-
Hindawi Publishing CorporationAdvances in Optical
TechnologiesVolume 2013, Article ID 738427, 8
pageshttp://dx.doi.org/10.1155/2013/738427
Research ArticlePhotonic Processing for Wideband Cancellation
and SpectralDiscrimination of RF Signals
David M. Benton
L-3 TRL Technology, Unit 19 Miller Court, Severn Drive,
Tewkesbury, Gloucestershire GL20 8DN, UK
Correspondence should be addressed to David M. Benton;
[email protected]
Received 9 July 2013; Revised 16 October 2013; Accepted 16
October 2013
Academic Editor: Augusto Beléndez
Copyright © 2013 David M. Benton. This is an open access article
distributed under the Creative Commons Attribution License,which
permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Photonic signal processing is used to implement common mode
signal cancellation across a very wide bandwidth utilising
phasemodulation of radio frequency (RF) signals onto a narrow
linewidth laser carrier. RF spectra were observed using
narrow-band, tunable optical filtering using a scanning Fabry Perot
etalon. Thus functions conventionally performed using digital
signalprocessing techniques in the electronic domain have been
replaced by analog techniques in the photonic domain. This
techniquewas able to observe simultaneous cancellation of signals
across a bandwidth of 1400MHz, limited only by the free spectral
range ofthe etalon.
1. Introduction
Photonic signal processing offers a new paradigm for pro-cessing
high bandwidth signals and opens up possibilities fordirectly
processing signals that are modulated onto an opticalcarrier. The
ability to process and manipulate ultrawidebandsignals enables
high-resolution, reconfigurable processingof potentially the entire
RF spectrum of signals. It is thispotential that has received a lot
of attention over the past fewdecades [1, 2] and is suitably
demonstrated through devicessuch as wideband adaptivemicrowave
filters enabled throughcoherence and phase control [3]. Photonic
technologies area natural source for delivering increased
bandwidth, butalso the low loss of fibre optic transmission systems
hasseen developments in transmitting Radio over Fibre (RoF)and
antenna remoting [4, 5]. As requirements for increaseddata
bandwidth develop and as spectral densities increase,with
congestion becoming an issue, the need for ever morepowerful
digital signal processing (DSP) is a constant driver.This has
inevitable consequences for size, weight, and power(SWaP)
considerations, especially in RF signal monitoringand
characterisation equipment. With the RF signal in pho-tonic form
and the inherent bandwidth benefits that ensue,there is a natural
opportunity for implementing analog signalprocessing (ASP)
techniques and making the processing aninherent part of the optical
link [6].
Signal cancellation is conventionally implemented byengineering
a version of the target waveform that is matchedin amplitude but
180∘ out of phase (negative feedback) suchthat the interference of
the target and engineered waveformsresult in cancellation. This
approach can be performed withanalog or digital signals but is
likely to be limited in itsoperational bandwidth by frequency
dependent phase shifts.Photonic carriers modulated by RF signals
are not limitedby these frequency dependent phase shifts in
circuitry andhence can operate across a much wider bandwidth.
Photonicsignal processing (the manipulation of light waves
ratherthan digitised signals) can be used to cancel out
commonwideband RF signals enabling weaker signals to be
identified.Whilst cancellation is common in photonic signal
processingit has tended to be targeted at sideband removal
[7–10].Suppression of modulation signals in photonic systems
withsome specific examples of signal cancellation was shownthrough
inversion in an optical amplifier [11]. Single sidebandsuppression
is a common technique in which the signal ismade to interfere with
a phase shifted version of itself, helpingto remove the power
residing in the (unmodulated) carriercomponent.
There have recently been reports of photonic based
RFcancellation systems that claim impressive levels of
cancel-lation [12–15], typically 40 dB for broadband signals and
a
-
2 Advances in Optical Technologies
very impressive 70 dB for single tones. These systems repr-esent
the current state of the art in photonic cancellationbut have
significant differences from the system discussedin this paper.
Applications of this technique have focussedon removal of cosite
interference where a close proximitytransmitter will pose
significant problems for a receiver,particularly relevant where
jammers are being used andcan overwhelm a receiver, significantly
reducing its abilityto observe weak signals. However, the pairwise
nature ofsignal generation for cancellation does not specifically
requireone signal to be the reference to be removed. Where thetwo
signal sources are independent cancellation will seek toreduce
signal contributions that are common to both sources.Conversely
signal contributions unique to one source onlywill be unaffected
and can emphasise the differences betweenthe two sources, which can
help with signal gradiometry andsource location. In this paper
photonic processing techniques(equivalent to ASP) are implemented
to demonstrate theproof of principle that not only the signal
cancellation butalso spectral analysis across a wide bandwidth is
attainableby optical means without significant DSP.
