Introduction Of Specialty Fibers For Optical Communication Systems

Optical fiber communications have changed our lives over the last 40 years. There is no doubt that low-loss optical transmission fibers have been critical to the enormous success of optical communications technology. It is less well known however, that fiber-based components have also played a critical role in this success.

 

Initially, fiber optic transmission systems were point to point systems, with lengths significantly less than 100 km. Then in the 1980s, rapid progress was made on the research and understanding of optical components including fiber components. Many of these fiber components found commercial applications in optical sensor technology such as in fiber gyroscopes and other optical sensor devices. Simple components such as power splitters, polarization controllers, multiplexing components, interferometric devices, and other optical components proved to be very useful. A significant number of these components were fabricated from polarization maintaining fibers (PMFs). You can buy the PM fiber patch cables from Fiberstore.

 

Although not a large market, optical fiber sensor applications spurred research into the fabrication of new components such as polarization maintaining components, other components such as power splitters were fabricated from standard multimode (MM) or single-mode telecommunication fiber. In the telecommunication sector, the so-called passive optical network was proposed for the already envisioned fiber-to-the-home (FTTH) network. This network relied heavily on the use of passive optical splitters. These splitters were fabricated from standard single-mode fibers (SMFs). Click here to get the price single mode cable fiber optic. Although FTTH, at a large scale, did not occur until decades later, research into the use of components for telecommunications applications continued.

 

The commercial introduction of the fiber optical amplifier in the early 1990s revolutionized optical fiber transmissions. With amplification, optical signals could travel hundreds of kilometers without regeneration. This had major technical as well as commercial implications. Rapidly, new fiber optic components were introduced to enable better amplifiers and to enhance these transmission systems. Special fibers were required for the amplifier, for example, erbium-doped fibers. The design of high-performance amplifier fibers required special considerations of mode field diameter, overlap of the optical field with the fiber active core, core composition, and use of novel dopants. Designs radically different from those of conventional transmission fiber have evolved to optimize amplifier performance for specific applications. The introduction of wavelength division multiplexing (WDM) technology put even greater demands on fiber design and composition to achieve wider bandwidth and flat gain. Efforts to extend the bandwidth of erbiumdoped fibers and develop amplifiers at other wavelength such as 1300nm have spurred development of other dopants. Codoping with ytterbium (Yb) allows pumping from 900 to 1090nm using solid-state lasers or Nd and Yb fiber lasers. Of recent interest is the ability to pump Er/Yb fibers in a double-clad geometry with high power sources at 920 or 975 nm. Double-clad fibers are also being used to produce fiber lasers using Yb and Nd.

 

Besides the amplication fiber, the EDFA (Erbium-Doped Fiber Amplifier) requires a number of optical components for its operation. These include wavelength multiplexing and polarization multiplexing devices for the pump and signal wavelengths. Filters for gain flattening, power attenuators, and taps for power monitoring among other optical components are required for module performance. Also, because the amplifier-enable transmission distance of hundreds of kilometers without regeneration, other propagation propeties became important. These properties include chromatic dispersion, polarization dispersion, and nonlinearities such as four-wave mixing (FWM), self-and cross-phase modulation, and Raman and Brillouin scattering. Dispersion compensating fibers were introduced in order to deal with wavelength dispersion. Broadband coupling losses between the transmission and the compensating fibers was an issue. Specially designed mode conversion or bridge fibers enable low-loss splicing among these thre fibers, making low insertion loss dispersion compensators possible. Fiber components as well as microoptic or in some instance planar optical components can be fabricated to provide for these applications. Generally speaking, but not always, fiber components enable the lowest insertion loss per device. A number of these fiber devices can be fabricated using standard SMF, but often special fibers are required.

 

Specialty fibers are designed by changing fiber glass composition, refractive index profile, or coating to achieve certain unique properties and functionalities. In addition to applications in optical communications, specialty fibers find a wide range of applications in other fields, such as industrial sensors, biomedical power delivery and imaging systems, military fiber gyroscope, high-power lasers, to name just a few. There are so many linds of specialty fibers for different applications. Some of the common specialty fibers include the following:

 

Active Fibers: These fibers are doped with a rare earth element such as Er, Nd, Yb or another active element, The fibers are used for optical amplifiers and lasers. Erblium doped fiber amplifiers are a goog example of fiber components using an active fiber. Semiconductor and nanoparticle doped fibers are becoming an interesting research topic.

Polarization Control Fibers: These fibers have high birefringence that can maintain the polarization state for a long length of fiber. The high birefringence is introduced either by asymmetric stresses such as in Panda, and bowtie design. If both polarization modes are available in the fiber, the fiber is called PMF. If only one polarization mode propagates in the fiber while the other polarization mode is cutoff, the fiber is called single polarization fiber.

Dispersion Compensation Fibers: Fibers have opposite chromatic dispersion to that of transmission fibers such as standard SMFs and nonzero dispersion shifted fibers (NZDSFs). The fibers are used to make dispersion compensation modules for mitigating dispersion effects in a fiber transmission system.

Highly Nonlinear Optical Fibers: Fibers have high nonlinear coefficient for use in optical signal processing and sensing using optical nonlinear effects such as the optical Kerr effect, Brillouin scattering, and Raman scattering.

Coupling Fibers or Bridge Fibers: Fibers have mode field diameter between the standard SMF and a specialty fiber. The fiber serves as an intermendiate coupling element to reduce the high coupling loss between the standard SMF and the specialty fiber.

Photo-Sensitive Fibers: Fibers whose refractive index is sensitive to ultraviolet (UV) light. This type of fiber is used to produce fiber gratings by UV light exposure.

High Numerical Aperture (NA) Fibers: Fibers with NA higher than 0.3. These fibers are used for power delivery and for short distance communication applications.

Special SMFs: This category includes standard SMF with reduced cladding for improved bending performance, and specially designed SMF for short wavelength applications.

Specially Coated Fibers: Fibers with special coating such as hermitic coating for preventing hydrogen and water penetration, metal coating for high temperature applications.

Mid-Infrared Fibers: Non-silica glass-based fibers for applications between 2 and 10 micron

Photonic Crystal Fibers (PCFs): Fibers with periodic structure to achieve fiber properties that are not available with conventional fiber structures.