OWNERS MANUAL | Cobolt 06-01 Series | D0136-K DECEMBER 2018
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When the laser is modulated from 0 to 1 V, the current through the laser diode is modulated from slightly above zero
to the current that gives the laser’s nominal power. The laser diode has a threshold current below which no laser light
is emitted, and above which the optical power is approximately linear with current. The figure below contains a typical
power vs. current graph for a Cobolt 06-MLD 405nm laser, showing this behavior. When modulating with an arbitrary
waveform, it is possible use a DC offset on the signal generator such that the laser is modulated from this threshold
point to the desired maximum signal level. The threshold level varies from laser-to-laser. To determine the threshold
level for a given laser apply a variable DC voltage to the analog modulation input and looking for the lowest voltage
where laser light is emitted. The amplitude and DC offset of the input signal should then be set so that it modulates
from this point up to 1 V. Note that although the diode does not emit laser radiation below threshold, it still emits
some light; modulating from
0
V will therefore give the best possible extinction ratio.
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
P
o
wer
(
m
W
)
Current (mA)
Power vs. Current for a typical 405 nm MLD laser
The user is given the choice of two impedance values
for the laser’s analog modulation circuit: 1 k
and 50
. The
impedance value can be toggled either in the Cobolt Monitor
™
software or by RS-232 commands. The default
impedance is set to 1 k
, and for most cases it is recommended to use this value. When using a 50
signal generator,
the rising and falling edges of the output may suffer ringing due to the impedance mismatch, a problem which can be
solved by setting the impedance to 50
. Using the low impedance will cause more of the power to be dissipated in
the signal generator, so in order to modulate up to full power the user should monitor the voltage across the laser
device to ensure that it reaches 1 V.
Modulation mode combinations
In addition to the modulation types described above, the Cobolt 06-MLD can be operated with a combination of
modulation signals.
On/off + another mode
Any mode can be operated in combination with on/off modulation. When the laser is in modulation mode and a signal
is input to the on/off modulation socket, the laser operates in both modes simultaneously. When the on/off input state
is high, the laser behaves as if it were in digital or analog mode, and when it is low the laser switches off regardless of
the digital or analog state. This is a useful way to produce a train of pulses.
D Analog
