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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an acoustooptic recording
apparatus, and, more particularly, to a high speed, wide bandwidth optical
system for reading out a serial electrical signal as parallel optical
data.
2. Description of the Prior Art
High speed recorders for recording a serial line of electrical data as a
spatial array of optical data are important in many fields. For example,
in imaging applications, they are used to reproduce a picture or image
that has been transmitted electrically over large distances. In addition,
they are used in a wide variety of signal processing systems including
systems for performing spectrum analysis.
One conventional prior art technique for high speed data recording is with
the use of a laser scanner. In such systems, a laser beam is caused to
scan across a recording medium by a suitable mechanical scanning apparatus
such as a system of rotating mirrors or prisms. By turning the laser beam
on and off or otherwise modulating it with the signal to be recorded as it
is scanned across the recording medium, the data can be optically
reproduced on the medium.
Such systems, being mechanical in nature, suffer from obvious limitations.
For one thing, they are somewhat limited in scanning speed as the mirrors
or prisms tend to distort at excessive speeds of rotation. Also, they have
bearings and other mechanical components which eventually wear out. In
addition, the scanning apparatus frequently must be maintained in a vacuum
to eliminate air turbulence problems caused by the mirror rotation, and
this makes the system unweildy and not very portable. Finally, it is
essentially impossible to make each facet of the scanner exactly the same,
and, as a result, as an array of lines are put down in a raster format,
banding and line jitter will occur, at least to some extent.
More recently, the prior art has developed a solid state scanner which
requires no moving parts and, therefore, does not suffer from many of the
above-described inadequacies. Typically, these systems employ an
acousto-optic cell to replace the rotating mirrors or prisms.
Specifically, a laser beam is first appropriately modulated by the
electrical signal to be spatially recorded and is then passed through the
cell and focused to a point on the recording medium. A sinusoidal acoustic
signal is also passed through the cell, and, as is understood in the art,
this will cause the laser beam to be diffracted at a given angle. By then
changing the acoustic signal frequency in a continuous manner, the laser
beam will be scanned across the recording medium to form a line of points
representative of the input electrical signal by which the beam is
modulated. By indexing the recording medium, an entire raster or series of
lines of data can be recorded.
This system, while having the advantage of being fully solid state, still
suffers from several shortcomings. For one thing, it is, for practical
purposes, limited in the number of spots that can be placed along a single
line. Specifically, in such systems, there is a trade-off between the
number of points that can be placed along a line and the scan rate (i.e.,
the so-called .beta..tau. product). For example, for a commercially
available 100 megahertz bandwidth device, although 1,000 resolvable spots
can be recorded at low data rates, only 500 spots can be recorded at 50
million spots per second, and only 2 resolvable spots can be recorded at
100 million spots per second. Also, in these systems, there is a loss in
duty cycle due to the time required for the first acoustic signal to pass
through the cell before the next signal can be sent. Further, the spot
position accuracy obtainable with such systems are not as great as might
be desired. Finally, the optical quality of the acousto-optic cells used
in this type of system must be quite good or distortions will result.
SUMMARY OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a novel technique for recording a
line of electrical data as a spatial array of optical data, is provided.
The system, according to the preferred embodiment, also utilizes an
acousto-optic cell, for example, a Bragg cell, and, thus, also has the
advantages of solid state operation. However, in the present invention as
distinguished from prior art systems, the electrical signal to be
recorded, is fed directly into the acoustic cell via appropriate
transducer means to produce an acoustic wave which can be directly imaged
onto a recording medium via laser or other light. In other words, in the
present invention, the cell itself is imaged onto the recording medium as
a line of optical data corresponding to the line of acoustic signals in
the cell at the moment of imaging.
This line of acoustic data will obviously move across the cell in a
continuous fashion, however, by pulsing the illuminating light just when
the cell is filled with a line of data, a single stationary line of
optical data can be recorded on the recording medium as a stationary
image. By vertically deflecting the line image, and by periodically
pulsing the light just as a new line of acoustic data fills the cell, a
series of lines of data can be recorded in a raster format.
Thus, in the present invention, the electrical signal to be recorded is
passed through the cell as a corresponding acoustic wave, which, in turn,
is imaged directly onto the recording medium. Such a technique provides
several advantages over the prior art systems described above. For one
thing, there will be essentially zero distortion in the recording because
the travelling wave will always move through the cell at a constant speed
of sound. Furthermore, the system can essentially operate with a 100% duty
cycle if the light is pulsed exactly in correspondence with the fill time
of the cell. Also, there will be no line banding problems as in rotating
mirror systems because the recorded line will be an image of the acoustic
cell. Further, line jitter can be very easily controlled because, in this
system, it is determined only by the accuracy of the laser pulse which can
be controlled very accurately. Finally, the present invention results in a
true high speed, high data system, because it is not necessary to
trade-off between the number of spots on a line and the scan rate. For
example, using the same commercially available device described
previously, 1,000 spots can be recorded on a line at a data rate of 100
million spots per second with no loss in duty cycle. Further details and
advantages of the invention will be set forth in greater detail
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates an acousto-optic snapshot recorder
according to a presently most preferred embodiment of the invention.
