Enhanced processing gain via pulse polarity switching in an RF photonic phase filter

Transcription

1 Vol. 24, o Oct 2016 OPTICS EXPRESS Ehaced processig gai via pulse polarity switchig i a RF photoic phase filter HYOUG-JU KIM* AD ADREW M. WEIER School of Electrical ad Computer Egieerig, Purdue Uiversity, 465 orthwester Aveue, West Lafayette, Idiaa , USA * sju27@gmail.com Abstract: Fast pulse polarity switchig is proposed ad demostrated to ehace processig gai i a comb-based radio-frequecy photoic phase filter. The polarity switchig scheme overcomes previous limits o time badwidth product ad processig gai based o the umber of optical frequecy comb lies. I a experimet with broadbad jammig oise, the peak sigal-to-oise ratio of compressed RF output pulses is improved by ~30 db compared to the iput average sigal-to-oise ratio Optical Society of America OCIS codes: ( ) Radio frequecy photoics; ( ) Pulse compressio. Refereces ad liks M. I. Skolik, Radar Hadbook (McGraw-Hill, 1990). M. K. Simo, J. K. Omura, R. A. Scholtz, ad B. K. Levitt, Spread Spectrum Commuicatios Hadbook (McGraw-Hill, 1994). R. Brocato, J. Skier, G. Wouters, J. Wedt, E. Heller, ad J. Blaich, Ultra-widebad SAW correlator, IEEE Tras. Ultraso. Ferroelectr. Freq. Cotrol 53(9), (2006). M. Bolea, J. Mora, B. Ortega, ad J. Capmay, Highly chirped sigle-badpass microwave photoic filter with recofiguratio capabilities, Opt. Express 19(5), (2011). M. Sog, V. Torres-Compay, R. Wu, A. J. Metcalf, ad A. M. Weier, Compressio of ultra-log microwave pulses usig programmable microwave photoic phase filterig with > 100 complex-coefficiet taps, Opt. Express 22(6), (2014). H.-J. Kim, A. Rashidiejad, ad A. M. Weier, Low-loss ultrawidebad programmable RF photoic phase filter for spread spectrum pulse compressio, IEEE Tras. Microw. Theory Tech. 62(12), (2015). M. Li, M. Atoio, L. Sophie, J. P. Yao, ad J. Azaa, Recofigurable ad sigle-shot chirped microwave pulse compressio usig a time-spectrum covolutio system, i It. Topical Meetig Microw. Photo. Cof. (IEEE, 2011), pp C. Wag ad J. P. Yao, Chirped microwave pulse compressio usig a photoic microwave filter with a oliear phase respose, IEEE Tras. Microw. Theory Tech. 57(2), (2009). E. Hamidi ad A. M. Weier, Phase-oly matched filterig of ultrawidebad arbitrary microwave waveforms via optical pulse shapig, J. Lightwave Techol. 26(15), (2008). Y. Li, A. Dezfooliya, ad A. M. Weier, Photoic sythesis of spread spectrum radio frequecy waveforms with arbitrarily log time apertures, J. Lightwave Techol. 32(20), (2014). A. Rashidiejad, D. E. Leaird, ad A. M. Weier, Ultrabroadbad radio-frequecy arbitrary waveform geeratio with high-speed phase ad amplitude modulatio capability, Opt. Express 23(9), (2015). H.-J. Kim, D. E. Leaird, A. J. Metcalf, ad A. M. Weier, Comb-based RF photoic filters based o iterferometric cofiguratio ad balaced detectio, J. Lightwave Techol. 32(20), (2014). C. Fiot, B. Kibler, L. Provost, ad S. Wabitz, Beeficial impact of wave-breakig for coheret cotiuum formatio i ormally dispersive oliear fibers, J. Opt. Soc. Am. B 25(11), (2008). V. K. Igle ad J. G. Proakis, Digital Sigal Processig Usig MATLAB (CEGAGE Learig, 2012). K.. Madse, T. D. Gathma, S. Daeshgar, T. C. Oh, J. C. Li, ad J. F. Buckwalter, A High-Liearity, 30 GS/s Track-ad-Hold Amplifier ad Time Iterleaved Sample-ad-Hold i a IP-o-CMOS Process, IEEE J. Solid-State Circuits 50(11), (2015). S. E. Turer, Jr., R. B. Elder, D. S. Jase, ad D. E. Kotecki, 4-bit adder-accumulator at 41-GHz clock frequecy i IP DHBT techology, IEEE Microw. Wirel. Compo. Lett. 15(3), (2005). Y. Li, A. Rashidiejad, J.-M. Wu, D. E. Leaird, J.-W. Shi, ad A. M. Weier, Photoic geeratio of W-bad arbitrary waveforms with high time-badwidth products eablig 3.9mm Rage Resolutio, Optica 1(6), (2014). E. Hamidi, D. E. Leaird, ad A. M. Weier, Tuable programmable microwave photoic filters based o a optical frequecy comb, IEEE Tras. Microw. Theory Tech. 58(11), (2010). # Joural Received 19 Aug 2016; revised 28 Sep 2016; accepted 10 Oct 2016; published 28 Oct 2016

