In this work, we demonstrate the existence of abnormal pulse propagation regimes in linear and passive multiple-beam interferometers, where the group velocity can be tuned from subluminal to superluminal values by simply changing the length of one of the interferometer's arms. Experiments are performed in the radiofrequency range by using coaxial transmission lines. The interferometers group delay is characterized both in the frequency and in the time domain. Group velocities of 2c and tunnelling with negative group velocity of -0.11c were measured for electromagnetic wave packets of 2 μs width travelling through a Mach-Zehnder interferometer. In a 4-beam interferometer, the group velocity of a 5 μs wide pulse was reduced to only 0.3c (compared to the usual value of 0.67c). The system is scalable to other frequency ranges and its implementation for narrowband optical pulses could be feasible by means of micromachining technologies. A scaling approach for advancing/delaying optical pulses at 1.55 μm is discussed. These systems are proposed as an alternative for controlling the group velocity without the need of using photonic crystals or periodically microstructuring, doping or using non-linear media.
In this work, we demonstrate the existence of abnormal pulse propagation regimes in linear and passive multiple-beam interferometers, where the group velocity can be tuned from subluminal to superluminal values by simply changing the length of one of the interferometer's arms. Experiments are performed in the radiofrequency range by using coaxial transmission lines. The interferometers group delay is characterized both in the frequency and in the time domain. Group velocities of 2c and tunnelling with negative group velocity of -0.11c were measured for electromagnetic wave packets of 2 μs width travelling through a Mach-Zehnder interferometer. In a 4-beam interferometer, the group velocity of a 5 μs wide pulse was reduced to only 0.3c (compared to the usual value of 0.67c). The system is scalable to other frequency ranges and its implementation for narrowband optical pulses could be feasible by means of micromachining technologies. A scaling approach for advancing/delaying optical pulses at 1.55 μm is discussed. These systems are proposed as an alternative for controlling the group velocity without the need of using photonic crystals or periodically microstructuring, doping or using non-linear media.
espanolDemostramos velocidades de grupo sub-, super-luminales y negativas en interferometros de multihaz lineales y pasivos. Los experimentos se realizan en el rango de radiofrecuencia con cables coaxiales. Se han medido velocidades de grupo de 2c y tunelado con un adelanto relativo de 0.125 para un pulso propagandose por un interferometro Mach-Zehnder. En un interferometro de cuatro haces se ha frenado un pulso hasta una velocidad de grupo de tan solo 0.3c (frente a los 0.67c usuales). Estos resultados concuerdan con las predicciones teoricas obtenidas de la funcion de fase en transmision. Este sistema supone una alternativa a los cristales fotonicos y a interferometros con ramas activas o microestructuradas para el control de la velocidad de grupo. Palabras clave: Propagacion de Pulsos, Velocidad de Grupo, Dispersion Anomala, Superluminal, Subluminal, Luz Rapida, Luz Lenta, Interferometros de Multihaz, Mach-Zehnder. EnglishWe demonstrate sub-, super-luminal and negative group velocities in linear and passive multiplebeam interferometers. Experiments are performed in the radiofrequency range by using coaxial cables. Group velocities of 2c and tunneling with a fractional advancement of 0.125 were measured for wave packets traveling through a Mach-Zehnder interferometer. In a 4-beam interferometer, the group velocity of pulse was reduced to only 0.3c (compared to the usual value of 0.67c). These results agree with theoretical predictions using the interferometer’s transmission phase function. This system is proposed as an alternative to photonic crystals and active or microstructured interferometers for controlling the group velocity.
In this work, we explore the generation of slow and fast light (SFL) in a linear and passive three-beam interferometer for center pulse frequency close to the transmission minima. We show that the group delay can be tuned from subluminal to superluminal values by simply changing the length of one of the interferometer's arms. Some figures of merit of this three-beam interferometer as an SFL system are discussed, namely fractional delay and pulse distortion. We build our interferometers using 50-Ω coaxial cables and 1x3 power splitters in order to perform the experiments in the radiofrequency range, where a full frequency-domain characterization of them can be performed with a vector network analyzer. Together with the frequency domain results, we present direct measurement of the group delay in time domain. Fractional delays of ±25% were measured for a train of sinusoidally modulated wavepacket with 300 kHz repetition rate. Additionally, a Lithium Niobate interferometer operative in the optical range (1.55 μm) was simulated and discussed. This system is proposed as an alternative to active systems and photonic band-gap structures for sustaining both slow and fast light, with special emphasis in sensing purposes.
We propose and demonstrate a method for manipulation of group velocity from slow-to-fast light (SFL) in a two-beam Mach–Zehnder interferometer in radio frequency (RF) regime. The subluminal, superluminal, and tunneling propagation regimes occur in a narrow frequency band around the resonance frequency of the interferometer where destructive interference occurs. Importantly, the group velocity at the resonance frequency can be controlled by simply adjusting the transmission magnitude of the signals from the two beams, without changing the length of the interferometer. Models that fully describe the SFL effects in the microwave interferometer system are developed and closed-form expressions for the prediction of group delays at the resonance frequency are obtained. The simple and effective method is verified through numerical calculations and experimental measurements. We also demonstrate that the proposed method applies to a microwave cavity interferometer. A proof-of-concept experiment is performed using a custom open-ended coaxial cable resonator (OE-CCR). Positive and negative group delays are experimentally measured when the resonator is set at different coupling states.
