Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2001-12-12
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S030000
Reexamination Certificate
active
06711188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength stabilizing unit and a laser module using the same wavelength stabilizing unit and, in particular, the present invention relates to a wavelength stabilizing unit capable of stabilizing a wavelength of emitted laser light with high accuracy, simplifying a structure thereof and reducing a size thereof and a wavelength stabilized laser module using the same wavelength stabilizing unit.
2. Description of the Prior Art
A semiconductor laser has been used as a light source of an optical fiber communication system. In particular, a single axial mode semiconductor laser such as DFB (distributed feedback) laser has been employed for optical fiber communication over distances of tens of kilometers or more in order to restrict wavelength dispersion. However, though the DFB laser oscillates at a single wavelength, its oscillation wavelength is changed depending on the temperature of the semiconductor laser device and/or an injected current. Moreover, in the optical fiber communication system, in which it is important to keep strength of light output of a semiconductor laser light source at a constant level, control is conventionally exercised so as to keep the temperature of the semiconductor laser device and the output light strength of the semiconductor laser light source in constant levels, respectively. Basically, by keeping the temperature of the semiconductor laser device and the injected current at constant levels, light output and oscillation wavelength of the semiconductor laser device is maintained constant. However, if the quality of semiconductor laser device is degraded due to longtime use, the injected current required for keeping the light output at constant level increases, causing the oscillation wavelength to change. However, since an amount of the change of the oscillation wavelength is slight, substantially no problem occurs in the conventional optical fiber communication system.
In recent years, a dense wavelength division multiplexing (DWDM) method in which multiple pieces of light each having a different wavelength are multiplexed in one optical fiber becomes mainstream in the conventional optical fiber communication system and the interval among a plurality of oscillation wavelengths used for the DWDM system becomes as narrow as 100 GHz or 50 GHz. In this case, the degree of wavelength stabilization required for the semiconductor laser device, which is used as the light source, is, for example, wavelength change within ±1 nm for a 2×10
5
hours (about 25 years) use. Therefore, the conventional wavelength stabilization by using the conventional constant element temperature-constant light output control becomes not sufficient to obtain the required degree of wavelength stabilization. Moreover, even if the temperature of the semiconductor laser itself is successfully controlled so as to remain constant, the oscillation wavelength is changed slightly when the ambient temperature around the semiconductor laser device changes and an amount of such slight change in the oscillation wavelength may become a problem in the recent conventional optical fiber communication system.
In order to restrict such change in the oscillation wavelength of the semiconductor laser light to thereby stabilize the oscillation wavelength, some wavelength stabilized devices have been proposed in such as JP H10-209546 A (Japanese Patent No. 2989775), JP H4-157780 A (Japanese Patent No. 2914748), JP H9-219554 A, JP H10-79723 and JP H9-121070. However, each of the proposed wavelength stabilized devices requires a large number of parts and a large space, so that it becomes difficult to house the wavelength stabilized device in the generally used casing of the conventional semiconductor laser module. Moreover, the setting of a reference wavelength to be stabilized is difficult and the fabrication cost becomes high.
On the other hand, Japanese Patent Application No. 2000-67606, which corresponds to U.S. patent application Ser. No. 09/804,499 assigned to the assignee of the present application and will be referred to as “prior application”, hereinafter, proposes a wavelength stabilized laser module capable of solving the above problems.
FIG. 10A
shows an example of a construction of a wavelength stabilized laser module proposed in the prior application and
FIG. 10B
shows a portion thereof in an enlarged scale. The wavelength stabilized laser module shown in FIG.
10
A and
FIG. 10B
includes a semiconductor laser
801
housed in a casing
809
, a lens
802
for converting rearward diverging light emitted from the semiconductor laser into parallel light bundle, a first photoelectric conversion element
805
, which directly receives a portion of the parallel light bundle passed through the lens
802
and converts the light portion into an electric signal, an etalon-type filter
831
, which receives another portion of the parallel light bundle passed through the lens
802
, and a second photoelectric conversion element
806
, which converts light passed through the etalon-type filter
831
into an electric signal.
The semiconductor laser
801
is mounted on a substrate
807
equipped with a Peltier element so that temperature thereof during operation can be regulated. An incident angle of light to the etalon type filter
831
can be regulated by an angle regulating mechanism, which is not shown. The first photoelectric conversion element
805
and the second photoelectric conversion element
806
are arranged in parallel on a supporting substrate
849
to form an array type optical detector
804
. The optical detector
804
is slanted with respect to an optical axis of the incident light in order to prevent light from being reflected back to the semiconductor laser.
The wavelength stabilized laser module constructed as mentioned above has is highly accurate, has small number of parts, has good space efficiency and has a size small enough to be housed in a casing of the semiconductor laser module, which has been used usually. Further, since the assembling of the wavelength stabilized laser module and the positional regulation thereof are easy, the fabrication cost thereof can be substantially reduced.
As mentioned above, it has been found that the considerable effects can be obtained by the wavelength stabilized laser module proposed in the prior art application. However, the inventors of the present invention conducted various experiments on the proposed wavelength stabilized laser module and have found that the latter wavelength stabilized laser module has some points to be improved.
The points to be improved will be described in detail with reference to
FIG. 12
, which shows graphs illustrating relations between an oscillation wavelength &lgr; of a semiconductor laser in abscissa and currents Im of a strength monitoring PD (photo diode) when laser light emitted from the semiconductor laser is incident directly on the photoelectric conversion element and a wavelength monitoring PD when the output light is incident on the photoelectric conversion element after passed through a predetermined filter such as an etalon-type filter, in ordinate. Further, as described in the prior art application, the oscillation wavelength of the semiconductor laser is changed depending on not only change of temperature of the photoelectric conversion element but also change of injected current of the semiconductor laser, as shown in FIG.
13
A and FIG.
13
B. On the contrary, the light output is changed by not only change of the injected current but also change of temperature as shown in the same figures. Considering a case where the oscillation wavelength of the semiconductor laser is controlled to a reference wavelength &lgr;o while keeping the optical output thereof constant, on the basis of the graph shown in
FIG. 12
, it is possible to simultaneously control the oscillation wavelength and the output light of the semiconductor laser by controlling the current I
pd1
detected by the strength monitoring PD
Ito Akihiro
Shimizu Jun-ichi
Hayes & Soloway P.C.
Ip Paul
NEC Compound Semiconductor Devices Ltd.
Nguyen Tuan
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