System and method for synchronizing an optical source and a router in a wavelength division multiplexed fiber optic network

invention field

the This invention relates to fiber optics, and particularly wavelength- fiber optic networks.

General State of the art

A common Type fiber optic network is the WDM network (Wavelength Division Multiplexed). In a typical WDM network a multi-frequency source is used, two or more signals with different wavelengths to produce. The signals are out a single fiber to a remote router, where them according to their wavelengths are distributed to network participants. Similarly, in networks that providing a two-way signaling, the router uses to be to combine signals generated by the participants.

Around to maximize the efficiency of a WDM network, the router must trace the source wavelength accurately. The Router is essentially a comb filter whose lobes in Ideally, centered on the source signal wavelengths. If the Lobes of the router are not aligned to the source wavelengths attenuates the Router according to the for the subscriber specific signals, wherein the degree of damping is proportional to the extent of Misalignment between the source and the router. In addition, misalignment of source and router in crosstalk between the router outputs lead, thereby the power is reduced even further. Misalignment may occur for example, when a temperature change at source drifting of the source frequencies caused or Similarly, when a temperature change the router caused a drift of the router filter bands.

At the Attempt to eliminate misalignment between sources and router network, Designers loopback WDM proposed. In a Prüfschleifensystem is a portion of the received signal from a subscriber via a separate feedback fiber sent back to a central office. The power level of the feedback signal Wil be inspected. A possible misalignment between the source and router appears as a drop in power level of the feedback signal, and Realignment can be achieved by adjusting the source wavelengths are to the feedback signal to maximize. However, such a system requires a separate Fiber line, to the feedback to send to the central office, and thereby increase the overall cost of the system used network.

Out US-A-4,839,614, a system and method for referencing Radiation frequencies from a number of sources known to the filter element a contains, to which the radiation is incident. The filter element has a copied Amount of pass bands on, which are spaced in frequency. detection means detecting through the filter element and passing radiation provides corresponding output signals. Control means responsive to the output signals from the detection means to the sources so as to control that transmitted through the filter element Radiation is held substantially constant. Optical signals However, in US-A-4,839,614 of fiber distributors in a first deflected amount of lenses, then focused so that they on fall Etalon with a copied set of pass bands, and then from out a second set of lenses to photodiodes which feed a comparator, with which a laser driver is controlled. One such system Network-use requires additional a filter element copied its own fiber cable for feedback, thereby reducing the overall system cost increases.

Summary the invention

systems and method according to the invention are as described in the independent claims forth. Preferred forms are set out in the dependent claims.

at a WDM network according to the present Invention is required to maintain an alignment between a WDM source and a WDM router feedback needed without the use of a loopback provided.

at a preferred embodiment, of the invention is a narrow band of light emitted by the source wavelengths as a monitoring channel designated. The monitoring channel is in terms of the wavelength adapted to a fiber grating which lies in the optical path between the source and the router. The Grating is designed such that it reflects all the light of the monitoring channel wavelength while all <?page 3?>pass other source wavelengths unhindered leaves. Prefers the grating is disposed in close proximity to the router, so that a any environment generated by the wavelength drift that is experienced by the router, and experienced by the grating. In this way, followed the drift in the grid to drift in the router.

Among normal working conditions remain the monitoring channel and the fiber grating in wavelength alignment is another, and the amount of reflected monitor channel light located. on its peak However, when an environmental disturbance to leads, that the Source, and the gratings become misaligned, the amount of reflected monitor channel light lower than the peak value, the decrease in reflected Light proportional to the misalignment between the source and the grid is. Since the grating tracks the router, the decrease the reflected light is also proportional to the misalignment between the source and the router. Thus, the decrease may occur when reflected light may be used as an error signal representing the extent of Misalignment between the source and router displays. This error signal is for the purpose of adjusting the source wavelengths to to compensate for the misalignment, returned to the WDM source.

Short description tHE dRAWINGS

<figref idrefs="S19">1</figref> is a block diagram of an optical fiber WDM-system (Wavelength Division multiplexed) according to a preferred embodiment, the invention.

<figref idrefs="S20">2A</figref>-<figref idrefs="S20">2C</figref> show Examples of reflectance characteristics realized by the fiber grating can be.

<figref idrefs="S19">3</figref> is a block diagram of an alternative WDM router that for use in a preferred embodiment the invention is useful.

<figref idrefs="S21">4</figref> is a block diagram of a fiber optic WDM system according to an alternative embodiment the invention.

<figref idrefs="S21">5</figref> is a graph showing the reflection characteristic of an in a test embodiment of representing invention fiber grating used.

