High-gain Laser Research Articles

Evaluation of amplified spontaneous emission in thin disk lasers using the spectral linewidth.

Amplified spontaneous emission (ASE) is important to power scaling of the large-scale, high-gain thin disk laser. In this paper, spectral properties of ASE in Yb:YAG thin disk lasers are deeply studied in both theory and experiment. The experimental results show that the ASE strength is much stronger when emitted from the edge surface than the pumping area. And the spectrum of ASE emitted from the coarsened edge surface is angle independent. Meanwhile, the reabsorption effect in the Yb:YAG crystal on spectral linewidth is analyzed and corrected. Finally, ASE spectral linewidths have been measured. We demonstrate that the spectral linewidths can evaluate ASE strength effectively.

Impact of temperature on the linewidth enhancement factor of chirped InAs/InP broadband quantum-dash lasers around 1610 nm

We report on the effect of temperature on the differential gain, differential refractive index, and linewidth enhancement factor (α-factor) of ∼1610-nm chirped barrier thickness multistack InAs quantum-dash (Qdash)-in-a-well laser diodes with an extended active region inhomogeneity. By employing Hakki–Paoli method, the performance is found to be comparable at a lower temperature region (16°C to 25°C), exhibiting higher differential gain and lower α-factor values of ∼0.7 ± 0.1 cm − 1 mA − 1 and ∼2.6 ± 1.0, respectively, at gain maximum. At higher temperatures of 25°C to 35°C, the performance degrades mainly due to drop of the differential gain at a rate of ∼0.03 cm − 1 mA − 1 ° C − 1 and α-factor values reaching ∼4.7 ± 2.0 at 40°C. The room temperature (20°C) measured values are in good agreement with the literature, and we qualitatively explain the temperature influence on these results from the highly inhomogeneous Qdash system viewpoint. This study will assist in further optimization of these nanostructure-based high active region gain laser devices that are promising candidates in L-band optical communications, capable of providing low-frequency chirp and high-performance operation.

Application of few-layer WSe2/Sapphire in passively Q-switched solid-state lasers as saturable absorber and output coupler

We demonstrate that it is possible for a few-layer WSe2/Sapphire to act in specific high-gain solid-state lasers as both saturable absorber for passive Q-switching and as etalon mirror for output coupling. A passively Q-switched Yb:LuPO4 microchip laser formed in this manner is realized. With 11.5 W of pump power absorbed, a maximum output power of 1.8 W is produced at a pulse repetition rate of 1.2 MHz with a slope efficiency of 25%; the resulting pulse energy, duration, and peak power are, respectively, 1.5 μJ, 30 ns, and 50 W. Our work provides a simple way of making compact, reliable, high-repetition-rate pulsed microchip or miniature solid-state lasers based on 2D saturable absorbers.

Structural, thermal, and luminescent properties of germanate glass containing heavily Dy2O3 content

The structural, thermal, and luminescent properties of germanate glass containing heavily Dy2O3 content are systematically investigated by means of differential scanning calorimetry, X-ray diffraction, Raman spectra, absorption and emission spectra, and dynamic lifetime decay curves. To seek a suitable composition, the optimal glass forming region is first predicted through the thermodynamic calculation method and their basic physical properties are further forecasted according to the phase diagram model. The experiment results show that the density, refractive index, characteristic temperature, and phonon energy of the glass host all exhibit a close connection with the rare earth concentration. Strikingly, when the amount of Dy2O3 is up to 10.5 mol%, the transition temperature of the glass reaches 747 °C, which is higher than any other known germanate glass and is much advantageous to achieve a higher laser damage threshold. Moreover, the high doping caused the refractive index to increase significantly, which also prompted the glass to obtain a larger emission section and quality factor, increasing the possibility of laser output. Besides, three visible luminescence peaks of Dy3+ at 480 nm (blue) and 575 nm (yellow), and 665 nm (red) are observed even at an extremely high Dy3+ concentration. All the results demonstrate that this kind of germanate glass containing significant amounts of RE could be of interest for implementation of high-gain fiber lasers.

Mode-locking Instabilities for High-Gain Semiconductor Disk Lasers Based on Active Submonolayer Quantum Dots

Optically pumped continuous-wave (cw) semiconductor disk lasers (SDLs) have an established commercial impact, and recent progress in ultrashort-pulse operation makes them attractive for applications in frequency metrology. There is, however, a tradeoff between femtosecond pulse lengths and average output power. Quantum-dot (QD) materials could potentially solve this problem---but not just any dots. The authors show that SDLs based on active submonolayer QDs produce record-high cw output power, but without stable mode locking, which is due to fundamental physical reasons.

