Hi everyone. I need some help in understanding the electrical parameters of a VCSEL I am working with (OPV300). The threshold current is rated at a min of .8mA and a max of 3mA. The Max forward current is rated at 12mA and the forward voltage is a max of 2.2V. I have it in a forward bias configuration with a 1k resistor in series with the Anode, and I am applying 5V to this, giving me a forward current of 5mA. Is it ok that I am applying more than the MAX forward voltage to get my 5mA current? Thank you.

 

The threshold current is the minimum current required to get the VCSEL lasing; below this level there can be a little output light, but it is incoherent, like an LED. This threshold current varies with laser diode model, individual device, and aging effects. It's a bit of a nuisance, and can necessitate manual setting up for each VCSEL.

It's quite possible to turn the VCSEL on from zero forward current, but this can often produce ringing on the edges of the output optical pulse, so it is generally better to turn on the VCSEL starting from a bias current slightly above the threshold current. However, given a 0.8 to 3.0 mA threshold current range together with a 7 mA normal operating current doesn't leave much to play with.

If it's any consolation, cheaper laser diodes are very awkward to drive; VCSELs are relatively friendly. Have a play around and see what sort of responses you get - as long as you don't exceed any maximum specs you'll be OK. As a rough rule of thumb, lower "off" current levels will increase the risk of the optical output ringing and the VCSEL will consume less power, while higher "off" current levels will decrease optical output ringing, but at the expense of a higher power consumption and lower optical output range.

Don't worry about the VCSEL forward voltage, as long as the driving current source has enough voltage compliance to drive the VCSEL.

 

Thanks Darren. My circuit is working fine with 5mA forward current. So long as the voltage is not a problem. I am all set. Thanks.