2. Photonic Processing andOptical Cancellation
The cancellation of signals common to two independentsignal
sources, such as two antennas, can be implemented bymodulating the
RF signal from each antenna onto an opticalphase modulator, each
incorporated into one arm of a MachZehnder interferometer (MZI) and
fed from a common laser.Conventional MZI modulators are widely
available but areused for applications such as single sideband
suppressionwhere only a single input signal is required and the
outputis amplitude modulated. Using separate phase modulators
ineach arm allows independent signals to be combined andindependent
phase control is available for each signal, priorto coherent
combination.
The simplest demonstration is where a phase shift of 𝜋is imposed
upon one of the modulated laser fields by theapplication of a bias
voltage and a photodiode detector canbe used to observe the
resulting intensity after the two armsare recombined.This setup is
shown schematically in Figure 1for the simplest case.
An electrooptic modulator subject to a sinusoidally oscil-lating
RF electric field of modulation frequency 𝜔
𝑚gives rise
to an output field consisting of a series of sidebands
centredaround the laser carrier frequency represented by (see
e.g.,[16])
𝐸 (𝑡) = 𝐸0
{
∞
∑
𝑘=0
𝐽𝑘
(𝛿) cos (𝜔 + 𝑘𝜔𝑚
) 𝑡
+
∞
∑
𝑘=0
(−1)𝑘
𝐽𝑘
(𝛿) cos (𝜔 − 𝑘𝜔𝑚
) 𝑡 } ,
(1)
where 𝐽𝑘(𝛿) represents a 𝑘th order Bessel function and 𝛿 is
a modulator characteristic parameter. In the case above
twomodulated fields 𝐸
1(𝑡) and 𝐸
2(𝑡) are produced in eachmodu-
lator, respectively, and then combined.Where onemodulator
Laser 2:2 fibre
RF source 2
RF source 1
Detector
DC voltage
splitter
Figure 1: A schematic diagram for observing common
modecancellation in the simplest case.
has a 𝜋 phase change added to the carrier frequency 𝜔, thenall
terms at frequency (𝜔+𝑘𝜔
𝑚) can be cancelled in principle.
In this arrangement all frequency components must be mea-sured
through conventional sampling and spectral powersdetermined through
the Fourier transform.
More generally the RF signals received at each modulatorwill
have a phase difference, due, for example, to time of
flightdifferences from source to antennas. The electric field
exitingeach modulator can be represented in an ideal set up by
𝐸1
(𝑡) = 𝐸0cos (𝜔𝑡 + 𝛿 sin (𝜔
𝑚𝑡 + 𝜃)) , (2)
𝐸2
(𝑡) = 𝐸0cos (𝜔𝑡 + 𝜙 + 𝛿 sin (𝜔
𝑚𝑡)) , (3)
where 𝜃 is the signal phase offset between the received
signals,𝜙 is the optical phase offset applied to the carrier by
themodulator, and each modulator is assumed to receive thesame
optical amplitude𝐸
0. After application of trigonometric
identities and ignoring harmonic terms above 𝑘 = 1 we canwrite
for modulator (1):
𝐸1
(𝑡) = 𝐸0
[𝐽0
(𝛿) cos (𝜔𝑡) + 𝐽1
(𝛿) cos ((𝜔 − 𝜔𝑚
) 𝑡 − 𝜃)
−𝐽1
(𝛿) cos ((𝜔 + 𝜔𝑚
) 𝑡 + 𝜃)] .
(4)
Similarly from modulator (2),
𝐸2
(𝑡) = 𝐸0
[𝐽0
(𝛿) cos (𝜔𝑡 + 𝜙) + 𝐽1
(𝛿) cos ((𝜔 − 𝜔𝑚
) 𝑡 + 𝜙)
−𝐽1
(𝛿) cos ((𝜔 + 𝜔𝑚
) 𝑡 + 𝜙)] .
(5)
The amplitudes from each modulator are combined, withone arm
experiencing a 𝜋 phase shift (due to being a MZI).
