Design of a colorless transmitter based on Fabry-Perot cavity for applications in WDM-PON systems

Abstract

ABSTRACT : In order to satisfy the exponential increase of data traffic demand in access networks (fixed or mobile), mainly due to the increasing of broadband services (as high-quality video applications), Passive-Optical-Networks (PON) based on Wavelength-Division-Multiplexing (WDM) techniques has been considered as a promising solution. However, the lack of low-cost optical sources represents a high implementation cost of WDM-PON systems in access networks. Hence, the WDM-PON systems have been proposed using a kind of optical sources, known as colorless transmitters, which act as tunable lasers with an emission wavelength controlled by external mechanisms (usually located in the central office side). The main idea of colorless technology is to reduce the inventory, operation, and maintenance cost by the implementation of universal transmitter devices, mainly at the end-user side. Currently, there are several approaches for implementing a colorless transmitter according to the network requirements (speed, distance range, and spectrum usage efficiency), however, the most common approaches can be classified according to the following 4 categories: (1) Based on Tunable Laser diodes (TLD), (2) Based on Reflective Semiconductor Optical Amplifiers (RSOA), (3) Based on Electro-Absorption Modulators (EAM), and (4) Based on Fabry-Perot Laser Diodes (FPLD). Nevertheless, even considering the recent advances, the colorless transmitter based on TLDs or EAMs are still considering high-cost solutions to be implemented in optical access networks. Besides, although the RSOA case is considered one of the best solutions for the deployment of low-cost and spectral efficient WDM networks, devices to ensure this low-cost condition have not been commercialized yet. Finally, the FPLDs are low-cost devices (usually two digits) that are available in the market and although their emission spectra are limited by the longitudinal modes in the cavity, these can be customized to be useful for applications in optical communications, making them a suitable option to deploy low-cost colorless WDM-PON systems. Thereby, in this work, we propose and numerically analyze a colorless transmitter based on a Fabry-Perot Laser Diode (FPLD) with an emission wavelength that is externally tuned using an ASE optical carrier. The Fabry-Perot based colorless transmitter is designed to support 100 GHz spaced longitudinal modes with a central wavelength of 1552.525 nm. Furthermore, we analyze the cavity’s end-facets reflectivity, to determine a FPLD’s config- uration that allows a good trade-off between the injection power requirement for seeding the emission wavelength with at least 30 dB Side Mode Suppression Ratio (SMSR) and the source chirp. Finally, we analyze the transmission performance of the colorless transmitter in a PON with the following configuration: Transmission distances up to 100 km, seeding carrier with a 3 dB bandwidth of 12.5 GHz, 25 GHz, and 50 GHz, and two optical channels spaced each other 400 GHz: (1) 1552.525 nm (central mode), and (2) 1550.125 nm. Moreover, for all cases was used a Return to Zero (NRZ) modulation format at 2.5 Gbps bit rate. The results show that reflectivities of 10 % and 90 % for the front-facet and end- facet, respectively, allows a good trade-off between the injection power requirement and the transmission performance, achieving SMSR above 30 dB for injection powers below to 300 μW and error-free transmissions (BER 10−9) for distances over 25 km. These results can only be achieved if the optical carrier used to seed the FPLD has a 3-dB bandwidth that is a fraction of the longitudinal mode spacing and the bias current is below 60 mA. Thus, in our case, for longitudinal modes spaced 100 GHz, the 3-dB bandwidth of this optical carrier must be less than 25 GHz. Furthermore, a comparison between the performance of the designed colorless transmitter and a commercial FPLD showed that our laser allows tuning at least 8 longitudinal modes with an SMSR of 30 dB, using one third of the power required by a commercial FPLD. Moreover, if we use a 300 μW seeding carrier to tune the commercial FPLD, it is not possible to achieve error-free transmission for the longitudinal modes far from the central. Hence, the proposed colorless source can be a suitable low-cost solution to implement optical access networks. Furthermore, if the bias current is set at a value between 35 mA (above the threshold current) and 60 mA, it is possible to guarantee an SMSR of 30 dB for at least 13 longitudinal modes, i.e., it is possible to obtain a WDM PON system with at least 13 optical carriers. Additionally, in order to improve the colorless transmitter performance, coherent type-sources such as Laser Diodes can be used as seeding source not only to reduce the ASE noise in the resulting optical carrier, but also to transmit information in a carrier reuse scheme. However, laser diodes typically present a narrow bandwidth spectrum, therefore it would be necessary to have a different laser diode for each longitudinal mode to be tuned in the FPLD. Therefore, to use Ultra Short Pulsed Lasers (USPL) as seeding sources in colorless WDM-PON system can a suitable option not only for its wide spectrum, but also due to its high stability and coherence. Hence, we experimentally design a USPL based on optical fiber a graphene saturable absorber that emits in the spectral region around 1558 nm, and presents a 3-dB bandwidth up to 21 nm, which is adequate for WDM applications. Furthermore, using experimental data from the USPL spectrum and introducing them into a simulated network using the software VPItransmissionMakerTM, we demonstrate that it is possible to use the USPL as a multi-wavelength source, for transmitting information in a WDM-PON system. Therefore, we propose as a future work the use of a USPL as a seeding source in colorless WDM-PON system with and without carrier reuse.