A Microwave Polarimeter Demonstrator for Astronomy with Near-Infra-Red Up-Conversion for Optical Correlation and Detection

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Article A Microwave Polarimeter Demonstrator for Astronomy with Near-Infra-Red Up-Conversion for Optical Correlation and Detection

Francisco J. Casas 1,*, David Ortiz 1, Beatriz Aja 2 , Luisa de la Fuente 2, Eduardo Artal 2 , Rubén Ruiz 3 and Jesús M. Mirapeix 3,4,5

1 Instituto de Física de Cantabria (IFCA), Avda. Los Castros s/n, 39005 Santander, Spain; [email protected] 2 Departamento Ingeniería de Comunicaciones (DICOM), Universidad de Cantabria, Plaza de la Ciencia s/n, 39005 Santander, Spain; [email protected] (B.A.); [email protected] (L.d.l.F.); [email protected] (E.A.) 3 Grupo de Ingeniería Fotónica, Universidad de Cantabria, Plaza de la Ciencia s/n, 39005 Santander, Spain; [email protected] (R.R.); [email protected] (J.M.M.) 4 Biomedical Research Networking Center in Bioengineering Biomaterials and Nanomedicine (CIBER-BBN), Plaza de la Ciencia s/n, 39005 Santander, Spain 5 Instituto de Investigacion Sanitaria Valdecilla (IDIVAL), Calle Cardenal Herrera Oria, 39011 Santander, Spain * Correspondence: [email protected]; Tel.: +34-942-200-892; Fax: +34-942-200-935

Received: 4 March 2019; Accepted: 16 April 2019; Published: 19 April 2019

Abstract: This paper presents a 10 to 20 GHz bandwidth microwave polarimeter demonstrator, based on the implementation of a near-infra-red frequency up-conversion stage that allows both the optical correlation, when operating as a synthesized-image interferometer, and signal detection, when operating as a direct-image instrument. The proposed idea is oriented towards the implementation of ultra-sensitive instruments presenting several dozens or even thousands of microwave receivers operating in the lowest bands of the cosmic microwave background. In this work, an electro-optical back-end module replaces the usual microwave detection stage with Mach–Zehnder modulators for the frequency up-conversion, and an optical stage for the signals correlation and detection at near-infra-red wavelengths (1550 nm). As interferometer, the instrument is able to correlate the signals of large-format instruments, while operating as a direct imaging instrument also presents advantages in terms of the possibility of implementing the optical back end by means of photonic integrated circuits to achieve reductions in cost, weight, size, and power consumption. A linearly polarized input wave, with a variable polar angle, is used as a signal source for laboratory tests. The receiver demonstrator has proved its capabilities of being used as a new microwave-photonic polarimeter for the study of the lowest bands of cosmic microwave background.

Keywords: instrumentation; astronomy; polarization; cosmic microwave background; microwave photonics; direct imaging; synthesized imaging; interferometry

1. Introduction Penzias and Wilson in 1964 measured a noise-like signal [1] that was ﬁnally identiﬁed as the cosmic microwave background (CMB). This radiation is the remaining footprint of the Big Bang and was postulated by Gamow, Alpher, and Herman in the late 1940s [2]. CMB intensity and polarization measurements have been an invaluable resource for testing cosmological models and fundamental physics, since the processes that operated in the early Universe, or acted on the photons during their passage to the Earth, have imprinted very weak but distinct features on the otherwise

Sensors 2019, 19, 1870; doi:10.3390/s19081870 www.mdpi.com/journal/sensors Sensors 2019, 19, 1870 2 of 17 uniform background. Space missions [3–6] and ground-based experiments [7–9] have been dedicated, with increasingly higher sensitivity, to the analysis of temperature and polarization anisotropies of the CMB, with the aim of measuring the B-mode polarization pattern predicted by inflationary models of the early Universe. B-mode signals may reveal crucial information about our Universe, such as if inflation really happened, the signal level of primordial gravitational waves predicted by inflation, the number of neutrino species and their mass, the existence of magnetic fields in the early Universe, and the origin of magnetic fields in galaxies and galaxy clusters. Accurate measurements of B-modes can also reveal (or place limits on) cosmic birefringence, a revolutionary departure from the standard model that allows one to probe the validity of fundamental symmetries, investigate the nature of dark matter, and test extensions of general relativity. B-modes can have different origins, arising from either the primordial gravitational waves produced at inflation, primordial magnetic fields, the gravitational lensing of the CMB, or astrophysical emissions (mainly synchrotron and thermal dust). B-mode signals are faint and can be easily contaminated. So, only with the use of ultra-sensitive instrumentation is it possible to detect and characterize both B-modes and foreground contamination that appear at different frequencies. The number of receivers or detectors required to measure the B-mode polarization depends on the frequency. The signal from the sky is formed mainly by contaminants as synchrotron and thermal dust, while the CMB represents only a very small fraction of the overall signal. Synchrotron dominates at low frequencies (1–80 GHz) while thermal dust is predominant at high frequencies (400–1000 GHz). Due to this, a lot of the experiments [8,9] designed to measure the B-modes are focused between these two ranges of frequencies (cosmological bands) presenting thousands of detectors (usually bolometers). To remove the contaminants from the CMB signal, it is required to characterize them with accuracy similar to the one of the experiments dedicated to the cosmological bands. At low frequencies, as the ones of the reported demonstrator, the synchrotron emission presents a much higher signal level than the CMB at the cosmological bands. So, in order to have similar accuracy, the required number of detectors or receivers is much lower. In the 10–20 GHz frequency band, an instrument with around 30–40 receivers should be enough, while at 30 or 40 GHz, 300–400 would be needed to characterize the synchrotron with the required accuracy. Most of the present active experiments operate as direct image telescopes whose number of receivers, and hence, sensitivity, is limited by the space available in the focal plane. This aspect is especially critical when operating at the lowest bands of the microwave range (1–20 GHz), where the size of the receiver horn antennas is bigger. Therefore, alternative ways to achieve better sensitivities must be considered. The use of an interferometer overcomes the space limitation of telescopes and avoids their high cost, by potentially correlating a much larger number (hundreds or even thousands) of receivers or detectors. However, the number of receivers for CMB low-frequency interferometers is typically less than 20, due to the limitations of traditional analog correlators in terms of phase controlling and routing of a high number of wide-band microwave signals. On the other hand, the use of digital correlators does not seem the most suitable option for this type of CMB experiments, due to the high cost (relative to the overall experiment budget) and power consumption of the large number of broadband digitizing cards needed to correlate a large number of wide-band signals in real time and during several months or even years. Other issues, as the requirement of a complex down-conversion and channelization structure and the digitalization noise added to the signals, must also be taken into account. However, for big experiments with as large budgets as the Square Kilometre Array (SKA) [10], all these aspects probably do not represent insurmountable issues. In fact, in that particular case, digital electronics is the technology used to correlate signals from their very high-resolution astronomical observations. The use of electro-optical correlators is an alternative solution already implemented in embedded instruments designed for security and defense [11], and also proposed for astronomy as a Michelson correlator [12]. They allow a drastic reduction in the complexity of correlating the large amount of wideband microwave signals for large-format instruments, in contrast to microwave correlators and, moreover, avoiding expensive telescopes. In this work, this concept is applied to a microwave Sensors 2019, 19, 1870 3 of 17

