A photonic-delay line is used as a frequency discriminator for measurement of the phase noise-hence the short-term frequency stability-of microwave oscillators. The scheme is suitable for electronic and photonic oscillators, including the optoelectronic oscillator, mode lock lasers, and other types of rf and microwave pulsed optical sources. The approach is inherently suitable for a wide range of frequency without reconfiguration, which is important for the measurement of tunable oscillators. It is also insensitive to a moderate frequency drift without the need for phase locking.

The microwave signal at the output of a photodiode that detects a modulated optical beam contains the phase noise ϕ(t) and the amplitude noise α(t) of the detector. Beside the white noise, which is well understood, the spectral densities Sϕ(f ) and Sα(f ) show flicker noise, proportional to 1/f . We report on the measurement of the phase and amplitude noise of high-speed p-i-n photodiodes. The main result is that the flicker coefficient of the samples is ∼ 10 −12 rad 2 /Hz (−120 dBrad 2 /Hz) for phase noise, and ∼ 10 −12 Hz −1 (−120 dB) for amplitude noise. These values could be observed only after solving a number of experimental problems and in a protected environment. By contrast, in ordinary conditions insufficient EMI isolation, and also insufficient mechanical isolation, are responsible for additional noise to be taken in. This suggests that if package and EMC are revisited, applications can take the full benefit from the surprisingly low noise of the p-i-n photodiodes.

We describe simultaneous generation of ultralow-noise optical pulses and microwave signal with a mode-locked fiber laser in a coupled optoelectronic oscillator configuration. We demonstrate 9.2-GHz optical and microwave signals with the measured phase noise of -140 dBc/Hz at 10-kHz offset frequency. We show that the mode-locked laser in the photonic oscillator serves as a high-Q filter and is responsible for the observed low phase noise.