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LDC500 Series: RF Modulation of Injection Current
Rev B. Sept 2012
Fig.1 LDC500 laser diode driver block diagram
LDC500 series laser diode controller has an analog capacitor to suppress 50/60Hz line noise. The laser anode is
modulation input which allows the output current to be tied to the LDC internal power rail, which is at low impedance
modulated up to 1MHz. To achieve higher frequency relative to internal LD ground. The laser cathode is
modulation, directly injecting RF current into the laser diode connected to the sinking current source, which has high
can be used. output impedance.
Fig.1 is a simplified schematic of the LDC500 series laser RF signal can be injected into either cathode or anode of
diode driver. The circuit is floating, with the internal "LD the laser diode as shown in Fig. 2.
ground" node ACcoupled to chassis ground through a 100 nF
Fig.2 Two ways of RF Signal Injection
Stanford Research Systems www.thinksrs.com (408)7449040 -1-
In cathode injection, the laser anode is grounded to RF mode. In this mode, the optical feedback loop has a
source chassis and the RF signal is injected to the cathode bandwidth of less than 5kHz, so any injection signal with
through R and C. Resistor R together with the laser diode 5kHz or higher won't affect the photocurrent. If the injection
internal AC resistance should match the RF cable impedance. signal frequency is within the feedback loop bandwidth, the
Capacitor C is used to block the possible DC signal from RF photodiode current will be suppressed. So the modulation
source. An inductor L is used to prevent the RF signal from effect will fade away as frequency goes lower.
flowing through the LDC500 source. If the current source ILD In all cases, be careful that the RF signal is not so large as
were ideal, inductor L would be unnecessary. But in reality, to reverse bias or overdrive the laser diode. Otherwise
the output impedance of the ILD becomes lower as frequency permanent damage may happen to the laser diode. When
increases. So inductor L is helpful. configured for external RF injection, the builtin protection
In anode injection, the cathode is grounded to chassis features of the LDC500 controller cannot protect the laser
and the RF signal is injected to the anode through R and C. diode from electrical stresses generated by the RF
The inductor L must be there to isolate the power rail which injection.
is low impedance to ground chassis through 100nF. Fig.3 shows an experiment of injecting RF signal to a DFB
Generally cathode injection is preferred, when laser and the result is shown on a scope. The red trace is the
consistent with constraints of the laser diode packaging. modulation signal directly from DS335 function generator.
RF injection works without complications when the The green trace is the optical output from the DFB laser. The
LDC500 is operating in Constant Current mode. The situation optical signal is converted to electric signal by a photo
is slightly more subtle when operating in Constant Power detector (Model 1181).
Fig.3 RF injection to a DFB laser and the Result
Stanford Research Systems www.thinksrs.com (408)7449040 -2-
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