WDM system that uses nonlinear temporal gratings

Details WDM system that uses nonlinear temporal gratings WDM system that uses nonlinear temporal gratings

2001-03-08

2002-06-11

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06404526

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to transmitting optical signals in optical fibers, and more particularly, to reducing pulse broadening in a nonlinear operating region of optical fiber transmission and ultra-fast optical switching.
BACKGROUND
FIG. 1
shows that an initial optical pulse
2
becomes a broader pulse
3
after traveling through an optical fiber
4
. One source of broadening of pulse
2
results from dispersion. One cause of dispersion is a variation in a fiber's refractive index with wavelength. The fiber's refractive index is defined as the ratio of speed of light in vacuum to speed of light in the fiber. The refractive index variations make longer and shorter wavelength components of pulse
2
travel at different speeds in optical fiber
4
. After traveling through a certain length of optical fiber
4
, the speed variations produce broader pulse
3
. Another cause of dispersion is waveguide dispersion, which is induced by the geometric configuration of fiber
4
.
Pulse broadening can affect the quality of digital data transmission in optical fiber
4
. Digital data is transmitted as a series of optical pulses. Each temporal interval for a pulse may represent one binary bit. For example, a data format called On-Off Keying (OOK) indicates the binary states “1” and “0” corresponding to the presence and absence of a pulse, respectively. As pulses broaden and overlap, a receiver may not be able to determine whether a pulse is present in a particular time interval or whether a detected optical signal is the tail of a previous or subsequent pulse. Inserting an amplifier
5
into optical fiber
4
can help to reduce receiver errors due to propagation weakening of pulse intensities. But, amplifier
5
does not help to reduce receiver errors caused by the dispersion-generated pulse broadening and overlap.
Present optical fiber communications typically use optical pulses having wavelengths of about 1.5 microns, because erbium-doped fibers can provide quality optical amplification at 1.5 microns. Unfortunately, many older optical fibers produce significant chromatic dispersion in optical signals at 1.5 microns. This chromatic dispersion produces significant pulse broadening, which limits transmission wavelengths and distances in contemporary optical networks.
The refractive index of fiber
4
also varies with the magnitude of electric field ∈ of pulse
2
. For symmetric molecules, such as silica glasses of which most optical fibers are made, the first-order ∈ dependent term in the refractive index vanishes. The higher-order terms of ∈, ∈
2
, in particular, in the refractive index produce most of the nonlinear effects in optical fibers. When the intensity in pulse
2
is low, the higher-order terms of ∈ in the refractive index only have negligible effects, and therefore pulse
2
is in a linear operating region of fiber
4
. When the intensity of pulse
2
is sufficiently high, the higher-order terms of ∈ become non-negligible and cause pulse
2
to enter a nonlinear region of operation of fiber
4
.
A notable manifestation of the nonlinear operation of fibers is self-phase modulation (SPM). SPM generally causes a pulse to broaden in spectrum while the pulse is propagating in the nonlinear operation region of a fiber. However, the effects of spectral broadening caused by SPM may counterbalance the effects of chromatic dispersion with the result that the pulse retains its shape.
The chromatic dispersion is characterized by a second order chromatic dispersion parameter &bgr;
2
, which is a function of the pulse's wavelength and derivatives of the fiber's refractive index with respect to the wavelength. If &bgr;
2
is negative, the pulse is said to be propagating in an anomalous dispersion regime of the fiber. In the anomalous dispersion regime, the SPM causes the leading edge of the pulse to travel slower than its trailing edge, thus effectively compressing the pulse and balancing out the pulse broadening induced by the second order chromatic dispersion.
A pulse propagating in the fiber with balanced SPM and chromatic dispersion is a form of solitary wave called a soliton. Ideally, a soliton may travel a long distance while retaining its shape and spectrum. However, a soliton is susceptible to amplitude fluctuations, which may be caused by, for example, the amplifiers that are required along the fiber. The amplitude fluctuations generate frequency shifts, which in turn cause Gordon-Haus time jitters due to different frequencies traveling at different velocities. The frequency shifts and Gordon-Haus time jitters are detrimental to a data transmission system. In a wave-length division multiplexing (WDM) system, frequency shifts produce undesired emissions outside of the allotted frequency band assigned to each channel, and the undesired emissions may interfere with other channels or other systems; while time jitters create problems of data clock recovery at a receiver or regenerator site, because data bits represented by the optical pulses may not be synchronized due to the timing uncertainties.
Time jitters can be reduced by inserting sliding filters in strategically chosen locations along the fiber span. Another method to reduce time jitters is a dispersion-managed soliton technique that uses dispersion compensating fibers, which have dispersion characteristics tuned to compensate for the time jitters along the fiber span. The overall average dispersion characteristicis, on the other hand, is designed to counterbalance the SPM.
Even with dispersion management, any soliton, when traveling far enough into a fiber, surrenders to an effect called third order dispersion (TOD). TOD causes the soliton to spread unsymmetrically in the temporal domain into a widened, non-symmetrical pulse.
FIGS. 2
illustrates the TOD effects on a soliton.
FIG. 2
shows a soliton pulse after being unsymmetrically spread by TOD.
Some implementations allow a soliton to travel over long distance with optical regenerators. The design of regenerators, for example, optical
2
R (re-shape and re-time) or
3
R (re-shape, re-time and re-amplify), involves complicated issues such as polarization sensitivities, cost and complexities.
SUMMARY
In general, in one aspect, the invention is a method of generating a signal pulse in an optical fiber characterized by dispersion and a refraction index that has a nonlinear regime of operation. The method involves generating a sequence of coherent optical pulses each of which has an associated energy; and introducing the sequence of pulses into the optical fiber, wherein the pulses in the sequence of pulses are sufficiently close in spacing so that after traveling a predetermined length down the optical fiber, the pulses of the sequence of pulses overlap and interfere to form an interference pattern. The associated energy of at least one of the pulses of the sequence of pulses is within the nonlinear regime of the optical fiber.
In general, in another aspect, the invention is a method of generating a signal pulse in an optical fiber that involves generating a sequence of coherent optical pulses each of which has an associated energy; and introducing the sequence of pulses into the optical fiber, wherein the pulses in the sequence of pulses are sufficiently close in spacing so that after traveling a predetermined length down the optical fiber, the pulses of the sequence of pulses overlap and interfere to form an interference pattern having a central lobe and multiple side lobes. The interference pattern is characterized by a contrast ratio, and the associated energy of each pulse of the sequence of pulses is sufficiently high relative to characteristics of the optical fiber so as to cause the contrast ratio of the interference pattern to increase as the interference pattern propagates further along the optical fiber.
In general, in still another aspect, the invention is a method of generating a signal pulse in an optical fiber that involves generating a sequence of coherent optical pulses each of which has a

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