Weiqi Xue

Wideband 360 degrees microwave photonic phase shifter based on slow light in semiconductor optical amplifiers

In this work we demonstrate for the first time, to the best of our knowledge, a continuously tunable 360 degrees microwave phase shifter spanning a microwave bandwidth of several tens of GHz (up to 40 GHz). The proposed device exploits the phenomenon of coherent population oscillations, enhanced by optical filtering, in combination with a regeneration stage realized by four-wave mixing effects. This combination provides scalability: three hybrid stages are demonstrated but the technology allows an all-integrated device. The microwave operation frequency limitations of the suggested technique, dictated by the underlying physics, are also analyzed.

Widely Tunable Microwave Photonic Notch Filter Based on Slow and Fast Light Effects

A continuously tunable microwave photonic notch filter at around 30 GHz is experimentally demonstrated and 100% fractional tuning over 360deg range is achieved without changing the shape of the spectral response. The tuning mechanism is based on the use of slow and fast light effects in semiconductor optical amplifiers assisted by optical filtering.

Enhancing light slow-down in semiconductor optical amplifiers by optical filtering

We show that the degree of light-speed control in a semiconductor optical amplifier can be significantly extended by the introduction of optical filtering. We achieve a phase shift of approximately 150 degrees at 19 GHz modulation frequency, corresponding to a several-fold increase of the absolute phase shift as well as the achievable bandwidth. We show good quantitative agreement with numerical simulations, including the effects of population oscillations and four-wave mixing, and provide a simple physical explanation based on an analytical perturbation approach.

Microwave phase shifter with controllable power response based on slow- and fast-light effects in semiconductor optical amplifiers

We suggest and experimentally demonstrate a method for increasing the tunable rf phase shift of semiconductor waveguides while at the same time enabling control of the rf power. This method is based on the use of slow- and fast-light effects in a cascade of semiconductor optical amplifiers combined with the use of spectral filtering to enhance the role of refractive index dynamics. A continuously tunable phase shift of approximately 240 degrees at a microwave frequency of 19 GHz is demonstrated in a cascade of two semiconductor optical amplifiers, while maintaining an rf power change of less than 1.6 dB. The technique is scalable to more amplifiers and should allow realization of an rf phase shift of 360 degrees.

Widely tunable microwave phase shifter based on silicon-on-insulator dual-microring resonator

We propose and demonstrate tunable microwave phase shifters based on electrically tunable silicon-on-insulator microring resonators. The phase-shifting range and the RF-power variation are analyzed. A maximum phase-shifting range of 0-600 degrees is achieved by utilizing a dual-microring resonator. A quasi-linear phase shift of 360 degrees with RF-power variation lower than 2dB and a continuous 270 degrees phase shift without RF-power variation at a microwave frequency of 40GHz are also demonstrated.

Tunable Microwave Phase Shifter Based on Silicon-on-Insulator Microring Resonator

We demonstrate microwave phase shifters based on electrically tunable silicon-on-insulator microring resonators (MRRs). MRRs with different quality factors are fabricated and tested. A continuously tunable phase shift of up to 336° at a microwave frequency of 40 GHz is obtained using a high-quality-factor ( ~28 000) MRR with only 1.6-mW power consumption. A quasi-linear phase shift in the range of 0° -204° at 40 GHz with a radio-frequency power variation less than 1.3 dB is also achieved by using a lower-quality-factor MRR.

Slow and Fast Light Effects and Their Applications to Microwave Photonics Using Semiconductor Optical Amplifiers

We provide a comprehensive review of the application of slow and fast light (SFL) techniques to the field of microwave photonics. Basic principles leading to the implementation of phase shifting and true time delay operations which are instrumental in this field are first considered. We then focus on the description of the main results obtained by our groups in the implementation of broadband, full 360 phase shifting using coherent population oscillations in semiconductor waveguides. Next, attention is given to the evaluation of the system impairments implied by these devices when included in analog links. Finally, the main results obtained for several microwave photonic applications such as filtering, arbitrary waveform generation and optoelectronic oscillators (OEOs) are reviewed, and other directions for future research in the field are discussed.

Fano resonance control in a photonic crystal structure and its application to ultrafast switching

We experimentally demonstrate a photonic crystal structure that allows easy and robust control of the Fano spectrum. Its operation relies on controlling the amplitude of light propagating along one of the light paths in the structure from which the Fano resonance is obtained. Short-pulse dynamic measurements show that besides drastically increasing the switching contrast, the transmission dynamics itself is strongly affected by the nature of the resonance. The influence of slow-recovery tails implied by a long carrier lifetime can thus be reduced using a Fano resonance due to a hitherto unrecognized reshaping effect of the nonlinear Fano transfer function. As an example, we present a system application of a Fano structure, demonstrating its advantages by the experimental realization of 10 Gbit/s all-optical modulation with optical control power less than 1 mW.

Threshold Characteristics of Slow-Light Photonic Crystal Lasers.

The threshold properties of photonic crystal quantum dot lasers operating in the slow-light regime are investigated experimentally and theoretically. Measurements show that, in contrast to conventional lasers, the threshold gain attains a minimum value for a specific cavity length. The experimental results are explained by an analytical theory for the laser threshold that takes into account the effects of slow light and random disorder due to unavoidable fabrication imperfections. Longer lasers are found to operate deeper into the slow-light region, leading to a trade-off between slow-light induced reduction of the mirror loss and slow-light enhancement of disorder-induced losses.

Optically fed microwave true-time delay based on a compact liquid-crystal photonic-bandgap-fiber device.

An electrically tunable liquid-crystal, photonic-bandgap-fiber-device-based, optically fed microwave, true-time delay is demonstrated with the response time in the millisecond range. A maximum electrically controlled phase shift of around 70 degrees at 15 GHz and an averaged 12.9 ps true-time delay over the entire modulation frequency range of 1-15 GHz are obtained.