Common cathode laser diode pin connections

Question

I have a question relating to the operation of a laser diode within an optical pickup head that includes both a laser diode and photodiode.

The Optical pickup head is the HOP-1200W and I am attempting to utilise the CD laser in the head, namely the RLD78MRA1. The laser diode's pin layout can be found in the datasheet of the HOP-1200W, http://dalincom.ru/datasheet/HOP-1200W.pdf , on pg 23. In this datasheet it can be seen that the laser is 'Type A' and shares a common cathode (according to Thorlabs).

My main area of confusion is coming from the current flow and the required reverse biasing of the photodiode for the feedback to be obtained. I currently think the current is supplied (from a driver circuit) into pin 15, the parallel cap removes any unwanted AC component and then powers the laser diode. However will the following not have happened/happen?

1) Will current not then flow directly into ground removing any current that can flow across the photodiode?

2) The photodiode needs to be reverse biased and by my interpretation of events there will be no potential difference at the photodiode left for this biasing after the laser diode?

Any help would be greatly appreciated!

Answer 1

The photodiode needs to be reverse biased and by my interpretation of events there will be no potential difference at the photodiode left for this biasing after the laser diode?

You need to use a split supply, so that you can apply a negative voltage to the PD anode.

Will current not then flow directly into ground removing any current that can flow across the photodiode?

You don't want laser diode current to flow through the photodiode. You want to the common connection tied to a very good constant voltage (like ground) so that changing laser diode current doesn't affect the PD at all.