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Optics Express

Optics Express

  • Editor: C. Martijin de Sterke
  • Vol. 19, Iss. 7 — Mar. 28, 2011
  • pp: 6980–6989
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Novel ROF/FTTX/CATV hybrid three-band transport system

Wen-Jeng Ho, Hsiao-Chun Peng, Hai-Han Lu, Cheng-Ling Ying, and Chung-Yi Li  »View Author Affiliations


Optics Express, Vol. 19, Issue 7, pp. 6980-6989 (2011)
http://dx.doi.org/10.1364/OE.19.006980


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Abstract

A novel cost-effective radio-over-fiber (ROF)/fiber-to-the-X (FTTX)/CATV hybrid three-band transport system based on direct modulation of a distributed feedback laser diode (DFB LD) with multi-wavelength output characteristic is proposed and experimentally demonstrated. Radio-frequency (RF) (1.25Gbps/6GHz) signal with direct modulation, as well as baseband (BB) (622 Mbps) and CATV (channels 2-78) signals with external remodulation are successfully transmitted simultaneously. Low bit error rate (BER) and clear eye diagram were achieved for ROF and FTTX applications; as well as good performances of carrier-to-noise ratio (CNR), composite second-order (CSO) and composite triple beat (CTB) were obtained for CATV signals over an 80-km single-mode fiber (SMF) transport.

© 2011 OSA

1. Introduction

2. Experimental setup

The schematic architecture of our proposed novel ROF/FTTX/CATV hybrid three-band transport systems based on direct modulation of a DFB LD with multi-wavelength output characteristic is shown in Fig. 1
Fig. 1 The schematic architecture of our proposed novel ROF/FTTX/CATV hybrid three-band transport systems.
. The hybrid three-band transport systems consist of one directly modulated DFB laser transmitter, and an 80-km SMF with two cascaded broadband erbium-doped fiber amplifiers (EDFAs). The output power of the DFB LD is 4.77 dBm, at a bias current of 25 mA. Furthermore, the optical characteristics of the DFB LD including threshold current and 3-dB bandwidth of frequency response are 17 mA and 7.8 GHz, respectively. The output power and noise figure of each EDFA are ~17 dBm and ~4.5 dB, at an input power of 0 dBm, respectively. For the transmitting site, it is composed of one DFB LD with a central wavelength of 1532.14 nm, three optical band-pass filters (OBPFs; OBPF1-OBPF3), two external modulators, and one EDFA. A 1.25-Gbps data stream is mixed with a 6-GHz RF carrier to generate the data signal. The resulting data signal is directly modulated into the DFB LD with a large OMI of 9% to generate the multi-wavelength output characteristic. The modulated optical signal is efficiently split into three parts by three OBPFs, directly transmitted and externally remodulated individually. Each OBPF is composed of one optical circulator (OC) and one fiber Bragg grating (FBG). The wavelength variation of the FBG with temperature controller is ~0.003 nm/°C. The OBPF1, with a 3-dB bandwidth of 0.06 nm, is used to pick up two modes (0 and + 1) from the output of the DFB LD. As to the OBPF2 and OBPF3, with a 3-dB bandwidth of 0.02 nm, each is employed to pick up one mode (−1 and −2) from the one. To ensure only one mode is pick up, OBPF2 and OBPF3 exhibit a sharp cutoff in the transmission spectrum. A data signal of 622 Mbps, with a pseudorandom binary sequence (PRBS) length of 223-1, is fed into an external modulator for remodulation. A multi-carrier generator (Matrix SX-16; NTSC) is employed to feed RF subcarriers (channels 2-78) into the other external modulator for remodulation. To compare with the three DFB LDs configuration, in which each DFB LD is directly modulated with ROF, FTTX, and CATV applications, it is noted that we have applied two additional external modulators in our proposed systems. DFB laser transmitter with direct modulation exhibits an optical frequency that varies with output power, referred to as chirp. As chirp is combined with fiber dispersion, signal degradation accumulates along the fiber length. For this reason, nearly all 1550 nm optical transmitters currently in operation use external modulation, a technique which provides minimal chirp. The composite signals are now a combination of ROF (1.25Gbps/6GHz, RF), FTTX (622 Mbps, BB), and CATV (CH2-78) signals. The hybrid three-band signals are multiplexed back into the EDFA-I through a 3 × 1 optical combiner. Since the optical power level of the CATV channel is much higher than that of the ROF and FTTX channels, yet the output of the ROF and FTTX channels are through a variable optical attenuator (VOA). Thereby, the ROF RF and FTTX BB signals compared to the CATV signal have only a very small effect on the input power of EDFA-I.

