Editing Opto-isolator (section)

===Bidirectional opto-isolators===

All opto-isolators described so far are uni-directional. Optical channel always works one way, from the source (LED) to the sensor. The sensors, be they photoresistors, photodiodes or phototransistors, cannot emit light.<ref group=note>Exception: Ternary and quaternary [[gallium arsenide phosphide|GaAsP]] photodiodes can generate light. - Mims, p. 102.</ref> But LEDs, like all semiconductor diodes,<ref group=note>"Even the garden variety signal diodes you use in circuits have a small photovoltaic effect. There are amusing stories of bizarre circuit behavior finally traced to this." - Horowitz and Hill McCoulny, p. 184.</ref> are capable of detecting incoming light, which makes possible construction of a two-way opto-isolator from a pair of LEDs. The simplest bidirectional opto-isolator is merely a pair of LEDs placed face to face and held together with [[heat-shrink tubing]]. If necessary, the gap between two LEDs can be extended with a [[optical fiber|glass fiber insert]].<ref name=M102>Mims vol. 2, p. 102.</ref>

[[Visible spectrum]] LEDs have relatively poor transfer efficiency, thus [[infrared#Regions within the infrared|near infrared spectrum]] [[gallium arsenide|GaAs]], [[gallium arsenide|GaAs:Si]] and [[aluminium gallium arsenide|AlGaAs:Si]] LEDs are the preferred choice for bidirectional devices. Bidirectional opto-isolators built around pairs of GaAs:Si LEDs have current transfer ratio of around 0.06% in either [[photodiode#Photovoltaic mode|photovoltaic]] or [[photodiode#Photoconductive mode|photoconductive]] mode — less than photodiode-based isolators,<ref>Photodiode opto-isolators have current transfer ratios of up to 0.2% - Mataré, p. 177, table 5.1.</ref> but sufficiently practical for real-world applications.<ref name=M102/>