U.S. patent application number 09/766683 was filed with the patent office on 2001-09-27 for opto-electronic frequency divider circuit and method of operating same.
This patent application is currently assigned to AGILENT TECHNOLOGIES, INC.. Invention is credited to Cisternino, Francesco, De Marchi, Andrea, Girardi, Raffaele, Roemisch, Stefania.
Application Number | 20010024315 09/766683 |
Document ID | / |
Family ID | 11416205 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024315 |
Kind Code |
A1 |
Cisternino, Francesco ; et
al. |
September 27, 2001 |
Opto-electronic frequency divider circuit and method of operating
same
Abstract
The circuit includes an electro-optical mixer, such as an
electro-optical Mach-Zehnder modulator, with non-linear behavior.
The modulator receives as input an optical signal (P.sub.in) at the
frequency to be divided, in addition to an electric signal
(e.sub.3) at a given frequency, usually corresponding to the
frequency deriving from such division. The output optical signal
(P.sub.out) from modulator exhibits a modulation spectrum
containing the frequency difference between the frequency to be
divided and at least one harmonic of the frequency of the above
electric signal. After having been converted into an electric
signal (e.sub.1), the output signal of the mixer is subjected to a
filtering action to extract the above frequency difference
component. This latter one is then used both as electrical signal
(e.sub.3) for the mixing, and as output signal from the divider
(e.sub.2). The preferred application is to OTDM systems, to extract
a synchronism signal as tributary signal frequency.
Inventors: |
Cisternino, Francesco;
(Torino, IT) ; De Marchi, Andrea; (Torino, IT)
; Girardi, Raffaele; (Mondovi, IT) ; Roemisch,
Stefania; (Borgaretto, IT) |
Correspondence
Address: |
THE FIRM OF KARL F ROSS
5676 RIVERDALE AVENUE
PO BOX 900
RIVERDALE (BRONX)
NY
10471-0900
US
|
Assignee: |
AGILENT TECHNOLOGIES, INC.
|
Family ID: |
11416205 |
Appl. No.: |
09/766683 |
Filed: |
January 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09766683 |
Jan 17, 2001 |
|
|
|
09199144 |
Nov 24, 1998 |
|
|
|
6204956 |
|
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Current U.S.
Class: |
359/329 |
Current CPC
Class: |
H04B 10/5053 20130101;
H04J 14/08 20130101; H04B 10/505 20130101; H04L 7/0075 20130101;
H04B 10/50577 20130101 |
Class at
Publication: |
359/329 |
International
Class: |
G02F 001/35 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 1997 |
IT |
TO97A001097 |
Claims
We claim:
1. A circuit for receiving an aggregate flow of optical signals
including a first optical signal (P.sub.in) obtained by optical
time division multiplexing of a plurality of tributary flows of
optical signals, each having a bit rate equal to a given frequency
(f.sub.1), and wherein a signal (e.sub.2) is produced by frequency
division which is synchronous with said tributary flows and forms a
synchronism signal for demultiplexing the aggregate flow, said
circuit comprising: nonlinear electro-optical mixer means receiving
an input said first optical signal (P.sub.in) at a frequency
(f.sub.0) to be divided in addition to an electrical signal
(e.sub.3) at said given frequency (f.sub.1) and to generate as an
output a second optical signal (P.sub.out) whose modulation
spectrum includes, due to mixing action, a frequency corresponding
to the difference between said frequency (f.sub.0) to be divided
and at least one harmonic of said given frequency (f.sub.1)
generated by nonlinear behavior of said mixer means; a feedback
path connected to said mixer means and comprising opto-electronic
converter means for converting said second optical signal
(P.sub.out) into an electrical conversion signal (e.sub.1) returned
to said electro-optical mixer means; filtering means along said
feedback path for extracting from said spectrum the component at
said difference frequency; and extracting means along said feedback
path for deriving from said feedback path a signal (e.sub.2) at
said difference frequency and resulting from frequency
division.