In the simple example above involving Nyquist samplingof the
detector signal, the amplitude at the modulationfrequency will be
observed through the mixing within thedetector between the
modulation signal and the unmodu-lated carrier. Whilst this
provides coherent gain to increasethe signal amplitude, it prevents
direct comparison of thesignal frequencies from each source and is
prone to variationas the two carrier components being combined can
becancelled independently from the modulation signals as thecarrier
phase is varied. It is also possible that a single sourceof
modulation in only one modulator can appear to becancelled solely
through its phase difference with the carrier.To overcome this, the
modulation components must be
-
Advances in Optical Technologies 3
DAQ
Optical
Fibre
RF source 2
RF source 1Phase modulators
Bias tee
RF splitter
Electrical connectionFibre optic connectionCoil of fibre
Offset voltage
Detector signal
DetectorData acquisition
Scanning etalon
splitter/combiner
Fibresplitter/combiner
isolator
and control system
Single frequencylaser 1550 nm
Etalon scancontroller
Experimental observations
Figure 2: System setup including a scanning Fabry Perot
etalon.
separated from the carrier. Separating the carrier frequencyfrom
the sidebands can in principle be achieved throughdispersion, such
as using a diffraction grating. However, amodulation component of
10MHz has a frequency differenceof only 1 part in 107 from the
unmodulated carrier and wouldthus require an impractical
propagation distance before themodulation component could be
spatially separated from thecarrier. Separation of the carrier
andmodulation componentsis best achieved through filtering. The
Fabry Perot etalon(FPE) is in optical terms (though not in RF
terms) a highresolution tunable filter, formed of two reflective
surfaces(mirrors) accurately separated by a known distance [17].
Thetransmission through the filter can be tuned by adjustingthe
spacing between the mirrors and the filter bandwidthwhich is
determined by the level of reflectivity of the mirrors.FPEs with a
bandwidth of 10MHz are commercially availableand capable of
discriminating between the laser carrier andmodulation sidebands. A
photodiode detector placed afterthe FPE measures the intensity
being transmitted and theparticular mirror separation during a scan
determines thefrequency to which this intensity relates. Thus the
spectralcharacteristics of the modulation are measured by
scanningthe FPE filter not by rapid temporal sampling and
processing.The spectral bandwidth limitations are those of the FPE
andare determined by the free spectral range which can be 10sof
GHz. It is also convenient to observe the intensity in boththe
positive and negative sidebands if required.Thus
spectralcharacteristics across an ultrawide bandwidth can be
accessedwithout recourse to very high speed DSP.
After filtering with the FPE the observed intensity ineach
sideband arises from the time average of the square ofthe total
amplitude from both modulators. In the positive
sideband𝜔+𝜔𝑚, the observed time averaged intensity ismade
up of the positive sideband contributions in (4) and (5):
⟨𝐸 (𝜔 + 𝜔𝑚
)2
⟩ = ⟨𝐸2
0
(−𝐽1
(𝛿))2
× [cos ((𝜔 + 𝜔𝑚
) 𝑡 + 𝜃)
+ cos ((𝜔 + 𝜔𝑚
) 𝑡 + 𝜙 + 𝜋)]2
⟩
= 𝐸2
0
𝐽1
(𝛿)2
⟨1 − cos (𝜃 − 𝜙)⟩ .
(6)
Similarly for the negative sideband 𝜔 − 𝜔𝑚,
⟨𝐸 (𝜔 − 𝜔𝑚
)2
⟩ = 𝐸2
0
𝐽1
(𝛿)2
⟨1 − cos (𝜃 + 𝜙)⟩ . (7)
Thuswith a suitable choice of carrier phase offset𝜙,
frequencycomponents common to both modulators can be minimisedin
one or both of the sidebands. Hence the “processing”of cancellation
and spectral transform are achieved throughonly optical means.
3. Experimental Observations
An experimental setup was constructed using
commerciallyavailable optical fibre based components and is shown
inFigure 2. A narrow linewidth laser (100KHz) operating at1550 nm
with up to 40mW of power was split into two armsusing a 2 : 2
single mode fibre splitter, each of which wasconnected to a fibre
coupled electrooptic phase modulator.The modulated outputs from the
phase modulators wererecombined with the use of a 2 : 1
splitter/combiner. The out-put from the recombiner was collimated
and then focussed
-
4 Advances in Optical TechnologiesPh
ase o
ffset
var
ying
with
tim
e
Carrierfrequency
+400 MHzcommon
signal
+500 MHzuniquesignal
−500 MHzuniquesignal
−400 MHzcommon
signal
Figure 3: A time evolving spectral plot (waterfall plot) showing
pos-itive and negative sidebands around the central carrier
frequency. A400MHz signal is common to both modulators whilst a
500MHzsignal is present in only one modulator and is unaffected by
phasechanges.
