Dadin
Mahmudin, Pamungkas Daud, Yusuf Nur Wijayanto
Telecommunication Division, Research Center for Electronics and
Telecommunication, Indonesian Institute of Sciences (LIPI)
ABSTRAK
Trafik data melalui jaringan nirkabel meningkat
setiap tahun. Untuk mengantisipasi kemacetan trafik data di kemudian hari,
kombinasi antara jaringan nirkabel dan serat optik dapat digunakan untuk menyelesaikan
masalah tersebut. Konverter antara gelombang mikro nirkabel dan cahaya diperlukan
pada jaringan kombinasi tersebut. Pada makalah ini, kami mengajukan konsep baru
berupa modulator optik terintegrasi dengan antenna berbentuk bowtie berbasis teknologi Silicon-On-Insolator (SOI) untuk jaringan
nirkabel dikombinasikan dengan jaringan serat optik. Alat yang diajukan dirancang
menggunakan struktur planar berbasis teknologi SOI dan disusun dari antenna
berbentuk bowtie, konektor, dan elektroda
modulasi optik. Alat ini dapat digunakan untuk menerima sinyal gelombang mikro nirkabel
dan mengubahnya ke sinyal gelombang optik. Sederhana dan kompak dan rugi transmisi
yang rendah dapat diperoleh untuk beroperasi pada frekuensi tinggi seperti gelombang
millimeter atau terahertz.
Kata kunci : Modulator optik, antenna bowtie, Silicon-On-Insulator,
jaringan micro wave nirkable dan serat optik
ABSTRACT
Data traffic
through wireless links increases every year. In order to anticipate bottleneck
of the data traffic in the future, wireless-fiber links can be used for solving
the problem. Converters between wireless microwave and lightwave are required
in the combination links. In this paper, we propose a new optical modulator
integrated with anbowtie antenna using Silicon-On-Insulator (SOI) technology
for microwave/ millimeter-wave wireless and optical fiber links. The proposed
device is designed with a planar structure based on SOI technology and composed
of a bowtie antenna, connection line, and optical modulation electrode. It can
be used for receiving a wireless microwave signal and converting wireless microwave
signal to a lightwave signal. Simple and compact and low transmission loss are
obtainable for high frequency operation i.e. millimeter-wave or terrahertz
regions.
Keywords: Optical
modulator, bowtie antenna, Silicon-On-Insulator, microwave wireless and optical
fiber link.
INTRODUCTION
Currently, microwave/millimetre- wave wireless and optical fiber links have
attracted a lot of interest for several applications such as for communication
and measurement [1]. The links are illustrated in Fig. 1. They are operated
when a wireless microwave/ millimeter-wave signal is distributed in a optical
fiber using lightwave as a carrier. The advantages of the links are low-loss
transmission about 0.2 dB/km when lightwave propagates in an optical fiber, no
inductance and crosstalk, huge bandwidth since high operation microwave/ millimeter-wave
is used, and low cost [2].
The microwave/ millimeter-wave wireless and optical fiber links are composed of two
domains: one is for optical signals in the lightwave frequency range and the
other is for electrical signals in the microwave/ millimeter-wave frequency
range. Therefore, a microwave/ millimeter-wave-lightwave converter is required
as a key device [3-6]. The conversion of microwave/ millimeter-wave signals to
optical signals can be obtained by using an external optical modulator. In
order to obtain direct conversion from wireless microwave/millimeter-wave to
lightwave signals, a device for receiving wireless microwave/- millimeter-wave signal
and converting it directly is designed.
Fig.
1 Typical of microwave wireless and optical fiber links.
Optical modulators for direct conversion of wireless
microwave/ millimeter-wave to lightwave signals can be composed of an antenna
as a wireless signal receiver and a resonant electrode for optical modulation
as a microwave/millimeter-wave-lightwave converter. Several optical modulators
have been proposed and developed. One interesting configuration is the
integration of a planar antenna and a modulation electrode on the same
substrate [5,6]. It can be operated with no external power supply and a simple
compact structure.
In this paper, we propose a newoptical modulator integrated
with antenna using Silicon-On-Insulator (SOI) for microwave/ millimeter-wave wireless
and optical fiber links. The bowtie antenna is used since it can be operated
for broadband application. The SOI is used for a substrate. Modulation
electrode is designed using coplanar stripline. Simple and compact device
structure can be obtained. Therefore, we expect that the proposed device can be
operated for receiving broadband wireless microwave/ millimetre-wave signals
and converting the microwave/millimeter-wave signals to lightwave signals.
