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Irish Signals and Systems Conference

Slow light based microwave photonic phase shifter using coherent population oscillations in a bulk tensile-strained semiconductor optical amplifier

Aidan Meehan, Michael J. Connelly

Published Jan 1, 2014• 2 citations
Publisher

Abstract

We experimentally demonstrate a microwave photonic phase shifter based on SFL in a commercially available tensile strained bulk SOA using CPO. Controllable phase shifts of up to 50 degrees are achieved through changes in both the bias current and optical power. Beat signal gain values of up to 40dB are also reported. The experimental data shows good agreement with previous simulation work also carried out by this group.

DOI
10.1049/cp.2014.0707

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We experimentally demonstrate a microwave photonic phase shifter based on SFL in a commercially available tensile strained bulk SOA using CPO. Controllable phase shifts of up to 50 degrees are achieved through changes in both the bias current and optical power. Beat signal gain values of up to 40dB are also reported. The experimental data shows good agreement with previous simulation work also carried out by this group.
The slow light effect in SOAs has many applications in microwave photonics such as phase shifting and filtering. Models are needed to predict slow light in SOAs and its dependence on the bias current, optical power and modulation index. In this paper we predict the slow light characteristics of a tensile-strained SOA by using a detailed time-domain model. The model includes full band-structure based calculations of the material gain, bimolecular recombination and spontaneous emission, a carrier density rate equation and travelling wave equations for the input signal and amplified spontaneous emission. The slow light effect is caused by coherent population oscillations, whereby beating between the spectral components of an amplitude modulated lightwave causes carrier density oscillations at the beat frequency, leading to changes in the group velocity. The resulting beat signal at the SOA output after photodetection, is phase shifted relative to the SOA input beat signal. The phase shift can be adjusted by controlling the optical power and bias current. However the beat signal gain is low at low frequencies, leading to a poor beat signal output signal-to-noise ratio. If the optical input and SOA drive current are simultaneously modulated, this leads to forced population oscillations that greatly enhance the low frequency beat signal gain. The model is used to determine the improvement in gain and phase response and its dependency on the optical power, bias current and modulation index. Model predictions show good agreement with experimental trends reported in the literature.
The enhancement of the low frequency gain response of a microwave phase shifter based on slow light in a bulk reflective semiconductor optical amplifier (RSOA), by using forced coherent population oscillations (FCPO), is experimentally demonstrated. FCPO is achieved by simultaneously modulating the input optical power and bias current. The beat signal gain improvement ranges from 45 to 0 dB over a frequency range of 0.5 to 2.5 GHz, thereby improving the noise performance of the phase shifter. Tunable phase shifts of up to 40º are possible over this frequency range.
In this talk we show how slow and fast light effects arising from the nonlinear gain and refractive index dynamics within semiconductor optical amplifiers can be used to control the phase shift of microwave frequency signals. Theoretical predictions with experimental conformation will be shown as to how continuous phase shifting can be achieved and pay particular attention to the noise issue that arises in such phase shifters. Finally, applications of such phase shifters, including realizations of tunable microwave notch filters and tunable optoelectronic oscillators will be presented.
This paper presents theoretical and experimental analysis of up-converted coherent population oscillations (Up-CPO) in semiconductor optical amplifiers (SOAs) for tunable optical phase shift applications at high frequencies. The study of Up-CPO frequency response is carried out by a small-signal analysis of a SOA modeled by two calculation sections in order to better take into account the nonlinear effects such as the SOA gain saturation. We demonstrate that we can fully determine the dynamic parameters of Up-CPO from the experimental frequency response of an optical probe signal obtained by cross-gain modulation mechanism. The Up-CPO frequency responses are measured for three SOAs with different cut-off frequencies up to 18.45 GHz. We finally validate experimentally the obtained analytical frequency responses for the three SOAs over different operating points, each characterized by the corresponding optical gain.
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.