Cascading second-order nonlinear processes in quasi-phase matching (QPM) nonlinear crystals can strongly affect the effective group velocity of a propagating optical pulse, leading to superluminal or even negative transit times. With reference to a QPM LiNbO 3 parametric amplifier, we propose a device in which the transit time of spectrally narrow optical pulses at signal wavelength of 1.55 μm can be controlled by the pump intensity and may be pushed to negative values under experimentally achievable pump power levels.
Cascading second-order nonlinear processes in quasi-phase matching (QPM) nonlinear crystals can strongly affect the effective group velocity of a propagating optical pulse, leading to superluminal or even negative transit times. With reference to a QPM LiNbO 3 parametric amplifier, we propose a device in which the transit time of spectrally narrow optical pulses at signal wavelength of 1.55 μm can be controlled by the pump intensity and may be pushed to negative values under experimentally achievable pump power levels.
Optical pulse propagation in photonic grating structures can show anomalous (i.e., superluminal or negative) group velocities under certain circumstances owing to the anomalous dispersive properties induced by the periodic grating structure. Such phenomena can be observed for either linear pulse propagation in passive dielectric grating structures, such as in fiber Bragg gratings (FBGs), as well as in frequency-conversion processes exploiting second-order cascading effects in quasi-phase-matched (QPM) nonlinear crystals. Engineering of the grating structure can be exploited to observe a wide variety of anomalous pulse transmission and reflection behaviors. In this article, we review the main recent experimental and theoretical achievements obtained by our group in this field. In particular, we report on superluminal propagation of picosecond optical pulses at the 1.5-μm wavelength of optical communications in FBGs, both in transmission and reflection configurations, with the observation of group velocities as large as ∼5c/sub 0/. We also show that the phenomenon of transparent pulse propagation at a negative group velocity in a gain doublet atomic amplifier, recently observed in cesium vapor by Wang and co-workers (L. J. Wang, A. Kuzmich, and A. Dogariu, Nature vol.406, p.277-9, 2000), can be achieved as well in a photonic parametric amplifier by exploiting the anomalous dispersive properties of the amplifier induced by a suitably designed QPM grating profile.
The existence of anomalous, i.e. superluminal or negative, group velocities [1] provides one of the most amazing phenomenon of wave propagation which is compatible with, and even a consequence of, causality [2]. Anomalous wave propagation generally relies on the resonant interaction of light with an atomic medium, i.e. it exploits the dispersive properties of absorptive [3] or inverted [4,5] atomic media near resonances. Evanescent wave propagation across photonic barriers [1] provides the other wide framework where anomalous group velocities have been observed, however in such cases the achievement of a negative group velocity seems unlikely. By exploiting the dispersive properties of a gain doublet, a narrow spectral region of transparent anomalous dispersion around the dip center of two Lorentzian gain lines, leading to negative transit times and no pulse attenuation, can be created [4]. A recent demonstration thereof has been indeed reported in an experiment by Wang and coworkers using Cs vapor [5].
This thesis describes the implementation of the all-optical wavelength converters based on quasi-phase-matched (QPM) lithium niobate (LiNbO3) crystals and some related optical nonlinear materials for optical fiber communication systems and other applications. Such devices would be the enabling technology to realize high-capacity and transparent communication networks required in wavelength-division-multiplexing systems and highly efficient laser sources at desired wavelengths. The wavelength converters discussed in this thesis are based on the three-wave mixing processes attributed to the second-order nonlinear interactions (x2,). The main challenge with a LiNbO3 QPM wavelength converter is to implement a highly efficient device, which is capable of operating at a power of hundreds of mW. For this purpose, several wavelength conversion schemes have been demonstrated and theoretically investigated, including the fiber ring resonators and a novel cascaded χ(2).∖ The former based on the conventional cascaded x(2) contains erbium-doped fiber amplifiers and other passive components exclusively as pump sources (no additional pump source is required) with excellent characteristics such as high black-box efficiency. While the encouraging properties of the latter are those pump wavelengths can be set outside the communication window, and same efficiency can be achieved by employing two pump sources with lower output power as compared with the conventional cascaded one. By taking into account the higher-order dispersion effects, pulse propagation in QPM second harmonic generation waveguides is also analyzed. In addition, the photorefractive effect (PRE) in QPM waveguides is systematically studied by using a pump-probe method, since the LiNbO3 crystals are extremely vulnerable to high-power shorter-wavelength light illumination. Both temperature and wavelength dependent results are helpful in further understanding the physical insight of the PRE, and providing a guideline for practical application of the QPM LiNbO3 waveguides.
Slow and fast light are ubiquitous in optical amplifiers. In this Letter, we show for the first time, to the best of our knowledge, that optical parametric amplification (OPA) in chirped quasi-phase-matching structures can act as a platform for group-velocity control in the femtosecond regime. The resonant cascaded nonlinear phase underlies the group-velocity control, which manifests an unusual effect that both slow and fast light can be achieved under the normal condition of signal amplification. As numerically demonstrated in the OPA based on the lithium niobate crystal, the signal and idler pulse can be significantly delayed in time comparable to the signal duration and can also keep high fidelity for durations down to 100 fs until the crystal dispersion becomes effective. The broad bandwidth, large group delay, and direct compatibility with integrated optics will make the proposed platform attractive to both fundamental research and applied science.
Three-wave nonlinear interactions in chirped quasi-phase-matched (QPM) gratings are shown to exhibit conversion efficiency approaching 100% with increasing input pump and signal intensities, evading backconversion, as long as the idler vanishes at the input and the QPM grating is sufficiently chirped. The signal phase is described in terms of Kerr-like self- and cross-phase modulations, in the cascade χ((3)) approximation. Achieving high gain and efficiency simultaneously can lead to a large nonlinear phase, and the resulting trade-off is discussed.
Cascaded second-order nonlinear interaction (&chi(2));based wavelength conversion technique has attracted much attention due to its unique characteristics such as low noise and broadband, which are critical in fiber communication networks. In this report, wavelength conversions based on the newly proposed SFG-DFG (sum frequency generation - difference frequency generation) and conventional SHG-DFG (second harmonic generation - difference frequency generation) are studied and compared both experimentally and theoretically in a LiNbO3 quasi-phase matched (QPM) waveguide. It is shown that the same conversion efficiency can be achieved by employing two pump sources with only half power each (P1, P2) in the SFG-DFG scheme as compared with the SHG-DFG scheme with a single higher power pump beam (P=P1+P2). It is shown that the cascaded SFG-DFG based wavelength conversion has a larger 3-dB pump tolerance bandwidth. The theoretical results are consistent well with the experimental ones. It is found that the pump wavelength difference can be separated by a span as large as 75 nm, while 3-dB signal conversion efficiency is retained in a 45 mm-long device. It is also exhibited that tolerance of temperature for the cascaded SFG-DFG remains the same as that of the cascaded SHG-DFG based devices. The results show that the SFG-DFG wavelength conversion scheme is very attractive for practical applications.