Sensors 2019, 19, x FOR PEER REVIEW 3 of 17 polarimeter [13] operating in two frequency sub-bands: From 10 to 14 GHz and from 16 to 20 GHz. In particular,correlators a and demonstrator, moreover, avoiding is developed expensive to testtelescopes the operation. In this work, and this viability concept of is the applied instrumental to a microwave polarimeter [13] operating in two frequency sub-bands: From 10 to 14 GHz and from 16 concept shown in Reference [11], applied to CMB astronomical experiments. Figure1 shows a to 20 GHz. In particular, a demonstrator is developed to test the operation and viability of the simplified block diagram of the polarimeter instrument. The prototype developed in this work is instrumental concept shown in Reference [11], applied to CMB astronomical experiments. Figure 1 composedshows ofa simplified a front-end block module diagram (FEM) of the connected polarimeter to twoinstrument. microwave The prototype receivers, devel operatingoped in in this the two sub-bandswork is previously composed of mentioned a front-end and module an electro-optical (FEM) connected back-end to two microwave module (EOBEM) receivers, withoperating a frequency in up-conversionthe two sub stage-bands (FUS) previously at the input,mentioned connected and anto electro an optical-optical correlation back-end m andodule detection (EOBEM) stage with (OCDS). a The microwavefrequency up receivers-conversion share stage the (FUS) conceptual at the input, design connected of the to onesan optical of QUIJOTE correlation experiment and detection [14 ,15], but somestage modifications (OCDS). The m areicrowave included receivers to split share the 10–20 the conceptual GHz input design signal of in the two ones sub-bands of QUIJOTE of interest (10–14experiment GHz and [1 16–204,15], but GHz). some On modifications the other hand,are included the detection to split the stage 10–20 of GHz QUIJOTE-like input signal receiversin two is replacedsub-bands here by of the interest EOBEM (10–14 with GHz an and input 16– microwave20 GHz). On to the near-infra-red other hand, (NIR) the detection FUS, composed stage of of a QUIJOTE-like receivers is replaced here by the EOBEM with an input microwave to near-infra-red laser and a set of commercial LiNbO3 Mach–Zehnder modulators (MZM), and an OCDS implemented (NIR) FUS, composed of a laser and a set of commercial LiNbO3 Mach–Zehnder modulators (MZM), basicallyand awithn OCDS a fiber implemented array, a pair basically of lenses, with and a fiber a camera. array, All a pair these of lenses components, and a camera. are already All these described in Referencecomponent [16s ].are An already advantage described of this in Reference concept [1 is6 that]. An the advantage same OCDS of this can concept be used is tha tot th operatee same both as a synthesized-imageOCDS can be used to interferometer operate both as and a synthesized also as a traditional-image interferometer imager, only and byalso changing as a traditional the optical configurationimager, only of theby OCDS.changing In the the optical first case, configuration the instrument of the providesOCDS. In athe synthesized first case, the image instrument of the polar parametersprovides in a synthesized the sky region image determined of the polar parameters by the instrument in the sky region field ofdetermined view (FoV), by the which instrument is mainly determinedfield of v byiew the (FoV), beam which of the is hornmainly antennas. determined In theby the second beamcase, of the the horn OCDS antennas. is basically In the second a NIR case detection, stagethe of OCDS the up-converted is basically a microwaveNIR detection signals stage of that the haveup-converted the polar microwave information signals of that the have CMB. the In the last case,polar ainformation telescope isof requiredthe CMB. toIn the focus last the case signal, a telescope from is the required sky to to the focus instrument, the signal butfrom the the potential sky to the instrument, but the potential implementation of the EOBEM and part of the receivers in implementation of the EOBEM and part of the receivers in narrow-band PIC (photonic integrated narrow-band PIC (photonic integrated circuits) technology, makes this concept also interesting, circuits)expect technology,ing reduction makess in cost, this conceptvolume, alsoweight interesting,, and power expecting consumption reductions with respect in cost, tovolume, wide-band weight, and powertraditional consumption microwave withtechnology respect. to wide-band traditional microwave technology.

Microwave Front-End Module and Receivers Electro-Op cal Back-End Module

g MZM n Cryo_LNA i h Microwave c MZM e t l

Polarizer i

u Correla on w & OMT d MZM S

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M MZM s

a Lens Lens h

P I+Q Cryo_LNA I-Q

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g MZM d1 d2 d3 n m i Cryo_LNA a h

Microwave Fiber C MZM c e t l Polarizer R i Correla on Bundles I u N

w MZM & OMT d

S Module o Horn_N -

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Frequency Up-conversion Stage (FUS) FigureFigure 1. Simpliﬁed 1. Simplified block-diagram block-diagram of the of microwave the microwave polarimeter polarimeter for N forreceivers N receivers with correlation with / detectioncorrelation/detection in the near-infra-red in the n (1550ear-infra nm).-red (1550 nm).

In thisIn this work, work a demonstrator, a demonstrator of of the the proposed proposed concept is is developed developed and and tested tested in the in laboratory the laboratory using a linearly polarized input wave with a variable polar angle as excitation. The operation of the using a linearly polarized input wave with a variable polar angle as excitation. The operation of polarimeter is tested in both modes of operation (interferometry and direct image). Due to the the polarimetercommercial MZM is tested bandwidth in both limitations, modes of only operation the signal (interferometry from 10 to 12 GHz and directis characterized image)., Duewhile to the commercialthe rest MZMof the bandwidthsetup allows limitations, for the measurement only the signalof the fromcomplete 10 to two 12 sub GHz-bands is characterized, of interest (10 while–14 the rest ofGHz the and setup 16–20 allows GHz). for the measurement of the complete two sub-bands of interest (10–14 GHz and 16–20This GHz). document is divided into five sections. The first one is an introduction, followed by a Thisdetailed document description is of divided the proposed into five polarimeter sections. in Section The first 2. In one Section is an 3, introduction,the polarimeter followedoperation by a detailedis presented description showing of the laboratory proposed measurement polarimeter results in Section. The2 . demonstrator In Section3, thepossible polarimeter enhancements, operation is presented showing laboratory measurement results. The demonstrator possible enhancements, potential, and future plans are discussed in Section4, and, finally, Section5 draws the general conclusions from this work. SensorsSensors2019 2019, 19, ,19 1870, x FOR PEER REVIEW 4 of4 17 of 17

potential, and future plans are discussed in Section 4, and, finally, Section 5 draws the general 2. Polarimeterconclusions from Description this work. In this section, the microwave polarimeter, shown in Figure1 for N receivers, is described. 2. Polarimeter Description As mentioned previously, the instrument is composed of a microwave FEM connected to receivers and an electro-opticalIn this section, back-end the microwave module with polarimeter the FUS, atshown the input, in Figure connected 1 for N to receivers, an OCDS. is As describ it is showned. As in Sectionmentioned 2.2, the previously, dual operation the instrument of the polarimeter is composed is determined of a microwave by the FEM distances connected between to receivers the fiber and array, an electro-optical back-end module with the FUS at the input, connected to an OCDS. As it is shown the lenses, and the camera (d1, d2, and d3). Due to the particular design of the microwave receivers, thein four Section output 2.2, the signals dual ofoperation the microwave of the polarimeter correlation is determined module are by proportional the distances to between a combination the fiber of thearray, Stokes the parameters lenses, and (I the+ Q, camera I Q, (d I1,+ dU,2, and I U). d3). TheirDue to values the particular are determined design ofby the the microwave polarization − − ofreceivers, the input wave the four [17 ,output18]. That signals is the of reason the m foricrowave having fourcorrelation fiber arrays module (bundles) are proportional and four images to a combination of the Stokes parameters (I + Q, I − Q, I + U, I − U). Their values are determined by the represented in the NIR camera. polarization of the input wave [17,18]. That is the reason for having four fiber arrays (bundles) and The reported demonstrator is down-scoped with respect to the polarimeter scheme shown in four images represented in the NIR camera. Figure1. First, the number of receivers (N) is only two, which is the minimum number required to be The reported demonstrator is down-scoped with respect to the polarimeter scheme shown in used as an interferometer. The operation of the two-receiver demonstrator is tested in direct image Figure 1. First, the number of receivers (N) is only two, which is the minimum number required to be andused also as in an interferometry interferometer. operation The operation mode. of the Only two one-receiver fiber arraydemonstrator with 46 fibersis tested is implementedin direct image for theand demonstrator also in interferometry so the interferometer operation mode mode. O ofnly operation one fiber isar testedray with by 46 correlating fibers is implemented two signals, for each onethe coming demonstrator from the so correspondingthe interferometer two mode receivers of operation set of fouris tested output by correlating signals. Therefore, two signals, only each one synthesizedone coming image, from providingthe corresponding the complete two receivers polar information set of four ofoutput the inputsignals. wave, Therefore is obtained, only one in the interferometrysynthesized imag tests,e, as providing it is explained the complete in References polar information [17,18] and of described the input in wave, the next is obtained sections. in the interferometry tests, as it is explained in References [17,18] and described in the next sections. 2.1. Front-End and Microwave Receivers 2.1.In Front this- section,End and Microwave some details Receivers about the microwave receivers are explained. As it was mentioned before,In the this receivers section, have some the details same about conceptual the microwave design of receivers the ones are of explained. QUIJOTE experimentAs it was mentioned [14,15] but withbefore some, the improvements receivers haveand the same optimization conceptual in design the design. of the Inones particular, of QUIJOTE the operationexperiment bandwidth [14,15] but is thewith same some of the improve QUIJOTEments multi-frequency and optimization instrument in the design. (MFI) In [ 15particular,], but with the a operation receiver designbandwidth using is an electronicthe same polarization of the QUIJOTE modulation multi-frequency similar to instrument thirty- and (MFI) forty-gigahertz [15], but with instruments a receiver (TGIdesign and using FGI) an [19 ]. Onelectronic the other polarization hand, while modulation MFI has two similar polarimeters to thirty covering- and forty the-g bandigahertz from instruments 10 to 14 GHz (TGI and and the FGI) other two[19 covering]. On the theother band hand, from while 16 toMFI 20 has GHz, two the polarimeters receiversof covering the proposed the band demonstrator from 10 to 14 split GHz the and full 10–20the other GHz two input covering signal the to obtainband from the Stokes16 to 20 parameters GHz, the receivers of the twoof the sub-bands proposed demonstrator in each polarimeter. split Therefore,the full 1 it0– would20 GHz be input possible signal to tooptimize obtain the fillingStokes of parameters the telescope of the focal two plane, sub-bands improving in each the sensitivitypolarimeter of. theTherefore resulting, it instrument.would be possible Figure to2 optimizeshows a the detailed filling diagram of the tele ofscope the front-end focal plane and, microwaveimproving receiver the sensitivity implemented of the resulting for this work. instrument. Front-end Figure and 2 shows first back-end a detailed elements diagram are of designedthe front- to end and microwave receiver implemented for this work. Front-end and first back-end elements are cover the overall 10–20 GHz bandwidth. Then, the diplexer splits the original bandwidth in the two designed to cover the overall 10–20 GHz bandwidth. Then, the diplexer splits the original bandwidth sub-bands of interest (10–14 GHz and 16–20 GHz) to feed two correlation modules, which combine the in the two sub-bands of interest (10–14 GHz and 16–20 GHz) to feed two correlation modules, which signals in such a way that, at the output, the microwave signals are proportional to a combination of combine the signals in such a way that, at the output, the microwave signals are proportional to a the corresponding Stokes parameters in each frequency band. combination of the corresponding Stokes parameters in each frequency band.

