The present invention generally relates to a passive optical network or PON and 5more precisely to a device and a method to finely adapt to relative wavelength drift due to temperature evolution in a network comprising a frequency splitter based on Mach-Zehnder frequency splitter or alike.
Passive optical networks are increasingly used to give network access to residential users or ensure mobile backhauling for instances.
In an attempt to increase the number of ONUS to be served by one OLT, WDM for Wavelength Division Multiplexing technologies have been developed. These technologies take advantage of multiplexing several signals using different wavelengths on a single Fiber. Frequency splitter, equipment 1.2 on the figure, is then necessary to separate the different wavelengths before the power splitter 1.3. This equipment is referred as a frequency splitter. Different techniques could be used to achieve the frequency splittering. We can cite thin films based systems, interference cavities as AWG for Array Wavelength Gratings and FBG for Fiber Bragg Gratings. We focus on this later in this document.
Wavelength extraction from an optical signal based on Fiber Bragg Gratings is done using the so-called Mach-Zehnder components. Such a component is illustrated on
Depending on the number of wavelength bands multiplexed in the signal, the optical filtering can be done by several Mach-Zehnder components daisy chained. Each of the components drops one of the wavelengths from the input signal depending on its own nominal wavelength. We call nominal wavelength of a Mach-Zehnder component or of a FBG, the wavelength for which the component is reflexive.
The nominal wavelength of a Mach-Zehnder component depends on the nominal wavelength of the two Fiber Bragg Gratings used inside. One can achieve good granularity of the extracted signal of a few GHz. The nominal wavelength of a FBG depends on the particular pattern imprinted in the core of the Fiber and the operating temperature of the component. Depending on the temperature the nominal wavelength of a FBG moves. Typically for a temperature in a range from −40° to 80° centigrade the nominal wavelength can moves from −0.6 nm to +0.6 nm which corresponds to a frequency jitter over a band of almost 200 GHz. The terminal equipment being also typically in a non controlled environment they could be subject to similar drifts.
For that reason, these components are usually used in a temperature-controlled environment. For flexibility in the network deployment and cost reason it would be advantageous to get rid of this constraint.
The invention aims to solve these problems by devices and methods to allow to estimate the drift induced by temperature variation in a network comprising a frequency splitter based on a Mach-Zehnder, or alike component, and to track it over time. It is based on adding mirrors to the unused port of the Mach-Zehnder components. Doing so, both OLT and ONU are able to scan a band of frequencies. The frequency corresponding to the nominal wavelength of the component will be reflected on the other port on the same side of the Mach-Zehnder while other frequencies will go through the Mach-Zehnder to be reflected by the mirror and come back to the emitter. By measuring the reflected signal while scanning frequencies, the actual nominal wavelength of the component can be determined.
The invention concerns a device aimed to be used as a frequency splitter comprising at least one component with a Mach-Zehnder topology. In case of several, the components are daisy chained; each component with a Mach-Zehnder topology comprising one input port to receive an input signal typically multiplexing several bands using different wavelengths; one extracting port to output the band of the signal corresponding to the nominal wavelength of said component; one output port to output the input signal but the extracted band, this output port being connected to the input port of next component if any or being unused for the last component of the chain or the only component and one add port which is unused wherein all unused ports are equipped with a reflecting means.
The invention further concerns a method for coupling an emitting device to a frequency splitter as described, said emitting device being connected to the input port or the extracting port of one of the components with a Mach-Zehnder topology, said emitting device operating using one operating level or a set of operating levels, comprising for the emitting device a step of initializing its operating level to a first wavelength; a step of sending a signature signal using its operating level; a step of measuring the power of the returned signal to estimate the presence of a reflection signal; these steps being repeated over a band of operating levels and further comprising a step of determining the operating level, or the set of operating levels, for which the power of the returned signal is minimum and a step of setting the operating level, or the set of operating levels, to the determined operating level, or operating levels, for which the power of the returned signal is minimum.