<figref idrefs="S22">6</figref> is a graphical representation of the transfer characteristic in a test embodiment represents the fiber grating used invention.

<figref idrefs="S22">7</figref> is is a graphical representation showing the transmission router with respect to the grid transmission for a Test embodiment of the invention.

<figref idrefs="S23">8</figref> is a graph showing the multi-frequency source output the opposite to the router port issue for a test embodiment of the invention.

Detailed description

<figref idrefs="S19">1</figref> is a block diagram of an optical fiber WDM-system (Wavelength Division multiplexed) according to a preferred embodiment, the invention. As can be seen from the figure, the WDM system a multi-frequency source <figref>10</figref>, The source light in N + 1 different Wavelength bands (or "channels") can create. There are many types of multi-frequency sources with the invention can be used. For example, the multi-frequency source <figref>10</figref> a multi-frequency laser be in the form of an integrated distributed feedback arrays. alternative , the source <figref>10</figref> consist of several individual lasers, each transmitting within an own wavelength bands.

Anyway, are the source of rooflights to transfer by an optical fiber link <figref>12</figref> combined, the the Light to a WDM router <figref>14</figref> passes. An optical junction<figref>18</figref> is in the optical path between the source and the fiber, namely to be described below purpose. As regards the source of for fiber <figref>12</figref> outgoing light allows the joint <figref>18</figref>. that all such light through the router passes, and directs little or no light in the fiber <figref>13</figref>, be arrived at router for use allocated by network participants rooflights for reception entkombiniert by the participants.

&lt;?page 4?&gt;

From N + 1 channels up to N channels intended for use by network participants. These channels referred to as "subscriber channels". Each individual of subscriber channels is used, optical transmissions to a certain network participants to wear, and therefore the Source of light for a particular subscriber channel according to the information modulated, the for determines the reception by this channel assigned participants are. The remaining one or more channels are determined as monitoring channels. (For purposes of illustration are in the following only single channel monitoring systems considered).

The monitoring channel is not receipt determined by network subscribers, but rather is for maintaining wavelength alignment between the Multifrequency source and the router used. Referring again on <figref idrefs="S19">1</figref> the monitoring channel in terms of wavelength to a fiber grating <figref>16</figref> adapted the in the optical Path between the source and the router is located. The grid is designed such that it reflects all the light of the monitoring channel wavelength and pass them with all other source wavelengths unhindered leaves. Prefers the grating is disposed in close proximity to the router, so that a any environment generated by the wavelength drift that is experienced by the router, and experienced by the grating. In this way, followed the drift in the grid to drift in the router.

Among normal working conditions remain the monitoring channel and the fiber grating in wavelength alignment is another, and the amount of reflected monitor channel light located. on its peak However, when an environmental disturbance to leads, that the Source, and the gratings become misaligned, the amount of reflected monitor channel light lower than the peak value, the decrease in reflected Light proportional to the misalignment between the source and the grid is. Since the grating tracks the router, the decrease the reflected light is also proportional to the misalignment between the source and the router. Thus, the decrease may occur when reflected light may be used as an error signal representing the extent of Misalignment between the source and router displays. This error signal is for the purpose of adjusting the source wavelengths to to compensate for the misalignment, returned to the WDM source.

After Reflection by the fiber grating, the monitor channel light from the Fiber link back to performed multifrequency source, until the tap <figref>18</figref> achieved. The tap, such as a directional coupler, directs the reflected light into the fiber <figref>13</figref>, which on the other hand the reflected light to a feedback circuit <figref>20</figref> directs. the Function of the feedback circuit is the reflected light by adjusting the source wavelengths to maximize. To a high dynamic range in the feedback circuit achieve, can Synchronous detection techniques (or "lock-in" techniques) may be used.

A important consideration in the system in <figref idrefs="S19">1</figref> and in the invention in general is the reflection curve of the fiber grating. An ideal grid reflected 100 of the supervisory channel light and permitted here that 100% the subscriber channel light can pass through unhindered. Practical Grid but allow the passage of some supervisory channel light and reflect some subscriber channel light.

the <figref idrefs="S20">2A</figref>-<figref idrefs="S20">2C</figref> illustrate Reflectivity characteristics of fiber gratings that used with the invention can be. The horizontal axis in the figures represents the wavelength in nanometers, and the vertical axis represents the reflectance in percent. <figref idrefs="S20">2A</figref> shows the reflectance characteristic of a narrow band grating. The graph overlaid and is indicated by a dashed line is an idealized Reflectance characteristic. <figref idrefs="S20">2 B</figref> shows the characteristic of a grating having a high reflectance in the Middle of his bands and a lower reflectance at the edge his bands. This characteristic may be alternatively realized by combining two gratings, a narrowband grating with a high reflectivity and a wide-band grating with a lower reflectance. <figref idrefs="S20">2C</figref> shows the reflectance characteristics of a grating filter with a zero in its center. This characteristic may be alternatively realized by combining two narrowband grille with offset wavelengths. If this type is used by the filter shape, synchronizes Feedback circuit with zero instead of the tip.

