Ultralong Faraday laser as an optical frequency standard

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In this letter, we introduce the concept and experimentally demonstrate an ultralong Faraday laser as an optical frequency standard in principle. The ultralong Faraday laser is based on the Faraday anomalous dispersion optical filter (FADOF) with ultra-narrow bandwidth and the ultralong fiber extended cavity of 800 m. The ultra-narrow FADOF is based on atomic transition line of isotope ⁸⁷Rb, which has an ultra-narrow bandwidth of 26.0 MHz and a transmission of 23.6% at 780 nm. Fibers of length 150 m and 800 m are used as ultralong fiber extended cavities, which provide optical feedback and give extremely small FSR of 0.667 MHz and 0.125 MHz, respectively. The mechanism of the proposed ultralong Faraday laser is to combine FADOF's ultra-narrow bandwidth and ultralong cavity's small free spectral range to limit the lasing frequency within FADOF bandwidth covered by the semiconductor gain. The active lasing frequency of the ultralong Faraday laser is determined by the center frequency of FADOF transmission, which is corresponding to atomic transition 5²S₁/₂,F = 2 →5²P₃/₂,F′ = 2,3 of isotope ⁸⁷Rb. The Allan deviation of the fractional frequency of the ultralong Faraday laser output signal in 0.1 s -- 1 s measuring time is around 1×10⁻¹¹.