Question : Is it ever correct to spell "laser" with a z ("lazer")?
Answer : The word "LASER" is an acronym. It stands for "Light Amplification By Stimulated Emission Of Radiation". The word "Laser" is as fine a term for it as "laser". But the word "lazer" is not a legitimate word. It must not be used to describe a laser ever. However, the word "Lazer" is employed in the stage name of a popular DJ.
Note : The acronym “LASER” is not a proper noun and must not be capitalised every time but only per context.