Chapter 4 Setup; Setting The Ld1100 Feedback Gain For A Given Monitor Photocurrent - THORLABS LD1100 User Manual

LD1100

Chapter 4 Setup

Laser Diodes are extremely static sensitive and can be easily damaged by
mishandling. Always use a grounded wrist strap and work on an anti-static
4.1. Setting the LD1100 Feedback Gain for a Given Monitor
Photocurrent
The LD1100 has a pin-programmable feedback gain to accommodate a wide
range of laser monitor photodiode currents. It is important that this gain be
matched to the specific laser so that the LD1100 operates properly. Please refer
to Figure 1 for identifying LD1100 features. For a detailed description of the
feedback circuit as well as instructions on fully optimizing the gain, refer to
Appendix.
1. Determine the monitor photodiode feedback current for your laser. This
is usually given in the laser manufacturer‟s data sheet.
2. Using Figure 3, look up the resistor combination that matches the
photodiode current. If an exact match is not found, select the
combination that yields the next hightest feedback current.
3. Read across Figure 3 to identify the gain resistors that need to be
enabled. The resistors that have an "ON" in their respective column
need to be enabled by connecting their pin to the power supply common
(J1 pin 2). The resistors that are labeled "OFF" should not be connected
to any other pin.
Example:
Laser is a CQL806/D (Iop = 70 mA, Imon = 400 µA). Looking up Table1, the
closest monitor current to that is 770 µA which is set by jumpering RD to the
power supply common and leaving RA-RC, & RE open.
In this example, the maximum feedback current of 770 µA occurs when the
output adjust pot is turned up to the full 12 turns. However, this particular laser
reaches a maximum output at 400 µA which occurs at an output adjust pot
setting proportionately lower then 12 turns (400 µA / 770 µA * 12 turns = 6¼
turns). If you turn the output adjust above this point you will overdrive the laser
and cause pre-mature laser failure.
When using the on-board gain settings, it is possible to overdrive the laser if
the output adjust pot is turned completely up. This may be avoided by
precisely matching the gain to your laser (see Chapter 6 for more details).
WARNING
mat when handling lasers.
WARNING
Chapter 4: Setup
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