Ultrafast laser inscribed Yb:KGd(WO4)2 and Yb:KY(WO4)2 channel waveguide lasers

Transition of pulsed operation from Q-switching to continuous-wave mode-locking in a Yb:KLuW waveguide laser

We report on the diverse pulsed operation regimes of a femtosecond-laser-written Yb:KLuW channel waveguide laser emitting near 1040 nm. By the precise position tuning of a carbon-nanotube-coated saturable absorber (SA) mirror, the transition of the pulsed operation from Q-switching, Q-switched mode-locking and finally sub-GHz continuous-wave mode-locking are obtained based on the interplay of dispersion and mode area control. The Q-switched pulses exhibit typical fast SA Q-switched pulse characteristics depending on absorbed pump powers. In the Q-switched mode-locking, amplitude modulations of the mode-locked pulses on the Q-switched envelope are observed. The radio-frequency spectrum represents the coexistence of Q-switching and mode-locking signals. In the purely mode-locked operation, the waveguide laser generates 2.05-ps pulses at 0.5 GHz.

Femtosecond-laser-written Ho:KGd(WO4)2 waveguide laser at 2.1 μm

We report on efficient laser operation of the first holmium monoclinic double tungstate waveguide laser fabricated by femtosecond direct laser writing. A depressed-index buried channel waveguide with a 60 μm diameter circular cladding was inscribed in 5 at.% Ho³⁺:KGd(WO₄)₂. It was characterized by confocal microscopy and μ-Raman and μ-luminescence spectroscopy, indicating well-preserved crystallinity of its core. Pumped by a thulium bulk laser, the holmium waveguide laser generated 212 mW at 2055 nm with a slope efficiency of 67.2%. The waveguide propagation losses were 0.94±0.2  dB/cm.

Fs-laser-written erbium-doped double tungstate waveguide laser

We report on the first erbium (Er³⁺) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er³⁺:KLu(WO₄)₂ crystal. A depressed-index buried channel waveguide formed by a 60 µm-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, µ-Raman and µ-luminescence mapping, confirming that the crystallinity of the core is preserved. The waveguide laser, diode pumped at 981 nm, generated 8.9 mW at 1533.6 nm with a slope efficiency of 20.9% in the continuous-wave regime. The laser polarization was linear (E || N_m). The laser threshold was at 93 mW of absorbed pump power.

Femtosecond-laser-written Tm:KLu(WO4)2 waveguide lasers

Depressed-index channel waveguides with a circular and photonic crystal cladding structures are prepared in a bulk monoclinic Tm:KLu(WO₄)₂ crystal by 3D direct femtosecond laser writing. The channel waveguide structures are characterized and laser operation is achieved using external mirrors. In the continuous-wave mode, the maximum output power of 46 mW is achieved at 1912 nm corresponding to a slope efficiency of 15.2% and a laser threshold of only 21 mW. Passive Q-switching of a waveguide with a circular cladding is realized using single-walled carbon nanotubes. Stable 7 nJ/50 ns pulses are achieved at a repetition rate of 1.48 MHz. This first demonstration of ∼2  μm fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm³⁺ and Ho³⁺  ions.

Waveguide lasers in ytterbium-doped tantalum pentoxide on silicon

A waveguide laser in an ytterbium-doped tantalum pentoxide film is reported. The waveguide is formed of a rib of sputtered tantalum pentoxide on top of oxidized silicon with an over-cladding of silica. Emission at a wavelength of 1025 nm was achieved with an absorbed pump power threshold and slope efficiency of ≈29  mW and 27%, respectively, for a cavity formed by a high reflector mirror and an estimated 12% Fresnel reflection from the bare end-face at the output.

2.8 W end-pumped Yb3+:LiYF4 waveguide laser

Continuous wave, room temperature laser oscillation around 1 μm is reported for the first time (to our knowledge) from an Yb(3+)-doped fluoride crystalline waveguide fabricated by using the liquid phase epitaxy technique. Maximum slope efficiencies of 76% and 41% were obtained for laser emissions at 994 and 1020 nm, respectively. A maximum output power of 2.8 W was obtained at 1020 nm by pumping around 980 nm with a high-brightness fiber laser in an 87 mm long V-shaped cavity.

