What is Fiber Optic Isolator?
Fiber optic isolator is a passive component used for fiber optic communications. As a magneto-optic device, the purpose of optical isolator is to allow light to be transmitted in only one direction. This helps prevent laser source from unwanted feedback which will damage the laser source or arouse unexpected laser problems, such as mode hop, amplitude modulate, frequency shift and so on. Therefore, isolator is an useful and indispensable device to reduce these effects. In the following parts, fiber optic isolator’s construction, operating principle and classifications will be discussed.
Definition of Fiber Optic Isolator
A fiber optic isolator lets light passing through in one direction with a low loss while blocking the light in the opposite direction with a high loss.
Isolators are placed in output circuits of devices with a high output light level such as laser diode transmitter and EDFAs, as shown in this picture.
Their function is to reduce the level of reflected light back into the laser diode or EDFA.
Most fiber optic isolators use the Faraday effect to achieve their function. Faraday effect governs the rotation of the polarization plane of the optical beam in a magnetic field. The rotation is in the same direction for light propagating either parallel or antiparallel to the magnetic field direction.
Optical isolators consist of a rod of Faraday material such as yttrium iron garnet (YIG), whose length is selected to provide 45° rotation. The Faraday material is sandwiched between two polarizers whose axes are tilted by 45° with respect to each other.
Light propagating in one direction passes through the second polarizer because of the Faraday rotation. By contrast, light propagating in the opposite direction is blocked by the first polarizer.
There are a few critical parameters that determine the performance of isolators.
Wavelength-dependence, especially for so-called narrowband isolators that are designed to operate in a spectral range narrower than 20nm. Isolators are described by peak reverse direction attenuation and by the bandwidth for which the isolation is within 3 dB of the peak value
Low insertion loss. The insertion loss should be less than 1 dB in the forward direction, and in excess of 35 dB (single-stage isolator) or 60 dB (double-stage isolator) in the reverse direction.
Polarization mode dispersion (PMD). Isolators are constructed using high birefringent elements; and they are very prone to PMD – typically 50 to 100 fs, especially for single-stage designs. Double-stage isolators can be designed so that the PMD induced by the first stage is largely cancelled by the second stage.
Polarization-dependent loss (PDL). This degrades the performance of an optical isolator.
Construction of Optical Isolator
Fiber optic isolator includes three main parts of an input polarizer, a Faraday rotator with magnet, and an output polarizer. Only linearly polarized light can pass through the input polarizer into the Faraday rotator. The function of the Faraday rotator is to rotate the input light by a certain angle before it reaches the output polarizer. This allows the light in the forward direction to pass unimpeded. However, the light in the reverse direction will not be able to pass the optical isolator and is either reflected or absorbed. These three components of optical isolator skillfully work together and ensure the normal transmission of light signals.
Working Principle of Optical Isolator
An optical isolator contains three components, an input polarizer, a Faraday rotator and an output polarizer. As showed in Figure 1, light traveling in the forward direction passes through the input polarizer and becomes polarized in the vertical plane. Upon passing through the Faraday rotator, the plane of polarization will have been rotated 45° on axis. The output polarizer, which has been aligned 45° relative to the input polarizer will allow the light to pass unimpeded. As Figure 2 illustrates, light traveling in the reverse direction will pass through the output polarizer and become polarized at 45°. The light will then pass through the Faraday rotator and experience an additional 45° of nonreciprocal rotation. The light is now polarized in the horizontal plane and will be rejected by the input polarizer which only allows light polarized in the vertical plane to pass unimpeded.
Types of Optical Isolator
According to the polarization characteristics, optical isolator can be divided into polarization independent type and polarization dependent type.
The polarization dependent isolator, or Faraday isolator, is made of three parts, an input polarizer, a Faraday rotator, and an output polarizer, called an analyser (polarized at 45°). Polarization dependent isolators are typically used in free space optical systems. This is because the polarization of the source is typically maintained by the system. In optical fiber systems, the polarization direction is typically dispersed in non polarization maintaining systems. Hence the angle of polarization will lead to a loss.
The polarization independent isolator is made of three parts, an input birefringent wedge, a Faraday rotator, and an output birefringent wedge. Typically collimators are used on either side of the isolator. In the transmitted direction, the beam is split and then combined and focused into the output collimator. In the isolated direction the beam is split, and then diverged, so it does not focus on the collimator.
Operation of Optical Isolator
The operation of optical isolator is based on the Faraday effect which was discovered by Michael Faraday in 1842. Faraday effect refers to a phenomenon that the plane of polarized light rotates while transmitting through glass (or other materials) that is exposed to a magnetic field. The rotation direction depends on the direction of the magnetic field instead of the direction of light transmission.
According to different light directions, there are two types of operation modes. One is the forward mode and the other is the backward mode. The forward mode enables light enter into the input polarizer and become linearly polarized. When laser light reaches the Faraday rotator, the Faraday rotator rod will rotate by 45° polarization. Thus, the light finally leaves the output polarizer at 45° polarization. However in the backward mode, the light first enters into the output polarizer with a 45° polarization. Next, as it passes through the Faraday rotator, it continues to be rotated for anther 45° in the same direction. Then the light of 90° polarization becomes vertical to the input polarizer and can not leave the isolator. As a result, the light will be either reflected or absorbed.
Types of Optical Isolator
1) Polarized Optical Isolator
Polarized optical isolator employs the polarization axis to keep light transmit in one direction. It allows light to propagate forward freely, but disallows any light to travel back. Also, there are dependent and independent polarized optical isolators. The latter is more complicated and often used in EDFA optical amplifier.
2) Composite Optical Isolator
Composite optical isolator is actually a type of independent polarized optical isolator. It is used in EDFA optical amplifier which consists of many other components, such as erbium-doped fiber, wavelength-division multiplexer, pumping diode laser and so on. Since there are many other components in EDFA module, this type of isolator is named as composite optical isolator.
3) Magnetic Optical Isolator
Magnetic optical isolator is essentially the polarized optical isolator in another expression. It stresses the magnetic part of a Faraday rotator. The Faraday rotator is generally a rod made of a magnetic crystal under strong magnetic field with Faraday effect.
Optical isolator is used in many optical applications in corporate, industrial, and laboratory settings. They are reliable devices when used in conjunction with fiber optic amplifiers, fiber optic ring lasers, fiber optic links in CATV applications, and high-speed and coherent fiber optic communication systems. Single polarization fiber optic isolators are also used with laser diodes, gyroscopic systems, optical modular interfaces, and a variety of other mechanical control and testing applications.
In summary, fiber optic isolator guarantees the stable function of laser transmitter and amplifiers by eliminating unnecessary lights. It also ensures a higher performance of light transmission. Using fiber optic isolator is no doubt a good choice for your network.