Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-11-10
2003-03-11
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Polarization without modulation
C385S018000, C359S281000, C359S282000, C359S484010, C359S494010, C359S490020
Reexamination Certificate
active
06532316
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization independent optical isolator and particularly to a bi-directional polarization independent isolator simultaneously and independently providing isolation to separate light-wave data channels propagating in opposite directions through an optical fiber.
2. Description of the Related Art
The use of optical fiber in long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity in the transmission of voice and/or data continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable being used as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. The latter practice of increasing the carrying capacity of existing fiber cable is sometimes referred to as the creation of “virtual fiber” and is clearly more cost effective than adding real fiber.
One of the ways in which “virtual” fiber is created is through the practice of Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” To increase data carrying capacity in a given direction, the number of such channels or bands must be increased.
Additionally, it is desirable to use existing fiber for bi-directional communications. Through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, both eastbound (northbound) as well as westbound (southbound) data. This bi-directional data-carrying capability of optical fiber further increases the need for additional channels. However, since all of the channels (wavelength bands) must reside within specific low-loss wavelength regions determined by the properties of existing optical fiber, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demands on WDM optical components—to transmit increasing numbers of more closely spaced channels with no interference or “crosstalk” between them and over long distances—becomes more severe.
Optical amplifiers are important components of fiber-optic communication systems. Traditionally, signal regeneration has been accomplished through the use of repeaters, which are combinations of demultiplexers, receivers, signal recovery electronics, transmitters (light sources together with optical modulators), and multiplexers. In a repeater, the signal for each channel is recovered electronically and transmitted anew. Unfortunately, the complexity and cost of repeater-based systems becomes unwieldy with the increase in the number of channels of WDM systems.
Optical amplifier systems have therefore become attractive alternatives to repeaters. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 &mgr;m transmission band.
Because of the indiscriminate and non-directional nature of optical fluorescence amplification, unless special precautions are taken, all signals will be amplified on transit through an EDFA and re-transmitted in both directions. These signals may include spurious signals caused by stray reflections or light scattering off of various optical components and propagating counter to the desired signal transmission direction.
To guard against amplification and subsequent transmission of such unwanted signals, optical amplifier systems generally include optical isolators on both sides of the optical gain element (the Er-doped fiber). As shown in the amplifier
100
of the prior art in
FIG. 1
, optical isolators, such as isolator
101
and isolator
102
, are disposed to either side of an Er-doped fiber
103
, and comprise part of a set of so-called “optical passive components” which are generally associated with optical amplifier systems. Other such optical passive components illustrated in
FIG. 1
are Wavelength Division Multiplexers (WDM's)
104
and
105
and bandpass filter
106
. Also included in the amplifier
100
of the prior art shown in
FIG. 1
are Co-Pump Laser
108
of 980 nm or 1480 nm and Counter-Pump Laser
110
of 980 nm or 1480 nm.
Optical isolators act as “one-way gates” which only permit signal transmission in the desired direction. This property, although essential, creates a problem for communications systems in which signals are carried in both directions within individual optical fibers, viz. the isolators would block one set of signals.
Therefore, in the current state of the art, separate amplifiers are used for eastbound (northbound) and westbound (southbound) communications channels as shown in the band bi-directional amplifier
200
of the prior art of FIG.
2
. In the band bi-directional amplifier
200
, the counter-propagating signals are respectively separated and re-combined on either side of the pair of optical amplifiers
206
and
207
.
For instance, in
FIG. 2
, if the “blue” or relatively short wavelength band
201
shown as solid lines represents westward propagating signals and the “red” or relatively long wavelength band
202
shown as dash-dot lines represents eastward propagating signals, then these two signals are separated and recombined by WDMs or circulators
203
A and
203
B. Between the two WDMs or circulators
203
A and
203
B, the blue and red signals propagate on separate physical optical fiber sub-paths
204
and
205
, respectively, but to either side of each WDM, the westbound blue and eastbound red signals co-propagate along the same physical fiber pathways
211
and
212
. Each of the fiber sub-paths
204
and
205
contains its own amplifier system,
206
and
207
, respectively. Optional second amplifiers
208
and
209
may be placed in each of the fiber sub-paths and the locations between each of the resulting sequential amplifiers
206
and
208
or
207
and
209
corresponds to multi-access ports
210
A and
210
B in the blue and red sub-paths, respectively. Generally, each of the optical amplifier systems,
206
and
207
and, optionally,
208
and
209
, shown in
FIG. 2
, comprises all of the optical passive and active components illustrated in FIG.
1
and possibly others. In particular, the amplifier
206
(and optionally
208
) contains optical isolators that only permit westbound light propagation and the amplifier
207
(and optionally
209
) contains optical isolators that only permit eastbound light propagation.
One example of the possible wavelength constitution of co-propagating bi-directional signals is illustrated in
FIG. 3
, showing the relative positions between light traveling in a “red” band and light traveling in a “blue” band through a band bi-directional polarization independent isolator. For the example shown in
FIG. 3
, the terms “red” band and “blue” band are meant as relative terms referring to light of a relatively longer wavelength (the “red” band) and light of a relatively shorter wavelength (the “blue” band) and may not correspond to actual colors of red or blue produced by that light.
Referring now to
FIG. 3
, as an example, the “blue” band
301
and the “red” band
302
occupy separate wavelength regions each wholly contained within the well-known fiber transmission band
303
centered near a wavelength of 1.55 &mgr;m. For instance band
301
might represent the wavelength constitution of the westbound signal channel(s)
201
of
FIG. 2
while band
302
might represent the wavelength constitution of the eastbound signal channel(s)
202
. This type of bi-directional lightwave transmission scheme is termed “band bi-directional” transmission herein. Other types of band bi-directional transmission schemes are possible. For instance, the “blue” band might correspond to all or a portion of the 1.3 &mgr;m fiber transmission band while the “red” band might correspond to all or a portion of the 1.55 &mgr;m transmission band, etc.
Ge
Avanex Corporation
Rojas, Jr. Omar
Sanghavi Hemang
Staas & Halsey , LLP
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