2 Vol. 24, o Oct 2016 OPTICS EXPRESS Itroductio Spread spectrum radio-frequecy (RF) systems are well kow ad widely applied [1,2]. I civilia cellular radio, spread spectrum eables multiple-access through its iterferece suppressio property. I defese electroics spread spectrum is valued for its low power spectral desity, which imparts covertess, ad for its ability to redistribute a arrowbad sigal over a much larger badwidth, thereby givig jammig resistace. I chirped radar spread spectrum sigals provide low peak power i both time ad frequecy, importat i the typical sceario of a peak-power-limited trasmitter, while retaiig both high pulse eergy (for good detectability) ad high badwidth (which coverts to high time ad rage resolutio with a pulse compressio receiver). The ability to spread a sigal over a large frequecy rage at low power spectral desity may provide a aveue toward operatio i a highly cogested RF spectrum with reduced iterferece. I geeral, large spectral spreadig factor (termed processig gai) is desirable for all spread spectrum applicatios. From a time domai perspective, the processig gai is closely related to the time-badwidth product (TBP) of the spread spectrum sigal ad the achievable pulse compressio ratio. Curret spread spectrum systems are typically costraied to RF badwidths of hudreds of MHz or below. Oe of the key bottleecks is the implemetatio of pulse compressio receivers at frequecies above approximately 1 GHz [3]. Photoics is fudametally capable of deliverig ad processig larger sigal badwidths compared to electroics ad with greater sigal itegrity. Recetly schemes based o RF photoics that achieve phase filterig ad compressio of high frequecy spread spectrum sigals have bee demostrated [4 9]. To date the highest TBP reported was 75 [4]. Scalig to substatially higher TBPs is difficult ad is limited, for example, by the umber of comb lies i RF photoics filterig schemes based o optical frequecy combs [5,6] as well as by the complexity provided by optical spectral shapig ad filterig devices [4,7 9]. I this paper we itroduce a ovel hybrid spread spectrum scheme that overlays coordiated photoic-electroic processig o top of our previously reported, comb-based, recofigurable RF photoic phase filterig approach [6]. The ew scheme preserves the advatages of photoic spectral processig while scalig to essetially ulimited processig gais. We experimetally demostrate a processig gai of ~103, which allows us to achieve 30 db sigal-to-iterferece improvemet i a experimet with broadbad oise jammig. 2. Hybrid processig cocept The proposed hybrid spread spectrum scheme is illustrated i Fig. 1. Figure 1(a) shows best curret practice: a waveform which we call a chip, with badwidth B, duratio T (a few s to perhaps tes of s) ad TBP below 100, is geerated at the trasmitter. The receiver employs a RF photoic phase filter to compress this chip ito a pulse with duratio ~B 1, with correspodig peak power ehacemet. Such operatio has bee demostrated previously, e.g [6]. I order ow to expad the TBP ad processig gai as required, we cocateate a series of chips ito a spread spectrum frame, i which idividual chips with temporal profile v c (t) are phase modulated accordig to a suitable pseudoradom code uder electroic cotrol. The RF waveform w(t) geerated at the trasmitter is ow writte wt () = av( t mt) (1) m c m= 1 where the am are a series of complex amplitudes correspodig to the pseudoradom code. Figure 1(b) depicts the case where the am take o biary phase shift values ± 1; however, other schemes (e.g., quadrature phase shiftig) are also possible. ote that pseudoradom phase codes such as P sequeces should be used for the a m i order to preserve the importat low power spectral desity property of the spread spectrum sigal; i cotrast,

