Millimetre-wave Electro-Optic Modulator with

Quasi-Phase-Matching Array of Orthogonal-Gap-Embedded

Patch-antennas on Low-k Dielectric Material

Yusuf Nur Wijayanto

, Atsushi Kanno

, Hiroshi Murata

, Sinya Nakajima

Tetsuya Kawanishi

and Yasuyuki Okamura

National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei,

Tokyo, 184-8795, Japan

Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, Japan

Keywords: Electro-optic Modulator, Patch-antenna Array, Quasi-phase-Matching, Low-k Dielectric, Millimetre-wave.

Abstract: In Fibre-Wireless (Fi-Wi) links, conversion devices between wireless microwave/ millimetre-wave and

lightwave signals are required. In this paper, we propose a wireless millimetre-wave-lightwave signal

converter using an electro-optic (EO) modulator with Quasi-Phase-Matching (QPM) array of orthogonal-gap-

embedded patch-antennas on a low-k dielectric material. Wireless millimetre-wave signals can be received

and converted directly to lightwave signals using the proposed device. It can be operated with no external

power supply and extremely-low millimetre-wave losses. The orthogonal-gap-embedded patch-antennas can

be used for receiving dual-linearized or circular polarizations of wireless millimetre-wave signals. The QPM

array structure can be adopted for enhancing modulation efficiency by transit-time effects consideration.

Structure, analysis, and experimental of the proposed device are discussed for 40GHz operational millimetre-

wave bands.

1 INTRODUCTION

Recently, wireless communication has been

implemented and used for transferring data to mobile

devices. Microwave bands are used widely for

carrying data through air medium with several

wireless communication standards such as Wi-Fi,

WiMAX, LTE, and so on (Abichar, 2006) (Akyildiz,

2010). Since demand of high quality data transfers

using mobile devices is always increase time by time,

the microwave bands will be saturated in the near

future. Scientists and researchers are looking for

solving the future problem by minimizing the data,

saving used frequency spectra, enlarging operational

bandwidth, and so on (Mendeiros, 2014) (Lu, 2014)

(Pi, 2011).

In order to enlarge the operational bandwidth,

high operational microwave frequency into

millimetre-wave or sub-millimetre-wave bands are

promising to use for carrying large data with high

transfer speed. The millimetre-wave bands have

relatively large propagation loss in air and metal

cables (Rec. ITU-R P.676-5, 2001) (Iezekiel, 2009).

Therefore, short distance wireless millimetre-wave

communication in pico/ femto-cells can be

developed. Since coverage area of pico/ femto-cells

is small, networking of pico/ femto-cells can be

adopted for enlargement of the coverage area. The

pico/ femto-cell networks can be connected using low

propagation loss optical fibres as backhaul networks

by adopting microwave-photonic technology (Shi,

2011).

Microwave-photonic technology is a combination

technology where microwave and lightwave bands

are operated simultaneously by considering their

advantages such as high mobility, large bandwidth,

no induction, and so on (Seed, 2002). The technology

can be implemented on Fiber-Wireless (Fi-Wi) links

since wireless microwave and optical fibre

communication are used together. In order to realize

the Fi-Wi link, converters between microwave and

lightwave signals are highly required. A high-speed

photo-detector can be used for converting lightwave

signals to microwave signals (Watanabe, 2000). As

the other one, microwave signals can be converted to

lightwave signals by use of a high-speed optical

modulator (Shinada, 2007).

Wijayanto Y., Kanno A., Murata H., Nakajima S., Kawanishi T. and Okamura Y..

Millimetre-wave Electro-Optic Modulator with Quasi-Phase-Matching Array of Orthogonal-Gap-Embedded Patch-antennas on Low-k Dielectric Material.

