Abstract
Recent progress in ultra low phase noise microwave generation indispensably depends on ultra low phase noise characterization systems. However, achieving high sensitivity currently relies on time consuming averaging via cross correlation, which sometimes even underestimates phase noise because of residual correlations. Moreover, extending high sensitivity phase noise measurements to microwaves beyond 10 GHz is very difficult because of the lack of suitable high frequency microwave components. In this work, we introduce a delayed self-heterodyne method in conjunction with sensitivity enhancement via the use of higher order comb modes from an electro-optic comb for ultra-high sensitivity phase noise measurements. The method obviates the need for any high frequency RF components and has a frequency measurement range limited only by the bandwidth (100 GHz) of current electro-optic modulators. The estimated noise floor is as low as −133 dBc/Hz, −155 dBc/Hz, −170 dBc/Hz and −171 dBc/Hz without cross correlation at 1 kHz, 10 kHz, 100 kHz and 1 MHz Fourier offset frequency for a 10 GHz carrier, respectively. Moreover, since no cross correlation is necessary, RF oscillator phase noise can be directly suppressed via feedback up to 100 kHz frequency offset.
Highlights
Microwave photonics[1, 2] is a rapidly expanding field for handling the explosive growth of data rates as found in broadband wireless communication, radar, satellite communication, and electric warfare systems, greatly improving the operational frequency and bandwidth
In addition to the device under test (DUT), a second low noise microwave reference source is required, which need to be synchronized to the DUT
The demonstrated sensitivity of −137 dBc/Hz at 10 kHz frequency offset for a 10 GHz carrier, which is probably limited by background noise such as electronic and shot noise, has not been as good as achievable with the photonics-based delayed self-homodyne method
Summary
By dividing the phase noise PSD of the output from the mixer by the delay transfer function for 100 m and 1 km fiber with the enhancement factor ((2n)2|H100m(jf)|2)or (2n)2|H1km(jf)|2)), the sensitivity limit by the background noise can be estimated. The +/−1st order sidemodes are used, not higher order sidemodes, because intrinsic phase noise of the DUT with limited feedback bandwidth determines the achievable phase noise In this experiment, the suppressed phase noise is measured out-of-loop, based on a conventional delayed self-homodyne method with a 1 km fiber delay. Note that because the proposed method does not rely on electronic and O/E components with high bandwidth, the phase noise reduction demonstrated here is compatible with microwave carrier frequencies limited only by the modulation bandwidth of the EOM, which can reach up to 100 GHz
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