First Generation Optical Transmission Windows

Fiber optic communication systems use different wavelength ranges for transmission of data. These wavelengths are called operating windows. The selection of wavelength from the spectrum of light depends on many factors such as the availability and cost of Light sources and other optical components, application etc. Multimode fibers operate in the 850nm and the 1300nm. Single mode fibers operate at 1310nm, 1550nm and 1625nm.

The first generation optical fibers, that is multimode fibers used 850nm window for transmission. When single mode fibers were introduced, they  were intended to be used in 1310nm. Semiconductor sources are designed to operate at wavelengths that minimize optical fiber absorption and maximize system bandwidth. Absorption from impurities in the optical fiber, such as hydroxyl ions (OH) can be minimized by designing an optical source to operate at specific wavelengths. Maximizing system bandwidth involves designing optical fibers and sources that minimize chromatic and intermodal dispersion at the intended operational wavelength.

Material properties of semiconductor optical sources were suitable for optical emission in the 850 nm wavelength region. Optical loss at 850nm is higher when compared to the loss at other transmission wavelengths. Since semiconductor LEDs that emit light at 850 nm only were available at the beginning, first generation optical systems had higher attenuation values. An 850 nm operational wavelength avoids fiber absorption loss from OH impurities near the 900-nm wavelength.

Light sources for 850 nm systems were originally semiconductor LEDs and lasers. Currently, most 850 nm systems use LEDs as a light source. LEDs operating at 850 nm provide sufficient optical power for short distance, low-bandwidth systems. However, multimode fiber dispersion, the relatively high fiber attenuation, and the LED’s relatively low optical output power prevent the use of these devices in longer distance, higher bandwidth systems.

The first development allowing the operational wavelength to move from 850 nm to 1300 nm was the introduction of multimode graded-index fibers. Graded index multimode fibers have substantially lower intermodal dispersion than multimode step index fibers. Systems operating at 850 nm cannot take full advantage of the fiber’s low intermodal dispersion because of high chromatic dispersion at 850 nm. However, the use of multimode graded index fibers allow 850-nm LEDs to operate satisfactorily in short distance, higher bandwidth systems.

Following the enhancements in multimode fiber design, next generation LEDs were designed to provide optical emission in the 1300-nm region. Multimode graded-index fiber systems using these LEDs can operate over longer distances and at higher bandwidths than 850 nm systems. Longer distances and higher bandwidths are possible because fiber material losses and dispersion are significantly reduced at the 1300-nm region.

Advances in single mode fiber design and construction pushed for the development of semiconductor LEDs and LDs (Laser Diodes) optimized for single mode fibers. Single mode fibers have very low dispersion values. However, existing LEDs were unable to focus and launch sufficient optical power into single mode fibers for long-haul, high-bandwidth communication systems. New semiconductor LEDs and LDs capable of operating with single mode fibers at 1300 nm were developed to take advantage of low values of dispersion and attenuation offered Singlemode fibers.

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