Robert A. Laff

Laser heating and melting of thin films on low‐conductivity substrates

A laser beam, incident on a highly absorbent thin film supported by a poorly conductive substrate, causes that film to heat and melt. The time required to reach the melting point and that required to complete the melting process are calculated as a function of the incident laser flux. The calculations neglect heat losses arising from lateral diffusion, convection, and thermal radiation, but they account for a possible reflectivity change at the melting point. They yield a criterion for the minimal absorbed flux necessary to maintain stable monotonic melting.

Thermal performance and limitations of silicon–substrate packaged GaAs laser arrays

Thermal resistance and crosstalk have been investigated for a source package consisting of a monolithic, multilaser heterojunction array mounted on a single crystalline silicon substrate, which is in turn laminated to a copper heatsink. Models for 2-D and 3-D heat spreading are used to calculate the heat flow distribution and to obtain upper and lower bounds for both resistance of single devices and crosstalk in arrays. Results for experimental five-laser arrays are shown to fall within these limits. Active cooling is required to maintain junctions at safe operating temperatures prerequisite to stable, long-lived operation.

Gallium arsenide laser-array-on-silicon package.

A monolithic array of AlGaAs lasers has been packaged together with an array of fiber lightguides on a substrate of silicon. The components have been optimized for maximum lightguide output radiance consistent with reliable cw laser operation. Coupling efficiencies up to 80% have been achieved between laser and lightguide. Output powers up to 70 mW cw have been observed from a 50-microm core diameter lightguide of 0.15 numerical aperture. Eight-device array multispot packages have been fabricated with 10 mW/spot, limited by laser quality and thermoelectric cooler capacity. Fabrication tolerances and device electrical and optical crosstalk are discussed.

GaAs laser array source package.

A GaAs laser array multispot source has been fabricated in which the array is bonded to a silicon substrate, which also contains an accurately aligned collimating lens and an array of fiber lightguides. The coupling efficiency of each laser to its corresponding lightguide is greater than 50%. The package is cooled by a thermoelectric cooler, allowing cw, room-temperature operation of the array at 5 mW output per fiber. This source is useful for multichannel optical-communication applications.

Photoeffects in Lead Telluride p‐n Junctions

The direct energy gap in PbTe has been deduced over the range 3.5°≤T≤300°K, from the spectral dependence of short circuit photocurrent in grown p‐n junctions. At temperatures above ∼30°K, the deduced absorption in the depletion region is found to be exponential, with the steepness of the edge dominated by thermal broadening. At lower temperatures, the effects of broadening due to the electric field within the junction depletion region have been observed. A third broadening mechanism is observed at low temperatures. At 5.8°K, this mechanism is of approximately equal strength to the thermal broadening. The energy gap, taken from the photon energy at which the photocurrent has fallen to one‐half its maximum value, at which point the absorption coefficient α=14 cm−1, is given by Eg=0.173+0.485×10−3T eV for temperatures 30°