into a scanning Fabry Perot etalon (FPE). The transmittedoutput
intensity was measured with an amplified photodiodesampled with an
ADC within a data acquisition unit (DAQ).Data was collected and
processed using a LabView program.RF signals were supplied from a
signal generator and directedto each modulator via a splitter. A DC
voltage was controlledfrom the LabView program and applied to one
modulatorvia a bias tee. The LabView program monitored the
spectralamplitude at the signal frequency as the DC voltage
wassystematically varied thereby varying the relative carrierphase
between the two arms.
A scanning FPE with a free spectral range of 1.5 GHz anda
finesse of 100 was incorporated into the system. This waschosen
primarily to give a reasonable compromise betweenbandwidth (wide in
RF terms), resolution (poor in RF terms),and physical size. Whilst
significantly higher finesse etalonscould be used they are large
and cumbersome devices notwellsuited to use outside of the
laboratory.Thus for any system tobe widely useful in the real
world, size matters. The etalonscan was set to cover a spectral
range of 750MHz either sideof the laser carrier frequency so as to
prevent confusion withsidebands from the neighbouring free spectral
range; thus theeffective spectral range is 750MHz.The scan
controller cycledthe etalon spacing at a rate of 100Hz and a
detector followingthe etalon measured the spectral intensity in
relation to thefilter position. The RF signal amplitude spectral
componentsare related to the optical amplitude seen by the
detector. Thefrequency is determined by the filter position and not
byany temporal modulation; thus the spectrum can be obtainedwith a
very low rate sampler, whilst covering a wide spectralrange
The detector was sampled at a rate of 10 kSps and relatedto the
scan position using timing gates issued from theetalon scan
controller. A time evolving spectral plot (waterfallplot) could be
produced to examine the evolution of spectralamplitudes with time
and hence with applied voltage (phase).
A common signal at 400MHz (5 dBm) was applied to bothmodulators
via a set of splitters. Another “unique” signal at500MHz was
applied to just one of the modulators. The plotof Figure 3 shows
how the amplitudes of the two signals varywith time as phase
changes. In this case a step change wasmade to the applied voltage
and the system was then left tostabilise. The phase varied slowly
with time due to thermaleffects and the common 400MHz signal is
affected by thisin both sidebands. The carrier frequency amplitude
is alsoaffected by this phase drift, but the unique 500MHz
signalremains unaffected, thus demonstrating that signal present
inonly one arm can be discriminated from pervasive
commonsignals.
Some characteristics of this plot need explanation. Firstlythe
upper and lower sidebands at 400MHz are out of phasein their
response to the phase change. In this case this arisesfrom a
difference in the length of the cables connecting thesignal
generator to the modulators equivalent to a time offlight phase
difference in the modulation signal. Secondlythe carrier power is
cancelled at a different phase to thesidebands. This arises due to
a lack of polarisation definitionbefore the laser light enters the
modulators. Modulationoccurs predominantly for one polarisation
state within themodulator and withoutmatching the input
polarisation thereexist two “modes”—one modulated and one
unmodulated—in the output field.The coherent addition of the
unmodulatedcarrier mode and the modulated (𝐽
0(𝛿)) carrier mode results
in a carrier field with an offset phase relative to the
sidebands.This situation will be rectified in future with the use
ofmodulatorswith integral polarisation stages.Nevertheless
theproposition that common mode signals can be cancelled
isadequately shown here. During the course of these investi-gations
it was observed that the phase applied to the carrierwas not linear
with applied voltage. This was determined tobe due to ohmic heating
in the modulator devices, whichare impedance matched for use at
10GHz.The thermal effectupon the refractive index of the modulator
material results ina nonlinear phase response, as outlined in the
appendix.
To demonstrate that this cancellation can be carriedout across a
wide instantaneous bandwidth a programmablesignal generator was
used to repeatedly scan the commonmodulation frequency in steps of
20MHz between 100MHzand 600MHz, whilst a unique frequency of 500MHz
wasmaintained in one of the modulators. The offset voltagewas then
adjusted for maximum suppression of the com-mon signal. In this
case the cable length connected to themodulators was matched so
that no frequency dependentmodulation arising from the RF offset
phase was present.The data showing the effect of common signal
suppressionacross a 500MHz bandwidth is shown in Figure 4 where
thecancellation level here is 11 dB across the entire
frequencyrange simultaneously. In this case simultaneous
cancellationoccurs due to zero relative phase (𝜃 = 0) of the
signals ateach modulator; thus (6) and (7) are minimised for the
samecarrier phase.
