True Time Delay Line For Antenna Beam Forming - IJSR
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391True Time Delay Line for Antenna Beam FormingPradnya Sutar1, Jyothi Digge21, 2Terna Engineering College, Navi MumbaiAbstract: This paper presents the different types of True Time delay lines used for antenna beam forming till date. Low losscapabilities of photonics are used for controlling the properties of microwave and millimeter waves, which are used in sensorynetworks, wireless access networks, radar and satellite networks. Hence we present the different types of True Time Delay(TTD) linessuch as Photonic crystal fiber(PCF) delay line, Silicon chip delay line, Polymer optical switch, Waveguide delay lines and Wavelengthdivision multiplexing for antenna beam forming.Keywords: True time delay line(TTD),Mach-Zehnder interferometer, photonic crystal fiber, Beam forming1. IntroductionIntegrated microwave photonics (IMWP) is a novel field inwhich the fast-paced progress in integrated optics isharnessed to provide breakthrough performance to wellestablished microwave photonic processing ctroniccomponents. Phased array antennas offer a number ofattractive characteristics, including conformal profile,electronic beam forming (beam shaping and beam steering),interference nulling and the capability to generate multiplesimultaneous antenna beams[1].With the development of radio over fibers, a growingattention is devoted to the use of photonics components forthe generation, processing and transport of microwave signal.Optoelectronic components are usually more compact andhave a much larger bandwidth than their radio-frequencycounterparts. For radar, electronic warfare or telecom (e.g.narrowband tunable filter) applications, reconfigurablemicrowave photonics delay lines are key elements. Especiallya true-time delay, is a desirable feature in phased arrayantenna to avoid squinting effects. True-time delay lines havebeen proposed on various platform such as fiber Bragggrating, dispersive fibers and switched structures[2].Recently, the technique of system in package (SiP), in whicha number of integrated circuit chips are enclosed in a singlepackage, has been used for reducing size and supportingmultiple functions. This has brought a number of benefitssuch as the shortening of development periods and thereduction of development costs compared to the technique ofsystem on chip (SOC)[3].Light propagation through an optical fiber causes a long,non-resonant (true) time delay used in numerousapplications. In contrast to how it is deployed in opticalcommunication systems, fiber is coiled in these applicationsto reduce footprint. This is a configuration better suited for achip-based waveguide that would improve shock resistance,and afford the possibility of integration for system-on-a-chipfunctionality.Recently chip based optical TTD lines for antenna beamforming has become very popular. Here chip based delay lineon silicon chip offers very low loss as compared to itscounter parts Silica filters and photonic crystal fiber. Scalingthis waveguide to integrated spans exceeding 250m andattenuation rates below 0.01 dB/m[6].1.1 Microwave PhotonicsMicrowave photonics is an interdisciplinary area that studiesthe interaction between microwave and optical signals. Themajor functions of microwave photonics systems includephotonic generation, processing, control and distribution ofmicrowave and millimetre-wave (mm-wave) signals. Thetopics covered by microwave photonics include photonicgeneration of microwave and mm-wave signals, photonicprocessing of microwave and mm-wave signals, opticallycontrolled phased array antennas, radio-over-fiber systems,and photonic analog-to-digital conversion. Techniquesdeveloped in the last few years in microwave photonics willbe reviewed with an emphasis on the systems. Challenges insystem implementation and new areas of research inmicrowave photonics are also discussed[5].1.2 Optical Generation of Microwave SignalsConventionally, a microwave or mm-wave signal is generatedusing electronic circuitry with many stages of frequencydoubling to achieve the desired frequency. The system iscomplicated and costly. In addition, for many applications,the generated microwave or mm-wave signal should bedistributed to a remote site. The distribution of a microwaveor mm-wave signal in the electrical domain is not practicaldue to the high loss associated with electrical distributionlines, such as coaxial cable. Therefore, the ability to generatea microwave or mm-wave signal in the optical domain wouldallow the distribution of the signal via optical fiber from acentral office to a remote site, greatly simplifying theequipment requirement.Usually, a microwave or mm-wave signal can be generated inthe optical domain based on optical heterodyning, in whichtwo optical waves of different wavelengths beat at a photodetector. An electrical beat note is then generated at theoutput of the photo detector with a frequency correspondingto the wavelength spacing of the two optical waves[1].Assume that we have two optical waves given byE1(t) E01 cos (ω1 t ϕ1)(1)E2(t) E02 cos (ω2t ϕ2)(2)Volume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702926
