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Terahertz Information and Signal Processing by RF-Photonics
 

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    Terahertz Information and Signal Processing by RF-Photonics Terahertz Information and Signal Processing by RF-Photonics Document Transcript

    • IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 167 Terahertz Information and Signal Processing by RF-Photonics S. J. Ben Yoo, Fellow, IEEE, Ryan P. Scott, Member, IEEE, David J. Geisler, Student Member, IEEE, Nicolas K. Fontaine, Member, IEEE, and Francisco M. Soares, Member, IEEE (Invited Paper) Abstract—This paper discusses THz bandwidth information and signal processing based on RF-photonic technologies. In particular, we will emphasize integrated circuit approaches to RF-photonics where highly stable THz bandwidth information and signal processing can take place. We will demonstrate THz signal generation by optical arbitrary waveform generation (OAWG), and will describe its inverse process, optical arbitrary waveform measurement (OAWM). Further, we will discuss an RF-photonic lattice filter useful for optical equalization and other THz signal processing replacing traditional electronic digital signal processing which cannot currently scale to THz. The paper will also cover future prospects for the THz information and signal processing by integrated RF-photonic methods. Index Terms—Integrated optoelectronics, optical modulation, optical pulse shaping, RF-photonics, THz science. I. INTRODUCTION TERAHERTZ science and technology have greatly bene- fited from advances in both high-speed electronics and photonics [1]–[5]. The desire to accurately generate, receive, and process information at high information rates, THz and be- yond, has spurred recent efforts in RF-photonics to synergisti- cally solve this recent challenge [6]–[8]. While both RF waves and optical waves are electromagnetic radiation, they often use dissimilar material and device technologies in different ways. Exploiting benefits from both RF and photonics can yield high- fidelity and high-precision information generation, detection, and processing at possibly THz bandwidths and beyond. A pow- erful combination of the optical technology offering wavelength parallelism, high bandwidth, and low energy consumption with electronic technology offers agile processing and storage capa- bilities as demonstrated by high fidelity optical arbitrary wave- form generation and measurement [optical arbitrary waveform Manuscript received December 21, 2011; revised January 18, 2012; accepted January 18, 2012. Date of publication February 10, 2012; date of current version March 02, 2012. This work was supported in part by DARPA and SPAWAR under OAWG Contract HR0011-05-C-0155, by DARPA under MTO Si-PhASER Project Grant HR0011-09-1-0013, by the National Science Foun- dation under ECCS Grant 1028729, and by the CISCO University Research Program. S. J. B. Yoo, R. P. Scott, and D. J. Geisler are with the Department of Electrical and Computer Engineering, University of California, Davis, CA 95616 USA (e-mail: sbyoo@ucdavis.edu). N. K. Fontaine was with the University of California, Davis, CA 95616 USA. He is now with Alcatel-Lucent Bell Laboratories, Holmdel, NJ 07733 USA. F. M. Soares was with the University of California, Davis, CA 95616 USA. He is now with Fraunhofer Heinrich Hertz Institute, 10587 Berlin, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2012.2185720 generation (OAWG) and optical arbitrary waveform measure- ment (OAWM)] at THz bandwidths [8]–[10]. Another benefit of RF-photonics is the ability to operate in the other extreme of MHz 10 GHz frequencies in which signal processing typically handled by RF electronics alone can be helped by optical lat- tice filters capable of creating reconfigurable filter shapes in the RF domain without involving power hungry RF electronics [7], [11]–[14]. The combination of the two regimes of RF and pho- tonics offers agility, precision, and high resolution to THz band- width signal generation, detection, and processing. Applications of such THz capabilities span a very wide range of new opportu- nities including LIDAR/LADAR [15], [16], THz analog-to-dig- ital conversion (ADC) [17]–[19], THz digital-to-analog conver- sion (DAC) [20], and coherent communications for free-space, satellite, and optical fiber networks [8], [21], [22]. This paper discusses THz bandwidth information and signal processing based on RF-photonic technologies. In particular, we will emphasize integrated circuit approaches to RF-photonics where highly stable THz bandwidth information and signal processing can take place. Section II provides an overview of optical arbitrary waveform generation (OAWG), and Section III covers an overview of its inverse process, optical arbitrary waveform measurement (OAWM). Section IV describes an RF-photonic lattice filter useful for optical equalization, and Section V compares optical versus electrical equalization methods. Section VI summarizes the THz information and signal processing by integrated RF-photonic methods and projects future directions. II. OAWG OVERVIEW A. Concept Fig. 1 illustrates how dynamic OAWG [8], [23]–[27] can generate spectral slices, each with bandwidth , to form an aggregate output waveform with a total bandwidth of . Dynamic OAWG begins with a coherent optical frequency comb (OFC), which is spectrally demultiplexed with narrow passbands placing each comb line at a separate spatial location. A set of in-phase and quadrature-phase modulators ( modulators) each with a bandwidth of apply tem- poral modulations to broaden the comb lines to create the spectral slices. Coherently combining the spectral slices using a gapless spectral multiplexer with broad overlapping passbands ensures a continuous bandwidth output waveform. Further, incorporating compensation for the multiplexer transmission as 2156-342X/$31.00 © 2012 IEEE
