JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22,NO. 2,FEBRUARY 2004 619 Hybrid Analog–Digital Variable Fiber-Optic Delay Line Nabeel A. Riza, Senior Member,IEEE, Fellow, OSA, Muzammil A. Arain, Student Member, IEEE, and Sajjad A. Khan, Student Member, IEEE Abstract—A variable fiber-optic delay line (VFODL) is in- troduced that, to the best of the authors’ knowledge, is the first time that a hybrid analog–digital VFODL is proposed to solve the dilemma of efficiently enabling many settable and long-duration time delays together with continuous and short time delays. In essence, this VFODL can provide near-continuous high-resolution time generation across an entire long-time-delay band. The VFODL is based on the concept of cascaded wavelength-sensi- tive and wavelength-insensitive time delays. A proof-of-concept VFODL built demonstrates near-continuous 0.5-ps-resolution time-delay control across an entire 25.6-ns time-delay band generating a total of 51 200 measurable time-delay bins. The experimental VFODL also gives a 4.95-dB total optical loss and a 1-ms time-delay control-setting speed. The proposed VFODL can be used in applications such as radio frequency photonic signal processing and radar testing. Index Terms—Fiber Bragg grating (FBG), fiber delay line (FDL), optical delay, variable delay. I. INTRODUCTION A VARIABLE fiber- optic delay line (VFODL) is a highly sought after component with applications ranging from microwave/millimeter-wave analog photonic signal processing to digital optical communication systems based on packet switching. The ideal VFODL is able to efficiently and con- tinuously generate time delays with high temporal resolution over any given long-time-delay range. Over the years, efforts have been made to realize these VFODLs, particularly for mi- crowave photonics applications where a radio-frequency (RF) signal riding on an optical carrier needs to be provided with a desirable delay. One way to efficiently generate many time delays over a long-time-delay range uses an -bit switched binary architecture that employs 2 2 digital switches to select given binary paths connected in a serial cascade (architecture) [1]. Here, based on the delay range required, free-space, solid-optic, and fiber-based delay paths have been deployed in both serial- and parallel-switched architectures using a variety of switching technologies such as liquid crystals [2], [3], lithium niobate integrated optics [4], [5], micromechanics [6], [7], acoustooptics [8], [9], gallium arsenide integrated optics [10], and indium phosphide and silica-on-silicon integrated optics [11]. Because of the digital switched nature of these VFODLs, time-delay resolution is quantized to a discrete value, Manuscript received June 26, 2003; revised November 17, 2003. The authors are with the Photonic Information Processing Systems (PIPS) Laboratory, School of Optics/Center for Research and Education in Optics and Lasers (CREOL), University of Central Florida, Orlando, FL 32816-2700 USA (e-mail: riza@creol.ucf.edu). Digital Object Identifier 10.1109/JLT.2004.824383 and there is a tradeoff between resolution and number of binary switched stages. In effect, getting smaller resolutions across larger time-delay ranges means adding more cascading, leading to higher losses and greater module complexity. Hence, a dilemma exists to get both high resolution and long-time-delay range. Another approach for generating large delays is via resonant optical devices, although this method has a tradeoff between delayed signal bandwidth and delay time [12]. A more recent and attractive technology for generating time delays involves the use of wavelength tuning and fiber Bragg gratings (FBGs). Initially, discrete FBGs positioned along specified fiber paths were used to produce discrete time delays based on the wavelength chosen [13], [14]. Later, the concept was extended to use a chirped FBG to generate near-continuous time delay but over short-delay range due to the fabrication size limitations of FBGs and the laser tuning range [15], [16]. To get more delay settings within an efficient structure, mul- tiwavelength fiber-time-delay processing was proposed using discrete FBG’s delay segments within a serial-optical-switched structure [17], [18]. In addition, wavelength tuning in com- bination with wavelength-division-multiplexer devices was also proposed to realize VFODLs [19], [20]. So far, all these efforts, to the best of the authors’ knowledge, have not realized a VFODL that can deliver near-continuous time delays over an arbitrary large-time-delay range. In this paper, we describe such a desired VFODL that solves the prior resolution-range dilemma. II. HYBRID VARIABLE FIBER-OPTIC-DELAY LINE DESIGN Fig. 1 shows one version of the proposed hybrid VFODL. The module has one electrical input port and one electrical output port from which emerges the given delayed electrical waveform riding on a delayed optical carrier. The module has two delay control ports: one to control the analog time delay, and the other to control the digitally switched optical delay. Hence, the pro- posed structure is a cascade of an efficient digitally switched optical delay line in combination with an analog-controlled op- tical delay line. This hybrid combination solves the earlier res- olution-range dilemma as the digital delay is excellent for pro- viding the long-time-delay range, while the analog delay is ex- cellent for providing the near-continuous high-resolution delay between the discretized delays of the switched delay line. In effect, a near-continuous time-delay control can be generated across a large-time-delay range. Reflective optical fibers (ROFs) are used at the output ports of the optical switch that elimi- nates the need for alignment-sensitive fiber-to-free-space cou- pling with bulk-mirror optics. This reflective nature of the pro- posed ROF-based VFODL reduces optical loss, packaging cost, 0733-8724/04$20.00 © 2004 IEEE