DOI: 10.5220/0005325200050013

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 5-13

ISBN: 978-989-758-093-2

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

Generally for wireless applications, a converter from

wireless microwave/ millimetre-wave to lightwave

signals is composed of wireless microwave/

millimetre-wave antennas and optical modulators

(Sheehy, 1993). The antennas are used for receiving

wireless microwave/ millimetre-wave signals. Then,

the received microwave/ millimetre-wave signals are

transferred to the optical modulators by a connection

line, such as a coaxial cable. Therefore, the

microwave/ millimetre-wave signals are modulated

in lightwave signals propagated on optical fibres.

However, microwave/ millimetre-wave signal

distortion and decay might occur in the connection

line when an operational frequency becomes high.

Integration of microwave/ millimetre-wave

antennas and optical modulators fabricated on an

electro-optic (EO) crystal are also developed for

reducing microwave/ millimetre-wave signal

distortion and decay (Bridge, 1991) (Murata, 2012).

They are composed of planar antennas, connection

lines, and resonant electrodes with simple and

compact device structures. Since several microwave/

millimetre-wave planar electrodes on the substrate,

completely impedance matching is required to obtain

effective microwave/ millimetre-wave resonance.

The tuning of them are rather difficult and low

microwave/ millimetre-wave losses might be still

induce along the connection line and their coupling.

New fusion of microwave/ millimetre-wave

antennas and EO modulators were proposed

(Wijayanto, 2011) (Wijayanto, 2012). Patch-antennas

embedded with a narrow-gap were fabricated on EO

crystal as the substrate. Displacement current and

microwave/ millimetre-wave electric field across the

gap can be used for EO modulation. Precise tuning is

not required since only patch-antennas on the

substrate. Therefore, extremely low microwave/

millimetre-wave signal distortion can be achieved

using the fusion structures with a simple and compact

structure. EO modulators using the fusion structures

in an array structure were also reported for enhancing

modulation efficiency by considering transit-time

effect (Yariv, 1989). In high operational frequency,

the patch-antennas becomes small. Therefore,

antenna gain becomes small and microwave/

millimetre-wave-lightwave electric field interaction

length becomes short. The reported devices are

operated effectively for a linear microwave/

millimetre-wave polarization.

In this paper, a millimetre-wave EO modulator

with a quasi-phase-matching (QPM) array of

orthogonal-gap-embedded patch-antennas on a low-k

dielectric material is proposed. By using low-k

dielectric material as an antenna substrate, a large

patch-antenna size can be realized for increasing

antenna gain and enhancing millimetre-wave electric

field strength across the gaps and enlarging

interaction length between millimetre-wave and

lightwave electric fields. Therefore, modulation

efficiency enhancement can be obtained. Further

enhancement of the modulation efficiency can be

achieved using the QPM array structure by

considering transit-time effect. Additionally, the

proposed device can be operated for dual linear or

circular millimetre-wave polarizations.

The device structure, operational principle,

analysis, fabrication, and measurement of the

proposed millimetre-wave EO modulator are

presented for operation with 40GHz millimetre-wave

bands.

2 EO MODULATOR

(a)

(b)

Figure 1: Structure of QPM array of patch-antennas with

orthogonal-gaps on low-k dielectric material stacked with

EO modulator, (a) whole and (b) top views.

Figure 1 shows a structure of the proposed device.

It consists of a QPM array of patch-antennas with

orthogonal-gaps fabricated on a low-k dielectric

material stacked with a thin LiNbO

optical

modulator. The patch electrodes are inserted between

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

the LiNbO

optical crystal and low-k dielectric

material. The patch electrode length, L, is set at a half

wavelength of the designed millimetre-wave. The gap

width, G, is set in micrometre order (<10μm). The

patch electrodes are set in an array structure by

considering a QPM method with a distance of D.

Since a z-cut LiNbO

optical crystal is used, optical

waveguides are located on a one side of the gap edge

as shown in cross-sectional view of Figure 2. The gap

position refer to the optical waveguide are slightly

shifted for satisfying the QPM method. A buffer layer

is also inserted between the LiNbO

optical crystal

and patch electrodes. The reverse side of the low-k

dielectric material is covered with a ground electrode.

