Frequency measurement of I2 lines in the NIR using Ca and CH4 optical frequency standards

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lines have been carried out using difference-frequency mixing of the emission of CH₄ and Ca frequency standards at 88 THz (or 3.4 μm) and 456 THz (or 657 nm), respectively. A power of ≈10 pW for the mixing product at 815 nm was obtained using a critically phase-matched KTP crystal and input powers of the order of mW. The relative uncertainty of the measurement was a few times 10^-11, limited by the frequency instability of the laser source locked to I₂.