EOT 500-1030nm LOW POWER FARADAY ISOLATOR USER'S GUIDE

Thank you for purchasing your Faraday Isolator from EOT. This user's guide will help answer questions you may have regarding the safe use and optimal operation of your Faraday Isolator.

TABLE OF CONTENTS

I. Faraday Isolator Overview
II. Safe use of your EOT Faraday Isolator
III. EOT Faraday Isolator
IV. Using your Faraday Isolator
V. Tuning your Faraday Isolator
VI. Warranty Statement and Repair
VII. Specifications

I. Faraday Isolator Overview

Your EOT Faraday Isolator is essentially a uni-directional light valve. It is used to protect a laser source from destabilizing feedback or actual damage from back-reflected light. Figure 1 below identifies the main elements of your Faraday Isolator.

Figure 1: EOT 500-1030nm Low Power Faraday

The Faraday Isolator is a cylindrically shaped magneto-optic device. Strong Neodymium Iron Boron permanent magnets are used to generate high (>10,000 Gauss) axially oriented fields within the magnet housing. The strong longitudinal field causes 45 degrees of non-reciprocal polarization rotation for propagating light via the Faraday Effect in the terbium gallium garnet ("TGG") crystal located within the magnet housing. In operation, the magnet housing is sandwiched between input and output polarizers that have their transmission axis oriented 45 degrees relative to each other to account for the 45 degrees of Faraday Rotation in the TGG crystal in the forward (transmission) direction. In the reverse (isolation) direction, the non-reciprocal Faraday rotation and the 45 degree polarizer transmission axis angle add so that the polarization transmitted by the output polarizer is rejected at the input polarizer.

Your EOT Faraday Isolator is labeled with a Serial Numbers on the base clamp of the device.

II. Safe use of your EOT Faraday Isolator

The operational hazards presented to operating personnel by the use of your EOT Faraday Isolator are listed below. An explanation of how the Faraday Isolator is designed together with procedures users can employ to eliminate or minimize these hazards is presented in italics.

1. Danger of sharp ferromagnetic objects being attracted to the residual permanent magnet fields outside of the Faraday rotator. This hazard is of most concern if such fields cause flying objects when being handled.

Your EOT Faraday Isolator requires strong internal magnetic fields to operate properly. Efforts have been made to minimize external fields from the device while still maintaining a relatively small and cost effective package. The external fields are designed to be well within Federal safety guidelines which limit external fields from magnetic devices to be less than 2KGauss at a radial distance of 5cm from the outside of the device. However, such fields can be sufficient to attract nearby objects such as knives and razor blades. Should attraction of such objects begin to occur there would be a strong attractive force directing these objects towards the interior of the magnet housing. This could be particularly likely to result in injury (e.g. a cut or puncture wound) if such attraction occurred while the device was being handled –particularly if a body part of the operating personnel is near a beam Aperture (i.e. end) of the device.

To minimize the above risks remove all loose ferromagnetic objects from the path over which your EOT Faraday Isolator is to be moved prior to attempting to move it. Do not pick up the isolator by its ends (i.e. apertures) where the attractive magnetic fields are strongest. Always pick the isolator up along its sides.

2. Failure of operating personnel to observe standard laser safety by sighting down through the Faraday Rotator when laser radiation is present.

The optical elements within the EOT Faraday Isolators can be transmissive throughout the visible and near infrared. Consequently it is never appropriate to view through the device in either the transmission or isolation direction when laser radiation is present –even with laser safety goggles.

Never sight through your EOT Faraday Isolator in either direction when there is any possibility of laser radiation being present.

3. Harm caused by external magnetic fields.

Your EOT Faraday Isolator has been designed to meet existing Federal safety guidelines for external fields as noted previously. Such guidelines could change in the future as more information becomes known or reviewed regarding the interaction between magnetic fields and human health. Since there exist various claims regarding the potential harmful (and beneficial!) effects of magnetic fields on humans it is prudent to limit interaction with these fields as much as possible.

Personnel with any magnetically sensitive implants such as pacemakers should present a copy of this report and consult their medical doctor regarding any potential complications which could arise from the isolator external magnetic fields.

4. Other non-health related hazards.

The Faraday Isolator external magnetic fields can draw ferromagnetic objects into the magnet housing which can damage the optical elements within the device. Keep a suitable area from the Faraday Isolator in all directions clear of any loose ferromagnetic objects. Ideally, use non-magnetic tools (such as stainless steel or titanium) and hardware to secure the Faraday Isolator. If only ferromagnetic tools are available use extreme care when using them around the Faraday Isolator. It is always helpful to bring such tools towards an aperture (or end) radially rather than along the optical beam path. Doing this ensures that the fields will tend to pull such objects into the magnet housing endplate rather than into the optical aperture. Where possible use two hands, one to hold the tool and the other to guide it to the desired destination.

Another concern regarding external magnetic fields is their effect on magnetically sensitive devices. The external fields are strong enough to induce a pulse of current in electronic devices (such as digital watches) that can destroy them. The fields can also disrupt the operation of other mechanical devices with ferromagnetic parts in them. Finally, the external fields can erase information from magnetic strips such as are found on credit and ID cards. Remove all magnetically sensitive materials and devices such as watches, computer hard drives and magnetic strips from operators prior to working in the proximity of an isolator.

