Features

• Minimize Feedback into Optical Systems
• Operating Wavelength Ranges of 1054 - 1074 nm or 1059 - 1069 nm
• 0.8 m to 1 m of PANDA-Style PM Fiber on Both Sides
• Available with 2.0 mm Narrow Key FC/APC Connectors or Unterminated
• Designed for CW Applications
• Each Unit is Individually Tested

Fiber isolators protect light sources from back reflections and signals that can cause intensity noise and optical damage. Optical isolators, also known as Faraday isolators, are magneto-optic devices that preferentially transmit light in the forward direction while absorbing or displacing light propagating in the reverse direction (see the schematic below). Please see the Isolator Tutorial tab for an explanation of the operating principles of a Faraday isolator.

Polarization-Dependent Isolator Schematic
IO-J and IO-L Isolators: Light propagating in the reverse direction is rejected by the input polarizer. For more detailed information, please see the Isolator Tutorial tab.

Thorlabs offers polarization-dependent isolators with polarization-maintaining (PM) fiber below. In contrast, our single-mode (SM) fiber polarization-independent Nd:YAG isolators are designed to connect to SM fiber. Our high-power units are built using a specialized fiber end face process that increases the maximum power. A length of at least 0.8 m of fiber is built in to each side of the isolator, and an arrow on the body indicates the transmission direction. In addition, each unit is tested before shipment to ensure compliance with our specifications and a complete test report comes with every serialized part.

Thorlabs also manufactures free-space isolators and fiber isolators designed for the infrared range. Please use the Selection Guide table above for more information. If you do not see an isolator that suits your application, please refer to the Custom Isolators tab for information on our build-to-order options, or contact Tech Support.

Optical Isolator Tutorial

Function
An optical isolator is a passive magneto-optic device that only allows light to travel in one direction. Isolators are used to protect a source from back reflections or signals that may occur after the isolator. Back reflections can damage a laser source or cause it to mode hop, amplitude modulate, or frequency shift. In high-power applications, back reflections can cause instabilities and power spikes.

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 β equals V x B x d, where V, B, and d are as defined below.

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Figure 1. Faraday Rotator's Effect on Linearly Polarized Light

Faraday Rotation

β = V x B x d

V: the Verdet Constant, a property of the optical material, in radians/T • m.

B: the magnetic flux density in teslas.

d: the path length through the optical material in meters.

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.

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Figure 2. A single-stage, polarization-dependent isolator. Light propagating in the reverse direction is rejected by the input polarizer.

Polarization-Dependent Isolators

The Forward Mode
In this example, we will assume that the input polarizer's axis is vertical (0° in Figure 2). Laser light, either polarized or unpolarized, enters the input polarizer and becomes vertically polarized. The Faraday rotator will rotate the plane of polarization (POP) by 45° in the positive direction. Finally, the light exits through the output polarizer which has its axis at 45°. Therefore, the light leaves the isolator with a POP of 45°.

In a dual-stage isolator, the light exiting the output polarizer is sent through a second Faraday rotator followed by an additional polarizer in order to achieve greater isolation than a single-stage isolator.

The Reverse Mode
Light traveling backwards through the isolator will first enter the output polarizer, which polarizes the light at 45° with respect to the input polarizer. It then passes through the Faraday rotator rod, and the POP is rotated another 45° in the positive direction. This results in a net rotation of 90° with respect to the input polarizer, and thus, the POP is now perpendicular to the transmission axis of the input polarizer. Hence, the light will either be reflected or absorbed.

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Figure 3. A single-stage, polarization-independent isolator. Light is deflected away from the input path and stopped by the housing.

Polarization-Independent Fiber Isolators

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.

In a dual-stage isolator, the light then travels through an additional Faraday rotator, half-wave plate, and birefringent beam displacer before reaching the output collimating lens. This achieves greater isolation than the single-stage design.

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.

General Information

Damage Threshold
With 25 years of experience and 5 U.S. patents, our isolators typically have higher transmission and isolation than other isolators, and are smaller than other units of equivalent aperture. 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 22.5 J/cm² at 1064 nm in 15 ns pulses (1.5 GW/cm²), and to 20 kW/cm² CW. However, Thorlabs does not bear responsibility for laser power damage that is attributed to hot spots in the beam.

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Figure 4. Pulse Duration Measurements Before and After an IO-5-780-HP Isolator

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. Many Thorlabs magnets are not simple one piece magnets but are complex assemblies. Thorlabs' modeling systems allow optimization of the many parameters that affect size, optical path length, total rotation, and field uniformity. Thorlabs' US Patent 4,856,878 describes one such design that is used in several of the larger aperture isolators for YAG lasers. Thorlabs emphasizes that a powerful magnetic field exists around these Isolators, and thus, steel or magnetic objects should not be brought closer than 5 cm.

Temperature
The magnets and the Faraday rotator materials both exhibit a temperature dependence. Both the magnetic field strength and the Verdet Constant decrease with increased temperature. For operation greater than ±10 °C beyond room temperature, please contact Technical Support.

Pulse Dispersion
Pulse broadening occurs anytime a pulse propagates through a material with an index of refraction greater than 1. This dispersion increases inversely with the pulse width and therefore can become significant in ultrafast lasers.

τ: Pulse Width Before Isolator

τ_(z): Pulse Width After Isolator

Example:
τ = 197 fs results in τ_(z) = 306 fs (pictured to the right)
τ = 120 fs results in τ_(z) = 186 fs

The following selection guide contains all of Thorlabs' Fiber Optical Isolators. Click the colored bars below to to see specifications and options for each wavelength range and isolator type. Please note that Thorlabs also offers free space optical isolators and custom optical isolators.