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391Where, E01, E02 are the amplitude terms ω1, ω2 are theangular frequency terms and ϕ1,ϕ2 are the phase terms of thetwo optical waves.Considering the limited bandwidth of the photo detector, thecurrent at the output of the photo detector is given byIRF A cos [( ω1 - ω2 ) ( ϕ1 - ϕ2 )](3)Where A is a constant which is determined by E01, E02 andthe responsibility of the photo detector.As can be seen from Eq.(3), an electrical signal with afrequency equal to the frequency difference of the two opticalwaves is generated. This technique is capable of generatingan electrical signal with a frequency up to THz band, limitedonly by the bandwidth of the photo detector. However, bybeating two optical waves from two free-running laser diodeswould lead to a microwave or mm-wave signal with highphase noise since the phases of the two optical waves are notcorrelated, which will be transferred to the generatedmicrowave or mm-wave signal. Numerous techniques havebeen proposed and demonstrated in the last few years togenerate low-phase-noise microwave or mm-wave signalswith the two optical waves being locked in phase. Thesetechniques can be classified into four categories: 1) Opticalinjection locking, 2) Optical phase-lock loop (OPLL), 3)Microwave generation using external modulation, and 4)Dual-wavelength laser source[5].2. Types of True Time Delay lines1)2)3)4)5)6)7)8)9)10)11)In 1991, the design and performance of the first microwavephased array antenna steered by optical delay lines. Usingsemiconductor laser switching to implement the delay times,demonstrated the absence of "beam squint" in the antennapattern as its frequency was switched from L to X band.The techniques can be classified into two categories: truetime delay beam forming based on free-space optics and truetime delay beam forming based on fiber or guided-waveoptics. In, a true-time delay beam forming system based onfree space optics was proposed and experimentallydemonstrated.Since the system was based on bulk optics, it has a large sizeand heavy weight. Most of the systems were implementedbased on fiber optics. The realization of tunable true-timedelays based on fiber-optic prism consisting of an array ofdispersive delay lines was demonstrated in 1993. To reducethe size of the fiber-optic prism, the dispersive delay linescould be replaced by Fiber Bragg Grating (FBG) delay linesin 2002. A FBG prism consisting of five channels of FBGdelay lines is shown in Fig.2[5].As can be seen the beam pointing direction can be steered bysimply tuning the wavelength of the tunable laser source.Since the grating spacing in the second delay line is verysmall, to simplify the fabrication, the discrete FBGs can bereplaced by a single chirped Bragg grating in 2003.In fact, ifall the discrete grating delay lines are replaced by chirpedgrating delay lines, a true time delay beam forming systemwith continuous beam steering would be realized.Fiber based delay lineOptical delay line on silicon chip2 2 - Optical MEMS based TTDLPolymer waveguide switch array based TTDLRing resonator based optical beam formingPhotonic crystal fiber(PCF) based TTDLPhotonic Microwave Delay line using Mach-ZehenderModulatorOptical Mux/Demux based delay linePCW based AWG Demux /TTDLSub wavelength grating enabled on-chipultra-compact optical true time delay line2.1 Fiber based delay lineTraditionally, feed networks and phase shifters for phasedarray antennas were realized using electronic components.This was the most intuitive approach since antennas operateon an electrical driving source. With the advancement oftechnology, severe limitations were observed in electricaldevices. For example, copper wires display high losses athigh frequencies resulting in a limited bandwidth for the feedsignals. Furthermore, electrical beam forming networks havea relatively high weight, thus limiting their use in airbornesystems.Optical components, with key advantages such as immunityto electromagnetic interference, low loss, small size and lightweight are being considered as a promising alternative forwideband phased array antennas.Figure 1: Array factor of a phased array antenna usingtrue-time delay components operating at frequenciesbetween 10–20 GH.[5]Figure 2: A photonic true-time delay beam forming systembased on a FBG prism[5]Volume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702927