Figure 2. Detailed schematic of the front-end and microwave receivers designed for this work. Figure 2. Detailed schematic of the front-end and microwave receivers designed for this work.

Sensors 2019, 19, 1870 5 of 17

Sensors 2019, 19, x FOR PEER REVIEW 5 of 17 In Figure2, the cryogenic microwave front-end (colored in blue) is cooled-down to 20 K in In Figure 2, the cryogenic microwave front-end (colored in blue) is cooled-down to 20 K in order order to have the required signal to noise ratio when making astronomical observations of the CMB. to have the required signal to noise ratio when making astronomical observations of the CMB. It is It is comprised of a feed-horn, a polarizer, and an orthomode transducer (OMT) to split the polar comprised of a feed-horn, a polarizer, and an orthomode transducer (OMT) to split the polar components of the signal, which are introduced into two cryogenic low noise ampliﬁers (LNA) [17,20]. components of the signal, which are introduced into two cryogenic low noise amplifiers (LNA) All these components cover the overall bandwidth from 10 to 20 GHz. Figure3a shows pictures of the [17,20]. All these components cover the overall bandwidth from 10 to 20 GHz. Figure 3a shows passive components of the microwave front-end. pictures of the passive components of the microwave front-end.

(a) (b)

Figure 3. (a)) Picture of the two feed-horns,feed-horns, orthomode transducer ( (OMT),OMT), and polarizers assembled together. ( (b) Picture Picture of of the the rack rack containing containing two two units units of of the the ambient ambient-temperature-temperature microwave microwave receivers receivers of Figure 22..

Also, in in Figure Figure 2, t thehe ambient ambient-temperature-temperature microwave microwave receiver receiver (colored (colored in in red) red) is iscompr comprisedised of twoof two gain, gain, polarizati polarizationon modulation modulation (phase (phase-switching),-switching), and and diplexer diplexer input input module moduless ( (citedcited as as BEM modules inin FigureFigure2 ),2) followed, followed by by two two microwave microwave correlation correlation output output modules. modules As the. As aim the of aim this of work this workis to demonstrate is to demonstrate the operation the of operation the proposed of the instrument proposed concept, instrument the prototype concept, implemented the prototype and implementedtested in the laboratory and tested for in thisthe laboratory work is an for ambient-temperature this work is an ambient version-temperature of the one shownversion in of Figure the one2, shownremoving in Figure only the 2, cryogenicremoving LNAonly andthe cryogenic without using LNA a and cryostat. without Consequently, using a cryostat. the laboratory Consequently polar, thetest laboratory signal is generated polar test with signal the requiredis generated power with to bethe detectable, required power presenting to be a detectable higher power, presenting level than a higherthe noise power added level by than the receiver the noise itself added [20 ].by Figure the receiver3b shows itself a picture [20]. Figure of a rack 3b shows containing a picture two of units a rack of containingthe ambient-temperature two units of the microwave ambient- receiverstemperature of Figure microwave2 assembled receivers and of operating Figure 2 in assembled the laboratory. and operatingOver the rack,in the a laboratory PCB with. anOver Arduino the rack system, a PCB that with is an used Arduino to control system the that polarization is used to modulation control the polarizatistage canon be modulation seen, composed stage of can the be two seen phase-switches, composed of also the two shown phase in Figure-switches2. These also shown phase-switches in Figure 2.are These composed phase- ofswitches two commercial are compo units,sed of CGY2173UHtwo commercial from units OMMIC,, CGY2173UH which is from a high-performance OMMIC, which isGaAs a high MMIC-performance 6–bit phase GaAs shifter MMIC operating 6–bit phase nominally shifter operating from 6 GHz nominally up to 18 from GHz 6 butGHz working up to 18 quiteGHz butwell working until 20 quite GHz. well The until CGY2173UH 20 GHz. The has CGY2173UH a nominal phase has a shifting nominal range phase of shifting 0◦–360 ◦rangein 5.625 of 0◦°–steps360° in(64 5.625° phase-states). steps (64 For phase our-states). tests, only For fourour tests, different only phase-states four different are phase used by-states shifting are used in 90 ◦bysteps shifting [17,18 in]. 90°The stepsrequired [17, 6-bits18]. The control required signals 6- arebits provided control signals by the previouslyare provided mentioned by the Arduinopreviously system. mentioned Arduino system. 2.2. Electro-Optical Back-End 2.2. ElectroAs it- isOptical shown Back in-End Figure 1, the electro-optical back-end is composed of the input frequency up-conversionAs it is shown stage in (FUS) Figure connected 1, the electro to the- OCDS,optical whichback-end presents is composed two diff erentof the optical input f configurationsrequency up- conversiondepending onstage the (FUS) type ofcon operationnected to (interferometer the OCDS, which/imager). present Thes two demonstrator different optical of this configurations work has only dependingtwo receivers, on whichthe type is theof operation minimum (interferometer/imager). number required to be used The as demonstrator an interferometer. of this The work commercial has only twoMach–Zehnder receivers, modulatorswhich is the (MZM) minimum used in number the FUS required have a nominal to be bandwidth used as an of 10interferometer GHz. Hence,. onlyThe commercialfour MZM are Mach selected–Zehnder from m theodulators initial set (MZM) of 16 units used tested in the in FUS the laboratoryhave a nomi showingnal bandwidth an acceptable of 10 GHz.response Hence to microwave, only four signals MZM are up toselected 12 GHz from with the enough initial level set of of 16 stability units astested well in as the optical laboratory carrier showingrejection [an16 ].acceptable Another diresfferenceponse to between microwave the demonstrator signals up to and 12 GHz the scheme with enough in Figure level1 is of that stability only one as wellfiber as array optical with carrier 46 fibers rejection is implemented [16]. Another instead difference of the fourbetween shown the in demonstrator that figure. Nevertheless, and the scheme this in is Figure 1 is that only one fiber array with 46 fibers is implemented instead of the four shown in that figure. Nevertheless, this is not a limitation to test the operation of the demonstrator, since only four

