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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 1 — Jan. 14, 2013
  • pp: 533–543
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SSII cancellation in an EAM-based OFDM-IMDD transmission system employing a novel dynamic chirp model

Dar-Zu Hsu, Chia-Chien Wei, Hsing-Yu Chen, Yi-Cheng Lu, Cih-Yuan Song, Chih-Chieh Yang, and Jyehong Chen  »View Author Affiliations


Optics Express, Vol. 21, Issue 1, pp. 533-543 (2013)
http://dx.doi.org/10.1364/OE.21.000533


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Abstract

We develop a novel subcarrier-to-subcarrier intermixing interference (SSII) cancellation technique to estimate and eliminate SSII. For the first time, the SSII cancellation technique is experimentally demonstrated in an electro-absorption modulator- (EAM-) based intensity-modulation-direct-detection (IMDD) multi-band OFDM transmission system. Since the characteristics of SSII are seriously affected by the chirp parameter, a simple constant chirp model, we found, cannot effectively remove the SSII. Therefore, assuming that the chirp parameter linearly depends on the optical power, a novel dynamic chirp model is developed to obtain better estimation and cancellation of SSII. Compared with 23.6% SSII cancellation by the constant chirp model, our experimental results show that incorporating the dynamic chirp model into the SSII cancellation technique can achieve up to 74.4% SSII cancellation and 2.8-dB sensitivity improvement in a 32.25-Gbps OFDM system over 100-km uncompensated standard single-mode fiber.

© 2013 OSA

1. Introduction

With the exponentially increasing of customer needs for broadband services, optical access networks have been considered as future-proof infrastructures, and a passive optical network (PON) is one of the most promising candidates to economically provide high bandwidth to end-users [1

1. T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE 94(5), 911–934 (2006). [CrossRef]

]. Recently, optically amplified large-split long-reach PON (LR-PON) is proposed to economically provide higher bandwidth and larger fiber coverage range [1

1. T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE 94(5), 911–934 (2006). [CrossRef]

6

6. D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol. 25(3), 685–693 (2007). [CrossRef]

], thus the capital and operational expenditures can be reduced considerably by consolidating the O/E/O conversion interfaces inside the existing networks. To reach the target of higher data rate over up to 100-km transmission distance without significant cost increase, optical intensity modulation and direct-detection (IMDD) scheme is still preferred, especially using cost-effective transmitters, such as directly modulated DFB lasers (DMLs) and electro-absorption modulated lasers (EMLs).

Similar to the reconstruction and elimination of subcarrier-to-subcarrier beating interference (SSBI) in a Mach-Zehnder modulator- (MZM-) based single-side band OFDM system [11

11. W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. 27(24), 5723–5735 (2009). [CrossRef]

], it is possible to reconstruct and mitigate SSII in an IMDD OFDM system [12

12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express 20(23), 25774–25789 (2012). [CrossRef] [PubMed]

]. However, in an MZM-based amplitude modulation system, OFDM signals are directly encoded in an optical field. Accordingly, SSBI is only composed of the beating terms among subcarriers due to the square-law photo-detection, but SSII includes additional intermixing terms caused by square-root-law amplitude modulation (i.e. linear intensity modulation) and chirp-induced phase modulation. Hence, unlike the case of SSBI, the amount of total SSII is a function of dispersion and chirp. In this work, the SSII cancellation technique is experimentally demonstrated in an IMDD OFDM transmission system for the first time. Based on the proposed SSII reconstruction scheme, the theoretical SSII can be calculated and used to eliminate the received SSII. To further improve the effectiveness of SSII cancelation, this work develops a dynamic chirp model that takes the voltage-dependent chirp [13

13. D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express 18(26), 27758–27763 (2010). [CrossRef] [PubMed]

] into consideration. While only 23.6% SSII is cancelled by the constant chirp model, 74.4% SSII cancellation and 2.8-dB sensitivity improvement at the FEC limit are experimentally demonstrated by incorporating the dynamic chirp model into the SSII cancellation technique in a 32.25-Gbps OFDM system over 100-km uncompensated SSMF.

