Lowest Order Transverse Mode Research Articles

Megawatt-Level Repetitively Pulsed Erbium 3-µm Laser with Strong Thermal Lens Compensation

We report on an increase in the average power of a repetitively pulsed (10 Hz) nanosecond 3-µm laser due to the compensation of a strong thermal lens in the Cr:Er:YSGG laser cavity maintaining the megawatt-level peak power. Thermally-induced beam distortions are measured in a set of 3-µm erbium crystals (Er:YAG, Er:YSGG, Cr:Er:YSGG), a scheme for compensating a thermal lens in a Cr:Er:YSGG Q-switched laser with a pulse energy of 23 mJ at a repetition rate of 10 Hz in with low-order transverse mode beam is proposed and experimentally implemented. The developed approach makes it possible to control the cavity mode in high-power nanosecond 3-µm lasers with flashlamp or diode pumping, which are used for a variety of scientific and technological applications, including pumping laser crystals in the middle infrared range, as well as laser-induced microstructuring and tissue engineering.

Experimental Investigation on Wideband Optical Frequency Comb Generation Based on a Gain-Switched 1550 nm Multi-Transverse Mode Vertical-Cavity Surface-Emitting Laser Subject to Dual Optical Injection

A scheme for generating wideband optical frequency comb (OFC) is proposed and experimentally investigated based on a master-slave system. In such a system, a tunable optical resource and a distributed feedback semiconductor laser are utilized as master lasers (MLs), and their outputs provide dual optical injection (DOI) into a slave laser (SL) which is a 1550 nm multi-transverse mode vertical-cavity surface-emitting laser (1550 nm-VCSEL) operating at gain-switched state. For the 1550 nm-VCSEL biased at a relatively high current, two low-order transverse modes LP 01 and LP 11 can simultaneously lase and possess comparable powers. After loaded a large signal modulation, the 1550 nm-VCSEL can be driven into gain-switched state, and two separated sub-combs originating from two transverse modes can be generated. Through further introducing DOI to the gain-switched 1550 nm-VCSEL, the two sub-combs can link up to form an overall OFC under suitable operation parameters. The experimental results show that, through selecting optimized operating parameters, the bandwidth of the integral OFC within 10 dB amplitude variation can achieve 150.0 GHz. Additionally, the effects of the wavelengths of the two injection lights on the bandwidth of the overall OFC are also analyzed.

Numerical investigations on multi-channel wideband chaotic signal generation by a multi-transverse mode vertical-cavity surface-emitting laser subject to chaotic optical injection.

A multi-channel wideband chaotic signal generation scheme is proposed and numerically investigated based on a slave multi-transverse mode vertical-cavity surface-emitting laser (SL) subject to chaotic optical injection from a master multi-transverse mode vertical-cavity surface-emitting laser (ML) with optical feedback. Taking two low-order transverse modes, LP01 and LP11, as an example for numerical calculations, the simulated results show that under suitable optical feedback both the LP01 and LP11 modes (two-channel) of a ML can be driven into the chaotic states where their bandwidths are relatively narrow at a level about 8 GHz. Further injecting the two chaotic signals into a SL, for the case of the globally chaotic optical injection, the SL can output two-channel chaotic signals with wide bandwidths above 20 GHz under appropriate operation parameters. Moreover, the case of SL with mode-selective chaotic optical injection is also analyzed.

Spectral engineering for circular-side square microlasers

Spectral engineering has been demonstrated for the circular-side square microlasers with an output waveguide butt-coupled to one vertex. By carefully optimizing deformation parameter and waveguide connection angle, undesired high-order transverse modes are suppressed while the mode Q factors and the transverse-mode intervals are enhanced simultaneously for the low-order transverse modes. Dual-mode lasing with pure lasing spectra is realized experimentally for the circular-side square microlasers with side lengths of 16 μm, and the transverse mode intervals can be adjusted from 0.54 to 5.4 nm by changing the deformation parameter. Due to the enhanced mode confinement, single-mode lasing with a side-mode suppression-ratio of 36 dB is achieved for a 10μm-side-length circular-side square microlaser with a 1.5μm-wide waveguide.

Selective excitation of lowest-order transverse ring modes in a quasi-stadium laser diode

For a two-dimensional quasi-stadium laser diode, we demonstrate stable excitation of the lowest-order transverse ring modes by optimally designing the confocal end mirrors of the laser cavity based on extended Fox-Li mode calculations. We observe kink-free light output versus injection current characteristics and highly directional single-peak emissions corresponding to the diamond-shaped trajectory in the cavity. These results provide convincing evidence for selective excitation of the lowest-order transverse modes.

