1076 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 5, MAY 2005 Wavelength-Controlled Photonic True Time Delay for Wide-Band Applications O. Raz, R. Rotman, and M. Tur Abstract—A radio frequency subcarrier modulated wavelength- controlled dense wavelength-division-multiplexing-based photonic true time delay module, with very low phase ( 4 ) and magni- tude ( 0.5 dB) ripples over a bandwidth of several gigahertz, is demonstrated to achieve wide-band linear frequency modulation transmission (600 MHz) with 37-dB sidelobe suppression and signal-to-noise exceeding 80 dB/MHz. Index Terms—Chirp radar, delay effects, optical modulation, op- tical transfer functions, phased array radar, photonic switching systems, pulse compression radar. I. INTRODUCTION P HOTONIC beamformers are being considered for wide-band radio frequency (RF) true time delay (TTD) systems, which use various pulse compression techniques to improve temporal resolution without the need to employ impractically high peak power short pulses [1], [2]. Of wide spread use is linear frequency modulation (LFM), defined by [3] , where is the pulse center frequency, is the total bandwidth of the signal, and is the pulse length. During the pulse life , the instantaneous frequency linearly scans the frequency range: . In receive, close-by targets will give rise to several overlapping LFM pulses, and the feasibility of optical links in the receiver is presently limited by intermodulations and the need for large spurious free dynamic range. However, in transmit, a single LFM pulse goes through the beamformer, and as a result of the mapping between the instantaneous frequency and time in the pulse [3], any existing nonlinearity will only produce out-of-range harmonics but not intermodulations. (This holds true for quite a few other pulse compression methods [3].) Thus, in transmit the link can work at high modulation indexes, alleviating several of the difficulties in practical implementations. The quality of an LFM signal, processed by a photonic beamformer, is mainly determined by its associated com- pressed form, which is derived from the output signal, through correlation with the undistorted original pulse shape, either via weighted matched-filter or dechirp processing [3]. The resulting narrow temporal pulse, the so-called impulse response, has a width which is inversely proportional to and whose peak sidelobe level (PSL) is ideally determined by the apodizing Manuscript received September 1, 2004; revised January 10, 2005. The authors are with the Faculty of Engineering, Tel-Aviv University, Tel Aviv 69978, Israel (e-mail: odedr@eng.tau.ac.il). Digital Object Identifier 10.1109/LPT.2005.844559 weighting filter (e.g., Hamming window or equivalent). How- ever, amplitude and phase distortions anywhere between the input and output of the photonic beamformer [3], [4] may either widen the main lobe and/or increase the PSL, above their ideal values, determined by and the apodizing window. A photonic-based TTD device comprises three basic mod- ules: a modulator with its driving circuitry to convert the elec- trical signal into some form of modulation of the optical carrier (E/O), an optical device which transmits/processes the signal in an advantageously prescribed way, and a photoreceiver (O/E) to retrieve the processed RF signal. While in general system performance must be numerically calculated based on the input LFM signal and the known or measured characteristics of the various system components [4], for small modulation index, an equivalent RF transfer function can be established from the RF input through the modulator and optical device and up to the output of the photoreceiver, and the impact of any deviations from a flat gain and linear phase on LFM performance can be studied using [4]. In practice, two sources of errors are to be ex- pected: those which occur in the E/O and O/E conversions, and those which are purely optical in nature as determined by the optical transfer function of the optical device (the wavelength demultiplexer and its optical circuitry in our TTD implementa- tion, see Section II) [5]. Several schemes for photonic TTD have been suggested, among which are dispersive fibers [6], [7], chirped Bragg gratings [8], and wavelength routing via an arrayed wave- guide grating (AWG) multiplexer [9]. While the first two TTD implementations may introduce significant amplitude/phase distortions [6], [10], optical-communication-grade wave- length-division demultiplexers, using AWG or thin film technology, can potentially pass the optically subcarrier modu- lated LFM pulse with minimum penalty. In this letter, we propose and demonstrate a wavelength-con- trolled photonic TTD based on a single thin film wavelength-di- vision demultiplexer to achieve very large RF bandwidths with minimum phase and magnitude distortions. Indeed, transmis- sion of a wide-band LFM pulse through the photonic TTD de- vice results in excellent resolution and sidelobe performance with practically no degradations. II. PHOTONIC TTD DEVICE An eight-channel thin-film optical demultiplexer (channels 23–37 on the ITU grid, emanating from a 4-dBm tunable laser) was used to accomplish the wavelength-controlled TTD operation (see Fig. 1). The input RF subcarrier modulated light went through a circulator and depending on its wavelength, was routed to a particular output port, where a different length 1041-1135/$20.00 © 2005 IEEE