Frequency-modulated multifunction lidar for anemometry, range finding, and velocimetry-1. Theory and signal processing.

Original waveforms and optimized signal processing are proposed for frequency-modulated continuous-wave lidar for range finding, velocimetry, and laser anemometry. For range finding, the aim of this signal processing is to extend lidar range and reduce ambiguities. Moreover, the effect of moderate atmospheric turbulence on lidar efficiency is analyzed for infinite and finite targets, taking into account wind-induced bistatism. For laser anemometry, the aim is to measure air speed at the shortest distance farther than the rotor-induced turbulent volume around the helicopter and to avoid parasitic echoes coming from clouds or hard targets in the vicinity of a helicopter.

Frequency-modulated multifunction lidar for anemometry, range finding, and velocimetry-2. Experimental results.

Frequency-modulated continuous-wave lidar is evaluated for range finding, velocimetry, and laser anemometry. An original signal processing and waveform calibration for range finding leads to a reduction of computational effort while preserving capability for long-range measurement. Multiple target distance measurement is also demonstrated. For laser anemometry, the aim is to avoid parasitic echoes in the vicinity of a helicopter and to measure the air speed at the shortest distance farther than the rotor-induced turbulent volume around the helicopter. Flight tests of this functionality and vortex ring state warning are demonstrated.

Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3.

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology-a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption--reminiscent of piezochromism--which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

Measurement of the coupling constant in a two-frequency VECSEL.

We measure the coupling constant between the two perpendicularly polarized eigenstates of a two-frequency Vertical External Cavity Surface Emitting Laser (VECSEL). This measurement is performed for different values of the transverse spatial separation between the two perpendicularly polarized modes. The consequences of these measurements on the two-frequency operation of such class-A semiconductor lasers are discussed.

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

We experimentally and theoretically analyze an original method based on two-wave mixing in an erbium-doped fiber amplifier for optical carrier reduction of microwave signals. 75% optical carrier attenuation has been observed, and a 10 dB modulation depth increase of the microwave signal is experimentally demonstrated. Moreover, calculated results are in good agreement with measurements and predict that up to 80% carrier attenuation is easily possible.

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

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.

Wideband versatile radio-frequency spectrum analyzer.

Operation of a wideband, versatile optical spectrum analyzer for radio-frequency (RF) signals is demonstrated. The device is based on spectral hole burning (SHB). The demonstration features 2.3-GHz instantaneous bandwidth, 500-kHz resolution, and a 32-dB dynamic range. A true RF signal, transferred to the optical carrier with the help of a Mach-Zehnder modulator, is analyzed with optical carrier suppression and zooming capabilities. This is to the authors' knowledge the largest instantaneous bandwidth ever demonstrated for a SHB-based processor in rare-earth-doped crystals.

Hypertrophy, hyperplasia, and infectious virus in gut-associated lymphoid tissue of mice after oral inoculation with simian-human or bovine-human reassortant rotaviruses.

Oral inoculation of infants with a vaccine that contains simian-human reassortant rotaviruses has been found to be a rare cause of intussusception. Because intussusception can be associated with enlargement of gut-associated lymphoid tissue, we studied the capacity of simian-human and bovine-human reassortant rotaviruses to cause lymphoid hypertrophy and hyperplasia of Peyer's patches (PP) of adult BALB/c mice. Neither hypertrophy nor hyperplasia was detected in PP after oral inoculation with simian-human or bovine-human reassortant rotaviruses. However, infectious virus was detected in PP and mesenteric lymph nodes after oral inoculation with simian, but not bovine, reassortant rotaviruses. Implications of these findings on the pathogenesis of intussusception are discussed.

Demonstration of a radio-frequency spectrum analyzer based on spectral hole burning.

Spectral hole-burning (SHB) technology is considered for >10-GHz instantaneous bandwidth signal-processing applications. In this context we report on what is believed to be the first demonstration of a SHB microwave spectrometer. A set of gratings engraved in a SHB crystal is used to filter one sideband of the optically carried microwave signal. The setup is confined to narrow-bandwidth operation, over a 35-MHz-wide interval. The first findings confirm the validity of the architecture in terms of spectral resolution, angular channel separation, and simultaneous detection of multiple spectral lines.

