I added a differential receiver circuit and negative supply voltage to Disha's latest current driver design and gave it a home in a more robust case. It now matches better with the CDS DAC's differential output and can actually turn all the way off while still being powered (since OP27 is not rail-to-rail there was always a sub-threshold creep current through the laser diode). The scaling is 10 mA/V on the modulation input, and the offset has to be supplied by CDS.

I also assembled a second unit so we can modulate both cavities at the same time. I went with a P-channel Mosfet for this one because in the N-channel design the laser diode cathode (which is connected to the case) is "hot" and we were seeing occasional problems with the laser diode case getting grounded in its mount. This would bypass the transistor and sense resistor and just push max current through the laser diode. In the P-channel design the laser diode cathode is at ground, however to match the behavior of the N-circuit I had to subtract the input voltage from the bias voltage, which required additional circuitry. This unit also provides 10 mA/V. In retrospect, I prefer the n-channel, and with proper mounting and cabling the shorting issue is easily avoided.

I configured one of the drivers for use with the red laser diodes (pin code A) and the other one for blue violet (pin code B). The package pins for cathode and diode are swapped for the two, so to change the color the driver has to be opened and the header flipped.

I installed both on the table and aligned them onto the cavities for maximum impact in the noise spectrum. Both diodes have a nominal operating current of 40 mA, so I found the threshold current for both laser diodes (blue: 24.5 mA - red: 34 mA) and set the modulation in CDS to range from just above the threshold to 40 mA. According to a Thorlabs PDA36A, using the manual values for TI gain and sensor responsivity (and correcting for PD reflection obtained from a secondary measurement) the modulations are 8 mWpp for red and 22 mWpp for blue violet. I attached some spectra with and without modulations, fast and slow frequencies. Both lasers evoke similar modulation depth despite the power difference, most likely because less blue violet light reaches the cavity.