# Patent

## Title
Optical line terminal transmitting device for next generation optical access networks

## Abstract

An Optical Line Terminal (OLT) transmitting device is provided, the device comprising an IQ modulator having a first input, a second input and an output, a first digital analog converter (DAC) directly coupled to the first input of the IQ modulator, a second digital analog converter (DAC) directly coupled to the second input of the IQ modulator; an optical filter directly coupled to the output of the IQ modulator.

## Background
RELATED APPLICATIONS
This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/EP2012/061747, filed on Jun. 19, 2012, which claims priority to and benefit of European Patent Application No. 11172272.4, filed Jun. 30, 2011, the contents of each of which are hereby incorporated by reference herein.
FIELD OF THE INVENTION
The invention relates to devices for optical communication networks. In particular, the invention relates to a Optical Line Terminal (OLT) transmitting device for Next Generation Optical Access networks.
BACKGROUND OF THE INVENTION
Recently, a lot of effort and energy is put into designing next generation optical access (NGOA) networks that can alleviate the data rate bottleneck that is present in current access networks. Many different next generation optical access (NGOA) networks architectures have been proposed. A very promising architecture has been recently been proposed in H. Rohde “Next Generation Optical Access: 1 Gbit/s for everyone”, invited talk at ECOC 2009.
Coherent detection is a key factor to enable the key parameters: 1 Gbit/s sustained symmetrical data rate per user, up to 100 km reach and a splitting factor of up to 1000.
FIG. 1 is a schematic representation of a high level architecture of a next generation optical access (NGOA) network.
FIG. 2 is a schematic representation of possible DAC based realization of an OLT transmitter and the optical spectrum at the output.
The data that is to be sent is first pre-processed by a DSP unit and digitally up-converted to intermediate frequencies. Subsequently the real and imaginary part are separated and fed to two DACs.
The outputs of the DACs are then used to modulate the complex signal on an optical carrier using an IQ modulator. On the right side of FIG. 2 the optical spectrum of the transmitter output is shown. In this illustration 3 subcarriers are shown.
In between the subcarriers a guardband is present in order to allow the signal from the ONU to be sent back to the OLT on the same fiber without being impaired by back-reflections. The spectral guardband that is required is at least as wide as the bandwidth of the subcarriers itself and this means that for the DACs less then half of the bandwidth can be used effectively for the generation of subcarriers.
The main disadvantage of this implementation is that the guardband required in between the subcarriers reduces the efficiency of the DAC (the number of subcarriers that can be generated with a single OLT).
In conventional DAC based realization of OLT transmitters, DAC are used in association with guardbands between the subcarriers, which may result in an inefficient use of the DACs
Alternatively analogue electrical IQ mixers could be used to up-convert all individual subcarriers. However, these analogue components are relatively expensive.
The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution which allows increasing the efficiency of an OLT transmitter in a cost effective way.
SUMMARY OF THE INVENTION
In order to overcome the above-described need in the art, the present invention discloses an Optical Line Terminal (OLT) transmitting device comprising an IQ modulator having a first input, a second input and an output, a first digital analog converter (DAC) directly coupled to the first input of the IQ modulator, a second digital analog converter (DAC) directly coupled to the second input of the IQ modulator; an optical filter directly coupled to the output of the IQ modulator.
In a next embodiment, the optical filter is a periodic filter.
In a further embodiment, the IQ modulator has a third input, and the tunable laser is directly coupled to the third input of the IQ modulator.
It is also an embodiment that the tunable laser is configured to be tuned with the optical filter.
In a next embodiment of the invention, the optical filter is configured to drift so that it can lock with the tunable laser.
The device provided, in particular, bears the following advantages:

    
    
        a) It allows increasing the efficiency of an OLT transmitter.
        b) It has relatively broad applications and it can be easy implemented.
        c) It effectively reduces the effective cost of the OLT per signal generated, since each DAC can generate more signals.
    
