EP1264425A1

EP1264425A1 - Method and system for amplitude modulation of an optical signal

Info

Publication number: EP1264425A1
Application number: EP01915009A
Authority: EP
Inventors: Erich Gottwald; Christoph Glingener
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2000-03-17
Filing date: 2001-02-15
Publication date: 2002-12-11
Also published as: DE10013197A1; WO2001069819A1; US20030030874A1

Abstract

The invention relates to a method and to a system for amplitude modulation of an optical signal (os) with a binary data signal (ds). To this end, the optical signal (os) is divided up into a first and a second adjustable optical signal (os1, os2). The first optical signal (os1) is supplied to a modulator (MZM) that outputs an optical transmission signal (ts) after amplitude modulation with a binary data signal (ds). A counter-phase second optical signal (gps) is produced from the second adjustable optical signal (os2) and the optical transmission signal (ts) and the counter-phase second optical signal (gps) are combined to a carrier-reduced optical transmission signal (rts).

Description

description

Method and arrangement for amplitude modulation of an optical signal

The invention relates to a method and an arrangement for amplitude modulation of an optical signal with a binary data signal that is fed to a modulator to generate an optical transmission signal.

In the case of optical transmission systems with data rates of 20 Gbit / s and more, with digital amplitude-modulated optical signals or transmission signals, a low extinction ratio, in particular in optical long-haul systems with optical amplifiers, contributes significantly to the fact that the optical signal to noise ratio required on the receiving end for the reconstruction of the data OSNR) deteriorates. The extinction ratio results from the power ratio between the logic “0” signal power ”and the logic“ 1 ”signal power, ie an amplitude-modulated signal with a high continuity portion or carrier portion which exceeds half the total power of the optical transmission signal and a low portion Extinction ratio and an almost completely "fully modulated" signal have a very high extinction ratio. If, for example, the extinction ratio of the amplitude-modulated optical signal is 3 dB in a transmission system with optical amplifiers, an OSNR that is higher by a factor of 10 than for an extinction ratio of 20 dB is required for the error-free reception of the amplitude-modulated optical signal. This results in a considerable shortening of the transmission path that can be bridged without regeneration by about an order of magnitude.

At extremely high data rates (40 Gbit / s and more), the achievement of a high extinction ratio in the current state of the art is only very limited and with high financial effort possible. Since such high data rates in the current state of the art neither direct modulation of a laser nor modulation with electroabsorption modulators is appropriate, Mach-Zehnder modulators (MZM) or Mach-Zehnder

Interferometer (MZI) used. In addition to the modulation signal, MZM modulators generally require a high drive voltage or driver voltage of 6 volts to 2 x 6 volts in order to achieve the high extinction ratio required for successful transmission. The driver circuits required to generate such high driver voltages are manufactured, for example, by manufacturers such as SHF-Design, Berlin - see product brochure "SHF 106P" or "SHF 103 CPA". However, driver amplifiers of this type have a signal quality that is just sufficient and are very cost-intensive. In addition, the procurement costs for such a driver amplifier many times exceed the procurement costs for an MZM modulator, which considerably worsens the competitiveness of the optical transmission system.

The object on which the invention is based is to improve the amplitude modulation of an optical signal such that the amplitude-modulated optical transmission signal has a reduced carrier component. The

Based on a method according to the features of the preamble of claim 1, the object is achieved by the characterizing features thereof.

The essential aspect of the method according to the invention can be seen in that the optical signal is divided into a first and second adjustable optical signal and the first adjustable optical signal is fed to the modulator, which emits the optical transmission signal after amplitude modulation with the binary data signal. Furthermore, a second antiphase optical signal is formed from the second adjustable optical signal and that optical transmission signal and the second antiphase optical signal are combined to form a reduced-carrier optical transmission signal. As a result of the inventive reduction of the carrier portion of the amplitude-modulated optical transmission signal following the modulation of the first optical signal, a lower modulation voltage is required to modulate the first optical signal in order to obtain an almost completely modulated optical transmission signal. Electrical driver amplifiers with a low control voltage can thus be used to control the modulator, which are inexpensive and additionally contribute to an improvement in the signal quality of the transmission signal due to the distortions in the optical transmission signal which are reduced by the lower modulation voltage. By reducing the carrier portion according to the invention with the aid of the adaptation of the signal level and the phase position of the amplitude-modulated optical transmission signal, the transmission range that can be bridged without regeneration can be increased particularly advantageously.

