Direct Detection frequency Discrimination optical receiver

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3234 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 18, SEPTEMBER 15, 2008

Direct Detection Frequency DiscriminationOptical Receiver for Minimum-Shift

Keying Format TransmissionLe N. Binh, Member, IEEE, Member, OSA, Thanh Liem Huynh, and Khee K. Pang, Life Senior Member, IEEE

AbstractAn incoherent detection scheme based on frequencydiscrimination for minimum-shift keying (MSK) optical transmis-sion systems is proposed to significantly extend the reach of un-compensated transmission distance. The receiver consists of dualnarrowband optical filters in association with an optical delay line.This photonic front end operates based on frequency discrimina-tion principles of matched filters, rather than relying on the phaseof the optical carrier. Hence, its performance is less sensitive to theall-pass quadratic phase transfer function of thefiber transmissionmedium and is thus less susceptible to fiber chromatic dispersion.The photonic front-end optical receiver for 40-Gb/s optical MSKoffers a dispersion tolerance of up to 340 ps/nm for 1-dB powerpenalty at a bit-error rate of 1 0 9 . This achievement is approx-imately five to seven times better than that of the existing valuebased on the MachZehnder delay interferometer (MZDI) opticalbalanced receiver. Furthermore, the receiver is shown to be robustto polarization-mode dispersion.

Index TermsMinimum-shift keying (MSK), modulation, op-tical modulation, optical receivers.

I. INTRODUCTION

THE TRANSMISSION of optical signals over long-hauluncompensated links without using inline dispersion

compensating fibers (DCFs) is particularly important formetropolitan optical networks which have transmission linkswithin a range of 300 to 500 km of standard single-mode fiber(SSMF). The key benefit from this network configuration is thepotential to reduce the number of inline optical amplifiers [e.g.,Erbium-doped fiber amplifiers (EDFAs), thus decreasing theamount of amplified spontaneous noise (ASE) noise injectedinto the network]. As a result, lower signal levels are requiredfor the same optical signal-to-noise ratio (OSNR), which

effectively mitigates effects of fiber nonlinearities. However,

Manuscript received November 23, 2007; revised March 9, 2008. Currentversion published December 19, 2008.

L. N. Binh is with the Centre for Telecommunication and InformationEngineering, Monash University, Clayton Victoria 3168 Australia. He is alsowith Lehrstuhl fr Nachrichen- und bertragungstechnik, Technische Fakultaetder Christian Albretchs Universitaet zu Kiel, Kiel D-24143, Germany (e-mail:[email protected]).

T. L. Huynh and K. K. Pang are with the Centre for Telecommunicationand Information Engineering, Monash University, Clayton Victoria 3168, Aus-tralia (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2008.925037

in order to implement this network configuration, next-gen-eration high-speed digital photonic transmission systems arerequired to be highly robust to fiber CD. Conventional residualdispersions of around 1000 ps/nm and 60 ps/nm, whichcorrespond to uncompensated links of approximately 60 kmand 4 km SSMF, are reported for OC-192 l0-Gb/s and OC-76840-Gb/s NRZ OOK optical systems, respectively [1], [2]. Itshould be noted that the dispersion tolerance is inversely pro-

portional to the square of the bit rate due to the parabolic phasecharacteristic of the optical fiber [1][4] (with assumptions oflinear transmission and negligible effects from the dispersionslope).

The minimum-shift keying (MSK) format possesses spec-trally efficient attributes of the continuous phase frequency-shiftkeying (CPFSK) family and, thus, is robust to fiber CD. Thegeneration of externally modulated optical MSK signals hasbeen reported [5][7] and, thus, mitigates problems of directmodulation of laser sources when generating CPFSK opticalsignals. The existing technique for incoherent detection ofoptical MSK signals is to utilize a MachZehnder delay in-

terfeometer (MZDI) balanced receiver [7][11]. This receiverserves as a differential phase comparator to detect the differen-tially coded phase information between every two adjacentlytransmitted symbols. The performance of an MZDI-balancedreceiver is severely degraded by the ISI induced from fiberdispersion impairments and by MZDI imperfections [12],[13]. As commonly used and reported in previous works [10],[11], 10-Gb/s and 40-Gb/s MZDI-based optical MSK systemssuffer the 2-dB penalty in the dispersion tolerance at a residualdispersion of approximately 1000 ps/nm and 100 ps/nm or,equivalently, to 55 km and 6 km SSMF, respectively. Therefore,it is found that by using the existing MZDI-based phase detec-tion technique, optical MSK slightly improves the dispersion

tolerance compared to other modulation formats.This paper proposes a realizable incoherent detection scheme

using frequency discrimination for MSK optical transmissionsystems that can significantly extend the reach of uncompen-sated transmission. The proposed optical frequency discrimina-tion receiver (OFDR) employs dual narrowband optical filtersin association with optical delay lines (ODLs). The operation ofthe receiver is based on frequency discrimination principles ofmatched filters, rather than relying on the phase of the opticalcarrier. Therefore, the receiver performance is less sensitive tothe phase profile of the optical fiber (i.e., less susceptible to thefiber CD). The detection of optical MSK signals using the fre-

quency discrimination principles was studied in the early 1990s;0733-8724/$25.00 2008 IEEE


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BINH et al.: DIRECT DETECTION FREQUENCY DISCRIMINATION OPTICAL RECEIVER 3235

Fig. 1. Optical frequency discrimination receiver for optical MSK.

mainly, however, for heterodyne coherent receivers [14][18].The frequency discrimination was conducted in the electrical

domain, at the intermediate-frequency (IF) range. The nonco-

herent frequency discrimination in the photonic domain for di-

rect detection of MSK lightwaves could not be effectively im-

plemented at that time due to the unavailability of optical fil-ters with very narrow bandwidths (as a fraction of signal band-

with). However, with recent advances in the design and fabrica-

tion of optical filters, in particular, micro-ring resonator types,the bandwidth of optical filters can be ultranarrow, around 2GHz [19][22]. The availability of such ultranarrow optical fil-ters thus enables the feasibility of noncoherent optical frequency

discrimination for detecting MSK-modulated lightwave carrier

frequencies.

This paper is organized as follows: The operational princi-

ples of OFDR are described in Section II, while Section III

presents the analysis via the modeling of the receiver. This anal-ysis provides the template functions that are used to calculate the

eye-opening (EO) metric for the assessment of the receiver per-

formance. Section IV then justifies effects of key receiver sub-system components: the filters bandwidth and center frequencyand the value of ODL, thus leading to the guidelines for opti-

mizing the receiver design. Optimum values of these key com-

ponents are derived analytically. The performance of OFDR-

based MSK optical transmission systems is investigated by sim-

ulation in Section V. The first part of this section shows simula-tion results validating the receiver design guidelines. The rest of

Section V presents performance characteristics of OFDR-based

optical MSK systems, including receiver sensitivity, CD toler-ance, PMD robustness, SPM resilience, and the transmission

limits imposed by fiber PMD. Finally, concluding remarks aregiven.