2. SSII theory

In [7

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express 19(18), 17546–17556 (2011). [CrossRef] [PubMed]

], the EAM model only considers the constant chirp parameter. However, in [13

13. D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express 18(26), 27758–27763 (2010). [CrossRef] [PubMed]

], we learned that the chirp parameter is a function of modulation voltage, and therefore, a new model that takes the dynamic chirp into consideration would be required. Similar to the deviation in [10

10. C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. 36(2), 151–153 (2011). [CrossRef] [PubMed]

], the envelop of the output optical field at the optical transmitter can be given by EVejΔϕ, where V is the modulation voltage, Δϕ is the chirp-induced phase modulation, approximated as Δϕαlog|E|, and α is the chirp parameter. The modulation voltage of OFDM signals is expressed as V=Vb+n=1N{vnejnωt}, where Vb is the bias voltage, N is the OFDM subcarrier number, ω/(2π) is the subcarrier spacing, and vn is the encoded complex information of the nth subcarrier, and {} represents the real part. The whole frequency response of electrical-signal-to-optical-power conversion is set as HT(n), composed of the responses of digital-to-analog conversion (DAC), electrical filter, electrical amplifier, and optical modulator, and HT(0) denotes the frequency response of the optical carrier. Thus, the power envelop can be written as P=HT(0)Vb+n=1N{HT(n)vnejnωt} or P=HT(0)Vb×[1+n=1N{xnejnωt}], where xn=[HT(n)/HT(0)][vn/Vb] stands for the normalized OFDM signal. Setting the AC power envelop as X1=n=1N{xnejnωt}, we could consider the power-dependent chirp parameter as α=m=0αmX1m by Taylor series, and therefore, the normalized envelop of the optical field E can be approximated as
E1+1jα02X11+α02+4jα18X2,
(1)
where X2=X12n=12N{x˜nejnωt} is the 2nd order term, and the third and higher order terms of xn are neglected such that only the first two terms of α are included. Here only considering α0 is the constant chirp model [7

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express 19(18), 17546–17556 (2011). [CrossRef] [PubMed]

, 10

10. C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. 36(2), 151–153 (2011). [CrossRef] [PubMed]

], and the dynamic chirp model will include both α0 and α1 so that the chirp parameter is linearly dependent on the optical power. Considering only chromatic dispersion, the response of fiber transmission after fiber of distance L with the dispersion parameter β2 can be simplified as HCD(n)=ejn2θD, where θD=β2Lω2/2. Accordingly, the optical field becomes
E(L)1+12×1+α02ejθαn=1N{xnejnωt}ejn2θDXt118×(1+α02)|secθA|ejθAn=12N{x˜nejnωt}ejn2θDXt2,
(2)
where θαtan1α0, θΑtan1(4α1/1+α02), Xt1 represents the transmitted OFDM signal, and Xt2 is the transmitted 2nd order nonlinear term. After square-law photo-detection, the received signal is proportional to |E(L)|21+{Xt1}+|Xt1|2/4{Xt2}/4, where the third order and the fourth order terms are also omitted, and |Xt1|2 can be represented as (1+α02)n=12N{x˜d,nejnωt}. If the combined frequency responses of the receiver, including photodiode, electrical amplifier, and analog-to-digital conversion (ADC), is denoted as HR(n), the normalized received signal is
R(L)1+1+α02n=1NHR(n){xnejnωt}cos(n2θDθα)+1+α024×[n=12NHR(n){x˜d,nejnωt}Beatingterms|secθA|n=12NHR(n){x˜nejnωt}cos(n2θD+θA)Intermixingterms].
(3)
The 2nd term at the RHS of R(L) corresponds to the desired OFDM signal; the 3rd term indicates the beating terms among subcarriers due to the square-law photo-detection, and the last term represents the intermixing terms caused by square-root-law amplitude modulation and chirp-induced phase modulation. Then, the combination of the 2nd order terms is the so-called SSII, and denoted as the theoretical SSIIT in this work. From the SSII theory, not only the chirp parameters of α0 and α1, and the responses of HT, HCD and HR, but also the complex information of vn are necessary to calculate the SSII. Although the transmitted data are never exactly known at the receiver, the received data after hard decision are used as vn to estimate SSII, and an iterative process could be employed to improve the accuracy of SSII estimation.

3. SSII cancellation technique

To mitigate the influence of SSII, we introduce the SSII cancellation technique at the receiver and the block diagram is shown in Fig. 1
Fig. 1 The block diagram of the SSII cancellation technique.
. The received OFDM data with SSII are demodulated and detected as the normal OFDM demodulation process, including fast Fourier transform (FFT), equalization, and decision. Then the detected data are used as vn to calculate the SSII based on the SSII theory, and the calculated result can be further feedback to carry out the SSII cancellation. Since the SSII of each subcarrier will be calculated individually, the SSII cancellation is performed after the FFT. Then the OFDM data after the SSII cancellation are demodulated again, and the iterative process would get the more correct detected data.

