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Obtaining Increased Radiance from Injection Lasers

IP.com Disclosure Number: IPCOM000077084D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Byrne, FT: AUTHOR [+2]

Abstract

Conventional semiconductor injection lasers radiate into a solid angle which is considerably larger than the theoretical limit set by diffraction. Therefore, the radiance is considerably lower than that of a perfectly spatially-coherent source. Diffraction-limiting has been achieved from injection lasers by making them very small with a consequently low-power output, or by disposing them in external resonant cavities. The present structure provides increased radiance at reasonable power outputs and without external mode selecting cavities.

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Obtaining Increased Radiance from Injection Lasers

Conventional semiconductor injection lasers radiate into a solid angle which is considerably larger than the theoretical limit set by diffraction. Therefore, the radiance is considerably lower than that of a perfectly spatially-coherent source. Diffraction-limiting has been achieved from injection lasers by making them very small with a consequently low-power output, or by disposing them in external resonant cavities. The present structure provides increased radiance at reasonable power outputs and without external mode selecting cavities.

Fig. 1 is a view of the laser in the plane of the junction. Laser material 2 has an index of refraction nl. Material 3, having a refractive index n(2), surrounds the end of the laser which has two mutually perpendicular surfaces 4 and 5. Surface 6 is perpendicular to the bisector of the angle between surfaces 4 and 5.

Fresnel's theory of reflection for plane waves at a plane boundary between two media of refractive index n(1) and n(2), indicates that the reflection coefficient at the boundary is given by:

(1) R(s) = (sin (alpha(1) - alpha(2))/sin (alpha(1) + alpha(2)))/2/ for waves polarized with the E vector perpendicular to the plane of incidence; and (2) R(p) = (tan (alpha(1) - alpha(2))/tan (alpha(1) + alpha(2)))/2/ for waves polarized with the E vector in the plane.

In the above formulas, alpha(1) is the angle of incidence and alpha(2) is the angle of refraction. For a wave incident from the higher index laser material, there will be total reflection for angles of incidence greater than alpha(t) = sin/- 1/(n(2)/n(1)(1). For values of alpha < alpha(t) the reflectivity decreases rapidly toward it...