Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2002-01-11
2004-01-06
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S016000, C385S022000, C372S020000, C359S199200
Reexamination Certificate
active
06674937
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, generally, to the field of apparatus and methods for use in optical telecommunication networks and, in its preferred embodiments, to the field of apparatus and methods for dynamically routing wavelength channels in optical fiber DWDM networks.
BACKGROUND OF THE INVENTION
In the modern age of information exchange, many companies depend upon telecommunications networks to carry out their daily business and often rely upon telecommunications providers to supply a fast and reliable network with a very high bandwidth. For today's large scale telecommunication applications, optical fiber dense wavelength division multiplexed (“DWDM”) networks appear to be a very good first generation solution which address and/or meet such requirements. The challenge, however, is to optimize the capabilities of optical networks to create a second generation network for use in the future.
The second generation of optical networks may use transparent optical routing of numerous wavelength channels. While it is desirable to route such wavelength channels entirely in the optical domain using integrated optics technology, such routing presents a number of technological challenges. One of the main challenges is that such routing may require the use of wavelength-selective filtering elements compatible with integrated optics. Unfortunately, such wavelength-selective filtering elements are, typically, not tunable over the entire wavelength range of interest. For instance, with the advent of optical DWDM, the number of wavelength channels requiring routing in a particular optical fiber application may be on the order of one hundred to a thousand, and the total optical bandwidth may range between ten nanometers and hundreds of nanometers. Building a tunable wavelength filter that can be tuned over such a wide DWDM bandwidth is difficult. Another technological challenge stems from the need to route such wavelength channels at high speeds on a microsecond or faster time scale. These requirements place great demands on any technology, and are difficult to achieve in concert.
Two important, “building-block” circuits for routing wavelength channels in second generation optical and/or DWDM networks are likely to be (1) the tunable add/drop, in which one of “N” incoming wavelengths is dropped, and (2) the 1×N tunable wavelength demultiplexer, in which “N” wavelengths on an input channel are separated into “N” independent output channels, as shown in
FIG. 1
(alternatively, the demultiplexer of
FIG. 1
may be reconfigured to drop multiple wavelengths onto a single output optical fiber by using additional tunable wavelength filters, as depicted in FIG.
2
). Due to reciprocity, these circuits may be used in reverse to perform wavelength add and N×1 wavelength multiplexing, respectively. In these circuits, 1 to “N” tunable wavelength filters are used to selectively “drop”, or direct, a selected wavelength channel to an output optical fiber. Each such tunable wavelength filter should be tunable over all wavelength channels.
Another important circuit for routing wavelength channels in second generation optical and/or DWDM networks is likely to be the N×N wavelength routing switch circuit illustrated in FIG.
3
. This circuit takes “N” inputs, each with “N” wavelengths, and routes one wavelength from each input to each output. Each output receives all “N” wavelengths, with each wavelength originating from a different input. As noted above, it is desirable for each of the N
2
wavelength filters to have the ability to independently access all “N” wavelength channels.
Many of the prior art tunable filter technologies suitable for dense integration on an optical chip cannot tune over the full DWDM bandwidth, thus making implementation of the above-described circuits (and a variety of other circuits) impractical. For instance, arrayed waveguide gratings, acousto-optical and electro-optical tunable filters, and Mach-Zehnder interferometer techniques are so limited. Similarly, while being suitable for dense integration, neither the traveling wave optical microcavity filter nor longitudinal Bragg gratings can tune over the full DWDM bandwidth.
The traveling wave optical microcavity filter, which includes a tuned optical cavity, has resonances that allow the transfer of specific wavelengths from an input optical channel to an output optical channel. The tuned optical cavity supports whispering gallery modes which behave very similarly to the longitudinal modes of a linear Fabry-Perot type cavity. Fabry-Perot type resonators may be implemented with optical fiber or integrated onto an optical chip using reflective interfaces or longitudinal Bragg gratings. The length of the cavity determines the resonance wavelengths; which are the wavelengths that pass from the input channel through the cavity to the output channel with high efficiency. These wavelengths, or frequencies, given by
v
i
=
i
c
nL
are periodic, with the period being given by the cavity free-spectral range (“FSR”), which is approximately
FSR
=
Δ
v
=
c
nL
,
where “c” is the speed of light, “n” is the effective index of the cavity mode, and “L” is the round trip path length through the cavity. In DWDM systems, it is generally beneficial to have &Dgr;&ngr; be greater than the total optical bandwidth, which is computed from the number of wavelength channels multiplied by the channel spacing. By doing so, a single wavelength channel may be operated upon without interference from other channels. Another condition that must be met is to have the resonance frequency passband, given by the expression
δ
v
=
v
Q
,
where “&ngr;” is the resonance frequency and “Q” is the quality factor of the cavity (i.e., which is related to the losses in the cavity), be approximately equal to the wavelength channel spacing &ngr;
ch
.
Additionally, it is desirable to have the ability to tune the resonance frequency by one free-spectral range, so that all wavelength channels may be operated upon by a single cavity. Such tuning may be achieved by varying the index of refraction. To tune over the entire free-spectral range by changing the index of refraction requires that
Δ
n
=
λ
2
L
or
Δ
n
n
=
Δ
v
v
be achieved. For more general situations in which the tuning range is less than the free-spectral range, the condition &Dgr;&ngr;/&ngr;=&Dgr;n
is still valid, where &Dgr;&ngr; now represents the tuning range. For a number of tuning mechanisms, such as the electro-optic effect and the thermo-optic effect, the maximum achievable fractional index change, &Dgr;n
, is of the order 0.01, meaning that the maximum cavity free-spectral range over which full tuning can be performed is &Dgr;&ngr;≈0.01&ngr;≈2 THz, which is much smaller than the optical bandwidth of interest such as that made available, for example, by optical fiber amplifiers, and therefore smaller than the total bandwidth that may be used by high capacity DWDM networks.
Alternatively, the length of the cavity could be changed by the amount
Δ
L
L
=
Δ
v
v
,
but again, large amounts of change, such as provided by the piezoelectric effect, are difficult to achieve. Utilization of both refractive index and cavity length changes may increase the tuning by about a factor of two, but such an increase may still not be enough to cover the desired wavelength range. However, it should be noted that MEMS type devices with moving parts may achieve this goal, but may be very difficult to stabilize to a specific wavelength channel, as a positioning accuracy of
δ
L
Δ
L
<
δ
v
Δ
v
or
δ
L
L
<
δ
v
v
≈
10
-
4
must be attained, where “&dgr;L” is the necessary positio
Blair Steven M.
Campbell Larry L.
Coursey, Esq. R. Steven
LC2I, Inc.
Palmer Phan T. H.
Troutman Sanders LLP
LandOfFree
Optical wavelength routing circuits does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical wavelength routing circuits, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical wavelength routing circuits will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3217761