Understanding Optical Communications:Optical Fibre - IMEDEA

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Understanding Optical Communications: Optical Fibre - IMEDEA

2.2.1.1 Attenuation (Absorption) Characteristics of Glasses

Figure 13 on page 31 illustrates the attenuation characteristics of contemporary fibre optics in the infrared spectrum. Light transitions to an invisible state (infrared) when wavelengths exceed approximately 730 nanometers (nm).

Note:  1 nm = 10 Å (Angstrom)

The variety of available glasses presents differing characteristics that hinge on their chemical composition. Considerable advancements have been made over the past few years in enhancing the transmission qualities of glass. Initially, the attenuation measurement for a silicon fibre stood at 20 dB/km. Subsequent research has dramatically reduced this figure to 1 dB/km, and modern figures suggest it can be as low as 0.2 dB/km. These variations in absorption highlight how significantly wavelength impacts performance. The displayed curves exemplify the disparity in the characteristics of various glasses.

Figure 13.  The typical Infrared Absorption Spectrum of Fibres. The lower curve represents a single-mode fibre composed of approximately 4% germanium dioxide (GeO2) dopant in its core, while the upper curve denotes the properties of a graded index multimode fibre. Notably, attenuation is greater in multimode fibres due to the elevated dopant levels, with peaks observed around certain wavelengths attributed to the influence of trace water in the glass.

Light scattering in fibre results primarily from minor density or composition variances within the glass, typically less than 1/10th of the wavelength. This phenomenon is termed “Rayleigh Scattering.” Interestingly, Rayleigh scattering also explains the blue color of the sky and the red hues of sunsets.

In fibre optics, it is important to note that Rayleigh scattering is inversely related to the fourth power of the wavelength! This factor alone accounts for nearly 90% of the pronounced disparities in light attenuation between 850 nm and other wavelengths. Unfortunately, optimizing fibre manufacturing techniques offers limited potential for addressing Rayleigh scattering.

Another scattering phenomenon, known as “Mie Scattering,” occurs due to imperfections within the fibre comparable in size to the wavelength. However, with modern manufacturing advancements, this issue is negligible.

The absorption peak displayed in Figure 13 is centered at a specific wavelength, but several factors, including ambient heat, contribute to its “broadened” nature. This absorption is attributable to the -OH atomic bond, which indicates the presence of water and resonates at a specific wavelength. The challenge of completely eliminating water during manufacturing means that current high-quality fibres typically retain a small residual peak, previously much more pronounced, reaching heights of up to 4 dB/km.

Historically, impurities in the glass were the primary source of attenuation in optical fibre communications. Elements such as iron (Fe), chromium (Cr), and nickel (Ni) can create significant absorption even in minuscule amounts. Thankfully, contemporary silica purification techniques have substantially mitigated concerns regarding impurities.

Additionally, certain dopants utilized to adjust the refractive index of the fibre may inadvertently increase absorption. This factor explains why single-mode fibres generally exhibit lower absorption compared to multimode fibres, as they contain lesser amounts of dopant. Analyzing the absorption spectrum indicates that specific wavelengths are far superior for transmission than others.

2.2.1.2 Fibre Transmission Windows (Bands)

Figure 14.  Transmission Windows. The upper curve showcases the absorption characteristics of fibre in the s. The lower one illustrates those of modern fibre.

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Initially, fibre attenuation regarding optical fibre communication was predominantly characterized by the upper curve displayed in Figure 14, signifying a significant evolution from past conditions. For historical context, three distinct “windows” or bands are recognized within the optical fibre transmission spectrum. The wavelength band employed by a system is a crucial defining aspect of that optical technology.

Short Wavelength Band (First Window)

This region encompasses wavelengths around 800-900 nm. It was the pioneering band employed in optical fibre communications during the late 1970s and early 1980s, favored due to a minor dip in the attenuation profile characteristic of fibres available at the time, in addition to the cost-effectiveness of the optical sources and detectors usable within this band.

Medium Wavelength Band (Second Window)

This band, spanning approximately 1300-1500 nm, gained prominence in the mid-1980s. It remains attractive due to the absence of fibre dispersion in single-mode fibres. Although the optical sources and detectors for this band are pricier than those for the short wave band, the fibre attenuation is merely about 0.4 dB/km. Consequently, this band comprises the majority of long-distance communication systems in operation today.

Long Wavelength Band (Third Window)

Wavelengths ranging from 1550 nm to 1625 nm represent the lowest attenuation achievable presently (about 0.26 dB/km). Moreover, optical amplifiers function effectively within this band. Nonetheless, crafting optical sources and detectors for this range poses significant challenges and expense, alongside the fact that standard fibre disperses signals in this band. By the late 1990s, this band became the primary operational spectrum for nearly all new communication systems.

2.2.2 Transmission Capacity

The potential transmission capacity of optical fibre is exceptional. By reviewing Figure 14 on page 32, both the medium and long wavelength bands exhibit remarkably low losses. The medium wavelength band (second window) spans approximately 100 nm, ranging from 1260 nm to 1360 nm (with a loss of about 0.4 dB per km). The long wavelength band (third window) extends roughly 150 nm, ranging from 1530 nm to 1680 nm (with a loss of about 0.2 dB per km). The loss peaks around 1450 nm and 1625 nm are attributed to water traces in the glass. This results in a useful, low-loss range of approximately 250 nm.

When expressed in terms of analogue bandwidth, a 1 nm-wide waveband at 1550 nm possesses a bandwidth of about 133 GHz, while a similar width at 1600 nm corresponds to 177 GHz. Combining these factors, the usable bandwidth can reach nearly 30 Tera Hertz (3 × 10¹² Hz).

Transmission capacity depends on the modulation technique employed. In the electronic domain, achieving a digital bandwidth of up to 8 bits per Hz of analogue bandwidth is typical. In optical systems, however, this target seems far off and slightly unnecessary. Yet, under the assumption that a modulation technique yielding one bit per Hz of analogue bandwidth is feasible, one could anticipate a digital bandwidth of around 3 × 10¹² bits per second.

As it stands, current technology restricts electronic systems to about 10 Gbps, with ongoing research aiming for higher rates. Current practical fibre systems, too, are limited to this speed due to the electronics required for transmission and reception. Given these insights, even maintaining current fibre quality could theoretically yield a throughput increase of up to 10,000 times compared to present practical limits.

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