Dual-wavelength synchronous operation of a mode-locked 2-μm Tm:CaYAlO4 laser

Synchronized multi-wavelength soliton fiber laser via intracavity group delay modulation

Locking of longitudinal modes in laser cavities is the common path to generate ultrashort pulses. In traditional multi-wavelength mode-locked lasers, the group velocities rely on lasing wavelengths due to the chromatic dispersion, yielding multiple trains of independently evolved pulses. Here, we show that mode-locked solitons at different wavelengths can be synchronized inside the cavity by engineering the intracavity group delay with a programmable pulse shaper. Frequency-resolved measurements fully retrieve the fine temporal structure of pulses, validating the direct generation of synchronized ultrafast lasers from two to five wavelengths with sub-pulse repetition-rate up to ~1.26 THz. Simulation results well reproduce and interpret the key experimental phenomena, and indicate that the saturable absorption effect automatically synchronize multi-wavelength solitons in despite of the small residual group delay difference. These results demonstrate an effective approach to create synchronized complex-structure solitons, and offer an effective platform to study the evolution dynamics of nonlinear wavepackets.

Self-Q-switched and multicolor operation of a Tm:LuAG laser

In this paper, we report self-Q-switched (SQS) and three-color operation of an all-solid-state Tm:LuAG laser for the first time to our knowledge. In the experiments, a low-cost 3 W AlGaAs laser diode was used to end-pump the Tm:LuAG crystal inside a four-mirror x cavity. In typical continuous-wave (cw) operation, as high as 754 mW output power was obtained with 49% power efficiency at 2023 nm. Three-color and SQS operations were initiated by fine tuning of the curved mirror separation within the stability range of the resonator. In the three-color regime, the Tm:LuAG laser produced two extra, i.e., 2019 and 2033 nm, wavelength oscillations in addition to on at 2023 nm. As high as 542 mW output power was observed in this regime. To the best of our knowledge, this is the first stable three-color laser operation obtained with an isotropic gain medium without having any birefringent elements in the cavity. Furthermore, the SQS operations were also observed at other curved mirror separations. In the SQS regime, the Tm:LuAG laser produced as fast as 13.3 kHz repetition rate pulses and as high as 42.5 µJ pulse energy. Analysis of power-dependent repetition rate data gave an estimated value of 2% for the round-trip saturable loss of the crystal. As far as we know, this is the first cw-pumped stable SQS 2 µm laser that contains an ordered isotropic gain medium as well as the highest pulse energy obtained in any cw-pumped SQS laser.

Passively Q-switched Tm:CaLu0.1Gd0.9AlO4 laser at 2 µm with hematite nanosheets as the saturable absorber

We report the first passive Q-switching operation at 1.95 µm utilizing the disordered Tm:CaLu_0.1Gd_0.9AlO₄ (Tm:CLGA) crystal and the hematite (α-Fe₂O₃) nanosheets as the saturable absorber. The nonlinear saturable absorption properties of the hematite nanosheets were investigated by the conventional Z-scan technology. The modulation depth of 14.3% with the low saturation intensity of 205 kW/cm2 was obtained, indicating that the hematite could be a suitable saturable absorber for the mid-infrared pulse generation. Using the disordered Tm:CLGA crystal as the gain medium, the passive Q-switching operation could be realized with the hematite nanosheets as the saturable absorber, producing the shortest pulse duration of 402 ns with a repetition rate of 76 kHz. The experimental results convinced us that the hematite nanosheets could be of great interest in the optical pulse generation in the mid-infrared region.

Watt-level passively mode-locked Tm:YLF laser at 1.83 µm

Passive, continuous-wave mode-locked (CWML) operation of a 1.83 µm Tm:YLF laser is experimentally demonstrated for the first time, to the best of our knowledge. Two specially selected output couplers are used to realize this operation. Stability of the CWML laser is obtained with a commercial semiconductor saturable absorber mirror. The maximum average output power is 1.04 W with a pulse duration of 107 ps and repetition rate of 54.1 MHz. Further, a 0.1 mm fused-quartz Fabry-Perot etalon is used to tune the central wavelength of the stable CWML laser at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm.

