6 electromagnetic field distribution measurements using an ... · an optically scanning...

143TAKAHASHI Masanori et al.

1 Introduction

Recent years have seen a dramatic expan-sion in the use of information and communica-tion devices such as cellular phones and PCs.Associated with this trend is increasing con-cern as to the adverse effects on other instru-ments of the inadvertent release of electro-magnetic radiation (referred to as “electro-magnetic field leakage”) from these electronicdevices, which may impair the performance ofthe affected devices. Furthermore, it has beennoted that the performance of the communica-tion devices themselves may also be impairedby their own leaked electromagnetic fields. Anumber of studies on technologies for sup-pressing such leakage from these electronicdevices have been carried out to date, and oneeffective method determined as a result con-sists of identification of the source of the leak-age based on measurement of the electromag-netic field distribution in the vicinity of theelectromagnetic device, for which counter-

measures are then implemented. The operational frequencies of devices are

continuing to increase from year to year asthese devices become more sophisticated.Accordingly we are now seeing the need todevelop technologies for measurement athigher frequencies to develop countermea-sures against leakage in this range.

Electromagnetic field probes, featuringimproved spatial resolution thanks to the reduc-tion in size of dipole antennas and loop anten-nas, have conventionally been used for electro-magnetic near-field distribution measurementsof electronic devices. In these probes, metaltransmission lines such as coaxial cables areused to transmit signals to the detectors, leadingto the problem of disturbance of the electro-magnetic field distribution under study by themetal components of the measuring instrumentitself. Furthermore, since the transmission linealso directly couples with the electromagneticfield when the detected signals are transmitted,the signals may be contaminated with unwant-

6 Electromagnetic Field DistributionMeasurements using an OpticallyScanning Probe System

TAKAHASHI Masanori, OTA Hiroyasu, and ARAI Ken Ichi

An optically scanning electromagnetic field probe system consisting of an electro-optic ormagneto-optic crystal substrate and a galvano scanner has been developed for high speed andlow-invasive electromagnetic field distribution measurements. In this report, we introduce someof the examples of measuring the electric field distribution using LiNbO3 or CdTe crystal sub-strate and the probe system. Furthermore, we have developed an optical magnetic field probearray for detecting magnetic fields in the gigahertz range. Using the probe array, we also mea-sured the magnetic field distributions above a patch antenna working at 2.49 GHz.

KeywordsProbe, Electromagnetic field, Electro-optic effect, Magneto-optic effect, Galvano scanner

144 Journal of the National Institute of Information and Communications Technology Vol.53 No.1 2006

ed signals. These problems have resulted in anoverall reduction in the precision of measure-ment［1］［2］.

To overcome these issues, efforts havefocused on the development of optical tech-nologies for electromagnetic field measure-ment［3］［4］. By applying these optical tech-nologies, it should become possible to sup-press the invasive effects of the measurementinstrument on the target electromagnetic fieldto be measured.

This paper provides a summary of an opti-cally scanning probe system for measurementof an electromagnetic field distribution; thisprobe conducts a high-speed scan within aplane using an optical beam. The device iscurrently under development within ourgroup, with the ultimate aim of developing asystem that will feature not only high preci-sion but also high speed［5］. We will also pro-vide examples of electromagnetic field distrib-ution measurements made using the developedprobe.

2 Optically scanning probe sys-tem

Figure 1 is a schematic diagram of theoptically scanning probe system［5］. The probemainly consists of a laser source, a galvanoscanner, an fθ lens, an electro-optic thin crys-tal plate, a polarization analyzer, a photore-ceiver, and a spectrum analyzer.

A 1,342 nm laser beam is emitted from the

laser source and enters the galvano scanner viathe optical circulator. The galvano scannerunit is equipped with two PC-controlled high-speed movable mirrors and controls the direc-tion of the incident laser beam to enable scan-ning. This function allows for high-speedmovement of the incident position of the laserbeam over the optical crystal placed below thegalvano unit. With this configuration, it is pos-sible to measure the intensity of the electro-magnetic field at the point of laser beam inci-dence, which can then be moved at high speedover the surface range of the optical crystal.

After passing through the galvano scanner,the laser beam then enters the fθ lens locatedbeneath the unit. The fθ lens concentrates thelaser beam to an extremely small spot, and atthe same time, maintains the incident beamperpendicular to the plane over a large area.

With the present probe, it is possible tofocus a perpendicular incident laser beam ontoa 50 mm×50 mm plane at focal length fromthe fθ lens. The beam diameter at the plane isapproximately 50μm. An optical crystal isplaced at the focal length from the fθ lens,and the device under test (DUT) is placedbelow the crystal. Figure 2 shows a photo of aLiNbO3 thin crystal plate placed directly onthe DUT (referred to below as the microstripline, or “MSL”). Here, the laser beam is trans-mitted from the galvano scanner to the opticalcrystal through air.

