1. Field of the Invention
The present invention relates to laser writing apparatus and, more particularly, to method and apparatus for using a laser beam to expose information on a recording material at a relatively high speed with high resolution.
2. Description Relative to the Prior Art
An acousto-optic cell is a device which can be used to provide controlled angular deflection and/or intensity modulation of a laser beam. A typical acousto-optic cell is comprised of an acousto-optic material having a transducer mounted thereto which converts an electrical drive signal into an acoustic wave. The acoustic wave then travels through the acousto-optic material and produces periodic variations in its index of refraction. Such periodic index variations cause a laser beam incident thereon to be diffracted into a fan-like array of beams wherein the angular positions and relative intensities of the diffracted beams depend on the frequency and amplitude content, respectively, of the acoustic wave.
It is well known that if the laser beam is incident at an angle known as the Bragg angle .theta..sub.B, it can be diffracted into only two beams, an undeflected zero order beam and a deflected first order beam. In this case, the first order beam is deflected relative to the zero order beam at an angle 2.theta..sub.B which is equal to twice the Bragg angle. Quantitatively, the deflection angle 2.theta..sub.B is given (for small angles) as EQU 2.theta..sub.B =.lambda.f/V.sub.s, (1)
where .lambda. is the wavelength of laser radiation, f is the frequency of the acoustic wave, and V.sub.s is the acoustic velocity, i.e., the speed of the acoustic wave in the acousto-optic material. See, for example, "Acousto-optic Scanning" by H. J. Aronson, Laser Focus, December 1976, p. 36. Because the deflection angle 2.theta..sub.B is dependent on the frequency of the acoustic wave, the angular position of the deflected beam can be made to vary by varying the frequency of the signal used to drive the acousto-optic cell.
In addition to deflecting a laser beam through an angular range, an acousto-optic cell can also be used to intensity modulate the deflected beam. This is done by varying the amplitude of the acoustic wave. Specifically, the ratio of beam energy which is diffracted into the deflected first order beam to that in the undeflected zero order beam is proportional to the amplitude of the acoustic wave. Amplitude modulation of the drive signal, therefore, results in a corresponding intensity modulation of the deflected first order beam. (Even though the zero order beam is also intensity modulated, it is generally not used for laser writing because, unlike the first order beam, it cannot be deflected by varying the frequency of the drive signal.)
In laser writing apparatus, acousto-optic cells are used to angularly deflect and intensity modulate a laser beam in accordance with an information signal derived from a document scanner or other information source. The deflected and modulated laser beam is then used to "write" (i.e., expose) such information on a recording material. The performance of an acousto-optic cell in such applications is measured in terms of writing speed and image resolution. The term writing speed refers to how fast information can be written on the recording material and is thus a direct measure of the number of documents per hour which can be produced. A parameter which is a measure of the writing speed of an acousto-optic cell used as a modulator is the access time .tau. which is a measure of the length of time required for an acoustic wavefront to traverse the incident laser beam. The access time .tau. is quantitatively defined as EQU .tau.=D/V.sub.s ( 2)
where D is the effective diameter of the incident laser beam and V.sub.s is the acoustic wave velocity. Because the intensity of the deflected laser beam is varied by changing the amplitude of the drive signal (and thus of the acoustic wave), the access time .tau. is a measure of how quickly the deflected laser beam can be modulated, i.e., turned "on" and "off."
The resolution capability of an acousto-optic cell is determined by the angular divergence of the deflected first order beam and is a measure of the number of resolved spots that can be written within a given range of angular deflection. Specifically, the number of resolved spots is determined by a resolution parameter, hereinafter denoted as N.sub.res, which is the ratio of the total angular range through which the first order beam is deflected to the angular divergence of the deflected beam. The resolution parameter N.sub.res is given analytically by the equation ##EQU2## where .tau. is the access time of the acousto-optic cell used for deflection (defined by equation 2), .DELTA.f is the frequency range of the drive signal, and .epsilon. is a constant referred to hereinafter as the beam separation constant. The beam separation constant .epsilon. depends on the intensity profile of the incident laser beam, the diffracting aperture of the acousto-optic cell, and the criterion used for resolution. In the case of a uniformly illuminated diffracting aperture wherein a resolution criterion known as Rayleigh's criterion (discussed below) is assumed, the beam separation constant has a value of 1.0. In applications wherein the illumination is non-uniform (such as produced by a gaussian profile laser beam), or where a different resolution criterion is assumed, the beam separation will have different values, generally in the range of 1.0 to 1.4. For example, the article by Aronson cited above gives, at page 38, values of .epsilon. for a uniform-intensity rectangular beam (.epsilon.=1.0), a uniform-intensity circular beam (.epsilon.=1.22), and a gaussian beam clipped at the 1/e.sup.2 intensity points (.epsilon.=1.34). It has been believed that the number of pixels (i.e., spots) N.sub.pix that can be written along a column which results from angularly deflecting the first order beam is equal to the number of resolved spots within the angular range, i.e., EQU N.sub.pix =N.sub.res
In the case where separate acousto-optic cells are used for deflection and modulation, the number of resolved spots N.sub.res defined by equation 3 and the access time .tau. defined by equation 2 are independent, and laser writing apparatus can be designed which has both high speed and high resolution. See, for example, "Laser-Optical System of the IBM 3800 Printer" by J. M. Fleischer et al, IBM J. Res. Develop., September 1977, page 479. In those applications (see, for example, U.S. Pat. No. 3,863,262) where a single acousto-optic cell is used for both deflection and modulation, however, the access time which determines the writing speed (equation 2) also affects the resolution parameter N.sub.res (equation 3). In particular, to maximize the resolution parameter N.sub.res, it is desirable to make the access time .tau. as large as possible (see equation 3); but to maximize the writing speed it is necessary to be able to rapidly modulate the deflected laser beam, which implies an access time .tau. (given by equation 2) that is as small as possible. Obviously, both conditions cannot be satisfied simultaneously.
A solution to this problem is given in U.S. Pat. No. 4,164,717 which discloses an acousto-optic cell of complex construction which, when used with a sophisticated optical system having cylindrical lens elements, enables high speed and high resolution to be obtained even though a single acousto-optic cell is used to both deflect and modulate an incident laser beam. Apart from the fact that the disclosed acousto-optic cell is relatively difficult to manufacture, the major benefits of single cell operation i.e., low-cost and inherent simplicity, are lost in that system because of its complexity.