Emission characteristics of high-gain GaN-based Vertical-Cavity Surface-Emitting Lasers

GaN-based vertical-cavity surface-emitting lasers (VCSELs) with high optical gain and short cavity lifetime are favorable for the generation of ultra-short pulses in the blue and green regions. In our previous works, 6 and 2 picosecond short-pulses have been generated from gain-switched InGaN VCSELs with 3- and 10-period InGaN/GaN quantum wells (QWs) in the active layers by using an up-conversion measurement system. To further increase the gain of the VCSEL for the generation of even shorter pulses, 20-period InGaN/GaN QWs samples were fabricated. The emission characteristics of these high-gain VCSELs were investigated and analyzed under the optical pumping at room temperature.

Intra-cavity round-trip loss measurement of all-solid-state single-frequency laser by introducing extra nonlinear loss

A scheme for measuring the intra-cavity round-trip loss of an all-solid-state single-frequency laser by inserting a type-I noncritical phase-matching nonlinear crystal introducing nonlinear loss into the resonator is presented. The intra-cavity round-trip loss is theoretically deduced by analyzing the dependence of the fundamental-wave (FW) and second-harmonic-wave (SHW) powers on the pump factor and the nonlinear conversion factor of the single-frequency laser and experimentally measuring them by recording different FW and SHW powers, which are decided by the temperature of the nonlinear crystal. The measured intra-cavity round-trip loss and pump factor are 4.84% and 6.91% W−1, respectively. The standard deviations of the measured intra-cavity round-trip loss and the pump factor are 0.26% and 0.07%, respectively. This scheme is very suitable for measuring the intra-cavity round-trip loss of a high-gain solid-state single-frequency laser.

Pulse laser imaging amplifier for advanced Ladar systems

Coherent image amplification is an important and difficult task that has a wide range of applications including surveillance, detection, and imaging of biological species. Earlier approaches to achieve a high-gain low-noise coherent amplification were met with little success in regards to gain and image quality. These methods included direct coherent beam propagation through the gain medium with inverted population levels as well as two- and four-beam coupling. This paper discusses a high-gain intracavity laser image amplifier. The approach enables time synchronization of the incoming and amplifying signals with an accuracy ≤1 ns. Matching the spectrum of the incoming signal to the cavity modes is no longer necessary with this method and is an essential advantage of the amplifier. Instead, the incoming signal is accepted within the spectral band of the amplifier’s gain material. We have gauged experimentally the performance of the amplifier with a 30-dB gain and an angle of view up to 30 mrad.

Innovative high-gain optically pumped far-infrared laser.

A new optically pumped far-infrared (FIR) laser with separate pump beam reflector and FIR output coupler has been developed. The new design greatly simplifies the tuning of the laser and enables the optimization of the pump beam absorption without affecting the laser alignment. Working with formic acid (HCOOH) vapor pumped by a 30W CO2 laser at the 9R20 line, a high laser gain of 3 dB/m and power conversion efficiency of 16.4% of the Manley-Rowe limit are achieved for the FIR wavelength of 433μm.

Dynamic properties of a pulse-pumped fiber laser with a short, high-gain cavity

Abstract We demonstrate a pulsed high-gain all-fiber laser without intracavity modulators, where a short and heavily Erbium-doped fiber is used as the gain medium in a ring cavity. By pulsed-pumping this short high gain cavity and tuning an intracavity variable optical coupler, the laser generates optical pulses with a pulse-width of μs at a repetition rate in the order of kHz down to one-shot operation. Furthermore, dynamic properties of this laser are investigated theoretically based on a traveling-wave-model, in which an adaptive-discrete-grid-finite-difference-method is applied. The simulation results validate the experimental results. The demonstrated pulsed laser is compact, flexible and cost-effective, which will have great potential for applications in all-optical sensing and communication systems.

MULTI-IFE—A one-dimensional computer code for Inertial Fusion Energy (IFE) target simulations

