Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2002-08-21
2004-10-19
Wong, Don (Department: 2828)
Coherent light generators
Particular temperature control
Heat sink
C372S034000
Reexamination Certificate
active
06807208
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a semiconductor laser module used for an optical communication apparatus, and more specifically relates to a laser module for optical signal transmission or for a pump light source used for a wavelength division multiplexing system (WDM).
2) Description of the Related Art
Semiconductor laser devices can obtain a high laser output power by increasing an injected current, but the heat output from the device itself generally increases, in proportion to the injected current. The increased heat affects the properties of the semiconductor layer or optical parts which constitute the semiconductor laser device, causing various problems such that the wavelength of the laser actually output is deviated from a desired wavelength or the life of the device is shortened.
Particularly, in a semiconductor laser device used for dense WDM, it is required that the wavelength of the optical signal is stable for a long period of time, and hence it is necessary to accurately perform wavelength control. Therefore, a technique of providing a wavelength monitoring function in a laser module with the semiconductor laser device embedded therein has been well known.
FIG. 28
is a sectional side view of a conventional laser module in a laser outgoing direction. In
FIG. 28
, in a conventional laser module
300
, a ferrule
12
for holding an optical fiber
11
is provided at an opening of a package
101
, that is, in a light outgoing portion. On the bottom of the package
101
, a first thermo-module
68
and a second thermo-module
69
are arranged close to each other. The first thermo-module
68
and the second thermo-module
69
are apparatus, the surface of which can be heated or cooled depending on the size and direction of the current to be passed, and are formed of a Peltier element or the like.
A base
30
formed of CuW or the like is arranged on the first thermo-module
68
. On the top of the base, a submount
34
on which a semiconductor laser device
20
is mounted, a focusing lens
33
which focuses laser beams output from the front end face of the semiconductor laser device
20
onto an optical fiber
11
, an optical isolator
32
which interrupts reflected return light from the optical fiber
11
side, and a collimator lens
35
which collimates the monitoring laser beams output from the rear end face of the semiconductor laser device
20
, are provided. The portion including the base
30
, the focusing lens
33
, the submount
34
, and the collimator lens
35
is referred to as a laser section.
On the other hand, a base
50
formed of CuW or the like is put on the second thermo-module
69
, and on the top of the base, a prism
51
that splits the monitoring laser beams output from the rear end face of the semiconductor laser device
20
into two directions at a predetermined angle, an optical filter
52
to which one of the beams split by the prism
51
enters, and a submount
53
, are provided. On the front face (a face in the laser outgoing direction) of the submount
53
, a first optical detector
41
which receives the other of the beams split by the prism
51
, and a second optical detector
42
which receives the beam passing through the optical filter
52
are provided on the same plane of the submount. A photo diode is used for the first optical detector
41
and the second optical detector
42
.
A thermistor
54
that monitors the temperature of the optical filter
52
is provided near a portion where the prism
51
is fixed. The portion including the base
50
and each component provided on the base
50
is referred to as a wavelength monitoring section.
In this laser module
300
having the configuration, stable laser emission is realized by controlling the temperature of the first thermo-module
68
and the second thermo-module
69
. The temperature control in this laser module
300
will be briefly explained below. The monitoring laser beam output from the rear end face of the semiconductor laser device
20
passes through the collimator lens
35
, and the beam is split into two directions by the prism
51
.
The one of the beams split by the prism
51
is converted into electric current by the first optical detector
41
, and is used as a reference voltage in a not-shown current-voltage converter. The other of the beams split by the prism
51
passes through the optical filter
52
, and the beam is converted into electric current by the second optical detector
42
, and is used as a signal voltage in the not-shown current-voltage converter. The optical filter
52
has a property such that the transmission factor thereof is different with respect to the wavelength of the incident beams, and is formed of for example etalon. Therefore, when it is assumed that a difference between the signal voltage obtained with beams having a desired wavelength passing through the optical filter
52
and the reference voltage is a standard voltage difference, a wavelength deviation can be found by comparing the voltage difference between the actual reference voltage and the signal voltage with the standard voltage difference.
Since the wavelength deviation can be corrected by changing the temperature of the semiconductor laser device
20
, the temperature of the submount
34
located below the semiconductor laser device
20
may be adjusted (cooled or heated) in order to correct the deviation. Therefore, a not-shown controller uses the voltage indicating the wavelength deviation obtained by the comparison as a control voltage for controlling the temperature of the first thermo-module
68
, to operate the first thermo-module
68
as a temperature adjuster. As a result, the semiconductor laser device
20
is feedback controlled so that the temperature thereof is adjusted via the first thermo-module
68
, the base
30
, and the submount
34
, to thereby suppress changes in the wavelength. That is, laser beams having a desired wavelength are output (hereinafter, this controlled state is referred to as wavelength locking).
However, since the optical filter
52
formed of etalon changes the property depending on the temperature, it is desirable to keep the temperature constant. Therefore, the not-shown controller calculates a difference between a desired temperature and the temperature detected by the thermistor
54
, to control the temperature of the second thermo-module
69
, designating the voltage corresponding to the difference as a control voltage. As a result, the optical filter
52
is heated or cooled via the second thermo-module
69
and the base
50
, and stabilized at a desired temperature.
In the conventional laser module, however, since the temperature of the semiconductor laser device
20
is controlled by only the first thermo-module
68
, there is a problem that a wavelength variable range, that is, a temperature variable range is not sufficient for realizing a so-called wavelength variable type laser module that selects the temperature of the semiconductor laser device
20
within a predetermined range and uses a laser beam having a wavelength emitted at the selected temperature. As the cause thereof, it can be considered that the cooling ability of the thermo-module unit is not sufficient, and the temperature of the package becomes high due to heat transmitted from the thermo-module.
Insufficient cooling ability of the thermo-module unit will be first explained. The temperature range that can be controlled in the normal thermo-module is about 60° C., and therefore, when temperature of from −5° C. to 70° C. are required as the temperature specification of the laser module package, the temperature variable range by the first thermo-module
68
becomes from 10° C. to 55° C., and hence it is possible to adjust the semiconductor laser device
20
in a range of about 45° C. The temperature dependency of the emission wavelength of the semiconductor laser device is determined by the material of the semiconductor, and it is known that the temperature dependency thereof is about 0.1 nm/° C. Therefore,
Hiratani Yuji
Nasu Hideyuki
Nomura Takehiko
Shimada Mamoru
Ueki Tatsuhiko
Menefee James
The Furukawa Electric Co. Ltd.
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