A Another important consideration consists in the invention therein, such as an environmental drift in the source (Instead of the router) affects performance. In this regard, Note that the Invention an environmental drift in the source in the same way compensated, as it compensates for a drift in the router. As above discussed causes a drift in the wavelength router, a decrease in reflected monitor channel light. Analog causes drift in the source wavelengths a decrease in reflected monitor channel light. Regardless of the source of the decrease tried the feedback circuit, To compensate for this, and therefore a drift in the source handled in the same manner as a drift ge the router&lt;?page 5?&gt;handles is.

It is also important to note that, so that the drift compensation function with maximum effectiveness, it is necessary that all Source wavelengths (including the monitoring channel) as a comb of wavelengths together drifting. This means, for a maximum effectiveness have to all source wavelengths drift by the same amount. Under such conditions the monitor channel drift compensation as determined by the error signal accurately, the compensation amount which is necessary for the source channels , whereby an optimal source channel compensation is permitted. However, if the drift in one or more of the channels from the source Drift in the monitoring channel is different, the compensation is applied to those channels something below the optimum. The larger the Difference between the drift in a specific source channel and the drift in the monitoring channel, the less effective the compensation for that source channel.

often depends on the Extent, in which a channel is drifting, on its frequency. In the system from <figref idrefs="S19">1</figref> For example, the channel with the highest frequency by a given amount drift, the channel with the second highest Frequency slightly less, the channel with the third highest frequency even less and so on. In such a system, the maximum degree of difference between monitoring channel drift and source channel drift be minimized by monitoring channel selected from a channel is operating at or near the middle of the frequencies is to be spanned by the N + 1 channels. Although the monitoring channel from any of the N + 1 channels can be selected, when He selected toward the center it is ensured that it by an amount drifts, lying in the center (or approximately in the center) between the drifts, the further outward lying source channels learn.

<figref idrefs="S19">3</figref> shows an alternative WDM router <figref>22</figref>Which for use in a preferred embodiment the invention is useful. The router is connected via an input port<figref>24</figref> at a fiber link <figref>12 '</figref> coupled. The fiber link is analogous to the route <figref>12</figref> in <figref idrefs="S19">1</figref> and as the route <figref>12</figref> the optical path defined by the Router is coupled to a multi-frequency source. This input port contains a fiber grating <figref>16 '</figref>, The fiber grating <figref>16 '</figref> leads the same function as the grating <figref>16</figref> from <figref idrefs="S19">1</figref> out; by directly writing the grid in the router deleted but the need for a separate fiber grating, and the region between the coupling Grid and the router is improved. This improved coupling receives to a better environment persecution between the grid and the Router, and therefore an improved overall system performance.

<figref idrefs="S21">4</figref> shows a further alternative embodiment the invention. Each element in both<figref idrefs="S19">1</figref> as well as <figref idrefs="S21">4</figref> appears leads two embodiments the same function and it is the same reference number assigned. Accordingly, the multi-frequency laser generates <figref>10</figref> from <figref idrefs="S21">4</figref> as the laser of <figref idrefs="S19">1</figref> Source light in N + 1 different Wavelength bands of which one, as a monitor channel is used. The source of light, which is intended for participants, is through the fiber link <figref>12</figref> in combination with the WDM router <figref>24</figref> at participants distributed. In addition, the monitor channel light is the multi-frequency laser reflected back and in the direction of the feedback control circuit <figref>20</figref> by tapping <figref>18</figref> diverted. The feedback control circuit used by the supervisory channel obtained information to maintain an alignment the source and the router.

in the Unlike the embodiment from <figref idrefs="S19">1</figref> but contains embodiment from <figref idrefs="S21">4</figref> no fiber grating for the purpose of selectively reflecting the supervisory channel light of. within the combined light source Instead, the monitor channel light is reflected, after it has passed through the router and has been isolated to the subscriber channel light. As shown in the figure, this can happen that a reflector <figref>28</figref> accordingly in the way of the router output the monitoring channel is arranged.