The code MULTI-IFE is a numerical tool devoted to the study of Inertial Fusion Energy (IFE) microcapsules. It includes the relevant physics for the implosion and thermonuclear ignition and burning: hydrodynamics of two component plasmas (ions and electrons), three-dimensional laser light ray-tracing, thermal diffusion, multigroup radiation transport, deuterium–tritium burning, and alpha particle diffusion. The corresponding differential equations are discretized in spherical one-dimensional Lagrangian coordinates. Two typical application examples, a high gain laser driven capsule and a low gain radiation driven marginally igniting capsule are discussed. In addition to phenomena relevant for IFE, the code includes also components (planar and cylindrical geometries, transport coefficients at low temperature, explicit treatment of Maxwell’s equations) that extend its range of applicability to laser–matter interaction at moderate intensities (<1016 W cm−2). The source code design has been kept simple and structured with the aim to encourage user’s modifications for specialized purposes. Program summaryProgram title: MULTI-IFECatalogue identifier: AEZR_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEZR_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 267428No. of bytes in distributed program, including test data, etc.: 3798542Distribution format: tar.gzProgramming language: Fortran 95.Computer: Any.Operating system: Any.RAM: 1.8 MBClassification: 19.7.Nature of problem: The implosion and ignition of inertial energy fusion capsules driven either directly by laser beams or indirectly by thermal radiation.Solution method: The one-dimensional equations of hydrodynamic motion, laser propagation, energy transport by thermal conduction and radiative transfer, deuterium–tritium thermonuclear burning, and α-particle diffusion are discretized on a Lagrangian grid, and advanced in time by a fractional step scheme. An implicit conservative method is used to treat hydrodynamics. Thermal and electrical conductivities as well as coefficients for laser absorption and electron–ion energy exchange are obtained from a two temperature (for electrons and ions) model. Laser propagation is treated by 3D tracing of the laser rays in the geometrical optics approximation. Radiation transfer is solved by using the forward–reverse method for a discrete number of frequency groups. Matter properties are interpolated from tables (equations-of-state, ionization, and opacities) generated by external codes.Restrictions: The code has been designed for typical conditions prevailing in inertial confinement fusion (ps–ns time scale, matter states close to local thermodynamic equilibrium, and negligible radiation pressure). A wider range of situations can be treated, as long as the above conditions are fulfilled. This includes, in particular, laser plasma experiments at moderate intensities (≤1016W cm−2).Unusual features: An optional graphical post-processing package is included in the distribution. This option requires a Linux/Unix operating system with the essential developing tools (C compiler; X11 libraries and include files; and the xterm command). Most of the figures in this paper have been created using this software.Additional comments: The source code design has been kept simple and structured, with the aim to encourage user’s modifications for specialized purposes. A technical manual is included in the package.Running time: 13–18 s for the examples discussed in the paper.

Spectral narrowing of Yb:YAG waveguide lasers through hybrid integration with ultrafast laser written Bragg gratings.

Laser written waveguides in crystalline materials can be used to make highly efficient, high gain lasers. The bi-directional emission from such lasers however is typically broadband with poor spectral control. Hybridizing a tapered, mode matched laser written Bragg grating with a broadband Yb:YAG crystalline waveguide laser, we demonstrate single longitudinal mode output from one end of the device. Careful control of the grating characteristics led to laser thresholds below 90 mW, slope efficiencies greater than 42% and output powers greater than 20 mW.

Er(3)/Yb(3)-codoped phosphate glass for short-length high-gain fiber lasers and amplifiers.

Er(3)/Yb(3)-codoped phosphate glass with compositions of (78.2-x)P(2)O(5)-14Al(2)O(3)-5Li(2)O-1K(2)O-1.8Yb(2)O(3)-xEr(2)O(3)(x=0.2,0.4,0.6) in mol. % were investigated. Judd-Ofelt (JO) intensity parameters have been calculated to predict radiative properties based on absorption spectra. The stimulated emission cross section (σ(e)) calculated according to McCumber theory was 1.50×10(-20) cm(2), almost twice larger than values reported before. The effective line width (Δ(eff)), full width at half-maximum (FWHM) and the quality parameters for designing optical amplifier devices were listed in the table compared with other types of phosphate glass matrices. A theoretical model of a Er(3)/Yb(3)-codoped system based on rate and power propagation equations was put forward to investigate the potential advantages of the materials applied for short-length, high-gain fiber amplifiers. A simulated gain of 32.2 and 2.6 dB/cm per unit length was achieved in 12.5-cm-long fiber.

Triturating versatile carbon materials as saturable absorptive nano powders for ultrafast pulsating of erbium-doped fiber lasers

Five triturating nano-scale carbon materials based saturable absorbers are compared for passively mode-locked high-gain erbium-doped fiber lasers (EDFLs), including few-layer graphene nano-sheets, graphite, graphene oxide, carbon black and charcoal nano-particles. Nonlinear transmittance analyses show that these nano-scale carbon powders with saturation intensity increased from 0.8 to 6.1 MW/cm2 can reduce their modulation depth (MD) from 60% (few-layer graphene) to 23% (charcoal nano-powder). Except few-layer graphene, other carbon materials exhibit Raman scattering spectra with enlarged D-peaks and attenuated 2D-peaks. Even with the nonlinear absorbance slightly decreased from 5.4 × 10−2 (few-layer graphene) to 3.7 × 10−2 (charcoal), these carbon material powders still start the mode-locking of the EDFLs with <500-fs pulsewidth at L-band. The effect of saturable absorber induced self-amplitude modulation (SAM) becomes weakened, whereas the intense self-phase modulation (SPM) dominates the pulse shortening force afterwards. These observations elucidate that the carbon based saturable absorber plays a role like the mode-locking initiator and stabilizer instead of serving as a strong pulse compressor.

