Wideband 0/� Retro-Reflective Photonic System Lior Yaron, Tomi Langer, Yuri Geiler, Shlomo Zach* and Moshe Tur

Tel-Aviv University, Tel-Aviv 69978, Israel; *Wales Ltd., Ramat-Gan 52522, Israel email:yarlior@gmail.com, Tel: +972-3-6409719, fax: +972-3-6409719

Abstract „ A novel 0/� wideband (a few GHz) retro-reflective photonic system is proposed and demonstrated, exhibiting high reciprocity, long delays, close-to-unity (~0.2 dB rms, ~1 deg rms) differential RF transfer function, which makes it suitable for demanding microwave applications.

Index Terms „ Optical delay lines, Optical processing of radar images, Microwaves.

I. INTRODUCTION

Optical analog links have been intensively studied in the last three decades thanks to their flat transfer function over wide RF bandwidths, high RFI/EMI tolerances, and low weight and volume [1], [2]. These advantages suggest that photonics could be used to benefit demanding microwave applications such as, photonic beamforming [3], and the processing of microwave signals [4]. One of the more challenging microwave applications is the implementation of wideband, long delay �-phase shift retro-reflective cross-eye systems [5], which require a high degree of reciprocity over a wide range of RF frequencies.

Cross-eye is an electronic warfare technique [5]- [9], which responds to a seeker radar by shaping its received phase front using a �-phase shift retro-reflective system, as shown in Fig. 1. Each of two spatially separated antennas receives the transmitter signal and then sends it to the other antenna for transmission, while inserting a �-phase shift difference between the counter-propagating signals. The seeker radar, which tries to align itself perpendicular to the returned field of a point reflector, Fig. 2a, will see a complex interference pattern, Fig. 2b, and will align itself in a wrong angle with respect to the retro-reflective system. The angular error can be estimated by [5]:

Fig. 1. The building blocks of a reciprocal �-phase shift retro-reflective system. T/R stands for Transmit/Receive Module

2

2

cos211

2cos

kkk

rL

error ���

��

�� (1),

When L is the distance between the two transmitting antennas, r the range to the seeking radar, � the angle between the normal to the system and the line between the system and the radar, k is the power ratio between the two transmitters (~1) and � the phase shift (~�). The reciprocity of the system ensures that the desired phase shift and power ratio are seen at the aperture of the seeking radar, regardless of its angle and position relative to the transmitters. In order to achieve a given angular error, the distance between the two antennas of the system should be appropriately long and may reach tens of meters [5], [6]. Additionally, the delay � of Fig. 1 is quite often required to be much longer than the physical distance between the two antennas [7]. These requirements make the cross-eye system implementation a rather challenging engineering task, and the first [8] successful demonstration of an electronic cross-eye system was published only in 2000 [7]. This demonstration, however, employed a Digital RF Memory (DRFM), thereby limiting the achievable RF bandwidth and/or the dynamic range.

(a)

(b)

Fig. 2. The field pattern at the radar apertutre for: (a) point reflector (b) retro-reflective photonic system with L=50m, r=5km and � =00.

In this paper we propose and demonstrate, for the first time to the best of our knowledge, a 0/� retro-reflective photonic system with a very wide RF bandwidth.

II. THE RETRO-REFLECTIVE PHOTONIC SYSTEM

The proposed photonic system (Fig. 3) replaces the delay element and phase inverter of the �-phase shift retro-reflective system of Fig. 1. The system comprises two (ideally) identical optical modules, to be named CW (Clockwise) and CCW (Counter Clockwise) according to the signal direction they control, Figs. 1 and 3. To enhance reciprocity, each photonic module is located as close as possible to its corresponding transmit/receive antenna. A spool of single mode fiber of length L provides a reciprocal common delay for both counter-propagating signals. Unlike the heavy and lossy coaxial cables used in RF implementations, the optical fiber can furnish a delay tens of microseconds long with negligible loss and predetermined (down to zero) dispersion.

The left-hand antenna of Fig. 1 feeds the RF-in port of the CCW photonic module, normally, after passing through a circulator (Fig. 1) or its switching equivalent, and after some RF amplification.

Concentrating on the CCW path, the electrical signal is then fed to a Mach-Zehnder Modulator (MZM), biased at its positive-slope quadrature point, Fig. 4a. The double-sided modulated optical signal continues through a high isolation optical circulator, which routes it through the common delay fiber to the CW module. In the CW module the signal is routed by a similar optical circulator to an optical detector (possibly after optical amplification), where it is converted back to an electronic signal. This electronic signal is then amplified by an optional RF power amplifier (not shown) and sent to the CW antenna through the RF circulator (or switch).

