This paper presents a novel SCM optical transmission system for next-generation WDM-PONs combining broadband optical sources and a Mach-Zehnder interferometric structure. The approach leds to transport RF signals up to 50 GHz being compatible with RoF systems since a second configuration has been proposed in order to overcome dispersion carrier suppression effect using DSB modulation. The theoretical analysis validates the potentiality of the system also considering the effects of the dispersion slope over the transmission window.
©2009 Optical Society of America
During the last decades, the advances on photonics technologies and the growing demand of end-users are the main reasons for the extraordinary increase in transmission capacity over the optical network. To attend the bandwidth requirements in the access network Passive Optical Networks (PONs) have arisen as promising architectures . A PON is a point-to-multipoint optical network, where an optical line terminal (OLT) at the central office (CO) is connected to many optical network units (ONUs) at remote nodes through passive optical splitters.
To provide multiple-access capability, PONs can be adapted using conventional multiplexing techniques such as time-domain multiplexing (TDM), wavelength-domain multiplexing (WDM), subcarrier multiplexing (SCM) and code-division multiplexing (CDM) . In fact, TDM-PON has emerged as the current-generation PON. The bandwidth provisioned by an optical channel and the hardware in the CO are shared among all users, which reduce significantly the cost of access network, although the increase of transmission speed is limited compared with the demand of future services. WDM-PON has appeared as the best promising solution for the next-generation PON , supporting high user services such as voice over IP, IPTV, video on demand (VoD), video conferencing, high-definition TV (HDTV), e-learning, high quality interactive games amongst and others. Nevertheless, SCM and CDM hybrid architectures are currently proposed to satisfy the future bandwidth demand combined with WDM technology [4, 5].
WDM-PON is attractive due to its large capacity, easy management, network security, and upgradability. From the perspective of practical deployment, the network complexity and its subsequent cost have been the most critical issues for WDM-PONs. To overcome this problem, different schemes have been proposed which are based on reducing the high cost of WDM light sources through wavelength-locked Fabry-Perot diodes (FPLDs) with external injection of a broadband sources , reflective semiconductor optical amplifiers (SOAs)  and the spectral slicing of incoherent light sources such as amplified spontaneous emission (ASE) sources  or light-emitting diodes (LEDs) . Although the spectrum-sliced WDM systems are interesting due to the cost effectiveness and the robustness to the optical crosstalk, the maximum operation bandwith per channel and transmission distance are seriously limited due to the beat noise and fiber dispersion, and therefore radio over fiber (RoF) transmission is not allowed.
Furthermore, WDM compatible SCM-PON is an alternative solution that has a great potentiality [10–11] since a new idea about RoF-PON convergence is to convey the RF subcarriers on the fiber plant of PON so that the baseband data stream and the data modulated RF signal can be simultaneously delivered to wireline and wireless users. Furthermore, optical broadband sources are interesting on optical networks based on spectrum-sliced WDM systems due to the cost effectiveness and the robustness to the optical crosstalk and the temperature-induced drift of WDM components such as the arrayed-waveguide gratings. Iniatially, the large slicing loss and low output power limited the development of WDM-PONs with broadband optical sources. However, we can find nowadays high-power LEDs and sensitive receivers to overcome this problem. In fact, new architerctures by using spectrum-sliced transmission have appeared in the last years. In this context, we demonstrate theoretically and experimentally a novel configuration based on a broadband optical source and a Mach-Zehnder interferometric structure which leads to transmit RoF signals in a 50 GHz frequency range using DSB modulation but avoiding the carrier suppression effect (CSE).
The paper is structured as follows: section 2 presents the main SCM signal transmission limitations, mainly due to the CSE and the broadband optical sources. Section 3 describes the system we propose to allow RoF signal transmission employing a broadband source, and section 4 presents the experimental results. Section 5 includes a discussion on the third order dispersion effects and section 6 summarizes the main conclusions of the work.