II. FREQUENCY DISCRIMINATION RECEIVER

A. Operational Principles

In MSK, binary logics are modulated in either the upper-side-

band (USB) frequency or lower-sideband (LSB) frequency

, given as and , where is the

lightwave carrier frequency, and is the frequency deviation

from and is equal to a quarter of the bit rate .

The block diagram of the proposed OFDR scheme is shown inFig. 1.

At the output of the optical fiber, MSK-modulated lightwavesare amplified by a low-noise EDFA before being split into twopaths by using a 3-dB optical power splitter. Two optical

narrowband filters F1 and F2 are utilized to discriminate theUSB and LSB frequencies, respectively. Center frequencies

and bandwidths of these two filtersF1 and F2are selected

so that F1 would capture most of optical MSK signals when a 1 is transmitted and F2 would capture most of MSK signalswhen a 0 is transmitted.

Regarding the tuning of the center frequency of the optical

filter, optical filters with a tuning time in the order of submil-liseconds using thermoelectric effects with electrodes are ap-

plied on a surface of silicon on high-index materials [1] of a

small section of the micro-ring resonator. An adaptive tuning

scheme can be used when necessary, for example, when there

is a degradation of the eye diagram. Alternatively, the feed-

back tuning signals can be derived from the output of an elec-

trical filter whose input signals can be coupled via a narrow-band 10-dB microwave coupler. The center frequency of this

narrowband electrical filter can be at 20 GHz for the 40-GHzNRZ pulse shape. The output power of this filter can then beamplified and appropriate signals can be conditioned to drivethe electrode for thermal tuning of the micro-ring resonator. A

feedback control electronic system can be designed to fulfill thisfunction. Alternatively, ultrafast tuning optical filters can be an

electrooptic TE-TM converter and can also be used for

fiber inline fast tuning [24]. A polarization controller must beplaced in front of the TE-TM converter to increase the distinc-

tion ratio at its output. Thetuning speed of this electrooptic filtercan reach several gigahertz.

An ODL is introduced in one path to compensate the differ-

ential delay between USB and LSB ,where , represents the GVD param-

eter of the optical fiber and is the fiber length. The selectionof which path to place ODL depends on the sign of . The

standard transmission fiber SSMF has a negative and, thus,in low-pass equivalent representation, positive frequencies (i.e.,

frequencies higher than the carrier) experience negative delays

and arrive at the photodiodes early compared to the carrier. In

contrast, negative frequencies (i.e., frequencies lower than the

carrier) experience positive delays and arrive delayed. Oppo-

site of SSMF, DCF has a positive and, hence, the aforemen-

tioned effects are reversed. Consequently, in the case of SSMF

, ODL is used on the path of the USB frequency. If

the differential delay is completely compensated, MSK pulses

from the output of the dual optical discrimination filters arriveat the balanced photodiodes simultaneously. MSK-modulated

lightwaves are then converted to the electrical domain and am-

plified by a broadband transimpedance electrical amplifier. Anelectrical filter is normally used to reduce the noise contributedby the electrical amplifier. At this stage, electrically filtered sig-nals produce pushpull eye diagrams from which the samplingand decision-making processes are carried out, leading to the

recovery of data information.

OFDR offers a number of advantages over the existing

MZDI-based detection scheme as follows: 1) OFDR mitigates

the ISI induced from fiber CD impairment more effectivelywith the use of narrowband optical discrimination filters; 2) the


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Fig. 2. Noise distribution following the Gaussian profile due to ultra-narrow-band optical filtering.

receiver sensitivity of OFDR can be controlled flexibly. This isbecause a preamp optical amplifier is compulsory in this OFDRscheme to compensate for the loss of signal power caused by

tight optical filtering. The optical gain of the preamp can be in-creased without having additional degradations from generated

ASE noise of EDFAs due to the high suppression of noise after

narrowband optical filtering; 3) In optical MSK systems, it isassumed that the ultra-narrowband optical filter can induce aGaussian noise distribution profile. The square-law detectionproperty of the photodetection may not produce a Gaussian

noise distribution; we have assumed a Gaussian distribution for

the sake of convenience of calculating BERs. This also assists

the electronic equalization process in the postdetection elec-

tronics. The Gaussian noise distribution of received electricalMSK signals can be shown in Fig. 2.

B. Receiver Modeling and Analysis

The low-pass representation of MSK signals at the th

interval is expressed as [23]

(1)

where is the bit energy, is the symbol period, and the phase

term is given by

(2)

where is the transmitted data symbol and

is the deviation from the carrier frequency. When

, the carrier frequency in the symbol period is shifted by

to and when , the carrier frequency is

shifted by to For the purpose of the analysis, the

low-pass equivalent MSK signal train is expressed as a sum of

MSK pulses in each symbol period, i.e.,

(3)

where is defined from (1) and (2) given by

(4)

with being a square pulse of duration (i.e.,

where is the unit step function). Thus,

describes the MSK signal within the time intervaland has zero amplitude elsewhere.

This single MSK-modulated pulse is then propagating through

the optical-fiber channel. For simplifying the analysis of ISIeffects to the MSK-modulated pulse, the optical fiber is as-sumed to be operating in linear domain and considering only

the second-order dispersion (fiber CD). This is a reasonable as-sumption because the fiber CD is usually dominant over the dis-persion slope for optical systems operating less than 100 Gb/s.

Therefore, the low-pass equivalent frequency response of the

optical fiber has a parabolic phase profile [3], [4] and it isexpressed as

(5)

where in which is the fiber length. In this anal-ysis, it is assumed that the optical carrier has a line spectrum.

