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ONET4201LD: Laser interfacing questions

juan abelaira
Prodigy 150 points
Part Number: ONET4201LD

I would like to use ONET4201LD to drive a QDFBLD-1550-5  (link) at 2 GHz. There is a couple of things I'm not sure of:

- The example schematic shows a dump resistor RD and inductor between the laser and this driver. My laser diode has already these elements in the module, so can I just connect them straight away? In other words, do these symbols in the schematic represent the built-in resistance and inductance?

- Again the example schematic shows the PD cathode and the LD anode connected together and to VCC at 3.3V. My PD seems to like 5V (still to be confirmed from them). It looks like the APC works on the current from the PD, not voltages, so could I have 5V for the PD and 3.3V for the LD?

- It surprises me that for a GHz chip there isn't even a mention for decoupling capacitors or ground planes. Of course, having them won't harm, but, does this mean they are not essential?

- Laser diodes are sensitive to voltage glitches and have the bad habit of blow up... Is there any specific requirement to supply the ONET4201LD? Will a common 3V3 linear regulator be safe enough? 

- I'll probably start with a kind of custom eval board where we would like to play with things, is it safe to have multiturn potentiometers instead of resistors for setting currents? (they tend to be 'crackling' in resistance because of the wiper and contact resistance). 

Regards

  • 0
    TI__Guru* 93670 points

     Hi Juan,

    1). The ONET4201LD provides 20-ohm-active back termination. In the frequency range of interest, the impedance is approximately resistive with a magnitude of 20 ohm.  The series RD resistor is used to damp reflections as well as the over/undershoot and ringing caused by the laser and stray inductance. RD also helps to present a stable load to the modulation output. as the laser diode impedance can vary significantly versus frequency. The value of the RD depends on the series resistance of the laser diode which is typically 2 to 7 ohm.  The suggestion is you can use a small value like 2 ohm for this resistor and then optimize this to get the optimum output eye diagram.

    2). The APC control loop uses laser photodiode to detect and maintain average output power. The APC control block modifies the laser diode bias current to maintain the average power. Theoretically using 5V for the laser while 3.3V for the ONET4201LD could work since photo diode current is used to monitor and maintain average power. However, all of our evaluation and testing used the same 3.3V for both the laser diode and ONET4201.

    3). Providing decoupling caps and ground plane are given and these are good design practices and should be followed.

    4). 3.3V Linear regulator with supply coupling caps like 47uF and 3.3uF caps should be sufficient. should be fine. 

    5). I think it would be better to have plain resistors and you can optimize the value. Potentiometer may not provide a good resolution.

    Regards,, Nasser