Ultralong Faraday laser as an optical frequency standard

TL;DR

This paper demonstrates an ultralong Faraday laser using a 800 m fiber cavity and a narrowband FADOF based on rubidium atoms, achieving a stable optical frequency standard with low Allan deviation.

Contribution

It introduces and experimentally demonstrates a novel ultralong Faraday laser that combines ultra-narrow atomic transition filtering with an extended fiber cavity for stable optical frequency standards.

Findings

Achieved an ultra-narrow FADOF with 26.0 MHz bandwidth at 780 nm.

Constructed an 800 m fiber cavity providing extremely small free spectral range.

Demonstrated a fractional frequency stability with an Allan deviation of 1×10⁻¹¹.

Abstract

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 $1\times10^{-11}$.