RESUMO
In this Letter we report on an all-optical-fiber approach to the generation of ultra-low-noise microwave signals. We make use of two erbium fiber mode-locked lasers phase locked to a common ultrastable laser source to generate an 11.55 GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz from the carrier. The residual frequency instability of the microwave signals derived from the two optical frequency combs is below 2.3x10(-16) at 1 s and about 4x10(-19) at 6.5x10(4) s (in 5 Hz bandwidth, three days of continuous operation).
RESUMO
Improvements on electronic technology in recent years have allowed the application of digital techniques in phase noise metrology, where low noise and high accuracy are required, yielding flexibility in system implementation and setup. This results in measurement systems with extended capabilities, additional functionalities, and ease of use. In most digital schemes, the Analog to Digital Converters (ADCs) set the ultimate performance of the system; therefore the proper selection of this component is a critical issue. Currently, the information available in the literature describes in depth the ADC features only at frequency offsets far from the carrier. However, the performance close to the carrier is a more important concern. As a consequence, the ADC noise is, in general, analyzed on the implemented phase measurement setup. We propose a noise model for ADCs and a method to estimate its parameters. The method retrieves the phase modulation and amplitude modulation noise by sampling around zero and maximum amplitude, a test sine-wave synchronous with the ADC clock. The model allows discriminating the ADC noise sources and obtaining the phase noise and amplitude noise power spectral densities from 10 Hz to one half of the sampling frequency. This approach reduces the data processing, allowing an efficient ADC evaluation in terms of hardware complexity and computational cost.
RESUMO
This article reports the design, the breadboarding, and the validation of an ultrastable cryogenic sapphire oscillator operated in an autonomous cryocooler. The objective of this project was to demonstrate the feasibility of a frequency stability of 3x10(-15) between 1 and 1000 s for the European Space Agency deep space stations. This represents the lowest fractional frequency instability ever achieved with cryocoolers. The preliminary results presented in this paper validate the design we adopted for the sapphire resonator, the cold source, and the oscillator loop.