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A hybrid WDM transport system based on mutually injection-locked Fabry–Perot laser diodes
Optics Communications 276 (2007) 87–92 www.elsevier.com/locate/optcom
A hybrid WDM transport system based on mutually injection-locked Fabry–Perot laser diodes Cheng-Ling Ying b
a,b
, Hai-Han Lu
b,*
, Shah-Jye Tzeng b, Hsien-Li Ma b, Yao-Wei Chuang
b
a Department of Electronic Engineering, Jin-Wen Institute of Technology, Taiwan Institute of Electro-Optical Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao East Road, Taipei 10608, Taiwan, ROC
Received 16 October 2006; received in revised form 19 March 2007; accepted 25 March 2007
Abstract A hybrid wavelength-division-multiplexing (WDM) transport system based on mutually injection-locked Fabry–Perot laser diodes (FP LDs) for CATV, 256-QAM and OC-48 transmission is proposed and demonstrated. Mutually injection-locked FP LDs as broadband light source could be relatively simple and cost-effective compared with other demonstrated light source schemes. The proposed hybrid WDM transport system employs four filtered wavelengths (modes) to transmit 111 AM-VSB channels, four 256-QAM digital passband channels, and one OC-48 digital baseband channel simultaneously. Since our proposed system does not use multiple distributed feedback (DFB) LDs, it reveals a prominent one with simpler and more economic advantages. 2007 Elsevier B.V. All rights reserved. Keywords: Broadband light source; Mutually injection-locked Fabry–Perot laser diode; Wavelength-division-multiplexing
1. Introduction Wavelength-division-multiplexing (WDM) transport systems provide many advantages for optical networks, including high transmission capacity, network flexibility and upgradability. A hybrid WDM transport system that uses different wavelengths to transmit AM-VSB analog video, 64/256-QAM digital passband and OC-48/OC-192 digital baseband signals is very useful for an optically amplified fiber network providing CATV, Internet and telecommunication services [1–3]. Hybrid WDM transport systems are envisioned to have a multiple number of distributed feedback laser diodes (DFB LDs) which are wavelength-selected for each channel and controlled to operate at a specific wavelength, this process will increase the cost and complexity of the systems. For a practical implementation of hybrid WDM transport systems, it is necessary to *
Corresponding author. Tel.: +886 2 27712171x4601/4621; fax: +886 2 87733216. E-mail address: [email protected] (H.-H. Lu). 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.03.052
develop low cost and low complexity light source. Several approaches have been proposed to solve these problems. WDM transport systems employing spectrum-sliced light sources such as light-emitting diodes (LEDs) or amplified spontaneous emission (ASE) sources were proposed [4–6]. However, the output power of LED is insufficient to accommodate many channels. Although the spectrumsliced ASE source provides much higher output power than that of the LED, it requires expensive erbium-doped fiber (EDF) and pumping LD. Recently, there has been a proposal of a broadband light source which is based on mutually injection-locked Fabry–Perot (FP) LDs [7]. When FP LD is injection-locked, the multiple longitudinal modes of FP LD can be changed into a similar single longitudinal mode characteristic, and it can be used as cheap light source in hybrid WDM transport systems [8,9]. In this paper, we proposed and demonstrated a potentially cost-effective hybrid WDM transport system based on mutually injection-locked FP LDs. By mutual injection between two FP LDs, we realized a broadband light source with flatness and multimode output spectrum. The
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C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
proposed multimode light source generates 15 modes, ranging from 1532 to 1544 nm, and with a flatness of <4 dB. This proposed hybrid WDM transport system used two wavelengths (modes) for AM-VSB video signal, one for 256-QAM digital passband and one for OC-48 digital baseband signals. System performances were investigated over a 100-km standard single-mode fiber (SMF) transmission. Since our proposed system does not use multiple expensive DFB LDs, it reveals a prominent one with simpler and more economic advantages than that of traditional hybrid WDM transport systems. 2. Experimental setup Fig. 1 shows the experimental setup of our proposed hybrid WDM transport systems based on mutually injection-locked FP LDs. The hybrid WDM transport systems consist of one broadband light source, four optical bandpass filters (BPFs), four external modulators, and 100 km of standard SMF with three cascaded erbium-doped fiber amplifiers (EDFAs). All the EDFAs have similar characteristics, with an output power of 17 dB and a noise figure of 4.5 dB, at an input power of 0 dB. The broadband light source is efficiently split into four optical channels by four optical BPFs with 3-dB bandwidth of 0.4 nm, externally modulated individually, and multiplexed back into the SMF by using two optical couplers. The filtered four wavelengths of k1, k2, k3 and k4 for externally modulated AMVSB analog video, 256-QAM digital passband and
OC-48 (2.5 Gb/s) digital baseband signals are 1540.4, 1538.8, 1537.2 and 1535.6 nm, with a channel spacing of 200 GHz. To be the best wavelength allocation, the longest wavelength is employed to transmit AM-VSB analog video signal, and the shortest wavelength is employed to transmit OC-48 digital baseband signal to avoid the stimulated Raman scattering (SRS) effect. If the OC-48 is located at the long wavelength and AM-VSB located at the short wavelengths, then the Raman-induced crosstalk degrades the carrier-to-noise ratio (CNR) performance and is of considerable concern for a hybrid WDM transport system. In view of the fact that the digital signal is in general more robust than analog signal with respect to noise and nonlinear distortions, the CNR value of the AM-VSB video channel for a hybrid WDM transport system is the most important limiting parameter [10]. In a typical hybrid WDM transport system for CATV/256-QAM/OC-48 signals transport, the optical power launched into the EDFAs (EDFA-I, EDFA-II, and EDFA-III) for CATV and 256QAM channels needs to be about 0 dB due to the systems’ CNR and bit error rate (BER) performance requirements. In contrast, the optical power launched into the EDFAs (EDFA-II and EDFA-III) for OC-48 channel is usually less than 30 dB due to the low optical receiver sensitivity. Optical output power of EDFA-I and the OC-48 optical channel through a variable optical attenuator (VOA) are coupled together by using an optical coupler. Note that the optical power level of EDFA-I is much higher than that of the OC-48 channel through a VOA so the OC-48 signal
CATV (CH2 ~ CH78)
Optical BPF
1540.4 nm
External Modulator
1
CATV (CH79 ~ CH112)
Optical BPF Broadband Light Source
1538.8 nm
External Modulator
2
Optical Splitter
EDFA-
Optical BPF
1537.2 nm
External Modulator
3
Optical Coupler
VOA
256-QAM (CH113 ~ CH116)
VOA
VOA
Optical Coupler
EDFA-
EDFA-
40km SMF
20km SMF VOA
Optical BPF
1535.6 nm
External Modulator
4
OC-48 (2.5Gb/s)
CH112
HP-8591C Tunable RF BPF 256-QAM Error Detector
Analog Optical Rx
1/
256-QAM DeModulator CH116
2/
3
Optical Tunable BPF 40km SMF
OC-48 Error Detector
2.5Gb/s
Digital Optical Rx
4
Fig. 1. The experimental setup of our proposed hybrid WDM transport systems based on mutually injection-locked FP LDs.
