Download Tutorial on Optomechanical Beam Steering Mechanisms

Tutorial on Optomechanical Beam Steering Mechanisms
OPTI 521 Tutorial
Doug Essex
62 N. Worth Ave.
Elgin, IL 60123
(847) 697-3286
[email protected]
Introduction
Crucial to any optical system is the ability to adjust the beam pointing, whether incoming
or outgoing. Beams need to be moved in position and angle, and adjustments are easier if
movements for each degree of freedom of the beam are kept separate. In this tutorial
some examples of common optomechanical beam steering elements will be shown and
their advantages and disadvantages compared. Additionally a new type of adjustable
prism will be described and compared. Spatial light modulators, acoustooptical and
electrooptical deflectors and other diffraction based devices will not be considered in
detail, because with the exception of micro opto electro mechanical (MOEM) chips they
tend to be less optomechanically intensive.
The application of these devices can be roughly broken down into two main areas: beam
or alignment within a system and tracking or pointing at the target. Pointing at a target is
conceptually very simple – all that is required is a 2 axis mirror to adjust tip and tilt
angles until the target is illuminated (for an active system) or brought into the field of
view (for a passive system). The complications tend to involve the subsidiary systems
required to keep the optics pointed at the target. These pointing systems are generally
electrically driven, and have feedback systems to assess target pointing, so bandwidths
and slew rates and the existence of good control laws or models are important. System
alignment requires moving the beam so that it is on or closely parallels to a required
optical path, rather than simply hitting a target. Achieving this requires mechanisms that
offset the beam position in addition to angle adjusting mechanisms. These mechanisms
tend to be manually driven or controlled with loosely coupled or ad hoc feedback. The
adjustments tend to be made once and then locked down and staked.
The functional requirements1 for a beam steering device are
Maximum steering angle or offset
Control/resolution of steering
Beam divergence or imaging
Aperture/vignetting
Spectral range
Throughput
We will provide tables with some semi-quantitative comparisons of these parameters at
the end of this paper.
1
Mechanisms to adjust beam position
Tilt Blocks
These are simple plane parallel plates of transparent material that offset the beam position
through refraction. The offset (for small ) is given by
y = t (n-1)/n

y
t
Figure 1 Schematic of a tilt block
In order to get large displacements the thickness and/or the refractive index must be as
high as possible and the tilt must be as high as practical also. When dealing with
polarized light if the tilt is out of the plane of incidence then the surfaces will have to
have high transmission for both polarization states. This is difficult to achieve for angles
~>45 degrees. High angles also either require use of oversize pieces or acceptance of
significant foreshortening of the clear aperture in the tilt axis direction. As with all
refractive systems dispersion limits the useable spectral bandwidth.
Periscopes
These elements are excellent for providing comparatively large offsets. As they are
reflective they can be very achromatic. However this also means that they are prone to
the common problem of mirrors: 2x magnification of angle errors. Using mirror matrices
it can be shown that in order for the output beam to still be parallel to the input the
second mirror must match any angle error in the first mirror.
If we start with the matrix for a mirror with its normal in the z direction and use the
rotation matrices to tilt it to an angle of /4+ where  is an angular error we have
1
0
0 cos

0

sin
4
0
sin

4

1

4

cos

4

1 0 0
 0 1 0
0 0
1
0 cos
0


sin
4
0 sin

4
1
0


0 cos
4

cos

0
1


4

0
4
2  cos
sin
1


2
2  cos
4
1
4
2
0
2

1
  sin  
4
1
1
  sin  
4

4

sin
1
4


2
cos
1
4



2
The second mirror of the periscope starts with the same matrix for a mirror with the
normal in the z direction, but now we rotate 5/4+
1
0
0
5
0 cos