The division of the power of the optical signal into a first and second adjustable optical signal can be controlled in a particularly advantageous manner by means of an adjustable fade switch via a first control signal. Alternatively, the power of the optical signal can be controlled into a first and second adjustable optical signal by means of a fixed one Cross-fade switch having a cross-fade switch, the power of the second adjustable optical signal being controllable via an adjustable optical attenuator via the first control signal - claim 3. Through the cross-fader switch adjustable according to the invention via a first control signal or the adjustable optical attenuator, the carrier portion of the amplitude-modulated optical is Transmission signal adjustable so that the power of the carrier can be reduced to almost half the total power of the amplitude-modulated optical transmission signal, ie the Ge total output of the amplitude modulating The optical transmission signal is ^" distributed almost uniformly by the method according to the invention on the carrier and on the two sidebands. Thus, a relatively low modulation voltage, for example the data signal emitted by a multiplexer, is sufficient for the complete modulation of the optical signal in order for the optical carrier-reduced according to the invention Generate transmission signal.

The phase position of the antiphase second optical signal can advantageously be controlled by an adjustable phase actuator via a second control signal. This makes it particularly advantageous to regulate the phase position of the antiphase second optical signal in order to generate an exact 180 ° phase shift with respect to the optical transmission signal. For this purpose, for example, the setting of the phase shift is changed by “wobble” and regulation is carried out according to the lock-in principle.

Advantageous further developments and developments of the method according to the invention, in particular an arrangement for amplitude modulation of an optical signal, are described in the further patent claims.

The invention is to be explained in more detail below with the aid of three circuit diagrams and two signal flow diagrams.

FIG. 1 shows the basic structure of the arrangement for amplitude modulation according to the invention, FIG. 2 shows a further variant of the arrangement for amplitude modulation according to the invention, FIG. 3 shows an amplitude-modulated optical transmission signal with a low extinction ratio, ie an excessively high proportion of carrier, FIG. 4 also shows the carrier-reduced optical transmission signal according to the invention a high extinction ratio and FIG. 5 shows the arrangement according to the invention for amplitude modulation integrated in a modulator module.

FIG. 1 shows, for example, an arrangement for amplitude modulation of an optical signal os with a binary data signal ds, which has an adjustable phase actuator PSG, an adjustable cross-fade switch OCU, a modulator MZM, a data source DQ, an optical coupler OC, and an optical transmitter unit CW and has a control unit Cü. The adjustable cross-fade switch OCU can be implemented, for example, as an optical coupler with an adjustable cross-fade ratio. The input i of the adjustable cross-fade switch OCU is connected to the optical transmission unit CW via an optical connecting fiber. Furthermore, the adjustable crossfader OCU has a first and second output el, e2 and a control input ri, the first output el being connected to the input i of the modulator MZM and the second output e2 being connected to the input i of the adjustable phase actuator PSG is. A Mach-Zehnder modulator can be provided as the modulator MZM, for example, which, in addition to an optical input i, has an electrical data input di and an optical output e, the data input di being connected to a data source DQ and the output e having the first input il of the optical coupler OC is connected.

The adjustable phase actuator PSG has an output e and a control input ri, the output e of the adjustable phase element PSG being connected to the second input i2 of the optical coupler OC via an optical connecting fiber. The optical coupler OC has, for example, a first output el and a second output e2, wherein in FIG. 1 the first output el is connected via a TAP coupler TAP to a remote optical receiving unit EU and the second output e2 optionally via one in FIG 1 dashed line is connected to the control unit CU. The optical TAP coupler TAP is connected to the control unit CU. The control unit CU has an optical converter OW, a first and second filter unit FU1, FU2, a phase controller PR and a power controller LR. Here, the optical converter OW is connected to the TAP coupler TAP and to the first and second filter units FU1, FU2, and the first filter unit FU1 is connected to the phase controller PR, which is connected via a first control line RL1 to the control input ri of the adjustable phase actuator PSG , The second filter unit FU2 is connected to the power regulator LR, which is connected via a second control line R2 to the control input ri of the adjustable crossfader OCU.