The amount of cancellation seen here is limited due toa number
of contributions. In particular the central carrierfrequency is
many orders of magnitude more intense thanthe sidebands and its
tail adds noise to the sideband spectral
-
Advances in Optical Technologies 5
200 400 600
1
2
Frequency (MHz)
Phot
odio
de si
gnal
(V)
No cancellation
Tim
e (10
0 ms s
teps
)
0
2
4
6
(a)
200 400 600
1
2
Frequency (MHz)
Phot
odio
de si
gnal
(V)
With cancellation
Tim
e (10
0 ms s
teps
)
0
2
4
6
(b)
Figure 4: Data plots showing one sideband with a frequency
scanning signal in both modulators and unique signal at 500MHz in
only onemodulator (a). Simultaneous cancellation of common mode
signals across a 500MHz bandwidth can be seen in the data on
(b).
locations. Residual signal can be seen in the suppressed
dataconfirming that cancellation is not complete. The lack
ofpolarisation stage before the modulators leads to differencesin
the amplitude of the modulated mode which will limit theamount of
cancellation that is possible.This can be addressedsomewhat by
variable attenuators placed after themodulatorsbut this leads to a
reduction in overall signal. The isolationoffered by the FPE is
also imperfect and this adds to thenoise level. Modelling has also
exposed that the cancellationlevel is very sensitive to the precise
amount of phase offsetapplied. In this particular case the voltage
step resolutioncorresponded to a phase change of around 0.5∘ which
wouldlimit the cancellation level to 30 dB and hence clearly
othercontributions were the dominant limitation to the amountof
cancellation achieved. Whilst the demonstrated level ofcancellation
is a little disappointing it is valuable whenconsidering that this
system has implemented the dual rolesof cancellation and spectral
transform without any recourseto high speed sampling or DSP.The
cancellation system setupapplies equally across a very wide
bandwidth, limited here bythe free spectral range of the FPE, but
in principle can operateacross the bandwidth of the modulator. Only
an adjustmentofmodulator offset voltage is required to tune the
cancellationfor any given frequency and offset signal phase.
Cancellationacross a range 100–1500MHz is shown in Figure 5
wherehigher frequencies are seen entering from right to left
andoriginate from a neighbouring spectral order. There is much
potential for improving the performance of this system
insubsequent development.
4. Sideband Behaviour
Both upper and lower sidebands can be observed (as seenin Figure
3) and the behaviour of the spectral power in eachsideband is
dependent upon the relative RF signal phasedifference in the two
modulators and the carrier phase. Theunique signal is however
unaffected by the carrier phase andtherefore cancellation of common
signal can be observed inwhatever sideband is most beneficial.
If each modulator is subject to an RF signal obtained viaan
antenna in a general setting, the distribution of sourcesin the
local environment is likely to be uncontrollable. Thereceived
signals will then be subject to a phase differencedependent upon
the signal frequency and the spacing of thereceiving antennas. Thus
cancellation will require a carrierphase setting specific for that
source. It can be seen from (6)and (7) that, whatever the phase
difference 𝜃 of the modula-tion signal, a value of carrier offset
phase can be chosen thatwill result in cancellation of that
modulation signal in eithersideband. This can be equivalently seen
when a mismatchin cable lengths between signal generator and
modulators isintroduced, which introduces a frequency dependent
phaseshift. A cable of length 1m was introduced before one
-
6 Advances in Optical Technologies
Frequency (MHz) Frequency (MHz)
(a) (b)
With cancellationNo cancellation
0
2
1
0
2
1
600400200 600400200
Unique signal at 600 MHz
100–750 MHz 100–750 MHz
750–1500 MH
z
750–1500 M
Hz
Tim
e (10
0 ms s
teps
)
Tim
e (10
0 ms s
teps
)
Figure 5: A wideband frequency scanning signal into both
modulators with a unique frequency at 500MHz. Time increases down
thediagram and frequencies 100–750MHz proceed left to right, whilst
750–1500MHz from neighbouring spectral order proceed right to
left.Cancellation is demonstrated in (b).
