Second order birefringent acousto-optic device

An acousto-optic deflector that utilizes second order diffraction in a birefringent crystal provides doubled resolution while maintaining high diffraction efficiency. For the second order birefringent deflector employing a paratellurite crystal, 1200 spot resolution was achieved with a 24 MHz bandwidth and 25 .mu. sec access time. This maximum number of spots is twice that obtainable in a conventional first order acousto-optic deflector.

BACKGROUND OF THE INVENTION 
This invention relates to the type of laser beam deflector that utilizes 
the diffraction of light by acoustic waves. In such acousto-optic 
deflectors an electrical input signal is fed to a transducer that is 
bonded to a deflector medium. The transducer can be excited and launches 
acoustic waves into the medium. Under appropriate conditions, an incident 
laser beam may be deflected by the acoustic wave to many diffraction 
orders. The angle of deflection can be scanned by changing the frequency 
of the acoustic waves. 
In a laser deflector, one of the most important performance parameters is 
the resolution, or maximum number of angular positions, which is defined 
as the ratio of total deflection angle to the optical divergence of the 
laser beam. For conventional acousto-optic deflectors where Bragg 
diffraction in the first order is always use, the resolution N is equal to 
the product of total scanning frequency bandwidth .DELTA.f and the 
acoustic transit time .tau. across the optical aperture, i.e., N = 
.DELTA.f.tau.. Bandwidth is limited by acoustic attenuation at higher 
frequency, and the acoustic transit time is usually limited by the access 
time requirement or spatial constraint. One method to improve the 
resolution is the use of optically cascaded deflectors which was reported 
by Watson and Adler in a paper published in 1969 IEEE Conference on Laser 
Engineering and Applications. For efficient operation of the deflectors, 
special beam steering had to be used and the cells had to be critically 
aligned. The complexity of this approach makes it less attractive. In a 
paper entitled "Continuous Deflection of Laser Beams," which appears in 
Applied Physics Letters, Vol. 10, of January, 1967, pages 48-51, Lean et 
al. reported the deflection of laser beam by acoustic waves in 
birefringent LiNbO.sub.3 crystal. At a specific acoustic frequency as 
determined by the crystal birefringence and optical wavelength, the 
momentum matching condition for the acousto-optic diffraction is 
approximately satisfied over a broad range of acoustic frequencies. This 
has been referred to as the 90.degree. phase matching birefringent 
diffraction since in this case the diffracted light wave vector is 
perpendicular to the acoustic wave vector. Based on the same concept. 
Warner et al. described the operation of a broadband acousto-optic 
deflector using the circular birefringence in paratellurite; this appeared 
in the Journal of Applied Physics, Vol. 43, November of 1972, pages 
4489-4495. The authors also reported that due to rediffraction the peak 
diffraction efficiency to first order is limited to below 50%. 
BRIEF SUMMARY OF THE INVENTION 
In the present invention a method is provided to achieve double resolution 
while maintaining high diffraction efficiency in a single laser beam 
deflector by utilizing second order acousto-optic diffraction in an 
anisotropic medium. The key point of this method is that the first and 
second order diffraction are nearly degenerately phase-matched; enabling 
high diffraction into the second order. Since the total angle for the 
second order of deflection is equal to twice that of the first order, the 
deflector resolution is doubled for the same scanning frequency bandwidth 
and access time. In a further embodiment of the present invention, the 
direction of the laser beam is chosen to be making an angle with respect 
to the optic axis of a birefringent crystal for the selection of midband 
frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown a schematic of a second order 
birefringent acousto-optic device. The device comprises an anisotropic 
medium 11, for example, a TeO.sub.2 crystal, provided with input and 
output end surfaces 12 and 13, respectively. The medium 11 is oriented 
with the [110] axis of the TeO.sub.2 crystal in the vertical direction and 
the [001] axis in the horizontal direction. An acoustical transducer 14 is 
mounted in intimate contact with the medium 11 and is connected to a 
suitable tunable signal generator 15. The RF output of the signal 
generator 15 is fed to the transducer 14 and excites an acoustic shear 
wave 16 which is propagated along the [110] axis of the TeO.sub.2 crystal. 