An optical signal os is generated in the optical transmission unit CW and output via an optical connecting fiber to the input i of the adjustable cross-fade switch OCU. With the help of the adjustable crossfade switch OCU, the optical signal os is divided into a first and second optical signal osl, os2 with regard to the set crossfade ratio. The first optical signal osl is fed to the first output el of the adjustable cross-fade switch OCU and fed to the input i of the modulator MZM via an optical connecting fiber or an optical waveguide. Furthermore, the second optical signal os2 is emitted at the second output e2 of the adjustable cross-fade switch OCU and is fed to the input i of the adjustable phase control element PSG via a further optical connecting fiber. The first optical signal os1 is amplitude-modulated in the modulator MZM with the aid of the data signal ds present at the data input di, and an optical transmission signal ts is thus generated. The optical transmission signal ts is emitted at the output e of the modulator MZM via an optical connecting fiber to the first input il of the optical coupler OC. In the adjustable phase actuator PSG, the second optical signal os2 is shifted by the preset phase amount in accordance with the set phase shift amount. inventions dung according to a phase shift of ^'loan amount selected in the range of 180 ° anti-phase, particularly to produce at the output e of the adjustable phase control element PSG a one with respect to the carrier portion of the optical transmission signal ts anti-phase, second optical signal gps. This antiphase second optical signal gps is transmitted from the output e of the adjustable phase actuator PSG via an optical connecting fiber to the second input i2 of the optical coupler OC.

With the aid of the optical coupler OC, the optical transmission signal ts and the antiphase second optical signal gps are superimposed or coupled, so that a carrier-reduced optical transmission signal rts arises due to destructive interference. The carrier-reduced optical transmission signal rts is controlled at the first output el of the optical coupler OC and from there via the optical TAP coupler TAP to the distant optical receiver unit EU, which is indicated in FIG. 1 by a dotted transmission fiber OF. In addition, when the optical transmission signal ts and the antiphase second optical signal gps are coupled or superimposed, an “inverted” optical transmission signal irt is generated, the carrier component of which has increased in comparison to the optical transmission signal.

Coupling of the optical transmission signal ts with the antiphase second optical signal gps reduces the carrier component of the optical transmission signal ts, whereby the extinction ratio, ie the ratio of the binary “1” signal power to the binary “0” signal power, of the carrier-reduced optical transmission signal rts is significantly increased becomes. In the ideal case, the carrier-reduced optical transmission signal rts has a carrier portion of 50% and an information or data signal portion of the remaining 50% distributed over the sidebands, ie the power of the carrier corresponds to half the total signal power of the carrier-reduced optical transmission signal rts. One of those The carrier-reduced optical transmission signal rts is also referred to in the technical field as a completely modulated transmission signal rts.

5. Furthermore, with the aid of the TAP coupler TAP, part, for example 10 percent, of the carrier-reduced optical transmission signal rts' is coupled out and fed to the control unit CU, in particular the optical converter OW, via an optical connecting fiber. In the optical converter OW, the part of the carrier-reduced optical transmission signal rts' which is decoupled is converted into an electrical signal es which is controlled by the first and second filter units FU1, FU2.

5 The first filter unit FU1 is designed, for example, as a bandpass, the pass band of the bandpass having a bandwidth which is approximately half the data transmission rate. With the aid of the first filter unit FU1, the electrical signal is filtered and the result of the filtering is delivered to the phase controller PR. In the phase controller PR, the filtered electrical signal is evaluated with regard to its signal amplitude or signal amplitude position, and a second control signal rs2 for regulating the adjustable phase actuator PSG is determined from the evaluation result with regard to the amount of the phase shift. The second control signal is controlled via the first control line RL1 to the control input ri of the adjustable phase actuator PSG. In this phase control, for example, the phase deviation or the sign of the phase deviation of the antiphase second optical signal os2 from the optical transmission signal ts is determined by “wobble” (periodic change of the phase shift by a small amount by wobble voltages) and a phase control according to the The phase position of the second optical signal os2, which is in phase opposition, is set by the adjustable phase actuator PSG in such a way that the measured amplitude of the electrical signal it assumes a maximum, ie the eye diagram of the carrier-reduced optical transmission signal rts has a maximum opening.

The second filter unit FU2 is implemented, for example, as a low-pass filter having a low cut-off frequency, with the aid of which the power of the electrical signal is determined. The result of the filtering by the second filter unit FU2 is delivered to the power regulator LR. The power of the filtered electrical signal es is evaluated in the power regulator LR and a first control signal rsl for regulating the adjustable cross-fade switch OCU is obtained on the basis of the evaluation result. The first control signal rsl is transmitted via the second control line RL2 to the control input ri of the adjustable optical crossover switch OCU. With such a control, for example, the measured signal power of the electrical signal es is regulated to a minimum, as a result of which the carrier power component in the carrier-reduced optical transmission signal rts is reduced to half the total signal power of the carrier-reduced optical transmission signal rts.

