Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-03-23
2004-12-28
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S038060
Reexamination Certificate
active
06836497
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser module and a method for forming the semiconductor laser module, and in particular, to a semiconductor laser module which converts an original center wavelength of stimulated emission of a semiconductor laser to emit a laser beam of a desired wavelength and a method for forming the semiconductor laser module.
2. Description of the Related Art
A method for converting a fundamental wave to a second harmonic by using an optical wavelength conversion element, in which a region where the spontaneous polarization (i.e., domain) of ferroelectrics having a non-linear optical effect is periodically inverted is provided, has been proposed by Bleombergen et al. (see Phys. Rev., Vol. 127, No. 6, 1918 (1962)). In accordance with this method, by setting the period &Lgr; of the domain inversion portion so as to be an integer multiple of the coherent length &Lgr;c defined by the following formula, phase matching (so-called pseudo phase matching) between the fundamental wave and the second harmonic can be carried out.
&Lgr;
c=
2
n/{&bgr;
(2&ohgr;)−2&bgr;(&ohgr;)}
In this equation, &bgr;(2&ohgr;) is the propagation constant of the second harmonic and &bgr;(&ohgr;) is the propagation constant of the fundamental wave. Attempts have been made to carry out phase matching efficiently by forming such a periodic domain inversion structure in an optical waveguide type optical wavelength conversion element which converts the wavelength of the fundamental wave which propagates in an optical waveguide made of a non-linear optical material.
As one example, a conventional optical waveguide type optical wavelength conversion element, in which a periodic domain inversion structure is formed, is such that, as shown in
FIG. 12
, a spontaneous polarization direction of a substrate
2
shown by the arrow P is perpendicular to one substrate surface
2
a
(i.e., the substrate surface along which an optical waveguide
1
extends) (see Japanese Patent Application Laid-Open (JP-A) No. 5-29207).
In the above-described art, in order to carry out wavelength conversion efficiently, a Z-cut substrate of LiNbO
3
, LiTaO
3
or the like is used as an optical element in art which uses a Z-cut substrate. In a voltage application method, when the periodic domain inversion region is formed, the inversion region tends to extend along the Z axis, and therefore, when using the Z-cut substrate, a deep periodic domain inversion structure can be formed. Accordingly, the periodic domain inversion structure is formed in the optical element and the optical waveguide is formed in a region where the periodic domain inversion structure is formed, and therefore, the wavelength conversion can be carried out with high efficiency.
In this optical wavelength conversion element, the domain inversion portion can be formed sufficiently deep from the substrate surface. However, in a case of using the optical wavelength conversion element with a semiconductor laser, an incident optical system of the fundamental wave becomes complicated. Namely, in the structure shown in
FIG. 12
, a beam pattern of the guided light is such that, as shown in the circle A in
FIG. 12
, a beam diameter in a direction parallel to the direction of the polarization vector shown by the arrow R is small and a beam diameter in a direction perpendicular to the direction is large. At this time, the direction of the polarization vector coincides with the spontaneous polarization direction of the substrate
2
(generally, in ferroelectrics such as LiNbO
3
or the like, the spontaneous polarization direction is parallel to the Z axis), and the guided mode is the TM mode. On the other hand, a beam pattern of a laser beam
4
emitted from a semiconductor laser
3
is such that, as shown in the circle B in
FIG. 12
, a beam diameter in the direction parallel to the direction of the polarization vector shown by the arrow Q is large and a beam diameter in a direction perpendicular to the direction is small. When respective polarization directions are aligned in order to input the laser beam
4
emitted from the semiconductor laser
3
into the optical waveguide
1
, beam shapes are mismatched and therefore, the laser beam
4
cannot be entered efficiently into the optical waveguide
1
. Accordingly, the intensity of the second harmonic is small.
In order to rotate the polarization direction of the laser beam
4
90 degrees without changing the beam pattern, a complicated fundamental wave incident optical system is needed, in which a &lgr;/2 plate
7
is disposed between a collimator lens
5
and a condenser lens
6
.
To solve the above-described problem, an optical element, in which an optical wavelength conversion element and a semiconductor laser are integrated, has been proposed (see Japanese National Publication No. 10-506724). In this art, the semiconductor laser, to which a wavelength tuning mechanism is applied, is directly mounted to the optical wavelength conversion element (waveguide SHG). Since the optical wavelength conversion element is excited by the semiconductor laser to which the wavelength tuning mechanism is applied, a center wavelength of stimulated emission of the semiconductor laser is made to coincide with a phase matching wavelength of the optical wavelength conversion element.
However, the semiconductor laser emits semiconductor laser light in a TE mode in which the polarization direction of the semiconductor laser light is parallel to a substrate. In contrast, in the optical wavelength conversion element, semiconductor laser light is emitted in a TM mode in which the polarization direction of the semiconductor laser light is perpendicular to the substrate. Thus, as mentioned above, in order to mount the semiconductor laser, to which the wavelength tuning mechanism is applied, directly to the optical wavelength conversion element, the polarization directions must be made to coincide with each other, and the substrate of the semiconductor laser and the substrate of the optical wavelength conversion element must be mounted so as to be perpendicular to each other. In the conventional optical element, alignment must be carried out with high accuracy in order to make the polarization directions coincide with each other.
SUMMARY OF THE INVENTION
In view of the aforementioned, an object of the present invention is to provide a semiconductor laser module by which high wavelength conversion efficiency can be obtained easily, and a method for forming the semiconductor laser module.
In order to accomplish the object, there is provided a semiconductor laser module comprising: an optical wavelength conversion element which is formed such that, on a ferroelectric crystal substrate having a non-linear optical effect, a TE mode optical waveguide which extends along a substrate surface and in which a polarization direction is parallel to the substrate is formed, and a domain inversion portion, where a spontaneous polarization direction of the substrate is inverted, is periodically formed in the optical waveguide, and the optical wavelength conversion element converts a wavelength of a fundamental wave which propagates in the optical waveguide in a direction along which the domain inversion portions are aligned; and a semiconductor laser which can emit a laser beam in the TE mode in which a polarization direction is parallel to the substrate and which can adjust a center wavelength of stimulated emission of the laser beam, and light emitted from the semiconductor laser is made to enter the optical waveguide, wherein the optical wavelength conversion element and the semiconductor laser are mounted such that the polarization directions of the TE mode coincide with each other and a light exit portion of the semiconductor laser and a light entrance portion of the optical wavelength conversion element coincide with each other.
As described above, because both the optical wavelength conversion element and the semiconductor laser can propagate light in the
Fuji Photo Film Co. , Ltd.
Nguyen Tuan N.
Sughrue & Mion, PLLC
Wong Don
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