Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-06-24
2001-05-08
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S011000
Reexamination Certificate
active
06229937
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to systems that use optical fiber and optical fiber components, and particularly to such systems that include circularly polarized waveguide fiber.
2. Technical Background
The optical non-linearities that affect light wave transmission systems fall into two general categories. In the first category are stimulated scattering phenomena, such as stimulated Brillouin scattering and stimulated Raman scattering. These effects are interactions between an optical signal and a phonon in the transmission material. The frequency of the phonon determines the type of scattering that occurs. In the second category, the nonlinear index of refraction gives rise to three effects, self phase modulation (SPM), cross phase modulation (XPM), and four wave mixing (4WM). Based on studies of long-distance, multi-wavelength systems, the second category of nonlinear interactions are most deleterious for wavelength division multiplexed (WDM) systems, especially those having electronic regenerator spacing greater than about 50 km. This second category of non-linear effects is the subject of the present application.
In SPM, the nonlinear index, which depends upon pulse intensity, leads to phase modulation of those pulses above a threshold intensity. The threshold intensity depends upon the material used in the waveguide but is generally of the order of 10 mW. One of the consequences of SPM is that the spectral width of signal pulses gradually increases as they propagate in the fiber. For operation near the zero dispersion wavelength of the waveguide, the spectral broadening of the signal will not degrade system performance. However, if there is sufficient group velocity dispersion, then the spectral broadening from SPM will result in temporal broadening of the pulses. Alternately, in densely spaced WDM systems, cross-talk will occur if the spectral broadening is large enough to cause spectra of a broadened signal to appear in adjacent channels to overlap those channels.
For WDM systems, the intensity variations in one channel can affect the other channels through XPM. For linearly polarized fiber, the XPM coefficient, which indicates the size of the effect, is about twice as large as the SPM coefficient. XPM does depend upon the length of waveguide over which interaction between pulses occurs so that change in spacing (walk-off) between channels due to group-velocity dispersion affects the interaction length and thus the amount of XPM. For sufficiently long systems, the group velocities of various channels will lead to complete walk-through between the channels. Thus, under loss-free conditions, the spectral broadening from XPM is virtually eliminated.
Four wave mixing also arises from the nonlinear index of refraction, but, unlike SPM and XPM, 4WM has a phase matching requirement. For signals at two different wavelengths, the intensity modulation at the beat frequency of the waveguide modulates the refractive index, thus producing a phase modulation at the difference frequency of the two signals. Consequently, in 4WM, side-band frequencies are generated at the original frequencies plus and minus the difference frequencies (the lower frequency side band is called the Stokes frequency, and the higher frequency side band is called the anti-Stokes frequency). The phase-matching requirement means that the index or speed at the two signal wavelengths must coincide with the index or speed of the Stokes and anti-Stokes waves. Therefore, 4WM depends strongly on total dispersion. For high total dispersion, the difference in propagation velocities at the different frequencies is large, and the efficiency of 4WM is poor. In fibers with the zero dispersion wavelength near the signal wavelengths, all waves are nearly coincident in index and speed and the 4WM process can be extremely efficient. In WDM systems, 4WM has two deleterious effects. First is the depletion of power from the signal wavelengths into the mixing products. Second, in systems that have equally spaced signal channels, the Stokes and anti-Stokes frequencies coincide with existing channels causing cross-talk. Also, the mixing products can interfere constructively or destructively with the existing channels, depending on the relative phases of the signals.
In high performance transmission systems, therefore, there is a need for a system configuration, which can include a particular type of optical waveguide fiber, that permits operation close to the zero dispersion wavelength, thus minimizing the linear dispersion penalty, but which still limits the non-linear effects, especially 4WM.
Glossary
The following terms are defined in accord with common usage in the art.
A quarter-wave retarder (QWR) converts linearly polarized light into circularly polarized light and conversely. For optimum efficiency, the linearly polarized light is incident upon the QWR with its polarization axis at 45° to the right or left of the fast axis of the QWR.
A half-wave retarder (HWR) rotates the polarization direction of linearly polarized light by 90 degrees. For optimum efficiency, the linearly polarized light is incident upon the HWR with its polarization axis at 45° to the right or left of the fast axis of the HWR. A HWR converts right-hand circularly polarized light (RHC) into left-hand circularly polarized light (LHC) and conversely.
Therefore, placing a QWR at the input and output of the CPF enables all linearly-polarized optics employed in the application in which the CPF is used.
Fiber QWR's and HWR's are implemented in fibers by folding the fiber into a number of loops and rotating the loops relative to one another. A fiber HWR is shown schematically as
33
in FIG.
7
. The birefringence induced by rotation of the fiber provides for the phase retardation between the two field vectors that mathematically define the polarization state of the light.
SUMMARY OF THE INVENTION
One aspect of the present invention is a circularly polarized single mode fiber (CPF). The CPF has at least a slight birefringence and an axial twist that is substantially continuous along the CPF length. The pitch of the axial twist is less than the beat length of the CPF so that circular polarization effects are large compared to linear polarization effects in the CPF. Beat length is the fiber length between repeats of a given polarization state.
The CPF is so called because it preserves propagated circularly polarized light in a state of circular polarization, given that the launch orientation of the light matches a polarization mode of the fiber. The required launch is assumed throughout this application. The CPF maintains the circular polarization of circularly polarized light (either right or left handed circularly polarized light) that is launched into the CPF.
In one embodiment of the CPF, the birefringence is &Dgr;n about 10
−5
, where &Dgr;n is the difference in refractive index of the two orthogonal polarization axes of the waveguide fiber. The fiber can be made to have birefringence by any of several methods known in the art. For example the core can be made elliptical in cross section or a non-uniform radially directed stress may be applied to the core.
In a further embodiment of the CPF, the applied twist has a right handed pitch over a portion of the fiber length and left handed pitch over another portion.
In another aspect, the present invention includes an optical transmission link for high data rate, multiplexed systems. The link makes use of CPF to suppress the non-linear effects that occur in systems using high power signals or make use of multiple wavelength channels. The transmission link is formed from a plurality of CPF's optically coupled to each other. The first CPF in the link is optically coupled to a multi-wavelength transmitter module and the last CPF in the link is optically coupled to a multi-wavelength receiver module. Alternating the pitch from right to left handed polarized light for alternating channels, effectively eliminates four wave mixing, the no
Islam Mohammed Nazrul
Nolan Daniel Aloysius
Chervenak William J.
Corning Incorporated
Ngo Hung N.
LandOfFree
Circularly polarized fiber in optical circuits does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Circularly polarized fiber in optical circuits, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Circularly polarized fiber in optical circuits will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2468035