Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2001-08-27
2004-02-03
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S326000
Reexamination Certificate
active
06687042
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the processing of optical signals in nonlinear optical frequency mixers, and in particular to the alleviation of the group velocity mismatch (GVM) occurring between interaction waves in such mixers.
BACKGROUND OF THE INVENTION
Development of high capacity optical networks has accelerated because of emerging demand for world-wide communications. Information, interactive multimedia service, electronic commerce, and many other services are efficiently delivered online through the Internet. Optical fiber communication serves as the enabling technology to realize those Internet activities. Today, several tens of gigabits-per-second of data traffic are carried over many thousands of kilometers through optical fiber communication systems.
Transmission of high capacity data and, more importantly, the management of that high capacity data are the keys to the realization of such global optical-fiber-based networks. This rapid evolution in communication systems is creating enormous demands for optoelectronic components with capabilities beyond those currently available. In particular, the requirements push some theoretical limitations of transmission of optical signals.
Today's optical communication systems rely on wavelength division multiplexing (WDM) as well as time division multiplexing (TDM) techniques to send optical signals in the form of pulses through optical fiber. The pulses are designed with pulse widths as narrow as 3×10
−12
s and the trend to narrower pulses and higher rates continues. One of the main physical limits to our ability to reduce the pulse width even further is the basic phenomenon of pulse lengthening due to the dependence of its group velocity on frequency. This phenomenon, called group velocity dispersion (GVD), affects every mode of light, with the exception of solitons and is often defined by the relation:
D≡L
−1
(dT/d&lgr;),
where T is the pulse transmission time through length L of the fiber and &lgr; is the wavelength of the light. This definition is related to the second order derivative of the propagation constant &bgr;(&ohgr;) of the mode with respect to its angular frequency &ohgr;by:
D
=
-
2
π
c
λ
2
(
ⅆ
2
β
ⅆ
ω
2
)
,
where c is the speed of light in vacuum. Meanwhile, group velocity v
g
is defined as:
1
v
g
=
ⅆ
β
ⅆ
ω
.
When a light pulse contains several wavelength components, GVD causes these to migrate within the pulse envelope producing a “chirp” and it also causes the pulse to broaden. In particular, the chirp causes the longer wavelengths to migrate to the front of the pulse envelope while the shorter wavelengths recede to the back. The effects of GVD are frequently expressed in terms of a group velocity mismatch (GVM) describing the rate at which pulses at different wavelengths slip off each other.
The prior art contains many teachings related to compensation of pulse broadening occurring when pulses travel through fiber by phase conjugation. In these schemes, a pulse travels a certain length of fiber and broadens while accumulating a chirp. A phase conjugator reverses the chirp of the pulse, typically by a nonlinear mixing operation relying on a nonlinear optical material exhibiting a third order susceptibility &khgr;
(3)
. The chirp reversed pulse travels through another length of fiber and experiences recompression. The recompression occurs because the longer wavelengths flipped to the back of the pulse will move forward and the shorter wavelengths flipped to the front of the pulse will move to the back.
In addition to the use of nonlinear materials for phase conjugation based on &khgr;
(3)
, nonlinear optical materials having a second order susceptibility &khgr;
(2)
are also used in optical frequency mixers to perform various mixing functions including second harmonic generation, difference frequency generation, sum frequency generation, parametric generation or parametric amplification. These functionalities can be used in an all-optical network at nodes for switching optical signals in different wavelength channels in different directions without ever converting the optical signals into electronic form. In addition, nonlinear optical mixers can be used to switch optical signals between different optical carrier wavelengths, either within the immediate network or when transferring to a neighboring network. Such wavelength switches can be used to build wavelength interchangers or wavelength interchanging cross-connects. More information about such switches can be found in S. J. B. Yoo, “Wavelength Conversion Technologies for WDM Network Applications”, Journal of Lightwave Technology, Vol. 14, No. 6, June 1995, pp. 955-66 as well as U.S. Pat. No. 5,825,517 to Antoniades et al. and the references cited therein.
The effects of GVD on short pulses, and especially on ultra-short pulses on the order of picoseconds, interferes not only with the propagation of such pulses through fiber but also with efficient nonlinear wavelength mixing of such ultra-short pulses. U.S. Pat. No. 5,815,307 to Arbore et al. and U.S. Pat. No. 5,867,304 to Galvanauskas et al. teach the use of chirped gratings to take advantage of second order susceptibility &khgr;
(2)
of the nonlinear material to adjust the shape of pulses. For example, Arbore et al. teach how to compress pulses during second harmonic generation (SHG) by taking advantage of the principles of GVD and nonlinear optical frequency mixing. To achieve efficient frequency conversion these devices employ quasi-phase-matching (QPM) to counteract the phase slip between the generating or pumping light and the generated or converted light as these two interaction waves propagate through the nonlinear optical material. In contrast to GVD, the phase slip is due to the fact that optical signals of different wavelengths, e.g. the pumping wave and the frequency doubled wave experience a different index of refraction in the nonlinear optical material. Thus, there is a phase velocity mismatch between the interaction waves. The QPM grating is employed in the nonlinear material to prevent the phase slip occurring between the generating and generated light signals or interaction waves due to phase velocity mismatch. Thus, by keeping the interacting waves in phase, QPM ensures efficient frequency mixing between the interaction waves.
Unfortunately, the effects of GVD are felt in nonlinear mixing processes irrespective of the type of nonlinear mixing process and phase matching technique used. GVD effects are especially pronounced when the interaction waves are short pulses and have very different wavelengths. In those situations a substantial walk-off is produced between the interaction waves over very short distances and the nonlinear mixing process stops.
The prior art describes several systems and devices which contend with dispersion problems. For example, U.S. Pat. Nos. 5,369,519 and 5,224,194 to Islam teach the use of a nonlinear material with negligible walk-off to achieve all-optical timing restoration function in optical switching and transmission systems. The negligible walk-off is realized by a hybrid solution that consists of a nonlinear chirper followed by a dispersive line. The scheme can be characterized as a hybrid solution, which needs a delay line with a dispersion sign different from the nonlinear chirper. In U.S. Pat. No. 5,696,614 Ishikawa et al. provide an optical wavelength multiplex transmission method to realize an optical communication system of an increased capacity which is not influenced by crosstalk by four-wave mixing (FWM). This patent also describes a dispersion compensation method for the WDM transmission link. Unfortunately, none of these references teach compensation for group velocity mismatch (GVM) in nonlinear frequency conversion based on material second order susceptibility &khgr;
(2)
.
In view of the above, it would be a significant advantage over the prior art, to provide nonlinear optical mixers which are c
Chou Ming-Hsien
Fejer Martin M.
Kurz Jonathan
Lee John D.
Lumen Intellectual Property Services Inc.
The Board of Trustees of the Leland Stanford Junior University
LandOfFree
Group-velocity mismatch compensation for optical signal... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Group-velocity mismatch compensation for optical signal..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Group-velocity mismatch compensation for optical signal... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3318695