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Invited Paper
All Optical OFDM Transmission Systems
June-Koo Kevin Rhee, Seong-Jin Lim, and Malaz Kserawi
Dept. of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, South Korea [email protected]
Abstract
All-optical OFDM data transmission opens up a new realm of advanced optical transmission at extreme data rates, as
subcarriers are multiplexed and demultiplexed by all optical discrete Fourier transforms (DFT). This paper reviews the principles of all optical OFDM transmission and its system application techniques, providing the generic ideas and the
practical implementation issues to achieve 100Gbps or higher data rates with a spectral efficiency of 1 bps/Hz or better.
This paper also include discussions on all-optical OFDM implementation variants such as an A WG-based OFDM
mUltiplexer and demultiplexer, a receiver design without optical sampling, a transmitter design with frequency-locked
cw lasers, an OFDM cyclic prefix designs, and a chromatic dispersion mitigation technique.
Keywords: optical OFDM, opical communication, optical modulation formamt, optical discrete Fourier transform
1. INTRODUCTION
Optical OFDM has been considered as the key enabling technology breaking through the 100-Gbps barrier of optical
single wavelength channel rate. There are many promising research reports on optical OFDM [1-7]. An optical OFDM
symbol can be generated either by an electronic FFT (fast Fourier transform) processor or by an all-optical DFT (discrete Fourier transform) processor [4-7]. In the former case, there is a great flexibility in generation and detection of an OFDM
symbol, as one can implement very delicate functions such as subcarrier equalization, advanced modulation formats, and
adaptive filtering. However, the OFDM data rate is limited by the throughput of electronics, and achieving 100 Gbps or
higher data rates is quite challenging and costly.
All-optical technology tends to provide, almost always, a one-step higher data rate in a transmission system than
a state-of-the-art electronic system. The electronic FFT and OFDM symbol modulation can be replaced by an all-optical DFT to overcome the electronics speed limit. All optical OFDM symbols can be generated by the following sequence as
shown in Fig.l; subcarrier optical data modulation, inverse discrete Fourier transform (iDFT), delay alignment, and
multiplex. The reference model of Fig.l is a one-to-one mapping of mathematical model to optical circuitry. The
detection process is an inverse of the generation process. The combination of symbol generation and detection in this
model ought to provide inter-subcarrier orthogonality as well as inter-symbol orthogonality. However, this model is not
an ideal realization of the mathematical model. In this paper we address the in-depth understandings of all optical OFDM, and review practical implementation variants that have been considered by many research groups.
2. ALL OPTICAL SAMPLING OFDM MODEL
The main benefit of OFDM is the inter-subcarrier orthogonality; there is no interference introduced by neighbor or far
subcarriers. This is achieved by numerical mUltiplexing and demultiplexing of subcarriers with FFTs in a typical
embodiment. Let us consider the forward DFT process in the frame of a time propagator e -2lrijl , where f and t denote
frequency and time, respectively:
sVJ= � II S(tk) e2lriJjlk N k=O N-I
S{tk)= L S(rJe -2mJjtk j=O
(1)
(2)
for forward and inverse DFTs, respectively. Here, S(.) and s( .) are the frequency and time domain representations of an
Optical Transmission Systems, Subsystems, and Technologies IX, edited by Xiang Liu, Ernesto Ciaramella, Naoya Wada, Nan Chi, Proc. of SPIE-OSA-IEEE Asia Communications and Photonics,
SPIE Vol. 8309, 83091W· © 2011 SPIE-OSA-IEEE . CCC code: 0277-786X/11/$18 . doi: 10.1117/12.900449
Proc. of SPIE-OSA-IEEENol. 8309 83091W-1
4x25 Gbps
100 Gbps
25GHz ! dock r------
A
subcarrier phase & amplitude
pre-emphasis
•• J
1 ..... 1
Fig.1 All-optical OFDM reference modeL Curves in the small insets indicate the pulse positions in a symbol time slot
100 Gbps
OFDM symboL In the multiplexing process, a vector of input values S = {S j I Sj = sVJ j = O .. (N -I)} represents the
modulation status of information to be transmitted from the transmitter with N subcarriers. This vector is inverse Fourier
transformed to obtain the time-domain representation s = {Sk I sk = S(tk ), k = O .. (N - l)} . Note that both representations
are sampled values of a frequency-domain spectrum and a time-domain waveform. Numerically, the sampled values can
provide absolutely no interference with other subcarriers. In an electronic embodiment of this transmission method, the complex values of waveform sequence s can be
used to create a waveform by I1Q (in- and quadrature-phase) modulation on a carrier. Here, the sampling space in the
time domain is r = 1/ N t5 where t5 is the subcarrier frequency spacing. At the receiving end, each subcarrier is
demultiplexed by demodulation followed by forward FFT to convert time-domain sequence s to the corresponding spectrum sequence S. Note that in these process of OFDM multiplexing and demultiplexing, the orthogonality is
preserved only at the sampling points in both time and space domains.
