Optical communication device

Details Optical communication device Optical communication device

2001-01-18

2003-05-20

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S014000, C385S088000, C385S092000

Reexamination Certificate

active

06567590

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical transmission device for the optical communication system or an optical transmission/receiving device combined with a receiving device.
This application claims the priority of Japanese Patent Application No.2000-12857 filed on Jan. 21, 2000 which is incorporated herein by reference.
This invention aims at an improvement of an inspection part of the light emitting device for monitoring the power of the light emitting device. The present invention can be widely applied to the communication systems making use of optical fibers as a medium. The optical signal transmission device includes an LD (laser diode) as a light emitting device. The LD power varies due to the change of temperature or the degradation by aging. The light source device has, in general, a PD (photodiode) for detecting the power of the LD and a controlling device for adjusting the driving current by feedback of the PD signal and for maintaining the output power of the LD. The PD is called a “monitoring PD”. This invention proposes an improvement for the coupling of the monitoring PD with the object LD.
2. Description of Related Art
An LD module which is employed for transmitting optical signals of optical communication systems is described by referring to FIG.
1
. The LD module
1
has a metallic round stem
2
with an erect mount
3
. An LD chip
4
is fixed on a side of the mount
3
. The LD chip
4
emits light in upward and downward directions at a certain rate. A monitoring PD chip
5
is fixed at a center of the stem
2
beneath the LD chip
4
. A metal cap
6
with an opening
7
covers the LD
4
, the PD
5
and the mount
3
on the stem
2
. The foot of the metal cap
6
is welded on the stem
2
. The light emitted upward from the LD
4
passes through the opening
7
of the cap
6
. A cylindrical metallic lens holder
8
having an opening is welded upon the stem
2
. The lens holder
8
supports a lens
9
at the opening. A metallic conical ferrule holder
10
is welded upon the lens holder
8
.
An optical fiber
11
which carries optical signals is held by a ferrule
12
at its end. The axial hole of the ferrule holder
10
seizes the ferrule
12
. Pins
13
downward project from the bottom of the stem
2
. In the assembling steps, the optimum position of the lens holder
8
is determined by displacing the lens holder
8
in the xy-plane, measuring the light power at the other end of the fiber, and seeking the spot which brings about the maximum power to the fiber. The optimum position of the fiber is determined by displacing the fiber in the axial direction (z-direction), measuring the power at the other end of the fiber and fixing the fiber at the spot which maximizes the power. The operation for seeking the optimum positions of the lens holder and the fiber is called “alignment”.
This invention pays attention to the relation between the monitoring PD
5
and the LD
4
. The monitoring PD
5
laid behind the LD
4
always monitors the rear light of the LD
4
. Thus, the monitoring PD
5
can detect the change of the rear light power of the LD
4
. The front light of the LD
4
is signal light which carries signals to another terminal. The front light of the LD
4
is in proportion to the rear light. The power of the LD can be maintained at a constant level by regulating the driving current for cancelling the long-term change of the laser power level obtained by the monitoring PD.
In the example, the LD chip
4
emits light in the z-direction vertical to the stem plane (xy-plane). The LD light (z-direction) is orthogonal to the plane (xy-plane) of the package (stem
2
). Thus, the structure of the LD module is three-dimensional. The rear LD light shoots the top of the PD chip
5
. The PD is a top surface-incidence type. The PD
5
receives almost all of the LD rear beam. The PD
5
can gather the rear beam at high efficiency due to the three-dimensional structure. The PD
5
can obtain the strong LD rear light in the arrangement. Conveniently, this type allows the PD
5
to lie directly upon the stem
2
. Since the package is made from metal, the LD module has strong points of high seal performance and low-noise property. The LD module of
FIG. 1
has some advantages such as the strong monitoring light, the low-noise and the tight hermetic sealing. The current optical communication employs this metal-can type LD module as a signal transmission device.
The three-dimensional LD module is still expensive due to the high cost of the parts and the high manufacturing cost. The direction of the beam emitted from the LD is upward, i.e., vertical to the stem plane. When the LD module is mounted upon a print circuit board at the bottom pins
13
, the cylindrical metal package is so tall that the LD module hinders efficient arrangement of the circuit boards in an apparatus.
Since the current LD modules have these difficulties, new two-dimensional, planar type LD modules have been intensively studied. The new planar type LD module determines the light path on a surface of a substrate and arranges devices on the surface of the substrate in two-dimensional arrangement. Since the devices and the paths are arranged on the plane, the type of the modules is called a “planar lightwave circuit (PLC)”. All the light paths and all the devices lie on the surface of the substrate in the PLC modules. Although the light path extends in the z-direction in prior modules, the light paths lie on the xy-plane in the planar type devices. Various kinds of PLC modules have been proposed.
FIG. 2
shows an example of a planar lightguide type LD module. A silicon (Si) substrate
14
is placed upon a package
15
. A laser diode (LD) chip
16
which makes transmission signals is mounted upside down (epi-down) upon the Si substrate
14
. A lightwaveguide
17
is formed along a center line on a forward half region of the Si substrate
14
. A flat submount
18
having a PD
19
on the front surface is erected upon the package
15
. The PD
19
is provided by mounting the PD
19
on the surface of the submount
18
and sticking the side of the submount
18
on the package
15
. The PD
19
is a monitoring PD for sensing the rear light beam of the LD
16
.
The lightwaveguide
17
is described by referring to
FIG. 3
which is a vertically sectional view of a part of the lightwaveguide
17
and the Si substrate
14
. An undercladding layer
24
of SiO
2
and a linear core
21
and an overcladding layer
25
of SiO
2
are formed on the silicon substrate (Si-bench)
14
by the sputtering or the CVD. The linear core
21
has a refractive index higher than the refractive index of the cladding layers
24
and
25
. The core
21
is a SiO
2
part doped with a dopant which raises the refractive index, e.g., germanium (Ge). The lightwaveguide is fabricated by making the undercladding SiO
2
layer
24
and the Ge-doped SiO
2
layer
21
by, e.g., sputtering, etching unnecessary sides of the Ge-doped SiO
2
layer away by lithography and piling the overcladding layer
25
on the Ge-doped stripe and the undercladding SiO
2
layer
24
by sputtering. The striped Ge-doped core
21
is buried in the overcladding SiO
2
layer
25
. The difference of the refractive indexes enables the core
21
to maintain the propagating light without dissipation. Since the substrate is silicon, the SiO
2
layers can be made by a thermal diffusion method instead of sputtering.
The core of the optical fiber
20
, the core
21
of the lightwaveguide
17
, the light emitting part (stripe)
22
of the LD
16
and the center of a sensing region
23
of the PD
19
lie on the same level. The monitoring PD
19
is a top surface incidence type PD. Since the submount
18
supports the PD
19
on the side, the top of the PD
19
faces the LD
16
. The vertical support enables the PD
19
to receive the rear light of the LD with high efficiency. The PD
19
can obtain strong monitoring light from the LD
16
. The top incidence type PD is a common PD which can be obtained easily on the market. This

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