A wavelength-division multiplexing (WDM) system is a promising candidate for large capacity lightwave networks. In such a system, a frequency standard light source is indispensable to define an absolute frequency reference for WDM channel allocation. Here, we demonstrate a frequency standard light source with an arbitrary optical frequency by offset-locking it to a gas absorption line. It is also needed to allocate WDM channels at optical frequencies which are separated from the reference frequency precisely. For this purpose, we use a semiconductor mode-locked laser diode, which has multi-mode output with precise optical frequency spacings.
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