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Influence of modulation formats performance of the combined modulation in WDM-PON system
Optik 124 (2013) 5660–5664
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Influence of modulation formats performance of the combined modulation in WDM-PON system Li Li ∗ , Hong-an Li, Ji-jun Zhang, Xiu-tai Zhang Department of Electronic Information and Electrical Engineering, Anyang Institute of Technology, Anyang 455000, China
a r t i c l e
i n f o
Article history: Received 7 November 2012 Accepted 2 April 2013
Keywords: WDM-PON Combined modulation IRZ code Extinction ratio DPSK
a b s t r a c t Different modulation modes of DPSK and OOK are separately employed in the upstream and downstream link, after the comparison with different modulation formats in the downstream including the codes of non return-to-zero (NRZ), return-to-zero (RZ) and inverse return-to-zero (IRZ), the symmetric rate of 10 Gbps with 20 km transmission is realized without the dispersion compensation. It can be shown that in the combined modulation mode with the downstream of IRZ code, higher extinction ratio in the downstream and better performance of dispersion tolerance in the upstream could be achieved, enhancing the overall property of the transmission system. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction With the rapid development of FTTH recently, the wavelengthdivision multiplexing passive optical network (WDM-PON) is a promising access network technology due to the better performance in the high capacity, manageability, scalability and reliable security [1]. But the cost of installation and complexity will be increased because of the employment in light sources with specified wavelength, so the scheme of “Colorless ONU” is proposed. This scheme use the centralized light sources in the optical line terminal (OLT) and the same wavelength is shared in the upstream and downstream for transmission, through the re-modulation toward the downstream optical carrier, the “Colorless ONU” transmission of upstream signal is realized, lowering the operation and maintenance costs. Because the “Colorless ONU” is carried out superposition modulation, the quality of the upstream signal is essential to the overall WDM-PON system. Two relatively independent modulation modes are separately employed in the upstream and downstream to make the different transmission information modulate in the same optical carrier, and the receiving signal of the upstream and downstream is detected by different photodiodes, realizing the non-interfering transmission between the upstream and downstream signals. Many different combined modulation modes in the upstream and downstream are emerged, such as DPSK/OOK, FSK/OOK, OOK/OOK, and DPSK/DPSK. There are some different shortcomings existed in the above mod-
∗ Corresponding author. E-mail address: [email protected] (L. Li). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.04.034
ulation modes [2–5]. In this paper, the combined modulation of downstream OOK and upstream DPSK is proposed with the introduction of different downstream transmission codes in the OLT, including IRZ, NRZ, and different duty cycles of RZ, in order to compare the upstream performance of different modulation codes in the combined modulation [6]. An IRZ signal is formed by inverting the intensity level of a conventional RZ signal, thus it carries optical power at both “0” and “1” bit period, which is helpful to the upstream signal re-modulation. Not only realize the higher extinction ratio of the downstream signal, but also enhance the anti-dispersion performance of the upstream signal. Without the re-modulation synchronization and dispersion compensation, the symmetric rate of 10 Gbps with 20 km transmission is realized to enhance the overall property of the transmission system. 2. Proposed combined modulation architecture of WDM-PON Fig. 1 is the established architecture of the combined modulation, researching the effects of different downstream modulation codes on the bidirectional WDM-PON system. In the OLT, the light source adopts the continuous wave-laser diode (CW-LD), and its operating wavelength is 1551 nm. Mach-Zehnder modulator is driven directly by the 10 Gbps downstream electrical signals (including the codes of IRZ, NRZ and different duty cycles of RZ), after the multiplexing of AWG, the modulated optical signal is transmitted to the remote node (RN) by the 20 km single mode fiber (SMF) without dispersion compensation. In the ONU, through the splitter with variable optical splitting ratio (this paper we choose the ratio between the upstream and downstream is 15:85), one
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Fig. 1. The structure of upstream and downstream signal in WDM-PON.
