D. Bacquet

Ultrawide-Bandwidth Single-Channel 0.4-THz Wireless Link Combining Broadband Quasi-Optic Photomixer and Coherent Detection

Free space communications with huge data capacity have become a key point for the development of mobile access, services, and network technologies convergence. Wireless links using emerging terahertz technologies, also referred to as T-ray communications, have become an intensive research field within last years since T-rays at the millimeter/submillimeter frontier remain more robust in terms of scintillation, dielectric obstacles, and fog compared with near-infrared signals. Using a passive THz hot-spot which can be fed by fiber optic networks, we investigate up bit rates up to 46 Gbps for a THz wireless transmission system at 400-GHz carrier frequency. Using a THz photomixer integrated with a specific broadband antenna and heterodyne electronic detection, we investigate the eye diagrams using 1-μW received power, highlighting the high sensitivity and ultra-wideband behavior of the whole THz system.

Highly coherent terahertz wave generation with a dual-frequency Brillouin fiber laser and a 1.55 μm photomixer

Thanks to a portable dual-frequency Brillouin fiber laser and a 1.55 μm photomixer, we report the generation of a highly coherent kilohertz level submillimeter wave emission. Low-cost telecommunications components are used to achieve very simple source architecture. The photomixer is composed of a unitravelling carrier photodiode integrated with an antenna. An emission at 316 GHz is observed and analyzed thanks to heterodyne detection with a signal-to-noise ratio >65 dB and a ~1 kHz linewidth. The phase noise of the proposed source has the same performance at 1.7 and 316 GHz. We show that this source has comparable or better phase noise compared to electrical oscillators and the tunability is much wider.

Vector Brillouin optical time-domain analyzer for high-order acoustic modes

Thanks to a double-frequency phase modulation scheme, we report a vector Brillouin optical time-domain analyzer (BOTDA). This BOTDA has a high immunity level to noise, and it features a phase spectrogram capability. It is well suited for complex situations involving several acoustic resonances, such as high-order longitudinal modes. It has notably been used to characterize a dispersion-shifted fiber, allowing us to report spectrograms with multiple acoustic resonances. A very high 57 dB dynamic range is also reported for 100-ns-long pulses simultaneously with a 16 cm numerical resolution.

Narrow Linewidth Tunable Terahertz Radiation By Photomixing Without Servo-Locking

A beatnote, tunable from dc to 1 THz, provided by a dual-frequency laser is used to feed an unitravelling carrier photodiode in order to produce a highly coherent THz signal radiated by a transverse-electromagnetic-horn antenna. The THz signal is detected and analyzed by a subharmonic mixer coupled to an electrical spectrum analyzer. All components involved in this experiment operate at room temperature without phase locking the beatnote. So far, the dynamic range evolves from 58 dB at 282 GHz to 15 dB at 1.026 THz, and the measured linewidth is better than 30 kHz. Linewidth narrowing using a Brillouin fiber laser pumped by the dual-frequency laser leads to a beatnote of 500-Hz linewidth at 1 THz.

Relative Intensity Noise Transfer Reduction in Raman-Assisted BOTDA Systems

Raman-assistance (RA) has been identified as a promising technique to extend the measurement range of Brillouin optical time-domain analysis (BOTDA)-based distributed sensors. Unfortunately, Raman amplification introduces a great amount of relative intensity noise (RIN) to the detected low-frequency probe wave. This RIN transfer problem has been widely identified as a major limitation in RA-BOTDA. In vector BOTDA (VBOTDA), the detected signal is transferred to a high-frequency carrier where the Raman RIN transfer turns out to be much less harmful. In addition, a VBOTDA can also provide information about the phase-shift induced by the local stimulated Brillouin scattering gain curve, paving the way for dynamic measurements. In this letter, we demonstrate, for the first time to our knowledge, the RA in a VBOTDA obtaining gain and phase measurements. Our results show a significant reduction of the RIN transfer effect in RA-VBOTDA compared with standard RA-BOTDA, making this type of scheme particularly interesting for long-range and dynamic distributed sensing.

Raman-assisted vector Brillouin optical time domain analysis

Raman-assistance (RA) has become a promising technique to enhance the sensing range of standard Brillouin Optical Time Domain Analysis (BOTDA) fiber sensors due to its ability to amplify in a distributed way all the interacting signals within the fiber. Unfortunately, Raman amplification introduces a great amount of Relative Intensity Noise (RIN) to the detected low-frequency probe wave. This RIN transfer problem has been widely identified as a major limitation in RABOTDA. In Vector Brillouin Optical Time Domain Analysis (VBOTDA) the detected signal is transferred to a highfrequency carrier where the Raman RIN transfer turns out to be much less harmful. In this work we demonstrate, for the first time to our knowledge, Raman-assistance in a VBOTDA. Our results show significant reduction of the RIN transfer effect in RA-VBOTDA compared to standard RA-BOTDA, making this type of scheme particularly interesting for long range distributed sensing.

Mode-hopping suppression in long Brillouin fiber laser with non-resonant pumping.

We propose a reliable method for stabilizing narrow linewidth Brillouin fiber lasers with non-resonant pumping. Mode-hopping is suppressed by means of a phase-locked loop that locks the pump-Stokes detuning to a local radio-frequency (RF) oscillator. Stable single-mode operation of a 110-m-long Brillouin fiber laser oscillating at 1.55 μm is demonstrated for several hours. The beat note between two independent Stokes waves presents a phase noise level of -60  dBc/Hz at 100 Hz with a -20  dB/decade slope, and a FWHM linewidth lower than 50 Hz.

Multicore fiber for cold-atomic cloud monitoring

Thanks to an all solid core photonic crystal fiber (PCF) used as a multicore fiber, we propose and experimentally demonstrate what is to our knowledge a new optical detection scheme for the spontaneous emission collection of cold atoms. A Magneto-Optical Trap (MOT) is placed in front of a polished PCF end-face. As they display a higher optical index than the surrounding cladding silica, the 108 rods (equivalent to a 108 pixels camera) of this PCF are light guiding and behave like an array of detectors. Both global and local properties of the trapped atoms are probed. A MOT lifetime is reported. We also take advantage of the multi-core geometry for a real time detection of the center-of-mass motion of the atomic cloud.

Terahertz wireless communications using photonic and electronic devices

With the fast increase of mobile data transfers, wireless communications carrier frequencies have entered in the millimeter wave region and now they enter in the submillimeter or terahertz region. In this context photonic-based emitters have several advantages, we will present our communication links results using photomixers at 0.2, 0.4 and 0.6 THz.

Enhanced structural sensitivity of hybrid-mode acoustic phonons in axially-varying photonic crystal fiber.

We report the observation of anti-crossings between hybrid-mode acoustic phonons in an axially-varying photonic crystal fiber. Our experimental results are analyzed using an electrostriction theory which reveals strong coupling between longitudinal and shear components of elastic wave. These anti-crossings are highly sensitive to the transverse fiber structure and thus could be potentially used for ultra-sensitive sensors and new opto-acoustic devices.