Features Free-Space Operation Fixed and Adjustable Narrowband and Fixed and Adjustable Broadband Versions Available Isolation up to 44 dB High Power Safe, Some Units up to 20 kW/cm2 CW (1 GW/cm2 Pulsed) Thorlabs offers a wide selection of free-space optical isolators. These isolators typically have higher transmittance and isolation compared to all other isolators on the market. Furthermore, because of certain proprietary features (covered by 25 years of experience and 5 US patents), OFR Isolators are smaller and have higher performance than any units of equivalent aperture available anywhere. An optical isolator is a passive magneto-optic device that only transmits light 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.

Thorlabs offers adapters to mount OFR isolator bodies into SM series components. Please see below for more information.

OFR, a company with a 35-year history, became a division of Thorlabs in January 2007. Shortly afterwards, the OFR group began a rapid expansion of its optical isolator production facilities with support from a team of Thorlabs lean manufacturing experts. Driven in large part by the growing demand created by the rapid expansion of the fiber laser market, we are now producing significant volumes of optical isolators.

To further develop our capabilities, in June 2008 we completed the installations of precision thin film coating facilities. OFR’s new coating facility will support the expansion of its world-class line of optical isolators and provide the flexibility needed to expand its on-demand production capabilities of custom or OEM optical isolators.

Page Organization

The isolators listed below are organized by operating wavelength, then by aperture size, and finally by maximum power level. The Selection tab offers a complete list of stocked isolators offered at this time and an easy way to view the specifications for each and compare them. On the Isolator Types tab, you can find a short tutorial delineating the four basic types of isolators offered and a graph showing typical isolation spectrum. We use five types of polarization set-ups in our isolators, each of which allows for a different maximum power level. The Polarizer Types tab gives a short tutorial on these polarization set-ups. Finally, the Isolator Tutorial tab gives a basic introduction on the principles behind the design of isolators and how they work.

Isolator Selection Table

Isolation and Transmission of Free-Space Optical Isolators

Below are the theoretical isolation and transmission graphs for our free-space optical isolators. To enlarge the graph, simply click on it. Please note that the central wavelength can be changed on all adjustable isolators by adjusting the relative positions of the polarizers. If you would like to obtain the appropriate graph for a central wavelength not shown below, please contact our technical support staff.
Above: The shaded region represents the tuning range of the IO-5-405-LP isoloator.	Above: The shaded region represents the tuning range of the IO-3D-633-PBS, IO-3-633-LP, and IO-5-633-PBS isolators.
Above: The yellow shaded region represents the tuning range of the IO-3D-780-VLP isolator, while the additional region shaded in gray represents the extended tuning range of the IO-3-780-HP and IO-5-780-HP isoloators.	Above: The yellow shaded region represents the tuning range of the IO-3D-780-VLP and IO-3D-850-VLP isolators, while the additional region shaded in gray represents the extended tuning range of the IO-3-850-HP and IO-5-850-HP isoloators.
Please Note: Since Thorlabs offers isolators with several different tuning ranges, those ranges are not denoted on the graph above.	Above: The yellow shaded region represents the tuning range of the IO-2.5-1310-VLP and IO-4-1310-VLP isolators.
Above: The yellow shaded region represents the tuning range of the IO-2.5-1550-VLP, IO-2.5-1550-HP, IO-4-1550-VLP, and IO-5-1550-HP isolators.	Please note that the entire region shown above represents the full spectral range of the IO-4-2050-HP isolator.

Polarizer Types and Power Limits

Several types of polarizers are available to meet specific application needs. All Glan, Cube, and Brewster polarizers are manufactured and assembled in the OFR optical shop using OFR-designed tooling. VLP polarizers are an absorptive film type used in compact low-power applications. PBS are low power cubes that are useful for monitoring and injection-locking applications. LP polarizers are a Glan-type polarizer with very high transmission and isolation values. HP polarizers are also a Glan-type polarizer, can handle high power, and have an escape window that can be used for injection-locking. VHP polarizers are Brewster window polarizers that offer very high damage thresholds.

Very Low Power absorptive film polarizer. Smallest package and usually least expensive.	Polarizing Beam Splitter cube. Low power applications. Good for injection locking applications.	Low Power Glan-type crystal polarizer. Larger package size but very high transmission.	High Power Glan-type crystal polarizer. Larger package size but very high transmission.	Very High Power Brewster window polarizer. Highest power possible, largest package, narrowband, lower cost than HP.
25 W/cm2 (CW)	50 W/cm2 (CW)	250 W/cm2 (CW)	500 W/cm2 (CW)	20 kW/cm2 (CW)
300 kW/cm2 (Pulsed)	-	25 MW/cm2 (Pulsed)	200 MW/cm2 (Pulsed)	1 GW/cm2 (Pulsed)

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 Q equals V x L x H, where V, L, and H are as defined below.

Faraday Rotation Q = 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.

The Forward Mode Laser light, whether or not polarized, enters the input polarizer and becomes linearly polarized, say in the vertical plane (0°). It then enters the Faraday rotator rod, which rotates the plane of polarization (POP) by 45°. Finally, the light exits through the output polarizer whose axis is at 45°. Therefore, the light leaves the isolator with a POP of 45°.

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.

General Information

Damage Threshold
Our isolators typically have higher transmittance and isolation compared to all other isolators on the market. Furthermore, because of certain proprietary features (covered by 25 years of experience and 5 US patents), OFR isolators are smaller and have higher performance than any units of equivalent aperture available anywhere. For visible to YAG laser Isolators, OFR's 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. OFR TGG Isolator rods have been damage tested to 22.5J/cm2 at 1064nm in 15ns pulses (1.5GW/cm2), and to 20kW/cm2 CW. However, OFR 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. Many OFR magnets are not simple one piece magnets but are complex assemblies. OFR's modeling systems allow optimization of the many parameters that affect size, optical path length, total rotation, and field uniformity. OFR's US Patent 4,856,878 describes one such design that is used in several of the larger aperture Isolators for YAG lasers. OFR 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: t = 197fs results in t(z) = 306fs (pictured to the right) t = 120fs results in t(z) = 186fs

Using the Isolator

Below are instructions on how to align and fine tune our free-space isolators. Normally, OFR aligns the isolator for horizontal input polarization. A customer can request a different input polarization orientation; please contact us directly for assistance in purchasing a device with this option.

Some of our isolators, depending on the body style, are normally shipped with the output polarizer fixed. If requested, we can ship the isolator with the output polarizers not fixed in place. Please contact us directly for more information on these devices, including how to align and optimize these units.

Note: If you have an LP model, do not place the isolator <6" from the laser. LP polarizers reject at 7° from the beam and the rejected feedback can strike the laser.

Initial Alignment

1. Remove the protective tape from the polarizer cells.
2. Place the isolator in the laser beam. Place a detector >8" from the output polarizer. Center the beam.
3. Loosen the setscrews on the polarizer cells.
4. Rotate the input polarizer until transmission is maximized, thus aligning it to the laser’s plane of polarization. Lock down its setscrew.
5. Rotate the output polarizer until transmission is maximized. This occurs nominally at 45° CW or CCW, depending on the model, relative to the input plane of polarization.

Fine Tuning to Optimize Isolation

1. Reverse the isolator, so the output polarizer faces the laser and the detector is a few inches from the input polarizer. Center the beam.
2. Adjust the output polarizer, now facing the laser, by 1° to 2° until transmission is minimized.
3. Lock down its setscrew.
4. Return the isolator to the operating position and center the beam.

The isolator is now aligned and ready for use.