Sensors 2019, 19, 1870 6 of 17

Sensors 2019, 19, x FOR PEER REVIEW 6 of 17 not a limitation to test the operation of the demonstrator, since only four fibers of the array are needed to test the imaging operation of each receiver (one per MZM of the set of four connected to the microwave fibers of the array are needed to test the imaging operation of each receiver (one per MZM of the set receiver outputs) and a minimum of only two fibers are needed to test the operation as interferometer. of four connected to the microwave receiver outputs) and a minimum of only two fibers are needed In this last case, the synthesized image resulting from the interference of two up-converted output to test the operation as interferometer. In this last case, the synthesized image resulting from the signals from each receiver is achieved, which corresponds to the same Stokes parameter combination interference of two up-converted output signals from each receiver is achieved, which corresponds (same output of each receiver). to the same Stokes parameter combination (same output of each receiver). Figure4a,b shows pictures of the MZM connected to the rear panel of the microwave receiver Figure 4a,b shows pictures of the MZM connected to the rear panel of the microwave receiver rack and the OCDS. Figure4b shows, from right to left, the fiber array, two lenses of 10 cm focal length, rack and the OCDS. Figure 4b shows, from right to left, the fiber array, two lenses of 10 cm focal an iris, and the near-infra-red (NIR) optical camera. Most of the components of the OCDS, except the length, an iris, and the near-infra-red (NIR) optical camera. Most of the components of the OCDS, camera, are described in Reference [16]. except the camera, are described in Reference [16].

(a)

(b)

FigureFigure 4.4.( a(a)) Picture Picture of of the the frequency frequency up-conversion up-conversion stage stage (FUS) (FUS composed) composed of a set of of a four set Mach–Zehnderof four Mach– modulatorsZehnder modulators (MZM). (b ()MZM Picture). (b of) Picture optical correlationof optical correlation and detection and stage detection (OCDS) stage composed (OCDS) ofcomposed the ﬁber arrayof the twofiber lenses array and two a lenses near-infra-red and a near (NIR)-infra camera.-red (NIR) camera.

In this work, the demonstrator is implemented with a 640 512-pixel resolution camera from In this work, the demonstrator is implemented with a 640 ×× 512-pixel resolution camera from Xenics (Leuven, Belgium), model Cheetah-640-CL, which presents a pixel size of 20 20 µm, a spectral Xenics (Leuven, Belgium), model Cheetah-640-CL, which presents a pixel size× of 20 × 20 μm, a bandspectral from band 0.9 to from 1.7 µ 0.9m, to and 1.7 a frameμm, and rate a at frame maximum rate at resolution maximum of resolution 865 Hz. This of last 865 characteristic Hz. This last ischaracteristic very important is very for important the reported for application,the reported sinceapplication, it is required since it to is acquirerequired the to imagesacquire atthe a images higher rateat a thanhigher the rate one than given the by one the given phase-switching by the phase frequency-switching of frequency the microwave of the receivers. microwave Since receivers. the 1/f kneeSince frequency the 1/f knee of the frequency cryogenic of LNA the cryogenic is expected LNA to be is around expected 10 to Hz, be the around phase-switching 10 Hz, the rate phase in- realswitching operation rate shouldin real operation be about 100should Hz tobe avoidabout the100 systemHz to avoid gain the variation. system Therefore,gain variation. a frame Therefore rate of, abouta frame 1 kHz,rate of which about can 1 kHz be easily, which achieved can be byeasily slightly achieved reducing by slightly the actual reducing acquisition the actual resolution acquisition of the camera,resolution is adequateof the camera, for this is adequate particular for application. this particular The polarimeterapplication. operation The polarimeter mode is operation determined mode by theis determined distances between by the thedistances fiber array, between the lenses, the fiber and array, the camera the lenses (d1, d, 2and, and the d3 camerain Figure (d11)[, d212, ].andf being d3 in theFigure focal 1) length [21]. f ofbe theing lenses,the focal in length order to of operate the lenses, as an in imaging order to instrument, operate as an a 4 imagingf optical configurationinstrument, a is4f used.optical In configuration this case, d1 is= used.d3 = fInand this d case,2 = 2 fd.1 On= d3 the = f and other d2 hand, = 2f. On in the order other to operatehand, in as order a synthesized to operate imageas a synthesized interferometer, image a 6interferometerf optical configuration, a 6f optical is used, configuration with d1 = is2f ,used d2 =, with3f and d1d =3 2=f, fd.2 This = 3f lastand cased3 = isf. This the one last shown case is in the Figure one 4shownb. The in reported Figure optical4b. The configurations, reported optical using config twourations, lenses, areusing well two intended lenses, forare thewell use intended of a transmission for the use optical of a transmission filter to remove optical the filter optical to carrierremove and the one optical of the carrier side bands and one from of thethe modulatedside bands from NIR signal the modulated at MZMoutputs. NIR signal However, at MZM the outputs. reported However, demonstrator the reported does not demonstrator implement does not implement such a filter, because an optimal filtering solution operating in transmission has not been found yet. As it is explained in Section 4, the use of a reflection filter is tested instead.

Sensors 2019, 19, 1870 7 of 17 such a filter, because an optimal filtering solution operating in transmission has not been found yet. As it is explained in Section4, the use of a reflection filter is tested instead. Nevertheless, by using the optimal operation point of the MZM and a stabilization method [16], it is possible to demonstrate the instrumental concept in both configurations, as the next section describes.

3. Polarimeter Operation This section is focused on the tests of the polarimeter in both modes of operation. Figure5 shows a sketch of the measurement test bench implemented in the laboratory. In order to test the polarimeter, a variable polarization source is used, which provides microwave signals in the 10 to 20 GHz frequency range. They are generated with a noise source 346B from Agilent (now Keysight Technologies, Santa Rosa, CA, USA) with excess noise ratio (ENR) 15 dB, amplified with a broadband microwave system amplifier HP83017A of gain 37 dB and noise figure 5 dB at 12 GHz, and transmitted through a broadband log-periodic antenna HL050 from Rohde and Schwarz (Munich, Germany) with almost rotation-symmetrical pattern. The amplifier and the antenna are connected with a coaxial rotary joint, allowing the rotation of the antenna and the generation of polarized signals with different polar angles. As the polar angle variation is implemented by means of a simple hand-controlled mechanical system, the estimated polar angle error of the source is around one degree. In order Sensors 2019, 19, x FOR PEER REVIEW 7 of 17 to test the demonstrator operation this error level is feasible, but for astronomical measurements, Nevertheless,a software control by using system the providing optimal operation lower angular point error of the is required.MZM and The a stabilization far-field distance method considering [16], it is possiblethe frequency to demonstrate and antenna the diameterinstrumental is 1.25 concept m. in both configurations, as the next section describes.

Microwave Front-End and Receivers Electro-Op?cal Back-End Module

Variable Polariza, on Signal Source

FUS Polarizer Microwave MZM OCDS Power Supply Receiver & OMT MZM Horn_1 Linear Polariza, on Rota, ng Antenna Noise Source Instrumenta, on d = 1.25 m Laser Amplifier

MZM Polarizer Microwave MZM & OMT Receiver Horn_2

FigureFigure 5. Sketch 5. Sketch of measurement of measurement test test-bench-bench implemented implemented to test to test the thepolarimeter polarimeter demonstrator demonstrator..