FIG. 2 schematically illustrates an alternative embodiment of the invention
to permit relaxation of the laser specifications.
FIG. 3 schematically illustrates a second alternative embodiment to permit
an increase in the number of resolvable spots recorded along a line.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 schematically illustrates an acousto-optic snapshot recorder
according to a presently most preferred embodiment of the invention. The
system is generally identified by reference number 10 and includes the
following basic structure: a laser 12, beam expanding optics 13,
cylindrical condenser lens 14, acousto-optic cell 16, imaging lenses 17,
18 and 19, demodulating slit 21, scanner 22 and recording medium 23.
The laser 12, which will be described in greater detail hereinafter,
preferably comprises a pulsed laser such as a cavity-dumped, mode-locked
argon laser or a Q switched frequency doubled Nd-Yag laser, and generates
a narrow collimated beam which is expanded by optics 13 and condenser lens
14 so as to substantially fill acousto-optic cell 16. The cell 16 is of
conventional type and may be constructed of any one of a variety of
well-known materials such as lead molybdate or tellurium dioxide. A
transducer 24 of, for example, lithium niobate, is epoxied or cold-well
bonded to the cell 16 and is used to create a travelling acoustic wave
which will propagate across the acousto-optic material to provide a
travelling wave phase grating which will diffract the light entering from
the side of the cell from laser 12. As is known in the art, light passing
through the acousto-optic cell will be diffracted at an angle proportional
to the frequency of the acoustic signal passed through the cell and the
amount of energy present in the diffracted first order beam can be made
quiite high. Furthermore, by feeding a multiple frequency acoustic signal
into the device, a set of diffracted first order beams can be produced,
each being proportional to a corresponding frequency.
In the present invention, the transducer is fed by a signal from input 26.
Specifically, the signal to be recorded is used to amplitude modulate (or,
alternatively, if desired, frequency modulate or phase modulate) a high
frequency carrier signal, and this modulated signal is then used to drive
the acousto-optic cell 16 via transducer 24. This signal,
S(t)cos.omega..sub.c t, crosses the cell as a travelling wave phase
variation, and is illuminated by light from laser 12. The light passing
through the cell after being diffracted by the acoustic wave, is then
imaged onto a recording medium 23 via imaging optics 17, 18 and 19. A
demodulating slit 21 is placed in the Fourier transform of the cell and is
centered on the carrier diffracted first order, such that the travelling
wave phase variation will appear as an intensity variation on the
recording medium with the intensity being proportional to the signal of
interest with the carrier removed. In other words, the resultant image on
the recording medium 23 will be a line of data points with the intensity
of each point being proportional to the phase variations of the travelling
wave in the acousto-optic cell at the time it is illuminated, which
travelling acoustic wave will, in turn, be proportional to the inputted
electrical signal of interest.
It should be recognized that the line of data will move across the
recording medium in correspondence with the movement of the travelling
wave across the cell. This motion can be stopped, however, to permit
recording of a stationary line of data on the recording medium 23 by
pulsing the laser with a very short pulse at the proper time.
Specifically, by pulsing the laser 12 with pulse control electronics 27
just at the instant when the acoustic wave fills the cell, an entire line
of optical data can be instantly recorded on medium 23. Vertical scanner
22, which, for example, may be a conventional polygonal mirror type
scanner, is then actuated to translate the image of the signal down by one
spacing during the transit time of the next set of data across the cell
and the laser is again pulsed to record a second line of data. By
continuing the process, an entire raster can be generated on the recording
medium. By pulsing the laser exactly in correspondence with the fill time
of the cell, substantially 100% duty cycle recording can be obtained.
In the design of the system, a relatively efficient acousto-optic cell is
required. Two good materials are tellurium dioxide and lead molybdate.
Assuming an efficiency of 70% with attenuation losses of 3bd across the
cell, devices can be fabricated from these materials with bandwidths of
500 MHz or more.
In addition to the acousto-optic cell, the laser used must also meet fairly
stringent requirements. It must have sufficiently short pulses to freeze
the travelling wave (e.g., 2 nanoseconds), and a sufficient repetition
rate (235,000 per second) to permit attainment of substantially 100% duty
cycle operation. A cavity-dumped, mode-locked argon laser marketed by
Spectra-Physics Inc., has been found to satisfy these requirements. As
will be amplified hereinafter, however, techniques can be provided to
relax these requirements to permit the use of less expensive lasers.