part of the downstream optical signal is sent into the PD of ONU for direct detection, and the other part is used for the upstream re-modulation. Due to the combined modulation of OOK/DPSK, the upstream signal is employed the phase modulator (PM) as the modulation mode. Firstly, the upstream 10 Gbps Pseudo Random Bit Sequence (PRBS) of the electrical NRZ signal is carried on the differential precoding, and the PM modulator is driven by the upstream precoding. Then after passing the 20 km SMF, the upstream signal is received in the OLT. The attenuator used before the Mach-Zehnder interferometer (MZI) is intended to set the different injection optical powers. Through the demodulation of 1 bit delay interferometer (DI) and balanced detection, the received signal is sent into the BER analyzer for measurement [7]. Fig. 1 we can see that, the architecture of receiver in the ONU downstream is simple, and the receiver located in the OLT is relatively complex,From which is helpful to decrease the installation cost and extend the construction of the access network. 3. Principle and characteristic of IRZ code production IRZ code called inverse RZ code is a special line coding of the OOK format. An IRZ signal is formed by inverting the intensity level of a conventional RZ signal, thus it carries optical power at both the mark levels and space levels in each period. This is helpful to accelerate the re-modulation of upstream signal and can eliminate the process of erasing the downstream data from the received optical carrier. Not only can maintain high extinction ratio of the downstream data, but also the receiving optical power of the ONU can be used for the directly re-modulation of the upstream NRZ code. Besides, in comparison with the traditional receiving circuit of NRZ code, the receiving circuit of IRZ code does not require any extra demodulation circuit, no more than inverting a post amplifier after the PD [8]. The operation principle of the IRZ code is illustrated in Fig. 2. The generator of electrical IRZ code is formed by the MZM driven by the RZ-shaped Radio Frequency (RF) signal [9]. The electrical driving signal for the transmitter can be composed by a logical AND operation between a NRZ signal and a RF clock signal. By biasing the modulator at the quadrature point of the negative slope of the MZM transmission curve, the electrical data “0” modulates the optical signal as the mark level, while the electrical data “1” as an inversedpulse waveform as the space level. Thus, the IRZ code carries optical power at both the mark levels and space levels in each bit period. The generation of IRZ signal is carried out the amplitude modulation mode, so the MZM is driven by the working condition of push-pull way. The below table is the parameter setting of the IRZ signal: V is the value of switch voltage, V1m and V2m are separately the RF modulation voltage and switch modulation voltage, VNRZ and VTime are the amplitude of electrical driven signal and clock signal, and the VBais1 and VBais2 are the value of bias voltage in the two arms of the MZM (Table 1). The generation of downstream signal is important to the overall system, especially in the combined modulation WDM-PON system.
Fig. 2. Schematic diagram of IRZ code. Table 1 Parameter value of IRZ production.
IRZ
V1m
V2m
VNRZ
VTime
VBais1
VBais2
V
V
V/2
V/2
0
0
Different downstream optical signals production is driven by the different electrical signals including the code of IRZ, RZ and NRZ. After the multiplexing of AWG and the transmission of 20 km SMF, the following eye diagrams are the receiving signal of ONUs from different downstream electrical signals. The electrical signal of RZ code is modulated by two levels MZMs. The first level modulation is the production of NRZ code, and different duty cycles of RZ codes are achieved by passing the different driven data of electrical clock signal [10,11]. Among the following figures we can see the code of NRZ, duty cycle 33% RZ, duty cycle 67% RZ and IRZ. In the Fig. 3(d) the IRZ code carries the optical power at both the space levels and the mark levels. This characteristic is not only helpful to the remodulation of the upstream electrical signals, but also enhancing the signal quality of receiver in the OLT. 4. The choices of extinction ratio (ER) in the different modulation codes As the parameter of the transmitter, ER is critical to the performance of transmitting and receiving in the optic fiber communication. ER is usually defined as the ratio between the
L. Li et al. / Optik 124 (2013) 5660–5664
(a)
NRZ downstream NRZ upstream 33RZdownstream 33RZ upstream 67RZdownstream 67RZ upstream
120
100
Q factor
80
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20
0 0
2
4
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8
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Extinction Ratio
(b)
IRZ downstream IRZ upstream 12
11
Q factor
5662
10
9
8
0
2
4
6
8
10
12
Extinction Ratio Fig. 4. Relationship between ER and Q factor.