Based on the SSII theory in Sec. 2, SSII is composed of the beating and intermixing terms, which are caused by the square-law photo-detection and the square-root-law amplitude modulation and chirp-induced phase modulation, respectively. Consequently, following the model of the whole optical transmission system, SSII could be estimated by iteratively replacing vn by the detected data, vn(i-1), where the superscript, i, denotes the number of iteration. The detail of the block of SSII calculation of Fig. 1 is shown in Fig. 2(a)
Fig. 2 The detail block diagrams of the (a) SSII calculation and (b) theoretical SSII.
, and SSII calculation is composed of three parts, transmitted signal reconstruction, emulated SSII calculation, and calculated SSII modification. In the block of the transmitted signal reconstruction, the detected OFDM data vn(i-1) is multiplied with the frequency response of optical transmitter, HT(n), the transmitted AC power envelop X1(i) of the OFDM signal can be reconstructed, and then be sent to the block of emulated SSII calculation.

As for the response measurements of HT(n), HCD(n) and HR(n), the response of the optical receiver HR(n) is assumed flatten in the beginning. Then, through measuring the responses of optical back-to-back signals and optical fiber transmission by the training symbols, the responses of HT(n) and HCD(n) can be obtained. Then, the inaccuracy of the assumed HR(n) is compensated by SSII weighting in the block of calculated SSII modification. However, since the SSII power is generally much lower than the power of subcarriers, SSII-to-noise ratio would be low [7

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express 19(18), 17546–17556 (2011). [CrossRef] [PubMed]

,12

12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express 20(23), 25774–25789 (2012). [CrossRef] [PubMed]

] and the presence of noise will degrade the accuracy of both chirp estimation and SSII weighting. Therefore, we need to increase the number of training symbols at the price of lower bandwidth efficiency. Nonetheless, since chirp estimation and SSII weighting are dependent on the operation conditions of the transmitter but almost irrelevant to the slowly-varying response of fiber channel, long training symbol can be just used in the beginning or used occasionally, and normal training symbol format can be used after the required parameters of SSII cancellation have been estimated.

4. Experimental set-up and results

Figure 5(a)
Fig. 5 (a) SNR of each subcarrier of optical back-to-back, one-band OFDM after 100-km SSMF, two-band OFDM after 100-km SSMF without SSII cancellation, and two-band OFDM after 100-km SSMF with SSII cancellation via considering the constant chirp and the dynamic chirp. (b) SNR improvement by SSII cancellation of three cases corresponding to those in (a).
shows the measured SNR of each OFDM subcarrier. The SNR after 100-km SSMF transmission without SSII cancellation is shown by red curve in Fig. 5(a), and the SNR shown by black curve is measured by transmitting band-1 and band-2 OFDM signals individually. Theoretically, if only the band-2 signal is transmitted, the signal will not suffer the SSII induced by itself, or equivalently, all the SSII is out-of-band. In addition, for the case of band-1 only, the influence of the SSII can be ignored, as shown in Fig. 4. Thus, the SNR difference between red and black curves can be regarded as the upper bound of SSII cancellation. After introducing the SSII cancellation technique with only considering the constant chirp parameter α0 of EAM, which is about 0.53 at the bias voltage of –1 V measured by the method in [15

15. F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol. 11(12), 1937–1940 (1993). [CrossRef]

], as shown by green curve in Fig. 5(a), we can find that the SSII cancellation is not obvious. After considering the dynamic chirp parameter including α0 and α1, as shown by blue curve in Fig. 5(a), the black and blue curves are almost overlapped and the SSII can be almost canceled. Thus, from both the SSII theory shown in Sec. 2 and the experimental results shown in Fig. 5(a), the dynamic chirp must be taken into consideration to improve the performance of SSII cancellation. Figure 5(b) shows the SNR improvement by SSII cancellation of three cases corresponding to those in Fig. 5(a).

