indeed exists in the actual reflectance of the optical thin films 211 and 202 thereby the fabry perot device of the invention may realize an expected fwhm value after the connection done by the solder 221 in addition after the connection done by the solder 221 in the fabry perot device of the invention because the second reflecting surface of the micro reflecting mirror 210 functions as a mobile reflecting surface the tunable gap d 2 may then be adjusted to modulate the filtering effect of light waves however one should notice that the gap d 2 is comparatively small to the space d 2 thus in the fabry perot device of the invention the degree of the adjustable space d 2 and the tunable gap d 2 are entirely different values to be more explicit the adjustable distance between the two reflecting surfaces of the fabry perot device of the invention is practically d 2 pluses d 2 the fabry perot device 100 in the above embodiment may further include an additional grin lens 301 or a photodiode not shown to become the fabry perot device 300 as shown in fig5 a in this case the grin lens 301 included in the embodiment is employed as a receiver for receiving light waves passed through the fabry perot device 100 other than this the rest part of the fabry perot device 300.