3. PolarimeterThis distance Operation is required only for the operation as interferometer (for direct imaging the source couldThis be closer)sectionbut is focused in order on to the simplify tests of the the setup, polarimeter the same in both distance modes for of the operati two operationon. Figure modes 5 shows is aused. sketch Consequently, of the measurement the power budgettest bench is calculated implemented to avoid in saturationthe laboratory in the. polarimeterIn order to receivers. test the polarimeterFigure6 shows, a variable some pictures polarization of the source measurement is used, which test-bench provides in the microwave laboratory, signals operating in the in 10 direct to 20 GHimagingz frequency mode (4 rangef optical. They configuration). are generated The with measurement a noise source procedure 346B from in both Agilent operation (now modesKeysight is Technologies,similar, because Santa an imagingRosa, CA, instrument USA) with performs excess noise the polarizationratio (ENR) 15 measurement. dB, amplified For with direct a broadband imaging, microwavethe power of system each receiver amplifier is up-converted HP83017A and of gaindetected 37 dBdirectly and by noise the NIRfigure camera. 5 dB For at interferometry, 12 GHz, and transmittedthe optical system through provides a broadband the synthesized log-periodic image antenna that isHL050 also detected from Rohde with theand camera.Schwarz In (Munich, order to Germany)have the same with optical almost power rotation level-symmetrical coming out pattern. of each The fiber amplifier of the bundle, and the the antenna power are level connected is tuned withusing a variablecoaxial rotary attenuators. joint, allowing The homogeneity the rotation in opticalof the antenna power is and required the generation in both operation of polarized modes, signals but withit is more different critical polar when angles. operating As the as polar an interferometer. angle variation In is this implemen last case,ted a tuningby means method of a simple for the hand phase- controlledis also required mechanical for instrument system, the calibration. estimated polar angle error of the source is around one degree. In orderThe to image test the synthesized demonstrator by only operation two receivers this is error basically level a group is feasible, of fringes, but which for astronomical is not good meenoughasurements to be used, a software in actual control observations. system Inproviding order to havelower a angular cleaner image,error is more required. receivers The placed far-field in distancean optimized considering configuration the frequency should and be usedantenna to provide diameter an is optimal 1.25 m. point spread function (PSF) (see ReferencesThis distance [11,16,21 is] required for more only details). for the On operation the other as hand, interferometer the polarization (for direct measurement imaging the method source is could be closer) but in order to simplify the setup, the same distance for the two operation modes is used. Consequently, the power budget is calculated to avoid saturation in the polarimeter receivers. Figure 6 shows some pictures of the measurement test-bench in the laboratory, operating in direct imaging mode (4f optical configuration). The measurement procedure in both operation modes is similar, because an imaging instrument performs the polarization measurement. For direct imaging, the power of each receiver is up-converted and detected directly by the NIR camera. For interferometry, the optical system provides the synthesized image that is also detected with the camera. In order to have the same optical power level coming out of each fiber of the bundle, the power level is tuned using variable attenuators. The homogeneity in optical power is required in both operation modes, but it is more critical when operating as an interferometer. In this last case, a tuning method for the phase is also required for instrument calibration.

Sensors 2019, 19, x FOR PEER REVIEW 8 of 17 Sensors 2019, 19, 1870 8 of 17 basically the same that the one explained in References [17,18] for the QUIJOTE experiment. For the measurement tests, the camera acquisition ratio (frame rate) is set to 100 Hz. The applied polarization modulation phase-switching frequency is 3 Hz, so the complete cycle with 16 phase -states provided by the phase switches (four sequences of 0, 90, 180, and 270 deg. phase-shift between receiver’s branches [17,18]) is repeated approximately every 5.4 s. In order to integrate the power of the Near-Infra-Red signals imaged with the camera, the detected signal level is calculated as the mean value in a certain number of camera pixels, where the up-converted signals are imaged (direct imaging) or the maximum of the synthesized image power is found (interferometry). Figure7 shows two (a) pictures of the images achieved when operating in both modes and the pixel areas selected to measure the average signal level. Sensors 2019, 19, x FOR PEER REVIEW 8 of 17

(b) (c)

Figure 6. Pictures of the measurement test bench. Overall system operating in direct imaging mode (a). Polarized signal source (b). Microwave front-end and receivers at forefront (c).

The image synthesized by only two receivers is basically a group of fringes, which is not good (a) enough to be used in actual observations. In order to have a cleaner image, more receivers placed in an optimized configuration should be used to provide an optimal point spread function (PSF) (see References [11,16,21] for more details). On the other hand, the polarization measurement method is basically the same that the one explained in References [17,18] for the QUIJOTE experiment. For the measurement tests, the camera acquisition ratio (frame rate) is set to 100 Hz. The applied polarization modulation phase-switching frequency is 3 Hz, so the complete cycle with 16 phase -states provided by the phase switches (four sequences of 0, 90, 180, and 270 deg. phase-shift between receiver’s branches [17,18]) is repeated approximately every 5.4 s. In order to integrate the power of the Near- Infra-Red signals imaged with the camera, the detected signal level is calculated as the mean value in a certain number of camera (pixels,b) where the up-converted signals are( cimaged) (direct imaging) or the maximum of the synthesized image power is found (interferometry). Figure 7 shows two pictures Figure 6. Pictures of the measurement test bench. Overall system operating in direct imaging mode Figureof the images 6. Pictures achieved of the measurementwhen operating test in bench. both modes Overall and system the operatingpixel areas in sele directcted imaging to measure mode the (a). (a). Polarized signal source (b). Microwave front-end and receivers at forefront (c). Polarizedaverage signal signal level. source (b). Microwave front-end and receivers at forefront (c). The image synthesized by only two receivers is basically a group of fringes, which is not good enough to be used in actual observations. In order to have a cleaner image, more receivers placed in an optimized configuration should be used to provide an optimal point spread function (PSF) (see References [11,16,21] for more details). On the other hand, the polarization measurement method is basically the same that the one explained in References [17,18] for the QUIJOTE experiment. For the measurement tests, the camera acquisition ratio (frame rate) is set to 100 Hz. The applied polarization modulation phase-switching frequency is 3 Hz, so the complete cycle with 16 phase -states provided by the phase switches (four sequences of 0, 90, 180, and 270 deg. phase-shift between receiver’s branches [17,18]) is repeated approximately every 5.4 s. In order to integrate the power of the Near- Infra-Red signals imaged with the camera, the detected signal level is calculated as the mean value in a certain number of camera pixels, where the up-converted signals are imaged (direct imaging) or the maximum of the synthesized image(a) power is found (interferometry).( bFigure) 7 shows two pictures of the images achieved when operating in both modes and the pixel areas selected to measure the FigureFigure 7. Pictures 7. Pictures taken taken from from the the camera. camera. (a(a)) TwoTwo up-convertedup-converted receiver receiver output output signals signals operating operating in in average signal level. directdirect imaging. imaging The. The mean mean amplitude amplitude value value in in 14 14 × 14 pixels pixels is is taken taken as asdetected detected signal signal.. (b) Synthesized (b) Synthesized × image fringes from the interference of two receiver up-converted output signals. The mean amplitude value in 140 150 pixels is taken as detected signal. ×

(a) (b)

Figure 7. Pictures taken from the camera. (a) Two up-converted receiver output signals operating in direct imaging. The mean amplitude value in 14 × 14 pixels is taken as detected signal. (b) Synthesized

Sensors 2019, 19, x FOR PEER REVIEW 9 of 17 Sensors 2019, 19, x FOR PEER REVIEW 9 of 17

imageimage fringesfringes fromfrom thethe interferenceinterference ofof twotwo receiverreceiver upup--convertedconverted outputoutput signalssignals.. TheThe memeanan amplitudeamplitude Sensors 2019, 19, 1870 9 of 17 valuevalue inin 140140 ×× 150150 pixelspixels isis takentaken asas detecteddetected signal.signal.

