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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