Method and apparatus for combined active and passive athermalization of an optical assembly

An athermalized optical assembly includes a laser beam source, such as a laser diode, and a collimator lens which are together mounted in an active thermally-compensated structure. The laser diode source is mounted in a beam source mount thermally isolated from a supporting base plate that serves as the assembly mounting surface. The collimating lens is mounted on a flexure plate having a kinematic hinge. The flexure plate is supported by concentric inner and outer compensation rings of dissimilar materials that are arranged to support the flexure plate at a predetermined distance above the base. The difference in the coefficient of thermal expansion (CTE) of each compensation ring is chosen such that the flexure plate kinematic hinge is passively operated to approximately compensate for thermal shifts in system focal length while maintaining radial and angular alignment of the lens relative to the laser diode source, so as to provide controlled axial movement of the collimating lens. A first thermal element, operatively connected to the beam source mount, and a second thermal element, operatively connected to the outer compensation ring and thermally isolated from the inner compensation ring, are controlled according to the inner compensation ring, outer compensation ring, and lens barrel temperatures. The thermal elements actively regulate the inner and outer compensation ring temperatures so as to supplant or correct any deficiencies in the accuracy or range of motion afforded by the passive operation of the kinematic hinge. Accordingly, the combination of passive and active athermalization provides more exact compensation for thermally-induced focal shifts than may be obtained by a passive or active system acting alone.

CROSS REFERENCE TO RELATED APPLICATION 
This patent application is related to copending, commonly-assigned U.S. 
patent application Ser. No. 860,936, filed in the names of O'Brien et al 
on Mar. 31, 1992, and entitled "COMT, PASSIVELY ATHERMALIZED OPTICAL 
ASSEMBLY", now U.S. Pat. No. 5,120,650 the disclosure of which is included 
herein by reference. 
BACKGROUND OF THE INVENTION 
The present invention relates to means for effecting 
temperature-compensation of focal length in an optical assembly. 
Certain optical assemblies such as those found in laser output scanners 
require a stable monochromatic collimated light beam provided by a laser 
diode and a collimating lens. For adequate optical performance, the beam 
source must maintain a predetermined beam quality over a wide ambient 
temperature range. In conventional apparatus, the beam source and lens are 
mounted in a mechanical structure that attempts to maintain the beam focal 
length while the apparatus undergoes temperature-induced structural 
changes. Hence, the athermalization (i.e., thermal compensation) is 
effected either passively or actively. 
Passive compensation systems typically rely on the differences in 
coefficients of thermal expansions of the various elements in the optical 
system such that there is minimal net focus shift with temperature. For 
example, the conventional approach is to employ concentric tube systems, 
which, if constructed from common materials, are too large or bulky. U.S. 
Pat. No. 4,730,335 discloses a series of interlocking tubes each carrying 
a single optical element of an optically-pumped solid-state laser. Active 
compensation systems typically rely on active temperature control of the 
various elements in the optical system such that there is minimal 
temperature change. For example, a thermoelectric cooler is employed in 
the apparatus disclosed in U.S. Pat. No. 4,604,753 to stabilize the output 
power and wavelength of a laser diode beam source; U.S. Pat. Nos. 
4,656,635 and 4,993,801 disclose a beam source wherein a thermoelectric 
cooler is employed to control the operating temperature of the entire 
head. 
However, the accuracy of a passive system will depend upon the CTE of the 
materials chosen for the construction of certain components of the system. 
Thus, some passive designs cannot be manufactured because the desired CTE 
is simply not exhibited by the materials suited for fabricating the 
requisite system components, or because the CTE is available but tends to 
be unstable. Further, the accuracy of known active systems is often 
dependant upon the construction of complex components in exacting 
tolerances. The foregoing approaches have accordingly been found to be 
more costly and complex, and offer less precision and less range of 
adjustment, than is desired for certain applications. 
SUMMARY OF THE INVENTION 
A preferred embodiment of an improved athermalized optical assembly may be 
constructed according to the present invention to include a laser beam 
source, such as a laser diode, and a collimator lens which are together 
mounted in an active thermally-compensated structure. The laser diode 
source is mounted in a beam source mount that is thermally isolated from a 
supporting baseplate that serves as the assembly mounting surface. The 
collimating lens is mounted on a flexure plate having a kinematic hinge. 
