This invention relates to the field of optical transmission systems and, more specifically, to improving the jitter tolerance for receiving optical signals in optical transmission systems.
A major limitation to the maximum reach of ultra-long haul on-off transmission systems, such as 10 Gb/s WDM transmission systems, is pulse timing jitter. Pulse timing jitter arises from inter-channel pulse collisions, which develop as a result of the chromatic dispersion of a fiber, resulting in a wavelength dependent signal propagation velocity. Other sources of timing jitter can be attributed to contributions from transmitter and receiver electronics, acoustic interaction effects, or especially, in the case of soliton transmission, to the Gordon-Haus effect.
Inter-channel pulse collisions occur when pulses of shorter wavelength channels, which have higher velocities than those of longer wavelength channels, overtake and pass through pulses in the longer wavelength channels. Since real data streams are substantially random, some pulses tend to experience more collisions and others experience fewer collisions in the course of traversing the system. The time displacements of the collisions, therefore, can result in considerable timing jitter.
The Gordon-Haus effect is caused by the interaction of soliton pulses with amplifier spontaneous emission (ASE) noise present along the transmission medium. J. P. Gordon et al. describe this effect in “Random Walk of Coherently Amplified Solitons in Optical Fiber Transmission,” Optic Letters, 11(10), pp. 665-7 (1986). ASE noise randomly alters both the amplitude and carrier or channel frequency of propagating soliton pulses. The frequency shifts result in jitter in pulse arrival times. Pulse timing jitter can subsequently cause a soliton pulse to shift into the time interval reserved for a neighboring soliton pulse. The result, often known as intersymbol interference, culminates as an error in the received information.
The above described sources of timing jitter in optical signals propagating along a transmission system, as well as others, can create errors when attempting to receive the propagating optical signals. In conventional receivers, an optical pulse is sampled once within its bit slot. Because clock recovery circuits of conventional receivers cannot track fast timing jitter, the adjustment of an optimal pulse sampling point on every bit to attempt to minimize the receiving errors due to timing jitter is impossible. As such, timing jitter can cause an increase in the bit-error rate of an optical transmission system because of the shifting of the frequency of an optical pulse.
The present invention advantageously provides a novel type of data receiver, which has superior performance compared to a standard receiver when an input signal is distorted by timing jitter.
In one embodiment of the present invention, a method for improved timing jitter tolerance includes sampling an input signal more than once within a bit slot of the input signal and determining, using logic circuitry, from a combination of at least a subset of the samples, a resulting logic state for an output signal.
In an alternate embodiment of the present invention, a receiver for improved jitter tolerance includes a sampling circuit, for sampling an input signal more than once within a bit slot of the input signal, and logic circuitry, for determining a resulting logic state of an output signal from a combination of at least a subset of the more than one samples.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
The present invention advantageously provides a method and apparatus for improved jitter tolerance. Although the present invention will be described within the context of a single-channel 10 Gb/s return-to-zero (RZ) optical transmission system, it will be appreciated by those skilled in the art that within the teachings of the present invention, the inventive concept can be advantageously implemented in various other transmission systems wherein it is desirable to improve a jitter tolerance.
In the optical transmission test setup 100, a 10 Gb/s 33% RZ signal (231−1 PRBS) is generated using pulse carver techniques. The optical signal propagates through jitter emulator 140 and is received by the receiver 160. In the receiver 160, a clock recovery circuit (not shown) generates a sequence of optical pulses with a frequency related to a pulse frequency of the optical pulses received from the transmission fiber. That is, the clock recovery circuit monitors the data rate of the optical pulses generated by the transmitter 110 and adjusts the sampling time of the receiver 160 to correct for any variation in the data rate of the optical signal due to drift in the transmitter 110. As such, the input optical signals are sampled by the receiver 160 at a rate substantially equal to the data rate of the modulated optical signal.
An optical signal from the optical transmission test setup 100 of
In the embodiment of
In the receiver 160 of
Due to timing jitter, a pulse can arrive early or late compared to the sampling point. The late arrival of the pulse causes an increase in the bit-error rate (BER) of the received optical signals (assuming that the pulse is superposed with signal noise). If the pulse arrives early, the intensity of the pulse may significantly decreased at the sampling point, thus the BER will increase due to the relatively stronger impact of the signal noise. Only pulses, which arrive with their center substantially at the sampling point, or at least close to it, will show no enhanced error probability (EP) for the logical highs (ONEs). In a conventional receiver, the sample averaged bit error probability (BEP) is given by equation (1) as follows:
where P1 and P0 represent the EPs of the logical highs (ONEs) and the logical lows (ZEROs), respectively. For the cases of no timing jitter (no timing jitter present in a transmission system) the EPs of the logical ONEs and the logical ZEROs of a conventional receiver are typically in the range of 10−9. In the cases involving timing jitter (timing jitter present in a transmission system), the EP for the logical ONEs will dramatically increase. For example, the EP for the logical ONEs in a conventional receiver involving timing jitter is typically measured to be approximately 10−5, which corresponds to a penalty of substantially 2 dB. The EP for the logical ZEROs, however, remains substantially the same or is even slightly reduced as compared to the case with no timing jitter.