Rotational symmetry breaking in small-area circular vertical cavity surface emitting lasers

We investigate theoretically the dynamics of three low-order transverse modes in a small-area vertical cavity surface emitting laser. We demonstrate the spontaneous breaking of axial symmetry of the transverse field distribution in such a device. In particular, we show that if the linewidth enhancement factor is sufficiently large dynamical regimes with broken axial symmetry can exist up to very high diffusion coefficients ~10μm2/ns.

Generation of hollow scalar and vector beams using a spot-defect mirror

Generation of both scalar and vector hollow beams was demonstrated by using a mirror with a low-reflectivity spot to suppress the oscillation of lower order transverse modes. As scalar beams, several hollow Laguerre-Gaussian beams have been observed from a side-pumped Nd:yttrium aluminum garnet laser cavity. The intensity profiles were in an excellent agreement with theoretical ones. The phase front variation around the optical axis was verified to be spiral. Furthermore, both Laguerre-Gaussian and Bessel-Gaussian vector beams have been also observed from the identical cavity. In addition to the verification of intensity profiles, polarization pattern measurement confirmed that the beams had revolving polarization distributions along the azimuthal direction as theoretically predicted.

1019 流れ方向に密な格子配列管群からの渦放出と音響共鳴現象に関する研究(OS10-4 流体関連振動・騒音,オーガナイズドセッション)

In the present paper the attention is focused on relationship between acoustic resonance and vortex shedding from closely arranged In-line tube banks which occurred in the two-dimensional model of boiler. We have examined the characteristics of vortex shedding, acoustic damping and acoustic resonance generated from in-line tube banks with tube pitch ratio of 1.33, 1.44, 1.56, 1.67, 1.76 and 2.0 in the flow direction. As the tube pitch ratio in the flow direction decreased, the acoustic damping increased and the vortex shedding frequency became broad-band. The resonance mode of low acoustic damping was generated in the broad-band vortex shedding frequency. It was easy to generate acoustic resonance of low order transverse mode as the tube pitch ratio in the flow direction increased.

Three-dimensional theory of the Smith–Purcell free-electron laser with side walls

We present an analytic theory for the operation of a Smith–Purcell free-electron laser with side walls that includes the effects of transverse diffraction in the optical beam. We allow the width of the electron beam and the width of the grating to vary independently and require the walls to be high compared to the wavelength of the evanescent wave. The results show that the side walls change the empty-grating dispersion relation in important ways. When the separation of the walls is not too large, it is sufficient to consider only the lowest-order transverse mode of the grating. In this case, we obtain excellent agreement with numerical simulations and experimental results.

Polarization Switching in Vertical-Cavity Surface-Emitting Lasers by Asymmetrical Current Injection

Polarization switching between two orthogonal linear polarizations was experimentally demonstrated in newly designed 980-nm vertical-cavity surface-emitting lasers ( VCSELs ). A square-shaped mesa structure was prepared in a p-type distributed Bragg reflector mirror. Four p-side electrodes were placed in contact with the four sides of the square mesa. When the same current was injected into the four electrodes, the VCSEL oscillated with 0 or 90 linear polarizations. When the current was asymmetrically injected (the same current for two electrodes on the opposite sides of the square, but different for neighboring electrodes), the direction of the linear polarization became perpendicular to the direction of the line connecting the two electrodes injected with the larger current. It was confirmed that the VCSELs oscillated with a single frequency and the lowest order transverse mode in all cases.

Enhancement of laser performance using an intracavity deformable membrane mirror

An intracavity deformable membrane mirror has been successfully used to optimise the brightness of solid-state lasers--a side-pumped Nd(3+):YAlO laser where the oscillation of a low-order transverse mode was obtained, and a grazing incidence Nd(3+):GdVO(4) laser where a brightness increase by an order of magnitude with negligible drop in power was achieved. Several search algorithms were also implemented in the system and compared with respect to intracavity optimisation.

A TE21 second-harmonic gyrotron backward-wave oscillator with slotted structure

Second-harmonic gyrotron backward-wave oscillator (gyro-BWO) with a reduced magnetic field strength is a tunable source in the millimeter wave regime, but it has long been impeded by the severe mode competition as a result of low efficiency and narrow bandwidth. This study employs a slotted structure functioning as a mode selective circuit to suppress the lower order transverse modes. In addition, a two-step tapered waveguide is adopted to stabilize the higher-order transverse modes and axial modes. Some important characteristics of the slotted gyro-BWO will be analyzed and discussed. As a calculated result, the interaction efficiency is improved and the stable tuning range is broadened. A stable, Ka-band, slotted second-harmonic gyro-BWO is capable of producing an efficiency of 23% with a 3dB tuning bandwidth of 9% at 5A and 100kV.