In a particular embodiment, the measure of the power of the returned signal is done by modulation and filtering.
In a particular embodiment, the measure of the power of the returned signal is done by synchronous detection.
In a particular embodiment, the step of measuring the power of the returned signal comprises a step of definition of a temporal sliding window; a step of measuring the power of the return signal on the temporal sliding window; a step of moving the sliding window in a range within the one from origin to a maximum corresponding to the round trip time of the total transmission path coupled with a gain to get the expected signal.
In a particular embodiment, the width of the temporal sliding window is chosen to be the duration of the emitted signature signal. In a particular embodiment, a power splitter being between the frequency splitter and the emitting device, a gain corresponding to the attenuation due to this power splitter is applied for position of the sliding window corresponding to reflexion point behind the power splitter.
The invention further comprises a method for tracking the variation of the nominal wavelength of components with Mach-Zehnder topology in a frequency splitter over time characterized in that a coupling method as described is applied on a regular basis. The characteristics of the invention will emerge more clearly from a reading of the following description of an illustrative embodiment, the said description being produced with reference to the accompanying drawings, among which:
There are many other alternatives to build an Optical Add-Drop Multiplexer having the same topology as the Mach-Zehnder component. A first example of such component is based on a Fiber Bragg gratings surrounded with two optical circulators like the one described in US patent published with U.S. Pat. No. 5,909,310. A second example is based on free space optics as illustrated by US patent published with U.S. Pat. No. 6,198,857. The present document focuses on frequency splitters based on Mach-Zehnder component but the invention applies on all frequency splitters based on components with the Mach-Zehnder topology.
There are two solutions for the return path or uplink. One can use the same wavelength in both directions. Signals coming from the ONU toward the OLT using the same wavelength will be added to the signal by the Mach-Zehnder components and multiplexed in the output signal that goes to the OLT.
Another solution is to use a different set of wavelengths for the uplink. In this case, one can use Mach-Zehnder components having two different nominal wavelengths. These components are built replacing the FBGs inside by two FBGs each having a nominal wavelength that fits one of the desired wavelengths.
Such passive frequency splitter works well as long as the different wavelengths are well defined and does not drift over time. This means that the frequency splitter should be set up in a temperature controlled environment to avoid the drift of the nominal wavelength of the components due to temperature changes.
To be able to relax the constraint of controlling the temperature, the invention proposes a device and a method to be able to estimate the nominal wavelength of each component inside the frequency splitter from each side of the transmission, namely from the OLT and the ONU.
To achieve this, it is proposed to set up a mirror on each unused port 3.8 of the Mach-Zehnder components.
It comes that an ONU that needs to find out the nominal wavelength of a Mach-Zehnder component can do a scan of the wavelengths. All the wavelengths but the nominal one will be reflected to the emitter. Then measuring the reflected signal allows determining the nominal wavelength of the component.
It comes that placing a mirror on 4.3 port of each Mach-Zehnder component of the passive frequency splitter enables all ONUs to determine the nominal wavelength of the component to which they are connected.
Symmetrically, placing a mirror on the unused 4.4 port of the last Mach-Zehnder component 3.3 of the frequency splitter allows the OLT to do the same. The only difference is that there will be as many holes in the spectral response of the reflected signal that there are Mach-Zehnder components in the frequency splitter. Only wavelengths that are different from all the nominal wavelengths of the Mach-Zehnder components will go through all of them to reach the mirror placed on the 4.4 port of the last one and be reflected to the emitter, here the OLT, while a signal having the wavelength of one of the Mach-Zehnder components will be extracted and will not be reflected toward the OLT.
