I have a 50W Raycus marking machine. The 50W Raycus source got a red laser diode inside source and in my machine the red line focus is about 10mm above fiber focus, it give me a thick line 
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The 50W laser includes red light inside. When you use the machine, you will see two red dots on the workpiece at the focus position (the converging point of the double red lights for focusing and the laser focus), and when you replace the field lens, the red light for focusing will be No longer works, you need to readjust or prepare a focusing scale.