FIG. 2 shows a further embodiment of the arrangement for amplitude modulation according to the invention, in which the adjustable optical cross-switch OCU shown in FIG. 1 is replaced by an optical cross-switch C having a preset cross-fade ratio, for example 50:50. The optical cross-fade switch C has an input e and a first and second output el, e2, the input e being connected to the optical transmitter unit CW and the first output el being connected to the input i of the modulator MZM. In addition, in the further embodiment of the arrangement for amplitude modulation, an adjustable optical attenuator A is provided, which has an input i, a control input ri and an output e. The input i of the adjustable optical attenuator A is connected to the second output e2 of the optical cross-fader switch C. connected and the output e of the adjustable optical attenuator A is connected to the input i of the adjustable phase actuator PSG. Furthermore, the control input ri of the adjustable optical attenuator A is connected to the power controller LR of the control unit CU via the second control line RL2.

The mode of operation of the arrangement for amplitude modulation shown in FIG. 2 differs in principle from the embodiment shown in FIG. 1 in that the optical fade switch C divides the power of the optical signal os into a first and a second optical signal by means of the fixed fade ratio osl, os2 takes place. The regulation of the power distribution between the carrier and signal components of the carrier-reduced optical transmission signal rts is carried out with the aid of the adjustable optical attenuator A connected to the power controller LR. In this case, the attenuation amount of the adjustable optical attenuator A is regulated, for example, on the basis of the first control signal rsl.

To illustrate the reduction of the carrier portion of the optical transmission signal ts according to the invention, for example in FIG. 3 in a diagram. the amplitude-modulated optical transmission signal ts, which has a low extinction ratio, and in FIG. 4, the optical transmission signal rts, reduced according to the invention, with a high extinction ratio, are shown in a further diagram; optical transmission signals ts, rts were shown in FIGS. 3 and 4 in FIGS - (No-Return-To-Zero) transmission signals selected. Furthermore, the diagram shown in Figure 3 and Figure 4 each have a horizontal and a vertical axis T, OSA, with the horizontal axis, the time course T and the vertical axis OSA, the amplitude OSA of the amplitude-modulated optical transmission signal ts and the slow - 00 ω IV) M t- ¹ π o (_π o Cπ o cπ

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Claims

claims

1. A method for amplitude modulation of an optical signal (os) with a binary data signal (ds) which is fed to a modulator (MZM) for generating an optical transmission signal (ts), characterized in that the optical signal (os) in a first and second adjustable optical signal (osl, os2) is divided so that the first adjustable optical signal (osl) is fed to the modulator (MZM), which emits the optical transmission signal (ts) after the amplitude modulation with the binary data signal (ds), that a second antiphase optical signal (gps) is formed from the second adjustable optical signal (os2) and that the optical transmission signal (ts) and the second antiphase optical signal (gps) are combined to form a reduced-carrier optical transmission signal (rts).

2. Method for amplitude modulation according to claim 1, characterized in that the division of the power of the optical signal (os) into a first and second adjustable optical signal (osl, os2) by an adjustable cross-fade switch (OCU) via a first control signal (rsl) is controllable.

3. A method for amplitude modulation according to claim 1, characterized in that the power of the optical signal (os) is divided into a first and second adjustable optical signal (osl, os2) by a cross-fade switch (C) having a fixed cross-fade ratio, the power of the second adjustable optical signal (os2) can be controlled via an adjustable optical attenuator (A) via the first control signal (rsl).

4. A method for amplitude modulation according to claim 1 to 3, characterized in that the phase position of the antiphase second optical signal (os2) can be controlled by an adjustable phase actuator (A) via a second control signal (rs2).

5. A method for amplitude modulation according to claim 2 or 3, characterized in that for generating the first control signal (rsl) a part of the reduced-carrier optical transmission signal is coupled out (rts') and a control unit (CU) is fed in which the decoupled part of the reduced-carrier optical transmission signal (rts ^Λ ) is converted into an electrical signal (es) and the signal power of the electrical signal (es) is determined by filtering and, depending on this, the first control signal (rsl) for controlling the adjustable cross-fade switch (OCU) or the adjustable attenuator ( A) is generated.

6. A method for amplitude modulation according to claim 5, characterized in that for generating a second control signal (rs2) in the control unit (CU) a frequency range of the electrical signal (es) is filtered out, the amplitude of the filtered electrical signal (es) is determined and depending on this, the second control signal (rs2) for controlling the adjustable phase control element (PSG) is generated according to the lock-in principle.