The invention is based on using such modified passive frequency splitter equipped with mirrors on all the otherwise unused ports of the Mach-Zehnder components. It is advantageously coupled with end communicating device equipped with means to scan a band of wavelengths to determine the effective nominal wavelength of each Mach-Zehnder components at the moment. The communicating device is also equipped with means to adjust the wavelengths used in communication, called its operating level, to those measured during the scan. For an emitting device emitting a signal over a single band of wavelength, this wavelength is called its operating level. For an emitting device emitting a signal multiplexing multiple bands using different wavelengths, the set of used wavelengths forms a set of operating levels. Advantageously the steps of measure and adjustment are done periodically for monitoring the drift due to temperature of the passive frequency splitter.
It should be understood that the emitter and the receiver could also be subject to drift. Due to the fact that the frequency splitter is passive and cannot therefore adjust its operating level, the adaptation of the operating level of the emitter and/or receiver will allow them to adjust themselves to the frequency splitter. In this process the drift of all the elements is corrected.
According to the actual setup of the network, some problems could occur due to the attenuation of the reflected signal to be measured and to dazzle operation due to splicing generating unwanted back reflection close to the emitter.
The propagation range of the reflected signal is in the worse case twice span of the network. As contemplated networks could be 40 km long and include a power splitter, attenuation could reach typically around 65 dB. Advantageously, the emitted power is the maximum emitted power allowed for an ONU while exchanging data. It comes that the reflected signal to detect is weak as compared to the noise sources in the detector. We have typically to face shot noise, Johnson noise that is a thermal noise dependent on the circuit and dark current of photodiode. Flicker noise can be neglected for frequencies under 10 KHz.
In such a situation, special attention should be paid to be able to detect the reflected signal and to discriminate it from eventual splicing back reflection. The direct detection is not reasonable. Instead, modulation-filtering or synchronous detection are two perfectly suited solutions. Filtering and discrete estimation can also be contemplated.
This optical signal 7.3 takes the form of Ps=cos(Ωt)ps, where psis the emitting power.
The signal is reflected by the mirror 7.4 and comes back to the emitter in attenuated form 7.5 s=Pse−αL-βN, where e−αL is due to propagation loss and e−βN due to other losses. This received signal is transformed back to an electrical signal 7.7 by the photodiode 7.6. The received electrical signal is in the form:
i(t)=e−αL-βN ps cos(Ωt)+b(t);
It comes that the problem is to detect the attenuated signal from the noise b(t).
The solution is based on using a modulation and filtering block 7.8. This block achieves an integration of the received signal by the integrator 7.9 to eliminate the mean of the black current. This is followed by an amplification 7.10 while the block 7.11 realizes an estimation. Advantageously this estimation is done over several realizations in case the level of the expected signal is low compared to noise.
A locking agent 7.12 which controls the emitter uses the filtered result of this block. This allows achieving the scan of the desired bandwidth of wavelength to determine the hole in the received signal and thus the nominal wavelength of the Mach-Zehnder component.
Alternatively a solution based on synchronous detection could be used as illustrated on
Ss=sscos(Ωt) cos(ωst+φs);
The output of the amplifier 8.10 is then multiplied by the cos(Ωt) signal before a low band filter 8.13 to achieve the synchronous detection.
Alternatively the module 8.13 could consist in an integrator.
In some embodiment of the invention, dazzle could become an issue. A typical PON as contemplated in this document is sketched on
Considering the dazzle for a signal emitted by the OLT, it comes that the reflected signal from a close splice and the reflected signal from the frequency splitter we need to detect are subject to similar values of attenuation. In order to allow discriminating between the reflected signal by the frequency splitter and unwanted back reflection due to splices, it is proposed to define a sliding temporal window for the detection of the reflected signal. The solution is based on the propagation time of the signal. The emitted signal is a signature of sufficiently short duration. A sliding temporal window is defined to detect the reflected signal. The beginning and the width of the temporal sliding window allow focusing on the detection of the reflected signal to reflexion located in a particular part of the transmission path depending on the propagation time of the signal. This is illustrated on
Advantageously, the gain will be adapted to compensate the attenuation due to the length of the path. This could be done by using a gain corresponding to the following formula :
with ∝ being the linear attenuation coefficient of the fiber, and η the material index, t1 being the moment of the beginning of the window.
