U.S. patent application number 10/043634 was filed with the patent office on 2002-05-16 for electro-optic modulator for generating solitons.
Invention is credited to Yang, Guangning.
Application Number | 20020057858 10/043634 |
Document ID | / |
Family ID | 22669201 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020057858 |
Kind Code |
A1 |
Yang, Guangning |
May 16, 2002 |
Electro-optic modulator for generating solitons
Abstract
A method and apparatus is provided for generating optical pulses
with an electro-optic amplitude modulator. The modulator includes
first and second waveguides that form an optical interferometer. At
least the first waveguide includes an electro-optic material such
as lithium niobate and an electrode extending along a portion
thereof. Input and output optical waveguides are respectively
coupled to input and output junctions of the interferometer. A
voltage source biases the electrode such that a modulation
switching curve arises that generates two optical pulses over a
complete voltage cycle.
Inventors: |
Yang, Guangning; (Matawan,
NJ) |
Correspondence
Address: |
John P. Maldjian
Senior Patent and Trademark Counsel
TyCom (US) Inc.
250 Industrial Way West, Rm. 2B-106
Eatontown
NJ
07724
US
|
Family ID: |
22669201 |
Appl. No.: |
10/043634 |
Filed: |
January 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10043634 |
Jan 10, 2002 |
|
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09182604 |
Oct 29, 1998 |
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Current U.S.
Class: |
385/2 |
Current CPC
Class: |
G02F 1/225 20130101;
G02F 2203/54 20130101 |
Class at
Publication: |
385/2 |
International
Class: |
G02F 001/035 |
Claims
1. A method for generating optical pulses with an electro-optic
amplitude modulator having a pair of waveguides and at least one
pair of electrodes for controlling a refractive index of at least
one of the waveguides, said method comprising the steps of:
receiving a cw optical signal at an input waveguide of the
modulator; applying at least one electrical pulse to the electrode
pair to modulate the cw optical signal so that an edge of the
electrical pulse yields an optical pulse at an output waveguide of
the modulator.
2. The method of claim 1 wherein said optical pulse has a temporal
width substantially equal to the temporal width of the edge.
3. The method of claim 1 wherein said modulator is a lithium
niobate modulator.
4. The method of claim 1 further comprising the step of applying at
least a second electrical pulse to a second electrode pair to
modulate the cw optical signal so that an edge of the electrical
pulse yields an optical pulse at an output waveguide of the
modulator, wherein said electrical pulse and said second electrical
pulse are 180 degrees out of phase with respect to one another.
5. The method of claim 1 wherein said optical pulse corresponds to
an optical bit of one, and further comprising the step of
maintaining a constant voltage level between the electrode pair to
generate an optical bit of zero.
6. The method of claim 1 wherein said optical pulse corresponds to
an optical bit of zero, and further comprising the step of
maintaining a constant voltage level between the electrode pair to
generate an optical bit of one.
7. The method of claim 1 wherein the step of applying at least one
electrical
15. The method of claim 9 wherein said optical pulse is a
soliton
16. An electro-optic amplitude modulator, comprising: first and
second waveguides forming an optical interferometer, at least said
first waveguide including an electro-optic material; an electrode
extending along a portion of said first waveguide; an input and
output optical waveguide respectively coupled to input and output
junctions of the interferometer; a voltage source biasing said
electrode such that a modulation switching curve arises that
generates two optical pulses over a complete voltage cycle.
17. The modulator of claim 16 wherein said electro-optic material
is lithium niobate.
18. The modulator of claim 16 wherein said voltage source generates
an electrical waveform that is sinusoidal.
19. The modulator of claim 16 further comprising a second electrode
extending along a portion of the second waveguide.
20. The modulator of claim 19 wherein said voltage source bias said
first and second electrodes 180 degrees out of phase with respect
to one another.
21. The modulator of claim 16 wherein said optical pulses have a
temporal width substantially equal to the time over which voltage
applied to the electrode by the voltage source changes from a first
to a second value.
