Hongliang Liu

Femtosecond laser inscribed cladding waveguides in Nd:YAG ceramics: Fabrication, fluorescence imaging and laser performance

We report on the fabrication of depressed cladding waveguide lasers in Nd:YAG (neodymium doped yttrium aluminum garnet, Nd:Y3Al5O12) ceramics microstructured by femtosecond laser pulses. Full control over the confined light spatial distribution is demonstrated by the fabrication of high contrast waveguides with hexagonal, circular and trapezoidal configurations. The confocal fluorescence measurements of the waveguides reveal that the original luminescence features of Nd3+ ions are well-preserved in the waveguide regions. Under optical pump at 808 nm, cladding waveguides showed continuous wave efficient laser oscillation. The maximum output power obtained at 1064.5 nm is ~181 mW with a slope efficiency as high as 44%, which suggests that the fabricated Nd:YAG ceramic waveguides are promising candidates for efficient integrated laser sources.

Femtosecond-laser inscribed double-cladding waveguides in Nd:YAG crystal: a promising prototype for integrated lasers

We report on the design and implementation of a prototype of optical waveguides fabricated in Nd:YAG crystals by using femtosecond-laser irradiation. In this prototype, two concentric tubular structures with nearly circular cross sections of different diameters have been inscribed in the Nd:YAG crystals, generating double-cladding waveguides. Under 808 nm optical pumping, waveguide lasers have been realized in the double-cladding structures. Compared with single-cladding waveguides, the concentric tubular structures, benefiting from the large pump area of the outermost cladding, possess both superior laser performance and nearly single-mode beam profile in the inner cladding. Double-cladding waveguides of the same size were fabricated and coated by a thin optical film, and a maximum output power of 384 mW and a slope efficiency of 46.1% were obtained. Since the large diameters of the outer claddings are comparable with those of the optical fibers, this prototype paves a way to construct an integrated single-mode laser system with a direct fiber-waveguide configuration.

Femtosecond laser written waveguides with MoS 2 as satuable absorber for passively Q-switched lasing

This work reports on the passively Q-switched waveguide laser system based on Nd:YAG crystal and MoS2 satuable absorber. A depressed cladding waveguide with circular cross-sectional geometry has been produced in Nd:YAG crystal by direct femtosecond laser writing at low-repetition rate. The confocal microscopic investigation of the structure reveals the well-preserved microphotoluminescence features in the waveguide core. With chemical-vapor-deposition (CVD) MoS2 membrane as satuable absorber, the passive Q-switching of the Nd:YAG waveguide system has been achieved under optical pump, reaching maximum average output power of 85.2 mW, corresponding to single-pulse energy of 112 nJ, at wavelength of 1064 nm. The repetition rate of the pulsed waveguide laser system is tunable from 0.51 to 1.10 MHz, and the obtained minimum pulse duration is 203 ns.

Graphene-based Y-branch laser in femtosecond laser written Nd:YAG waveguides

We report on Q-switched waveguide lasers on the graphene-based crystalline Y-branch platform. By applying direct femtosecond laser writing of Nd:YAG laser crystal, a surface waveguide splitter with Y-branch geometry has been fabricated with depressed cladding configuration. The Q-switched lasing operation at 1064 nm is achieved in transmission mode, by attaching a two-layer graphene on the resonator output mirror, as well as by using interaction between the evanescent field and a few-layer graphene that was positioned right above the Y-type waveguide. Q-switched laser with a maximum average power of 173 mW, pulse energy and duration of 63 nJ and 90 ns is obtained. This work opens a way for laser-written crystalline devices as compact, direct-pump laser sources for diverse applications.

Microdomain Patterns Recorded by an Electron Beam in He-Implanted Optical Waveguides on X-Cut LiNbO 3 Crystals

We present the results of studies in planar optical waveguides fabricated by He-ion implantation with the energy of 500 keV in X-cut LiNbO3 crystals. The thickness of the formed waveguide layer confined by the depth D of the implanted layer is of about 1.06 μm. The refractive indices as well as differences in refractive indices were evaluated for wavelengths λ = 445, 626.5, and 650 nm. Domain gratings with the period λ = 4 μm were recorded in these samples by electron beam irradiation with acceleration voltages U in the range from 5 to 25 kV. Gratings characteristics measured by the PFM method were obtained for different domain thicknesses Td determined by U. The optimum grating regularity is achieved when the domain growth occurs beyond the He-implanted damaged barrier, i.e., at Td ≤ D, which in the given case corresponds to U = 10 and 15 kV. Otherwise, (Td > D), the domain evolution is affected by the structurally damaged layer and the gratings become irregular.