By contrast, a receiver in accordance with the present invention advantageously provides a method for increasing a timing jitter tolerance of a receiver in an optical transmission system.
In the embodiment of
where P1JD and 2P0 represent the EPs of the logical highs (ONEs) and the logical lows (ZEROs), respectively, for the case of dual sampling in a transmission system having timing jitter. The inventor determined that the EP for the logical highs (ONEs), P1JD, of an optical pulse for the case of dual sampling in a transmission system containing jitter, is orders of magnitudes smaller than the EP for the logical highs, P1, of an optical pulse in a conventional receiver implementing a single sampling technique (P1JD≈[P1]2). The inventor further determined that the EP for the logical lows (ZEROs), 2P0, of an optical pulse for the case of dual sampling in a transmission system containing jitter, only increases slightly and does not have a significant negative effect on the EP of the received optical pulse. Additionally, to further optimize the improvement in jitter tolerance in accordance with the present invention, the predetermined threshold level of the decision circuit (i.e. the decision circuit 240 of the receiver 160) can be adjusted to lower the EP of the receiver. By adjusting the threshold level of the decision circuit 240, the determination of a logical high (ONE) and the logical low (ZERO) can be further controlled.
In an experiment, a 10 Gb/s 33% RZ is launched through a jitter emulator and detected by a pre-amplified receiver, whose optical front end has an optical and electrical bandwidth of approx. 0.3 nm and 10 GHz, respectively. The converted optical signal is sampled by a commercially available 40 Gb/s 1:4 DeMux. The two center outputs of the 40 Gb/s 1:4 DeMux were processed by an OR gate after the relative delay (25 ps) between the outputs was compensated by means of tunable delay lines. The 40 GHz clock for the DeMux was generated by recovering the 10 GHz clock tone and using a 1:4 frequency multiplier.
In order to keep the experiment free of side effects such as pulse broadening due to non-linear propagation, PMD and other non-jitter related pulse distortions, a polarization scrambler was used in front of a PMD emulator to emulate timing jitter instead of generating timing jitter by propagating an optical pulse through a WDM transmission line. The State of Polarization (SOP) of the optical pulse changes in the micro second (μs) range as the polarization scrambler's output is translated into arriving time fluctuations (jitter) at the receiver. It should be noted, though, that the timing jitter generated by the PMD emulator has a different characteristic in terms of statistical properties when compared to timing jitter generated in a transmission line, however, the important aspects of timing jitter, which are, for this purpose, when the pulse center arrives with a time offset with respect to the sampling point in a receiver, are well reproduced by the PMD emulator.
For substantially small timing jitter offsets (DGD of the emulator), the single sampling of a conventional receiver and the dual sampling method of the present invention both produce substantially similar performances. When larger amounts of timing jitter offset (DGD) are applied, the dual (or multiple) sampling method of the present invention significantly outperforms single sampling.
An optical signal is received by the pre-amplified receiver 550 of the receiver 500, wherein the optical signal is converted into an electrical signal. After optical to electrical conversion, the signal is split into two substantially identical versions of the optical signal and launched into the two decision circuits 5601, 5602. The clock signals of the decision circuits 560 have a phase offset with respect to each other ΔT, which causes the sampling of the optical pulse at two different times. The outputs of the decision circuits 560 are input into the OR logic gate 570. The OR logic gate 570 operates substantially identically to the OR logic gate 160 described above with reference to
In accordance with the present invention and to further improve the jitter tolerance of a receiver, the electrical signal applied to a decision circuit can be filtered to ensure proper pulse shape of the input signals. Even further, because delays can cause variations in pulse shapes, different filters or filter values can be used to filter the input signals to the decision circuit, each filter chosen to act upon a specific input signal containing a variation in pulse shape due to a delay associated with that input signal.
While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.
This patent application claims benefit of U.S. Provisional Patent Application No. 60/362,113 filed Mar. 6, 2002, which is herein incorporated by reference in its entirety. Furthermore, this patent application is related to U.S. Provisional Patent Application No. 60/364,644 filed Mar. 15, 2002, which is herein incorporated by reference in its entirety.
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Number | Date | Country | |
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20030170022 A1 | Sep 2003 | US |
Number | Date | Country | |
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60362113 | Mar 2002 | US | |
60364644 | Mar 2002 | US |