What is MTTF(Mean Time to Failure) in laser diodes?

There are a few possible answers here, depending on exactly what you are asking:

1. It is a very misleading estimate of the expected mean life of the devices, worked out by marketing managers and based on the careful use of statistics to say something which is statistically true, but false from every other point of view. Sort of how politicians and governments 'tell the truth'. See here: http://www.zdnet.com/blog/hardware/m...ilure-mttf/310

2. See the datasheet for your particular device, then search google for other people using that part in their designs, and see what they have to say. You could even try talking to the engineers at the manufacturer. Ask them for the actual testing methodology and raw results.

3. Anything from 3000 hours to 1,000,000 hours, and probably beyond (see note 1).