Highly efficient Yb3+-doped channel waveguide laser at 981 nm

Channel waveguide lasers operating at 981 nm are demonstrated in KY(1-x-y)Gd(x)Lu(y)(WO4)2:Yb3+ waveguides grown by liquid phase epitaxy onto undoped KY(WO4)2 substrates and microstructured by Ar+ beam etching. Under pumping at 934 nm of samples with different waveguide geometry and outcoupling degree, a record-high slope efficiency of 76% versus absorbed pump power and a record-high output power of 650 mW for rare-earth-ion-doped microstructured channel waveguide lasers is achieved. The laser performance is compared to that of the same devices when pumping at 981 nm and lasing near 1025 nm.

Two-photon excited fluorescence in the LYB:Eu monoclinic crystal: towards a new scheme of single-beam dual-voxel direct laser writing in crystals

We report on two-photon excited fluorescence in the oriented Eu(3+)doped LYB monoclinic crystal under femtosecond laser tight focusing. Due to spatial walk-off, the two polarization modes of the incident femtosecond beam simultaneously provide the independent excitation of two distinct focuses, leading to a single-beam dual-voxel nonlinear excitation of fluorescence below material modification threshold. These observations emphasize on the anisotropy of both two-photon absorption as well as fluorescence emission. They demonstrate the localized control of the nonlinear energy deposit, thanks to the adjustment of both the input power and polarization, by properly balancing the injected energy in each voxel. Such approach should be considered for future direct laser writing of waveguides in propagation directions out of the dielectric axes, so as to optimally cope with the highly probable anisotropy of laser-induced material modification thresholds in these crystals. These results open new ways for further potential developments in direct laser writing as the simultaneous inscription of double-line structures for original waveguides processes.

Tunable, continuous-wave Ti:sapphire channel waveguide lasers written by femtosecond and picosecond laser pulses

Fabrication and cw lasing at 798.25 nm is reported for femtosecond (fs) and picosecond (ps) laser-inscribed channel waveguides in Ti:sapphire crystals. Lasing in channels written by fs (ps) pulses was obtained above a threshold of 84 mW (189 mW) with a maximum output power and a slope efficiency of 143 mW (45 mW) and 23.5% (7.1%), respectively. The emission wavelength was tuned over a 170 nm range by using a birefringent filter in an external cavity.

Compact, highly efficient ytterbium doped bismuthate glass waveguide laser

Laser slope efficiencies close to the quantum defect limit and in excess of 78% have been obtained from an ultrafast laser inscribed buried channel waveguide fabricated in a ytterbium-doped bismuthate glass. The simultaneous achievement of low propagation losses and preservation of the fluorescence properties of ytterbium ions is the basis of the outstanding laser performance.

Beam shaping of laser diode radiation by waveguides with arbitrary cladding geometry written with fs-laser radiation

Waveguides with arbitrary cross sections are written in the volume of Al(2)O(3)-crystals using tightly focused femtosecond laser radiation. Utilizing a scanning system with large numerical aperture, complex cladding geometries are realized with a precision around 0.5 µm and a scanning speed up to 100 mm/s. Individual beam and mode shaping of laser diode radiation is demonstrated by varying the design of the waveguide cladding. The influence of the writing parameters on the waveguide properties are investigated resulting in a numerical aperture of the waveguides in the range of 0.1. This direct laser writing technique enables optical devices which could possibly replace bulky beam shaping setups with an integrated solution.

Crystalline Pr:SrAl12O19 waveguide laser in the visible spectral region

We fabricated waveguides in Pr:SrAl(12)O(19) crystals by direct femtosecond laser writing. The propagation losses were calculated to be as low as 0.16 dB/cm at a wavelength of 633 nm. Laser oscillation in a diode-pumped waveguide at a wavelength of 643.5 nm was realized. The output power of the waveguide laser was 28.1 mW at a slope efficiency of 8%.

Channel waveguide lasers in Nd:GGG crystals fabricated by femtosecond laser inscription

Buried channel waveguides have been fabricated in Nd:GGG crystals by using the femtosecond laser inscription. The waveguides are confined between two filaments with propagation losses of 2.0 dB/cm. Stable continuous wave laser oscillation at ~1061 nm has been demonstrated at room temperature. Under 808 nm optical excitation, a pump threshold of 29 mW and a slope efficiency of 25% have been obtained.