2. A circuit according to claim 1 wherein said electro-optical
mixer means includes a Mach-Zehnder electro-optical modulator.
3. A circuit according to claim 1 wherein said electro-optical
mixer means is provided with a control input (V.sub.bias) to select
an order of said at least one harmonic.
4. A circuit according to claim 1 wherein said electro-optical
mixer means has substantially sinusoidal transmittivity/input
voltage characteristics, and said electro-optical mixer means
operate next to one of the intermediate points in said
characteristics, so that said at least one harmonic is an odd-order
harmonic.
5. A circuit according to claim 1 wherein said at least one
harmonic is the third harmonic of said given frequency
(f.sub.1).
6. A circuit according to claim 1 wherein said feedback path
includes at least one delay element.
7. A circuit according to claim 6 wherein said feedback path
includes, as said delay element, an optical waveguide interposed
between said electro-optical mixer means and said opto-electronic
converter means.
8. A circuit according to claim 6 wherein said feedback path
includes, as said delay element, a delay line operating on
electrical signals and located, along the feedback path, downstream
of said opto-electronic converter means.
9. A circuit according to claim 1 wherein within said feedback
path, said filtering means is located downstream of said
opto-electronic converter means.
10. A circuit according to claim 1 wherein in said feedback path
includes gain control means to keep said electrical signal
(e.sub.3) at such a level as to ensure the non-linear behavior of
said electro-optical mixer means.
11. A circuit according to claim 1 wherein said filtering means are
connected in said feedback path, so that said component at said
difference frequency is used as said electric signal (e.sub.3) fed
to said electro-optical mixer means.
12. A circuit according to claim 1 wherein said extracting means is
located downstream of said filtering means, so that said component
at said difference frequency is used as signal (e.sub.2) resulting
from the frequency division action.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of Ser. No. 09/199,144
filed Nov. 24, 1998, now U.S. Pat. No. ______.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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
[0007] 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.
[0008] According to the invention, opto-electronic frequency
divider circuit which comprises:
[0009] electro-optical mixer means with a nonlinear behavior,
adapted to receive as input a first optical signal (P.sub.in) at a
frequency to be divided (f.sub.0) in addition to an electric signal
(e.sub.3) at a given frequency (f.sub.1) and to generate as output
a second optical signal (P.sub.out) whose modulation spectrum
includes, due to the mixing action, the frequency corresponding to
the difference between the frequency to be divided (f.sub.0) and at
least one harmonic of the given frequency (f.sub.1) generated due
to the nonlinear behavior,
[0010] a feedback path comprising opto-electronic converter means
to convert the second optical signal (P.sub.out) into an electrical
conversion signal (e.sub.1) adapted to be sent back to the
electro-optical mixer means,
[0011] filtering means associated with the feedback path to extract
from the spectrum the component at the difference frequency,
and
[0012] extracting means to derive from the feedback path as signal
(e.sub.2) resulting from the frequency division action, a signal at
the difference frequency.
[0013] The electro-optical mixer means can include a Mach-Zehnder
electro-optical modulator.
[0014] The electro-optical mixer means can be provided with a
control input (V.sub.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.
[0015] 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.sub.3) at such
a level as to ensure the nonlinear behavior of the electro-optical
mixer means.
[0016] 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.sub.3) fed to the electro-optical mixer
means.
[0017] The extracting means can be located downstream of the
filtering means so that the component at the difference frequency
is used as signal (e.sub.2) resulting from the frequency division
action.
[0018] The first optical signal (P.sub.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.sub.1), and the signal
(e.sub.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.
[0019] The invention also comprises a method of extracting from an
optical signal (P.sub.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.sub.1) of the optical tributary signal. The method
includes the following operations:
[0020] subjecting the optical signal (P.sub.in) to a nonlinear
electro-optical mixing operation with an electric signal (e.sub.3)
at the frequency (f.sub.1) of the optical tributary signals in
order to generate a further optical signal (P.sub.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.sub.0) and at least one harmonic of the
frequency (f.sub.1) of the optical tributary signals, generated due
to the nonlinear behavior of the mixing operation,
[0021] converting the further optical signal (P.sub.out) into an
electrical conversion signal (e.sub.1) that can be used to generate
the electrical signal (e.sub.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.
