Polaris® Kinematic Mirror Mounts with Piezoelectric Adjusters

Features

• Machined from Heat-Treated Stainless Steel with Low Coefficient of Thermal Expansion (CTE)
• Fully Integrated Piezoelectric Elements Provide Step Sizes Down to 0.5 µrad
• Matched Actuator and Back Plate Provide Smooth Manual Kinematic Adjustment
• Hardened Stainless Steel Ball Contacts with Sapphire Seats for Durability and Smooth Movement
• Passivated Stainless Steel Surface Ideal for Vacuum and High-Power Laser Cavity Applications
• Extensive Testing Guarantees Less than 6 μrad Deviation After 15 °C Temperature Cycling (See Test Data Tab)
• Custom Mount Configurations are Available by Contacting Tech Support

The Polaris^® Kinematic Mirror Mounts with Piezoelectric Adjusters are the ultimate solution for applications requiring stringent, actively monitored, long-term alignment stability. We recommend driving the piezo actuators using our benchtop or Kinesis^® K-Cube™ piezo controllers.

Replacement piezoelectric adjusters for the mounts are also available separately below.

Optic Retention
These Polaris mounts use either a monolithic flexure arm or a flexure spring and setscrew combination to hold the optic. These designs provide high holding force and pointing stability with minimal optic distortion. For more details, see the Test Data tab.

Polaris optic bores are precision machined to achieve a fit that will provide optimum beam pointing stability performance over changing environmental conditions such as temperature changes, transportation shock, and vibration. These mounts are not designed to be used with optics that have an outer diameter tolerance greater than +0/-0.1 mm, such as metric mirror sizes (Ø12.5 mm, Ø25 mm, or Ø50 mm). To order a mount designed for metric optics, please contact Tech Support.

Design
Machined from heat-treated stainless steel, Polaris mounts utilize precision-matched adjusters with ball contacts and sapphire seats to provide smooth kinematic adjustment. As shown on the Test Data tab, these mounts have undergone extensive testing to ensure high-quality performance. The Polaris design addresses all of the common causes of beam misalignment; please refer to the Design Features tab for more information.

Our design incorporates high-adjustment-sensitivity piezoelectric stacks directly into the hardened stainless steel adjusters, enabling smooth coarse and fine movements along each axis. When used in an external feedback loop, as illustrated on the Applications tab, these mounts combine the exceptional thermal stability of our standard Polaris kinematic mirror mounts with the ability to actively correct the beam pointing. Like all Polaris mounts, they are machined from heat-treated stainless steel, utilize precision-matched adjusters, and incorporate ball contacts and sapphire seats at all contact points.

Each Polaris piezo mount utilizes snap-on SMB connectors. This model is specifically designed to minimize cable twisting as the mount is actuated. Furthermore, the design of SMB cables prevents incidental movement of the mirror when the cord is attached. One 36" SMB-to-BNC cable per axis is included with each piezo mount sold below. If different cable lengths or connctors are desired, additional SMB cable configurations can be purchased separately below. Please note that the piezo actuators do not include a strain gauge.

Post Mounting
The Polaris mirror mounts are equipped with #8 (M4) counterbores for post mounting. Select mounts also include Ø2 mm alignment pin holes around the mounting counterbore, allowing for precision alignments when paired with our posts for Polaris mirror mounts. See the Usage Tips tab for more recommendations about mounting configurations.

Vacuum Compatibility
All Polaris mounts on this page are designed to be compatible with cleanroom and vacuum applications. See the Specs tab and the Design Features tab for details.

Polaris^® Mirror Mount Test Data

Polaris Kinematic Mirror Mounts undergo extensive testing to ensure high-quality performance.

After mounting the Polaris to a Ø1" stainless steel post, the mirror and post assembly was secured to a stainless steel optical table in a temperature-controlled environment. The mirror was secured using the flexure spring, not glued; see the Usage Tips tab for additional mounting recommendations. A beam from an independently temperature-stabilized laser diode was reflected by the mirror onto a position sensing detector.

Angular Tuning Range of Piezoelectric Adjusters

Purpose: This test was done to determine the full adjustable range of the piezoelectric stack inside the adjuster screw.

Procedure: Each axis was connected to one of our previous-generation KPZ101 Single-Channel K-Cube Piezo Controllers (set in manual mode) or to a single channel on our MTD693B Three-Axis Benchtop Controller. The voltage was increased from 0 to 150 V, the maximum control voltage, in 5 V steps (blue line). Then the voltage was decreased from 150 V to 0 V, again using 5 V steps (red line).

Results: The maximum deflection that can be imparted by the piezoelectric stack is shown in the plots in the expandable tables below. This maximum deflection is >490 µrad for the Ø1/2" mount, >500 µrad for the Ø1" mounts, and >280 µrad for the Ø2" mounts. The well-known hysteresis response of piezoelectric materials is also evident. That is, the displacement at a given voltage depends upon whether the applied voltage is increasing or decreasing.

