Lowest Order Transverse Mode Research Articles

Optimizing KTP and LiTaO/sub 3/ channel waveguides for quasi-phase-matched second harmonic generation with high conversion efficiency

The conversion efficiency of quasi-phase-matched second harmonic generation (QPM-SHG) in waveguides can be enhanced significantly by optimizing the linear properties of the guiding structure. We describe here a method for fast and accurate computation of the modal properties of continuous, periodically poled, ferroelectric channel waveguides and a multiparameter optimization algorithm which we have used to maximize normalized internal SHG conversion efficiency. We present the phase-matching characteristics of these designs and discuss the possibility of high SHG conversion efficiency between lowest order transverse modes at the fundamental and second harmonic wavelengths near noncritically phase-matched regimes of operation.

Laser-diode-pumped, electro-optically tunable Nd:MgO:LiNbO_3 microchip laser

We have demonstrated continuous-wave operation of a microchip laser fabricated from Nd:MgO:LiNbO3 and pumped by a laser diode. Incident pump power thresholds of 32 mW with slope efficiencies of 6.5% were observed. The microchip laser operated in the lowest-order transverse mode at all output powers up to and including the maximum output power of 13 mW. Application of an electric field transverse to the cavity axis permitted tuning of the oscillating cavity modes to be obtained at a rate of 8.9 MHz V−1 mm.

Transverse mode selection in a convectively cooled cw-CO2 laser with a cyclonic flow configuration

Strong cyclonic flow induced discharge confinement has been used for the first time to efficiently select the lower order transverse modes in a fast axial flow high power cw-CO2 laser. The laser performance with and without the cyclonic flow has been compared. Comparison has also been made with the conventional aperture method of selection of lower-order transverse modes.

Optical layout for single-transverse-mode operation of 2-D arrays of vertical cavity surface-emitting lasers

An intracavity spatial frequency filter is proposed as a method of locking a 2-D array of vertical cavity surface emitting lasers on a single, low-order transverse mode. Such a filter, which requires the interconnection of nearest-neighbour emitters, could be fabricated as a multilayer structure containing a number of multiplexed volume holographic optical elements.

Optimal transverse mode control of waveguide array lasers by discrete transform techniques

A general approach for the control of the transverse mode characteristics of waveguide array lasers is proposed, based on the discrete Fourier transform. This allows the design of intracavity spatial frequency filters which have, for example, 100% transmission of the lowest-order transverse mode (which has a uniform amplitude distribution) and zero transmission of all higher-order modes.

Single-mode resonator incorporating an internal multimode optical fiber and a phase-conjugate reflector

Phase-conjugate mirrors (PCM’s) can be used to correct for phase distortion introduced by optical elements included within laser resonators, enabling diffraction-limited output beams to be produced. A severe example of a phase-distorting medium that could be included within a resonator is a multimode optical fiber. We describe the operation of a resonator that contains such a fiber and uses a PCM to restrict the output from the fiber to the lowest-order transverse mode. The system thereby enables the output from a laser to be transported to a remote location through the multimode fiber without loss of mode quality. The PCM was a high-gain passive PCM made from a barium titanate crystal pumped by a 60-mW single-mode argon-ion laser.

Single transverse mode operation at 1345 nm wavelength of a diode-laser pumped neodymium:ZBLAN multimode fiber laser

A diode-laser pumped multitransverse-mode neodymium-doped fluoride fiber laser is shown to oscillate in the lowest-order transverse mode when the cavity conditions are optimized. The combination of good pump coupling efficiency, laser operating efficiency, and good overlap of the HE/sub 11/ mode to that of a single-mode fiber, allows up to 12 mW of continuous wave (CW) power to be coupled into a standard single-mode fiber at a 1345-nm wavelength. Improved lens design should significantly increase the power injected into the single-mode fiber from the multimode fiber laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High frequency buried heterostructure 1.5 μm GaInAsP/InP lasers, grown entirely by metalorganic vapour phase epitaxy in two epitaxial growth steps

A new high frequency buried heterostructure laser is described and fabricated using two epitaxial growth steps, grown entirely by atmospheric pressure metalorganic vapour phase epitaxy. The lasers are made with non-re-entrant mesas with the active layer narrowed to 1–2 μm by a selective etch to ensure operation in the lowest order transverse mode. The regrowth with Fe doped semi-insulating InP fills in the etched slots adjacent to the active layer and planarises the structure. The non-re-entrant mesa eliminates the anomalous rapid growth in MOVPE at the edges of the re-entrant mesas. The lasers have thresholds of 20 mA and quantum efficiencies of 46% with good linearity up to 10 mW. The laser shows a small signal microwave bandwidth of 5.8 GHz at 5mW.

