1) Laser Beam Stabilization
Beam Stabilizer Layout, Dual Fast Steering
Beam stabilizers are used to correct for dynamic laser beam pointing errors
in optical systems. These pointing errors can be a combination of slow varying
(thermal) and higher frequency error (mechanical vibration from fans, water
A beam stabilizer uses active mirrors to compensate for beam pointing
errors. By sampling a small percentage of the beam, the active mirrors can
eliminate beam motion by using feedback from position sensing detectors.
Active mirrors fall into the following groups:
Piezo driven screw mirrors
Actuator motor driven mirrors (steppers and dc
Galvometric scanning mirrors
Voice coil actuated scan mirrors
Here is a brief overview of the various active mirror
Piezo mirrors make use of the inherent piezo effect
of certain crystals and ceramics. The thickness of these materials change
by applying a high voltage across a section of the material. In order to
limit the magnitude of the applied voltage, most manufactures stack thin
sections to form a cylinder. The range of travel of these materials is
typically 15um per 20mm section. Piezo mirror have the advantage of a
large pushing force over a small range. The piezo actuators push against a
flexure to obtain the desired mirror tilt.
See the Physik website for information on piezo mirrors
Note: combining piezo technology and using mechanical
levers, can increase the angular range of piezo mirrors for example Physik model
"s-224 ultra-fast minature piezo tiltplatform with mirror" has a 2.2 mrad
angular range. And a company called Mirrortech has a line of mechanical
advantage piezo mirrors
www.mirrortech.com these mirror have up to a +/-20mrad angular range.
Key features: small angular range, high bandwidth, high
Piezo driven screw mirrors, one company uses a piezo
to drive a fine pitch screw thread acutator. This approach can result in
very fine motion increments (company claims <30 nm of incremental motion).
These units are great for laboratory mount replacements where adjustments are
done very seldom. But they fall short when it comes to active control.
Because of the way these actuators work, they create a large noise during motion
(high pitched whine) and they have a very limited lifetime. Because the
actuator relies on friction between the moving piezo actuated part and the lead
screw, repetitive motion will cause the actuator to stop working. The
manufacturer claims a 2500 cycle (motion of 3mm) life for these actuators.
In addition the maximum speed of the acutator is 40um/second which limits the
frequency/amplitude of the mirrors disturbance capability to slow drifts such as
For more information on these actuators see
Key features: great for occasional operation and
replacement of manual actuators.
Note: a similar product that has been around for years is
the Burleigh Inchworm. This product uses as series of clamping piezos to
slowly inch an actuator forward. See
http://documents.exfo.com/appnotes/AnoteBurleigh010-ang.pdf for more
Actuator motor driven mirrors, for very slow speed
applications, standard kinematic mirror mounts with motorized actuators can be
used. These actuators are available as stepper motors or dc servo motors.
Because most of these actuators use fine pitch threaded lead screw, wear is a
consideration for repetitive usage.
Galvometric scanning mirrors, these are single axis
mirrors cantilever mounted on shafts. These can scan large angular ranges
+/-20degrees. Very low priced versions of these mirror are simple stepper
motors with mirrors mounted to the rotary shafts. High end galvo mirrors
have built in position sensors and are linear to better than 0.1%. The
galvo rotary shaft is usually supported by a ball bearing, but flexure suspended
shafts are available. The disadvantage of this mirror is that since it is
only a single axis device, two scanners need to be mounted together get a two
axis scan. In addition, when using a scanning head (f-theta lens), an
additional optic is needed between the scanners to eliminate beam walk on the
Voice coil mirrors, are two axis mirrors with
flexure suspension. The voice coils are mounted in a push/pull
configuration to apply torque to rotate the mirror. Most mirrors have
magnets mounted to the moving mirror substrate and the voice coils mounted on
the fixed base. This arrangement allows for better heat-sinking of the
coils and eliminates the problems associated with moving wire connections.
Proper design will result in an infinite life mirror, limited only by the
electrical component lifetimes.
Beam Stabilizer Configurations:
A beam stabilizer is made up a number of the following components:
1) Fast steering mirror - in our case a flexure suspended two axis voice coil
2) Optical Beam-splitter - a half silvered flat mirror that reflects some
percentage of a laser beam while also transmitting a percentage.
3) Position Sensing Detector - either a lateral effect cell that outputs a
voltage proportional to beam placement, or a quad cell that splits up the
detector into 4 quadrants.
4) Imaging or focusing lenses Ė used either to transform angular error into
displacement or to re-image an angular source.
Why would you use a lateral effect cell instead of a quad cell? A
lateral effect cell measures the power centroid of a beam regardless of the beam
size. A quad cell only measures position when the beam is split up into the
quadrants of the cell. If the beam is fully on one quadrant the output will
rail (either positive or negative)
If a lateral effect cell is used, the angular gain of the beam stabilizer
mirror loop is dependent on the detector size and the distance of the mirror to
the detector. If a quad cell is used, the angular gain is dependent on the
detector size, the distance from the mirror to detector, and the beam size. See
the graphs bellow to see how beam size effects the mirror angular gain.
Note: Changes in beam size do not affect the lateral effect cell
scale factor but they do affect the quad cell scale factor. (in volts/beam
There are other advantages for using a quad cell in a beam stabilizer system.
The first is that the quad cell is less likely to drift center position with
aging. The other advantage is that for the same size detectors the quad
cell has a lower capacitance and lower noise.
Single FSM Stabilizers
It is possible to configure a beam stabilizer with a single FSM however there
are drawbacks to this approach. In order to eliminate beam motion 4 degrees of
freedom are necessary (to compensate for two displacements, and two tilts). A
single FSM allows only 2 degrees of freedom, so proper placement of the error
sources must be considered.