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
compared to the combined AM-VSB and 256-QAM signals has only a very small effect on the input power of EDFAII. For CATV channel performance, there is a trade-off between CNR and composite second order (CSO)/composite triple beat (CTB) values for different received optical power level. Higher (lower) received optical power level results in better (bad) CNR value, but degrades (improves) the CSO/CTB values. The actual operating point of the CNR/CSO/CTB parameters is chosen by maximizing the CNR value while maintaining CSO/CTB distortions below acceptable levels. To satisfy the fiber optical CATV systems requirements (CNR >50 dB, CSO <60 dB and CTB <60 dB), the received optical power level needs to be kept at 0 dB. For 256-QAM (OC-48) channel performance, the BER performance can be improved by operating the 256QAM (OC-48) channel at a higher received optical power level. However, higher received optical power level results in higher crosstalk to affect the adjacent channel performance [11]. A multiple carrier generator (Matrix SX-16; NTSC) was used to feed RF sub-carriers into the first and second external modulators. Channels 2–78 (55.25–547.25 MHz) were fed into the first external modulator, and channels 79– 112 (553.25–721.25 MHz) were fed into the second one. Four 256-QAM digital passband channels (channels 113– 116) were fed into the third external modulator. The transmitted data (40.2 Mb/s) is up-converted from the 44 MHz IF frequency to RF frequencies between 727.25 and 745.25 MHz. The fourth external modulator is externally modulated by a 2.5 Gb/s pseudo-random bit sequence (PRBS). The composite signals are now a combination of the 111 AM-VSB video channels, four 256-QAM digital passband channels and one OC-48 digital baseband channel. Fiber span between EDFA-I and the EDFA-II is 20 km, between EDFA-II and the EDFA-III is 40 km, and between EDFA-III and the optical receiver is also 40 km. Between the EDFA-III and the optical receiver is an optical tunable BPF to select the appropriate wavelength. If the selected wavelength is k4, then we use a digital optical receiver to detect the received wavelength. Otherwise, the optical receiver used in this experiment is an analog optical receiver. For CATV and 256-QAM signals, the detected signals were sent through a tunable RF BPF to select the channel for measurement. All CATV RF parameters were measured using an HP-8591C CATV analyzer at channel 112. The selected 256-QAM channel (channel 116) is down-converted to an IF frequency, demodulated, and then fed into a 256-QAM error detector for BER analysis. After receiving, the OC-48 signal is directly fed into an OC-48 error detector for BER analysis. The configuration of the broadband light source is illustrated in Fig. 2. It consists of two FP LDs, a 50:50 optical coupler, and two optical isolators. The reflectivity of the front facet and the mode spacing of the FP LD are 1% and 100 GHz, respectively. The FP LD is manipulated at continuous wave (CW) mode, with a threshold current of
89 Isolator
PC
Isolator
PC
F-P LD1 50:50 Coupler F-P LD2
PBC
Fig. 2. The configuration of the broadband light source.