22. The modulator of claim 21 wherein said first and second values
are minimum and maximum voltage levels, respectively.
23. The modulator of claim 16 wherein said optical pulses are
solitons.
Description
TECHNICAL FIELD
[0001] This invention relates generally to amplitude modulators and
more particularly to an electro-optic modulator for generating
solitons from a continuous wave signal.
BACKGROUND OF THE INVENTION
[0002] Long distance optical transmission using optical amplifiers
can provide greater bandwidth at lower cost than that using
electronic regeneration. Erbium doped optical fiber amplifiers can
easily handle several channels simultaneously, and do so with low
crosstalk. For long distance transmission, it is necessary to use a
transmission mode which is resistant to the various dispersive
effects of the fiber. In an optical fiber transmission path, the
optical fiber's chromatic dispersion, acting by itself, attempts to
broaden pulse signals in time. The fiber's index, which also
depends on the intensity of light, acting by itself through the
process of self phase modulation, always serves to broaden the
pulse's frequency spectrum. Thus, for long distance transmission,
an optical signal which is resistant to the various dispersive
effects of the optical fiber can result in an increase in the
spacing between optical amplifiers in the optical transmission
path.
[0003] Under certain conditions such as, for example, zero loss or
loss periodically compensated by optical gain, a soliton is
nondispersive in the time domain. Thus, the waveshape of a soliton
is independent of the distance that it travels along an optical
fiber. In addition, a soliton is also nondispersive in the
frequency domain. Thus, for a range of soliton pulse widths, such
as 50-80 ps for a data rate of 2.5 G b/s, and fiber group delay
dispersion parameters of approximately 0.7-2 ps/nm/km, the distance
that a soliton can be transmitted before serious dispersive effects
occur is typically 500 km or greater.
[0004] Creation of soliton pulses is dependent upon proper launch
and transmission characteristics such as pulse power, pulse width,
center frequency, and fiber dispersion. Of particular concern for
the present purposes, creation of solitons require the generation
of temporally narrow pulses, typically on the order of 1-10
picoseconds. These characteristics of solitons are well known to
those skilled in the art and will not be discussed further herein.
For additional information concerning soliton generation and
soliton transmission, see Optical Fiber Telecommunications U, ed.
S. E. Miller et al., p.90 et seq. (Academic Press 1988).
[0005] One device for generating solitons consists of a high speed
amplitude modulator such as an electro-optic waveguide modulator.
One class of electro-optic modulators are made of ferroelectric
materials, such as z-cut lithium niobate (LiNbO.sub.3) or lithium
tantalate (LiTaO.sub.3). These modulators convert an applied
voltage to an optcal signal. Typically, an electric pulse is used
to generate an optical pulse. Lithium niobate modulators are
commonly employed because they offer high speed, a high extinction
ratio, and a controllable (or zero) chirp. However, one problem
with such modulators is that it is difficult to generate extremely
narrow electrical pulses that can be translated into optical pulses
of sufficiently narrow temporal width to form solitons.
[0006] Therefore, it is desirable to provide an electro-optic
amplitude modulator with an electric signal that allows the
modulator to generate temporally narrow optical pulses.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, a method and
apparatus is provided for generating optical pulses with an
electro-optic amplitude modulator. The modulator includes first and
second waveguides that form an optical interferometer. At least the
first waveguide includes an electro-optic material such as lithium
niobate and an electrode extending along a portion thereof. Input
and output optical waveguides are respectively coupled to input and
output junctions of the interferometer. A voltage source biases the
electrode such that a modulation switching curve arises that
generates two optical pulses over a complete voltage cycle.
[0008] In accordance with another aspect of the invention, a method
is provided for generating optical pulses with an electro-optic
amplitude modulator having a pair of waveguides and at least one
pair of electrodes for controlling a refractive index of at least
one of the waveguides. In particular, a cw optical signal is
received at an input waveguide of the modulator. At least one
electrical pulse is applied to the electrode pair to modulate the
cw optical signal so that an edge of the electrical pulse yields an
optical pulse at an output waveguide of the modulator. The optical
pulse may have a temporal width substantially equal to the temporal
width of the edge of the electrical pulse.