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.3910.1-0.2dB/cm. Another advantage of SOI is its compatibilitywith silicon integrated circuit technology which implies lowcost and high yield manufacturing [7].Figure 3: A photonic true-time delay beam forming systemusing a chirped Bragg grating.[8]Fig.5 shows integrated optical time-delays in the silicon-oninsulator (SOI) waveguide technology. In order to select aparticular time-delay during antenna steering the mostobvious choice is a switch that can be used to select thedesired time-delay. This can in general be a very expensiveand lossy technique. The alternative is to use a self-routingscheme which is switchless and the desired delay line isselected by changing the optical wavelength, as shown inFig.5(a). This requires a fast tunable laser source. Fig.5(b),which uses an electrically tunable grating and completelyeliminates the tunable source while preserving the selfrouting concept[7].Figure 4: Power distribution as a function of microwavefrequency [5]The architecture shown in Fig.2 has the advantage of using asingle tunable laser source, which is easy to implement withfast beam steering capability by tuning the wavelength of thetunable laser source. However, the prism consists of manydiscrete FBGs, which may make the system bulky,complicated and unstable. A solution is to use a singleChirped Bragg Grating[8].As shown in Fig.3 a singlewideband Chirped Bragg Grating is used. Different timedelays are achieved by reflecting the wavelengths from atunable multi wavelength laser source at different locations ofthe chirped Bragg grating. To achieve tunable time delays,the wavelength spacing should be tunable. Therefore, a multiwavelength laser source with tunable wavelength spacing isrequired[5].2.2 Optical delay line on Silicon chipFiber-optic waveguides for true time delay are used inrotation sensing, radio frequency photonics, high-stabilitymicrowave oscillators and all-optical signal processing.Transfer of these applications to a wafer imposes newchallenges on micro photonic fabrication. Waveguide lossmust be reduced to unprecedented, low levels and maintainedover a broadband spectral region [6].Silicon-on-insulator (SOI) represents an attractive alternativeto the silica technology. SOI combines large area and lowcost silicon substrate technology with a high delay per lengthratio available in a semiconductor waveguide technology(n 3.46). Further, the SOI waveguides exhibit low losses of(a)(b)Figure 5: (a) Scheme for self-routed true-time-delay usinga tunable laser source[7] (b)Integrated self-routed electrooptic tunable time-delay unit[7]2.3 Optical MEMS based TTDA optical true time-delay (TTD) feeder for X-band linearphased array antennas (PAAs), which possesses high-speedbeam scan capability by selecting different lengths of fiberdelay lines with fast 2 2 optical micro electromechanicalsystem switches. For proof of concept, a 3-bit optical TTDhas been built for a 10-GHz linear PAA composed of twoantenna elements. Experimental results show that themaximum time-delay error is less than 0.2ps, correspondingto a radiation angle error of less than 0.84, which is withinthe equipment resolution. Design of a 10-GHz linear PAAVolume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702928
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391composed of eight micro strip patch antenna elements drivenby the proposed TTD. The radiation patterns of this PAAhave been obtained by simulation [8].An optical true time-delay (TTD) feeder for phased-arrayantennas (PAAs) has advantages such as small size, low loss,no electromagnetic interference, large instantaneousbandwidth, high resolution, squint-free beam scanning over abroad range of frequencies, and multibeam capability,etc.Several schemes for optical TTD feeder can be used in thefiber-optic prism using high dispersion compensation fibers(DCFs), integrated silica waveguide switches, fiber Bragggratings, and chirped fiber gratings (CFGs). Among these,FBGs, integrated silica waveguides, and DCFs can offerdiscrete beam scanning capability. On the other hand, CFGscan offer continuous beam scanning capability. Most of theoptical TTDs described above, however, require tunable ormulti wavelength sources to operate, resulting in highersystem costs, longer time to reach a steady-state, and extracontrol for wavelength adjustment, etc.The optical micro electromechanical system (MEMS) switchshows features such as low insertion loss, fast switching time,and easy electronic control. Therefore, MEMS switches incombination with fiber delay lines can be an economical andefficient alternative to implement a fast TTD feeder.resulting in reduced system costs and enhanced resistance toharsh environments. Another delay device structurecomposed of optical micro electromechanical system(MEMS) switches and fiber delay lines shows desirablefeatures such as low insertion loss. A novel design based onMEMS and free-space white cell also shows potential interms of speed and scalability. However, either the fiberlength has to be precisely cut or the mirrors have to beexactly assembled to achieve accurate delays.A more attractive approach is to integrate waveguideswitches and delay lines on a single chip by the planar lightwave circuit technique. A previously reported 2-bit TTDconfiguration using polymer optical switches and waveguidedelay