This is a valid assumption considering state-of-the-art laser

sources these days with very narrow linewidth and particularly,

the utilization of external modulators for data modulation. The

low-pass equivalent channel impulse response of the optical

fiber is derived from (5) and has a parabolic phase profile

(6)

where is the fiber attenuation. When the optical MSK pulsepasses through the optical channel, the output signal (at the

fiber output) is thus given by the convolution of the signaland the channel impulse response given as

(7)

The signal at the output of the fiber is then input into ODLsand then through the optical narrowband filters whose impulseresponses are denoted as and , respectively. In

this analysis, two ODLs with the same magnitude of the de-

lays but opposite signs are used. However, for a practical im-plementation, these two ODLs are replaced with a single ODL

placed on only one arm of OFDR, as described in Section II. The

signaloutputsarrivingat the photodiodes,denotedas and

, can be modeled as a convolution of the channel output

with the impulse responses of each of the filters and thedelayed delta pulses of ODLs, , as given in the fol-

lowing expressions:

and

(8)


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From (7) and (8) and in order to simplify the convolutions in

(8), two template functions for filter F1 andfor filter F2 are defined as

(9)

(10)

These template functions represent the correctly delayed out-

puts offilters F1 and F2 for each of the 1 and 1 MSK-modulated pulse inputs. When these template functions are sub-

stituted into (8), the expression for the output of the delay lines

is compactly expressed as

(11)

The outputs of ODLs are then converted to the electrical do-main by the photodiodes connected in a balanced configuration.The electrical signal is then amplified, sampled, and sent to thedecision device. The decision device selects 1 as the trans-mitted symbol if the input is positive or 1, otherwise. Inorder to express the sampled values going to the decision de-

vice, let us express the delay line outputs at the sampling instant

by

(12)

where the phase offset is given by

if

if

(13)

The last term in (12) need not be considered furtheras it is lost when the signal is passed through the photodiodes.

The output of the balanced receiver at sampling instant

is then given by .

The photodiode introduces the quadratic point nonlinearity

into the system and complicates the analysis of the receiver

design. Therefore, a semianalytical method is implemented to

evaluate the receiver performance. This method takes into ac-

count mutual interactions among the receiver key parameters

and, thus, provides design guidelines for selection of their op-

timum values. In this analysis, the two optical discrimination fil-ters F1 and F2 are modeled as Gaussian filters having the same

product and having the center frequencies at and ,

respectively. The selection of the two parameters and hasa significant impact on the optimum system performance.

When the system performance is limited by ISI, the EO gives

a good indication of system performance. The proposed method

relies on obtaining EOs for different values of the filters param-eters. The parameters that give maximum EO provide the best

performance. Thus, the eye spectrum is necessary to find outexactly the optimum parameters. However, due to the fact that

most of the errors occur when the eye diagram is at its minimum,the consideration for the EO at its minimum is accurate enough

for most purposes. To obtain EOs, we first consider the trans-mission of a train of sequences at the symbol interval 0th. Wethen find out the minimum sample value for 1, denoted as

, over all of the input train sequences that have 1 at its0th symbol, i.e.,

(14)

Similarly, we obtain the minimum negative sample value when

1 is transmitted, i.e.,

(15)

The EO is then given by . It is noted that car-

rying out the minimization over infinite length sequences is pro-hibitive [refer to (14) and (15)]. The length of the sequence is

thereforelimited to (i.e., symbols oneithersideof the

0th symbol). The value for is selected such that the max-imum time span over which the template functions has a sig-

nificant absolute value of less than . In other words,it is assumed that the currently transmitted symbol is under ISI

effects caused by the precursor and postcursor symbols.

Hence, the time span should cover enough of the long-tailed

spreading of the ISI so that the minimization operations in (14)

and (15) are carried out without sacrificing the accuracy of thecalculated EO values. The value of is usually small (e.g., 3

to 6) when the ISI distortion is small. When the value of gets

large (i.e., under severe ISI distortion), the eye will be closed

and, thus, it is not necessary to carry out the aforementioned

minimization over all the sequences. When a sequence with a

negative sample value for EO (i.e., the complete closure of the

eye diagram) is obtained, the minimization operation is stopped.

Fig. 3 illustrates EOs given by the sample template functions

when the optical fiber has a length of 25km; optical filters have and center frequencies of thefilters are . The axis of Fig. 3 has an arbitrary

unit whereas the axis is normalized to the bit period. It canbe observed that the amplitudes of all the template functions

fall within four symbol periods on either side of the center

sample. Thus, taking for the minimization operation to

calculate EO values would produce accurate results. To obtain

the optimum values for the key receiver components, the EO is

obtained for a different set of parameters and the set giving the

maximum value for the EO is the optimum set. This provides

the receiver design guidelines, which are discussed in the next

section.

The aforementioned analysis and modeling provide a tech-

nique to analytically obtain optimum values for the key receiver

components based on EOs. The receiver design in the next sec-

tion justifies the significance of these key receiver components,leading to guidelines for optimizing receiver performance.


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Fig. 3. EOs given by template functions of the discrimination filters F1 andF2 for each symbol + 1 and 0 1.

III. RECEIVER DESIGN

A. Design Guidelines

The proposed OFDR relies on the principles of narrowband

frequency discrimination as well as on the alignment of signal

arrivals at two balanced photodiodes. Thus, the key parame-

ters of the optical receiver include the bandwidths and center

frequencies of the optical discrimination filter and the valueof ODL. In this section, intuitive justifications of the effectsof these parameters on the receiver performance are provided.

These justifications are based on the EOs given by the tem-

plate functions presented in the previous section. Theselection of optimum values for these key parameters is sought

from this discussion. The two discrimination filters F1 and F2used in this analysis are modeled by Gaussian filters, havingthe same product and their center frequencies are located at

and , respectively (written in the low-pass

equivalent representation).

1) Bandwidth of Optical Filter: The receivers performancecritically depends on the bandwidth of the optical discrimi-

nation filter. When designing the filters bandwidth, two signifi-cant aspects are under consideration: the signal power discrimi-

nation and the ISI that involves the fiber CD and filters impulse

response itself. For simplicity and clarity, effects of these twoaspects are separated in the following discussion.

The bandwidth has to be sufficiently narrow compared to thesignal bandwidth in order to discriminate the USB and LSB fre-

quencies and effectively. However, the switching of fre-

quencies at the rate of results in the spectrum of MSK

spilling over a range of frequencies centered around and

corresponding to either 1 or 1 being transmitted. Thisspectral spilling causes the leakage problem and in the MSK

modulation scheme, where the two modulated frequencies are

close to each other, the leakage terms are substantial. For ex-

ample, when considering the transmission of a 1 pulse, thebulk of energy of a 1 MSK pulse would be mostly cap-

tured by filter F1 and a small amount of the leakage energywill be captured by filter F2. For the system to discriminate

Fig. 4. EOs for different BT products to demonstrate the effects of powerleakage when using dual optical-frequency discrimination filters.

the 1 pulse effectively, the leakage term should be compar-atively small. The effect would be reversed for a 0 modulatedpulse.