"Mixed" Tm:Ca(Gd,Lu)AlO4 - a novel crystal for tunable and mode-locked 2 µm lasers

We report on the crystal growth, spectroscopy characterization and first laser operation of a new tetragonal disordered "mixed" calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO₄. The introduction of Lu³⁺ leads to an additional inhomogeneous broadening of Tm³⁺ absorption and emission spectra compared to the well-known Tm:CaGdAlO₄. The maximum stimulated-emission cross-section for the ³F₄ → ³H₆ Tm³⁺ transition is 0.91 × 10^-20 cm² at 1813 nm for σ-polarization, and the emission bandwidth is more than 200 nm. A continuous-wave diode-pumped Tm:Ca(Gd,Lu)AlO₄ laser generates 1.82 W at 1945 nm with a slope efficiency of 29%. Under Ti:Sapphire laser pumping, a continuous tuning of the laser wavelength from 1836 to 2083 nm (tuning range: 247 nm) is demonstrated. The Tm:Ca(Gd,Lu)AlO₄ crystal is promising for tunable/femtosecond lasers at ~2 μm.

Widely tunable dual-wavelength operation of Tm:YLF, Tm:LuAG, and Tm:YAG lasers using off-surface optic axis birefringent filters

In this work, we have demonstrated dual-wavelength continuous-wave laser operation in diode-end-pumped Tm:YLF, Tm:LuAG, and Tm:YAG lasers. A 3-mm-thick quartz birefringent filter with an optical axis 45° to the surface plane was exploited for achieving broadly tunable two-color laser operation. By using the different orders of the filter with varying filter width and free spectral range values, dual-wavelength operation has been achieved in 11, 12, and 8 different wavelength pairs in Tm:YLF, Tm:LuAG, and Tm:YAG, respectively. Fine tuning of the rotation angle of the birefringent filter enabled control of laser power in each line. To our knowledge, this is the first report of multicolor laser operation in these gain media, and the technique used is applicable to other laser operation regimes including mode-locking.

Spectroscopic characteristics, continuous-wave and mode-locking laser performances of Tm,Y:CaF2 disordered crystal

The spectroscopic characteristics, continuous-wave (CW) and mode-locking laser performances of Tm,Y:CaF₂ disordered crystal were studied. A maximum CW output power of 586 mW was obtained with a slope efficiency of 26%. The Tm,Y:CaF₂ mode-locked laser could operate in two states: single-wavelength mode locking or dual-wavelength synchronous mode locking. The single-wavelength mode-locked laser generated pulses with pulse duration of 22 ps, repetition rate of 99 MHz, and pulse energy of 1.15 nJ at 1887 nm. Alternatively, the laser could also be mode-locked simultaneously at 1880.7 nm and 1889.0 nm wavelengths. The beating modulation in autocorrelation trace shows that the dual-wavelength pulses were temporally synchronous.

Sellmeier equations, group velocity dispersion, and thermo-optic dispersion formulas for CaLnAlO4 (Ln = Y, Gd) laser host crystals

We studied the refractive index and dispersive properties of the tetragonal rare-earth calcium aluminates, CaLnAlO₄ (Ln=Gd or Y). Sellmeier equations were derived for the spectral range of 0.35-2.1 μm. The group velocity dispersion (GVD) in CaGdAlO₄ is positive at ∼1  μm, 95  fs²/mm and negative at ∼2  μm, -40  fs²/mm. The GVD values for CaYAlO₄ are similar. In addition, thermo-optic coefficients, dn/dT, and thermal coefficients of the optical path were determined for CaYAlO₄. dn/dT is negative at ∼1  μm, dn_o/dT=-7.8, and dn_e/dT=-8.7×10^-6  K^-1. Thermo-optic dispersion formulas were constructed. The obtained data are of key importance to the design of high-power mode-locked oscillators at ∼1 and ∼2  μm based on such laser hosts.

