Rev A Nov 2017
Page 14
Chapter 3
Getting Started
Fig. 3.7 Laser Diode Power v Current Characteristics
Fig. 3.7 shows a typical laser diode power v current be havior. At currents be low a
certain ‘threshold current’, the laser diode produces very little optical power. Once a
threshold current is exceeded, the output power starts increasing very rapidly. At a
particular current the optical power reaches a maximum value specified for the
device, and any further increase can easily damage the laser diode, even if it occurs
only for a very short length of time (a few % above the maximum operating current for
a microsecond can cause a catastrophic failure).
The threshold current and maximum operating current vary from device to device,
therefore the operating conditions for individual laser diodes must b e set very
carefully. The threshold and maximum operating current values are also very
sensitive to te mperature, therefore it is recomme nded that laser diodes are n ot
operated above 80% of their rated maximum output power.
Generally, unless the electrical parameters of the given laser diode are already known
(i.e. the diode has been characterized), the drive current limit must be d erived by
experiment. This involves connecting an optical power meter to the laser diode and
continually monitoring the optical power whilst gradually increasing the drive current,
taking care to stay within the maximum optical power quoted by a safe margin (usually
20%).
In general, setting the maximum current must be performed with extreme caution. The
process is complicated by the temperature dependence of the laser power: as the
laser diode chip warms up, the optical power decreases. This can create a potentially
dangerous situation during current limit adjustment. The problem is that since during
the current limit adjustment the laser chip warms up, the ma ximum laser c urrent
adjustment will be made for a warm laser diode chip. If now the laser diode is turned
off, allowed to cool down and then turned back on again with the maximum la ser
current applied, the cold laser diode chip will produce a much higher optical power
40
30
20
10
10
20
30
40
50
O/P Power
(mW)
Forward current (mA)
25°C
60°C
limit