3.1. Operation as a Direct Imaging Instrument 3.1.3.1. Operation Operation as as a a Direct Direct Imaging Imaging Instrument Instrument In this section, the polarimeter demonstrator measurement results when operating in direct InIn this section,section, the the polarimeter polarimeter demonstrator demonstrator measurement measurement results results when when operating operating in direct in directimage image mode is described. The polarized signal source, shown in Figure 6b, is used to generate 100% imagemode ismode described. is described The polarized. The polarized signal source,signal source, shown shown in Figure in6 Figureb, is used 6b, tois generateused to generate 100% linearly 100% linearly polarized signals with polar angles between 0° and 180° in 10° step. For each polar angle, a linearlypolarized polarized signals withsignals polar with angles polar between angles between 0 and 180 0° andin 10 180step.° in 10 For° step each. For polar each angle, polar a complete angle, a complete sequence of 16 phase-states, provided◦ by the ◦phase ◦switches in the microwave receivers, is completesequence sequence of 16 phase-states, of 16 phase provided-states, provided by the phase by the switches phase switch in thees microwave in the microwav receivers,e receivers, is used tois used to characterize the amplitude and phase of the input signal polarization vector. usedcharacterize to characterize the amplitude the amplitude and phase and of phase the input of the signal input polarization signal polarization vector. vector. Figure 8a shows the resulting NIR detected signal levels of one receiver, for a 0° input polar FigureFigure 88aa showsshows the the resulting resulting NIR NIR detected detected signal signal levels level ofs one of one receiver, receiver, for a 0for inputa 0° input polar polar angle, angle, taking the mean values in square pixel areas as the one shown in Figure 7◦a. For each phase- angle,taking taking the mean the mean values values in square in square pixel areaspixel areas as the as one the shown one shown in Figure in Figure7a. For 7a. each For phase-state,each phase- state,state, thethe meanmean valuevalue ofof thethe detecteddetected signalssignals isis calculated,calculated, andand waveformswaveforms areare shownshown inin FigureFigure 88b.b. theThe meansignal value level of values the detected are given signals in A.D.U. is calculated, (A/D converter and waveforms units of are the shown camera). in Figure The amplitude8b. The signal and Thelevel signal values level are values given inare A.D.U. given in (A /A.D.U.D converter (A/D converter units of the units camera). of the Thecamera). amplitude The amplitude and phase and of phasephase ooff thethe polarpolarizationization vectorvector isis extractedextracted fromfrom thethe waveformwaveform fundamentalfundamental harmonicharmonic calculatedcalculated thethrough polarization an FFT [1 vector8]. With is extractedthis procedure from, the it is waveform possible to fundamental measure the harmonic polarization calculated of the incoming through throughan FFT [a18n]. FFT With [18 this]. With procedure, this procedure it is possible, it is possible to measure to measure the polarization the polarization of the incomingof the incoming signal, signalsignal,, achievingachieving fourfour valuesvalues ofof polarpolar angleangle andand polarizationpolarization percentagepercentage forfor eacheach receiver.receiver. TThehe twotwo achievingreceivers of four the values demonstrator of polar angletested andin the polarization laboratory percentage show very for similar each receiver.results, therefore The two receiversonly the receiversof the demonstrator of the demonstrator tested in the tested laboratory in the showlaborato veryry similarshow very results, similar therefore results, only therefore the results only of onethe resultsresults ofof oneone ofof themthem areare presentedpresented.. FigureFigure 99aa––dd showsshows thethe measuredmeasured polarpolar anglesangles andand theirtheir errorserrors,, ofand them also arethe presented.achieved polarization Figure9a–d percentages shows the measured (or degree) polar and angles their errors and their for each errors, one and of alsothe four the andachieved also the polarization achieved percentagespolarization (or percentages degree) and (or their degree) errors and for their each errors one of thefor each four detectedone of the signals. four detecteddetected signals.signals.

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Sensors 2019, 19, 1870 10 of 17

Sensors 2019, 19, x FOR PEER REVIEW 10 of 17

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ItIt can canFigure be be seen seen 9. Polarization that that the the polar polar measurement angle angle error errors resultss are are of around around the demonstrator: 6 6°◦ andand the the Polar polarization polarization angles (a) anddegree degree errors errors errors (b). present present values aroundaroundPolarization 50%.50%. degree Clearly, Clearly, (c) theand the mosterrors most important(d ) importantcalculated error with error isrespect in is the intopolarization athe 100% polarization polarized degree wave degree. so in the so nextin the section, next the source of these errors and some instrumental modification proposals to reduce them are discussed. section, theIt can source be seen of thesethat the errors polar andangle some error sinstrumental are around 6° modificationand the polarization proposals degree to errors reduce present them are discussedNevertheless,values. Nevertheless,around a calibration 50%. Clearly, a methodologycalibration the most methodologyimportant [22,23] applied error [2is2 specificallyin,2 3the] applied polarization to thisspecific degree kindally of so instrumentationto in thisthe next kind of instrumentationcan reducesection, thethe systematicsource can reduce of these errorsthe errors systematic to and the some levels errors instrumental required to the levels bymodification the required new cosmicproposals by the microwave new to reduce cosmic them background microwave are b(CMB)ackgrounddiscussed experiments. (CMB). Nevertheless, experiments The calibration a calibration. The method calibration methodology is based method on [22 polar, 2is3 ]based applied vector on error specificpolar fitting allyvector to as error thisa function kind fitting of of as the a functionmeasuredinstrumentation of polar the measured angle, can to reduce removepolar the angle, thatsystematic errorto remove directlyerrors thatto fromthe error levels the directly required observed from by polar the the new vectors. observed cosmic The m polaricrowave method vectors. will Thebe explained methodbackground will in detail (CMB)be explained in experiments a future in detail work.. The incalibration a future methodwork. is based on polar vector error fitting as a function of the measured polar angle, to remove that error directly from the observed polar vectors. 3.2. Operation OperationThe method as as awill a Synthesized Synthesized be explained Imaging Imaging in detail Interferometer Interferometer in a future work. TThe3.2.he Operationmeasurement as a Synthesized setup setup when Imaging operating Interferometer in in synthesized image mode use usess the same polarized test signal as in the previous section with 1.25 m far-field distance. The polarization modulation is test signalT heas measurementin the previous setup section when withoperating 1.25 inm synthesizedfar-field distance image .mode The polarizationuses the same modulationpolarized is also similar except for the particularity of modulating the polarization synchronously with the two also similartest signal except as in forthe theprevious particularity section with of modulating 1.25 m far-field the distancepolarization. The polarization synchronously modulation with the is two receiversreceiversalso similarof the the demonstrator demonstrator.except for the .particularity One One important important of modulating difference difference the with polarization the direct direct synchronously imaging imaging instrument, instrument, with the in two which which each receiverreceivers receiver ofis is usedthe used demonstrator to to obtain obtain independent. independent One important measurements measurements difference with of the of the thedirect polarization polarization imaging instrument, (requiring (requiring in the which the use use ofof a telescopea telescope),each ),receiver is isthat that isnow used now four to four obtain synthesized synthesized independent image images measurementss can can be beachieved achievedof the polarization in inthe the NIR NIR, (,requiring making making theinterference interferenceuse of a of theofthe fourtelescope four pairs pairs), ofis that ofup up-converted -nowconverted four synthesized signals signals in image in the the sOCDS can OCDS. be. achievedThe The operation operation in the NIR of of , the themaking instrument instrument interference concept concept of is demonstratedthe four pairs using of o onlyupnly-converted one of the signals four possible in the OCDS synthesized . The operation images images. . of Figure Figure the instrument 1010aa shows shows concept the the resulting resulting is demonstrated using only one of the four possible synthesized images. Figure 10a shows the resulting NIR detected detected signal level levels,s, for a 0 °◦ inputinput polar polar angle, angle, taken taken the the mean mean values values in in the the selected selected region region in in NIR detected signal levels, for a 0° input polar angle, taken the mean values in the selected region in Figure 77b.b. ForFor eacheach phase-state,phase-state, the the mean mean value value of of the the signals signals is is calculated, calculated, achieving achieving the the waveform waveform of FigureFigure 10b. 7b. For each phase-state, the mean value of the signals is calculated, achieving the waveform of Figureof Figure 10b. 10b.

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l 8.0E+05 8.0E+05 m S1 p m 6.0E+05 MV_S1

6.0E+05 p A A m

6.0E+05 m 6.0E+05 A 4.0E+05 4.0E+05 A 4.0E+05 2.0E+05 24..00EE++0055 2.0E+05 0.0E+00 02..00EE++0005 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0.0E+00 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 0.0E+00 Phase-State Number 0.0 0.4 0.8 1.2 1.6 2.0 2.4 T2im.8e (3S.e2c on3d.6s) 4.0 4.4 4.8 5.2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Time (Seconds) Phase-State Number (a) (b) (a) (b) FigureFigure 10. ( 10a). NIR(a) NIR detected detected signal signal levels levels with with polarizationpolarization modulation modulation given given by bythe thephase phase-switching-switching Figurestagestage of 10 the. of(a receivers.)the NIR receivers. detected (b) (b Resulting) signalResulting levels waveform waveform with polarization fromfrom each each phase phase-state modulation-state mean mean given value value. by. the phase-switching stage of the receivers. (b) Resulting waveform from each phase-state mean value.

