Electro-optic modulator with acoustic damping

There is provided by this invention an electro-optic modulator for damping acoustical energy produced by an electro-optic crystal. The modulator couples acoustic energy from the electro-optic crystal to an acoustic coupler due to the matching acoustic impedances of the coupler and the crystal. The coupler transmits the acoustic wave to the acoustic damper which dampens the acoustic energy. The coupler linearly decreases in height in a direction away from the crystal so that the acoustic wave is reflected through more than one damper prior to returning to the crystal, thus further decreasing acoustical energy. Another feature of this invention is the geometrical design of the electro-optic crystal which has two of its faces, which are not attached to a coupler, positioned not to be perpendicular with a face attached to a coupler. This design of the crystal allows for all acoustic waves, even if directed toward a face which does not have an attached coupler, to be reflected toward a face having an attached coupler so that they can be coupled from the cavity and damped. A further feature of this invention is the use of material from the damper in the bonding material between the coupler and the damper so that a gradient interface is formed so that more efficient transmission to and from the damper is obtained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to electro-optic modulators with 
acoustic damping and more particularly to electro-optic modulators with 
acoustic damping which utilize an acoustic coupler to link the modulator 
crystal to an acoustic damper wherein the acoustic coupler and the 
acoustic damper are shaped to insure acoustic reflection and scattering 
within the acoustic damper. 
2. Description of the Prior Art 
In modern laser apparatus, the laser cavity emission is controlled by 
regulating the voltage applied to an electro-optic modulator, such as a 
LiNbO.sub.3 crystal. The voltage change across the electro-optic crystal 
will cause the power within the laser cavity to either be emitted or 
accumulated depending upon the laser's design due to the polarization 
change of the light traveling through the crystal. Additionally, the 
voltage change across the electro-optic crystal generates an undesirable 
acoustic wave which also alters the polarization of the light traveling 
through the crystal. 
In a typical electro-optic modulator, the acoustic wave, generated by the 
voltage change, continues to reverberate within the electro-optic crystal 
even after the voltage applied to the crystal reaches its desired value. 
This continued reverberation modulates the polarization of the light, thus 
perturbing the desired laser response. This perturbation may be in the 
form of light leakage during a period of power buildup prior to emitting a 
pulse or it may alter the amount of radiation leaving the laser cavity. 
The acoustic waves also limit the pulse rate at which the laser can emit 
uniform pulses and distort the temporal profile of the laser pulse. 
A typical method of damping acoustic waves in an electro-optic modulator is 
to immerse the modulator crystal in an appropriate liquid to dampen the 
acoustic waves. The liquid enveloping the crystal is chosen to match the 
acoustic impedance of the modulator crystal. This method suffers, however, 
from several deficiencies including the potential for leakage of the fluid 
or vaporization or decomposition of the fluid upon the occurrence of a 
laser pulse with sufficiently high energy. The tendency of liquid to not 
support shear waves, a property desirable in order to damp acoustic waves, 
further limits the use of liquid in damping acoustic waves. 
An alternative apparatus for damping acoustic waves is the electro-optic 
device disclosed by Kiefer, et al. in U.S. Pat. No. 3,653,743 which has an 
acoustic energy absorbing material bonded thereto. However, the device is 
limited to utilizing dampers which have their planar faces parallel to the 
electro-optic crystal's face and which have cross-sectional dimensions 
which are substantially equivalent to those of the crystal in order to 
maintain best operation. Thus, while this device does couple acoustic 
waves from the electro-optic crystal and dampen those waves, these 
limitations cause the device to be inadequate in several respects. The 
damper's geometry is not designed to prevent reflections from the damper's 
edge from returning to the crystal following only one pass through the 
damper and disturbing laser modulation upon their reentry into the 
electro-optic crystal. Furthermore, when the dampers couple acoustic waves 
at a resonant frequency, the acoustic energy is stored in the dampers as a 
standing wave which requires a lengthy time in which to dissipate. 
