A laser ranging method dedicated to path lengths equalization in diluted telescopes

International audienceWhen implementing a diluted telescope with large dimensions, one has to reach the equal path condition to the different segments of the primary mirror. In this work we suggest a way to implement a fast laser ranging method able to provide the error signal, using phase detection of the microwave modulation of a laser beam

Comments on "Frequency Response of the Noise Conversion from Relative Intensity Noise to Phase Noise in the Photodetection of an Optical Pulse Train"

International audienceIt is shown that the results presented in the above letter, and attributed by the authors to amplitude-to-phase conversion in the photodetector, do not originate in the photodetector under test, due to the power dependence observed. More recent results show that conversion effects with the same size and frequency dependence are observed in passively mode-locked nonlinear polarization rotation mode-locked lasers

Elimination of systematic errors in two-mode laser telemetry

4p.International audienceWe present a simple two-mode telemetry procedure which eliminates cyclic errors, to allow accurate absolute distance measurements. We show that phase drifts and cyclic errors are suppressed using a fast polarization switch that exchanges the roles of the reference and measurement paths. Preliminary measurements obtained using this novel design show a measurement stability better than 1 micron. Sources of residual noise and systematic errors are identified, and we expect that an improved but still simple version of the apparatus will allow accuracies in the nanometre range for absolute measurements of kilometre-scale distances

Noise spectroscopy with large clouds of cold atoms

Noise measurement is a powerful tool to investigate many phenomena from laser characterization to quantum behavior of light. In this paper, we report on intensity noise measurements obtained when a laser beam is transmitted through a large cloud of cold atoms. While this measurement could possibly investigate complex processes such as the influence of atomic motion, one is first limited by the conversion of the intrinsic laser frequency noise to intensity noise via the atomic resonance. This conversion is studied here in details. We show that, while experimental intensity noise spectra collapse onto the same curve at low Fourier frequencies, some differences appear at higher frequencies when the probe beam is detuned from the center of the resonance line. A simple model, based on a mean-field approach, which corresponds to describing the atomic cloud by a dielectric susceptibility, is sufficient to understand the main features. Using this model, the noise spectra allow extracting some quantitative informations on the laser noise as well as on the atomic sample

IMPLEMENTATION OF TWO TELEMETER DESIGNS FOR HIGH ACCURACY LASER RANGING OF KILOMETER SCALE DISTANCES IN SPACE

We present two different laser ranging systems under development, both based on the use of a high frequency modulated beam. The first range meter makes no use of interferometry: only the phase of the return beam is detected, in a way that rejects cyclic errors due to optical and electronic crosstalk. An Allan deviation slightly better than 10nm has been obtained with this simple system. The other range meter should provide better resolution, at the expense of a somewhat more sophisticated procedure, as it involves both time-of-flight and interferometry measurements

ABSOLUTE DISTANCE MEASUREMENTS USING TWO-MODE LASER TELEMETRY

A novel laser ranging method is described that uses a two-mode laser source, and detection of the phase of the return beam. The design eliminates the cyclic error usually associated with phase measurements and provides unambiguous, absolute distance determination. Measurements of an 8m path are obtained at a beat frequency of 13 GHz. We analyse the 1µm stability of the data obtained with this preliminary implementation, and expect that an improved version will allow accuracies well below 1 µm, for the kilometer-scale distances involved in satellite formation flight

Nanometer-scale absolute laser ranging: exploiting a two-mode interference signal for high accuracy distance measurements

International audienceAbsolute distance measurement with accuracy below the micron scale is important in astronomical optical interferometry. We present here an absolute laser rangefinder which relies on the detection of a two mode interference signal. We exploit the specific signature of the signal to extract both the interferometric and synthetic phase measurements, leading to distance measurement with nanometric accuracy. A resolution of 100 pm has been achieved in 75 μs with a relatively simple laser source. Amplitude to phase coupling in the detection chains turns out to be the largest source of systematic errors. A specific detection scheme is implemented, using optical demodulation of the microwave optical signal, to reduce amplitude-to-phase related systematic errors to below the required level

Dynamics of the Amplitude-to-Phase Coupling of 1.5µm High Bandwidth Commercial Photodiodes

National audienceWe 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. These results are important in optimising RF optical links

Phase measurement of a microwave optical modulation: characterisation and reduction of amplitude-to-phase conversion in 1.5 µm high bandwidth photodiodes

En attente de publication papier; Lien vers l'éditeur: http://ieeexplore.ieee.org/xpl/articleDetails.jsp? arnumber=6775285&sortType%3Dasc_p_Sequence% 26filter%3DAND%28p_IS_Number%3A4357488%29International audienceHigh 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

Long distance absolute laser ranging at the nanometer precision level: the two-mode interference measurement in the Iliade rangemeter

International audienceSome measurement schemes have been proposed and realized for the absolute measurement of long distances with an accuracy better than 10 nm. Published measurement setups use one or even two laser frequency combs. But significant engineering will be required to space qualify such a system. Simple methods, less technology-demanding would be valuable in the perspective of embedded instrumentation. We have designed and implemented a two-mode interference measurement scheme that allows sub-nanometer scale resolution in long distance measurement. The two-mode interference signal contains both - (sub-µm) interferometric information: the scale is the optical wavelength - (sub-15mm) modulation phase information: the scale is the "synthetic wavelength" corresponding to the frequency of the beat-note of the two modes. With the addition of a time-of-flight (ToF) measurement, the method allows to combine the three data (ToF, synthetic wavelength and interferometric) in a single, high-resolution, high accuracy length measurement, obtained every 50 ms. A measurement update rate of 100 µs is possible, but may rely on the availability of significantly higher data processing rates in the FPGA phase-meter. Implementation of this scheme has required that systematic errors on the phase and amplitude of the microwave optical signal be kept at a level well below 10^-4 cycle and 10^-4 respectively. One consequence of this requirement is the replacement of any parallel optics in the optical setup by wedged optics, so that multiple reflections do not interfere with the measurement and reference beams to better than 10^-8 in optical power. Systematic errors of electronic origin are more difficult to deal with because the amplitude-to-phase (AM-to-PM) couplings effects at 20 GHz appear to have, not only an instantaneous contribution, but also a transient contribution. This contribution, related to the heating of the photodiode junction under the dissipated Joule power, exceeds the limit of 10^-4 cycle by roughly two orders of magnitude. This thermal behaviour is not purely exponential with time and cannot be accurately corrected for. We will present the implementation of the setup, and the way we have suppressed, by 3 orders of magnitude, the AM-to-PM coupling effect by modifying the detection scheme of the 20 GHz beatnote. This last point is important, not only for the range meter presented, but also for in high accuracy and low phase noise microwave optical links