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Optics Express

Strong tunable slow and fast lights using a gain-clamped semiconductor optical amplifier

Scott Moon, J. Park, Jung Mi Oh, N. J. Kim, Dong-Han Lee, Shu‐Wei Chang, David Nielsen, S. L. Chuang

Published Nov 6, 2009• 4 citations
Publisher

Abstract

Previously demonstrated slow light is still far from applications, particularly due to the limited bandwidth and control speed. Although semiconductor-based slow light has the high bandwidth and sub-nanosecond control speed, slow light was observed only in the absorption regime with attenuation, while fast light observed in the gain regime with amplification. The large power difference in two regimes makes the use of the optical delay impractical. We report novel slow light in the gain regime, with a high power comparable to that of fast light, utilizing the anomalous gain characteristic in a gain-clamped semiconductor optical amplifier. The slow light is tunable to fast light with the current as the only variable. Additional high speed operation, fast delay control, and wide range of operation wavelength make the present approach practical.

DOI
10.1364/oe.17.021222

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Previously demonstrated slow light is still far from applications, particularly due to the limited bandwidth and control speed. Although semiconductor-based slow light has the high bandwidth and sub-nanosecond control speed, slow light was observed only in the absorption regime with attenuation, while fast light observed in the gain regime with amplification. The large power difference in two regimes makes the use of the optical delay impractical. We report novel slow light in the gain regime, with a high power comparable to that of fast light, utilizing the anomalous gain characteristic in a gain-clamped semiconductor optical amplifier. The slow light is tunable to fast light with the current as the only variable. Additional high speed operation, fast delay control, and wide range of operation wavelength make the present approach practical.
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.
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.
We report on experimental work investigating slow- and fast-light in a ring-laser cavity that utilizes a semiconductor optical amplifier (SOA) as its primary gain medium. By allowing a pulse to propagate around the loop multiple times, we are able to achieve a tunable delay-bandwidth product near 10 for a 10-ps pulse. Our results show a shift between fast-light, as expected in a typical SOA, to slow-light as the pulse propagates around the loop. This slow-light phenomenon in a gain medium is not expected from an SOA, but arises due to the interaction between the lasing mode and the signal mode.
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.