The flexure plate is supported by concentric inner and outer compensation 
rings of dissimilar materials that are arranged to support the flexure 
plate at a predetermined distance above the base. The difference in the 
coefficient of thermal expansion (CTE) of each compensation ring is chosen 
such that the flexure plate kinematic hinge is passively operated to 
approximately compensate for thermal shifts in system focal length while 
maintaining radial and angular alignment of the lens relative to the laser 
diode source, so as to provide controlled axial movement of the 
collimating lens. A first thermal element, operatively connected to the 
beam source mount, and a second thermal element, operatively connected to 
the outer compensation ring and thermally isolated from the inner 
compensation ring, are controlled according to the inner compensation 
ring, outer compensation ring, and lens barrel temperatures. The thermal 
elements actively regulate the inner and outer compensation ring 
temperatures so as to supplant or correct any deficiencies in the accuracy 
or range of motion afforded by the passive operation of the kinematic 
hinge. Accordingly, the combination of passive and active athermalization 
provides more exact compensation for thermally-induced focal shifts than 
may be obtained by a passive or active system acting alone. 
The invention, its objects, and advantages, will become more apparent in 
the detailed description of the preferred embodiments presented below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will now be described with 
reference to the accompanying drawings, wherein like reference numerals 
refer to like components. 
As shown in FIG. 1, a preferred embodiment of the present invention may be 
constructed as an athermalized optical assembly 20 for use in providing a 
collimated laser beam 22 of essentially constant quality over a wide range 
of operating temperature. The thermally-compensated structure includes 
inner and outer compensation rings 24, 26 each attached between a base 
plate 28 and a flexure plate 30. A kinematic hinge, preferably in the form 
of circular notches 32, 34 in the flexure plate, allows localized 
deformation at the flexure plate at one or more portions of the plate 30. 
The base plate 28 is of sufficient thickness and rigidity that any 
deformation therein is insignificant relative to the deformation 
experienced by the flexure plate 30. A collimating lens 36, located in the 
flexure plate 30, is aligned with the optical axis of the light beam 22. 
The beam source is preferably in the form of a laser diode 38 fixed in the 
base plate 28. Radial and angular alignment of the lens 36 to the diode 38 
is maintained due to the cylindrical geometry of the optical assembly 20. 
The assembly is contemplated as being initially aligned and focused during 
manufacturing by techniques known in the art. 
The contemplated passive response of the compensation rings 24, 26 to a 
temperature shift will then produce an axial motion of the lens 36 with 
respect to the beam source so as to approximately correct for focal length 
shifts that would otherwise occur. Materials for the compensation rings 
24, 26 are chosen to have different coefficients of thermal expansion 
(.alpha..sub.1 and .alpha..sub.2), such that each compensation ring 
experiences change in height as the temperature changes. Because the 
kinematic hinge is operable in the upper flexure plate 30 for small 
deflections, the lens 36 will move axially due to the "lever" action about 
a fulcrum point 37 located at the top of the inner compensation ring 24. 
The inner compensation ring 24 is formed as an integral part of the laser 
diode mounting block 39 which together are cooled by a first thermal 
element, preferably in the form of a thermo-electric cooler (TEC) module 
40 and heatsink 41. The outer compensation ring 26 is contiguous with, or 
extended to form, an integral radial heat sink 42 on its exterior wall. A 
second thermal element, preferably in the form of an electrical heater 43, 
is operatively connected to the interior wall of the outer compensation 
ring 26. The flexure plate 30 is formed of a material selected for its low 
thermal conductivity so as to thermally isolate the inner and outer 
compensation rings 24, 26. The inner compensation ring 24 and diode mount 
39 are thermally isolated from the base plate 28 by a thermal isolation 
spacer 44. Insulation 48 is positioned to fill the cavity between the 
inner and outer compensation rings. Thermistors 46 are located on the 
inner compensation ring, outer compensation ring, and lens barrel to 
monitor the temperatures of those components. 
Important dimensions for modelling the thermo/mechanical operation of the 
assembly 20 are illustrated in FIG. 2. The degree of kinematic movement of 
the collimating lens can be controlled by several factors. A variety of 
temperature compensation effects can be obtained for a given outer 
compensation ring height by varying the dimensions of the hinged sections 
of the flexure plate 30 and the locations of the inner and outer 
compensation rings 24, 26. Because the stiffness of the flexure plate 30 
at the hinge points (notches 32, 34) is very low compared to the stiffness 
of the bulk material in the remainder of the flexure plate, the radial 
level arm input and output values are set by the radial amounts R.sub.4 
and R.sub.3, respectively. A fulcrum position 37 is established by 
choosing an inner compensation ring radius R.sub.1, at a predetermined 
point between R.sub.3 and R.sub.4. This levering arrangement provides an 
amplification or reduction of mechanical displacement that depends upon 
the ratio of (R.sub.4 -R.sub.1) to (R.sub.1 -R.sub.3). Kinematic hinge 
operation is independent of the outer compensation compensation ring 
radius R.sub.2 because the flexure plate 30 bulk thickness is selected 
such that the bending moment is transmitted directly to the hinge area at 
R.sub.4. Also, because the compensation rings 24, 26 have different 
coefficients of thermal expansion (.alpha..sub.1 and .alpha..sub.2), an 
input to the lever system is proportional to the inner and outer 
compensation ring lengths L.sub.1 and L.sub.2, the CTE difference 
(.alpha..sub.1 -.alpha..sub.2), and the change in temperature DT 
experienced by the structure. 
