Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-03-19
2004-10-12
Porta, David (Department: 2878)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S020000, C372S034000, C372S029011, C372S038020, C372S038070
Reexamination Certificate
active
06804273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates generally to optical sources and in particular to compensation of thermal characteristics of a frequency stabilized optical source.
2. Description of Related Art
Recently, the channel density of commercial Wavelength Division Multiplexing (WDM) systems has increased dramatically, resulting in narrower frequency spacing between channels. Narrow channel spacing, on the order 25 GHz or 12.5 GHz, is often very sensitive to crosstalk caused by frequency drifts in which a channel interferes with an adjacent channel. To address this frequency drift and facilitate wavelength locking, optical device suppliers have integrated wavelength monitors with the optical source.
FIG. 1A
is a block diagram of an optical device
10
with a frequency controller
60
(also referred to as a locker). The optical device
10
includes an optical source
20
with an integrated frequency reference element
30
. The optical source
20
may include, but is not limited to, a distributed feedback (DFB) laser, other lasers, and the like, as well as combinations including the foregoing. The frequency reference element
30
is a component that translates the frequency of the output of the optical source
20
to an amplitude. An exemplary frequency reference element
30
is a Fabry-Perot etalon filter.
Light emitted from the rear facet of the optical source
20
is transmitted via a beam splitter to a first detector
40
and a frequency reference element
30
and thereafter to a second detector
50
. The first detector
40
and second detector
50
may include but not limited to photo detectors, photodiodes, phototransistors, and the like, as well as combinations including the foregoing. The first detector
40
produces a current indicative of the total optical output power denoted I
pf
. The second detector
50
produces a current indicative of a wavelength dependent optical power denoted I
&lgr;
. The optical power, as measured by the first detector
40
and second detector
50
, is transmitted as currents I
pf
and I
&lgr;
respectively, to controller
60
.
FIG. 1B
is a side view depicting one arrangement of components in the optical device
10
. As shown in
FIG. 1B
, the optical source
20
and the frequency reference element
30
may be positioned on the thermal electrical cooler (“TEC”)
72
. As described in further detail, the frequency reference element
30
experiences a temperature gradient due to a difference between the temperature of TEC
72
and the case
11
(or alternatively called a housing) of the optical device
10
. This temperature gradient causes the output frequency of the optical source
20
to vary.
The optical output from frequency reference element
30
varies with wavelength so that the current I
&lgr;
is indicative of the wavelength output by optical source
20
.
FIG. 2
depicts an exemplary discriminator curve when an etalon filter is used for frequency reference element
30
. The discriminator curve illustrates that the ratio of I
&lgr;
to I
pf
is indicative of the output frequency of the optical source
20
. The frequency processing module
62
executed by controller
60
translates currents I
pf
and I
&lgr;
into an error signal that is used by a temperature compensator
70
. The temperature compensator
70
adjusts the temperature of the optical source
20
to control the output frequency of the optical source
20
.
The temperature compensator
70
includes, but is not limited to, a thermoelectric cooler (TEC)
72
, temperature sensor
74
and temperature driver module
64
. The temperature driver module
64
is preferably, but not necessarily, integrated with controller
60
to control temperature of the optical source
20
. The error signal is received by the temperature driver module
64
which adjusts the temperature of the optical source
20
to reduce the error signal.
As described above, the existing wavelength-locking scheme is primarily composed of a feedback loop where the ratio (I
&lgr;
/I
pf
) is monitored. Referring to
FIG. 3
, the desired frequency is established with a particular reference point (I
&lgr;
/I
pf
)
REF
102
on the discriminator curve corresponding to a selected magnitude of the ratio (I
&lgr;
/I
pf
) and resulting in the desired frequency f
REF
. The feedback functionality implemented in frequency processing module
62
and temperature driver module
64
then adjusts the optical source
20
parameters to ensure that the ratio (I
&lgr;
/I
pf
) is maintained at the reference point, (I
&lgr;
/I
pf
)
REF
102
. The optical source parameter that is adjusted can be the drive current, the temperature, or both. In the implementation depicted in
FIG. 1
a temperature driver module
64
adjusts the temperature of the optical source
20
to maintain the desired frequency f
REF
.
FIG. 4B
depicts conventional wavelength locker processing with which the operating frequency is detected at step
190
. The operating frequency is compared to a reference frequency at step
192
and operating parameters of the optical source
20
are adjusted at step
194
.
A drawback to the existing systems is that the characteristics of the frequency reference element
30
change with temperature. Since the frequency reference element
30
may be distanced from the optical source
20
, monitoring the temperature through temperature sensor
74
may not accurately reflect the temperature of the frequency reference element
30
. As noted above, the frequency reference element
30
may experience a temperature gradient due to a temperature differential between the TEC
72
and the case
11
(or housing) of optical device
10
. Variations in the temperature of the frequency reference element
30
shifts the discriminator curve. Thus, locking the frequency based on the same reference point (I
&lgr;
/I
Pf
)
REF
104
on the shifted discriminator curve will shift the locked frequency value to f
SHIFT
. As a result of this temperature dependence for the frequency reference element
30
, the operational frequency of a frequency-locked optical source
20
drifts as the case temperature of the optical device
10
is changed. This drift is depicted in FIG.
4
A. Such drift in the output frequency of the optical source
20
can result in deleterious effects such as crosstalk between channels.
Therefore, there is a need for a mechanism to reduce the temperature effects on the frequency of the optical device
10
.
SUMMARY OF THE INVENTION
An embodiment of the invention is a controller for use with an optical device having an optical source and a frequency reference element. The controller includes a frequency processing module coupled to the optical device. The frequency processing module generates an error signal indicative of a deviation between the output frequency of the optical source and a reference frequency corresponding to a reference point. A driver module communicates with the optical device and the frequency processing module. The driver module adjusts a parameter of the optical source in response to the error signal. An offset processing module is coupled to the frequency processing module. The offset processing module derives an offset signal based on an estimate of a temperature of the frequency reference element. The offset processing module provides the offset signal to the frequency processing module which updates the reference point in response to the offset.
REFERENCES:
patent: 5373515 (1994-12-01), Wakabayashi et al.
patent: 6359918 (2002-03-01), Bielas
patent: 6560253 (2003-05-01), Munks et al.
patent: 6580513 (2003-06-01), Akahoshi
“Optical Components for Existing and Next-generation Networks,” NEC Electronics (Europe) GmbH, Global Optical Communications, Jul. 2001, pp. 1-4.
“Tunable LD Module with Wavelength Locker”, Fijitsu Compound Semiconductor, Inc., Global Optical Communications, Edition 1.0; Feb 2001, pp. 1-4.
“Wavelength Monitor Integrated 1550nm DFB laser Module”, Fitel Technologies, Inc., Preliminary Data Sheet, Jun. 2001, pp. 1-5.
Cammarata Michael
Ciena Corporation
Fox David
Monbleau Davienne
Soltz David
LandOfFree
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