DEVICE
STRUCTURE
Figure 1shows the basic structure of the proposed optical
modulator integrated with antenna. A bowtie antenna and coplanar stripline are
fabricated on SOI where size of them depends on operational frequency of the
designed microwave/ millimeter-wave signals. Length of the antenna is set to a
half wavelength of the designed microwave/ millimeter-wave signals. The
coplanar stripline is operated for a one wavelength standing-wave of the
designed microwave/ millimeter-wave signals. A straight optical waveguide with
silicon core material is located along and under the coplanar strip line as a
modulation electrode. A buffer layer can be inserted between the substrate and
metal structure of the coplanar stripline. The reverse side of the substrate is
covered with a ground electrode.
Illustration of operational principle of the proposed device is shown in
Fig. 3. When a wireless microwave/ millimeter-wave signal at the designed frequency is
irradiated to the proposed device. A resonant standing-wave microwave/ millimeter-wave
current is induced on surface of the bowtie antenna. The induce microwave/
millimeter-wave current is transferred to the coplanar stripline, where the two
striplines have opposite polarity of microwave/ millimeter-wave electric fields.
As a result, the strong electric field are induced across two stripline with
one wavelength standing-wave. The strong electric field can be used for optical
modulation by using electro-optic (EO) effect of the EO substrate. Therefore,
the lightwave, which propagates through a strightoptical waveguide located
under the coplanar stripline, is modulated by the induced electric field across
the stripline and wireless microwave/ millimeter-wave to lightwave signal
conversion can be obtained. Therefore, a wireless microwave/ millimeter-wave
signal can be received and converted directly to a lightwave signal using the
proposed device with simple and compact structure.
(a)
(b)
Fig.
2 Basic structure of the proposed device, whole device structure (a) and cross-sectional
view (b).
ANALYSIS
The proposed device is composed of a bowtie antenna
and coplanar stripline fabricate on a SOI substrate. The bowtie antenna has a
length with half wavelength of the designed microwave/ millimeter-wave signal
[7]. The length of the antenna can be expressed as
L_ant=c/(2f_m √(ε_eff ))
|
(1)
|
wherec is the lightwave speed in vacuum, fm is the
frequency of the designed microwave/ millimeter-wave signal, and Ɛeff
is the effective dielectric constant of the substrate. Since a SOI substrate is
used, the effective dielectric constant of the substrate is below 10. The
antenna size is quite large for applications of microwave/ millimeter-wave region.
The modulation electrode is designed using coplanar
stripline with a one standing-wave of microwave/ millimeter-wave signal. Length
of the coplanar stripline can be represented as [8]
L_ele=c/(f_m √(ε_eff ))
|
(2)
|
The stripline and gap widths of the coplanar stripline
are designed by considering matching condition in antenna feeding. By using
impedance matching between the antenna and coplanar stripline, completely field
transfer can be obtained with very low distortion. As a result, strong electric
field can be induced along the coplanar stripline as the modulation electrode
for optical modulation.
Optical waveguide is designed with channel type for
single mode of infra-red region (1550 nm), core material using silicon
material. The waveguide core is located between the coplanar stripline. By
using the configuration, large overlapping between microwave/ millimeter-wave
electric field and optical electric field are obtainable.
Based on the analysis, optical modulation by wireless
microwave/ millimeter-wave signal can be obtained using the proposed device
through EO effect. The optical modulation induced by the proposed device can be
transformed using equation below,
Δϕ=(πr_x n_e^3)/λ Γ∫_0^(L_ele)▒〖E_m0 sin(k_m n_g
y)dy〗
|
(3)
|
where rx is the coefficient of the EO effect of the EO
material, ne is the extraordinary refractive index, is the
wavelength of the designed lightwave signal, is the overlapping factor
between the microwave/ millimeter-wave and optical electric field, Lele
is the electrode length as the interaction length between microwave/
millimeter-wave and lightwave electric fields, km is the wave number
of the microwave/ millimeter-wave signal, and ng is the group
velocity.
Detail analysis of the proposed device will be done
using 3D electromagnetic analysis software such as HFSS, CST Microwave Studio,
Ansoft Designer, or so on. The antenna and coplanar stripline are analyzed
separately first for obtaining precise structure. Then, integration of the
antenna and coplanar stripline are also analysis simultaneously. Beside that,
channel optical waveguides will be also designed and analyzed using Marcatili’s
method with their modal dispersion and field distribution.
APPLICATIONS OF MICRO- WAVE WIRELESS
AND OPTICAL FIBER LINKS
Based on this proposal of the new optical modulator integrated with
bowtie antennas using SOI, various promissing applications can be realized especially
for communication and sensing.