In a first step 11.1 the system is initialized. The OLT, for example, sets its operating level, meaning the wavelength of the emission signal at the lowest level, and the beginning of the temporal sliding window is also set to its minimum, typically the end of the signature emission. In some embodiments, especially in case of tracking where the temporal location of the frequency splitter is already known, the extent of the search could be advantageously reduced around this known location.
In a second step 11.2, the device emits the signature.
In a third step 11.3, the returned signal power is measured over the temporal sliding window. We estimate the presence of the expected signal. Typically the width of the temporal sliding window is set to the duration of the signature, but other values could be used.
In a step 11.4, the position of the temporal sliding window is updated. If the position of the temporal sliding has reached its extent, it is reinitialized to its minimum and we go to step 11.5. Else, the temporal sliding position is incremented, the used step is typically of the width of the window, but lower values could be used.
In step 11.5, when a complete range of temporal sliding window has been explored for a given wavelength, this wavelength is incremented. The position of the temporal sliding window is reinitialized. Then the process of temporal exploration is resumed by going back to step 11.2. In some embodiments, especially in case of tracking where the former value of the wavelength is already known, the extent of the search could be advantageously reduced around this known value.
While the complete range of wavelength has been explored, the spectral location of the minimum is estimated. This could be done by linear regression for example. The operational level of the device is set to this found value and the emitting device and the frequency splitter are coupled. In case of the OLT that is emitting a multiplexed signal comprising bands on several wavelengths, we can talk of a set of operating levels. These operating levels correspond to multiple minimums detected in the power of the return signal as illustrated by
As illustrated by
This allows to apply a different value of gain to explore the first part between the ONU 9.4 and the power splitter 9.3 and the second part between the power splitter 9.3 and the frequency splitter 9.2. Therefore a gain corresponding to the attenuation due to this power splitter is applied for position of the sliding window corresponding to reflexion point behind the power splitter.
Advantageously, for tracking the variation of the nominal wavelength of the Mach-Zehnder components over time, this coupling method is applied on a regular basis.
The invention could be used in various cases as soon as an optical transmission is set up between at least two devices. This transmission using WDM technologies, these wavelengths being extracted by a frequency splitter with a topology similar to the topology of the said Mac Zhender component.
Number | Date | Country | Kind |
---|---|---|---|
10195695 | Dec 2010 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2011/072365 | 12/9/2011 | WO | 00 | 6/12/2013 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2012/080138 | 6/21/2012 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
5978114 | Clark et al. | Nov 1999 | A |
20020067881 | Mathis | Jun 2002 | A1 |
20030035446 | Griffel | Feb 2003 | A1 |
20040022493 | Takiguchi et al. | Feb 2004 | A1 |
20040208523 | Carrick et al. | Oct 2004 | A1 |
Number | Date | Country |
---|---|---|
101 324 443 | Dec 2008 | CN |
1 380 865 | Jan 2004 | EP |
99 12296 | Mar 1999 | WO |
Entry |
---|
Li. Y. P. et al., “Silica-based Optical Integrated Circuits”, IEE Proceedings: Optoelectronics, Institution of Electrical Engineers, vol. 143, No. 5, pp. 263-280, XP 006006672, ISSN: 1350-2433 (Oct. 22, 1996). |
Yamada, H., et al., “Si Photonic Wire Waveguide Devices”, IEEE Journal of Selected Topics in Quantum Electronics, IEEE Service Center, vol. 12, No. 6, pp. 1371-1379, XP011151878, ISSN: 1077-260X, (Nov. 1, 2006). |
International Search Report issued Jan. 19, 2012 in PCT/EP11/072365 filed Dec. 9, 2011. |
Number | Date | Country | |
---|---|---|---|
20130279906 A1 | Oct 2013 | US |