2.3 Operating the Laser Diode Driver
2 – 17
configured to trip-o
ff
if this limit is exceeded, but there is no
active “clamping” circuit to limit the photodiode current.
•
Press [ ] repeatedly to select “PD I:”. This is the operating point
setting in CP mode (using
μ
A units for photodiode current).
Enter the desired initial operating point, and press [Enter] to
accept.
The laser can now be turned on by pressing [LASER On
/
O
ff
].
If the laser was on and in CC mode, the LD driver can be switched to
CP mode directly without turning o
ff
the laser first. To do this, press
the [Current
/
Power] button; a confirmation message will appear,
requesting you to press the button “again to confirm.” If you press
[Current
/
Power] within 2 seconds, the LD driver mode will change
between CC and CP “on-the-fly”, presetting the CP setpoint based
on the most recent measurement of the photodiode current. This
feature of switching “on-the-fly” can be disabled by setting the “LD
Cfg” menu tree item, “CC
/
CP lock when on:” to “Y.”
2.3.5
Modulation
Analog modulation of the laser diode current is allowed in CC and
CP modes when the “Modulate enable:” item in the LD Cfg menu
tree (reached by the [Config] button) is set to “Y.” The front-panel
BNC input accepts
±
10 V inputs, which imposes a modulation on the
laser current in addition to the DC setpoint. See the specifications
table on page ix for DC transfer functions (in mA
/
V) for the di
ff
erent
models.
To achieve the highest bandwidth for modulation, the cable connect-
ing the LDC500 to the laser should be as short as possible. Best
results are obtained if the laser is electrically floating; the bandwidth
can be a
ff
ected if the laser cathode is grounded. The bandwidth for
CC mode is set by “LD Config: CC BW”, and can be either “0)low”
(DC – 10 kHz), or “1)high” (DC – 1.2 MHz). The bandwidth for CP
mode is set by “LD Config: CP BW”, and can be either “0)low”
(DC – 100 Hz), or “1)high” (DC – 5 kHz). Note that the actual band-
width is strongly a
ff
ected by physical characteristics of the laser, the
photodiode (in CP mode), and the interconnect cabling.
2.3.6
Monitoring the LD
The LD driver’s status may be monitored in the alphanumeric dis-
play. Press the LD [Monitor] button, and then use [ ] and [ ] to step
between monitoring Ild (the laser diode current), Ipd (the photodi-
ode current), Ppd (the photodiode power,
=
Ipd
/
responsivity), Vld
(the laser diode operating voltage), and the external interlock status.
LDC500 Series Laser Diode Controllers
