Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-10-31
2002-09-03
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S015000, C359S326000
Reexamination Certificate
active
06445848
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a circuit for recovering the shape and spectrum of an optical signal after the signal has traversed a length of optical waveguide fiber. In particular, the circuit is used to preserve the shape of solitons over long lengths of waveguide fiber without use of electronic regeneration.
The value of soliton transmission of information over optical waveguide fibers is recognized in the art. The possibility of essentially dispersion-free transmission of pulses over long fiber lengths without electronic regeneration has encouraged work in the area of maintaining soliton signal integrity in extended transmission lines. With the introduction of waveguide fibers having attenuation in the range of a few tenths of a decibel per kilometer and optical amplifiers, solitons have become even more attractive as the transmission method of choice in very high bit rate systems or those that make use of wavelength division multiplexing.
A problem to be addressed in using soliton signals is how to control changes in the soliton time window, often called timing jitter, to avoid overlap with neighboring pulses. In addition, one should provide for filtering of noise that arises from the energy shedding of the soliton pulse as it undergoes shape changes during propagation along the fiber. Noise also originates in amplified spontaneous emission in the optical amplifiers
At bit rates >50 Gb/sec, the soliton pulse width must be less than about 5 ps to avoid overlap between adjacent soliton time windows. This pulse width is in general sufficient to minimize errors at the receiver end of the transmission. At the same time, the soliton period for these short pulse widths must be kept short relative to the preferred optical amplifier spacing, about 25 km. Thus there is a need to remove the energy lost by the soliton so that signal to noise ratios are at a desired level, inter-symbol interference is eliminated, and optical amplifiers do not become saturated by presence of noise signals.
Another consideration is the self-frequency shift of a narrow time width soliton due to differential Raman amplification of the pulse wavelength spectrum. This shift should be compensated in order to maintain the soliton wavelength within the desired low attenuation operating window and within the gain spectrum of the optical amplifiers.
The control of soliton timing jitter using a non-linear optical loop mirror (NOLM) is described in U.S. Pat. No. 5,757,529, Desurvire et al ('529). In the '529 patent, a loop mirror is used as a switch that rejects system noise by preferential switching of the soliton signal through the NOLM. The switching is brought about my means of a stream of control pulses introduced into the NOLM. The overlap of the signal pulses and the control pulses in time determines the switching characteristics of the NOLM. Because relative timing difference between the signal pulse and control pulse are critical, the '529 patent proposes a clock extraction or recovery circuit that produces a clock signal from the signal solitons. Such clock recovery circuits add considerable cost and complication to an optical circuit employing a NOLM. In some clock circuits, electo-optical devices are employed.
Thus, there is a need for a relatively simple and low cost means for recovering the shape of soliton signal after it has traversed a span of about 25 km of waveguide fiber. Also there is a need to address the problem of selffrequency shifting of the soliton signals without resort to elaborate, expensive optical or electo-optical circuits. The invention disclosed and described in this application incorporates simplicity and low cost into an optical circuit including a NOLM that simultaneously removes transmission circuit noise and recovers the original spectrum of the soliton signals.
SUMMARY
The present invention is an optical circuit for noise filtering and frequency modulation of soliton pulses. The circuit includes a NOLM having its ends optically joined to the output ports of an NXN or first coupler, where N is at least two. Signal pulses are optically coupled to one of the input ports of the coupler and the signal pulses are thus divided into a clockwise (CW) and a counter-clockwise (CCW) stream of pulses propagating in the NOLM. A tap coupler is optically coupled to a point along the length of the NOLM. A stream of control pulses are input into the tap coupler which then couples the control pulses to the NOLM. Both the signal pulses and the control pulses can originate from a single optical soliton source. A source optically couples a stream of solitons to the input port of a splitting coupler which divides the stream of solitons into a stream of control pulses and a stream of signal pulses. Depending upon the direction along the loop mirror in which the tap coupler couples the control pulses, the control pulses will co-propagate with either the CW or CCW propagating signal pulses. The optical circuit is symmetric in the sense that the circuit may be configured to cause interaction between the control pulses and either the CW or CCW propagating signal pulses. It will therefore be understood that the description of the circuit given herein applies equally to the interaction of CW or CCW propagating signal pulses and the control pulses.
The optical path between the splitting coupler and the tap coupler is an optical fiber optically connected between the two couplers. So too, the optical path between the splitting coupler and the input of the first coupler is an optical fiber. The amount of interaction between the control pulses and the signal pulses depends upon the amount of overlap of the two sets of pulses as they travel through the optical loop mirror. This amount of overlap is controlled by selecting the lengths other two fibers that connect the respective pairs of coupler ports to provide a pre-selected lead or lag time of the signal pulses relative to the control pulses.
The interaction of control pulses with co-propagating signal pulses produces both a shift in centroid wavelength and phase (relative to the counterpropagating signal pulses) of the signal pulses. The centroid wavelength may be shifted up or down depending upon whether the control pulses lead or lag the co-propagating signal pulses. The amount of phase shift of the co-propagating signal pulse depends upon the magnitude of the lead or lag time between the control and signal pulses.
The selection of the lengths of the connecting waveguide fibers thus allows one to select the amount of centroid and phase shift of the co-propagating signal pulses in the NOLM.
In addition, the NOLM reflects the stray energy waveforms (noise), due to power shedding of the solitons or due to amplified spontaneous emission. Because the amplitude of the stray pulses is below the level at which nonlinear phase shifting occurs, no phase shift occurs between the CW and CCW propagating noise so they are not switched through the first coupler.
Thus the optical circuit disclosed and described herein serves to shift the centroid wavelength of the co-propagating signal pulses, shift the phase of the co-propagating signal pulse to switch the signal pulses thought the first coupler, and also to remove the low amplitude noise accumulated in the optical circuit. The shift in centroid can be chosen to offset any centroid shift of the solitons caused by differential Raman gain across the soliton spectrum. These functions are accomplished without use of clock extraction circuits or synchronized control pulses from a source separate from the signal soliton source.
In one embodiment of the optical circuit, the lead or lag time of the control pulses relative to the co-propagating signal pulses is within the range of about three times
T
. Here
T
is the soliton pulse width expressed as a time interval between the half maximum power points of the soliton.
An embodiment of the invention includes polarization adjusting components in one or both of the waveguides joined to the tap and first coupler. The control pulse is
Islam Mohammed N.
Nowak George A.
Xia Tiejun
Chervenak William J.
Ngo Hung N.
The Regents of the University of Michigan
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