Tunable Ytterbium-Doped Fiber Laser Based on a Mechanically Induced Long Period Holey Fiber Grating

We describe a tunable double-clad Yb-doped fiber laser based on a long period fiber grating mechanically induced in a section of single mode holey fiber inserted into the laser cavity. The mechanically induced long period holey fiber grating acts as a wavelength-selective fiber filter whose central wavelength, linewidth, and strength can be tuned by changing the period, the length of the grating, and the applied pressure. The fiber laser gives a ∼12.6 nm tuning range, from ∼1079:4–1092nm, with slope efficiencies of 18.7–26.3% at this wavelength range, with respect to the launched pump power.     

Synthesis of LaB6 micro/nano structures using picosecond (Nd:YAG) laser and its field emission investigations

Micro/nano structures have been obtained by laser surface treatment on sintered LaB₆ pellets employing a picosecond pulsed Nd:YAG laser at a pressure of ∼1×10⁻³ mbar. The X-ray diffraction pattern of the laser treated pellet shows a set of well defined diffraction peaks, indexed to the cubic phase of LaB₆ only. The scanning electron microscope studies reveal formation of micro and nano structures upon laser treatment and the resultant surface morphology is found to be strongly influenced by the laser fluence. Field electron emission studies made on the LaB₆ pellet, treated with optimized laser fluence, have been performed in a planar diode configuration under ultra high vacuum conditions. The threshold field required to draw an emission current density of ∼10 μA/cm² has been found to be ∼2.3 V/μm and a current density of ∼530 μA/cm² has been drawn at an applied field of 5.2 V/μm. The Fowler-Nordheim plot is found to be linear in accordance with the quantum mechanical tunneling phenomenon, confirming the metallic nature of the emitter. The emission current at the pre-set value ∼10 μA shows very good stability over a period of more than 3 hours. The present results emphasize the effectiveness of a picosecond laser treatment towards fabrication of a nano metric LaB₆ emitter for high current density applications.     

Distributed feedback dye laser holographically induced in improved organic–inorganic photocurable nanocomposites

Distributed feedback (DFB) lasing in permanent volume transmission gratings formed in a laser dye-doped organic–inorganic nanocomposite has been investigated. DFB laser cavities were fabricated using one-step two-beam holographic exposure of Pyrromethene 567 (PM567) doped photopolymerizable acrylate monomers containing inorganic (LaPO₄) nanoparticles. Compared to the formulation previously utilized, the material composition presented provides longer lifetime of the laser. Spectral and polarization properties, input–output and stability characteristics of the laser output have been investigated by varying the material composition and the patterning parameters. DFB lasing emission of the second and the third diffraction orders has been demonstrated. The spectral linewidth of ∼0.08 nm has been observed at a pump energy threshold of about 0.2 μJ/pulse for the second-order DFB lasing when pumped with 532 nm 500 ps laser pulses. Spectral tuning of the lasing output over ∼56 and ∼7 nm was obtained by varying the grating period and the content of inorganic nanoparticles in the polymer matrix, respectively.     

Rapid fabrication of optical volume gratings in Foturan glass by femtosecond laser micromachining

We report on rapid fabrication of optical volume gratings in Foturan glass using a modulated femtosecond laser focused with cylindrical lenses. An optical volume grating with an area of 2 mm ×3 mm and ∼2 mm thickness can be achieved within 10 min by use of this method. Optical micrography confirms the volume nature of the gratings and shows that they consist of 10 μm-thickness planes with a period of 15 μm. The diffraction efficiency is examined to be ∼56%. The limitations and future implementations of the fabricated volume gratings are discussed.     

Physics and technology of tunable pulsed single longitudinal mode dye laser

Design and technology demonstration of compact, narrow bandwidth, high repetition rate, tunable SLM dye lasers in two different configurations, namely Littrow and grazing incidence grating (GIG), were carried out in our lab at BARC, India. The single longitudinal mode (SLM) dye laser generates single-mode laser beams of ∼400 MHz (GIG configuration) and ∼600 MHz (Littrow configuration) bandwidth. Detailed performance studies of the Littrow and GIG dye laser resonators showed that GIG dye laser results in narrower linewidth and broad mode hop free wavelength scanning over 70 GHz. In this paper we present experimental studies carried out on the high repetition rate SLM dye laser system.     