The CW signal goes through an identical path in the opposite direction, except that the CW Modulator is biased at its negative quadrature point, Fig. 4b. This operation achieves a wideband phase inversion between the two signals [10]. If both modulators are biased at quadrature with the same slope, a 0-phase shift retro-reflector is implemented. Excluding the

reciprocal fiber, all non-reciprocal signal paths are kept to a minimum and must be carefully equalized to ensure reciprocity.

Full reciprocal behavior of the fiber requires the CW and CCW lasers to operate at exactly the same optical wavelength, thereby running the risk of generating linear (Rayleigh) and nonlinear (Brillouin) penalizing back-reflections [11]. Using dispersion shifted fibers (DSF) and non-zero dispersion shifted fibers (NZDSF) can maintain the practical reciprocity even with slightly different CW/CCW wavelengths. Furthermore, non-zero dispersion and laser tunability can be advantageously invoked to compensate for residual parasitic delay mismatches between the two counter-propagating signals. Finally, to ensure flat and reciprocal RF response, matched pairs of MZMs and detectors are to be preferred, although the experimental results shown below were obtained with off-the-shelf components.

Fig. 4. The optical power transfer function of an MZM as a function of the applied electrical voltage. Phase-inversion is obtained by biasing the CW modulator to work at the positive slope (a), while the CCW modulator is biased at a negative slope (b).

Fig. 3. The photonic retro-reflective system. The heavy (optical path) and dashed (electronic path) blue lines designate the path of the CCW signal (see Fig.1), while the red lines describe the CW signal path. The light lines denote optical taps for bias control. O/E: photo-detector.

III. EXPERIMENT AND RESULTS

The photonic retro-reflective system (Fig. 3) was built to support C-Band (4-8 GHz) RF operation using two MZMs with a bandwidth of ~12GHz, high isolation optical circulators (~55 optical dB transmit/receive isolation), 1 km of SMF fiber and 24GHz wideband detectors. The propagation delay between the two counter-propagating signals was optically matched down to ~ 80psec (1.6 cm) by fiber splicing, and then further equalized using wideband RF Line Stretcher, connected at the CW MZM input. The relative magnitude of the signals was adjusted by controlling the intensities of the CW and CCW lasers, and the RF transfer functions of the CW and CCW paths was measured using an RF vector network analyzer. The difference between the CCW and CW transfer functions, CWCCW SS ,21,2110 /log10 and

� CWCCW SS ,21,21 / , shown in Fig. 5 for both the 0-phase and a

�-phase cases, exhibits a flat magnitude (~0.2 dB rms) and very uniform RF phase inversion (~1 degree rms) over the measured 4 GHz bandwidth. The small linear trend in Fig. 5b and Fig. 5d is due to ~ 1.4psec residual delay mismatch of the counter-propagating signals. This trend shows how important it is to maintain all the non-reciprocal signals paths as short and as stable as possible, in order to prevent temperature drifts. However, a fully packaged system can control the temperature of the CW and CCW units, leaving only the reciprocal delay fiber un-stabilized. Moreover, as was mentioned in Sec. 2, using tunable lasers and the common

fiber dispersion enables to continuously reduce the residual parasitic delay. Higher bandwidth matched MZMs are expected to provide a very wide retro-reflecting bandwidth performance.

VII. CONCLUSION

A wideband retro-reflective photonic system was built and measured. With a delay of 5 microseconds, the system exhibits high reciprocity, and a very flat differential RF transfer function with a uniform zero or �-phase shift over 4 GHz. The performance can be extended both in bandwidth and other ways to meet the stringent demands of advanced systems such as cross-eye.

REFERENCES

[1] C. Cox, Analog Optical Links, Theory and Practice,

Cambridge, U.K.: Cambridge Univ. Press, 2004 [2] H. Zmuda and E. H. Toughlian, Photonic Aspects of Modern

Radar, Norwood, MA: Artech House, 1994. [3] M.Tur, L.Yaron and O.Raz, "Photonic Tecnology for Antenna

Beamforming", OFC/NFOEC conference, 2011. [4] R. A. Minasian, •Photonic signal processing of microwave

signals,Ž IEEE Trans. Microw. Theory Tech., vol. 54, no. 2, pp. 832 846, Feb. 2006.

[5] F. Neri, Introduction to Electronic Defense Systems. Boston: Artech House, 1991.

[6] R.N. Lothes et al, Radar Vulnerability to Jamming