Optical decision circuit and use thereof

Details Optical decision circuit and use thereof Optical decision circuit and use thereof

1998-08-27

2002-04-02

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S349000

Reexamination Certificate

active

06366382

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to an optical decision circuit, based on semiconductor optical amplifiers, which can be used for signal regeneration in optical communication systems. Such a circuit can easily be implemented as an integrated circuit or an OEIC.
BACKGROUND OF THE INVENTION
In optical long-haul communication systems, signal regeneration at regular distances is a prerequisite because several mechanisms deteriorate the optical signals. Examples of such mechanisms are pure transmission losses and different noise and signal distortion sources in communication components. Therefore there is a common interest in simple high quality optical regeneration circuits which can easily be integrated.
Several signal regeneration mechanisms are known, as illustrated in
FIG. 1
, to improve the signals at regular distances or times. A generated signal, like e.g. a periodic Return to Zero pulse signal (
FIG. 1
,
a
) or any other signal comprising a number of pulses which are at regularly spaced points in time (e.g. :t
0
, t
1
, t
2
), enters an optical fiber. After a certain distance, the signal is deteriorated as illustrated e.g. in
FIG. 1
,
b
and needs to be regenerated. The simplest regeneration system is the so-called 1R regeneration (
FIG. 1
,
c
), which is in fact an amplification system. The input signal (
FIG. 1
,
b
) is amplified in order to bring the signal power level sufficiently above the noise level as illustrated in
FIG. 1
,
c.
A more advanced system is the so-called 2R regeneration (
FIG. 1
,
d
). An optical input signal (
FIG. 1
,
b
) which is presented to a 2R regeneration system is set to a fixed high output signal level if the power of the input signal is above a certain threshold level and is set to a fixed low output level if otherwise. This regeneration allows to choose a more optimum decision threshold, i.e. a certain optical power level, at the receivers but in many cases this is still not satisfying.
An even more advanced system is the so-called 3R regeneration (
FIG. 1
,
e
). An optical input signal (
FIG. 1
,
b
) which is presented to a 3R regeneration system is set to a fixed high output signal level if the power of the input signal is above a certain threshold level and is set to a fixed low output level if otherwise. Furthermore a clock signal (
FIG. 1
,
e
), having the same period as the generated signal (
FIG. 1
,
a
), is used to retime the input signal pulses in order to coincide exactly with the clock pulses. The retiming allows to remove the signal jitter, i.e. fluctuations in the start instant of the pulses. Since the sampling at the receiver side occurs at instants defined by a periodic clock signal, this time jitter can cause additional detection errors, especially for very high bit rates.
So far, regeneration in optical communication has mainly been 1R regeneration using Erbium-doped fibre amplifiers. 3R regeneration has however been under investigation for several years and has been reported in the literature, e.g. in J. K. Lucek, K. Smith, ‘All-optical signal regenerator’, Optics Letters, Vol. 18, pp. 1226-1228, 1993. The clock signal is typically extracted from the signal using a mode locked laser as e.g. in P. B. Hansen et al., ‘All-optical clock recovery using a mode-locked laser’, El. Lett., Vol. 29, pp. 739-741, 1993, and regeneration is based on a non-linear fibre loop mirror, in which the clock signal is modulated by the signal. This technique however involves a complicated combination of fibre based components, making the regeneration circuit spacious and unsuitable for integration. Besides, it only works for certain modulation formats.
SUMMARY OF THE INVENTION
An important part of an optical regeneration system can be a so-called optical decision circuit. An optical decision circuit can be used both for 2R and 3R regeneration. Consequently, for 2R regeneration an optical decision circuit should give a low output power level, i.e. ideally this low level is a zero output power level, if the input power is below a certain threshold value and should give a high output power level, i.e. ideally this high level is a constant predetermined high output power level, if the input power is above the threshold value and there should be a steep transition between the low level and the high level. In a practical implementation, the low level may be small, but different from zero, the high level may slightly vary with input power and the transition between low and high level may be more gradual.
According to the present invention a device with an optical signal at its input and an optical signal at its output for optical signal regeneration is disclosed, where said optical output signal has a predetermined output power level, said device comprising:
a beam splitter for splitting said optical input signal in at least a first and a second signal;
at least a first and a second gain clamped optical amplifier, said first amplifier amplifying said first signal, said second amplifier amplifying said second signal, said first and said second amplifier having a different saturation input power level;
a phase shifting element for shifting the phase of either one of said first signal or said second signal;
a combiner for combining said first and said second signal. Preferably said phase is shifted such that the phase difference between said first signal and said second signal is essentially 180 degrees.
In an embodiment of the invention an optical decision circuit is disclosed, comprising a Mach-Zehnder interferometer (MZI) and two gain clamped semiconductor optical amplifiers (GCSOA's) and where said interferometer further comprises a splitter, a phase shifting element and a combiner. The GCSOA's are located in the branches of the interferometer.
In an embodiment of the invention an optical decision circuit is configured such that the beamsplitter for the incident signal as well as the combiner for the output signal are substantially symmetric giving a splitting and combining ratio of 50/50. When the input power of the incident signal is below a certain threshold value the optical decision circuit should return a low output power level, i.e. ideally a zero output power level. In this configuration, the amplitude of a first signal incident on a first GCSOA and the amplitude of a second signal incident on a second GCSOA is the same due to the 50/50 splitting ratio. After amplification of these signals, at least the phase of one of the signals is shifted such that the phase difference between the first signal and the second signal is essentially 180 degrees. Consequently, thereafter combining the first and the second signal using a combiner with a combining ratio of 50/50 is in fact substracting the first and the second signal one from another. Therefore, in order to establish a low output power level the amplification factor for input values below the threshold value should be substantially the same. To accomplish this, preferably, the optical decision circuit should further comprise two essentially identical GCSOA's, i.e. two GCSOA's having essentially the same structure, essentially the same dimensions and being composed of essentially the same materials.
In an embodiment of the invention an optical decision circuit is configured such that either the beamsplitter for the incident signal or the combiner for the output signal or the beamsplitter and the combiner are asymmetric giving a splitting ratio of &agr;/(100-&agr;), &agr; being different from 50 and 0<&agr;<100, and/or a combining ratio of &bgr;/(100-&bgr;), &bgr; being different from 50 and 0<&bgr;<100. When the input power of the incident signal is below a certain threshold value the optical decision circuit should return a low output power level, i.e. ideally a zero output power level. In this configuration, the amplitude of a first signal incident on a first GCSOA and the amplitude of a second signal incident on a second GCSOA can be substantially different due to the asymmetric splitting ratio. After amplification of these s

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