It would be desirable to develop a electro-optic modulator with acoustic 
damping whose geometry is such that a plurality of reflections will occur 
within the acoustic damper to scatter the acoustic wavefront before the 
acoustic energy is reflected towards the crystal. Furthermore, it would be 
desirable for the electro-optic crystal to be shaped so that acoustic 
energy traveling within the crystal in a direction where there is no 
acoustic damper attached is directed towards an interface where there is 
an acoustic damper attached. 
SUMMARY OF THE INVENTION 
There is provided by this invention an electro-optic modulator for damping 
acoustical energy produced by an electro-optic crystal. The modulator 
efficiently couples acoustic energy from the electro-optic crystal to an 
acoustic coupler due to the matching acoustic impedances of the coupler 
and the crystal. The coupler serves to transmit the acoustic wave to the 
acoustic damper which dampens the acoustic energy. Due to the design of 
the coupler, constituting another feature of this invention, which 
linearly decreases in height in a direction away from the crystal, the 
acoustic wave is reflected through more than one damper prior to returning 
to the crystal, thus insuring a greater reduction in its acoustical energy 
than in prior art systems. Another feature of this invention is the 
geometrical design of the electro-optic crystal which has two of its 
faces, which are not attached to either coupler, positioned so as to not 
be perpendicular with a face attached to a coupler. This design of the 
crystal allows for all acoustic waves, even if directed toward a face 
which does not have an attached coupler, to be reflected toward a face 
having an attached coupler so that they can be coupled from the cavity and 
damped. A further feature of this invention is the use of material from 
the damper in the bonding material between the coupler and the damper so 
that a gradient interface is formed so that more efficient transmission to 
and from the damper is obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of an electro-optic modulator 10 with acoustic 
damping, shown in FIGS. 1 and 2, comprises an electro-optic crystal 12, 
two acoustic couplers 14, and two pairs of acoustic dampers 16. The 
crystal 12 may be composed of any material exhibiting the electro-optic 
effect, such as lithium niobate (LiNbO.sub.3). 
A pair of couplers 14 are bonded to opposite faces of the crystal 12. In 
order to apply an electric field across the crystal 12 for electro-optic 
applications, an electrode 60 is attached to each of the two acoustic 
couplers 14 in a manner which is well known to one skilled in the art. The 
couplers 14 should be designed so that their acoustic impedance equals the 
acoustic impedance of the crystal 12. For the previously mentioned 
exemplary crystal of LiNbO.sub.3, a coupler 14 with an equivalent acoustic 
impedance as that of LiNbO.sub.3 is silver. While the bond between the 
crystal 12 and the coupler 14 may be of composed of various materials as 
is well known in the art, an exemplary bond for use between a LiNbO.sub.3 
crystal 12 and a silver coupler 14 is an indium and tin solder which is 
applied to adjacent crystal 12 and coupler 14 faces which are to be bonded 
after the adjacent faces have been metallized with a Ti-Pt-Au combination. 
For most efficient damping, the bond between the crystal 12 and the 
coupler 14 should be less than one-quarter of the wavelength of the 
acoustic energy to be damped in thickness. For bond thicknesses which are 
greater than one-quarter of the wavelength of the acoustic energy to be 
damped, the acoustic energy will not be coupled as efficiently from the 
crystal resulting in increased perturbations in the laser modulator's 
output. 
As shown in the sectional view of FIG. 2, the face 18 of the coupler 14 
adjacent to the crystal 12 is the same height as the adjoining face of the 
crystal 12. The height of the coupler 14 is then increased as shown by 
face 20 until it is equal to the maximum height of the crystal 12 so that 
the maximum amount of acoustic energy is coupled from the crystal 12. From 
this maximum height, the coupler 14 linearly decreases in height as the 
distance from the crystal 12 increases so that the coupler 14 forms a 
triangular shape. The acute angle 22 is critical to the efficient 
operation of the electro-optic modulator 10 with acoustic damping as it 
determines the number of times which the acoustic energy is reflected 
through the acoustic dampers 16 as hereinafter explained. 