3 Vol. 24, o Oct 2016 OPTICS EXPRESS repeatig idetical copies of the spread spectrum chip (settig all the a m = 1) would lead to udesirable peakig i the RF spectrum. The time aperture of the spread spectrum frame is ow T, ad its TBP is BT, i.e., expaded times compared to the TBP of a sigle chip. Such spread spectrum frames composed of cocateated phase switched chips ca be geerated either by a electroic waveform geerator or through optical geeratio schemes such as [10,11]. Fig. 1. Hybrid spread spectrum scheme employig photoic spectral processig with polarity switchig for large processig gai. (a) Chirped pulse compressio for a sigle chip, (b) Polarity-switched chirped waveform sequece, (c) Chirped pulse compressio without polarity switchig, (d) Chirped pulse compressio with polarity switchig for despreadig the phasecoded waveform. ow at the receiver, we may cosider processig to ivolve two steps. I the first step, Fig. 1(c), the waveform is agai preseted to the RF photoic spectral phase filter which achieves compressio of the idividual chips. This results i a series of RF pulses of duratio 1/B, but the polarities remai modulated. At this poit we ca write the received ad partially processed RF waveform r 1 (t) as r1 () t = a v ( t mt) (2) m comp m= 1 where v comp (t) is the waveform of a sigle chip after compressio. I the secod step, Fig. 1(d), the idividual chips are modulated by the cojugate of the pseudoradom spreadig code, which despreads the spread spectrum frame such that the polarities of the compressed chips are ow all the same. At the this stage of processig, the waveform is writte

4 Vol. 24, o Oct 2016 OPTICS EXPRESS * r2 () t = a a v ( t mt) (3) m m comp m= 1 (For a pseudoradom phase spreadig code, we have the coefficiets a m 2 = 1.) These two processig steps could be performed i either order or eve simultaeously. I our experimet the steps show i Figs. 1(c) ad 1(d) are performed simultaeously i the RF photoic phase filter, which is modified as discussed below to accomplish this task. The series of compressed pulses may ow be sampled; the additioal processig gai is achieved by itegratig the sampled sigal either i hardware or i digital sigal processig. I this paper we use a simple tapped delay lie filter implemeted via off-lie processig, where to obtai the fial processed sigal r fial (t),we compute the simple sum fial 2 (4) m= 1 () = ( ) r t r t mt I this way the processig gai attaiable with the sigle chip photoic-assisted compressio (BT) is multiplied up by the umber of chips () to achieve processig gai BT. The umber of chips ad the ehacemet of the processig gai ca potetially be very large. 3. Comb-based RF photoic phase filter Figure 2 shows the cofiguratio of the RF photoic phase filter with pulse polarity switchig. The broadbad optical frequecy comb is directed to the optical iterferometer. Oe arm is set to a erbium-doped fiber amplifier (EDFA) through a optical phase modulator drive by the cotrol sigal ad a programmable pulse shaper, which are used to switch the pulse polarity ad program the filter taps, respectively; The other passes through a Mach-Zehder modulator (MZM) drive by the RF iput sigal of iterest. The MZM is biased at the miimum trasmissio poit, which suppresses both the optical carriers ad optical oise [12]. The outputs of the EDFA ad MZM are combied by a 2 2 optical coupler. The two outputs of the 2 2 optical coupler are coected to the balaced photodetector (BPD) through a bidirectioal fiber cofiguratio icludig a sigle dispersive fiber ad two optical circulators [12]. This simultaeously matches the dispersio ad the time delay of the paths betwee the 2 2 coupler ad the BPD. Fig. 2. Cofiguratio of the RF photoic phase filter. (EDFA: erbium-doped fiber amplifier; MZM: Mach Zehder modulator; BPD: balaced photodetector) The cofiguratio is similar to that employed i our previous RF photoic phase filterig experimet [6], except for the additio of the phase modulator i the upper arm. The filter trasfer fuctio ca be expressed as 2 RF ( ωrf, ) H t P e j [ Δ ωψω ( ± τ) ϕ + θ( t) ] (5)

5 Vol. 24, o Oct 2016 OPTICS EXPRESS where P is the RF amplitude of th tap produced by beatig the th optical carrier with its correspodig sidebad geerated at the MZM; ω is the comb spacig; ω RF is the agular RF frequecy; ψ 2 is the coefficiet for the secod-order spectral phase imparted by the dispersive fiber; τ is the delay differece betwee the two iterferometer arms; ad the φ s are the phases applied to each optical comb lie with the pulse shaper. θ(t) ( = πv C (t)/v π ) is the optical phase shift itroduced by the phase modulator, where V C (t) is the cotrol voltage to the phase modulator ad V π is the half-wave voltage of the phase