WO2012110080A1

WO2012110080A1 - Optical modulator

Info

Publication number: WO2012110080A1
Application number: PCT/EP2011/052209
Authority: WO
Inventors: Erich Gottwald
Original assignee: Nokia Siemens Networks Oy
Current assignee: Nokia Solutions and Networks Oy
Priority date: 2011-02-15
Filing date: 2011-02-15
Publication date: 2012-08-23
Anticipated expiration: 2013-08-15

Abstract

An optical modulator is provided comprising a first branch and a second branch, both being connectable to an input, in particular to a light source; wherein the first branch comprises a first modulator; wherein the second branch comprises a second modulator in series connected with a polarization transformer; comprising a combining unit with two inputs that combines the optical fields of the first branch and the second branch; wherein the first modulator is controlled by a first electrical signal comprising a first group of data channels, each utilizing a different carrier frequency; wherein the second modulator is controlled by a second electrical signal comprising a second group of data channels, each utilizing a different carrier frequency; and wherein the carrier frequencies of the first group and the carrier frequencies of the second group are each shifted by a predetermined offset frequency. Also a demodulator is suggested for processing signals conveyed by such optical modulator. Furthermore, methods for modulating and demodulating these signals are provided.

Description

Optical Modulator The invention relates to an optical modulator and to a de^¬ modulator as well as to a method for providing an optically modulated signal and to a method for processing an opti^¬ cally modulated signal. A passive optical network (PON) is a promising approach regarding fiber-to-the-home (FTTH) , fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) scenarios, in particu^¬ lar as it overcomes the economic limitations of traditional point-to-point solutions.

Several PON types have been standardized and are currently being deployed by network service providers worldwide. Con^¬ ventional PONs distribute downstream traffic from the opti^¬ cal line terminal (OLT) to optical network units (ONUs) in a broadcast manner while the ONUs send upstream data pack^¬ ets multiplexed in time to the OLT. Hence, communication among the ONUs needs to be conveyed through the OLT involv^¬ ing electronic processing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network.

In fiber-optic communications, wavelength-division multi^¬ plexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry dif^¬ ferent signals. This allows for a multiplication in capac^¬ ity, in addition to enabling bidirectional communications over one strand of fiber. WDM systems are divided into different wavelength patterns, conventional or coarse and dense WDM. WDM systems provide, e.g., up to 16 channels in the 3rd transmission window (C- band) of silica fibers of around 1550 nm. Dense WDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system may use 40 channels at 100 GHz spacing or 80 channels at 50 GHz spac- ing. Some technologies are capable of 25 GHz spacing. Am^¬ plification options enable the extension of the usable wavelengths to the L-band, more or less doubling these num^¬ bers . Optical access networks, e.g., coherent Ultra-Dense Wave^¬ length Division Multiplex (UDWDM) networks, are deemed to be a promising approach for future data access.

Data transmission of spectrally densely spaced wavelengths is utilized by applications as Next Generation Optical Ac^¬ cess (NGOA) systems allowing high data rates of, e.g., 100 Gbit/s .

In these optical scenarios, a multitude of optical wave- lengths are required in order to be individually modulated. Such optical wavelengths may have a spectral distance of a few gigahertz and can be used either for a ultra dense wavelength grid optical access system like NGOA where each user may be assigned a wavelength of his own or for a transmission of high data rates such as 100 Gbit/s where a multitude of wavelengths are bundled and are transmitted over a small spectral range.

Providing these individual wavelengths by several discrete lasers leads to a huge amount of laser sources that require a significant amount of precision and thus involve high costs. As an alternative, modulating a multitude of single sidebands on an optical carrier also leads to significant costs because of the electronics involved needing to cope with high frequencies required. The problem to be solved is to provide an efficient mecha^¬ nism supplying a multitude of individually modulated opti^¬ cal wavelengths in particular at a spectral distance amounting to a few GHz from a single laser source utilized for an optically coherent UDWDM access network.

This problem is solved according to the features of the in^¬ dependent claims. Further embodiments result from the de^¬ pending claims.