3. Experimental results and discussions

The optical spectra for a directly modulated DFB LD at various OMI are present in Fig. 3(a), (b), and (c)
Fig. 3 The optical spectrum for a directly modulated DFB LD at various OMI (a) 3.8% (b) 5.7% (c) 9% .
, respectively. As OMI values are 3.8% (Fig. 3(a)) and 5.7% (Fig. 3(b)), the optical spectra possess only a few wavelengths. However, as OMI value is increased up to 9% (Fig. 3(c)), the optical spectrum possesses multiple wavelengths with adequate flatness (−2, −1, 0, and + 1 modes). Figure 4
Fig. 4 The measured SMSR values under different OMI for mode 0.
shows the measured side-mode suppression ratio (SMSR) values under different OMI for mode 0. The SMSR value is defined as a power level comparison between the mode 0 and the other mode. It is clear that the SMSR is inversely proportional to the OMI. Moreover, the marked solid triangle (a), (b), and (c) three cases in Fig. 4 are correspondent with Fig. 3(a), (b), and (c), respectively. Large OMI not only results in small SMSR, but also results in directly modulated DFB LD with flat multi-wavelength output characteristic. The spectrum of the output light from a directly modulated DFB LD using a small signal approximation is given by [9

9. H. Olesen and G. Jacobsen, “A theoretical and experimental analysis of modulated laser fields and power spectra,” IEEE J. Quantum Electron. 18(12), 2069–2080 (1982). [CrossRef]

]:
S(f)=Σ|Jn(Δffm)M4{Jn+1(Δffm)ejφ+Jn1(Δffm)ejφ}|2δ(f(f0+nfm)),
(1)
where Jn(x) is the nth order Bessel function of the first kind, n is the number of side modes, M is the OMI, f0 is the optical frequency under CW operation, Δf is the peak frequency deviation caused by the modulation, fm is the modulation frequency, Δf/fm is the FM index, and φis the phase delay between the intensity and the phase modulation. It is clear that, from the Eq. (1), the amplitude of each sideband is mainly affected by the OMI value. As we observed in the Fig. 3(a)-(c), the sideband optical power is proportionally increased with the increasing OMI value. A large OMI allows a directly modulated DFB LD to obtain a multi-wavelength output with flat power level.

The measured BER curves for the ROF (1.25Gbps/6GHz) and the FTTX (622 Mbps) applications as a function of received optical power level are plotted in the Fig. 5
Fig. 5 The measured BER curves for the ROF (1.25Gbps/6GHz) and the FTTX (622 Mbps) applications as a function of the received optical power level.
. At a BER of 10−9, for ROF RF transmission, the received optical power level is −23 dBm; for FTTX BB transmission, the received optical power level is −24.2 dBm. Good BER performances are achieved over an 80-km SMF transport, it verifies that the proposed ROF/FTTX transport systems can be constructed by employing by a DFB LD with direct modulation. The back-to-back (BTB) BER curves are also given in Fig. 5. At a BER of 10−9, there exists a large power penalty of 14.6 dB (ROF) between BTB case and optical DSB scheme due to RF power degradation induced by fiber dispersion. And at a BER of 10−9, there exist small power penalties of 6.8 (ROF) and 6.1 (FTTX) dB between BTB cases and only one optical sideband schemes due to the suppression of RF power degradation induced by fiber dispersion [10

10. W. I. Lin, H. H. Lu, H. C. Peng, and C. H. Huang, “Direct-detection full-duplex radio-over-fiber transport systems,” Opt. Lett. 34(21), 3319–3321 (2009). [CrossRef] [PubMed]

]. These >6 dB power penalties are the results of the fiber dispersion-induced distortions. Fiber dispersion is one of the most severe limiting factors in long-haul lightwave transport systems. If the fiber transmission length exceeds several tens of kilometers, dispersion effect can cause intolerable amounts of distortion. In lightwave transport systems with chirp parameter α, the received RF power Pf can be stated as [11

11. G. H. Smith and D. Novak, “Broad-band millimeter-wave (38 GHz) fiber-wireless transmission system using electrical and optical SSB modulation to overcome dispersion effects,” IEEE Photon. Technol. Lett. 10(1), 141–143 (1998). [CrossRef]

]:
Pfcos2{πLDλc2fc2c[12πtan1(α)]},
(2)
where L is the fiber length, D is the fiber dispersion coefficient, λc is the optical carrier wavelength, fc is the frequency for which the power fading is evaluated, and c is the light velocity in vacuum. Over an 80-km SMF transport, fiber dispersion causes severe power degradation due to optical DSB scheme. The RF power degradation because of fiber dispersion degrades the BER performance.