modulator. The first phase terms idicate that the secod-order phase from the dispersive fiber itroduces liear group delay betwee the various filter taps, which eables implemetatio of a fiite impulse respose filter. The differetial tap delay (T) is Δω ψ 2. The amplitude ad phase of the filter taps ca be programmed by the pulse shaper iside the iterferometer. I the curret paper, the optical carrier power is cotrolled i the pulse shaper to provide taps with a flat amplitude profile, which icreases the TBP. Pulse compressio actio depeds o the ability to cotrol the phase of the taps. These may be programmed by applyig the desired optical phases (φ ) i the pulse shaper; iterferece betwee carriers from the upper brach of the iterferometer ad sidebads from the lower brach of the iterferometer trasfers the optical phases ito the electrical domai. For example, a quadratic RF phase respose (or liear chirp delay respose) of the filter ca be achieved by applyig a quadratic spectral phase fuctio to the optical carriers, as follows [6]: ϕ 2 = β (6) The pulse compressio fuctioality described thus far is idetical to that i [6]. The ew feature here is that the pulse polarity i the phase filter ca be switched by usig the optical phase modulator iside the iterferometer. For example, whe the cotrol voltage to the phase modulator is zero or V π, the filter trasfer fuctio ca be expressed as j [ Δ ωψω ( 2 RF ± τ) ϕ ] P e for VC() t = 0 H ( ω ). RF j [ Δ ωψω ( 2 RF ± τ) ϕ ] P e for VC() t = V π (7) As show i Eq. (7), the trasfer fuctio with a cotrol voltage of V π has the opposite sig, compared to the filter trasfer fuctio with zero cotrol voltage. This idicates that the iput waveform polarity of the filter ca be chaged via the cotrol voltage. I priciple, by usig appropriate cotrol sigals to the phase modulator, other phase factors beyod 0 ad π ca also be implemeted; however, oly 0 ad π levels will be demostrated i the experimets that follow. It is worth otig that polarity switchig has recetly bee demostrated i the cotext of photoic-assisted RF arbitrary waveform geeratio (RF-AWG) [10,11]. I oe paper the polarity switchig overcomes the time aperture limitatios usually ecoutered i photoicsbased RF-AWG. This eables geeratio of waveforms with orepeatig features over essetially ulimited time apertures, importat to avoid distace ambiguities i ragig experimets [10]. I a secod paper, a phase modulator ad pulse shaper are icorporated together ito a iterferometer i a cofiguratio similar to that of Fig. 2 to overlay two-level ad multi-level phase shift keyig oto ultrabroadbad RF spread spectrum waveforms [11]. Here for the first time we modify a RF photoic phase filter to receive ad process such polarity-switched spread spectrum waveforms. 4. Experimet ad results A broadbad ad flat-topped frequecy comb is geerated i the same setup used i our previous work o phase filterig [6]. It cosists of a electro-optic frequecy comb icludig oe itesity modulator ad oe phase modulator, a pulse shaper, ad a legth of highly

6 Vol. 24, o Oct 2016 OPTICS EXPRESS oliear fiber. A seed electro-optic frequecy comb with the comb spacig of 12.5 GHz is shaped i the programmable pulse shaper to yield a secat hyperbolic shaped pulse. This iput pulse shape to the highly oliear fiber results i self-phase modulatio-based spectral broadeig with a approximately flat-topped shape [13]. The broadeed comb after a highly oliear fiber has ~29 m optical badwidth withi a 5-dB power variatio. The broadeed comb show i Fig. 3(a) is directed to the iterferometer show i Fig. 2. The pulse shaper (Fiisar WaveShaper 1000P) i the upper brach of the iterferometer selects ad shapes 293 lies of the broadeed comb to produce costat amplitude filter taps. The phases applied to the comb lies ca be recofigured to provide essetially arbitrary tap-depedet phases; however, i the experimets here we focus o oe specific quadratic phase fuctio. The halfwave voltage of the phase modulator (EOSPACE PM-5K1-20) is 2.8 V at 1 GHz. I the lower iterferometer arm, the MZM (EOSPACE AZ-0K5-10) has a half-wave voltage of 3V at 1 GHz ad a extictio ratio of 20 db. The total output photocurret at the output of the BPD (Discovery Semicoductors DSC720-HLPD) is 8.2 ma. The dispersive fiber of the bidirectioal fiber cofiguratio is implemeted by a dispersio compesatig fiber (DCF) with total dispersio of 800 ps/m. The differetial tap delay is 79.3 ps. The filter frequecy respose is evaluated by a etwork aalyzer (Agilet 5230C). Figures 3(b) ad 3(c) show the frequecy respose of the phase filter. The pulse shaper is programmed to realize a quadratic phase fuctio correspodig to β = (see Eq. (6)). As explaied i [6], the value of β realized is equal to the sum of the quadratic phase programmed oto the pulse shaper (0.0027) ad a additioal quadratic phase cotributio of arisig from the higher order dispersio of the DCF. The 3-dB badwidth ad chirp rate measured for this filter are 4.2 GHz ad 206 MHz/s, respectively. The product of filter badwidth ad delay aperture is equivalet to a TBP of ~86, to our kowledge the highest yet reported for a RF photoic pulse compressio filter. The measured filter amplitude ad group delay resposes agree closely with the resposes simulated based o Eq. (5); the measured chirp rate agrees closely with the calculated value (202 MHz/s) based o Eq. (4) of ref. 6, usig β = Fig. 3. (a) Broadbad ad flat-topped comb (resolutio = 0.01 m), (b) RF filter amplitude respose, ad (c) RF group delay respose. I (b) ad (c), colored solid ad black dashed curves show measuremet ad simulatio, respectively. For pulse compressio experimets a iput dow-chirp waveform is geerated from a RF arbitrary waveform geerator (Tektroix AWG7122C) with a samplig rate of 24 GS/s. The chirp waveform has a RF badwidth of 4 GHz at the ceter frequecy of 4 GHz ad time aperture of 19.4 s (TBP = 77.6). The peak-to-average power ratio (PAPR) of the chirp waveform is approximately 5 db withi the time aperture. The guard period betwee adjacet iput chirp waveforms is set equal to the time aperture of a idividual chirp, yieldig a overall repeat period of 38.8 s. The pulse polarity cotrol sigal is geerated from a fuctio geerator (Agilet 33250A) sychroized with the RF-AWG. At the output of the filter, a low-oise amplifier ad a badpass filter (with a filter ceter frequecy of 4 GHz ad a filter badwidth of 4 GHz) are used. The, the output waveforms are recorded by a real-time

7 Vol. 24, o Oct 2016 OPTICS EXPRESS oscilloscope (Tektroix DSA72004B) with 8 bit aalog-to-digital coverter ad 50 GS/s samplig rate. Figure 4 shows chirped pulse compressio ad pulse polarity switchig of the iput dowchirp waveform. With a costat cotrol sigal, as show i Fig. 4(b), all compressed pulses at the filter output have the same positive peaks. I aother experimet the cotrol iput to the phase modulator is drive by a amplitude V π square wave repeatig at 12.9 MHz, which is half the repetitio rate of the iput RF chirps. As show i Fig. 4(c), this causes the polarity of the compressed pulses at the output of RF photoic filter to alterate betwee positive ad egative o successive pulses (we emphasize that all the iput chirps have the same polarity for the data of Fig. 4). ote that all waveforms are obtaied through sigle-shot measuremets because the filter has good oise performace ad thus does ot require averagig of multiple traces. As expected, the compressed pulses are well-matched to the autocorrelatio of the iput waveform both with ad without polarity cotrol, Figs. 4(e) ad 4(f). The PAPR of the compressed pulses is approximately 21 db ad is improved by 16 db compared to the iput chirp waveform. The output pulse width defied as the full width at half maximum is ~250 ps, which is the iverse of the RF badwidth. The compressio ratio betwee iput ad output RF waveforms is comparable to the TBP of the filter respose. Fig. 4. Compressio of costat polarity chirped iputs with pulse polarity switchig i the filter. (a) Sequece of costat polarity iput dow-chirp waveforms; (b) Sequece of output waveforms with costat polarity cotrol to the filter; (c) Sequece of color-coded output waveforms with alteratig polarity cotrol applied to the filter for successive waveforms; the black lie shows the cotrol waveform. (d), (e), ad (f) are zoom-i views of (a), (b), ad (c), respectively. I (e) ad (f), black dash lies