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Browse Prior Art Database

Light Modulator and Optical Logical Devices

IP.com Disclosure Number: IPCOM000094747D
Original Publication Date: 1965-May-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 4 page(s) / 88K

Publishing Venue

IBM

Related People

Gunn, JB: AUTHOR

Abstract

Light modulation and logical functions are obtained through change in the index of refraction n for light propagating in a semiconductor due to changes in the concentration of free carriers in it. Large and reversible changes in the index of refraction are made in a semiconductor having a relatively long carrier lifetime. This is effected either by an injection of minority carriers or by photoionization of impurities. By changing the refractive index of a semiconductor homogeneously, gradients of refractive index are used to deflect a beam of light and to phase modulate it. Amplitude modulation is obtainable from both beam deflection and beam phase modulation.

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Light Modulator and Optical Logical Devices

Light modulation and logical functions are obtained through change in the index of refraction n for light propagating in a semiconductor due to changes in the concentration of free carriers in it. Large and reversible changes in the index of refraction are made in a semiconductor having a relatively long carrier lifetime. This is effected either by an injection of minority carriers or by photoionization of impurities. By changing the refractive index of a semiconductor homogeneously, gradients of refractive index are used to deflect a beam of light and to phase modulate it. Amplitude modulation is obtainable from both beam deflection and beam phase modulation.

Deflection modulation permits construction of extremely versatile logical devices not obtainable by simple phase-modulation techniques. For deflection modulation, collimated beam of light 10 is incident on the face at x = 0 of rectangular block 12 of a semiconductor having dimensions X, Y and Z as in drawing A. Emergent beam 14 from the face at x = X of block 12 is not perfectly collimated but exhibits diffraction effects. This is because the face has finite dimensions. There is an angular spread of 0 between the central maximum of the diffraction pattern and its first minimum.

A uniform gradient dn/dz of refractive index, resulting from a distribution of free carriers due to current 1 applied to the face at z = 0 of block 12, rotates the diffraction pattern as a whole by an amount 0 = X dn/dz resulting from changes of phase along the face at x = X. A measure of the deflection is expressed by the deflection ratio R = theta/theta(0). For a linear concentration gradient, the deflection ratio equals the difference in the number of wavelengths of light in the optically shortest and longest paths through block
10.

The deflection is converted into amplitude modulation by semi-infinite screen 18 placed in the path of emergent beam 14. If screen 18 is placed to intercept the undeflected maximum of the diffraction pattern and has its edge along the first minimum, most of the undeflected beam for R = 0 is cut off. For the condition R = 1, 50% transmission of the emergent light beam is obtained. For R = 2, only that part of beam 14 beyond the first minimum is intercepted by screen 18 while most of it is transmitted. A gradient of the index of refraction dn/dz is obtained by injecting minority carriers over the face at z = 0 of block 12 by injection current I and having them recombine on the face at z = Z which is a surface of high recombination velocity. If dimension Z of block 12 is much less than the diffusion length of minority carriers in it, the gradient of refractive index dn/dz is essentially constant between the face at z = 0 and the face at z = Z. Minority carriers are injected at the face z = 0 if it is a PN junction by injection current 1 passed through it in the forward direction. For N-type silicon at room temperature for a wavel...