Region Of Laser Research Articles

Routes to chaos in a class-B laser coupled to a neutral resonator

We study the dynamics of a class-B semiconductor microring laser coupled to a neutral microring resonator for common fabrication parameters. For zero detuning between the resonators, we identify five dynamical regions controlled by the normalized pump rate of the laser and coupling strength between the resonators. These regions show stable lasing with phase locking for either sufficiently small or large coupling strength parameter values. Multistability and chaos arise in between these stable regions where at least one fixed point is attractive. The transition from stable to unstable lasing regions in the parameter space phase diagram is a crossover.

Elimination of catastrophic optical mirror damage in continuous-wave high-power laser diodes using multi-section waveguides.

One of the persistent obstacles for high-power laser diodes (LDs) has been the catastrophic optical mirror damage (COMD), which limits the operating power level and lifetime of commercial high-power LDs. The output facet of LD reaches a critical temperature resulting in COMD, which is an irreversible device failure. Here, we fabricate multi-section LDs by tailoring the waveguide structure along the cavity that separates the output facet from the heat-generating lasing region. In this method, the LD waveguide is divided into electrically isolated laser and window sections along the cavity. The laser section is pumped at a high current to achieve high output power, and the window is biased at a low current with negligible heat generation. This design restricts the thermal impact of the laser section on the facet, and the window section allows lossless transport of the laser to the output facet. The lasers were operated continuous-wave up to the maximum achievable power. While standard LDs show COMD failures, the multi-section waveguide LDs are COMD-free. Our technique and results provide a pathway for high-reliability LDs, which would find diverse applications in semiconductor lasers.

Mid-infrared tunable pulsed laser based on Cr2+-doped II–VI chalcogenide

Cr2+-doped II–VI chalcogenides are attractive laser media for direct access to the mid-infrared (IR) region from 2 to 3 μm. We have developed a mid-IR tunable laser using Cr:ZnSe and Cr:CdSe as laser media. We have used a Q-switched Tm:YAG laser as a pump source. For the Cr:ZnSe laser, the tuning range of 2.18–2.83 μm and the maximum pulse energy of 4.98 mJ at 2.38 μm were obtained with a pump energy of 22 mJ. For the Cr:CdSe laser, the tuning range of 2.22–3.04 μm and the maximum pulse energy of 2.42 mJ at 2.54 μm were achieved with a pump energy of 22 mJ. In addition, we have demonstrated a method of extending the mid-IR lasing region by a combined active medium technique with Cr:ZnSe and Cr:CdSe. We observed the broad tuning range from 2.15 to 3.04 μm and the maximum pulse energy of 5.96 mJ at 2.44 μm with a pump energy of 22 mJ.

Controllable X-Ray Pulse Trains from Enhanced Self-Amplified Spontaneous Emission.

We report the demonstration of optical compression of an electron beam and the production of controllable trains of femtosecond, soft x-ray pulses with the Linac Coherent Light Source (LCLS) free-electron laser (FEL). This is achieved by enhanced self-amplified spontaneous emission with a 2 μm laser and a dechirper device. Optical compression was achieved by modulating the energy of an electron beam with the laser and then compressing with a chicane, resulting in high current spikes on the beam which we observe to lase. A dechirper was then used to selectively control the lasing region of the electron beam. Field autocorrelation measurements indicate a train of pulses, and we find that the number of pulses within the train can be controlled (from 1 to 5 pulses) by varying the dechirper position and undulator taper. These results are a step toward attosecond spectroscopy with x-ray FELs as well as future FEL schemes relying on optical compression of an electron beam.

Dependence of static K DPAL performance on addition of methane to He buffer gas: 3D CFD modeling and comparison with experimental results

Recently it was observed that adding a small amount of methane to the He buffer gas of a static potassium diode pumped alkali laser (K DPAL) increases considerably the laser power. Further increase in the amount of methane leads to a moderate decrease in power. In the present work the effect of methane addition was investigated theoretically applying a 3D computational fluid dynamics (CFD) and potassium kinetics model, which was supplemented by the analysis of the electron temperature and K ion ambipolar diffusion. It was found that for a pure He buffer the K DPAL power is lower than for ${\rm He}/{{\rm CH}_4}$He/CH4 mixtures due to slow ion-electron recombination and high electron temperature exceeding 3000 K. The high electron temperature in pure He results in fast ambipolar diffusion of K ions to the wall and depletion of the neutral K atoms in the lasing region. These effects are mitigated when methane is added to the buffer gas. The calculated results for the normalized laser power are in satisfactory agreement with the experimental ones.