sin
5
4
0
sin
1
1 0 0
5
0 cos
 0 1 0 
4

4
5

cos
4
5
0
0 0

1
0 sin
4
5
1
0

5
sin

0
0 cos
4



sin
4
5
cos
4
1
0
2



2  cos
4
2  cos
0
4
1
2
1

4
1
1
  sin  
4

4
1
  sin  
4

sin
1


2
1
cos
4



4
Now multiplying the matrices for mirror 1 and mirror 2 we get
1
0
0 cos
1


4
0
2  cos
0
2
sin
1


2
2  cos
4
1
4

1
  sin  
4
1
1
  sin  
4

4

sin
1


1
2
cos
4
1


0
0 cos
1


4

2
0
4
2  cos
0
2
sin
1


2
2  cos
4
1
4

1
  sin  
4
1
1
  sin  
4

4

sin
1
4


2
cos
1
4
Which results in
1 0 0
0 1 0
0 0 1
Showing that the output beam is travelling in the same direction as the input beam. A
popular method of assuring that the two mirrors are parallel is to make the periscope a
monolithic rhomboid prism. The parallelism of the mirrors can be checked
interferometrically to a fraction of an arcsecond and is fixed in the glass of the finished
prism. If the offset of the periscope is made adjustable then straightness of the linear
motion must be very good to ensure that no angle is introduced between the two mirrors.
Mechanisms to adjust beam angle
Fast steering mirrors2,3,4
These are typically the final output mirrors in active (beam emitting) systems. They are
typically lightweight mirrors steered by electromechanical actuators on the back of the
mirror. The choice of actuation mechanism is driven by the required frequency response
of the mirror – high frequency, high load with small tilts or translations indicate using a
piezoelectric driver, while medium frequency, low load or large tilts or translations
indicate use of a voice coil. While angular errors still have a 2x effect on the output
beam, since these systems generally have a sensor and feedback system the beam can
generally be held on the target as long as the frequency response and actuator resolution
limits are not reached.
3



2
2
Risley prisms5,6,7
Low angle wedges of transparent material deviate a beam by an angle  = (n-1) where 
is the prism apex angle. One prism can only move the beam in a circle. Addition of a
second prism moves the beam in a superposition of circles.



n





Maximum deviation case
Minimum deviation case,
distance between prisms
exaggerated for clarity


