Optical: systems and elements – Optical modulator
Reexamination Certificate
1999-10-12
2001-05-15
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C385S003000
Reexamination Certificate
active
06233082
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical transmitters and, more particularly, to laser-based transmitters for use in wavelength division multiplexed (WDM) systems.
BACKGROUND OF THE INVENTION
In optical transmitters that utilize semiconductor lasers as the optical source, the laser may be either directly modulated or externally modulated. In the directly modulated case the drive current to the laser is modulated in accordance with an information signal in order to produce a corresponding modulation of a parameter (e.g., intensity) of the output beam of the laser. In the externally modulated case the laser is operated in a continuous wave (cw) mode, and the output beam of the laser is coupled to an optical modulator that is external to the laser. An information signal is applied to the modulator so as to modulate a parameter of the output beam.
In a typical externally modulated optical transmitter the modulator is a semiconductor electroabsorption (EA) modulator. The EA modulator relies on the Quantum-Confined Stark Effect (in MQW semiconductors) or the Franz-Keldysh Effect (in bulk semiconductors) to alter the absorption of the laser beam. That is, a voltage bias applied to the EA modulator causes the bandgap of the modulator to shift relative to the wavelength of the beam which, in turn, changes the absorption of the beam.
The difference between the wavelength of the laser beam and the wavelength corresponding to the bandgap of the EA modulator is defined as the detuning. Detuning controls many important transmission parameters such as output power, extinction ratio and dynamic chirp of the modulator.
In a single wavelength transmitter, such as a monolithically integrated DFB laser/EA modulator, detuning is tightly controlled by the design of the devices; e.g., by varying the bandgap of the EA modulator during epitaxial growth to match the desired DFB wavelength on a particular wafer. One or two nanometers of variability can be compensated by adjusting the voltage bias to the modulator. The adjustment range is limited, however, by the voltage at which the EA modulator characteristics become degraded.
In WDM systems the transmitter is modified so that it is capable of generating an output beam at a any one of a multiplicity of wavelengths. One such modification is to replace the single-channel DFB laser with either a broadband wavelength selectable laser (WSL), such as a tunable DBR laser, or an array of DFB lasers coupled to a passive combiner network. In this case, however, the detuning of each channel wavelength from the EA modulator bandgap would be different from channel to channel, with the undesirable consequence that the transmission performance of each channel would be different.
SUMMARY OF THE INVENTION
An optical transmitter for generating any one of N carrier signals for use in an M-channel WDM system (M≧N), each channel operating at a different carrier wavelength &lgr;
s
(s=1, 2 . . . M), includes an optical source for generating the carrier signals at any one of multiplicity of N wavelengths &lgr;
i
(i=1, 2 . . . N), where (1≦N≦M). A first controller selects a particular one of the wavelengths &lgr;
i
at which the source operates. An optical modulator receives the carrier signal corresponding to the selected wavelength &lgr;
i
and impresses information on the received signal. The modulator has a characteristic electronic bandgap and a wavelength &lgr;
g
corresponding thereto. At a given temperature, &lgr;
g
is offset from each &lgr;
i
by an amount &Dgr;&lgr;
i
. For each wavelength &lgr;
i
there is a predetermined value of &Dgr;&lgr;
i
which delivers preferred (e.g., optimum) transmission performance. In general, however, the actual &Dgr;&lgr;
i
may not be equal to the predetermined &Dgr;&lgr;
i
for all values of i. The difference between the actual &Dgr;&lgr;
i
and the predetermined &Dgr;&lgr;
i
is termed the detuning error. In accordance with one aspect of my invention, the transmitter includes a second controller for minimizing the detuning error. In a preferred embodiment, the second controller changes the temperature of the modulator as the wavelength of the source is changed (i.e., as different &lgr;
i
are selected) so that &Dgr;&lgr;
i
is controlled according to the above criterion. In one embodiment, in which the wavelength of the source can be tuned independent of the temperature of the modulator, the second controller maintains the detuning error essentially zero for all values of i selected. In another embodiment, in which the wavelength of source is not tuned independent of the temperature of the modulator, &lgr;
i
is detuned from &lgr;
s
by a predetermined amount related to the rate of change of &lgr;
i
and &lgr;
g
with respect to temperature.
REFERENCES:
patent: 5325225 (1994-06-01), Suzaki et al.
patent: 5347601 (1994-09-01), Ade et al.
patent: 5365361 (1994-11-01), Noll et al.
patent: 5373382 (1994-12-01), Pirio et al.
patent: 5926297 (1999-07-01), Ishikawa et al.
patent: 6078414 (2000-06-01), Iwano
patent: 6141140 (2000-10-01), Kim
Ben Loha
Lucent Technologies - Inc.
Urbano Michael J.
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