The ratio of the signal energy captured by filter F1 to theleakage energy at filter F2 increases with the decrease of thefilters product. However, if the bandwidths of the filters aretoo narrow, the signal energy captured by F1 and F2 goes down,

reducing the discrimination property. As a result, there should

be an optimal product that gives the best performance for

the detection. This power leakage analysis is isolated from the

ISI effects. The aforementioned justifications are demonstratedin Fig. 4. It is found that the values of within the range of

0.4 to 0.5 give the maximum EOs and, thus, are the optimum

values for the optical discrimination filters. Increasing theproduct (i.e., increasing the filter bandwidth over 0.5) doesnot further open the eye.

When propagating through the optical fiber, different spec-tral components of the optical pulse experience different delays

and, thus, arrive at the receiver at different times. This leads to

pulse spreading in the time domain, causing the ISI. When op-

tical signals pass through the discrimination filter, the filter at-tenuates frequencies away from its center frequency. The nar-

rower the bandwidth is, the greater the frequency componentsare suppressed at the tail of the signal spectrum, thus reducing

ISI effects caused by fiber CD. As a result, when the length offiber increases (i.e., the fiber CD is getting more severe), thebandwidth of the optical discrimination filter should be reducedequivalently. However, when the filters bandwidth is reduced,its impulse response starts spreading in the time domain, hence

introducing the ISI caused by the filters impulse response it-self. Thus, there should be an optimum bandwidth which is the

intercept point of the two causes of the ISI.

2) Center Frequency of Optical Filter: Another way to re-

duce the leakage terms is by offsetting the filters center fre-quency away from the nominal modulation frequencies used

by the MSK format. However, this frequency offset also re-duces the signal power captured by discrimination filters and,


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hence, the discrimination property is reduced. Hence, the op-

timum offset (from the nominal value) for the filter center fre-quency is achieved when the reduction in the leakage term is

comparable to the reduction in the signal energy. It should be

highlighted that when offsetting the filters center frequency, thebandwidth of the filters should also increase accordingly.

3) Optical Delay Line: ODL is introduced to compensatefor the differential delay caused by GVD in order that opti-

cally filtered pulses arrive at the photodiodes simultaneously(refer to Section II). The ODL value can be estimated from

intuitively. This calculation is accurately ap-

plied to the delay between two line frequencies. However, the

spectrum of MSK-modulated signals is not a line. In addition,

when CD-induced ISI effects are getting severe (i.e. long trans-

mission fiber, the leakage from an optical discrimination filterto the other can skew detected eye diagrams. Thus, a maximum

push-pull EO requires ODL value to be different from the in-

tuitive value of . Due to the fact that the leakage occurs at

the inner tail of the signal frequency spectrum, optimum ODL

values are expected to be smaller than the intuitive calculation.In MZDI-based receiver configurations, a preamp EDFA is

commonly followed by an optical filter having a large band-width, normally several times larger than the signal spectral

main lobe. The effects of additional ASE noise infected from

preamp EDFA considerably degrade the receivers perfor-mance. This problem is overcome in OFDR as the ASE noise of

EDFA is greatly filtered after passing through the narrowbandoptical filters.

The receiver sensitivity of OFDR can be adjusted by control-

ling the optical gain of EDFA. In the case of small residual dis-

persions, the requirement of very narrow bandwidth can be re-

laxed. As a result, larger bandwidths, which are still many timessmaller than those used for other formats, allow higher energy

for the detected signals. On the other hand, for long uncompen-

sated optical links (i.e. very high residual dispersions), a very

narrow bandwidth is required for the optical filter. As shownin the next section and in the simulation results in Section IV, a

bandwidth as narrow as 5.2 GHz is required for noncoherent de-

tection of 40-Gb/s optical MSK signals. To compensate for the

signal loss after such tight optical filtering, the optical gain ofthe preamp EDFA is boosted to its saturation region (e.g., 25-dB

gain). Thus, the optical gain of the EDFA is adjusted depending

on the severity of the CD effects.

B. Receiver Optimization

This section presents analytical results on EOs which are

obtained from the template functions (refer

to (7) and (8)). These EOs take into account CD-induced ISI

effects caused by precursor and postcursor symbols. Larger

values of the EOs indicate better performance for the receiver

whereas zero values imply the complete closure of the eye

diagram. Thus, the optimized values of ODL and the key

filters parameters, including the filters bandwidth and centerfrequency, can be achieved.

1) Optical Delay Line and Filter Bandwidth: In this paper,

EOs are obtained for different products of the two opticaldiscrimination filters and for different delay mismatch values

Fig. 5. EO versus mismatches of ODL with differentB T

products of optical

discrimination filters

of ODL with for various lengths of SSMF with the dispersion

factor of 17 ps/(nm.km). The zero mismatch represents the

value of which is the nominal delay when

there is no ISI at all. Delay mismatches are obtained from this

nominal value as a percentage with respect to the pulsewidth

( for a 40-Gb/s bit rate).

The 3-D mesh in Fig. 5 shows the simulated EOs versus

products and delay mismatches for a length of 25-km SSMF.This length of SSMF can be referred to as a residual dispersion

of 425 ps/nm. At this dispersion value, ISI effects induced from

fiber CD become quite severe for a 40-Gb/s bit rate. The meshthus provides the guidelines for optimizing the receiver perfor-

mance as well as indicating whether the system has an error

floor, for a particular setting of ODL, , and fiber length.It is found that for the 25-km SSMF length, high values of

EO are obtained on the curve of . Thus, a more

comprehensive view of the mesh is shown in Fig. 6 via the X-Y

cross section at . It can be observed that in the case

of a 25-km SSMF uncompensated optical link for the 40-Gb/s

transmission bit rate, the optimum value of ODL decreases byan amount of 5 ps or 20% of the pulse period from the value

of the intuitive calculation .

2) Filter Bandwidth and Center Frequency: The filter band-width has a tradeoff between the discrimination properties and

the mitigation of ISI. Our analysis is obtained for EOs versus

different products and for various fiber lengths. Analyticalresults are shown in Fig. 7. The filters center frequency is setat the nominal values of 10 GHz for 40-Gb/s MSK signals. It

can be seen from Fig. 7 that for a long span of SSMF (e.g., 25

km SSMF), the optimum values are stable within the range

of 0.13 to 0.18. On the other hand, , 0.22 and 0.21 is

equivalent to the length of 5, 12, and 20 km SSMF, respectively.

It can also be observed that with the nominal delay of ODL, theEO value is slightly greater than zero at in the case


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Fig. 6. EO versus mismatches of ODL on an X-Y cross section at B T = 0 : 1 3obtained from Fig. 8.