Lasing action at around 1.9 μm from an ultrafast laser inscribed Tm-doped glass waveguide

We report optical guiding and laser action at around 1.9 μm in a Tm³⁺-doped fluorogermanate glass waveguide fabricated using ultrafast laser inscription. A monolithic laser cavity was constructed by directly butt coupling dielectric mirrors to each facet of the 6.0 mm long Tm³⁺-doped waveguide. When the waveguide was pumped by a Ti:sapphire laser tuned to 791 nm, laser oscillation was achieved at around 1.91 μm. This waveguide laser exhibited a maximum slope efficiency of 6% and a maximum output power of 32 mW when pumped with 620 mW of incident laser power.

Mirrorless buried waveguide laser in monoclinic double tungstates fabricated by a novel combination of ion milling and liquid phase epitaxy

Buried channel waveguides were fabricated by liquid phase epitaxial growth of a lattice-matched KY(0.58)Gd(0.22)Lu(0.17)Tm(0.03)(WO4)2 film on a microstructured KY(WO4)2 substrate. Channels were transferred to the substrates by standard photolithography and Ar-ion milling. The bottom and sidewalls of the milled channels were smooth enough (rms roughness = 70 nm and 20 nm, respectively) to favour the epitaxial growth of the active layer without defects at the boundary of substrate/epitaxial layer. The refractive index contrast was sufficient to enable light confinement and guided modes with low scattering losses were observed at wavelengths between 1440 nm and 1640 nm. CW laser operation at 1840 nm at room temperature was observed with feedback provided only by Fresnel reflection at the end faces, with slope efficiencies of 4% and 9% for TE and TM polarizations, respectively.

High-power, broadly tunable, and low-quantum-defect KGd(1-x)Lu(x)(WO(4))(2):Yb(3+) channel waveguide lasers

In KGd(1-x)Lu(x)(WO(4))(2):Yb(3+) channel waveguides grown onto KY(WO(4))(2) substrates by liquid phase epitaxy and microstructured by Ar+ beam etching, we produced 418 mW of continuous-wave output power at 1023 nm with a slope efficiency of 71% and a threshold of 40 mW of launched pump power at 981 nm. The degree of output coupling was 70%. By grating tuning in an extended cavity and pumping at 930 nm, we demonstrated laser operation from 980 nm to 1045 nm. When pumping at 973 nm, lasing at 980 nm with a record-low quantum defect of 0.7% was achieved.

Ultrafast laser inscription of near-infrared waveguides in polycrystalline ZnSe

We report the successful fabrication of a low-loss near-IR waveguide in polycrystalline ZnSe using ultrafast laser inscription. The waveguide, which was inscribed using the multiscan fabrication technique, supported a well-confined mode at 1.55 μm. Propagation losses were characterized at 1.55 μm using the Fabry-Perot technique and found to be 1.07 dB · cm(-1) ± 0.03 dB · cm(-1).

70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser

We report high efficiency continuous wave laser oscillations at 1063.6 nm from an ultrafast laser written Nd(3+):GdVO4 channel waveguide under the 808 nm optical excitation. A record 17 mm·s(-1) writing speed was used while the low propagation loss of the waveguide (~0.5 dB·cm(-1)) enabled laser performance with a threshold pump power as low as 52 mW and a near to quantum defect limited laser slope efficiency of 70%.

Highly efficient Yb:YAG channel waveguide laser written with a femtosecond-laser

Using a femtosecond Ti:Sapphire laser, micro-tracks of material damage were written into Yb:YAG crystals. Waveguiding was achieved in a channel between pairs of tracks with guiding losses of 1.3 dB/cm at a wavelength of 1063 nm, due to a stress induced change of the refractive index. Pumped at a wavelength of 941 nm, highly efficient laser oscillation in a Yb:YAG channel waveguide at a wavelength of 1030 nm was demonstrated. An output power of 0.8 W at 1.2 W of launched pump power was achieved, resulting in a record slope efficiency of 75%.

Microstructured KY(WO(4))(2):Gd(3+), Lu(3+), Yb(3+) channel waveguide laser

Epitaxially grown, 2.4-microm-thin layers of KY(WO(4))(2):Gd(3+), Lu(3+), Yb(3+), which exhibit a high refractive index contrast with respect to the undoped KY(WO(4))(2) substrate, have been microstructured by Ar beam milling, providing 1.4-microm-deep ridge channel waveguides of 2 to 7 microm width, and overgrown by an undoped KY(WO(4))(2) layer. Channel waveguide laser operation was achieved with a launched pump power threshold of only 5 mW, a slope efficiency of 62% versus launched pump power, and 76 mW output power.