Low-threshold lasing in ZnO microtetrapods

Lasing of ZnO microtatrapods in the spectral range of exciton recombination was studied at room temperature. Samples were produced by a method of pyrolytic carbothermal synthesis. With nanosecond pumping in Al-doped ZnO sample the record low value of the lasing threshold was observed. The threshold pumping energy was 0.13 μJ/cm2. The dependences of the integrated intensity on the pumping level were obtained. As expected, they appeared to be linear in the lasing region. It was shown that with pumping level increase the intensities of laser lines is red shifted, i.e. long wavelength lines become more intensive. This is suggested to be related to the band-gap renormalisation as a result of possible heating or electron-hole plasma formation. The possible nature of the laser resonator in the tetrapod was discussed. Most likely that the resonator is formed at the bottom of the tetrapod leg where its cross-section is hexagonal. The appearance of the whispering gallery modes is possible in such resonator.

Conservation of quantum efficiency in quantum well intermixing by stress engineering with dielectric bilayers

In semiconductor lasers, quantum well intermixing (QWI) with high selectivity using dielectrics often results in lower quantum efficiency. In this paper, we report on an investigation regarding the effect of thermally induced dielectric stress on the quantum efficiency of quantum well structures in impurity-free vacancy disordering (IFVD) process using photoluminescence and device characterization in conjunction with microscopy. SiO2 and SixO2/SrF2 (versus SrF2) films were employed for the enhancement and suppression of QWI, respectively. Large intermixing selectivity of 75 nm (125 meV), consistent with the theoretical modeling results, with negligible effect on the suppression region characteristics, was obtained. SixO2 layer compensates for the large thermal expansion coefficient mismatch of SrF2 with the semiconductor and mitigates the detrimental effects of SrF2 without sacrificing its QWI benefits. The bilayer dielectric approach dramatically improved the dielectric–semiconductor interface quality. Fabricated high power semiconductor lasers demonstrated high quantum efficiency in the lasing region using the bilayer dielectric film during the intermixing process. Our results reveal that stress engineering in IFVD is essential and the thermal stress can be controlled by engineering the dielectric strain opening new perspectives for QWI of photonic devices.

Resonance energy transfer process in nanogap-based dual-color random lasing

The resonance energy transfer (RET) process between Rhodamine 6G and oxazine in the nanogap-based random systems is systematically studied by revealing the variations and fluctuations of RET coefficients with pump power density. Three working regions stable fluorescence, dynamic laser, and stable laser are thus demonstrated in the dual-color random systems. The stable RET coefficients in fluorescence and lasing regions are generally different and greatly dependent on the donor concentration and the donor-acceptor ratio. These results may provide a way to reveal the energy distribution regulars in the random system and to design the tunable multi-color coherent random lasers for colorful imaging.

Dynamic characteristics of two-state lasing quantum dot lasers under large signal modulation

Large signal modulation characteristics of the simultaneous ground-state (GS) and excited-state (ES) lasing quantum dot lasers are theoretically investigated. Relaxation oscillations of ‘0 → 1’ and ‘1 → 0’ in the GS lasing region (Region I), the transition region from GS lasing to two-state lasing (Region II) and the two-state lasing region (Region III) are compared and analyzed. It is found that the overshooting power and settling time in both Regions I and III decrease as the bias current increases. However, there exist abnormal behaviors of the overshooting power and settling time in Region II owing to the occurrence of ES lasing, which lead to fuzzy eye diagrams of the GS and ES lasing. Moreover, the ES lasing in Region III possesses much better eye diagrams because of its shorter settling time and smaller overshooting power over the GS lasing in Region I.