The polarization of the laser beam enteringinside the optical crystal changes depending

Fig.1 Optically scanning probe systemFig.2 Exterior view of the optically scan-

ning probe system


on the intensity of the electromagnetic fieldgenerated above the DUT. The bottom surfaceof the optical crystal is fitted with a reflectorfilm, which returns the laser beam with thechanged polarization to the polarization ana-lyzer unit through the same path. The polar-ization analyzer unit is composed of the wave-plates and the polarizer, and here the changein polarization of the laser beam is convertedinto a change in light intensity. This opticalsignal is then converted into electrical signalsat the photoreceiver, and these signals arefinally measured by the spectrum analyzer.

3 Electric field distribution mea-surement using an electro-opticcrystal

3.1 Electric field distribution measure-ment on the microstrip line (MSL)filter by LiNbO3 thin crystal plate

The present probe was used to measure theelectric field distribution on the MSL filter. A40 mm×40 mm×1 mm LiNbO3 thin crystalplate was used as the electric field detector.Figure 3 shows the filter pattern for measure-ment. The filter was produced using a glassepoxy substrate 1.6 mm in thickness, with areadimensions of 60 mm×60 mm. The LiNbO3

thin crystal plate was placed 0.5 mm above thefilter surface. Figure 4 shows the results ofmeasurement of the transmission characteris-tics of this filter using a vector network ana-lyzer. In this experiment, the field distribution

was measured at a pass band of 1.2 GHz andan elimination band of 2.1 GHz. Additionally,measurements were performed by aligning theoptical axis of the LiNbO3 crystal in the x-direction and in the y-direction as shown inFig. 5. For this experiment, a 17 dBm beamwas output from the laser source, and a10 dBm sinusoidal signal was input to the fil-ter from the signal generator. The 40 mm×40 mm area was scanned by moving the laserbeam at 0.2 mm intervals and measuring theelectric field intensity at each point. Approxi-mately 3 minutes were required to measure the40,401 points in this case, which proves thatthis system can perform extremely high-speedmeasurements of approximately 4 millisec-onds per point.

Figures 6(a) and (b) show the results ofelectric field distribution measurement at1.2 GHz for the electric field components inthe y and x directions, respectively. Here wesee a region of strong electric field intensity

Fig.3 Microstrip line filter

Fig.4 Transmission characteristic of filter

Fig.5 Alignment of the optical axis of theLiNbO3 crystal


along the edges of the filter conductor pattern.Furthermore, a region of strong electric fieldintensity is also present on the output side,indicating that the input signal has been trans-mitted to the output side.

Figures 7(a) and (b) show the results ofelectric field distribution measurement at2.1 GHz for the electric field components inthe y and x directions, respectively. With theseresults, a region of strong electric field intensi-ty is not observed near the output side, indicat-ing that the input signal has been stopped andthat transmission to the output side has beenblocked at this frequency.

In order to compare the results of measure-ment using this probe, the electric field distri-bution in the upper region of the filter was cal-culated using the FDTD (finite-differencetime-domain) method. The results of calcula-tion are shown in Figs. 8(a) and (b) and Figs.9(a) and (b). Figures 8(a) and (b) show theresults of calculation at 1.2 GHz, with (a) and(b) corresponding to the y- and x-components,respectively, and Figures 9(a) and (b) indicatethe results of calculation at 2.1 GHz, with (a)and (b) corresponding to the y- and x-compo-nents, respectively. Here we see that theresults of calculation are extremely consistentwith the results of measurement using theprobe.

3.2 Electric field distribution measure-ment above a five-branch trans-mission line using a CdTe thin crys-tal plate

The spatial resolution of the present probeis dependent upon the beam diameter of theincident laser beam and the thickness of theoptical crystal. To evaluate the spatial resolu-tion of this probe, we measured the electricfield generated above a five-branch transmis-sion line. The substrate pattern for evaluationused in this measurement is shown in Fig. 10.Five conductors with pattern widths of0.2 mm each were aligned on a 0.6-mm thickglass epoxy substrate. The gaps between thelines varied from 400μm to 100μm as shownin Fig. 10. The widths of the sloping lines

passing through the transition region betweenthe areas of different gap widths were con-stant. The electric field distribution was mea-sured by applying a 1 GHz signal to the evalu-ation substrate. A 10 mm×10 mm×1 mmCdTe thin crystal was used for electric fielddetection in this measurement. Unlike theLiNbO3 crystal, use of the CdTe crystalenables measurement of the electric field com-ponent perpendicular to the surface of theevaluation substrate.

Figure 11 shows the results of field distrib-ution measurement in this case. For the region(or segment) with an interval of 0.4 mmbetween adjacent lines, regions of strong elec-tric fields were observed above the five lines,and the observed intensities between themwere small.

The narrowing down of the intervals from0.4 mm to 0.1 mm was observable in theresults, and even at intervals of 0.1 mm thefive lines remained discernable.

4 Magnetic field distribution mea-surement using an optical mag-netic field probe array

4.1 Structure of the optical magneticfield probe array

The present probe may also be used formeasuring the magnetic field distributionusing a magneto-optic crystal. However, com-pared to the electro-optic crystals, magneto-crystals such as magnetic garnets generallyfeature reduced detection sensitivity at highfrequencies in or above the GHz band.

