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Reduction of Optical Feedback Effects during Reading and Writing in Optical Recording

IP.com Disclosure Number: IPCOM000040910D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Lenth, W: AUTHOR [+2]

Abstract

A special optical head assures substantial reduction of optical feedback during both reading and writing. The basic concept is to place an optical delay line of appropriate length between the laser and the disk so that the laser is turned off when the back reflected light impinges on the laser front facet. For writing a suitable effective delay path length L (including dispersion effects of the optical components in the head) is chosen so that 2L/c>Wtw, where Wtw is the duration of the writing pulse. The carrier concentration in the laser waveguide channel decays in no more than 3 ns either by spontaneous emission or by carrier diffusion in lasers with no lateral carrier confinement. By choosing 2L slightly larger than c x Wtw the back reflected laser light will experience no gain.

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Reduction of Optical Feedback Effects during Reading and Writing in Optical Recording

A special optical head assures substantial reduction of optical feedback during both reading and writing. The basic concept is to place an optical delay line of appropriate length between the laser and the disk so that the laser is turned off when the back reflected light impinges on the laser front facet. For writing a suitable effective delay path length L (including dispersion effects of the optical components in the head) is chosen so that 2L/c>Wtw, where Wtw is the duration of the writing pulse. The carrier concentration in the laser waveguide channel decays in no more than 3 ns either by spontaneous emission or by carrier diffusion in lasers with no lateral carrier confinement. By choosing 2L slightly larger than c x Wtw the back reflected laser light will experience no gain. When the duty cycle for writing is no larger than 50%, all the back reflected light impinges on the laser facet while the diode is turned off. For a writing pulse of 20 ns the required delay path is L = 2 m (assuming a refractive index n = 1.5). Such a delay line can best be implemented using a polarization-conserving fiber or waveguide. For such a short fiber length optical losses are minimal; coupling losses are kept low using appropriate optics or suitable butt-coupling. Since there is a trend towards the use of shorter writing pulses, such a delay line can be even shorter in the future. Multiple reflections between the laser and the disk are insignificant since the reflectivities of these surfaces are rather low (...