−0.5
0
0.5
1
1.5
2
−700 −500 −300 −100 100 300 500 700
Frequency (MHz)Measured sideband intensityPhase dependent
envelope
Figure 6: Frequency dependent sideband intensity variations
inupper and lower sidebands for a fixed path length difference
en-route to the modulators.
modulator. The frequency dependent phase difference 𝜃(𝑓)is
then
𝜃 (𝑓) =
𝑑𝜀𝑟
𝑓
𝑐
, (8)
where 𝑑 is the path difference and 𝜀𝑟is the relative permit-
tivity of the cable. Data was captured encompassing
bothsidebands where a sequentially varying frequency input
waspresented to the modulators. The strong, saturated
centralcarrier component was removed from the data by fittinga
Lorentzian lineshape (with a half-width of 1.9MHz andamplitude of
7592). Frequency calibration was carried outwith a linear fit to
each sideband independently.The envelopeof the sideband data was
then modelled using the expectedfrequency dependent phase and is
shown in Figure 6. In thiscase the modelling revealed that the
carrier offset phase wasset to 60∘, which could not be determined
independently dueto the thermal effects upon the modulators.
5. Improvements and Extensions
Subsequent developments have shown that sharing the
offsetvoltage across both modulators in a push-pull configura-tion
improves stability significantly for two reasons. Firstlywhilst the
electrooptic phase change is parity dependent, thethermal phase
change is not; hence the thermal effects inboth modulators have a
similar size and the differential effectupon the output phase from
the pair of modulators is muchreduced. Also because the magnitude
of the applied voltageto each modulator is half the required
voltage, the powerdissipated in each modulator is reduced by a
relative factorof 4. A relative test applying a 2V offset change
(from 0V)was applied comparing application to a singlemodulator
withapplication across the pair. This has seen the thermal
phasedrift reduced from 4∘/s when applied to a single modulatorto
0.8∘/s in a push pull-configuration, with the settling timereduced
from 200 s to 50 s.
In this paper the intended application has been thecomparison of
signals from two separate antennas, withthe capability of removing
signals common to both. Inother photonic cancellation work [12, 13]
the usage has beenpredominantly removing a known transmission
signal whichcan be easily obtained and used to provide the
feedbackreference signal. Using 2 signals from spatially
separatedantenna would clearly introduce a signal dependent
phaseoffset which is related to the source location relative to
theantenna and is the source of the 𝜃 term in (6) and (7).Therefore
to observe cancellation effects for any particularsignal, the
carrier phase offset must be adjusted and itis straightforward to
conceive of capturing spectra whilstscanning the carrier phase.
Indeed given the uncontrollablenature of source locations in the
real world, it is necessaryto make measurements with at least 2
phase values in orderto observe changes in the amount of
cancellation takingplace.These two measurements should differ by
180∘ to allowthe possibility of maximum and minimum cancellation
to
-
Advances in Optical Technologies 7
be observed. Knowing the source frequency, the separationof the
receiving antennas and relative phase obtained fromcomparing the
sideband strengths allows two possibilitiesfor the angle of arrival
to be determined and a measureof source location to be performed.
Unambiguous sourcedirection finding would require 3 receivers.
Preliminaryattempts have shown an angle-of-arrival influence upon
thesideband strengths but this requires more investigation
anddevelopment.
6. Conclusion
This work has demonstrated the basic principle that
photonic(analog) processing can be used to implement widebandsignal
cancellation between two signal sources modulatedonto coherent
laser carriers using phase modulation. Furtherto this photonic
processing can additionally be used to imple-ment frequency
spectrum generation across an ultrawidebandwidth without recourse
to high speed sampling, thusreplacing the digital signal processing
required to performa Fourier transform. In the case given here
filtering is usedto separate the sidebands from the carrier in
order thatunambiguous cancellation effects can be observed, but
thisis due in part to an inadequacy of the phase modulatorsused
here. In principle significant carrier cancellation canalso take
place which will help in reducing noise in the finalfiltered output
by reducing the height of the Lorentzian tailof the residual
carrier. These are improvements that can beincorporated in future
developments, all of which shouldhelp to increase the level of
cancellation observed at theoutput detector.The system is suitable
for incorporation ontoan integrated waveguide system which would
significantlyreduce the system size and complexity.The current
limitationto the system performance is the Fabry Perot etalon
wherethe finesse of 100 limits the isolation between carrier
powerand sidebands, and the FSR is a limit to the
unambiguousbandwidth that can be observed. Etalons with higher
finesseand FSR are available but are large, bulky, and
cumbersomeand hence not well suited to usage outside of
laboratoryconditions. There are clearly a number of developments
that,when implemented, could lead to significant improvementsin
performance and would form part of a developmentprogramme.
