Help designing a circuit to drive a 1550nm DFB LD and use it to modulate an RF signal
What modulation scheme do you intend to use? (Amplitude modulation, Frequency modulation, Pulse-density modulation, etc.). 

What type of waveform is your input signal? (sinewave, pulse, triangle, etc.). Are you trying to convey the input's amplitude, frequency, duty-cycle, and/or phase?

For OOK and other pulse-modulation schemes, the laser-diode/LD current would switch from full off to full on (0mA and 75mA for your LD. The 90mA is an absolute max. spec.), at a frequency or width determined by the modulation signal. For FM, the LD would be driven with a pulse train whose frequency changes according to the modulation signal. For AM, the LD would be biased at 50%, and DC-modulated by the input signal.  

For OOK, I would use a simple switched current source:







simulate this circuit &ndash; Schematic created using CircuitLab