Application of the optical fiber to generation and measurement of low phase noise microwaves

International audienceThe optical delay line proved to be a method to measure the phase noise of microwave oscillators with high sensitivity. The delay, inherently, turns frequency fluctuations into phase fluctuations. Hence, a mixer is used to compare the phase of the oscillator signal to a delayed copy, from which we measure the oscillator phase noise. This article reports on the progress in this type of instruments in our laboratory. For practical reasons, the delay is implemented with an optical-fiber channel, either at 1.3 or 1.55 µm wavelength, modulated in intensity. The laser Relative Intensity Noise (RIN) turns out to be a critical parameter because it converts the RIN into near DC noise through the mixer offset sensitivity to power. The best semiconductor lasers we can find show a RIN of about -155 dB/Hz. Additionally, in our experience the simple microwave photodiodes are to be preferred to the photodiodes with integrated transconductance amplifier because of the lower noise. This seems to be a technical issue, rather than a general property. Thus, we used a separate amplifier, based on SiGe technology for lowest flicker. The optical fiber is temperature stabilized by an Aluminum mass and a sub-milliKelvin electronic control. Other components, like the Mach Zehnder intensity modulator, seem to be less critical for noise. A single channel version was realized and tested with a microwave synthesizer and with a sapphire whispering gallery oscillator at 10 GHz carrier frequency. Using a 2 km delay line (Τ=10 µs) the measured 1/f3 noise is b-3 = -12 dB.rad²/Hz, which matches the a-priori knowledge of the oscillator 1/f3 noise. This means that the instrument sensitivity is higher than this value, thus it is sufficient to measure a room-temperature sapphire oscillator without need of correlation. At higher Fourier frequencies, the instrument background noise is of -145 dBrad²/Hz at 10 kHz off the carrier. Unfortunately, with other source types (eg, a synthesizer) the background noise can be higher because of the effect of am noise. Another test of the noise floor consists of shortening the optical delay to a negligible value. In this way, the oscillator noise is rejected. Unfortunately, this is only a qualitative test because it hides the noise of the optical fiber, due to Rayleigh scattering and other optical phenomena. The background noise is reduced proportionally to 1/√m, where m is the number of averaged spectra, by correlation and averaging on two fully independent channels that measure the same oscillator. Using 2 km optical fibers and averaging on 200 spectra, the background noise is of -110 dBrad²/Hz at 100 Hz off the carrier, and of -160 dBrad²/Hz at 10 kHz. Re-using the parts of the two-channel system, we assemble single-channel system with matched 10 µs optical delays at the two inputs of the mixer, which rejects the noise of the oscillator. The 1/f noise measured in this condition, b-1 = -113 dBrad²/Hz referred to one channel, is the background noise of the system without correlation, which includes amplifiers, detector and optical fiber

Applications of the optical fiber to the generation and to the measurement of low-phase-noise microwave signals

The optical fiber used as a microwave delay line exhibits high stability and low noise and makes accessible a long delay (>100 microseconds) in a wide bandwidth (about 40 GHz, limited by the optronic components). Hence, it finds applications as the frequency reference in microwave oscillators and as the reference discriminator for the measurement of phase noise. The fiber is suitable to measure the oscillator stability with a sensitivity of parts in 1E-12. Enhanced sensitivity is obtained with two independent delay lines, after correlating and averaging. Short-term stability of parts in 1E-12 is achieved inserting the delay line in an oscillator. The frequency can be set in steps multiple of the inverse delay, which is in the 10-100 kHz region. This article adds to the available references a considerable amount of engineering and practical knowledge, the understanding of 1/f noise, calibration, the analysis of the cross-spectrum technique to reduce the instrument background, the phase-noise model of the oscillator, and the experimental test of the oscillator model.Comment: 23 pages, 13 figures, 41 reference

FFREE: a Fresnel-FRee Experiment for EPICS, the EELT planets imager

The purpose of FFREE - the new optical bench devoted to experiments on high-contrast imaging at LAOG - consists in the validation of algorithms based on off-line calibration techniques and adaptive optics (AO) respectively for the wavefront measurement and its compensation. The aim is the rejection of the static speckles pattern arising in a focal plane after a diffraction suppression system (based on apodization or coronagraphy) by wavefront pre-compensation. To this aim, FFREE has been optimized to minimize Fresnel propagation over a large near infrared (NIR) bandwidth in a way allowing efficient rejection up to the AO control radius, it stands then as a demonstrator for the future implementation of the optics that will be common to the scientific instrumentation installed on EPICS.Comment: 12 pages, 15 figures, Proceeding 7736120 of the SPIE Conference "Adaptive Optics Systems II", monday 28 June 2010, San Diego, California, US

Spectral density of phase noise inter-laboratory comparison final results

This paper reports main results of the phase noise comparison that has been performed between october 2005 and december 2006, using two oscillators at 5 and 100 MHz and un DRO at 3.5 GHz. The problem is not to compare the performances of several oscillators, but to compare and to make an evaluation of the uncertainties, and of course the resolution and the reproducibility of the measurements. This comparison allow us to determine the ability to get various systems traceable together in order to increase the trust that one can have in phase noise measurements

Polarized light scattering by inhomogeneous hexagonal monocrystals. Validation with ADEOS-POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements

The E-ELT first light spectrograph HARMONI: capabilities and modes

Trabajo presentado en SPIE Astronomical Telescopes, celebrado en San Diego (California), del 26 de junio al 1 de julio de 2016HARMONI is the E-ELT's first light visible and near-infrared integral field spectrograph. It will provide four different spatial scales, ranging from coarse spaxels of 60 × 30 mas best suited for seeing limited observations, to 4 mas spaxels that Nyquist sample the diffraction limited point spread function of the E-ELT at near-infrared wavelengths. Each spaxel scale may be combined with eleven spectral settings, that provide a range of spectral resolving powers (R 3500, 7500 and 20000) and instantaneous wavelength coverage spanning the 0.5 - 2.4 ¿m wavelength range of the instrument. In autumn 2015, the HARMONI project started the Preliminary Design Phase, following signature of the contract to design, build, test and commission the instrument, signed between the European Southern Observatory and the UK Science and Technology Facilities Council. Crucially, the contract also includes the preliminary design of the HARMONI Laser Tomographic Adaptive Optics system. The instrument's technical specifications were finalized in the period leading up to contract signature. In this paper, we report on the first activity carried out during preliminary design, defining the baseline architecture for the system, and the trade-off studies leading up to the choice of baseline

Calibration and precompensation of noncommon path aberrations for extreme adaptive optics

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