Optical delay lines have some important applications, notably in optical communication systems and in phased arrays. These devices are based on the concept of optical group delay, which, in turn, can be understood as the property of an optical filter. Optical filters are well-understood devices and, in particular, their dispersive properties determine the group delay response. We review these dispersive properties and point out some of the inherent tradeoffs involved in generating large group delay. Fiber Bragg gratings and recent results on optical all-pass filters are used as examples.

Optical delay lines based on optical filters

A compact variable all-optical buffer using semiconductor quantum dot (QD) structures is proposed and analyzed. The buffering effect is achieved by slowing down the optical signal using an external control light source to vary the dispersion characteristic of the medium via an electromagnetically induced transparency effect. We present a theoretical investigation of the criteria for achieving slow light in semiconductor QDs. A QD structure in the presence of strain is analyzed with the inclusion of polarization-dependent intersubband dipole selection rules. Experimental methods to synthesize and the measurements of coherent properties in state-of-the-art QDs are surveyed. Slow-light effects in uniform and nonuniform QDs are compared. Finally, optical signal propagation through the semiconductor optical buffer is presented to demonstrate the feasibility for practical applications.

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Variable optical buffer using slow light in semiconductor nanostructures

Microwave photonics is a rapidly-growing interdisciplinary field linking microwave and optical technologies. This paper intends to trace the evolution of microwave photonics over the last three decades emphasizing some major research areas and application fields. Currently, microwave photonics is a vivid research area with several practical applications. In this paper, we attempt to project potential future trends as well.

Microwave Photonics—A Historical Perspective

A continuously variable true-time delay beamformer based on a multichannel chirped fiber grating (MCFG) is demonstrated. The beamformer is capable of wide-band squint-free operation and demonstrates beam steering of a four-element array over /spl plusmn/40/spl deg/. The use of an MCFG provides a significant reduction in the length of the chirped grating required, eliminates spurious signal transmission, and utilizes narrow-band tunable laser sources. In addition, the effect of grating errors on the antenna pattern is examined and the tolerance on performance is presented.

Demonstration of a continuously variable true-time delay beamformer using a multichannel chirped fiber grating

In this letter, we demonstrate a new five-element true-time-delay (TTD) system that uses only a single tunable linear chirped fiber grating (TLCFG) delay line for continuous TTD beamforming. The system avoids the complicated wavelength tuning and synchronal control of the tunable lasers as well as the tunable bandpass filters. Different time delays are achieved by tuning the chirp rate of the TLCFG, to which a multiwavelength laser source that carries the RF signal is applied. The time delay responses of the TTD system are measured experimentally, which agree well with the theoretical analysis. The beampointing angle can be controlled continuously by changing the chirp rate of the TLCFG. The minimum time delay step can be much smaller than 1 ps thanks to the high position resolution of the actuator.

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Continuous true-time-delay beamforming for phased array antenna using a tunable chirped fiber grating delay line

In this paper, we present and demonstrate a novel approach of a true time-delay (TTD) optical feeder for phased-array antennas (PAAs). A continuously variable TTD is achieved by employing tunable lasers and a wide-bandwidth chirped fiber Bragg grating (FBG) as the dispersive element. The results show that a very high resolution performance (equivalent to a 6-b microwave phase shifter) is obtained for an L-band PAA employing narrow-tuning bandwidth lasers with a wavelength stability of 0.005 nm and a 4-nm bandwidth chirped grating with a dispersion of 835 ps/nm.

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J.M. Foster

Papers

Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber grating