    • 168 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 Fig. 1. Dynamic optical arbitrary waveform generation principle [23]. Fig. 2. (a) Mask and (b) photograph of a 100 GHz (10 channel 2 10 GHz) OAWG device showing both waveguide and metallization layers. a pre-emphasis of the modulation signals ensures high-fidelity waveform generation after the multiplexer [8], [28]. B. Device Implementation OAWG device implementation involved two configurations. The first relies on electrical excitation of optical modulators and the second uses optical excitation of optical modulators. Fig. 2 shows the mask layout and photograph of a 100 GHz (10 channel 10 GHz) InP-based OAWG [29]. This device uses a matched pair of arrayed-waveguide gratings (AWGs) to imple- ment the demultiplexing and multiplexing operations. Fabrica- tion was performed on a semi-insulating InP substrate to enable high-speed operation. The chip employed two electrical layers: a DC layer to handle phase-error correction, and an RF layer to drive amplitude and phase modulators via the electro-optic ef- fect. The devices underwent a 14-mask-stop fabrication process Fig. 3. (a) Mask layout of 10-channel traveling-wave phase modulator (TW-PM) array. (b) Zoom-in view of one of the TW-PM as part of the 10-channel–10 GHz OAWG transmitter chip including spectral demux, spectral mux, and arrays of 10 amplitude modulators and 10 phase modulators. (c) S measurement of electrical excitation and optical response (in dB) with >30 GHz bandwidth. (d) Electrical reflection (S in dB) obtained from a 4 mm long test phase modulator fabricated in the same way as the TW-PM in OAWG transmitter chip. to accommodate the optical waveguides, high-speed RF trans- mission lines, and DC electrical lines. Fig. 2 illustrates the first type of device where a pair of ar- rayed grating waveguides (AWGs) of 10 channel and 10 GHz spacing demultiplexes and multiplexes the signals with 10 sets of amplitude and phase modulators placed in between them to provide dynamic coherent modulation. Fig. 3(a) and (b) shows the detail of the traveling-wave op- tical phase modulator array where the phase velocity of the RF excitation and the group velocity of the optical wave are matched to achieve optimal interactions along the length of the modulator. Segmented H ion implantation suppresses power losses by blocking the current flow along the 2 mm length of the waveguides. Fig. 3(c) shows measurement of electrical ex- citation and optical response (in dB) with 30 GHz bandwidth and Fig. 3(d) shows electrical reflection [in decibels (dB)] ob- tained from a 4 mm long test phase modulator [30]. RF excitation of all the amplitude and phase modulators cast challenges towards high-speed electrical interconnection and packaging while achieving low crosstalk. To provide scalability, we considered RF-over-fiber excitation into optical-to-optical Michelson modulators interfacing with a fiber ribbon cable on silicon V-groove. Fig. 4 shows a schematic drawing of the 1 THz (100 channel 10 GHz) InP-based OAWG device with optically induced modulation. The device structure incorporates a reflec- tion-based geometry in which a single AWG performs both the spectral demultiplexing and multiplexing operations. Here, the AWG passband transmission (Fig. 4 inset on the right bottom) is optimized for isolation of individual comb lines for demulti- plexing, and for overlapping of spectral slices for multiplexing. After demultiplexing the individual comb lines of an OFC onto separate waveguide outputs, reflective-mode Michelson mod- ulators apply amplitude and phase modulation using a push-
    • YOO et al.: TERAHERTZ INFORMATION AND SIGNAL PROCESSING BY RF-PHOTONICS 169 Fig. 4. Schematic of dynamic-OAWG using a reflective-mode device and off-chip optical control signals. DEMUX: demultiplexer; MUX: multiplexer; LD: laser diode; DAC: digital-to-analog converter. Fig. 5. Device layout of a single 100-channel 210-GHz OAWG device. Inset shows two fabricated InP devices from a single 50-mm diameter wafer with a flex circuit attached to one device. FPR: free-propagation region , [31]. pull configuration to each comb line. External optical control signals (e.g., 1310 nm) drive the reflective-mode modulators through the high-reflection/antireflection (HR/AR) coated right facet configured for HR at 1550 nm and AR at 1310 nm. Next, the spectral multiplexer combines the spectral slices (i.e., mod- ulated comb lines) to form the output waveform that has a con- tinuous gapless spectrum. Fig. 5 shows the actual device layout for a single 100 channel 10 GHz InP-based OAWG device that has dimen- sions of 30 mm 35 mm [10], [31]. The inset of Fig. 5 shows a photo of the fabricated device that consists of two devices on a single 50-mm wafer in which the upper and lower devices are mirror images. The cleaved left facet yields 2 1 multi-mode interference (MMI) coupler to serve as the 1550-nm light input and output. This MMI coupler introduces a 6-dB penalty, but avoids the need to use a circulator or other method in order to separate the input and output signals. As Fig. 5 indicates the integrated OAWG contains 1200 independently addressable components and is currently the largest integrated photonic circuits on an InP platform. The AWG has a total of 400 array arms, and each pair of arms has an electro-optic Mach-Zehnder modulator (MZM). High-reso- lution AWGs are especially susceptible to phase errors that can result from imperfections in the fabrication process causing variations in array arm path length [32]. The MZMs enable im- plementing both phase-error correction [33] and the passband shaping [34] necessary to ensure an AWG passband profile suitable for dynamic OAWG such as that shown in the inset of Fig. 4. C. Device Characterization Measurement of the complex transmission (i.e., amplitude and phase) of any component requires using a phase sensitive measurement technique. Here, an optical vector network an- alyzer (OVNA) enabled complex transmission measurement by employing polarization-sensitive, swept-wavelength inter- ferometry with only a single scan of a fast-sweeping tunable laser [35], [36]. The use of an OVNA with high optical spectral resolution (100 MHz here) provides the capability to measure phase errors with an OAWG device in addition to providing a high dynamic range due to its balanced detection [37]. Additionally, the OVNA can be configured to provide real- time updates at several hertz for ease of measuring dynamic transmission variations or applying phase-error correction. The OAWG device supports propagation in both the quasi-transverse-electric (TE) and quasi-transverse-magnetic (TM) polarizations, but full functionality is only available in the TE polarization. This is due to the fact that the [001] ori- entation of the InP crystal symmetry combined with the linear electro-optic effect for a vertical electric bias field influences only the TE polarization. Many components within the OAWG device such as the MMI couplers and QM modulators were also optimized for TE polarization. The spectral response of the two polarizations differed slightly since the TM polarization is faster than the TE polarization by about 0.8 ps/cm due to