Fig. 7. EOs for different lengths of SSMF and different bandwidths (via B Tproducts) of the optical discrimination filters. Thecenterfrequencyis nominallyset at 6 10 GHz.

of 30-km SSMF. This implies that 30 km is the noncompensa-

tion limit of 40-Gb/s OFDR-based optical MSK systems with

the nominal center frequency ( 10 GHz).The effects of offset filtering versus different bandwidths ofthe optical filters are investigated in the case of 25-km SSMF oreffectively 425 ps/nm of residual dispersion. The EO analytical

results are shown in Fig. 8.

It is found that offset filtering can significantly improve thereceiver performance when the bandwidths of optical discrim-

ination filters are properly selected. For larger offset frequen-cies, the fiber bandwidth is increased. Obtained from the meshin Fig. 8, is optimum at about 0.22 and the optimum center

frequency is shifted from 10 GHz toward 15 GHz (i.e. an offset

of 5 GHz from the nominal value of 10 GHz). The analysis of

40-Gb/s optical MSK systems over an uncompensated length of

35 km SSMF is also conducted. The EO mesh is demonstratedin Fig. 9(a) whereas Fig. 9(b) shows the X-Z cross-section by

Fig. 8. EO versus different bandwidths (via BT products) and center frequen-cies (from the optical carrier) of optical discrimination filters over a length of25-km SSMF.

Fig. 9. (a) EOs versusdifferent bandwidths (via BT products) and center fre-quencies of optical filters for an SSMF length of 35 km. (b) EOs versus centerfrequencies in the X-Z cross section of the mesh results shown in (a).

this mesh result. The negative (and also zero) eye openings (EO)

simply indicate the failure of the receiver in the detection ofMSK signals.


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Fig. 10. Power penalty atB E R = 1 0

versus the center frequency offset fordifferent

B T

products over a length of 25-km SSMF.

By offsetting the filters center frequency, OFDR still obtainsa positive EO at a and for the

transmission over 35 km SSMF noncompensating length. This

suggests that there is a possibility that the transmission disper-

sion tolerancelimit of 40-Gb/s OFDR-based optical MSKcan be

achieved up to 35-km SSMF or equivalently 595 ps/nm. In terms

of the10-Gb/s rate, this corresponds to a 560-kmSSMF noncom-

pensating optical link with a residual dispersion of 9250 ps/nm.

ThisstatementisalsobasedontheinterpolationoftheBERversus

the receiver sensitivity as shown later in Section III-B-1. This is

basedontheinterpolationoftheBERresultsandyetremainstobe

proven experimentally and the performance evaluation is based

on the eye-diagram statistical property.3) Validating Receiver Design: The design guidelines and

the analytical results reported in the previous sections can now

be validated with simulation results shown in Figs. 10 and 11.

In Fig. 10, the receiver sensitivity penalties for different filterscenter frequencies and bandwidths are recorded for a 25-km

fiber SSMF length. It is found that for narrow bandwidths (e.g.,to 0.17), the receiver sensitivity varies gently for a

variation of the center frequency within the range of 11 to 12.5

GHz. However, the receiver performance degrades considerably

for frequencies falling outside this range. On the other hand,

when considering relatively large bandwidths ( and

0.25), the optimum center frequency shifts towards 14.5 GHz.Simulation results shown in Fig. 10 validate the guidelines of

the receiver design relating to the filters bandwidth and centerfrequency. In addition, these results also agree with the analyt-

ical results given in Figs. 7 and 8.

The receiver sensitivity penalty against different ODL

mismatches for different lengths of SSMF are numerically

obtained as shown in Fig. 11. is fixed for opticaldiscrimination filters in order to combat severe ISI effectseffectively. Zero mismatch represents the nominal ODL value

of and the mismatches are identified from thiszero value as a percentage of the pulsewidth .

When the distance increases, the optimum delays tend to shift

to smaller values. For example, for 5-km SSMF, the optimumODL value is yet unchanged (i.e. at 0), whereas for 25 km

Fig. 11. Receiver sensitivity penalty atB E R = 1 0

versus delay mis-matches of ODL from the nominal value (at 0) for different lengths of SSMFwith

B T = 0 : 1 3

.

SSMF, it is 12.5%. Furthermore, the receiver is less sensitive

to delay mismatches at short fiber lengths or small residual dis-persions. At 1-dB power penalty, the case of 5-km SSMF obtains

a tolerance range of approximately 45% ( 20% to 25%) com-

pared to 22% in the case of 25-km SSMF. The obtained simula-

tion results verify the intuitive justification of the receiver design(refer to Section IV) and they closely agree with the analytical

results shown in Figs. 5 and 6.

IV. TRANSMISSION PERFORMANCE

A. Receiver Sensitivity

The receiver sensitivity of OFDR-based 40-Gb/s optical

MSK is numerically investigated and compared to that of

MZDI-based 40-Gb/s optical MSK and CS-RZ DPSK formats.

BER values are obtained by the Monte Carlo method and are

shown in Fig. 12. The receiver sensitivity of the OFDR can

be adjusted by controlling the optical gain of the preamp

EDFA (refer to Section II). Additional ASE noise does not

significantly affect the performance of the OFDR due to nar-rowband filtering whereas severe degradations occurs to thereceiver performance of other formats. In addition, depending

on the severity of the fiber CD, the bandwidth of the opticaldiscrimination filters can be enlarged accordingly.From Fig. 12, it is found that at dB, the receiver

sensitivity of is comparable to that of

when dB. At , the receiver sensitivity

of 40 Gb/s OFDR in the case of and dB

is approximately 34 dBm, providing improvements of about

4 dB and 5.5 dB to the sensitivities of MZDI-balanced receivers

for CS-RZ DPSK and MSK, respectively. It can be inferred that

at a moderate residual dispersion, the product can be relaxed

and, therefore, the OFDR may achieve high receiver sensitivity

without pushing the preamp EDFA to its maximum operations.

Another notable remark is that BER curves of for

various gain have slower rolloff slopes compared to otherformats. This is due to the fact that in the case of ,


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Fig. 12. Receiver sensitivities of the OFDR-based 40-Gb/s MSK with different

B T

and optical gainG

in comparison to the sensitivities of MZDI-based40-Gb/s MSK and CSRZ DPSK systems.

Fig. 13. Simulation setup to study the CD tolerance of the 40-Gb/s OFDR-based optical MSK.

waveform distortions caused by tight optical filtering play thedominant role compared to noise effects as in the cases of larger

bandwidth and other formats.

