OSA / OFC/NFOEC 2010 a2380_1.pdf OWJ4.pdf
A Programmable Optical Filter Unit Cell Element for High Resolution RF Signal Processing in Silicon Photonics P. Toliver*, R. Menendez*, T. Banwell*, A. Agarwal*, T. K. Woodward*, N-N Feng#, P. Dong#, D. Feng#, W. Qian#, H. Liang#, D. C. Lee#, B. J. Luff#, M. Asghari# *
#
Telcordia Technologies, Red Bank, New Jersey 07701, Kotura, Inc. Monterey Park, CA, 91754
[email protected]
Abstract: We demonstrate the first reconfigurable, cascadable silicon photonics unit cell fabricated in commercial wafer-scale processes using a single 500ps delay line and allowing independent FIR and IIR response for high-resolution RF filtering and correlation applications. © 2010 Optical Society of America OCIS Codes: (060.5625) Radio frequency photonics; (070.1170) Analog Optical Signal Processing; (130.0130) Integrated Optics
1. Introduction Despite decades of progress in microelectronics technology, the demand for ever-greater RF signal processing performance in civilian and military applications continues to stress the bandwidth and resolution capabilities of digital signal processing (DSP) and analog-to-digital conversion (ADC) technologies. Accordingly, opportunities to optimize the signal processing channel prior to DSP abound [1,2]. Here, we apply integrated optics technology to solve challenging RF signal processing tasks using reconfigurable, cascadable integrated optical elements. Building on significant prior work [3,4] and leveraging silicon photonics technology, we present the first experimental realization of a programmable optical filtering element engineered specifically as a building block for this task.
Figure 1. (a) Schematic of a unit cell capable of realizing an independent pole and a zero for use in reconfigurable optical filters for RF signal processing. : tunable couplers, : phase shifters, : delay element, : pole/zero path matching delay. (b) Photo of a silicon photonics realization of the unit cell. Bond pads are 200 x 200 m.
The ability to apply filtering within the analog optical path can significantly benefit the overall signal processing chain, in part by reducing the demands on subsequent ADC operations, as well as providing wideband agility and space-saving benefits. These benefits can be significantly enhanced with reconfigurable, generic optical processing elements that be programmed on demand [5]. Such elements, when realized in integrated arrays with appropriate gain and control systems, can realize applications such as wide-band programmable filters, correlators, and beam steering elements. Design techniques based on Z-transforms have been employed with success in the design of optical filters for fiber-optic communications [6]. To extend these concepts to a generic unit cell capable of high resolution (~ 50 MHz) for RF filtering and correlator applications, it is desirable for the element to provide fully independent zero and pole positioning, allowing for flexible finite and infinite impulse filtering responses (FIR and IIR). To support correlation, true time delay zeros may require as much as 20 ns of delay, while notch filters or channelizers can require narrow (~50 MHz) frequency response. Both are difficult to achieve without narrow (1-10 GHz) free spectral range (FSR) elements that we believe will be important parts of an ultimate system solution. 2. Design and Fabrication The architecture and realization of our unit cell is shown in Figure 1. The shared delay element () permits both pole and zero responses, facilitating a variety of filtering functions while re-using the space-consuming time delay element. By configuring the feedback coupler (c) to a pure bar state, thereby preventing a feedback path through the lower phase shifter (b), an FIR response can be realized through proper adjustment of the input/output couplers (a ,b). Conversely, when the input coupler (a) is configured for the cross state, thereby preventing the feedforward path through upper phase shifter (a), an IIR response can be obtained by adjusting the output and feedback couplers (b ,c) appropriately. In the general case, a mixed FIR/IIR response results, with a field transfer function equal to:
H11
z 1 e i e i sin a cos c i cos a cos b sin c e i sin a sin b a
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OSA / OFC/NFOEC 2010 a2380_1.pdf OWJ4.pdf
where z e i and is the loss associated with the delay line ( ). An additional fixed delay () is used to ensure that the path lengths associated with the FIR and IIR responses are appropriately matched. Also, while a and b provide full generality, it is desirable to introduce a third phase shifter (c) into the design permitting simultaneous adjustment of both pole and zero positions without changing their relative alignment, a feature convenient for control of multi-cell elements. Unlike designs in which the pole and zero have a relatively complex coupled relationship to one another [4, 6], the design of Figure 1 offers fully independent adjustment of pole and zero positions (subject to constraints arising from non-zero losses) and provides a true time delay suitable for narrow-band zero response and correlator operation. This freedom is particularly important when delay line loss is considered, since arbitrary zero positions on the ‘Z’-plane unit circle are possible; such generality is not the case for ‘all-pass’ designs in which the zero locations are derived from multi-pole interference [6]. The frequency domain response of the unit cell is directly related to the length of the time delay element. A frequency range between 1 and 10 GHz is of particular interest for current RF signal processing, suggesting an FSR scale of between 1 and 10 GHz, or a delay of 0.1 to 1 ns. For wide addressable bandwidth, a 10 to 20 GHz FSR is desirable, which implies that longer delay