Causality and negative group delays in a simple bandpass amplifier

Abstract

We demonstrate a bandpass amplifier which can be constructed from common electronic components and has the surprising property that the group delay is negative in most spectral regions. A pulse propagating through a chain of such amplifiers is advanced by several milliseconds: the output waveform precedes the input waveform. Although striking, this behavior is not in conflict with causality, as demonstrated by experiments with pulses which start or end abruptly.

Transparent anomalous dispersion and superluminal light-pulse propagation at a negative group velocity

“…We place two Raman gain peaks closely to obtain an essentially lossless anomalous dispersion region that results in a superluminal group velocity. The group velocity of a pulse in this region exceeds c and can even become negative [22,23], while the shape of the pulse is preserved. We measured a negative group velocity index of n g = −315(±5).…”

“…In a series of papers [7,18,19,[22][23][24][25][26], Chiao and coworkers showed theoretically that anomalous dispersion can occur inside a transparent material, particularly on the DC side of a single gain line. It was predicted that by using a gain doublet [22], it is possible to obtain a transparent anomalous dispersion region where the group velocity of a light pulse exceeds c with almost no pulse distortion.…”

“…More recently, it has been considered [7][8][9][10][11][13][14][15][16][17][18][19][21][22][23][24]] to use various schemes including the anomalous dispersion near an absorption line [8][9][10][11], nonlinear [13] and linear gain lines [14][15][16][17][18][19][22][23][24], active plasma medium [17,21], or in a tunneling barrier [25,26] to observe these effects. Sometimes it is applied [9][10][11]25,26] to observe superluminal propagation of light pulses.…”

Neutralization of LC- and RC-Disturbances with Left-Handed and NGD Effects

“…(4) Since the early 1990s, the possibility to generate negative group delay (NGD) phenomenon has been confirmed in electronic domain thanks to the topology of circuit proposed by Chiao and his co-workers [19][20][21][22]. The first NGD circuit composed of an operational amplifier in feedback with passive networks mainly composed of resistor(s), inductance(s) and capacitance(s) as described in [23][24].…”

“…However, such NGD circuit operates only up to some hundreds kHz [23][24][25]. It was stated that the NGD effect does not contradict the causality principle [19][20][21][22][23][24][25][26]. In addition, the left-handed (LH) lumped passive circuit capable to generate NGD effect for microwave signals up to GHz has been introduced by the group of Mojahedi [15][16].…”