The simplest implementation of a beam stabilizer consists of a FSM, a
beam-splitter, and a position sensing detector. The laser is reflected off the
FSM and then passes through a beam-splitter. The majority of the energy is
reflected and a small percentage is passed through the beam-splitter. The
energy that passes through the beam-splitter is directed onto a quad cell.
Figure 1. Simple Single FSM
As the laser beam drifts, the spot on the quad cell moves off center.
Feedback from the quad cell causes the FSM to correct this motion and move the
beam back to the center of the quad cell. The result of this correction is that
the beam is held fixed at a point in space (the center of the quad cell). This
may be an acceptable condition, but the beam angle is not controlled. In fact
depending on the distance from the FSM of the angular error source compared to
the distance from the quad to the FSM, the angular error may even be magnified.
For example, if you wanted the angular error to be reduced by a factor of 100,
then the distance from the FSM to the quad cell must be 100 times greater than
the distance from the error source (usually the laser) to the FSM.
To eliminate this angular error, we can add a focusing lens in front of the
quad cell. This lens is located one focal length away from the quad cell. This
lens has the effect of eliminating beam translation errors from the quad cell
output. Only beam angle change causes the spot to move on the quad cell. In
addition, the spot on the quad cell is now focused, this has several affects on
the beam stabilizer which will be covered in the setting gain section of this
Now this simple beam stabilizer will correct for beam angle errors. However,
beam translation errors will not be corrected. The magnitude of the translation
error depends on the distance from the error source to the FSM. Another benefit
of this design is that the distance from the FSM to the focusing lens can be a
small as possible, larger distance does not improve the angular resolution of
the stabilizer. The angular range and resolution is set by the choice of the
quad cell and the focusing lens.
Figure 2. Single FSM Stabilizer
with Focus Lens
In order to eliminate the translation effect, we need to insure that the
angular error is relayed onto the surface of the FSM. If desired, the focusing
optic can be eliminated and the stabilizer range and resolution will be a
function of the distance from the FSM to the quad cell. This method works if
there is no beam translation of the laser source, only angular errors.
Since the angular error occurs at the laser head, we need to add a set of
relay optics that image the error source onto the face of the FSM.
Figure 3. Single FSM Stabilizer
with Relay Lenses
The distance from the first relay optic to the laser error source is one
focal length of the relay lens. The relay lenses are spaced 2 focal lengths
apart, and the FSM is one focal length from the second relay lens. Any angular
errors at the laser source are relayed onto the face of the FSM at a single
point. The laser beam then reflects off the FSM and through the beam-splitter
and onto the quad cell. Since the spot on the face of the FSM is fixed, the
quad cell reads only angular error which is fed back to the FSM to correct the
A simple test can be run to determine the effectiveness of this beam
stabilizer configuration. By installing a relay lens set and placing a position
sensing detector at the location of the FSM face, the resultant motion sensed by
the PSD should be from any beam translation. If the magnitude of this
translation is acceptable then this approach should work. Remember the
resultant translation divided by the distance to the quad cell will be the
resultant angular error in the system.
Dual FSM Stabilizer
A dual FSM stabilizer is more versatile than the single FSM stabilizers.
This lends itself to simpler implementation. Systems can be built which act as
a black box, with a laser beam input and a compensated laser beam output.
Correction of both angle and displacement will be handed by the stabilizer
without adding additional constraints to the optical system.
Two steering mirrors are needed to correct angular errors
originating from a point at any distance from the front of the beam stabilizer
unit. FSM1 corrects for errors in displacement of the beam from the reference
line. FSM2 corrects for errors in angle of the beam from the reference line.
A schematic of a standard beam stabilization module layout
is shown in figure 1. The heart of the system is the two FSMs and two position
detectors. The position detectors are silicon quadrant detectors, which give
feedback to the FSMís controller to keep the beam locked at the center. The
beam input is at the bottom of the figure. Two 90 degree bends reproduce the
beamís original direction with an offset. FSM1 corrects for angle due to the
feedback it receives from Quad Cell1. FSM2 corrects for position due to the
feedback it receives for Quad Cell 2. The beam position on Quad Cell 2 is an
image of the spot on the front of FSM1. This is done by the lens which has a
focal length of f/2 where f is the distance between FSM1 and the lens and also
the distance between the lens and Quad Cell 2. This arrangement creates a relay
lens which produces a spot on the detector that follows the spot on the front of
FSM1. Since the two FSMs are operating independently they simultaneously
correct for angle and position. The beam sampler takes a small percentage of
the output beam (usually 1% to 5%).
Figure 4. Dual
FSM Beam Stabilizer
Figure 5: Alternate Dual FSM
The alternate dual FSM beam stabilizer shown in figure 5 uses two
beam-splitters, and two quad cells. The distance from FSM 1 to FSM 2 is equal
to the distance from FSM 1 to Quad Cell 1. This insures that the beam is
stationary on the surface of FSM 2. An additional beam-splitter samples the
beam and sends it to Quad Cell 2. FSM 2 removes the angular beam error. The
advantage of this layout is that it does not need a lens, but the main beam has
to pass through two beam-splitters.
Beam Stabilizer Performance Analysis:
If the PSD were a lateral effect cell, assume a UDT
Quad cell size is 2 mm, focusing lens is 500 mm.
The angular range is 2/500 or 4mrad. (or +/- 2mrad).
If the beam moves outside the angular range of the quad cell the stabilizer
will become open loop and will not function.
The angular resolution is more difficult. First, it depends on the spot size
on the quad cell. The other factors are the detector noise, and the electronic
noise in the quad cell electronics.
See the "Beam Stabilizer
Users Manual" for more information and getting started notes.