A prototype high resolution, adjustable, lockable mirror mount is described, suitable for use as a resonator end mirror mount in fieldable lasers. The prototype was vibrated to 15g levels, 10-2000 Hz, and was shown to be stable to within 1 arc second and settable to an accuracy of 10 arc seconds. Improvements to be made to the prototype are outlined which will significantly improve the accuracy without sacrificing the other attributes of the prototype

Chadwick, C. H.

English

NASA Technical Reports Server

I-w
 7  o8283
A HIGH RESOLUTION, ADJUSTABLE, LOCKABLE LASER MIRROR MOUNT*
By Dr. Curt H. Chadwick
GTE SYLVANIA, INC.
P. O. Box 188
Mountain View, California 94042
ABSTRACT
A description is given of a prototype high resolution, adjustable,
lockable mirror mount suitable for use as a resonator end mirror mount in
fleldable lasers. The prototype was vibrated to 15g levels, 10-2000 Hz, and
was shown to be stable to within 1 arc second and settable to an accuracy of
i0 arc seconds. Improvements to be made to the prototype are outlined which
will significantly improve the accuracy without sacrificing the other at-
tributes of the prototype.
INTRODUCTION
The advent of fleldable laser systems, such as satellite communications
systems has mandated the construction of lasers that are capable of "HANDS . ..
OFF" operation even in the presence of severe vibration and temperature
cycling. Since the efficiency of a laser is highly dependent upon the mainten-
ance of alignment of the two cavity end mirrors to a high degree of accuracy,
the cavity end mirror mounts are critical items. Depending upon the optical
design of the particular laser, this accuracy ranges from approximately 1 arc
second to about 50 arc seconds. This means that these end mirrors must be
adjustable to a very precise angular orientatiou and then locked with a
minimal positional change during the locking process.
To satisfy these stringent requirements, a set of goals was formulated
for the mirror mount. A prototype lockable mirror mount was then designed,
fabricated, and tested. Most of the defined requirements, or design goals,
were either met or exceeded by the prototype design.
I
* This mirror mount was developed for the Air Force Avionics Lab., Air Force
Systems Command, Wright-Patterson Air Force Base, Ohio, McDonnell-Douglas
i prime contract number F33615-74-C-I028, GTE Sylvania sub-contract with
McDonnell-Douglas number Y3E-247R.
i
.. _ 116 JPL Technical Memorandum 33-777
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1976021184-123
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r
DESIGN GOALS
The design goals are listed below in approximate order of decreasing
importance.
Stability
i As stated previously, the primary goal of the design was retention of
aligranentafter severe vibration and temperature cycling.
Accuracy
The accuracy of an adjustment is defined as the precision to which the
adjustment can be set in a single trial without overshooting the set point,
plus the lockability (the maximum amount the adjustment may be disturbed when
it is locked), plus the maximum amount that it may be disturbed when the ad-
Justment mechanism is removed (in the case of removable adjustment mechanisms).
In orOer to make the mount usable on as many lasers as possible, the mount was
designed to a 1 arc second accuracy limit.
!
; Adjustability
Adjustability is defined as the ease of adjustment of mirror oxlentation
before locking. Durln8 _he alisnment of a laser, many mirror adjustments are
required, especially in the case of the more developmental versions. This
fact makes it very important to design the mount so that it is easily adjust-
able. To ensure this, it was decided that the mount would provide two per- L
pendlcular pivot axes intersectin8 at the center of the mirror face. This ._
arransement of pivot axes is the ,us provldlnl the minimum theoretlcal"cross _
talk" between the axes of adjustment. In practice, ocher factors (mechanlcal _ •
tolerances for example) increase cross talk. The 8oai was set at IZ cross
talk, a value deteruimed to be very accepteble.
Another 8o81 adopted to enhance adJust•billty was Co minimize adjustment
hysteresis to ensure • nearly one t_ one position relationship between the
mirror position and adJustmJnt knob s•ttimg.
_ Size and Weisht
The advantages inherent in a light_tight and compact desISn for
fleld•ble applications •re obvious. However, there •re tradeoffe tc be umd•
beWeen this 8o•1 and the previous two 8o•18, for • compact and lightvei|ht
i design is commonly achieved by compromlsln8 accuracy and adjustability. With
this in mind. • size of 2.5 inches in d/meter by 1.0 inch thick wss defined
J as the ntaximunenvelope size which would alloy the mouz.t to conveniently sate
--_ vith the 8retreat variety of laser structures, iv eight of 8 ounces was
arbitrarily chosen aJ an upper limit.
JPL Technical Memorandum33-777 I17
1976021184-124
Range of Adjustment
