  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 $^{87}$Rb, 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 $5^{2}S_{1/2},\ F = 2 \ \rightarrow\ 5^{2}P_{3/2},\ F^{\prime} = 2,\
3$ of isotope $^{87}$Rb. The Allan deviation of the fractional frequency of the
ultralong Faraday laser output signal in 0.1 s -- 1 s measuring time is around
$1\times10^{-11}$.