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Positional Repeatability After Thermal Shock

Purpose: This testing was done to determine how reliably the mount returns the mirror to its initial position so that the alignment of the optical system is unaffected by the temperature shock.

Procedure: The ambient temperature was raised by 15 °C over a period of at least 45 minutes. Then the temperature was allowed to return to near room temperature. During these tests, the piezo actuators were not connected to a voltage source. The worst-case results obtained by this method are shown below.

Results: The thermal shock data for each Polaris mount is shown in the plots in the expandable tables below. As shown in the plots, when the Polaris mount was returned to its initial temperature, the mirror position returned to within 6 µrad of its initial position for the Ø1/2" mount, within 1 µrad for the Ø1" mounts, and within 2 µrad for the Ø2" mount.

For Comparison: To get a 1 µrad change in the mount's position, the 100 TPI adjuster on the Ø1/2" Polaris mount needs to be rotated by only 0.025° (1/14400 of a turn). A highly skilled operator might be able to make an adjustment as small as 0.6° (1/1200 of a turn), which corresponds to 12 µrad.

Conclusions: These Polaris mirror mounts are high-quality, ultra-stable mounts that will reliably return a mirror to within several µrad of its original position after cycling through a temperature change in an open-loop setup. With a large adjustable range on each axis and a typical step size of 0.5 µrad (for a 0.1 V change in applied voltage), the Polaris mounts are capable of providing sub-µrad alignment stability in a closed-loop setup, even under punishing experimental conditions.

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Click to Enlarge The plot above shows the final angular position of a second POLARIS-K05P2 for 20 consecutive thermal shock tests. The change in temperature is the difference between the starting temperature and the temperature at the end of the test and includes factors such as the variation in room temperature. This shows the repeatability of the tests.

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Click to Enlarge The plot above shows the final angular position of a second POLARIS-K1S3P for 10 consecutive thermal shock tests. The change in temperature is the difference between the starting temperature and the temperature at the end of the test and includes factors such as the variation in room temperature. This shows the repeatability of the tests.

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Click to Enlarge The plot above shows the final angular position of a second POLARIS-K2S2P for 10 consecutive thermal shock tests. The change in temperature is the difference between the starting temperature and the temperature at the end of the test and includes factors such as the variation in room temperature. This shows the repeatability of the tests.

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Polaris Kinematic Mirror Mount as Part of a Closed-Loop System

Polaris^® Mirror Mount in a Beam Stabilization Setup

Active beam stabilization is often used to compensate for beam drift (unintended beam pointing deviations) in experimental setups. Drift can be caused by insecurely mounted optics, laser source instabilities, and thermal fluctuations within an optomechanical setup. In addition to correcting for setup errors, active stabilization is frequently used in laser cavities to maintain a high output power or used on an optical table to ensure that long measurements will take place under constant illumination conditions. Setups with long beam paths also benefit from active stabilization, since small angular deviations in a long path will lead to significant displacements downstream.

An example of a beam stabilization setup is shown in the schematic to the left. A beamsplitter inserted in the optical path sends a sample of the beam to a position sensor that monitors the displacement of the beam relative to the detector's center. (For optimal stabilization, the beamsplitter should be as close as possible to the measurement.) The position detector outputs an error signal in X and Y that is proportional to the beam's position. Each error signal is fed into a channel of a piezoelectric controller that steers the beam back to the center of the sensor.

The setup illustrated here stabilizes the beam to a point in space. In order to stabilize the beam over a beam path (i.e., over two points in space), two piezoelectric mirror mounts, as well as the associated electronics, are required. Suggested electronics for a beam stabilization setup are given in the table below.

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Details of the Polaris Design. The POLARIS-K05P2 uses a flexure spring and setscrew combination instead of the monolithic retention arm shown here.

Several common factors typically lead to beam misalignment in an optical setup. These include temperature-induced hysteresis of the mirror's position, crosstalk, drift, and backlash. Polaris mirror mounts are designed specifically to minimize these misalignment factors and thus provide extremely stable performance. Hours of extensive research, multiple design efforts using sophisticated design tools, and months of rigorous testing went into choosing the best components to provide an ideal solution for experiments requiring ultra-stable performance from a kinematic mirror mount.

Thermal Hysteresis
The temperature in most labs is not constant due to factors such as air conditioning, the number of people in the room, and the operating states of equipment. Thus, it is necessary that all mounts used in an alignment-sensitive optical setup be designed to minimize any thermally induced alignment effects. Thermal effects can be minimized by choosing materials with a low coefficient of thermal expansion (CTE), like stainless steel. However, even mounts made from a material with a low CTE do not typically return the mirror to its initial position when the initial temperature is restored. All the critical components of the Polaris mirror mounts are heat treated prior to assembly since this process removes internal stresses that can cause a temperature-dependent hysteresis. As a result, the alignment of the optical system will be restored when the temperature of the mirror mount is returned to the initial temperature.

