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Chinese Optics Letters

Slow and fast light in quantum-well and quantum-dot semiconductor optical amplifiers

Piotr Konrad Kondratko, Akira Matsudaira, Shu‐Wei Chang, and Shun Lien Chuang Shun Lien Chuang

Published Jan 1, 2008• 3 citations
Publisher

Abstract

Slow and fast light in quantum-well (QW) and quantum-dot (QD) semiconductor optical amplifiers (SOAs) using nonlinear quantum optical effects are presented.We demonstrate electrical and optical controls of fast light using the coherent population oscillation (CPO) and four wave mixing (FWM) in the gain regime of QW SOAs.We then consider the dependence on the wavelength and modal gain of the pump in QW SOAs.To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple amplifiers.A model for the number of QW SOAs in series with variable optical attenuation is developed and matched to the experimental data.We demonstrate the scaling law and the bandwidth control by using the serial cascade of multiple QW SOAs.Experimentally, we achieve a phase change of 160 • and a scaling factor of four at 1 GHz using the cascade of four QW SOAs.Finally, we investigate CPO and FWM slow and fast light of QD SOAs.The experiment shows that the bandwidth of the time delay as a function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime variation.The tunable phase shift in QD SOA is compared between the ground-and first excited-state transitions with different modal gains.

DOI
10.3788/col20080610.0736

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Slow and fast light in quantum-well (QW) and quantum-dot (QD) semiconductor optical amplifiers (SOAs) using nonlinear quantum optical effects are presented.We demonstrate electrical and optical controls of fast light using the coherent population oscillation (CPO) and four wave mixing (FWM) in the gain regime of QW SOAs.We then consider the dependence on the wavelength and modal gain of the pump in QW SOAs.To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple amplifiers.A model for the number of QW SOAs in series with variable optical attenuation is developed and matched to the experimental data.We demonstrate the scaling law and the bandwidth control by using the serial cascade of multiple QW SOAs.Experimentally, we achieve a phase change of 160 • and a scaling factor of four at 1 GHz using the cascade of four QW SOAs.Finally, we investigate CPO and FWM slow and fast light of QD SOAs.The experiment shows that the bandwidth of the time delay as a function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime variation.The tunable phase shift in QD SOA is compared between the ground-and first excited-state transitions with different modal gains.
Abstract : Slow and fast light schemes are attractive for many applications including optical signal processing and RF phased array antennas. Semiconductor based schemes offer electrical control of velocity at very high bandwidths in an extremely compact device. Further they operate at room temperature and can be easily integrated into various optical systems. In this program, we used ultrafast nonlinearities in semiconductor optical amplifiers (SOAs) to achieve tunable time shifts at THz frequencies. In particular, we leveraged the spectral hole burning phenomena and demonstrated fast light in quantum well (QW) and quantum dot (QD) SOAs. The research included both theoretical and experimental efforts. Theoretical work was developed to understand and simulate experimental results and to optimize device designs. We introduced a novel pulse compression technique to reduce the pulse width to 100 fs to efficiently use ultra-fast nonlinearities in SOAs to achieve a large and cascadable delay. We implemented a novel chirped pulse scheme and obtained the largest advance-pulse-product of 10.7 with THz bandwidth. We demonstrated slow and fast light with GHz RF signal in QW/QD SOAs using coherent population oscillation and four-wave mixing. We also demonstrated cascaded SOAs in a loop to scale up the delay/advance.
Controlling the group velocity of light has great potential applications in future optical communication, photonic computing, and microwave signal processing. These applications have recently triggered immense experimental and theoretical interest in the research community. Numerous studies report large optical delays while sacrificing many important aspects of engineering feasibility: bandwidth, device compactness, room temperature operation, ease of integration, and minimized signal insertion loss. Slow and fast light in the quantum-dot and quantum-well semiconductor optical amplifiers (SOAs) address many of these characteristics. First, this dissertation presents variable slow light in quantum-dot SOAs using population oscillation in an absorptive medium. Bandwidth tuning of slow light is observed with forward and reverse bias of quantum-dots. Second, the study explores four-wave mixing and population oscillation in a quantum-well gain medium. Control of optical advance and bandwidth is achieved by the medium's electrical bias or the optical pump power. Moreover, the fast light in the cascade of N number of quantum-well SOAs is experimentally and theoretically investigated. The cascaded scheme uses SOA-to-SOA attenuators to achieve active delay-bandwidth control. Finally, a large delay-bandwidth scheme is presented in which slow-to-fast light is observed by means of absorption to gain switching. More than a half cycle of tunable delay at 1 GHz bandwidth is achieved at optical frequencies.
Recent progress in the field of quantum dot/dash based semiconductor optical amplifiers (SOAs) for slow and fast light is discussed. Room temperature fast light has been obtained in InAs/InP QDash based SOAs by means of coherent population oscillation and four wave mixing (FWM) effects. Typical optical delays amount to 55 ps at 2 GHz. Growth optimization of the QDashes allowed us to achieve high modal gain, leading to very similar performances, e.g. gain and FWM efficiency, to those of a bulk SOA. A novel approach based on linear spectrograms is also introduced to measure the phase shift induced by wave mixing in an SOA.
Room temperature quantum-well semiconductor optical amplifier with large input power is utilized in both the absorption and gain regime as an optical group delay and advance (slow and fast light), respectively. Material resonance created by coherent population oscillation and four wave mixing is tuned by electrical injection current, which in turn controls the speed of light. The four-wave mixing and population oscillation model explains the slow-to-fast light switching. Experimentally, the scheme achieves 200 degrees phase shift at 1 GHz, which corresponds to 0.56 delay-bandwidth product. The device presents a feasible building block of a multi-bit optical buffer system.
Coherent population oscillations (CPO) in quantum dots (QDs) have been utilized in this article to achieve slow and fast light effects in a semiconductor optical amplifier. These phenomena have been achieved based on a pump-probe scheme and tuning the input current source. Coupled rate equations have been developed to determine the carrier dynamics and occupation probabilities. The effects of barrier, wetting layer and excited state densities have been included for CPO phenomenon occurring between the ground states. A quantum disc model embedded in another disc has been assumed for modeling the QDs and density matrix elements are used to obtain induced polarization at the signal frequency after determining the energy states and state probabilities. Tunable absorption dip, corresponding refractive index variation and group index, exhibit widely tunable slow and fast light characteristics by changing the bias condition.