The maximum range of adjustment needed in lasers is about 1.5 ° in
either direction about each adjustment axis.
Proximity of Mirror Face to Front Surface of Mount l
Since it is often desirable to mount other optical elements between the
end mirrors, but close to a mirror face, it was determined that the mirror
face should be located as close as po3sible to the front surface of the mount.
THE PROTCTYPE DESIGN
The prc.totype mirror mount is shown in Figures i and 2. The basic desist
approach used two gimbals and a frame connected by torsion bar pivots. The
inner gimbal is connected to the outer glmbal by two torsion bars, while the
outer gimbal is in turn connected to the frame by two more torsion bars
(Figure 2). The frame is usually bolted directly to the laser structure.
However in the case of the prototype, the frame was attached to the structure
via four special flexures which allow for differential thermal expansion be-
tween the frame and the laser structure.
Torsional flexure pivots were used for their advantages of having no
mechanical play, wear, or friction. These properties are essential in order
to achieve low adjustment hysteresis and stability. The limited angular
motion required p_rmltted the use of these torsion bar pivots. An added
benefit of the torsion bar pivots is that the adjustment return springs are
Frov_ded by the torsion bars themselves. In the interest of stability, the
two gimbals, the frme, and _be four torsion bars are constructed as a
un_tary structure, machined from one piece of bery11Ium copper 811oy, chosen
for its good spring propertles.
The adjusting screws (80 threads/inch) are threaded into bosses at-
tached to the outer irene (Fisure 1). The outer 81mbal adJusti.nS screw bear8
directly against the outer 81nbal. The Inner 8hnbal 8dJuement screw passes
through the glmbals to bear on en ad_us_nt tab (Figure 2) that is bolted
dlrectly to the inner glnbal. This is done to allow the point of contact to
be nonlnally on the pivot axis of the outer 81nbal, to ulnlnlza the cross
talk. If the inner g/mbal adjusting screw were su_mted on the outer 81mbal In-
stead of the irma, there would theoretlcally be no cross talk. However, in
practice, this cause8 severe cross _alk due to tendon forces frou the operator's
hand being tran_Itted to the outer slabs1 during adJuo_nt.
After fln81 uachlaing, each pair of torsion bars is siren a _raanent
two degree twist rota_£n8 uch _inbal toward its adJuemant screw. Thie pro-
rides the eprin8 bi_s necessary for +1.5 o rotation about the strgtght ahead
position.
1 18 RF_RODUC_31L[I Y O_ T_
c_l_i'_AL_'t=,_.a'_!g :.' _3R JPL Technical Memorandum 33-'777
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1976021184-125
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ADJU$TMINTKJ_.._| ADJU6TMINTKNOB
AND _EEWe AND _JEWwOUTEE
_NEm GUdEAL .. GIM_,,.AL
LOCK.
u,u_lG_L.,[ A. f
LOCKING_..REW r_ "_ • =._INNERGIMBAL .... MmO| MODULE
(2PLACES)- __
MOUNTU_G •
FULXUIIE 11_ _ OUTERGIMBAL
(4 PLACES)- _ ,,_
• _,
LOCK, _. "_G_.)_ _ _ ".OUTEIIGIMIIAL
(2 PLACES)---- _, = '- TOIISIONIA]I,
•_ OUTll GE_EAL
• V _ _ FLAME
LOCICI_GSOEWe -.
OUTU GIMIA| / _. -I_,AC,.I$)-- - _ - _- INNIil GE_EAL
-/
Figure 1. Back Side of Prototype M_rror Hount.
]
• _
3PL Technical Memora.dum 3]-777 I Ig _
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1976021184-126
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i •
Figure 2. Front SLde of Prototype Mirror ,_unt.
IZO JPL Teclmical Memorandum 33-777
1976021184-127
The torsion bars are 0.125 inch long with a rectangular areas section.
This cross section is .050 inch by .025 inch with the longer din:ension paral-
lel to the adjusting screws. These dimensions were chosen to llm_t the axial
translation of the mirror during adjustment to approximately 10-5 inch,
while providing 3.5° of rotation and an acceptable spring rate. The maximum
axial translation of the mirror over 3° of rotation is about 1.2 x 10-5 inch,
while the torsional spring rate for a pair of torsion bars is 2.7 in.oz./deg.
This design results in the ,_ivotaxes being only 0.025 inch behind the front
face of the mou,lt itself.
L
There are four lock_ng mechanisms on the mirror mount, two to lock the
inner gimbal to the outer glmbal, and two to lock the outer gimbal to the
frame (Figure i). Since all four mechanisms are essentially identical, only
one, an inner glmbal lock, will be described in detail. This lock is shown
in Figure 3. It consists of only three parts, a boss bolted to the inner
gimbal, a clamp bolted to the outer glmbal, and a locking screw.
As the inner gimbal rotates within the outer gimbal, the boss translates
up and down between the wings of the clemD as shown in Figure 3. When the
locking screw is loose, the boss fits between the wings with approximately
.0001 inch clearance on each side. This allows adjustments to be made with
no friction being introduced by the locking mechanisms. The locking screw