The eye diagrams of the transmitted RF and BB (after 1 GHz LPF) signals at the receiving site are demonstrated in Fig. 6(a) and (b)
Fig. 6 The eye diagrams of the transmitted (a) RF and (b) BB (after 1 GHz LPF) signals at the receiving site.
, respectively. In Fig. 6(a), the corresponding jitter and SNR are 3.9 ps and 30.5 dB; in Fig. 6(b), the corresponding jitter and SNR are 3.5 ps and 31 dB. In addition, the corresponding jitter and SNR for RF and BB signals (BTB) are 0.4ps/36dB and 0.3ps/36.3dB, respectively. Although little and undesired jitter and amplitude fluctuations are introduced; nevertheless, clear and open eye diagrams for both RF and BB signals are still observed. Furthermore, the corresponding jitter and SNR for RF signal (DSB) are 7.8 ps and 24.4 dB. More undesired jitter and amplitude fluctuations are induced because of fiber dispersion-induced distortion.

Figure 7
Fig. 7 The measured CNR, CSO and CTB values under NTSC channel number.
shows the measured CNR, CSO and CTB values under NTSC channel number, respectively. Since CNR value results from the relative intensity noise of LD, thermal and shot noise of optical receiver, as well as signal-spontaneous and spontaneous-spontaneous beat noise of EDFA; CNR values (>50 dB) of systems with optical DSB and SSB scheme are almost the same due to the use of an identical LD, the same input optical power levels of EDFA and analog receiver. For CSO and CTB performances, the CSO (<-70 dBc) and CTB (<-69 dBc) values of systems with optical SSB scheme can be improved significantly. The improved results seen are due to the use of optical SSB filter (OBPF6) to decrease the linewidth of the optical signal, in which leading to the reduction of the fiber dispersion. The dispersion coefficient D follows that
D=1Lτωωλc,
(3)
where τ is the group delay, and ω is the angular frequency. Since τ/ω=Lβ¨, ω/λc=2πc/λc2(β¨ is the second order dispersion coefficient in the frequency expansion), thus

D=2πcλc2β¨.
(4)

Finally, the differential group delay (Δτ) can be expressed by
Δτ=DLΔλc,
(5)
where Δλc is the linewidth of the optical signal. It is effective to introduce an optical SSB filter to reduce the optical linewidth so that total fiber dispersion is reduced. The second and third order harmonic distortions (2HD and 3HD) due to fiber dispersion are [12

12. M. R. Phillips, T. E. Darcie, D. Marcuse, G. E. Bodeep, and N. J. Frigo, “Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals,” IEEE Photon. Technol. Lett. 3(5), 481–483 (1991). [CrossRef]

]:
2HD=10log[mDλc2Lf4c16(Δτ)2+4λc4L2π2f6c2],
(6)
3HD=10log[9m2D2λc4L2f24c(4(Δτ)2+4π2f)],
(7)
where m is the OMI, and f is the RF carrier frequency. Moreover, according to the analysis in [13

13. W. I. Way, Broadband Hybrid Fiber/Coax Access System Technologies, San Diego: CA: Academic, ch. 2, 33–37 (1999).

], CSO and CTB distortions can be stated as:
CSO=2HD+10logNCSO+6,
(8)
CTB=3HD+10logNCTB+6,
(9)
where NCSO and NCTB are the product counts of CSO and CTB. It can be concluded that, from these above equations, CSO and CTB degradations are proportional to the linewidth of the optical signal. The use of an optical SSB filter makes the optical linewidth to change from a broad linewidth into a narrow one. Then there would be significant reductions in the CSO and CTB distortions, since the CSO and CTB distortions are due to fiber dispersion. From the experimental results we can see that large CSO and CTB improvements of about 6 dB have been achieved compared to optical DSB system.

OBPFs with tight and sharp cutoff characteristics are used in systems. Probably, the cost of OBPFs will increase the cost of systems. However, they are worth employing because a multiple number of DFB LDs are replaced by a single DFB LD at the transmitting site, as well as only one optical sideband and optical SSB formats are obtained at the receiving site. For ROF RF signal with only one optical sideband format, since optical carrier and one of the sidebands are eliminated before detecting, the RF power degradation induced by fiber dispersion can be cancelled. In this way, the BB data signal is obtained directly from the optical sideband. It is shown to be a promising solution since expensive and sophisticated RF devices (for example, local oscillator for RF signal down-conversion) are not required at the receiving site. For CATV signal with optical SSB format, since one of the sidebands is deleted before receiving, the RF power degradation induced by fiber dispersion can be suppressed. In this way, the optical spectral efficiency is improved and the fiber dispersion-induced distortion is reduced.

