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Laser Drive Techniques to Realize Short Coherence Behavior

IP.com Disclosure Number: IPCOM000102623D
Original Publication Date: 1990-Dec-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 4 page(s) / 114K

Publishing Venue

IBM

Related People

Ainspan, HA: AUTHOR [+2]

Abstract

Disclosed are several circuit techniques utilizing GaAs technology which will enable conventional, long-coherence-length lasers to operate with multi-mode fiber at data rates of several 100 Mb/s without the presence of optical noise or 'speckle patterns', by disrupting the laser light's coherence without disturbing the data signal behavior.

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Laser Drive Techniques to Realize Short Coherence Behavior

       Disclosed are several circuit techniques utilizing GaAs
technology which will enable conventional, long-coherence-length
lasers to operate with multi-mode fiber at data rates of several 100
Mb/s without the presence of optical noise or 'speckle patterns', by
disrupting the laser light's coherence without disturbing the data
signal behavior.

      The problem of optical noise known as 'modal noise' in
multi-mode fibers when using lasers is well known (1,2). One solution
has been the introduction of a 'self-pulsating' characteristic into
the device behavior (3). Another approach is to perturb the device
electrically at very high frequencies disturbing the equilibrium in
the laser cavity and thereby decreasing the source's coherence (1).
This article describes specific circuit and technology techniques to
realize short coherence laser behavior at 100's of Mb/s data rates.

      The basic concept is to provide a small GaAs chip located
physically very close to the laser (i.e., in the same optical
assembly as the laser) which contains not only the circuits which
drive the laser but also circuits which generate the very high
frequency dithering modulation onto the laser drive current.  The
magnitude of the high frequency dithering modulation current should
be adequate to assure disturbance of the equilibrium of the laser
cavity. Hence, the magnitude of the current may have to be equal to
or a significant percent of the data modulation current. The
frequency of the dithering modulation should be at least a factor of
5 higher than the data rate to assure that high frequency intensity
modulation is not detected by the data receiver which would have a
high frequency filter. As the laser is normally biased on for both
signal levels, it may also be necessary to dither the laser at high
frequency at both the logical zero and logical one states.  The
implementation approaches include both alternatives.

      Utilizing GaAs digital technology, the most straightforward
method to generate a high frequency signal is to utilize an odd
number of inverters in a loop.  This concept is illustrated in Figs.
1 and 2.  Fig. 1 illustrates the basic concept with the one stage
providing the highest clock frequency possible with the techn...