    • 170 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 Fig. 6. Measured relative phase of the array waveguides (arms) in the 100- channel210-GHz AWG before and after phase-error correction. birefringence in InP. This provided a means to optimize polar- ization into the OAWG device using a polarization controller at the input and measuring the delay of the impulse response on the OVNA [31]. Successful operation of high-resolution OAWG devices requires that the fabricated AWG has low channel crosstalk. In order to achieve a minimum of channel crosstalk, the phases between the array arms of the AWG need to be matched, which occurs when the path lengths all have a constant difference in length. Any deviations from the desired path length lead to phase errors between the array arms. The application of a reverse bias voltage to the phase-error correction MZMs produces a large electric field across the intrinsic region of the p-i-n diodes. The electric field causes a change to the effective index along the affected array arms causing the total accumu- lated phase on those arms to change. Using the electro-optic effect for phase-error correction is based on a reverse bias diode that only draws a small amount of leakage current on the order of several microamperes ( A). Typically, a radian phase change can be achieved using only an 8 V reverse bias over 4-mm long InGaAsP/InP waveguides with 1.5- m thick depletion regions [31]. Fig. 6 shows the relative phase for the central 200 array waveguides where the phase errors can be most relevant to the AWG crosstalk. The phase values between adjacent array arms are shown modulo 2 to emphasize the slow phase fluctua- tions that are important for AWG performance. The integrated OAWG device is capable of electro-optically phase tuning by more than 2 for phase-error correction. Without phase-error correction the relative phase deviates significantly from the desired zero relative phase across all array arms. However, variations in core-layer composition and thickness and fluctua- tions in the waveguide width as a result of practical fabrication leads to the accumulation of phase errors. After phase-error correction the relative phase across the array arms is reduced, most notably in the range of array waveguides 150–200 in Fig. 6. The standard deviation of the phase errors also reduced from 1.03 radians to 0.33 radians after phase-error correction as a result of removing most of the linear phase and mitigating the slowly varying phase errors. Reducing the AWG phase errors has the effect of improving the crosstalk performance of all 100 OAWG channels. Fig. 7. Single pass transmission of the 100-channel 210-GHz OAWG device after the phase-error correction. After applying phase-error correction the passband profile is reduced to a single peak with 17 dB of crosstalk suppression. This is especially good for high-resolution AWGs that are more susceptible to phase errors due to their larger physical size [10], [38]. Measurements of all 100 channels were performed by mea- suring each channel separately using an OVNA. The average channel bandwidth was 6 GHz and the crosstalk between adja- cent channels measured from the desired transmission peak to the next highest peak varied from a best case of 17 dB to a worst case of 11 dB with an average value of 15.1 dB that has a standard deviation of 1.2 dB. Fig. 7 shows the single pass TE polarization transmission of the full 100-channel 10 GHz OAWG device. All 100 chan- nels are distinguishable, but have slightly different amplitudes since no electrical or optical control signals were present. The lack of control signals results in the Michelson interferometers having random attenuations. Using the Fabry-Perot resonance method [39] to measure waveguide loss in test samples from the same wafer enabled estimation of the total loss through the OAWG device. The doping profile is graded from 10 cm to 5 10 cm to minimize optical losses while providing suf- ficient reverse bias. The loss was estimated to be 1.7–2 dB/cm in which 1 dB/cm is from doping p- and n-types for the p-n junction. This result agrees with the measured round trip inser- tion loss at a channel peak of 43 dB for the TE polarization after accounting for losses from all device components: 11 cm round-trip path through the device ( 22 dB), 2 3 dB loss for the MMI input/output coupler ( 6 dB), 2 6 dB of coupling loss from the lensed fibers ( 12 dB), and a minimal amount of excess loss ( 3 dB). The dynamics of the plasma dispersion effect primarily de- termine the performance of high-speed optically-driven modu- lators. In multiple QWs the modulation depth is mostly indepen- dent of the modulator length and instead the total phase change is proportional to the number of carriers. The phase modulation can also be increased as a result of the step-like density of states [40]. As an example, the penetration depth of 1310 nm light into the QW region is only a few , and is completely ab- sorbed within . As a result, the reflective mode geom- etry enables modulators of 10 gigahertz before the modulation efficiency is significantly degraded from a velocity mismatch
    • YOO et al.: TERAHERTZ INFORMATION AND SIGNAL PROCESSING BY RF-PHOTONICS 171 Fig. 8. Response of the 1310 nm pump laser directly modulated by electrical excitation from 0  10 GHz. (i.e., bidirectionality of optical signal propagation). Currently, optical-to-optical modulation response for a Michelson interfer- ometer is limited by the photo-generated carrier lifetime and its sweepout from the waveguide region. The modulation speed can be improved through H ion implantation which affects the InP crystal lattice by adding defects and vacancies. These additional imperfections serve to reduce the carrier lifetime by providing additional recombination centers for free carriers. Researchers have already demonstrated the potential for carrier lifetimes of less than 1 ps at very high doses of H ion implantation [41]. In our InP OAWG, the H ion implantation at 8 10 ions/cm into the Michelson device 3-dB bandwidth exceeding 1 GHz. Higher doses of H ion implantation can further improve the 3-dB bandwidth, however, the modulation efficiency degrades since the number of carriers is still constant. As a result, a com- promise between modulation efficiency and bandwidth deter- mines the optimal dose. The Michelson modulators are pumped by the 1310 nm pump lasers directly modulated by the RF elec- trical signal. Fig. 8 shows the modulation response extending to 7 GHz. For modulation, Nyquist bandwidth of 5 GHz on each channel is sufficient to create THz dynamic OAWG signal