10 mA. As FP LD1 is served as an optical injection source, it injects light into FP LD2 via a 2 · 2 optical coupler. The output of each FP LD was passed through an optical isolator, with a 55-dB isolation. Two optical isolators were used in mutual injection-locked branch in order to keep the injection lightwave direction. We realized an un-polarized broadband light source by polarization-multiplexing with two polarization controllers (PCs) and one polarization beam combiner (PBC). Two PCs were used to adjust the polarization states of the light. The output spectra of FP LDs were measured using an optical spectrum analyzer (OSA). 3. Experimental results and discussions The outer boundary of the locking range for laser under light injection is given by [12] rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kc Si d< ð1 þ a2l Þ ð1Þ 2p S where the frequency detuning d = finj ffree (finj is the frequency of injection source laser and ffree is the frequency of free running injection-locked laser), kc the coupling coefficient, SSi the injection ratio, and al is the linewidth enhancement factor. An optimum locking can be achieved if the frequency of the injection source laser (master laser) is lower than that of the free running injection-locked laser (slave laser), i.e., negative detuning. Within the locking range, the frequency of the injection-locked laser is locked nearly to that of the injection source laser. However, outside the locking range, severe oscillation occurs. Fig. 3 shows the measured optical spectra of two FP LDs before mutual injection. The 3-dB linewidth of each FP LD is less than 0.06 nm. The pyramidal profile spectra of free running FP LDs are unsuitable for using in multi-wavelength light source, because of un-flat spectra. As mutual injectionlocking happens, the central modes of the FP LD1 are injected into the side-modes of the FP LD2. The output powers of the side-modes are increased largely, and the output powers of the central modes are increased limited. Thereby, a flat mutually injection-locked spectrum can be obtained. The optimal injection-locking condition is found when the detuning between two FP LDs wavelength is 0.12–0.14 nm. Flat spectrum due to polarization-multiplexing
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AM optical modulation index (OMI) for back-to-back and over a 100-km SMF transmission is shown in Fig. 5. The performance of the AM-VSB channel is analyzed in the presence of four 256-QAM channels and one OC-48 channel. The theoretical expression for CNR is [13]: CNR ¼ 10.0 dB/D
1520.86
1535.860nm
2.00nm/D
1550.86
1535.860nm
2.00nm/D
1550.86
10.0 dB/D
1520.86
Fig. 3. The measured optical spectra of two FP LDs before mutual injection.
m2 I 2 ðN laser þ N shot þ N th þ N sig–sp þ N sp–sp Þ B
ð2Þ
where m is the OMI, I the photocurrent, Nlaser the laser noise (Nshot (shot noise) + Nth (thermal noise)) are noise associated with the optical receiver (Nsig–sp (signal–spontaneous beat noise) + Nsp–sp (spontaneous–spontaneous beat noise)) are noise associated with the optical amplifier, and B is the bandwidth in which the noise is measured. It can be seen that, from Eq. (2), the CNR value is proportional to the square of OMI, and inverse proportional to the noise associated with laser, optical receiver, and optical amplifier. Over a 100-km SMF transmission, laser source and optical receiver have not been changed in this system, the CNR value is inverse proportional to the noise associated with optical amplifier. Fig. 5 verifies that the CNR value is proportional to the square of OMI, and also shows that the 100 km of SMF transmission induced a penalty of about 3 dB on the CNR of the AM-VSB channel, due to the noise accumulation from three cascaded EDFAs. The measured CSO and CTB values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI for back-to-back and over a 100km SMF transmission are shown in Fig. 6. Both the induced CSO and CTB penalties for the proposed hybrid WDM transport systems are shown in Fig. 6. It is obvious that the CSO and CTB penalties increase with OMI levels. For fiber optical CATV systems, CNR value is proportional to the OMI value, a higher CNR value can be acquired from a higher OMI value. However, CSO and CTB performances will be degraded from a higher OMI. According to Figs. 5 and 6, CNR >50 dB, CSO <61 dB
57 back-to-back 100 km
Fig. 4. Flat spectrum due to polarization-multiplexing.
is present in Fig. 4. The proposed broadband light source generates a 100 GHz spaced comb with 15 modes, ranging from 1532 to 1544 nm and with a flatness of <4 dB. This proposed low cost broadband light source could provide flat spectrum profile to satisfy signal transmission performance in hybrid WDM transport systems. The measured CNR value at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of
CNR (dB)
54
51
48
45 2.3
2.9
3.4
4.0
4.5
5.1
5.6
AM OMI (%) Fig. 5. The measured CNR values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI.
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92 -30
10
-35 -40
10
BER
-45
10
-50
10
-55
10
-60
10
-65
10
-70
10
-75 2.3
-3 -4 -5
back-to-back
-6
100 km
-7
-8
-9
-10 -11
0.17 2.9
3.4
4.0
4.5
5.1
AM OMI (%) Fig. 6. The measured CSO and CTB values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI.
and CTB <60 dB can be obtained at the end of the WDM link with an OMI of 3.9%, in which these values meet the fiber optical CATV system demands. Nonlinear distortion (/NL) can be expressed as [14]: /NL ¼ ðn2 =Aeff Þ Leff P
0.20
0.23
ð3Þ
where n2 is the nonlinear refractive index, Aeff the effective core area, Leff the effective fiber length, and P is the injected optical power. Since /NL is proportional to P; we place all VOAs at the start of each optical link so that the optical power launched into the fiber section is lower, then there would be a reduction in the nonlinear distortion, leading to better CSO and CTB performance. CSO and CTB values are very small in back-to-back conditions, while fiber dispersion provides large degradations in CSO and CTB performances. System link with a transmission length of 100-km SMF (with a dispersion coefficient of 17 ps/nm km) has a positive dispersion of 1700 ps/nm (17 ps/nm km · 100 km), resulting in bad CSO and CTB performance. EDFAs placed along the fiber link may cause the generation of second and third order harmonics. However, all EDFAs have flat amplifier gain (with a flat gain within 1 dB over 30 nm) in the hybrid WDM transport systems. Flat amplifier gain in a wavelength range of a few tens of nanometers can be very useful in reducing the CSO and CTB distortions caused by amplifier gain tilt in an AM-VSB CATV link [15]. The CSO and CTB performances can be improved by using dispersion compensation devices such as chirped fiber grating, and large effective area fiber [16,17]. Fig. 7 shows the measured BER of the 256-QAM channel (channel 116) as a function of the QAM OMI for back-to-back and over a 100-km SMF transmission. The performance of the 256-QAM channel is analyzed in the presence of 111 AM-VSB video channels with an OMI of 3.9% and one OC-48 channel. For an equal BER, the QAM OMI level over a 100-km SMF transmission was found to be higher than that of back-to-back case. The BER can be expressed as [18]
0.26
0.29
0.32
0.35
0.38
QAM OMI (%)
5.6
Fig. 7. The measured BER of the 256-QAM channel (channel 116) as a function of the QAM OMI.
