Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-06-07
2001-06-12
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S140000, C385S024000, C385S034000
Reexamination Certificate
active
06246818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a tunable optical filter, and more particularly to a tunable optical filter having a variable stop band.
2. Description of the Related Art
As a technique for constructing a flexible optical fiber network, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The optical carriers are individually modulated to obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is launched into an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into a plurality of individual optical signals by an optical demultiplexer to reproduce transmitted data according to each optical signal.
In such a system adopting WDM, an add/drop function is important. The add function is a function of joining an optical signal of a specific wavelength channel into WDM signal light, and the drop function is a function of extracting an optical signal of a specific wavelength channel from WDM signal light.
As an optical device for effecting the add/drop function, a tunable optical filter is known. In the case that the tunable optical filter is a bandpass filter, its pass band is variable, whereas in the case that the tunable optical filter is a notch filter, its stop band is variable.
Referring to
FIGS. 1A and 1B
, there are respectively shown the configuration and operating characteristic of a conventional tunable optical filter having a variable pass band. As shown in
FIG. 1A
, this tunable optical filter includes a first port
2
, an optical filter member
4
having a characteristic changing according to a position in a predetermined direction (e.g., a direction parallel to the plane of the sheet of FIG.
1
A), a second port
6
optically coupled to the first port
2
by a transmitted light path concerning the optical filter member
4
, and a mechanism
8
for displacing the optical filter member
4
in the above predetermined direction.
The optical filter member
4
may be configured by changing the thickness of a multilayer dielectric film in the predetermined direction. When the optical filter member
4
is displaced by the mechanism
8
, the pass band is shifted along a wavelength axis as shown in FIG.
1
B. In
FIG. 1B
, the vertical axis represents transmittance (T) between the ports
2
and
6
, and the horizontal axis represents wavelength (&lgr;).
Referring to
FIGS. 2A and 2B
, there are respectively shown the configuration and operating characteristic of a conventional tunable optical filter having a variable stop band. This tunable optical filter includes a first port
10
, an optical filter member
12
having a characteristic changing according to a position in a predetermined direction (e.g., a direction parallel to the plane of the sheet of FIG.
2
A), a second port
14
optically coupled to the first port
10
by a transmitted light path concerning the optical filter member
12
, a third port
16
optically coupled to the first port
10
by a reflected light path concerning the optical filter member
12
, and a mechanism
18
for displacing the optical filter member
12
in the above predetermined direction.
In the case that the optical filter member
12
has a characteristic similar to that of the optical filter member
4
shown in
FIG. 1A
, the wavelength characteristic of transmittance between the first port
10
and the second port
14
is variable along a wavelength axis as in FIG.
1
B. Further, as shown in
FIG. 2B
, the wavelength characteristic of reflectance between the first port
10
and the third port
16
is variable. In
FIG. 2B
, the vertical axis represents reflectance (R), and the horizontal axis represents wavelength (&lgr;). The characteristic shown in
FIG. 2B
is reverse to the characteristic shown in FIG.
1
B.
In the optical coupling by the transmitted light path as shown in
FIG. 1A
, the transmittance is almost stable irrespective of an angular deviation of the optical filter member
4
due to play or the like in the mechanism
8
. In the case that the transmitted light path is provided by a parallel beam, for example, shifting of the parallel beam due to the angular deviation has almost no effect on a coupling efficiency between the ports
2
and
6
.
However, in the case that the first port
10
and the third port
16
are optically coupled by the reflected light path as shown in
FIG. 2A
, an angular deviation of the optical filter member
12
has a direct effect on the reflectance, causing instability of the operation. In the case that the reflected light path is provided by a parallel beam, for example, the angular deviation has a profound effect on a coupling efficiency between the ports
10
and
16
. Thus, the prior art tunable optical filter having a variable stop band has a problem that the operation becomes unstable.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a tunable optical filter having a variable stop band and a stable characteristic.
In accordance with an aspect of the present invention, there is provided a tunable optical filter comprising a first port; an optical filter member having a characteristic changing according to a position in a predetermined direction; a second port optically coupled to the first port by a transmitted light path concerning the optical filter member; a reflecting unit optically coupled to the first port by a reflected light path concerning the optical filter member; and a mechanism for displacing the optical filter member in the predetermined direction.
The reflecting unit may be provided by a cat's eye or a corner cube both generally known in the field of optics. The cat's eye is composed of a lens provided on the reflected light path and a mirror having a reflecting surface at a focal point of the lens. By using such a reflecting unit, reflected light from the reflecting unit travels back the same optical path as that of incident light irrespective of an angle of incidence on the reflecting unit, so that there is no possibility that the characteristic of the optical filter member may be changed by an angular deviation of the optical filter member.
In the case that the optical filter member has such a characteristic as shown in
FIG. 1B
, for example, the function of a bandpass filter can be obtained between the first port and the second port. Further, since the reflecting unit is provided, the first port can also function as an input/output port of a notch filter. Accordingly, in a preferred embodiment wherein the function of only a notch filter is required, the second port may be eliminated.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.
REFERENCES:
patent: 5646399 (1997-07-01), Fukushima et al.
patent: 5966987 (1999-10-01), Yoon et al.
patent: 5-281480 (1993-10-01), None
patent: 7-92530 (1995-04-01), None
Fujitsu Limited
Ngo Hung N.
Staas & Halsey , LLP
LandOfFree
Tunable optical filter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tunable optical filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tunable optical filter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2437588