2. SCM signal transmission limitations
Consider an optical transmission system depicted in Fig. 1. An optical signal ES(t) is modulated by using a conventional electro-optic modulator (EOM). The optical signal, which is modulated by an electrical sinusoidal signal VIN RF(t) of angular frequency Ω = 2π·f, is transmitted through an optical transmission link which optical transfer function can be defined as following:
After detection of the optical signal, the electrical transfer function HRF(Ω) = Vout RF(Ω)/Vin RF(Ω) can be obtained from :
where ℜ and ZL are the responsivity and the load impedance of the detector, respectively, mi are the normalized modulation amplitudes and ∣ES(ω)∣2 is the optical power spectrum of the input optical signal.
Provided the signal is transmitted through an L long optical fibre link which losses are represented by α and the propagation constant is β(ω), the optical transfer function corresponding to the optical transmission link results:
where the propagation constant β(ω) is expanded as a third-order Taylor series around the central angular frequency ωo of the input optical signal.
Note that the effects of dispersion slope β3 over the transfer function will be more relevant for optical sources with a given bandwith δω. In this way, we consider that the optical bandwith is sufficiently small to neglect the last term in Eq. (4). However, in the section 5 we analyze its influence on the transmission system.
As observed in Eq. (5), there are some constant values which determine the modulation and detection efficiency. A term appears corresponding to the total optical power PS of the input optical signal and another term that includes the losses and the delay τ=β1/L caused by the signal propagation through the fibre link. In addition, inherently to the signal propagation when double sideband modulation format is employed, a term reflecting the carrier suppression effect appears, which generates a reduction on the available transmission bandwidth. Finally, the last term of Eq. (5), takes into account the effects of the dispersion β2 over the spectral distribution of the input optical signal.
As it is well known, the limitations due to the dispersion β2 will be significant in third window used for the downlink when standard fiber is employed . However, in this fiber, the second window used for uplink in PONs does not suffers the CSE and the last term of equation (5) can be neglected since β2 = 0 at 1300 nm. Therefore, our analysis is focused on the use of optical broadband sources at 1550 nm where the dispersion is a limitating factor.
2.1. Carrier suppression effect.
If a coherent laser is used as the optical source and modulation indexes in a DSB modulation technique satisfying m = m1 = m2, the following transfer function is obtained:
and Po is the optical power of the coherent laser. Note that in this case, the last term of Eq. (5) is the unity.
Figure 2 shows the normalized transfer function of a 10 km long optical fiber link which presents a dispersion β2L = 226.2ps2 around 1550 nm with a first transmission null at a relatively low frequency (f1 = 18.44 GHz). As it is well known, the carrier suppression effect is caused by phase difference on the sidebands due to the propagation through the fibre. Fibre dispersion is wavelength dependent, thus, each sideband undergo different delays and therefore, when the sidebands are in counterphase, a null at fk is presented in the link frequency response. In particular, the module of the transfer function for the radiofrequency signal is suppressed according to:
Although there are several techniques to avoid the dispersion effects such as single sideband modulation (SSB) as shown in Fig. 2 , in the following section we will propose a simple transmission system avoiding this effect, without using any extra device.
2.2. Broadband optical sources.
Optical sources have a significant impact on the transfer function of SCM transmission through the last term of Eq. (5). Considering that the optical input signal comes from an optical source with a power spectral distribution ∣ES(ω)∣2 =S(ω), the component HRF o(Ω)of the electrical transfer function is given by the expression:
We consider three different profiles of optical spectrum: Lorentzian, Gaussian and Rectangular profile, as good for describing typical optical sources as DFB laser, LEDs, and the combination of LEDs with optical channel selectors, respectively. In table 1, we show the optical spectrum of each profile where Po is the total power of the optical source and δω3dB is the 3 dB optical bandwidth, together with the corresponding term HRF o(Ω).
