Optical: systems and elements – Lens – With support
Reexamination Certificate
2001-04-19
2002-11-26
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With support
C372S032000, C385S092000
Reexamination Certificate
active
06487027
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No. 2000-121650 filed on Apr. 21, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor module having a capability of temperature regulation.
In a large-capacity optical-fiber transmission system that uses the technology of optical wavelength multiplexing for modulating optical information, it is necessary to use a large number of stabilized optical sources that produce respective optical beams with stabilized wavelengths.
Thus, it has been practiced to use an optical semiconductor module that uses a temperature regulation mechanism called wavelength-locker for the optical source of optical-fiber transmission systems, wherein the wavelength-locker uses a feedback control for maintaining the desired oscillation wavelength.
FIG. 1
shows the construction of an optical semiconductor module
10
having a conventional wavelength-locker.
Referring to
FIG. 1
, the optical semiconductor module
10
includes a package body
2
and a laser diode
1
, wherein the laser diode
1
is mounted on the package body
2
by way of a temperature regulation block
3
provided on the package body
2
and a carrier member
4
provided further on the temperature regulation block
3
. The laser diode
1
is fed with a driving current via a bonding wire
1
C connected to an electrode on the package body
2
and produces an output optical beam
1
A such that the optical beam
1
A is injected into an end of an optical fiber coupled to an optical window
2
A on the package body
2
.
The laser diode
1
further produces another optical beam
1
B in a direction opposite to the direction of the optical beam
1
A, wherein the optical beam travels consecutively through a beam splitter
5
and an optical wavelength-filter
6
and reaches a first photodiode
7
. Further, the optical beam
1
B which is split by the beam splitter
5
is directed to a second photodiode
8
. Further, there is provided a thermister
4
A on the carrier member
4
for measurement of the temperature of the laser diode
1
. The thermister
4
A thereby supplies an output signal thereof indicative of the temperature of the laser diode
1
to an external control circuit provided outside the package body
2
via a bonding wire
4
B and a corresponding terminal.
Further, the-optical semiconductor module
10
includes a first collimator lens (not illustrated) on the optical path of the optical beam
1
A between the laser diode
1
and the window
2
A, and a second collimator lens (not illustrated) is provided on the optical path of the optical beam
1
B between the laser diode
1
and the beam splitter
5
.
As noted previously, the optical semiconductor module
10
cooperates with an external control circuit, and the external control circuit controls the laser diode
1
, in response to the output of the thermister
4
A, in such a manner that the optical power of the optical beam
1
B detected by the second photodiode
8
is maintained constant. In other words, the photodiode
8
forms a part of an APC (automatic power control) loop.
The external control circuit further controls, in response to the output signal of the first photodiode
7
indicative of the optical power of the optical beam
1
B after being passed through the optical wavelength-filter
6
, a Peltier element that constitute the temperature regulation block
3
so as to compensate for the variation of the oscillation wavelength of the laser diode
1
by changing the operational temperature thereof.
FIG. 2
shows an example of the dependence of the oscillation wavelength on operational temperature of the laser diode
1
.
Referring to
FIG. 2
, it can be seen that the oscillation wavelength of the laser diode
1
shifts in the direction of longer wavelength side with increasing operational temperature as a result of the change of the effective length of the optical cavity and the change of the refractive index caused as a result of the temperature change.
FIG. 3
shows a transmittance curve of the optical filter
6
. It should be noted that the optical filter
6
is formed of an optical medium defined by a pair of parallel surfaces such as a glass slab and has the nature of etalon.
Referring to
FIG. 3
, it can be seen that the transmittance curve shows a sinusoidal change with the wavelength of incoming optical beam, and thus, it becomes possible to achieve a sensitive detection of the wavelength change of the laser diode
1
by way of the photodetector
7
, by designing the optical filter
6
such that the sloped region of the transmittance curve coincides with the oscillation wavelength of the laser diode
1
.
In view of the fact that the output of the laser diode
1
is maintained constant as a result of the APC control achieved by using the photodiode
8
, any decrease of the optical power of the optical beam
1
B detected by the photodetector
7
indicates an increase of the operational temperature of the laser diode
1
according to the relationship of FIG.
2
. Thus, the external control circuit controls the temperature regulation block
3
and causes a decrease of the operational temperature of the laser diode
1
. Thus, the temperature regulation block
3
and the components provided thereon constitute a wavelength-locker.
Thus, the optical semiconductor module
10
of
FIG. 1
successfully detects and compensates for any change of oscillation wavelength of the laser diode
1
caused by the temperature change.
On the other hand, in the case the temperature of the environment in which the optical semiconductor module
10
is used has been changed, there is a possibility that a temperature difference caused between the laser diode
1
, of which temperature is regulated by the temperature regulation block
3
, and other components on the temperature regulation block
3
such as the optical wavelength-filter
6
, may lead to an erroneous operation of the wavelength-locker.
In more detail, because of the fact that the temperature regulation block
3
has only a finite thermal conductivity, there can be a case, when a thermal effect arising from an environmental temperature change is transferred to the temperature regulation block
3
via the package body
2
, in that a temperature difference may arise between the temperature regulation block
3
, and hence the laser diode
1
subjected to temperature control by the temperature regulation block
3
, and the wavelength filter
6
provided on the temperature regulation block
3
.
FIG. 4
shows the transmittance of the optical filter
6
at 25° C. and 75° C., wherein the continuous line represents the transmittance at 25° C. while the broken line represents the transmittance at 75° C.
Referring to
FIG. 4
, it can be seen that the transmittance curve for the temperature of 75° C. has been shifted in the longer wavelength side, due to the thermal expansion or change of refractive index of the filter
6
, with respect to the transmittance curve for the temperature of 25° C.
Thus, there can occur a situation in the optical semiconductor module
10
of
FIG. 1
in that the photodiode
7
produces an increased output power when the optical filter
6
is at the temperature of 75° C., even in such a case the temperature of the laser diode
1
is controlled properly on the carrier member
4
by the temperature regulation block
3
. Thereby, the control circuit erroneously judges that the temperature of the laser diode
1
has decreased and activates the temperature regulation block
3
so as to raise the temperature of the laser diode
1
. When such an erroneous activation of the temperature regulation block
3
occurs, the oscillation wavelength of the laser diode shifts from the desired wavelength &lgr; to a longer wavelength &lgr;′.
In order to eliminate the foregoing problem of erroneous operation of the temperature regulation block in the optical semicondu
Machida Toyotoshi
Ninomiya Nobuhiro
Yamauchi Yasuyuki
Armstrong Westerman & Hattori, LLP
Epps Georgia
Fujitsu Quantum Devices Limited
Spector David N.
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