[0009] In contrast to known biasing arrangements in which an
electrical pulse was required to produce an optical pulse, the
present invention advantageously produces an optical pulse upon a
change in voltage levels. Since it is relatively easy to produce
sharp voltage transitions (as opposed to narrow electrical pulses),
the invention is capable of produces extremely narrow optical
pulses, such as solitons, for example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a known lithium niobate amplitude
modulator.
[0011] FIG. 2 shows a known modulation switching curve for the
modulator shown in FIG. 1.
[0012] FIG. 3(a) shows a modulation switching curve in accordance
with the present invention and FIG. 3(b) shows a complete voltage
cycle applied to the modulator and the resulting optical signal
levels corresponding thereto.
DETAILED DESCRIPTION
[0013] Referring to FIG. 1, there is illustrated an example of a
known lithium niobate (LiNbO.sub.3) high-speed amplitude modulator
for modulating an optical signal with an electrical signal to form
a soliton. It should be noted that the present invention is
applicable to a wide variety of electro-optic amplitude modulators
and that the modulator of FIG. 1 is shown for illustrative purposes
only. As shown, an electro-optic material substrate 20 such as
lithium niobate (LiNbO.sub.3) or the like, which can convert an
electrical potential into optical phase shifts, includes an optical
waveguide 22. The waveguide 22 may be formed, for example, by
diffusing titanium (Ti) into the substrate. Alternatively, the
waveguide 22 may be formed in the substrate by a proton exchange
process. The optical waveguide 22 is constructed to include two
parallel paths 26 and 28 positioned between two optical Y junctions
30 and 32, which are respectively coupled to two end sections 23
and 25 of waveguide 22. The LiNbO.sub.3 substrate, including the
optical Y junctions, the parallel paths and the end sections,
supports an SiO.sub.2 buffer layer which forms a common ground
plane and at least one pair of electrodes. The ground plane and the
electrodes can be electroplated onto the buffer layer and may be
formed from aluminum, silver, gold or the like. One pair of
electrodes can comprise a ground plane 40 and an elongated
electrode 36 positioned over optical waveguide 28. Electrode 36 can
extend along the waveguide 28 for a distance of approximately 1 cm.
Longer or shorter lengths can be chosen depending on the desired
bandwidth. If another pair of electrodes is employed, it can
compromise a ground plane 38 and an elongated electrode 34
positioned over optical waveguide 26. Electrode 34 can extend along
the waveguide for a distance comparable to the length of electrode
36. A common ground plane 33 can be included to cooperate with
electrodes 34 and 36. The assemblage of the LiNbO.sub.3 substrate,
the optical Y junctions and associated optical waveguides, and the
set of electrodes is one manifestation of an interferometer
normally identified as a Y junction Mach-Zehnder interferometer.
The specific example of a double pair of electrodes to provide one
set of electrodes is applicable to z-cut LiNbO3, which is a
commonly used crystal orientation. For x-cut LiNbO.sub.3, a single
pair of electrodes can be used in place of the double pair of
electrodes.
[0014] In a Y junction interferometer, a change in the index of
refraction of one or both waveguides 26 and 28, which is directly
proportional to the voltages applied to the individual pairs of
electrodes, causes an optical signal in the waveguides 26 and 28 to
experience an optical phase shift. It is this optical phase shift
which causes the optical signal to undergo an amplitude change. In
operation, optical energy in the form of a continuous wave (cw) of
optical energy from, for example, a laser via a single mode
waveguide, is directed into end section 23 of waveguide 22, where
it is divided into two equal optical signals by Y junction 30. At
this instant, an electrical signal having a specific waveshape is
applied to the pair of electrodes 36 and 40. If a second pair of
electrodes is employed, an electrical signal having a phase which
is 180 degrees out of phase with the first signal is applied to the
second pair of electrodes 34 and 38. The electrical signal applied
to the first pair of electrodes causes a change in the index of
refraction of the waveguide 28. (If the second pair of electrodes
is employed, the electrical signal applied to the second pair of
electrodes causes a change in the index of refraction of the
waveguides 26). The second Y junction 32 combines the two signals
from the waveguides 26 and 28 into a single signal which causes an
amplitude change to the optical signals in the waveguide 25. This
signal advances along the end section 25 of waveguide 22 to an
outgoing single mode fiber 42.
