Dec. 30, 2024
Extensive investigations into optical fiber technology have revealed that the effects of absorption and scattering, which contribute to the attenuation of fiber, tend to diminish as the wavelength increases. Notably, in a specific spectral range around 1300 nm, attenuation losses can be reduced to 1.5 dB/km when utilizing multimode fibers. This advancement has led to substantial cost savings by negating the need for expensive repeaters or regenerators. In the late 1980s and early 1990s, breakthroughs in high-performance photodetectors, edge-emitting LEDs, and solid-state laser diodes provided the crucial optical components needed for this transformative period, during which the term "second window" was coined, suggesting that the 850 nm range was the initial window.
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The second "window" was established at 1300 nm and indicated a spectral zone surpassing previous standards, specifically defined as 1300 nm +/- 50 nanometers (1200 nm - 1350 nm). In the late 1980s, the high expenses associated with amplifiers required for single-mode transatlantic spans, such as TAT-8, prompted the adoption of laser transmitters with a center wavelength of approximately 1310 nm. This adjustment significantly cut costs and minimized the number of amplifiers needed. To simplify terminology, 1310 nm became widely accepted to describe single-mode fiber systems, as opposed to the 850 nm designation commonly used for multimode fibers. Nonetheless, both 850 nm and 1310 nm fall within the broader spectral band of the second window.
NTT announced the third optical window in 1995, characterized by a center wavelength of 1550 nm, which facilitated even lower attenuation rates of approximately 0.5 dB/km. The introduction of Distributed Feedback (DFB) Lasers and erbium-doped fiber amplifiers further reduced optical dispersion, paving the way for high-speed, Dense Wavelength Division Multiplexing (DWDM) systems.
Though the fourth window centered around 1625 nm experienced higher optical attenuation, it notably expanded the range of usable wavelengths available for FTTx and WDM systems. Nowadays, this window is also officially recognized for the management of both live and dark fiber systems as per the specifications of the International Telecommunications Union (ITU).
In our upcoming article, we will explore how the ITU established the concept of "Bands" to categorize specific wavelengths and their implications for modern and future fiber optic transmission systems.
Key point: The practice of rounding 1310 nm up to 1320 nm continues to define single-mode transmission methodologies today.
Signal attenuation as light travels through optical fibers is a critical parameter when designing optical communication systems, as it greatly influences the maximum transmission distance achievable between a transmitter and a receiver, or inline amplifier.
The greater the length of the fiber and the distance the light must cover, the higher the level of signal attenuation experienced. Therefore, attenuation is quantified in decibels per kilometer (dB/Km), also referred to as the attenuation rate or attenuation coefficient.
Attenuation levels fluctuate based on the fiber type and the operational wavelength.
The first optical window spans from 800-900 nm, where the loss is measured at 4 dB/km. This window was utilized primarily in the early 1990s for operational optical sources and detectors.
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Related links:By decreasing the presence of hydroxyl ions and metallic impurities within the fiber material throughout the 1990s, manufacturers successfully produced optical fibers exhibiting extraordinarily low losses in the 1300 nm region, identified as the long wavelength range.
The second optical window is centered around 1310 nm, also known as the O-band, which offers attenuation of 0.5 dB/km.
The third optical window resides at 1550 nm, known as the C-band, promoting a loss rate of 0.2 dB/km.
In system designs targeting long-distance applications, the 1550 nm wavelength is prioritized due to its lower associated losses compared to other wavelengths.
In silica-based optical fibers, single-mode fibers demonstrate reduced attenuation compared to their multimode counterparts. Generally, the longer the wavelength, the smaller the attenuation becomes, a trend evident across the typically 800 nm operational range of conventional datacom and telecom optical fibers.
Factors Influencing Attenuation:
The primary mechanisms that contribute to attenuation in fibers include;
Absorption: Linked directly to the fiber material itself.
Scattering: Associated with both the fiber material and any structural imperfections present within the optical waveguide.
Bending (radiative losses): Attenuation from radiative effects occurs due to disturbances in fiber geometry (incorporating both microbending and macrobending).
Dispersion: Resulting from mode divergence.
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