Alignment procedures for optical breadboard assemblies#

Aligning optical components to a pre-existing laser beam path#

Aligning an optical (Faraday) isolator#

Readout: Laser power transmitted by the device in forward and reverse direction.

Alignment goal: Maximum transmission in forward direction (input polarizer, wavelength tuning); 45° rotation (if adjustable); minimum transmission in reverse direction (output polarizer).

Degrees of freedom: X, Y, yaw, roll (pitch can usually be fixed, assuming beam parallel to table); angle between polarizers; rotation angle (some models)

These generic instructions assume a polarization-dependent Faraday isolator that rotates the polarization by 45°. Not all optical isolators are designed this way, so make sure to check the manufacturer documentation.

These optical isolators consist of a Faraday rotator flanked by two polarizers (often polarizing beam splitters). One or both of the polarizers may be rotated (roll axis) relative to the Faraday rotator; the roll angle of the Faraday rotator itself does not affect alignment.

Some optical isolators (the “adjustable” or “tunable” type) have an additional adjustment for the angle of rotation by the Faraday rotator; this can be used to tune the isolator for specific wavelengths (because the angle of rotation is wavelength-dependent).

The following procedure assumes that fine adjustment of the X, Y, pitch, and yaw is not necessary, due to the device’s aperture being large enough compared to the laser beam. If the beam diameter is closer to the aperture size, it may be necessary to have fine and independent adjustments for these DoFs (for example, use a pair of kinematic mirrors before the isolator to stear the beam).

  • Set up a power meter on the output side of where the isolator will be placed.

  • Place the isolator in the beam path, in the forward orientation, such that the beam is not clipped (X, Y, (pitch,) yaw).

  • With the output polarizer removed, adjust the roll of the input polarizer (this may be the roll of the whole device if the input polarizer is not independently adjustable; in this case there is no need to remove the output polarizer) for maximum power transmission.

  • Set the roll of the output polarizer to be at 45° to the input polarizer. (Of the two possible 45° positions, choose the one resulting in higher transmission; however, set it to 45°, not the point of highest transmission.)

  • If the isolator has a tuning adjustment, adjust it for maximum transmission. (This should set the Faraday rotator’s angle of rotation to be 45°, which is required for isolation behavior.)

  • Flip the isolator to the reverse orientation (but ensure that the laser passes through the device).

  • Fine-adjust the roll of the output polarizer for minimum power transmission (that is, maximum extinction).

  • Return the isolator to the forward orientation (and ensure that the beam is centered through the device).

  • Set up beam dumps to block the rejected light exiting from the two polarizing beam splitters.

Aligning an EOTech broadband Faraday isolator#

Readout: Laser power transmitted by the device in forward direction.

Alignment goal: Maximum transmission in forward direction.

Degrees of freedom: X, Y, yaw (for pitch and roll, we rely on the mechanical accuracy of the device and mount)

Unlike standard Faraday isolators that rotate the polarization by 45°, EOTech braodband isolators designed for Ti:Sapphire lasers (BB8 or BB9 series) have a fixed input polarization at 0° (horizontal) and output polarization at 90° (vertical).

For these devices, there is no need to adjust the polarizers; alignment consists simply of positioning the device such that the laser beam passes through the device without being clipped.

Aligning an electro-optic modulator (EOM)#

Readout: Laser power transmitted by the device.

Alignment goal: Maximum extinction ratio of laser power through the device.

Degrees of freedom: X, Y, pitch, yaw, roll; bias voltage

Note: Alignment procedure will likely differ depending on the manufacturer of the device. For now we describe alignment for Conoptics EOMs. Further research is needed to derive aspects of alignment (if any) that are common to other EOMs.

Aligning a Conoptics EOM#

Here we describe how to align a Conoptics 350-80 EOM to a pre-existing optical axis defined by a laser beam. We assume an EOM configured for amplitude modulation and with minimal transmission near zero bias voltage.

Make sure to read the manufacturer manual first, as incorrect handling can damage the EOM. The (linear) polarization angle of incoming laser beam must be known, at least approximately.

  • Set up a power meter on the output side of where the device will be placed.

    • With the device out of the beam path, align the power meter to the beam; note down the measurement.

  • Place the alignment tool (Conoptics Model 103) on the EOM mount.

  • Initialize the mount’s alignment screws (see mount instructions).

  • Coarse-adjust and place the mount such that the beam passes through the alignment tool; clamp down to table (see mount instructions).

    • For this step, it may be easier to place an alignment screen in front of the power meter and observe the beam profile.

    • Goal of coarse alignment: beam should be minimally clipped by the alignment tool.

  • Using the power meter, fine-adjust the X, Y, pitch, and yaw for maximum power transmission (see mount instructions).

  • With the laser off, swap the alignment tool for the actual EOM, noting correct direction (hole for rejected beam on the output end).

  • Roughly set the roll angle of the device such that the crystal axes are at 45 degrees to the incoming laser’s polarization angle.

    • The square crystal should be visible through the input aperture; the incoming laser polarization should match the diagonal of the crystal

  • Set up a beam dump at about 30 mm away from the EOM rejected beam port. The rejected beam will be tilted by 22.5° toward the input end.

  • With the driver turned off, connect the cables.

  • Turn on the driver and set the bias voltage to zero.

  • Fine-adjust the roll by iteratively optimizing for minimum transmission, alternating between adjusting the bias voltage (near zero) and adjusting the roll angle.

  • Fine-adjust the X, Y, pitch, and yaw to maximize the extinction ratio, which is the ratio of the maximum and minimum transmission.

    • Do this by adjusting in small increments to reduce the minimum transmission, and checking that you have not reduced the maximum transmission.

  • Iterate over the last two steps (fine adjustment of roll; X, Y, pitch, and yaw) until no further improvement can be made to the extinction ratio.

Aligning a laser beam path to optical components#

TODO: General: pair of mirrors.

Aligning a laser beam to a beam expander#

TODO.

Apendix: Instructions for specific optomechanical mounts#

Conoptics Model 102 EOM mount#

TODO: Photograph.

This mount offers non-independent adjustments for the X, Y, pitch, and yaw. The roll adjustment is independent but with no fine adjustment screw.

  • Initialization

    • Set the 4 thumb screws to abut the cylinder radially (i.e., each normal to the cylinder surface). (TODO: good vs bad diagram)

    • Set the pitch angle close to zero: using a beam height ruler (or similar) to measure the height above the table of each end of the cylinder, ensure that the cylinder is parallel to the table to within ~0.2 mm.

  • Coarse adjustment and positioning

    • The lab jack (bottom portion of the mount) Y adjustment is used for coarse positioning.

    • X and yaw coarse positioning done by sliding on table.

      • Clamp down once correctly positioned.

  • Fine adjustment of X, Y, pitch, and yaw with the 4 thumb screws:

    • None of these screws are independent from the other 3, so iterative adjustment is necessary.

    • The two thumb screws at the same end of the mount are 120° (not 90°) apart, so adjustment is relatively interdependent. In contrast, the two screws on one side of the mount mostly affect the input and output apertures of the device, and are less interdependent. (TODO verify).

    • Therefore, it makes sense to alternate between adjusting the pairs of thumb screws at each end of the mount; for each pair, also alternate between the two screws to iteratively optimize.

  • The upper half-rings should be closed and the top screws tightened only after all alignment is complete. Use only reasonable force, and check that alignment is not affected while tightening.