) are measured to be 1.04 in both the x and y axes. A tuning range of 123.4 nm, from 2017.8 nm to 2141.2 nm, is achieved in the CW regime by using a birefringent filter (BF). A stable passively Q switched Tm,Ho:CYLA laser employing Cr2+:ZnSe as a saturable absorber (SA) is realized for the first time, delivering the shortest pulse width of 560 ns with a transmittance of 1%. The results indicate that the Tm,Ho:CYLA crystal is an excellent laser medium for generating high-efficiency laser at ∼2 µm and has a potential in ultrafast laser generation.

Mode-locking of a Tm,Ho:CALYGO laser delivering 50 fs pulses at 2.08 µm.

We study the polarization-dependent laser performance of a novel, to the best of our knowledge, "mixed" Tm,Ho:CaYGdAlO4 crystal in the continuous-wave (CW) and mode-locked regimes. Both in terms of the CW tunability range (261 nm) and the minimum pulse duration (50 fs at 2078 nm, spectral width of 95 nm) in the mode-locked regime, σ-polarization is superior. With extended inhomogeneous spectral broadening due to structural and compositional disorder, Tm,Ho:CaYGdAlO4 is promising for few-optical-cycle pulse generation around 2 µm.

Efficient continuous wave and broad tunable lasers with the Tm:GdScO3 crystal.

The spectroscopic properties and tunable laser performances of the orthorhombic perovskite Tm:GdScO3 crystal grown by the Czochralski method are comparatively studied for polarization along different crystallographic axes. The polarized emission spectrum of Tm:GdScO3 along the b-axis exhibits, to the best of our knowledge, the broadest bandwidth among all the single Tm3+-doped bulk gain media, indicating the strong inhomogeneous line broadening of Tm3+ ions in GdScO3 and thus leads to a broad and smooth gain spectrum. Tunable laser operation with a tuning range as broad as 321 nm from 1824 nm to 2145 nm is achieved, which indicates its potential for few-optical-cycle pulse generation in the 2-µm spectral range.

44-fs pulse generation at 2.05 µm from a SESAM mode-locked Tm:GdScO3 laser.

Pulses as short as 44 fs (6 optical cycles) with a spectral width of 120 nm are generated from a mode-locked solid-state laser near 2 µm employing an orthorhombic Tm:GdScO3 perovskite crystal. The average power amounts to 188 mW at a repetition rate of ∼77.6 MHz. The strong inhomogeneous line broadening in GdScO3 suggests optimum conditions for few-optical-cycle pulse generation of rare-earth ion doped GdScO3 bulk lasers.

63 W wing-pumped Tm:YAG single-crystal fiber laser.

We present a high-power continuous-wave (CW) Tm:YAG single-crystal fiber (SCF) laser wing-pumped by laser diodes at 791 nm. A maximum output power of 63.3 W is achieved at ∼ 2.01 µm, corresponding to a slope efficiency of 34.2%. This is, to the best of our knowledge, the highest power obtained from the SCF laser in the 2 µm spectral range. In addition to the wing pumping scheme, the large surface-to-volume ratio of such fiber-geometry crystalline rod with diffusion-bonded undoped YAG end caps are benefited for the spatial uniform distribution of pump intensity and thermal load, and thus improving the power scalability.

Diode-pumped SESAM mode-locked Yb:(Y,Gd)AlO3 laser.

We report on the continuous-wave (CW) and mode-locked (ML) laser performance of an Yb3+-doped yttrium-gadolinium orthoaluminate crystal, Yb:(Y,Gd)AlO3. Pumping by a single-transverse-mode fiber-coupled 976 nm InGaAs laser diode, the maximum output power in the CW regime amounted to 429 mW at 1041.8 nm corresponding to a slope efficiency of 51.1% and a continuous wavelength tuning across 84 nm (1011-1095 nm) was achieved. The self-starting ML operation of the Yb:(Y,Gd)AlO3 laser was stabilized by a semiconductor saturable absorber mirror. Soliton pulses as short as 43 fs were generated at 1052.3 nm with an average output power of 103 mW and a pulse repetition rate of ∼70.8 MHz. To the best of our knowledge, our result represents the first report on the passively mode-locked operation of a Yb:(Y,Gd)AlO3 laser, and the shortest pulse duration ever achieved from any Yb3+-doped orthorhombic perovskite aluminate crystals.

Soliton mode-locked Yb:Ca3Gd2(BO3)4 laser.

We report on a soliton mode-locked Yb:Ca3Gd2(BO3)4 laser at ∼1.06 µm stabilized by a semiconductor saturable absorber mirror. Pumping with a single-transverse mode, fiber-coupled laser diode at 976 nm, the Yb:Ca3Gd2(BO3)4 laser delivers soliton pulses as short as 39 fs at a central wavelength of 1059.2 nm with an average output power of 70 mW and a pulse repetition rate of ∼67.3 MHz.

SESAM mode-locked Yb:Sr3Y2(BO3)4 laser.

We demonstrate the first sub-40 fs soliton pulse generation from a diode-pumped Yb:Sr3Y2(BO3)4 laser passively mode-locked by a semiconductor saturable absorber mirror. Pulses as short as 38 fs at a central wavelength of 1051.7 nm were achieved with an average output power of 115 mW and a pulse repetition rate of 67.7 MHz. The maximum average output power reached 303 mW at 1057.8 nm with a slightly longer pulse duration of 52 fs, which corresponded to a peak power of 76.9 kW and an optical efficiency of 25.3%.

Polarized spectroscopy and SESAM mode-locking of Tm,Ho:CALGO.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho3+ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.

SESAM mode-locked Tm:Y2O3 ceramic laser.

We demonstrate a widely tunable and passively mode-locked Tm:Y2O3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y2O3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y2O3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

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.

Watt-level femtosecond Tm-doped "mixed" sesquioxide ceramic laser in-band pumped by a Raman fiber laser at 1627 nm.

We report on a semiconductor saturable absorber mirror mode-locked Tm:(Lu,Sc)2O3 ceramic laser in-band pumped by a Raman fiber laser at 1627 nm. The nonlinear refractive index (n2) of the Tm:(Lu,Sc)2O3 ceramic has been measured to be 4.66 × 10-20 m2/W at 2000 nm. An average output power up to 1.02 W at 2060 nm is achieved for transform-limited 280-fs pulses at a repetition rate of 86.5 MHz, giving an optical efficiency with respect to the absorbed pump power of 36.4%. Pulses as short as 66 fs at 2076 nm are produced at the expense of output power (0.3 W), corresponding to a spectral bandwidth of 69 nm. The present work reveals the potential of Tm3+-doped sesquioxide transparent ceramics for power scaling of femtosecond mode-locked bulk lasers emitting in the 2-µm spectral range.

Ho:LuAG single crystal fiber: growth, spectroscopy and laser characteristics.

Lutetium aluminum garnet single-crystal fiber (SCF, ∼ Φ 0.9 mm - 165 mm) doped with 0.5 at.% Ho3+ has been grown by the micro-pulling-down (µ-PD) technique. The room-temperature absorption and emission spectra exhibit similar features to the bulk crystal. Laser performances of the SCFs with two different pump configurations, i.e., pump guiding and free-space propagation, are studied by employing a 1.9-µm laser diode and a high-brightness fiber laser, respectively. Laser slope efficiencies obtained with both pump configurations can be higher than 50%, and a maximum output power of 6.01 W is achieved at ∼ 2.09 µm with the former pump. The comparable efficiency to the high-brightness pump is an indication of that high laser performance can also be expected through pump-guiding in the SCF even with a low pump beam quality.