If embodiment from <figref idrefs="S21">4</figref> with the other presented embodiments is compared, two important differences should be noted. First occurs in the embodiment from <figref idrefs="S21">4</figref> the supervisory channel light through the router and is therefore due to the fiber-router coupling loss steamed. damping is amplified by the fact that the supervisory channel light a Tour of the router takes, even at transfer from the source and once at the reflection in the direction of the Feedback circuit. In contrast to this need in the embodiment of <figref idrefs="S19">1</figref> the supervisory channel light at all not pass through the router and will not be by the fiber-router coupling loss affected. Accordingly, in Applications in which the level of the monitor channel light critically is, the embodiment of from <figref idrefs="S19">1</figref> prefers. Second, is the embodiment from <figref idrefs="S21">4</figref> the monitoring channel a route outputs, whereby the at&lt;?page 6?&gt;number of router outputs is reduced, which are used for operator control of subscribers can. In contrast to this need in the embodiment of<figref idrefs="S19">1</figref> the monitoring channel not pass through the router and can therefore be selected, that he between Router channels lies. In this way interferes the monitoring channel not the full utilization of the router.

Around embodiment from <figref idrefs="S19">1</figref> and to test the feasibility of the invention to demonstrate an attempt has been made. was for testing purposes the from the fiber grating <figref>16</figref> any returned error signal measured, but not for adjusting the wavelengths of the multi-frequency source used. Instead, the source wavelengths were temperature adjustment set. The test is described in more detail below.

the in the multi-frequency source test used was an integrated optical 24-channel multi-frequency laser. The light emitted from the laser light was in 1555nm-wavelength region with a channel spacing of 100 GHz and a fiber-joint-total output power of -9.90 dBm. The laser 7 user channels have a channel spacing of generates 200 GHz. The channels were with pseudorandom 50 Mb / s NRZ data (nonreturn-to-zero) modulated.

the for the Test fiber link used was a 6.3 km-distance from 5 to connectors provided sections. The track was a fiber grating with a at 1557.0 nm centered bandwidth of 13.6 GHz and a reflection of 3 dB (temperature = 20 ° C) coupled. The reflection characteristic of the grating and its transfer characteristic are in the <figref idrefs="S21">5</figref> or. <figref idrefs="S22">6</figref> shown.

Of the in the router experiment used was an 8-channel arrayed waveguide grating router with a channel spacing of 200 GHz and an average insertion loss of 8.9 dB. The router was coupled to the output of the fiber grating, and both the router and the grid were within a temperature-controlled furnace arranged. During the tests, the left output ports of the router without a diploma to a worst-case condition to simulate. The transfer characteristic the router in comparison to that of the grating is in <figref idrefs="S22">7</figref> shown. As can be seen from the figure, the fiber grating wavelength towards the wavelength the next Router channel by about 0.6 nm are added (temperature = 19.3 ° C).

The monitoring channel for the Test was chosen so that he was in the area of ​​1558 nm. He was 100 GHz of the next subscriber channel spaced. The overall frequency plan of the multi-frequency laser is in<figref idrefs="S23">8</figref> shown. Also shown is the output of the router port, which with regard to the wavelength the monitoring channel the next lies. As you can see, there is no significant interference between the monitoring channel and the router port issue.

the Results of the tests are documented in Table I below. The in Table shown measurements of the normalized router output were to the with respect to the wavelength the monitoring channel the next lying router port made. The following test procedure was called.

table I <img img-content="tb" img-format="tif" he="46" wi="145" file="00130001.tif" />

First the router and grating were stabilized at room temperature (19.3 ° C) and the multi-frequency laser was tuned in terms of temperature, to obtain the maximum error signal. As indicated in line 1 of the table it can be seen, a maximum error signal of 42.5 microvolts was at a Multifrequency laser temperature of 14.5 ° C and a supervisory channel wavelength of achieved 1558.65 nm. The router output under these conditions was as a reference for the remainder of the test used, and is thus the normalized router output row 1 of the table 0 dB.

&lt;?page 7?&gt;

As next the temperature of the furnace increased to 73.0 ° C, so that the router and the grating a Temperature of 73.0 ° C would receive. This led the loss of both the router output signal and the error signal (Row two of the Table). Finally, the error signal was restored by the multifrequency laser temperature was increased from 14.5 ° C to 19.3 ° C. Since the error signal was restored, the normalized router output was recovered (third line of the table). By adjusting the Temperature of the source according to the monitoring channel feedback was thus between the source and the router output from the wavelength ago the monitoring channel the next located scored. Because the source was a multi-frequency laser, also conducted an Alignment between the source and the monitoring channel nearest Router output to an alignment between the source and all other router outputs. This is demonstrated that a Wavelength alignment can be maintained between the source and the router by observing the monitoring channel reflection and correspondingly adjusting the source.

Although the best way to run the invention in detail has been described, the skilled artisan will recognize in the art to which the invention relates, many alternative designs and embodiments to exercise of the invention as defined by the following claims.