We address an experimental Stokes imaging setup allowing one to explore the polarimetric properties of a speckle light field with spatial resolution well beyond the speckle grain scale. We detail how the various experimental difficulties inherent to such measurements can be overcome with a dedicated measurement protocol involving a careful speckle registration step. The setup and protocol are then validated on a metallic reference sample, and used to measure the state of polarization (SOP) of light in each pixel of highly resolved speckle patterns (>2000 pixels per speckle grain) resulting from the scattering of an incident coherent beam on samples exhibiting different polarimetric properties. Evolution of the SOP with spatial averaging and across adjacent speckle grains is eventually addressed.
International audienceOptoelectronic down-conversion of a terahertz optical beatnote to a RF intermediate frequency is performed with a standard Mach-Zehnder modulator followed by a zero dispersion-slope highly nonlinear fiber. The two interleaved optical combs obtained by four-wave mixing are shown to contain more than 75 harmonics enabling to conveniently recover the phase noise of a beatnote at 770 GHz at ~500 MHz. This simple four-wave mixing assisted down-conversion architecture is implemented to a two-frequency solid-state laser in order to directly phase-lock its frequency difference. This is illustrated on a beatnote at 168 GHz directly phase locked to the local oscillator at 10 MHz
Optoelectronic down-conversion and phase locking of mm-wave and THz optical beatnotes using interleaved combs is presented. The generation of the required combs using either nonlinear modulation or four-wave-mixing in highly nonlinear fibers is detailed. These down conversion techniques are then implemented to phase-lock widely tunable mm-wave beatnotes well below the laser linewidth itself. Such achievements are made possible through a thorough trade-off between laser characteristics, optoelectronic arrangement, and electronicfeedback design. These advanced techniques give a new trend for bridging the gap between microwave and THz photonics.
The introduction of a buffer reservoir mechanism with optimized time-constants and cross sections in a laser system enables breaking any resonant exchange between the population inversion and photon population over an extremely wide bandwidth. The associated noise cancellation, including the excess noise at relaxation oscillations and spontaneous-signal beating, is experimentally evidenced up to 16 GHz in an Er,Yb laser comprising a GaAs two-photon absorber. Such approach is shown to preserve the laser linewidth quality and is advantageously implemented for optical distribution of frequency references.
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