III. The EOT 500-1030nm Low Power Faraday Isolators

Figure 2: View of Faraday Isolator and Tuning Features.

The low power Faraday Isolator uses dichroic sheet polarizers at both the input and output of the device. These broadband polarizers absorb the rejected transmission and are therefore only effective for low power applications. The inscribed arrow on the baseclamp displays the transmission direction. The output polarizer is seen to be oriented with its transmission axis rotated 45 degrees relative to the input polarization. The input polarization shown is vertical. The central magnet housing together with the TGG crystal residing in its center forms a Faraday Rotator. The Faraday Rotator rotates the input transmission axis by 45 degrees so that transmitted light has a polarization aligned with the output transmission axis. The input and output polarizers work in conjunction with the central Faraday Rotator to form a Faraday Isolator as described previously in Section I. Though the overall size of the device varies depending on the wavelength dependent model, the size and operation of the polarizer mounts are identical.

Figure 3: Horizontal Input Polarization

Figure 4: Vertical Input Polarization

Figure 3 shows a device aligned for a horizontal input polarization. The screw-hole located on the side of the polarizer mount indicates the approximate angular position of the polarizer transmission axis and maximum transmission is observed when the input polarization is aligned to this angle, the default orientation is horizontal unless customer specified.

Figure 4 shows a device aligned for a vertical input polarization, with the transmission axis rotated 90 deg with respect to that depicted in figure 3.

The second isolator available for 500-1030nm operation is a medium power version that uses polarizing beam splitting cubes and therefore may be operated at higher power levels and the rejected beams may be directed out of the device for analysis if desired. Refer to our document "EOT 500-1030nm MEDIUM POWER FARADAY ISOLATOR
USER'S GUIDE" for further information.

IV. Using your Faraday Isolator

Observe the guidelines for safe use of your Faraday Isolator found in Section II above when removing your isolator from its shipping container. Do not remove the protective dust-cover endcaps from the polarizers until the device is in a clean, relatively dust free environment. Save the protective endcaps, packaging material and containers in the event that the device should ever need to be returned to EOT.

Verify that the Input and Output polarization states are consistent with the intended mode of operation as described by the Purchase Order Model Number. If not, either send the device back to EOT (see Section VI) or, if desired, re-adjust the isolator as required (see Section V).

With the source laser off, or running at very low power (less than 100mW), position the Faraday Isolator such that the source laser beam can be directed through the Input Aperture.

Critical alignment of the Faraday Isolator should be done at low power (less than 100mW) in order to prevent optical damage to your isolator or laser source.

Use IR cards or viewers to ensure that the source laser beam is centered on the input and Output Apertures. The clear aperture of this device is 4mm, centered on the circular cross section of the magnet body. There are different mounting options for establishing appropriate beam height. It is also preferable to use an IR viewer to ensure that weak reflections from AR coated optical surfaces in the Faraday Isolator are not being directed back into the source laser. The Optical surfaces in the Faraday Isolator are angled slightly to reduce these reflections. Increasing the distance between the Faraday Isolator and the source laser can also help ensure that no reflections couple back into the source laser if necessary. Alternatively, if the beam used is smaller than the aperture of the device by a reasonable margin, the device may be slightly tilted.

At this point, the Faraday Isolator should be secured to the work surface with two (2) ¼ - 20 or M6 screws –one for each slot in the baseplate flanges. Alternatively, the baseplate may be removed from the baseclamp by removing two 8-32 screws on the bottom side of the baseplate. Then, the baseclamp/isolator assembly may be mounted to a standard laboratory post with an 8-32 set screw or may be conveniently mounted into laser systems, minimizing the required footprint of the device. Steel (ferromagnetic) ball drivers or other such wrenches will be attracted to the external magnetic field surrounding the device. If possible use anti-magnetic stainless steel or titanium tools. If ferromagnetic tools are used it is desirable to introduce them slowly towards the device from the sides along the direction of the baseplate flange slots.

One important consideration is that this isolator is intended to be used at low power. The device is very compact given the physical size of the polarizers used in its construction, however, one drawback of these is that the rejected power is absorbed in the polarizers themselves and therefore damage thresholds are limited to 25W/cm^2 CW.

V. Tuning your Faraday Isolator

A. Adjusting Input Polarization

The transmission axis of the input polarizer is indicated by the radial position of the screw hole located on the input polarizer mount. If the linear polarization of the laser source is geometrically known, aligning the input polarization of the Faraday Isolator to that of the laser source is straightforward. Simply loosen the #2-56 socket head Baseclamp Screw in the Baseclamp until the magnet housing rotates freely. Continue to rotate the magnet Housing until the Input Polarizer transmission axis is aligned to that of your laser source. Following alignment procedures, rotating the magnet body inside the clamp until maximum transmission is observed through the isolator will indicate that the input polarization is parallel to the input polarizer axis, then re-tighten the Baseplate Clamp Screw.

