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Optical Isolator Tutorial![]() ![]() Please Wait Thorlabs manufactures a wide selection of narrowband and broadband free-space optical isolators (Faraday isolators) that operate in spectral ranges from 365 nm to 4550 nm, including high-power options, as well as fiber isolators designed for wavelength ranges from 650 to 2010 nm for polarization-independent versions and 770 nm to 2010 nm for polarization-dependent versions. For customers whose needs are not met by our stock selection, we offer a custom isolator service that can combine the best features across our entire isolator family. ![]() Fixed Narrowband IsolatorThe isolator is set for 45° of rotation at the design wavelength. The polarizers are non-adjustable and are set to provide maximum isolation at the design wavelength. As the wavelength changes the isolation will drop; the graph shows a representative profile.
![]() Adjustable Narrowband IsolatorThe isolator is set for 45° of rotation at the design wavelength. If the usage wavelength changes, the Faraday rotation will change, thereby decreasing the isolation. To regain maximum isolation, the output polarizer can be rotated to "re-center" the isolation curve. This rotation causes transmission losses in the forward direction that increase as the difference between the usage wavelength and the design wavelength grows.
![]() Adjustable Broadband IsolatorThe isolator is set for 45° of rotation at the design wavelength. There is a tuning ring on the isolator that adjusts the amount of Faraday rotator material that is inserted into the internal magnet. As your usage wavelength changes, the Faraday rotation will change, thereby decreasing the isolation. To regain maximum isolation, the tuning ring is adjusted to produce the 45° of rotation necessary for maximum isolation.
![]() Fixed Broadband IsolatorA 45° Faraday rotator is coupled with a 45° crystal quartz rotator to produce a combined 90° rotation on the output. The wavelength dependences of the two rotator materials work together to produce a flat-top isolation profile. The isolator does not require any tuning or adjustment for operation within the designated design bandwidth.
![]() Tandem IsolatorsTandem isolators consist of two Faraday rotators in series, which share one central polarizer. Since the two rotators cancel each other, the net rotation at the output is 0°. Our tandem designs yield narrowband isolators that may be fixed or adjustable.
Optical Isolator TutorialFunction An isolator's function is based on the Faraday Effect. In 1842, Michael Faraday discovered that the plane of polarized light rotates while transmitting through glass (or other materials) that is exposed to a magnetic field. 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. The amount of rotation Q equals V x L x H, where V, L, and H are as defined below.
![]() Figure 1. Faraday Rotator's Effect on Linearly Polarized Light Faraday RotationQ = V x L x H V: the Verdet Constant, a property of the optical material, in minutes/Oersted-cm. L: the path length through the optical material in cm. H: the magnetic field strength in Oersted. 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. The Faraday element rotates the input light's polarization by 45°, after which it exits through another linear polarizer. The output light is now rotated by 45° with respect to the input signal. In the reverse direction, the Faraday rotator continues to rotate the light's polarization in the same direction that it did in the forward direction so that the polarization of the light is now rotated 90° with respect to the input signal. 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.
![]() Figure 2. A polarization-dependent isolator. Light propagating in the reverse direction is rejected by the input polarizer. Polarization-Dependent IsolatorsThe Forward Mode The Reverse Mode
![]() Figure 3. A polarization independent isolator. Light is deflected away from the input path and stopped by the housing. Polarization-Independent Fiber IsolatorsThe Forward Mode The Reverse Mode
General InformationDamage Threshold ![]() Figure 4. Pulse Dispersion Measurements Before and After an IO-5-780-HP Isolator Magnet Temperature Pulse Dispersion τ: Pulse Width Before Isolator τ(z): Pulse Width After Isolator Example:
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