Christophe Alexandre

Photonic microwave signals with zeptosecond-level absolute timing noise

Ultralow-noise microwave signals are generated at 12 GHz by a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. The microwave signals have a fractional frequency stability below 6.5 × 10–16 at 1 s and a timing noise floor below 41 zs Hz–1/2. Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 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 outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.

High resolution kilometric range optical telemetry in air by radio frequency phase measurement

We have developed an optical Absolute Distance Meter (ADM) based on the measurement of the phase accumulated by a Radio Frequency wave during its propagation in the air by a laser beam. In this article, the ADM principle will be described and the main results will be presented. In particular, we will emphasize how the choice of an appropriate photodetector can significantly improve the telemeter performances by minimizing the amplitude to phase conversion. Our prototype, tested in the field, has proven its efficiency with a resolution better than 15 μm for a measurement time of 10 ms and distances up to 1.2 km.

Phase Measurement of a Microwave Optical Modulation: Characterisation and Reduction of Amplitude-to-Phase Conversion in 1.5 μm High Bandwidth Photodiodes

High accuracy and low noise measurement of the phase of a microwave signal requires that spurious contributions are adequately dealt with. In this paper we investigate the power-to-phase coupling in two commercial high bandwidth P-I-N, near-IR photodetectors. We observe that a sudden change of the optical power induces a transient of the phase of the 20 GHz signal, at different time scales. The temperature rise of the photodetector junction is likely to be involved in this dynamical behaviour. The value of the bias voltage applied to the photodetector appears to control the size of the phase transients, as well as the optical power for which the slope of the amplitude-to-phase coupling cancels. The most efficient way to reduce amplitude to phase couplings consists in implementing optical demodulation, instead of electrical demodulation, of the microwave signal.

The influence of residual errors on a digital satellite TV decoder

Abstract In spite of the use of convolutional and Reed-Solomon codes in satellite broadcasting to protect the MPEG-2 bitstream, residual errors will be found at the input of audio and video decoders. Concealment methods must be elaborated to preserve image quality. In this paper, we simulate the complete transmission system in order to design residual error models. Then, using these models, errors are injected in audio and video bitstreams and their influence on quality is determined with subjective tests. We show that the audio decoding process is much more robust than the video one. So we focus our attention on video and elaborate two concealment strategies. In the first one, the decoder uses the information provided by the system layer to conceal false packets. In the second, a smart decoder is designed which uses bitstream properties to detect and conceal residual errors. We note that the smart decoder gives better image quality than that of the first decoder. Finally, the benefits of concealment strategy are evaluated in terms of C N .

Characterization and reduction of the amplitude-to-phase conversion effects in telemetry

We are developing a telemeter based on the measurement of the phase accumulated by an RF sine wave during its propagation in air. This wave is carried by a laser beam by an intensity modulation. The main limitation of this technique lies in amplitude-to-phase conversion occurring in the detection of this modulation. Therefore, we characterize this phenomenon for a given telemetric system and discuss how to reduce its effects on the resolution and the accuracy of the distance measurement. Finally, a solution is implemented and tested outdoors in real conditions of use.

SDR based prototype for filter bank based multi-carrier transmission

This paper investigates the design of a potential Software Defined Radio (SDR) based prototype for Filter Bank based Multi-Carrier Transmission. It allows for implementing and testing transmitter and receiver in real-time on Universal Software Radio Peripheral (USRP) by using two USRP boards. Additionally, an implementation of some classic channel models in a separate USRP board has been used for emulating a propagation channel. The presented prototyping system supports FBMC waveform but also Orthogonal Frequency Division Multi-carriers(OFDM) one using MATLAB routines. With this motivation, we propose a study on real-time decoding of Offset Quadrature Amplitude Modulation FBMC (OQAM-FBMC) radio frames. In this system, data recovery at the receiver is very sensitive to time and frequency synchronization of the frames. This problem was addressed in many theoretical research works. However, considering its practical aspects in the context of a realtime radio transmission, a signal processing block is required to satisfy a trade-off between the accuracy and the latency. We especially recognize different limits, imposed by both hardware and software entities, to the real-time system.

Accurate control of optoelectronic amplitude to phase noise conversion in photodetection of ultra-fast optical pulses

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.

Laser diodes based absolute distance meter

We present our first results in the realization of a laser diodes based telemeter. We aim at developing an air index compensated distance meter taking advantage of the dependence of the chromatic dispersion of the refractive index with air temperature. This will be realized by two simultaneous distance measurements at two different wavelengths: each measurement is based on the measurement of the phase accumulated by a radio wave propagated in air by an intensity modulated laser beam.

DSP implementation of interference cancellation algorithm for a SIMO system

This paper presents an implementation on the fixed-point DSP chip of an interference cancellation algorithm containing a linear system. Difficulty in matching the algorithms performance obtained in full precision (IEEE float-point) implementation to those given by a finite precision tends to increase when the algorithms contains one or several linear system to solve. Gauss and Cholesky methods are implemented. Their performance results are showed and compared. The global implementation margins are discussed.

Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation

Phase noise or frequency noise is a key metric to evaluate the short-term stability of a laser. This property is of great interest for the applications but delicate to characterize, especially for narrow linewidth lasers. In this Letter, we demonstrate a digital cross-correlation scheme to characterize the absolute phase noise of sub-hertz linewidth lasers. Three 1542 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. To the best of our knowledge, 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.