Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2000-04-21
2001-04-24
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
Reexamination Certificate
active
06222959
ABSTRACT:
TECHNICAL FIELD
Pulse narrowing technology is combined with nonlinear optical loop mirrors to enhance management of optical pulses.
BACKGROUND
Nonlinear optical loop mirrors (NOLMs), which are fiber versions of Sagnac interferometers, can perform many different functions in optical systems including multiplexing and demultiplexing, switching, amplifying, logical operations, pulse shaping, filtering, and signal regeneration. Processing speeds are extremely fast, accommodating bit rates approaching terabit per second speeds.
Pulse routing through nonlinear optical loop mirrors is controlled by the mechanism of interference. A two-by-two directional coupler divides a data (signal) pulse train into two pulse trains that counter propagate around a common loop of fiber. Phase modulation induced by intensities within the nonlinear optical regime of the fiber (the Kerr effect) alters the relative phases of the paired counter propagating pulses. Upon return to the directional coupler, the recombined pulses are switched between the input and output of the coupler in accordance with their interference properties. Constructively interfering pulses reflect back through the coupler's input, and destructively interfering pulses transmit through the coupler's output.
Normally, the directional coupler is a 3 dB coupler, splitting input pulse intensities equally between the counter propagating directions. If the optical properties exhibited by the fiber loop are symmetric in both directions of propagation, the returning pulses interfere constructively and reflect back through the coupler input. Asymmetries resulting in a “&pgr;” phase shift cause the returning pulses to interfere destructively and to transmit through the coupler output. Other phase shifts divide the intensities of individual pulses between the coupler input and output in accordance with their relative amounts of constructive and destructive interference.
The asymmetries can be arranged to affect all data pulses equally or to have a differential effect on selected data pulses such as on data pulses with certain characteristics, on particular patterns of data pulses, or even on individual data pulses. For example, my recently issued U.S. Pat. No. 5,655,039, which is hereby incorporated by reference, constructs the loop of a non-linear optical loop mirror with dispersion-tapered fiber to produce a differential effect on data pulses having different widths or intensities. Individual data pulses or patterns of data pulses can be differentially affected by using specially timed control (clock) pulses, which limit asymmetric effects to periods of overlap with selected data pulses.
In a preferred mode of operation, the control pulse starts just ahead or just behind the expected position of the selected data pulse and ends in the opposite position. Between the two positions, the control pulse overlaps the selected data pulse for a sufficient duration and with a sufficient intensity to produce a &pgr; phase shift along the entire selected data pulse. The phase shift is induced by temporary changes in the effective refractive index of the fiber loop in the presence of the control pulse, which has an intensity within the nonlinear optical regime of the fiber.
However, a variety of differential effects on data pulses causes them to drift from their expected positions—a phenomenon referred to as “timing jitter”. Any delay between the expected arrivals of the control and selected data pulses reduces the transmission efficiency of the selected data pulse because less of the selected data pulse undergoes the required &pgr; phase shift. The effects of timing jitter on transmission efficiency can be reduced by starting the control pulse farther behind the selected data pulse and ending the control pulse farther ahead of the selected data pulse (i.e., increase the so-called “walkoff distance”). However, the increased walkoff distance (as it approaches the bit period) can also contribute to increased crosstalk caused by the unintended transmission of adjacent pulses.
A convenient measure of how well timing jitter can be accommodated is the so-called “switching window”, having a width defined as the full width at half maximum (FWHM) of a transmission efficiency curve plotted as a function of the relative delay between the control and selected data pulses. The optimum window width balances the need to accommodate timing jitter (i.e., minimize intensity variations between transmitted data pulses) with the need to avoid crosstalk (i.e., minimize transmission of unselected data pulses). The shape of the switching window can also be optimized to further these objectives. The top of the switching window is preferably flattened to reduce intensity variations within a limited range of timing jitter, and the sides are preferably steepened (i.e., approach a more nearly vertical slope) to reduce crosstalk. The ideal shape of a switching window is described by a rectangle function.
SUMMARY OF THE INVENTION
My invention improves performance of nonlinear optical loop mirrors (NOLMs) by adiabatically compressing pulses that are input to the loop. Such pulse compression reshapes a switching window to reduce both intensity variations among data pulses selected for transmission and crosstalk with the remaining data pulses. Both control pulses and data pulses can be adiabatically compressed before entering the loop, and the data pulses selected for transmission beyond the loop can be re-expanded after leaving the loop. Dispersion-tapered fibers (DTFs) or chirped fiber Bragg gratings are preferred for compressing and re-expanding the pulses.
One embodiment of my invention is a fiber optic device arranged for processing data pulses. The device includes a loop of optical fiber and a directional coupler joining two ends of the fiber to a common input and output. A pulse compressor connected to the coupler input reduces the width of data pulses entering the fiber loop.
The loop of fiber together with the directional coupler can be constituted as a nonlinear optical loop mirror. As such, the directional coupler (a) divides a train of the data pulses into two counter propagating trains of data pulses, (b) recombines paired pulses from the two trains, and (c) directs the recombined pulses between the input and output of the directional coupler in accordance with interference characteristics of the recombined pulses. Control pulses entering the loop vary the interference characteristics of the recombined pulses by an optical nonlinear mechanism known as “cross-phase modulation”.
The nonlinear loop mirror receives both control pulses and data pulses and further transmits selected data pulses in accordance with relative amounts of overlap between individual control and data pulses. A switching window is defined in terms of pulse transmission efficiency as a function of relative delays between the overlapping control and data pulses. The pulse compressor compresses at least one of the control and data pulses for reshaping the switching window to reduce transmission efficiency variations among the selected pulses.
The control and data pulses can enter the loop through the same or different couplers. Similarly, a single pulse compressor can be used to compress both the control and data pulses, or two pulse compressors can be used to compress the control and data pulses separately. A single pulse compressor can also be used to compress just one or the other of the control and data pulses. A pulse expander can be connected to the output of the directional coupler for restoring the width of the data pulses selected for further transmission.
Both the pulse compressors and the pulse expanders can be constructed using axially varying fiber. Preferably, dispersion-decreasing fiber is used to compress pulses and dispersion-increasing fiber is used to expand pulses both using the mechanism of self-phase modulation. Chirped fiber Bragg gratings can also be used for these purposes.
My invention also provides for temporal filtering of the data pulses. The nonlinear optical loop mirr
Chervenak William J.
Corning Incorporated
Ullah Akm E.
LandOfFree
Nonlinear optical loop mirror with adiabatic pulse compression does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Nonlinear optical loop mirror with adiabatic pulse compression, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nonlinear optical loop mirror with adiabatic pulse compression will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2477282