Circuit structure for dispersion compensation in optical communication systems by means of an optical filter
Abstract
For dispersion compensation in optical transmission systems, an optical transversal filter is provided as a negative dispersion filter, which has a series of directional couplers each operating at a distance τ / 2 in succession and operated as a branch and a series of directional couplers likewise operating in each case at a distance τ / 2 , wherein the second output of each directional coupler of the directional coupler operated as a branch leads to the second input of the corresponding directional coupler operated as a combiner.

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5 claims: 5 independent, 0 dependent
- 1Circuit arrangement for dispersion compensation in optical transmission systems using an optical filter,characterized,that an optical transversal filter (k0', k1', ..., kN-1 ';k0'', k1'', ..., kN-1 '') is provided as a negative dispersion filter. Schaltungsanordnung zur Dispersionskompensation in optischen Übertragungssystemen mittels eines optischen Filters, dadurch gekennzeichnet, daß ein optisches Transversalfilter (k0', k1', ..., kN-1';k0'', k1'', ..., kN-1'') als Filter negativer Dispersion vorgesehen ist.
- 2Circuit arrangement according to claim 1,characterized,that the optical transversal filter is a series of directional couplers (k0', k1', ..., kN-1 ') and a series of directional couplers (k0'', k1'', ..., kN-1 ''), with the second output of each directional coupler of the directional couplers (k0', k1', ..., kN-1 ') to the second input of the corresponding directional coupler (k0'', k1'', ..., kN-1 '') leads. Schaltungsanordnung nach Anspruch 1, dadurch gekennzeichnet, daß das optische Transversalfilter eine Reihe von jeweils in einem Abstand τ/2 aufeinanderfolgenden, als Verzweiger betriebenen Richtungskopplern (k0', k1', ..., kN-1') und eine Reihe von ebenfalls jeweils im Abstand τ/2 aufeinanderfolgenden, als Vereiniger betriebenen Richtungskopplern (k0'', k1'', ..., kN-1'') aufweist, wobei jeweils der zweite Ausgang jedes Richtungskopplers der als Verzweiger betriebenen Richtungskoppler (k0', k1', ..., kN-1') zu dem zweiten Eingang des jeweils entsprechenden, als Vereiniger betriebenen Richtungskopplers (k0'', k1'', ..., kN-1'') führt.
- 3Circuit arrangement according to claim 2,characterized,that the two corresponding directional couplers (k0', k0'';k1', k1'';..., kN-1 ', kN-1 '') have at least approximately the same coupling factors. Schaltungsanordnung nach Anspruch 2, dadurch gekennzeichnet, daß jeweils die beiden einander entsprechenden Richtungskoppler (k0',k0'';k1',k1'';..., kN-1', kN-1'') zumindest angenähert gleiche Koppelfaktoren aufweisen.
- 4Circuit arrangement according to one of claims 1 to 3,characterized,that in the branches of the optical transversal filter (k0', k1', ..., kN-1 ';k0'', k1'', ..., kN-1 '') Heating elements are provided. Schaltungsanordnung nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß bei den Zweigen des optischen Transversalfilters (k0', k1', ..., kN-1';k0'', k1'', ..., kN-1'') Heizelemente vorgesehen sind.
- 5Circuit arrangement according to one of claims 1 to 4,characterized,that tunable directional couplers (k0', k1', ..., kN-1 ';k0'', k1'', ..., kN-1 '') are provided. Schaltungsanordnung nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß abstimmbare Richtungskoppler (k0', k1', ..., kN-1';k0'', k1'', ..., kN-1'') vorgesehen sind.
Independent claims5
33 paragraphs, as filed
In the case of optical message transmission with data rates in the Gbit / s range via an optical waveguide, the fiber dispersion is decisive for the distance that can be bridged. This is especially true in the wavelength window around 1.55 µm, since here the attenuation can be eliminated by means of optical amplifiers, while the dispersion of a standard fiber with around 17 ps / nm / km has quite large positive values. There is therefore an interest in components which have a negative dispersion and can thus form a dispersion-free transmission medium together with the standard fiber. For very broadband applications such as wavelength division multiplexing (WDM) it would also be desirable to also use the (at 1.55 µm about 0.06 ps / nm<sup>2</sup>/ km) slope of the dispersion of the standard fiber. In addition, components may be of interest which have a variable dispersion (also in the sign), in order, for example, to be able to compensate for the residual dispersion of a dispersion-shifted fiber at the transmitter wavelength.