In order to measure the magnetic field dis-tribution in the GHz band, a probe array wasdeveloped［6］with optical-magnetic fieldprobes arranged in a two-dimensional configu-ration, allowing for magnetic field distribu-tions even at high frequencies. The configura-tion of the probe array is shown in Fig. 12.

The optical magnetic field probe array ismainly composed of three parts–a glass epoxysubstrate with several loop-element patterns,minute LiNbO3 crystals, and a quartz substrateto hold both of these components. The quartz


(a) Y-direction component (b) X-direction component

Fig.6 Results of electric field distribution measurement on the MSL filter (1.2 GHz)


Fig.7 Results of electric field distribution measurement on the MSL filter (2.1 GHz)


Fig.8 Results of calculation of electric field distribution above filter by the FDTD method (at 1.2 GHz)


Fig.9 Results of calculation of electric field distribution above filter by the FDTD method (at 2.1 GHz)


substrate is used to fix the positions of theLiNbO3 crystals and the glass-epoxy substratefeaturing the loop-element patterns. In theactual preparation of the array, an LiNbO3

crystal substrate is affixed to a quartz substrateof equivalent dimensions; a precision cutter isthen used to cut away sections of only theLiNbO3 crystal substrate, leaving only the por-tions of the LiNbO3 substrate required for theoptical magnetic probe array. The effectiveresult is the generation of numerous lines ofminute LiNbO3 crystals on the quartz sub-strate. In contrast, the tops of the loop ele-ments on the glass epoxy substrate featuregaps for the insertion of LiNbO3 crystals. Theglass epoxy substrates are placed onto thequartz substrates such that the LiNbO3 crystalsfit into these gaps, and the LiNbO3 crystals arefixed onto the respective loop elements with aconducting adhesive. On the glass epoxy sub-strate, there are 16 loop elements with loop

conductor widths of 0.1 mm and loop open-ings of 1.5 mm×1.5 mm aligned in rows at3.2-mm intervals. The glass epoxy substratesare placed in a multiple crisscross pattern toform the probe array with 16 lines in both thex and y direction.

During actual measurements, the side withthe loop element is placed toward the DUT,and the laser beam is irradiated from thequartz substrate side and into the LiNbO3

crystals to measure the magnetic field distrib-ution.

Figure 13 shows the reflected light intensi-ty distribution obtained with the optical-mag-netic field probe array. This image confirmsthat the reflected light is being returned fromthe LiNbO3 crystals. Compared to the centerof the probe array, the reflected light intensityfalls by a maximum of nearly 6 dB at theedges. This is thought to be due to the limits inthe precision of the fθ lens, and so corrections

Fig.10 Five-branch transmission line

Fig.11 Electric field distribution above thefive-branch transmission line(1 GHz)

Fig.12 Structure of an optical-magneticfield probe array

Fig.13 Reflected light intensity distributionobtained with the optical-mag-netic field probe array


will be made in measurement to account forthis drop in intensity.

4.2 Magnetic field distribution abovea patch antenna using the opticalmagnetic field probe array

The optical magnetic field probe array wasapplied to the scanning probe system to mea-sure the magnetic field distribution above apatch antenna. Figure 14 shows the patchantenna used in this measurement. A 2.49 GHzsinusoidal signal was input to the patch anten-na, and the magnetic field distribution abovethe antenna was measured in the excited state.The results are shown in Figs. 15 and 16 forthe y- and x-direction components, respective-ly. Approximately 4 seconds were required tomeasure a total of 512 points for the two direc-tion components, demonstrating the system’sability to measure circuits displaying moderatevariations over time.

5 Concluding remarks

This paper described an optically scanningprobe system developed for high-speed mea-surements of electromagnetic field distribu-tion. We found that high-speed electric fielddistribution measurements above electric cir-cuit substrates are possible with the combina-tion of a laser-beam scanning mechanism andthin plates of LiNbO3 or CdTe crystals. Thispaper also described the structure of an opti-cal-magnetic field probe array having multiplealignments of electro-optic crystals andminute loop elements, and we also introducedan example of magnetic field distributionmeasurement in the GHz band above a patchantenna. In the future, we plan to devoteefforts to the development of a measuring sys-tem featuring higher precision and speed rela-tive to the system introduced in this paper, aswell as to investigate production methods forsystems using magneto-optic crystals aimed athigh-frequency magnetic field distributionmeasurements.

Fig.14 Photo of patch antenna

Fig.15 Magnetic field distribution abovepatch antenna (y-direction com-ponent)

Fig.16 Magnetic field distribution abovepatch antenna (x-direction com-ponent)


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TAKAHASI Masanori

Guest Researcher, Sendai Electromag-netic Field Measurement TechnologyResearch Center, CollaborativeResearch Management Department

High Frequency Measurement

ARAI Ken Ichi, Dr. Eng.

Project Leader, Sendai Electromagnet-ic Field Measurement TechnologyResearch Center, CollaborativeResearch Management Department

Electronic material, High FrequencyMeasurement

OTA Hiroyasu

Expert Researcher, Sendai Electromag-netic Field Measurement TechnologyResearch Center, CollaborativeResearch Management Department

High Frequency Measurement

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