Nevertheless this is step towards analog signalprocessing for
spectral analysis.
This proof of concept has shown that it is feasible toconsider
the general case of common mode cancellationof signals across a
very wide bandwidth, rather than justthe specific case of a
cancellation of a single known signal.All that is required is the
control of a single voltage levelapplied to a phase modulator
enabling phase scanning andthe subsequent observation of the
spectrum will reveal therelative phases of common signal
components. Further tothis there is significance in the observation
that signals thatare not common to both receivers are unaffected by
thecancellation process. This, coupled with observations of
thesideband intensities on the positive and negative sides,
canreveal information about the spatial distribution of
commonsources. Converting RF signals into the photonic domain
can
therefore offer attractive benefits in terms of reducing
DSPrequirements and acquiring signal source information.
Appendix
The phase offset Δ𝜙 from an electrooptic modulator has
2contributions, the electrooptic effect Δ𝜙EO and a thermaleffect
Δ𝜙
𝑇:
Δ𝜙 = Δ𝜙EO + Δ𝜙𝑇 =2𝜋𝑓𝐿
𝑐
(Δ𝑛EO + Δ𝑛𝑇) , (A.1)
where Δ𝑛 is a change in refractive index, 𝑓 is the
appliedfrequency, 𝐿 is the length of the modulator, and 𝑐 is the
speedof light.
The electrooptic effect gives a refractive index change:
Δ𝑛EO =𝑛3
0
𝑟 𝑉 ⋅ 𝐿
2𝑑
, (A.2)
where 𝑟 is the electrooptic tensor value for the material, 𝑉
isthe applied voltage, 𝑑 is the electrode gap, and 𝐿 is the
lengthof the crystal.
The change in temperature Δ𝑇 of the device is related tothe
amount of energy 𝑄 deposited through the well-knownrelation:
𝑄 = 𝑚𝐶Δ𝑇, (A.3)
where 𝑚 is the mass of the device and 𝐶 is the specific
heatcapacity.
The energy deposited is proportional to the square of theapplied
voltage 𝑄 ∝ (𝑉2/𝑅) leading to
Δ𝑛𝑇
=
𝑑𝑛
𝑑𝑇
Δ𝑇 =
𝑑𝑛
𝑑𝑡
𝜅
𝑉2
𝑚𝐶𝑅
, (A.4)
where 𝜅 is a constant.Thus the phase change has the form
Δ𝜙 =
2𝜋𝑓𝐿
𝑐
(
𝑛3
0
𝑟33
𝑉 ⋅ 𝐿
2𝑑
+
𝑑𝑛
𝑑𝑡
𝜅
𝑉2
𝑚𝐶𝑅
) . (A.5)
For LiNbO3
𝑟33= 30 pm/V, 𝑑𝑛/𝑑𝑇 = 37 × 10−6/∘C.
This form will have the effect of reducing the spacingbetween
consecutive voltages providing a 𝜋 phase offset. Theprecise device
characteristics are not known and so accuratemodelling cannot be
performed. However, estimating theelectrode gap to be of order 10
𝜇m, the EO component isof the order 𝑛3
0
× 10−7 which is similar in scale to 𝑑𝑛/𝑑𝑇.This broadly agrees
with the voltage squared dependenceobserved, but the net result is
that thermal effects causeslowly varying phase instability until
thermal equilibrium isreached.
References
[1] A. J. Seeds, “Microwave photonics,” IEEETransactions
onMicro-wave Theory and Techniques, vol. 50, no. 3, pp. 877–887,
2002.
-
8 Advances in Optical Technologies
[2] R. A. Minasian, “Photonic signal processing of microwave
sig-nals,” IEEE Transactions on Microwave Theory and
Techniques,vol. 54, no. 2, pp. 832–846, 2006.
[3] F. Zeng and J. Yao, “Investigation of phase-modulator-based
all-optical bandpass microwave filter,” Journal of Lightwave
Tech-nology, vol. 23, no. 4, pp. 1721–1728, 2005.
[4] E. I. Ackerman andA. S. Daryoush, “Broad-band
externalmod-ulation fiber-optic links for antenna-remoting
applications,”IEEE Transactions onMicrowaveTheory and Techniques,
vol. 45,no. 8, pp. 1436–1442, 1997.
