Photonic Ultrawideband Millimeter Wave Beamformer

D.A. Tulchinsky
Optical Sciences Division

Introduction: Electronically steered radar systems are currently used in a variety of military and commercial applications. The speed and agility of electronically steered beams is far superior to those from mechanical steered antennas. These applications include electronic countermeasure and surveillance radars, Improving array systems to transmit and/or receive across an ultrawide frequency bandwidth (percentage bandwidths greater than 25%) would upgrade high-resolution mapping/target identification radar systems. Furthermore, the military currently needs radar systems capable of operating within the millimeter wave (MMW) frequency range (30 to 300 GHz). Trying to satisfy all these performance goals has historically proved to be a significant challenge. For example, the development of MMW radar systems has been slowed because of the lack of high-power sources and low-loss millimeter wave components. Current electronically steered systems normally have narrow bandwidths (percentage bandwidths less than 1%) for reasons of cost and performance.

Array antennas are steered by delaying the RF signal at each antenna feed by an appropriate amount of time (Fig. 8). When the radiated waves combine, constructive interference occurs and the signal power adds in the desired pointing direction, forming the main beam. In narrowband systems, these time shifts are produced using RF phase shifters. However, phase shifters are not suitable for ultrawideband systems because phase control is not capable of steering wide frequency bandwidths to the same point in space. Hence, ultrawideband systems require true-time delay (TTD) techniques to avoid frequency-dependent steering effects. The Optical Science Division of NRL has developed a fiber-optic beamforming system that implements TTD.¹ In this article, we show that this photonic TTD architecture is extendable into the MMW frequency range and is capable of steering ultrawideband arrays.²

FIGURE 8
True time-delay beam steering.

Beamformer Design: Figure 9 is a schematic of the photonic millimeter wave beamformer. A wavelength tunable semiconductor laser serves as the optical source for the system, after which an erbiumdoped fiber amplifier (EDFA) boosts the laser's output power. Optical fiber directs the light into a 40 GHz electro-optic Mach-Zehnder intensity modulator (MZM). The MZM takes the input RF signal and impresses its signature onto the amplitude of the light. The modulated optical signal is fed into a four-channel fiber-optic dispersive prism. The optical dispersion gradient in the dispersive prism translates a wavelength change of the laser into an optical path length difference among the prism's channels. This change in path length provides the time-delays between the prism's four output channels that are required to steer the RF beam. After the prism, the four optical paths continue through separate fiber-optic length and amplitude trimmers (not shown) before being directed onto the photodetectors. The photodetectors, sensitive to modulations in optical power up to 50 GHz, convert the optical signals into RF electrical signals- replicas of the input RF signal. Each resulting RF path is amplified by a low-noise MMW amplifier and fed into the input plane of a MMW antenna array.

FIGURE 9
Fiber-optic beamformer.

Demonstration and Results: After construction and testing in the laboratory, further experiments on the photonic beamformer were performed in a MMW anechoic radar range within the Tactical Electronic Warfare Division's Radar Range Facilities. In the range, the beamforming system is mounted on a multi-axis positioning stage, and the RF radiation from the transmitting antenna array is focused onto a receive antenna by an off-axis parabolic microwave mirror. While the positioner rotates the photonic beamformer in the azimuthal direction through a ±70° sweep, a microwave network analyzer records the frequency response of the system between 20 and 45 GHz in the forward-looking direction. Figure 10 shows examples of the transmitted intensity patterns with the laser tuned to steer the beam to -15°, 0°, and +30° away from the forward looking direction for frequencies across the entire Ka (26.5 to 40 GHz) frequency band. These intensity plots show that the main intensity of the transmitted RF beam is at the expected steered angle and the steering is independent of frequency. On either side of the mainbeam lobe, RF intensity is expected and present in two beam sidelobes as well as one grating lobe. Note how the sidelobes and the grating lobes exhibit the expected frequencydependent steering.

FIGURE 10
Transmitted intensity plot as a function of mechanical angle and frequency with the laser adjusted for optical steering to (a)-15°, (b) 0°, and (c) 30° in azimuth. The black vertical lines are guides to the eye to indicate at what azimuthal angle the beam is steered.

Summary: NRL has successfully demonstrated a photonically controlled ultrawideband beamformer for millimeter wave transmit arrays. The high speed and large bandwidth potential of photonics make this system an attractive alternative to conventional array beamformers.

Acknowledgments: I thank my collaborators, Dr. P.J. Matthews and N. Matovelle. Special thanks to P.D. Boran for assistance with the MMW radar range.

[Sponsored by ONR]

References
¹ R.D. Esman, M.Y. Frankel, J.L. Dexter, L. Goldberg, M.G. Parent, D. Stilwell, and D.G. Cooper, "Fiber-optic Prism True Time-delay Antenna Feed," IEEE Photon. Technol. Lett. 5, 1347 (1993).
² D.A. Tulchinsky and P.J. Matthews, "Ultrawide-Band Fiber-Optic Control of a Millimeter-Wave Transmit Beamformer," IEEE Trans. Microwave Theory Tech 49, 1248 (2001).