In this paper, we surveyed potential wavelength references in U-band wavelength region. The hollow-core photonic bandgap fiber (PBGF) filled with methane (CH$-4$) gas is used as a gas cell. The experimental results clearly indicate that the weak absorption lines of methane can be used as U-band wavelength division multiplexing (WDM) channel references due to the long interaction path length provided by PBGF. The absorption lines of CH 4 are useful as wavelength references in the 1625-1700 nm region. In addition, we have measured the pressure broadening and the wavelength stability of absorption lines, and find that absorption lines are particularly insensitive to external perturbation. This technique is useful for wavelength calibration of components of communication system, or as monitor the wavelength of the channels. It may be implemented in next-generation WDM communication systems.
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