Semiconductor laser module

Details Semiconductor laser module Semiconductor laser module

2000-09-14

2004-01-27

Jackson, Jerome (Department: 2815)

Coherent light generators

Particular resonant cavity

Specified cavity component

C372S027000, C372S102000, C372S098000, C372S006000, C372S019000, C385S097000

Reexamination Certificate

active

06683902

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor laser module and, in particular, to a semiconductor laser module which comprises an external resonator having a stable oscillating condition.
BACKGROUND OF THE INVENTION
In recent years, semiconductor lasers have come to be used extensively as light sources for signals and as light sources for pumping optical fiber amplifiers in the field of optical transmission. When a semiconductor laser is used as a light source for signals and as a light source for pumping an optical fiber amplifier for optical transmission, the semiconductor laser is often used in a semiconductor laser module which is a device for optically coupling laser beams from the semiconductor laser (semiconductor laser element) with an optical fiber by an optical coupling means.
In order to stabilize the wavelength of the light emitted from the semiconductor laser element, for example, this semiconductor laser module optically feed backs the light from the semiconductor laser element by a fiber Bragg grating (hereinafter, simply referred to as “FBG”).
However, when an external resonator comprises the FBG, conditions of deflected light fed back may have a component different from a polarization direction that can be amplified by the semiconductor laser depending on form conditions of the optical fiber such as a winding method, and the amount of light fed back effectively changes and, as a result, oscillation conditions may change.
As a countermeasure therefor, use of a birefringence fiber can be considered. By means of the birefringence fiber, a plane of polarization is maintained, the amount of light fed back can almost uniformly be maintained, and it becomes possible to suppress the fluctuation in oscillation conditions caused by the change in form such as the winding method of the optical fiber.
However, it was found that use of the birefringence fiber has a harmful influence in that peaks at same intervals arise on the spectrum and the oscillation conditions become unstable as time elapses.
Herein, where the length from the incident end of the optical coupling means side to the center of the reflective portion in the optical fiber is defined as L, the birefringence amount of the optical fiber is defined as &Dgr;n, and the oscillation wavelength is defined as &lgr;, according to the characteristics of the birefringence fiber, the propagation coefficient of light is different between eigen axes, that is, between X axis and Y axis, therefore, when the light returns after reflected by the FBG, the phase difference 4&pgr;·&Dgr;n·L/&lgr; occurs between the light propagated through the respective eigen axes.
On the other hand, the semiconductor laser element oscillates in TE (transverse electric) mode, whereas a very little amount of TM (transverse magnetic) mode also exists.
Also, when the eigen axes of the birefringence fiber are slightly shifted from the direction of TE mode of the semiconductor laser element and fixed to the module, light in is made incident onto both the X axis and Y axis in the birefringence fiber.
Also, there is no possibility that the crosstalk between the eigen axes of the birefringence fiber becomes absolutely 0.
Due to the abovedescribed factors, when light is slightly made incident into both of the X axis and Y axis and the light, which created interference due to the phase difference at propagation of each of the light, is fed back to the semiconductor element in the polarization direction that can be amplified, it is assumed that peaks at intervals of &lgr;
2
/(2·&Dgr;n·L) arise on the spectrum.
Such excessive peaks have been one of the causes whereby the harmful influence such that time instability occurs in the oscillation conditions of the abovedescribed semiconductor laser element with the FBG.
FIG. 6
is a schematic view of a semiconductor laser module
10
according to the prior art.
In
FIG. 6
, the semiconductor laser module
10
comprises a semiconductor laser element
12
a
and an optical coupling means
12
b
composed of a lens, etc. in a package
12
.
Then, in the package
12
, one end (incident end) of an optical fiber
13
a
composed of a birefringence fiber which receives light emitted from the semiconductor laser element
12
a
via the optical coupling means
12
b
is disposed and which is extended to the outside of the package
12
.
An optical connector
15
is provided on the other end side of the optical fiber
13
a
.
14
denotes an FBG which is provided in the optical fiber
13
a
and comprises an external resonator.
FIG. 7
is an explanatory chart showing the relationship between the output intensity and oscillation wavelength of the semiconductor laser module
10
. As shown in
FIG. 7
, stable oscillation with a high mode suppressing ratio cannot be performed at the center wavelength of reflection of the FBG &lgr;
FBG
, and the peaks are generated at intervals of &lgr;
2
/(2·&Dgr;n·L).
OBJECT AND SUMMARY OF THE INVENTION
The present invention is made for solving such problems as the unstable oscillation conditions of the semiconductor laser module according to the prior art, and the object thereof is to provide a semiconductor laser module in which stable oscillation conditions can be provided regardless of the fluctuations in the form such as a winding method of the optical fiber.
In order to achieve the object as described above, the present invention provides a semiconductor laser module having the following construction. That is, a semiconductor laser module according to the present invention is the semiconductor laser module comprising a semiconductor laser element and an optical fiber optically coupled by an optical coupling means, wherein on the optical fiber, a reflective portion for reflecting light emitted from the semiconductor laser toward the side of the semiconductor laser element and a birefringence fiber having eigen axes and provided with birefringent characteristics are provided, said birefringence fiber is provided at least between the incident end on the optical coupling means side of the optical fiber and immediately before the reflective portion, the birefringence fiber has, at least, a connection portion where birefringence fibers are connected to each other, and at the said connection portion, eigen axes of the connected birefringence fibers are in a condition where the eigen axes are shifted in relation to each other by an established rotating angle &thgr;.
The connection portion between the birefringence fibers is usually fusion-connected and preferably, where the length from the incident end on the optical coupling means to the center position of the reflective portion of the optical fiber is set as L, the connection portion between the birefringence fibers locates within the length of L/2±L/3 from the optical coupling means side.
In addition, the established rotating angle &thgr; is preferably within 90±10° or within 45±10°.
The reflective portion can comprise a fiber Bragg grating, an optical connector, or a fiber cut section.
In addition, the birefringence fiber comprises an optical fiber where axially asymmetric stress is applied to the core, that is, any of a PANDA fiber whose stress applying part has a circular section, a bow-tie fiber whose stress applying part has a fan-shaped section, or an oval jacket fiber whose stress applying part has an oval section, or an oval core optical fiber where the core is oval and the waveguide structure of the core is axially asymmetric.
The optical fiber can comprise a birefringence fiber having an established length L
3
provided from the center position in the longitudinal direction of the reflective portion toward the side of the optical transmission direction, and further comprise a polarization independent fiber having an established length L
4
connected on the tip of the birefringence fiber having the said established length L
3
.
In one aspect of the present invention, the length from the incident end to the connection portion of the birefringence fiber provided between the incident end on the opt

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