Solid state solvation effect and reduced amplified spontaneous emission threshold value of glass forming DCM derivative in PMMA films

Molecule crystallization is one of the limitations for obtaining high-gain organic laser systems. One of the examples is well known red laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The lowest threshold value of amplified spontaneous emission was achieved by doping 2wt% of DCM molecule in tris-(8-hydroxy quinoline) aluminum (Alq3) matrix. Further increase of the DCM dye concentration makes the system less efficient as its threshold value increases. It is due to large intermolecular interaction, which induces photoluminescence quenching. Compounds with reduced intermolecular interaction could be prospective in organic laser systems due to higher possible doping.In this work photoluminescence and amplified spontaneous emission properties of modified DCM molecule in poly(methyl methacrylate) (PMMA) matrix were investigated. Bulky trityloxyethyl groups were attached to the donor part of DCM. These groups increase intermolecular distance wherewith reduce photoluminescence quenching. More than one order of magnitude lower excitation threshold energy of the amplified spontaneous emission was achieved in doped polymer films with investigated compound in comparison to doped polymer with DCM. It means that the investigated compound is more perspective as a laser material compared to the previously studied. In addition, amplified spontaneous emission maximum could be tuned within 15nm by changing concentration from 0.1wt% to 10wt% DWK-1 in PMMA matrix due to solid state solvation effect.

High-brightness X-ray free-electron laser with an optical undulator by pulse shaping

A normal-incident flattop laser with a tapered end is proposed as an optical undulator to achieve a high-gain and high-brightness X-ray free electron laser (FEL). The synchronic interaction of an electron bunch with the normal incident laser is realized by tilting the laser pulse front. The intensity of the flattop laser is kept constant during the interaction time of the electron bunch and the laser along the focal plane of a cylindrical lens. Optical shaping to generate the desired flattop pulse with a tapered end from an original Gaussian pulse distribution is designed and simulated. The flattop laser with a tapered end can enhance the X-ray FEL beyond the exponential growth saturation power by one order to reach 1 Gigawatt as compared to that without a tapered end. The peak brightness can reach 1030 photons/mm2/mrad2/s/0.1% bandwidth, more than 10 orders brighter than the conventional incoherent Thompson Scattering X-ray source.

High-gain X-ray free electron laser by beat-wave terahertz undulator

The THz undulator has a higher gain to realize a much brighter X-ray at saturation, compared with the optical undulator under the same undulator strength and beam quality. In order to fill the high-power THz gap and realize the THz undulator, two superimposed laser pulses at normal incidence to the electron-beam moving direction form an equivalent high-field THz undulator by the frequency difference to realize the high-gain X-ray Free electron laser. The pulse front tilt of lateral fed lasers is used to realize the electron-laser synchronic interaction. By PIC simulation, a higher gain and a larger X-ray radiation power by the beat wave THz undulator could be realized, compared with the optical undulator for the same electron beam parameters.

Inversion density threshold for Rabi oscillations and modified small-signal gain in extreme-ultraviolet lasers

An expression is proposed for a threshold discriminating between classical quasi-steady-state amplification and a strong pumping regime where a new dynamic behavior shows up in plasma-based extreme-ultraviolet (XUV) lasers. The criterion derived is applied to laser systems currently investigated in various laboratories. It is found that only high-gain XUV laser systems can be driven in that dynamic regime, associated with Rabi oscillations. For those systems, the small-signal dynamic gain accounting for the exponential amplification of the laser intensity is shown to be significantly smaller than the adiabatic value usually inferred from numerical simulations.

Frequency noise of high-gain phosphate fiber single-frequency laser

Frequency noise in single-frequency high-gain phosphate fiber lasers is investigated theoretically and experimentally. An analytical expression relating the pump fluctuations to the laser frequency noise is established and verified by a homemade single-frequency phosphate fiber laser. The theoretical and experimental results indicate that the frequency noise of the output laser can be reduced by lowering the intensity noise of pump source. This will help improving the laser performance and progress the wide application of this type of fiber laser.

Growth of high-density 1.06-μm InGaAs/GaAs quantum dots for high gain lasers by molecular beamepitaxy

We have progressed the growth procedure for high-density highly-uniform In(Ga)As/GaAs quantum dots (QDs) by using molecular beam epitaxy and have demonstrated high-gain 1.06-μm QD lasers for consumer electronics. The structural and optical properties of QD crystals have been improved by optimizing the growth temperature, employing InGaAs QDs in place of InAs QDs, and utilizing a height-limiting growth method, which is called the Indium-flush procedure. Lasers containing ten-layer height-limited InGaAs QDs provided a net modal gain as high as 60cm−1, which is one of the largest values for InAs/GaAs QD lasers. These results will promise the development of optoelectronic devices with high-density multiply-stacked QDs in the 1-μmband.