1 BER ¼ erfc 2
rffiffiffiffiffiffiffiffiffiffiffi! SNR 2
ð4Þ
where SNR is systems’ signal-to-noise ratio. Over a 100-km SMF transmission, the accumulated ASE noise from three cascaded EDFAs leads to decreasing the SNR value, results in BER performance degradation. For QAM OMI of about 0.37%, the measured BER was <109 over a 100-km SMF transmission. Fig. 8 shows the performance of the OC-48 digital baseband channel for the 100 km hybrid WDM links. The performance of the OC-48 channel is analyzed in the presence of 111 AM-VSB channels with an OMI of 3.9%, and four 256-QAM channels with an OMI of 0.37%. It presents that the 100 km of SMF transmission induced a penalty of 0 dB on the BER of the OC-48 channel. A received signal level of 33.5 dB or higher is needed for a BER of 109; the received optical power for the digital optical receiver was about 30 dB, so a BER <109 over 100 km SMF transmission was readily obtained. The optical signal might be contaminated by the accumulation of ASE noise generated by the in-line EDFAs. However, the BER of <109 could be achieved because of the low receiver sensitivity of the OC-48 optical receiver (36 dB).
BER
CSO/CTB (dBc)
10
CSO (back-to-back) CSO (100 km) CTB (back-to-back) CTB (100 km)
91
10
-5
10
-6
10
-7
10
-8
10
-9
back-to-back 100 km
-10
10
-11
10
-12
10
-35
-34.5
-34
-33.5
-33
-32.5
-32
Received Optical Power (dBm) Fig. 8. The performance of the OC-48 digital baseband channel for the 100 km hybrid WDM links.
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4. Conclusions We proposed and demonstrated a hybrid WDM transport system based on mutually injection-locked FP LDs. By mutual injection between two FP LDs, we realized a broadband light source with flatness and multimode output spectrum. The proposed hybrid WDM transport systems use two filtered wavelengths for AM-VSB CATV, one for 256-QAM digital passband and one for OC-48 digital baseband signals. Our proposed hybrid WDM transport system can simultaneously transmit 111 AM-VSB channels with CNR >50 dB, CSO <61 dB and CTB <60 dB; four 256-QAM digital passband channels and one OC-48 digital baseband channel with BER <109. Since our proposed system does not use multiple expensive DFB LDs, it reveals a prominent one with simpler and more economic advantages than that of traditional hybrid WDM transport systems. Acknowledgements The authors would like to thank the financial support from the National Taipei University of Technology and National Science Council of the Republic of China under Grant NSC 95-2221-E-027-095-MY3. References [1] S. Diaz, G. Lasheras, M. Lopez-Amo, Opt. Express. 13 (2005) 9666.
[2] H.H. Lu, P.C. Lai, H.C. Chen, Y.L. Chen, Opt. Eng. 42 (2003) 1370. [3] K.P. Ho, H. Dai, C. Lin, S.K. Liaw, H. Gysel, M. Ramachandran, IEEE Photon. Technol. Lett. 10 (1998) 297. [4] S. Gao, C. Yang, X. Xiao, Y. Tian, Z. You, G. Jin, Opt. Express. 14 (2006) 2873. [5] K.H. Han, E.S. Son, K.W. Lim, H.Y. Choi, S.P. Jung, Y.C. Chung, in: Proceedings of the Optical Fiber Communication Conference, vol. 1, 2004, p. 23. [6] D.K. Jung, H. Kim, K.H. Han, Y.C. Chung, Electron. Lett. 37 (2001) 308. [7] K.M. Choi, J.S. Baik, C.H. Lee, IEEE Photon. Technol. Lett. 17 (2005) 2529. [8] S.M. Lee, K.M. Choi, S.G. Mun, J.H. Moon, C.H. Lee, IEEE Photon. Technol. Lett. 17 (2005) 1579. [9] S.J. Park, C.H. Lee, K.T. Jeong, H.J. Park, J.G. Ahn, K.H. Song, J. Lightwave Technol. 22 (2004) 2582. [10] H. Dai, S. Ovadia, C. Lin, IEEE Photon. Technol. Lett. 8 (1996) 1713. [11] K.P. Ho, S.K. Liaw, IEEE Photon. Technol. Lett. 10 (1998) 737. [12] C.H. Chang, L. Chrostowski, C.J. Chang-Hasnain, IEEE J. Sel. Top. Quant. Electron. 9 (2003) 1386. [13] C. Bonang, C.Y. Kuo, Long Distance 1550 nm Fiber Optic CATV Supertrunking, Harmonic Lightwaves Inc., Sunnyvale, CA, 1997. [14] T. Yamamoto, E. Yoshida, K.R. Tamura, K. Yonenaga, M. Nakazawa, IEEE Photon. Technol. Lett. 12 (2000) 353. [15] K. Kikushima, J. Lightwave Technol. 12 (1994) 463. [16] H.H. Lu, W.S. Tsai, W.J. Wang, H.H. Huang, Opt. Eng. 42 (2003) 1901. [17] C.C. Lee, Y.K. Chen, C.H. Chang, K.M. Feng, S.L. Tzeng, S. Chi, IEEE Photon. Technol. Lett. 14 (2002) 230. [18] G.P. Agrawal, Fiber-optic Communication Systems, third ed., Wiley– Interscience, New York, 2002.
