OXFORD, Mass.--(BUSINESS WIRE)--IPG Photonics Corporation (NASDAQ: IPGP) today reported financial results for the first quarter ended March 31, 2015.           Three Months Ended March 31, (In millions, except per share data) 2015     2014 % Change Revenue $ 199.0 $ 170.6 17 % Gross margin 54.2 % 52.3 % Operating income $ 82.0 $ 57.8 42 % Operating margin 41.2 % 33.9 % Net income attributable to IPG Photonics Corporation $ 57.4 $ 40.5 42 % Earnings per diluted share $ 1.08 $ 0.77 40 %   Managem