First I want to thank everyone for your help, I am clearly not getting what is needed communicated here however :)
This is for a training device and more than one rider may or may not use one at the track, I am wanting the entire device to be on board the bike, the only thing that is at the finish line is some kind to marker that the onboard system can reliably detect to stop the timer and say the run is complete.
So I envisioned a laser line, like those used by laser levels, projected across the finish line from above, I don't know why it would need any kind of modulation, it either touches the sensor for a split second or it does not.
On the bike the sensor that simply has a wire that is at 0 unless the laser light touches the sensor then a pulse is sent out, the on board Arduno nano would use this pulse to set a condition in the program.
there is little chance of the sensor not receiving the laser physically, yes I am worried about daylight interfering and causing false signals or failing to sense the laser. I will have to work out this in proto.
using another Arduino fixed at the finish line to send out a wireless 'stop' signal is not out of the question. but this unit would not be able to distinguish between one bike and another, since the system I am working on is for a BMX track the problem at hand would be that all bikes are within radio range, so the first bike across the line would stop the timer on every bike on the track, this would make the entire system pointless as logging performance related to the training would be corrupted.
my understanding of RFID is that it is proximity based very short range, so I don't see how the RFID tag on the bike would get close enough to the sensor to trigger, and if the sensor was sensitive enough and the tag signal strong enough then instead of a finish line we would have a finish zone :)
this is why the idea was born to have everything on board the bike and simply have something (anything does not have to be laser) that could be sensed at the finish line with some accuracy, 1/10 sec is good.