Multi-branch ultra-low noise Er:Fiber frequency comb comparison (2017)

Summary form only given. The Optical Frequency Comb (OFC) represents a cornerstone of precision metrology, because it provides a clockwork to down-convert hundreds of THz of optical frequency standards to the countable and traceable radio frequency (RF) domain. In particular, for a future redefinition of the Coordinated Universal Time by an optical atomic transition [1], it will be an essential ingredient. Nowadays, narrowband optical transitions show relative stability and accuracy on the level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-18</sup> [2, 3] and the comb has to keep pace in order not to add noise or instability. While there are schemes to mix out noise related to the comb by appropriate RF techniques, modern ultra-low noise frequency combs may advantageously be used with much reduced complexity.To address the comb instability, in particular when serving multiple spectral regions as possible outputs, we have compared a multi-branch ultra-low noise (ULN) OFC with a similar ULN OFC and investigated the short term noise processes and long-term instability of the system. The femtosecond fiber lasers, the combs' core device, employ only polarization-maintaining components, operate at 1.5 μm, and are mode-locked using a nonlinear amplifying loop mirror (NALM) [4]. In figure 1, the scheme of the comparison setup is given. Both OFCs are commonly referenced to a cavity-stabilized CW laser (ORS1500, Menlo Systems) generating optical beats at the same mode-number, to have the same repetition rate, approximately set at 250 MHz. The reference frequencies for the phase-lock loops are chosen to obtain an offset of 10 MHz between the two comb spectra in order to realize four RF signals at this carrier frequency, specifically at ~1063, 1157, 1397 and 1559 nm. To simultaneously generate the beats, synchronous overlap of the pulses for the four different optical frequencies is necessary; therefore, fiber links of appropriate lengths have been prepared using precision splicing. The fiber lengths between the comb outputs and the beat detection units are similar only up to a few meters for the pairs generating the optical beats, therefore, temperature fluctuations induce changes of differential optical path length on the order of approx. 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> /(m·K), ultimately becoming the limiting factor of the stability and accuracy of the involved OFC. We measure an overlapping ADEV of <;5x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> at 1 s (PI-type counter, 1 MHz measurement bandwidth), dropping below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-18</sup> at 100 s, as shown in figure 2, together with the optical phase evolution and room temperature variation over time. In Lambda-mode, the high-resolution counter shows already few parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-18</sup> in 1 s for the modified Allan deviation. At the given environmental conditions and measurement time, all optical frequencies' systematic uncertainties were found to be in the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-20</sup> level.