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EOT 500-1030nm MEDIUM 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 Medium Power Faraday Isolator
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.
- 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.
- Reflection of rejected beams from the input and output polarizer.
The polarizer covers have been rotated at the factory to block all beams rejected from the polarizers. In the event that your Faraday Isolator will be used with transmitted average powers in excess of 25W, or will block backward propagating light in excess of 0.5W average power, these polarizer covers must be rotated to allow rejected beams to exit through the Escape Ports (see Figure 1) onto user supplied beam dumps. These rejected beams can represent a hazard to users and/or their colleagues. Care must be exercised to ensure that all rejected beams (both transmission and isolation directions) are accounted for and terminated into functional beam dumps. Wherever possible keep the strongest rejected beams in the horizontal plane of the table or otherwise safest direction (typically down into the table). Always wear laser safety glasses/goggles consistent with all laser frequencies and power levels present. See Sections III and IV for further details.
- 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.
- 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.
- 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 Faraday Isolator
Figure 2: View of Faraday Isolator and Tuning Features.
With the polarizer cover off a polarizing beamsplitter cube ("PBSC") can be seen inside the input polarization MountThe inscribed arrow on the baseclamp shows the transmission direction. The output PBSC is seen to be oriented with its transmission axis rotated 45 degrees relative to the input PBSC. The input polarization shown is horizontal. The central magnet housing together with the TGG crystal residing in its center forms a Faraday Rotator. The Faraday Rotator rotates the input horizontal transmission axis by 45 degrees so that transmitted light has a polarization aligned with the output transmission axis. The input and output PBSC's 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 PBSC mounts are identical.
Figures 3 and 4 below show Input views of an EOT 500-1030nm Faraday Isolator.
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 PBSC mount indicates the direction of polarization and the direction of the rejected beam. This is the beam that is reflected off of the input polarizer, having originating at the output of the device, traveling in the reverse direction, opposite the direction of the arrow.
Figure 4 shows a device aligned for a vertical input polarization.
The second isolator available for the 500-1030nm is a low power version that uses dichroic polarizers. The rejected polarization is absorbed in the polarizer and therefore, there is no rejected beam and the device is shorter in length. Please consult our document "EOT 500-1030nm LOW POWER FARADAY ISOLATOR USER'S GUIDE".
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 250mW), 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 250mW) 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. 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.
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.
If the Faraday Isolator will be used with average powers in excess of 12W transmitted or 0.5W rejected backward propagating radiation (please consult damage threshold specifications for operating range) the Polarizer Covers will need to be rotated so that the Escape Ports allow rejected polarization light to be safely dumped onto a beam dump. Failure to allow these rejected polarizations to escape can cause the device to heat up. Such heat can degrade the performance of the Faraday Isolator, or in severe cases, cause damage to optical components in the isolator. While working with low alignment level power and wearing safety glasses, physically grasp the Polarizer Cover and rotate it by 90 degrees. Any rejected polarized beams (in either the forward or backward propagating directions) can now exit the Polarizer Cover. Use an IR viewer or IR card to locate these beams. Ensure that they are terminated on beam dumps consistent with the maximum amount of power that may be in such termination points. In addition to high rejection (>27dB) of any un-desired linear polarization component in transmission, the Input and Output PBSC may reflect as much as 3% of the desired tranmitted polarization. Backward propagating rejected beams will exit from the PBSC from the side of the PBSC mount containing the screw hole (on the input polarizer). Forward transmission rejected beams will exit from the other side of the PBSC and are minimized when the input beam is polarized parallel to the input polarizer. If the Faraday Isolator is used in application where strong reflections and/or optical gain elements (amplifiers) exist there may be very high power rejected beams for backward propagating light at the input polarizer. If the average power levels used do not exceed 12W transmitted or 0.5W of backward propagating power then the Polarizer Covers may be kept in their factory positioned orientation –that is, with all rejected beams blocked by the Polarizer Cover. However, if the Faraday Isolator is to be used with very high peak intensities it is prudent to allow rejected beams to escape on to external beam dumps to prevent any ablation damage to the Nickel plated Polarizer Covers. Follow the same procedure above as for high average powers in order to safely terminate all rejected beams. Note that the direction of the screw hole on the output polarizer indicates the direction of the component of rejected power from a backward propagating beam that is not parallel to the output polarizer.
V. Tuning your Faraday Isolator
- 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. This is most easily accomplished by observing the beam exiting the escape port on the opposite side of the screw hole on the mount. Recall that the direction of a rejected beam entering in the reverse direction through the output will exit in the direction of the screw hole on the polarizer mount. As always, use caution when allowing rejected beams to escape from the Polarizer Covers. Confirm the direction and safe termination of these beams at low power. Wear laser Safety Glasses or Goggles. Protect your colleagues by safely terminating beams and ensuring that they are wearing Safety Glasses/Goggles as appropriate. At the desired operating power level minimize the power in the rejected input polarization with a power meter or other suitable power indicator. When the rejected power is minimized this corresponds to maximum transmission, re-tighten the Baseplate Clamp Screw.
- Fine Wavelength Adjustment
Each of the 532, 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 26nm at 532nm to 46nm 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 0.5W 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 0.05 mW (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 minimal reading is indicated on the Power Meter. Re-tighten the Input Polarizer Clamp Ring Screws. The minimal reading should be at least 27dB 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.
- Changing the direction of Output Polarizer PBSC Rejected Beam Steering
Section A above described how to orient the Input Polarizer PBSC to steer rejected beams as desired. If the Output Polarizer rejected beam steering direction needs to be changed, loosen the three Output Polarizer Clamp Screws and rotate the Output Polarizer Mount by 180° before re-tightening the Screws. Depending upon the isolation requirements, follow Procedure B from above in order to fully optimize the isolation of the device with this Output Polarizer orientation.
- 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
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 Medium Power Faraday Isolators
Figure 5: Following Removal of Baseplate
Figure 6: Mounted to Lab Post
Notes:
- Transmission: This is measured from the center wavelength, tunable transmission will be wavelength dependent, please consult the spectra below. These values are based on absolute minimum allowable transmission and on average will be >90%.
- Damage Threshold: Damage threshold is limited by cemented optics and broadband AR coatings for wavelength tunability, for higher damage threshold applications, please contact EOT.
- 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.
- Center Wavelength: This refers to the wavelength specified for 45 +/-2 deg and is not necessarily the median of the wavelength range.
- Tunable Transmission: The below graphs show the transmission of the device following tuning as per instructions in section V B.
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