using 10 GHz spacing optical frequency comb. Various integrated OAWG devices based on InP electro-optic modulators, InP optical-optical modulators, and silica thermo-optical modulators have been successfully fabricated. High-quality device implementations allows generation of various waveforms carrying information in various modulation formats, data rates, and spectral widths. Fig. 9 shows OOK and DPSK waveforms generated and de- tected at 360 Gb/s data rates. The agreement between the tar- geted (dots) versus measured (solid) data are reasonably good. Fig. 10 shows 1.2 Tb/s 16 QAM signal with 3 b/s/Hz spectral efficiency using a 120 bit sequence. III. OPTICAL ARBITRARY WAVEFORM MEASUREMENT (OAWM) Inverse process of OAWG allows detection of THz arbi- trary waveform of full amplitude and phase information (or in-phase and quadrature information) by OAWM. OAWM is a waveform characterization technique analogous to OAWG utilizing inverse Fourier transform and coherent detection (in- stead of Fourier transform and coherent modulation in OAWG). Fig. 12(a) shows a schematic of OAWM which is essentially Fig. 9. (b) OOK and (d) DPSK waveforms generated and detected at 360 Gb/s using line-by-line amplitude (‘x’) and phase (‘o’) manipulation of optical fre- quency comb lines for (a) OOK and (c) DPSK. The agreement between the targeted (dots) versus measured (solid) data is reasonably good [28]. Fig. 10. (a) 1.2 Tb/s 16 QAM signal with 3 b/s/Hz spectral efficiency. The con- stellation generated using a limited number of bits (120 bit) showed a reasonably good agreement between (b) targeted constellation and (c) measured constella- tion [28]. The limitation in the constellation chart is due to the limited number of bits (120) of the word length used in the waveform generation. the inverse of dynamic-OAWG, where instead of producing a waveform by creating and combining spectral slices, OAWM splits the signal into spectral slices that are coherently mea- sured. A key element in OAWM is the gapless demultiplexer for spectral slicing of the THz waveform signal so that no infor- mation is lost. The reference OFC is split into isolated lines by spectral demultiplexer with high isolation. Then, these two sig- nals provide the inputs to an array of optical coherent receivers. A digital signal processing (DSP) algorithm reconstructs the signal waveform from the individual slice signals after removing the effects of optical and electrical filters in the measurement system (i.e., spectral and frequency responses of demultiplexers, photoreceivers, etc.). If the waveform data is recorded continuously, the spectral resolution is limited only by the reference OFC stability and the optical coherent receiver performance. Integrated circuit implementation provides the stability required for the coherent process. Just as for OAWG, in OAWM it is critical that the spectral slices remain coherent with one another. Therefore the first generation of OAWM employed a specially designed silica planar lightwave circuit (PLC) which integrates all passive
    • 172 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 Fig. 11. Optical arbitrary waveform measurement (OAWM). BPD: balanced photodiodes. Fig. 12. (a) Mask layout and (b) photograph of the planar lightwave integrated circuit (PLC) with 640 GHz spectral bandwidth (16 240 GHz) containing two spectral demultiplexers and 16 sets of optical hybrids for coherent detection. Balanced detectors are placed external to the silica PLC. optical components necessary to implement the spectral slicing and optical coherent detection. Balanced detection (i.e., homo- dyne down-conversion) and digitizing occurs off-chip using dc-coupled high-speed balanced photodetectors connected to the inputs of high-speed digitizers. As Fig. 12(a) shows, the OAWM PLC incorporates two arrayed-waveguide grating (AWG) spectral demultiplexers, both aligned so that their transmission peaks are matched to the reference OFC. The OAWM PLC can accommodate a total of 640 GHz of optical bandwidth. The AWGs have 16 outputs with a 40-GHz spacing and there are sixteen 90 -optical hybrids. Single-mode fiber arrays couple signals in and out of the PLC. The gapless AWG Fig. 13. OAWM measurement of a train of transform-limited pulses with an overall sawtooth amplitude envelope (500 ns period). (a) The reconstructed signal waveform showing the full 2-s record length. (b) Detail of a 400 ps portion from (a) showing the intensity (solid) and phase (dashed). slices the signal into slightly overlapping spectral slices and directs each slice to a different optical hybrid. The high-iso- lation AWG demultiplexes and directs each comb line to the reference (LO) input of the corresponding optical hybrid. Then, each hybrid combines a spectral slice in four quadrature phases with a reference line. The hybrid outputs are coupled off chip to balanced photodiode pairs which generate the and signals. Then the signals are digitized at 20 GHz electrical bandwidth. OAWM enables measurements with high temporal resolu- tion over long record lengths. Using the fabricated 0.64 THz bandwidth Silica OAWM device shown in Fig. 12(b) coupled to eight pairs of balanced photodetectors and eight 50 GS/s digitizers we demonstrated 160 GHz (i.e., four spectral slices) of instantaneous bandwidth for 2- s record lengths. Fig. 13(a) shows the temporal intensity of a long waveform composed of a -like waveform (160 GHz of optical bandwidth) repeating every 100 ps, modulated with a relatively slowly varying saw- tooth intensity envelope. The envelope is created by driving the 2 2 MZM with a voltage ramp from an electronic arbitrary waveform generator which is switched on in less than 100 ps and then slowly switched off over a 500-ns duration. Fig. 13(b) shows the intensity and phase of the waveform for a 400-ps window of the time record at a fast rising transition of the ramp. Within the window, the waveform abruptly switches on. Each pulse has sub-pulses with phase alternating between 0 and rad (i.e., alternating between negative and positive fields). The 6.25-ps sampling interval (320 GS/s aggregate) determines the minimum resolvable temporal feature, as shown by the 6-ps wide pulse peak. This measurement demonstrates a record-length-to-resolution ratio of over 320 000 (6.25-ps resolution and a 2- s record length). IV. RECONFIGURABLE RF-PHOTONIC FILTERS One of the key challenges in THz signal synthesis and analysis is overcoming the nonuniform frequency response of the modulators and multiplexers in OAWGs and detectors and demultiplexers in OAWMs. Fig. 14 illustrates the effect.