In order to overcome this problem, an optical modulator is provided

- comprising a first branch and a second branch, both being connectable to an input, in particular to a light source;

- wherein the first branch comprises a first modula^¬ tor;

- wherein the second branch comprises a second modu^¬ lator in series connected with a polarization transformer;

- comprising a combining unit with two inputs that combines the optical fields of the first branch and the second branch;

- wherein the first modulator is controlled by a

first electrical signal comprising a first group of data channels, each utilizing a different carrier frequency;

- wherein the second modulator is controlled by a

second electrical signal comprising a second group of data channels, each utilizing a different car^¬ rier frequency;

- wherein the (corresponding) carrier frequencies of the first group and the carrier frequencies of the second group are each shifted by a predetermined offset frequency. The input source and/or signal could be realized as a la^¬ ser, in particular as a local oscillator laser.

The electrical signal fed to the modulator in particular comprises several modulated channels, i.e. signals modu^¬ lated at carrier frequencies that are separated, e.g., by a few GHz. Hence, a (virtual) point-to-point connection can be established via each such channel. Due to the modulation onto the optical carrier, each frequency band associated for each channel corresponds to a wavelength range to be used individually for an optical point-to-point connection.

The first group of data channels and the second group of data channels may in particular use the same frequency car- riers enabling the same spacing of channels separated by, e.g., a few GHz. However, the single carriers for the first group of data channels and for the second group of data channels are shifted by said offset frequency. This approach allows for generating and detecting of a multitude of individually modulated optical wavelengths with a spectral distance of a few GHz based on a single laser source in an effective cost saving way that could be used at an OLT site in an optical (coherent) UDWDM access net- work for establishing virtual point to point connections.

In an embodiment, the polarization transformer is a 90 degree polarization transformer. The polarization transformer could be realized by a λ/4 plate of a birefringent material.

In another embodiment, the offset frequency is in the range of lHz to 10MHz, in particular in the range between 10Hz and 1MHz. In a further embodiment, the offset frequency is below a lower cut-off frequency of the signal of the data channels.

This enables utilizing the offset frequency at the receiver in an efficient manner to determine the signals of orthogo^¬ nal polarizations.

In a next embodiment, the data channels are distributed evenly among the first group and the second group of data channels.

Hence, N/2 data channels (i.e. data channel 1 to N/2) can be processed via the first modulator and also N/2 data channels (i.e. data channels N/2+1 to N) can be processed via the second modulator.

It is also an embodiment that the first group of data chan^¬ nels and the second group of data channels are modulated at different polarizations, in particular at polarizations that are orthogonal to each other.

Hence, polarization multiplexing can be used.

Pursuant to another embodiment, the first modulator and/or the second modulator is at least one of the following:

- a Mach-Zehnder modulator;

- a two-beam interferometer;

- a Michelson interferometer. According to an embodiment, the optical modulator is ar^¬ ranged in an optical unit, in particular in a centralized or a decentralized optical unit, in an optical line termi^¬ nal or in an optical network unit. The problem stated above is also solved by a demodulator for processing a signal provided by the optical modulator as described herein, comprising a processing unit, in par- ticular a software defined radio, that is arranged to at least partially compensate a rotation and/or time shift of the signal received by providing a re-alignment of the or^¬ thogonally polarized signals. A software defined radio is a synonym for a signal processing unit that's functionality is configurable by software.

According to an embodiment, the re-alignment of the or^¬ thogonally polarized signals is achieved by minimizing a beat between the two signals received.

This can be done by analyzing the incoming signal at the receiver and minimizing the frequency difference based on the frequency offset.

The problem cited above could also be solved by a method for providing an optically modulated signal,

- wherein the input signal is fed by a splitter to a first branch and to a second branch;

- wherein the input signal is modulated by the first branch via a first electrical signal comprising a first group of data channels, each utilizing a dif^¬ ferent carrier frequency;

- wherein the input signal is modulated via a second electrical signal comprising a second group of data channels, each utilizing a different carrier fre^¬ quency and the polarization of the modulated signal is transformed, in particular by 90 degrees;

- wherein the optical fields of the first branch and the second branch are combined by a combining unit;

- wherein the (corresponding) carrier frequencies of the first group and the carrier frequencies of the second group are each shifted by a predetermined offset frequency.

