This invention relates to apparatus for producing a relatively noiseless information modulated laser output light beam.

In many optical printing systems the intensity of a laser light beam focused on a two—dimensional photosensitive surface is modulated as the beam is moved relative to such surface to provide a two—dimensional output image. Such systems often use an output scanner which may include a gas laser which produces a beam of light at a predetermined wavelength and a deflector such as a rotating polygon mirror which line scans this light beam. ^" The intensity of this laser light beam is information modulated by an acoustooptic modulator device.

^■ This type of modulator includes a transparent cell which may be made of an acoustooptic material such as glass or TeO 2 crystal and a piezoelectric transducer bonded to the cell. An RF signal, usually in the range of 40—300 MHz, is applied to the transducer. The transducer launches acoustic waves in the cell which produces sonic compression waves that create a diffraction wave grating. This diffraction grating causes a portion of the input laser light beam passing through the cell to be diffracted out of its original path. Amplitude changes of the RF signal cause intensity modulation of the diffracted (first-order) and undiffracted (zero—order) beams. The intensity of the modulated diffracted light beam varies in direct proportion to RF signal amplitude. The modulated diffracted light beam, rather than the undiffracted beam, is utilized, e.g. applied to a deflector which converts the information modulated light beam into a line scan.

In some printing applications, it is desirable that variations in light beam intensity about a desired constant intensity level be kept on a very low level. For example, with laser printers there are applications where it is very important that the DC laser power variations at the image zone be kept at less than about + .5% from a desired level to prevent banding in prints.

U.K. Patent Application, GB, A, 2,054,888, discloses a device for stabilizing the intensity of an acoustically deflected light beam. In this device, the intensity of a diffracted light beam from an acoustooptic deflector is monitored, and the control signal to the acoustooptic deflector is changed in accordance with the monitored beam intensity. One problem associated with such a correction system is that relatively large amo.unts of laser power are used in monitoring the intensity of the diffracted beam. It is an object of the present invention to overcome the problems of prior-art devices and to provide apparatus having .substantially reduced AC and DC noise components in an output laser beam at an image zone. In accordance wj.th the present invention there is provided apparatus for stabilizing a laser beam which may be applied to a utilization device, the apparatus comprising an acoustooptic cell for receiving an input laser light beam and producing a zero—order beam (I ) and a first—order diffracted light beam (I.) in response to an input RF signal, changes in the amplitude of the RF signal causing intensity changes in the diffracted and zero—order beams characterized in that the apparatus includes means for producing an electrical signal which is a function of time—varying changes in the intensity of the zero—order beam, means for sampling an

unmodulated output beam from the utilizaton device and for producing a reference signal representing the instantaneous intensity thereof, and adjustable means responsive to the electrical signal and the reference signal for adjusting the amplitude of the RF signal to maintain the intensity of the zero—order beam substantially constant and thereby reduce noise in the output beam from the utilization device.

A principal advantage of applying the zero—order beam produced by an acoustooptic cell to the utilization device rather than the first-order beam is that the zero—order beam permits highly efficient throughput of laser energy since only a small amount of laser power needs to be diverted to the first—order beam to control beam intensity. In commonly—assigned PCT Application, Publication No. WO 86/00148, published January 3, 1986, there is disclosed beam intensity controlling apparatus in which the zero—order beam from an acoustooptic modulator is maintained at a relatively constant intensity and is used as an input to a utilization device. The present invention differs from this apparatus as a result of the recognition that, whereas the AC beam intensity variations at the image zone are substantially caused by noise in the input laser beam to the utilization device, DC beam intensity variations at the image zone are caused not only in the input beam but also by the utilization device which information modulates the beam. Thus, means are provided in the present invention for producing a signal indicative of the intensity of an unmodulated output beam from the utilization device and for using this signal in the stabilization of the zero—order beam. Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings in which:

Fig. 1 shows a diagram, partially in block and partially in schematic form, of noise reducing apparatus coupled to an output scanner in accordance with the invention; Fig. 2 is a graph of the intensity of the light beam delivered to the deflector of Fig. 1, showing that the intensity of such beam includes AC and DC noise components; and

Fig. 3 is a detailed schematic diagram of portions of the circuit 20 of Fig. 1.

In Fig. 1, there is shown laser noise reducing apparatus 10 constructed in accordance with the presen .invention. Apparatus 10 includes a conventional acoustooptic cell 12. The apparatus 10 receives a noisy input laser light beam at a predetermined wavelength from a laser 14. This beam

Has AC and DC noise components (see Fig. 2). The apparatus 10 delivers a zero—order beam Io" to an output scanner utilization device which includes an acoustooptic modulator 17 which information modulates the output beam and a deflector 19 that line scans the modulsled T?sam across an image zone. It should be noted that the modulator 17 delivers a modulated first-order output beam Ii.r to the deflector 19. This output bifl. I. has variations in its AC and DC intensity components reduced in accordance with the invention. The- deflector 19 may be, for example, a continuously rotating polygon which line scans the modulated output beam across a moving photosensitive medium M disposed-at the image zone. When the output beam Ii.r is not information modulated by modulator

17, then it should be at the desired intensity shown in Fig. 2.

