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OWNERS MANUAL | Cobolt 08-01 Series | D0141-K SEPTEMBER 2020
06-MLD
Which modulation type to use?
The Cobolt 06-MLD has three different modulation types: on/off, digital and analog. These three modulation types
aim to cover most applications the user may have, and they each have very different specifications.
•
On/off modulation
is quick to set up and requires no computer connection. It allows for the highest
extinction ratio. Use this mode if you are modulating at speeds below 500 kHz in a square wave.
•
Digital modulation
allows for extremely fast modulation speeds of up to 150 MHz with a rise time of under
2.5 ns. Use digital modulation if you are modulating at high speeds in a square wave.
•
Analog modulation
is not as fast as digital (2 MHz), but has the advantage that the user can drive the laser
with arbitrary waveforms. Use this mode if you require arbitrary waveform or DC signal to control the ON
power level in CW operation or in combination with digital modulation and On/off.
These types of modulation can be used simultaneously in any combination to give complicated output shapes such
as pulse bursts. These combinations are dealt with separately at the end of this section
On/Off modulation
On/off modulation is the simplest modulation type and can be used out of the box with no connection to a computer.
It allows the user to completely shut off the diode between pulses, giving it a theoretically infinite extinction ratio. It
does however have the narrowest bandwidth and slowest switching times of the three modulation types. The on/off
modulation feature can be accessed via a socket on the key control box (CDRH model) or pin 11 on the laser head’s D
-
SUB (OEM model).
The on/off modulation socket is connected to a floating voltage line. When this line is grounded or forced to zero, the
laser automatically enters on/off modulation mode in the off state. The laser will remain in on/off mode until the laser
is restarted. Note that only a very small curr
ent (< 500 μA) must flow in order to ground the circuit. A 0
-5 V TTL signal
should be applied to modulate the laser, and the duty cycle is set by the input signal. The on/off modulation socket
takes a 3.5 mm stereo audio plug. The plug should be connected as shown in the figure below.
In Cobolt Monitor
™
there is a tick box “On/Off Modulation”, which indicates when on/off modulation is active. When
this is the case, all other options are greyed out, although the previous mode remains checked, as the laser remembers
which state it was in previously.
The laser automatically detects when an on/off modulation signal is present and enables on/off modulation, so there
is no need for the user to check the “On/Off Modulation” box manually. In the box next to this tick box the user can
set the peak power level tha
t the laser will modulate up to. This is set in the factory to the laser’s nominal maximum
power level. If analog or digital modulation is enabled when On/Off modulation is enabled, the laser will operate in
both simultaneously. See below for more information about modulation mode combinations.
Ground
+5V