B. Dispersion Tolerance of 40-Gb/s OFDR-Based Optical

MSK Systems

1) Nominal CD Tolerance: Fig. 13 shows the simulation

testbed to investigate the dispersion tolerance of 40-Gb/s

OFDR-based optical MSK systems. Various residual disper-sions are obtained by varying lengths of SSMF from 0 to 30

km or, equivalently, from 0 up to 540 ps/nm.

In this setup, the input power into fiber is set at 3 dBmwhich is lower than the fiber nonlinear threshold so that thesystem is operating in linear transmission domain and nonlinear

effects are negligibly small. EDFA1 is a booster optical ampli-

fier used to set designated input powers into fiber and EDFA2provides optical gains to completely compensate the attenua-

tion of signals when propagating through SSMF. The received

optical power is measured at the input of the OFDR by

adjusting the variable optical attenuator. In this study, a narrow-

band Gaussian filter with is used as the optical dis-

crimination filter. The ODL value is based on the differentialdelay between and , calculated as . The

TABLE I

KEY PARAMETERS USED IN THE SIMULATION

modeling of receiver noise sources is comprised of the signal-

dependent quantum shot noise, the total equivalent noise current

density of 20 at the input of the transimpedance elec-

trical amplifier and dark current of 10 nA for each of the twophotodiodes in balanced structure. The thermal noise, as nor-

mally mentioned in optical receivers, is already included in the

total equivalent noise current density as seen at the input port

of the electronic preamplifier. This includes the thermal noiseof the equivalent input impedance or the transfer impedance of

the amplifiers, the shot noise currents due to biasing current,and the base resistance or channel resistance for the metal semi-

conductor field-effect transistor (MESFET). All currents as seenat the output of the electronic amplifier are also referred to theinput stage of the amplifier.

A fifth-order Bessel electrical filter with a bandwidth of 36GHz is used. Key simulation parameters used in the experiment

are summarized in Table I. BERs versus received optical powers

for different lengths of fiber are shown in Fig. 14. The numer-ical results are obtained via Monte Carlo simulation with low

BERs obtained by linearly extrapolated. It is found that there are

negligible diffrences in the BER performance when the SSMF

length varies from 0 to 12 km. The receiver sensitivity of OFDR

is found to be approximately 23 dBm at BER . An-

other important point noted from Fig. 14 is the capability of

OFDR-based optical MSK transmission systems to reach a BER

at in the case of 30-km uncompen-

sated SSMF length.

From the obtained BER curves in Fig. 14, the dispersion

tolerance of the 40-Gb/s OFDR-based optical MSK system is

achieved in Fig. 15, showing power penalties versus the residualdispersions. It can be observed that the 1-dB and 2-dB power

penalties of the 40-Gb/s optical MSK systems are obtained

when the transmission length is 15-km and 20-km SSMF,

respectively, which equivalently corresponds to the residual

dispersions of 225 ps/nm and 340 ps/nm, respectively.

2) Optimized CD Tolerance by Offset Filtering: The pre-

vious section studies the dispersion tolerance of the 40-Gb/s

OFDR-based optical MSK in the case of nominal values of

the subsystem components. This means that the filter centerfrequencies are set at 10 GHz, and the ODL value is calcu-

lated with the nominal value of . The optimum

values of ODL and offset frequency are obtained from the re-

ceiver design guidelines as well as from the obtained analyt-ical results presented in Section IV. It should be highlighted


10/14


Fig. 14. Transmission performance of OFDR-based optical MSK systems overan uncompensated SSMF length of 0 to 30 km.

Fig. 15. Dispersion tolerance of the 40-Gb/s OFDR-based optical MSK.

that these optimum values depend on the residual dispersions

(i.e., lengths of the transmission link). The filter bandwidth inthis optimized case is still set at since it is best

to combat severe ISI effects induced from long SSMF. Fig. 16

shows the comparison of the nominal and the optimized disper-sion tolerances of the proposed 40-Gb/s OFDR-based optical

MSK system. The power penalties for these two cases are ob-

tained at .

It can be seen that there is no advantage for offset filtering atlow residual dispersion values of up to 170 ps/nm or, equiv-

alently, 10-km SSMF in 40-Gb/s transmission. However, offset

filtering offers significant improvements in the receiver sensi-tivity at high residual dispersions. In the optimized case, the

1-dB power penalty occurs at the residual dispersion of 340

ps/nm. In addition, offset filtering enables improvement gainsof 1 dB and 12 dB for the receiver sensitivity at residual dis-

persions of 340 ps/nm and 510 ps/nm or, equivalently, to 20

km and 30 km of SSMF, respectively. Another notable remarkis that the system can operate up to 595 ps/nm or, effectively,

Fig. 16. Dispersion tolerance of 40-Gb/s OFDR-basedoptical MSKsystems in1) thenominal case (blue,triangle markers anddashedline)and 2) theoptimizedcase (red, square markers, solid line).

Fig. 17. PMDtolerance of OFDR-based optical MSKsystems compared to theMZDI-based MSK and CSRZ-DPSK counterparts.

35-km SSMF whereas nominal filtering fails when residual dis-persion exceeds 510 ps/nm or 30-km SSMF. This simulation

result clarifies the analytical results given in Figs. 7 and 9.3) PMD Tolerance: The robustness to fiber PMD of the

40-Gb/s OFDR-based optical MSK is investigated in this

section via the EO metric which is plotted as a function ofthe normalized mean PMD with respect to bit pe-

riod . The obtained EOs are compared to those

of MZDI-based MSK and CS-RZ DPSK optical systems in

Fig. 17.

It can be seen that PMD tolerance performances of these

three optical systems are similar and the receiver performance

degrades by a 1-dB power penalty at the normalized ratio of

or .

4) Performance of 10-Gb/s Uncompensated Long-Haul

Transmission: This section explores the potential of a 10-Gb/s

optical MSK format for long-haul and particularly metropolitan

transmission without inline dispersion compensation. The

testbed of simulation of this study is arranged such that the fiberSSMF in Fig. 13 is now replaced with seven spans of 80-km


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Fig. 18. BER versus received optical powers for 10-Gb/s OFDR-based opticalMSK transmission systems overuncompensated optical links from 0-to-560-kmSSMF.

SSMF and no dispersion compensating fibers are used. Thus,the total distance is 560 km of SSMF fibers.

A multispan optically amplified uncompensated link of560-km SSMF is utilized. The fiber input power is set at

3 dBm and key simulation parameters in this simulation

experiment can be referred to Table I. It is emphasized that

is utilized while the ODL value and the offsetcenter frequency are optimally selected. Fig. 18 shows the

transmission performance of 10-Gb/s OFDR-based optical

MSK systems over different uncompensated lengths of SSMF.