A hybrid WDM transport system based on mutually injection-locked Fabry–Perot laser diodes Cheng-Ling Ying b
a,b
, Hai-Han Lu
b,*
, Shah-Jye Tzeng b, Hsien-Li Ma b, Yao-Wei Chuang
b
a Department of Electronic Engineering, Jin-Wen Institute of Technology, Taiwan Institute of Electro-Optical Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao East Road, Taipei 10608, Taiwan, ROC
Received 16 October 2006; received in revised form 19 March 2007; accepted 25 March 2007
Abstract A hybrid wavelength-division-multiplexing (WDM) transport system based on mutually injection-locked Fabry–Perot laser diodes (FP LDs) for CATV, 256-QAM and OC-48 transmission is proposed and demonstrated. Mutually injection-locked FP LDs as broadband light source could be relatively simple and cost-effective compared with other demonstrated light source schemes. The proposed hybrid WDM transport system employs four filtered wavelengths (modes) to transmit 111 AM-VSB channels, four 256-QAM digital passband channels, and one OC-48 digital baseband channel simultaneously. Since our proposed system does not use multiple distributed feedback (DFB) LDs, it reveals a prominent one with simpler and more economic advantages. 2007 Elsevier B.V. All rights reserved. Keywords: Broadband light source; Mutually injection-locked Fabry–Perot laser diode; Wavelength-division-multiplexing
1. Introduction Wavelength-division-multiplexing (WDM) transport systems provide many advantages for optical networks, including high transmission capacity, network flexibility and upgradability. A hybrid WDM transport system that uses different wavelengths to transmit AM-VSB analog video, 64/256-QAM digital passband and OC-48/OC-192 digital baseband signals is very useful for an optically amplified fiber network providing CATV, Internet and telecommunication services [1–3]. Hybrid WDM transport systems are envisioned to have a multiple number of distributed feedback laser diodes (DFB LDs) which are wavelength-selected for each channel and controlled to operate at a specific wavelength, this process will increase the cost and complexity of the systems. For a practical implementation of hybrid WDM transport systems, it is necessary to *
Corresponding author. Tel.: +886 2 27712171x4601/4621; fax: +886 2 87733216. E-mail address: [email protected] (H.-H. Lu). 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.03.052
develop low cost and low complexity light source. Several approaches have been proposed to solve these problems. WDM transport systems employing spectrum-sliced light sources such as light-emitting diodes (LEDs) or amplified spontaneous emission (ASE) sources were proposed [4–6]. However, the output power of LED is insufficient to accommodate many channels. Although the spectrumsliced ASE source provides much higher output power than that of the LED, it requires expensive erbium-doped fiber (EDF) and pumping LD. Recently, there has been a proposal of a broadband light source which is based on mutually injection-locked Fabry–Perot (FP) LDs [7]. When FP LD is injection-locked, the multiple longitudinal modes of FP LD can be changed into a similar single longitudinal mode characteristic, and it can be used as cheap light source in hybrid WDM transport systems [8,9]. In this paper, we proposed and demonstrated a potentially cost-effective hybrid WDM transport system based on mutually injection-locked FP LDs. By mutual injection between two FP LDs, we realized a broadband light source with flatness and multimode output spectrum. The
88
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
proposed multimode light source generates 15 modes, ranging from 1532 to 1544 nm, and with a flatness of <4 dB. This proposed hybrid WDM transport system used two wavelengths (modes) for AM-VSB video signal, one for 256-QAM digital passband and one for OC-48 digital baseband signals. System performances were investigated over a 100-km standard single-mode fiber (SMF) transmission. Since our proposed system does not use multiple expensive DFB LDs, it reveals a prominent one with simpler and more economic advantages than that of traditional hybrid WDM transport systems. 2. Experimental setup Fig. 1 shows the experimental setup of our proposed hybrid WDM transport systems based on mutually injection-locked FP LDs. The hybrid WDM transport systems consist of one broadband light source, four optical bandpass filters (BPFs), four external modulators, and 100 km of standard SMF with three cascaded erbium-doped fiber amplifiers (EDFAs). All the EDFAs have similar characteristics, with an output power of 17 dB and a noise figure of 4.5 dB, at an input power of 0 dB. The broadband light source is efficiently split into four optical channels by four optical BPFs with 3-dB bandwidth of 0.4 nm, externally modulated individually, and multiplexed back into the SMF by using two optical couplers. The filtered four wavelengths of k1, k2, k3 and k4 for externally modulated AMVSB analog video, 256-QAM digital passband and
OC-48 (2.5 Gb/s) digital baseband signals are 1540.4, 1538.8, 1537.2 and 1535.6 nm, with a channel spacing of 200 GHz. To be the best wavelength allocation, the longest wavelength is employed to transmit AM-VSB analog video signal, and the shortest wavelength is employed to transmit OC-48 digital baseband signal to avoid the stimulated Raman scattering (SRS) effect. If the OC-48 is located at the long wavelength and AM-VSB located at the short wavelengths, then the Raman-induced crosstalk degrades the carrier-to-noise ratio (CNR) performance and is of considerable concern for a hybrid WDM transport system. In view of the fact that the digital signal is in general more robust than analog signal with respect to noise and nonlinear distortions, the CNR value of the AM-VSB video channel for a hybrid WDM transport system is the most important limiting parameter [10]. In a typical hybrid WDM transport system for CATV/256-QAM/OC-48 signals transport, the optical power launched into the EDFAs (EDFA-I, EDFA-II, and EDFA-III) for CATV and 256QAM channels needs to be about 0 dB due to the systems’ CNR and bit error rate (BER) performance requirements. In contrast, the optical power launched into the EDFAs (EDFA-II and EDFA-III) for OC-48 channel is usually less than 30 dB due to the low optical receiver sensitivity. Optical output power of EDFA-I and the OC-48 optical channel through a variable optical attenuator (VOA) are coupled together by using an optical coupler. Note that the optical power level of EDFA-I is much higher than that of the OC-48 channel through a VOA so the OC-48 signal
CATV (CH2 ~ CH78)
Optical BPF
1540.4 nm
External Modulator
1
CATV (CH79 ~ CH112)
Optical BPF Broadband Light Source
1538.8 nm
External Modulator
2
Optical Splitter
EDFA-
Optical BPF
1537.2 nm
External Modulator
3
Optical Coupler
VOA
256-QAM (CH113 ~ CH116)
VOA
VOA
Optical Coupler
EDFA-
EDFA-
40km SMF
20km SMF VOA
Optical BPF
1535.6 nm
External Modulator
4
OC-48 (2.5Gb/s)
CH112
HP-8591C Tunable RF BPF 256-QAM Error Detector
Analog Optical Rx
1/
256-QAM DeModulator CH116
2/
3
Optical Tunable BPF 40km SMF
OC-48 Error Detector
2.5Gb/s
Digital Optical Rx
4
Fig. 1. The experimental setup of our proposed hybrid WDM transport systems based on mutually injection-locked FP LDs.
