Optical-to-electrical frequency conversion with attosecond timing

The most frequency-stable electromagnetic radiation is produced optically, with lasers locked to passive reference cavities reaching a fractional frequency instability of 10⁻¹⁶, and optical clock instabilities approaching 10⁻¹⁸ at 10⁴ s. These references can find new utility when their stability is transferred to the electronic domain, such as in microwave spectroscopy, radar, and synchronization at kilometer-scale scientific facilities, such as free-electron lasers and phasedarray telescopes. High fidelity optical-to-electrical conversion is achieved by locking an optical frequency comb (OFC) to an ultrastable optical reference, transferring the frequency stability to the pulse repetition rate. Direct detection of the ultrashort optical pulse train from the OFC then generates ultrastable electrical signals at the pulse repetition rate and harmonics thereof. Using this technique, 10 GHz microwave signals have been generated with instability at one second of 8×10⁻¹⁶, matching the stability of the optical reference (T. Fortier, et al, Nat. Photon., 5, 2011, p. 425). With continued improvements to the stability of optical frequency references, extremely high fidelity photodetection across timescales ranging from less than a microsecond to hours will be required. Here we report optical-to-electrical conversion performance using modified uni-traveling carrier (MUTC) photodetectors for frequency transfer of current state-of-the-art and next-generation optical references. Our work demonstrates residual fractional frequency instability at 1 second averaging of 1.4×10⁻¹⁷ (F. Baynes et al., arXiv:1410.7461), and photocurrent timing noise spectral density reaching 25 zeptoseconds/√Hz (F. Quinlan, et al, Nat. Photon., 7, 2013, p. 290).