I need to modulate a diode laser with a RF source. I've found parts on Minicircuits that can do this, but I'd like to have the Bias-T and amp on my own PCB. So, I need to design a bias-T circuit that will mix my constant current source with the RF source to drive the laser.
I'm having a hard time choosing the capacitor and inductor values for the Bias-T, since I don't know the load impedance of the diode laser. How can I find this out? Would it just be better to put a 50 ohm resistor in parallel with the diode and then design the Bias-T for a 50 ohm load?
Update: After some thought, it seems reasonable to use the I/V characteristics of the diode, after turn-on, to approximate the resistance. This yields a very low value – 5-10 ohms. This, however, doesn't approximate the load while the diode is off – I assume this would be very high. But, the purpose of the Bias-T would be to bias the diode so it's always on, so I presume just the on characteristics would be sufficient. Sound reasonable?
Update 2: My design needs to supply ~200 mA to the diode. It's a 405 nm diode, so the operating voltage is ~4-5.5 v. I have a wide bandwidth, so anywhere from a Fc of 5 Mhz to 50 MHz will probably work. My transmit bandwidth is about 2 MHz.