(a)

(b)

Figure 2: Structure of meandering gaps for polarization

inversed structure, (a) top and (b) cross-sectional views.

When a wireless millimetre-wave signal is

irradiated to the device, standing-wave currents are

induced on the patch electrode surface (Lefort, 1997)

(Gupta, 2004). By embedding orthogonal-gaps at the

centre of the patch electrodes, millimetre-wave

displacement current and strong electric field are

induced across the orthogonal-gaps due to current

flow continuity (Rodriguez-Berral, 2011). The

induced millimetre-wave electric field can be used for

optical modulation thru the Pockels EO effect of

LiNbO

optical crystal. When lightwave propagating

in the orthogonal optical waveguides located on the

orthogonal-gaps are modulated by the radiated

wireless millimetre-wave signal. Since the gap width

is relatively much smaller than the patch electrode

size, the patch-antenna characteristics are not

changed. Generally, modulation efficiency

improvement can be obtained using an array

structure. However, the spacing between patch

electrodes is required relatively large owing to the

transit-time effect (Yariv, 1989). QPM methods can

be adopted to reduce the spacing of about half.

Therefore, twice modulation efficiency can be

obtained using QPM array structure in the same

device length, since the patch electrodes number

becomes double.

3 ANALYSIS

3.1 Millimetre-wave Analysis

3.1.1 Displacement Current

Figure 3: Millimetre-wave current and electric field profiles

of the proposed device.

When a wireless millimetre-wave signal is

irradiated to the standard patch-antennas with no gap,

a standing-wave millimetre-wave surface current is

induced on the patch electrodes. Then, orthogonal

narrow gaps are introduced at the centre of the patch

electrode as shown in Figure 3. Owing to the

requirement of current continuity on the patch

electrode, millimetre-wave displacement current and

strong electric field are induced across the gap. The

induced millimetre-wave electric field across the gap

is obtained by time integration of the displacement

current. Therefore, it can be expressed as





(



)

=



cos

(







)

(1)

The millimetre-wave current and electric field

profiles of the proposed device are illustrated in

Figure 3.

Millimetre-waveElectro-OpticModulatorwithQuasi-Phase-MatchingArrayofOrthogonal-Gap-EmbeddedPatch-antennas

onLow-kDielectricMaterial

3.1.2 Patch-antenna Size

In order to consider for obtaining large antenna gain,

it can be achieved by enlarging patch electrode

fabricated on a low-k dielectric material as the

antenna substrate. In general, a patch electrode size is

inversely proportional to the designed operational

frequency of wireless millimetre-wave signals. It is

also inversely proportional to square root of substrate

effective dielectric constant (Gupta, 2004).

The patch electrode size can be enlarged by

reducing the effective dielectric constant. In order to

reduce the effective dielectric constant of the

substrate, it can be realized using a thin high-k EO

crystal bonded with a low-k dielectric material as

shown in Figure 1. By using a bonded material

structure with a thin EO crystal, the effective

dielectric constant becomes low.

3.1.3 EO Crystal Types

The EO crystal orientation, the distribution of the

millimetre-wave electric field across the orthogonal-

gaps, and position of the optical waveguide must be

taken into account for achieving effective operation.

The orthogonal optical waveguides should be set on

one side of the gap edge as shown in Figure 2, since

a z-cut LiNbO

optical crystal is used in the analysis.

Figure 4: The calculated millimetre-wave electric field

across the gap for z-cut LiNbO

(solid-line) and LiTaO

(dashed line).

