The use of laser guide star (LGS) in daytime expands the operational time of telescope for AO instruments, especially for thermal infrared wavelengths, because the sky brightness is not so different from night to daytime. However, the daytime sky background at 589nm is about 10 6 times brighter than a bright night. However, as the bandwidth of sodium D2 line is a few GHz, an extremely narrow bandpass filter at this wavelength makes it possible to use LGSs during the day. Begin with developing

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... interference filter, we found that the spectral bandwidth could not be achieved at the level of 1nm. Then we started to investigate etalon technology to achieve an extremely narrow band filter. We focused on two different technologies, one is an air-gap etalon and another is a thin etalon layer sandwiched by the high reflective coating. We present the prototype fabrication of etalons based on both technologies and report the results of measurement of the peak transmission and spectral bandwidth.