Since a photodiode and the laser diode is connected in parallel in a laser device, measuring the voltage across the photodiode can tell us how much power the laser is emitting.

The photodiode and laser diode is connected in parallel, the voltage produced in the photodiode should reduce the voltage across the laser diode, because the photodiode only allows current in one direction(?), I guess it is not just simply to measure the voltage as shown in the figure.

So, how can I get the output voltage of the photodiode?

enter image description here

  • \$\begingroup\$ There is probably a great reason why anyone would want to do this of course???? \$\endgroup\$ – Andy aka Apr 11 '16 at 8:15
  • \$\begingroup\$ Are you trying to design an ad-hoc luminosity regulator? \$\endgroup\$ – Dmitry Grigoryev Apr 11 '16 at 8:46
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    \$\begingroup\$ "Since a photodiode and the laser diode is connected in parallel in a laser device" - what makes you think that this is the case? \$\endgroup\$ – brhans Apr 11 '16 at 12:57
  • \$\begingroup\$ Sorry, I don't have circuit training before, so my understanding may be aa little naive. \$\endgroup\$ – W.Wown Apr 11 '16 at 18:27

To measure the photodiode response, you need to disconnect one end of the photodiode or the other from the laser. Typically, laser diodes packaged with monitor diodes are connected either "common cathode" or "common anode" (although other arrangements are possible), meaning the two cathodes or two anodes are connected to one pin, but the other two pins are separate.

With the arrangement you show, any photodiode current will just be a tiny offset to the amount the source has to supply to the laser, so it will be very difficult to measure accurately. Furthermore, the photodiode voltage will not be meaningful because it will not be able to supply enough current to change the voltage across the laser significantly.

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  • \$\begingroup\$ Thank you for replying, Do you mean disconnect the photodiode from the ground? \$\endgroup\$ – W.Wown Apr 11 '16 at 18:41
  • \$\begingroup\$ It would probably be easier to disconnect the cathode, but either way would let you measure the pd voltage independent of the operation of the laser. \$\endgroup\$ – The Photon Apr 11 '16 at 18:50
  • \$\begingroup\$ I think it is hard, because the pin connected to the laser also connects to the photodiode. Can I measure the current flowing through the photodiode instead by connecting a resistor to get voltage? \$\endgroup\$ – W.Wown Apr 11 '16 at 18:55
  • \$\begingroup\$ @W.Wown, the usual thing to do would be connect the PD to a transimpedance amplifier. For the LD-PD connection you have it would be most convenient to have both positive and negative supply voltages available. \$\endgroup\$ – The Photon Apr 11 '16 at 20:28

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