Upstream vertical cavity surface-emitting lasers for fault monitoring and localization in WDM passive optical networks
Introduction
The ever-increasing bandwidth demand from end users will soon outgrow the capacity of first generation access networks based on the passive optical network (PON) topology. By employing wavelength division multiplexing (WDM), many wavelengths can be simultaneously supported to transport data downstream towards optical network units (ONUs) that reside at end user premises and upstream from the users back to the central office (CO), thereby providing higher capacity, ease of upgradeability, security guarantee and protocol, format and bit-rate transparency. As WDM-PONs are expected to be first deployed by business customers to support high bandwidth applications and services in both upstream and downstream directions, real-time knowledge of a fiber fault and the location of the fault will ensure rapid rectification and restoration of services [1]. However, despite all its advantages, a typical WDM-PON architecture does not allow for easy diagnostic of fiber failures due to its tree and branch topology. The feeder fiber from the CO to the arrayed waveguide grating (AWG) in the remote node can be considered as the trunk of the tree network whilst the distribution fibers from the remote node to at each ONU can be equated to the branches of the tree network. Conventional optical time-domain reflectometry (OTDR) based on a single wavelength source at the CO is not applicable in a WDM-PON due to the wavelength-routing characteristics of the AWG in the remote node [2]. Likewise, OTDR with a broadband optical source is unsuitable as reflections from different distribution fibers after the AWG will overlap back at the CO thus making the reflected pulses from different distribution fibers indistinguishable. Solutions based on tunable OTDR methods add cost and complexity due to the requirement of a tunable laser source at the CO and the need to schedule OTDR pulse transmission sequentially in a round-robin fashion amongst the branches of the WDM-PON [3]. An alternative proposal uses the optical transmitter at the CO to transmit an OTDR pulse upon detecting the absence of upstream signals [4].
Aside from multiwavelength OTDR, recent solutions have been proposed to combine the use of broadband light that is spectrally sliced at the remote node into multiple monitoring channels, and wavelength/waveband-dependent optical reflectors at each ONU to reflect the monitoring channel back to the CO for detection using either a series of optical power meters [5] or an optical spectrum analyzer (OSA) [6]. The broadband light is centered at a waveband different from that of the upstream and downstream wavelengths. At each ONU, the sliced broadband light (i.e. monitoring channel) is reflected at the input via an optical reflector comprising of either a wavelength dependent component such as a fiber Bragg grating [5] or a combination of a wavelength coupler and a wideband mirror centered on the emission waveband of the broadband source [6]. Aside from the cost and complexity of highly-sensitive monitoring modules, these solutions require one or more high-power broadband sources to compensate for the round-trip loss incurred by the monitoring channels and also an upgrade of all ONUs to include the optical reflectors. As the access network segment is particularly cost sensitive due to the relatively small number of end users it services without the benefits of large cost-sharing, minimizing costs especially at the ONU end whilst having the ability to provide added features is important for network operators.
In this work, a simple and potentially cost-effective fault monitoring and localization scheme is proposed and demonstrated. The scheme is based on using a low bandwidth and low output power fault monitor in a WDM-PON in conjunction with optical injection-locked (OIL) VCSEL upstream transmitters at the ONU. Without requiring additional narrowband/broadband injection-locking sources in the CO, the VCSELs are optically injection-locked by modulated downstream signals from the CO to align the upstream wavelengths to that of the downstream, and hence to the WDM grid without active temperature control. The benefits and detailed operation of OIL-VCSELs in a WDM-PON can be found in [7]. Here, the fault monitor is added only to the CO without the need for modifications to be made to the ONUs. Once the monitor is in place, monitoring can be implemented individually and on-demand by each customer depending on their service level agreement. In comparison to existing WDM-PON monitoring schemes that use optical power measurements to detect fiber failure e.g. [5], [6], the proposed scheme relies on simple electronics through a lock-in module and a series of low-bandwidth detectors. To the best of our knowledge, this is the first proposal that exploits the inherent mirror structure of VCSELs to reflect monitoring channels back to the CO for monitoring and localization of fiber and device faults in a passive optical network. In this work, the feasibility of the proposed scheme are experimentally demonstrated with detailed characterizations of the fault monitor showing high sensitivity (∼−67 dB m), low bandwidth requirements (∼2 kHz) and potentially low output power (∼−7 dB m). Additionally, measurements of bit-error-rate (BER) show that the monitoring scheme incurs a negligible penalty (∼0.5 dB) on the transmissions of the WDM-PON.
Section snippets
Principle of operation
The schematic of a WDM-PON implementing the proposed fault monitor is shown in Fig. 1. Central to the fault monitor are a low-power broadband source, a lock-in module, and a series of low-bandwidth detectors. The broadband light source is centered on a waveband distinct from the downstream and upstream wavelengths of the network and is modulated with an ac signal at a reference frequency, fref, from the lock-in module. Likewise, transceivers in the CO are temperature-tuned to emit distinct
Experimental setup
The demonstration of the feasibility of the proposed scheme and the characterization of the fault monitor is performed using a non-polarization maintaining experimental setup as shown in Fig. 2. At the CO, CW light at 1576.1 nm from a distributed feedback (DFB) laser is externally modulated with a 223 − 1 pseudorandom bit sequence (PRBS) non-return-to-zero (NRZ) data at 2.5 Gb/s from a bit-error-rate testset (BERT). The downstream signal is coupled with light from a broadband optical source and
Results and discussion
The detailed characterization of the fault monitor showing high sensitivity (∼−67 dB m) and potentially low output power (∼−7 dB m) is described in this section. Additionally, results from bit-error-rate measurements will be discussed, showing that the monitoring scheme incurs a negligible penalty (∼0.5 dB) on the transmissions of the WDM-PON.
Summary
A novel fault monitoring and localization scheme was proposed and experimentally demonstrated for use in a WDM-PON with a wavelength-dependent passive tree and branch topology. Using a highly-sensitive but low-bandwidth and potentially low-cost monitor comprising a low output power broadband source and low bandwidth detectors, in conjunction with the high reflectivity mirror of VCSELs, the monitoring channel can be fedback and detected with high sensitivity at the central office, incurring
Acknowledgments
The authors gratefully acknowledge Prof. Ivan P. Kaminow and Forrest Sedgwick for helpful discussions, Dr. Boh Ruffin of Corning, Inc., for loaning the fibers used in the experiments, and the financial support by DARPA (Grant Number F30602-02-2-0096).
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