Chaos synchronization of unidirectionally injected vertical-cavity surface-emitting lasers with global and mode-selective coupling

Chaos synchronization in two unidirectionally coupled vertical-cavity surface-emitting lasers (VCSELs) with open-loop configuration is studied numerically. We consider two low-order transverse modes for weakly index-guided VCSELs and investigate the generalized type of chaos synchronization for both global coupling and mode-selective coupling configurations. It is found that the synchronization can be obtained between each mode of the transmitter and the receiver lasers when all modes of the transmitter are injected into the receiver equally and coupled with the corresponding modes of the receiver laser. For mode-selective cases, the modes with strong optical injection can achieve the synchronization; while the rest modes without coupling can not obtain the synchronization any more even though the injection is very strong. The results afford an opportunity to multichannel optical secure communications.

Iterative method for the design of a dual-phase-conjugation-mirror resonator with multiple apertures.

Apertures have been used to select the low-order transverse modes in resonators. The additional diffraction losses result in a change in the transverse-mode structure, and the presence of apertures inside a resonator generally distorts the mode shape. The optimization of a multiple-aperture resonator demands an approach that differs from the conventional method in which the mode theory is used. We demonstrate an iterative design method to find optimal phase profiles for the reflector surfaces to build a resonator with multiple apertures to produce a lowest-order mode with much smaller diffraction loss and to satisfy the phase-conjugation conditions at the mirrors. The results are compared with conventional stable resonators, and we show that substantial improvement in round-trip loss and beam quality can also be obtained.

Laterally periodic resonator for large-area gain lasers

Laterally periodic resonators, which can be constructed by use of transversely periodic phase- or amplitude-modulating elements in a cavity, are proposed for stabilization and generation of transversely coherent output from large-area gain. Lasers with periodic resonators have the combined features of conventional cavities and laser arrays. Significant low-order transverse modes and mode discrimination of a sample resonator with intracavity periodic phase elements are investigated numerically by the iteration method. Wave-propagation calculations are carried out by use of a fast Fourier transform, and a modified Prony method is used to evaluate wave functions and losses of transverse modes. Results of numerical calculations are consistent with expectations.

Studies of advanced soft X-ray laser and coherent higher harmonic generation using intense subpicosecond laser produced high density plasma

Current activities on our research of soft X-ray lasers and higher harmonic generations using intense, subpicosecond lasers are reviewed. Especially for soft X-ray lasers we experimentally demonstrate that by longitudinally pumping 2-mm-long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-X-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires pump laser energy of 150 mJ . Simulations show that the pedestal in the main pump pulse can generate electron density and gain profiles with large spatial gradients, which result in the selective amplification of low-order transverse modes. The present result is the demonstration of an efficient and alternative method of improving the spatial coherence of X-ray lasers with amplified spontaneous emission medium, with possibilities of becoming an excellent tool to explore various application experiments. In higher harmonic generation, topics related to observation of blue shift due to collisionless absorption process is described.

Near-field spectroscopy of selectively oxidized vertical cavity surface emitting lasers

Selectively oxidized vertical cavity surface emitting lasers (VCSELs) have been studied by spectrally resolved near-field scanning optical microscopy. We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near-field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and somewhat different from those measured in the far field.

High-power end-pumped CW Nd:GdVO 4 laser formed with a flat–flat cavity

Formed with a flat–flat resonator, a diode-laser-array end-pumped CW Nd:GdVO 4 laser at 1.06 μm, capable of generating 8.6 W of TEM 00 output power with optical conversion efficiency of 43% and slope efficiency of 48%, has been developed. The laser beam was nearly diffraction limited, with the beam quality factor measured to be M 2=1.22. Under the conditions of multi-mode operation, the laser was able to produce 11.2 W of low-order transverse mode radiation ( M 2<2) at the incident pump power of 22 W, giving an optical conversion efficiency of 51%, and a slope efficiency of 55%.

Transverse modes in gain-guided vertical-cavity surface-emitting lasers

An analytical approximate form of the lowest-order transverse modes is proposed for gain-guided vertical-cavity surface-emitting lasers for the first time. Central intensity maxima of higher-order modes are found to disappear. Therefore the number of intensity maxima on experimental near-field intensity profiles cannot be used as an indicator of the mode order.

Vector vortices and polarization state of low-order transverse modes in a VCSEL

Transverse pattern selection and spatio-temporal dynamics in the competition among and superposition of first order Gauss-Hermite modes in a quantum well VCSEL are sensitive to birefringence and astigmatism. When modes of different spatial pattern and/or polarization state are frequency degenerate, the only stable solutions are those with doughnut intensity patterns. Patterns with orthogonal polarizations for the two constituent spatial modes are far more stable than plane polarized doughnuts. For weak anisotropies (birefringence and/or dichroism) the pattern is stationary with a locally time independent intensity (though the polarization state differs from position to position). When the anisotropies are strong, the emission becomes time dependent with two spectral components in the optical spectrum, each with a different pattern and polarization.