Also, a method is provided for processing a signal that has been modulated according to the approach presented herein, wherein a rotation and/or time shift of the signal received is at least partially compensated by providing a re^¬ alignment of the orthogonally polarized signals.

Furthermore, the problem stated above is solved by a commu^¬ nication system comprising at least one device as described herein .

Embodiments of the invention are shown and illustrated in the following figures:

Fig.l shows a modulator structure comprising two modula^¬ tor branches, each comprising an optical modulator that is controlled by an electrical signal with several data channels;

Fig.2 shows a more generalized structure compared to

Fig.1; shows an optical spectrum comprising six channels Chi to Ch6 conveyed at orthogonal polarizations (channels Chi to Ch3 are of a first and channels Ch4 to Ch6 are of a second polarization, wherein the first and the second polarizations are orthogo nal to each other) around a frequency fO; shows an exemplary diagram of an N-carrier transceiver with LO-carrier suppression.

The approach presented suggests generating and/or detecting of a multitude of individually modulated optical wave^¬ lengths with a spectral distance of, e.g., a few GHz from a single laser source. Promising applications are, e.g., ultra dense wavelength grid optical access systems (also referred to as NGOA) pro^¬ viding for each subscriber or user (or service) a separate wavelength (i.e. at least one wavelength range) . Also, a particular wavelength (i.e. wavelength range) can be assigned to at least one subscriber, user or service.

The approach presented enables an arbitrary split between broadcast data and individual data and allows for a pair- wise splitting between the two adjacent channels at the same carrier frequency in down-stream direction.

The approach allows generating optically modulated carriers twice via dual side-band spectra using, e.g., polarization multiplexing to enable a separation between two channels at (substantially) the same optical frequency (wavelength) .

Advantageously, no single side-band modulation is neces- sary. Instead of a single side band modulator (or an IQ modulator) that may typically comprise two nested Mach Zehnder modulators, two individual Mach Zehnder modulators can be used, wherein the two signal spectra can be set to orthogonal polarizations by optical means and can be super- posed by, e.g., a polarization combiner.

Fig.l shows a modulator structure 120 comprising two modu^¬ lator branches 118 and 119, each comprising an optical modulator that is controlled by an electrical signal 116, 117. The modulator structure 120 also shows a coupler 104 (also referred to as splitter) to which an input signal (light) 101 is fed. The coupler 104 is further connected to a coupler 105 and to a coupler 106, thereby conveying the input signal 101.

The coupler 105 conveys the incoming light via a phase ad^¬ justment 110 and a phase modulator 111 to a coupler 107 and also directly via a phase modulator 112 to said coupler 107.

Accordingly, the coupler 106 conveys the incoming light via a phase adjustment 113 and a phase modulator 114 to a cou^¬ pler 108 and also directly via a phase modulator 115 to said coupler 108.

The output of the coupler 107 is fed to a coupler 109 and the output of the coupler 108 is fed via a polarization transformer 103 to the coupler 109. The polarization transformer 103 could be realized by a λ/4-plate of a birefrin- gent material. The coupler 109 (that could be realized as a polarization combiner or splitter) provides an output signal 102.

The electrical signal 116 comprises several data channels 1 to N/2, wherein each data channel uses a different carrier frequency f_c, e.g., 2GHz, 5GHz, 8GHz, 11GHz, etc.

The electrical signal 117 comprises several data channels N/2+1 to N, wherein each data channel uses a different car^¬ rier frequency f_c, e.g., 2GHz+s , 5GHz+s , 8GHz+s , llGHz+ε , etc.

The modulator 111 and the modulator 112 are supplied by the same electrical signal with different prefix. The electri^¬ cal signal 116 is in particular modulated with carrier fre- quencies (utilizing, e.g., amplitude modulation, quadrature amplitude modulation or phase modulation) onto the optical carrier provided by the input signal 101. The same applies to the modulators 114, 115 and the electrical signal 117 accordingly .

Fig.3 shows an optical spectrum comprising six channels Chi to Ch6 conveyed at orthogonal polarizations (channels Chi to Ch3 are of a first and channels Ch4 to Ch6 are of a sec^¬ ond polarization, wherein the first and the second polarizations are orthogonal to each other) around a frequency f0. The channels Chi and Ch4 use substantially the same frequency, but are shifted by said frequency offset ε. This applies to the channels Ch2, Ch5 and the channels Ch3, Ch6 accordingly .

Hence, the modulator branch 118 modulates N/2 data channels at a first (parallel) polarization and the modulator branch 119 modulates N/2 data channels at a second (orthogonal) polarization. A total of N data channels can be transmitted accordingly. The data channels of the electrical signal 116 use the same carrier frequencies as the data channels of the electrical signal 117, only that the carrier frequen^¬ cies are shifted by a predetermined frequency offset amounting to ε. This frequency offset ε may be in the range from lHz (or 10Hz) to lMHz . The frequency offset ε may be less than a lower cut-off frequency of the data signal (the signal of the electrical data channels) .

Subscribers to broadband access services may each use a separate carrier to convey their individually assigned data channel. The modulator branches 118, 119 allow a modulation of the data channels onto the optical carrier provided by the input signal 101. The split into different polariza^¬ tions provided by the modulator branches 118, 119, in par^¬ ticular due to the polarization transformerl03, allows to efficiently use the spectrum for different data signals.

The modulator shown in Fig.l may be implemented in an OLT that serves several ONUs via at least one optical fiber. The ONU attached to a fiber receives the optical signal comprising both polarizations, i.e. the optical signal com- prising all electrical channels that have been modulated onto this optical signal. The ONU may be configured to process at least one such channel of at least one polariza^¬ tion .

Fig.4 shows an exemplary diagram of an N-carrier trans- ceiver with LO-carrier suppression. The transceiver comprises a transceiver module 401 and a digital processing unit 402. An optical input signal "Optical In" is conveyed to a receiver 405. A laser serves as a local oscillator LO 403 and conveys a signal via a splitter 404 to the modula- tor structure 120 (see Fig.l) and to the receiver 405. The receiver 405 conveys two analog signals to A/D converters 406, 407. The digital signal provided by the A/D converters 406, 407 is processed via a software defined radio (SDR) 408 and further via a processing unit 409, which produces N digital streams (or channels) at a data rate amounting to, e.g., lGbit/s. The processing unit 409 provides in particu^¬ lar framing, a general frame procedure, a forward error correction, OAM services, and a broadcast capability. In the opposite direction for producing an optical output signal "Optical Out" 102, N digital data streams (or chan^¬ nels) are each fed to a processing unit 417 to 419 and fur^¬ ther via a QPSK modulation unit 414 to 416 to a carrier generation and modulation unit 413 providing N/2 channels on orthogonal polarizations. The output of each orthogonal polarization is converted to an analog signal by D/A con^¬ verters 411, 412 and are forwarded to the modulation struc^¬ ture 120 as analog electrical signals 116 and 117 (see also Fig.l) . The modulation structure 120 provides the optical output signal "Optical Out" 102 utilizing the input signal 101 as described above with regard to Fig.l.

The SDR 408 receives both polarizations of the signal 102 (conveyed from a different sender comprising the same modu- lation structure 120 as described) , wherein the polariza^¬ tions may be out of alignment. Hence, the SDR 408 needs to correct this displacement and to turn it back into align- ment such that the different polarizations can be further processed separately from each other.

The receiver 405 may provide two signals

wherein I i and I ₂ are the signals of orthogonal polariza^¬ tions .

The matrix M corresponds to the rotation and the time shift of the transmission network. The S DR 408 determines the inverse matrix M^_1, which compensates the rotation and the time shift and allows further processing of the separate orthogonal polarizations. Then, the parameters a and φ are determined such that S i=I i and S2= I ₂ .

The frequency offset ε added to the signal prior to its transmission allows the S DR 408 on the receiving side to determine a beat that could be minimized in order to sepa^¬ rate the orthogonally polarized signals.

This approach does not require a single sideband modulator with a phase locked coupling; instead, a common MZM modula- tor can be used. It is also an advantage that only two in^¬ stead of four HF signals are to be processed.

This approach allows flexibly adjusting a downstream capacity. At the receiver, e.g., an ONU, only minor software changes are required.

Fig.2 shows a more generalized structure compared to Fig.l. Instead of a MZM mentioned, different modulator schemes may apply as indicated by blocks 201 and 202. A first branch 201 provides a modulation with the electrical signal 116. A second branch 202 provides a modulation with the electrical signal 117. In addition, the second branch 202 provides a 90 degrees polarization shift by the polarization transformer 103.

The first and/or the second branch may comprise a single MZM or an OFDM modulator via, e.g., two MZMs . The same ap^¬ plies to the second branch. Instead of the MZM, a Michelson interferometer, a two-beam interferometer or the like could be used.

List of Abbreviations:

A/D Analog-to-Digital

Ch Channel

D/A Digital-to-Analog

DQPSK Differential QPSK

FEC Forward Error Correction

FTTB Fiber-to-the-Business

FTTC Fiber-to-the-Curb

FTTH Fiber-to-the-Home

GFP General Frame Procedure

HF High Frequency

LO Local Oscillator

MZM Mach-Zehnder Modulator

NGOA Next Generation Optical Access

OAM Operation, Administration and Maintenance

OLT Optical Line Terminal

ONU Optical Network Unit

PolMux Polarization Multiplex

PON Passive Optical Network

PSK Phase Shift Keying

QPSK Quadrature PSK

RX Receiver

SDR Software Defined Radio

SSB Single Sideband

SSBM SSB Modulator

SW Software

UDWDM Ultra Dense WDM

WDM Wavelength Division Multiplexing

Claims

An optical modulator

- wherein the first branch comprises a first modula^¬ tor;

- wherein the first modulator is controlled by a

- wherein the second modulator is controlled by a

- wherein the carrier frequencies of the first group and the carrier frequencies of the second group are each shifted by a predetermined offset frequency.

The optical modulator according to claim 1, wherein the polarization transformer is a 90 degree polariza^¬ tion transformer.

The optical modulator according to any of the preced^¬ ing claims, wherein the offset frequency is in the range of lHz to 10MHz.

The optical modulator according to any of the preced^¬ ing claims, wherein the offset frequency is below a cut-off frequency of the signal of the data channels. The optical modulator according to any of the preced^¬ ing claims, wherein the data channels are distributed evenly among the first group and the second group of data channels.

The optical modulator according to any of the preced^¬ ing claims, wherein the first group of data channels and the second group of data channels are modulated at different polarizations, in particular at polariza^¬ tions that are orthogonal to each other.

The optical modulator according to any of the preced^¬ ing claims, wherein the first modulator and/or the second modulator is at least one of the following:

- a Mach-Zehnder modulator;

- a two-beam interferometer;

- a Michelson interferometer.

The optical modulator according to any of the preced^¬ ing claims, wherein the optical modulator is arranged in an optical unit, in particular in a centralized or a decentralized optical unit, in an optical line ter^¬ minal or in an optical network unit.

A demodulator for processing a signal provided by the optical modulator according to any of the preceding claims, comprising a processing unit, in particular a software defined radio that is arranged to at least partially compensate a rotation and/or time shift of the signal received by providing a re-alignment of the orthogonally polarized signals.

The demodulator according to claim 9, wherein the realignment of the orthogonally polarized signals is achieved by minimizing a beat between the two signals received .

A method for providing an optically modulated signal, - wherein the input signal is fed by a splitter to a first branch and to a second branch;

- wherein the input signal is modulated by the second branch via a second electrical signal comprising a second group of data channels, each utilizing a different carrier frequency and the polarization of the modulated signal is transformed, in particular by 90 degrees;

A method for processing an optically modulated signal that has been modulated according to the method of claim 11, wherein a rotation and/or time shift of the signal received is at least partially compensated by providing a re-alignment of the orthogonally polarized signals .

The method according to claim 12, wherein the realignment of the orthogonally polarized signals is achieved by minimizing a beat between the two signals received .

A communication system comprising at least one device according to any of claims 1 to 10