MICROWAVE PHOTONICS-CONCEPTS
AND APPLICATIONS
Vaibhav Srivastava
Dept. Of Electronics & Telecommunication
College Of Engineering Roorkee, Uttarakhand, India
Abstract: The low-loss wide bandwidth capability of opto-electronic systems
makes them attractive for distributing and controlling micro and millimeter
wave signals, while the development of high capacity optical communication
system has required the use of microwave techniques in optical transmitter
and receivers. These two strands have led to development of research area of
microwave photonics.
Microwave photonics can be generally defined as the study of high speed
photonic devices operating at microwave or millimeter wave frequencies and
their use in microwave or photonic system.
This paper is intended to give an overview of multidisciplinary field of
microwave and millimeter wave photonics.
Keywords: Antenna Remoting, Antenna Beam Forming, Intensity Modulation,
Microwave Photonics, Millimeter-Wave, Radio Frequency, Radio Over Fiber
Introduction
In 1960s, photonics was seen by some visionaries as a possible alternative to microwave for
future high-speed communication. Towards the end of that decade it was not clear which one
of these two technologies would eventually prevail for terrestrial telecommunications indeed,
these were trails of a long-distance ‘Millimetric Waveguide’ system by the UK post
office(which would eventually be abandoned in favour of optical fiber). It was always clear,
however that the two technologies are complementary to one another in many ways and it is
not surprising that they should overlap and merge to form a new interdisciplinary topicmicrowave photonics.
Microwave photonics combines technology developed for both the microwave and optical
part of spectrum (Figure 1) [8]. It was first introduced in 1991. In the following years, it was
foreseen that the merging of microwave and photonic technologies would develop and
become a new approach to future fiber-radio communication system for example in the
transmission of radio frequency (RF) signal over optical carriers [3]. Microwave photonics is
today an innovative multi and interdisciplinary field combining and transforming different
technologies. In particular microwave technologies are used and employed in photonics and
vice-versa. The general field of optic microwave interaction initially called RF
optoelectronics, involves the study of high speed photonic device operating at microwave
millimeter wave frequency and their corresponding use in microwave or photonic system[7].
Fig: 1 Microwave Spectrum
Early Work
The fundamental elements of a microwave photonic link are devices that offers signal
modulation or control or detection at very high frequencies. A key component in a
communication system is a sinusoidal oscillator. One method of producing coherent
oscillations is through stimulated emission, and in early 1950s, the first MASER (microwave
amplification by stimulated emission) was demonstrated. It was to be the optical maser,
however better known as the laser- that would eventually revolutionize communication. The
first solid- state laser was developed in 1960 including both the pulsed ruby laser at Hugher
Research Laboratories, Murry Hills, NJ and the continuously operating helium neon laser at
Bell Laboratories Mailbu, CA. The semiconductor laser offers greater compatibilities and
development of double hetro-junction devices in 1970[1], this became the preferred source
for optical communications. In early days the transmission was based on the free space optics
and gas lenses, but due to realization of low loss transmission in silica optical fiber, this
rapidly became the preferred transmission medium. For detection of signal fast depletion and
avalanche detectors were developed at an early stage, and subsequently developed to give
useful microwave band- width response[5].
Anatomy Of A Basics Microwave Photonic System
Many microwave photonic system use laser diode for the electrical to optical source module.
The laser diode is basically an oscillator, which upon application of a bias current will
produce light wave with an optical frequency(ω) of about 200 THz (assuming a typical
emission wavelength such as 1550nm). If a perfectly monochromatic laser exists and there is
no noise then the electric field at a fixed point can be represented by a time varying complex
quantity: E(t) = E expj(ωt+ϕ). In principle the amplitude (│E│), frequency(ω) or phase(ϕ) of
the light wave may be modulated. Once modulation has been applied to the light wave, it will
be guided by the optical fiber and be subjected to attenuation, dispersion and possible
changes in polarization. At the fiber output, a photo-detector is used to recover the original
modulated signal. A photo diode can therefore directly detect intensity modulation(i.e.
modulation of │E│^2). Intensity modulation can be achieved directly or externally. Direct
modulation of a laser simply means adding a time varying current which results in the
intensity (i.e. optical power) tracking changes in the current. Direct modulation up to about
30GHz is possible, but one disadvantages of this scheme is chirp i.e. optical frequency
inadvertently modulated. This can be overcome by operating the laser in CW (continuous
wave) mode and using an external modulator instead; these are voltage-driven devices and
have larger modulation band-width[10].
Fig: 2 Microwave Photonic Link Architecture For Direct And External Modulation
Application Of Microwave Photonics
There are large number of system and devices that can be said to involve micro wave
photonics. The main system function can be divided in to: (i) transport of microwave and
millimeter wave signals over optical fiber (ii) filtering and processing of micro wave signal in
to optical domain (iii) generation of microwave, millimeter wave and THz signal using
photonic techniques.
