Pulsed Laser Operation Research Articles

DFB Laser Diodes Based on GaN Using 10th Order Laterally Coupled Surface Gratings

Single longitudinal mode emission of laterally coupled distributed-feedback (LC-DFB) laser diodes (LDs) based on InGaN/GaN multiquantum-well structures containing 10th-order surface Bragg gratings with V-shaped grooves is demonstrated. The gratings were fabricated alongside a 2-μm-wide contact stripe by i-line stepper lithography and inductively coupled plasma etching. A single peak emission at 404.6 nm with a fullwidth at half-maximum of 0.04 nm was achieved at an output power of about 46 mW under pulsed laser operation. The shift of the lasing wavelength of LC-DFB LDs in the temperature range from 22 °C to 45 °C was around three times smaller than that of comparable ridge waveguide Fabry-Perot LDs.

Multi-operational tuneable Q-switched mode-locking Er fibre laser

A wavelength-spacing tuneable, Q-switched mode-locking (QML) erbium-doped fibre laser based on non-linear polarization rotation controlled by four waveplates and a cube polarizer is proposed. A mode-locked pulse train using two quarter-wave plates and a half-wave plate (HWP) is obtained first, and then an extra HWP is inserted into the cavity to produce different operation regimes. The evolutions of temporal and spectral dynamics with different orientation angles of the extra HWP are investigated. A fully modulated stable QML pulse train is observed experimentally. This is, to the author’s best knowledge, the first experimental work reporting QML operation without adding an extra saturable absorber inside the laser cavity. Multi-wavelength pulse laser operation, multi-pulse train continuous-wave mode-locking operation and pulse-splitting operations are also reported at certain HWP angles. The observed operational dynamics are interpreted as a mutual interaction of dispersion, non-linear effect and insertion loss. This work provides a new mechanism for fabricating cheap tuneable multi-wavelength lasers with QML pulses.

Continues-wave Brillouin-Raman fiber ring laser using 7.7 km long dispersion compensating fiber at 1563 nm wavelength

We demonstrate the generation of continues-wave (CW) laser based on Brillouin-Raman fiber laser (BRFL) by incorporating a 7.7 km long dispersion compensating fiber (DCF) as a nonlinear gain medium. The 1455 nm Raman pump (RP) was launched into the DCF via a wavelength division multiplexing (WDM). At 645 mW pump power, stable CW laser presence at 1563.5 nm (1st stokes), 1563.4 (2nd stokes) nm and 1563.3 nm (3rd stokes). Those peak wavelengths have produced most identical output power. This finding proves our ring laser cavity has the capability to generate stable CW BRFL. Thus, it potentially to generate pulse laser operation in the future work.

Laser micropolishing of AISI 304 stainless steel surfaces for cleanability and bacteria removal capability

In this work, laser micropolishing (LμP) was employed to reduce the surface roughness and waviness of cold-rolled AISI 304 stainless steel sheets. A pulsed fibre laser operating in the ns regime was used and the influence of laser parameters in a N2-controlled atmospheres was evaluated. In the optimal conditions, the surface remelting induced by the process allowed to reduce the surface roughness by closing cracks and defects formed during the rolling process. Other conditions that did not improve the surface quality were analysed for defect typology. Moreover, laser treatments allowed the production of more hydrophobic surfaces, and no surface chemistry modification was identified. Surface cleanability was investigated with Escherichia coli (E. coli), evaluating the number of residual bacteria adhering to the substrate after a washing procedure. These results showed that LμP is a suitable way to lower the average surface roughness by about 58% and average surface waviness by approximately 38%. The LμP process proved to be effective on the bacteria cleanability as approximately five times fewer bacteria remained on the surfaces treated with the optimized LμP parameters compared to the untreated surfaces.

Simulation of Manipulating Various Pulsed Laser Operations Through Tuning the Modulation Depth of a Saturable Absorber

Simulation of Manipulating Various Pulsed Laser Operations Through Tuning the Modulation Depth of a Saturable Absorber

Graphene oxide-COOH as a new saturable absorber for both Q-switching and mode-locking fiber lasers

Graphene oxide carboxylic acid (COOH), a novel two-dimensional (2D) layered material with its unique optical and electronic properties, is discovered to exhibit the saturation of optical absorption under laser illumination. Applying the liquid-phase exfoliation method, we prepare graphene oxide-COOH dispersions with deionized water and fabricate graphene oxide-COOH polyvinyl alcohol polymer composite film. We further obtain stable Q-switching pulse and mode-locked laser operation with a 22.7 MHz repetition rate and a 1.5 ps pulse duration by incorporating the graphene oxide-COOH-based saturable absorbers into the all-fiber erbium-doped fiber laser cavity. The experimental results show that the proposed graphene oxide-COOH material can act as an effective absorber for pulsed fiber lasers, which demonstrate potential applications in the area of ultrafast optics.

