Group Velocity Manipulation Research Articles

All-optical group velocity manipulation in Mach-Zehnder fiber interferometers incorporating with Brillouin random lasing resonance

We proposed and experimentally demonstrated a novel scheme of light group velocity control via a Mach-Zehnder fiber interferometer incorporated with Brillouin random lasing resonance in a half-open linear cavity. A theoretical model of the proposed Brillouin random lasing resonance-based Mach-Zehnder interferometer (BRLR-MZI) for the light group velocity manipulation was derived, predicting optical power-dependent transmittance spectrum shift as well as the enhancement of the temporal advancement. Thanks to Brillouin random lasing-induced fast light effect, the proposed scheme not only achieves a flexible all-optical manipulation of its transmittance spectrum but also overcomes the saturation effect for the light speed control based on a traditional MZI. In experimental validation, sinusoidally modulated signals with the modulation frequency of 160 kHz ultimately experienced a maximum advancement of 4031 ns (corresponding to the group index of 0.875), which exhibits an advancement improvement of 1364 ns compared with the traditional MZI-based scheme. It suggests a promising platform for exploring applications in fields of all-optical signal processing, microwave photonic, and ultra-sensitive fiber sensing.

Manipulating multi-frequency light in a five-level cascade EIT medium under Doppler broadening

An electromagnetically induced transparency (EIT) medium consisting of five-level-cascade atoms is proposed to control the group velocity of multi-frequency light under Doppler broadening. An analytic expression for the group index of probe light is derived as a function of the parameters of coupling light and temperature of the medium. It is shown that by adjusting intensity and/or frequency of the solely coupling light one may manipulate simultaneously group velocity of the probe light in three separated frequency regions, each of which enables to switch between the subluminal and superluminal modes. On the other hand, the effect of Doppler broadening increases the group velocity and enlarges the tuning range of the coupling intensity to switch between the superluminal and subluminal modes. The model is helpful to find applications related to the group velocity manipulation of multi-frequency light, and to serve future experimental studies at various temperature conditions.

Cascaded superluminal propagation via Brillouin lasing resonance in optical fibers

All-optical group velocity manipulation of light, so-called slow and fast light, has drawn extensive attention due to its potential applications in the fields of optical signal processing, light storage, light-matter interaction enhancement, hypersensitive sensing and temporal cloak. Tremendous approaches of slow and fast light generation have been widely demonstrated by the creation of the sharp spectral resonances in the past decade. Specifically, superluminal propagation (larger-than-light velocity c ) through the optical media has been proposed and validated in anomalous dispersion media. One effective approach is employing stimulated Brillouin scattering to harness the narrowband spectral resonance and thus generate slow and fast light in the optical fiber. In particular, Brillouin lasing oscillation has been experimentally demonstrated to generate fast and superluminal propagation in ten-meter optical fibers. Afterwards, the propagating distance over hundreds of meters has been experimentally demonstrated by the suppression of multiple-longitudinal-mode operation. However, the delay-bandwidth product or the fractional advancement is still fundamentally limited in conventional fast and slow light system. For example, the maximum advancement as well as the fractional temporal advancement are practically restricted due to the Brillouin gain saturation and multiple longitudinal mode lasing operation under high pump power in the Brillouin lasing resonance-based fast and superluminal system. In this paper, we experimentally validate the extension of time advancement of superluminal propagation based on Brillouin lasing oscillation for the first time. By employing two stages of single-frequency Brillouin lasing fiber ring resonators, Stokes lasing resonance was effectively created in each fiber ring cavity to deliver Brillouin-induced loss resonance at the center frequency of pump, which essentially leads to fast light and superluminal propagation of the pump light. Consequently, Gaussian signal pulses with the pulse width of 500 ns experiences cascaded Brillouin-induced fast light and even negative group-velocity superluminal propagation. The temporal advancement was basically extended to 365.8 ns which is twice than that of single stage fast light and superluminal system. Correspondingly, the fractional advancement was increased from 0.36 to 0.73. The maximum group velocity was 6.3 times larger than the light speed c in vacuum while negative-group- velocity propagation can be found in the range of −9.9 to −0.2 by increasing the total input power from 593.2 to 812.0 mW. Furthermore, the group index can be manipulated in the range of 1.5 to −4.0 by simply varying the total input pump power. It indicates superluminal signals can be cascaded to essentially extend the propagation distance and enlarge the temporal advancement. The scheme exhibits robustness in cascading Brillouin spectral loss resonance for temporal advancement enhancement as well as propagating distance extension. More importantly, the cascading design could basically eliminate the fundamental limitation introduced by Brillouin gain saturation and multiple longitudinal lasing mode under high power operation in single stage Brillouin fast and superluminal system. Further optimization could also be implemented by utilizing the nonstandard specialty fibers as Brillouin gain medium to achieve an efficient Brillouin lasing resonance for an enhanced advancement efficiency. Additionally, the combination with the cascading technique and long-cavity single-frequency Brillouin lasing resonance could enable the feasibility of distance-limitless and large-advancement superluminal propagation, paving a way for promising applications in sensing and temporal cloak.