According to the theory, the transfer function HRF o(Ω) of the Lorentzian profile has an exponential dependence while the Gaussian profile gives a Gaussian dependence. In case of a rectangular profile, the transfer function corresponds with a sinc function. All of these profiles could be used to describe theoretically a coherent laser when the width δω3dB tends to the limit zero. In this case, the function HRF o(Ω) is the unity as we considered in section 2.1.
Considering an optical fiber link of 10 km, Fig. 3 shows the transfer function HRF o(Ω) obtained for the different optical sources with different 3 dB-bandwidth values detailed in table 2 in order to perform a theoretical analysis of the impact of the profile over the SCM transfer function by changing the bandwidth of the optical source.
As shown in Fig. 3, the bandwidth of the optical source generates a low pass filtering response which limits the range of operating frequencies in all cases, whereas the bandwidth of the transfer function decreases when the optical source width increases.
In Fig. 3(d), the SCM transmission bandwidth is plotted as a function of the optical source bandwidth for the previous profiles. There is a slight difference in the way that the amplitude responses decrease with the frequency considering each profile. Indeed, the rectangular profile lets to transmit higher frequencies than the Gaussian and Lorenztian profiles, respectively, for a given optical bandwidth δω3dB. In addition, we can check that for a laser with typical bandwidth of 125 MHz corresponding to δωo there is no problem to transmit electrical signals at high frequencies, but when a broadband source is used with typical bandwidth of several nanometers, the SCM transmission is drastically limited.
Therefore, we can conclude that SCM transmission through a fiber link using broadband sources is not possible due to dispersion effects. Apart from limitation due to CSE when using conventional DSB modulation, the degradation of the optical signal through the fiber link is a hard limitation.
3. SCM transmission system description
We propose the use of a Mach-Zehnder Interferometer (MZI) for obtaining advanced transmission features in an SCM transmission system, such as avoiding the bandwidth transmission limitation imposed by the optical source bandwidth, and also, the carrier suppression effect.
The transfer function of a MZI interferometer based on two 50:50 couplers and two fiber arms with a different delay τ1 and τ2 is the following:
For the sake of simplicity, the theoretical analysis considers the same states of the polarization for the two branches and no losses in the MZI.
3.1 Configuration 1.
In the first configuration we propose, as shown in Fig. 8, the optical transmission system is composed by an optical source, a MZI including a variable delay line (VDL), an EOM, the optical fibre link and an optical detector.
In this case, the optical signal at the input of the EOM is the product of the optical source signal S(ω) and the transfer function of the MZI, HMZ(ω), so the optical power at the MZI output results:
In Eq. (12), apart form the first factors corresponding to the EOM and detector efficiency and the delay caused by the signal propagation through the fibre, a term corresponding to the CSE appears due to the DSB modulation. There is also a noticeable low pass filtering effect HRF o(Ω) in baseband identical to Eq. (9) that would correspond when the MZI is not included in the transmission setup. However, as it can be seen, there is a transmission window HRF o(Ω - Ωo) featuring the same structure as the baseband, but centered at Ωo, given by:
where ∆τ = τ1 - τ2 is the differential delay between the interferometer fiber arms.
Therefore, this first configuration allows the tuning of the transmission window in the transfer function at the operating frequency by controlling this delay as it can be observed in the Eq. (13).
3.2 Configuration 2.
Figure 9 shows the general scheme for configuration 2, where in this case, the signal emitted by the optical source ∣Es(ω)∣2 = S(ω) is directly launched into the EOM. The MZI is allocated behind the EOM in such a way that the transmission link is now composed of a MZI and the dispersive fiber. Therefore, the optical transfer function can be defined as the product of the optical transfer function of the fibre link, HFIBER(ω) , and the optical transfer function of the MZI, HMZ(ω) , as:
Note that the electrical transfer function of Eq. (3) is valid for a general optical transfer function HOPT(ω).