It can be observed from Fig. 18 that the 1- and 2-dB power

penalties (at ) of the 10-Gb/s optical MSK are

obtained when the transmission lengths are approximately 320

and 390 km SSMF, respectively, corresponding to residual dis-

persions of 5440 ps/nm and 6630 ps/nm. It is noted that in

the case of transmission over the 560-km SSMF uncompensated

optical link, the performance of the 10-Gb/s optical MSK is able

to reach the mark at . Therefore, the

error-free transmission for the 10-Gb/s OFDR-based MSKoptical system achieving an uncompensated distance of up to560 km or equivalently to 9520 ps/nm has been obtained. It

is noted here that these results are interpolated from simulated

results by the Monte Carlo method and are yet to be proven in

experiments and statistical evaluation from the simulated eye

diagram.

5) SPM Resilience: The transmission resilience to fiber SPMof 40-Gb/s OFDR-based optical MSK systems is investigated

and compared to that of the RZ/CS-RZ DPSK, CS-RZ OOK and

MZDI-based MSK systems (RZ implies the RZ33 pulse type).

The simulation testbed for the study is structured similar to the

arrangement ofFig. 13 but the dispersion-managed transmission

link consists of eight spans of 100-km SSMF and SumitomoDCF100 associated with optical amplifiers at the end of either

Fig. 19. Comparison of SPM resilience performance of OFDR-based 40-Gb/soptical MSK to CS-RZ, RZ DPSK, and CS-RZ OOK.

SSMF or DCF100. Thus, a total length of 880 km is the trans-

mission distance. In the cases of DPSK or OOK formats, the

optical transmitter and receiver are replaced accordingly. The

OOK format employs a single photodetector whereas a MZDI

balanced receiver is used for the incoherent detection of DPSK

signals.

The fiber CD is fully compensated while fiber input poweris gradually increased as the focus of this study is on the

SPM nonlinear transmission performance. The optical gains of

EDFA1 and EDFA2 are set to be 10 dB and 19 dB, respec-

tively, to compensate completely for the fiber attenuation. Thefiber input power into these optical amplifiers is also ensured to

be greater than their required minimum values. Therefore, theinput power into each span remains constant and the fiberSPM nonlinearity mainly occurs in the SSMF span rather than

in the DCF. Due to the fact that the effect of fiber CD is negli-gible, the bandwidth of optical discrimination filters is relaxed.A or GHz, which gives a maximum

eye opening (refer to Section IV) is utilized. The center fre-

quency is set at 10 GHz and the ODL takes its nominal value

. The SPM effect is numericallystudied by in-

creasing values from 6 dBm. BERs versus the average input

power are shown in Fig. 19 in comparison to CS-RZ, RZ

DPSK and CS-RZ OOK systems, and BERs are obtained via

the Monte Carlo method.It is found that the OFDR-based optical MSK is more sensi-

tive to SPM nonlinear effects than CS-RZ/RZ DPSK and suffers

the power penalties of approximately 3 dB and 4 dB, respec-

tively, at . However, OFDR-based optical MSK is more

robust to fiber SPM than the CS-RZ/OOK counterpart. It doesnot significantly enhance the nonlinearity resilience over theMZDI-based optical MSK systems. The better SPM resilience

of RZ/CS-RZ DPSK originates from the advantages of the re-

turn-to-zero pulses.

6) Transmission Limits: One common study for a new mod-

ulation format or for a new detection scheme in photonic com-

munications is to explore the transmission limits without regen-

erating optical signals. Such a study is carried out in this sectionfor 40-Gb/s OFDR-based MSK optical transmission systems


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Fig. 20. Performance of the OFDR-based optical MSK systems over differenttransmission distances ranging from 1760- to 2640-km SSMF.

in comparison with the MZDI-based MSK and CS-RZ/DPSK

counterparts.

The simulation testbed is similar to the setup but the number

of optical spans is increased. Since the dispersion-managed

scheme is utilized, the primary shortfall of the transmission

limit is expected from either effects of the fiber PMD or lowOSNR margin caused by a high value of accumulated optical

ASE noise. In this simulation experiment, Gaussian optical

filters with or GHz are utilized. The centerfrequencies are set at 10 GHz; the nominal ODL value is

selected, and the fiber input power into SSMF is approximately3 dBm to avoid impacts of the SPM nonlinear effect. The

system is thus considered to be operating in the linear trans-mission domain.

The normalised PMD parameter is set to be 0.5

to consider the old fiber vintages. The BER perfor-mance of the OFDR-based MSK optical systems are shown in

Fig. 20 for various lengths, ranging from 1760 km to 2640 km

SSMF of the optically amplified dispersion-managed multispanlink.

It can be observed that there are slow rolloff trends in the

BERs for lengths in the range from 2090- to 2640-km SSMF.

This means that ISI effects induced from the PMD dynamic

of the optical fiber degrade the system performance consid-

erably. When the accumulated value of PMD gets small (i.e.,shorter transmission distances), steeper slopes are observed.

Particularly in the case of back-to-back setup, a steep linear

trend of the BER curve indicates that there is no effect of

PMD. Therefore, a length of 1980-km SSMF can be considered

as the transmission limit for the ORDR-based optical MSK

systems over transmission links employing old fiber vintages. The performance of BER curves for

this transmission limit can be significantly improved with ahigh-performance forward error correction (FEC) module.

This transmission limit of the OFDR-based optical MSK

system is then compared to that of the MZDI-based MSK and

of the CSRZ-DPSK system, also for the old fiber vintages

. The simulation results are shown inFig. 21. It is found that the CS-RZ DPSK format is most robust

Fig. 21 Performance comparison between OFDR-based MSK, MZDI-basedMSK, and CS-RZ DPSK formats over a dispersion-managed optical link of

1980-km SSMF withh i = 0 : 5 p s =

p

k m

.

to effects of PMD for low BERs, which is reflected by its fastrolloff trend of the BER curve. In this case of 40-Gb/s trans-

mission, more than 1980-km SSMF and ,

the average accumulated PMD is approximately 22.5 ps, which

is significantly large with respect to the 25-ps pulsewidth ofthe optical pulse. It can be observed that the BER curve of the

MZDI-based optical MSK system also has a slow rolloff trend,

indicating severe effects of PMD. In short, the error-freetransmission above a distance of 1500-km SSMF has demon-

strated the potential of MSK format for ultra-long-haul optical

systems.

