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T-Cube Laser Diode Driver
3.3.5 Connecting the Photodiode with Bias
There are 2 modes of operation for photodiodes; zero bias (photovoltaic mode) or
reverse bias (photoconductive mode). When operating with zero bias, light falling on
the diode causes a current across the device, leading to forward bias which in turn
induces "dark current" in the opposite direction to the p hotocurrent. The normal
operating mode for the TLD001 is photovoltaic (zero bias) mode.
With reverse bias, a b attery is connected in series with the photodiode to hold the
device in reverse bias. This is normally done to improve the transient response (i.e.
speed) of the photodiode. If the photodiode is to be operated with reverse bias, the
battery can be connected as shown in Fig. 3.6 below.
Fig. 3.6 Battery connections for biassed operation
3.3.6 Laser Diode Drive Current Limit
Setting the maximum current the unit can output under any operating conditions
ensures that the laser diode cannot be damaged by accidental overdriving. Individual
laser diodes show a very large production spread, and the maximum current that a
given device can be operated at varies from device to device.
From an electrical point of view, laser diodes are similar to LEDs, in that both devices
are semi conductor diodes that emit light. However, while LEDs are ve ry simple to
use, require no device-to-device characterization, and are rel atively resilient to
damage, laser diodes need far more sophisticated driver electronics.
For example, most common small LEDs work reasonably well at 10mA. Laser diodes
on the other hand, need to be individually set to an operating current. Furthermore,
because of their inherent sensitivity to the operating conditions, some devices may
not produce any laser light at a particular current, while others may become damaged
at the same current.
In addition, it is helpful to consider the output powe r versus drive current
characteristics of laser diodes as shown on the next page.
AG with Bias
CG with Bias
LD OUT PIn No.
2
_
+
4
+_