To evaluate the performance of the SSII cancellation technique, the average amount of SSII elimination is to be defined and calculated. For the nth subcarrier, if the received signal power, the noise power and the SSII power without the SSII cancellation are denoted as PS(n), PN(n) and PSSII,W(n), respectively, the corresponding SNR could be written as ΓW(n) = PS(n)/[PSSII,W(n) + PN(n)]. Since we assume no SSII for the case of the upper bound, its SNR will be ΓU(n) = PS(n)/PN(n). Furthermore, the power of residual SSII after SSII cancellation is set as PSSII,C(n) or PSSII,D(n), where the subscript C or D indicates the cancellation based on the constant chirp model or the dynamic chirp model. Hence, the SNR after the SSII cancellation will be ΓC(n) = PS(n)/[PSSII,C(n) + PN(n)] or ΓD(n) = PS(n)/[PSSII,D(n) + PN(n)]. Consequently, the power of SSII could be estimated by the SNRs and the signal power,
PSSII,ζ(n)=PS(n)Γζ(n)PS(n)ΓU(n),
(4)
where the subscript ζ{W,C,D}. Moreover, the amount of SSII elimination could be obtained by ΔPSSII,C(n) = PSSII,W(n)–PSSII,C(n) or ΔPSSII,D(n) = PSSII,W(n)–PSSII,D(n). Then the average amount of SSII elimination in percentage can be written as
ΔP¯SSII,ζ=n=1NΔPSSII,ζn=1NPSSII,W.
(5)
If the constant chirp is considered, the proposed SSII cancellation technique can only achieve 23.6% SSII elimination. However, employing the new dynamic chirp model, 74.4% SSII elimination can be achieved which demonstrates a considerable effectiveness of the new model.

Based on the dynamic chirp model, Fig. 6
Fig. 6 SNR improvement with different numbers of training symbol, NTS.
shows the measured SNR improvement with different numbers of training symbols. With only 10 training symbols, the estimation of SSII weighting is inaccurate and the SNR improvement is much less than those with more training symbols. As the number of training symbols increasing, the SNR improvement will be improved. However, 50 training symbols are sufficient to accurately estimate SSII weighting, and further increment of training symbols does not improve the signal performance much, as shown in Fig. 6. Thus, in order to optimize the performance of SSII cancellation and also maintain the bandwidth efficiency, 50 training symbols are adopted in this work. Moreover, according to Eq. (5), 66.8%, 73.5%, 75.0% and 75.1% SSII eliminations are achieved by using 10, 30, 70, and 90 training symbols, respectively.

Furthermore, the BER performances of the EAM-based OFDM signal with five cases are plotted in Fig. 7(a)
Fig. 7 (a) BER and (b) constellations of OFDM signals before and after SSII cancellation.
. In Fig. 7(a), the cases (i) and (ii) shown by the black and green solid circles indicate the cases of optical back-to-back (O B-t-B) and 100-km SSMF without SSII cancellation, respectively, while the cases (iii) and (iv) shown by the red and blue solid circles indicate the cases of using the SSII cancellation technique with considering the constant and dynamic chirp parameters of EAM, respectively. The BER of ~3.8 × 10−3 (the FEC limit) of cases (ii), (iii) and (iv) can be obtained at the received powers of –8.4 dBm, –10.2 dBm, and –11.2 dBm. Accordingly, after using the SSII cancellation technique with considering the constant and dynamic chirp parameters, the receiver sensitivities are improved by 1.8 dB and 2.8 dB, respectively. In addition, assuming the full knowledge of the transmitted data is known and used for the SSII estimation and cancellation, the BER performance with such ideal SSII cancellation is also plotted as the case (v) in Fig. 7(a). Compared with the ideal SSII cancellation, the proposed SSII cancellation without iteration almost shows identical performance, and therefore, the iteration process can be dropped in this experiment. Moreover, corresponding to the cases (ii) and (iv) at the received power of –6 dBm, the constellations of the 32-QAM and QPSK in the 2nd passband before and after SSII cancellation are shown in Fig. 7(b) for comparison.

5. Conclusion

References and links

1.

T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE 94(5), 911–934 (2006). [CrossRef]

2.

G. Talli, C. W. Chow, E. M. MacHale, C. Antony, R. Davey, P. D. Townsend, T. De Ridder, X. Z. Qiu, P. Ossieur, H. G. Krimmel, D. W. Smith, I. Lealman, A. Poustie, S. Randel, and H. Rohde, “Long reach passive optical networks,” in The 20th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2007. LEOS 2007, (IEEE-LEOS, 2007), pp. 868–869.

3.

R. Lin, “Next generation PON in emerging networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America (2008), paper OWH1.

4.

R. P. Davey, D. B. Grossman, M. Rasztovits-Wiech, D. B. Payne, D. Nesset, A. E. Kelly, A. Rafel, S. Appathurai, and S. H. Yang, “Long-reach passive optical networks,” J. Lightwave Technol. 27(3), 273–291 (2009). [CrossRef]

5.

K. Y. Cho, K. Tanaka, T. Sano, S. P. Jung, J. H. Chang, Y. Takushima, A. Agata, Y. Horiuchi, M. Suzuki, and Y. C. Chung, “Long-reach coherent WDM PON employing self-polarization-stabilization technique,” J. Lightwave Technol. 29(4), 456–462 (2011). [CrossRef]

6.

