Quest for Compact Blue Lasers for Optical Memories
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QUEST FOR COMPACT BLUE LASERS FOR OPTICAL MEMORIES R.N. BHARGAVA Philips Laboratories, North American Philips Corporation, 345 Scarborough Road, Briarcliff Manor, New York 10510 ABSTRACT A short wavelength laser offers unique opportunities in high density optical recording as well as in laser printing. To achieve a compact blue laser, the current effort worldwide is primarily concentrated on achieving well-conducting p-type ZnSe and fabricating quantum well heterostructures so as to achieve a low threshold laser at room temperature. The recent milestone 'an injection blue laser below room temperature' gives us confidence that indeed we are on the right path. In photopumped lasers in various II-VI heterostructures, thresholds at room temperature comparable to the theoretical limit have been reported. This not only reinforces that indeed a room temperature injection laser is possible but when combined with earlier electronbeam pumped laser results, a scanned compact laser is also feasible. In recent years several breakthroughs have demonstrated that an infrared emitting GaAs laser can be used to generate blue light through efficient second harmonic generation (SHG) in certain non-linear optical materials. Recent exciting results on SHG of GaAs lasers in KTP grating waveguides resulted in a blue laser with output power in the range of several milliwatts. Alternative schemes such as upconversion lasers and SHG in III-V quantum wells structures are presented. In summary, various efforts to achieve compact blue lasers and their availability in the near future are presented. 1.
INTRODUCTION
An infrared emitting laser from III-V semiconductors is the key component in optical communication, laser printers and optical memories (e.g. video and compact discs). Significant technological advantages are possible in the enhancement of optical readout density and printing speed in laser printers if the wavelength of the semiconductor laser can be altered from infrared to blue. In the case of optical readout memories such as compact disc the storage bit density, in diffraction limited optical systems, increases as the square of the spatial cut off frequency f,-2NAA where NA is the numerical aperture and X is the laser wavelength [1]. For the change of wavelength from 800 nm to 400 nm, an order of magnitude increase in the data rate can be achieved [1]. This increase in optical density will allow us to find more applications beyond the current possibilities in video/compact disc optical information systems. The other major application of blue lasers is in laser printers. The present laser printers use electrophotography [2] where photoconductive properties are modulated by a scanned laser. The improved sensitivity of certain photoconducting materials from infrared/red region to blue region allows one to increase printing speed by one to two orders of magnitude. Besides use of blue lasers in electrophotography, they may play a critical role when we use them in color hard copy printers where low sensitivity photographic polaroid-like
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