All Optical OFDM Transmission Systems

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.


(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.


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].

REFERENCES

[1] W. Shieh and C. Athaudage, "Coherent optical orthogonal frequency division multiplexing," Electron. Lett., vo1.42, pp. 587-589, 2006.

[2] W. Shieh, H. Bao, and Y. Tang, "Coherent optical OFDM: theory and design," Opt. Express, vo1.l6, pp.841-859, 2008.

[3] A. Sano, et. al., "30 x 100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes," ECOC 2007, PDP, Berlin, 2007.

[4] D. Hillerkuss, et. at. "Single Source Optical OFDM Transmitter and Optical FFT Receiver Demonstrated at Line Rates of 5. 4 and 10. 8 Tbitls," OFC 2010, PDP C, San Diego, 2009.

[5] K. Lee, C.T. D. Thai, and J.-K. K. Rhee, "All optical discrete Fourier transform processor for 100 Gbps OFDM transmission," Opt. Express, 16,4023-4028,2008.

[6] Y. Huang, D. Qian, R. E. Saperstein, P. N. Ji, N. Cvijetic, L. Xu, and T. Wang, "Dual-Polarization 2x2 IFFT/FFT Optical Signal Processing for 100-Gb/s QPSK-PDM All-Optical OFDM," in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2009), paper OTuM4.

[7] H. Chen, M. Chen, and S. Xie, "All-optical sampling orthogonal frequency-division multiplexing Scheme for High-Speed Transmission System," J a/Lightwave Techno!. vo1.27, pp.4848-4854, 2009.

[8] M.E. Marhic, "Discrete Fourier transforms by single-mode star networks," Opt. Lett., vo1.l2, pp.63-65, 1987.


[9] G. Cincotti, N. Wada, and K. Kitayama, "Characterization of a full encoder/decoder in the A WG configuration for code-based photonic routers," J. of Lightwave Technol., vo1.24, pp.1 03-112, 2006.

[10] W. Li, X. Liang, W. Ma, T. Zhou, B. Huang, and D. Liu, "A planar waveguide optical discrete Fourier transformer design for 160 Gb/s all-optical OFDM systems," Opt. Fiber Technol., vol.l6, pp.5-ll, 2010.

[II] AI. Lowery, "Design of arrayed-waveguide grating routers for use as optical OFDM demultiplexers," Opt. Express vol.l8, pp.14129-14143, 2010.

[12] K. Takiguchi, T. Kitoh, A Mori, M. Oguma, and H. Takahashi, "Optical orthogonal frequency division multiplexing demultiplexer using slab star coupler-based optical discrete Fourier transform circuit," Opt. Lett., vo1.36, pp.1 140-1 142, 2011.

[13] Z. Wang, K. S. Kravtsov, Y.-K. Huang, and P. R. Prucnal, "Optical FFT/IFFT circuit realization using arrayed waveguide gratings and the applications in all-optical OFDM system," Opt. Express, vol.l9, pp.4501-4512, 2011.

[14] S.-I. Lim and l-K.K. Rhee, "System tolerance of all-optical sampling OFDM using AWG discrete Fourier transform," Opt. Express, vol.l9, iss.14, pp.13590-13597, 2011.

[15] H. Chen, M. Chen, and S. Xie, "All-optical sampling orthogonal frequency-division multiplexing scheme for high-speed transmission system," J. Lightwave Techno., vol. 27, pp.4848-4854, 2009.

[16] X. Liang, W. Li, W. Ma, and K. Wang, "A simple peak-to-average power ratio reduction scheme for all optical orthogonal frequency division multiplexing systems with intensity modulation and direct detection," Opt. Express, vol. 17, issue 18, pp.15614-15622, 2009.

[17] S. Yamamoto, et al. , "Achievement of subchannel frequency spacing less than symbol rate and improvement of dispersion tolerance in optical ofdm transmission," J. Lightwave Techno., vol. 28, no. I, pp.157-163, 2010.


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All Optical OFDM Transmission Systems