show the autocorrelatio of the iput waveform. ow we perform experimets o a sequece of chirped sigals which are polarity switched upo geeratio accordig to a legth-15 pseudo-oise (P) sequece [14]. Broadbad additive oise is superimposed with the iput sequece of chirps; the combied chirp sequece ad oise are the coected to our RF photoic filter. Figure 5(a) shows the phase-coded dow-chirp iput sequece as well as the polarity cotrol waveform which will be applied to phase modulator i the RF photoic filter. Here these waveforms are geerated from chael 1 ad 2 of the RF arbitrary waveform geerator, respectively, each with a samplig rate of 12 GS/s. As before, each dow-chirp waveform has a RF badwidth of 4 GHz at the ceter frequecy of 4 GHz ad time aperture of 19.4 s. Figure 5(d) shows a overlay of the idividual iput chirps with positive ad egative polarities, respectively; the π phase shift is clearly evidet. The ultrawidebad jammig oise, geerated by cascaded RF amplifiers ad RF filters, is characterized by 10-dB badwidth of 1.5 GHz at ~5.1 GHz ceter

8 Vol. 24, o Oct 2016 OPTICS EXPRESS frequecy. The geerated chirp sequece ad oise are combied by a 3-dB power combier ad directed to the RF photoic phase filter. As show i Fig. 5(b), with a low iput average sigal-to-oise ratio (SR) of 1.6 db, the dow-chirp waveforms are largely obscured at the filter iput. At the output of the filter, compressed pulses are clearly observed, with amplitudes well above the oise, Fig. 5(c). Furthermore, because the polarity cotrol iput of the RF phase filter compesates the polarity reversals icorporated o the iput chirp sequece, the compressed output sequece show i Fig. 5(c) has oly positive polarities. Therefore, at this poit the processed sigal already has the form of Eq. (3). The measured output peak SRs are ~23 db. The ratio of the output peak SR to the iput average SR is 21.4 db. This is i good agreemet with the 21.9 db SR improvemet, equal to the pulse compressio gai (18.9 db) plus 3 db, expected for jammig oise limited operatio [6]. At this poit the idividual spread spectrum waveforms (the chips) have bee compressed via the RF photoic phase filter ito a sequece of isolated peaks. I this first stage of processig, the compressio comprises a aalog chirp correlatio operatio implemeted via the RF photoic phase filter. I the ext stage of processig, we perform correlatio at the frame level usig a tapped-delay-lie filter cosistig of delay-lies, weightig, ad a summer [1]. The tapped delay lie filter is implemeted via a combiatio of aalog hardware ad digital processig. The weightig fuctio is performed i hardware by pulse polarity switchig of our RF photoic filter, as has already bee described. The remaiig operatios (delays ad summatio) are accomplished through off-lie digital processig of the sigal from the BPD recorded o the real-time oscilloscope accordig to Eq. (4). The receiver is assumed to have accurate kowledge of the spacig T of the waveform chips makig up the spread spectrum frame; here the spacig of the iput chirp waveforms is 38.8 s. Figure 5(e) shows a sigle waveform recorded after the RF photoic phase filter but before subsequet digital processig. After the digital processig, the peak voltage of the compressed pulse show i Fig. 5(f) is icreased by a factor of The icrease is somewhat lower tha the ideal value of 15 which correspods to the legth of the P sequece. The differece betwee measured ad ideal values is attributed to timig errors. The stadard deviatio of the timig error i the peaks is ~18.6 ps, which is cosistet with the timig jitter of the electroic RF- AWG used to geerate the iput spread spectrum sigal. However, the root-mea-square oise voltage is oly icreased by a factor of ~3.9. This is very close to the factor of 15 expected upo averagig of ucorrelated oise sigals. As a result, the peak SR value is icreased up to 31.4 db. Istead of the offlie tapped-delay lie processig, electroic methods with aalog circuits with high samplig rates o the order of tes of gigasample-persecod ca be used [15,16]. Here, the peak voltage values of compressed pulses would be sampled by a sample ad hold circuit ad the iput to a summatio circuit.