High spectral resolution analysis of tunable narrowband resonant grating waveguide structures

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high-performance dye laser with ∼ 0.15 cm^-1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths in the 694 nm and 633 nm ranges. A reflectivity of ∼ 91% and a line width of ∼ 0.55 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. For a second grating waveguide structure, designed for the helium-neon laser emission wavelength 632.8 nm, we observed a thermal shift of its spectral resonance position of several nanometers, when increasing the sample temperature by some degrees. An inverse thermal shift was observed when the structure was subsequently cooled down to room temperature. Our results suggest implementation of grating waveguide devices combining a narrow line width with a tunability of the resonant response into innovative concepts for reflection filter and sensor applications. PACS 42.62.-b; 42.79.Dj; 42.79.Gn     

Modelling of a simple Dy<Superscript>3+</Superscript> doped chalcogenide glass fibre laser for mid-infrared light generation

A simple Dy³⁺-doped chalcogenide glass fibre laser design for mid-infrared light generation is studied using a one dimensional rate equation model. The fibre laser design employs the concept of cascade lasing. The results obtained demonstrate that efficient cascade lasing may be achieved in practice without the need for fibre grating fabrication, as a sufficient level of feedback for laser action is provided by Fresnel light reflection at chalcogenide glass fibre–air interfaces. Further enhancement of the laser efficiency can be achieved by terminating one of the fibre ends with a mirror. A numerical analysis of the effect of the Dy³⁺ doping concentration and fibre loss on the laser operation shows that with 5 W of pump power, at 1.71 μm wavelength, output powers above 100 mW at ∼ 4.5 μm wavelength can be achieved with Dy³⁺ ion concentrations as low as 3 × 10¹⁹ cm⁻³, when fibre loss is of the order 1dB/m.     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

Self-grown fiber fabrication by two-photon photopolymerization

We demonstrate a new fiber growth mechanism in a photocurable resin by ultrafast laser illumination. A high-repetition rate (∼1 MHz) ultrafast laser beam at the wavelength of ∼523 nm was focused into an ultraviolet photocurable resin to trigger two-photon photopolymerization process. Time-resolved shadowgraphs and scattered light imaging revealed that the curing commenced in the neighborhood of the geometric focal point of the laser beam and that the fiber growth progressed mostly towards the laser source. The cured fiber was thinner and longer than the profile of the focused laser beam, facilitated by nonlinear propagation and absorption of the ultra-fast laser beam. The achieved aspect ratio of the fiber was higher than 180 with ∼10 μm mean diameter, and the average growth rate was up to ∼2 mm/s.     

Stable output, high power diode-pumped Tm:YLF laser with a volume Bragg grating

A stable output, high power diode-pumped Tm: YLF laser operating at 1908 nm with FWHM line width less than 0.1 nm is reported. Using a volume Bragg grating, 41.1 W of output with M ²∼2 under an incident pump power of 111.7 W was achieved, corresponding to an optical-to-optical conversion efficiency of 36.8% and a slope efficiency of 43.2%. A laser wavelength shift of only 0.3 nm with the incident pump power varying from 13.1 W to 111.7 W was observed.     

Controlling aspect ratios of suspended nanorods fabricated by multi-photon polymerization

Nanorods are building blocks of three-dimensional photonic crystals and other nanostructures fabricated by multi-photon polymerization with femtosecond laser pulses. The aspect ratios of their cross sections are critical to the in-plane and the interlayer rod distances, which greatly affect the performance. Here we demonstrate the control of aspect ratios from ∼3 to 0.85. At a high scanning speed, aspect ratios can be smaller than unity with a lateral size of ∼150 nm. The results indicate that cylindrical nanorods can be polymerized by the commonly used transverse scanning method to improve the qualities of three-dimensional nanostructures.     