Fig. 3. Different eye diagrams of the downstream in ONU.
average optical powers P1 when all the bits are “1” and the average optical powers P0 when all the bits are “0”, that is represented by the equation: ER = 10 log(P1 /P0 ) dB [12]. The receiving of the ONU toward the downstream is directly influenced by the ER of the downstream optical signal in the OLT, but more than that, the upstream receiving performance of the re-modulation in the OLT is closely decided by the ER in the downstream data. The ER in the downstream is directly reflected by the optical power difference between all the bits “0” and “1”. The choice of the ER is closely connected with the combined modulation mode. The combined modulation mode of downstream OOK and upstream DPSK is employed in this paper. Due to the upstream modulation mode DPSK, if the ER of the downstream is too big, the receiving decision of the upstream will go wrong and increase the BER. In the previously proposed downstream DPSK upstream OOK, the phase modulation is used in the downstream data, thus the downstream optical carriers with constant amplitude is for the OOK re-modulation. This modulation mode although reduce the requirement to the ER of the downstream signal, the transmission of the upstream signal is affected by the chirp signal generated from the phase modulation of downstream. Moreover, the upstream signal is vulnerable to the effects of dispersion, causing the performance degradation of the upstream signals. In comparison with different modulation codes, we will choose one code type, not only meet the demand with better Q factor in the transmitter and receiver in the downstream and upstream, but also decrease the requirement of ER in the downstream, thus enhance the transmission performance both at the downstream and upstream signals. The relationship between ER and Q factor in different code types is illustrated through simulations in Fig. 4. The comparison
L. Li et al. / Optik 124 (2013) 5660–5664
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NRZ 33RZ 67RZ IRZ
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Power Penalty(dB)
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3
2
1
0 -1
0
1
2
3 2
4 4
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6
7
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R DL[10 (Gb/s) ps/nm]
Fig. 5. Anti-dispersion contrast with different code types in upstream.
Fig. 6. Contrast of sensitivity in downstream.
5. Performance analysis of the transmission system 5.1. Anti-dispersion performance analysis of the upstream signals The comparison with anti-dispersion performance after a 20 km symmetrical rate 10 Gbps WDM-PON transmission can be seen in the Fig. 5. Not like the ordinary discussion of the anti-dispersion analysis in the single channel, faced with combined modulation the anti-dispersion performance in the upstream signal is critical to the overall optical transmission in the WDM-PON system. As can be seen in Fig. 5 the upstream re-modulation signal generated by the downstream NRZ code is the biggest variation for the power penalty, but the IRZ code is the smallest. An interesting phenomenon appeared at the simulation. In the single channel transmission, the anti-dispersion performance of code type is closely related to the spectrum width of the optical signal. The main lobe width of the duty cycle 33% RZ code is bigger than the 67% RZ, causing more transmission delay and BER in different frequency components. But in this paper, the upstream re-modulation signal generated by the downstream 33% RZ code has a better dispersion tolerance performance than the duty cycle 33% RZ code. This is closely connected with the combined modulation mode. In the combined modulation mode downstream OOK upstream DPSK, there is a requirement for the amplitude of the downstream signal. The smaller variation of the amplitude of the downstream signal is, the better receiving performance of the upstream signal has. In comparison with the NRZ and 67% RZ code, the 33% RZ code has the smaller amplitude variation and cause the lower effects to the upstream phase re-modulation, representing the better performance on the anti-dispersion. Toward to the upstream signals used the IRZ downstream, the re-modulation code in the upstream carries the optical power both at space levels and mark levels in each bit period. Due to the smallest amplitude variation after modulation in the downstream, the upstream signal with the modulation of IRZ code has the best performance of anti-dispersion.
