Wavelength stabilized laser module

Details Wavelength stabilized laser module Wavelength stabilized laser module

2001-03-12

2004-09-07

Thomas, Tom (Department: 2815)

Coherent light generators

Particular beam control device

Having particular beam control circuit component

C372S032000, C372S034000, C372S036000, C372S038100, C372S038010, C372S038020, C372S029011, C372S029020, C356S452000, C356S454000, C356S460000, C356S506000

Reexamination Certificate

active

06788717

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength stabilized laser module and more particularly to the wavelength stabilized laser module capable of emitting laser light whose wavelength is stabilized with high accuracy and of being so configured as to be simple in structure and being made smaller in size.
The present application claims priority of Japanese Patent Application No.2000-067606 filed on Mar. 10, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, a semiconductor laser device is used as an optical source for optical fiber communication. A single axial mode semiconductor laser such as a DFB (Distributed FeedBack laser) laser is employed, in particular, for optical fiber communication over distances of tens of kilometers or more in order to prevent an adverse effect on chromatic dispersion. However, though the DFB laser oscillates at a single wavelength, its oscillation wavelength is changed depending on a temperature of the semiconductor laser device and/or an injected current. Moreover, in the optical fiber communication system, since it is important to keep output strength 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 strength of the semiconductor laser light source at the constant level. Basically, by keeping the temperature of the semiconductor laser device and the injected current at the constant level, a light output and oscillation wavelength of the semiconductor laser device can remain constant. However, if the semiconductor laser device is degraded due to longtime use, the injected current required for keeping the light output at the constant level increases, as a result, causing the oscillation wavelength to change. However, since changed amounts of the oscillation wavelength are slight, substantially no problem has occurred in the conventional optical fiber communication system.
In recent years, a DWDM (Dense Wavelength Division Multiplexing) 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 an interval among a plurality of the oscillation wavelengths used for the DWDM system becomes as narrow as 100 GHz or 50 GHz. In this case, a degree of wavelength stabilization required for the semiconductor laser device being used as the light source is, for example, ±50 pm, which means that conventional method in which temperatures of the semiconductor laser device and outputs of the semiconductor laser light are controlled so as to be kept at the constant level is not sufficient to obtain the required degree of wavelength stabilization. Moreover, even if the temperature of the semiconductor laser itself can be successfully controlled so as to remain constant, every time an ambient temperature surrounding the semiconductor laser device changes, the oscillation wavelength is also changed slightly and cases are increasing in which such a slight change in the oscillation wavelength causes a problem in the recent conventional optical fiber communication system.
In order to prevent such changes in the oscillation wavelength of the semiconductor laser light and to stabilize the oscillation wavelength, some conventional methods for stabilization have been proposed. A first example of the conventional wavelength stabilizing device (hereinafter being referred to as a first conventional example) is disclosed in, for example, Japanese Patent No. 2989775 (JP. Appln. Laid-open No. Hei 10-209546, in which, as shown in
FIG. 19
, a wavelength stabilizing device
128
is housed in a case mounted separately from a semiconductor laser module. A part of laser light is branched through a coupler
109
from an optical fiber transmission path
108
and introduced into the wavelength stabilizing device
128
. In the wavelength stabilizing device
128
, a filter
103
serving as a band pass filter is embedded, and an optical detector
111
used to detect light transmitted through the filter
103
and an optical detector
110
used to detect light reflected off the filter
103
are placed opposite to each other.
FIG. 20
is a diagram showing optical current spectra to explain operations of the wavelength stabilizing device
128
. As shown in
FIG. 20
, the transmitted light detected by the optical detector
111
and the reflected light detected by the optical detector
110
are in opposite phase relative to the oscillation wavelength of the semiconductor laser light. By calibrating the filter
103
and the optical detectors
110
and
111
so that a point of intersection of the reflected light and transmitted light, which is indicated by an arrow in
FIG. 20
, becomes a targeted wavelength for stabilization and by feeding back the transmitted light and reflected light to a temperature controlling unit (not shown) attached to the semiconductor laser device so that the transmitted light and reflected light become equal in light strength, the stabilization of oscillation wavelength of the semiconductor laser is achieved. Moreover, a slide adjusting unit
112
to set a reference wavelength to be used as a target for the stabilization is mounted on the filter
103
.
The conventional wavelength stabilizing device
128
has problems in that, since it is basically housed in the case mounted separately from the semiconductor laser module, additional space for its installation is required, thus causing an increase in costs. Moreover, since a part of the signal light is branched by the coupler
109
, the branched light is attenuated. Though the targeted reference wavelength can be set only by adjusting a position of the filter
103
using the slide adjusting unit
112
, a specially-fabricated expensive filter is required which is so worked as to change its transmission characteristics in a direction of its plane by gradually changing internal thickness of the filter. Furthermore, since transmission characteristics of a filter, in general, are changed depending on a temperature of the filter itself, a separate process of adjusting the temperature of the filter
103
or a special electric circuit that can compensate for changes in transmission characteristics caused by the temperature is required.
A second example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser light and to stabilize its wavelength is disclosed in Japanese Patent No. 2914748 (JP. Appln. Laid-open No. Hei 4-157780, which is shown in
FIG. 21. A
basic principle of stabilizing the wavelength in this wavelength stabilizing device is the same as provided in the first conventional example; that is, a part of signal light is branched and incident on a filter
103
, and reflected light and transmitted light from the filter
103
are detected by the optical detectors
110
and
111
respectively, which are then fed back to the temperature controlling unit (not shown) of the semiconductor laser. The second conventional example differs from the first conventional example in that a frequency setting section
113
is provided, which is used to adjust an angle of the filter
103
.
However, there is a problem in that, when the angle of the filter
103
is adjusted, since a direction of the reflected light from the filter
103
is also changed, a position of the optical detector
110
used to detect the reflected light has to be adjusted accordingly. In the second conventional example, though a method for adjusting a temperature of the filter
103
, method for changing an electro-optic effect of the filter
103
or a like are also disclosed, it is actually difficult to put these methods to practical use.
A third example of the conventional wavelength stabilizing device adapted to prevent changes in an oscillation wavelength of a semiconductor laser and to stabilize its wavelength is disclosed in Japane

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