We have demonstrated a new photonic system suitable for RF phased array sensors. This system uses a serial-feed concept that represents a major simplification in both optical and microwave components compared with conventional parallel systems [[1]-[6]]. Our system provides both phase delay and true time delay (TTD) for beam steering and requires only a single tunable laser, optical modulator, and time delay element [7]. In our design, the use of fiber optic techniques, such as long and low-loss delay lines, is intrinsic to the operation of the system. It is predicated upon using the pulsed nature of most microwave sensors in a manner similar to clocked systems used in digital configurations. Precise timing control of photonic signals is used to distribute RF pulses with time/phase delay information to each element in a radiating/receiving array.
TRANSMIT MODE
The timing information is obtained via the fiber grating which yields
a wavelength-selective propagation delay for each gated pulse. Each serially-fed
optical pulse has a unique time delay relative to the RF pulse gate timing.
It is these TTDs that control the pointing direction of our phased array.
Figure 1 - (a) Basic XMIT mode implementation for array of n elements. It shows only one laser, one optical modulator and one delay element are needed. (b) Tapped optical delay line used as the distribution network to demultiplex serial pulses into parallel ones.
There is complete freedom in choosing this direction constrained only by the number of available laser wavelengths and associated Bragg grating reflectors. Chirped Bragg gratings can allow for continuous tuning of TX angle.
The serially fed receiver consists of a timing unit and a serial to parallel
conversion distribution network as shown in Fig.2. The timing unit sequentially
generates delays designated for a given direction of observation. The distribution
network then transforms these delays into parallel signals and feeds them
to the antenna elements. Although, as in our initial transmit experiment
a two-laser switch system was used, we have utilized a linearly chirped
grating, which along with a tunable DBR laser provides almost continuous
scanning for the ultimate system.
Figure 2 - Basic serially fed receive mode configuration for an array of n elements. A chirped fiber grating in conjunction with a tunable laser provides the necessary phased delays for virtually continuous directional operation. A second tapped delay line in parallel can be added to achieve almost continuous temporal operation.
References:
[1] D. Dolfi, P. Joffre, J. Antoine, J.-P. Huignard, et al., "Photonics
for phased array radars," Proceedings of the SPIE - The International
Society for Optical Engineering 2560, 158-165 (1995).
[2] H. R. Fetterman, Y. Chang, D. C. Scott, S. R. Forrest, et al., "Optically
controlled phased array radar receiver using SLM switched real time delays,"
IEEE Microwave and Guided Wave Lett. 5, 414-416 (1995).
[3] L. Xu, R. Taylor and S. R. Forrest, "True time-delay phased-array
antenna feed system based on optical heterodyne techniques," IEEE Photon.
Technol. Lett. 8, 160-162 (1996).
[4] R. D. Esman, M. Y. Frankel, J. L. Dexter, L. Goldberg, et al., "Fiber-optic
prism true time-delay antenna feed," IEEE Photon. Technol. Lett. 5,
1347-1349 (1993).
[5] L. J. Lembo, T. Holcomb, M. Wickham, P. Wisseman and J. C. Brock, "Low-loss
fiber optic time-delay element for phased-array antennas," Proceedings
of the SPIE - The International Society for Optical Engineering 2155, 13-23
(1994).
[6] Molony, C. Edge and I. Bennion, "Fibre grating time delay element
for phased array antennas," Elect. Lett. 31, 1485-1486 (1995).
[7] G. A. Ball, W. H. Glenn and W. W. Morey, "Programmable fiber optic
delay line," IEEE Photon. Technol. Lett. 6, 741-743 (1994).
[8] Phased-array optically controlled receiver
using serial feed, B. Tsap, Y. Chang, H. Fetterman, A. F. J. Levi, D. A.
Cohen, I. Newberg, IEEE Photonics Technol. Lett. 10, 267-269 (1998).
[9] Optically-controlled serially-fed phased
array transmitter, Y. Chang, B. Tsap, H. Fetterman, D. A. Cohen, A. F. J.
Levi, and I. Newberg, IEEE Microwave and Guided Wave Lett. 7, 69-71 (1997).
[10] Optically-controlled serially-fed phased
array sensor, D. Cohen, Y. Chang, A. F. J. Levi, H. Fetterman, and I. Newberg,
IEEE Photonics Technol. Lett. 8, 1683-1685 (1996).