Note: Descriptions are shown in the official language in which they were submitted.
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FIBER OPTIC SYSTEM WITH Sl1\11JLTANEOUS
SWITCHING AND RANIAN
FIELD OF THE INVENTION
This invention relates generally to a fiber optic system and more
particularly, to a fiber optic system which can concurrently amplify and switch
an input signal via a pump signal.
BACKGROUND OF THE IN~ENTION
Historically, fiber optic systems have separately addressed the
problems of switching of an input signal and amplifying the input signal.
For example, one type of optical switch for an input signal uses a
birefringent fiber switch which has an optical fiber with a birefringence that
preserves polarization. When an input signal is coupled into the birefringent
optical fiber, the input signal propagates in one of two, perpendicular,
polarized modes in the optical fiber. To switch the input signal to propagate inthe other polarized mode in the birefringent optical fiber, a gating signal of
sufficient energy is coupled into the optical fiber to propagate in the same
polarized mode as the input signal initially. The gating signal induces a non-
linear birefringence in the optical fiber which causes the input signal to switch
and propagate in the other polarized mode. However, the gating signal in the
birefringent fiber switch does not amplify the inpui. signal.
One example of a type of optical amplifier for amplifying an input
signal uses an optical fiber and a phenomenon known as Raman gain. Again,
the input signal propagates in the optical fiber. To amplify the input signal
using Raman gain, a pump signal whose wavelength is less than the
wavelength of the input signal is coupled into the optical fiber which is carrying
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the input signal. The pump signal amplifies the input signal, but does not
switch the polarization or propagation of the input signal.
SUMMARY OF THE INVENTION
A fiber optic system in accordance with the present invention
includes an optical fiber, a coupling device, and a system for generating a
pump signal. The optical fiber has a birefringence which preserves
polarization and has a first polarization mode which is substantially
perpendicular to a second polarization mode. The coupling device couples an
input signal having an input wavelength into the optical fiber so that the inputsignal propagates in the first polarization mode in the optical fiber. The system
for generating a pump signal generates a pump signal with a pump wavelength
which is within a range of 50 nm to 300 nm less than the input wavelength and
which and has a first amount of pump power which is enough power to switch
the input signal polarization, but not enough power to create higher order
solitons. The coupling device also couples the pump signal into the optical
fiber so that the pump signal propagates in the first polarization mode in the
optical fiber. The pump signal alters the birefringence of the optical fiber which
causes the input signal to switch polarization modes and propagate in the
second polarization mode and amplifies the input signal as it propagates
through the optical fiber. Accordingly, with the fiber optic system of the
invention, an input signal can be amplified and switched concurrently, and thus
separate stages for amplification and switching are not required.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a fiber optic system in accordance
with the present invention; and
FIG. 2 is a cross-sectional view of the optical fiber taken along
lines 2-2 of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of a fiber optic system 10 in accordance with
the present invention is illustrated in FIG. 1. Fiber optic system includes an
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optical fiber 12, a coupler 14, and a pump laser 18. With fiber optic system 10,an input signal can be both amplified and switched concurrently, and thus
separate stages for amplification and switching are not required.
Referring more specifically to FIG. 1, fiber optic system '10
includes optical fiber 12 which has a birefringence that preserves polarization.Since optical fiber 12 is birefringent, optical fiber 12 has a first polarization
mode which is substantially perpendicular to a second polarization mode. An
input signal coupled into one end 19 of optical fiber 12 will propagate in either
the first or second polarization mode.
Referring to FIG. 2, optical fiber 12 has an elliptical, cross
sectional shape with a short or fast axis 20, along which one of the polarization
modes runs, and a long or slow axis 22, along which the other one of the
polarization modes runs. Although an optical fiber 12 with an elliptical shape
is shown, other types of birefringent optical fibers~ such as an optical fiber with
stress rods, could be used. Optical fiber 12 has a core 24, which in this
particular embodiment is made from silica, and a cladding 26.
Fiber optic system 10 also includes ,a pump laser 18 which
generates a pump signal which has a pump wavelength that is about 50 nm to
300 nm less than the input wavelength for the input signal. The pump
wavelength and gain are chosen to amplify the sic~nal, but not to the point of
creating higher order solitons. The pump signal is also generated by laser
pump 18 to have sufficient energy to induce a non-linear birefringence in
optical fiber 12 and cause the input signal to switch between the two
polarization modes. The amount of energy or power needed to induce a non-
linear birefringence in optical fiber 12 and cause the input signal to switch
polarization modes depends upon the birefringence of optical fiber. Pump
laser 18 outputs the pump signal on optical fiber 29. Although in this particular
embodiment a pump laser 18 is used, any type of system for generating the
pump signal could be used.
Fiber optic system 10 also includes coupler 14 which couples the
input signal and the pump signal into the optical fiber 12 to propagate in the
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first polarization mode. Coupler 14 is located between an optical fiber 28 and
one end 19 of optical fiber 12 and couples the input signal and pump signal
into optical fiber 12. In a preferred embodiment, coupler 14 is a wavelength
division multiplexer (WDM). Although only one coupler 14 is shown, separate
couplers could be used to input the input signal and pump signal.