5.2. The sensitivity analysis of the receiver The sensitivity of the optical receiver is an important parameter in the receiving performance [13], after passing a symmetrical rate 10 Gbps 20 km SMF, the sensitivity comparison of the receiver in the downstream and upstream are illustrated in Figs. 6 and 7. From Fig. 6 we can see the receiver performance of the NRZ downstream has the best sensitivity, and the duty cycle of 33% RZ code has the worst performance. In the signal channel optical communication system, the paramount effect on the transmission performance is the chromatic dispersion, which causes the pulse broadening and degrades the receiving performance. Because the code type with the large main lobe spectrum width is more vulnerable to the chromatic dispersion, the code type of 33% RZ has the worst performance in the receiver sensitivity. The contrast with receiving sensitivity of upstream remodulation can be seen in Fig. 7. The receiving sensitivity of upstream used the IRZ downstream modulation has the best performance, and the condition of the NRZ code is the worst. The performance of upstream signal is closely related to the downstream modulation code type in the combined modulation. IRZ code carries the optical power both at the “0” and “1” in each period. But the power of the amplitude variation is large, causing the degradation of the receiving sensitivity performance in the upstream optical signal.
IRZ 33RZ 67RZ NRZ
-3
-4
-5
log10(BER)
of Q factor from upstream and downstream is carried out by the downstream code types, including NRZ, 33% RZ, 67% RZ and IRZ. As can be seen from Fig. 4, the bidirectional transmission system with the 67% RZ code is the most sensitive to the variation of the ER, but for the code of IRZ the sensitivity is not evident. That’s why we choose IRZ code as the downstream transmitter. Faced with different code types are employed different ER to analyze. The ER chosen by the NRZ and RZ codes are separately 1.5 dB and 1 dB, whereas the ER of the IRZ code is 10 dB.
-6
-7
-8
-9 -45
-44
-43
-42
-41
-40
-39
-38
-37
-36
Received Optical Power(dBm) Fig. 7. Contrast of sensitivity in upstream.
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6. Concluding remarks The combined modulation system of WDM-PON is constructed through simulations. Selecting the different downstream modulation codes as the upstream re-modulation optical carrier, it is showed that the downstream IRZ code not only lower the requirement to the ER of downstream and extend the transmission distance, but also enhance the anti-dispersion performance of the upstream re-modulation and realize the transmission of symmetrical rate 10 Gbps without dispersion compensation. In the future, we can carry out further research on the following aspects: the analysis of the new code type and multi-levels modulation mode. With increasing the dispersion tolerance, enhance the efficiency of the optical spectrum and improve the transmission performance of the overall system. Acknowledgement This work was supported by the tackle key problems in Science and Technology of Henan Provincial (122102210017) and fund of Anyang Institute of Technology. References [1] A. Banerjee, Y. Park, H. Song, et al., Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access: a review [Invited], J. Opt. Network. 4 (11) (2005) 737–758.
