A Symmetric RZ-DPSK Based Colorless NG-PON using Optical Carrier Suppression Scheme

In this paper a simultaneous transmission of a 10 Gbps RZ-DPSK data signal for downstream as well as for upstream is proposed and successfully simulated. An OCS (Optical Carrier Suppression) scheme for generation of second order dual side-band optical carrier is utilized by quadrupling a 10 GHz clockfrequency with a 10 GHz LN-MZM (Lithium-Niobate Mach-Zehnder-Modulator). The upper side second order band is used to generate a RZ-DPSK (Return to Zero-Differential Phase Shift Keying) data signal at the OLT (Optical Line Terminal). At the receiving ONU (Optical Network Unit) 50 km away from the OLT the unmodulated lower side second order band coupled with the downlink transmitted signal is utilized for the uplink modulation of 10 Gbps data in RZ-DPSK format. The simulation results show that the performance of the single-tone RZ-DPSK data modulation format is a suitable choice for the WDMPON (Wavelength Division Multiplexing-Passive Optical Network) link with a transmission span of 50 km. The proposed architecture eliminates the need of any pulse carver and mid-span power amplifiers along with the requirement of any power splitting device used in the ONU for colorless uplink transmission. In this scheme, high data rate transmission over long distance is achieved. This scheme merges the boundaries of local access networks and MAN (Metropolitan Area Networks). The proposed scheme is a highly robust, cost effective, backward compatible as well as future proof WDM-PON architecture.

SOAs (Semiconductor Optical Amplifiers) on the other hand, have several applications and benefits from the fact that they can function at any wavelength of interest including the O band along with improved gain dynamics than the EDFA, as established in [5][6]. The high level of noise and inability to provide simultaneous amplification across multiple channels make them to be rarely used as in-line amplifiers. The low production cost, wide bandwidth, small insertion loss as well as fast time response enable them as a potential candidate to be used as an electro-optic transceiver at the user premises in photonic access network.
DRAs (Distributed Raman Amplifiers) execute seamless band-separated, bidirectional amplification due to simple configuration, but their fundamental drawback is the incidence of high-power pump beams inside the feeder fiber that could lead to significant eye safety issues. Therefore, highly sensitive detection modules must be acquired for rapid shutdown of amplifiers and protection switching upon fiber failure detection [7].
Moreover, for the amplification of optical wavelength bands in a full duplex transmission, the mid-span bidirectional optical amplifiers must be constructed to have a band separated configuration with two independent amplifiers for amplification of each downlink as well as uplink. The deployment of any such type of optical amplifiers along the feeder fiber or in the ONU stage for any reason converts the network from passive to active and enhances the network cost.
Another major task for the wide exploitation of WDM-PON is the need of a cost-effective wavelength specific light source for each ONU. Among many proposed techniques so far, the centralized light source scheme appears to be the most promising with low cost for the upstream transmission [8]. Carrier distribution [9][10] and downstream wavelength re-modulation [11] are types of centralized light source scheme. The carrier distribution scheme is based on provision of two individual wavelengths for each ONU, one for the downlink and the other for the uplink. Hence provision of wavelength management is the major issue of concern. In the downstream wavelength re-modulation technique, the downlink carrier can be modulated at either baseband or subcarriers producing a colorless wavelength independent ONUs. It is a cost effective scheme and required only one wavelength channel per ONU as no seed light is required from OLT for uplink.
DPSK modulation format with balance detection is greatly advantageous in high speed transmission systems due to its superior performance against nonlinear effects, increased receiver sensitivity and enhanced tolerance to coherent crosstalk [12]. Similarly, return to zero coding format offers high receiver sensitivity and high immunity to inter-symbol interferences compared to non-returnto-zero format. Therefore, RZ-DPSK modulation format is the ideal choice for high speed long reach access  While the frequency separation of the generated dual tone optical signal is 40 GHz. The above phenomenon can be expressed mathematically as [16]: In the Equation (2) "α" represents the insertion loss of the modulator and "m h " denotes the modulation index with a value = 2.405, defined as: From the Equation (2), it can be seen clearly that the optical carrier and the odd numbered sidebands are suppressed completely and the output OCS signal can be formulated as: The higher order second-order sideband thus generated is separated by using an optical filter and subjected to be

TRANSMITTER DESIGN AND WORKING PRINCIPLE
The drive circuitry includes a clock signal generator AND gates, inverters, power combiner and a LN-MZM driven by light source as shown in Fig. 2. A differentially encoded binary data signal of 10 Gbps is combined with a sinusoidal clock signal of 10 GHz using a high speed AND gate to produce a two level positive electronic RZ pulse shape for each "1" bit, and zero pulse for each "0" bit as shown in the Inset-I of Fig. 1. Similarly inverted or inverse of the input data signal along with a copy of the sinusoidal clock signal (10GHz) are combined together using a second high speed AND gate. The output thus produced is inverted to generate a negative electronic RZ pulse shape for each "0" bit and zero pulse for each "1" bit respectively as shown in the Inset-II of Fig. 2.
Both outputs thus produced are then combined together using a power combiner to generate a 3-level RZ pulse signal as shown in the Inset-III of Fig. 2.
Where v 1 (t) and v 2 (t) are the time varying applied voltages on the two arms of the MZM, and V π is the voltage needed to introduce an optical phase change of π-degree on the optical wave passing through one arm of the MZM . As, v 2 (t) = -v 1 (t), so the output optical field can be written as: Δv 12 (t) is the applied voltages difference between the two arms of the modulator. The optical intensity transfer function is expressed below: The optical phase across each bit is always identical, i.e. either "0" or "π", so the generated RZ-DPSK signal is intrinsically chirp free or have no phase variations across each transmitted bit period. Hence there required no need for a second modulator to be used as a pulse carver so the transmitter design is very simple and cost effective. Therefore, such novel transmitter design can easily be utilized in a colorless WDM-PON for implementation of an RZ-DPSK uplink with improved transmission budget loss for extended reach as compared to OOK uplink transmission.

SIMULATION MODEL SETUP
A simulation model is designed in Opti-system software generating a 10Gbps RZ-DPSK shape data signal as shown in Fig. 4(b-d) respectively. Afterwards an optical coupler is utilized to re-unite the two sidebands before transmitting toward the ONU.
It can be easily observed from the optical spectrum in Fig. 4(e) that the upper sideband gets phase modulated having -8dBm power, while the lower sideband with an optical power level of -4dbm remained blank being unmodulated. Fig. 4(f) is showing the multiplexed optical spectrum of both downlink channels. At the receiving ONU at a distance of 50 km away from the OLT an optical interleaver is used to separate the two side bands. The upper side band is delivered to the downlink receiver for data retrieval, whereas the empty lower side band signal is subject to the uplink transmitter to be modulated with a 10Gbps data for the uplink direction as shown in

PERFORMANCE ANALYSIS
To which can also be cleared from the eye diagrams. Fig. 9 gives the eye diagrams of the simulated results. In Fig. 9(a-b), eye diagrams of channel 1 for B2b and 50 km in the downlink directions are given in Fig. 9(c-d) are for channel 2 in the downlink for B2B and 50 km respectively.
In the last Fig. 9(e-f), eye diagrams for the uplink channel