I find that very hard to believe. What were you using to measure it, a thermal or optical sensor? Is your meter reading to the nW level or is that the answer that comes out when you do the math? 500nW is amazingly small, even for very high end lasers. To put that in perspective, that is 0.000005% stability. I do not believe that, I think it was a mistake either in your math, measurements, or meter.