We take advantage of III-V VeCSEL technology integrating at-photonics for the generation of new coherent states, thanks to insertion of intracavity functions based on at photonics. These new kinds of coherent light states target many applications including optical tweezers, telecommunications, fundamental physics, sensors For this purpose, we extended the VeCSEL semiconductor technology, designing active sections, sub-wavelength metallic masks and photonic crystal, enabling to control the electrical field inside the cavity. This leads to the generation and control of highly coherent single high-order Laguerre- or Hermite-Gauss mode, VORTEX beam carrying controlled orbital-angular-momentum, as well as of coherent dual-frequency wave for THz, and of coherent continuum modeless source.
Nanometer size field effect transistors can operate as efficient resonant or broadband terahertz detectors, mixers, phase shifters and frequency multipliers at frequencies far beyond their fundamental cut-of frequency. This work is an
overview of some recent results concerning the low temperatures operation, linearity, and circular polarization studies of
nanometer scale field effect transistors for the detection of terahertz radiation. Also first results on graphene transistors
are discussed.
We report on measurements of radiation transmission in the 0.220-0.325 THz and 0.75-1.1 THz
frequency ranges through GaN quantum wells grown on sapphire substrates at nitrogen and room
temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is
found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional
electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility
and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for
the development of electro-optic modulators operating in the sub-THz frequency range.
Both theoretical and experimental analysis of a new efficient method to measure the number and type of modes
propagating in optical fibers are presented. This consists in measuring the intensity of the near field image at the end of a
fiber while scanning the wavelength with a laser source signal. Modes are extracted from Fourier transformation of the
spectral data at each point (x,y) of the images. A novel technique which is referred to as scalar product technique is
implemented in order to reconfirm real modes and exclude spurious modes. The technique is based on the orthogonality
of different modes. A standard multimode fiber has been measured to verify the technique. Three real modes LP01, LP11, and LP02 are discovered and reconfirmed by the orthogonality with the minimum values of the scalar products. One
spurious mode, which comes from the dependence of the power of the laser source on the wavelength, is thus excluded as it is not orthogonal either to the LP11 mode or the LP02 mode.
KEYWORDS: Multimode fibers, Optical fibers, Single mode fibers, Near field optics, Tunable lasers, Modal analysis, Light wave propagation, Near field scanning optical microscopy, Structured optical fibers, High power fiber lasers
We propose and demonstrate two methods for modal decomposition in multi-mode fibres. Linearly polarized modes
propagating in a slightly multi-mode fibre are easily retrieved from intensity measurements at the fibre output surface.
The first method is an improvement of the so-called spectrally and spatially imaging technique, which is limited to largemode-
area optical fibers. The second method is a new, simpler and faster solution for the characterization of any kind of
optical fibre, thus attractive in comparison to previously reported methods, which are cumbersome, time-consuming
and/or limited to large-more-area fibres. Different kinds of multi-mode optical fibres are characterized. A large-modearea
photonic-bandgap fibre, a photonic-crystal small-core non-linear fibre, and a standard index-stepped multi-mode
fibre are characterized successfully.
We propose using a frequency-modulated laser in a fiber-optic gyroscope (FOG) in a minimum configuration.
Compared to the traditional broadband source, a narrow-band laser offers two significant advantages, namely the
elimination of excess noise and thus improved sensitivity to rotation, and a more stable mean wavelength, hence a
greater scale-factor stability. We show that the strong back-reflection and coherent backscattering noise introduced by
the use of a laser is greatly reduced by modulating the laser frequency. In both a conventional FOG and in a FOG using
an air-core fiber, we demonstrate experimentally that this technique reduces these two sources of noise by at least a
factor of 4 compared to the same gyros operated with an unmodulated laser.
Large-scale acoustic fiber sensor arrays consisting of hundreds of hydrophones distributed along kilometers-long fiber buses are required for applications such as undersea oil exploration. Sagnac-based Sensor Arrays (SSAs) exhibit attractive performance; however the acoustic wave incident on the buses generates a pickup signal that can swamp the signals from the hydrophones. We propose, model, and demonstrate a simple technique for reducing this unwanted pickup by periodically inserting null sensors in the buses instead of hydrophones, so the pickup signal is measured at different locations along the buses. The signal from the null sensor (pickup only) is then subtracted from the signal seen by the adjacent hydrophone (true signal and pickup) to recover the true hydrophone signal. This paper describes in detail the signal processing steps used to perform this recovery. Applied to an experimental two-rung SSA, this technique produced a -18.6 dB pickup suppression for a pickup amplitude as large as 0.44 rad and signal amplitudes up to 0.44 rad, and -15 dB for a signal as large as 0.88 rad. These values are limited by the accuracy of the 8-bit data acquisition and/or electronic noise. With a low-noise 12-bit data acquisition, the pickup suppression for small signal amplitudes is predicted to be -35 dB. This work makes headway towards practical SSAs.
Feeding light inside a semiconductor laser gives rise to a wealthy set of dynamics that have been largely described. The two fundamental parameters are the detuning and the injected power. We present here theoretical and experimental evidence that other parameters may play a role. For instance, injecting with an orthogonal polarization is not equivalent to nullify the effect of optical injection as demonstrated by our experimental map. We will focus the main part of the talk on the influence of coherence. Some part of this study has been made possible by the use of a single-frequency fibre laser which enables us to establish an enlightening comparison between these two different types of lasers and also to surround the influence of coherence.
We investigate the threshold-crossing of a single-mode semiconductor laser, with special emphasis on its transfer function. Spontaneous emission, looked upon as the driving source of the radiation, is described in a semi-classical way in the spectral domain. The internal and emitted fields are filtered into the resonance modes of the whole structure: their spectral density are described by the generalized Airy-like transfer function, well approximated by a Lorentzian, which contains all essential mechanisms at work in a laser oscillator: Gain, losses and sources. The active zone is saturated through Amplified Spontaneous Emission, integrated over its whole spectral range. Continuously valid across threshold, the method enables one to derive in a simple way the main steady-state properties of the laser oscillation. Most specifically, we obtain analytical expressions, in normalized units, for emitted power, linewidth sharpening, carrier clamping and frequency shift, with the pumping rate as the only external parameter. In this approach, the optical properties of the active medium (the gain, the source and the refractive index) are supposed to be uniquely determined by the steady-state values of the carrier and photon density, obtained within the framework of the rate equation formalism and assumed uniform along the active zone.
The response of a single mode semiconductor laser to an injected external signal has been studied. The control parameters are the power and the frequency of the injected signal together with the gain of the laser. The injected power varies from 6 down to -120 dBm. Following the magnitude of these control parameters many phenomena can be observed. When both injected field and laser eigenfield are of the same order of magnitude they compete in a non-linear way, leading to frequency generation, push-pull effects, hysteresis phenomena and chaos. For weak dBm the laser behaves with the injected field in the same way as it does for the spontaneous emission which is its natural source. We describe the role of the injected laser as a filter and an amplifier in this case. It follows that the laser can be used to process information in ways that are not yet completely exploited.
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