Filamentary cavity formation in poly(methyl methacrylate) by single femtosecond pulse

A single femtosecond laser pulse creates a filamentary structural change along the optical axis inside bulk poly(methyl methacrylate). The filamentary structural change was revealed to be a cylindrical cavity based on scanning electron microscope examination, the presence of capillary action, and analysis of diffraction by an embedded diffraction grating. The cavity had a diameter of 0.8 μm and a length of 125 μm. PACS 42.65.Jx; 42.70.Jk     

Effective pulse recompression after nonlinear spectral broadening in picosecond Yb-doped fiber amplifier

We report the performance of a picosecond master-oscillator power amplifier (MOPA) system based on a diode-pumped solid-state (DPSS) seed laser and Yb-doped fiber amplifier. An average power of 28 W at ∼200 MHz repetition rate is achieved by using only one amplification stage. We found that positive nonlinear phase shift induced by nonlinear effect in the active fiber can be effectively compensated by a grating pair. A pulse duration of ∼1.6 ps is shown after recompression.     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

Localized insulator-conductor transformation of graphene oxide thin films via focused laser beam irradiation

By scanning a focused laser beam over graphene oxide (GO) film deposited on SiO₂/Si substrates, conductive strips as small as 1 μm can be patterned directly either as a channel in the insulating matrix, or as a stand-alone micro belt. The conductivity was increased by at least two orders of magnitude with the mobility estimated in the range of 1–10 cm²/V s. Raman mapping and X-ray photoelectron spectroscopy studies demonstrated the reduction of GO in the laser-irradiated area. The conductance of the patterned channel was independent of the change in oxide-electrode contact resistance of the graphene, and increased linearly with increasing channel width. Increasing irradiation power by repeated scanning initially increased the conductivity of the irradiated area and saturated at a conductivity of ∼36 S/cm. Partial oxidative burning combined with photothermal reduction was identified as the underlying mechanism for the enhancement of the conductivity after laser irradiation on the GO film. Oxidative burning can be controlled by varying the film thickness and laser power.     

Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

High-efficiency mid-infrared laser from synchronous optical parametric oscillation and amplification based on a single MgO:PPLN crystal

This paper presents a specially designed optical parametric oscillator (OPO) which achieved high-efficiency mid-infrared laser of 2.83 μm. The cascaded nonlinear interactions of OPO and optical parametric amplifier (OPA) were simultaneously realized in a single MgO:PPLN crystal. The signal oscillation of 1.70 μm was used to pump a secondary parametric process that resulted in amplification of the idler laser of 2.83 μm. When the MgO:PPLN crystal with a grating period of 31.2 μm was pumped by a 1.064 μm laser and operated at 148°C, the quasi-phase-matching of both OPO and OPA could be simultaneously achieved. Average output power of 7.68 W at 2.83 μm was obtained for 25 W of pump at 7 kHz. The power conversion efficiency of 2.83 μm laser was 30.7%, which was evidently higher than common OPOs.     

A novel-configuration multi-wavelength Brillouin erbium fiber laser and its application in switchable high-frequency microwave generation

A novel configuration of compound-cavity multi-wavelength Brillouin erbium fiber laser is proposed and experimentally demonstrated. With an incident optical carrier power of 8 dBm, at least 14 lasing lines are obtained with a wavelength spacing of ∼0.08 nm. Stability and power uniformity of the multi-wave-length lasing are ensured by the flat hybrid gain of Brillouin and erbium, the compound-cavity structure, and the four-wave mixing suppression using a long (10 km) single-mode fiber. A stable and frequency-switchable microwave can be achieved by incorporating a fiber Bragg grating filter to select the desired nth-order Stokes wave and beating it with the optical carrier at a photodetector. In our experiment, the 1st-4th-order Stokes waves are filtered respectively and hence a high-quality microwave with a switchable frequency from ∼10 to ∼40 GHz and a tuning step of ∼10 GHz is achieved. The signal-to-noise ratio is measured to be >25 dB.     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr     

Room-temperature continuous wave laser oscillations in Nd:YAG ceramic waveguides produced by carbon ion implantation

We report on the generation of continuous wave lasers at a wavelength of ∼1064 nm in a Nd:YAG ceramic waveguide at room temperature. The waveguide was fabricated by using 6 MeV carbon ion implantation at a fluence of 3×10¹⁴ ions/cm². Laser operation has been realized with a slope efficiency as high as ∼11%. The pump threshold of an 808-nm laser beam for the waveguide laser oscillation is 19.5 mW.