A right-hand circularly polarized laser beam 17 is incident on the input 
surface 12 and transmitted into the medium 11 near the optic axis and is 
diffracted by the acoustic wave. Under appropriate conditions, efficient 
diffraction of the laser beam occurs in the second order. The light in the 
second order diffraction appears as a deflected laser beam 18 that exists 
at the output surface 13. When the frequency of the signal generator 15 is 
varied, the direction of the deflected laser beam is accordingly changed. 
The device then acts as a laser deflector. The resolution of this 
configuration is equal to twice that of the conventional acousto-optic 
deflector since the second order diffraction is employed, i.e., 
EQU N = 2.DELTA.f.tau. (1) 
where N = the resolution, .DELTA.f = total scanning frequency bandwidth, 
and .tau. = acoustic transit time across the optical aperture. 
The difference between conventional acousto-optic devices and the second 
order birefringent device described here is best illustrated by the 
diagram of wave vectors construction. FIG. 2(a) shows the wave vector 
diagram for an acousto-optic diffraction in an isotropic medium. Although 
exact wave vector matching is achieved at the band center in second order, 
there is a substantial wave vector mismatch for the intermediate first 
order process. The overall diffraction efficiency to the second 
diffraction order is low, hence the use of second order diffraction in an 
isotropic medium is impractical. On the other hand, the overall 
diffraction efficiency for a second order birefringent deflector can be 
high. FIG. 2(b) shows the wave vector diagram for the acousto-optic 
diffraction near 90.degree. phase matching condition in a birefringent 
crystal. Note that in this case the acoustic wave vectors for the midband 
frequency are degenerate in magnitude and direction, thus enabling high 
overall diffraction efficiency into the second order. 
A complete wave analysis of the second order birefringent diffraction 
process was made. At midband frequency the first and second order process 
are exactly phase matched. The diffraction efficiency in second order as a 
function of drive power is then given by 
EQU I.sub.2 /I.sub.o = Sin.sup.4 [.pi..sup.2 P/4P.sub.m ].sup.1/2(2) 
where P is the acoustic power, P.sub.m = .lambda..sup.2.sub.o H/M.sub.2 L, 
.lambda..sub.o is the free space wavelength, H is the transducer height, L 
is the interaction length and M.sub.2 is the acousto-optic figure of 
merit. At P = P.sub.m, 100% diffraction efficiency into the second order 
is obtainable. 
Experiments were performed on a second order birefringent deflector using 
TeO.sub.2 as the medium. The arrangement is the same as shown in FIG. 1 
with a He-Ne laser at 6328fA. Maximum light intensity was obtained at the 
90.degree. phase-matching frequency of 37.4 MHz. The measured diffracted 
efficiency I.sub.2 /I.sub.o is shown in FIG. 3 as a function of 
(P/P.sub.m).sup.1/2, which is in good agreement with Eq. (2). The 
resolution of the second order birefringent deflector was also measured. 
The device had a total scanning frequency bandwidth of 24 MHz and a 25 
.mu.sec access time. The resolution of the deflector was found to be 1200 
spots, which is twice that obtainable in the first order. 
It should be noted that the mid-band acoustic frequency f.sub.o of the 
deflector for optical wavelength .lambda..sub.o is 
EQU f.sub.o = V/.lambda..sub.o .sqroot.2n.DELTA.n (3) 
where V is the acoustic velocity, n is the refractive index and .DELTA.n is 
the effective birefringence of the crystal. In the configuration shown in 
FIG. 1, the circular birefringence in TeO.sub.2 is utilized. If the linear 
birefringence of a crystal such as TeO.sub.2 is utilized, the mid-band 
acoustic frequency is in general too high for practical applications. For 
an incident laser beam is incident at an angel .theta. with respect to the 
optic axis. The effective birefringence is given by 
EQU .DELTA.n = (n.sub.e - n.sub.o)sin.sup.2 .theta. (4) 
where n.sub.e and n.sub.o are indices for the ordinary and extra-ordinary 
light propagated perpendicular to the optic axis. This permits the choice 
of lower mid-band frequency of the second order birefringent deflector.