DE202006000197U1 - Optical IQ-transmitter for producing high value modulation signals, has phase modulators that are supplied with control pulses of two different electrical conditions for realizing of phase shifts of specified degree - Google Patents

Abstract

The transmitter has a serial IQ-modulator with which a phase modulator (PM1) is arranged in an in-phase-arm of an IQ-modulator in series to a Mach-Zehnder-modulator (MZM1) and another phase modulator (PM2) is arranged in a quadrature arm is series to another Mach-Zehnder-modulator (MZM2). The phase modulators are supplied with control pulses of two different electrical conditions for realizing of phase shifts of 0 degree or 180 degree.

Description

The Innovation refers to an optical IQ transmitter for generation of high-level modulation signals with a carrier signal source and an optical IQ modulator with an in-phase arm and a quadrature arm between a 3dB branch point and a 3dB coupling point, wherein in the in-phase arm a first Mach-Zehnder modulator and in-line in the quadrature arm to a 90 ° phase shifter another Mach-Zehnder modulator are arranged, both with Control signals with modulation-specific number of electrical conditions be charged.

In the modern optical transmission technology are used for the efficient use of the optical bandwidth and the Improvement of transmission characteristics complex, high-grade Modulation method applied. Here, special constellation points encode in a chosen one Constellation system symbols with a certain number of bits (For example, 2 or 4) and have the optical carrier a certain amplitude (length the vector to the constellation point) and phase (angle of the vector) to. In M-ary Quadrature Amplitude Modulation (M-QAM) the M constellation points on concentric circles (several amplitude and phase states) around the origin of the constellation system. In the M-valent phase shift (M-PSK), all constellation points lie on one and the same Circle (one amplitude state, M phase states).

For example the Quadrature PSK (QPSK) names 4 constellation points, while the Square 16 QAM names already 16 constellation points.

For the generation M-valued optical quadrature amplitude modulation (M-QAM) and phase-keying signals (M-PSK) used in, for example, optical This is the case for access, metro and wide area networks called optical "IQ modulator" the key component the IQ station, because it allows the direct conversion of complex data symbols to an optical one carrier frequency and thus accepting the special constellation states in an IQ constellation system. The individual constellation points become an in-phase signal component and assigned a quadrature signal component. When applied in an IQ constellation system with the in-phase signal component as ordinate and the quadrature signal component as abscissa a representation of the vectors to the individual constellation points by a simple addition of the vectors that are perpendicular to each other two signal components possible. The required number of electrical states of the drive signals to Generation of all possible Constellation points corresponds to the number for this IQ splitting the possible conditions the in-phase and quadrature signal components. IQ modulators are needed two orthogonal, monofrequent carriers for the function: a sine and a cosine signal in the carrier frequency range, with the help of a 90 ° phase shifter from one outside within the IQ transmitter monofrequent carrier be generated.

State of technology

to Formation of the M-valued optical modulation signals (M-PSK / M-QAM) can basically a serial structure of a Mach-Zehnder modulator and a phase modulator be used. An in-phase and a quadrature arm will be here not formed. It can these two components are also summarized in a dual-drive MZM become. From the carrier signal source, for example, a continuous wave laser, becomes an optical carrier signal constant wavelength generated that first in its intensity, i.e. in its brightness, and then modulated in its phase. The laser light can optionally be used for zero-return pulses (return-to-zero RZ pulses). be formed by means of a RZ-MZM. In the serial arrangement it is a visually very simple structure. However, it is a high number of different electrical states the drive signals for the optical components required, which in practice for high data rates difficult to realize.

In contrast, the drive signals in a conventional optical IQ transmitter with an IQ modulator with an in-phase and a quadrature arm already have a lower number of different electrical states. Such a structure with an IQ modulator is disclosed, for example, in the publication by Keang-Po Ho et al. entitled "Generation of Arbitrary Quadrature Signaling Using One Dual-Drive Modulator" (Journal of Lightwave Technology, Vol. 23, No.2, Feb. 2005, pp 764-770), of which the present innovation is the closest prior art The conventional optical IQ transmitter includes a carrier signal source and an IQ modulator that begins with an optical 3dB branch to form an in-phase and a quadrature arm and a 3dB optical coupling point for optical addition of the two In addition, care must be taken to ensure that both sub-signals have the same polarization. In the in-phase arm, a first Mach-Zehnder modulator is arranged in the quadrature arm. (or -90 °) phase shifter and another Mach-Zehnder modulator, so the two-arm structure characteristic of an IQ transmitter is the pure amplitude mode dulation of two, by the phase shift of 90 ° orthogonal to each other optical carriers before. The laser light is split evenly between both arms, phase-shifted by 90 ° in one arm, and amplitude-modulated in both arms by a Mach-Zehnder modulator. The required number of different electrical states of the drive signals in this structure corresponds to the projection of the constellation points on the two axes, ie in QPSK are drive signals with two different electrical states (two-stage control signals), in Square-16-QAM already control signals with four different electrical states (four-stage drive signals) required for the in-phase and the quadrature arm. For even higher-valued modulation methods, the required number of different electrical states continues to increase. In the case of square M-QAM methods, the required number of different electrical states is, for example, log ₂ M. The conventional IQ structure therefore also requires higher-level electrical drive signals with a greater number of different electrical states, the generation of which in practice is particularly advantageous high data rates is problematic.