[0015] FIG. 2 shows a typical modulation switching curve for the
modulator shown in FIG. 1. Normalized optical output power is shown
on the ordinate and voltage is shown on the abscissa. The electrode
pair or pairs is normally biased so that a pulse in the electrical
domain is translated into a pulse in the optical domain. That is,
an electrical bit of "1" (represented by maximum voltage) is
translated into an optical bit of "1" (represented by maximum
optical output power). Likewise, an electrical bit of "0"
(represented by minimum voltage) is translated into an optical bit
of "0" (represented by minimum optical output power). As FIG. 2
shows, an optical bit of "1" will yield an optical bit of "0" when
the voltage changes by one-quarter of a complete voltage cycle
(i.e., from V.sub.a to V.sub.b in FIG. 2). As a consequence, only a
quarter of the voltage cycle is employed to generate the optical
bits.
[0016] In accordance with the present invention, the lithium
niobate amplitude modulator is biased in such a way that a change
in voltage level (from "1" to "0" or visa versa) is translated into
an optical bit of "1" and a constant voltage level is translated
into an optical bit of "0." This is accomplished by initially
biasing the modulator at a voltage that produces a maximum optical
power output. In other words, the voltage bias is initially placed
at a value that would translate into an optical "1" in the known
arrangement shown in FIG. 2.
[0017] FIG. 3(a) shows a modulation switching curve in accordance
with the present invention in which the modulator is initially
biased at V.sub.I, which is intermediate to voltages V.sub.c and
V.sub.d defining the lower and upper limits of the voltage applied
to the modulator. FIG. 3(b) shows a complete cycle of the voltage
(curve 30) as it is applied to the modulator (left-most side of
FIG. 3(b) ) and the corresponding optical signal levels that are
produced (right-most side of FIG. 3(b) ). In FIG. 3(b) time is
indicated on the ordinate. As the applied voltage is changed from
V.sub.c to V.sub.d along curve 30 during the time interval between
t.sub.0 and t.sub.1, the optical output power changes in accordance
with the modulation switching curve shown in FIG. 3(a). That is,
the change in voltage from V.sub.c to V.sub.d is translated into
optical pulse 32 shown on the rightmost portion of FIG. 3(b). Pulse
32 corresponds to an optical bit of "1."
[0018] Next, the voltage remains constant at V.sub.d for a
prescribed time interval between t.sub.1 and t.sub.2, producing an
optical bit of "0." During the time interval between t.sub.2 and
t.sub.3 the voltages changes from V.sub.d to V.sub.c, yielding a
second optical pulse 34. Once again, the voltage remains constant
(at the level V.sub.c) for the time interval between t.sub.3 and
t.sub.4, producing an optical bit of "0."
[0019] FIG. 3(b) shows that over the course of a complete voltage
cycle, which occurs between time t.sub.0 and t.sub.4, two optical
pulses or bits are produced. In contrast, known biasing
arrangements such as discussed in connection with FIG. 2 generate
one pulse over a quarter of a voltage cycle. The present invention
thus allows more refined control over the generation of optical
bits. Another advantage of the inventive biasing arrangement is
that a change in voltage level (i.e., the edge of the voltage pulse
defined between times t.sub.0 and t.sub.1 in FIG. 3(b)) produces an
optical pulse, whereas in the prior arrangement an electrical pulse
was required to produce an optical pulse. This is advantageous
because it is easier to produce a sharp voltage transition than it
is to produce a narrow electrical pulse. The present invention is
therefore capable of producing extremely narrow optical pulses,
thus facilitating the generation of solitons, which require such
narrow optical pulses.
* * * * *