lines, which are defined by photolithography, canprecisely deliver four TTDs. The fully integrated photoniccircuit eliminates the discrete between optical switches andfibers; therefore, it provides a more stable throughput andoccupies less space. Additionally, the fabrication cost issignificantly reduced[8].A 4-bit polymer TTD device containing five fully integrated2 2 thermo-optic switches based on the total internalreflection (TIR) effect. Compared with other optical switchstructures, such as Mach–Zehnder interferometers ordirectional couplers, TIR switches have a much lower crosstalk, which significantly decreases the RF signal phase error.Additionally, the switching operation of TIR switches is notsensitive to the wavelength, thus providing a large opticalbandwidth. Compared with digital optical switches, TIRswitches are more compact, with lengths of only several millimetres and require lower driving powers.Figure 6: shows the configuration of the proposed TTDfeeder using 2 2 optical MEMS switches and fiber delaylines for a linear PAA composed of antenna elements.[8]An optical TTD feeder for PAAs, which consists of a fixedwavelength laser diode, 2 2 optical MEMS switches, andfiber delay lines. This system provides advantages overexisting ones such as low cost, fast operation, and reliability,etc. A 3-bit TTD feeder for 10-GHz two-element linearPAAs has been implemented and the time-delay differencebetween the antenna elements has been measured for all thepossible radiation angles.2.4 Polymer waveguide switch arrayOptical true time delay (TTD) for phased-array antennasoffers many advantages over electrical phase delays such aswide bandwidth, immunity to electromagnetic interference,and compact size. Compared with the wavelength tuningconfiguration, an optically switched waveguide delay linedevice structure needs no tunable wavelength sources,Figure 7: (Color online) (a) Schematic of the 4-bit TTDdevice using TIR optical switches (b) Schematic of the TIRoptical switches [8]Volume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702929
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391provides large bandwidths, RF frequency transparency, TrueTime Delay (squint-free) characteristic over the band ofinterest, EMI immunity, compactness and light weight, thusallowing critical size and weight applications (e.g.aerospace).True time delays (TTD) realized with optical ring resonators.For an OBFN it is desirable to have two basic features: asquint-free behaviour, achievable by using true time delays,and a continuously tunable delay operation. For this reason,optical ring resonators (ORR) appear to be good candidatesto realize the delay elements. Ideal lossless ORRs are opticalall-pass filters, characterized by a unity magnitude responseand continuously tunable group delay response, whichrepresents the effective delay to the radiofrequency (RF)signal that is modulated on the optical carrier.Figure 8: (Color online) Chip die of the 4-bit TTD devicewith an enlarged view of the optical switch.[8]The configuration of the proposed TTD device is shown inFig.7(a). The device is composed of input–outputwaveguides, five 2 2 TIR thermo-optic switches, fourreference lines, and four delay lines, which give 16(24) delaycombinations. The TIR switches have a 250μm waveguideseparation and 4 half branch angle, as Fig.4.7 (b) shows.Fig.4.7 shows the fabricated 4-bit TTD device with anenlarged view of the TIR thermo-optic switch. The chip diedimension is 21.7 mm 13.7 mm.ORR-based OBFN using multiple wavelengths- The basicidea is to create multiple signal paths on the same beamformer. In this way, a single delay line carries the signal ofdifferent antenna elements, thus significantly reducing thenetwork complexity and, in turn, the number of rings andheaters required. This idea is made possible by theexploitation of the frequency-periodic behaviour of the ORRsused as units.Then, by using multi-wavelength lasers and fast integratedmodulators, it is possible to multiplex signals from differentantenna elements on a single path, delaying them of the sameamount. This multiplexing technique is an optical wavelengthdivision multiplexing (WDM).The fully-integrated 4-bit TTD module using TIR switchesexhibits accurate delays, low power consumption,wavelength insensitivity, small chip size and low fabricationcosts. The insertion loss at the wavelength of 1550nm is 13.2to 16.6dB, depending on the delay paths and the states ofoptical switches. The switching time of the TIR switches isbelow 3ms. Design of n bits time delays can be possiblewhere n is a finite integer[8].2.5 Ring Resonator based optical beam formingIntegrated optical beam forming networks (IOBFNs) offermany advantages for phased array applications. ORR-basedtrue-time-delay units can be cascaded in a binary treetopology and tuned for continuously-adjustable broadbandtime delay. Nonetheless, with large number of antennaelements, the IOBFN may become very complex. A novelidea is proposed to exploit the frequency periodicity of theORRs and the WDM technique to achieve multiple-signalpaths on a single beam former, thus reducing complexity andcosts.