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OWNERS MANUAL | Cobolt 08-01 Series | D0141-K SEPTEMBER 2020
NOTICE
The on/off modulation feature should never be used as remote interlock connector. A
remote interlock socket is provided for this purpose on the laser head.
Digital modulation
Digital modulation is the fastest modulation type; it has the largest bandwidth and shortest rise time. Digital
modulation requires a 0-5V TTL input signal applied to the digital modulation input female SMA connector on the
laser head, and the duty cycle is set by the input signal. The diode current is modulated in a square wave.
To enable digital modulation using the Cobolt Monitor
™
software, select “Modulation Mode”, and “Digital” under
Modulation Type. You can set the peak power level that the laser will modulate up to by first pressing the “More”
button which opens up a new control window and then enter your preferred value in the box to the right of the
“Modulation Mode” check box.
Analog modulation
Analog modulation allows direct control of the laser power by an input signal. This allows the laser to be modulated
with arbitrary waveform at limited bandwidth. To enable analog modulation, use the Cobolt Monitor software, select
“Modulation Mode”, and “Analog” under Modulation Type. The input signal should be connected to the Anal
og BNC
connector on the key control box (CDRH model) or the dedicated pins on the 15-pin D-SUB (OEM model).
The laser is calibrated so that a 1 V input gives 100 % of the laser’s nominal power level (for frequencies above 1 MHz
it may be necessary to increase the amplitude of the analog signal up to 1.5 V). Note that the laser may give more
power if a voltage larger than 1 V is used. Although the current is clamped to a safe current limit to prevent damage,
the user must nevertheless avoid overdriving the diode so that it gives more than the nominal power level.
Specifications and diode lifetime are not guaranteed above nominal power, which is stated on the
manufacturer’s
identification label on the laser head. Measure the input voltage before connecting to the laser head.
When the laser is modulated from 0 V 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 determining 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.
