Alignment method

A method for optical axis alignment between a laser oscillator and an optical fiber by using a laser light evaluator, the laser light evaluator including a light-receiving unit having a light-receiving surface configured to receive laser light emitted from the laser oscillator and guided by the optical fiber, the method comprising:

• receiving laser light on the light-receiving surface of the light-receiving unit, the laser light having been emitted from the laser oscillator and guided by the optical fiber via an axis alignment mechanism situated between the laser oscillator and the optical fiber;
• and
• aligning an axis of the laser oscillator with an axis of the optical fiber using the axis alignment mechanism based on a light intensity distribution of the laser light received on the light-receiving surface of the light-receiving unit, the aligning being performed such that the laser light radiated from the optical fiber has a minimum numerical aperture, the numerical aperture being a function of a radiation angle of the laser light radiated from the optical fiber, the numerical aperture being derived from at least one of following formulas indicating an intensity profile in an X direction and an intensity profile in a Y direction:
• NA=sin(x/f′), and
• NA=sin(y/f)
• where
• NA represents a numerical aperture,
• x represents a length between a first position where an intensity has a peak in the intensity profile in the X direction and a second position where the intensity is 1/e2 of the peak in the intensity profile in the X direction,
• y represents a length between a third position where the intensity has a peak in the intensity profile in the Y direction and a fourth position where the intensity is 1/e2 of the peak in the intensity profile in the Y direction, and
• f represents a parameter determined by an optical system.