A press-in fastening apparatus and method couples a screw with a nut by pushing the screw through an opening of the nut along a slip axis and rotating the nut to align a lock axis with the inserted screw. The device uses a Mach-Zehnder interferometer operating as a square law device and features a housing for maintaining the interferometer at constant temperature using a temperature control unit. The first reflecting image forming optical system has a first reflecting mirror M1, a second reflecting mirror M2, a third reflecting mirror M3, and a fourth reflecting mirror M4. The guide string hanger is configured to support a guide string.