LED Modulation

Since the light ouput from an LED has a region where it is linearly proportional to the forward current through the diode, it is useful for producing a light level which is proportional to some signal. That signal can then be send through a fiber optic cable and detected on the other end. The basic need for modulation is then a device which produces a current proportional to the input voltage of the applied signal. This can be done with an op-amp arranged as a voltage-to-current converter.

Detailed Modulation Circuit
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LED Modulation Circuit

This op-amp application is a summing amplifier where the signal input has a gain of -1 and the 12V supply is amplified by -3/7 to provide enough voltage to turn on the LED to get it into its linear range. The change in light output is then proportional to the signal voltage.

Modulation of an LED is one common way of getting a signal into a fiber optic transmission line.

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LED Characteristics

When an LED is forward biased to the threshold of conduction, its current increases rapidly and must be controlled to prevent destruction of the device. The light output is quite linearly proportional to the current within its active region, so the light output can be precisely modulated to send an undistorted signal through a fiber optic cable.

Light Output Pattern
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LED Radiation Patterns

An LED is a directional light source, with the maximum emitted power in the direction perpendicular to the emitting surface. The typical radiation pattern shows that most of the energy is emitted within 20 of the direction of maximum light. Some packages for LEDs include plastic lenses to spread the light for a greater angle of visibility.

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Reference
Floyd, Electronic Devices
 
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Light Emitting Diodes

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