Looking down the beam axis at the
path the beam traces out with prism
rotation. Deviation ranges between
 and 2
Figure 2 Risley prisms
Generally restricted to low wedge angles to keep the angle of incidence relatively low
and therefore transmission relatively constant for polarized beams and to avoid dispersion
therefore they have a low field of regard. The beam path follows a Lissajou type pattern
dependant on wedge angle and rotation rate. If the wedges are not matched exactly there
can be dead zones that cannot be pointed to. It is difficult to formulate control laws to
position wedges to give predictable result – generally used manually to set angle and then
fixed in place or in constant motion to scan.
Metastable adjustable prism8,9
Described on www.metastableinstruments.com and in U.S. patent 6320705 this new type
of beam adjuster uses a doublet with a lubricating fluid filled interface between matching
spherical surfaces.
4
Figure 3 Page from the patent describing the Metastable beam steering mechanism
One of the elements is decentered thereby creating an effective block with a wedge angle
tan  = T/R where T is the amount of translation of one of the elements and R is the
radius of curvature. The diameter of the clear aperture is given by DCA = D-2T. Because
of this reduction of clear aperture as the device is adjusted it is preferable to use an
oversize lens for the moving element Dmoving lens = Dnominal + 2T to make up for the 2T
reduction when the mechanism is adjusted. The decentering can be performed on either
axis independently, making easy orthogonal adjustments of beam angle possible. The
entire assembly can be tilted much like a tilt block to translate the beam, thus combining
2 functions into one optical element. The surface tension of the lubricating film holds the
lenses together, eliminating any need for guides to control the moving element.
This mechanism can also be adapted to make an extremely finely adjustable mirror
mount. In conventional mounts the actuator translation distance divided by the lever arm
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between the pivot point and the actuator determines the increment of adjustment. Since
mirror mounts are seldom much bigger than the mirror itself this lever arm distance is
limited and consequently if fine adjustments are required then more precise actuators are
required. If the flat surface of the moving lens of the Metastable mechanism is made
reflective the change in angle of this surface is T (the actuator translation)/R (radius of
the curved surface). This radius can be extremely accurately made up to a few meters
long, potentially giving a fine adjust mirror 2 centimeters in diameter with a coarse
threaded actuator an angular adjustment resolution 50x better than a conventional mirror
mount.
Figure 4 Page from the Metastable patent describing the use of the mechanism for a mirror mount
Disadvantages of this mechanism include the need for a lubricating index matching fluid.
This restricts the use to the visible and NIR with glasses, where index matching fluids
exist. No fluids that match ZnSe (n=2.4) or Ge (n=4) are known, meaning the technique
is not applicable in the mid to far IR. Decentered lenses systems have been described for
use in the infrared but they rely on mechanical methods to keep the surfaces separated
and moving in the correct orientation10. A common index matching fluid used in the UV,
3M Fluorinert FC-40 has too low a viscosity to lubricate well. The UV transparency of
other fluids would have to be investigated before committing to using this mechanism in
the UV. Laser damage threshold of the fluid may restrict the range of possible
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applications. The patent uses microscope immersion oil for best results, but used a
variety of different lubricants, including 5W-30 motor oil!
Summary
Beam offset
mechanisms
Max Offset
Tilt Block
Solid Periscope
t(n-1)/n
Control/Resolution
Actuator controls
this/orthogonal
As large as practical
length of material
allows
N/A
Beam div/imaging
Image
shifted/aberration
Dcos
Limited by
dispersion/ AR
coatings/material
transparency
~99.5%, possibly
less if high angle
AR’s required
Aperture/Vignetting
Spectral Width
Throughput
Increased path
length
Separate mirror
Periscope
As large as a
straight path can be
made
Fixed or governed
by straightness of
path. Mirror angles
must be controlled.
Increased path
length
D/ 2
D/ 2
Limited by AR
coatings/ material
transparency
Limited by reflector
coatings – for
metals very wide.
~99.5% (AR’s +
TIR bounces)
R2 ~ 98%
Table 1 Comparision of beam offset mechanisms
Beam angle
adjustment
mechanisms
Max deviation
Control/Resolution
FSM
Risley
Many degrees
Typically minutes to
a few degrees
Orthogonal/actuator Not
controlled resolution orthogonal/resolution
controlled by actuator
Beam div/imaging
Limited by mirror
flatness
Aperture/Vignetting
Spectral Width
~ D/ 2
Limited by reflector
coatings – metallic
is very wide
Metastable
T(n-1)/R
Orthogonal
control/resolution
controlled by
actuator
Limited by
Limited by
transmitted wavefront transmitted
wavefront
D
D-2T
Limited by
Limited by
dispersion/ AR
material
coatings material
disperision/AR
transparency
coatings/material
(lube fluid?)
transparency
7
Throughput
~99%
~96%
~98%
Table 2 Comparision of beam steering mechanisms
References
Tholl, “Novel Laser Beam Steering Techniques” SPIE 6397 Technologies for Optical Countermeasures
III (2006)
2
Sweeney, et. al. “Design Considerations for Fast Steering Mirrors (FSM’s)”, SPIE Vol. 4773 Optical
Scanning 2002 (2002).
3
Berta, et. al. “Development of a Commercial Line of High Performance Fast Steering Mirrors” SPIE Vol.
3787 Optical Scanning: Design and Application (1999)
4
Zook, “Light Beam Deflector Performance: A Comparative Analysis”, Applied Optics 13 (4) April 1974
p. 875
5
Marshall, “Risley Prism Scan Patterns”, SPIE Vol. 3787 Optical Scanning: Design and Application
(1999)
6
Ostaszewski et. al. , “Risley Prism Beam Pointer”, SPIE Vol. 6304 Free-Space Laser Communications VI
(2006)
7
Sanchez, et. al. ,“Control Laws for a Three Element Risley Prism Optical Beam Pointer”, SPIE Vol. 6304
Free-Space Laser Communications VI (2006)
8
U.S. Patent 6320705
9
www.metastableinstruments.com
10
Gibson, et. al. “Wide Angle Decentered Lens Beam Steering for Infrared Countermeasures
Applications”, Optical Engineering 43 (10) Oct. 2004 p. 2312.
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