The induced millimetre-wave electric field across

the gaps was calculated using electromagnetic

software analysis. The optical crystal thickness was

set 80μm and low-k dielectric material thickness was

set 130μm with dielectric constant of 3.5. The length

and width of the patch electrodes with gold metal

were set 1.6mm. A gap with 10μm-wide was located

at the centre of the patch electrodes. Ultraviolet

adhesive glue as a buffer layer was also inserted

between the bonded structures. The calculated

electric fields across the gap are shown in Figure 4 for

z-cut LiNbO

(solid-line) and LiTaO

(dashed line).

The millimetre-wave operational frequencies are

shifted due to different effective dielectric constant of

the devices with bonded structure between

anisotropic LiNbO

/ LiTaO

crystal and low-k

material. Based on the result, we expected that the

proposed device using z-cut LiNbO

crystal can be

used for enhancing modulation efficiency.

3.1.4 EO Crystal Thickness

Figure 5: The calculated millimetre-wave electric field

across the gap for z-cut LiNbO

crystal dependences.

Figure 5 shows the calculated millimetre-wave

electric field magnitude across the gap as a function

of operational frequency for several z-cut LiNbO

optical crystal thicknesses. The peak frequency is

shifted due to different effective dielectric constant of

the proposed device by changing the LiNbO

optical

crystal thickness. When the z-cut LiNbO

crystal

thickness becomes thin, operational frequency

becomes high and millimetre-wave electric field

strength becomes large. It is promising also for

obtaining large modulation efficiency.

3.1.5 Millimetre-wave Polarization

Figure 6 shows the calculated electric field

distributions in the z-component for the top view. We

can see that the strong millimetre-wave electric field

is induced across the gaps. The strongest millimetre-

wave electric field is induced when the millimetre-

wave polarization is perpendicular to the gaps.

Almost no millimetre-wave electric field is induced,

when the millimetre-wave polarization is parallel to

the gaps. When the millimetre-wave polarization is

not completely perpendicular or parallel to one of the

gap, the millimetre-wave electric field is induced

across the two orthogonal-gaps. The magnitude of

millimetre-wave electric field across the gap depends

on the millimetre-wave polarization condition.

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

(a)

(b)

(c)

Figure 6: Calculated millimetre-wave electric field

distribution in the z-component for (a) x-, (b) y-, and (c) xy-

polarizations (diagonal).

3.2 Optical Modulation

3.2.1 Transit-time Effect

For an array of gap-embedded patch electrodes, the

temporal phases of the millimetre-wave signal

supplied to the gap-embedded patch electrodes are

changed according their distance, D, and the wireless

irradiation angle,

and

. When a lightwave

propagates in the optical waveguide, the millimetre-

wave electric field as would be observed by the

lightwave can be expressed by following equation

with taking into account the transit-time of the

lightwave







,



=



cos









+2ℎ(







sin



+)

(2)

where k

is the wave number of the millimetre-wave,

is the group refractive index, h denotes the number

of the gap-embedded patch electrodes, D is a distance

of the patch electrodes (D =

/2n

), n

is the

refractive index of the millimetre-wave in air (=1),

and

is an initial phase of the lightwave (

= k

y’). The millimetre-wave electric fields as would be

observed by the lightwave are shown by the

sinusoidal-curve in Figure 7.

The proposed QPM device is an optical phase

modulator, therefore the modulation efficiency,

from the wireless millimetre-wave signal to the

optical signal is proportional to power ratio between

lightwave carrier and sidebands, when Δ

<< 1. The

modulation efficiency is calculated by the integration

of millimetre-wave electric field as would be

observed by the lightwave along the gap-embedded

Figure 7: Operational principle of QPM EO modulators

under irradiation of a wireless millimetre-wave signal angle

x,y

degrees. Modulation efficiency corresponds to the

shaded-areas.

patch electrodes, it can be expressed as following

equation for optical waveguide along y-axis,

Δ(



)













Γ()











(,



)







