Browse Prior Art Database

Self-Tuning Optical External Modulator

IP.com Disclosure Number: IPCOM000103819D
Original Publication Date: 1993-Feb-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 4 page(s) / 118K

Publishing Venue

IBM

Related People

Choy, MM: AUTHOR [+3]

Abstract

In fiber optic communication systems operating at very high bit rates (around 1 gigabit per second or higher), external on-off keying (OOK) of light source is often required. If direct modulation of the laser diode producing the light were used instead, the frequency chirping of the laser would spill over into adjacent channels (in the case of a dense wavelength division system) or produce undesired pulse smearing due to dispersion in the fiber transmission medium (in the case of long fiber links).

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Self-Tuning Optical External Modulator

      In fiber optic communication systems operating at very high bit
rates (around 1 gigabit per second or higher), external on-off keying
(OOK) of light source is often required.  If direct modulation of the
laser diode producing the light were used instead, the frequency
chirping of the laser would spill over into adjacent channels (in the
case of a dense wavelength division system) or produce undesired
pulse smearing due to dispersion in the fiber transmission medium (in
the case of long fiber links).

      Thus, external modulation is required for such cases, that is,
the laser diode operates CW and the light is turned on and off by
some external device, such as a Mach-Zehnder interferometer, through
which the CW light has been passed on its way into the fiber
transmission path.  The available external modulators suffer from at
least two disadvantages.  They are expensive (around $8-10,000) and
they do not have a very low extinction ratio (ratio of "off" light
intensity to "on" light intensity).

      Recently, Barnsley left lbracket 1 right rbracket has proposed
using a Fabry-Perot laser diode amplifier (LDA) as an external
modulator.  G(.la), the gain as a function of optical wavelength for
a typical LDA is shown in FIG. 1, and Barnsley's proposal is that the
injection current on the LDA be varied directly by the modulation, so
that one of the gain peaks (due to Fabry-Perot cavity resonance
within the LDA) be brought into or out of coincidence with .la sub 0,
the wavelength of the incident CW light for a "1" or a "0" bit,
respectively.  (Solid and dashed curves, respectively).

      Fabry-Perot laser diodes cost only a few hundred dollars and
there is no reason to believe that they cannot be made into equally
cheap LDAs.  Also, by controlling the product of mirror reflectivity
and internal gain, the extinction ratio can be made very large.  In
fact, the only real expense of LDAs comes when one wants to minimize
the extinction ratio, i.e., the Fabry-Perot ripple.

      However, as the proposal now stands, left lbracket 1 right
rbracket, no means for compensating and tracking frequency drift of
the LDA or the CW light source has been available.  The present
invention solves this problem.

      The present invention, shown in Fig. 2, involves measuring
either the instantaneous output light power P(t) or the voltage V(t)
developed across the LDA to indicate whether or not the LDA has been
tuned so that a "1" bit corresponds to .la sub 0, the frequency of
the incoming CW light.  If it has been properly tuned, either P(t) or
V(t) will be at its maximum value.

      The measured variable is used as the input to a servo loop that
operates to assure this tuned condition.  A bipolar error signal is
developed by dithering the tuning of the LDA about the peak at a
frequency of, i.e., several tens of KHz, which is out of band with
respect to the gigabit modulation r...