The acoustooptic cell 12 includes a transparent member formed for example from glass or TeO crystal and a piezoelectric transducer 16

bonded to the cell. Apparatus 10 also includes RF generator 22 which provides an electrical signal of a predetermined frequency to the transducer 16, which translates the RF electrical signal into acoustic waves that propagate through the acoustooptic cell. Also bonded to the cell 12 is a conventional acoustic absorber 16a. The acoustic waves launched within the cell correspond to the predetermined frequency of the RF signal and form a phase diffraction grating causing a first—order light beam I. which passes through the cell to be diffracted out of the zero-order beam Io to a beam stop.

As will be shortly described, the zero—order beam I is sampled to determine the AC component intensity variations in the output beam I. from a desired constant intensity level. The beam I. , is also sampled to determine DC component intensity level in beam Ii.r and the beam Io is sampled to determine the DC component variations at the image zone. The amplitude of the RF signal applied to transducer 16 is adjusted to correct for both AC and

DC component errors at ;he image zone and change the intensity of the output beam at the image zone to reduce noise as will be described later. The zero-order beam Io is divided by a beam splitter 13 into two zero-order beams Io ^■ and

Io". The beam Io ^• is a small sample of the zero—order beam I and will be used in producing an

AC component error signal and a DC component error signal which will be discussed later. The beam I ^■■ is delivered by the apparatus 10 as an input to the modulator 17.

The modulator 17 may be a conventional single—beam acoustooptic modulator utilization device. A video signal drives a gain control and power amplifier 17a which provides a control signal to the modulator 17 which information modulates the

output beam I. in accordance with information in the video signal. When the output beam I. is unmodulated, then the power amplifier 17a should provide a control signal to the modulator causing the beam to be at a desired intensity (see Fig. 2).

The modulator 17 produces the first order output laser beam I. which is diffracted and an undiffracted beam (zero-order beam) Ior. The deflector 19, which may be a rotating polygon mirror, converts the stationary output light beam I. to a line scan which is recorded on the medium M at the image zone. The zero—order beam Ior is applied to a beam stop and is not used for recording. The operation of the modulator 17 and deflector 19, as used in output scanners, is well understood in the art. For a more complete description of modulators and deflectors, see Urbach et al, "Laser Scanning for

Electronic Printing", Proceeding of the IEEE. 597

(June 1982). A beam splitter 30 divides the beam Ii.r into two first order beams. A small sample is delivered to a photocell 31 which detects and converts the amplitude of this beam to an electrical voltage signal. This signal is provided at the junction of photocell 31 and a resister 32. This voltage signal is applied to a conventional sample, hold and- compare circuit 34. This circuit 34 is periodically operated as shown by a clock signal at a time when the unmodulating control signal is being applied to the modulator 17 by the power amplifier 17a. Such a time could conveniently be selected between line scans. This circuit 34 periodically samples and holds the level of the electrical signal which represents the intensity of the I. beam, then compares it with a fixed DC intensity reference which represents the desired intensity of the DC component and calculates a reference signal for the

error signal producing circuit 20. This comparison function could be provided by an operational amplifier. The operation of the circuit 34 can be performed either in a continuous mode or intermittently simply by changing the frequency of the clock signal.

Returning now to the output of the cell 12, the beam splitter 13 divides the zero—order beam Io into two zero—order beams, Io' and Io". Beam I ^• is detected by a photocell 18. Photocell 18 is shown as being biased by a battery V\.. The current produced by the photocell 18 contains a component which accurately represents AC variations in the intensity of beam I. and a component which represents DC variations in intensity of beam I. . Fluctuations in the photocell current cause a varying voltage signal between the photocell 18 and resister- 21. This voltage signal is applied to error signal producing circuit 20. The RF generator 22 which may be a conventional device well understood in the art, produces an electrical signal having a fixed frequency at a predetermined amplitude. The amplitude of this signal is controlled by a network which is shown in Fig. 1 only as a resister 24 for simplicity of illustration. The RF signal is actually impressed upon the transducer 16 through a circuit 28 which includes gain control and power amplifier portions. The circuit 28 receives an error signal E from circuit 20. In response to the error signal, the circuit 28 adjusts the amplitude of the RF signal applied to the transducer 16 so that noise in the beam I. is reduced. The acoustic wavelength of the compression wave launched within the cell 12 corresponds to the fixed frequency electrical signal produced by the RF generator. This causes a diffraction grating which produces the

first-order beam I. with an intensity directly proportional to the amplitude of the RF signal. The acoustic wavelength (Λ) can be calculated from the following expression