In this case, the obtained equation presents again a transmission window HRF(Ω) at baseband together with the CSE defined in Eq. (9) and an additional term corresponding to the propagation of the electrical signal carried optically trough the MZI. Moreover, a new component HRF o(Ω-Ωo) appears defining a transmission window generated by the MZ. This transmission window is affected with a modified version of the CSEmod component due to the optical transmission link composed by the MZI and the dispersive fiber. The CSEmod term corresponds to the unity at the central frequency Ωo where the transmission window is allocated.
Therefore, in this configuration the CSE is not suppressed but it is not affecting the spectral region of the transmission window. By tuning the transmission window central frequency , the resulting CSEmod is also tuned, in this way, it is assured that the impact of the CSE is minimized on the transmission window when a DSB modulation is used.
4. Experimental results
In this section, we present the experimental results obtained from both previous configurations detailed in Figs. (8) and (9). The experimental setup uses a 80 nm ASE optical source combined with an optical channel selector in order to obtain an equivalent optical source with variable bandwidth in steps of 0.8 nm from 1527.99 to 1565.50 nm. Moreover, the amplitude of each channel can be attenuated up to 20 dB. The ASE source has a fixed 13dBm optical power which is centered at 1565 nm. In Fig. 10(a), the output optical power of the ASE source is plotted and Fig. 10(b) shows the optical spectra after the optical channel selector with 20 enabled channels. The MZI is composed of two 50:50 optical coupler with a VDL in one of the branches. The VDL has a 1 dB insertion losses with a maximum optical delay around 330 ps. Figure 10(c) plots the optical spectra after the MZI with a time delay correspondin to 3.1 ps.
Finally, the EOM used in the experimental setup has a 3.5 dB optical losses and 30 GHz bandwitdh which is biased in the quadrature point ( Vπ = 4.5V ). The employed optical detector has a 50 GHz electrical bandwidth with a responsivity of 0.53 A/W.
4.1. Configuration 1.
Figure 11 shows the experimental measurements of the transfer function response when a rectangular profile 5.6 nm around 1550 nm width optical source is used to transmit over 10 km fiber link, together with theoretical predictions when the variable delay line in the MZI has been adjusted in order to obtain the transmission band at different frequencies. As expected, a common baseband pass transmission window is found, but a new transmission window appears at a frequency which can be calculated according to Eq. (11), and can be tuned by changing the delay difference in the MZ interferometric structure.
In Fig. 12(a) we can observe the linear dependence between the tuned central frequency and the corresponding delay ∆τ as predicted the Eq. (13). The experimental slope is 0.718 GHz/ps giving a good agreement with the theoretical dependence. Moreover, we have obtained a transmission window with 1 GHz bandwidth at the 50 GHz operation range, although some variations can be observed due to the CSE affecting the transmission window. This fact can be verified in Fig. 12 (b) where the peak amplitude of the transmission window is represented for different tuning frequencies (delays) showing higher variations than 10 dB. Indeed, the filter is totally degraded when it is tuned close to one of the nulls of the CSE. In addition, Fig. 12 (b) plots the ratio between the main lobe and the left (MSLR1) and right (MSLR2) secondary lobe. An asymmetry is found in the transmission window when it is tuned close to the nulls of CSE.
4.2 Configuration 2.
Figure 13 shows the experimental results obtained when the 5.6 nm optical source around 1550 nm is used to transmit signals over a 10 km fiber link. As expected from the theoretical analysis, the new transmission window is not affected by the CSE under this configuration.
The complete experimental characterisation showing tunability of the central frequency, the 3 dB bandwidth, the peak amplitude and the MSLR as a function of the delay (∆τ) is depicted in Fig. 14.
Again, a linear dependence is found in Fig. 14(a) between the central frequency and the delay with a slope of 0.714 GHz/ps, and the 3 dB bandwidth of the transmission window around 1 GHz at the 50 GHz operation range. Figure 12 (b) verifies that the amplitude of the transmission window is not affected by the CSE, which is also shown as a higher symmetry between the left and right MSLR than previously shown in configuration 1.