A pair of acoustic dampers 16 are attached to each coupler 14 to damp the 
acoustic energy coupled from the crystal 12. The dampers 16 may be 
constructed from any material which dampens acoustic waves, such as lead, 
titanium, or tungsten. In the preferred embodiment, the dampers consist 
essentially of 97% tungsten by weight and 3% epoxy by weight. The 
attachment of the acoustic dampers 16 to the couplers 14 in the preferred 
embodiment is accomplished by means of a mixture of 50% tungsten by weight 
and 50% epoxy by weight which is applied in a thin layer 15 between the 
coupler 14 and the damper 16. The use of the damper material, tungsten, 
within the bonding mixture allows for the formation of a gradient 
interface between the damper 16 and the coupler 14 so that acoustic energy 
is more efficiently transferred to or from the damper 16. 
As shown in FIGS. 1 and 2, the electro-optic modulator 10 with acoustic 
damping may be attached on a mounting surface 24 which in turn may be 
connected to a source of heat 62. By applying heat from the heat source 62 
to the electro-optic modulator 10 by means of the thermally conductive 
mounting surface 24, the components of the electro-optic modulator each 
expand based upon their individual coefficients of expansion such that 
increased contact is maintained between the components to permit improved 
acoustic energy coupling and damping. For a modulator composed of the 
exemplary materials previously discussed, heat is typically applied such 
that the modulator is held at 40.degree. C. for best operation. The 
mounting surface 24 may be constructed from Beryllium Oxide (BeO) or any 
other suitable material as is well known in the art. 
The operation of the electro-optic modulator 10 with acoustic damping is 
initiated by the application of a voltage differential to the electrodes 
60 which is then transmitted from the electrodes 60 to the coupler 14 and 
then to the electro-optic crystal 12. Once energized, the crystal 12 
produces acoustic waves which are transmitted from the electro-optic 
crystal 12 to the coupler 14. As previously discussed, the acoustic 
impedance of the coupler 14 is nearly matched to the acoustic impedance of 
the crystal 12 so that the acoustic energy will exit the crystal 12 
without being reflected internally by the crystal faces due to a mismatch 
in acoustic impedances in the different materials. Thereafter, the 
acoustic energy is drawn away from the crystal 12, where it could cause 
destructive interference, by being conducted through the coupler 14 to the 
dampers 16. 
The design of the couplers 14 is such that all acoustic waves which exit 
the crystal 12, either perpendicular to the face of the crystal or in a 
typical diverging fan configuration, are transmitted to at least one 
damper 16. This effect is due to the linear decrease in the couplers' 
height in the direction away from the crystal 12. The rate of the linear 
decrease is dependent on the angle 22. As the acoustic wave is transmitted 
away from the crystal 12 it will eventually contact a damper 16. The 
acoustic wave, after having been refracted upon entry to the damper 16 due 
to the different acoustic impedances in the damper 16 and coupler 14 
materials, is conducted through the damper 16 until it reaches the 
damper/external environment interface 26 where the acoustic wave is 
reflected for another pass through the damper 16. During the transmission 
of the acoustic wave through the damper 16, the amplitude of the acoustic 
wave is decreased due to a transfer of energy from the acoustic wave to 
the damper 16 in the form of heat due to the increased vibrations within 
the matrix of material forming the damper 16 caused by the acoustic wave. 
The damped acoustic wave is refracted once more as it exits the damper 16 
and reenters the coupler 14 through which it is again transmitted. Due to 
the coupler's linear decrease in height, the majority of acoustic waves 
will be transmitted from one damper 16 to the other damper 16 of the pair. 
This process will be repeated as the acoustic wave is reflected between 
the dampers 16 as the acoustic wave travels away from the crystal 12. Upon 
the acoustic wave's reflection at the damper/external environment 
interface 28 at the far end of the electro-optic modulator 10, the 
acoustic wave will proceed to return toward the crystal 12 while again 
being reflected between the pair of dampers 16. Due to the repeated 
reflections through the dampers 16, each of which decreases the acoustic 
energy of the wave, the amount of acoustic energy which may return to the 
crystal 12 is extremely small and thus will not deleteriously effect the 
crystal's performance. 
