Prospects for increasing the precision of optical frequency standards based on transitions in the methane molecule

An analysis is made of the feasibility of increasing further the precision of the 3-μm frequency standards in which lines in the ν3 band of the methane molecule are used as the reference points. It is proposed to use stronger absorption lines of methane, cooled to cryogenic temperatures, as new reference lines. Numerical estimates are obtained of the absorption coefficients of lines in the ν3 band as a function of temperature. Experimental observations, made with a colour-centre laser, of lines in the R branch (λ = 3.2–3.3 μm) at room and liquid nitrogen temperatures are reported.