B. Fine Wavelength Adjustment

Each of the 650, 780, 850, and 980 nm isolators may be tuned over a wavelength range specified in section VII. Tuning is achieved by adjusting the relative angle between the input and output polarizers. For wavelengths longer than the central wavelength, the faraday rotation is less than 45 deg., for wavelengths shorter than the central wavelength, the faraday rotation is more than 45 deg. and therefore, the device may be tuned for maximum extinction with a small transmission loss, illustrated by the wavelength tuning curves in section VII. The 3dB bandwidth of the isolators ranges from 12 nm at 650 nm to 20 nm at 980nm, in increasing value. For maximum extinction, manually tune the device to the appropriate wavelength.

With the source laser operating at an average power of 100 mW or less (attenuate the beam if necessary to achieve such a low power level) direct the source laser beam through the Faraday Isolator in the reverse direction –through the Output Polarizer first and then through the Input Polarizer. Use an IR viewer to view the transmitted radiation to ensure that it is directed onto a Power Meter. The Power Meter should be sensitive enough to detect power levels below 0.01mW (or 40dB of the input signal used). As a reference, 40 dB is a factor of 1:10000 and 10dB is a factor of 1:10. If necessary, loosen the Baseplate Clamp Screw to allow the Output Polarizer transmission axis to be rotated parallel to the source laser polarization axis, re-tighten this screw when complete.

Loosen the button head Input Polarizer Clamp Ring screws just enough so that the Input Polarizer Mount may be rotated (the Input Polarizer is opposite to the laser source at this point). Rotate the Input Polarizer Mount until a minimum reading is indicated on the Power Meter. Re-tighten the Input Polarizer Clamp Ring Screws. The minimum reading should be at least 30dB (1:1000) of the input signal. If not, call EOT for assistance(see Section VI below). The Faraday Isolator is now optimized to operate at the new laser source operating wavelength. It may now be installed for operation in transmission with the laser source as per Section IV and the procedure outlined in Section V A.

C. Waveplate Option and Adjustment

As described in Section III above in the "Numbering Fields and Coordinate System" description it is possible to order a Faraday Isolator with a half-waveplate on the output. Should any of the above adjustments become necessary, or if the desired output polarization changes, the waveplate will need to be adjusted. To re-align the waveplate loosen the radially oriented set-screw in the output polarizer mount and rotate the waveplate until the desired output polarization is achieved. Re-tighten the waveplate set-screw. Do not over-tighten. A waveplate may also be used in the input of the device if necessary.

VI. Warranty Statement and Repair

EOT warrants its Faraday rotators/isolators to be free of defects in materials and workmanship for a period of one year after date of shipment. Any unauthorized modifications made by the customer to EOT's Faraday rotators/isolators will render the warranty null and void. If the customer believes there is a problem with the rotator/isolator, they should immediately contact EOT's Sales/Customer department at 231-935-4044 or sales@eotech.com. EOT's customer service department will either issue an RMA for the device, or provide the customer with a procedure and authorize the customer do modify the device. All returns reference the RMA No. on the outside of the shipping container and should be sent to:

Electro-Optics Technology, Inc.
Attn: Sales/Customer Service
5835 Shugart Lane
Traverse City, MI. 46984 USA

EOT reserves the right to inspect rotators/isolators returned under for warranty to assess if the problem was caused by a manufacturer defect. If EOT determines the problem is not due to a manufacturer defect (an example would be damage to an optical element caused by impact from a loose balldriver or exceeding the damage threshold of the device), repairs will be done at customer expense. EOT will always provide a written or verbal quote prior to peroforming repairs at customer expense. Never attempt to disassemble the magnetic housing of your Faraday rotator/isolator. Injury could result. Any indications that an attempt to disassemble the magnetic housing was made will render the warranty null and void.

VII. Specifications: 500-1030 nm Low Power Faraday Isolators

Two-Stage Option
Faraday Isolators may be used in series to obtail 60+dB isolation, mounted on a common baseplate.

Mounting Options
Magnet housings are inserted into a clamping device that provides 8-32 screw holes and may be mounted onto a standard post. All devices include a base for mating with 1/4-20 screw holes 1" spaced breadboard. Figure 5 below shows the isolator with the baseplate removed and Figure 6 shows the isolator mounted to a standard lab post.

Figure 5: Baseplate Removal

Figure 6: Mounted to Lab Post

Notes

1. Transmission: This is measured from the center wavelength, tunable transmission will be wavelength dependent, please consult the spectra below.
2. Damage Threshold: Damage threshold is limited by cemented optics and broadband AR coatings for wavelength tunability, for higher damage threshold applications, please contact EOT.
3. Operating Temperature: Performance of EOT's Faraday rotators/isolators is related to operating temperature. For information on the effect of operating temperature on EOT's Faraday rotators/isolators, please review our technical bulletin, Effects of Temperature on EOT's Faraday Rotators/Isolators.
4. Center Wavelength: This refers to the wavelength specified for 45 +/-2 deg and is not necessarily the median of the wavelength range.
5. Tunable Transmission: The below graphs show the transmission of the device following tuning as per instructions in section V B.