The authors propose a novel, multifunctional approach to the problem of demultiplexing closely spaced channels sourced from an optical frequency comb. This solution, based on an externally injected laser, combines the functionality of a tunable demultiplexer, an ultra-low noise amplifier and a modulator, using a single device. Such a device can serve as an optimum transceiver for next generation broadband access (wired and wireless) networks.
The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive
index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index
of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the
transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these
perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the
unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be
manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced
below that of the other cavity modes. Here we review the latest results obtained from devices containing such features.
These results clearly demonstrate that these devices exceed the specifications required for a number of FTTH and
Datacomms applications, such as GEPON, LX4 and CWDM. As well as this we will also present initial results on the
linewidth of these devices.
KEYWORDS: Dispersion, Fiber Bragg gratings, Single sideband modulation, Radio over Fiber, Optical filters, Photodiodes, Scattering, Modulation, Receivers, Power meters
With the increasing demand for broadband services, it is expected that radio-over-fiber (RoF) systems may be employed to provide high capacity wireless access networks. A major problem with the system is that standard amplitude modulation of the optical carrier generates Double Side-Band (DSB) signals. When these signals are transmitted over fiber, chromatic dispersion causes each side band to experience a different phase shift. If the phase difference at the receiver equals pi, the two side bands interfere destructively causing fading of the received signal. Using an optical filter to filter out one side band can reduce chromatic dispersion effects. Another problem in RoF systems may come from Stimulated Brillouin Scattering (SBS). SBS is an interaction between light and sound waves in the fiber, which causes frequency conversion and reversal of the propagation direction of the light. SBS severely limits the optical power that could be transmitted through a fiber and it is detrimental for optical communication systems. It can also be reduced by the filtration carried out at the transmitter (pre-filtering). Since the optical filer functions to eliminate one sideband and induces insertion loss, the optical power to be launched into the transmission fiber falls sharply and so does the risk of SBS. In this paper, we investigate the possibility of performing SSB filtering to simultaneously overcome the effects of dispersion and SBS in the transmission fibre. The results show that the pre-filtering can filter out one side band as well as reduce the effect of SBS.
It is obvious that the development of a wavelength tunable source of short optical pulses will be of paramount importance for future Wavelength Division Multiplexing (WDM), optical time division multiplexed (OTDM), and hybrid WDM/OTDM optical communication systems. Self-seeding of a gain-switched Fabry-Perot (FP) laser is one of the most reliable techniques available to generate wavelength tunable optical pulses. An important characteristic of these self-seeded gain-switched (SSGS) sources is the variation in the side-mode-suppression-ratio (SMSR) as the wavelength is tuned, as this may ultimately affect their usefulness in optical communication systems. A quantitative analysis of how mode partition noise and side mode suppression ratio affects a wavelength division multiplexed system is investigated here. Bit Error Rate measurements have been carried out on a 4-channel wavelength division multiplexed set-up using tunable self-seeded gain-switched pulse sources. These measurements demonstrate the degradation in overall system performance, due to mode partition noise, as the side mode suppression ratio of the self-seeded gain-switched optical pulse sources is reduced. The results also show that the constraints on the minimum side mode suppression ratio required increase with the number of channels in the system.
The authors present the idea of using direct modulation of a laser for generating multiple RF carriers for hybrid radio/fiber systems. External light injection is used in order to increase the modulation bandwidth of the laser and at the same time to improve the overall system operation. The experiments show a 16 dB enhancement in the performance of the system used for distributing 155 Mb/s data signals on 18.6 and 19 GHz carriers.
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