Inner and outer compensation ring lengths L.sub.1, L.sub.2 and the 
coefficients of thermal expansion (.alpha..sub.1 and .alpha..sub.2), as 
well as the CTE of the base (.alpha..sub.3) may be varied, and once the 
change D.sub.f in the optical system focal length (optical and mechanical) 
with temperature has been determined, the values of L.sub.1, L.sub.2, 
.alpha..sub.1, .alpha..sub.2, and .alpha..sub.3 may be optimized based on 
the relationship: 
##EQU1## 
where: L.sub.1 =inner compensation ring length 
L.sub.2 =outer compensation ring length 
R.sub.1 =inner compensation ring radius 
R.sub.3 =inner hinge radius 
R.sub.4 =outer hinge radius 
.alpha..sub.1 =inner compensation ring CTE 
.alpha..sub.2 =outer compensation ring CTE 
.alpha..sub.3 =base material CTE 
For the case where the inner and outer compensation rings are of equal 
length (L.sub.1 =L.sub.2 =L.sub.o): 
##EQU2## 
Generally, the above-listed thermal coefficients will be defined by the 
materials and manufacturing methods selected in producing the assembly. 
R.sub.3, R.sub.4, and L.sub.2 are typically defined according to the lens 
characteristics and available space, leaving L.sub.1 and R.sub.1 
selectable for establishing the desired degree of thermal compensation. 
Further, it is believed that to reduce effects of manufacturing 
tolerances, one would: 
a. maximize (R.sub.4 -R.sub.3)/(R.sub.1 -R.sub.3); 
b. maximize (.alpha..sub.2)/(.alpha..sub.1); and 
c. set (R.sub.4 -R.sub.3)/(R.sub.1 
-R.sub.3)=(.alpha..sub.2)/(.alpha..sub.1) 
From the geometric relationships of the components, the total lens movement 
.delta. is calculated as: 
EQU .delta.=[(R.sub.1 -R.sub.3)/(R.sub.4 -R.sub.1)][.alpha..sub.2 (T.sub.2 
-T.sub.0)-.alpha..sub.1 (T.sub.1 -T.sub.0)]L.sub.0 [ 3] 
where 
T.sub.0 =nominal system temperature at initial assembly of the system 
T.sub.1 =temperature of inner compensation ring 
T.sub.2 =temperature of outer compensation ring 
The change in system focal length D.sub.f is linearly proportional to 
temperature and is expressed as: 
EQU D.sub.f =K(T.sub.3 -T.sub.0) [4] 
where T.sub.3 =lens temperature; K=the measured focal shift constant for 
the lens. (The units must be assembled and focused at a constant initial 
temperature.) Since the outer compensation ring temperature T.sub.2 must 
be controlled and the other system parameters are fixed, the control 
equation becomes: 
EQU T.sub.2 =T.sub.0 +(.alpha..sub.1 /.alpha..sub.2)[T.sub.1 -T.sub.0 
]+[K/.alpha..sub.2 L.sub.0 ][T.sub.3 -T.sub.0 ] [5] 
It is contemplated that those skilled in the art will optimize the geometry 
of the optical assembly 20 by varying the foregoing parameters, to 
minimize sensitivity to manufacturing tolerances, or for other reasons. 
One embodiment of the optical assembly 20 could be constructed wherein 
.alpha..sub.2 is greater than .alpha..sub.1, to provide an increase in 
focal distance with an increase in ambient temperature. A second 
alternative embodiment of optical assembly 20 could be constructed to 
provide a decrease in focal distance with an increase in ambient 
temperature. In yet another alternative embodiment, the flexure plate 
response may be made nonlinear by use of a flexure plate having a 
stiffness that varies according to its radial dimension. 
