1.2 Introduction to the Instrument
1 – 3
CPU and
Display
TEC
Controller
TEC
sensor
LD Driver
P/S
P/S
P/S
P/S
LD
Modulate
PD
GPIB, etc
Sync Out
PD bias
Figure 1.1: The LDC500 power supply block diagram.
Vpower
+
Modulate
Laser Diode
Current Source
SVLM = user setting for LD Voltage limit
SILM = user setting for LD Current limit
Vpower = SVLM + (2.8ohm * SILM) + 0.55V
Photodiode
Bias/Amp
LD ANODE(+)
LD CATHODE (-)
PD CATHODE (-)
PD ANODE (+)
LD
LD
LD
J250
BNC
Photodiode
2k
Modulate OFF
U250A
2
3
6
Laser
Figure 1.2: The laser diode drive circuitry (simplified).
or 1000 mA and 2000 mA (LDC502). For both ranges, there are two
control modes for the magnitude of the current source output: con-
stant current (CC), and constant optical power (CP). A simplified
circuit diagram is in Figure 1.2.
Operating in CC mode, the LDC500 programs the desired current
magnitude, in milliamps.To operate in CP mode, a monitor pho-
todiode must be used to measure the output intensity of the laser
diode. In CP mode, the LDC500 measures this photodiode current,
and continuously adjusts (servos) the LD current source to maintain
a constant
photodiode
current. Since the photodiode current is pro-
portional to optical power, this operating mode stabilizes the laser’s
LDC500 Series Laser Diode Controllers