[0022] 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:
[0023] (1) delaying propagation of the further optical signal
(P.sub.out) within the feedback path,
[0024] (2) delaying propagation of the electrical conversion signal
(e.sub.1) within the feedback path, and
[0025] (3) delaying propagation of both the further optical signal
(P.sub.out) and the electrical conversion signal (e.sub.1) within
the feedback path.
[0026] In the preferred but not exclusive application to an OTDM
transmission system, this circuit allows performing a division of
the aggregate bit rate. The practical embodiment of the frequency
divider therefore does not require a technological development
degree higher than the one necessary to make the transmitter and
receiver of the single channel in the transmission system
BRIEF DESCRIPTION OF THE DRAWING
[0027] The above and other objects, features, and advantages will
become more readily apparent from the following description,
reference being made to the accompanying drawing in which:
[0028] FIG. 1 shows, in block diagram form, the general arrangement
of an OTDM optical transmission system to which a frequency divider
circuit according to the invention can be applied;
[0029] FIG. 2 shows also in block diagram form--the general
structure of the divider circuit according to the invention;
and
[0030] FIG. 3 is a diagram showing an operating characteristic of
one of the components of the circuit in FIG. 2.
SPECIFIC DESCRIPTION
[0031] In FIG. 1 reference T globally designates an optical link
realized according to the OTDM technique.
[0032] System T, operating according to criteria known per se,
allows conveying on an optical fiber line W (with associated
respective amplification/realization units schematically
represented by blocks AE1 and AE2) an RZ optical signal intensity
modulated according to RZ (return to zero) code and having a bit
rate f.sub.0 equal, for example, to 40 Gbit/s.
[0033] Such optical signal is obtained by aggregating in a
multiplexing unit (optical coupler) OC a number of optical signals
(four in the example), defined as "tributary", each one with bit
rate f.sub.1 equal to 10 Gbit/s.
[0034] It is clearly apparent that reference to an aggregate flow
with bit rate equal to 40 Gbit/s, obtained through multiplexing
four 10 Gbit/s tributary flows, must be deemed purely as an
example. Both the bit rate for tributary signals, and the number of
such multiplexed signals, and--consequently--the bit rate of the
resulting aggregate flow are design parameters adapted to be widely
modified as a function of the specific application needs of the
components being used, without departing from the scope of the
present invention.
[0035] In particular, the tributary flows (hereinbelow in the
present specification four tributary flows will always be referred
to, as an example) are generated starting from the RZ pulse train
issued by a pulse source S, typically composed of a laser source
with sufficiently narrow pulses that are time-domain limited. The
signal from source S is split by a separator SP1 into a plurality
of replicas each fed to a respective modulator M1 to M4.
[0036] The transmitter input data, in the form of electric signals
ideally coming from a data source SD synchronously driven with
source S due to the common slaving to a synchronism oscillator SYN
(also operating, in the disclosed embodiment, at a frequency of 10
Ghz, equal to the frequency of source S), are organized in a
corresponding number of channels C1-C4, each with a bit rate of 10
Gbit/s. The signals present on channels C1-C4 drive the respective
optical modulators M1-M4. The latter ones "write" the information
on the related pulse trains and the signals obtained are sent to
fiber W through optical coupler OC after having been mutually
time-offset by a time interval .DELTA.t, for example in respective
adjustable optical delay lines L1-L4.
[0037] The aggregate flow injected into fiber W thereby shows a
typical time-domain multiplexed structure. In practice, the
aggregate flow conveyed by fiber W is cyclically composed of a
symbol corresponding to a datum coming from channel C1, a symbol
corresponding to a datum coming from channel C2, a symbol
corresponding to datum coming from channel C3, a symbol
corresponding to a datum coming from channel C4, etc.
