Optical device

Details Optical device Optical device

2000-07-07

2003-03-11

Ullah, Akm E. (Department: 2874)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

C359S199200

Reexamination Certificate

active

06530698

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical device for an optical transmitting device, an optical receiving device, an optical transmitting/receiving device or other optical parts for constructing same and an assembly of the devices for optical communication. This invention, in particular, aims at reducing the ORL (Optical Reflection Loss).
This application claims the priority of Japanese Patent Application No.11-196468 (196468/1999) filed Jul. 9, 1999 which is incorporated herein by reference.
2. Description of Related Art
Practical development of optical communication accelerates miniaturization and cost-reduction of optical transmitting devices, optical receiving devices or so. Recent endeavors are made for investigating very tiny optical devices called PLC (planar lightwave circuit) type which makes use of passive alignment. For example, the followings suggested PLC devices.
{circumflex over (1)} T. Nishikawa, Y. Inaba, G. Tomon, T. Uno, Y. Matsui, “Surface Mounting LD Module on a Silicon Substrate”, 1997 IEIC C-3-63, p248(1997).
{circumflex over (2)} Jun-ichi Sasaki, Masataka Itoh, Hiroyuki Yamazaki, Masayuki Yamaguchi,“Si bench for highly efficient optical coupling using passively-aligned spot-size converter integrated laser diode”, 1997 IEIC C-3-65, p250(1997).
{circumflex over (3)} A. Hirai, R. Kaku, T. Maezawa, K. Takayama, T. Harada, “Silicon V-Groove Substrate for Optical Modules”, 1997 IEIC C-3-66, p251(1997).
FIG.
1
and
FIG. 2
show prior PLC type optical receiving modules (PD module).
FIG. 1
is a plan view of the PLC type PD module and
FIG. 2
is a sectional of the same module. An optical receiving module (PD module)
1
has an Si bench
2
including a lower step
4
and a higher step
3
. The higher step
3
sustains an end of a fiber
9
and the lower step
4
holds a PD
5
. The PD
5
is a waveguide type PD which has an light sensing waveguide
12
. The light going into the PD from the side is sensed by the waveguide
12
. The Si-bench
2
has a smaller V-groove
7
and a bigger V-groove
6
made by anisotropic etching on the upper step
3
. A ferrule
8
and the fiber
9
are supported in the V-grooves
6
and
7
. The ferrule
8
encloses an end of the fiber
9
. The ferrule
8
can be attached to or detached from an external optical device (not shown in FIGS.
1
and
2
). The end surface of the fiber
9
is orthogonal to the central optical axis. Outgoing light
11
from an end
10
passes a narrow gap and reaches the light sensing waveguide
12
of the PD
5
. The fiber is also fixed to the same Si-bench
2
. Mounting both the fiber and the PD on the same Si-bench enables the PD module to reduce its size. There is no joint requiring alignment. No alignment (passive alignment) alleviates the fabrication time and the cost. The omission of a lens reduces the cost also. Then, the PLC type PD module of FIG.
1
and
FIG. 2
would be a cheap, miniaturized PD module.
The prior art of FIG.
1
and
FIG. 2
disposes optical devices (PD
5
, ferrule
8
and optical fiber
9
) on t he Si-bench
2
for joining the fiber directly to the light sensing device (photodiode; PD
5
) without lens. The butting joint between the fiber and the PD allows the PD module to decrease parts and reduce the size, which would lead to a low-cost PD module. Here the optical fiber
9
is shown as a light introducing part by way of example. A light waveguide can be employed instead of the fiber
9
. The waveguide type PD which allows the incidence light to enter the side as an example. The side incidence waveguide type can also be replaced by a top incidence type PD or a bottom incidence type PD in accordance with the design of the optical system.
The V-grooves
6
and
7
are formed by anisotropic etching based on photolithography on an Si wafer. The positioning marks are formed also by photolithography on the Si wafer for predetermining the spot of a PD on a bench. The V-grooves and the positioning mark enable the module to place the fiber and the PD at exactly predetermined positions. The rigorous positioning by the grooves and the marks without positive alignment is called “passive alignment”. The passive alignment allows the PLC module denoted by
FIG. 1
or
FIG. 2
to reduce the assembling cost. The PLC module has advantages of low part cost and low assembling cost.
The end of the fiber is orthogonal to the light axis. The orthogonality is considered to be indispensable to the passive alignment. If the end surface were to be oblique to the light axis, the beam emanating from the fiber would bend sideward and would require a time-consuming positive alignment for coupling with the PD. It is a common sense that the passive alignment should inherently request the orthogonal end of the fiber.
As a matter of fact, the reflection at the fiber end causes a problem in the PLC prior art of FIG.
1
and FIG.
2
. The end
10
of the fiber is cut in a plane vertical to the light propagating direction (axial direction). Another end of the fiber faces an LD (laser diode) as a light source (not shown in the figures). The vertical end reflects the laser light backward. The reflected beam propagates in the fiber in the reverse direction to the LD and induces instability of the LD oscillation. The LD makes use of mirror surfaces of both ends for reciprocating light as a resonator. If the light reflected at the fiber end returned to the LD, the LD would have two resonators. The existence of two resonators changes the oscillation wavelength or the frequency and the power. The instability would incur inconveniences. The returning light should be fully suppressed for maintaining the stability of the laser oscillation-wavelength and power. The light receiving surface of the PD which is coated with an antireflection film does not reflect the LD light. But the fiber end which is not coated with the antireflection film would cause the serious problem by reflecting the laser light.
The light which is reflected by the fiber end and is returned to the LD is called “reflection returning light” here. The light emanating from the LD is called “input light”. The rate of the reflection returning light to the input light is called ORL (Optical Reflection Loss).
ORL
=10 log(
P
r
/P
in
)(dB).  (1)
Here, log means logarithm, P
r
is the light power which is reflected at the fiber end and is returned to the LD and P
in
is the light power which is produced by the LD and is progressing to the fiber end. ORL is defined in a unit of dB. Since P
r
is always smaller than P
in
(P
r
<P
in
), ORL is negative. ORL is a measure of the influence of the returning light to the LD. Smaller ORL is better for the PD module. Too big ORL is a drawback of the prior art of FIG.
1
and FIG.
2
. The ORL of the PLC module is now calculated.
The power reflection rate R
ef
at an interface from a medium of a refractive index n
1
to another medium of a refractive index n
a
is denoted by
R
ef
={(
n
1
−n
a
)/(
n
1
+n
a
)}
2
.  (2)
In the case of the prior PD module shown by FIG.
1
and
FIG. 2
, light goes out from a quartz fiber of a refractive index n
1
=1.46 to air of a refractive index n
a
=1.00. The returning light is ORL=−14.6 dB which is a large value. Namely, the reflected light is strong. The large difference of refractive index between the fiber and air leads to such a big ORL.
How small ORL is required for practical PD modules? The requirements and characteristics depend upon the kinds of optical communication systems. The allowable maximum ORL is contingent upon the systems. More sophisticated system requires a smaller ORL. An optical receiving device requires a small ORL of less than −27 dB. The fabrication margin is about −3 dB. Then, less than −30 dB is required for the ORL in practice. This is why the laser is perturbed even by small reflection returning light.
Furthermore, transmission of multichannel analog signals, e.g., optical CATV, requires a very small ORL of

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