Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2002-05-31
2004-07-06
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S140000
Reexamination Certificate
active
06760504
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to the switching of optical signals and, more particularly, to an optical switching system that facilitates output power balancing.
In an optical communication network based on Dense Wavelength Division Multiplexing, signals carried on carrier waves of different wavelengths are liable to have different optical powers, for several reasons. One reason is that such a network uses optical amplifiers to maintain signal power. The optical gain of an optical amplifier is not flat, as a function of wavelength. Therefore, even if the incoming multiplexed signals are equal in power, the outgoing multiplexed signals generally are not equal in power. A second reason is that the multiplexed signals typically have different origins, and so have suffered different propagation losses, as a result of having traveled different distances, by the time these signals reach an optical amplifier. If the range of signal powers among the multiplexed signals entering an optical amplifier is too great, the amplifier becomes saturated, resulting in unacceptable data loss.
Two different approaches have been used to solve this problem. The first approach is to flatten the response curve of the system (which is a composite of the response curves of the optical amplifier and of any other wavelength-dependent component, such as filters) by introducing a loss curve that is reciprocal to the response curve. This can be done passively (Y. Li, “A waveguide EDFA gain equalizer filter”,
Electronics Letters
, vol. 31 pp. 2005-2006, 1995) or dynamically (M. C. Parker, “Dynamic holographic spectral equalization for WDM”, IEEE Photon
Technology Letters
, vol. 9 pp. 529-531, 1997; J. E. Ford and J. A. Walker, “dynamic spectral power equalization using micro-opto mechanics”,
IEEE Photon Technology Letters
, vol. 10 pp. 1440-1442, 1998). In this approach, the signals remain multiplexed on a common optical waveguide. The second approach demultiplexes the signals to respective channels and attenuates each channel using an optical attenuator.
Optical switches such as 2×2 and 1×2 Mach-Zehnder interferometers can be used as attenuators.
FIG. 1
shows a Mach-Zehnder interferometer
10
. Interferometer
10
is based on two more-or-less parallel waveguides, an upper waveguide
12
and a lower waveguide
14
. Waveguides
12
and
14
are coupled to each other in a first 3 dB directional coupler
16
and in a second 3 dB directional coupler
18
. In-between directional couplers
16
and
18
, each waveguide
12
and
14
passes through a respective phase shifter
20
and
22
. Left end
24
of upper waveguide
12
serves as an input port of interferometer
10
. Right end
26
of upper waveguide
12
serves as an output port of interferometer
10
. Right end
28
of lower waveguide
14
is an idle port.
The operation of interferometer
10
is as follows. Coherent light entering interferometer
10
at input port
24
is split by directional coupler
16
, with half the light continuing rightward in upper waveguide
12
and the other half of the light propagating rightward in lower waveguide
14
. Phase shifters
20
and
22
are used to change the relative phases of the light in waveguides
12
and
14
. Directional coupler
18
then causes some or all of the light to emerge from interferometer
10
via output port
26
and/or idle port
28
, depending on the phase difference, between the light in upper waveguide
12
and the light in lower waveguide
14
, that is induced by phase shifters
20
and
22
.
FIG. 2
shows the power leaving a specific Mach-Zehnder interferometer
10
via output port
28
, relative to the power entering this interferometer
10
via input port
24
, in dB, versus the heating power applied to either phase shifter
20
or phase shifter
22
. This specific Mach-Zehnder interferometer
10
was fabricated using SiO
2
on Si technology, for light of a wavelength of 1.55 microns. Maximum attenuation, of 35 dB, is obtained at point I (approximately 50 mW heating power). Minimum attenuation is obtained at point II (approximately 610 mW heating power). This Mach-Zehnder interferometer
10
therefore is capable of a 35 dB attenuation range. When this Mach-Zehnder interferometer
10
is used as a switch, point I corresponds to the switch being OFF, with almost all power leaving the switch via output port
26
, and point II corresponds to the switch being fully ON, with almost all power leaving the switch via output port
28
.
The resolution of the attenuation depends on the resolution of the heating power used in phase shifters
20
and
22
.
SUMMARY OF THE INVENTION
2×2 and 1×2 optical switches also are used as elements in optical switch matrices, such as those taught in PCT application WO 99/60434 and U.S. Pat. No. 6,285,809, for switching optical signals from input waveguides to output waveguides. The present invention is an optical switching system based on an optical switch matrix that combines the switching functionality of optical switches such as Mach-Zehnder interferometer
10
with the attenuation functionality of such optical switches in a single unit.
Therefore, according to the present invention there is provided an optical switching system, for switching optical energy from a plurality of input waveguides to a plurality of output waveguides, including: (a) for each output waveguide: for each input waveguide: at least one respective attenuator for diverting an adjustable portion of the optical energy entering via the each input waveguide to the each output waveguide.
Furthermore, according to the present invention there is provided a method of switching each of a plurality of optical signals, that travel on respective input waveguides, from the respective input waveguide thereof to a desired one of a plurality of output waveguides, including the steps of: (a) providing an optical switch matrix including: for each output waveguide: for each input waveguide: at least one respective attenuator for diverting an adjustable portion of the signal that travels on the each input waveguide to the each output waveguide; (b) selecting the attenuators that divert the optical signals to the desired output waveguides; and (c) adjusting the selected attenuators to balance powers of the optical signals in the output waveguides.
The optical switching system of the present invention is based on an optical switch matrix that includes, for each input waveguide and for each output waveguide, a set of one or more optical switches for diverting an adjustable portion of the optical energy in the input waveguide to the output waveguide. At least one of the optical switches in each set is an attenuator, preferably a Mach-Zehnder attenuator. Preferably, the switches are 2×2 switches. If there are two switches per set, one for input and the other for output, then the input switch has an idle input port and the output switch has an idle output port. The input switch of the last switch set of each input waveguide also has an idle output port, and the output switch of the first switch set of each output waveguide also has an idle input port.
Preferably, the optical switching system of the present invention includes a feedback mechanism for adjusting the attenuators to balance the output powers in the output waveguides. The feedback mechanism includes a power measurement device such as a spectrum analyzer, a set of taps for diverting fixed portions of the optical energy from either the input waveguides or the output waveguides to the spectrum analyzer, and a control unit that receives signals from the spectrum analyzer that indicate the power levels in the tapped waveguides and that adjusts the attenuators on the basis of these signals. Most preferably, each tap includes a directional coupler that is coupled to a respective input or output waveguide.
By “balancing” the output powers in the output waveguides is meant adjusting the output powers in the output waveguides to facilitate the accurate transmission of signals downstream from the o
Friedman Mark M.
Lee John D.
Lynx Photonic Networks Inc.
Stahl Mike
LandOfFree
Optical switching system with power balancing 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 switching system with power balancing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical switching system with power balancing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3258571