The method by which the mirror is secured in the mount is another important design factor for the Polaris; these Polaris mounts offer excellent performance without the use of adhesives. Instead, they use a monolithic retention arm or a flexure spring that is pressed onto the edge of the mirror using a setscrew. Setscrews, when used by themselves to hold an optic, tend to move as the temperature changes. In contrast, the holding force provided by the Polaris design is sufficient to keep the mirror locked into place regardless of the ambient temperature.

Crosstalk
Crosstalk is minimized by carefully controlling the dimensional tolerances of the front and back plates of the mount so that the pitch and yaw actuators are orthogonal. In addition, sapphire seats are used at all three contact points. Standard metal-to-metal actuator contact points will wear down over time. The polished sapphire seats of the Polaris mounts, in conjunction with the hardened stainless steel actuator tips, maintain the integrity of the contact surfaces over time.

Drift and Backlash
In order to minimize the positional drift of the mirror mount and backlash, it is necessary to limit the amount of play in the adjuster as well as the amount of lubricant used. When an adjustment is made to the actuator, the lubricant will be squeezed out of some spaces and built up in others. This non-equilibrium distribution of lubricant will slowly relax back into an equilibrium state. However, in doing so, this may cause the position of the front plate of the mount to move. The Polaris mounts use adjusters matched to the body that exceed all industry standards so very little adjuster lubricant is needed. As a result, alignment of the Polaris mounts is extremely stable even after being adjusted (see the Test Data tab for more information). In addition, these adjusters have a smooth feel that allows the user to make small, repeatable adjustments.

Integrated Piezoelectric Stacks
Each adjuster screw incorporates a piezoelectric stack inside the body of the actuator, seated directly behind the tip. This in-line design keeps the number of ball contact points to a minimum, reduces the number of moving parts, and ensures that the low crosstalk guaranteed by our precision-machined components is not compromised. Whether being moved by hand or by the piezoelectric stack, the adjusters make use of the same sapphire-to-hardened-steel contact points to provide precise motion for the life of the mount.

Compatible with Vacuum Environments
Each component has been carefully selected to ensure vacuum compatibility. The epoxy used to bond the sapphire seats in place is baked using a NASA-approved low-outgassing procedure. DuPont LVP High-Vacuum (Krytox) Grease, an ultra-high-vacuum-compatible, low-outgassing PTFE grease is used with the adjusters. The piezoelectric stack is sheathed by a thin, low-solvent-content acrylic material that supports vacuum environments down to at least 10^-5 Torr. For the piezo connector, we chose PTFE, a non-hygroscopic plastic, as the insulating layer. All electrical contacts are made by a low-flux solder; the flux is burned off during the soldering process.

Please note that a high-voltage, coaxial feedthrough vacuum flange is required to pass the piezo control voltage into vacuum.

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Optic distortion of a BB1-E02 mirror mounted in a Ø1" Polaris mount. At zero torque, the sample mirror's flatness was λ/10 over the clear aperture
(λ = 633 nm). The shaded region indicates the recommended amount of torque.

Through thermal changes and vibrations, the Polaris kinematic mirror mounts are designed to provide years of use. Below are some usage tips to ensure that the mount provides optimal performance.

General Usage Tips

Match Materials
Due to its relatively low coefficient of thermal expansion, stainless steel was chosen as the material from which to fabricate the Polaris mount. When mounting the Polaris, we recommend using components fabricated from the same material.

Use a Wide Post
The Polaris' performance is optimized for use with a Ø1" post for Polaris mounts. These posts are made of heat-treated stainless steel and provide two planes of contact with the mount, which help confine the bottom of the mount during variations in the surrounding temperature, thereby minimizing potential alignment issues.

Optic Mounting
Since an optic is prone to movement within its mounting bore, all optics should be mounted with the Polaris out of the setup to ensure accurate mounting that will minimize misalignment effects. We recommend using a torque wrench when installing an optic in a Polaris mount. Over-torquing the flexure arm optic retainer can result in dramatic surface distortions. The graph to the right shows surface distortions that result from increasing torque values delivered by the TD24 Torque Wrench for the front plate used in the POLARIS-K1S3P and POLARIS-K1S2P with a Ø1", 6 mm thick BB1-E02 mirror mounted in it. The test was stopped once the distortion was greater than 0.2 waves.

Front Plate's Position
To achieve the best performance, it is recommended that the front plate be kept as parallel as possible to the back plate. This ensures the highest adjustment stability.