Λ - _ ^v !s_ (1) ^f s wherein vs is the acoustic velocity in cell 12 and f is the RF frequency. Diffraction in the acoustooptic cell 12 is explained by Robert Alder in an article in the IEEE Spectrum, May 1967, pp. 42-55 entitled "Interaction of Light and Sound". The transducer 16 can be designed to have a 50-ohm impedance. The RF electrical signal in the range of 40—300 MHz is applied to the transducer. The transducer launches acoustic waves which produce a diffraction grating. The amplitude of the RF signal is adjusted by the circuit 28. Peak—to—peak voltages of the RF signal applied to the transducer 16 are in a range of about 2—3 volts. The error signal E produced by circuit 20 is used to adjust the gain control portion of circuit 28 which amplitude modulates the RF signal to give rise to intensity changes in the zero and first—order beams.

Turning now to Fig. 2, we see a graphical representation of variations in the intensity of the output light beam I. before the deflector 19. Since the deflector 19 does not distort the output beam, this representation also represents the intensity of the output beam at the image zone. The intensity of this light beam as discussed above has two noise components, a DC component shown as a dotted line which follows the slow drift of changes of the beam intensity and an AC component which is the faster time variations in the beam intensity. The AC beam intensity variations at the image zone

are substantially caused by noise in the input beam produced by the laser 14. The DC beam intensity variations at the image zone are caused not only by the input beam but also by other system components downstream of the cell 12. The acoustooptic modulator 17 can be a source of DC noise.

The delay time of cell 12 limits the high frequencies over which corrections due to changes in the AC component can be made. The delay time of the cell 12 results from the propagation time of an acoustic wave within the cell 12 to the beam position within the cell. A typical delay time in a TeO 2 crystal with a beam located 300 microns from the transducer 16 would result in about 0.07 microseconds delay. Typically, intensity variations at AC component frequencies over about 1 MHz cannot be corrected.

As shown in Fig. 3, the error signal producing circuit 20 actually consists of two separate stages, a DC^stage and an AC stage. The DC stage produces the DC component of the error signal corresponding to difference between the output beam (without modulation) DC intensity component and a desired constant intensity level and the AC -s.tage produces the AC component of the error signp.I Fcr minimizing the output beam AC intensity component. A voltage signal produced at the junction of photocell 18 and resister 21 is applied as an input to both the AC and DC stages. The DC voltage component has low frequency constituents and the. AC voltage component has higher frequency time varying constituents. The DC stage is adapted to compare the reference voltage signal from the circuit 34 to an output from an inverter 40 and feedback a DC component of the error signal E to the gain control and power amplifier 28 in order to maintain a long term average intensity of the beam I. at

the desired intensity level. The DC stage corrects for slow drifts of the beam intensity while the AC stage minimizes higher frequency changes. The AC stage preferably has 40 db gain at low frequency up to about 10 KHz. When the frequency increases above about 10 KHz, the gain decreases at 20 db per decade to 0 db at about 1 MHz. Because of the delay time in acoustic wave propagation, correction cannot be made for high frequency changes (greater than 1 MHz) in the intensity of beam Io".

As shown in Fig. 3, the DC stage of circuit 20 includes an inverter 40. The output signal of inverter 40 is applied through an adjustable resister 41 to the inverting input of an operational amplifier of an integrator 44. The reference voltage signal from circuit 34 is applied through adjustable resister 42, to the inverting input of the operational amplifier. By adjusting the resistance of the resisters 41 and 42, the output DC beam component intensity level can be adjusted. The integrator 44 produces a DC component error signal which represents the negative of the difference in the DC intensity component from the desired intensity level. The frequency response of the integrator 44 can be adjusted by changing its capacitor and/or input resistance. The output signal of integrator 44 is applied to a summing resister 48.

The AC stage of circuit 20 includes a coupling capacitor 43 which permits only the AC component of the voltage signal from photocell 18 to be delivered to the noninverting input of a variable gain amplifier 45. A low pass filter 46 receives the output of the amplifier 45. For frequency constituents above 10 KHz, the filter gain gradually decreases until at about 1 MHz it is about 0 db. The AC component error signal of filter 46 is

applied to a summing resister 49. The summing resisters 48 and 49 combine both AC and DC component error signals and apply them to an output buffer amplifier 60. Amplifier 60 produces the error signal E which is fed back to the gain control portion of circuit 28. Circuit 28 adjusts the amplitude of the RF signal to thereby change the intensity of the beam Ii.r to reduce noise at the image zone. Apparatus, aa disclosed herein, for producing a relatively constant intensity laser beam is useful anywhere a constant noise free intensity laser beam is desired, such as for example, in an output laser printer which makes prints of photographic negatives and in electrostatic copiers and duplicators.