FTTX networks are usually implemented with different fiber lengths. To show a more direct association with our proposed systems and the fiber lengths, we measure the BER/CNR/CSO/CTB values at different fiber lengths, and the results are given in Table 1

Table 1. Measured BER/CNR/CSO/CTB Values at Different Fiber Lengths

table-icon
View This Table
. It is obvious that longer fiber length leads to worse system performances, due to the accumulations of noise and fiber dispersion.

4. Conclusions

We have proposed a novel cost-effective ROF/FTTX/ CATV hybrid three-band transport system based on direct modulation of a DFB LD with multi-wavelength output characteristic. From the serious discussions and the demonstrated experimental results, we can declare that such a novel system not only reveals simple and economic advantages by employing a DFB LD with direct modulation to replace multiple LDs, but also demonstrates an efficient and practical architecture to serve broadband integrated services. With the assistance of only one optical sideband and optical SSB schemes, low BER and clear eye diagram were achieved for ROF and FTTX applications; as well as good performances of CNR, CSO and CTB were obtained for CATV signals. Such a hybrid three-band transport system would be very attractive for trunk applications in advanced optical fiber transport and distribution networks, it reveals a prominent alternative with advantages in simplicity and cost to provide triple-play services for wide areas.

References and links

1.

C. H. Chang, H. H. Lu, H. S. Su, C. L. Shih, and K. J. Chen, “A broadband ASE light source-based full-duplex FTTX/ROF transport system,” Opt. Express 17(24), 22246–22253 (2009). [CrossRef] [PubMed]

2.

R. Llorente, T. Alves, M. Morant, M. Beltran, J. Perez, A. Cartaxo, and J. Marti, “Ultra-wideband radio signals distribution in FTTH nettworks,” IEEE Photon. Technol. Lett. 20(11), 945–947 (2008). [CrossRef]

3.

H. H. Lu, H. C. Peng, W. S. Tsai, C. C. Lin, S. J. Tzeng, and Y. Z. Lin, “Bidirectional hybrid CATV/radio-over-fiber WDM transport system,” Opt. Lett. 35(3), 279–281 (2010). [CrossRef] [PubMed]

4.

A. Murakoshi, K. Tsukamoto, and S. Komaki, “High-performance RF signals transmission in SCM/WDMA radio-on-fiber bus link using optical FM method in presence of optical beat interference,” IEEE Trans. Microw. Theory Tech. 54(2), 967–972 (2006). [CrossRef]

5.

T. F. Fent, S. Shaari, and B. Y. Majlis, “Distributed CATV inputs in FTTH-PON system,” IEEE International Conf. on Semiconductor Electron. (ICSE). 58–61 (2006).

6.

S. Gao, C. Yang, X. Xiao, Y. Tian, Z. You, and G. Jin, “Wavelength conversion of spectrum-sliced broadband amplified spontaneous emission light by hybrid four-wave mixing in highly nonlinear, dispersion-shifted fibers,” Opt. Express 14(7), 2873–2879 (2006). [CrossRef] [PubMed]

7.

K. H. Han, E. S. Son, K. W. Lim, H. Y. Choi, S. P. Jung, and Y. C. Chung, “Bi-directional WDM passive optical network using spectrum-sliced light-emitting diodes,” Opt. Fiber Commun. 1, 23–27 (2004) (OFC).

8.

M. Yoshino, N. Miki, N. Yoshimoto, and K. Kumozaki, ““Multiwavelength optical source for OCDM using sinusoidally modulated laser diode,” IEEE/OSA J,” J. Lightwave Technol. 27(20), 4524–4529 (2009). [CrossRef]

9.

H. Olesen and G. Jacobsen, “A theoretical and experimental analysis of modulated laser fields and power spectra,” IEEE J. Quantum Electron. 18(12), 2069–2080 (1982). [CrossRef]

10.

W. I. Lin, H. H. Lu, H. C. Peng, and C. H. Huang, “Direct-detection full-duplex radio-over-fiber transport systems,” Opt. Lett. 34(21), 3319–3321 (2009). [CrossRef] [PubMed]

11.