Self-filtering unstable resonator: An approximate analytical model with comparison to computed and XeCl laser experimental results

Approximate analytical solutions for the lowest-order transverse mode of empty self-filtering unstable resonator have been deduced for magnification M 》 1, and compared with some experimental measurements on an X-ray preionized XeCl discharge laser.

High power, AlGaAs buried heterostructure lasers with flared waveguides

High power, buried heterostructure laser with flared waveguide horns have been grown by a two-step metalorganic chemical vapor deposition process. Powers up to 120 mW cw in a single longitudinal and transverse mode have been obtained from a single stripe laser. The flared waveguides decrease the lateral far-field divergence from 25° to 11° while supporting only the lowest order transverse mode.

High-power (AlGa)As strip-buried heterostructure lasers

The characteristics of index-guided strip-buried heterostructure lasers are described. The lasers are grown by a two-step process, consisting of the initial 5-layer planar growth by MOCVD, and after etching to define the strip buried wave guide, recovery by Al 0.65 Ga 0.35 As using LPE. The devices are highly uniform, having 20 mA thresholds, quantum efficiencies of 0.65, and T 0 of 163°C. The TE polarized emission was in the lowest order transverse mode and consisted of ∼ 6 longitudinal modes. CW output powers of 50 mW per facet were obtained from lasers with reduced output reflectivity, and pulsed powers of 95 mW per facet from uncoated lasers have been obtained. The far-field angular widths were \sim 23\deg and \sim 50\deg in, and perpendicular to the junction plane, respectively. The total optical conversion efficiency of the coated laser emitting 50 mW CW was 41.5 percent.

The high-gain regime of the free electron laser

The main emphasis of this paper is on the role played by diffraction in the small-signal, high-gain regime of the FEL. The paper begins by reviewing the one-dimensional theory of the high-gain FEL. This is followed by an analysis of diffractive effects in an axisymmetric cw FEL. It is shown that exponentially growing modes exist which have a profile independent of the longitudinal coordinate. A dimensionless parameter a ̂ , which is proportional to the radius of the electron beam and independent of the interaction length, determines whether diffraction is important. For a ̂ > 20 , one-dimensional theory gives the correct gain. For small a ̂ the lowest-order transverse mode has a gain substantially higher than the other modes. Maximum gain occurs for a positive value of the energy detuning, though the gain on resonance is nearly maximal. Positive energy detuning broadens the laser modes, while negative detuning concentrates them within the electron beam. The paper concludes with an appendix on small-signal pulse propagation in the FEL and its high-gain limit.

Cavity mode analysis of plasma frequency waves in inhomogeneous cylindrical plasmas

A linear, electromagnetic analysis is presented of the cavity mode structure and dispersion relations for waves with frequency near the electron-plasma frequency in cylindrical plasmas with monotonically decreasing electron density and finite magnetic field. Analytic solutions are obtained which are exact in the limits when either the ratio of electron cyclotron to plasma frequency or the ratio of plasma size to parallel wavelength tends to infinity. Comparison with numerical solutions confirms that the analytic results are highly accurate even for fairly modest values of these ratios. Thermal effects are incorporated, including Landau (damping or) gain, which show the lowest-order transverse modes to have greatest gain. These modes are highly localized near the cylinder axis so that the plasma itself acts as a cavity, regardless of edge boundary conditions. The theory thus enables an interpretation to be made of maser action in quasicylindrical plasmas such as tokamaks.

Multilongitudinal mode operation in angled stripe buried heterostructure lasers

The various index-guided lasers have superior device performance over gain-guided lasers but operate in single longitudinal mode rendering them unsuitable for use as optical sources in multimode optical fiber transmission systems because of the increased modal noise produced. In this experiment we show that multilongitudinal mode operation can be obtained in buried-heterostructure (BH) lasers throughout the entire output power range from threshold to catastrophic damage by angling the buried active stripes so that they are nonorthogonal to the cleaved facets by angles, θ≳5°. These lasers also exhibit the various desirable characteristics of BH lasers with θ=0° such as linearity in light-current, low spontaneous emission level, and abrupt turnon of lasing. The angles buried stripe caused no noticeable degradation of threshold current, Ith, or external differential quantum efficiency, ηD. The averaged Ith ranged between 16 to 20 mA in all lasers and ηD increased with θ. In all cases, the lasers operated stably in the lowest order transverse mode.

