Expandable interleaving optical add/drop filter module

Details Expandable interleaving optical add/drop filter module Expandable interleaving optical add/drop filter module

1999-09-24

2001-07-03

Epps, Georgia (Department: 2873)

Optical waveguides

With optical coupler

Plural

C359S199200, C385S017000

Reexamination Certificate

active

06256433

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical filters and, in particular, to optical add/drop filters.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. One way to increase the capacity of an existing fiber optic link without modification to the fiber itself is by multiplexing multiple signals via wavelength division multiplexers (WDMs). The use of WDMs provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. The demultiplexed signals can then be routed to the final destinations.
Dense WDMs (DWDMs) can be utilized to further increase information transmission capacity. In a DWDM system, multiple optical signals, each having a different channel or wavelength, are multiplexed to form an optical signal comprised of the individual optical signals. The signal is transmitted over a single waveguide and demultiplexed at a receiving end such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, optical channels can be directly amplified simultaneously, thereby facilitating the use of DWDM systems in long-distance optical systems. DWDMs can be made using techniques, such as disclosed in commonly-owned U.S. Pat. No. 5,809,190, which is incorporated by reference in its entirety.
Because DWDMs can multiplex and demultiplex large numbers of communication channels, e.g., 8, 16, or even 32 discrete communication channels onto a single optic fiber, and transmit these channels over long distances, some of the channels may be desired at intermediate nodes before demultiplexing. Selected channels from the multiplexed signal are extracted or “dropped” and routed to desired nodes for use, such as for transmission to users coupled to the node. However, other nodes along the single optic cable path or at the demultiplexing node may also want to utilize the extracted signals. In addition, intermediate nodes may generate signals for transmission along the single optic cable. Accordingly, extracted or newly generated signals are inserted or “added” into the multiplexed signal. This extracting and inserting of optical signals is generally referred to as add/drop multiplexing and is typically carried out with devices such as optical add/drop filters (OADFs) or OADF modules.
FIG. 1
shows a generalized WDM system having an OADF module
100
. Signals having wavelengths &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
, are multiplexed onto a single optical fiber
110
. OADF module
100
drops a signal at the selected wavelength, e.g., &lgr;
2
, for routing to desired destinations. OADF module
100
also adds back the signal at wavelength &lgr;
2
to the multiplexed signal for continued transmission. The multiplexed signal is then demultiplexed into individual signals at wavelengths &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
. Note that any number of OADF modules
100
can be inserted along the multiplexed signal.
One type of OADF module
100
is shown in
FIG. 2
, which utilizes two non-absorbing interference filters
210
and
220
. Filters
210
and
220
comprise dielectric layers or coatings having refraction indices and thicknesses so that filters
210
and
220
transmit a certain portion of the spectrum of the incident radiation and reflect the remaining portion. For example, filter
210
receives a multiplexed optical signal having wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
, the dielectric coating transmits the signal to be extracted, e.g., at &lgr;
2
, and reflects all other signals, e.g., at &lgr;
1
, &lgr;
3
, and &lgr;
4
, to filter
220
. Filter
220
also receives the signal to be inserted, e.g., at &lgr;
2
, and combines the signal with the multiplexed signal.
FIG. 3
shows another type of OADF module utilizing two optical circulators
310
and
320
and fiber Bragg grating
330
coupled between circulators
310
and
320
. The multiplexed optical signal, e.g., at wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
, enters optical circulator
310
at an input port
311
and is transmitted along an optical fiber
340
toward fiber grating
330
via an input/output port
312
. Fiber grating
330
reflects the signal to be extracted, e.g., at &lgr;
2
, back to optical circulator
310
via input/output port
312
, where the signal is dropped at an output port
313
. Meanwhile, the unreflected portion of the multiplexed signal, i.e., channels at &lgr;
1
, &lgr;
3
, and &lgr;
4
, travel through fiber grating
330
and enter optical circulator
320
via input/output port
321
. A signal to be inserted, such as a signal at &lgr;
2
, is inserted to optical circulator
320
at input port
322
. This signal is transmitted back along fiber
340
to fiber grating
330
, where it is reflected back to circulator
320
and inserted into the multiplexed signal. Thus, a signal with all channel components at &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
, is transmitted out of circulator
320
at an output port
323
.
While the OADF modules discussed above are effective for dropping and adding a single optical channel, problems arise when more than one optical channel is dropped and added from the multiplexed signal. One way to add and drop multiple optical signals is to add additional OADF modules to the single OADF modules of
FIGS. 2 and 3
. For example, each of the additional OADF modules can have dielectric coatings that transmit signals at a distinct wavelength or fiber gratings that only reflect signals at a particular wavelength. Thus, another OADF module can be coupled to the output of the single OADF module of
FIGS. 2 and 3
, where the second OADF module drops the channel at &lgr;
1
at one filter or circulator and adds a channel at &lgr;
1
at another filter or circulator. Accordingly, by cascading N OADF modules, N channels can be dropped and added along the multiplexed signal. However, where N is large, such as with DWDM applications, the system can be large and costly, requiring large numbers of interference filters or optical circulators and fiber gratings. In addition, the dielectric coatings and fiber gratings do not completely reflect or pass signals. Therefore, the resulting multiplexed signal out of each OADF module experiences some signal loss, which compounds as the signal travels through each subsequent OADF module. As a result, systems adding and dropping many channels can experience substantial signal degradation.
FIG. 4
shows an OADF module capable of adding and dropping multiple channels, but having only two optical circulators
410
and
420
. The OADF module is similar to that of
FIG. 3
, except that each optical circulator drops or adds multiple channels via an exit port
411
or input port
412
, respectively, and that multiple fiber gratings are coupled along the optic fiber between the two circulators. For example, optical circulator
410
drops channels at &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
from an N-channel multiplexed signal, and optical circulator
420
adds channels at &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
. Fiber Bragg gratings
431
-
434
are tuned to reflect signals at &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
, respectively. Because fiber gratings
431
-
434
do not completely reflect signals at the tuned wavelengths, leakage components at the tuned wavelengths pass through the associated fiber gratings. These adverse effects increase as the number of channels to be added and dropped increase

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