Patent GB-1304776-A (GB)
IPC: G02F1/35
Filed: 1970-08-05
Abstract: 1304776 Modulating light INTERNATIONAL BUSINESS MACHINES CORP 5 Aug 1970 [5 Sept 1969] 37698/70 Heading H4F Binary connectives for processing coherent optical pulse inputs may be constructed using a first element formed of a resonant material, i.e. exhibiting resonant absorption at the optical frequency of the pulses, and which is responsive to an input pulse so as to vary its transmission characteristic for a successive pulse, together with a further element of resonant material for analyzing the output of the first element to give a pulse output in accordance with the desired logic operation and presence or absence of the input pulses. A coherent pulse at a point z in a resonant medium can be described in part by an angle #(z) where # represents the angle through which an induced effective dipole movement rotates about the direction of the electric field vector of the pulse -see, for example, Physical Review Letters, Vol. 18, No. 21, at page 908. As indicated in Fig. 1B, when the resonant material is in the ground state and is "thick", i.e. is over 4-5 absorption units long along the optical path, # evolves in the positive direction of z and any pulse (#<SP>-</SP>) for which 0 < # < # is converted into a # = 0 pulse (zero physical output) while if # < # < 2# (a #<SP>+</SP> pulse) it is converted to a # = 2# pulse (physical output). If the material is in the excited state # evolves in the negative z direction and input pulses are converted to #(#<SP>+</SP> or #<SP>-</SP>) pulses. If the resonant material is "thin", i.e. about “ absorption unit, a # pulse will effect the conversion ground # excited, or vice versa, for the whole element. Thus in the "AND" connective, Figs. 2A and 2B, # pulses A, if present, convert athin ground state element 1 to its excited state. Depending on whether the element 1 is excited or not, 0 for an ensuing #<SP>-</SP> pulse B tends to become more positive (#<SP>+-</SP>) or more negative (#<SP>--</SP>) after transmission therethrough. Lenses L 1 , L 2  decrease the pulse area so that the pulse C is either a #<SP>+</SP> or #<SP>-</SP> pulse, from the #<SP>+-</SP> and #<SP>--</SP> pulses respectively, which, after passing through a thick ground state element 2 become 2# or 0 pulses D (positive or zero pulse output). A further thick excited element 3 merely serves to convert the pulses D to # or 0 pulses for inputs to further logic stages. In a similar "NOT" connective, Fig. 4 (not shown), the pulses B are 2# pulses. For zero input A, 2# pulses B are transmitted uncharged through element 1 by self induced transparency, whereas for an excited element 1 (# pulses A), # for pulse B decreases giving a 2#<SP>-</SP> pulse. Lenses L 1  L 2  are reversed in order to increase the pulse area so that 2# and 2#<SP>-</SP> pulses are converted to #<SP>+</SP> and #<SP>-</SP> pulses C and 2# and 0 pulses D. The output pulse E are then # or 0, i.e. negative A. In the "OR" connective, Figs. 3A and 3B, medium 1 is thin enough so that one # pulse excites most of the atoms therein, but not so thin that # pulses from opposite ends nullify the effects of each other. Therefore # pulses A and/or B excite medium 1 so that a following #<SP>-</SP> pulse C becomes a #<SP>+-</SP> pulse D (or a #<SP>--</SP> pulse if A and B are zero). Lenses L 1  L 2  and thick ground state medium 2 convert #<SP>+-</SP> and #<SP>--</SP> pulses via #<SP>+</SP> and #<SP>-</SP> pulses E to 2# and 0 pulses F respectively. If medium 1 is thinner so that the effects of # pulses A and B cancel, an "EXCLUSIVE OR" connective results, Fig. 5 (not shown). The different elements may be portions of an integral piece of resonant material e.g. in Fig. 2, elements 2 and 3 could be the front and back halves of a single piece of material to which is fastened lens system L 1  L 2  and the element 1. In Fig. 6 (not shown) is a binary half adder using an "AND" and an "EXCLUSIVE OR" connective, all the inputs being derived from a single laser source via beam splitters and mirrors, and appropriate optical switches and delay elements. The combination of a ruby resonant medium and a Q-switched liquid nitrogen cooled ruby lazer tuned to the 4A 2  (Œ¢) # E (2E) transition thereof is suitable for use in these logic connections.