Although the main advantage of the second configuration is the avoidance of the CSE, and also permits the duplication of the transmitted signal since the EOM is located previously to the MZI with a reduced number of components, the first configuration permits to use the same optical source to transmit the same RF frequency with different data signals since two differents EOMs can be used.
5. Third order dispersion effects.
Since a broadband optical source is used in our system, the dispersion slope is considered to experimentally evaluate the effect on the SCM transmission system transfer function. Figure 15 shows the central frequency and the 3 dB bandwidth of the transmission window as a function of optical source width. In this case, the optical channel selector is controlled to change the effective width of the input optical signal spectrum for a constant delay in the MZI corresponding to a transmission window located at 25 GHz (see inset of Fig. 15(a)).
Figure 15(a) shows that third order dispersion effect does not affect the central frequency of the transmission window but Fig. 15(b) shows an increment on the 3 dB bandwidth of the transmission window for optical sources with width values higher than 12 nm. The experimental points differ from the solid line around 12 nm corresponding to the theoretical prediction when the dispersion slope is neglected.
In addition, Fig. 15(b) permits to observe the transmission bandwitdh when CWDM and DWDM components are used. Since WDM-PONs employ WDM WDM technology with optical bandwidths per channel from 0.8 nm to 16 nm corresponding to DWDM and DWDM componentes, respectively. In any case, Fig. 15(b) guarantees a transmission window with a bandwidth higher than 1 GHz.
Note that WDM-PONs employ WDM technology with optical bandwidths per channel from 16 nm to 0.8 nm corresponding to CWDM and DWDM thin film filters or AWGs. In any case, figure 15(b) guarantees a transmission window with a bandwidth higher than 1 GHz.
From Eq. (4), we can consider that the phase contribution between the optical carrier and the optical RF bands has to be minimized over the bandwidth of the optical broadband source to neglect the effects of the dispersion slope. From the last term of Eq. (4), we can found that the maximum width of the optical source δωc that allows neglecting degradations on the electrical transfer functions is given by the following expression:
The above expression shows that we can select an optical bandwidth δωc that leads to avoid the effects of the dispersion slope for a given fiber length L for RF frequencies where the transmission window is located around Ωo. Therefore, 10 km fiber length leads to a maximum spectral width of the optical source limited to 12 nm satisfying that the second order dispersion effect can be neglected, which validates the theoretical analysis presented in this paper. Therefore, a good agreement is found between the theoretical prediction and experimental measurements by using a 5.6 nm optical broadband source..
A new SCM optical transmission system using optical broadband sources has been proposed to transport RF signal up to 50 GHz. The system is based on a Mach Zehnder interferometric structure which allows to transmit in a radio frequency band, and furthermore, not affected by the carrier suppression effect in DSB conventional modulation systems when this interferometric structure is part of the fiber transmission link. The paper shows a complete experimental characterisation of the optical system response, also validated by the theoretical analysis. The tunability of this new transmission window, the main to side lobe ratios and the constant 3 dB bandwidth of this band along the 50 GHz range are also shown in the paper, opening new perspectives for next generation low cost WDM-SCM-PONs using broadband optical sources. Further work on the implementation of a real bidirectional WDM-PON network employing this new SCM transmission technique to evaluate the degradation of different services is currently performed.
The authors wish to acknowledge the European Comission FP7 under project ALPHA (grant no. 212352) and the national project TEC 2005-08298-C02-01 (ADIRA) funded by the Ministerio de Educación y Ciencia and also the “Ayuda Complementaria para proyectos de I+D+I” ACOMP2007/207 and the “Ajudes per a la realització de projectes precompetitius de I+D per a equips d’investigació” GVPRE/2008/250 supported by the Generalitat Valenciana.
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