[9].Phased arrays antennas offer a number of advantages:electronic beam forming (beam shaping and beam steering),multi beaming and interference nulling capability. Inpractice, their performances are limited by the characteristicsof the beam forming network (BFN) used. A possibleimprovement to the limitations of all-electronic BFNs can beachieved integrating electronics and photonics by realizingan optical beam forming network (OBFN). This, in principle,Figure 9: Optical ring resonator (ORR) used as true timedelay (TTD) unit. Tuning parameters (a) and spectralcharacteristics (b)[9]Figure 10: OBFN binary-tree topology[9]Volume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702930
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391As can be seen in the schematic, this technique allows adramatic reduction in the number of rings required,especially in the case of large number of elements N. As adirect consequence, we achieve a reduction in complexity, inarea occupation (the ring dimensions are the limiting factor),in power and heat dissipation (drop in the number of heaters).A novel idea towards the simplification of an existing OBFNhas been proposed. Exploiting the frequency-periodicbehaviour of the ORR-based delay units and filters, it ispossible to realize a WDM-based multi-signal-path OBFN,thus reducing system complexity and cost and makingpossible an integrated realization of a single-chip OBFN forlarge arrays or multiple-beam applications[9].concentric-core PCF with a cross section shown in Fig.11[16], can support two super modes just like in a directionalcoupler, which are designed to be nearly phase matched at awavelength of 0 .The high dispersion in such a PCFarises from the fact that when 0,most of the mode energyis strongly confined in the inner core, and when 0, mostof the mode energy stays in the outer core. Near the phasematched wavelength, there is a strong coupling between thetwo modes and a part of mode energy is in the inner core anda part of it is in the outer core. This redistribution of modeenergy causes the refractive index to change rapidly withwavelength leading to a very high dispersion value near thephase matched wavelength.2.6 Photonic crystal fiber(PCF) based delay linePHASED-ARRAY ANTENNA (PAA) systems have manyadvantages over mechanically steered antenna arrays in termsof speed, sensitivity, and size. However, most of phased arrayantenna radar architectures suffer from problems of beingbulky, sensitive to electromagnetic interference (EMI), beamsquint effect, and limited bandwidth due to the installation oflarge amount of electrical cables and microwave phaseshifting devices. Today‟s phased-array radar technologiescall for frequency independent beam steering, compact andlight weight systems, large instantaneous bandwidth, andEMI free performance [10]. These features can be realized byusing optical true-time delay (TTD) techniques.Furthermore, systems with TTD have the intrinsic capabilityof multi beam operation due to the fact that the opticalsignals with different optical wavelengths can propagatethrough a fiber without interfering with each other, for whichthe widely used dense wavelength division multiplexing(DWDM) system is an illustrative example. Many opticalschemes have been proposed to take advantages of aphotonic feed for true-time delay (TTD), including acoustooptic (AO) integrated circuit technique, Fourier opticstechnique, bulky optics technique, dispersive fiber technique,fiber grating technique, and substrate guided wave techniqueof these techniques, the dispersive fiber technique can reducethe size and weight of the overall system by a significantfactor. Conventional systems use single-mode fibers (SMF)as delay lines to implement the dispersive fiber technique.Since the dispersion coefficient D, of SMF is small (18ps/nm/km @1550 nm), longer lengths of fiber are generallyrequired to generate large time delay values. One alternativeto solve this problem is to use highly dispersive photoniccrystal fibers (PCF), which can be designed to have verylarge dispersion values compared to a conventional SMF.By using such highly dispersive photonic crystal fibers asdelay lines, we can reduce the length of the fiber dramaticallycompared to conventional SMF based systems.Working principle of highly dispersive PCF –The PCF structure is based on a dual concentric coreconfiguration. The inner and outer cores are made up ofdoped silica rods which have a higher refractive indexcompared to background silica. The refractive index of theinner core is slightly greater than that of the outer core. ThisFigure 11: Structure for simultaneous transmission ofmultiple beams, EOM: EDFA, TTD. The cross-sectionalschematic view of the PCF is also shown.