In general, with the present invention, many of the inadequacies of the
prior art recorder systems have been obviated. For one thing, the scan
rate will always be at the speed of sound in the acoustic cell so that
there will be no scan distortion. Also, there will be no trade-off between
the number of scan points and the scan rate as is found in the
conventional prior art acousto-optic scanner described previously. With
the present invention, the complete number of points accepted by the
acoustic cell is always used, and the data rate will only be limited by
the frequency properties of the acoustic materials. In signal processing,
recording rates of 150 megacycles (or 300 megapixels/second in the case of
image recording) is clearly possible with greater rates being
contemplated.
FIG. 2 illustrates an alternative embodiment of the invention.
Specifically, in the FIG. 1 embodiment, a laser is required which will
give extremely fast pulses with enough energy and a high enough repetition
rate to freeze each line of travelling data. Although as described above,
lasers are available to do this, they are quite expensive, and,
accordingly, it is desirable to provide a means for relaxing the
requirements on the laser pulse width. It is toward this goal that the
embodiment of FIG. 2 is directed.
Specifically, in FIG. 1, it should be recalled that as the input acoustic
signal moves through the acousto-optic cell as a travelling wave, the
resultant image of the cell will move across the imaging medium in a
similar manner. Accordingly, to freeze the image on the medium, a very
fast pulse is required from the laser at the precise instant that a line
of data just fills the cell. The system of FIG. 2 relaxes this precision
by compensating for the image motion of the travelling wave by deflecting
the image in the opposite direction with an acousto-optic deflector.
Specifically, in FIG. 2, an acousto-optic deflector 31 is positioned in
the path of the output of the cell 16. As is known in the art, a
continuous signal which changes linearly with time is passed through the
deflector from source 32 via transducer 33 so as to just compensate for
the motion of the travelling wave passing through cell 16 such that the
output from deflector 31 will be a stationary image which can be recorded
on imaging medium 23 via lens 19 (the scanner has been omitted from this
FIG. for clarity).
As mentioned above, the acousto-optic recording apparatus of the present
invention can readily record a line of data having, for example, 1,000
resolvable spots at high data rates. By employing the slightly modified
setup illustrated in FIG. 3, however, the number of resolvable spots along
a single line can be made even larger. In the embodiment shown in FIG. 3,
light from laser 12 is directed through acousto-optic cell 16 via
appropriate optics including lens 11, cylindrical condenser lens 14 and
anamorphic beam expander 15. Also, and as before, cell 16 is fed by the
serial electrical signal to be recorded via appropriate transducer means
(not shown in FIG. 3). The image of the acoustic wave is thereafter imaged
onto recording plane 41 via appropriate imaging optics 42 and 43. In FIG.
3, the imaging plane comprises a film recording system which moves a strip
of film in the direction indicated by arrow 44 to record consecutive lines
of data thereon. acousto-optic film recording system is interchangeable
with and may be used instead of the stationary recording plane and scanner
arrangement illustrated in FIGS. 1 and 2, if desired.
Also illustrated in FIG. 3 is a mirror assembly 46 positioned to receive
the image from cell 16 and reflect it onto the film 41. Specifically, the
mirror assembly 46 comprises a plurality of mirrors 46a, 46b, etc., five
being shown, with each being aligned to direct the light to a different
location on the film and more particularly, to a different position along
a line, e.g., line 47 on the film. Also, in the system is an acoustooptic
deflector 47 of conventional type to direct the laser beam light through
the acousto-optic cell 16 at the proper orientation to impinge upon one or
another of the mirrors 46a, 46b, etc.
The system operates as follows. When the acousto-optic cell 16 is just
filled with a set of data, the laser will be pulsed as before. Also,
deflector 47 will be driven to direct the image of the cell 16 onto mirror
46a which, in turn, will direct the image to position 47a along line 47.
When the next group of data fills the cell 16, and laser 12 is again
pulsed, deflector 47 will direct that image onto the film via mirror 46b
onto location 47b of the film. This is continued through the entire mirror
assembly and, in this way, a single long line of 10,000 or more data
points can be recorded on film 41.
Although five mirrors are shown in the FIG., a larger number can obviously
be used if desired. Also, if preferred, the mirror assembly could be
replaced by an appropriate wedge assembly.
While what has been described above are the presently most preferred
embodiments of the invention, it should be understood that many additions
and modifications could be made if desired. For one thing, the systems
described could be utilized in a wide variety of different applications.
They may, for example, be used merely as image recorders. Alternatively,
they could be used in optical data processing applications, for spectrum
analysis or the like. Because many additions, modifications or omissions
can be made, it should be clearly understood that the invention should be
limited only insofar as required by the scope of the following claims.
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