[2] W. Hung, C.-K. Chan, L.-K. Chen, et al., An optical network unit for WDM access networks with downstream DPSK and upstream remodulated OOK data using injection-locked FP laser, Photonics Technol. Lett., IEEE 15 (10) (2003) 1476–1478. [3] N. Deng, W. Hung, C.-K. Chan, et al., A novel wavelength modulated transmitter and its application in WDM passive optical networks, in: OFC2003, 2003, PaperMF79. [4] L.Y. Chan, C.K. Chan, D.T.K. Tong, F. Tong, L.K. Chen, Upstream traffic transmitter using injection-locked fabry–perot laser diode as modulator for WDM access networks, Electron. Lett. 38 (1) (2002) 43–45. [5] C.W. Chow, Wavelength remodulation using DPSK down-and-upstream with high extinction ratio for 10-Gb/s DWDM-passive optical networks, Photonics Technol. Lett., IEEE 20 (1) (2008) 12–14. [6] A. Yin, L. Li, X. Zhang, Analysis of modulation format in the 40Gb/s optical communication system, Optik 121 (2010) 1550–1557. [7] J. Wang, J.M. Kahn, Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection, J. Lightwave Technol., IEEE 22 (2) (2004) 362–371. [8] N. Deng, C.-K. Chan, L.-K. Chen, C. Lin, A WDM passive optical network with centralized light sources and multicast overlay, Photonics Technol. Lett., IEEE 20 (2) (2008) 114–116. [9] Y. Lua, X. Honga, J. Liua, L. Xub, et al., IRZ-manchester coding for downstream signal modulation in an ONU-source-free WDM-PON, Opt. Commun. 284 (March (5)) (2011) 1218–1222. [10] T. Tsuritani, A. Agata, I. Morita, et al., Ultra-long-haul 40-Gb/s-based DWDM transmission using optically prefiltered CS–RZ signals, IEEE J. Quantum Electron. 10 (2) (2004) 403–411. [11] B. Zhu, L.E. Nelson, S. Stulz, A.H. Gnauck, et al., High spectral density long-haul 40-Gb/s transmission using CSRZ–DPSK format, J. Lightwave Technol. 22 (4) (2004) 208–214. [12] J.-M. Kang, S.-K. Han, A novel hybrid WDM/SCM-PON sharing wavelength for up-and-down-link using reflective semiconductor optical amplifier, Photonics Technol. Lett., IEEE 18 (3) (2006) 502–504. [13] J. Yu, Y.-K. Yeo, A. Chowdhury, G.-K. Chang, Novel techniques for optical packet generation with high-spectral efficiency and high receiver sensitivity, in: OFC2006, 2006.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Influence of modulation formats performance of the combined modulation in WDM-PON system Li Li ∗ , Hong-an Li, Ji-jun Zhang, Xiu-tai Zhang Department of Electronic Information and Electrical Engineering, Anyang Institute of Technology, Anyang 455000, China
a r t i c l e
i n f o
Article history: Received 7 November 2012 Accepted 2 April 2013
Keywords: WDM-PON Combined modulation IRZ code Extinction ratio DPSK
a b s t r a c t Different modulation modes of DPSK and OOK are separately employed in the upstream and downstream link, after the comparison with different modulation formats in the downstream including the codes of non return-to-zero (NRZ), return-to-zero (RZ) and inverse return-to-zero (IRZ), the symmetric rate of 10 Gbps with 20 km transmission is realized without the dispersion compensation. It can be shown that in the combined modulation mode with the downstream of IRZ code, higher extinction ratio in the downstream and better performance of dispersion tolerance in the upstream could be achieved, enhancing the overall property of the transmission system. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction With the rapid development of FTTH recently, the wavelengthdivision multiplexing passive optical network (WDM-PON) is a promising access network technology due to the better performance in the high capacity, manageability, scalability and reliable security [1]. But the cost of installation and complexity will be increased because of the employment in light sources with specified wavelength, so the scheme of “Colorless ONU” is proposed. This scheme use the centralized light sources in the optical line terminal (OLT) and the same wavelength is shared in the upstream and downstream for transmission, through the re-modulation toward the downstream optical carrier, the “Colorless ONU” transmission of upstream signal is realized, lowering the operation and maintenance costs. Because the “Colorless ONU” is carried out superposition modulation, the quality of the upstream signal is essential to the overall WDM-PON system. Two relatively independent modulation modes are separately employed in the upstream and downstream to make the different transmission information modulate in the same optical carrier, and the receiving signal of the upstream and downstream is detected by different photodiodes, realizing the non-interfering transmission between the upstream and downstream signals. Many different combined modulation modes in the upstream and downstream are emerged, such as DPSK/OOK, FSK/OOK, OOK/OOK, and DPSK/DPSK. There are some different shortcomings existed in the above mod-
∗ Corresponding author. E-mail address: [email protected] (L. Li). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.04.034