Sensors 2019, 19, 1870 11 of 17 Sensors 2019, 19, x FOR PEER REVIEW 11 of 17 Again, the amplitude and phase of the polarization vector are extracted as in direct imaging Again, the amplitude and phase of the polarization vector are extracted as in direct imaging operation mode. Figure 11a–d shows the measured polar angles and their errors, and also the achieved operation mode. Figure 11a–d shows the measured polar angles and their errors, and also the polarization percentages and their errors. It can be seen that the polar angle errors and the polarization achieved polarization percentages and their errors. It can be seen that the polar angle errors and the degree errors are around 2.6 degrees and 35.5%, respectively. It is noticed that the errors are lower polarization degree errors− are around −2.6 degrees and 35.5%, respectively. It is noticed that the errors than in the direct image operation mode, and this is one of the most important advantages of this are lower than in the direct image operation mode, and this is one of the most important advantages type of operation with respect to the previous one. Nevertheless, the polarization degree error is still of this type of operation with respect to the previous one. Nevertheless, the polarization degree error important, but, as in the previous case, a calibration methodology with speciﬁc application to this kind is still important, but, as in the previous case, a calibration methodology with specific application to of instrumentation needs to be applied. this kind of instrumentation needs to be applied.

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Figure 11.11. Polarization measurement results of the demonstrator operating as a synthesizedsynthesized image interferometer: Polar Polar angles angles ( (a)) and errors (b). Polarization degree ( c) and errors (d) calculated with respect to a 100% polarizedpolarized wave.wave.

4. Discussion In the previous previous section, section, the polarimeter operation results are shown when measuring the polarization of a 10 10–20–20 GHz bandwidth microwave signal in two modes of operation operation—direct—direct imaging and synthesizedsynthesized image image interferometry. interferometry In. terms In terms of systematics, of systematics, the reported the reporteddesign has design the advantages has the givenadvantages by the given typical by interferometrythe typical interferometry such as reduced such as sensitivity reduced tosensitivity atmospheric to atmospheric variations, variations, no optical aberrationsno optical aberration on the edges on of the receiver edge ofarray, receiver or lower array sensitivity, or lower to sensitivity beam ellipticity, to beam but ellipticity presents, but an operationpresents an mode operation similar mode to the similar typical to imagers. the typical Additional imagers. systematics Additional of systematics the reported of systemthe reported come mainlysystem come from themainly fiber from dependence the fiber ofdependence temperature of temperature variations and variations misalignments and misa oflignments the fiber of array the thatfiber providesarray that errors provides in theerrors homothetic in the homothetic mapping mapping between between microwave microwave receivers receivers and optical and optical fibers. Allfiber theses. All issues these impact issues theimpact relative the phaserelative between phase between signals, which signals, is awhich fundamental is a fundamental factor for thefactor image for synthesis;the image therefore, synthesis; atherefore, phase tuning a phas ande tuningcontrolling and system controlling must system be implemented must be implemented to operate as to a practicaloperate as interferometer. a practical interferometer. Temperature Temperature control systems control and systems variable and phase variable shifters phase can beshifters included can be in included in the optical chains to control this kind of systematics. However, for the case of direct imaging operation, this is not an important issue, since the optical system basically acts as signal

Sensors 2019, 19, 1870 12 of 17 the optical chains to control this kind of systematics. However, for the case of direct imaging operation, this is not an important issue, since the optical system basically acts as signal detection stage. Hence, the instrument would be affected by the typical imager systematic errors such as optical aberrations from telescope and atmosphere and gain variations from 1/f noise. On the other hand, the frequency up-conversion performed by the MZM also provides additional systematics due to the optical carrier and additional lateral band of the up-converted signal. The optical carrier can be assumed as an interfering signal that limits the instrument sensitivity and also the polarization efficiency (errors in the measured polarization degree). The lateral band, while in direct imaging operation mode is not an issue, in interferometry operation mode provides optical aberrations and lateral lobes to the synthesized image. These issues can be solved with the use of an optical band-pass filter and also by setting the optimal operation point of the MZM. A thermal control system for the stabilization of the MZM operation point is commonly used in typical astronomical instruments. In our particular case, a feedback technique is also applied to provide the required operation point stability.

4.1. MZM Bandwidth The measured amplitude (polarization percentage or degree) and phase (polar angle) systematic errors of the instrument are shown in Figures9 and 11, respectively. Taking into account that the polar angle error of the input signal source is around 1◦, the measured instrumental phase error does not seem very significant. However, the polarization degree error seems important as only half of the signal polarization is detected in direct imaging while something more is detected with interferometry. The reason for that could come from the loss of signal introduced by the Mach–Zehnder electro-optical modulators (MZM) implementing the frequency up-conversion stage (FUS). The frequency up-conversion from microwaves to the near-infra-red allows for the operation of wide-band microwave signals, as they were narrow-band. Then, technically, the main bandwidth limitation of the proposed instrumental concept would come from the microwave receivers and the MZM. In the reported demonstrator, the bandwidth is limited by the low-cost 10 GHz nominal bandwidth MZM used to implement the frequency up-conversion stage. These MZM provide very important losses for signals from 10 to 14 GHz and are too high for 16 to 20 GHz, and only a frequency band from 10 to 12 GHz presents the required level to be distinguished from the noise floor determined mainly by the not-rejected optical carrier [16]. In order to overcome this issue, higher bandwidth MZM are commercially available to cover bandwidths of more than 100 GHz [24]. However, due to the cost of those MZM, an instrument with hundreds of receivers, as required, for instance, at 30 or 40 GHz, would be unaffordable. On the other hand, since at the frequency band of the demonstrator (10–20 GHz) the required number of receivers is an order of magnitude lower, the economic viability of such an instrument is improved. The development of wide-band MZM implemented with technologies, such as InP or Si, allowing the integration of several units in one compact module, is probably the most viable solutions in terms of cost and performance. Additionally, the integration of the microwave receiver functions in optical technology could optimize the design and reduce the number of required MZM.

4.2. Optical Carrier Rejection In the reported demonstrator, a carrier rejection optical filter has not been included for simplicity and because, with only two receivers, it is not possible to achieve a good quality synthesized image (only a group of fringes can be obtained). However, for a general case with a higher number of receivers, the use of optical filters to reject the undesired part of the modulated optical signals is required, not only to improve the polarization degree characterization, but also to be able to generate synthesized images with the required quality to be used in the actual CMB experiments. Figure 12a shows a sketch of the measurement test bench implemented to test a volume bragg optical filter (VBF) at “Instituto de Astrofísica de Canarias” (IAC) laboratory. It can be seen that the filter operates in reflection with a nominal deflection angle that can be slightly modified to tune the filter central wavelength (wC) or frequency (f C). Sensors 2019, 19, x FOR PEER REVIEW 13 of 17

Sensorsof2019 frequency, 19, 1870 offset are added to the x-axis of the graph to show that the 3-dB bandwidth of the filter13 of 17 is around 5 GHz (40 pm), resulting in enough selectivity for this particular application. Sensors 2019, 19, x FOR PEER REVIEW 13 of 17

