There are many, many factors that impact and effect the efficiency of a gas laser and a laser diode. In a perfect world, with a perfect manufacturing process, it may be possible to predict the output of a beam, based upon the input.
Lasers are among the few devices in the world that are measured by what they produce, not the amount of current it takes to drive them. Unlike light bulbs, for example. Light bulbs are rated for what you put into them, not what you get out of them.
Ten (10) identically manufactured laser devices, no matter how hard we try, will output at different levels with the same level of input power.
As Helium Neon lasers, for example, progress through their 20,000 hour (typical) lifetime, they slowly produce less and less output for the same input current. I have heard reports that laser diodes, too, are subject to degregation over time.
The only absolute, sure-fire way to tell what a laser device is outputting, is to measure it.
Now, with that said, with laser diodes, we can get a close estimate of output power given input current. This presumes the diode is new, and hasn't lost much efficiency in its lifetime. There are several threads on this site that discuss measured output compared to input current.
but if you want precision and accuracy, you'll need to measure it.
