Optoisolators are classified into three types which include Polarized, Composite, and Magnetic optical-isolator. This isolator uses the polarization axis to keep light transmit in one direction.
It allows light to transmit in forwarding direction, however, prohibits every light beam to transmit back. Also, there are dependent and independent polarized optical-isolators. The latter is more complicated and often used in EDFA optical amplifier. This is an independent polarized type optical-isolator, which can be used in EDFA optical amplifier which includes different components like wavelength-division multiplexer WDM , erbium-doped fiber, pumping diode laser , etc..
In the case of the two-stage cascaded isolator shown in figure , the amount of the Faraday rotation can alternatively be controlled by varying the separation between the two rotators to adjust the influence of the stray magnetic field from each stage on the rotator rod of the other.
The performance of an isolator also varies with temperature because of the temperature dependence of the Faraday rotator. To some extent, this temperature dependence can be compensated by applying similar techniques. Polarization-independent isolators. Although the isolators discussed above extinguish light of any polarization in the reverse direction, their transmission in the forward direction is polarization dependent because of the input polarizer.
In some applications, the sensitivity of an isolator to the polarization of the input optical signal is a major drawback. For instance, the polarization state of an optical wave transmitted through a non-polarization-preserving optical fiber not only is uncertain but also changes with environmental conditions.
It is therefore highly desirable that polarization-independent isolators be used in fiber-optic transmission systems, as well as in the applications of in-line isolation for fiber amplifiers.
In some other instances, light sources are capable of emitting in different polarization states, sometimes for very useful applications such as in the cases of polarization-switching and polarization-bistable lasers. Clearly, for optical isolation in systems containing such sources, polarization-independent isolators are absolutely necessary. The function of an isolator inherently relies on the manipulation of the polarization state of an optical wave using a Faraday rotator.
Therefore, the way to construct a polarization-independent isolator is not to avoid manipulating the polarization. Because an optical wave of any polarization state can be decomposed into two orthogonal linearly polarized components, the basic idea behind any polarization-independent isolator is to separate these two components at the input, manipulate them separately through the nonreciprocal Faraday rotator, and then combine them at the output.
This device consists of two birefringent plates functioning as polarizing beam splitters based on the phenomenon of spatial beam walk-off discussed in the propagation in an anisotropic medium tutorial , two half-wave plates for orienting the polarization in proper directions, and a polarization-dependent isolator that maintains the input polarization direction at the output, such as the one shown in figure b or that shown in figure In the forward direction, an input wave of any polarization state is split by the input birefringent plate into two orthogonal linearly polarized components.
The first half-wave plate rotates the polarization of the lower beam into the same direction as the upper beam, which is the proper polarization direction for transmission through the polarization-dependent isolator. The output birefringent plate then combines the two beams into one of the same polarization state as that of the input wave. The isolation function of this device in the reverse direction is self-evident from the function of the polarization-dependent isolator inside the device.
For a polarization-independent isolator used in a fiber transmission line, the isolation function can be accomplished by sufficiently displacing the backward-propagating optical wave, instead of extinguishing it, so that it does not couple into the input fiber core in the reverse direction. Using this principle, the structure of a polarization-independent isolator can be substantially simplified.
Also illustrated is the principle of operation of this device. Polarization-dependent circulators. Figure below shows the structure of a polarization-dependent optical circulator.
A polarizing beam splitter cube splits s- and p-polarized waves by transmitting and reflecting them, respectively, at the interface of the prims that constitute the cube. Therefore, an s-polarized wave entering port 1 exits port 2 s polarized; an s-polarized wave entering port 2 exists port 3 p polarized; a p-polarized wave entering port 3 exits port 4 p polarized; finally, a p-polarized wave entering port 4 exists port 1 s polarized.
It can be seen that these four ports are nonreciprocal because wave propagation in the reverse sequence is forbidden. For each of the these four nonreciprocal ports, the input and output polarizations are the same. Ports 5 and 6 in this particular device are reciprocal ports and are not part of the circulator. If an optical wave of the wrong polarization direction enters a particular nonreciprocal port, it cannot enter the loop of the circulator but is lost through one of the reciprocal ports.
For example, if a p-polarized wave enters port 1, it is lost through port 5. The direction of rotation is dependent on the direction of the magnetic field and not on the direction of light propagation; thus, the rotation is non-reciprocal. An optical isolator consists of an input polarizer, a Faraday rotator with magnet, and an output polarizer.
The input polarizer works as a filter to allow only linearly polarized light into the Faraday rotator. This light's polarization is now perpendicular to the transmission axis of the input polarizer, and as a result, the energy is either reflected or absorbed depending on the type of polarizer. Laser light, either polarized or unpolarized, enters the input polarizer and becomes vertically polarized. Hence, the light will either be reflected or absorbed. The Forward Mode In a polarization independent fiber isolator, the incoming light is split into two branches by a birefringent crystal see Figure 3.
A Faraday rotator and a half-wave plate rotate the polarization of each branch before they encounter a second birefringent crystal aligned to recombine the two beams. The Reverse Mode Back-reflected light will encounter the second birefringent crystal and be split into two beams with their polarizations aligned with the forward mode light.
The faraday rotator is a non-reciprocal rotator, so it will cancel out the rotation introduced by the half wave plate for the reverse mode light.
When the light encounters the input birefringent beam displacer, it will be deflected away from the collimating lens and into the walls of the isolator housing, preventing the reverse mode from entering the input fiber.
Damage Threshold With 25 years of experience and 5 U. For visible to YAG laser Isolators, Thorlabs' Faraday Rotator crystal of choice is TGG terbium-gallium-garnet , which is unsurpassed in terms of optical quality, Verdet constant, and resistance to high laser power.
Thorlabs' TGG Isolator rods have been damage tested to However, Thorlabs does not bear responsibility for laser power damage that is attributed to hot spots in the beam. Magnet The magnet is a major factor in determining the size and performance of an isolator. The ultimate size of the magnet is not simply determined by magnetic field strength but is also influenced by the mechanical design. The insertion loss will typically be somewhat higher with a dual-stage device.
For operation of an isolator at very high optical power levels, various effects have to be considered: For high average powers, there is thermally induced depolarization, which can affect the degree of isolation. In addition, the parasitic absorption in the Faraday rotator can cause substantial thermal lensing , which distorts the beam profile.
High-power devices usually have exit ports for the rejected light, rather than internal absorbers, in order to avoid the associated heating inside the device. For high peak powers , optical damage may occur. To prevent this, a sufficiently large input aperture is required. High peak powers may also lead to self-focusing.
The power limit depends not only on the input beam radius but also on the length of the device. Faraday isolators and circulators find many applications in laser technology: In many cases, they are used to protect some laser or amplifier against back-reflected light. Amplifier chains sometimes contain several isolators between the different amplifier stages, preventing not only back-reflected light but also amplified spontaneous emission from having detrimental effects.
In optical fiber communication systems, polarization-insensitive fiber-coupled isolators are frequently used e. Various kinds of interferometers and other devices e. This allows e. The polarizing property of a polarization-dependent isolator within a laser resonator e. Questions and Comments from Users Here you can submit questions and comments.
Bibliography [1] E. Turner and R. Klein and T. Ballato and E. Khazanov et al. Quantum Electron. Yu and S. Sun et al. Express 18 12 , , doi
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