Alan H. Paxton

Gamma-ray-induced effects in erbium-doped fiber optic amplifiers

Four Erbium doped fiber optic amplifiers (EDFAs) were irradiated by gamma-rays to dose levels of 40 Krad(Si) and 100 Krad(Si) at dose rates of 0.25 rad(Si) sec-1 and 1.0 rad(Si) sec-1, respectively. All EDFAs were observed to incur radiation induced permanent decreases to their pre-irradiated signal gains. The paper will discuss the influence of gamma-ray irradiations on EDFA parameters such as gain, noise figure, and integrated amplified spontaneous emission. A brief discussion of how changes to these parameters evolve is presented.

Surface emitting laser technology and its application to the space radiation environment

Present and future space-based applications such as sensors, low-weight and low-power data links for satellites, communication between electromagnetically-shielded modules, and short-distance cross-links within satellite constellations may benefit from the inclusion of small, low-power, and high-efficiency lasers such as the recently-developed Vertical Cavity Surface-Emitting Laser (VCSEL). Many factors influence the application of these devices to space. Temperature response, operational lifetime and reliability, and power consumption are all important considerations for space applications. In addition, the space radiation environments must be considered. In this work, the effects of ionizing radiation on VCSELs are studied with an emphasis on proton damage, and with comparisons to related neutron and gamma-induced phenomena. The influence of proton irradiation is studied in-depth for selected VCSEL structures by the use of an ion microbeam. The experiments indicate that VCSELs exhibit much less threshold current shift for a given radiation dose, compared to the more traditional edge-emitting semiconductor lasers, but that self-heating is a more important consideration for VCSELs. The high current densities associated with VCSELs also lead to a strong influence from forward-bias annealing. These effects are common to various VCSEL types (780 nm and 850 nm) and their magnitude at a given dose is strongly dependent on device size. This indicates that, while VCSELs appear to be very insensitive to ionizing radiation when compared with alternative technologies, there are a number of factors that must be taken into account when optimizing for the space environment.

Radiation-induced effects research in emerging photonic technologies: vertical cavity surface emitting lasers, GaN light-emitting diodes, and microelectromechanical devices

High quality optical fiber to OEIC pigtailing, using non-conventional technology, is required to create a real integrated optical system for optical communications, computing, signal processing, control, and sensing. In this paper, Physical Optics Corporation (POC) presents a novel singlemode fiber to singlemode GaAs channel waveguide pigtailing approach. This pigtailing approach involves two key technologies. First, a fiber end-face lensing technology was used to improve modeprofile matching between singlemode fiber and singlemode channel waveguide, so fiber to waveguide coupling efficiency could be improved. Second, resistance layer assisted dual-carrier-soldering (RLADCS) technology was introduced to facilitate fiber and waveguide chip alignment and fixing, so accurate, convenient, and reliable fiber to optoelectronic integrated cicuit (OEIC) pigtailing could be achieved. By using radiation hardened fiber and special OEIC, this pigtailing and packaging technology has potential applications in a space environment. This publication addresses all aspects of this pigtailing approach, including theoretical analysis, design, fabrication, testing, and measurement results.

Rotating-disk solid state lasers: thermal properties

Approximate calculations of the temperature distribution of a rotating-disk solid-state laser are presented. The surfaces of the Nd:YAG or Nd:glass rotating disk pass close to two water-cooled plates. A thin gap, filled with gas, separates each plate from the disk. For an Nd:YAG disk, temperature distributions are given for a 50 μm gap filled with He, for a 50 μm gap filled with air, and for the case in which the thermal conductivity of the Nd:YAG dominates the problem. Calculated results for an Nd:glass disk are compared with a temperature profile obtained from a rotating-disk laser.

Grating surface-emitting diode laser with lenslike medium

A concept for a grating surface emitting semiconductor laser with a lenslike medium that causes divergence along the lateral direction is described. In principle, the output beam can be diffraction limited. A second-order grating is fabricated with straight teeth which are perpendicular to the directions of propagation of the beams along the laser axis. The constant radius of curvature of the mode results in the matching of the reflected beam with the laser mode. The output beam will exhibit mild astigmatism which can easily be corrected. The astigmatism is the same for the waves traveling in both directions. The divergence of the mode discourages the breakup of the counterpropagating beams into filaments. Surface emission decreases the optical intensity of the laser mode and can essentially eliminate the output power through the facets. This type of laser is a candidate for high power operation.

Laser simulation applying Fox-Li iteration: investigation of reason for non-convergence

Fox-Li iteration is often used to numerically simulate lasers. If a solution is found, the complex field amplitude is a good indication of the laser mode. The case of a semiconductor laser, for which the medium possesses a self-focusing nonlinearity, was investigated. For a case of interest, the iterations did not yield a converged solution. Another approach was needed to explore the properties of the laser mode. The laser was treated (unphysically) as a regenerative amplifier. As the input to the amplifier, we required a smooth complex field distribution that matched the laser resonator. To obtain such a field, we found what would be the solution for the laser field if the strength of the self focusing nonlinearity were α = 0. This was used as the input to the laser, treated as an amplifier. Because the beam deteriorated as it propagated multiple passes in the resonator and through the gain medium (for α = 2.7), we concluded that a mode with good beam quality could not exist in the laser.

On-chip unstable resonator cavity 2-μm quantum well lasers

Focused ion beam milling was used to fabricate on-chip unstable resonator cavity quantum well laser devices. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Pérot cavity device, the unstable resonator cavity device exhibits a 2x diffraction limited beam. The preliminary results demonstrate that a much higher brightness can be reached in this class of broad area devices.

On-chip unstable resonator cavity GaSb-based quantum well lasers

The focused ion beam milling tool was used to convert a GaSb-based broad area gain-guided quantum well laser device with a standard Fabry-Perot cavity into one with an unstable resonator cavity. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Perot cavity device, where the coherency of the beam is severely disrupted by filamentation, the unstable resonator cavity device exhibits an ∼2× diffraction limited beam. The relatively small penalty in slope efficiency demonstrates that a much higher brightness can be reached in this class of broad area devices.

The finite-difference matrix for beam propagation: eigenvalues and eigenvectors

The partial differential equation for the three dimensional propagation of a light beam may be solved numerically by applying finite-difference techniques. We consider the matrix equation for the finite-difference, alternating direction implicit (ADI), numerical solution of the paraxial wave equation for the free-space propagation of light beams. The matrix is tridiagonal. It is also a Toeplitz matrix; Each diagonal descending from left to right is constant. Eigenvalues and eigenvectors are known for such matrices. The equation can be solved by making use of the orthogonality property of the eigenvectors.

Stable ring resonator with bidirectional passes through the gain medium

Ring resonators have unique properties that are sometimes desirable. Spatial hole burning is eliminated. Beam transformation, such as image rotation which may reduce the magnitude of certain aberrations, can be implemented in a traveling-wave region. There is a drawback, however. As usually constructed, a ring resonator has half as many passes through the gain medium as can be achieved with a standing-wave resonator. This may have a detrimental effect on laser efficiency. We have constructed a type of ring resonator that allows counterpropagating collinear passes through the gain medium, while there is also a section with a unidirectional beam. The resonator includes a polarizing beam splitter. The linear polarization is transformed to the orthogonal state by optical elements at the two ends of the region with counter-propagating beams. The beams passing through the gain medium in opposite directions are linearly polarized with orthogonal states.