YAG and Y2O3 laser ceramics from nonagglomerated nanopowders

Undoped and Nd- or Yb-doped laser-grade yttrium aluminum garnet and (Y,La)2O3 ceramics with a transmittance above 80% in the 1-μm lasing region have been prepared by solid-state reactions using nonagglomerated Y2O3 and Al2O3 nanopowders. The Y2O3 nanopowders were prepared via laser evaporation, chemical precipitation from urea solutions, and grinding of commercially available Y2O3 in a purposedesigned laboratory-scale attritor at stirrer rotation rates of up to 1500 rpm. The YAG ceramics were prepared using commercially available Al2O3. After grinding, all of the powders had a particle size on the order of a hundred nanometers. Green compacts produced from the nanopowders were sintered in a vacuum furnace between 1615 and 1750°C to give highly transparent ceramic samples.

Optically and electrically pumped random lasing from ZnO films annealed at different temperatures

We have investigated the optically and electrically pumped random lasing (RL) actions in ZnO films annealed at low and high temperatures respectively. While the optically pumped RL threshold for the ZnO film annealed at a low temperature, which features stronger light scattering and larger optical loss, is far higher than that for the ZnO film annealed at a high temperature, the electrically pumped RL threshold currents for the two ZnO films are almost the same with the electrical pumping scheme of metal–insulator–semiconductor structure. It is suggested that if the lasing region within the ZnO film is narrow enough in the case of electrical pumping, the strong light scattering can substantially alleviate the adverse effect of large optical loss on the onset of RL.

Spectral Dynamical Behavior in Passively Mode-Locked Semiconductor Lasers

In this paper, we present an experimental and theoretical study of passive mode-locking in semiconductor Fabry–Pérot, quantum-well, lasers operating at around 1550 nm and producing picosecond pulses at a repetition frequency of 40 GHz. The different regimes that occur as the reverse bias voltage applied to the saturable absorber (SA) section or the bias current injected into the amplifier section are characterized both in the time and frequency/wavelength domains. Our results reveal that the lasers display spectral competition between the gain of the amplifier section and the absorption of the SA, with variations of the lasing wavelength up to 25 nm as the bias conditions are changed. These wavelength variations result from the thermal drift of the SA band-edge due to Joule heating by the generated photocurrent and from the competition between two possible lasing regions placed either at the amplifier gain peak or near the band-edge of the SA. The experimental observations are satisfactorily reproduced and explained in the framework of a Traveling Wave Model complemented by a time-domain description of the semiconductor susceptibility.

Collisional broadening and shifting of saturated absorption resonances in silicon tetrafluoride

The broadening and shifting of 12 saturated absorption resonances of the SiF4 gas induced by molecular collisions have been measured in the lasing region of a low-frequency CO2 laser at the P(30) line of the 9.4-μm band. Values of the collisional broadening are identical accurate to the measurement uncertainty. The collisional shift does not exceed 10% of the collisional broadening. This means that collisions with changes in the internal energy of molecules prevail over dephasing collisions.

Possibilities of using modern infrared imaging devices for measuring the temperature of the lasing regions of solid-state lasers pumped with high-power laser diodes

The modern infrared imaging technique is applied to studies of the temperature fields and heat loads of Er:YAG laser crystals under conditions of optical pumping of these crystals with high-power (20 W) radiation from diode arrays with fiber outputs. It is shown that state-of-the-art infrared imagers operating in the range 8–14 μm can be efficiently used to monitor the near-surface temperatures in the lasing regions of solid-state lasers under high-power diode pumping. The possibilities of modern infrared imagers for evaluating the degree of heating of the lasing region in a laser crystal, the temperature distribution in this region, and the efficiency of operation of cooling systems and for preliminarily rejecting active laser elements have been demonstrated.

