Optics: measuring and testing – By light interference – Having partially reflecting plates in series
Reexamination Certificate
1999-09-01
2001-10-30
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having partially reflecting plates in series
C359S199200
Reexamination Certificate
active
06310690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic networks, and more particularly to fiber optic dense wavelength division multiplexers.
BACKGROUND OF THE INVENTION
Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. This totality of multiple combined wavelengths comprises a single transmitted composite signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component channels, typically by a wavelength division multiplexer. This separation must occur in order for the exchange of channels between signals on “loops” within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging channels.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data involves the exchanging of matching channels from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
FIG. 1
illustrates a simplified optical network
100
. A fiber optic network
100
could comprise a main loop
150
which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop
110
which connects with loop
150
at connector point
140
. Thus, if local loop
110
is Sacramento, channels at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point
140
, and the new signal will travel forward to New York. Within loop
110
, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop
110
to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal in the composite signal into its component channels is typically performed by a dense wavelength division multiplexer.
FIG. 2
illustrates add/drop systems
200
and
210
with dense wavelength division multiplexers
220
and
230
. An optical signal from Loop
110
(&lgr;
1
−&lgr;
n
) enters its add/drop system
200
at node A (
240
). The signal is separated into its component channels by the dense wavelength division multiplexer
220
. Each channel is then outputted to its own path
250
-
1
through
250
-n. For example, &lgr;
1
would travel along path
250
-
1
, &lgr;
2
would travel along path
250
-
2
, etc. In the same manner, the signal from Loop
150
(&lgr;
1
′−&lgr;
n
′) enters its add/drop system
210
via node C (
270
). The signal is separated into its component channels by the wavelength division multiplexer
230
. Each channel is then outputted via its own path
280
-
1
through
280
-n. For example, &lgr;
1
′ would travel along path
280
-
1
, &lgr;
2
′ would travel along path
280
-
2
, etc.
In the performance of an add/drop function, for example, &lgr;
1
is transferred from path
250
-
1
to path
280
-
1
. It is combined with the others of Loop
150
's channels into a single new optical signal by the dense wavelength division multiplexer
230
. The new signal is then returned to Loop
150
via node D (
290
). At the same time, &lgr;
1
′ is transferred from path
280
-
1
to path
250
-
1
. It is combined with the others of Loop
110
's channels into a single optical signal by the dense wavelength division multiplexer
220
. This new signal is then returned to Loop
110
via node B (
260
). In this manner, from Loop
110
's frame of reference, channel &lgr;
1
of its own signal is dropped to Loop
150
while channel &lgr;
1
′ of the signal from Loop
150
is added to form part of its new signal. The opposite is true from Loop
150
's frame of reference. This is the add/drop function.
Conventional methods used by wavelength division multiplexers in separating an optical signal into its component channels include the use of filters and fiber gratings as separators. A “separator,” as the term is used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is their limitation to channels with identical spacings and/or bandwidths. However, networks typically transmit multiple signals with different modulations or data transfer rates. These different modulation rates lead to different effective bandwidths, which are herein referred to an information bandwidths. For instance, greater modulation rates are associated with occupation or utilization of greater optical bandwidth (defined either in frequency or in wavelength) than are slower modulation rates. However, optical components within conventional wavelength division multiplexed (WDM) optical communications systems are associated with certain fixed channel band pass widths. In WDM systems in which different signals are transmitted at different data transfer rates along different channels, the available fiber bandwidth can be utilized most efficiently when hardware bandwidths match the information bandwidths dictated by the channel data transfer rates. This requires optical hardware with uneven or asymmetric channel pass bands. Conventional channel separators have fixed channel bandwidths and are not able to separate channels in such an asymmetric fashion. Thus, in conventional WDM systems, either the pass bands must be made as wide as the widest information bandwidth or else the information bandwidth on each channel is restricted to the available hardware band pass. In either case, this can lead to inefficient use of overall optical system bandwidth.
Accordingly, there exists a need for an optical channel separation mechanism which would allow a wavelength division multiplexer to separate channels in an asymmetric fashion. The mechanism should allow the hardware band pass to correlate with the channel information bandwidth, and allow more efficient use of optical bandwidth in WDM systems in which signals with different data transfer rates propagate simultaneously. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides an improvement in the separation mechanism to be used in a dense wavelength division multiplexer. The separation mechanism in accordance with the present invention includes an asymmetric pass band interferometer. The interferometer includes a first glass plate optically coupled to a second glass plate, forming a space therebetween; a first reflective coating with a first reflectivity residing inside the space and on the first glass plate; a second reflective coating with a second reflectivity residing inside the space and on the second glass plate; a first waveplate with a first optical retardance residing inside the space; and a second waveplate with a second optical retardance, optically coupled to the first glass plate and residing outside the space, where a combination of values for the first reflectivity, the first optical retardance, and the second optical retardance effect a separation of channels in at least one optical signal into at least two sets, and where the at least two sets have asymmetrically interleaved pass bands. The asymmetric pass band interferometer in accordance with the present invention is capable of separating channels from signals with different data transfer rates. With the present invention, a maximum use of available bandwidth on a
Cao Simon X. F.
Mao Xiaoping
Avanex Corporation
Font Frank G.
Natividad Phil
Sawyer Law Group LLP
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