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Fast Light in Quantum Dot and Well Semiconductor Optical Amplifiers

Connie J. Chang-Hasnain, P. Bhattacharya, S. L. Chuang

Published Oct 31, 2010• 0 citations
OpenAlex

Abstract

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.

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Optics Express•Jan 1, 2007•14 citations
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.
Slow and fast light enables key functionality in various rf applications and all-optical networks. Semiconductor-based schemes offer electrical control of velocity at very high bandwidths in an extremely compact device. Furthermore, they operate at room temperature and can be easily integrated into various optical systems. Ultrafast nonlinear processes in semiconductor optical amplifiers (SOAs) have been used to achieve tunable slow and fast light in the terahertz bandwidth. For a 700 fs pulse, we show an electrically and optically controllable advance of 1.9 ps corresponding to an advance-bandwidth product (ABP) of 2.5. Furthermore, by leveraging self-phase modulation in these devices, we extend the performance to an ABP of 3.7. We develop comprehensive theory using a density matrix approach to explain the experimental results. Our results show that an ultrashort pulse propagating through the SOA experiences nonlinear index change due to spectral-hole burning and wave mixing between different spectral components. We derive analytical expressions for the nonlinear index induced by these ultrafast processes and numerically solve the propagation of an ultrashort pulse through the SOA. Our theoretical predictions agree very well with our experimental results. Finally, we show fast light for two ultrashort pulses separated by 7.2 ps, which demonstrates the feasibility of this scheme at high bit rates.
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.
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.
Chirped pulse scheme is shown to be highly effective to attain large tunable time shifts via slow and fast light for an ultra-short pulse through a semiconductor optical amplifier (SOA). We show for the first time that advance can be turned into delay by simply reversing the sign of the chirp. A large continuously tunable advance-bandwidth product (ABP) of 4.7 and delay-bandwidth product (DBP) of 4.0 are achieved for a negatively and positively chirped pulse in the same device, respectively. We show that the tunable time shift is a direct result of self-phase modulation (SPM). Theoretical simulation agrees well with experimental results. Further, our simulation results show that by proper optimization of the SOA and chirper design, a large continuously tunable DBP of 55 can be achieved.