passes through a small clearance hole in one wing of the clamp, through a
large clearance hole in the boss, and is threaded into the opposite wing of
the clamp. When the locking screw is tightened, the wings are drawn together
to clamp the boss between them. Each wing has a thin section to provide flex-
ibility in the direction parallel to the locking screw. If the clearance be-
tween the wings and the boss is the same everywhere, there will be no net
moments, or forces, exerted between the inner and outer gimbals as the lock- "'
ing screw is tightened. The only forces exerted on the boss will be the equal
and opposite forces acting on each side of the boss in a direction parallel
to the adjusting screw. For thermal stability, the clamps and boss are made
of beryllium copper to match the gimbal and frame. The locking procedure
consists simply of tightening the two outer gimbal locking screws followed by
the two inner gimbal locking screws. After the locking screws are tightened,
the adjusting screws are removed to create a smaller package and to avoid the
screws shaking loose during vibration.
With the adjusting screws removed, the mount weighs about 5 oz., and
is 2.5 inches in diameter and 0.7 inch thick overall.
_ The prototype mount met or exceeded nearly all design goals. It was
vibrated at 158 levels from 10-2000 _z and temperature cycled through a lO0°C
temperature range with no change in setting observed using apparatus with a
resolution of about 1 arc second. It is very compact (2.5 inches diameter,
0.7inch thick) and light (5 ounces). It has a 3 degree range of adJustlnent
and the cross talk was determined to be less than 1% with a one to one rela-
tionship between adjusting knob position and the mirror settings. In addition
JPL Technical Memorandum 33-777 121
.... _,,wnw,wnJwmml nm Im
1976021184-128
Figure 3. Locking Mechanism, Inner Gimbal.
12Z JPL Technical Memorandum 33-777
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_ ............................ L
1976021184-129
!the mirror face is very close (.025 inch) tc _he front surface of the mount.
The adjustability of the mount is good and it is possible to set it to
within one arc second without overshoot. However, when it is locked, a change
in setting occurs which is greater than that desired, especially when locking
the inner gimbal. In addition, when the adjusting screws are removed, further
motion occurs. These effects combine to reduce the total accuracy of the
mount to about I0 arc seconds. This accuracy makes the mount usable in most
lasers but prevents its use in those requiring very accurate mirror settings.
The motion upon locking has been determined to be caused primarily by
misalignment between the locking clamps and their respective bosses. The
reduced performance of the inner gimbal locks as opposed to the outer gimbal
locks is caused by the closer spacing of the inner gimbal torsion bars and
locks. The torsion bars for the inner gimbal are only 1.2 inches apart and
the two locking screws are only 1.0 Inch apart. The problem is made evident
when it is noted that, with the locks this close together, a motion of only
2.5 millionths of an inch at the lock causes the mirror to rotate about i arc
second.
The motion resulting from the removal of the adjusting screw occurs as
the load previously supported by the adjusting screw is taken up by the pre-
viously unloaded locks.
PROPOSED IMPROVEMENTS
A new revised design has been created that should improve the accuracy
of the mount without compromising a significant number of the other goals.
This design employs Bendix flexure pivots instead of torsion bars. These have
a torsional spring rate about one order of magnitude smaller than the torsion
! bars used in the prototype. This will considerably reduce the load trans-
ferred to the locks upon removal of the adjustment mechanism. In addition
the locks will be reconfigured to make them stiffer when locked and much
easier to align.
Changes will be made in the configurations of the gimbal and the frame
to allow all four pivots and locks to be located at the extreme edges of the
mount. This at l_-astdoubles the spacing of each pair of locks and pivots. •
This change is allowed by the use of separate flexures which allows greater
latitude in the design of the gimbals and frame. This change alone should
increase the accuracy by at least a factor of two.
L
i: JPL Technical Memorandum 33-777 123
1976021184-130
1CONCLUSIONS
The prototype mirror mount discussed in this paper has passed severe
environmental tests. It is lightweight, compact and easy to adjust so long
as the accuracy required is no better than I0 arc seconds. The goal of one
arc second accuracy should be reached by a new prototype that will be under-
going test at the time of publication of this paper.
;T
,: 124 JPL Technical Memorandum 33-777
1976021184-131


A high resolution, adjustable, lockable laser mirror mount

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