Jean-Michel Guerin

Dual-beam modulation subsystem for high-speed digital copier

The Xerox Docutech High-Speed Digital Copier achieves an output print speed of 135 (14 in X 8.5 in) pages/minute, at 600 dots/ inch resolution. Its digital optical image is generated by a laser scanner whose dual-channel modulation subsystem (AOM/D) enables this performance capability through crystal beam splitting/recombination optics, polygon facet tracking, pulse-imaging, and a dual-electrode Bragg cell. The AOM/D features: 86% throughput enabling the use of a standard 5 mW package size 633 nm Helium-Neon laser. Facet-tracking, enabling the use of a 63.5 mm diameter 18-facet polygon rotating at 22 KRPM. The splitting optics generate two cross-polarized beams which are then modulated independently by the dual-channel Bragg cell. The recombination optics enable tight separation control between the two adjacent scan line spots at the photoreceptor with excellent stability. Symmetrization of path lengths insures that the scan and cross-scan waists and image structures are imaged at the same plane. The Bragg cell and associated driving electronics accommodate modulation of 66 Mbits/sec per channel and >= 20:1 contrast ratio of pulse-imaged pixels. Design and manufacturing aspects are discussed.

Liquid crystal variable attenuator for high-speed printing

Proper exposure of the photoconductor in an electrophotographic printing system is essential for high quality printing. Gas lasers such as helium-neon and helium-cadmium lasers do not lend themselves to easy electronic power control. To set proper exposure in an electrophotographic system, it is necessary to deal with a number of time varying factors including laser intensity, acousto-optic modulator efficiency, photoreceptor sensitivity, and toner developability. A nematic liquid crystal based variable laser attenuator was developed as a means of setting exposure accurately in a closed-loop exposure control system. This paper describes some of the design considerations and performance parameters for the variable attenuator. The system is compact, low cost, has high throughput, provides an exposure range > 100:1, and has sufficient speed to reset exposure between consecutive prints. The liquid crystal variable attenuator is used in a variety of Xerox printing products including the DocuTech Production Publisher and the Xerox 4850 Highlight Color Laser Printer.

Tunable electrooptic rf filter

A real time tunable electrooptic rf filter is demonstrated using a multichannel total internal reflection modulator to spatially filter the output of a Bragg cell. The demonstrated filter has a bandwidth to resolution ratio of 28. This ratio can potentially be increased to over 5000 with a like number of individually resolved frequencies.

RF Spectral And Temporal Analysis With A Real Time Tunable Frequency Filter

This paper reports on a set of experiments involving an acousto-optic RF spectrum analyzer, an electro-optic tunable frequency filter, and a slit detection configuration. The system extracts in real time any wanted signal out of a multifrequency RF population and recovers its temporal modulation information. Its rise time is faster than the acoustic pulse transit time through the optical beam in the Bragg cell, yet its frequency resolution remains as fine as its A/O spectrum analyzer parameters dictate. The E/0 tunable frequency filter operates as a real time optical processor, hence significantly reducing digital processing of the RF spectrum. First, a conceptual system using the 5376 electrode Xerox linear Total Internal Reflection (TIR) spatial light modulator is described. Experimental results using a 10 channel TIR spatial light modulator are then presented which verify the concepts, demonstrate the retrieval of a weak signal, > 20 dB dynamic range, and real time selection of desired frequency signals. The recovery of temporal information is demonstrated using an imaging technique showing 5 1 μs modulation pulse rise time using a narrow slit at the photodetector. Tradeoffs exist between dynamic range, recovered rise time, laser power, Bragg cell efficiency, TIR efficiency and channels cross-talk, and photodetector/amplifier noise. These aspects will be discussed.

Sound Diffraction And The Optimizing Of Acousto-Optic Modulator Efficiency

The sound diffraction profile due to the rectangular electrode of an acousto-optic cell has been computer modeled, with specific attention to the sound profile in the direction orthogonal to light and sound propagation. The profile is uniform only immediately adjacent to the transducer, which is too close for effective light interaction. Effective interaction can occur only when the light beam is at least 1 or 2 millimeters away from the transducer, where the sound field exhibits Fresnel near field fluctuations in sound intensity, corresponding to regions of constructive and destructive interference. The light beam should fit within a region of constructive acoustic interference to optimize the acousto-optic coupling. A deflector requires changing acoustic carrier frequency and therefore changing near-field acoustic profile. The light beam must be located in a region of reasonably uniform sound intensity for all carrier frequencies. A simple experimental method has been developed to test these results, consisting of Schlieren imaging onto a CCD linear array.