FIG. 3 graphically illustrates an example of the focal shift effected by a 
proper selection of materials and an appropriate construction and 
operation of the assembly 20. The uncompensated response of the 
collimator's focal length to temperature is shown by line (A), which 
reflects the uncompensated shift in the system optical focal length due to 
the effects of changes in the lens focal length and the relative beam 
source position. The purely passive response of the assembly is indicated 
by line (B). In a conventional passive athermalization scheme, lines (A) 
and (B) would be expected to have equal and opposite slopes for perfect 
compensation. As mentioned in the Background of the Invention, however, 
the limited selection of .alpha..sub.1 and .alpha..sub.2 from the 
materials suited for manufacturing would heretofore result in a 
less-than-desired overall system response, represented by line (C). 
However, and according to a feature of the present invention, the 
remaining correction necessary for accurate athermalization is obtained by 
controlling the inner and outer compensation ring temperatures such that 
slope of line (D), that is, the active system focal shift, is made equal 
and opposite to the slope of line (C). The resulting (compensated) total 
system response is line (E), which indicates that the effective system 
focal length is more accurately athermalized (i.e., made constant with 
temperature). Furthermore, it is contemplated that the response indicated 
by line (E) is most stable when .alpha..sub.1 and .alpha..sub.2 are 
selected to provide all available passive compensation, thereby minimizing 
the amount of active compensation necessary via the control of the inner 
and outer compensation ring temperature differential (T.sub.2 -T.sub.1). 
The active athermalization response of the system can thereby make up for 
the shortcomings of the passive athermalization response. 
A particularly preferred embodiment of an athermalized optical head 
assembly 60, constructed according to the present invention for use in a 
high resolution output writer, is illustrated in FIG. 4. The beam source 
38 is a laser diode commercially available as a Hitachi Model HL7806G 
laser diode. The internally cooled diode mount 39 and inner compensation 
ring 24 may be formed of yellow brass. The thermoelectric cooler (TEC) 
module 40 (commercially available as the Marlow Industries Model SD1507) 
is attached to the diode mount 39 along with the associated heat sink 41 
to maintain the beam source 38 and inner compensation ring 24 at a 
constant temperature. The thermoelectric cooler (modified to include a 
center hole for wire routing) 40 is clamped between the diode mount 39 and 
a black anodized aluminum radial fin heat sink 41. The inner compensation 
ring/diode assembly is mounted to the baseplate 28, and thermally isolated 
from it, with a 15% glass-filled polycarbonate isolation washer 44 that 
has nearly the same CTE as the inner compensation ring 24. The 
thermoelectric cooler module 40 is thereby required to remove heat from 
the diode mount 39 and inner compensation ring 24 only. 
The flexure plate 30 is preferably molded from 20% glass filled 
polycarbonate to minimize the heat transfer from the outer compensation 
ring 26 to the inner compensation ring 24. The inner and outer 
compensation rings 24, 26 are cemented with an appropriate adhesive to the 
underside of the flexure plate. A bore in the flexure plate 30 is 
configured to hold the collimating lens 36A (commercially available from 
Eastman Kodak Company as the Model Q-28). 
The outer compensation ring 26 is also cemented to the baseplate 28. As 
illustrated, the outer compensation ring 26 is preferably formed as a 
finned aluminum cylinder with an electrically-isolated nichrome heating 
wire wound in a non-inductive fashion along the inside wall of the outer 
compensation ring 26. Fiberglass wool is used as the insulation 48 that 
fills the cavity between the inner and outer compensation rings 24, 26. 
Linearized thermistors 46 are employed to measure the pertinent 
temperatures at the center of the inner compensation ring wall, and near 
the flexure plate on the lens barrel. The thermistors 46, which are 
commercially available from Yellow Springs instruments as model YSI-44018 
thermistors, are preferably mounted with a thermally-conductive adhesive 
such as that commercially available as Loctite OUTPUT 384. It is also 
contemplated that the assembly may be filled with an inert gas before 
final lens alignment to thus achieve both a degree of mechanical 
protection and a sealed environment for the beam source. 
An athermalized optical assembly constructed according to the present 
invention affords the following, and other, benefits and advantages. The 
components of the assembly may be easily produced and can be successfully 
designed to minimize component sensitivity to manufacturing tolerances. 
The assembly is suitable for addressing a wide range of focal length 
shifts that otherwise could not be compensated. In contrast to the lesser 
number of parameters available for compensation in prior art passive 
compensation schemes, the present invention allows optical thermal 
compensation by varying three sets of passive control parameters (L, R, 
.alpha.) plus two active control parameters (the temperatures T.sub.1 and 
T.sub.2 of the inner compensation ring 24 and outer compensation ring 26, 
respectively). In addition, any second-order nonlinearities in the passive 
athermalization can be corrected by proper control of the active 
athermalization. Therefore, the contemplated optical assembly offers very 
accurate athermalization, one that is substantially more accurate than 
achievable by purely passive designs. 
The invention has been described in detail with particular reference to the 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.