Group velocity manipulation using cross gain modulation in erbium-doped fibers with co-direction structure

Abstract We experimentally demonstrated tunable delays using mutrally modulated cross-gain modulation (MM XGM) in erbium-doped fiber (EDF) with a new co-direction structure. The group velocity of the signal light can be controlled by the saturating light at an arbitrary wavelength in the EDF gain bandwidth. We have observed slow light propagation with a group velocity as low as 146.4 m/s, which means a 88.8 ms delay, and an advancement of 11.2 ms after a 13 m EDF at the same time. This provides a greater enhancement on the delay or advancement compared to the previous experiment. Furthermore, we confirmed the influence on delay or advancement by varying the pump power, the input power of the saturate light. Also, we discussed the influence of recovery time, the modulation depth of the saturating light and the modulation frequency on the modulation gain. This attractive approach could be one of the suitable solutions for real applications.

Group velocity manipulation of far off-resonant pulse-pair in atomic system

Based on the far off-resonant four-wave mixing process, the slow light propagations of the amplified probe and generated conjugate pulses are obtained experimentally. Simultaneous manipulations of group velocity are realized by changing the two-photon detuning between the pump light and the probe light. The dependences of gains of the injected probe and generated conjugate light on the one-photon detuning for continuous wave mode are studied at different cesium vapor temperatures and pump light powers. It is shown that the maximum of gains occurs at the proper Raman one-photon detuning. The dependence of delay time on the two-photon detuning is measured using the 6 μs and 365 ns probe pulses, respectively. For the 6 μs input probe pulse, the maximum delay times of the probe and the conjugate pulses are 2.1 μs and 1.9 μs with the fractional delays of 0.35 and 0.32, respectively, corresponding to 0.000119 c and 0.000132 c group velocity. The high fractional delays of 2.07 and 1.83 with the maximum delay times of 756 ns and 670 ns for the 365 ns input pulse are obtained.

Group Velocity Manipulation of 10 Gb/s Signal by Mutually-Modulated Cross-Gain Modulation in Semiconductor Optical Amplifiers

We propose and experimentally demonstrate the propagation of slow/fast-light via mutually modulated cross-gain modulation in a semiconductor optical amplifier (SOA). Owing to very short recovery time in SOAs, the group velocity of 10-Gb/s signal light can be controlled by manipulating the saturating light. The maximum controlled time of one pulse period has been observed, whereas the signal light is modulated by either the sinusoidal signal or the real telecom signal.

Group velocity manipulation in active fibers using mutually modulated cross-gain modulation: from ultraslow to superluminal propagation

We propose and experimentally demonstrate the propagation of slow/fast light in an erbium-doped fiber (EDF) using mutually modulated cross-gain modulation. The group velocity of the light signal can be manipulated by the effect of gain cross-saturation modulation by a saturating light at an arbitrary wavelength in the gain bandwidth of the EDF. The ultraslow propagation with a small group velocity of 5.6 × 10⁻³c (c is the light speed in free space) and superluminal propagation with a negative group velocity of -1.1 × 10⁻³c has been observed under different modulation phases.

Gain-Assisted Slow to Superluminal Group Velocity Manipulation in Nanowaveguides

We study the energy propagation in subwavelength waveguides and demonstrate that the mechanism of material gain, previously suggested for loss compensation, is also a powerful tool to manipulate dispersion and propagation characteristics of electromagnetic pulses at the nanoscale. We show theoretically that the group velocity in lossy nanowaveguides can be controlled from slow to superluminal values by the material gain and waveguide geometry and develop an analytical description of the relevant physics. We utilize the developed formalism to show that gain-assisted dispersion management can be used to control the transition between "photonic-funnel" and "photonic-compressor" regimes in tapered nanowaveguides. The phenomenon of strong modulation of group velocity in subwavelength structures can be realized in waveguides with different geometries and is present for both volume and surface modes.

Group velocity manipulation in simple acoustic band gap filters

A simple experimental configuration is presented in which the group velocity of acoustic pulses is adjusted from much slower to much faster than the speed of sound. The experiment is an acoustic analog of two much studied optical phenomena: the superluminal group velocity achieved when optical pulses are tunneled through regions of high absorption or attenuation, and slow (or even stopped) light propagation in the vicinity of strong material dispersion, typically realized using electromagnetically induced transparency in atomic vapors. For the acoustic experiments described here, attenuation and dispersion were created using periodically structured waveguides. The periodicity results in one-dimensional acoustic band gaps, frequency intervals in which propagation of sound is strongly suppressed. The introduction of defects in the perfect periodicity leads to narrow transmission bands within the band gaps. There is strong normal dispersion in the vicinity of these defect modes leading to very slow group velocities. By tuning the carrier frequency of acoustic pulses through the forbidden and defect regions the group velocity can be adjusted from 0.22 to 1.8 times the speed of sound in the straight unstructured waveguide. These results are shown to be consistent with a straightforward theoretical model.