Distance detector for focus control in laser beam machine

A distance detector for use in a laser beam machine, said laser beam machine detecting a distance between a focal position of a laser beam and a workpiece by use of a capacitance, then irradiating a condensed laser beam on said workpiece for laser beam machining, while controlling said distance between said focal position and said workpiece, said distance detector comprises:

• a sensing electrode forming a capacitance relative to said workpiece;
• an input signal generator generating a first alternating signal defining an input signal, said input signal being input into said sensing electrode;
• a signal detector detecting a second alternating signal defining a detection signal, said detection signal being generated at said sensing electrode and having a value varied in accordance with said capacitance between said sensing electrode and said workpiece;
• a computing unit receiving said input signal from said input signal generator and said detection signal from said signal detector, said computing unit extracting a component having a same frequency as a frequency of said input signal from said detection signal, thereby computing said extracted component to generate a distance information output representing said distance between said focal position and said workpiece.