Fiber optic system 10 may also include a filter 30 which can be
coupled to the other end 32 of optical fiber 12. Filter 30 removes unwanted
wavelengths after the input signal has been amplified and switched, including
any of the pump signal which remains.
Fiber optic system 10 operates by coupling input and pump
signals into optical fiber 12 via coupler 14. In this particular embodiment, a
soliton signal is used as the input signal and as the pump signal, although
other types of input and pump signals could be used. The input signal and the
pump signal propagate in a first polarization mode with their electric fields
aligned. In this particular example, input signal and pump signal are input to
propagate in the short or fast axis 20.
When the pump signal is coupled into optical fiber 12 as
described above, part of the energy of the pump signal converts to and
combines with the input signal to amplify the input signal. Effectively, the
pump signal converts its energy to the input signal wavelength. The
amplification of the input signal is the result of a phenomenon called Raman
gain. For Raman gain to occur in optical fiber 12, there needs to be a
difference between the wavelength of the input signal and the wavelength of
the pump signal from pump laser 18. Preferably in systems using soliton
signals, the difference between the two wavelengths should be such that the
gain occurs without the creation of higher order solitons. Accordingly, in this
particular embodiment the gain is on the order of ten to avoid creating higher
order solitons. To keep the amplification factors on the order of ten, the
wavelength of the pump signal is kept within a range of 50 nm to 300 nm less
than the wavelength of the input signal when the wavelength of the input signal
is near 1550 nm.
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The specific amount of amplification or Raman gain experienced
by the input signal in optical fiber 12 is determine~l by the following equation:
P = P e [9 P2 L'A]
where P represents the power of the input signal after amplification, P0 is the
power of the input signal before amplification, P2 is the power of the pump
signal, A is the effective area of optical fiber, g is the gain coefficient which is a
function of the difference between the pump wavelength and the input
wavelength, and L is the length of the optical fiber.
By way of example, for an optical fil~er with an effective area of
50,um2 and a length of 10 meters which receives a one kilowatt input signal
and a one kilowatt pump signal from pump laser 18 and where the input and
pump wavelengths are separated by 100 nm and the gain coefficient is 1x10-
cmlwatt, the gain of the input signal will be approximately e23. If the
wavelength of the pump signal is 300 nm from the wavelength of the input
signal, then the amplification factor will be e2, or a gain cf about seven.
While the input signal is being amplified in optical fiber 12 as
described above, the input signal is also being switched in op~ical fiber 12.
The pump signal from pump laser 18 coupled into and propagating in the first
polarization mode of optical fiber 12 has sufficient energy to induce a non-
linear birefringence in optical fiber 12 and thus cause the input signal to switch
from propagating in the first polarization mode to propagating in the second
polarization mode.
As discussed above, the amount of energy or power required to
switch the input signal in optical fiber 12 depends upon the birefringence of
optical fiber 12, as discussed in M.N. Islam, Ultrafast Fiber Switching Devices
and Systems, Cambridge, University Press, 1992, which is herein incorporated
by reference. The birefringence of the optical fiber 12 can be determined by
the equation:
~ ~N = 0.33N2 (Ix- Iy)
where N2 is the index of refraction of the core of optical fiber 12, lx is the
intensity of the input signal along the x-axis, and Iy is the intensity of the input
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signal along the y-axis. Once the birefringence (~N) is known, then the
amount of power or energy required to switch the input signal in optical fiber
12 can be determined by first substituting for lx with the following equation:
Ix= PX/Ax
where Px is the power of the pump signal and Ax is the effective area of opticalfiber 12 and then by solving for Px By way of example, when the core 24 of
optical fiber 12 is glass, then N2 = 3.2 x 1 o-16 P/A and assuming lx is 10 W/,um,
and Iy is 0, ~N is on the order of 1 o-6, and Ax is 50 ,um2, then Px equals a one
kilowatt pump signal.
Once the amplified and switched input signal reaches the other
end 32 of optical fiber 12, filter 30 removes any of pump signal which still
remains. Filter 30 is designed to only permit certain wavelengths of light
signals to pass through. If the undesired wavelengths are not removed, the
undesired wavelengths will broaden the input signal as the input signal
propagates since the dispersion can be significantly different for the two
wavelengths. Accordingly, with the present invention an input signal may
concurrently be amplified and switched by a pump signal.
Having just described the basic concept of the invention, it will be
readily apparent to those skilled in the art that the forgoing detailed disclosure
is intended to be presented by way of example only, and is not limiting.
Various alterations, improvements and modifications will occur and are
intended to those skilled in the art, though not expressly stated herein. These
modifications, alterations, and improvements are intended to be suggested
hereby, and are within the spirit and scope of the invention. Accordingly, the
invention is limited only by the following claims and equivalents thereto.
