1720 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 16, AUGUST 15, 2006 Active Integrated Photonic True Time Delay Device Jian Tong, J. K. Wade, Member, IEEE, Duncan L. MacFarlane, Member, IEEE, Hanxing Shi, Scott McWilliams, Gary A. Evans, Fellow, IEEE, and Marc P. Christensen, Senior Member, IEEE Abstract—A photonic true time delay cell with two waveguide couplers and five semiconductor optical amplifiers is demon- strated. The five semiconductor optical amplifiers provide gain in the 1550-nm region and port selection to determine the time delay. With the amplifiers off the signal are blocked with extinction ratios of more than 10 dB. In the experiments reported here, the delay of 20 ns was provided by an optical fiber. Because of the potential for nanosecond switching times, the device has application in very agile phased array antenna applications, in optical switching and routing, and in optical filtering. Index Terms—Multiple quantum-well, semiconductor optical amplifier, true time delay, waveguide splitter. I. INTRODUCTION I NCREASED integration of photonic devices is an impor- tant frontier for optical signal processing. Very large scale systems require active components for signal regeneration in order to compensate for loss and fan-out. In photonics this re- quires optical amplification, and two prime candidates are para- metric amplifiers [1] and semiconductor optical amplifiers [2]. The present level of photonic integration of about five compo- nents is decades behind electronics. Economic forces impact the progress of photonic integrated circuitry. The market for appli- cations must be large enough to support considerable research and development costs. Alternatively, the photonic integrated circuit developed may be flexible enough to address a multi- plicity of moderately sized applications through reuse of core structures or through user programmability. One example where both programmability and market size coexist is in photonic true time delays for phased array an- tennas in high performance radar and communication systems. For wideband phased array antennas, a constant phase shift re- sults in a far field pattern dependence of frequency, a phenom- enon called beam squinting. Beam squinting can be eliminated by using time delays rather than phase delays. Unfortunately, conventional microwave true time delay elements are heavy, in- flexible, and expensive. The use of photonic techniques, how- ever, offers wide bandwidth, compact size, reduced weight and low radio frequency interference. Approaches to photonic true time delay include wavelength- division-multiplexing techniques [3], substrate-guided waves [4]–[6], all-pass optical filters [7], high dispersion fibers or Manuscript received March 2, 2006; revised May 15, 2006. J. Tong, J. K. Wade, and D. L. MacFarlane are with Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas, Richardson, TX 75083 USA (e-mail: dlm@utdallas.edu). H. Shi and S. McWilliams are with Photodigm Inc., Richardson, TX 75080 USA. G. A. Evans and M. P. Christensen are with the Department of Electrical Engineering, Southern Methodist University, Dallas, TX 75275 USA. Digital Object Identifier 10.1109/LPT.2006.879943 Fig. 1. SEM image of the integrated photonic true time delay cell based on five semiconductor optical amplifiers and two couplers machined using an FIB. The five large (130 130 m ) squares are the bonding pads for the pumping of the amplifiers. The first coupler is located in the region enclosed by the dashed box, shown in Fig. 3. waveguides [8], [9], optical switching matrices [10], laser diode bias switching [11], acoustooptics [12], integrated polymer switches [13], and switching of variable length free-space sections [14]. Most of these true time delay modules consist of only passive optical components. The reconfiguration times of these approaches are typically on the microsecond scale. In this letter, we demonstrate a photonic true time delay cell that utilizes semiconductor optical amplifier gates and waveguide splitters to route the signal to different optical delay paths. The gates also provide gain in the “ON” state which compensates for the insertion, coupling and branching losses. The gates can be switched with high-speed electronic circuits to achieve recon- figuration speeds of less than 1 ns [15]. Consequently, antenna beam shaping and optimization can occur on time scales much faster than the changing RF signal of the antenna, resulting in an extremely agile phased array antenna. In addition, this device also has applications in optical switching, routing and filtering. II. PHOTONIC TRUE TIME DELAY DEVICE The photonic true time delay cell based on two compact split- ters and five semiconductor optical amplifier gates [16], [17] is shown in Fig. 1. The waveguide splitters employ a 45 total internal reflection (TIR) mirror that partially obstructs a ridge waveguide, routing a portion of the light into a perpendicular ridge guide while allowing another fraction of the light to pass through the waveguide. A fraction of the light continues to prop- agate in waveguide while another portion is bent 90 into waveguide . By applying current only to waveguide ( ) and not to ( ), the light is amplified in ( ) and attenu- ated in ( ). Waveguides and have current continuously applied and serve as a preamplifier and a postamplifier. If the 1041-1135/$20.00 © 2006 IEEE