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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Comparing slow-light properties of 10Gbps RZ data in dispersion shifted fibers and highly nonlinear fibers based on Raman-assisted optical parametric amplification

Zhaoyang Hu, Daniel J. Blumenthal

Published Feb 8, 2007• 1 citation
Publisher

Abstract

We compared experimentally the properties of optically controlled tunable time delay based on a Raman-assisted optical parametric amplifier (OPA) with different gain media of normal dispersion shifted fibers (DSF) and highly nonlinear fibers (HNLF). The Raman-assisted OPA using the DSF has previously been demonstrated to obtain large light delay for one single pulse. Position optimization of pump wavelength according to zero dispersion wavelength of the fibers results in narrow band gain spectrum through a parametric process. Stimulated Raman scattering further modifies the shape of the gain spectrum. The modified gain spectrum with sharper transitions leads to a large group index change so that large time delay can be achieved. In the paper, we experimentally demonstrated propagating 10Gbps return-to-zero (RZ) optical packets through slow-light fiber delay line based on the Raman-assisted OPA with above 100ps time delay. We also experimentally compared the gain, the bandwidth and the time delay versus variable pump powers for the Raman-assisted OPA by using 1km-long DSF and 1km-long HNLF as the gain medium, respectively. While smaller time delay was obtained in the slow-light scheme using HNLF due to its wider gain spectrum, lower pump power threshold was found compared to that using DSF. In the experiment, we found that the slow-light scheme has THz gain bandwidth and indicates that slow light could be achieved at higher bit rates with further optimized system parameters.

DOI
10.1117/12.716395

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We compared experimentally the properties of optically controlled tunable time delay based on a Raman-assisted optical parametric amplifier (OPA) with different gain media of normal dispersion shifted fibers (DSF) and highly nonlinear fibers (HNLF). The Raman-assisted OPA using the DSF has previously been demonstrated to obtain large light delay for one single pulse. Position optimization of pump wavelength according to zero dispersion wavelength of the fibers results in narrow band gain spectrum through a parametric process. Stimulated Raman scattering further modifies the shape of the gain spectrum. The modified gain spectrum with sharper transitions leads to a large group index change so that large time delay can be achieved. In the paper, we experimentally demonstrated propagating 10Gbps return-to-zero (RZ) optical packets through slow-light fiber delay line based on the Raman-assisted OPA with above 100ps time delay. We also experimentally compared the gain, the bandwidth and the time delay versus variable pump powers for the Raman-assisted OPA by using 1km-long DSF and 1km-long HNLF as the gain medium, respectively. While smaller time delay was obtained in the slow-light scheme using HNLF due to its wider gain spectrum, lower pump power threshold was found compared to that using DSF. In the experiment, we found that the slow-light scheme has THz gain bandwidth and indicates that slow light could be achieved at higher bit rates with further optimized system parameters.
We propose and demonstrate the use of narrow band optical parametric amplification for tunable slow and fast light propagation in optical fibers. The parametric gain is coupled to the Raman process which changes the gain value moderately but modifies the gain spectral shape. Consequently, the delay is enhanced at short wavelengths while it is moderated at long wavelengths. The maximum delay and tuning range can be optimized with respect to each other considering saturation effects in long fibers. The proposed scheme offers tunable delay in the presence of gain and with a bandwidth which is sufficiently wide to process digital data streams at tens of Gbit/s rates as well as picoseconds pulses.
Slow and fast light (SFL) in nonlinear media has attracted considerable interest for several years and has been observed in plenty of configurations. It generally requires a sharp spectral resonance which in turn results in a strong change of the group velocity at the resonance, thus leading to optical delaying or advancement. SFL has also been observed in optical fibres by taking advantage of the parametric gain, stimulated Brillouin or Raman scattering, leading to the development of all-optical tunable fiber-based delay lines at telecommunication wavelengths. But the implications of SFL go well beyond pulse delaying or advancement. As the group and phase velocities have to be distinguished one from another in a slow-light medium, this can have other important implications, in particular on the pulse walk-off or on the frequency chirp. We recently demonstrated in a Raman medium with normal dispersion that pulse walk-off between the pump and the signal can be cancelled out despite the large frequency detuning, leading to an increasing interaction length and enhancement Raman conversion efficiency.
We theoretically study parametric amplification in highly birefringent optical fibers and show that large tunable optical delay or advancement can be achieved via slow and fast light propagation. We provide a clear derivation of the formula for the optical delay that originates from the imaginary part of the parametric gain. We also perform numerical simulations in both normal and anomalous dispersion regimes. In the latter case, results show that large nanosecond optical delay could, in principle, be obtained at 1550 nm in a 1-km-long polarization-maintaining fiber. We further demonstrate that the optical delay and advancement rely on a group-velocity locking between the two cross-polarized signal and idler pulses.
Narrowband fiber parametric amplification is known to provide slow and fast light capabilities. Here, an analytical expression of the maximum slow light time delay achievable in ideal, isotropic fibers is derived. Then, the effects of random birefringence on the slow and fast light ability are numerically investigated by integrating the governing equations over a large number of statistical realizations of the fiber. The different polarization rotation along the fiber for the signal, the idler, and the pump reduces the amplifier mean gain and the mean time delay. For small random birefringence, the decrease of the mean delay can be directly imputed to the reduction of the mean gain. For large random birefringence, severe pulse distortion occurs, the mean delay is further reduced and the delay uncertainty highly enhanced. The influence of the stimulated Raman scattering is finally addressed.
Over the past few years, researchers have directed a significant amount of effort towards realizing tunable all-optical devices using nonlinear optical methods. It is now possible to exercise dynamic control of the group velocity of light traveling through a wide variety of material systems. The slow and fast light refer to situations in which the group velocity íg of an optical pulse through a dispersive material can be made to be smaller and larger, respectively, than the phase velocity vp = c/n. This ability could overcome the remaining challenge in current optical networks of storing and manipulating an optical signal directly in optical domain so as to avoid a bottleneck due to optical-to-electrical (O/E) and electrical-to-optical (E/O) conversions. The overall purpose of the dissertation is to study novel slow-light systems that provide controlled generation of large pulse delays relative to the pulse width with minimal pulse shape distortion by optimally design resonance profiles of such systems. The system design studies utilize several measures of performance such as the fractional delay, power throughput, and signal distortion under the limited system resource constraints. To this end, powerful data fidelity metrics are required to quantify the performance of tunable delay devices. Here, a new framework for measuring an information velocity and throughput is described and implemented using Shannon mutual information concepts. This new technique is used to investigate trends, trade-offs, and limits in slow light devices, which are physically sensible and in good agreement with analyses obtained using a conventional eye-opening(EO) metric. Using these information-theoretic and/or conventional metrics, we present the quantifying performance of gain-based stimulated Brillouin scattering (SBS) system in optical fibers as well as optical passive devices such as Fabry-Perot, fiber Bragg gratings, and ring resonators. It is shown that combining the SBS gain medium with these passive devices can compensate their respective disadvantages and thus increase delay performance without using additional resource of SBS pump power. The results show the possibility of achieving a fractional delay up to 10 at a signal bandwidth up to tens of GHz.