    • YOO et al.: TERAHERTZ INFORMATION AND SIGNAL PROCESSING BY RF-PHOTONICS 173 Fig. 14. Synthesis of THz waveforms overcoming frequency multiplexer’s non-uniform responses. Fig. 15. (a) Schematic and photo of a single unit cell, red boxes indicate phase shifter electrodes. (b) Schematic of a four-unit-cell filter. Traditional methods rely on frequency independent power splitters/combiners in place of optical frequency demux/muxes, and sophisticated digital signal processing (DSP), but these methods are power hungry and limited to low bandwidth ( THz) applications. Fig. 15 illustrates photonic lattice filters [42]–[44] to achieve RF domain signal processing using optical filtering of arbitrary shape via reconfiguration of the various components in the pho- tonic lattice filter. The Unit Cell shown in Fig. 15(a) with micro- Fig. 16. (bottom) Measured transmission of a single unit cell configured as an optical equalizer for the H output (H is complementary). Inset shows fitted pole and zero. (top) Corresponding impulse response from time domain measurements [42]–[44]. resonator with 10 GHz free spectral range (FSR) is repeated in series to synthesize a programmable cascaded lattice filter shown in Fig. 15(b). The inset photo of Fig. 15(a) illustrates the actual device fabricated on a silicon photonic platform. Similarly to a digital filter, the optical lattice filter can be pro- grammed and reconfigured by tuning each element on each unit cell, which will in turn control the positions of zeros and poles. Fig. 16 shows (a) measured transmission of a single unit cell configured as an optical equalizer for the output ( is complementary). Inset shows fitted pole and zero, and (b) corre- sponding impulse response output ( is complementary). Instead of using electronic DSP, it is possible to utilize the programmable lattice filter which has the matching FSR (10 GHz) with the OFC spacing of OAWG to achieve channel equalization without consuming power in electronics. V. OPTICAL VERSUS ELECTRICAL EQUALIZATION The variation of factor as a function of DAC ENOB for both optical and electrical equalization was investigated via simula- tions. These simulations started with a 5 ns, 100 GHz BPSK waveform (1500 bits) generated using a raised cosine modula- tion filter with . Next, Gaussian white noise was added to both the in-phase and quadrature-phase com- ponents, which resulted in an SNR of 15 dB. At this point, the total 100 GHz spectrum was divided into ten 10 GHz spectral slices. For the electrical equalization case, multiplexer and modulator pre-emphasis was applied, the in-phase and quadra- ture-phase components of the waveform were quantized, and then the inverse multiplexer and modulator preemphasis was applied. The optical equalization case involved only quan- tization of the spectral slices. The -factor was then estimated from the constellation diagrams after reassembling the quan- tized spectral slices into the aggregate waveform. Fig. 17 shows the simulation results for both the optical and electrical equalization cases. For all DAC ENOB values, the optical equalization shows a higher -factor. This is since the
    • 174 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 Fig. 17. Simulation result showing Q-factor as a function of DAC ENOB for optical and electrical equalization. Simulations assume a 10 channel 210 GHz OAWG device and BPSK modulation format. electrical equalization case required accounting for the mul- tiplexer and modulator preemphasis before quantization, which lowered the dynamic range achievable with the DAC outputs. The advantage of using optical equalization is more pronounced at lower ENOB values, but is still present at high ENOB values. The presence of a ‘knee’ around a DAC ENOB of 5 indicates a reduction in the improvement of the -factor for increases in DAC ENOB. Insets show the progression of the constellation diagrams for the optical and electrical equalization cases. VI. CONCLUSION Exploiting benefits from both RF and photonics can yield high-fidelity and high-precision information generation, detec- tion, and processing at possibly THz bandwidths and beyond. A powerful combination of the optical technology offering wavelength parallelism, high bandwidth, and low energy con- sumption with electronic technology offers agile processing and storage capabilities as demonstrated by high fidelity optical arbitrary waveform generation and measurement (OAWG and OAWM) at THz bandwidths [8]–[10]. RF-Photonic signal processing utilizing optical lattice filter allows precise and low power RF processing at MHz 10 GHz frequencies covering THz total bandwidth. The optical lattice filters are capable of creating reconfigurable filter shapes in the RF domain without involving power hungry RF electronics. The combination of the two regimes of RF-photonics offers agility, precision, and high resolution to THz bandwidth signal generation, detection, and processing. Applications of such THz capabilities span a very wide range of new opportunities including LIDAR/LADAR [15], [16], THz ADC, THz DAC [20], and coherent communi- cations for free-space, satellite, and optical fiber networks. ACKNOWLEDGMENT The author is grateful for the work from the Next Generation Networking Systems Group at University of California at Davis, in particular, T. Su, S. Djordjevic, S. Ibrahim, X. Cai, B. Guan, J. Baek, and the collaborators at Multiplex, Royal Institute of Technology, Sweden, and Nistica. REFERENCES [1] H.