[5] D.Novak, T. Clark, S. O’Connor, D. Oursler, and
R.Waterhouse,“High performance, compact RF photonic transmitter
withfeedforward linearization,” in Proceedings of the IEEE
MilitaryCommunications Conference (MILCOM ’10), pp.
880–884,October-November 2010.
[6] T. K. Woodward, T. C. Banwell, A. Agarwal, P. Toliver, and
R.Menendez, “Signal processing in analog optical links,” in
Proc-eedings of the IEEE Avionics, Fiber-Optics and Photonics
Tech-nology Conference (AVFOP ’09), pp. 17–18, September 2009.
[7] M. Y. Frankel and R. D. Esman, “Optical single-sideband
sup-pressed-carrier modulator for wide-band signal
processing,”Journal of Lightwave Technology, vol. 16, no. 5, pp.
859–863, 1998.
[8] A. Loayssa, J. M. Salvide, D. Benito, and M. J. Garde,
“Noveloptical single-sideband suppressed-carrier modulator using
abidirectionally-driven electro-optic modulator,” in Proceedingsof
the International Topical Meeting on Microwave Photonics(MWP ’00),
2000.
[9] A. Loayssa, J. M. Salvide, D. Benito, and M. J. Garde,
“Noveloptical single-sideband suppressed-carrier modulator using
abidirectionally-driven electro-optic modulator,” in Proceedingsof
the International Topical Meeting on Microwave Photonics(MWP ’00),
pp. 117–120, 2000.
[10] A. Siahmakoun, S.Granieri, andK. Johnson, “Double and
singleside-band suppressed-carrier optical modulator implementedat
1320 nm using LiNbO3 crystals and bulk optics,” in Opto-electric
andWireless Data Management, Processing, Storage, andRetrieval,
vol. 4534 of Proceedings of SPIE, pp. 86–92, August2001.
[11] F. Coppinger, S. Yegnanarayanan, P. D. Trinh, and B.
Jalali, “All-optical rf filter using amplitude inversion in a
semiconductoroptical amplifier,” IEEE Transactions on Microwave
Theory andTechniques, vol. 45, no. 8, pp. 1473–1477, 1997.
[12] M. Alemohammad, D. Novak, and R. Waterhouse,
“Photonicsignal cancellation for co-site interference mitigation,”
in Pro-ceedings of the IEEEMilitary Communications Conference
(MIL-COM ’11), pp. 2142–2146, November 2011.
[13] M. Lu, J. Bruno, Y. Deng, P. R. Prucnal, and A. Hofmaier,
“Co-site interference mitigation using optical signal processing,”
inEnabling Photonics Technologies for Defense, Security,
andAerospace Applications VIII, vol. 8397 of Proceedings of
SPIE,June 2012.
[14] J. Suarez, K. Kravtsov, and P. R. Prucnal, “Incoherent
method ofoptical interference cancellation for radio-frequency
communi-cations,” IEEE Journal of Quantum Electronics, vol. 45, no.
4, pp.402–408, 2009.
[15] J. Suarez andP. R. Prucnal, “System-level performance and
char-acterization of counter-phase optical interference
cancellation,”Journal of Lightwave Technology, vol. 28, no. 12, pp.
1821–1831,2010.
[16] A. Yariv, Optical Electronics, chapter 9, HRW
international, 3rdedition, 1985.
[17] E. Hecht, Optics, chapter 9, Addison-Wesley, Reading,
Mass,USA, 1987.
-
International Journal of
AerospaceEngineeringHindawi Publishing
Corporationhttp://www.hindawi.com Volume 2014
RoboticsJournal of
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Active and Passive Electronic Components
Control Scienceand Engineering
Journal of
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
International Journal of
RotatingMachinery
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Hindawi Publishing Corporation http://www.hindawi.com
Journal ofEngineeringVolume 2014
Submit your manuscripts athttp://www.hindawi.com
VLSI Design
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Shock and Vibration
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Civil EngineeringAdvances in
Acoustics and VibrationAdvances in
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Electrical and Computer Engineering
Journal of
Advances inOptoElectronics
Hindawi Publishing Corporation http://www.hindawi.com
Volume 2014
The Scientific World JournalHindawi Publishing Corporation
http://www.hindawi.com Volume 2014
SensorsJournal of
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Modelling & Simulation in EngineeringHindawi Publishing
Corporation http://www.hindawi.com Volume 2014
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
Navigation and Observation
International Journal of
Hindawi Publishing Corporationhttp://www.hindawi.com Volume
2014
DistributedSensor Networks
International Journal of