The main bandwidth bottleneck in today's networks is in the access segment. To address that bottleneck, broadband fiber access technologies such as passive optical networks (PONs) are an indispensable solution. The industry has selected time-division multiplexing (TDM) for current PON deployments. To satisfy future bandwidth demands, however, next-generation PON systems are being investigated to provide even higher performance. In this paper, we first review current TDM-PONs; we designate them as generation C. Next, we review next-generation PON systems, which we categorize into C+1 and C+2 generations. We expect C+1 systems to provide economic near-term bandwidth upgrade by overlaying new services on current TDM-PONs. For the long term, C+2 systems will provide more dramatic system improvement using wavelength division multiplexing technologies. Some C+2 architectures require new infrastructures and/or equipment, whereas others employ a more evolutionary approach. We also review key enabling components and technologies for C+1 and C+2 generations and point out important topics for future research.

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Next-Generation Optical Access Networks

This paper reviews the future directions of next generation passive optical networks. A discussion on standardized 10 Gb/s passive optical network (PON) systems is first presented. Next, new technologies that facilitate multiple access beyond 10 Gb/s time division multiple access (TDMA)-PONs will be reviewed, with particular focus on the motivation, key technologies, and deployment challenges. The wavelength division multiplexed (WDM) PON will be discussed and in combination with TDMA, the hybrid WDM/TDMA PON will be reviewed in the context of improving system reach, capacity, and user count. Next, discussions on complementary high-speed technologies that provide improved tolerance to system impairments, capacity, and spectral efficiency will be presented. These technologies include digital coherent detection, orthogonal frequency division multiple access (OFDMA), and optical code division multiple access (OCDMA).

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Next-Generation Broadband Access Networks and Technologies

A potentially inexpensive light source for multichannel WDM applications is proposed The high-power amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), which is already in the single-mode fiber, can be efficiently divided into many channels by using an integrated optic wavelength-division-multiplexing (WDM) demultiplexer This spectrum-sliced ASE can be used as light sources for WDM systems in place of several wavelength-selected DFB lasers To demonstrate the principle, the 40-nm-wide ASE spectrum of an EDFA was sliced using a narrow optical filter (3-dB bandwidth: 13 nm), and the resulting source was used for the transmission of up to 17 Gb/s of data The problem of spontaneous-spontaneous beat noise in these sources is dealt with It is estimated that the total capacity would be about 40 Gb/s, realistically, since the channel spacing should be at least three times the optical bandwidth of each channel to avoid crosstalk >

Spectrum-sliced fiber amplifier light source for multichannel WDM applications

A method of implementing wavelength division multiplexed-passive optical network (WDM-PON) upstream channels based on spectral slicing with wavelength-seeded travelling-wave reflective semiconductor optical amplifier (SOA) modulators is described. A dense WDM comprising eight 1 nm slices, each modulated at a data rate of 1.25 Gbit/s are shown to operate over 25 km of standard fibre with negligible crosstalk and <1 dB dispersion penalty.

Spectral slicing WDM-PON using wavelength-seeded reflective SOAs

The invention is a rotary joint for singlemode optical fibers, having a fixed and a rotating part to permit the transmission of optical signals across a rotational interface (such as a winch or turret) with minimal insertion loss and, in particular, low reflections (good return loss). There is no need of conversion to electrical signals; the device is passive. It may be use an oil of refractive index matched to that of the glass fibers and to that of fiber tapers or lenses used to expand the beam emitted from one fiber and contract it for transmission into the other fiber. The device is bidirectional. By use foil, through precision techniques for building and mounting the optical and mechanical components, and by use of advanced bearings, both the insertion loss and unwanted reflections (return loss) can be minimized, thereby making it suitable for use with singlemode fiber. Insertion loss can be further reduced in conjunction with index-matching fluid by using optical elements (lenses, tapers, fibers) having angled or curved facets rather than perpendicular facets. Oil filling has the further advantage of pressure compensation allowing the device to operate at any ambient pressure. Lenses having curved surfaces can be accommodated by the use of fluid having a refractive index different from that of the lens material.

Fibre optic single mode rotary joint

An approach towards a single-mode local loop is presented in which each customer is given a dedicated wavelength channel by taking different wavelength slices from identical LEDs. This is accomplished by means of wavelength division multiplexing (WDM) components. Results from a fourchannel system operating at 2 Mbit/s over 2.2 km, together with a theoretical analysis of a ten-channel system, are presented.

LED spectral slicing for single-mode local loop applications

A method of obtaining tapered single mode fibre cores by controlled thermal diffusion of the dopant is described. The mechanical fibre dimensions are maintained during this process. Extended tapers have been produced, which show an increased tolerance to longitudinal and transverse offsets. Such tapers will prove useful for a range of optical fibre components.

Tapers in single-mode optical fibre by controlled core diffusion

We demonstrate a significant reduction in delay sensitivity against temperature in optical fibres, using a novel tight jacketed fibre structure. This technique has the cabability of producing fibre in which the optical delay is insensitive to temperature. This has potential in the field of sensors and fibre devices.

Temperature desensitisation of delay in optical fibres for sensor applications

A coupling device couples low power levels of optical radiation out of an optical fibre (3) for maintenance and fault-finding purposes. The assembled coupling device acts to clamp a fibre (3), with its primary plastics coating still in place, against the inner side of a gently curved silica rod (1). The rod (1) is strongly waveguiding and, picking up radiation leaked from the fibre (3), directs it to a photodetector (6). The device finds particular application in optical communications systems.

Optical coupling device

An optical fibre has a coating of, for example, liquid crystal polymer which causes temperature-dependent stress-induced changes in the optical fibre such as to counteract temperature-induced changes in the transmission delay of the fibre

Stephen Hornung

Papers

LED spectral slicing for single-mode local loop applications

Tapers in single-mode optical fibre by controlled core diffusion

Temperature desensitisation of delay in optical fibres for sensor applications

Optical coupling device

Temperature desensitization of delay in optical fibres