INSTRUMENTATION
will then oscillate at the same fre quency. If the mechanical system fails, the pure fluid oscillator will continue to operate and drive the counter assembly. Corning Glass reports on a binary-todecimal pure fluid converter. It needs only two input OR-NOR gates and can easily be laid out on 10 thin glass plates only a few inches on a side. It is stated that the binary-to-decimal converter has demonstrated that the fabrication of reasonably complex NOR logic circuits in multilayer diffusion sealed blocks can be accomplished. Optoelectronics
Intensive research continues in the field of optoelectronics. Most of the interest lies in attempts to obtain faster and less expensive logic and switching but the ultimate uses will be many and varied. Electronics 38, No. 22, 58 (1905) begins a four part discussion of the subject with an article by T. E. Bray of General Electric Co. From our previous accounts in this column, it will be recalled that the subject deals, in general, with optical coupling between one circuit element and another rather than direct electrical or magnetic cou
pling. Many complex and useful ex tensions of this simple principle already exist. It takes a little reflection to digest the terminology of this new tech nique, for example, one thinks of green light as something of the order of 550 millimicrons but to have it referred to as a high frequency carrier of the order of 100,000 gigacycles requires transla tion. This makes good sense in the lingua jranca of communications and also looks impressive as far as big num bers go (10 14 ). The first illustration shows an array of optical sources (nine rectangles) focussed upon a screen by means of a lens. With approximately unit magnification, the resultant image is a precise delineation of the object with its nine rectangles neatly resolved. This is good elementary physics, but the communications expert delights in pointing out that the nine signals are all mixed up as they pass through the lens but at the receiving screen they are sorted out again. From this point of view, we suppose the lens and its im mediate vicinity can bo viewed as a multiplex transmission line. Low speed circuits are easily achieved by a combination of a neon or electro luminescent lamp closely coupled to a
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photo-conductive cell such as CdS. For use as visible light injection diodes, many compounds are being in vestigated—gallium phosphide (orange or green) depending on the doping; gallium phosphide arsenide combina tions in various proportions (red or orange) ; boron nitride (white) ; alumi num phosphide (blue) and silicon (white plasma). A most useful device is the light sen sitive silicon controlled rectifier which is used as a solid state relay. The input or triggering energy required for this SCR is small yet, when the device turns on, its impedance decreases by several orders of magnitude so that the output energy is large. Much attention is being directed to optical beam switching. The object hero is to deflect a beam of light much as the fine beam of electrons is deflected on an oscilloscope. Most "steering" methods modify a medium's index of refraction, either electrically, magneti cally, or acoustically. These things have all been done electromechanically ; the present problem is to achieve them with liquid or solid state devices and at extremely high speed. This series of papers should bo very informative.
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Circle No. 23 on Readers' Service Card
Circle No. HO on Readers' Service Card V O L . 3 8 , NO. 1, JANUARY 1966 ·
109 A