Dual-wavelength Q-switched Er:SrF2 laser with a black phosphorus absorber in the mid-infrared region.

An efficient Er:SrF2 crystal, lightly Er-doped to a concentration of 4at.%, was successfully grown by the traditional Bridgman method and displayed excellent spectral properties. A diode-end-pumped passively Q-switched dual-wavelength laser, operating at 2.79 μm wavelength, was demonstrated with this crystal by using black phosphorus as the saturable absorber (BP-SA). In the compact passively Q-switched Er:SrF2 laser, the maximum average output power of 180 mW was achieved at an absorbed pump power of 2.47 W, with a pulse duration of 702 ns and a repetition rate of 77.03 kHz. To the best of our knowledge, this is the first reported application of BP-SA to dual-wavelength pulse laser operation in the mid-infrared region.

Black phosphorus based saturable absorber for Nd-ion doped pulsed solid state laser operation

In this paper, the use of black phosphorus (BP) as a saturable absorber in a Q-switched Nd-ion doped solid state laser is presented. Few layers of BP in isopropyl alcohol are obtained by liquid phase exfoliation. The BP nanosheets with thicknesses in the range of 15–20 nm are deposited onto a K9 glass substrate. By inserting the BP nanosheets into a diode pumped Nd-ion doped solid state laser, stable Q-switched lasing at 0.9, 1.06, 1.3 μm is obtained. Using this approach, we have achieved a short pulse duration down to 219 ns, a high pulse energy of up to 6.5 μJ, and the corresponding peak power of 30 W. Our results show that the BP saturable absorber functions well in a Nd-ion doped solid state laser for pulsed laser generation.

On second harmonic generation and multiphoton-absorption induced luminescence from laser-reshaped silver nanoparticles embedded in glass

Spherical silver nanoparticles (NPs) of 30 nm diameter embedded in soda-lime glass were uniformly reshaped (elongated) after irradiation by a linearly polarised 250 fs pulsed laser operating within the NPs’ surface plasmon resonance band. We observed second harmonic generation (SHG) and multiphoton-absorption-induced luminescence (MAIL) in the embedded laser-reshaped NPs upon picosecond (10 ps) pulsed laser excitation at 1064 nm. A complementary study of SHG and MAIL was conducted in soda-lime glass containing embedded, mechanically-reshaped silver NPs of a similar elongation ratio (aspect ratio) to the laser-reshaped NPs. This supports the notion that the observed difference in SHG and MAIL in the studied nanocomposite systems is due to the shape modification mechanism. The discrete dipole approximation method was used to assess the absorption and scattering cross-sections of the reshaped NPs with different elongation ratios.

Effect of the filler amount on the optical absorption properties and the surface features of polymeric joints based on biomedical UHMWPE welded by a Nd:YAG laser

Polymeric materials can be welded by the laser transmission welding technique in order to join two or more sheets. At the interface of the polymer sheets, the released energy induces melting that is assisted by pressure, producing a fast and resistant welding. In this research, single-lap polymeric joints of biomedical ultra-high molecular weight polyethylene sheets were realized; one of the two polymer sheets was doped, at different concentrations, with carbon nanomaterials as a laser absorbent filler. A pulsed neodymium-doped yttrium aluminium garnet laser operating in the wavelength of 532 nm and of 1064 nm with an intensity of the order of 109 W/cm2 was employed to obtain a joint at the interface between the transparent polymer and the absorbent one. The mechanical shear resistance of the prepared joints was related to the optical absorption feature of the polymeric materials. Besides, surface analyses such as hardness, roughness and wettability in different inorganic and organic fluids were presented and discussed. The result of characterization analyses indicated the best filler amount in order to have a mechanically resistant joint with surface properties also suitable for the biomedical employment.