7. Arrangement for amplitude modulation of an optical signal (os) with a binary data signal (ds) by a modulator (MZM), at whose data input (di) the binary data signal (ds) is guided and at its output (e) an optical transmission signal ( ts) is delivered, characterized in that that an adjustable optical crossfade unit (OCU) is provided for dividing the optical signal (os) into a first and second adjustable optical signal (osl, os2), that a first output (el) of the adjustable optical crossfade unit (OCU) with the Input (i) of the modulator (MZM) and a second output (e2) is connected to the input (i) of an adjustable phase actuator (PSG) that the output (e) of the modulator (MZM) and the output (e) of the phase actuator (PSG) are each connected to an input (el, e2) of a coupler unit (OC), at the at least one output (el) of which a reduced-carrier optical transmission signal (rts) is emitted that for decoupling a part of the reduced-carrier optical transmission signal (rts ') a TAP coupler unit (TAP) is connected to the output (el) of the coupler unit (OC), that the TAP coupler unit (TAP) is connected to a control unit (CU) for obtaining at least one control signal nals (rsl, rs2) from the decoupled part of the reduced-carrier optical transmission signal (rts ^Λ ) is connected and that the control unit (CU) is connected to the adjustable phase actuator (PSG) and to the adjustable optical cross-fader unit (OCU) for controlling it ,

8. Arrangement for amplitude modulation of an optical signal (os) with a binary data signal (ds) by a modulator (MZM), at whose data input (di) the binary data signal (ds) is guided and at its output (e) an optical transmission signal ( ts) is given, characterized in that an optical crossfade unit (C) for dividing the optical signal (os) into a first and second optical signal (osl, os2) is provided, that a first output (el) of the optical crossfade unit with the Input (i) of the modulator (MZM) and a second output (e2) is connected to the input (i) of an adjustable attenuator (A), that the output (e) of the adjustable attenuator (A) is connected to the input (i) of an adjustable phase actuator (PSG), that the output (e) of the modulator and the output (e) of the adjustable phase actuator (PSG) each to one Input (il, i2) of a coupler unit (OC) are connected, at the at least one output (el) of which a reduced-carrier optical transmission signal (rts) is emitted, that a TAP for decoupling a part of the given reduced-carrier optical transmission signal (rts') -

Coupler unit (TAP) is connected to the output (el) of the coupler unit (OC), that the TAP coupler unit (TAP) to a control unit (CU) for obtaining at least one control signal (rsl, rs2) from the decoupled part of the reduced-carrier optical transmission signal (rts') is connected and that the control unit (CU) is connected to the adjustable phase actuator (PSG) and to the adjustable attenuator (A) for controlling them.

9. Arrangement for amplitude modulation according to claim 7, characterized in that the control unit (CU) for generating at least one first control signal (rsl) for adjusting the distribution of the power of the optical signal (os) into a first and second adjustable optical signal (osl, os2) with the aid of the adjustable optical crossfade unit (OCU) or for adjusting the attenuation of the second adjustable optical signal (os2) with the aid of the adjustable attenuator (A).

10. Arrangement for amplitude modulation according to claim 7 to 9, characterized in that the control unit (CU) is provided for generating at least one second control signal (rs2) for controlling the amount of the phase shift of the adjustable phase actuator (PSG).

11. Arrangement for amplitude modulation according to claim 7 or 10, characterized in that a further output (e2) of the coupler unit (OC) is connected to the control unit (CU), via which an inverted optical transmission signal (irt) to the control unit (CU) is delivered.

12. Arrangement for amplitude modulation according to claim 7 to

11, characterized in that in the control unit (CU) an optical converter (OW), a first and second filter unit (FU1, FU2) and at least one phase controller (PR) and a power controller (LR) are provided, with the optical converter (OW) the decoupled part of the carrier-reduced optical transmission signal (rts ^Λ ) is converted into an electrical signal (es), that the electrical signal (es) is delivered to the first and second filter units (FU1, FU2), that by the first filter unit (FU1) the electrical signal (es) is filtered to measure its amplitude, that the electrical signal (es) is filtered by the second filter unit (FU2) to measure its signal power, depending on the measured power of the electrical signal (es ) is formed by the power controller (LR), the first control signal (rsl) for controlling the adjustable fade switch (OCU) or the adjustable attenuator (A) and that depending on the g measured amplitude of the electrical

Signal (es) by the phase controller (PR), the second control signal (rs2) for controlling the adjustable phase actuator (PSG) is formed according to the lock-in principle.

13. Arrangement for amplitude modulation according to claim 7 to 12, characterized in that that a Mach-Zehnder modulator operated in push-pull arrangement is provided as the modulator (MZM).

14. Arrangement for amplitude modulation according to claim 7 to 13, characterized in that the arrangement for amplitude modulation is integrated in a modulator module (MM) that at least one signal input (smi), at least one data input (dmi), at least one control input (rmil, rmi2 ) and has at least one signal output (eml, em2).