D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol. 25(3), 685–693 (2007). [CrossRef]

7.

D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express 19(18), 17546–17556 (2011). [CrossRef] [PubMed]

8.

D. Z. Hsu, C. C. Wei, H. Y. Chen, Y. C. Lu, and J. Chen, “A 40-Gbps OFDM LR-PON system over 100-km fiber employing an economical 10-GHz-based transceiver,” in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2012), paper OW4B.

9.

A. Gharba, P. Chanclou, M. Ouzzif, J. L. Masson, L. A. Neto, R. Xia, N. Genay, B. Charbonnier, M. Hélard, E. Grard, and V. Rodrigues, “Optical transmission performance for DML considering laser chirp and fiber dispersion using AMOOFDM,” in 2010 International Congress on Ultra Modern Telecommunications and Control Systems and Workshops (ICUMT) (2010), pp. 1022–1026.

10.

C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. 36(2), 151–153 (2011). [CrossRef] [PubMed]

11.

W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. 27(24), 5723–5735 (2009). [CrossRef]

12.

C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express 20(23), 25774–25789 (2012). [CrossRef] [PubMed]

13.

D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express 18(26), 27758–27763 (2010). [CrossRef] [PubMed]

14.

E. O. Brigham, Fast Fourier Transform and Its Applications, 1st ed. (Wiley, New York, 1997).

15.

F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol. 11(12), 1937–1940 (1993). [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: November 1, 2012
Revised Manuscript: December 14, 2012
Manuscript Accepted: December 21, 2012
Published: January 7, 2013

Citation
Dar-Zu Hsu, Chia-Chien Wei, Hsing-Yu Chen, Yi-Cheng Lu, Cih-Yuan Song, Chih-Chieh Yang, and Jyehong Chen, "SSII cancellation in an EAM-based OFDM-IMDD transmission system employing a novel dynamic chirp model," Opt. Express 21, 533-543 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-533


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References

  1. T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE94(5), 911–934 (2006). [CrossRef]
  2. G. Talli, C. W. Chow, E. M. MacHale, C. Antony, R. Davey, P. D. Townsend, T. De Ridder, X. Z. Qiu, P. Ossieur, H. G. Krimmel, D. W. Smith, I. Lealman, A. Poustie, S. Randel, and H. Rohde, “Long reach passive optical networks,” in The 20th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2007. LEOS 2007, (IEEE-LEOS, 2007), pp. 868–869.
  3. R. Lin, “Next generation PON in emerging networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America (2008), paper OWH1.
  4. R. P. Davey, D. B. Grossman, M. Rasztovits-Wiech, D. B. Payne, D. Nesset, A. E. Kelly, A. Rafel, S. Appathurai, and S. H. Yang, “Long-reach passive optical networks,” J. Lightwave Technol.27(3), 273–291 (2009). [CrossRef]
  5. K. Y. Cho, K. Tanaka, T. Sano, S. P. Jung, J. H. Chang, Y. Takushima, A. Agata, Y. Horiuchi, M. Suzuki, and Y. C. Chung, “Long-reach coherent WDM PON employing self-polarization-stabilization technique,” J. Lightwave Technol.29(4), 456–462 (2011). [CrossRef]
  6. D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol.25(3), 685–693 (2007). [CrossRef]
  7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express19(18), 17546–17556 (2011). [CrossRef] [PubMed]
  8. D. Z. Hsu, C. C. Wei, H. Y. Chen, Y. C. Lu, and J. Chen, “A 40-Gbps OFDM LR-PON system over 100-km fiber employing an economical 10-GHz-based transceiver,” in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2012), paper OW4B.
  9. A. Gharba, P. Chanclou, M. Ouzzif, J. L. Masson, L. A. Neto, R. Xia, N. Genay, B. Charbonnier, M. Hélard, E. Grard, and V. Rodrigues, “Optical transmission performance for DML considering laser chirp and fiber dispersion using AMOOFDM,” in 2010 International Congress on Ultra Modern Telecommunications and Control Systems and Workshops (ICUMT) (2010), pp. 1022–1026.
  10. C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett.36(2), 151–153 (2011). [CrossRef] [PubMed]
  11. W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol.27(24), 5723–5735 (2009). [CrossRef]
  12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express20(23), 25774–25789 (2012). [CrossRef] [PubMed]
  13. D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express18(26), 27758–27763 (2010). [CrossRef] [PubMed]
  14. E. O. Brigham, Fast Fourier Transform and Its Applications, 1st ed. (Wiley, New York, 1997).
  15. F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol.11(12), 1937–1940 (1993). [CrossRef]

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