-W. Hubers, “Terahertz technology: Towards THz integrated pho- tonics,” Nature Photon., vol. 4, pp. 503–504, 2010. [2] P. H. Siegel, “Terahertz technology,” IEEE Trans. Microw. Theory Techn., vol. 50, no. 3, pp. 910–928, Mar. 2002. [3] P. H. Siegel, “Terahertz technology in biology and medicine,” IEEE Trans. Microw. Theory Techn., vol. 52, no. 10, pp. 2438–2447, Oct. 2004. [4] C. Jansen, S. Wietzke, O. Peters, M. Scheller, N. Vieweg, M. Salhi, K. Krumbholz, C. Jordens, T. Hochrein, and M. Koch, “Terahertz imaging: Applications and perspectives,” Appl. Opt., vol. 49, pp. E48–E57, 2010. [5] M. C. Wanke, E. W. Young, C. D. Nordquist, M. J. Cich, A. D. Grine, C. T. Fuller, J. L. Reno, and M. Lee, “Monolithically integrated solid-state terahertz transceivers,” Nature Photon., vol. 4, pp. 565–569, 2010. [6] R. C. Williamson and R. D. Esman, “RF photonics,” J. Lightw. Technol., vol. 26, no. 9, pp. 1145–1153, May 1, 2008. [7] S. Ibrahim, N. K. Fontaine, S. S. Djordjevic, B. Guan, T. Su, S. Cheung, R. P. Scott, A. T. Pomerene, L. L. Seaford, C. M. Hill, S. Danzinger, Z. Ding, K. Okamoto, and S. J. B. Yoo, “Demonstration of a fast-reconfigurable silicon CMOS optical lattice filter,” Opt. Express, vol. 19, pp. 13245–13256, 2011. [8] D. J. Geisler, N. K. Fontaine, R. P. Scott, and S. J. B. Yoo, “Demonstra- tion of a flexible bandwidth optical transmitter/receiver system scalable to terahertz bandwidths,” IEEE Photon. J., vol. 3, no. 6, pp. 1013–1022, Dec. 2011. [9] R. P. Scott, N. K. Fontaine, Z. Linjie, F. M. Soares, J. P. Heritage, and S. J. B. Yoo, “Continuous, real-time, full-field waveform measure- ments via spectral slicing and parallel digital coherent detection,” in 2010 Conf. Opt. Fiber Commun.—OFC 2010. Collocated Nat. Fiber Opt. Eng. Conf. (OFC/NFOEC 2010), 2010, p. 3. [10] F. M. Soares, N. K. Fontaine, R. P. Scott, J.-H. Baek, X. Zhou, T. Su, S. Cheung, Y. Wang, C. Junesand, S. Lourdudoss, K. Y. Liou, R. A. Hamm, W. Wang, B. Patel, L. A. Gruezke, W. T. Tsang, J. P. Her- itage, and S. J. B. Yoo, “Monolithic InP 100-channel 210-GHz optical arbitrary waveform generation device,” IEEE Photon. Technol. Lett., no. , pp. –, 2011. [11] S. S. Djordjevic, L. W. Luo, S. Ibrahim, N. K. Fontaine, C. B. Poitras, B. Guan, L. Zhou, K. Okamoto, Z. Ding, M. Lipson, and S. J. B. Yoo, “Fully reconfigurable silicon photonic lattice filters with four cascaded unit cells,” IEEE Photon. Technol. Lett., vol. 23, no. , pp. 42–44, 2011. [12] E. M. Dowling and D. L. MacFarlane, “Lightwave lattice filters for optically multiplxed communication systems,” J. Lightw. Technol., vol. 12, no. 3, pp. 471–486, Mar. 1994. [13] J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave pho- tonic filters,” J. Lightw. Technol., vol. 24, no. 1, pp. 201–229, Jan. 2006. [14] R. A. Minasian, “Photonic signal processing of microwave signals,” IEEE Trans. Microw. Theory Techn., vol. 54, no. 2, pp. 832–846, Feb. 2006. [15] P. Adany, C. Allen, and R. Q. Hui, “Chirped Lidar using simplified ho- modyne detection,” J. Lightw. Technol., vol. 27, no. 16, pp. 3351–3357, Aug. 15, 2009. [16] S. M. Beck, J. R. Buck, W. F. Buell, R. P. Dickinson, D. A. Kozlowsky, N. J. Marechal, and T. J. Wright, “Synthetic-aperture imaging laser radar: Laboratory demonstration and signal processing,” Appl. Opt., vol. 44, pp. 7621–7629, 2005. [17] G. C. Valley, “Photonic analog-to-digital converters,” Opt. Express, vol. 15, pp. 1955–1982, 2007. [18] J. Kim, M. J. Park, M. H. Perrott, and F. X. Kartner, “Photonic sub- sampling analog-toigital conversion of microwave signals at 40-GHz with higher than 7-ENOB resolution,” Opt. Express, vol. 16, pp. 16509–16515, 2008. [19] Y. Han and B. Jalali, “Photonic time-stretched analog-to-digital con- verter: Fundamental concepts and practical considerations,” J. Lightw. Technol., vol. 21, no. 12, pp. 3085–3103, Dec. 2003. [20] Y. Peng, H. Zhang, Y. Zhang, and M. Yao, “Photonic digital-to-analog converter based on summering of serial weighted multiwavelength pulses,” IEEE Photon. Technol. Lett., vol. 20, no. , pp. 2135–2137, 2008. [21] V. W. S. Chan, “Free-space optical communications,” J. Lightw. Technol., vol. 24, no. 14, pp. 4750–4762, Jul. 15, 2006.