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In4.3. Figure Synthesized 12b, Beam the(a) measured filter characteristic is shown. The(b) amplitude of the filtered signal is representedFigureT 12.he as (mosta a) Sketch function relevant of the of additional themeasurement wavelength factors test oinfluencing ffbenchset with implemented wtheC asquality a reference. to oftest the the synthesized Theoptical corresponding filter image. (b) O areptical values the of number and distribution of the receivers and the correct homothetic mapping between the horns and frequencycharacteristic offset measur are addeded at to Instituto the x-axis de Astrofísica of the graph de Canarias to show ( thatIAC) thelaboratory 3-dB bandwidth as a function of of the the filter is the fiber array. The homothetic mapping refers to the common geometry of the antenna and fiber aroundwavelength 5 GHz (40and pm),frequency resulting offsets in respect enough to selectivitywC and fC. for this particular application. arrays being related by a simple scaling factor. The similarity of both geometries is not perfect due to 4.3. Synthesizedsome misalignments Beam in the fiber array. An antenna mechanical support precisely reproducing the 4.3. Synthesized Beam fiber array configuration can be implemented to enhance this situation. Finally, regarding the number The most relevant additional factors influencing the quality of the synthesized image are the Tandhe most distribution relevant of receivers,additional Figure factors 13 shows influencing synthesized the qualitybeam (SB) of simulation the synthesized results. image are the number and distribution of the receivers and the correct homothetic mapping between the horns and number and distribution of the receivers and the correct homothetic mapping between the horns and the fiber array. The homothetic mapping refers to the common geometry of the antenna and fiber the fiber array. The homothetic mapping refers to the common geometry of the antenna and fiber arrays being related by a simple scaling factor. The similarity of both geometries is not perfect due to arrays being related by a simple scaling factor. The similarity of both geometries is not perfect due to some misalignments in the fiber array. An antenna mechanical support precisely reproducing the fiber some misalignments in the fiber array. An antenna mechanical support precisely reproducing the array configuration can be implemented to enhance this situation. Finally, regarding the number and fiber array configuration can be implemented to enhance this situation. Finally, regarding the number distribution of receivers, Figure 13 shows synthesized beam (SB) simulation results. and distribution of receivers, Figure 13 shows synthesized beam (SB) simulation results.

(a)

Figure 13. Cont.

Sensors 2019, 19, 1870 14 of 17 Sensors 2019, 19, x FOR PEER REVIEW 14 of 17

(b)

(c)

FigureFigure 13. 13. SynthesizedSynthesized beam beam simulation simulation results results.. White White circles circles of of top top-left-left panels panels represent represent active active receiverreceiver horn horn antennas antennas and and bottom bottom panel panel shows shows the the X Xand and Y Y profiles profiles of of the the synthesized synthesized beam beam (SB (SB)) shownshown in in top-right top-right panel. panel. Two-receiver Two-receiver configuration configuration of the of reported the reported demonstrator demonstrator (a); fully ( populateda); fully populatedarray configuration array configuration with 46 receivers with 46 (receiversb); optimized (b); o arrayptimized configuration array configuration with 20 receivers with 20 ( creceivers). (c). Figure 13a shows the simulation results of the demonstrator configuration with only two receivers. In thatFigure case, 13 thea shows synthesized the simulation beam (SB) results presents of a the group demonstrator of fringes similar configuration to that achieved with only with two the receivers.laboratory In measurements. that case, the synthesized The difference beam in the(SB) power present ofs thea group fringes of forfringes angles simi higherlar to than that 10achieved degrees withare duethe laboratory to the used measurements. primary beam The model difference that does in the not p perfectlyower of the fit thefringes actual for beam angles of higher the horns than at 10those degrees angles. are Additionally,due to the used the primary number beam of fringes model achieved that does by simulationnot perfectly is lower fit the than actual that beam achieved of the in hornsthe laboratory at those angles because. Additionally, of the lack of the the number optical filterof fringes and theachieved resulting by interferencesimulation is between lower than both that side achievedbands of in the the modulated laboratory signals because [25]. of On the the lack other of hand, the optical simulating filter a and situation the resulting with a fully interference populated betweenarray of both 46 receivers side ba (seends Figureof the 13modulatedb), the SB signals presents [2 better5]. On optical the oth characteristics,er hand, simulating since the a situation side lobes withare low a fully and the populated resulting array alias of free 46 field receivers of view (see and Figure resolution 13b), are the similar SB present to thes ones better required optical in characteristicsexperiments, as since QUIJOTE the side (around lobes are 15). low However, and the the resulting number alias of receiversfree field canof v beiew reduced and resolution using an areoptimized similar arrayto theconfiguration ones required as in the experiments one shown inas Figure QUIJOTE 13c, where(around 20 receivers15). However form, athe SB number with similar of receivers can be reduced using an optimized array configuration as the one shown in Figure 13c, where 20 receivers form a SB with similar characteristics to the fully populated array one. In any case,

Sensors 2019, 19, 1870 15 of 17 characteristics to the fully populated array one. In any case, one disadvantage of the interferometer concept is that the large angular scales cannot be well characterized [26]; therefore, the synthesized images lose the information from those scales.

5. Conclusions In this work, it is shown that the performance of a microwave polarimeter demonstrator based on the implementation of a near-infra-red frequency up-conversion stage allowing both optical correlation, to operate as a synthesized-image interferometer, and signal detection, to operate as a direct-image instrument. The demonstrator is a down-scoped version of a large-format instrument, with tens to thousands of receivers, to be applied in low-frequency CMB polarization experiments. The demonstrator has only two receivers, which are enough to prove the interferometer concept, but on the other hand, the sensitivity and quality of the achieved synthesized images are not enough to be used as an interferometer in actual CMB experiments. Nevertheless, the results achieved in the laboratory show promising performance in both operation forms. The polar angle errors of the demonstrator are around 6◦ in direct imaging operation mode and around 2.6◦ in interferometry operation mode. The obtained error values are not an issue for the proposed application, taking into account that the source polar angle error is 1◦ and that the measured errors can be corrected through a dedicated calibration method. The highest-level systematic error characterized in the laboratory is the polarization percentage or degree. While in direct image the error values are around 50%, in interferometry, the error values improve until around a 30%. The main factor behind the reported performance is the reduced bandwidth of the electro-optical modulators implementing the frequency up-conversion stage. In order to solve this issue, higher bandwidth modulators could be used to implement an optimized version of that stage, expecting such significant error reductions. Other improvements can be applied, such as the addition of an optical filter to the demonstrator to get better rejection of the optical carrier and lateral band from modulated signals, or the improvement of the homothetic mapping between the receiver horns and the fiber array. The optical quality of the polarization parameter images is expected to be better with the use of a direct imaging instrument assembled in the focal plane of a telescope mainly because of the side lobes of the synthesized beam and the loss of large angular scales that is produced when operating as an interferometer. The beam side lobes provided by a telescope are usually lower than 30 dB, which is much better than the levels achieved until now with synthesized interferometry. − In this sense, work towards a receiver array configuration presenting an optimal synthetized beam for astronomy applications is ongoing. Additionally, in contrast to direct imaging with telescopes, synthesized image interferometry provides loss of large angular scales and makes it harder to perform mosaicking of astronomical images. However, the systematic errors obtained with interferometry are lower than those obtained with direct imaging, making the reported instrumental concept very interesting to be developed and tested with observations from an astronomical observatory. As a first step, the reported demonstrator could be adapted and installed in one of the QUIJOTE experiment telescopes to demonstrate the performance and reliability of the frequency up-conversion instrumental concept in actual direct image observations.

Author Contributions: F.J.C.: Conception of the demonstrator system functional design, demonstrator implementation, laboratory tests conception and implementation, measurements analysis, and writing of the paper. D.O.: Demonstrator implementation, laboratory tests implementation, measurements analysis, and writing of the paper. B.A., L.d.l.F., and E.A.: Microwave polar source and receivers’ design, implementation, laboratory tests, and writing of the paper. J.M.M. and R.R.: MZM laboratory test, writing, and advising with photonic technology. Funding: The authors would like to thank Spanish Ministry for Economy and Competitiveness (currently Ministry of Science, innovation and Universities) for the ﬁnancial support provided under the projects with references ESP2015-70646-C2-1-R, AYA2015-72768-EXP, ESP2017-83921-C2-1-R, TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R and CIBERBBN all co-ﬁnanced with EU FEDER funds. Acknowledgments: The authors would also like to thank Félix Gracia from IAC for his assistance and help in the optical tests, J.C. Hamilton from APC (Paris) for his assistance and help in the SB simulations and J.L. Cano, E. Villa and Eva Cuerno (DICOM) for their assistance in receivers design, manufacture and assembly. Sensors 2019, 19, 1870 16 of 17

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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