7) Spectral Efficiency: Fig. 22 shows the power spectra of

two modulation formats: 40-Gb/s linear MSK and 40-Gb/s NRZDPSK. Referring to the spectra, the following can be observed:

1) The main-lobe spectral widths of the signal power spectrum

of the 40-Gb/s MSK format are narrower than that of NRZ

DPSK. More specifically, the basewidth takes a value of approx-imately 32 GHz on either side compared to 40 GHz in the

case of DPSK. Hence, it is expected that the MSK-based for-

mats are more tolerant offiber CD and more robust to tight op-tical filtering than NRZ DPSK and 2) high suppression of sidelobes and the confinement of signal energy in the main spectrallobe of 40-Gb/s MSK optical power spectra significantly miti-gate interchannel crosstalk between DWDM channels.

We compare MSK and NRZ-DPSK spectra. The zeroes ofthe spectra are approximately 40 GHz and 50 GHz for

MSK and DPSK, respectively. Thus, it is reasonable to expect

that 42.3-Gb/s line-rate MSK signals can be accommodated

into 100-GHz spacing dense-division-multiplexed (DWDM)

optical transmission systems without any crosstalk between

adjacent channels. Small crosstalk between channels would be

expected if 50-GHz channel spacing is used for the 42.3-Gb/s

line-rate NRZ MSK channels. Thus, the MSK modulation

format can be considered efficient modulation for upgrading afew wavelength channels in 100-GHz spacing DWDM systems.

V. CONCLUSION

This paper has proposed a novel optical receiver configu-ration for noncoherent detection of optical MSK signals. The


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Fig. 22. Spectral properties of three modulation formats: 40-Gb/s MSK (red

and black, dash) as well as 80-Gb/s and 40-Gb/s NRZ DPSK (circle dot).

modeling of the receiver and the signal-processing analysis

based on the eye openings are presented. The significance ofthe subsystem key parameters, including the bandwidth and

the center frequency of the optical discrimination filters andthe optical delay line, are analyzed. As a result, the guidelines

for optimizing the receiver design are provided, leading to the

achievement of the optimum values for the key receiver param-

eters. These optimum values and the receiver design guidelines

are confirmed by the simulation results. It has been found thatsignificant improvements in the receiver performance can be

achieved by properly offsetting the center frequencies of theoptical discrimination filters from their nominal values. Thisoffsetting technique relaxes the requirement on optical filters tohave a very narrow bandwidth.

The proposed receiver for 40-Gb/s optical MSK is found to

offer a dispersion tolerance of up to 340 ps/nm for a 1-dB

power penalty at . This achievement is approxi-

mately 5 to 7 times better than that of the existing value based

on the MZDI-balanced receiver. The receiver is also shown to

be robust to PMD. More interestingly, the proposed receiver

enables the possibility for transmitting 10-Gb/s optical MSK

signals over uncompensated 560-km SSMF. This provides a

promising potential for metropolitan optical networks. We havealso examined the SPM resilience of 40-Gb/s OFDR-based op-

tical MSK systems to SPM nonlinearity in comparison with that

of MZDI-based MSK, DPSK, and OOK counterparts. It is found

that the OFDR-based optical MSK has better SPM resilience

performance than the OOK format but suffers about 3-dB and

4-dB penalties compared to CS-RZ and RZ DPSK. Therefore,

a tradeoff exists between the dispersion tolerance and the SPM

nonlinearity resilience. However, the latter problem can be over-

come by reducing the input power into the optical fiber. Thus,with careful consideration on the power distribution, OFDR-

based optical MSK with the uncompensated transmission dis-

tance offers great potential for long-haul and metropolitan op-

tical networks. Finally, the feasibility of the OFDR-based op-tical MSK systems for ultra-long-haul +transmission was also

proven numerically. For the case of the 40-Gb/s transmission

over the old installed fiber having a high PMD coefficient, atransmission limit of up to 1980-km SSMF could be achieved.

ACKNOWLEDGMENT

The authors would like to thank the reviewers for their com-ments that have significantly assisted the clarification of thepapers content.

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communication system with limiter-discriminator-integrator detector,Electron. Lett., vol. 26, 1990.

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Le N. Binh (M82) received the B.E. and Ph.D. degrees in electronic engi-neering and integrated photonics from theUniversity of Western Australia, Ned-lands, Australia, in 1975 and 1980, respectively.

In 1980, he becamea Research Scientistwiththe Departmentof Electrical En-

gineering of Monash University, Clayton Victoria, Australia, after a three-yearperiod with CSIRO Australia, developing parallel microcomputing systems. He

was appointed Reader at Monash University in 1995. He has worked for theDepartment of Optical Communications of Siemens AG Central Research Lab-oratories, Munich, Germany, and the Advanced Technology Centre of NortelNetworks, Harlow, U.K. Currently, he is a Visiting Professor of the Facultyof Engineering of Christian Albretchs University, Kiel, Germany. He has pub-lished more than 250 papers in leading journals and refereed conferences and

two books in the field of photonic signal processing and digital optical commu-nications, one is on Photonic Signal Processing (CRC, 2007) and the other is

Digital Optical Communications (2008).Dr. Binh is a member of the Institute of Electrical and Electronic Engineers

and the Optical Society of America, SPIE, and the Institution of Engineers and

Technologist (U.K.). He has served on technical programs committee of manyinternational conferences and as a Reviewer for several leading international

journals and numerous Australian and international research grants. He was

awarded the Colombo Plan Scholarship of the Australian Government to un-dertake his university education in Australia.

Thanh Liem Huynh received the B.Eng. degree from the Royal Melbourne In-stitute of Technology (RMIT), Australia, in 2004 andthe Ph.D. degree in opticalcommunications engineering from Monash University, Clayton Victoria, Aus-tralia, in 2008.

His research interests are in advanced modulation formats, fiber impairmentmitigation techniques, and electronic equalizers for optical communications.

Khee K. Pang (M69SM87LSM07) is an Hon-orary Research Associate with Monash University,Clayton Victoria, Australia. He was formerly theDeputy Director of the Centre for Telecommuni-cations and Information Engineering (CTIE) withMonash University. He has more than 30 yearsof teaching and research experience in Australia,

the U.S., and Japan, including the University ofCalifornia, Berkeley; Stanford Research Institute;Tohoku University, Japan; and the University ofNotre Dame, Notre Dame, IN. As Principal Inves-

tigator of three consecutive large research projects, funded by the IndustryResearch and Development Board, Commonwealth Department of Industry,Technology and Commerce, Australia, he conducted research and developmentin several areas, which include video and multimedia communications, sourceand channel coding, wireless communications, and digital signal processing.

Direct Detection frequency Discrimination optical receiver

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