Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-07-02
2003-05-13
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S037000, C385S049000, C372S006000
Reexamination Certificate
active
06563983
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser diode module (hereinbelow, called an “LD module”) used as a light source for exiting an optical fiber amplifier.
2. Description of the Background Art
FIG. 15
is a diagram showing the schematic structure of a conventional LD module disclosed in U.S. Pat. No. 5,724,377. The LD module has a laser diode hereinbelow, called an “LD”)
21
and an optical fiber
22
disposed on an optical path of outgoing light of the LD
21
. The LD
21
is an LD that oscillates at a wavelength of 982 nm in a state where there is no optical feedback. The optical fiber
22
has a micro lens processed portion
23
at its tip on the light incident side, and has a fiber grating (hereinbelow, called an “FG”)
24
having a reflection center wavelength of 980 nm in its part. Generally, the FG denotes a portion formed so that the refractive index periodically changes in the optical fiber and, by the cycle of the change in refractive index, has a property that only light having a specific wavelength is reflected by only a predetermined quantity in accordance with the degree of a change in the refractive index. The FG can be formed by photosensitizing the optical fiber with ultraviolet rays.
A laser beam as outgoing light from the LD
21
enters the microlens processed portion
23
, thereby being converted from diffusion light to parallel rays. The parallel rays propagate through the optical fiber
22
. A part of the laser beam propagating through the optical fiber
22
to the FG
24
is reflected by the FG
24
and re-enters the LD
21
. In the structure where a compound cavity is formed in such a manner, the oscillation wavelength of the LD
21
is not 982 nm as stated above but 980 nm as the reflection center wavelength of the FG
24
and is locked at 980 nm. That is, irrespective of a forward current passed to the LD
21
and temperature environment in which the LD is disposed, a light output at the wavelength of 980 nm can be stably obtained.
FIG. 16
shows the relation between a fiber end light output Pf at a laser beam outgoing end of the optical fiber
22
and an LD forward current If with respect to the case where the FG
24
is provided in the optical fiber
22
to make the laser beam re-enter the LD
21
as described above and the case where the FG
24
is not provided. The broken line indicates the characteristic in the case where no FG
24
is provided, and the solid line indicates the characteristic in the case where the FG
24
is provided. As obviously understood from
FIG. 16
, the maximum value of the light output which can be emitted without causing a kink (hereinbelow, called a “kink-free light output”) deteriorates considerably and there is a problem such that only the light output of about ⅔ of the case where no FG
24
is provided can be used. The “kink” denotes a phenomenon that due to occurrence of beam steering, the linearity between the light output of the optical fiber and the forward current passed to the LD is lost. The details will be described hereinlater.
SUMMARY OF THE INVENTION
An object of the present invention is to obtain an LD module capable of sufficiently increasing a kink-free light output even under the situation that the wavelength of outgoing light is fixed to a predetermined value by using an FG.
According to one aspect of the invention, a laser diode module according to the invention has: a laser diode of which oscillation wavelength is fixed at a predetermined wavelength; a fiber disposed on an optical path of outgoing light of the laser diode, in a part of which a fiber grating is formed; and a coupling optical system for optically coupling the laser diode and the fiber. An optical axis of the coupling optical system is inclined with respect to an optical axis of outgoing light of the laser diode by a predetermined angle in a plane parallel to an active layer of the laser diode.
With such a configuration, the coupling between the first-order lateral mode and the propagation mode of the fiber can be suppressed, and a kink-free light output can be increased.
In an embodiment of the laser diode module, a lens is used as the coupling optical system, and the optical axis of the coupling optical system is inclined by the predetermined angle by displacing a position of the lens with respect to the laser diode by a predetermined distance in a direction perpendicular to the optical axis of the outgoing light of the laser diode and parallel to the active layer of the laser diode.
With such a configuration, difficult angle adjustment and angle fixing can be carried out by distance adjustment in the x-direction which is relatively easy, so that the angle adjustment can be relatively easily realized at low cost.
Preferably, the optical axis of the coupling optical system is inclined by an angle so that a light output which can be emitted from the fiber without causing a kink becomes the maximum. Thus, a kink-free light output can be set to the maximum.
According to another aspect of the invention, a laser diode module of the invention has: a laser diode of which oscillation wavelength is fixed at a predetermined wavelength; a fiber disposed on an optical path of outgoing light of the laser diode, in a part of which a fiber grating is formed; and a coupling optical system for optically coupling the laser diode and the fiber. A position of the laser diode with respect to the coupling optical system is displaced only by a predetermined distance in a direction perpendicular to an optical axis of outgoing light of the laser diode and parallel to an active layer of the laser diode.
With such a configuration as well, the coupling between the first-order lateral mode and the propagation mode of the fiber can be suppressed, and the kink-free light output can be increased.
Preferably, a position of the laser diode with respect to the coupling optical system is displaced only by a distance so that a light output which can be emitted from the fiber without causing a kink becomes the maximum. Thus, the kink-free light output can be set to the maximum.
The oscillation wavelength of the laser diode is, preferably, fixed to a wavelength at which reflectance by the fiber grating becomes the maximum. Consequently, the wavelength of outgoing light can be fixed to a predetermined value.
As the laser diode, for example, a laser diode of a ridge waveguide type is employed. While using the most of the high output as an advantage of the ridge waveguide type, a lateral mode controllability as a drawback is overcome, so that a kind-free light output can be increased.
In a preferred embodiment of the invention, an inclined polished face is provided as an end face on the laser diode side of the fiber, and a normal of the polished face and that of the active layer of the layer diode are in the same plane. With the configuration, symmetry with respect to the optical axis in the x direction of the laser diode module is improved and the laser oscillation is performed symmetrically in the lateral direction. Consequently, generation of a high-order lateral mode can be suppressed, and a kink-free light output can be increased.
Preferably, a polarization-preserving optical fiber having a mode-enlarged end portion on the laser diode side of the fiber is used. By the mode-enlarged portion of the polarization-preserving optical fiber, the optical axis can be adjusted. Thus, angle and position can be adjusted and fixed with very high precision. An effect such that matching between the flat shape of outgoing light of the laser diode and the propagation mode of the fiber and fine adjustment for enlargement of an optical path can be realized by a single part at low cost is produced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5485481 (1996-01-01), Ventrudo et al.
patent: 5699377 (1997-12-01), Pan
pate
Hatta Tatsuo
Tamura Satoshi
Connelly-Cushwa Michelle R.
Mitsubishi Denki & Kabushiki Kaisha
Ullah Akm E.
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