Newport 6000 - Photodiode Feedback Amplifier; Constant Current, High Bandwidth Mode; Constant Current, Low Bandwidth Mode; Figure 15 - Constant Current - High Bandwidth Mode

26 Chapter 3 Principles of Operation
3.2.8 Photodiode Feedback Amplifier
Photodiode feedback is amplified by a precision instrumentation amplifier. When
constant Power mode is selected, the photodiode feedback signal is used to control
the laser output.
3.2.9 Constant Current, High Bandwidth Mode
This mode of laser operation is shown in Figure 15. In this mode, current feedback is
used to control the laser output. The bandwidth is between 50 kHz and 500 kHz,
depending on the model.
3.2.10 Constant Current, Low Bandwidth Mode
This mode of laser operation is shown in Figure 16. In this mode, current feedback is
used to control the laser output.
In this mode, capacitors are switched into the circuit. These capacitors act as a filter
and therefore prevent the laser output from changing too rapidly. This gives added
laser diode protection. This also limits the laser output bandwidth to about 10 kHz.
In the Low Bandwidth - CW mode, the bandwidth is further limited to 30 Hz.
Unregulated DC
Voltage
Regulator
Pass
Transistor
Modulation
Input Port
Current Set Point
Laser Diode
Current
Sense
Output
Shorting
Figure 15 - Constant Current - High Bandwidth Mode