9 Vol. 24, o Oct 2016 OPTICS EXPRESS Fig. 5. Compressio of P polarity switched chirped iputs i the presece of ultrawidebad jammig oise. (a) Color-coded iput chirp sequece, without the jammig oise. The red ad blue idicate flipped polarity of the dow-chirp; black idicates the matchig sigal which will be applied to the polarity cotrol of the RF photoic phase filter. (b) Iput waveform with the jammig oise. (c) Output waveform from the RF photoic phase filter; compressio of the idividual chirps ad polarity compesatio are accomplished simultaeously. (d) Overlay of the iput chirp waveforms with positive ad egative polarities, without jammig oise. (e) Sigle compressed pulse at the filter output. (f) Compressed pulse with improved SR after digital tapped-delay lie. 5. Discussio ad coclusio We itroduce a ovel pulse polarity switchig techique to ehace the processig gai attaiable with ultrawidebad RF photoic phase filters. Our demostratio improves o a previously itroduced RF photoic filterig cofiguratio based o a optical frequecy comb source, a iterferometric pulse shapig arragemet, dispersive frequecy-to-time coversio, ad balaced detectio by icorporatig a phase modulator ito oe of the iterferometer arms to provide polarity switchig capability. Our experimets start with a phase filter with a 3-dB badwidth of 4.2 GHz cofigured for compressio of frequecy modulated iput sigal with 206 MHz/s chirp rate. This provides a time-badwidth product of ~86, comparable to but slightly higher tha our previous experimets. By overlayig a additioal hybrid sigal processig step cosistig of polarity switchig ad off-lie digital filterig, processig gai ad oise suppressio are ehaced for iput spread spectrum sequeces with matchig polarity modulatio. I a experimet with strog additive broadbad oise, fully aalog pulse compressio via the RF photoic phase filter delivers 23 db output SR, 21.4 db higher tha the 1.6 db iput SR. For a polarity-switched, legth-15

10 Vol. 24, o Oct 2016 OPTICS EXPRESS iput sequece, our ew scheme provides peak output SR of 31.4 db, a additioal 8.4 db improvemet. Although the curret experimets utilize two-level phase modulatio (correspodig to switchig of the polarities of spread spectrum chips), our scheme is geeral ad is applicable to arbitrary multi-level phase modulatio without ay modificatio of the hardware. Furthermore, i our scheme the ehacemet i the SR is expected to scale liearly with the legth of the phase modulatio sequece. Although the specific example of a legth-15 polarity switchig sequece was demostrated above, fudametally there is o limit to the legth of the phase modulatio sequece. Practically, the legth of the phase modulatio sequece will be costraied by timig jitter withi the waveform frame. To reduce such jitter, it may be desirable to utilize photoics-based geeratio of polarity (or multi-level phase) switched RF spread spectrum iput sequeces [10,11], as photoic approaches to high badwidth RF arbitrary waveform geeratio have bee show to deliver lower RF phase oise tha their electroic arbitrary waveform geerator couterparts [17]. A importat attribute of RF photoic phase filters for spread spectrum pulse compressio is that they are asychroous [9,18], avoidig serious challeges i acquirig sychroism with high badwidth sigals of iterest. It is importat to ote that although our polarity switchig scheme is o loger completely asychroous, sychroism requiremets are substatially relaxed compared to traditioal time domai spreadig ad despreadig at multi- GHz rates. The first receiver processig step, chip level pulse compressio, remais fully asychroous. This provides a iitial processig gai which ehaces the desired sigal compared to the oise or RF backgroud, facilitatig timig acquisitio. For the secod step, the phase demodulator oly eeds to sychroize at the frame level (~s) to iitiate further processig gais. Although time aligmet at the highest time resolutio is evetually desired, the processig gai is expected to degrade gracefully with slight timig misaligmets. Ackowledgmets We thak Dr. Daiel E. Leaird, Amir Rashidiejad, ad Bog Cha Kim for their valuable discussios. This work was supported i part by the Defese Advaced Research Projects Agecy (DARPA) uder grat HR This research was also supported i part by the Basic Sciece Research Program through the atioal Research Foudatio of Korea (RF) fuded by the Miistry of Educatio (RF-2014R1A6A3A ).