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Developments of optical frequency standards for wavelength

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					            Developments of optical frequency standards for
  wavelength-division-multiplexing (WDM) optical fiber communication

                  Ken’ichi Nakagawa   Member (University of Electro-Communications,
       Atsushi Onae      Non-member (National Metrology Institute of Japan,

    Keywords : optical frequency standard, WDM (wavelength-division-multiplexing), optical fiber
  We have developed optical frequency standards for         We have also developed an accurate frequency
wavelength-division-multiplexing      (WDM)      optical   measurement system in the 1.5 µm region. We used a
communication systems based on absorption lines of         two-color mode-locked (ML) fiber laser as optical
acetylene molecules in the 1.5 µm region. In the           frequency combs at both 1.56 µm and 0.78 µm, and
WDM systems, hundreds of channels are allocated at         we realized a direct frequency link between the
different optical frequencies around 1.5µm. To avoid      acetylene transition at 1.5 µm and a Rb two-photon
cross talk between neighboring channels, each channel      transition at 0.778 µm which was used as an absolute
frequency should be monitored and controlled with a        frequency reference. We carefully calibrated the
high accuracy. The acetylene absorption lines in the       absolute frequency of this Rb reference, and we could
1.5 µm region can be used as accurate frequency            finally determine the absolute optical frequency of an
references for the calibration of frequency monitor        acetylene transition (13C2H2 P(16)) with a total
systems in the WDM systems.                                uncertainty of 12 kHz (6×10-11). As a result, this
  We have developed acetylene-stabilized diode             acetylene transition has been adopted as a new optical
lasers using a Doppler-free saturation absorption as a     frequency standard in the 1.5 µm region by
frequency reference. As a result, the frequency            Consultative Committee for Length (CCL) of
stability and reproducibility of these lasers reached to   International Committee for Weights and Measures
                       -13                          -11
about 400Hz (2×10 ) and 10 kHz (5×10 )                     (CIPM). Using the present acetylene optical frequency
respectively. We have also developed a transportable       standard, a traceable optical frequency standard
acetylene-stabilized laser, and using this transportable   system in the 1.5 µm region is now available and it
laser, we have compared the frequency between two          will contribute a reliable frequency management in the
acetylene    lasers   independently     developed     at   WDM optical communications.
University of Electro-Communications (UEC) and
National Metrology Institute of Japan (NMIJ)
respectively. As a result, the frequency difference
between two lasers was measured to be within 10

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