The angle 22 between the dampers 16 determines the number of times which 
the acoustic wave will be reflected through the dampers 16. As the angle 
22 is decreased, the acoustic wave will be reflected through more dampers 
16 and will thus lose a greater percentage of its energy. Likewise, as the 
angle 22 is increased, the acoustic wave will be reflected through fewer 
dampers 16 and will thus retain a greater percentage of its energy. 
However, as the angle 22 is decreased to increase damping, the length of 
the electro-optic modulator 10 will be increased for a crystal 12 of the 
same height. Thus, the modulator designer is faced with a tradeoff between 
the size of the modulator 10 and the amount of damping desired, which was 
resolved in the preferred embodiment with an angle 22 of approximately 
21.degree.. 
An additional feature of the electro-optic modulator 10 with acoustic 
damping is the geometrical design of the crystal 12. The crystal 12 has 
six faces; two of which 18 and 19 are bonded to the couplers and four of 
which 30 are not attached to another surface. As shown in FIG. 1, two of 
the unattached faces 30 are designed to not be perpendicular to the faces 
18 and 19 bonded to the coupler so that the acoustic energy which reflects 
from the two nonperpendicular crystal/external environment faces 30 is 
next transmitted toward a coupler/crystal interface 18 or 19 where it may 
be coupled from the crystal 12 to be subsequently damped. The remaining 
two unattached faces 30 are perpendicular to the faces 18 and 19 attached 
to the couplers and parallel to each other in order to allow the crystal 
to function properly. In prior art electro-optic crystals, the faces of 
the crystal were all mutually perpendicular so that acoustic energy would 
be continuously reflected between a pair of opposite faces and either 
would not be reflected toward an adjoining face where it could be coupled 
from the crystal and damped as was previously discussed, or, 
alternatively, the prior art electro-optic device would require a coupler 
and damper to be attached to each crystal face to effectively dampen the 
acoustic energy resulting in a bulkier and more expensive electro-optic 
device. 
An alternative embodiment of the electro-optic modulator 40 with acoustic 
damping is shown in FIGS. 3 and 4 in which the modulator 40 comprises a 
electro-optic crystal 42, one acoustic coupler 44, and one pair of 
acoustic dampers 46. Additionally, as previously discussed, the 
electro-optic modulator 40 may be attached to a mounting surface 48 which 
may in turn be thermally connected to a heat source 64 so as to heat the 
electro-optic modulator 40 and increase its acoustic damping efficiency. 
The modulator 40 shown in FIGS. 3 and 4 utilizes only one coupler 44 which 
is bonded, by a thin gradient interface layer 45, to one set of dampers 46 
since the crystal face 50 located opposite to the crystal face 52 
adjoining the sole coupler 44, which in the previously discussed 
embodiment was bonded to a coupler, is attached to an electrode 54 which 
serves to reflect the acoustic energy arriving at its electrode/crystal 
interface 50 toward the crystal face 52 adjacent to the coupler 44 to 
permit subsequent coupling from the crystal 42 and damping of the acoustic 
energy as previously discussed. An additional electrode 56 is attached to 
the coupler 44 so that a voltage differential between electrodes 54 and 56 
can be applied to the electro-optic crystal 42 as previously discussed. 
The coupler 44, dampers 46, and electro-optic crystal 42 of the alternative 
embodiment of the electro-optic modulator 40 with acoustic damping are 
designed and operate in an identical fashion to those previously 
described. Thus, by the elimination of one coupler and one pair of dampers 
from the modulator design discussed previously, the alternative embodiment 
performs the same function of damping acoustic energy but is smaller and 
requires less complicated fabrication. 
Although there has been illustrated and described specific detail and 
structure of operations, it is clearly understood that changes and 
modifications may be readily made therein by those skilled in the art 
without departing from the spirit and the scope of this invention.