Article comprising a tunable filter

Details Article comprising a tunable filter Article comprising a tunable filter

2000-02-10

2004-04-06

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling function

C385S015000, C385S024000, C359S199200

Reexamination Certificate

active

06718086

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to optical communications. More particularly, the present invention relates to a tunable filter for use in conjunction with optical communications systems.
BACKGROUND OF THE INVENTION
FIG. 1
a
depicts a simplified schematic diagram of atypical WDM network
100
in the prior art. WDM network
100
includes a plurality of transmitters TX-
1
through TX-n. Each of the transmitters includes an optical source for generating an optical signal &lgr;-i, i=1, n. Each optical signal &lgr;-i is characterized by a unique peak wavelength onto which information may be modulated in well-known fashion. The plurality of optical signals &lgr;-
1
through &lgr;-n are combined into a single “multiplexed” signal m-&lgr; by wavelength multiplexer
102
, and the multiplexed signal m-&lgr; is then launched into optical fiber
104
.
A plurality of subscriber terminals (e.g.,
106
-S
1
,
106
-S
2
and
108
-S
1
through
108
-Sn) are in optical communication with network
100
. Each such subscriber terminal includes a receiver(s) (not shown) for receiving information that is carried over network
100
via multiplexed signal m-&lgr;. An individual subscriber terminal may subscribe to the information contained on only a single channel (i.e., on a single optical signal &lgr;-i) of multiplexed signal m-&lgr;.
Subscriber terminals
108
-S
1
through
108
-Sn located at end terminal
108
require, collectively, most or all of the individual channels &lgr;-
1
through &lgr;-n multiplexed signal m-&lgr;. To provide such channels to subscriber terminals
108
-S
1
through
108
-Sn, multiplexed signal m-&lgr; is typically demultiplexed, fully resolving it into its constituent channels. Demultiplexer
110
is used for that purpose.
Subscriber terminals
106
-S
1
and
106
-S
2
are located at “small” intermediate node
106
. Node
106
requires only a few of the channels of multiplexed signal m-&lgr; (ie., terminal
106
-S
1
receives only channel &lgr;-
1
and terminal
106
-S
2
receives only channel &lgr;-
3
). As a consequence, rather than fully demultiplexing multiplexed signal m-&lgr; at node
106
, only the required channels are dropped (i.e., removed or separated) from multiplexed signal m-&lgr; and delivered to the appropriate subscriber terminal. One or more “wavelength “(add)/drop” filters (i.e., filters
106
-WAD
1
,
106
-WAD
2
), which are operable to drop a single channel, are advantageously used for this purpose.
For example, in network
100
at node
106
, add-drop filter
106
-WAD
1
separates and drops channel &lgr;-
1
from multiplexed signal m-&lgr;. Channel &lgr;-
1
is then transmitted to subscriber terminal
106
-S
1
. Also, add-drop filter
106
-WAD
2
separates and drops channel &lgr;-
3
, which is then transmitted to subscriber terminal
106
-S
2
. As the name implies, in at least some embodiments, wavelength add-drop filters are operable to add a single channel having the same characteristic wavelength as the drop channel. For example, in network
100
, transmitter
106
-T
1
generates signal &lgr;-
1
that is added to multiplexed signal m-&lgr; via
106
-WAD
1
. Alternatively, such a channel may be added to the multiplexed signal elsewhere in network
100
.
It will be clear to those skilled in the art that a typical WDM optical communications network will have many more nodes and typically includes many other elements (e.g., amplifiers for maintaining signal strength, etc.) than are depicted in
FIG. 1
a
. These other nodes and other elements are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention.
FIG. 1
b
depicts a known wavelength add-drop filter. The particular filter depicted in
FIG. 1
b
is a Fabry-Perot etalon filter, well known in the art. Etalon filter
150
consists of a pair of highly reflective dielectric mirrors M
1
and M
2
that are separated by a precisely defined gap G. An optical cavity OC is defined between opposed surfaces SM
1
and SM
2
of the final dielectric layer of each mirror.
A multiple-wavelength signal MWS-IN from input waveguide (e.g., an optical fiber) F-IN is collimated by lens L
1
and illuminates the mirrors M
1
and M
2
. Most of wavelengths of signal MWS-IN are reflected from the filter and couple into output waveguide F-OUT. Signals D&lgr;
1
-D&lgr;
j
having a wavelength within a very narrow range or “passband” are, however, transmitted through the mirrors, pass through lens L
2
, and couple into drop waveguide F-D. Any signals A&lgr; having a wavelength within the narrow pass band of the filter can be delivered to filter
150
from “add” waveguide F-A and coupled into output waveguide F-OUT.
Performance parameters of the etalon filter
150
, such as reflectivity/transmissibility, passband, center transmission wavelength of the passband and finesse are readily calculable and are dependent on properties of the optical cavity OC (i.e., gap G) and mirror reflectivity and the coupling efficiency into output waveguides.
Returning to illustrative network
100
, to “drop” two channels (e.g., &lgr;-
1
and &lgr;-
3
) from multiplexed signal m-&lgr;, two add-drop switches (e.g., implemented as described above) can be used. Alternatively, it is possible to drop the same two channels using a single “tunable” etalon filter having an adjustable passband “center” wavelength. The “center” wavelength is the predominant wavelength of the passband (hereinafter “center transmission wavelength”).
In such tunable etalon filters, one of the two mirrors is typically placed on a translation actuator (e.g., a piezoelectric transducer) that is under electrical control. Moving the actuator changes the size of the gap between the mirrors. Since the gap (size) controls the center transmission wavelength of the filter, moving the actuator changes that center transmission wavelength.
A problem exists, however, with existing tunable filters. As explained above, to change the center transmission wavelength, the size of the gap between the two mirrors is altered. In doing so, the gap will assume a number of intermediate sizes until the desired size is attained. At such intermediate gap sizes, the optical cavity will tune to channels or signals having intervening wavelengths (hereinafter “intervening channels” or “intervening signals”). Such intervening signals will be transmitted by the filter, delivered to the drop fiber and passed to the subscriber terminal rather than to the intended destination. To prevent intervening signals from being delivered to a subscriber terminal in this manner, those signals must be disadvantageously temporarily interrupted while tuning the filter to a new center transmission wavelength.
The art would therefore benefit from a tunable filter that, during tuning, does not disrupt intervening channels.
SUMMARY OF THE INVENTION
Some embodiments of the present invention provide a tunable filter without some of the disadvantages of the prior art. In particular, the illustrative embodiment of the present invention is a tunable filter that does not interrupt intervening channels during tuning.
In accordance with the illustrative embodiment of the present invention, a tunable filter includes an optical cavity, a tuning device and a filter-disabling device. The length of the optical cavity defines the center transmission wavelength of the filter. Other attributes of the optical cavity and the mirrors define the finesse of the filter.
As used herein, the term “passband” refers to the range of wavelengths that are transmitted or passed by a filter, the term “center transmission wavelength” refers to the predominant or peak wavelength in the passband, and the term “finesse” refers to the transmissibility of the filter. The term “finesse” is also properly considered to be a measure of the “sharpness” of the transmission peak of the filter. And, as will be appreciated by those skilled in the art, the term “finesse” also has mathematical definitions (e.g., assuming equal reflectivity mirrors: finesse=4r/(1−r
2
), where

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