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Journal of the Optical Society of America B

Slow optical pulse propagation in an amplifying ring resonator

Makoto Tomita, Takamichi Ueta, Parvin Sultana

Published Jun 6, 2011• 5 citations
Publisher

Abstract

We experimentally investigated optical pulse propagation through an amplifying ring resonator, in which a semiconductor optical amplifier was used as a loss or gain medium. Initially, the transmission intensity showed resonance dips as a function of laser frequency. These dips were gradually filled and transformed into resonance peaks as the amplification gain increased. A Gaussian-shaped temporal pulse was either advanced by −7.3 ns at low amplification gain or delayed up to 45 ns at high gain. System operation was classified into a complete set of the four regions depending on the gain parameter, and experimental delay times were analyzed based on relevant system dispersion.

DOI
10.1364/josab.28.001627

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We experimentally investigated optical pulse propagation through an amplifying ring resonator, in which a semiconductor optical amplifier was used as a loss or gain medium. Initially, the transmission intensity showed resonance dips as a function of laser frequency. These dips were gradually filled and transformed into resonance peaks as the amplification gain increased. A Gaussian-shaped temporal pulse was either advanced by −7.3 ns at low amplification gain or delayed up to 45 ns at high gain. System operation was classified into a complete set of the four regions depending on the gain parameter, and experimental delay times were analyzed based on relevant system dispersion.
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We experimentally examined the propagation of a Gaussian-shaped temporal pulse through a serial array of ring resonators, truncating the pulse before its peak. We observed the process whereby a smooth pulse peak related to the Gaussian pulse emerged even when the incident pulse was truncated before its peak, as the pulse passed through an increasing number of ring resonators. The peak emerged when the negative group delay accumulated beyond the pulse truncation time point. This result demonstrated that the earlier portion of the input pulse envelope developed into a smooth Gaussian-shaped pulse peak in the superluminal pulse in good agreement with the analyses based on the instantaneous spectrum. We also discussed the propagation of truncated pulses in a slow light system, where the peak disappeared in the output pulse despite the peak in the input pulse.
The nonlinear process of stimulated Raman scattering is important for silicon photonics as it enables optical amplification and lasing. To understand the dynamics of silicon Raman amplifiers (SRAs), a numerical approach is generally employed, even though it provides little insight into the contribution of different SRA parameters to the signal amplification process. In this paper, we solve the coupled pump-signal equations analytically under realistic conditions, and derive an exact formula for the envelope of a signal pulse when picosecond optical pulses are amplified inside a SRA pumped by a continuous-wave laser beam. Our solution is valid for an arbitrary pulse shape and fully accounts for the Raman gain-dispersion effects, including temporal broadening and group-velocity reduction (a slow-light effect). It can be applied to any pumping scenario and leads to a simple analytic expression for the maximum optical delay produced by the Raman dispersion in a unidirectionally pumped SRA. We employ our analytical formulation to study the evolution of optical pulses with Gaussian, exponential, and Lorentzian shapes. The ability of a Gaussian pulse to maintain its shape through the amplifier makes it possible to realize soliton-like propagation of chirped Gaussian pulses in SRAs. We obtain analytical expressions for the required linear chirp and temporal width of a soliton-like pulse in terms of the net signal gain and the Raman-dispersion parameter. Our results are useful for optimizing the performance of SRAs and for engineering controllable signal delays.
A nonlinear process between co-propagating signal and control pulses has been proposed and demonstrated numerically, based on semiconductor optical amplifiers (SOA). Results show that a Gaussian signal pulse with picosecond duration can be simultaneously amplified and compressed in a SOA by utilizing a co-propagating high-intensity control pulse. To obtain high quality signal pulse with high peak power level and short pulse width, the center carrier wavelengths of two pulses should locate in the gain region of the SOA. The initial delay time between both pulses should be tuned suitably before entering the SOA.