Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-09-25
2003-11-25
Spector, David N. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S260000, C398S080000
Reexamination Certificate
active
06654152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to frequency guiding filters for a dispersion managed soliton transmission system, and more particularly pertains to sliding frequency guiding filters using a wavelength locked feedback loop for a dispersion managed soliton transmission communication system.
The present invention uses wavelength locked feedback loops in frequency guide filters, and particularly in sliding frequency guide filters. The wavelength locked feedback loops offer various advantages over conventional methods. The main benefit is that the wavelength locked feedback loops allow precise control over the location of the filter peak center wavelength with respect to the transmitted signal peak center wavelength, to compensate for factors such as the filter rolloff, signal spectral width, and changes in transmission line properties due to temperature, microbending, aging and other effects. This approach allows the construction of very inexpensive frequency guiding filters, and also provides a new degree of freedom in the guide filter design (the guide filter can be designed with a dynamically controllable offset and sliding range). Taken together, these advantages make it possible to design new types of dispersion managed soliton transmission networks.
2. Discussion of the Prior Art
Long distance fiber optic communication systems have made increasing use of soliton transmissions to avoid dispersion and nonlinear effects that can limit both the distance and the bandwidth (maximum achievable data rate) thereof.
FIG. 1
a
illustrates a fiber optic transmission system which typically consists of a laser source
1
and an optical receiver
2
connected by strands of glass fiber
3
. The fiber attenuates optical signals from the transmitter; since the receiver has a limited sensitivity, signals can only be detected beyond a certain signal-to-noise ratio. The fiber also induces dispersion or pulse spreading which further degrades the receiver signal levels. Both attenuation and dispersion increase with distance and are more pronounced at higher data rates, which limits both the distance and data rate of a linear transmission system. One brute force approach to increasing the distance is to launch higher optical power levels into the fiber; beyond a certain point, this induces nonlinear effects which once again limit distance and bandwidth.
However, by inducing a controlled form of nonlinearity, it is possible to create optical pulses called solitons which do not change shape as they propagate. Solitons have become widely used in many long distance telecommunication systems, including dense wavelength division multiplexing (DWDM) systems, and in other data communication systems as well. However, various problems are associated with soliton transmission, in particular timing and frequency jitter.
Timing jitter can result from fluctuations in the frequency components of a soliton optical pulse; this imposes severe limitations on the signal to noise ratio. By controlling the frequency of the solitons, it is possible to control timing jitter as well. One approach to controlling the frequency of solitons is to periodically insert narrowband filters in a fiber optic link, usually at optical amplifier locations; these are known as frequency-guiding filters. If for some reason the center frequency of a soliton is shifted from the filter peak, the filter-induced differential loss across the pulse spectrum adjusts the pulse frequency. As a result, the pulse spectrum returns to the filter peak over some characteristic damping length L. If the damping length is considerably less than the transmission distance, then guiding filters can dramatically reduce the timing jitter. Although even linear fiber optic transmission systems will exhibit similar effects, guiding filters of this type can only be used in soliton based transmission systems. Every time an optical pulse passes through a guiding filter, its spectrum narrows; solitons can quickly recover their bandwidth through the fiber nonlinearity, whereas in a linear transmission system the filter acts to continuously degrade the signal.
Another variation is the sliding frequency gain filter in which the transmission peak of each guiding filter is shifted in frequency with respect to the transmission peak of the previous filter, so that the center frequency slides with distance at a predetermined rate. Because of their nonlinearity, solitons can follow the filters and slide in frequency, while linear noise is suppressed. By introducing sliding frequency guiding filters periodically positioned along the length of the transmission line, shown at
4
-
1
,
4
-
2
in
FIG. 1
a
, an optical transmission line using solitons becomes effectively an all-optical passive regenerator, compatible with DWDM networks. All nonsoliton components of the signal pulse are absorbed by the filters; while input pulses which are close to the optimal soliton profile are reshaped by the transmission line filters into propagating solitons. Note also that the filters act to remove energy fluctuations from the input optical signal, with a damping length close to the frequency damping length. This also acts to self-equalize the energy of different channels in a WDM transmission system. Feedback from the frequency guiding filters locks the energy of individual soliton channels to values that do not change with distance, even if optical amplifiers in the path have different gains at different wavelengths. Sliding frequency guide filters also reduce the timing and frequency drifts associated with effects such as soliton collisions and four wave mixing; they can also be used to construct hybrid transmission lines containing a mix of optical amps, positive dispersion fiber, negative dispersion fiber, and other optical elements which are designed to counterbalance each other and result in nearly flat average dispersion over long distances. Theoretical performance of 10 Gbit/s signals over 40,000 km or 20 Gbit/s signals over 14,000 km have been suggested using these techniques.
Conventional designs for sliding frequency filters have met with some success, but continue to face performance problems; in particular, etalon filters are not a good approximation of ideal parabolic filters, especially when large frequency excursions of solitons are involved; the curvature of the filter response reduces with the deviation of the frequency from the filter peak.
The following is additional background information about soliton transmission. Optical signals propagating in a glass fiber experience dispersion; an optical pulse of width t has a finite spectral bandwidth 1/t. When the pulse is transform limited, all of the spectral components have the same phase. In the time domain, all of the spectral components overlap in time. Because of dispersion, different spectral components propagate in the fiber with different group velocities; thus as the pulse propagates its frequency components spread out in time. The direction of this spreading, or chirp, depends on the sign of the group velocity dispersion (GVD), either positive or negative. There is also a nonlinear effect, self-phase modulation (SPM) resulting from the interaction between the light intensity and the nonlinear portion of the fiber's refractive index (also known as the Kerr effect). This produces a frequency shift determined by the time derivative of the pulse shape. In silica based fibers SPM always produces a positive chirp (shifts the leading edge of the pulse to the red spectral region). If both GVD and SPM are applied to an optical pulse with opposite signs, the two effects cancel each other out to yield a pulse which does not change shape as it propagates. The resulting pulse is known as a soliton, and is a nondispersive solution of the nonlinear Schrodinger equation. Pulses injected into a fiber which are close in shape to a soliton will be adjusted by the nonlinear effects to reform into stable soliton pulses.
One source of error in soliton systems is the fluctu
DeCusatis Casimer M.
Jacobowitz Lawrence
Scully, Scoot, Murphy & Presser
Spector David N.
Townsend Tiffany L.
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