These three areas in turn can be subdivided into specific application industry: radio over
fiber, antenna remoting for radar system, antenna beam forming, local oscillator generation
for radio astronomy arrays and THz spectroscopy[4].
A. Radio Over Fiber (ROF)
The wireless networking has attracted much interest in past decades, owing to its high
mobility. People can connect their devices such as PDAs, mobile phones or computers to a
network by radio signals anywhere in home, garden or office without the need for wires. The
global growth of mobile subscribers is much faster than wire-line ones, as the Figure 3 shows
the number of mobile subscribers worldwide has increased from 215 million in 1997 to 946
million (15.5% of global population) in 2001. It is predicted that by the year 2015 there will
be 6.4 billion terrestrial mobile subscribers worldwide. Now a days people have higher
requirements for the services, such as video, multimedia and other new value-added services.
In order to offer these broadband services, wireless systems will need to offer higher data
transmission capacities. By increasing operating frequencies of wireless system, a broader
bandwidth can be provided to transmit data with higher transmission speed. In Millimeterwave (mm-wave) band (30-GHz ~300GHz), about 270-GHz bandwidth can be utilized,
which is ten times the bandwidth in Centimeter wave band (3-GHz~30-GHz). Many research
works have been done to transmit mm-wave over the fiber-optic links, which exploit the
advantages of both optical fibers and mm-wave frequencies to realize broadband
communication systems and contribute a lot to the development of mm wave Radio over
Fiber (ROF) systems[9].
Radio over Fiber (ROF) refers to a technology whereby light is modulated by a radio signal
and transmitted over an optical fiber link to facilitate wireless access, such as 3G and Wi-Fi
simultaneous from the same antenna. In other words, radio signals are carried over fiber optic
cable. Thus, a single antenna can receive any and all radio signals (3G, Wi-Fi, cell, etc..)
carried over a single fiber cable to a central location where equipment then converts the
signals, this is opposed to the traditional way where each protocol type (3G, Wi-Fi, cell)
requires separate equipment at the location of the antenna. In ROF systems, wireless signals
are transported in optical form between a central station and a set of base stations before
being radiated through the air. Each base station is adapted to communicate over a radio link
with at least one user's mobile station located within the radio range of said base station.
Fig: 3 Global Growth of mobile and Wire-line subscribers
Fig: 4 Architecture of mm Wave ROF System
B. Antenna Remoting For Radar Communication
Antenna assets for military and commercial satellite communications as well as radar
applications must often be located at a distance from ground stations. Losses in coaxial cables
or waveguides limits the interconnection distance of earth station antenna to 100m or less if
the transmission frequencies are above 4GHz. Optical fiber offers transmission distance of
several ten kilometers at, or even above, such frequencies without regeneration. Another
example of antenna remoting is the test of radar system, where receiving or emitting dipoles
can be put in front of the main radar antenna and connected to the measuring equipment
through a fiber optic all dielectric link without perturbating the antenna field[3].
In military settings, deployed antenna assets, as potential targets, are remotely located to
protect personnel. Commercial cellular communications likewise may include system
architectures where a single base station serves multiple cell sites. Satellite downlinks and
GPS receivers also often require remote antenna locations.
Fig: 5 Basics Of Antenna Remoting
C. Antenna Beam Forming
The signal induced in different elements of an antenna arrays are combined to form a single
output of the array. This process of the combining the signal from different elements is
known as beam forming[6]. By phase steering or phase rotation the complex target echoes
from all antenna elements are aligned in phase for the desired direction to produce a
maximum sum signal as shown in Figure 6[1]. Along with the recent trends in electronics
circuits for wireless devices, even the antenna beam forming network (BFM) has employed
digital based architectures. The digital beam forming (DBF) architectures offers several
impressive functionalities, including programmable control of antenna radiation beam and
nulls to enhance the signal to interference noise ratio (SINR). It is generally organized that
these advantages can generally be carried out by the digital technology. The analog approach,
on the other hand, is re-emerging to create an alternative architecture of adaptive arrays
antennas. The concepts of analog beam forming itself was proposed more than forty years
ago, but it is consider practically impossible for analog system to provide the smart
functionalities that DBF does. If analog beam forming is available in the RF stage of adaptive
array antennas, this approach should provide drastic improvement in both dc power
dissipation and fabrication costs[9].
Fig: 6 Basic Principle of Beam Forming
D. Other Applications
With the availability of sub-millimeter-wave bandwidth photo-detectors capable of milli-watt
level output power, there are attractive possibilities for optical local-oscillator generation and
distribution in systems such as radio telescope arrays. For these applications, phase noise
requires careful attention [5]. Signals at frequencies 1 THz locked to a microwave reference
can be generated using optical comb generators. Combining optical comb generation with
injection-locked comb-line selection has allowed optical synthesis of signals from 10 to 110
GHz to be demonstrated.
Conclusion
Microwave photonics is a field in which many advances have been made since 1960s, and the
high level of research activity continues unabated. This paper gives a brief flavour of the
topic from the perspective of device and their various system applications.
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