Highly efficient (GaAl)As buried-heterostructure lasers with buried optical guide

A new buried-heterostructure laser with a buried optical guide has been developed. A threshold current around 20 mA and a differential quantum efficiency as high as 85% are obtained under cw operation. This results in highly efficient laser operation, i.e., 35% power conversion efficiency at an optical output of 15 mW per facet, at which no degradation is observed over 3000 h at room temperature. Stable lowest-order transverse mode and nearly isotropic laser beam (∼25°×35°) are obtained. Second-order harmonic distortion is 55–65 dB below the fundamental level for 30% modulation with a 100-MHz sinusoidal signal at 5 mW.

Determination of optical irradiance in nonlinear processes

A method has been developed for experimentally measuring the irradiance (energy per unit area) of optical radiation required to stimulate nonlinear processes. The method involves examining the spatial dependence of the process in the vicinity of a laser beam focus. A laser operating in the lowest order transverse mode is required. The treatment is exact for multiphoton processes in the absence of relaxation of recombination and is approximate for other nonlinear processes.

Frequency degeneracy of modes in a cw solid-state laser with a planospherical resonator

An investigation was made of the influence of the resonator configuration and of distortions of a thermally induced lens in an active element on the output radiation power in the case of degeneracy of he frequencies of transverse modes of different orders. Two types of resonator were investigated in which the competition between the transverse modes, resulting in a considerable reduction in the output power, was manifested most clearly. This should make it possible to avoid such configurations in those cases when a high output multimode radiation power is required and to use these configurations when the low-order transverse modes have to be separated.

Lateral current confinement by reverse-biased junctions in GaAs-AlxGa1−xAs DH lasers

Two methods are described for fabrication of a DH stripe-geometry laser where lateral current confinement is obtained with reverse-biased junctions on both sides of the active layer. Threshold current densities, comparable in values to optimum values achieved in other stripe geometry lasers, are obtained as a function of top channel widths for lasers with single and double current confinement. The lasers exhibit clean stable mode patterns with excellent linearity of the optical output power as a function of injection current. Lasers with channel widths ≲14 μm operate in the lowest-order transverse mode in the junction plane for currents up to ∼two times the threshold (an output power of 16 mW per mirror).

Single laser beam of spatial coherence from an array of GaAs lasers: Free-running mode

Spatially coherent radiation from a monolythic array of three GaAs lasers in a free-running mode is reported. The lasers, with their mirror faces antireflection coated, are operated in an external optical cavity built of spherical lenses and plane mirrors. The spatially coherent beam formation makes use of the Fourier transformation property of the internal lenses. The transverse mode control is accomplished by a spatial filter. The optical cavity is similar to that used for the phase-controlled mode of spatially coherent beam formation, which had been reported; only the spatial filters are different. The spatial filter in the free-running mode controls the fields of the lasers to be in the lowest-order transverse mode, but it does not control their phase angles. Random amplitude and phase variations of the longitudinal modes prevent the formation of a stationary interference waveform. In the far field (when restored by an external lens), the intensities of the lasers in the array are concentrated in a single laser beam of spatial coherence, without any grating lobes. The far-field distribution is not influenced by the spacings among the lasers. The far-field distribution of the laser array in the free-running mode differs significantly from the interference pattern of the phase-controlled mode. The modulation characteristics of the optical waveforms of the two modes are also quite different. This is because the modulation is related to the interaction of the spatial filter with the longitudinal modes of the laser array with the optical cavity. The modulation of the optical waveform of the free-running mode is nonperiodic, confirming that the fluctuations of the optical fields of the lasers are random. Experiments were performed at room temperature using a monolythic array of three GaAs lasers. The far-field distribution of the laser array and the optical waveform were investigated.

Single longitudinal mode operation of a transversely excited CO2laser

High peak power output has been obtained from a transversely excited CO2 laser in a single longitudinal mode, operating on a single rotational line and the lowest order transverse mode at 9.3, 9.6 and 10.6 mu m. The laser was designed to operate with high efficiency at reduced pressures and low voltage. Mode selection was by means of an intracavity germanium etalon.