Gold nanobipyramid Q-switched Nd:LGGG eye-safe laser operating at 1423.4 nm.

The performance of a laser-diode-pumped passively Q-switched Nd:LGGG laser at 1423.2nm with gold nanobipyramids (Au-NBPs) as a saturable absorber was demonstrated. An average output power of 125mW was obtained at a pump power of 12.2W, corresponding to an optical-to-optical conversion efficiency of 1.36% and a slope efficiency of 1.78%. A minimum pulse width of 514ns at a pulse repetition rate of 98.6kHz was obtained at a pump power of 12.2W. To the best of our knowledge, this is the first report focusing on the application of Au-NBPs as a saturable absorber for pulse laser operation in the eye-safe region.

Optically controlled in-line graphene saturable absorber for the manipulation of pulsed fiber laser operation.

We demonstrate an optically tunable graphene saturable absorber to manipulate the laser operation in pulsed fiber laser system. Owing to the strongly enhanced evanescent field interaction with monolayer graphene, we could realize an efficient control of modulation depth in the graphene saturable absorber by optical means through cross absorption modulation method. By integrating the tunable graphene saturable absorber into the fiber laser system, we could switch the laser operation from Q-switching through Q-switched mode-locking to continuous wave mode-locking by adjusting only the optical power of the control beam. In addition, we realized a hybrid Q-switching of fiber laser by periodical modulation of the absorption of the graphene saturable absorber, where we observed that the repetition rate of the Q-switched laser could be continuously tuned according to the modulation frequency of the applied external signal.

Improved Pulsed Laser Operation with Engineered Nanomaterials

The power efficiency of a diode-pumped solid state laser was improved by encasing the Nd:YAG lasing medium with gold nanorod-doped epoxy. Gold nanorods were synthesized with a specific aspect ratio tuned to absorb at the Nd:YAG lasing wavelength of 1064 nm. The surfactant-stabilized nanorods in aqueous solution were then treated with a sequential, two-step functionalization in order to improve nanorod solubility in organic solvents. This process required treating the gold nanorods with a thiol-containing polyethylene glycol (PEG) polymer followed by replacing the PEG-SH polymer with a multidentate thiol containing block copolymer synthesized using RAFT synthesis. With a multidentate polymer, the nanorods were soluble in traditional epoxies that could be used to coat the outside of Nd:YAG rods. By absorbing excess lateral 1064 nm emission, the gold nanorod coating attenuates amplified spontaneous emission (ASE), a parasitic, incoherent process that limits power produced by Q-switch laser designs. Laser power increased by nearly 2-fold with addition of the 1064 nm absorbing gold nanorod coating. Gold nanorod epoxy coatings stand out as attractive materials for attenuating ASE by avoiding the fabrication difficulties of samarium oxide ceramics and the photostability limitations of organic dyes.

Polyethylene laser welding based on optical absorption variations

Polymeric materials, both pure and containing nanostructures, can be prepared as thin sheets in order to produce joints with an interface between an optically transparent sheet and an optically absorbent substrate to be welded by infrared pulsed laser irradiation. The Laser Transmission Welding (LTW) technique has been successfully applied in order to join two or more thermoplastic polymeric sheets that must have a similar chemical composition. In this research work, polymeric joints of Ultra High Molecular Weight Polyethylene sheets were realized, characterized and welded. Some polymer sheets were doped, at different concentrations, with carbon nano-particles absorbent the laser radiation. A pulsed laser operating in the wavelength region 532 nm with intensity of the order of 109 Watt/cm2 was employed to be transmitted by the transparent polymer and to be absorbed by the carbon enriched surface. At the interface of the two polymers the released energy induces melting, that is assisted by pressure, producing a fast and resistant welding zone. Mechanical and optical characterizations and surface analyses are presented and discussed.

Free charge carrier absorption in silicon at 800 nm

The transmission of a Ti:sapphire laser beam (c.w. and fs pulsed operation at 800 nm) through a 10-μm-thin oxidized silicon membrane at 45° angle of incidence at first increases with the incident laser power, then shows a maximum, and finally decreases considerably. This nonlinear transmission behavior is the same for c.w. and pulsed laser operation and mainly attributed to free charge carrier absorption (FCA) in Si. A simple FCA model is developed and tested.

