Page 137 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

130 Chapter Six

transistor current source. Photocurrent flows through the variable resistor to
give a voltage which is compared with a reference voltage. The set point is pro-
vided by a 1.24-V two-terminal band-gap reference IC (e.g., Zetex ZRA124Y or
National Semiconductor LM385Z-1.2). The opamp adjusts its output to pass suf-
ficient current through the laser to generate this photocurrent and reduce the
-
voltage difference at its inputs to zero. If the photocurrent increases, V will drop
and the opamp output will rise, cutting off the transistor and stabilizing the
laser output.
Easy-to-use chips are also available for this function. For example the Sharp
IRC301 is good for cathode-to-cathode package configurations. Due to the dif-
ferences among lasers, their packaging polarity, and the widely differing
monitor photodiode sensitivities, the reader should study the manufacturers’
literature for circuits and precautions. That done, there is no reason why you
shouldn’t wire up your own. It is just important to think through the circuit
operation, both during regulated operation and during power-up and switch-
off. The use of a single supply rail, as in Fig. 6.6, avoids problems with the order
in which bipolar rails appear. This is a frequent source of problems in home-
made designs. Arrange for a slow turn-on set-point, and make sure that the
main power supply is free from transient interference. The electromagnetic
compatibility (EMC) regulations have spawned a huge variety of power-line
filters, which can keep the worst interference from getting to your laser.
The most important aspect, however, is to evaluate and protect against
dangers from visual damage. Although the output powers of laser and LEDs
may be similar, the danger from lasers is much higher due their ability to be
focused, either by instrument optics or the eye itself, onto a tiny spot of high-
power density. Even a few milliwatts can cause permanent damage, especially
with near infrared lasers which are almost invisible. If you can see the deep red
light of a 780nm laser, the density is probably far too high! Wear proper eye
protection rated for the laser power and wavelength in use, even if it is uncom-
fortable. Check the protection wavelength every time you put them on—many
different types look the same.
To modulate the laser’s light output you normally bias the device just above
the threshold knee and then apply small current pulses to drive up to the optical
power limit. Hence two control loops are ideally needed to stabilize both the off-
and the on-state powers. More often you will see just the off-state stabilized at
threshold, with fixed-current modulation applied. This is acceptable, as the
main variation with temperature is the threshold current increase. The slope
above threshold is relatively constant, although it does decrease slightly with
temperature. Some diode lasers show kinks in the PI characteristic above
threshold, as different optical modes compete for the material’s gain. This can
make power stabilization at a high level difficult. Modulation from zero current
is possible, but without prebiasing just above threshold, modulation speeds will
be reduced by a significant turn-on delay.
Note that the power stabilization is primarily there to protect the laser
from overload, not to give high precision in the output power. If you want

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