FOTL - Tutorial 009. Passive Components FOL

fiber, волс, компоненты

Passive Components Fiber-optic communication lines are divided into the following types:

  • optic connectors and splitters;
  • optical converters;
  • optical isolators;
  • optical switches.


The most important passive components are FOL optic connectors and splitters, which are used for consolidation or separation of optical signals.

Functional diagram of the splitter

Features splitters

1. coupling loss, which is a ratio of the optical power at one of the splitter output to the common output

2. excessive loss equal to the total input power with respect to the total output power

3. isolation (insulation) - the influence of the reflected power branch 4 branch 3 (for P 1,2 = 0)

Optical Power Splitters

Non-selective splitters are divided into two main types:

  • T shaped, built on the principle of branch terminals from the main trunk line,
  • star.

Losses in the distribution of the radiation power in the system with a T shaped connector increases in proportion to the number of subscribers in the system and with star couplers - the logarithm of the number of terminal devices N. Thus the system with 20 terminals in total loss is 130 dB the first case, and in the second - 28 dB. Therefore, a large number of subscribers systems appropriate to use star-shaped connectors.

By design splitters are divided into two main groups:

  • biconical, in which the radiation is transmitted through the side surface of the optical fibers;
  • end, in which the radiation is transmitted through the optical fiber end face.
Biconical coupler
Socket Splitter:
1 - input optical fiber;
2 and 3 - the output optical fibers
Coupler with a branching structure:
1 - Input optical fiber,
2 and 3 - the output optical fibers

The splitters with supporting elements are widely used dielectric cylindrical lens, is a segment of the gradient fiber with a parabolic refractive index profile.

Splitters with auxiliary elements:
and - with a partially reflective coating;
b - without a reflective coating;
Light guides:
1 - input optical fiber;
2 and 3 - the output optical fibers.

The star coupler is composed of a cylindrical body with a glass mixing rod. One end of the mixing rod is a spherical mirror, and on the other end applied an antireflection coating.

The star coupler:
1 - a bundle of optical fibers;
2 - mixing rod;
3 - spherical mirror;
4 - anti-reflection coating.

Directional coupler

Directional coupler

The directional coupler can transmit power in one direction (from 1 to 3), whereas the power on the same wavelength is taken from a different direction is transmitted and from 3 to 2.

Also, directional couplers are used as filters in reflectometers for measuring parameters of optical fibers.

The use of a directional coupler in the OTDR

Optical multiplexers and demultiplexers

Multiplexing allows to increase the data capacity fiber optic link. Lines used in an apparatus for combining signals with different carrier wavelengths (multiplexers) and separation (demultiplexers) must have a low insertion loss in a narrow bandwidth, and provide a high degree of isolation between the channels.

The main parameters that are currently used to describe the multiplexer WDM networks seals:

  • channel bandwidth
  • 0.5 dB bandwidth
  • deviation bandwidth
  • maximum insertion loss
  • tolerance in the central wavelength band
  • adjacent channel isolation
  • timeslot
  • 30 dB bandwidth
  • 30 dB Quality Score or FOM

Next, methods for forming the multiplexers and their components.

The main methods of forming the multiplexers:
and - a diffraction grating;
b - with an interference filter;
a - with a prism;
g - with an absorbing filter;
Components multiplexers:
1 - graded cylindrical lens;
2 - grating;
3 - Chromatic filter;
4 - prism;
5 - reflection coating;
6 - selective photodetector.

Demultiplexers laid light beam into its component wavelengths by a diffraction grating.

The five-channel demultiplexer on the grating:
1 - input optical fiber;
2 - output optical fibers;
3 - the lens;
4 - grating.

Optical fiber filters with fixed characteristic

Such filters may implement any limitation on one side or both sides of the wavelength range. In the latter case, the bandwidth is from 1 nm (narrow bandwidth) to 60 nm (bandwidth).

Optical fiber filters with fixed characteristic

Configurable optical fiber filters

Wavelength selection filter in such a change may be the inclination angle of the filter plane, its linear movement or rotation.

Configurable optical fiber filters

Fiber Optic WDM interference filters

Based bipolar interference filters, a number of multi-input selectors discussed above, which are used for multiplexing and demultiplexing light waves in the near infrared region of the optical range. They are based on the three-pole divider (nepoglaschayuschego interference filter), running to an angle of 45°, so that you could use both transmitted and reflected light.

interference filters WDM
Two-way optical fiber interference filter WDM

Multiplexers Dense Wave Division Multiplexing (DWDM)

Multiplexers Dense Wave Division Multiplexing have a much smaller step between wavelengths than conventional WDM multiplexer. A beam of light passes through data multiplexers demultipleskora diffractive structure and is decomposed into its constituent wavelengths and also reversed several wavelengths multiplexed into a single beam.

Multiplexers DWDM:
and - on one plate waveguide;
b - two waveguide plates.

Wave converters

In some cases, in addition to filtering and multiplexing signals having different wavelengths, it is necessary converting one wavelength to another, which is called wavelength converting. The principle of action of the device carrying out this transformation is based on the use of nonlinear effects in optical fibers, for example, fibers with zero dispersion shifted, which leads to the phenomenon of four-wave mixing.

In this case, the converted wavelength is determined from the expression:

λ 0 - the length of the primary radiation;
λ N - the length of the primary radiation;

Another way to implement the optical converter based on the effects of the nonlinear interaction of two optical signals of different wavelengths, resulting in the formation of the new signal wavelength. This principle can be implemented using segnetodielektrikov for example, lithium niobate crystal formed therein with a periodic polarization providing increased interaction optical waves.

Optical converter on a lithium niobate crystal

Optical isolators

In the high-speed fiber-optic data transmission systems to protect laser diodes from the spurious distortion optical isolators used on the cable side.

Optical Isolator:
1 - polarizer;
2 - Faraday cell;
3 - analyzer.

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