Optical waveguides – With optical coupler – With alignment device
Reexamination Certificate
2002-12-16
2003-09-16
Kim, Robert H. (Department: 2871)
Optical waveguides
With optical coupler
With alignment device
C372S034000, C372S075000
Reexamination Certificate
active
06621962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical waveguide device integrated module with a semiconductor laser and an optical waveguide device that are mounted on a submount and to a method of manufacturing the same.
2. Related Background Art
In the optical communication field, it is considered important to develop a hybrid integrated optical module including a semiconductor laser, an electronic element, an optical fiber, and the like that are integrated on a quartz-based lightwave circuit platform. This is an indispensable technique for reducing the size and cost of modules. In the technique, it is important to fix each element with high precision to minimize transfer-loss.
A surface mounting optical module has been proposed in which a semiconductor laser and a single mode fiber are bonded directly using a V-groove Si substrate (IEICE (The Institute of Electronics, Information and Communication Engineers) Conference 1997, C-3-63).
FIG. 12
shows a structural view. Alignment keys
26
are formed in a Si substrate
24
and a semiconductor laser
25
. The alignment keys
26
are subjected to image recognition, so that the center of a V-groove
27
and a position of an emission point of the semiconductor laser
25
are detected. Thus, position adjustment is carried out with high precision. Mounting variations of about ±0.61 &mgr;m in the x direction and about ±1 &mgr;m in the z direction are achieved with respect to the V-groove
27
of the Si substrate
24
. An optical fiber
28
is mounted in the V-groove
27
accurately. The V-groove
27
is formed with high precision by anisotropic etching of Si. Similarly, the optical fiber
28
is produced with its outer dimension and core center controlled with high precision. Therefore, the fiber
28
is fitted into and is fixed to the V-groove
27
, so that the optical fiber
28
can be fixed with respect to the semiconductor laser
25
with high precision.
On the other hand, in order to achieve the increases in density of optical disks and in definition of a display, a small short-wavelength light source is required. Techniques for obtaining short wavelength light include blue light generation using a semiconductor laser and an optical waveguide second harmonic generation (hereinafter referred to as “SHG”) device employing a quasi-phase-matched (hereinafter referred to as “QPM”) system (Yamamoto et al., Optics Letters Vol. 16, No. 15, p1156, (1991)).
FIG. 13
shows a schematic structural view of a blue light source using an optical waveguide QPM-SHG device. A wavelength variable semiconductor laser having a distributed Bragg reflector (hereinafter referred to as “DBR”) region (hereinafter referred to as a “wavelength-variable DBR semiconductor laser”) is used as a semiconductor laser. Numeral
29
is a 100-mW class AlGaAs-based wavelength-variable DBR semiconductor laser in a 0.85-&mgr;m range. The semiconductor laser includes an active layer region and a DBR region. An amount of current applied to the DBR region is varied, so that the emission wavelength can be varied.
An optical waveguide QPM-SHG device
30
as a wavelength conversion device includes an optical waveguide and a region whose polarization is reversed periodically, which are formed on a x-cut Mg-doped LiNbO
3
substrate. A SiO
2
protective film
31
is formed on the surface at which the optical waveguide is formed. The wavelength-variable DBR semiconductor laser
29
and the optical waveguide QPM-SHG device
30
are fixed so that the active layer and the surface at which the optical waveguide is formed are in contact with a submount
32
, respectively (hereinafter referred to as “face down mounting”). A laser beam obtained from an emission surface (from which a beam leaves the laser
29
) of the wavelength-variable DBR semiconductor laser
29
is coupled directly to the optical waveguide of the optical waveguide QPM-SHG device
30
.
The optical coupling adjustment is carried out with the semiconductor laser emitting a beam, and with respect to a 100-mW laser output, a 60-mW laser beam was coupled to the optical waveguide. The amount of current applied to the DBR region of the wavelength-variable DBR semiconductor laser is controlled and thus the emission wavelength is set within a tolerance of the phase matched wavelength of the optical waveguide QPM-SHG device. Currently, about 10-mW blue light with a wavelength of 425 nm has been obtained.
In an optical module in which a semiconductor laser and an optical fiber are integrated, the optical fiber is mounted in a V-groove formed in a Si submount and the semiconductor laser is mounted using the V-groove as a reference position. The optical fiber has a cylindrical shape and has a core portion (an optical propagation region) formed in its center. The optical fiber is formed with its diameter controlled with high precision. In addition, the V-groove in the Si submount also is formed with high precision using the anisotropic etching of Si. Therefore, the optical fiber is mounted with its core portion as the center of the optical fiber being adjusted with respect to the Si submount with high precision. On the other hand, the alignment keys used for positioning the semiconductor laser also are formed in reference to the V-groove and therefore the semiconductor laser also can be mounted with high accuracy.
In a planar optical waveguide device with an optical waveguide formed on a surface of a LiNbO
3
substrate by proton exchange or Ti diffusion (devices other than optical waveguide devices with an optical waveguide layer (core) in the coaxial center like an optical fiber are referred to as “planar optical waveguide devices” in the present invention), the distance from the substrate surface to the optical waveguide is controlled with high precision. In an integrated module including a semiconductor laser and a planar optical waveguide device, the semiconductor laser is fixed with a solder material and the optical waveguide device is fixed with an adhesive by face down mounting. In the semiconductor laser, generally, an active layer is formed on an n-type substrate, and a P-type clad layer and further a p-side electrode are formed thereon. Therefore, the distance from the p-side surface to the active layer is about 3 &mgr;m. The solder material has a thickness of about 1 to 2 &mgr;m. Consequently, the distance from the submount to the active layer after mounting is about 4 to 5 &mgr;m. This distance can be controlled to be about ±0.2 &mgr;m through the adjustment of the amount of pressure applied to the semiconductor laser during the mounting.
On the other hand, since the optical waveguide portion of the planar optical waveguide device is formed at the substrate surface, the distance from the substrate to the optical waveguide portion is about 1 &mgr;m. Therefore, there is a difference in level of about 3 to 4 &mgr;m between the active layer of the semiconductor laser and the optical waveguide portion of the optical waveguide device. Consequently, it has been difficult to carry out the adjustment without allowing the semiconductor laser to emit a beam (hereinafter referred to as “passive alignment mounting”).
A method has been proposed in which a thick film is formed on a planar optical waveguide device to allow the levels of an active layer of a semiconductor laser and an optical waveguide portion of the planar optical waveguide device to coincide with each other. This method, however, has the following problems.
(1) Conditions for manufacturing the optical waveguide vary due to the increase in temperature of a substrate during the formation of the thick film. Particularly, in the case of a SHG device employing the QPM system, a phase matched wavelength may vary and the wavelength conversion characteristics may deteriorate with the variation in refractive index of the optical waveguide.
(2) After being formed, the thick film shrinks and therefore, a substrate may warp. The warping makes it difficult to mount the device on a submount.
(3) The thick f
Kitaoka Yasuo
Yamamoto Kazuhisa
Yokoyama Toshifumi
Caley Michael H.
Kim Robert H.
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
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