will require suppression of in-band FSR resonances, achievable in a variety of methods; some of which (e.g. vernier design) have been previously described [6]. We have realized designs with both 250 and 500 ps delay elements, and present results from a 500 ps device here. The unit cell of Figure 1 has been realized in integrated silicon photonics, fabricated on 6-inch wafers in a CMOS-compatible process. Ridge-based waveguides are fabricated on SOI wafers employing a thin (250 nm) active Si layer over a thick (3000 nm) oxide buffer. Unit cells having either 1 or 2 m wide waveguides were built, and the results of 2 m guides are presented here. Calibration wafers from early in the process yielded waveguide loss of 0.7(0.4) dB/cm for 1(2) m wide waveguides, and the waveguide loss increased after full processing to ~ 0.9 dB/cm, due to correctable processing-induced effects. Owing to narrow waveguides and lack of beam expanding design elements at the facets, a coupling loss of ~ 7 dB/ interface when packaged with narrow-core (~ 4 m) fiber was obtained. Since beam expansion techniques in silicon have previously resulted in fiber coupling losses of < 1 dB per interface [7], and we have achieved waveguide loss of 0.1 dB/cm in wider (4 m) waveguides, we do not believe the losses in these unit cells are representative of future efforts. 2. Modeling and Experimental Results The advantages of a coupled ‘hybrid’ unit cell having both pole and zero response are apparent when considering how poles and zeros interact to create a composite response illustrated in Figure 2. The behavior of a coupled unit cell is distinct from the product of individual FIR and IIR responses, as shown in Figure 2(a). This demonstrates that an optimized notch response, possible by alignment of zero and pole, provides significantly lower loss than a straightforward product of the two cascaded elements. Further examples of simulations in which pole response is accentuated by coupling varying amounts of zero response are shown in Figure 2(b). A delay line loss of 1 dB was assumed. 0
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Figure 2. (a) Comparison of response from a single unit cell in hybrid state vs. two cascaded cells configured in pure FIR and pure IIR states, illustrating 23 dB loss difference. (b) Simulated response for relative pole/zero angle of 45/90/180 degrees. Assumed delay line loss is 1 dB.
Experimental validation of unit cell operation is shown in Figure 3. In (a), we illustrate the configuration for an FIR response, while in (b), the IIR configuration is shown. In (c) – (g), the performance for various configurations of the unit cell is shown, demonstrating a range of possible pole/zero positions. The resulting responses were taken with a high resolution (20 MHz) optical spectrum analyzer and swept laser. The ability to position a pole near the
OSA / OFC/NFOEC 2010 a2380_1.pdf OWJ4.pdf
unit circle in the Z-plane is limited by propagation loss (~4 dB) in the 500 ps delay line and is expected to improve. In all cases, excellent agreement of experiment (solid lines) and theory (dashed lines) is obtained. The rich set of potential interactions is very useful in programmable multi-stage filters. Simulations show that cascading as few as 8 of these unit cells can yield passband filters having 200 MHz passband with ~ 60 dB stopband extinction. ‐15
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Figure 3. Unit cell configuration for (a) FIR response and (b) IIR response. (c) Measured (solid) and modeled (dashed) FIR response with pole (x) and zero (o) positions indicated. (d) IIR response. (e), (f), and (g) present mixed unit cell response for 0/90/180 degree relative angle between pole/zero. In each inset the outer circle represents the unit circle, while the inner qualitatively indicates maximum loss-limited pole amplitude.
4. Conclusions We have presented the first realization of a programmable unit cell in silicon photonics utilizing long optical delay (~500 ps) suitable for high resolution RF signal processing that can realize a fully independently programmable pole and zero response. Such elements, when combined with the appropriate phase control systems, photonic analog link designs [8], and loss management techniques (e.g. optical gain) are key building blocks to realize generic programmable optical filters and correlators suitable for diverse optical signal processing applications. This material is based upon work supported by the DARPA PhASER program under Contract No. HR0011-08-C-0026. The views, opinions, and/or findings contained in this article are those of the authors and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.
References [1] RF photonic technology in optical fiber links, ed. W.S.C. Chang, Cambridge University Press, 2002. [2] Jose Capmany, Beatrice Ortega, Daniel Pastor, “A Tutorial on Microwave Photonic Filters,” J. Lightwave Tech., v.24, pp. 201-229, 2006. [3] R. A. Soref, “Silicon-based optoelectronics,” Proc. IEEE, vol. 81, no. 12, pp. 1687–1706, Dec. 1993. [4] Mahmoud S. Rasras, et al., “Demonstration of a Fourth-Order Pole-Zero Optical Filter Integrated Using CMOS Processes,” J. Lightwave Tech., v.25, pp. 87-92, 2007. [5] See, for example, DARPA BAA 07-17 PhASER solicitation: https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=30c6f6b5d5ebfbf40f8f61111eda0483&_cview=0 [6] Christie K. Madsen, J. H. Zhao, Optical Filter Design and Analysis, a Signal Processing Approach, Wiley, ISBN 0471183733, 1999. [7] I. Day, et al., “Tapered silicon waveguides for low insertion loss highly-efficient high-speed electronic variable optical attenuators,” OFC 2003, paper TuM5 (2003). [8] T. Banwell et al., “Exact expression for large signal transfer function of an optically filtered analog link,” Optics Express, v.17. pp.1544915454, 2009.