    • YOO et al.: TERAHERTZ INFORMATION AND SIGNAL PROCESSING BY RF-PHOTONICS 175 [22] A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using diversity-combining techniques,” Opt. Express, vol. 17, pp. 12601–12611, 2009. [23] N. K. Fontaine, “Optical arbitrary waveform generation and measure- ment,” Ph.D., Dep. Elect. Comput. Eng., Univ. of California, Davis, 2010. [24] N. K. Fontaine, R. P. Scott, J. Cao, A. Karalar, W. Jiang, K. Okamoto, J. P. Heritage, B. H. Kolner, and S. J. B. Yoo, “32 phase 2 32 am- plitude optical arbitrary waveform generation,” Opt. Lett., vol. 32, pp. 865–867, Apr. 2007. [25] R. P. Scott, N. K. Fontaine, J. Cao, K. Okamoto, B. H. Kolner, J. P. Heritage, and S. J. B. Yoo, “High-fidelity line-by-line optical waveform generation and complete characterization using FROG,” Opt. Express, vol. 15, pp. 9977–9988, Aug. 6, 2007. [26] R. P. Scott, N. K. Fontaine, J. P. Heritage, and S. J. B. Yoo, “Dynamic optical arbitrary waveform generation and measurement,” Optics Ex- press, vol. 18, pp. 18655–18670, Aug. 2010. [27] N. K. Fontaine, R. P. Scott, and S. J. B. Yoo, “Dynamic optical ar- bitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications,” Opt. Commun., vol. 284, pp. 3693–3705, Jul. 2011. [28] S. J. B. Yoo, “Flexible bandwidth terabit coherent optical communica- tion networks by optical arbitrary waveform generation and measure- ment,” IEEE Photon. Soc. Newsletter, vol. 25, pp. 5–12, Oct. 1, 2011. [29] N. K. Fontaine, R. P. Scott, and S. J. B. Yoo, “Dynamic optical ar- bitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications,” Opt. Commun., vol. 284, pp. 3693–3705, 2011. [30] S.-W. Seo, J. Yan, J.-H. Baek, F. M. Soares, R. Broeke, A.-V. Pham, and S. J. B. Yoo, “Microwave velocity and impedance tuning of traveling-wave modulator using ion implantation for monolithic integrated photonic systems,” Microw. Opt. Technol. Lett., vol. 50, pp. 2151–2155, Aug. 2008. [31] F. M. Soares, N. K. Fontaine, R. P. Scott, J. H. Baek, X. Zhou, T. Su, S. Cheung, Y. Wang, C. Junesand, S. Lourdudoss, K. Y. Liou, R. A. Hamm, W. Wang, B. Patel, L. A. Gruezke, W. T. Tsang, J. P. Heritage, and S. J. B. Yoo, “Monolithic InP 100-channel 2 10-GHz device for optical arbitrary waveform generation,” IEEE Photon. J.., vol. 3, no. 6, pp. 975–985, Dec. 2011. [32] H. Yamada, K. Yakada, and S. Mitachi, “Crosstalk reduction in a 10-GHz spacing arrayed-waveguide grating by phase-error compensation,” J. Lightw. Technol., vol. 16, no. 3, pp. 364–371, Mar. 1998. [33] S.-W. Seo, F. M. Soares, J.-H. Baek, W. Jiang, N. K. Fontaine, R. P. Scott, C. Yang, D. J. Geisler, J. Yan, R. G. Broeke, J. Cao, F. Olsson, S. Lourdudoss, A. Pham, and S. J. B. Yoo, “Monolithically integrated InP photonic micro systems on a chip for O-CDMA and OAWG appli- cations,” in Proc. Photon. in Switching, San Francisco, CA, 2007. [34] N. K. Fontaine, J. Yang, W. Jiang, D. J. Geisler, K. Okamoto, R. Huang, and S. J. B. Yoo, “Active arrayed-waveguide grating with amplitude and phase control for arbitrary filter generation and high-order dispersion compensation,” in 34th Eur. Conf. Opt. Commun. (ECOC 2008), 2008, p. Mo.4.C.3. [35] G. D. VanWiggeren and D. M. Baney, “Swept-wavelength interfero- metric analysis of multiport components,” IEEE Photon. Technol. Lett., vol. 15, no. , pp. 1267–1269, 2003. [36] D. K. Gifford, B. J. Soller, M. S. Wolfe, and M. E. Froggatt, “Op- tical vector network analyzer for single-scan measurements of loss, group delay, and polarization mode dispersion,” Appl. Opt;, vol. 44, pp. 7282–7286, 2005. [37] K. Takada and S. Satod, “Method for measuring the phase error dis- tribution of a wideband arrayed waveguide grating in the frequency domain,” Opt. Lett., vol. 31, pp. 323–325, 2006. [38] W. Jiang, K. Okamoto, F. M. Soares, S. Lourdudoss, and S. J. B. Yoo, “5 GHz chanel spacing InP-based 32-channel arrayed-waveguide grating,” in Proc. of OFC 2009, 2009. [39] T. Feuchter and C. Thirstrup, “High precision planar waveguide prop- agation loss measurement technique using a Fabry-Perot cavity,” IEEE Photon. Technol. Lett., vol. 6, no. , pp. 1244–1246, 1994. [40] S. Jong-In, M. Yamaguchi, P. Delansay, and M. Kitamura, “Refractive index and loss changes produced by current injection in InGaAs(P)- InGaAsP multiple quantum-well (MQW) waveguides,” IEEE J. Sel. Topics Quantum Electron., vol. 1, no. , pp. 408–415, 1995. [41] K. F. Lamprecht, S. Juen, L. Palmetshofer, and R. A. Hopfel, “Ultra- short carrier lifetimes in H+ bombarded InP,” Appl. Phys. Lett., vol. 59, pp. 926–928, 1991. [42] S. Ibrahim, N. K. Fontaine, S. S. Djordjevic, B. Guan, T. Su, S. Cheung, R. P. Scott, A. T. Pomerene, L. L. Seaford, C. M. Hill, S. Danziger, Z. Ding, K. Okamoto, and S. J. B. Yoo, “Demonstration of a fast-recon- figurable silicon CMOS optical lattice filter,” Opt. Express, vol. 19, pp. 13245–13256, Jul. 4, 2011. [43] S. S. Djordjevic, L. W. Luo, S. Ibrahim, N. K. Fontaine, C. B. Poitras, B. Guan, L. Zhou, K. Okamoto, Z. Ding, M. Lipson, and S. J. B. Yoo, “Fully reconfigurable silicon photonic lattice filters with four cascaded unit cells,” IEEE Photon. Technol. Lett., vol. 23, pp. 42–44, Jan. 1, 2011. [44] S.-E. Ibrahim, L.