An optical embodiment of the all-optical OFDM shown in Fig. 1 of a 4-subcarrier example achieves exactly the
same functions to multiplex and demultiplex OFDM symbols but by the optical circuitry. In order to perform inverse
DFT, one need start with a short pulse that has a spectrum broad enough to cover the whole OFDM symbol spectrum.
Each pulse in the different spectrum input port is then phase shifted by the amount defined by e -21ri!A . Here the phase
is defined in reference to the zero-th subcarrier frequency 10. The key idea how to generate an OFDM subcarrier is
illustrated Fig. 2.
OFDM Transmitter
A ke-zmMA
I A A
A A
A A "-r--'
/':t:f)A t] '2 t3 '4
OFDM
A A
A A �
---. /':t:f)A '1 12 t3 '4
Receiver A xe2lriJ3lk A
A / A A A
� I
A \ A A A A x e2ffi Iz lk
' 0 '
A Fig. 2 Schematic illustration of 4-subcarrier all-optical sampling OFDM multiplexing and demultiplexing that show how
inter subcarrier orthogonality is preserved. The number values inside pulse shapes indicate the phasor notation of
Proc. of SPIE-OSA-IEEENoL 8309 83091W-2
instantaneous phase with respect to frequency 10.
At a receiver, the OFDM symbol is sampled at every I , and separated into individual fiber paths. At this point
the spectrum of each sampled pulse is broadened to cover the whole spectrum of the OFDM symbol. In other words, the
frequency information is smeared by sampling but the instantaneous phase information is kept. Then each sampled
optical pulses are time aligned at the input of forward DFT that shifts phase of each pulse by e27tif/k • The spectrum
representation at an output port of the forward DFT is formed by constructive interference for the designated subcarrier frequency, and by destructive interference for other subcarrier frequencies. The performance of an all-optical OFDM receiver is critically affected by how accurately destructive interference cancellation is achieved.
The DFT realization with optical circuitry can be attained with the state-of-the-art planar lightwave circuit (PLC)
technology. There are two major PLC solutions: 2x2-coupler-based DFT and arrayed-waveguide-(A WG)-based DFT.
Fig. 3(a) depicts an optical circuit diagram that constructs a 4x4 inverse DFT [6,8]. This design is scalable with an N log2N cost similarly to an FFT. However, this construction is extremely sensitive to phase errors in the waveguide as a
large number of interference terms appear at an output port. Fig. 3(b) is another implementation scheme utilizing an A WG device technique. This configuration consists of
the free-space phase shifter for the DFT and the waveguide array for the delay. The optical DFT is simply achieved by
placing ports at carefully design positions on the free space region, such as reported in [9-12]. Combination of the DFT
with an array of waveguide for the time delay step forms a standard architecture of an A WG mUltiplexer. The
corresponding transfer function of each port is presented in Fig. 3(c), which is the same for a coupler-based DFT
combined with a delay step array. The transfer function shows zero power spectrum crosstalk in the neighbor and other
subcarrier channels. As a matter of fact, the implementation issue of an OFDM multiplexer is how one can achieve this filter function with various device technologies. The manufacturability is known to be better with the A WG device than with the coupler based device [14]. The same transfer functions can be attained by fiber Bragg grating (FBG) technology
[ 15].
(b)
phase shift by -i -i
inverse DFT
For 10FT
S3 = [(SO-S2) + i(SI-S3)]l2 SI = [(SO-S2) - i(SI-SJ)]l2 S2 = [(SO+S2) - (SI+S3)]/2 So = [(SO+S2) + (SI+S3)]12
(c)
1 .2 r---;---;-----r--r---r----r---r---,
-
::l .i 0.8 c.i § 0.6
LL c 0.4 e I- 0.2
o ������������ -75 -50 -25 0 25 50 75
Frequency (GHz) Fig. 3 All-optical OFDM multiplexer designs and the ideal transfer function for 4x4 example; (a) coupler based inverse
DFT, (b) A WG based inverse DFT and delay step array, and ( c) transfer functions of 4 subcarrier ports.
An ideal OFDM symbol with on-off keying has a spectral efficiency of 1 bps/Hz. The spectrum of 4x25Gbps all optical OFDM obtained by numerical modeling is shown in Fig. 4(a). When this symbol is demultplexed at an OFDM
receiver of Fig. 1, an RZ output can be found as presented in Fig. 4(b). The performance achieved by an ideal transmitter
and receiver pair is a little better than that of a single-carrier 100 Gbps RZ on/off keying transmission.
The OFDM demultiplexer design is nothing but a reverse of the optical circuits, except for the sampling process. In principle, the sampling creates amplitude and phase information of an OFDM symbol, in a way that each sample is
time demultiplexed into a different path. However, sampling can be moved to after the forward DFT, or even after the
photodetector in Fig. 1. The schematic illustration of the demultiplexing process is presented in Fig. 5. In this receiver
design, the bandwidth of all components, especially, the photodetector, has to be broad enough to sample the narrow
autocorrelation feature as shown in Fig. 5(b), and hence the optical and electrical noise filtering is not as effective as optically sampling model of Fig. 1.