[0038] At the receiving side, the aggregate flow is sent to an
optical demultiplexer DMPX that orderly extracts from the received
aggregate flow the signals corresponding to the tributary channels
and routes them towards corresponding receivers RX1-RX4 to recover
at the output the flows corresponding to channels C1-C4.
[0039] To be able to correctly extract the different tributary
signals, the demultiplexer device DMPX requires a driving signal
e.sub.2 (which is supplied by a synchronism recovery circuit
globally designated as 1 and which is to be also sent to receivers
RX1-RX4) at the tributary frequency and phase locked with the
aggregate (or multiplexed) flow frequency.
[0040] When operating in RZ format, a line at the aggregate flow
bit rate is always present in the multiplexed flow spectrum, while
the spectral component at tributary frequency is absent (it
disappears due to the multiplexing operation). In order to
correctly operate, the synchronism recovery circuit must perform a
division of the aggregate bit rate by a factor equal to the
multiplexing factor, while keeping the phase locking between the
original frequency and the divided one.
[0041] In the specific case, the signal corresponding to aggregate
bit rate is extracted from the signal coming form fiber W through a
component like separator SP2.
[0042] Synchronism recovery circuit 1, that is the subject matter
of the present invention, is therefore a circuit that, starting
from the optical signal at the aggregate frequency, is able to
generate an electric signal with a frequency corresponding to the
tributary signal frequency.
[0043] The circuit according to the invention ideally refers to a
circuit known as a Miller frequency divider. Such divider circuit,
also known as regenerative divider since it is composed of a
feedback system, is characterized by very low added noise levels.
It found thereby use in generating sources with high spectral
purity, obtained by dividing high frequency references. For a
general description of the features of such known circuit,
reference can be made to the paper by R. C. Miller
"Fractional-frequency Generators Utilizing Regenerative Modulation"
in Proceedings, IRE, Vol. 27, pages 446-457, July 1939.
[0044] Similarly to the Miller divider, circuit 1 according to the
invention uses a component adapted to perform a mixer function.
Here a multiplication is performed between the component at the
frequency corresponding to the pulse repetition rate of the signal
to be divided, and the harmonics of the electrical division signal
coming from the feedback loop. Such harmonics originate inside such
component, designated with reference 2 and have a non-linear
behavior.
[0045] Any component showing this type of features can then be used
in the invention. In the currently preferred embodiment, component
2 is an electro-optical Mach-Zehnder modulator having
transmittivity characteristics as shown in the diagram in FIG.
3.
[0046] In such a diagram, the ordinate axis shows transmittivity T
(normalized to unit as maximum value) versus a parameter that can
be expressed as (V.sub.bias+V.sub.RF)/V.sub..pi., and that is
characteristic of the driving conditions for modulator 2.
[0047] In particular, V.sub.bias represents a bias voltage applied
to a first driving input of modulator 2, while V.sub.RF is a
radiofrequency driving signal applied to a corresponding input. In
the specific embodiment shown in FIG. 2, the concerned
radiofrequency signal is designated as e.sub.3. Parameter
V.sub..pi. is a normalization parameter; in practice, it is the
voltage to be applied to move from maximum to minimum
transmittivity. Obviously the transmittivity refers to the
input/output behavior of the optical signal passing through the
modulator. In the example shown, the optical input signal is
represented by signal P.sub.in while the output signal, designated
as P.sub.out, is equal to the input signal multiplied by
transmittivity. With modulator 2 it is therefore possible to
perform a multiplication between the spectral components in optical
input power modulation and the driving signal harmonics.
[0048] If the radiofrequency signal V.sub.RF is sufficiently strong
sinusoidal signal, transmittivity includes components with
frequencies corresponding to the frequencies of the harmonics of
signal e.sub.3. The relative amplitudes of such harmonics depend on
the working point position on the modulator characteristics (see
FIG. 3), and therefore on bias voltage V.sub.bias. In particular,
it is possible to have even harmonics only, by placing the working
point next to the maximum or minimum transmittivity, or only the
odd ones, by operating at the characteristics center.
