Opto-electronic frequency divider circuit and method of...

Details Opto-electronic frequency divider circuit and method of... Opto-electronic frequency divider circuit and method of...

2001-01-17

2001-11-27

Lee, John D. (Department: 2874)

Optical: systems and elements

Optical frequency converter

Harmonic generator

C359S199200, C359S238000, C359S328000

Reexamination Certificate

active

06323991

ABSTRACT:

FIELD OF THE INVENTION
The present invention refers to frequency divider circuits and, in particular, to their possible application to optical transmissions based on the so called OTDM (Optical Time Division Multiplexing) technique.
BACKGROUND OF THE INVENTION
The OTDM technique is particularly interesting in all situations where the need arises to increase the transmission capacity of an optical link and is an alternative to other solutions based on sharing the same physical carrier among multiple channels, typically by employing WDM (Wavelength Division Multiplexing) techniques, or on increasing the number of optical fibers available for the link.
This latter solution generally requires intervention on installation (such as laying of new cables and performing excavation) and does not completely exploit the extremely wide band made available by optical fibers.
The simultaneous transmission of many different channels on the same optical fiber, for example according to wavelength division multiplexing techniques, allows using low speed opto-electronic components both in the transmitter and in the receiver, while obtaining a high overall capacity on the link. Wavelength division multiplexing further allows implementing at the optical level of some network functions like channel removal and insertion, dynamic routing, link protection, with a high reduction of the processing load for the electronic part in network nodes. The major inconveniences of such method are linked to the need for selecting and stabilizing the wavelengths for transmitters and optical filters used for channels selection, to the possible inter-channel interference due to non-linear phenomena in fiber propagation (for example the phenomenon known as Four Wave Mixing) or to the spectral nonuniformity of optical amplifier gain.
In OTDM systems, many optical signals, intensity modulated according to an RZ (return to zero) code, are interleaved into a single flow by acting on the relative delay of the pulse sequences. This solution retains most of the advantages of WDM techniques related to the possible use of low speed opto-electronic components both in the transmitter and in the receiver, further avoiding the onset of some of the above-mentioned negative phenomena. A basic condition for the proper operation of an OTDM system is however that the different optical tributary flows must be well synchronized and composed of sufficiently narrow pulses in order to avoid interference among channels. Moreover, it is essential that a driving signal at tributary frequency and synchronous with the multiplexed flow is available at the demultiplexing device.
SUMMARY OF THE INVENTION
The present invention provides a solution to this latter need and, more generally, provides a particularly simple opto-electronic frequency divider circuit, adapted to operate at very high frequencies (typically with input frequencies of the order of several tens of Gbit/s) with good performance as regards the stability of frequency and phase locking between the signal resulting from the division and the input signal.
According to the invention, opto-electronic frequency divider circuit which comprises:
electro-optical mixer means with a nonlinear behavior, adapted to receive as input a first optical signal (P
in
) at a frequency to be divided (f
0
) in addition to an electric signal (e
3
) at a given frequency (f
1
) and to generate as output a second optical signal (P
out
) whose modulation spectrum includes, due to the mixing action, the frequency corresponding to the difference between the frequency to be divided (f
0
) and at least one harmonic of the given frequency (f
1
) generated due to the nonlinear behavior,
a feedback path comprising opto-electronic converter means to convert the second optical signal (P
out
) into an electrical conversion signal (e
1
) adapted to be sent back to the electro-optical mixer means,
filtering means associated with the feedback path to extract from the spectrum the component at the difference frequency, and
extracting means to derive from the feedback path as signal (e
2
) resulting from the frequency division action, a signal at the difference frequency.
The electro-optical mixer means can include a Mach-Zehnder electro-optical modulator.
The electro-optical mixer means can be provided with a control input (V
bias
) to select the order of the at least one harmonic. The electro-optical mixer means can exhibit a substantially sinusoidal transmittivity/input voltage characteristics, and in that the electro-optical mixer means are made to operate next to one of the intermediate points in the characteristics, so that the at least one harmonic is an odd-order harmonic. This harmonic can be the third harmonic of the given frequency.
The feedback path can include at least one delay element which may be an optical waveguide interposed between the electro-optical mixer means and the opto-electronic converter means. Alternatively the delay element can be a delay line operating on electrical signals and located, along the feedback path, downstream of the opto-electronic converter means. Within this feedback path, the filtering means are located downstream of the opto-electronic converter means. The feedback path can include gain control means to keep the electrical signal (e
3
) at such a level as to ensure the nonlinear behavior of the electro-optical mixer means.
The filtering means can be connected in the feedback path, so that the component at the difference frequency is used as the electric signal (e
3
) fed to the electro-optical mixer means.
The extracting means can be located downstream of the filtering means so that the component at the difference frequency is used as signal (e
2
) resulting from the frequency division action.
The first optical signal (P
in
) can be a signal belonging to an aggregate flow obtained by optically time division multiplexing a plurality of tributary flows, each one having a bit rate equal to the given frequency (f
1
), and the signal (e
2
) resulting from the frequency division action can be a signal synchronous with the tributary flows that forms a synchronism signal for demultiplexing the aggregate flow.
The invention also comprises a method of extracting from an optical signal (P
in
) conveying an aggregate flow of a given number (N) of optical tributary signals interleaved according to an optical time division multiplexing scheme, a synchronism signal at the frequency (f
1
) of the optical tributary signal. The method includes the following operations:
subjecting the optical signal (P
in
) to a nonlinear electro-optical mixing operation with an electric signal (e
3
) at the frequency (f
1
) of the optical tributary signals in order to generate a further optical signal (P
out
) having a modulation spectrum including, due to the mixing operation, a frequency corresponding to the difference between the frequency of the aggregate flow (f
0
) and at least one harmonic of the frequency (f
1
) of the optical tributary signals, generated due to the nonlinear behavior of the mixing operation,
converting the further optical signal (P
out
) into an electrical conversion signal (e
1
) that can be used to generate the electrical signal (e
3
) for the electro-optical mixing operation, according to a general feedback path to which a filtering operation is associated to extract from the spectrum the component at the difference frequency, the signal thereby extracted being the synchronism signal.
The method, when applied to an aggregate flow of N optical interleaved tributary signals is characterized in that the electro-optical conversion operation is performed with such a nonlinearity degree that the at least one harmonic is an (N−1) th-order harmonic. The method can include the operation of delaying propagation of at least one of the stops of:
(1) delaying propagation of the further optical signal (P
out
) within the feedback path,
(2) delaying propagation of the electrical conversion signal (e
1
) within the feedback path, and
(3) delaying propagation of both the further optical signa

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