Wavelength monitor and laser module

Details Wavelength monitor and laser module Wavelength monitor and laser module

2002-08-21

2004-04-20

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular resonant cavity

Mirror support or alignment structure

C372S097000, C372S098000, C372S108000

Reexamination Certificate

active

06724800

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Filed of the Invention
The present invention relates to a semiconductor laser module that is utilized in an optical transmitter. Particularly, the invention relates to a wavelength monitor inside an optical signal transmission module that is utilized in a wavelength division multiplexing (WDM) system, and a laser module with integrated wavelength monitor.
2) Description of the Related Art
A semiconductor laser device can obtain large laser output power when an injection current is increased. In general, the calorific value of the device itself increases in proportion to the injection current. The increase in heat affects the characteristics of semiconductor layers and optical parts that constitute the semiconductor laser device. The increase in heat generates various inconveniences. For example, the wavelength of an actual laser output is deviated from a desired wavelength, and the life of the device is shortened.
Particularly, in the semiconductor laser device that is used in a high-density WDM system, it is necessary to carry out wavelength control precisely. Therefore, it is necessary that the wavelength of an optical signal is stable over a long period of time. For this purpose, there has been developed a technique of providing a wavelength monitoring function inside a laser module that is built in with a semiconductor laser device.
FIG. 15
is a top plan cross-sectional view of a laser module that has been proposed by the applicant of the present invention in U.S. patent application No. 10/032,612 (a first conventional example). In a laser module
200
shown in
FIG. 15
, the front end of an optical fiber
11
is fixed to a package
201
with a ferrule
12
, in order to emit a laser beam generated by a semiconductor laser device
20
into the optical fiber
11
.
On the bottom surface of the package
201
, there are adjacently disposed a first thermo-module and a second thermo-module not shown that can be heated or cooled based on a control from the outside. A base
30
made of CuW or the like is mounted on the first thermo-module. On this base
30
, there are disposed a sub-mount
34
that is mounted with the semiconductor laser device
20
and a thermistor
21
that measures the temperature of the semiconductor laser device
20
, a condenser lens
33
that connects a laser beam output from a front end surface of the semiconductor laser device
20
to the optical fiber
11
, an optical isolator
32
that interrupts a return light reflected from the optical fiber
11
, and a parallel lens
35
that makes parallel a monitoring laser beam that is output from a back end surface of the semiconductor laser device
20
. Sections including the base
30
, the condensing lens
33
, the sub-mount
34
, and the parallel lens
35
will be collectively called a laser section.
On the other hand, a base
50
made of CuW or the like is mounted on the second thermo-module. On this base
50
, there are disposed a prism
51
that divides a monitoring laser beam that is output from the back end surface of the semiconductor laser device
20
, into two directions at a predetermined angle, an optical filter
52
into which one of the lights divided by the prism
51
is incident, and a sub-mount
53
. On the front surface (the surface of a laser emission direction) of the sub-mount
53
, there are disposed on the same plane a first optical detector
41
that receives the other light divided by the prism
51
, and a second optical detector
42
that receives the light that has been transmitted through the optical filter
52
. Photodiodes are used for the first optical detector
41
and the second optical detector
42
. The prism
51
is constructed of light incident surfaces
51
a
and
51
b
mutually formed at a predetermined angle to which the monitoring beam is incident, and a light emission surface
51
c
from which lights that have been divided within the prism
51
are emitted.
In the vicinity of the portion at which the prism
51
is fixed, there is provided a thermistor
54
that monitors the temperature of the optical filter
52
. The base
50
and sections including the various constituent elements provided on the base
50
will be collectively called a wavelength monitor.
Based on the above structure, the laser module
200
controls the temperatures of the first thermo-module and the second thermo-module, thereby to realize a stable laser oscillation. The temperature control carried out by this laser module
200
will be briefly explained below. First, the monitoring laser beam that is output from the back end surface of these miconductor laser device
20
passes through the parallel lens
35
, and is divided into two directions by the prism
51
.
One of the lights obtained by dividing by the prism
51
is converted into a current by the first optical detector
41
, and this current is converted into a voltage by a current-voltage converter not shown. This voltage is used as a reference voltage. The other light obtained by the dividing by the prism
51
passes through the optical filter
52
, and is converted into a current by the second optical detector
42
, and this current is converted into a voltage by a current-voltage converter not shown. This voltage is used as a signal voltage. The optical filter
52
has characteristics of different transmittances for the wavelengths of the incident light. This optical filter
52
is formed with an etalon, for example. A difference between the signal voltage obtained by passing the light of a desired wavelength through the optical filter
52
and the reference voltage will be called a reference voltage difference. Then, it is possible to know a wavelength deviation by comparing a voltage difference between the actual reference voltage and the signal voltage with the reference voltage difference.
This wavelength deviation is due to the heating of the semiconductor laser device
20
. Therefore, in order to correct this deviation, the sub-mount
34
beneath the semiconductor laser device
20
may be cooled. The voltage that shows the wavelength deviation that is obtained based on the above comparison is used as a control voltage for a controller not shown to control the temperature of the first thermo-module disposed beneath the base
30
. The first thermo-module is operated as a cooler. With this arrangement, the semiconductor laser device
20
is cooled via the first thermo-module, the base
30
, and the sub-mount
34
, and is feedback controlled to output the laser beam of a desired wavelength. This will hereinafter be referred to as a wavelength locking. When excessive cooling is obtained based on the feedback control, the first thermo-module operates as a heater.
The characteristic of the optical filter
52
that is formed with etalon changes depending on the temperature. Therefore, it is preferable to keep constant the temperature of the optical filter
52
. The controller not shown calculates a difference between a desired temperature and the temperature detected by the thermistor
54
, and controls the temperature of the second thermo-module disposed beneath the base
50
, by using the voltage corresponding to this difference as a control voltage. With this arrangement, the optical filter
52
is heated or cooled via the second thermo-module and the base
50
, and is stabilized at the desired temperature.
FIG. 16
is a top plan cross-sectional view of a laser module which shows a second conventional example. In
FIG. 16
, sections that are common to those shown in
FIG. 15
are attached with identical reference symbols, and explanation of these sections will be omitted. A laser module
210
shown in
FIG. 16
is different from the laser module
200
shown in
FIG. 15
in only the structure of the wavelength monitor.
Specifically, on abase
50
, there are disposed sub-mounts
61
and
62
that are separated from each other so that their main surfaces form a right angle, a half-mirror
71
that transmits a monitoring laser beam output from the back end surface of a semiconductor laser device
20
to a sub-mou

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