Thermally balanced acousto-optic modulator

Details Thermally balanced acousto-optic modulator Thermally balanced acousto-optic modulator

1999-08-23

2001-09-25

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S305000

Reexamination Certificate

active

06295157

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to modulation of light beams and more specifically to an improved acoustooptic modulator.
2. Description of the Prior Art
Multi-channel laser beam systems are used, for instance, in laser writing applications, such as imaging patterns onto photo-resist using multiple laser beams for purposes of creating electronic circuit substrates. Such systems employ the well known acousto-optic modulator (AOM). In such a modulator, electrical energy is converted to acoustic waves by a piezoelectric transducer, and the acoustic waves modulate the incident laser (light) beams. The acoustic waves distort the optical index of refraction of the modulator body, which is typically crystalline material or glass, through which the laser beams pass. This distortion is periodic in space and time and thus provides a three dimensional dynamic diffraction grating that deflects or modulates the laser beams. Such acousto-optic devices are well known in broadband signal processing.
An example of such a modulator
10
is shown in
FIG. 1A
illustrating the exterior of the modulator body
14
. The light beam
16
enters from the left surface of the body
14
and passes through the body
14
. The horizontal lines are intended to suggest diffraction grating properties; it is to be understood that the molecules in the modulator body, compressed or stretched by the presence of acoustic waves, provide the effect of a three dimensional dynamic phase grating and this is not a conventional diffraction grating.
The electrical input signal (“input”) is applied via input electrode
21
to the surface transducer electrode
20
on the modulator body
14
. Electrode
20
is made of a thin platelet of piezoelectric material bonded to the surface of the modulator body
14
. Electrode
20
also provides acoustic impedance matching. Light beam
16
enters the body
14
through a surface of body
14
orthogonal to the surface to which the piezoelectric electrode
20
is bonded. The frequency and power of this electrical input signal determines to what extent light beam
16
is deflected by passing through modulator body
14
due to the presence of the resulting acoustic wave. Conventionally an acoustic termination such as an acoustic absorber
22
is provided on the surface of the modulator body
14
opposite to the surface on which the electrode
20
is bonded and the electrical signal is applied. Alternatively, the surface of the modulator body opposite to the surface on which electrode
20
is bonded may be cut at an angle causing incident acoustic waves to reflect off-axis and eventually be absorbed by the modulator body.
Thus the electrical connection
21
with electrode
20
and ground electrode
24
is an electrical input port and the voltage (signal) applied thereto creates a spatially uniform electric field in the piezoelectric active regions of electrode
20
to cause the generation of a uniform acoustic wave traveling down the modulator body
14
, which in turn, causes the intended deflection of the light beam
16
. Due to photo-elastic coefficients of the modulator material
14
, the actual effect is caused by appreciable variations in the refractive index of the modulator body
14
which in effect creates a moving (dynamic) diffraction grating traveling at the speed of sound with a grating strength determined by the input electrical power. The angle of deflection of the output light beam and its magnitude as produced by the moving diffraction grating depends on the frequency and the amplitude of the acoustic wave.
FIG. 1A
shows only a single electrode
20
for modulating a single incident light beam
16
. “Light beam” in this context refers to any electromagnetic radiation which may be so modulated, including not only visible light but also ultraviolet light and other frequencies including infra-red, etc., from a laser or other source.
In multi-channel laser beam systems, a plurality of laser (light) beams
16
a
,
16
b
,
16
c
,
16
d
are incident on a single modulator body (see FIG.
1
B). The modulator body
14
has formed on its surface a corresponding number of electrodes
20
a
,
20
b
,
20
c
,
20
d
, there being one such electrode for each beam
16
a
, . . . ,
16
d
to be modulated. Such a device has a plurality of electrodes
20
a
,
20
b
,
20
c
,
20
d
on the surface of the modulator body
14
. Typically there are 4 or 8 or more such electrodes, each deflecting a corresponding incident beam. The physical size of each electrode can be very small for the case of a high speed modulator array, about a few hundred micrometers by a few millimeters each for modulator bandwidth on the order of tens of megahertz. It is a common practice to form such modulator electrode arrays using conventional photo-lithographic means to define the small electrodes. To provide electrical and acoustic isolation, the electrodes are made with a finite gap in between.
In laser imaging systems the intent is to form an array of tiny laser beam dots, modulated in time, on the imaging medium, the dots having a typical packing density of 300 to 300,000 or more dots per inch. Moving the modulated optical dot array in a direction nominally orthogonal to the dot array orientation, i.e. raster scanning, on an optically sensitive medium produces a recorded image of the modulating signal. Obviously, in order to print a continuous quality pattern, there should be no noticeable gap between adjacent laser beam dots on the optically sensitive medium.
Since the desired laser beam dots tends to be substantially smaller in diameter than the laser beams in the modulator array, optical imaging techniques are employed to reduce the laser beam diameters and to eliminate the gaps between adjacent modulated laser beams from a modulator array.
For adequate efficiency, acousto-optic modulators as described above are typically operated, in terms of power input, at watts or fractions of a watt RF power. RF power refers to the amount of applied electrical power at the input terminal. This power is partly converted into heat near the transducer region and causes pattern dependent thermal gradients in the interaction medium, which is the body of the acousto-optic modulator. Undesirably, these gradients may deflect the incident light beam, or beams, away from the optimal Bragg angle condition and cause the amount of light transmitted to change, depending on the recent modulation state history of the modulator. Pointing of the diffracted light rays coming from the modulator may also be adversely affected.
SUMMARY
In accordance with this invention, operation of an acousto-optic modulator is modified to obtain a constant thermal condition within the modulator body and its electrode layer regardless of the prior modulation levels. Desirably this improves transmission and pointing stability. This is accomplished by driving the modulator using input signals having two different RF frequencies such that the sum of the load power over any given time provided at the two frequencies to the modulator transducer region is at least approximately constant. This need not be exactly the same level of power, but such that over a particular duration of time any pattern (writing pattern) dependent thermal gradients are minimized. Thermal effects occur over time scales of t=X
2
/D where D is the thermal diffusivity of electrode body and X is the distance from the electrode interface to the light beam. For a fused silica modulator body, D=0.85 mm
2
/sec, and typical separations are 0.1 to 1 mm, giving time constants around 1 sec.
The power level of only one of the two signals is modulated in order to print the pattern, and the total power level is constant. An optical stop is provided to block the light output from the modulator which is the zero order undiffracted beam as well as the light diffracted by the modulation at the other of the two frequencies. The two frequencies are chosen so as to provide adequate beam separation and so that the acoustical impedance of the transducer is approximately the same

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