Photonic synthesis of radiofrequency revived the quest for unrivalled microwave purity by its seducing ability to convey the benefits of the optics to the microwave world 1-11 . In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fiber-based frequency comb and cuttingedge photo-detection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 x 10 -16 at 1 s and a timing noise floor below 41 zs.Hz -1/2 (phase noise below -173 dBc.Hz -1 for a 12 GHz carrier). This outclasses existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation

When illuminating a photodiode with modulated laser light, optical intensity fluctuations of the incident beam are converted into phase fluctuations of the output electrical signal. This amplitude to phase noise conversion (APC) thus imposes a stringent constraint on the relative intensity noise (RIN) of the laser carrier when dealing with ultra-low phase noise microwave generation. Although the APC vanishes under certain conditions, it exhibits random fluctuations preventing efficient long-term passive stabilization schemes. In this paper, we present a digital coherent modulation-demodulation system for automatic measurement and control of the APC of a photodetector. The system is demonstrated in the detection of ultra-short optical pulses with an InGaAs photodetector and enables stable generation of ultra-low phase noise microwave signals with RIN rejection beyond 50 dB. This simple system can be used in various optoelectronic schemes, making photodetection virtually insensitive to the RIN of the lasers. We utilize this system to investigate the impact of the radiofrequency (RF) transmission line at the output of the photodetector on the APC coefficient that can affect the accuracy of the measurement in certain cases.

Phase noise or frequency noise is a key metrics to evaluate the short term stability of a laser. This property is of a great interest for the applications but delicate to characterize, especially for narrow line-width lasers. In this letter, we demonstrate a digital cross correlation scheme to characterize the absolute phase noise of subhertz line-width lasers. Three 1,542 nm ultra-stable lasers are used in this approach. For each measurement two lasers act as references to characterize a third one. Phase noise power spectral density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each laser by a mere change in the configuration of the lasers. This is the first time showing the phase noise of sub-hertz linewidth lasers with no reference limitation. We also present an analysis of the laser phase noise performance.Laser phase noise describes how the phase of a laser output electrical field deviates from an ideal sinusoidal wave. This quantity which is defined to evaluate the short term stability of a laser can also be used to estimate the line-width or coherent length of a laser. In many applications, such as coherent optical communication [1], LIDAR [2], optical fiber-based interferometeric sensors [3], high resolution spectroscopy [4], ultra-low phase noise photonic microwave generation [5], and optical atomic clock [6], the laser phase noise can profoundly impacts the limitation of a system. Thus, lasers with ultra-low phase noise are actively studied [7][8][9][10]; while the precise characterization of such ultrastable laser is becoming more important.Phase noise characterization is a comparison process. Generally, laser phase noise measurement approaches can be divided into two categories according to the comparison method. The first one is comparing the laser under test with itself through the schemes of delayed self-homodyne, delayed self-heterodyne [11,12] or Michelson interferometer [13]. Several kilometers of fiber are usually used for these optical delay line methods in order to make the delay time longer than the laser coherent time. It is difficult to characterize the phase noise of lasers with hundreds-hertz linewidth or narrower via these delay line technique as thousands of kilometers of fiber would be required, and the fiber noise itself can also become a limitation. The second category, which is called the beat-note method [14][15][16][17], implies comparing the laser under test with a reference laser whose phase noise is much lower than that of the one under test. When the phase noise of the laser under test is lower than that of any available reference, two similar lasers must be built and compared. Assuming statistical independence and equal contribution of both lasers, the phase noise is revealed after division of the beat-note phase noise by √2. However, to realize two equally good lasers is not straightforward. Cross correlation is a well know approach to characterize the phase noise of RF and microwave oscillators with ultra-low level [5,[18][19][20][21][22]. Here, we extend ...

Cooperative diversity and orthogonal frequency division multiplexing (OFDM) are two key technologies for future wireless communication systems. One of the main problems of OFDM systems is the high peak-to-average power ratio (PAPR) of the transmitted signals, which may cause the introduction of intercarrier interference due to the presence of nonlinear power amplifiers (PAs). In this paper, a theoretical analysis of the outage probability of an amplifyand-forward (AF) cooperative diversity OFDM system accounting for nonlinear distortions introduced by a nonlinear PA is developed. It is assumed a frequency-selective Rayleigh fading and a downlink transmission, with the base station having a linear PA and the relay having a nonlinear PA. Our analysis shows how the PA parameters affects the outage probability for different SNR levels. The validity of the proposed outage analysis is verified by means of computer simulations.C. A. R. Fernandes is supported by the FUNCAP/BR agency (Grant No. BP1-0031-00106.01.00/10) and Ceara State Government.