ulation modes [2–5]. In this paper, the combined modulation of downstream OOK and upstream DPSK is proposed with the introduction of different downstream transmission codes in the OLT, including IRZ, NRZ, and different duty cycles of RZ, in order to compare the upstream performance of different modulation codes in the combined modulation [6]. An IRZ signal is formed by inverting the intensity level of a conventional RZ signal, thus it carries optical power at both “0” and “1” bit period, which is helpful to the upstream signal re-modulation. Not only realize the higher extinction ratio of the downstream signal, but also enhance the anti-dispersion performance of the upstream signal. Without the re-modulation synchronization and dispersion compensation, the symmetric rate of 10 Gbps with 20 km transmission is realized to enhance the overall property of the transmission system. 2. Proposed combined modulation architecture of WDM-PON Fig. 1 is the established architecture of the combined modulation, researching the effects of different downstream modulation codes on the bidirectional WDM-PON system. In the OLT, the light source adopts the continuous wave-laser diode (CW-LD), and its operating wavelength is 1551 nm. Mach-Zehnder modulator is driven directly by the 10 Gbps downstream electrical signals (including the codes of IRZ, NRZ and different duty cycles of RZ), after the multiplexing of AWG, the modulated optical signal is transmitted to the remote node (RN) by the 20 km single mode fiber (SMF) without dispersion compensation. In the ONU, through the splitter with variable optical splitting ratio (this paper we choose the ratio between the upstream and downstream is 15:85), one
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Fig. 1. The structure of upstream and downstream signal in WDM-PON.
part of the downstream optical signal is sent into the PD of ONU for direct detection, and the other part is used for the upstream re-modulation. Due to the combined modulation of OOK/DPSK, the upstream signal is employed the phase modulator (PM) as the modulation mode. Firstly, the upstream 10 Gbps Pseudo Random Bit Sequence (PRBS) of the electrical NRZ signal is carried on the differential precoding, and the PM modulator is driven by the upstream precoding. Then after passing the 20 km SMF, the upstream signal is received in the OLT. The attenuator used before the Mach-Zehnder interferometer (MZI) is intended to set the different injection optical powers. Through the demodulation of 1 bit delay interferometer (DI) and balanced detection, the received signal is sent into the BER analyzer for measurement [7]. Fig. 1 we can see that, the architecture of receiver in the ONU downstream is simple, and the receiver located in the OLT is relatively complex,From which is helpful to decrease the installation cost and extend the construction of the access network. 3. Principle and characteristic of IRZ code production IRZ code called inverse RZ code is a special line coding of the OOK format. An IRZ signal is formed by inverting the intensity level of a conventional RZ signal, thus it carries optical power at both the mark levels and space levels in each period. This is helpful to accelerate the re-modulation of upstream signal and can eliminate the process of erasing the downstream data from the received optical carrier. Not only can maintain high extinction ratio of the downstream data, but also the receiving optical power of the ONU can be used for the directly re-modulation of the upstream NRZ code. Besides, in comparison with the traditional receiving circuit of NRZ code, the receiving circuit of IRZ code does not require any extra demodulation circuit, no more than inverting a post amplifier after the PD [8]. The operation principle of the IRZ code is illustrated in Fig. 2. The generator of electrical IRZ code is formed by the MZM driven by the RZ-shaped Radio Frequency (RF) signal [9]. The electrical driving signal for the transmitter can be composed by a logical AND operation between a NRZ signal and a RF clock signal. By biasing the modulator at the quadrature point of the negative slope of the MZM transmission curve, the electrical data “0” modulates the optical signal as the mark level, while the electrical data “1” as an inversedpulse waveform as the space level. Thus, the IRZ code carries optical power at both the mark levels and space levels in each bit period. The generation of IRZ signal is carried out the amplitude modulation mode, so the MZM is driven by the working condition of push-pull way. The below table is the parameter setting of the IRZ signal: V is the value of switch voltage, V1m and V2m are separately the RF modulation voltage and switch modulation voltage, VNRZ and VTime are the amplitude of electrical driven signal and clock signal, and the VBais1 and VBais2 are the value of bias voltage in the two arms of the MZM (Table 1). The generation of downstream signal is important to the overall system, especially in the combined modulation WDM-PON system.
