LED Visible Light encoding
I found two issues that were affecting the response of the circuit:
- The CSHUNT (capacitor in parallel with the LED) that should act as ripple filter didn't allow to follow correctly the PWM. So I removed and now works as I was expecting: 
- While encoding with digitalWrite() and delayMicroseconds() I was printing the iteration Serial.println(): this was making extra delays that were also affecting the output PWM.
- I tested now in the range of my requirements and it works fine: Bit_time= 166 usec, including a 95% duty cycle. That means 8 usec RTZ and the circuit responds fine