| 摘要 |
PROBLEM TO BE SOLVED: To provide a tunable filter which can have its bandwidth made narrow by increasing the selectivity of the wavelength of outputted light, at low cost. SOLUTION: The tunable filter includes a wavelength selecting means 3 and a reflecting means 4 which reflects transmitted light selected by the selecting means and has its wavelength selectivity increased by transmitting the light reflected by the reflecting means through the wavelength selecting means again.
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