Mount as Close to the Table's Surface as Possible
To minimize the impact of vibrations and temperature changes, it is recommended that your setup has as low of a profile as possible. Using short posts will reduce the Y-axis translation caused by temperature variations and will minimize any movements caused by vibrations.

Polish and Clean the Points of Contact
We highly recommend that the points of contact between the mount and the post, as well as the post and the table, are clean and free of scratches or defects. For best results, we recommend using a polishing stone to clean the table’s surface and a polishing pad (LFG1P) for the top and bottom of the post as well as the bottom of the mount.

Piezo Usage Tips

Disconnect Unused Channels
If active stabilization of a given axis is not needed, then it is not necessary to connect the unused axis to a piezoelectric controller. Any noise on the electrical connection may cause an undesired movement in the beam.

Connect Cables only when Power Supply is Off
The piezoelectric stacks may be irreversibly damaged by abrupt changes in the applied voltage. In order to prevent such damage, ensure that the power supply is off before connecting the SMB cable.

Use Gentle Voltage Steps
Abrupt charging or discharging of the piezoelectric actuator may cause irreversible damage to the piezo.

Stay within the 0 - 150 V Control Voltage Range
Applying voltages less than 0 V (negative voltages) or greater than 150 V may cause irreversible damage to the piezo.

Handle with Care
Piezoelectrics are ceramic materials, and therefore relatively brittle as compared to common optomechanics. Hence, they are sensitive to shock forces. Do not drop the mount or place it in a box without protective cushioning.

Not Recommended

We do not recommend taking the adjusters out of the back plate, as it can contaminate the threading. This can reduce the fine adjustment performance significantly. Also, do not pull the front plate away as it might stretch the springs beyond their operating range, crack the sapphire seats, or break the piezoelectric stacks. Finally, do not overtighten the retaining screws that secure the flat spring that holds the optic in place; only slight force is required to secure the optic in place.

Thorlabs offers several different general varieties of Polaris mounts, including kinematic side optic retention, SM-threaded, low optic distortion, piezo-actuated, vertical drive, and glue-in optic mounts, a fixed monolithic mirror mount and fixed optic mounts, XY translation mounts, 5-axis kinematic mount, and a kinematic platform mount. Refer to the tables below for our complete line of Polaris mounts, grouped by mount type, optic bore size, and then arranged by optic retention method and adjuster type (or intended application in the case of fixed mounts). We also offer a line of accessories that have been specifically designed for use with our Polaris mounts; these are listed in the table to the lower right. Note that the tables below list Item # suffixes that omit the "POLARIS" prefix for brevity. Click the photos below for details.

Polaris Mount Optic Retention Methods
Side Lock	SM Threaded	Low Distortion	Glue-In

Polaris Mount Adjuster Types
Side Hole	Hex	Adjuster Knobs	Adjuster Lock Nuts	Piezo Adjusters	Vertical-Drive Adjusters

Polaris XY Translation Mounts for Round Optics
Optic Retention Method	SM Threaded	Representative Photos
Ø1/2" Optics
2 Hex Adjusters (X/Y)	-05XY
3 Adjuster Knobs (Tip/Tilt/Z) & 2 Hex Adjusters (X/Y)	-K05XY
Ø1" Optics
2 Hex Adjusters (X/Y)	-1XY
3 Adjuster Knobs (Tip/Tilt/Z) & 2 Hex Adjusters (X/Y)	-K1XY
Ø1.5" Optics
2 Hex Adjusters (X/Y) & 3 Adjuster Knobs (Tip/Tilt/Z)	-K15XY

Polaris Fixed Mounts for Round Optics
Optic Retention Method	Side Lock	Low Distortion	Glue-In	Representative Photos
Ø1/2" Optics
Optimized for Mirrors	-	-B05F	-C05G
Optimized for Beamsplitters	-B05S	-	-B05G
Optimized for Lenses	-	-	-L05G
Ø19 mm (Ø3/4") Optics
Optimized for Mirrors	-19S50/M	-	-
Ø1" Optics
Optimized for Mirrors	-	-B1F	-C1G
Optimized for Beamsplitters	-B1S	-	-B1G
Optimized for Lenses	-	-	-L1G
Ø2" Optics
Optimized for Mirrors	-	-B2F	-C2G
Optimized for Beamsplitters	-B2S	-	-

Polaris Kinematic 1.8" x 1.8" Platform Mount
Optomech Retention Method	Tapped Holes & Counterbores
2 Adjuster Knobs	-K1M4(/M)

Accessories for Polaris Mounts
Description	Representative Photos
Ø1/2" Posts for Polaris Mounts
Ø1" Posts for Polaris Mounts
Non-Bridging Clamping Arms
45° Mounting Adapter