Far-field Beam Divergence Research Articles

Defining the effective radius of curvature for a nonideal optical beam

A simple expression is given for evaluating the effective or average radius of curvature R(z) of an arbitrarily wrinkled, distorted, or nondiffraction-limited optical beam. This real-beam radius of curvature obeys the same free-space propagation equation as does the wavefront curvature of an ideal Gaussian beam. Extracting this correctable curvature from the beam at any plane simultaneously collimates the beam, makes that location a waist for the real beam, and minimizes the far-field beam divergence starting from that position.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduced-confinement GaAlAs tapered waveguide antennas for enhanced far-field beam directionality

Reduced-confinement tapered waveguide antennas are shown to produce reductions of >35% in the transverse far-field beam divergence for radiation emitted from GaAlAs slab waveguides, resulting in far-field beams as narrow as 8.2 degrees full width at half-maximum (FWHM) along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular-beam-epitaxy growth technique to produce a longitudinal reduction in the refractive index and thickness of the waveguide film. The reduced-confinement geometry has the distinct advantage over horn antennas of being compatible with two-dimensional antenna development. A numerical simulation is used to verify and predict the performance limitations of reduced-confinement antennas. Requirements on both the amplitude and phase profiles of the electric field at the antenna output are shown to impose a stringent lower limit on the acceptable taper length for optimal antenna performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Demonstration of InGaAs/AlGaAs strained-layer distributed-feedback grating-surface-emitting lasers with a buried second-order grating structure

We have fabricated InGaAs/AlGaAs strained-layer distributed-feedback grating-surface-emitting lasers with a buried grating structure. The device consists of a pumped distributed-feedback section terminated on both sides by unpumped distributed Bragg reflector sections. cw operation in a stable single longitudinal mode is achieved up to six times threshold. The threshold current density is ∼600 A/cm2. The far-field beam divergence is predominantly single lobed and diffraction limited. The spectral linewidth is 1.0 MHz at an output power of 8 mW.

Reduced-confinement antennas for GaAlAs integrated optical waveguides

Monolithically integrated reduced-confinement antennas are shown to produce reductions of &amp;gt;35% in the far-field beam divergence for radiation emitted from single-mode GaAlAs slab waveguides, yielding far-field beams as narrow as 8.2° FWHM along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular beam epitaxy growth technique to produce a longitudinal variation in the refractive index and thickness of the waveguide film. Unlike present horn antennas, the reduced-confinement geometry has the distinct advantage of being compatible with two-dimensional antenna development.

Variable focusing due to refractive-index gradients in a diode-array traveling-wave amplifier

Focusing of beams injected into a diode-array traveling-wave amplifier has been observed. The gradient in the complex index of refraction responsible for this lensing is well described by a quadratic profile centered over the array. Measurements both resonant and nonresonant with the gain spectrum indicate that the real part of the refractive index associated with a thermal gradient is the dominant source of the index gradient. The strength of the quadratic refractive-index coefficient varies from 0 to 0.19 mm−2 for amplifier currents between 0 and 550 mA. Adjustment of the focusing of the injected signal by translating a single cylindrical lens in an anamorphic telescope allows for near-diffraction-limited performance at any current. A far-field lateral beam divergence of 0.37° was observed for a 110-μm aperture (the calculated diffraction limit is 0.36°).

Impurity-induced layer-disordered buried heterostructure AlxGa1−xAs-GaAs quantum well edge-injection laser array

A laser array is described that makes use of edge injection into two sides (two ‘‘edges’’) of a stack of three AlxGa1−xAs-GaAs multiple quantum well active regions. The edge-injection array is realized by impurity-induced layer disordering, which forms a higher gap Si-doped n-type emitter that edge injects electrons into either side of a stack of three lower gap multiple quantum well p-type active regions. The far-field beam divergence in the vertical direction (θ⊥) of the array diode is reduced from 45° to 15° as determined by the laser operation, for comparison, of one of the quantum well active regions (oxide-defined stripe geometry diode).

Supermode analysis of phase-locked arrays of semiconductor lasers

The optical characteristics of phase-locked semiconductor laser arrays are formulated in terms of the array supermodes, which are the eigenmodes of the composite-array waveguide, by using coupled-mode theory. These supermodes are employed to calculate the near fields, the far fields, and the difference in the longitudinal-mode oscillation wavelengths of the array. It is shown that the broadening in the far-field beam divergence, as well as the broadening of each of the longitudinal modes that were observed in phase-locked arrays, may arise from the excitation of an increasing number of supermodes at increasing pumping levels.

Lateral beam collimation of a phased array semiconductor laser

The lateral near-field and far-field radiation patterns of a phase-locked array of gain-guided semiconductor injection lasers are studied. These lasers exhibit a lateral far-field beam divergence full width at half-maximum (FWHM) of 1.5° up to 200 mW of output power per facet. Greater than 60% of the laser output power is concentrated in a low divergence beam at this high optical power. Catastrophic mirror degradation of the laser occurs at ≊250–270 mW cw per facet.

High-output power InGaAsP (λ=1.3 μm) strip-buried heterostructure lasers

An InGaAsP (λ=1.3 μm) strip-buried heterostructure laser with active layer strip widths of 5 μm is described. These devices show stable fundamental-transverse-mode operation with linear light-current characteristics to pulsed output powers over 100 mW. Output powers as high as 500 mW are observed without catastrophic damage, which corresponds to twice the power output at which catastrophic mirror damage occurs for similar GaAlAs SBH lasers. Far-field beam divergence is approximately 10° and 30° in the directions parallel and perpendicular to the junction, respectively. cw operation with a threshold current of 170 mA has been achieved at room temperature.

Mode structure and beam divergence of a large-bore high-power argon ion laser

Experimental observations on the mode structure and beam divergence of a large-bore high-power argon ion laser are discussed. The gain medium, 185 cm in length and 12 mm in diameter, is capable of generating a steady-state multiline multimode laser output of more than 125 W in the blue-green. Various stable resonance cavity configurations have been investigated. Littrow prism and small aperture have also been used to obtain single-line and low-order transverse mode operations, respectively. The beam intensity profiles at two widely separated distances from the output coupling mirror are measured by scanning the beam across a small aperture affixed to a photodetector, using a rotating mirror. The two measured beam diameters are found to increase with the transverse mode volume (or output power) at a nearly constant ratio. The far-field beam divergence angle deduced from these measurements is found to vary linearly with the initial beam diameter and to be of the order of 1 mrad at the highest output power.

Beam steering in confocal unstable resonators

The effects of mirror misalignment in a confocal unstable resonator are interpreted in terms of beam steering or deflection in the output beam. Exact mode solutions for a resonator with a Gaussian output mirror lead to an expression for the far-field beam-steering angle as a function of the magnification and Fresnel number of the resonator. This solution reduces to the geometric result when the Fresnel number exceeds 10, for output couplings greater than 50 percent. An expression is derived that relates the beam diameter, the mirror vibration amplitude, and the far-field beam divergence. Numerical values are presented that show that in order to prevent the far-field intensity of a 10.6-μm laser beam from being degraded by a factor of two, the product of the mirror vibration angle and the beam diameter should typically not exceed 220 μrad-cm.