This Thesis lies within the field of Slow and Fast Light (SFL) technologies, which are currently receiving much attention because of their interesting applications, ranging from optical information processing to enhanced precision sensing and interferometry. These technologies are based on systems that exhibit steep positive dispersion to propagate a light pulse at group velocity well below the speed of light in vacuum (slow light) or steep negative dispersion to achieve pulse propagation at superluminal or even at negative group velocity (fast light). SFL effects thus arise in a myriad of materials exhibiting spectral resonances. Current efforts in this field are mainly focused on the manipulation of material gain or absorption resonances by nonlinear optical processes (material SFL) or on the optimization of photonic band-gap structures, without substantial material dispersion, but where structural dispersion comes as a result of the coupling between the light wavelength and the characteristic length of the system (structural SFL). This Thesis focuses on the theoretical and experimental analysis of electromagnetic pulse propagation with abnormal group velocities in two kind of linear and passive devices. In the first part of the Thesis a new system exhibiting structural SFL is demonstrated. It deals with multiple-beam interferometers and provides a comprehensive study of the arising of SFL in this system devoid of photonic band gaps. A theoretical model that fully describes the allowed pulse propagation regimes and its performance in terms of both the interferometer's and the pulse characteristics is developed. Considering amplitude modulated pulses, the capabilities and limitations of SFL effects in this kind of system are retrieved by quantifying typical figures of merit like fractional delay, pulse distortion and Delay-Bandwidth Product. The theoretical framework is valid for any frequency region and the model predictions are probed by performing experiments in the radiofrequency range and through exact numerical simulations in the optical range. The simplest interferometer, with only two branches, is first considered since it is widely used in actual communication systems. It is analytically demonstrated that slow light cannot possibly be sustained and that the total attenuation drives the changes in the pulse propagation regimes. By increasing the number of branches, group delay tuning from slow to fast light regimes occurs if the optical length of one of the branches is slightly changed. In the second part of the Thesis, the ability of mono- and multilayer structures to speed up or slow down electromagnetic pulses is investigated, with special emphasis on their reflection properties. Namely, Distributed Bragg Reflectors (DBRs) and Fabry-Perot filters, common in today's communication systems, are examined. These structures were fabricated to operate in the microwave and in the radiofrequency range with the aim of confirming theoretical predictions. Experimental results of their frequency- and time-domain characterization are compared with simulations. An advantage of operating in these frequency ranges is that the transmission and reflection phase function can be measured with a two port vector network analyzer. Such a simple measurement of this key function that determines the pulse propagation regime cannot be directly performed in the optical range. Moreover, microstrip DBRs were designed and their potential application as a negative group-delay circuit to improve the efficiency of feedforward amplifiers, which are commonly used for cancelling inherent distortion in microwave amplifiers, is explored.
We demonstrate the generation of slow and fast light (SFL) in a linear and passive three-beam interferometer. Such propagation regimes occur for narrowband pulses with center frequency close to the transmission minima. A model that fully describes SFL effects in this system is developed and an analytical approximate expression for the group delay at the minima is derived. We demonstrate that slow light is not possible if the length difference between adjacent branches of the interferometer is a constant. If a small length detuning ($\ensuremath{\xi}$) in one of the branches is introduced, slow light at one of the two minima can be obtained as long as $\ensuremath{\xi}$ exceeds a critical value. Simultaneously, tunneling, superluminal, or normal regime is sustained at the other minimum, depending on the system's length. A proof-of-model experiment is performed in the radiofrequency range using coaxial cables and $1\ifmmode\times\else\texttimes\fi{}3$ power splitters. The possible realization and performance of such a system in the optical range is also discussed. This system is proposed as a simple alternative to active systems and photonic band-gap structures for sustaining both slow and fast light.
An analytical approach is reported that describes previously observed fast-light regimes in linear and passive Mach-Zehnder interferometers (MZI) where the optical path difference is due to a different length of the branches. Approximate expressions are developed for the transmission coefficient and group delay spectral functions valid for frequencies close to the transmission minima ${\ensuremath{\omega}}_{\mathrm{min}}$, where these regimes occur. It is found that the group delay at ${\ensuremath{\omega}}_{\mathrm{min}}$ verifies a simple scaling law. We demonstrate that slow light cannot arise in this system, and that tunneling and superluminal regimes appear only for low-loss devices, where the attenuation drives the change in the propagation regimes. The propagation of a sinusoidally modulated pulse train through the MZI is described, and relevant figures of merit, which are intrinsic to the system and universal for any operative spectral range, are determined. The theoretical approach is illustrated by simulations of a silicon-based interferometer designed for advancing pulses at 1.55 \ensuremath{\mu}m. Also, previously reported experimental results in the radiofrequency range are interpreted in the framework of the model.