    





BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained by way of example in more detail below with the aid of the attached drawings.
FIG. 1 is a schematic representation of a high level architecture of a next generation optical access (NGOA) network.
FIG. 2 is a schematic representation of possible DAC based realization of an OLT transmitter and the optical spectrum at the output.
FIG. 3 is a schematic representation of possible DAC based realization of an OLT transmitter and the optical spectrum at the output according to an embodiment of the invention.



DESCRIPTION OF THE INVENTION
Illustrative embodiments will now be described with reference to the accompanying drawings to disclose the teachings of the present invention. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
As regards the description of FIGS. 1 and 2, reference is made to the background of the invention.
FIG. 3 is a schematic representation of possible DAC based realization of an OLT transmitter and the optical spectrum at the output according to an embodiment of the invention.
The data that is to be sent is first pre-processed by a DSP unit and digitally up-converted to intermediate frequencies. Subsequently the real and imaginary part are separated and fed to two DACs.
The outputs of the DACs are then used to modulate the complex signal on an optical carrier using an IQ modulator.
An optical periodic filter is used in conjunction to the IQ modulator in order to increase the usage of the DACs.
The optical periodic filter has two outputs (Direction 1 and Direction 2), each of them provides 3 WDM signals with the required guardbands. On the right side of FIG. 3 the optical spectrum of the transmitter output is shown. In this illustration 6 subcarriers are shown.
The main advantage of the proposed method is that the by adding the periodic filter to the output of the OLT, more signals can be generated per DAC. This effectively reduces the effective cost of the OLT per signal generated.
One challenge that arises is that the tunable laser must be tuned to the periodic filter.
This can be realized by fixing the filter and laser to a certain grid.
However, more cost effective might be to let the filter drift and lock the laser of the OLT to the periodic filter.
This allows the periodic filter to drift as the laser follows the filter.
The present invention is not limited to the details of the above described principles. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalents of the scope of the claims are therefore to be embraced by the invention. Mathematical conversions or equivalent calculations of the signal values based on the inventive method or the use of analogue signals instead of digital values are also incorporated.
LIST OF ABBREVIATIONS
ONU: Optical Network Unit
OLT: Optical Line Terminal
DAC: Digital Analog Converter

## Claims

1. An Optical Line Terminal (OLT) transmitting device comprising:
an IQ modulator having a first input, a second input and an output;
a first digital analog converter (DAC) directly coupled to the first input of the IQ modulator;
a second digital analog converter (DAC) directly coupled to the second input of the IQ modulator;
an optical filter directly coupled to the output of the IQ modulator, wherein the output of the IQ modulator provides a wavelength-division multiplexing (WDM) signal including a plurality of subcarriers which are not separated by guardbands wider than the bandwidth of the subcarriers,
wherein the optical filter is a periodic filter, having two outputs each destined to serve a different transmission fiber, wherein the periodic filter is configured to provide different optical signals at each of the two outputs, and
wherein each of the two outputs of the optical periodic filter provides a different wavelength-division multiplexing (WDM) signal including subcarriers separated by guardbands which are at least as wide as the bandwidth of the subcarriers.

2. The Optical Line Terminal (OLT) transmitting device of claim 1 wherein the IQ modulator has a third input, and
wherein a tunable laser is directly coupled to the third input of the IQ modulator.

3. The Optical Line Terminal (OLT) transmitting device of claim 2, wherein the tunable laser is configured to be tuned with the optical filter.

4. The Optical Line Terminal (OLT) transmitting device of claim 2, wherein the optical filter is configured to drift to lock with the tunable laser.

5. A method of modulation in an Optical Line Terminal (OLT), comprising:
generating an IQ modulated output signal based on a first digital-to-analog (DAC) converted signal and a second digital-to-analog (DAC) converted signal such as to provide a wavelength-division multiplexing (WDM) signal including a plurality of subcarriers which are not separated by guardbands wider than the bandwidth of the subcarriers; and
filtering the generated IQ modulated output signal using a periodic optical filter having two outputs each destined to serve a different transmission fiber, such as to provide different optical signals at each of the two outputs of the periodic optical filter,
wherein each of the two outputs of the optical periodic filter provides a different wavelength-division multiplexing (WDM) signal including subcarriers separated by guardbands which are at least as wide as the bandwidth of the subcarriers.