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
compared to the combined AM-VSB and 256-QAM signals has only a very small effect on the input power of EDFAII. For CATV channel performance, there is a trade-off between CNR and composite second order (CSO)/composite triple beat (CTB) values for different received optical power level. Higher (lower) received optical power level results in better (bad) CNR value, but degrades (improves) the CSO/CTB values. The actual operating point of the CNR/CSO/CTB parameters is chosen by maximizing the CNR value while maintaining CSO/CTB distortions below acceptable levels. To satisfy the fiber optical CATV systems requirements (CNR >50 dB, CSO <60 dB and CTB <60 dB), the received optical power level needs to be kept at 0 dB. For 256-QAM (OC-48) channel performance, the BER performance can be improved by operating the 256QAM (OC-48) channel at a higher received optical power level. However, higher received optical power level results in higher crosstalk to affect the adjacent channel performance [11]. A multiple carrier generator (Matrix SX-16; NTSC) was used to feed RF sub-carriers into the first and second external modulators. Channels 2–78 (55.25–547.25 MHz) were fed into the first external modulator, and channels 79– 112 (553.25–721.25 MHz) were fed into the second one. Four 256-QAM digital passband channels (channels 113– 116) were fed into the third external modulator. The transmitted data (40.2 Mb/s) is up-converted from the 44 MHz IF frequency to RF frequencies between 727.25 and 745.25 MHz. The fourth external modulator is externally modulated by a 2.5 Gb/s pseudo-random bit sequence (PRBS). The composite signals are now a combination of the 111 AM-VSB video channels, four 256-QAM digital passband channels and one OC-48 digital baseband channel. Fiber span between EDFA-I and the EDFA-II is 20 km, between EDFA-II and the EDFA-III is 40 km, and between EDFA-III and the optical receiver is also 40 km. Between the EDFA-III and the optical receiver is an optical tunable BPF to select the appropriate wavelength. If the selected wavelength is k4, then we use a digital optical receiver to detect the received wavelength. Otherwise, the optical receiver used in this experiment is an analog optical receiver. For CATV and 256-QAM signals, the detected signals were sent through a tunable RF BPF to select the channel for measurement. All CATV RF parameters were measured using an HP-8591C CATV analyzer at channel 112. The selected 256-QAM channel (channel 116) is down-converted to an IF frequency, demodulated, and then fed into a 256-QAM error detector for BER analysis. After receiving, the OC-48 signal is directly fed into an OC-48 error detector for BER analysis. The configuration of the broadband light source is illustrated in Fig. 2. It consists of two FP LDs, a 50:50 optical coupler, and two optical isolators. The reflectivity of the front facet and the mode spacing of the FP LD are 1% and 100 GHz, respectively. The FP LD is manipulated at continuous wave (CW) mode, with a threshold current of
89 Isolator
PC
Isolator
PC
F-P LD1 50:50 Coupler F-P LD2
PBC
Fig. 2. The configuration of the broadband light source.
10 mA. As FP LD1 is served as an optical injection source, it injects light into FP LD2 via a 2 · 2 optical coupler. The output of each FP LD was passed through an optical isolator, with a 55-dB isolation. Two optical isolators were used in mutual injection-locked branch in order to keep the injection lightwave direction. We realized an un-polarized broadband light source by polarization-multiplexing with two polarization controllers (PCs) and one polarization beam combiner (PBC). Two PCs were used to adjust the polarization states of the light. The output spectra of FP LDs were measured using an optical spectrum analyzer (OSA). 3. Experimental results and discussions The outer boundary of the locking range for laser under light injection is given by [12] rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kc Si d< ð1 þ a2l Þ ð1Þ 2p S where the frequency detuning d = finj ffree (finj is the frequency of injection source laser and ffree is the frequency of free running injection-locked laser), kc the coupling coefficient, SSi the injection ratio, and al is the linewidth enhancement factor. An optimum locking can be achieved if the frequency of the injection source laser (master laser) is lower than that of the free running injection-locked laser (slave laser), i.e., negative detuning. Within the locking range, the frequency of the injection-locked laser is locked nearly to that of the injection source laser. However, outside the locking range, severe oscillation occurs. Fig. 3 shows the measured optical spectra of two FP LDs before mutual injection. The 3-dB linewidth of each FP LD is less than 0.06 nm. The pyramidal profile spectra of free running FP LDs are unsuitable for using in multi-wavelength light source, because of un-flat spectra. As mutual injectionlocking happens, the central modes of the FP LD1 are injected into the side-modes of the FP LD2. The output powers of the side-modes are increased largely, and the output powers of the central modes are increased limited. Thereby, a flat mutually injection-locked spectrum can be obtained. The optimal injection-locking condition is found when the detuning between two FP LDs wavelength is 0.12–0.14 nm. Flat spectrum due to polarization-multiplexing
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C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
AM optical modulation index (OMI) for back-to-back and over a 100-km SMF transmission is shown in Fig. 5. The performance of the AM-VSB channel is analyzed in the presence of four 256-QAM channels and one OC-48 channel. The theoretical expression for CNR is [13]: CNR ¼ 10.0 dB/D
1520.86
1535.860nm
2.00nm/D
1550.86
1535.860nm
2.00nm/D
1550.86
10.0 dB/D
1520.86
Fig. 3. The measured optical spectra of two FP LDs before mutual injection.
