D. Dolfi

Dual-Frequency Laser at 1.5

We describe the stabilization of the beatnote of an Er,Yb:glass dual-frequency laser at 1.5 mum with and without an external microwave reference. In the first case, a classical optical phase-locked loop (OPLL) is used, and absolute phase noise levels as low as -117 dBrad2/Hz at 10 kHz from the carrier are reported. In the second case one or two fiber-optic delay lines are used to lock the frequency of the beatnote. Absolute phase noise levels as low as -107 dBrad2/Hz at 10 kHz from the carrier are measured, fairly independant of the beatnote frequency varying from 2 to 6 GHz. An analysis of the phase noise level limitation is presented in the linear servo-loop theory framework. The expected phase noise level calculated from the measurement of the different noise sources fits well with the predictions.

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We experimentally demonstrate efficient optical carrier reduction of microwave signals with a single-mode 1.5-microm wavelength Brillouin all-fiber ring laser. Because of the tunable optical coupling, the lasing threshold of the short-length (20-m) fiber cavity is lower than 5 mW, and high conversion efficiencies (up to 60%) are obtained at any pump power up to approximately 200 mW. Using the single-mode Stokes beam as a seed for the stimulated Brillouin scattering process allows up to 40-dB optical carrier depletion with almost no added noise for an optically carried microwave signal at 6 GHz. In addition, using this resonator, we provide evidence of generation of high-spectral-purity beatnotes.

m for Optical Distribution and Generation of High-Purity Microwave Signals

A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical pre-amplification is demonstrated at 1.06 microm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.

Efficient single-mode Brillouin fiber laser for low-noise optical carrier reduction of microwave signals

New optoelectronic architectures are presented, based on parallel delay lines, performing programmable filtering of microwave signals. According to current performances of optoelectronic components, they can process optically carried microwave signals over frequency bandwidths as large as 20 GHz, with a time-frequency product up to 10/sup 3/. The operating principle of these structures is detailed and followed by the preliminary experimental demonstration at 1.3 GHz of a 53-dB rejection filter.

Building blocks for a two-frequency laser lidar-radar: a preliminary study

We present and experimentally demonstrate a new method for enhancing the signal-to-background ratio of two-wave mixing in photorefractive crystals. The method uses a mutually incoherent third beam to suppress the fanning in a dark ring-shaped region in which the amplified signal is located. A 20-fold improvement of the signal-to-background ratio is measured in BaTiO(3) at lambda = 514 nm. The extension of this principle to wide-field-of-view heterodyne detection is discussed.

Optical architectures for programmable filtering and correlation of microwave signals

We experimentally and theoretically analyze an original method based on two wave mixing in erbium doped fiber amplifier for optical carrier reduction of microwave signals. 75 % selective optical carrier attenuation are experimentally demonstrated.

ENHANCEMENT OF THE SIGNAL-TO-BACKGROUND RATIO IN PHOTOREFRACTIVE TWO-WAVE MIXING BY MUTUALLY INCOHERENT TWO-BEAM COUPLING

We investigate the utilisation of the dual-frequency lidar-radar concept for high resolution range measurements. Experimental results, leading to 17 cm resolution with a 200 /spl mu/s laser pulse width, are presented.

Two-wave mixing in an erbium-doped fiber amplifier for modulation depth enhancement of optically carried microwave signals

We present a new optoelectronic architecture, based on parallel canceled delay lines, that performs programmable filtering of microwave signals. The new architecture can process optically carried microwave signals over frequency bandwidths as large as 20 GHz, with a time-frequency product up to 10(3). The operating principle of this structure is detailed, followed by the preliminary experimental demonstration at 1.2 GHz of a 40-dB rejection filter.

High resolution range measurement using lidar-radar concept

An optical architecture enabling time delay analysis of large bandwidth microwave signals is presented. It permits to determine time delays between microwave signals with a resolution better than 10 ps.

Optical architecture for programmable filtering of microwave signals

We report on the experimental demonstration of a single-axis two-frequency Nd:YAG laser, in which the frequency difference is tunable from 11 GHz up to 120 GHz, thanks to a PLZT anisotropic electro-optically tunable etalon.