Passively Q-switched waveguide lasers based on two-dimensional transition metal diselenide

We reported on the passively Q-switched waveguide lasers based on few-layer transition metal diselenide, including molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂), as saturable absorbers. The MoSe₂ and WSe₂ membranes were covered on silica wafers by chemical vapor deposition (CVD). A low-loss depressed cladding waveguide was produced by femtosecond laser writing in a Nd:YAG crystal. Under optical pump at 808 nm, the passive Q-switching of the Nd:YAG waveguide lasing at 1064 nm was achieved, reaching maximum average output power of 115 mW (MoSe₂) and 45 mW (WSe₂), respectively, which are corresponding to single-pulse energy of 36 nJ and 19 nJ. The repetition rate of the Q-switched waveguide lasers was tunable from 0.995 to 3.334 MHz (MoSe₂) and 0.781 to 2.938 MHz (WSe₂), and the obtained minimum pulse duration was 80ns (MoSe₂) and 52 ns (WSe₂), respectively.

Continuous wave laser operation in Nd:GGG depressed tubular cladding waveguides produced by inscription of femtosecond laser pulses

Depressed tubular cladding waveguides have been produced in Nd:GGG crystals by using multiple inscription with femtosecond (fs) laser pulses. The guiding cores are located inside the tubular regions with cross-section diameters of 90-150 μm, which are surrounded by fs-laser induced low-refractive-index tracks. At room temperature continuous wave (cw) laser oscillations at wavelength of ~1063 nm have been realized through the optical pump at 808 nm. The slope efficiency of the cladding waveguide lasers is as high as 44.4% and the maximum output power at 1063 nm is 209 mW, which shows superior laser performance to the Type II stress induced Nd:GGG waveguides.

Efficient laser emission from cladding waveguide inscribed in Nd:GdVO 4 crystal by direct femtosecond laser writing

We report on the fabrication of depressed cladding waveguides in Nd:GdVO(4) laser crystal by using femtosecond laser inscription. The cross section of the structure is a circular shape with a diameter of 150 μm. Under the optical pump at 808 nm, the continuous wave (cw) as well as pulsed (Q-switched by graphene saturable absorber) waveguide lasing at 1064 nm has been realized, supporting guidance of both TE and TM polarizations. The maximum output power of 0.57 W was obtained in the cw regime, while the maximum pulse energy of the pulsed laser emissions was up to 19 nJ (corresponding to a maximum average output power of 0.33 W, at a resonant frequency of 18 MHz). The slope efficiencies achieved for the cw and pulsed Nd:GdVO(4) waveguide lasers were as high as 68% and 52%, respectively.

Femtosecond Laser Inscribed Y-Branch Waveguide in Nd:YAG Crystal: Fabrication and Continuous-Wave Lasing

Rectangular Y-branch cladding waveguides have been fabricated in Nd:YAG crystal by femtosecond laser inscription. Such novel configurations are fabricated with depth of 50 μm, supporting multimode guidance in both TM and TE polarizations. Continuous wave laser oscillations at wavelength of 1.06 μm have been achieved under the optical pump at 808 nm. The maximum output power is 0.2 W with a slope efficiency of 20% in the device with splitting angle of 0.5°.

Fabrication of Microdomains and Microdomain Patterns by AFM Method in He-Implanted Optical Waveguides on Strontium-Barium Niobate Crystals

For the first time microdomains and microdomain patterns were recorded with the aid of AFM method in planar optical waveguides formed by helium-ion implantation in SBN-Nd crystals. The dependences of isolated domains formation on the recording conditions were investigated. The specific feature of domain formation in implanted crystals is a drastic unipolarity of switching both in FC (poled) and ZFC (unpoled) state. This unipolarity of switching, not observed in unimplanted crystals we relate to domain pinning by the damaged layer formed under He-irradiation.