Laser source

Details Laser source Laser source

2002-07-16

2004-10-12

Le, Thein M. (Department: 2876)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

Reexamination Certificate

active

06802655

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a laser source with a semiconductor laser and an optical waveguide device that are mounted on a submount.
2. Description of the Related Art
In order to achieve increases in the density of optical disks and in definition of display, a small short-wavelength light source is required. As the small short-wavelength light source, a coherent source has been attracting attention that is provided with a semiconductor laser and an optical waveguide type second harmonic generation (hereinafter, referred to as “SHG”) device employing a quasi-phase-matching (hereinafter, referred to as “QPM”) system (see Yamamoto et al., Optics Letters Vol. 16, No. 15, p. 1156, (1991)). Hereinafter, the optical waveguide type SHG device employing the QPM system is referred to as an “optical waveguide type QPM-SHG device”.
FIG. 11
shows a schematic configuration of a bluish purple light source using an optical waveguide type QPM-SHG device. A wavelength-variable semiconductor laser
44
having a distributed Bragg reflection (hereinafter, referred to as “DBR”) region (hereinafter referred to as a “DBR semiconductor laser”) is used as a semiconductor laser. The DBR semiconductor laser
44
is a 100-mW class AlGaAs-based wavelength-variable DBR semiconductor laser in a 820-nm range. The DBR semiconductor laser
44
includes an active region
45
, a phase adjustment region
46
, and a DBR region
47
. By controlling a current injected into the phase adjustment region
46
and the DBR region
47
at a certain ratio, an oscillation wavelength can be varied successively.
An optical waveguide type QPM-SHG device
48
as a wavelength conversion device includes an optical waveguide
50
and a region
51
whose polarization is reversed periodically, which are formed on an X-cut MgO-doped LiNbO
3
substrate
49
as a ferroelectric substrate. The optical waveguide
50
is formed by proton exchange. The DBR semiconductor laser
44
and the optical waveguide type QPM-SHG device
48
are fixed to a submount
52
in such a manner that the surface of the DBR semiconductor laser
44
on which an active layer is formed and the surface of the optical waveguide type QPM-SHG device
48
on which the optical waveguide
50
is formed face the submount
52
. A laser beam emitted from a laser beam emission portion
53
of the DBR semiconductor laser
44
is coupled directly to a laser beam entrance portion
54
of the optical waveguide
50
of the optical waveguide type QPM-SHG device
48
. By carrying out an optical coupling adjustment with a laser beam being emitted from the DBR semiconductor laser
44
, a 60-mW laser beam is coupled to the optical waveguide
50
with respect to a 100-mW laser output. Further, by controlling an amount of the current injected into the phase adjustment region
46
and the DBR region
47
of the DBR semiconductor laser
44
, the oscillation wavelength is fixed within the allowable range of the phase matching wavelength of the optical waveguide type QPM-SHG device
48
, which allows about 20-mW bluish purple light having a wavelength of 410 nm to be obtained.
Hereinafter, a configuration and a method of fabricating a laser source will be described with reference to FIG.
8
.
The laser source is fabricated by disposing an optical waveguide type QPM-SHG device (optical waveguide type wavelength conversion device)
48
and a DBR semiconductor laser
44
on a submount
52
. In this case, the DBR semiconductor laser
44
is fixed onto the submount
52
using solder
55
as an adhesive in such a manner that the surface of the DBR semiconductor laser
44
on which an active layer
56
is formed faces the submount
52
. On the other hand, the optical waveguide type QPM-SHG device
48
is fixed onto the submount
52
with an adhesive
57
in such a manner that the surface of the optical waveguide type QPM-SHG device
48
on which the optical waveguide
50
is formed faces the submount
52
. The height of the optical waveguide type QPM-SHG device
48
in the vertical direction is adjusted with spacers
58
.
When fabricating this module, a device is used that mounts the optical waveguide type QPM-SHG device
48
and the DBR semiconductor laser
44
on the submount
52
by recognizing alignment markers formed on the optical waveguide type QPM-SHG device
48
and the DBR semiconductor laser
44
through image processing and positioning them using the alignment markers thus recognized. In this case, an important factor to be considered is how efficiently a laser beam emitted from the DBR semiconductor laser
44
is coupled to the optical waveguide
50
. Particularly, in the short-wavelength light source including the DBR semiconductor laser
44
and the optical waveguide type QPM-SHG device
48
, the power of harmonic light obtained is proportional to the square of a power of the fundamental wave to be coupled to the optical waveguide
50
. Therefore, improving the optical coupling efficiency and reducing variations in the coupling efficiency among samples are particularly important.
In order to achieve high-efficiency optical coupling, it is important that the distance between a laser beam emission portion
59
of the DBR semiconductor laser
44
and a laser beam entrance portion
60
of the optical waveguide
50
of the optical waveguide type QPM-SHG device
48
is short and that the positions of the laser beam emission portion
59
and the laser beam entrance portion
60
in the horizontal direction (i.e., Y-direction) and the vertical direction (i.e., Z-direction) coincide with each other. Particularly, the position accuracy in the vertical direction is important. The optical coupling efficiency decreases significantly when there is misalignment in the vertical direction. Therefore, it is necessary that the misalignment of the laser beam entrance portion
60
of the optical waveguide
50
of the optical waveguide type QPM-SHG device
48
with respect to the emission portion
59
of the DBR semiconductor laser
44
is within ±0.2 &mgr;m.
The distance d
a
from the lower surface of the DBR semiconductor laser
44
to the laser beam emission portion
59
is controlled with high precision when fabricating the DBR semiconductor laser
44
. Also, the position of the laser beam entrance portion
60
of the optical waveguide
50
is controlled with high precision since the optical waveguide
50
of the optical waveguide type QPM-SHG device
48
is formed on the surface of a LiNbO
3
substrate
49
. In addition, since the variations in size of the spacers
58
used for the position adjustment of the optical waveguide type QPM-SHG device
48
in the vertical direction are not more than ±0.1 &mgr;m, the position adjustment of the laser beam entrance portion
60
in the vertical direction can be carried out with high precision. However, in order to achieve the position adjustment in the vertical direction with high precision so that a laser beam emitted from the DBR semiconductor laser
44
is coupled to the optical waveguide
50
with high efficiency, it is necessary to control the thickness of the solder
55
as an adhesive for fixing the DBR semiconductor laser
44
.
FIGS. 9A and 9B
show a conventional submount.
FIG. 9A
is a plan view and
FIG. 9B
is a front view. A submount
52
is formed using a Si substrate. On the submount
52
, an electrode portion is formed that includes an electrode
61
for an active region, an electrode
62
for a phase adjustment region, an electrode
63
for a DBR region, and an electrode
64
for a ground. Films of the solder
55
as an adhesive portion for fixing a DBR semiconductor laser are formed on the electrodes
61
to
63
(not on the electrode
64
for a ground), respectively. Alignment markers
65
used for the position adjustment of a DBR semiconductor laser and an optical waveguide device fixing portion
66
further are formed on the submount
52
. The electrode
61
for an active region, the electrode
62
for a phase adjustment region, the electrode
63
for a DB

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