Lasing in single ZnO nanorods after fs‐ and ns‐pulsed excitation

AbstractIn this work we examine the lasing properties of single ZnO nanorods under ns‐ and fs‐pulsed excitation. By measuring the photoluminescence of individual nanorods excited by a single ns‐pulse we demonstrate that several stable lasing modes are emitted by the nanorod. In this quasi‐stationary excitation regime lasing up to room temperature can be achieved. Measurements of the temperature dependent laser threshold and of the spectral lasing region are presented. The excitation with fs‐pulsed excitation combined with a time resolved photoluminescence measurement gives furthermore an insight in the lasing dynamics. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An experimental setup for exciting semiconductors and dielectrics with picosecond electron-beam and electric-field pulses

An experimental facility for forming high-voltage pulses with amplitudes of 30–250 kV and durations of 100–500 ps and electron beams with a current density of up to 1000 A/cm2 is described. The facility was built using the principle of energy compression of a pulse from a nanosecond high-voltage generator accompanied by the subsequent pulse sharpening and cutting. The setup is equipped with two test coaxial chambers for exciting radiation in semiconductor crystals by an electron beam or an electric field in air at atmospheric pressure and T = 300 K. Generation of laser radiation in the visible range under field and electron pumping was attained in ZnSSe, ZnSe, ZnCdS, and CdS (462, 480, 515, and 525 nm, respectively). Under the exposure to an electric field (up to 106 V cm−1), the lasing region was as large as 300–500μm. The radiation divergence was within 5°. The maximum integral radiation power (6 kW at λ = 480 nm) was obtained under field pumping of a zinc selenide sample with a single dielectric mirror.

Near-field modulation of laser diode emissions by an aperture probe of near-field optical microscope

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LDs) observed by a near-field scanning optical microscope (NSOM) probe with an intentionally enlarged aperture shows a single broad elliptic profile, whereas the detected wavelength hops among those of the longitudinal multimodes observed in the far-field. This hopping suggests the near-field coupling between the aperture and the LD cavity, which is proved by the observation of strong modulation of laser intensity by the optical coupling only in the near-field regime. Probes with an intact aperture are found not to couple with the lasing TE 00 mode, but, therefore, couple only with a near-field longitudinal mode. Around the lasing region, emissions detected by an intact aperture are ascribed to amplified spontaneous emissions at wavelengths far from those of the lasing multimode emissions. These near-field modes, with spatial profile different from both the far-field TE and TM modes, are detectable only by the NSOM.

Diffraction phenomena in ring gas lasers

The frequency and amplitude diffraction nonreciprocity of counterpropagating waves in a ring gas laser is studied. The self-consistent eigenmode problem is solved within the framework of the model of Gaussian beams for a ring optical resonator containing a nonlinear gaseous medium and a model aperture. The expressions for frequency-dependent losses, frequencies and intensities of counterpropagating waves are obtained and analysed. Experimental data on the influence of diffraction phenomena on the frequencies and intensities of counterpropagating waves reported in the literature are analysed. Nonreciprocal effects related to diffraction are classified by comparing the experimental data with theoretical results. The mechanisms of the asymmetry of the lasing region with respect to the central frequency of the transition and asymmetry of the laser line are explained.

Near-field optical microscopy of AlGaInP laser diode emissions and comparison with far-field observation

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LD's) observed by a near-field scanning optical microscope (NSOM) probe with an intentionally enlarged aperture shows a single broad elliptic profile, whereas the detected wavelength hops among those of the longitudinal multimodes observed in the far-field. This hopping suggests the near-field coupling between the aperture and the LD cavity, which is proved by the observation of strong modulation of laser intensity by the optical coupling only in the near-field regime. Probes with an intact aperture are found not to couple with the lasing TE00 mode, but, therefore, couple only with a near-field longitudinal mode. Around the lasing region, emissions detected by an intact aperture are ascribed to amplified spontaneous emissions located at wavelengths far from those of the lasing multimode emissions. These near-field modes, with spatial profile different from both the far-field TE- and TM-modes, are detectable only by the NSOM.

Thermal Characterization of Electrically Injected Thin-Film InGaAsP Microdisk Lasers on Si

We have performed a numerical and experimental analysis of the thermal behavior of electrically injected microdisk lasers that are defined in an InGaAsP-based thin film bonded on top of a silicon wafer. Both the turn-on as well as the pulsed-regime temperature evolution in the lasing region was simulated using the finite-element method. The simulation results are in good agreement with experimental data, which was extracted from the broadening of the time-averaged emission spectra. Lasing at room temperature was only possible in pulsed regime due to the high thermal resistance (10 K/mW). Some strategies to decrease the thermal resistance of the microdisk lasers are proposed and discussed.