G. H. Smith and D. Novak, “Broad-band millimeter-wave (38 GHz) fiber-wireless transmission system using electrical and optical SSB modulation to overcome dispersion effects,” IEEE Photon. Technol. Lett. 10(1), 141–143 (1998). [CrossRef]

12.

M. R. Phillips, T. E. Darcie, D. Marcuse, G. E. Bodeep, and N. J. Frigo, “Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals,” IEEE Photon. Technol. Lett. 3(5), 481–483 (1991). [CrossRef]

13.

W. I. Way, Broadband Hybrid Fiber/Coax Access System Technologies, San Diego: CA: Academic, ch. 2, 33–37 (1999).

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(350.4010) Other areas of optics : Microwaves

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: February 16, 2011
Manuscript Accepted: March 15, 2011
Published: March 25, 2011

Citation
Wen-Jeng Ho, Hsiao-Chun Peng, Hai-Han Lu, Cheng-Ling Ying, and Chung-Yi Li, "Novel ROF/FTTX/CATV hybrid three-band transport system," Opt. Express 19, 6980-6989 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-7-6980


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References

  1. C. H. Chang, H. H. Lu, H. S. Su, C. L. Shih, and K. J. Chen, “A broadband ASE light source-based full-duplex FTTX/ROF transport system,” Opt. Express 17(24), 22246–22253 (2009). [CrossRef] [PubMed]
  2. R. Llorente, T. Alves, M. Morant, M. Beltran, J. Perez, A. Cartaxo, and J. Marti, “Ultra-wideband radio signals distribution in FTTH nettworks,” IEEE Photon. Technol. Lett. 20(11), 945–947 (2008). [CrossRef]
  3. H. H. Lu, H. C. Peng, W. S. Tsai, C. C. Lin, S. J. Tzeng, and Y. Z. Lin, “Bidirectional hybrid CATV/radio-over-fiber WDM transport system,” Opt. Lett. 35(3), 279–281 (2010). [CrossRef] [PubMed]
  4. A. Murakoshi, K. Tsukamoto, and S. Komaki, “High-performance RF signals transmission in SCM/WDMA radio-on-fiber bus link using optical FM method in presence of optical beat interference,” IEEE Trans. Microw. Theory Tech. 54(2), 967–972 (2006). [CrossRef]
  5. T. F. Fent, S. Shaari, and B. Y. Majlis, “Distributed CATV inputs in FTTH-PON system,” IEEE International Conf. on Semiconductor Electron. (ICSE). 58–61 (2006).
  6. S. Gao, C. Yang, X. Xiao, Y. Tian, Z. You, and G. Jin, “Wavelength conversion of spectrum-sliced broadband amplified spontaneous emission light by hybrid four-wave mixing in highly nonlinear, dispersion-shifted fibers,” Opt. Express 14(7), 2873–2879 (2006). [CrossRef] [PubMed]
  7. K. H. Han, E. S. Son, K. W. Lim, H. Y. Choi, S. P. Jung, and Y. C. Chung, “Bi-directional WDM passive optical network using spectrum-sliced light-emitting diodes,” Opt. Fiber Commun. 1, 23–27 (2004) (OFC).
  8. M. Yoshino, N. Miki, N. Yoshimoto, and K. Kumozaki, ““Multiwavelength optical source for OCDM using sinusoidally modulated laser diode,” IEEE/OSA J,” J. Lightwave Technol. 27(20), 4524–4529 (2009). [CrossRef]
  9. H. Olesen and G. Jacobsen, “A theoretical and experimental analysis of modulated laser fields and power spectra,” IEEE J. Quantum Electron. 18(12), 2069–2080 (1982). [CrossRef]
  10. W. I. Lin, H. H. Lu, H. C. Peng, and C. H. Huang, “Direct-detection full-duplex radio-over-fiber transport systems,” Opt. Lett. 34(21), 3319–3321 (2009). [CrossRef] [PubMed]
  11. G. H. Smith and D. Novak, “Broad-band millimeter-wave (38 GHz) fiber-wireless transmission system using electrical and optical SSB modulation to overcome dispersion effects,” IEEE Photon. Technol. Lett. 10(1), 141–143 (1998). [CrossRef]
  12. M. R. Phillips, T. E. Darcie, D. Marcuse, G. E. Bodeep, and N. J. Frigo, “Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals,” IEEE Photon. Technol. Lett. 3(5), 481–483 (1991). [CrossRef]
  13. W. I. Way, Broadband Hybrid Fiber/Coax Access System Technologies, San Diego: CA: Academic, ch. 2, 33–37 (1999).

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