Optoelectronic module

Details Optoelectronic module Optoelectronic module

2001-03-09

2002-08-20

Healy, Brian (Department: 2874)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

C385S092000, C385S094000

Reexamination Certificate

active

06435734

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optoelectronic modules. The optoelectronic module mean an LD module, a PD module, an LD/PD module, a set of them.
This application claims the priority of Japanese Patent Application No. 2000-117896 filed on Apr. 19, 2000 which is incorporated herein by reference.
2. Description of Related Art
Current optical communication systems utilize three-dimensionally shaped metal-packaged optoelectronic modules having a cylindrical metal package, a PD chip, an LD chip, a lens, a ferrule and a fiber which align along the central axial line perpendicular to the metallic stem. The current module requires a lens for converging light since the light propagates for a long distance which diverges the light beam in the module. The currently used optoelectronic modules excel in the sealing property, the noise-resistance and the reliability. The current LD, PD or LD/PD modules, however, have drawbacks of complicated alignment, large size and high-cost. The bulky prior modules occupy a wide area in a print circuit board.
A further prevalence of the optical communication networks requires smaller sizes and lower cost for LD, PD or LD/PD modules. Recently contrived PLC (Planar Lightguide Circuit) is promising technology for accelerating the size-reduction and cost-curtailment of optoelectronic modules. In the PLC devices, the direction of the light propagation is in parallel with the surface of the substrate. The PLC has a small two-dimensional structure instead of the prior three-dimensional cylindrical structure. The PLC modules have strong points of the omission of alignment and the omission of the lens. Eliminations of the alignment and the lens are advantages of the PLC.
An example of a PLC type PD module is depicted in
FIG. 1
(plan view) and
FIG. 2
(vertical section. The PLC module is built upon a flat Si bench instead of the tall, cylindrical metallic package. A flat silicon bench
1
has a longitudinal bigger V-groove
2
and a smaller V-groove
3
along the center line. A ferrule
4
is inserted into the bigger V-groove
2
and the fiber
5
is inserted into the smaller V-groove
3
. An adhesive fixes the ferrule
4
and the fiber
5
to the grooves. An end of the Si bench is cut to make a lower stage
6
for revealing the front end of the V-groove
3
. The lower stage
6
is provided with an optoelectronic device which is a waveguide type PD
7
in the example. The waveguide type PD
7
which has a horizontally spreading sensing layer
8
and a waveguide layer upon the sensing layer allows the light to go into the PD via a front end. The waveguide type simplifies the structure of the PD module.
The example in
FIGS. 1 and 2
shows the waveguide type PD upon the substrate because of the simplicity of the structure. The PLC structure allows also a bottom incidence type PD or a top incidence type PD to build a PD module on a flat substrate. The PDs take slightly different relations to the optical fiber axis in the case of the bottom incidence type PD or the top incidence type PD. Someone proposed transmitting modules disposing light sources LED or LD) at extensions of optical fibers upon substrates. A PD module, an LD/PD module and an LD module take similar flat PLC structures. Sometimes the optical fibers are replaced by light waveguides formed upon the substrates (Si-benches) and coupled to outer optical fibers at the end of the substrate.
In any cases, the PLC structure has a substrate, at least an optical fiber or a waveguide on the substrate and at least one optoelectronic device. (PD, LD, LED or APD) disposed at an extension of the fiber or the waveguide upon the substrate. Without a lens, the optical fiber or the waveguide is directly butted to the optoelectronic device chips (PD, LD, LED or APD), which reduces the number of parts and decreases the size of the module.
The prior art of
FIGS. 1 and 2
determines the position of the fiber by etching a V-groove upon the substrate and exactly determines the position of the PD by putting the PD at characteristic marks printed on the substrate. Without active alignment, the exact positions of the fibers and the optoelectronic devices are determined by the V-grooves and the characteristic marks designated upon the substrate. The facile positioning in the PLC is called “passive alignment” in contrast with the “active alignment” in the prior art current three dimensional metal packaged modules.
The PLC allows the passive alignment to the optical communication modules, which alleviates the cost of mounting. The PLC further will give low-cost modules by reducing the material cost and the parts cost in addition to the mounting cost. The reduction of the size will facilitate the mounting of the modules on a print circuit board. The PLC is a promising type of optical communication modules. However, the PLC must conquer several difficult problems for being put in practice. The current problems are here described with regard to the PD module shown in FIG.
1
and FIG.
2
. Similar problems accompany the LD modules or the LD/PD modules.
The reflection at the end of the fiber is one of the problems of the receiving (PD) module of
FIGS. 1 and 2
. The end of the fiber is rectangular to the fiber axis. The difference of the refractive indexes between the fiber and the outer space reflects the light at the end of the fiber. The reflected light goes back in the fiber
5
and returns to the laser at the signal-sending port. The returning light induces instability on the oscillation of the laser by perturbing the stimulated oscillation. The quantity of the returning light depends upon the refractive index difference between the fiber and the outer space. The front end of the PD
7
is coated with an antireflection film which annihilates the reflection at the PD front end. What matters is the reflection at the fiber end.
The light reflected at the fiber end is called “reflection light”. The rate of the reflection light to the original incidence light is named “Optical Return Loss (ORL)”. The ORL is defined by
ORL=10 log(
P
r
/P
in
), (
dB
)  (1)
where P
in
is the light power travelling in the fiber to the end and P
r
is the light power reflected at the end. The light power which goes out of the fiber is (P
in
−P
r
). Since P
r
<P
in
, the ORL is always negative. An ideal ORL is minus infinitive. In actual cases, the ORLs take definite minus values.
Even in vertical-cut end fibers, the ORL varies as a function of the refractive index difference between the fiber and the outer medium. In the case of silica (SiO
2
) fibers (n
1
=1.46), large reflection occurs when the outer medium is air (n
0
=1.0). The vertical reflection ratio R
ef
at the interface of the media of n
0
and n
1
is given by
R
ef
={(n
1
−n
0
)/(n
1
+n
0
)}
2
+0.035  . (2)
The air/silica interface brings about ORL=−14.6 dB which is too big for practical use.
The allowable upper limit of ORL depends upon the systems (LD, PD or LD/PD modules). PD modules require an ORL less than −27 dB as a whole. Assuming that the manufacturing margin is −3 dB, the ORL should be less than −30 dB at the interface. Such a small ORL (e.g., ORL≦−30 dB) is required in a wide temperature range between −40° C. and +85° C. for the PD modules. The less than −30 dB ORL in the wide temperature range is a quite rigorous condition.
The prior PD module of
FIGS. 1 and 2
cannot suffice the severe requirement. The use of the PD module is narrowly restricted. Some contrivance is requested for enhancing the utility of the
FIG. 1
module.
Eq. (2) notifies us that a reduction of the refractive index difference should be effective for reducing the reflection loss at the fiber end. FIG.
3
and
FIG. 4
show an improvement of potting a transparent resin having a refractive index n
2
similar to the fiber into the gap between the fiber
5
and the PD chip
7
. The transparent potting resin
9
alleviates the ref

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