-W. Luo, S. S. Djordjevic, C. B. Poitras, L. Zhou, N. K. Fontaine, B. Guan, S. Cheung, Z. Ding, K. Okamoto, M. Lipson, and S. J. B. Yoo, “Fully reconfigurable silicon photonic lattice filters with four cascaded unit cells,” presented at the Optical Fiber Commun. Conf., San Diego, CA, 2010. S. J. Ben Yoo (S’82–M’84–SM’97–F’07) received the B.S. degree in electrical engineering (with dis- tinction), the M.S. degree in electrical engineering, and the Ph.D. degree in electrical engineering with minor in physics, all from Stanford University, Stan- ford, CA, in 1984, 1986, and 1991, respectively. He currently serves as Professor of Electrical Engineering at University of California at Davis (UC Davis). His research at UC Davis includes RF-pho- tonic devices, systems, and networking technologies for the future generation Internet and computing systems. In particular, he is conducting research on architectures, systems integration, and network experiments related to all-optical label switching routers, terahertz optical arbitrary waveform generation, flexible bandwidth networking, and optical interconnect technologies. Prior to joining UC Davis in 1999, he was a Senior Research Scientist at Bell Communications Research (Bellcore), leading technical efforts in optical networking research and systems integration. His research activities at Bellcore included optical-label switching for the next-generation Internet, power transients in reconfigurable optical networks, wavelength interchanging cross connects, wavelength converters, vertical-cavity lasers, and high-speed modulators. He also participated in the advanced technology demonstration network/multiwavelength optical net- working (ATD/MONET) systems integration, the OC-192 synchronous optical network (SONET) ring studies, and a number of standardization activities which led to documentations of Generic Requirements, GR-2918-CORE (1999), GR-2918-ILR (1999), GR-1377-CORE (1995), and GR-1377-ILR (1995) on dense WDM and OC-192 systems. Prior to joining Bellcore in 1991, he conducted research at Stanford University on nonlinear optical processes in quantum wells, a four-wave-mixing study of relaxation mechanisms, and ultrafast diffusion-driven photodetectors. During this period, he also conducted research on lifetime measurements of intersubband transitions and on nonlinear optical storage mechanisms at Bell Laboratories and IBM Research Laborato- ries, respectively. Prof. Yoo is a Fellow of IEEE Photonics Society, a Fellow of the Optical So- ciety of America (OSA), and a Member of Tau Beta Pi. He is a recipient of the DARPA Award for Sustained Excellence in 1997, the Bellcore CEO Award in 1998, the Outstanding Mid-Career Research Faculty Award (UC Davis) in 2004, and the Outstanding Senior Research Faculty Award (UC Davis) in 2011. Prof. Yoo also served as Associate Editor for IEEE PHOTONICS TECHNOLOGY LETTERS, Guest Editor for IEEE/OSA JOURNAL OF LIGHTWAVE TECHNOLOGY and IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, and Gen- eral Co-Chair for Photonics in Switching conference 2007, 2010, and 2012. Ryan P. Scott (S’93–M’03) received the B.S. degree in laser electrooptics technology from the Oregon Institute of Technology, Klamath Falls, in 1991, the M.S. degree in electrical engineering from the University of California, Los Angeles (UCLA), in 1995 and the Ph.D. degree in electrical and computer engineering from the University of California, Davis (UC Davis) in 2009. He is currently a Post-Doctoral Scholar at UC Davis. His present research interests include optical code-division multiple-access technologies, optical arbitrary waveform generation and measurement, and optical comb generation.
    • 176 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 2, NO. 2, MARCH 2012 David J. Geisler (S’02) was born in New York in 1982. He received the B.S. degree in electrical engi- neering from Tufts University, Medford, MA, in 2004 and the M.S. degree in electrical engineering from the University of California, Davis, in 2009, where he is currently working toward the Ph.D. degree in elec- trical and computer engineering. His research interests include optical arbitrary waveform generation and measurement, enabling technologies for flexible bandwidth networking, and advanced modulation formats. Nicolas K. Fontaine (S’02–M’10) received double B.S. degrees in electrical engineering and also in op- tical engineering, from the University of California, Davis (UC Davis) in 2004, the M.S. degree in elec- trical and computer engineering from UC Davis, in 2007, and the Ph.D. degree in electrical and computer engineering from UC Davis in 2010. He is currently a Member of Technical Staff at Bell Laboratories, Alcatel-Lucent, Holmdel, NJ. Francisco M. Soares (S’99–M’05) received the M.S. degree in electrical engi- neering from the Delft University of Technology, the Netherlands, in 2000, and the Ph.D. degree from the Technical University of Eindhoven, the Netherlands, in 2006. The topic of his Ph.D. dissertation was on the design, fabrication and characterization of InP-based photonic-integrated devices and circuits. After that, he worked 4 years as a Post-Doctoral Researcher at the University of California at Davis. He is currently working in the Photonic-Components Department of the Fraunhofer Heinrich-Hertz Institute, Berlin, Germany, where his main efforts are devoted to large-scale integration of InP-based photonic devices.