Acousto-optic scanning system with fast non-linear scan

Details Acousto-optic scanning system with fast non-linear scan Acousto-optic scanning system with fast non-linear scan

2001-12-13

2003-07-01

Ben, Loha (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave directional modulation

C359S306000, C359S310000, C359S312000, C359S238000, C359S285000, C359S287000, C372S028000, C333S150000

Reexamination Certificate

active

06587255

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of acousto-optical scanners, especially those capable of generating fast non-linear scans, and/or fast longitudinal focus scanning.
BACKGROUND OF THE INVENTION
Acousto-optic scanners (AOS's) have no moving parts and are thus capable of scanning laser beams much faster than mechanical scanners. The limitation on the scanning speed of acousto-optic scanners arises from the transition or access time T
access
of the acoustic wave across the width of the laser beam. In most present applications, the requirement is for a scan angle &agr; which is linear with time &agr;(t)=at. This is achieved by linear chirping of the acoustic wave frequency f(t). For such linear scans the scan rate can approach 1/T
access
, since the effect of a linear chirp can be described as an effectively constant, time independent cylindrical lens, as shown by A. VanderLugt, in the book “Optical Signal Processing” published by John Wiley & Sons, 1992. Because of its time independence, such a lensing effect can be readily compensated for, by the addition of an external lens of identical and opposite power. There is consequently virtually no reduction in the number of resolvable points (NRP) obtainable from the scanner. This is shown in the article entitled “Design relationships for acousto-optical scanning systems” by A. VanderLugt, and A. M. Bardos, published in Applied Optics, Vol. 31, pp. 4058-4068 (1992).
In order to determine the limitations on the performance of such AOS's, the deflection of a uniform laser beam with a diameter D and a wavelength &lgr; is considered. Though the ensuing analysis deals with the case of a uniform laser beam, it is readily adapted to other beam shapes, with small changes in the numerical constants. The beam is Bragg deflected by a perpendicular acoustic wave with frequency f(t) and velocity &ngr; in an acousto-optic element. The angular scan-span in the first diffraction order is given by &Dgr;&agr;=&Dgr;f&lgr;/&ngr;, where &Dgr;f is the acoustic frequency span. Dividing &Dgr;&agr; by the diffraction limited angular spread &lgr;/D yields the so called static, or low scan-rate NRP as:
NRP
static
=&Dgr;fD/&ngr;=&Dgr;fT
access
.  (1)
Equation (1) indicates that to achieve a large value of NRP
static
, large values of &Dgr;f and T
access
are required. Scanners with &Dgr;f of more than 100 MHz are typically very expensive and suffer from reduced diffraction efficiencies and increased acoustic-wave absorption and heating. Hence, scanners with large NRP
static
mostly rely on the use of large values of T
access
, which is achieved by a combination of a slow acoustic velocity (using shear-mode acoustic waves) and a large laser beam diameter.
Next, the effects of the scan time, T
scan
, on the resolution are included. For a linear frequency chirp, which, as mentioned above, acts as a constant focal length cylindrical lens, T
scan
can approach T
access
, and hence the dynamic (or fast-scan) resolution limit for linear scans is simply given by:
NRP
dynamic,linear
~≦&Dgr;fT
scan
.  (2)
However, for some applications, it would be useful to have non-linear scans, namely scans with non-constant rate or a variable span. Non-linear acousto-optic scanners have recently attracted much attention for a variety of such applications. They have been used for generating two-dimensional circular scans to form dark optical dipole traps for ultra cold atoms, as described by N. Friedman et al, in Physical Review A, Vol. 61, page 031403(R) (2000), for stirring Bose-Einstein condensates, as described by R. Onofrio et al, in Physical Review Letters, Vol. 84, p. 810 (2000), and for rotating Bose-Einstein condensates, as described by K. W. Madison et al, in Physical Review Letters, Vol. 84, p. 806 (2000).
In addition, they are almost essential for ultra-fast laser vector plotters where arbitrary (and hence non-linear) scans are required to efficiently plot sparse information over a large area. For example, in order to plot a ring whose line width is 1000 times thinner than its diameter, 1000&pgr; resolvable points are required using a vector-plotter with a circular scan, as compared to 1,000,000 resolvable points using a conventional two-dimensional raster-mode scanner. Other applications include ultra-fast switching use in optical communication networks.
However, such non-linear scans must inherently use a non-constant frequency chirp, and since the focal length of the effective cylindrical lens is proportional to the chirp rate, the resulting effective lens is thus of constantly changing power, and so cannot be simply compensated for by the addition of an external lens. The result is a high level of aberrations and a drastically reduced NRP for fast scans.
An analysis of the combined limitations on speed and resolution for a non-linear AOS, similar to that performed above for the linear case, shows that such non-linear AOS's are indeed significantly inferior to linear scanners in these respects, as is now shown hereinbelow.
For arbitrary or totally random scans, which include NRP
dynamic
resolution points in a random order, T
scan
must be NRP
dynamic
×T
access
. For a given scan time, there is an optimal value for the access time, (T
access
)
opt
.=(T
scan
/&Dgr;f)
½
, which results in a significantly worse limitation on the optimal resolution than that for a linear scan:
NRP
dynamic,random
~≦(
&Dgr;fT
scan
)
½
.  (3)
Using typical values of parameters of &Dgr;f=100 MHz and T
scan
=10 &mgr;sec, the linear scan has an NRP value of the order of ~1000 as compared to a value of the order of ~30 for the random scan. Such a level of NRP is unacceptable for many high resolution applications.
There therefore exists a serious need for a acousto-optical scanner capable of performing high-speed, non-linear scanning, while maintaining levels of NRP which are close to those typically attainable with equivalent linear scanners of similar specifications.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new acousto-optic scanner capable of high scanning speed, by the use of two acoustic waves with the same frequency modulation, propagating in opposite directions through one or more acousto-optical media disposed in the path of the beam to be scanned. This scheme preferably completely suppresses the linear frequency chirp, and thus enables the generation of fast non-linear scans and non-constant linear scans, with only a limited reduction of the NRP as compared to the linear scan case. In addition, by changing the phase between the modulating signals, such a scanning system also preferably provides fast longitudinal scans of the focal point of the beam along the optical axis. The use of two counter propagating acoustic waves with the same frequency modulation, is applicable for providing one-dimensional scans, but the method can be readily generalized to two-dimensional scans by cascading two of such one-dimensional scanners orthogonally. One and two dimensional non-linear scans can also preferably be obtained with two and four acoustic transducers, respectively, attached to a single crystal. Three dimensional scans can preferably be obtained by means of combinations also involving scans of the center frequencies of the frequency modulated acoustic waves in the previously described embodiments.
There is further provided, in accordance with a preferred embodiment of the present invention, an acousto-optic scanner consisting of at least one acousto-optic element disposed in the path of a beam to be scanned, the element supporting at least two frequency-modulated, counter-propagating acoustic waves, such that the frequency chirp across the beam is essentially suppressed.
In the acousto-optical scanner described above, the counter-propagating acoustic

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