Stable passively harmonic mode-locking dissipative pulses in 2µm solid-state laser

We report on stable passively harmonic mode-locking dissipative pulses with high repetition rate and narrow bandwidth in 2µm Tm: CaYAlO₄ laser. At the large intracavity intensity, the laser generated 1st-order to 5th-order passively harmonic solitons with fundamental repetition rate of ~198 MHz and 5th-order repetition rate up to 0.98 GHz, which was mainly caused by the peak power clamp effect. The solitons yielded a tunable central wavelength from 1940nm to 1950 nm, and a narrow optical spectrum bandwidth of 60 pm without any active optical filter. At low intracavity intensity, the laser operated on the typical SESAM-guided mode-locking mode, with the scaled output average power up to 1.15 W. To our knowledge, this is the first observation of passively harmonic mode locking in 2µm solid laser system, and the first Watt-level output average power in Tm: CYA mode locking laser.

Dual-wavelength synchronously mode-locked Nd:LaGGG laser operating at 1.3 μm with a SESAM

A diode-end-pumped synchronously dual-wavelength mode-locked Nd:LaGGG laser operating at 1.3 μm with a semiconductor saturable absorber mirror was demonstrated for the first time.

Wavelength-spacing controllable, dual-wavelength synchronously mode locked Er:fiber laser oscillator based on dual-branch nonlinear polarization rotation technique

A wavelength-spacing controllable, dual-wavelength synchronously mode locked Er:fiber laser oscillator based on dual-branch nonlinear polarization rotation (NPR) technique was presented. The center wavelengths were at 1542 nm and 1561 nm, which had pulse durations of 1.38 ps and 1.70 ps, respectively. Experimentally, the synchronous mode locking was achieved by precisely adjusting the cavity length of one branch. A tolerance in the cavity length mismatch of 0.46 mm for synchronous mode locking was demonstrated. The frequency difference of the two pulse trains was measured to be less than 1 mHz. Additionally, this synchronously mode locked dual-wavelength laser had a wavelength tunable range of about 5.6 nm, and a controllable wavelength spacing from 10.5 nm to 28.2 nm, corresponding to a tunable frequency difference from 1.32 THz to 3.26 THz. To the best of our knowledge, this is the first demonstration of synchronously mode locked dual-wavelength output directly from a Er:fiber laser oscillator, using dual-branch NPR technique.

Broadly tunable mode-locked Ho:YAG ceramic laser around 2.1 µm

A passively mode-locked Ho:YAG ceramic laser around 2.1 µm is demonstrated using GaSb-based near-surface SESAM as saturable absorber. Stable and self-starting mode-locked operation is realized in the entire tuning range from 2059 to 2121 nm. The oscillator operated at 82 MHz with a maximum output power of 230 mW at 2121 nm. The shortest pulses with duration of 2.1 ps were achieved at 2064 nm. We also present spectroscopic properties of Ho:YAG ceramics at room temperature.

Broadly, independent-tunable, dual-wavelength mid-infrared ultrafast optical parametric oscillator

We demonstrate a two-crystal mid-infrared dual-wavelength optical parametric oscillator, synchronously pumped by a high power femtosecond Yb:fiber laser. The singly-resonant ring cavity, containing two periodically poled lithium niobate crystals, is capable of generating two synchronized idler wavelengths, independently tunable over 30 THz in the 2.9 - 4.2 μm wavelength region, due to the cascaded quadratic nonlinear effect. The independent tunability of the two idlers makes the optical parametric oscillator a promising source for ultrafast pulse generation towards the THz wavelength region, based on different frequency generation. In addition, the observed frequency doubled idler within the crystal indicates the possibility to realize a broadband optical self-phase locking between pump, signal, idler and higher order generated parametric lights.

1.21 W passively mode-locked Tm:LuAG laser

A watt-level output passively mode-locked Tm:LuAG bulk laser with an InGaAs semiconductor saturable absorber mirror (SESAM) is demonstrated for the first time. A maximum average output power of 1.21 W at 2022.9 nm has been achieved with a pulse duration of 38 ps and a repetition rate of 129.2 MHz. The results indicate the potential of Tm:LuAG crystals as candidate for realizing high power ultrafast lasers at 2 μm.