task and solution according to the invention

The Task for the present innovation is therefore to be seen on the basis of the generic optical IQ transmitter of the type described above such an IQ modulator for disposal to ask for whose optical components are the higher quality of the required electrical states for the drive signals is significantly reduced. The renewal solution for this task is therefore characterized by a serial IQ modulator, in which in the in-phase arm in series with the first Mach-Zehnden modulator and in the quadrature arm in Row to the other Mach-Zehnder modulator each have a phase modulator is arranged, wherein the two Mach-Zehnder modulators with drive signals with across from an IQ modulator without phase modulators halved number of different electrical states and the two phase modulators with drive signals of two different electrical states be applied for the realization of phase shifts of 0 ° or 180 °.

Of the IQ modulator of the inventive IQ transmitter has a serial structure of a Mach-Zehnder modulator in each arm (for intensity modulation) and a phase modulator, therefore, the new IQ modulator also referred to as "serial IQ modulator". Amplitude modulation in a conventional IQ modulator becomes in the new IQ modulator by a serial intensity and Phase modulation replaced. Now in each case in the Mach-Zehnder modulators only the intensity level of the current constellation point, i. it will only the positive values on the in-phase axis and quadrature axis are controlled. The negative values on the in-phase axis and the quadrature axis are then achieved by a separate subsequent phase modulation. By phase shifts of 0 ° or ± 180 °, the Constellation points in all three other quadrants in which the in-phase and / or the quadrature portion of the current constellation signal is negative, can be achieved. The opposite to the conventional IQ modulator because of the two additional ones Phase modulators more complex optical structure can in particular be accepted in a monolithic integration design.

by virtue of The series connection of Mach-Zehnder modulator and phase modulator must be included the new serial IQ modulator basically only-more drive signals with half Number of electrical states for the opposite to both MZM a conventional one IQ modulator can be generated. The two phase modulators in the Both arms of the IQ modulator are basically only with two-stage control signals for setting a phase shift from 0 ° and 180 ° applied. This is especially interesting for Square 16 QAM modulation, because then exclusively binary Drive signals with two different electrical states for each Mach-Zehnder modulator and every phase modulator needed become. For the generation of electrical drive signals from the binary signal data stream For example, a simple 1: 4 demultiplexer is sufficient.

by virtue of the changed Signal generation by the two phase modulators occurs in the IQ transmitter after the innovation of digital signals that during the Symbol period can not be reduced to zero (non return-to-zero NRZ), to overshoots and to frequency chirp with the effect of an extension of the optical Spectrum. overshoots However, frequency chirp does not occur when in a continuing education of the inventive IQ modulator is provided that between the carrier signal source and the 3dB branch point or behind the 3dB coupling point an additional Mach Zehnder modulator for one pulse-forming signal zero reset (Return-to-Zero RZ) is arranged.

Finally, it can also be provided in the IQ modulator after the innovation in a development that in the in-phase arm and in the quadrature arm each of the Mach-Zehnder modulator and the phase modulator as a common component in the form of a dual-drive Mach -Zehnder modulator are formed. However, a more complex electrical control is required again. The control of the DD-MZM is then carried out simultaneously for intensity modulation in push-pull mode and for phase modulation in common mode. The control signals are electrically combined in a defined manner before they are fed to the inputs of the DD-MZM.

Further Details as well as the information technology basics to the optical IQ transmitters with a serial IQ modulator after the innovation are the publication "Multi-format Transmitters for Coherent Optical M-PSK and M-QAM Transmission "inventor M. Seimetz, within the applicable for a utility model Novelty period on 03.07.2005 at the "7th International Conference on Transparent Optical Networks "(ICTON 2005) in Barcelona, Spain in the second conference proceedings has been. Through her quote, the said publication is completely to Disclosure of the present utility model include.

embodiments

forms of training The novelty will be used to further understand them below with reference to the schematic Figures closer explained. Showing:

1 a serial optical transmitter of the prior art,

2 a conventional optical IQ transmitter of the prior art,

3 an optical IQ transmitter with serial IQ modulator after the innovation,

4 a constellation diagram for Square-16-QAM,

5 a 1: 4 demultiplexer for Square 16 QAM,

6 Eye diagrams for different stations,

7 IQ constellation plots for different stations and

8th an IQ transmitter with two DD-MZMs.