[0049] In the specific embodiment shown here, it is desired to
perform a frequency division by four (N=4). Therefore, the choice
has been operating at the characteristics center, that is next to
one of the inflexion points shown by arrows in FIG. 3, in order to
generate and use the third harmonic (N-1=3) of the radiofrequency
signal.
[0050] In other words, the following results are obtained:
[0051] generation (due to non-linear behavior), starting from
signal e.sub.3 at frequency f.sub.1, of the N-1)th order harmonic
at frequency (N-1)f.sub.1, and
[0052] generation (due to the typical multiplying behavior of
modulator 2) of an output signal whose frequency is equal to the
difference the input signal frequencies (in addition to a signal
with sum frequency).
[0053] In general, since frequency f.sub.0 conveyed by input signal
P.sub.in is N times frequency f.sub.1 (f.sub.0=Nf.sub.1), the
output difference signal will have the following frequency
Nf.sub.1-(N-1)f.sub.1=f.sub.1
[0054] this latter one being the desired frequency.
[0055] Obviously, by choosing a different working point and/or a
component with different non-linear characteristics, it is possible
to generate a different-order harmonic and therefore, to perform a
division by a different factor. In any case it is important to note
that, since harmonic generation occurs inside modulator 2, it is
not necessary that the passband of radiofrequency input of
modulator 2 itself should extend beyond the frequency of signal
with frequency f.sub.1.
[0056] In summary, in the diagram in FIG. 2, the optical input
signal P.sub.in intensity modulated at the frequency f.sub.0 to be
divided (40 Gbit/s in the mentioned example) originates, through
electro-optical modulator 2, optical signal P.sub.out. This latter
signal is received, possibly through a section of optical fiber 3
operating as a delay line, by a photodiode 4. In photodiode 4 the
optical signal P.sub.out is converted into an electric signal
e.sub.1. After passing through a variable electric delay line 5
located downstream of photodiode 4, an amplifying stage 6 and a
variable attenuator 7 (whose function will be better described
below), signal e.sub.1 is filtered in a pass-band filter 8 tuned
around frequency f.sub.1. The filtered signal thus obtained,
designated as e.sub.3, is sent back to input V.sub.RF of modulator
2, thus providing a feedback. Usually, when passing from filter 8
(also located downstream of photodiode 4 within the feedback path)
to modulator 2, the signal is also made to pass through an
amplitude-adjusting amplifying stage 9 and an extracting device 10,
such as for example a directional coupler. The latter device
extracts a fraction of the feedback signal intended to build the
output signal e.sub.2 for the device.
[0057] The circuit performs a phase locking between input signal
P.sub.in and oscillating signal e.sub.3 and this makes the circuit
itself useful for applications like the tributary synchronism
recovery device in the diagram in FIG. 1.
[0058] Experiments carried out by the Applicant have demonstrated
the advantages of employing, within the general diagram in FIG. 2,
some particular application embodiments. These embodiments are to
be considered generally preferred, even if not mandatory per
se.
[0059] In particular , the input signal P.sub.in is preferably
injected into modulator 2 through a polarization controlling
member, of a known type and not specifically shown.
[0060] The use of optical fiber 3, responsible for the majority of
delays incurred by signals in the feedback path, is generally
deemed preferred in order to stabilize the circuit operation, when
necessary. The variable delay line 5 provided in the electrical
section of the feedback loop is necessary for the fine control of
the global phase shift. Preamplifier 6 and variable attenuator 7
allow accurately adjusting the circulating power.
[0061] The pass-band filter 8 does not require particular
selectivity properties. Amplifier 9 is to raise signal e.sub.3
supplied to input V.sub.RF of modulator 2 to a sufficiently high
level (for example about 25 dBm) so that such signal, inside
modulator 2, is subjected to the necessary distortion to produce
the desired harmonic.
[0062] Obviously, without changing the principle of the invention,
component parts and embodiments can be largely modified with
respect to what is described and shown, without departing from the
scope of the present invention.
* * * * *