Page 134 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

Useful Electronic Circuits and Construction Techniques to Get You Going 127

10000
Duty
t/T = 0 0.005 cycle
Peak Allowable Current (mA) 1000 0.1

0.05

0.2

0.5

I t

T
100
10m 100m 1m 10m 100m 1
t
Pulse Width (s)
Figure 6.3 A reduced duty-cycle pulsed drive allows LEDs to be driven well
beyond their normal maximum current to obtain higher peak outputs.

rather than the instantaneous current that is being applied. So, if the current
is applied only for a short time, the LED has no time to heat up and much
higher currents can often be applied. The technique of applying higher currents
in short pulses is widely used with infrared diodes used in remote controls and
in visible multiplexed displays. In addition, physiological effects increase the
apparent brightness of visible displays operated in this way, so efficiency
improves too. The impulse loading information is normally available as a family
of curves showing the allowable peak current versus the pulse width t, for a
range of duty cycles t/T. Figure 6.3 shows an example, similar to the Siemens
LD242 infrared LED. Typically, with a 100ms pulse and 1 percent duty-cycle the
current could be ten times the DC maximum value.

6.5 Laser Drive and Modulation
Although laser diodes are in many ways similar to LEDs, with the same mate-
rials, electrical diode characteristic, current requirements, power dissipation,
and optical power output, their operation is much more complex. This is because
in addition to specified limits on drive current and reverse voltage, they have a
maximum permissible optical power output. If this is exceeded, the very high
power density at the laser facets is likely to degrade or completely destroy the
laser. This process can happen in microseconds or less. Hence much of the
design of laser drivers is concerned with avoiding even transient current drive
overloads.

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