E. Connolly

Fast Wavelength Switching Lasers Using Two-Section Slotted Fabry–PÉrot Structures

Fast wavelength switching of a two-section slotted Fabry-Perot laser structure is presented. The slot design enables operation at five discrete wavelength channels spaced by 10 nm by tuning one section of the device. These wavelengths operate with sidemode suppression ratio in excess of 35 dB, and switching times between these channels of approximately 1 ns are demonstrated

Cross-Channel Interference Due to Wavelength Drift of Tunable Lasers in DWDM Networks

The authors present an investigation of the wavelength stability of a tunable laser (TL) transmitter and its impact on the performance of a 2.5-Gb/s dense wavelength-division-multiplexed (DWDM) system. Performance of a DWDM system, employing such a TL, is characterized by examining the cross-channel interference caused by this drift when the channel spacing is set to 12.5 and 25 GHz. Results obtained show that the wavelength drift affects the system performance by introducing an error floor in the case of 12.5-GHz spacing. This error floor can be mitigated by increasing the blanking time of the TL during the channel transition, in order to reduce the wavelength drift

Cross Channel Interference due to Wavelength Drift of Tuneable Lasers in DWDM Networks

The authors present an investigation of wavelength stability of a tunable laser transmitter and its impact on the performance of a 2.5 Gb/s dense wavelength division multiplexed system. The wavelength drift of the laser after switching between channels is accurately measured by using a self-heterodyning technique and an optical filtering technique. Performance of a DWDM system, employing such a tunable laser, is characterised by examining the inter-channel interference caused by this drift when the channel spacing is set to 12.5 and 25 GHz. Results obtained show that the wavelength drift affects the system performance by introducing a power penalty in the case of 25 GHz spacing and an error floor in the case of 12.5 GHz spacing

Demonstration of Wavelength Packet Switched Radio-Over-Fiber System

The authors present a novel concept of employing optical wavelength packet switching in radio-over-fiber access networks. In such a system, the tunable laser (TL) would be employed as a transmitter in the central station. The optical carrier generated by this device would be externally modulated with the data signal upconverted to an RF frequency before being sent to the appropriate base station (BS). If each of the BSs is assigned a unique wavelength, the addressing and routing of traffic could be performed on a packet-by-packet basis with the TL switching between the wavelength assigned to different BSs

Effects of Crosstalk in WDM Optical Label Switching Networks Due to Wavelength Switching of a Tunable Laser

Crosstalk caused by switching events in fast tunable lasers in an optical label switching (OLS) system is investigated for the first time. A wavelength-division-multiplexed OLS system based on subcarrier multiplexed labels is presented which employs a 40-Gb/s duobinary payload and a 155-Mb/s label on a 40-GHz subcarrier. Degradation in system performance as the transmitters switch between different channels is then characterized in terms of the frequency drift of the tunable laser

Fast Tunable Lasers in Radio-over-Fiber Access Networks

The authors present a novel concept of employing optical wavelength packet switching (WPS) in radio-over-fiber (RoF) access networks. The Central Station is equipped with a fast tunable laser (TL), which is externally modulated with a data signal upconverted to a radio frequency. The information transmitted over the network is encoded onto different wavelengths depending on the destination base station (determined by an optical band-pass filter at that BS). Routing of traffic could be performed on a packet-by-packet basis. In such a system dynamic bandwidth allocation could be realised by varying the time the TL transmits on a particular wavelength, depending on the amount of data that needs to be sent from/to the BS. The feasibility of employing TLs in the realisation of such a system is verified by building a basic WPS RoF system. The measurements of the cross-channel interference due to the TL wavelength instability and drift are also presented. No power penalty was observed due to switching of the laser, suggesting that RoF systems based on TLs are a feasible solution to the last mile problem

Spectrally Compact Optical Subcarrier Multiplexing for Label-Switched Networks with 42.6 Gb/s Duobinary Payload and 2.5Gb/s NRZ Labels

A novel approach to optical subcarrier multiplexing with compact spectrum is demonstrated using a 42.6 Gb/s duobinary payload and 2.5 Gb/s NRZ label. The label causes ≪1 dB penalty on the payload receiver sensitivity.

Adjacent Channel Interference due to Wavelength Drift of a Tunable Laser in Base-Band and Subcarrier Multiplexed System

In this paper, we examine the amplitude and the duration of a wavelength drift of the tunable lasers at the most crucial moment, which is the time after the wavelength switch and measure the impact of this drift on the performance of the DWDM system spaced by 12.5 GHz. The adjacent channel interference is examined for two cases: firstly when the TL is modulated with base-band (BB) data, secondly when subcarrier multiplexing (SCM) is used

SCM optical label switching scheme in a WDM packet transmitter employing a switching SG-DBR laser

We demonstrate an SCM optical label switching scheme with spectral efficiency of 0.4bit/s/Hz and a spectral density figure of merit of 1. We then employ this scheme in a WDM transmitter subsystem and we investigate the detrimental effects that switching events in tunable lasers can have on such systems. The label performance is shown to hit an error floor when the tunable laser switches channels and this would cause incorrect routing of packets. The bit error rate is time resolved in order to better understand the problem and some possible solutions are discussed.

Frequency Drift Characterisation of Directly Modulated SGDBR Tunable Lasers

Tunable Lasers (TL) are rapidly becoming key components in Dense Wavelength Division Multiplexed (DWDM) systems, packet switched schemes and access networks. They are being introduced as alternatives to fixed wavelength sources to provide a greater degree of flexibility and to reduce large inventory [1]. The SGDBR laser is an ideal candidate due to its large tuning range (40 nm), high output power (10 dBm), large Side Mode Suppression Ratio (>30 dB) and its ability to be monolithically integrated with other semiconductor devices. Such integration could comprise of a Semiconductor Optical Amplifier (SOA), allowing for extended reach tunable operation, in a very compact and low cost footprint [2]. Thus far, external modulation has been the most popular modulation technique used with TLs. However, the addition of the modulator introduces loss to the transmitted signal due to high insertion and coupling losses. Addressing these short comings would result in increased cost and complexity of the transmitter. Alternatively, direct modulation is one of the simplest and cost efficient ways to modulate the lightwave signal. Hence, it is rational to investigate the performance of a directly modulated SGDBR laser in order to verify its usefulness in a WDM based access network scenario. Previous work in this area has mainly focused on bandwidth characterisation and transmission experiments [3, 4]. In this paper, we characterise the frequency drift associated with a directly modulated SGDBR laser incorporating a wavelength locker. Focus is placed on investigating the magnitude and settling time of this drift. In addition, we also demonstrate how the frequency drift has a detrimental effect on DWDM system performance when the modulated channel is passed through a narrow Optical Band-Pass Filter (OBPF) centred at the target emission frequency.