(3)

where

is the wavelength of lightwave propagating

in the optical waveguides, r

is the EO coefficient, n

is the extraordinary refractive index of the substrate,

Γ is a factor expressing the overlapping between the

induced millimetre-wave and the lightwave electric

field, W are the width of the patch electrodes as the

interaction length of the millimetre-wave and

lightwave electric field, and N is the number of gap-

embedded patch electrodes in an array structure. P(y)

expresses the polarization of the millimetre-wave

electric field in the z-component under the gap edge

along the optical waveguide. The modulation index

of the QPM structure corresponds to the sum of the

shaded areas of the millimetre-wave electric field

observed by lightwave in Figure 7. Since the

modulation index is also a function of wireless

irradiation angle,

, the directivity in the modulation

efficiency can be also calculated using Equation (3).

3.2.2 QPM Array

The millimetre-wave electric fields in the z-

component between two-edges of gaps have different

polarities (Rodriguez-Berral, 2011). The different

polarities enable us to obtain the polarization-

inversed structure for the QPM condition. The

polarization-inversed structures on a z-cut EO

crystals are obtainable by switching spatial

relationship between the gap edge and optical

waveguide along the gap-embedded patch electrodes.

In order to obtain polarization-inversed structure,

meandering gaps can be adopted as shown in Figure

Millimetre-waveElectro-OpticModulatorwithQuasi-Phase-MatchingArrayofOrthogonal-Gap-EmbeddedPatch-antennas

onLow-kDielectricMaterial

2. The gaps are meandered on straight optical

waveguide by considering transit-time effect.

Therefore, it can be used for recovering optical

modulation degradation due to miss-matching

between millimetre-wave and lightwave electric

fields.

3.2.3 Wireless Irradiation Angle

Figure 8: Typical patterns of patch electrodes with

meandering gaps.

Figure 9: Calculated irradiation angle dependence of the

designed QPM EO modulator for several patterns of

meandering-gaps.

The meandering-gaps are promising for wireless

irradiation angle or beam-forming controls. Several

patterns of meandering gaps are designed as shown in

Figure 8. The designed meandering gaps are set by

considering transit-time effect for receiving several

irradiation angles of wireless signals.

The directivity of wireless millimetre-wave

signals in the proposed device can be calculated using

Equation (3). The calculated directivities in the

designed device for several patterns of meandering

gaps are shown in Figure 9. Clearly, the meandering

gaps can be used for controlling irradiation angle or

beam-forming of wireless millimetre-wave signals.

4 EXPERIMENT

4.1 Fabrication Process

The designed device was fabricated. First, a z-cut

LiNbO

crystal with a thickness of 250μm was

prepared. Then, single-mode orthogonal channel

optical waveguides were fabricated on the EO crystal

using titanium diffusion method (Hu, 2010). The

titanium were diffused with 1100

C for 10 hours.

After that, a 0.2μm-thick SiO

buffer layer was

deposited on the EO crystal. An array of patch

electrodes embedded with orthogonal-gaps was also

fabricated on the EO crystal. The patch electrodes

were fabricated using a 2μm-thick gold film on the

EO crystal through thermal vapor deposition,

standard photo-lithography, and a lift-off technique.

The optical waveguides were aligned onto one side of

the gap edge.

A ground metal was deposited to the bottom

surface of a low-k dielectric material. Then, the top

surface of a low-k dielectric material was covered

with an optical adhesive for next bonding process.

Figure 10: A photograph of the fabricated devices with an

array of gap-embedded patch-antennas, (a) with QPM

structures and (b) with no QPM structure.

In bonding process, the EO crystal was flipped

over with 180 degrees. So as the metal antennas

become on the bottom surface of the EO crystal.

Then, the flipped EO crystal was bonded to the low-k

dielectric material by exposing ultraviolet (UV) light

to the UV-cured optical adhesive (Uddin, 2006).

Finally, the 250μm-thick EO crystal was polished to

the designed thickness of 80μm using a polishing

machine with diamond slurry. A photograph of the

fabricated device is shown in Figure 10.

