I am experiencing difficulties with the gain circuit of Thorlabs' MLD203P1 laser driver. Despite adhering to the datasheet and evaluation manual, my laser diode is burning out, and the current readings are excessively high. I have experimented with various resistor values for the gain setting without success. Interestingly, using a photodiode and LED as a makeshift laser diode works fine. However, when I attach a laser diode, it instantly draws over ten times the current of the LED and burns out immediately.
My goal is to adjust the laser power using PSET and make slight changes to a potentiometer controlling the gain to slightly reduce or increase the power. However, I aim to keep the minimum resistance within acceptable levels to prevent damaging the lasers.
Curiously, I had previously wired this the same way, and it worked perfectly. It also functions correctly on the development boards. I wired it identically this time, even referring to a photo I took, yet I am encountering this issue.
Here is the configuration of my last test setup:
PSET at 2.2-2.5V.
Different RCOMP values:
20kΩ for 120 µA photodiode current.
11.5kΩ for 200 µA photodiode current.
8.33kΩ for 300 µA photodiode current.
The photodiode and LED setup in place of a laser diode works correctly and shows acceptable current ranges. However, as soon as I connect a laser diode, it instantly burns out, regardless of whether I use a 50k resistor or any other value.
I am using the L650P007 laser diode from Thorlabs. Here is some more useful information relating to these chips/diode from the MLD203P1 datasheet:
Power Control by Photodiode Current
The MLD203P1 operates in Constant Power Mode. The criteria for constant power is a constant photo current, delivered by the build-in to the laser housing photodiode (AKA monitor diode). Therefore, the photodiode current is measured (actual value) and converted into a voltage, which is compared with a set value.
The photodiode is connected between PDA and GND_LD. Its current is applied to the input of a VVA (Voltage-to-Voltage Amplifier). The output of this stage delivers a voltage equivalent to the optical power and is applied to one of the comparator inputs.
The feedback resistor connected to the COMP1 and COMP2 pins determines the gain of the VVA, and this way the maximum possible photodiode current. The relation between maximum photodiode current IPD,MAX and RCOMP is expressed by the formulas
From the L650P007 specification sheet:
From the MLD203P1 datasheet:
