. INTRODUCTIONF photonics is a new field of research that extendsptical-based processing techniques to RF and radar de-ection problems involving RF phased-array sensors1,2
nd filters.3 Photonic RF filters have much potential be-ause of their capability for high dynamic range, tunabil-ty, and reconfiguration. Several configurations have beenuggested that employ highly dispersive fibers,4 fiberratings,5 fiber optic prisms,6 or arrayed waveguideratings.7
In many RF warfare applications an external radar sig-al is received. The carrier of such a signal may vary inime within the range of less than 1 GHz up to 20 GHzhile the bandwidth of the information itself is approxi-ately 100 MHz. The information within the 100 MHz
andwidth has signal to noise ratio (SNR) of 10 dB, butince the noise is spread over the entire spectrum the re-ulting SNR when the signal is received without filterings approximately −13 dB. Such a low SNR does not allowetection. Thus prior to detection the signal must be fil-ered. However, the RF carrier is unknown. Thus the com-on approach in electronic warfare is to split the incom-
ng signal into many RF filters performing simultaneousltering with each filter tuned to a different bandwidth.8,9
he cost (many hundreds of thousands of dollars) and theize (many cubic meters) of such a configuration make itery unattractive.
In this paper we propose a different, integrated electro-ptic approach allowing the extraction of the RF carriernd the RF information using photonic processing: Theeceived RF signal modulates an electro-optical modula-or. Even the maximum possible carrier frequency of0 GHz is a more or less typical modulation frequencyate in the optics communication field (the common modu-ation rates are 2.5 GHz, 10 GHz, and 40 GHz). Thus theF signal is now converted into an optical signal. Then anll-optical monitoring unit based on optical fibers and ul-rafast optical switches serves as an all-optical, ultrafastpectral filter. Since the filtering is performed prior to theetection process, the RF carrier may be extracted. Theutput of the monitoring unit is sent as a control signal tohe detection unit, which is tuned to the correct spectralocation of the RF information, and the RF signal is ex-racted. The suggested all-optical configuration is small inimensions (less than 100 cm3) and cost (a few thousandsf dollars). The ultrafast switches are based on the 1�2ubmicrosecond, electro-optical switching concept10 ofivcom Devices and Systems, Inc.
005 Optical Society of America
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Section 2 presents the explanation of the suggested sys-em including some design analysis. A mathematicalnalysis is presented in Section 3. Computer simulationss well as experimental results are described in Section 4.he paper is concluded in Section 5.
. TECHNOLOGICAL OVERVIEWhe problem that we face in our photonic design is to de-ect the carrier of an RF signal embedded in strong wide-pectrum noise. The signal is restricted in time and itsidth is only 2 �s, while it appears periodically every.4–1 ms. Its spectral position is unknown. During theime slots where there is no signal the receiver detectsnly noise. The schematic sketch of the setup is depictedn Fig. 1. The optical input signal is split into two pathssing a 1 � 2 ultrafast optical switch that acts also as ahutter. The upper path includes the original signal thatasses through a delay of 2 �s (corresponding to a fiberength of 400 m), and the lower path is split into two sub-aths that are eventually recombined. Those two sub-aths compose the suitable optical filter since the upperubpath includes a delay relative to the lower subpath.he information for the proper delay that is required forenerating the optical filter according to the correct RFarrier is obtained from the lower path, which is usednly to extract the RF carrier information. The 1�2witch also acts as a shutter for the lower optical path.he optical shutter is intended to block the energy after00 ns �2 �s/20�. Owing to the blocking, 1/20 of the infor-ation slot is to be used to map the carrier frequency
done on the lower path). The carrier mapping is done inhe following manner. The signal is equally split into thewo optical subpaths. The upper subpath includes a 14 switching module. Each one of its outputs corresponds
o a different delay (relative to the lower optical subpath).he outputs of the switch are recombined and then splitgain such that part of the energy goes to the electronicampling unit and part is returned through a 20 m longber (corresponding to a delay of 2 �s divided by 20, sincee aim to obtain 20 spectral resolution points) to the in-
Fig. 1. Schemat
ut of the switch. This actually means that the input sig-al is replicated several times each time it is passedhrough the 1�4 switch that applies to it a different rela-ive delay. The lower optical subpath includes the replica-ion of the signal as well, by passing it through the 20 meedback fiber, but there no delay is applied. The uppernd the lower subpaths are recombined without energyosses using polarizing beam splitters (the polarizations ofhe upper and the lower subpaths are orthogonal in ordero obtain intensity and not field summation of the opticalnformation from the two recombined subpaths). The re-ombination of the lower and the upper subpaths gener-tes a two-term finite-impulse-response filter (FIR) thatxtracts the RF carrier. After that, this information issed as the control signal to the upper path in which alow tunable filter is positioned. The filter is slowly tunedrate of ms) to the required spectral band in order to ex-ract the information of the incoming signal. Its output isampled by a 1 GHz sampling analog/digital (A/D) cardaving eight sampling bits.
ch of the system.
ig. 2. Example of the obtained FIR filter. The figure depicts thebsolute value of the spectral response of a two-term FIR filter.
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ig. 3. (a) Spectrum of the signal and the noise for RF carrier of 15 GHz. (b) Obtained readout intensity at the detector as function ofhe delay (controlled by the 1�4 switch). (c) Same as (a) but for carrier of 8 GHz. (d) Same type of readout as (b) but for carrier as in (c).
e) Same as (a) but for carrier of 3 GHz. (f) Same type of readout as (b) but for carrier as in (e).
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Let us examine the lower optical path that is respon-ible for the RF carrier monitoring. As a result of thehutter operation, the 2 �s/20 time slot that passeshrough the lower path is duplicated M times (M shoulde the ratio between the spectral range of the RF carriernd the sampling rate, i.e., 20) in the upper and the lowerptical subpaths. The duplication is done by the 20 m fi-er feedback. The signal passing through the upper sub-ath is delayed relative to that in the lower subpath; inhat way, their optical combination in intensities (ob-ained since they are in orthogonal states of polarization)enerates an FIR filter. Since the combination is per-ormed between two paths the filter has two terms [N inq. (2)]. Each one of the M duplications exhibits a differ-nt delay (generated by the 1�4 switch) that is respon-ible for the optical FIR filter having a different spectralosition (see Fig. 2). Sampling the M duplications per-orms the spectral scanning and mapping of the positionf the RF carrier. Note that as a result of the given SNR,iscrete spectral sampling in the specified resolution isufficient to detect the noise-embedded signal.
Every 2 �s another time slot is coming in. Because ofhe shutter only 2 �s/20 of the information is used foronitoring, and thus the 20 duplications do not create
ny overlapping with the next-in-line incoming sequence.thermal control circuit connected to the two optical sub-
aths that generate the FIR filter is responsible for per-orming the proper synchronization of delays. The delay isbtained by expanding one optical subpath relative to thether. To generate a delay of 5 ps, for instance, one needso enlarge one optical free-space path by
�L = c�t = �3 � 108��5 � 10−12� = 1.5 mm. �1�
The delay quanta that are inserted into the 1�4 switchthat is responsible for selecting the different delay �that yields the spectral scanning of the FIR) are �12.5 ps, �2=5 ps, �3=10 ps, and �4=50 ps. A combinationf those delay quanta creates the spectral notch (see Fig.) located at the desired position. The 1�4 switch doeshe proper combination as follows: Since the delay is cu-ulative, for each cycle the signal passes through the
witch, an additional delay is added. The switch tunes itselay (�1, �2, �3, or �4) for each cycle such that the accu-ulated delay will be as close as possible to the desired
elay at that point in time (which is determined by Eq. (4)
Fig. 4. Schematic sketch of improved static sy
s described in the next section). Thus basically the over-ll delay after K cycles (i.e., replications) sums to
�t = �i=1
K
�i��1,�2,�3,�4�,
here �i is the delay that was tuned by the switch inycle i, and it equals �1, �2, �3, or �4. Basically this ap-roach resembles the way a decimal number is repre-ented on a binary basis. Four binary digits (each with aifferent weight corresponding to its position within theepresentation) may represent a decimal number of up to5.The RF carrier is extracted by finding the minimum of
he energy rendered by the detector for the M duplica-ions (each corresponds to a different spectral position ofhe filter). After extracting the carrier, the spectral foldingf information may be resolved and the narrowband infor-ation completely extracted in the upper path, which is
onnected to a detection circuit that includes a low-rateocal oscillator and an A/D card. An RF high-pass filterHPF) is required since the optical detector senses the in-ensity and not the optical field. The HPF also filters thec of the incoming signal.
aving high information efficiency throughput.
Fig. 5. Constructed optical configuration.
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. MATHEMATICAL ANALYSISssuming that an incoming sequence s�t� is duplicated N
imes and that each duplication is summed after delayingt by �t, then
sT�t� = �n=0
N−1
ans�t − n�t�. �2�
he Fourier transform of sT equals
ST��� =� sT�t�exp�− 2�i�t�dt
=� �n=0
N−1
ans�t − n�t�exp�− 2�i�t�dt = S���F���,
S��� =� s�t�exp�− 2�i�t�dt,
F��� = �n=0
N−1
an exp�2�i�n�t�. �3�
he filter F is an FIR filter. By tuning the delay betweenhe sequences the position of the filter may be changed.
ig. 6. Experimental results for optical filter generated over RFb) and (c) modulation at 34.682 MHz; (d) lower optical path is d
igure 2 presents the absolute value of the spectral re-ponse of the filter for N=2 duplications, �t=100 ps, andith equal coefficients an. For N=2 the position of the
pectral notch is
�FIR =1
2�t. �4�
he minima will be responsible for the energy fluctua-ions at the detector when the spectral scanning is ap-lied by using different delays. Since we have two sub-aths we realize a two-termed FIR filter, N=2.
. EXPERIMENTAL RESULTSigure 3 presents experimental simulations that follow
he setup of Fig. 1 including the generation of proper SNR10 dB� and performing of the sampling (spectral folding).igures 3(b), 3(d), and 3(f) depict the energy fluctuations
or various delays (spectral scanning) when the RF car-ier is 15, 8, and 3 GHz, respectively. As one may see, theinimum is obtained in the proper spectral locations and
his position is detectable with the 8-bit A/D card. In theimulations we used a 1 GHz A/D card. Figures 3(a), 3(c),
at the first replication: (a) First replication without modulation;ected.
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nd 3(e) present the spectrum of the 100 MHz bandwidthignal embedded with white, 20 GHz, spectrally wideoise.Although the system described in the previous sections
ill solve the problem, it suffers from a large disadvan-age. The switches that realize the suggested delays oper-te at a rate of 100 ns, but this is not fast enough sincehe spectral scanning process occupies 2 �s in that case.
ince this is the length of the signal sequence, there is a0% probability of losing a significant portion of the infor-ation (on average half of the information will be lost).he spectral scanning process must be at least ten times
aster so that the amount of lost information will be neg-igible. To obtain that speed, the first feedback fiber ishortened to 2 m instead of 20 m (corresponds to 10 ns)nd the 1�4 switch is replaced by two fixed delays. The
ig. 7. Additional experimental results for optical filter gener-ted over RF signal at the first replication: (a) first replicationithout modulation; (b) and (c) modulation frequency is below
he spectral position of the filter; (d) modulation frequency is4.682 MHz; (e) modulation frequency is higher than the filterrequency.
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rst delay is 12.5 ps, and through it the light is passednly once. The second delay is placed inside the opticaleedback loop of the upper subpath, and it is also 12.5 ps.his means that the first replication will have a notch fil-er at 20 GHz, the second at 13.3 GHz, then 10 GHz, 8,.67, 5.7, 5, etc. This is due to the fact that the second de-ay is additive, and each replication accumulates a delayhat corresponds to the replication serial number. A delayf 12.5 ps is realized by free-space propagation withinlass for a distance of 1.25 mm. A schematic sketch of thisrocess is depicted in Fig. 4. This is a fixed system exceptor the shutter that must generate the bit bursts, of 10 nshat is used for the monitoring.
The described system was constructed; it is depicted inig. 5 (this is just an illustrative figure; the schematicketch may be seen in Fig. 4). The shutter was realized byascading two Civcom 1�2 switches. The problem is thatlthough the switches have fast optical response (approxi-ately 200 ns), their electrical driver is limited to an op-
ration frequency of 10 KHz. Thus in order to be able toenerate short input bit burst sequences we inserted anC circuit between the electrical drivers of both switchesuch that we actually constructed an optical AND gate.
ig. 8. Experimental results for optical filter generated over Rodulation frequency is below the frequency of the optical filter; (
han the optical filter frequency.
ince the first switch’s driver was connected to a pulseenerator and the second one’s driver was connected tohe same generator but had a delayed control command,nly when both switches were opened could the light comehrough into the optical system. By playing with the RCarameters we could generate a very short pulse se-uence (corresponding to the RC delay). The fibers wereonnected by using 50%–50% (to separate and combinehe upper and the lower subpaths) and 95%–5% splittersfor the optical feedback loop). Manual polarization con-rollers were used to calibrate the system.
For the demonstration of the operational principle wesed fiber of 200 m for the optical feedback loop (corre-ponds to replication length of up to 1 �s). We started byxamining the system at low frequencies having relativeelay between the upper and the lower subpaths (follow-ng the description for Fig. 4) of 2.75 m for the first replicand 5 m for the second. Such a configuration should gen-rate filters at approximately 35 MHz, 20 MHz, etc. Inig. 6 we observe the first replication. The two lighterurves that resemble the exponential curve of a chargingapacitor are the voltage control commands to the driversf the two switches. As we described, an RC circuit is
al at the second replication: (a) Signal without modulation; (b)ulation is at 20.682 MHz; (d) modulation is at a frequency higher
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Zalevsky et al. Vol. 22, No. 8 /August 2005 /J. Opt. Soc. Am. A 1675
laced between the switches create delay (this is why theower exponential curve of Fig. 6(b) has a more pro-ounced exponential nature; it basically represents a ca-acitor charging circuit with large RC). The curves ofulses (dark curves) represent the measurement capturedt the first replication after connecting the output of thehotodiode to the second channel of the scope. In Fig. 6(a)ne sees the first replication without inserting modula-ion. In Figs. 6(b) and 6(c) we see how the replicationooks when modulation at 34.682 MHz is inserted. No in-ormation is passed since, as we mentioned, the optical fil-er is tuned to approximately 35 MHz. For Fig. 6(d) weisconnected the lower optical subpath, and as predictedrom theory the optical filter no longer exists and thus theignal may be detected (it is no longer filtered out).
In Fig. 7 we examine the first replication again. In Fig.(a) one may see the first replication without modulation.n Figs. 7(b) and 7(c) it is modulated at a frequency that iselow the spectral position of the filter of the first replica-ion (about 30 MHz). As can be seen, the signal is not fil-ered. In Fig. 7(d) the modulation frequency is4.682 MHz and thus it is filtered out. In Fig. 7(e) the
ig. 9. Experiments at high frequencies: (a) modulation fre-uency does not match the optical filter frequency, (b) modulationrequency of 800 MHz.
odulation frequency is higher than that of the filterabout 36 MHz) and thus it is detected at the scope.
In Fig. 8 we examine the optical filter applied over theecond replication (around 20 MHz). In Fig. 8(a) weresent the signal without modulation. In Fig. 8(b) theodulation frequency is below the frequency of the opti-
al filter (about 18 MHz). In Fig. 8(c) the modulation is at0.682 MHz and indeed the signal is filtered out. In Fig.(d) we modulate at a frequency higher than that of theptical filter (about 22 MHz).
Then we changed the detector to a faster detector andhanged the relative length between the two optical sub-aths. We tuned the filter to 800 MHz (12.5 cm of relativeelay). In Fig. 9(a) the modulation is at a frequencyigher than 800 MHz (about 810 MHz) and the signal isot filtered. In Fig. 9(b) we modulate at 800 MHz, and in-eed the signal is not detected.In order to allow a sufficient number of discrete spec-
ral positions for the notch filter, we need more replica-ions since each replication experiences a different spec-ral filter. To do that we added an erbium-doped fiber
ig. 10. Detected replications with EDFA at the optical feed-ack loop: (a) replications without modulation, (b) replicationsith modulation at 414.2 MHz.
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1676 J. Opt. Soc. Am. A/Vol. 22, No. 8 /August 2005 Zalevsky et al.
mplifier (EDFA) to each of the two optical subpaths. Asan be seen in Fig. 10(a), more than 16 replications canow be detected (before adding the amplifier only threeeplications could be seen). In Fig. 10(a) no modulation isnserted. In Fig. 10(b) a modulation of 414 MHz is ap-lied.In order to obtain measurements close to the real-case
cenario, we insert a noise modulation of 60 MHz band-idth. The modulation is added on top of the optical sig-al. The added noise is approximately 20 dBm/10 MHz.
ig. 11. Modulation frequency is (a) below the optical notch, (b)t the optical notch, (c) above the optical notch.
he information is a single varied frequency of powerielding a SNR of approximately 15 dB. We tune the fre-uency of the information modulation while keeping theptical filter constant (we use only the first replication)nd exhibit the results obtained when the modulation fre-uency is below, equal to, and above the position of theotch of the optical filter. Figure 11 depicts the results ob-ained. The spectral position of the optical notch is tunedo 42.6 MHz. In Fig. 11(a) the frequency of the informa-ion is 23.6 MHz (below the position of the notch), in Fig.1(b) is equal to that of the notch, and in Fig. 11(c) is2.8 MHz (above the optical notch). The upper curvesresent the digital spectrum computed by the digitalcope. One may observe the peak of the informationodulation appearing in Fig. 11(a) and 11(c) and not in
1(b), since there it is filtered by the optics. The meaneadout computed by the digital scope is approximately50 for Fig. 11(a) and 11(c) and 410 for Fig. 11(b). Thushe low-frequency readout at the output of the optical sys-em indeed provides the position of the information car-ier in the presence of noise as predicted by the presentedheory.
. CONCLUSIONSn this paper we have presented a new approach for all-ptical compact extraction of the carrier and the RF infor-ation of a radar signal embedded in noise. The main
ontribution of this paper is actually the realization of anltrafast (ns rate) and ultraprecise (less than 1 GHz) op-ical tunable filter. Such filters do not exist in the opticsommunication field today. The suggested configuration isased on realizing a finite impulse response filter by gen-rating delays between two optical paths. Generation ofultiple optical replications may allow obtaining spectral
canning for the RF carrier also without incorporating dy-amic elements into the system. In the final configurationhat was experimentally investigated, the only dynamiclement was an optical shutter/switch that generated theemporal bit bursts that were later replicated by the op-ical system. Numerical simulations as well as experi-ental results demonstrated the feasibility of the sug-
ested configuration.
Corresponding author Z. Zalevsky may be reached byhone, 972-3-5317055, and by e-mail, zalevszeng.biu.ac.il.
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