In communication, it can be used since wireless communication is always
required by supporting broadband and low propagation loss optical fiber
communication [9]. Recently, optical fibers are installed to the end user
(home/ room) as shown in Fig. 3. This technique is called Fiber-To-The-Home
(FTTH) for connecting to small coverage wireless microwave networks with low
power consumpsion. The optical fiber networks can be used for covering blank
spot areas of wireless microwave networks such as underground, isolated area,
and so on as shown in Fig. 3(b). Therefore, mobile devices can be used properly
to connect in the networks.
Fig. 3. Illustration of combination between wireless
microwaveand optical fiber links for communication.
In sensing, the proposed device can be used with low enviromental noises
and high speed processing [10]. Precise and accurate measurment results are
required in Electro-Magnetic Compatible (EMC) chamber by reducing the
microwave/ millimeter-wave noises since optical fiber links are used to carry
the data as shown in Fig. 4. Beside that, we can observe microwave
electromagnetic spectra in the air using the proposed device. Therefore, the
microwave electromagnetic spectra can be monitored by regulator/ government especially
the licenced microwave electromagnetic spectra include unpermitted (ilegal) transmision and over
limitation of transmition power.
Fig. 4. Illustration of combination between wireless microwaveand optical
fiber links for sensing.
CONCLUSION
A new optical modulator integrated with antenna using
Silicon-On-Insulator (SOI) for microwave/ millimeter-wave wireless and optical fiber
links was proposed. The proposed device is composed of a bowtie antenna and coplanar
stripline fabricated on SOI substrate. The proposed device can be operated with
no external power supply (passive operation) with simple and compact structure.
The main function is for receiving wireless microwave/ millimeter-wave signal
and converting it to lightwave signal. Furthermore, the proposed device is promising
for applications of the microwave/ millimeter-wave wireless and optical fiber
link in broadband communication and precise sensing.
Now, we are still trying to analyzein detail using 3D
electromagnetic analysis software. The fabrication and characterization of the
proposed device would be also done later.
ACKNOWLEDGMENTS
We would like thanks to Dr. Y. Wahyu
and Dr. F. Oktafiani from Research Center for Electronics and Telecommunication
(PPET) in the Indonesian Institute of Sciences (LIPI) for their kind advices on
design of the planar antenna and resonant electrode. Thanks to Dr. N. Armiand
Dr. P. Adhi from PPET-LIPI for their supports during discussion on wireless microwave
technology and its applications. Also thanks to Dr. M. Wahab from PPET-LIPI for
his valuable comments during discussion on sensing applications using radar
technology.
This research activity is financially
supported in Indonesian Institute of Sciences (LIPI) Indonesia thru the
competitive project of “Converters from Wireless Microwave to Lightwave
Signals”.
REFERENCES
[1]
C. H. Lee, Microwave
Photonics, CRC Press Taylor & Francis Group, New York - USA, 2007.
[2]
S. Iezekiel, Microwave
Photonic: Device and Applications, John Wiley & Sons, Ltd., Chichester -
UK, 2009.
[3]
R. B. Waterhouse and D.
Novak, "Integrated Antenna/
Electro-Optic Modulator for RF photonic Front-End," in International
Microwave Symposium, Baltimore, 2011.
[4]
Y. N. Wijayanto, et.
al, “Novel Electro-Optic Microwave-Light- wave Converters Utilizing a Patch
Antenna Embedded with a Narrow Gap,” IEICE Electronics Express, vol. 8, no. 7,
pp. 491-497, April 2011.
[5]
Y. N. Wijayanto, et.
al., “Wireless Microwave-Optical Signal Conversion in Quasi-Phase-Matching
Electro-Optic Modulators Using Gap-Embedded Patch-Antennas,” IEICE Transaction
on Electronics, vol. E96-C, no., pp. 212-219, Feb. 2013.
[6]
F. Gardes, et. al.,
"Evolution of optical modulation in silicon-on-insulator devices",
SPIE News- room, 10.1117/2.1200712.0985, 2007.
[7]
Y. Tawk, et. al.,
"A Simple Multiband Printed Bowtie Antenna", IEEE Antenna and
Wireless Propagation Letters, vol. 7, pp. 557.560, 2008.
[8]
K. C. Gupta, R. Garg and I. Bahl, Microstrip Lines and Slotlines, Norwood: Artech House, Inc., 2001.
[9]
A. Ng’oma and M. Sauer,
“Radio-over-Fiber Systems for Multi-Gbps Wireless Communication, in Proc. of
SPIE-OSA-IEEE Asia Communications and Photonics,” SPIE vol. 7632, 76321I, 2009.
[10]
A. Chen, E. Murphy,
Broadband Optical Modulators: Science, Technology, and Applications, CRC Press,
2011.