Fig. 2. Schematic diagram of IRZ code. Table 1 Parameter value of IRZ production.
IRZ
V1m
V2m
VNRZ
VTime
VBais1
VBais2
V
V
V/2
V/2
0
0
Different downstream optical signals production is driven by the different electrical signals including the code of IRZ, RZ and NRZ. After the multiplexing of AWG and the transmission of 20 km SMF, the following eye diagrams are the receiving signal of ONUs from different downstream electrical signals. The electrical signal of RZ code is modulated by two levels MZMs. The first level modulation is the production of NRZ code, and different duty cycles of RZ codes are achieved by passing the different driven data of electrical clock signal [10,11]. Among the following figures we can see the code of NRZ, duty cycle 33% RZ, duty cycle 67% RZ and IRZ. In the Fig. 3(d) the IRZ code carries the optical power at both the space levels and the mark levels. This characteristic is not only helpful to the remodulation of the upstream electrical signals, but also enhancing the signal quality of receiver in the OLT. 4. The choices of extinction ratio (ER) in the different modulation codes As the parameter of the transmitter, ER is critical to the performance of transmitting and receiving in the optic fiber communication. ER is usually defined as the ratio between the
L. Li et al. / Optik 124 (2013) 5660–5664
(a)
NRZ downstream NRZ upstream 33RZdownstream 33RZ upstream 67RZdownstream 67RZ upstream
120
100
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80
60
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Extinction Ratio
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IRZ downstream IRZ upstream 12
11
Q factor
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10
9
8
0
2
4
6
8
10
12
Extinction Ratio Fig. 4. Relationship between ER and Q factor.
Fig. 3. Different eye diagrams of the downstream in ONU.
average optical powers P1 when all the bits are “1” and the average optical powers P0 when all the bits are “0”, that is represented by the equation: ER = 10 log(P1 /P0 ) dB [12]. The receiving of the ONU toward the downstream is directly influenced by the ER of the downstream optical signal in the OLT, but more than that, the upstream receiving performance of the re-modulation in the OLT is closely decided by the ER in the downstream data. The ER in the downstream is directly reflected by the optical power difference between all the bits “0” and “1”. The choice of the ER is closely connected with the combined modulation mode. The combined modulation mode of downstream OOK and upstream DPSK is employed in this paper. Due to the upstream modulation mode DPSK, if the ER of the downstream is too big, the receiving decision of the upstream will go wrong and increase the BER. In the previously proposed downstream DPSK upstream OOK, the phase modulation is used in the downstream data, thus the downstream optical carriers with constant amplitude is for the OOK re-modulation. This modulation mode although reduce the requirement to the ER of the downstream signal, the transmission of the upstream signal is affected by the chirp signal generated from the phase modulation of downstream. Moreover, the upstream signal is vulnerable to the effects of dispersion, causing the performance degradation of the upstream signals. In comparison with different modulation codes, we will choose one code type, not only meet the demand with better Q factor in the transmitter and receiver in the downstream and upstream, but also decrease the requirement of ER in the downstream, thus enhance the transmission performance both at the downstream and upstream signals. The relationship between ER and Q factor in different code types is illustrated through simulations in Fig. 4. The comparison
L. Li et al. / Optik 124 (2013) 5660–5664
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NRZ 33RZ 67RZ IRZ
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Power Penalty(dB)
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R DL[10 (Gb/s) ps/nm]
Fig. 5. Anti-dispersion contrast with different code types in upstream.
Fig. 6. Contrast of sensitivity in downstream.