m2 I 2 ðN laser þ N shot þ N th þ N sig–sp þ N sp–sp Þ B
ð2Þ
where m is the OMI, I the photocurrent, Nlaser the laser noise (Nshot (shot noise) + Nth (thermal noise)) are noise associated with the optical receiver (Nsig–sp (signal–spontaneous beat noise) + Nsp–sp (spontaneous–spontaneous beat noise)) are noise associated with the optical amplifier, and B is the bandwidth in which the noise is measured. It can be seen that, from Eq. (2), the CNR value is proportional to the square of OMI, and inverse proportional to the noise associated with laser, optical receiver, and optical amplifier. Over a 100-km SMF transmission, laser source and optical receiver have not been changed in this system, the CNR value is inverse proportional to the noise associated with optical amplifier. Fig. 5 verifies that the CNR value is proportional to the square of OMI, and also shows that the 100 km of SMF transmission induced a penalty of about 3 dB on the CNR of the AM-VSB channel, due to the noise accumulation from three cascaded EDFAs. The measured CSO and CTB values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI for back-to-back and over a 100km SMF transmission are shown in Fig. 6. Both the induced CSO and CTB penalties for the proposed hybrid WDM transport systems are shown in Fig. 6. It is obvious that the CSO and CTB penalties increase with OMI levels. For fiber optical CATV systems, CNR value is proportional to the OMI value, a higher CNR value can be acquired from a higher OMI value. However, CSO and CTB performances will be degraded from a higher OMI. According to Figs. 5 and 6, CNR >50 dB, CSO <61 dB
57 back-to-back 100 km
Fig. 4. Flat spectrum due to polarization-multiplexing.
is present in Fig. 4. The proposed broadband light source generates a 100 GHz spaced comb with 15 modes, ranging from 1532 to 1544 nm and with a flatness of <4 dB. This proposed low cost broadband light source could provide flat spectrum profile to satisfy signal transmission performance in hybrid WDM transport systems. The measured CNR value at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of
CNR (dB)
54
51
48
45 2.3
2.9
3.4
4.0
4.5
5.1
5.6
AM OMI (%) Fig. 5. The measured CNR values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI.
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92 -30
10
-35 -40
10
BER
-45
10
-50
10
-55
10
-60
10
-65
10
-70
10
-75 2.3
-3 -4 -5
back-to-back
-6
100 km
-7
-8
-9
-10 -11
0.17 2.9
3.4
4.0
4.5
5.1
AM OMI (%) Fig. 6. The measured CSO and CTB values at AM-VSB channel highest frequency of 721.25 MHz (channel 112) as a function of AM OMI.
and CTB <60 dB can be obtained at the end of the WDM link with an OMI of 3.9%, in which these values meet the fiber optical CATV system demands. Nonlinear distortion (/NL) can be expressed as [14]: /NL ¼ ðn2 =Aeff Þ Leff P
0.20
0.23
ð3Þ
where n2 is the nonlinear refractive index, Aeff the effective core area, Leff the effective fiber length, and P is the injected optical power. Since /NL is proportional to P; we place all VOAs at the start of each optical link so that the optical power launched into the fiber section is lower, then there would be a reduction in the nonlinear distortion, leading to better CSO and CTB performance. CSO and CTB values are very small in back-to-back conditions, while fiber dispersion provides large degradations in CSO and CTB performances. System link with a transmission length of 100-km SMF (with a dispersion coefficient of 17 ps/nm km) has a positive dispersion of 1700 ps/nm (17 ps/nm km · 100 km), resulting in bad CSO and CTB performance. EDFAs placed along the fiber link may cause the generation of second and third order harmonics. However, all EDFAs have flat amplifier gain (with a flat gain within 1 dB over 30 nm) in the hybrid WDM transport systems. Flat amplifier gain in a wavelength range of a few tens of nanometers can be very useful in reducing the CSO and CTB distortions caused by amplifier gain tilt in an AM-VSB CATV link [15]. The CSO and CTB performances can be improved by using dispersion compensation devices such as chirped fiber grating, and large effective area fiber [16,17]. Fig. 7 shows the measured BER of the 256-QAM channel (channel 116) as a function of the QAM OMI for back-to-back and over a 100-km SMF transmission. The performance of the 256-QAM channel is analyzed in the presence of 111 AM-VSB video channels with an OMI of 3.9% and one OC-48 channel. For an equal BER, the QAM OMI level over a 100-km SMF transmission was found to be higher than that of back-to-back case. The BER can be expressed as [18]
0.26
0.29
0.32
0.35
0.38
QAM OMI (%)
5.6
Fig. 7. The measured BER of the 256-QAM channel (channel 116) as a function of the QAM OMI.