Nanosecond passively Q-switched thulium/holmium-doped fiber laser based on black phosphorus nanoplatelets

The generation of nanosecond pulses in a thulium/holmium-doped fiber laser passively Q-switched by a black phosphorus saturable absorber (BP-SA) was experimentally demonstrated. The high quality BP-SA with a modulation depth of ~24% was fabricated by depositing ~23 nm thickness of BP nanoplatelets onto a fiber ferrule. By inserting the BP-SA into a thulium/holmium-doped fiber laser cavity, stable Q-switched operation was achieved with the maximum pulse energy up to 632.4 nJ, the shortest pulse width of 731 ns and pulse repetition rates varying from 69.4 to 113.3 kHz. These results suggested that BP could be developed as an effective SA for pulsed laser operation at 2 μm.

Thulium/holmium-doped fiber laser passively mode locked by black phosphorus nanoplatelets-based saturable absorber.

By coupling black phosphorus (BP) nanoplatelets (NPs) with a fiber-taper evanescent light field, a saturable absorber (SA) based on the BP NPs has been successfully fabricated and used in a thulium/holmium-doped fiber laser as the mode locker. The SA had a modulation depth of ∼9.8% measured at 1.93 μm. A stable mode-locking operation at 1898 nm was achieved with a pulse width of 1.58 ps and a fundamental mode-lock repetition rate of 19.2 MHz. By increasing the pump intensity, phenomena of multi-pulsing operations, including harmonic mode-locked states and soliton bunches, were obtained in the experiment, showing that the BP NPs possess an ultrafast optical response time. This work suggests that the BP NPs-based SA is potentially useful for ultrashort, pulsed laser operations in the eye-safe region of 2 μm.

Synthesis, characterization, spectroscopy, and laser operation of SrMoO4 crystals co-doped with Tm3+ and Ho3+

We report on the crystal growth, optical spectroscopy, and 2-µm laser operation of co-doped Tm3+/Ho3+:SrMoO4 scheelite crystals. High optical quality crystals co-doped with Tm3+ (7.9×1019cm−3) and Ho3+ (8.0×1019cm−3) were grown from the melt in air by the Czochralski method. We characterized the spectroscopy of Tm3+ and Ho3+ ions in SrMoO4 in terms of polarized optical absorption, photoluminescence, and lifetime. Pulsed diode-pumped laser operation at room temperature at a wavelength of 2.06μm was achieved with a maximum output power of 1.62mW and a slope efficiency of 3%. The results showed that Tm3+/Ho3+:SrMoO4 crystals can be a promising material for developing a diode pumped mid-IR laser with Raman self-conversion of radiation in the active medium.

Well-aligned single-walled carbon nanotubes for optical pulse generation and laser operation states manipulation

We report operation switchable fiber lasers by using well-aligned (2,2) single-walled carbon nanotubes (SWCNTs) as saturable absorber. These SWCNTs are fabricated in the elliptical nanochannels of ZnAPO4-11(AEL) zeolite single crystal, which features anisotropic optical absorption. Thus, these SWCNTs can act as an excellent polarization sensitive saturable absorber for the wide-band optical pulse generation and laser operation states manipulation. By putting a SWCNTs@AEL single crystal into erbium-, and ytterbium-doped fiber laser cavities, stable mode-locking and Q-switching pulses can be switched by simply adjusting a polarization controller under a fixed pump power, respectively.

Tunable S-Band Q-Switched Fiber Laser Using Bi<sub>2</sub>Se<sub>3</sub> as the Saturable Absorber

This paper describes a successful demonstration of S-band region wavelength output generated via a tunable Q-switch fiber laser with the topological insulator (TI) Bi2Se3 as a saturable absorber (SA). The modulation depth of the TI was measured as 11.1%, whereas the utilized 980-nm laser source operated at 100 mW pump power. By inserting the TI-SA between two ferrules and utilizing a tunable bandpass filter (TBPF), a stable and tunable Q-switching operation was achieved over the S-band wavelength region from 1493.6 to 1508.9 nm, wherein pulsewidth was 7.6 $\mu{\rm s} $ , and the tunable repetition rate ranged from 26.1 to 36.6 kHz. The Q-switched pulses had maximum pulse energy of 6.1 nJ, as measured from 2.5% of the cavity output. The results provide evidence that a TI-based SA is suitable for pulsed laser operation in the S-band wavelength region and offers potential for further development as an ultrabroadband photonics device.