Two-dimensional optical architectures for the receive mode of phased-array antennas.

We propose and experimentally demonstrate two optical architectures that process the receive mode of a p x p element phased-array antenna. The architectures are based on free-space propagation and switching of the channelized optical carriers of microwave signals. With the first architecture a direct transposition of the received signals in the optical domain is assumed. The second architecture is based on the optical generation and distribution of a microwave local oscillator matched in frequency and direction. Preliminary experimental results at microwave frequencies of approximately 3 GHz are presented.

Optical architecture for programmable filtering of 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.

Experimental demonstration of a phased-array antenna optically controlled with phase and time delays.

The experimental demonstration and the far-field pattern characterization of an optically controlled phased-array antenna are described. It operates between 2.5 and 3.5 GHz and is made of 16 radiating elements. The optical control uses a two-dimensional architecture based on free-space propagation and on polarization switching by N spatial light modulators of p × p pixels. It provides 2(N-1) time-delay values and an analog control of the 0 to 2π phase for each of the p × p signals feeding the antenna (N = 5, p = 4).

Enhancement of the signal-to-background ratio in photorefractive two-wave mixing by mutually incoherent two-beam coupling.

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.

Anomalous sidelobes and birefringence apodization in acousto-optic tunable filters.

Integrated-optic acousto-optic filters have large asymmetric sidelobes that have not been explained in previous publications. We show that this is caused by waveguide birefringence nonuniformity. For the devices described here, the dominant cause of the nonuniformity is a thermal gradient associated with the generation and absorption of the surface acoustic wave. When the thermal birefringence gradient is included in the coupled-mode model, asymmetric sidelobes in good agreement with observations are predicted. In addition, we show that waveguide inhomogeneities also lead to birefringence nonuniformity and can be used to tailor the birefringence to supress sidelobes in a two-section acousto-optic filter.

Reduced sidelobe integrated acousto-optic filter with birefringence apodization.

A two-section integrated acousto-optic tunable filter with significantly reduced sidelobes has been fabricated. The sidelobe reduction was achieved with birefringence apodization in the form of variable-width Ti-stripe waveguides. The design and fabrication of the device, which includes integrated proton-exchanged TE and TM-pass polarizers, are described. Experimental tuning curves for each filter section and the overall device are presented and compared with calculated results. The largest sidelobes were found to be 24 dB below the peak transmission, which represents a 5.3-dB improvement over an ideal filter without birefringence apodization. The FWHM of the filter is 1.36 nm, which is only 16% larger than predicted for its interaction length of 17 mm/section.

Reset-free system for real-time polarization control and synthesis.

A reset-free polarization controllsynthesis system is described. The desired polarization state can be selected by keyboard control with no insertion or rearrangement of optical components.

Two-dimensional optical architecture for time-delay beam forming in a phased-array antenna.

We propose and experimentally demonstrate a new two-dimensional optical architecture based on polarization switching by N spatial light modulators that provides 2(N) time delays to a phased-array antenna of p x p radiating elements. Moreover the 0 to 2pi phase of the microwave signals is optically controlled by a liquid-crystal spatial light modulator that operates in the birefringent mode. The microwave signal originates from the coherent detection of a dual-frequency laser beam obtained with an acousto-optic frequency shifter. Preliminary experimental results obtained at a microwave frequency f = 1.85 GHz are presented.

Incoherent frequency modulated cw optical reflectometry with centimeter resolution.

An incoherent, frequency modulated continuous wave optical reflectomer is described which utilizes a broadband LiNbO(3) electrooptic modulator and slow photodiode for optical downconversion and detection, and a waveform recorder for digital spectrum analysis of the incoming signal. A two-point resolution of 6.8 cm in fiber is achieved. The dynamic range, defined as the location of the system noise floor relative to the maximum producible signal, is 50-dB optical.