Proc. of SPIE-OSA-IEEENol. 8309 83091W-3
(a) o .--�------�---, c _ -10 \I) E 0.. s:: -20 CU .-4 .� ci -30 .... -.!!:! a:I -40 CU'tJ a:: - -50
-60
- 5 0 -25 0 25 50 Frequency Detuning (GHz)
(b)
o
•
20 40 60 Time (ps)
80
Fig. 4 Typical example of all-optical sampling OFDM symbol, (a), and the demultiplexed subcarrier output at a receiver (b).
(a) (b) OFDM Receiver '. {�. ' . } ----f-
with post sampling f;jJj)j). � IYJfJ{;:. }X e7'" 131, fY)j)j).
.�.': , ' . "
IYJfJ{;:. � �{� ..
' . . } �s'mplm'
� xe2mf2 tk f;jJj)j). , received f\f\f\/\ OFDM symbol � IYJfJ{;:. " I, I, I,
Fig. 5 Schematic illustration of an OFDM receiver (a) and its demultiplexing process (b) with post sampling at a receiver.
The upper and lower eye patterns correspond to the auto-correlated and cross-correlated subcarriers. At the designated
sampling position, both patterns together show OFDM orthogonality.
3. PRACTICAL ApPLICA TION CONSIDERATIONS
The OFDM transceiver reference model of Fig. 1 requires quite complex optical circuits and phase sensitive operation conditions. There are many variants in transmitter and receiver designs. First, the OFDM subcarriers may not need to be
mode10cked in the linear propagation limit. When subcarriers are modelocked, one can introduce phase control in subcarriers to control instantaneous peak power [16]. However, how to control the transmission performance againt non
linear fiber impairment is not known clearly. Besides, uncertainty or randomness in subcarrier phases does not break orthogonality of OFDM as long as the frequencies are accurately controlled. Hence the embodiment of the conceptual circuit design of Fig.1 can be replaced by a simple model if accurate optical frequency control is provided for the
continuous wave (cw) laser sources, as illustrated in Fig. 6. In this case, the output spectrum of a modulated optical signal needs to be carefully controlled to match with the transfer function of Fig. 3( c).
Similarly to advanced OFDM transmission techniques used in radio wireless communication systems, cyclic
prefix can mitigate channel impairments. In optical transmission, chromatic dispersion, polarization modal dispersion
(PMD) impairment, and other inter-symbol interference (lSI) can be mitigated [2]. Fig. 7 presents a reference design to generate an OFDM symbol with cyclic prefix with ratio of 4:5. The inverse DFT is designed for 4x25 GHz forward DFT
but with a sampling rate of 20GHz. Within one period of sampling, the OFDM demultiplexer takes 4 samples instead of
5 samples at every T. Cyclic prefix can be generated with a simple similar modification in an FBG based OFDM
multiplexer [15]. The same effect can be achieved if one reduces the modulation rate of an OFDM multiplexer of Fig. 6 while
keeping the subcarrier spacing the same, 25 GHz, the same effect as cyclic prefix can be achieved. This has been used in
early experimental reports without clear understandings. In this application the modulation spectrum is critically
important to have no power at the center frequencies of neighbor subcarriers.
Proc. of SPIE-OSA-IEEENol. 8309 83091W-4
4x25 Gbps
100 Gbps
freq. ·locked cw lasers
spectrum tailored modulators
Jl
Fig. 6 cw-Iaser OFDM transmission schematic optical circuit
diagram. The coupler can be replaced by an OFDM
multiplexer, or other types of DWDM mux if the transfer
function is carefully tailored to match with Fig. 3(c).
Fig. 7 OFDM transmission schematic with cyclic prefix. The
corresponding receiver OFDM is the same as in Fig. 1 but with
sampling rate of 20 GI-Iz.
The penalty from chromatic dispersion can be mitigated by applying optical bandpass filters right after data
modulators, because the modulation bandwidth is limited and less sensitive to chromatic dispersion penalty. Another
technique to mitigate the chromatic dispersion penalty is to adjust the time delay at each sub carrier port of iDFT because
chromatic dispersion causes a different group delay to different subcarriers, and the delay adjustment can detune the
overlap of crosstalk away from the signal position in the time domain [17].
4. SUMMARY
In this paper, we review the fundamental principles of all optical OFDM transmission technologies. An ideal embodiment
of the reference model may require relatively complex and phase-sensitive optical circuitry and optical sampling at the
receiver. However, there are many variants that can achieve OFDM mUltiplex and demultiplex, with even simpler and
reliable device designs. Such examples discussed in this paper include an A WG-based OFDM multiplexer and
demultiplexer, a receiver design without optical sampling, a transmitter design with frequency-locked cw lasers, an OFDM cyclic prefix designs, and a chromatic dispersion mitigation technique. This review paper can excite intriguing questions
and new implementation ideas that come from the clear understandings of all-optical sampling OFDM fundamental
principles.
Acknowledgment
This work was supported in part by the IT R&D program of MKE/IITA [2008-FOI7-02, IOOGbps Ethernet and optical
transmission technology development].
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Proc. of SPIE-OSA-IEEENol. 8309 83091W-6