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

4.2 Measurement Setup

Figure 11: Measurement setup for characterization of the

fabricated devices.

Performance of the fabricated device were

measured experimentally with a measurement setup

as shown in Figure 11. Lights of 1.55μm wavelength

from laser were propagated to optical fibres and

coupled to the fabricated device. Millimetre-wave

signal in 40GHz bands from a signal generator was

amplified and irradiated to the fabricated device using

a horn antenna with an irradiation power of 20mW.

The output lightwave signals were measured using an

optical spectrum analyser (OSA).

Typical of the measured output light spectra from

two orthogonal waveguides are shown in Figure 12,

where a 34GHz wireless millimetre-wave signal was

irradiated at the device at a normal irradiation angle

and polarization of 45 degree. The optical sidebands

were observed clearly. The intensity ratio between the

sidebands and optical carrier were about 37dB.

Figure 12: Typical measured output light spectra from the

orthogonal waveguides.

The measured modulation efficiency as a function

of millimetre-wave frequency is shown by the dots in

Figure 13, when the irradiation angle of the wireless

millimetre-wave signal was set to be normal to the

device. The unit is expressed for power ratio between

carrier and sidebands per millimetre-wave irradiation

power and distance between the horn antenna and

fabricated device. The measured peak frequency was

about 34GHz. It is slightly shifted than the designed

operational frequency due to fabrication error such as

EO crystal thickness, UV-adhesive glue thickness,

and other parameters.

The dots in Figure 14 show the measured

modulation efficiency as a function of wireless

irradiation angle in the yz-plane, when the frequency

of the wireless millimetre-wave signal was set at

34GHz. The largest modulation efficiency for certain

wireless irradiation angle depends on meandering-

gap patterns. The measured directivities are in good

agreement with the calculation results.

Figure 13: Measured modulation efficiency as a function of

the millimetre-wave operational frequency.

Figure 14: Measured modulation efficiency as a function of

wireless millimetre-wave irradiation angles.

5 CONCLUSIONS

Optical modulators with a QPM array of orthogonal-

gap-embedded patch-antennas on a low-k dielectric

material were proposed. A wireless millimetre-wave

signal can be received and converted directly to a

lightwave signal with the proposed device through

EO modulation using the Pockels effect. Performance

of the proposed device for operation in the

millimetre-wave band was demonstrated

experimentally. The proposed device can be operated

for dual linear polarization or circular polarization of

wireless millimetre-wave signals since orthogonal-

gaps are used. Double modulation efficiency can be

Millimetre-waveElectro-OpticModulatorwithQuasi-Phase-MatchingArrayofOrthogonal-Gap-EmbeddedPatch-antennas

onLow-kDielectricMaterial

obtained also since a QPM array structure is used.

The proposed device has a compact structure and can

be operated with a low millimetre-wave signal loss

and no external electrical power supply.

The proposed device is promising for broadband

wireless communication such as for Multi-Input

Multi-Output (MIMO) and Space-Division-

Multiplexing-Access (SDMA) (Wijayanto, 2013). It

can be used also for precise measurement/ sensing

applications such as high-frequency Electromagnetic

Compatibility (EMC) chamber and Radio Detecting

and Ranging (RADAR).

ACKNOWLEDGEMENTS

The authors would like thank to Dr. T. Umezawa

from National Institute of Information and

Communication Technology (NICT) Japan and Dr.

H. Shiomi from Osaka University Japan for their

constructive comments and suggestions during

discussion. Thanks to Dr. Y. Ogawa from NICT

Japan for his helpful supports during device

fabrication.

Y. N. Wijayanto, A. Kanno, S. Nakajima, and T.

Kawanishi would like thank to the Ministry of

Internal Affairs and Communications, Japan, for the

financial support partly thru the project entitled

“Research and Development of high-precision

imaging technology using 90 GHz band linear cells”

funded by the “Research and Development to Expand

Radio Frequency Resources.”

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