In the 1 In the prior art, a serial transmitter SS for generating high-level modulation signals is shown, with which each constellation point is connected by discrete adjustment of the corresponding vector in amplitude by means of a Mach-Zehnder modulator MZM and phase by means connected in series to the Mach-Zehnder modulator MZM Phase modulator PM is achieved. Optionally, another Mach-Zehnder modulator RZ-MZM for zero feedback may be provided for pulse shaping.

The 2 also shows in the prior art a conventional optical IQ transmitter KIQS with a conventional IQ modulator KIQM for generating high-level optical modulation signals. The carrier signal generated by a carrier signal source TSQ, in this case a continuous-field laser CWL, is divided equally at an 3dB branching point AS into an in-phase arm IPA and a quadrature arm QA. In the in-phase arm IPA it passes through a Mach-Zehnder Modulaton MZM1, which is controlled by means of an electrical drive signal EAS according to the specification of the selected high-grade constellation. In the quadrature arm QA, the carrier signal is first phase-shifted in a phase shifter PS by 90 ° and then passed through another Mach-Zehnder modulator MZM2, which is also controlled by means of a dependent of the current symbol electrical drive signal EAS. There arise in both arms IPA, QA two modulated optical signals, which are optically combined in a 3dB coupling point KS again, that is added. The addition results in a modulated optical carrier having an amplitude and phase corresponding to the current constellation point.

to Avoidance of undershoots and frequency chirp assigns the conventional optical IQ transmitter KIQS between the carrier signal source TSQ and the 3dB branch point AS still an additional Mach Zehnder modulator RZ-MZM, which optionally pulses the signals during the symbol period returns to zero. alternative can the extra Mach-Zehnder modulator RZ-MZM also be arranged behind the 3db coupling point KS.

In the 3 is an IQ transmitter IQS with a serial IQ modulator IQM after the innovation shown. Same, in the 3 not mentioned reference numerals are the 2 refer to. The significant difference of the optical serial IQ modulator IQM after the innovation to the conventional IQ optical modulator KIQM is the serial intensity and phase modulation in each arm IPA, QA. For this purpose, the two Mach-Zehnder modulators MZM1, MZM2 each have a phase modulator PM1, PM2 arranged downstream in series.

In the 4 is a constellation diagram for square-16 QAM with M = 16 different constellation points, each assigning four bits to a modulation symbol to be transmitted. Each constellation point can be represented by a vector VE, which always provides the optical carrier with a certain amplitude (Vek gate length) and assigns a certain phase (vector angle). In the case of the serial IQ modulator IQM after the innovation, the two Mach-Zehnder modulators MZM1, MZM2 in the in-phase arm IPA and in the quadrature arm QA only in each case with binary drive signals EAS, which between two different electrical states with a high level Switch H and low-level L, controlled. This achieves all constellation points in the positive quadrant of the constellation diagram (constellation points 1000, 1001, 0000 and 0001 assuming Gray coding of all constellation points with exactly one bit deviation between adjacent constellation points). The two phase modulators PM1, PM2 are each also driven with a binary drive signal EAS between two different electrical states with a high-level H and low-level L, which is decided by whether the default constellation point is already correct or by a sign change of I In the second quadrant, the sign of the Q signal component is correct, the sign of the I signal component is negative, and in the third quadrant, the Q component is transferred to one of the other three quadrants of the constellation system by a 180 ° phase modulation both signs negative, in the fourth quadrant only the sign of the Q signal component is negative). Thus, in the square-16 QAM, all modulators MZM1, MZM2, PM1, PM2 can each be driven with a binary drive signal, which considerably simplifies the generation of the electrical drive signals. The same applies to the generation of other higher-valued optical modulation signals.

In the 5 a demultiplexer DEM is shown for generating the aforementioned binary electrical drive signals EAS for square 16 QAM modulation in a binary transmission data SDS. In square 16 QAM modulation, the demultiplexer DEM has four outputs. Evident is the binary control of all four optical components MZM1, MZM2, PM1, PM2 with a 0 for the electrical drive signal EAS with a low electrical state low-level L and a 1 for the electrical drive signal EAS with a high electrical state high level H and a 0 for a 0 ° phase shift and a 1 for a 180 ° phase shift.

