Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-10-26
2004-06-01
Jackson, Jerome (Department: 2815)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S020000, C372S028000, C372S075000, C372S038060, C372S029011
Reexamination Certificate
active
06744792
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to photonic devices in general, and more particularly to tunable lasers.
BACKGROUND OF THE INVENTION
In pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER, and in pending prior U.S. patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by Peidong Wang et al. for SINGLE MODE OPERATION OF MICROMECHANICALLY TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS, which patent applications are hereby incorporated herein by reference, there are disclosed tunable Fabry-Perot filters and tunable vertical cavity surface emitting lasers (VCSEL's).
More particularly, and looking now at
FIG. 1
, there is shown a tunable Fabry-Perot filter
5
formed in accordance with the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318. Filter
5
generally comprises a substrate
10
, a bottom mirror
20
mounted to the top of substrate
10
, a bottom electrode
15
mounted to the top of bottom mirror
20
, a thin support
25
atop bottom electrode
15
, a top electrode
30
fixed to the underside of thin support
25
, a reinforcer
35
fixed to the outside perimeter of thin support
25
, and a confocal top mirror
40
set atop thin support
25
, with an air cavity
45
being formed between bottom mirror
20
and top mirror
40
.
As a result of this construction, a Fabry-Perot filter is effectively created between top mirror
40
and bottom mirror
20
. Furthermore, by applying an appropriate voltage across top electrode
30
and bottom electrode
15
, the position of top mirror
40
can be changed relative to bottom mirror
20
, whereby to change the length of the Fabry-Perot cavity, and hence tune Fabry-Perot filter
5
.
Correspondingly, and looking next at
FIG. 2
, a tunable vertical cavity surface emitting laser (VCSEL)
50
can be constructed by positioning a gain medium
55
between bottom mirror
20
and bottom electrode
15
. As a result, when gain medium
55
is appropriately stimulated, e.g., by optical pumping or by electrical pumping, lasing can be established between top mirror
40
and bottom mirror
20
. Furthermore, by applying an appropriate voltage across top electrode
30
and bottom electrode
15
, the position of top mirror
40
can be changed relative to bottom mirror
20
, whereby to change the length of the laser's resonant cavity, and hence tune laser
50
.
The present invention is directed to tunable lasers of the type disclosed in the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318.
Tunable lasers of the type disclosed in the aforementioned U.S. patent application Ser. Nos. 09/105,399 and 09/543,318 are highly advantageous since they can be quickly and easily tuned by simply changing the voltage applied across the top electrode and the bottom electrode.
However, it has been found that tunable lasers of the type disclosed in the aforementioned U.S. patent application Ser. Nos. 09/105,399 and 09/543,318 can suffer from vibrational problems.
The aforementioned vibrational problems may be due to a variety of factors, such as thermal noise; or noise in the tuning voltage of the laser; or, in the case of an electrically pumped laser, shot noise in the injection current; etc.
Regardless of the cause, the effect of these vibrational problems is to cause the laser to move out of tune. In other words, these vibrational effects cause the output frequency of the laser to change even though the tuning voltage of the laser is held constant. While the extent of this vibration-related frequency shift may be relatively modest (e.g., a 300 MHz shift in the lasing frequency from a 100 MHz vibration frequency), this frequency shift may nonetheless create significant problems in certain types of systems, e.g., WDM communication systems.
See, for example,
FIG. 3
, which schematically illustrates how the aforementioned vibrational problems may cause a relatively periodic modulation of the lasing frequency; and
FIG. 4
, which schematically illustrates how the aforementioned vibrational problems may cause a relatively irregular modulation of the lasing frequency.
As a result, an object of the present invention is to provide a method and apparatus for stabilizing the wavelength of tunable lasers affected by the aforementioned vibrational problems.
SUMMARY OF THE INVENTION
The present invention provides a fast and easy way to compensate for the aforementioned vibrational problems in tunable lasers, by correspondingly adjusting the electrooptical performance of the laser's gain medium, whereby to eliminate the frequency shift due to vibrational factors.
The electrooptical performance of the laser's gain medium is adjusted, in the case of an electrically pumped laser, by changing the injection current used to pump the laser; and the electrical performance of the laser's gain medium is adjusted, in the case of an optically pumped laser, by changing the intensity of the pump laser used to energize the laser.
The system is implemented with a feedback mechanism. A wavelength measuring module detects the difference between the instantaneous wavelength of the laser and the desired wavelength of the laser, and generates a voltage signal which is representative of this difference. This voltage signal is then used to appropriately modify the electrooptical performance of the laser's gain medium, either by appropriately adjusting the injection current applied to the gain medium (in the case of an electrically pumped laser), or by appropriately adjusting the intensity of the pump laser applied to the gain medium (in the case of an optically pumped laser).
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Gorecki John C.
Jackson Jerome
Landau Matthew
Nortel Networks Ltd.
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