5. Performance analysis of the transmission system 5.1. Anti-dispersion performance analysis of the upstream signals The comparison with anti-dispersion performance after a 20 km symmetrical rate 10 Gbps WDM-PON transmission can be seen in the Fig. 5. Not like the ordinary discussion of the anti-dispersion analysis in the single channel, faced with combined modulation the anti-dispersion performance in the upstream signal is critical to the overall optical transmission in the WDM-PON system. As can be seen in Fig. 5 the upstream re-modulation signal generated by the downstream NRZ code is the biggest variation for the power penalty, but the IRZ code is the smallest. An interesting phenomenon appeared at the simulation. In the single channel transmission, the anti-dispersion performance of code type is closely related to the spectrum width of the optical signal. The main lobe width of the duty cycle 33% RZ code is bigger than the 67% RZ, causing more transmission delay and BER in different frequency components. But in this paper, the upstream re-modulation signal generated by the downstream 33% RZ code has a better dispersion tolerance performance than the duty cycle 33% RZ code. This is closely connected with the combined modulation mode. In the combined modulation mode downstream OOK upstream DPSK, there is a requirement for the amplitude of the downstream signal. The smaller variation of the amplitude of the downstream signal is, the better receiving performance of the upstream signal has. In comparison with the NRZ and 67% RZ code, the 33% RZ code has the smaller amplitude variation and cause the lower effects to the upstream phase re-modulation, representing the better performance on the anti-dispersion. Toward to the upstream signals used the IRZ downstream, the re-modulation code in the upstream carries the optical power both at space levels and mark levels in each bit period. Due to the smallest amplitude variation after modulation in the downstream, the upstream signal with the modulation of IRZ code has the best performance of anti-dispersion.
5.2. The sensitivity analysis of the receiver The sensitivity of the optical receiver is an important parameter in the receiving performance [13], after passing a symmetrical rate 10 Gbps 20 km SMF, the sensitivity comparison of the receiver in the downstream and upstream are illustrated in Figs. 6 and 7. From Fig. 6 we can see the receiver performance of the NRZ downstream has the best sensitivity, and the duty cycle of 33% RZ code has the worst performance. In the signal channel optical communication system, the paramount effect on the transmission performance is the chromatic dispersion, which causes the pulse broadening and degrades the receiving performance. Because the code type with the large main lobe spectrum width is more vulnerable to the chromatic dispersion, the code type of 33% RZ has the worst performance in the receiver sensitivity. The contrast with receiving sensitivity of upstream remodulation can be seen in Fig. 7. The receiving sensitivity of upstream used the IRZ downstream modulation has the best performance, and the condition of the NRZ code is the worst. The performance of upstream signal is closely related to the downstream modulation code type in the combined modulation. IRZ code carries the optical power both at the “0” and “1” in each period. But the power of the amplitude variation is large, causing the degradation of the receiving sensitivity performance in the upstream optical signal.
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of Q factor from upstream and downstream is carried out by the downstream code types, including NRZ, 33% RZ, 67% RZ and IRZ. As can be seen from Fig. 4, the bidirectional transmission system with the 67% RZ code is the most sensitive to the variation of the ER, but for the code of IRZ the sensitivity is not evident. That’s why we choose IRZ code as the downstream transmitter. Faced with different code types are employed different ER to analyze. The ER chosen by the NRZ and RZ codes are separately 1.5 dB and 1 dB, whereas the ER of the IRZ code is 10 dB.
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6. Concluding remarks The combined modulation system of WDM-PON is constructed through simulations. Selecting the different downstream modulation codes as the upstream re-modulation optical carrier, it is showed that the downstream IRZ code not only lower the requirement to the ER of downstream and extend the transmission distance, but also enhance the anti-dispersion performance of the upstream re-modulation and realize the transmission of symmetrical rate 10 Gbps without dispersion compensation. In the future, we can carry out further research on the following aspects: the analysis of the new code type and multi-levels modulation mode. With increasing the dispersion tolerance, enhance the efficiency of the optical spectrum and improve the transmission performance of the overall system. Acknowledgement This work was supported by the tackle key problems in Science and Technology of Henan Provincial (122102210017) and fund of Anyang Institute of Technology. References [1] A. Banerjee, Y. Park, H. Song, et al., Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access: a review [Invited], J. Opt. Network. 4 (11) (2005) 737–758.
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