The functionality and properties of the serial IQ modulator IQM after the innovation were investigated in simulations. Some of the results are in the 6 for square-16 QAM modulation as eye diagrams (normalized intensity versus time in s). It can be seen that in addition to the slight undershooters in the conventional IQ transmitter KIQS (left, KIQS / NRZ, prior art) in the IQ transmitter IQS with the serial IQ modulator IQM after the innovation (center, IQM / NRZ) still overshoots occur at symbol transitions. However, all overshoots and undershoots disappear when the optical signals are subjected to additional RZ pulse shaping (right, KIQS / IQS / RZ). This is also evident in 7 to recognize the left of an IQ plot (Imaginärteil In the complex signal amplitude (= Q modulation signal component) as a function of time t on real part Re of the signal amplitude (= in-phase modulation signal component) as a function of the time t) for Square-16 -QAM for a conventional IQ transmitter KIQS, in the middle for an IQ transmitter IQS with serial IQ modulator IQM after the innovation and on the right for both embodiments with a zero feedback RZ points. The latter graph shows that with appropriate RZ pulse shaping no difference between the known conventional IQ transmitter KIQS and the IQ transmitter IQS with the serial IQ modulator IQM after the innovation can be seen, so that caused in the innovation advantageous halving the Number of required electrical states of the drive signals has no effect on the behavior of the optical IQ transmitter IQS with the serial IQ modulator IQM after the innovation has.

The 8th Finally, an embodiment of the IQ transmitter IQS with a serial IQ modulator IQM after the innovation, in which each of the Mach-Zehnder modulator and the phase modulator (MZM1 / PM1, MZM2 / PM2) as a dual-drive Mach Zehnder Modulator DD-MZM1, DD-MZM2 are formed. Not mentioned reference numerals can be found in the preceding figures. The two dual-drive Mach-Zehnder modulators DD-MZM1, DD-MZM2 are controlled simultaneously in opposite and in common mode with defined combined electrical control signals.

AS

3dB branching point

CWL

Continuous wave laser

DD-MZM

Dual-drive Mach-Zehnder modulator

THE

demultiplexer

EAS

electrical control signal

H

High-Level

IQS

IQ transmitter

IQM

serial IQ modulator

IPA

In-phase arm

KIQM

conventional IQ modulator (prior art)

KIQs

conventional IQ transmitter (prior art)

KS

3dB coupling point

L

Low Level

M

valence the chosen one modulation

MZM

Mach-Zehnder modulator

NRZ

none Signal feedback

PM

phase modulator

PS

90 ° phase shifter

QA

Quadrature arm

RZ-MZM

Mach-Zehnder modulator for signal zero feedback

SDS

binary transmit data stream

SS

serial optical transmitter (prior art)

TSQ

Carrier signal source

VE

vector

Claims (4)

Optical IQ transmitter for generating high-order Modulation signals with a carrier signal source and an optical IQ modulator with an in-phase arm and a quadrature arm between a 3dB branch point and a 3dB coupling point, with the in-phase arm a first Mach-Zehnder modulator and in the quadrature arm in series with a 90 ° phase shifter another Mach-Zehnder modulator are arranged, both with drive signals with modulation-specific number electrical states be acted upon by a serial IQ modulator (IQM), in the in-phase arm (IPA) in series with the first Mach-Zehnden modulator (MZM1) and in quadrature arm (QPA) in series with the further Mach-Zehnden modulaton (MZM2) in each case a phase modulator (PM1, PM2) is arranged, wherein the two Mach-Zehnder modulators (MZM1, MZM2) with drive signals (EAS) with respect to an IQ modulator (KIQM) without phase modulators (PM1, PM2) halved number of different electrical states and the two phase modulators (PM1, PM2) with drive signals two different electrical states for the realization of phase shifts from 0 ° or 180 ° applied become. Optical IQ transmitter according to claim 1, characterized in that that between the carrier signal source (TSQ) and the 3dB branch point (AS) or behind the 3dB coupling point (AK) an additional Mach Zehnder modulator (RZ-MZM) for pulse-forming signal zero reset is arranged. Optical IQ transmitter according to claim 1 or 2, characterized characterized in that each in the in-phase arm (IPA) and in the quadrature arm (QA) the Mach-Zehnder modulator (MZM1 / MZM2) and the phase modulator (PM1 / PM2) as a common component in the form of a dual-drive Mach-Zehnder modulator (DD-MZM1 / DD-MZM2) are formed. Optical IQ transmitter according to one of claims 1 to 3, characterized in that for generating square-wave optical modulation signals the two Mach-Zehnder modulators (MZM1, MZM2) and the two Phase modulators (PM1, PM2) exclusively with binary control signals (EAS) with two different electrical states (H, L) and the generation of the binary control signals (EAS) from a binary Transmission data stream (SDS) with a demultiplexer (DEM) with a division ratio of 1: 4 he follows.