1 BER ¼ erfc 2
rffiffiffiffiffiffiffiffiffiffiffi! SNR 2
ð4Þ
where SNR is systems’ signal-to-noise ratio. Over a 100-km SMF transmission, the accumulated ASE noise from three cascaded EDFAs leads to decreasing the SNR value, results in BER performance degradation. For QAM OMI of about 0.37%, the measured BER was <109 over a 100-km SMF transmission. Fig. 8 shows the performance of the OC-48 digital baseband channel for the 100 km hybrid WDM links. The performance of the OC-48 channel is analyzed in the presence of 111 AM-VSB channels with an OMI of 3.9%, and four 256-QAM channels with an OMI of 0.37%. It presents that the 100 km of SMF transmission induced a penalty of 0 dB on the BER of the OC-48 channel. A received signal level of 33.5 dB or higher is needed for a BER of 109; the received optical power for the digital optical receiver was about 30 dB, so a BER <109 over 100 km SMF transmission was readily obtained. The optical signal might be contaminated by the accumulation of ASE noise generated by the in-line EDFAs. However, the BER of <109 could be achieved because of the low receiver sensitivity of the OC-48 optical receiver (36 dB).
BER
CSO/CTB (dBc)
10
CSO (back-to-back) CSO (100 km) CTB (back-to-back) CTB (100 km)
91
10
-5
10
-6
10
-7
10
-8
10
-9
back-to-back 100 km
-10
10
-11
10
-12
10
-35
-34.5
-34
-33.5
-33
-32.5
-32
Received Optical Power (dBm) Fig. 8. The performance of the OC-48 digital baseband channel for the 100 km hybrid WDM links.
92
C.-L. Ying et al. / Optics Communications 276 (2007) 87–92
4. Conclusions We proposed and demonstrated a hybrid WDM transport system based on mutually injection-locked FP LDs. By mutual injection between two FP LDs, we realized a broadband light source with flatness and multimode output spectrum. The proposed hybrid WDM transport systems use two filtered wavelengths for AM-VSB CATV, one for 256-QAM digital passband and one for OC-48 digital baseband signals. Our proposed hybrid WDM transport system can simultaneously transmit 111 AM-VSB channels with CNR >50 dB, CSO <61 dB and CTB <60 dB; four 256-QAM digital passband channels and one OC-48 digital baseband channel with BER <109. Since our proposed system does not use multiple expensive DFB LDs, it reveals a prominent one with simpler and more economic advantages than that of traditional hybrid WDM transport systems. Acknowledgements The authors would like to thank the financial support from the National Taipei University of Technology and National Science Council of the Republic of China under Grant NSC 95-2221-E-027-095-MY3. References [1] S. Diaz, G. Lasheras, M. Lopez-Amo, Opt. Express. 13 (2005) 9666.
[2] H.H. Lu, P.C. Lai, H.C. Chen, Y.L. Chen, Opt. Eng. 42 (2003) 1370. [3] K.P. Ho, H. Dai, C. Lin, S.K. Liaw, H. Gysel, M. Ramachandran, IEEE Photon. Technol. Lett. 10 (1998) 297. [4] S. Gao, C. Yang, X. Xiao, Y. Tian, Z. You, G. Jin, Opt. Express. 14 (2006) 2873. [5] K.H. Han, E.S. Son, K.W. Lim, H.Y. Choi, S.P. Jung, Y.C. Chung, in: Proceedings of the Optical Fiber Communication Conference, vol. 1, 2004, p. 23. [6] D.K. Jung, H. Kim, K.H. Han, Y.C. Chung, Electron. Lett. 37 (2001) 308. [7] K.M. Choi, J.S. Baik, C.H. Lee, IEEE Photon. Technol. Lett. 17 (2005) 2529. [8] S.M. Lee, K.M. Choi, S.G. Mun, J.H. Moon, C.H. Lee, IEEE Photon. Technol. Lett. 17 (2005) 1579. [9] S.J. Park, C.H. Lee, K.T. Jeong, H.J. Park, J.G. Ahn, K.H. Song, J. Lightwave Technol. 22 (2004) 2582. [10] H. Dai, S. Ovadia, C. Lin, IEEE Photon. Technol. Lett. 8 (1996) 1713. [11] K.P. Ho, S.K. Liaw, IEEE Photon. Technol. Lett. 10 (1998) 737. [12] C.H. Chang, L. Chrostowski, C.J. Chang-Hasnain, IEEE J. Sel. Top. Quant. Electron. 9 (2003) 1386. [13] C. Bonang, C.Y. Kuo, Long Distance 1550 nm Fiber Optic CATV Supertrunking, Harmonic Lightwaves Inc., Sunnyvale, CA, 1997. [14] T. Yamamoto, E. Yoshida, K.R. Tamura, K. Yonenaga, M. Nakazawa, IEEE Photon. Technol. Lett. 12 (2000) 353. [15] K. Kikushima, J. Lightwave Technol. 12 (1994) 463. [16] H.H. Lu, W.S. Tsai, W.J. Wang, H.H. Huang, Opt. Eng. 42 (2003) 1901. [17] C.C. Lee, Y.K. Chen, C.H. Chang, K.M. Feng, S.L. Tzeng, S. Chi, IEEE Photon. Technol. Lett. 14 (2002) 230. [18] G.P. Agrawal, Fiber-optic Communication Systems, third ed., Wiley– Interscience, New York, 2002.