[10]The PCF has a chromatic dispersion coefficient of -600ps/nm/km, measured at a wavelength of 1550 nm. Thedispersion value of the PCF is 33 times larger compared tothat of a conventional SMF which is 18 ps/nm/km at 1550nm. This means that we can shrink the length of the fiberused in this system by a factor of 33 compared to the systemusing SMF alone, making the system compact and lightweight.An optical beam former in the transmitting and receivingmode, employing highly dispersive PCF as TTD elements.Dual-beam operation in the transmitting and receiving modesare demonstrated for RF frequencies of 8.4 and 12 GHz.Utilization of short lengths of highly dispersive PCF in theTTD lines makes the overall system compact and lesscomplex. Such a compact, lightweight system, with thecapability of multiple beam transmission and reception, ions[10].The structure and working demonstration of multiple beamtransmission:Using the PCF-based TTD module, multiple-beamtransmission can be realized by using the scheme as shown inFig.11.A general system is shown wherein a multiple number(M) of RF signals are transmitted simultaneously using anantenna array having N elements. A single set of TTD linesgenerates the required time delay values for each element inthe antenna array. External cavity tunable lasers are used togenerate a multiple number of optical carrier waves withVolume 6 Issue 3, March 2017www.ijsr.netLicensed Under Creative Commons Attribution CC BYPaper ID: 9031702931
International Journal of Science and Research (IJSR)ISSN (Online): 2319-7064Index Copernicus Value (2015): 78.96 Impact Factor (2015): 6.391wavelengths 1 to M. RF signals with different frequenciesare modulated onto these optical carrier waves using electrooptic modulators (EOM). After passing through the EOMs,the optical carrier waves are combined together through anM-to-1 combiner and amplified using an erbium-doped fiberamplifier (EDFA). A 1-to-N optical power splitter divides theamplified optical signal to N TTD lines. Each TTD line hasan equal length and consists of different lengths of PCF andSMF segments. The lengths are chosen in such a way that ata wavelength of 0, the nominal delay through each TTD lineis the same and the beam is radiated broadside at the antennaarray. For wavelengths greater than or less than 0 , differenttime delays are induced in each TTD line, with a constanttime delay difference between adjacent channels at eachwavelength, and the beam is steered at an angle θ given by(4)For N TTD lines having PCF segments of lengths L1, L2,L3.and LN , respectively, as shown in Fig.4.11 Theadditional time delay generated in a delay line having PCFsegment of length Li is given by(5)The first term is contributed by the PCF section and thesecond term by the conventional single mode fiber. Since thedispersion coefficient of the PCF is much larger compared tothat of the SMF, for a fixed wavelength , the difference oftime delays between different channels are only determinedby the lengths of the PCF segments.Since the dispersion coefficient of the PCF is much largercompared to that of the SMF, for a fixed wavelengthdifference of time delays between different channels are onlydetermined by the lengths of the PCF segments. Therefore,by making the lengths of the PCF an arithmetic sequence, wecan achieve equal time delay differences between adjacentTTD lines at any given wavelength, thus, forming awavelength-tuned TTD line. After the optical signals passthrough the TTD lines, they are converted back to electricalsignals at the photo detector bank. These electrical signalsnow provide the phase information for the antenna array.Since for a fixed optical wave length, the time delay is onlyrelated to the lengths of PCF segments, the delay time of theeach output electrical signal is controlled continuously bytuning the optical wavelengths. Each optical wavelengthcreates a time delay set corresponding to a specific steeringangle as given by Eq.(4). By injecting laser beams withmultiple wavelengths simultaneously, one can generateequivalent number of independently steered RF far fieldpatterns at the same time due to the squint-free nature of theTTD lines.2.7 Ph
7) Photonic Microwave Delay line using Mach-Zehender Modulator 8) Optical Mux/Demux based delay line 9) PCW based AWG Demux /TTDL 10) Sub wavelength grating enabled on-chip 11) ultra-compact optical true time delay line . 2.1 Fiber based delay line . Traditionally, feed networks and phase shifters for phased
Bruksanvisning för bilstereo . Bruksanvisning for bilstereo . Instrukcja obsługi samochodowego odtwarzacza stereo . Operating Instructions for Car Stereo . 610-104 . SV . Bruksanvisning i original
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the phase delay x through an electro-optic phase shifter, the antennas are connected with an array of long delay lines. These delay lines add an optical delay L opt between every two antennas, which translates into a wavelength dependent phase delay x. With long delay lines, this phase delay changes rapidly with wavelength,
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