The use of passive, linear principles is of interest for dispersion compensation because, as long as there are no non-linear effects in the transmission, they allow the use of compensation components at any point on the optical transmission path. There is also the prospect of inexpensive and compact components, particularly in the case of passive principles.
In connection with dispersion compensation, various components have already been presented in addition to (now also commercially available) dispersion-compensating fibers: Fabry-Perot interferometers, ring resonators, cascaded Mach-Zehnder interferometers, cascaded birefringent crystals, free-beam optics with gratings, chirped gratings. In contrast, the invention shows another way to dispersion compensation in optical transmission systems.
The invention relates to a circuit arrangement for dispersion compensation in optical transmission systems by means of an optical filter; this circuit arrangement is characterized in that an optical transversal filter is provided as a negative dispersion filter; In a further embodiment of the invention, the optical transversal filter can have a number of directional couplers each operating at a distance τ / 2 in succession, operated as a branch, and a series of directional couplers likewise operating in each case at a distance τ / 2, each operating as a combiner, the second output in each case each directional coupler of the directional couplers operated as a branch to the second input of the corresponding, leads as a combiner operated directional coupler.
The invention brings with it the advantage of largely freely selectable courses of dispersion and transmission, the product of the dispersion and the square of the bandwidth determined with the filter being able to be increased almost linearly with the number of filter branches.
Further special features of the invention will become apparent from the following detailed explanation of the invention with reference to the drawings. Show<dl id="dl0001" compact="compact"><dt>FIG 1</dt><dd>a schematic diagram of a transversal filter,</dd><dt>FIG 2</dt><dd>an embodiment of an optical transversal filter,</dd><dt>FIG 3</dt><dd>a course of the transfer function of a filter for dispersion compensation,</dd><dt>FIG 4</dt><dd>the impulse response to this transfer function,</dd><dt>FIG 5</dt><dd>Transmission and dispersion of a filter for dispersion compensation,</dd><dt>FIG 6</dt><dd>Transmission and dispersion of an optimized filter for dispersion compensation.</dd></dl>
1 schematically shows a transversal filter with a delay chain consisting of N delay elements (τ) of delay time τ and N + 1 with coefficient elements a that can be set to a respective filter coefficient (sample value)<sub>0</sub>, ..., a<sub>N</sub> provided tap branches that lead to a summator Σ. Transversal filters are generally known (eg Bocker: data transmission, Berlin - Heidelberg - New York 1976, Volume 1, Chap. 5.3.2) and do not require any further explanation here; the implementation of optical transversal filters in planar form on a silicon substrate is also known per se (from J. Lightwave Technol., Vol.12 (1994), pp 664 ... 669), so that no further explanation is required here either . 2 shows a simple structure of an optical transversal filter.
According to FIG. 2, the optical transversal filter has a series of directional couplers k, each operating at a distance τ / 2, which is operated as a branch<sub>0'</sub>, k<sub>1'</sub>, ..., k<sub>N-1 '</sub> and a series of directional couplers k, also operated at a distance τ / 2 in succession, operated as a combiner<sub>0''</sub>, k<sub>1''</sub>, ..., k<sub>N-1 ''</sub> , with the second output of each directional coupler of the directional couplers k<sub>0'</sub>, k<sub>1'</sub>, ..., k<sub>N-1 '</sub> to the second input of the respective directional coupler k operated as a combiner<sub>0''</sub>, k<sub>1''</sub>, ..., k<sub>N-1 ''</sub> leads. In the individual branches of the optical transversal filter, heating elements W can be attached, which will be discussed further below.
The amplitudes of the individual filter coefficients are realized by the coupling factors of the individual branches and combiners, the dimensioning also taking into account the effect of the couplers located in front of the respective branch or behind the respective combiner. The filter structure is expediently symmetrical, ie The same coupling factors are provided for coupling and decoupling, so that additional losses when combining the power components are avoided.
For a transversal filter, the complex transfer function H (jω) for the electric field strength and the impulse response h (t) to a δ pulse look as follows:<maths id="math0001" num="(1)"><math display="block"><mrow><mtext mathvariant="italic">H</mtext><mtext>(</mtext><mtext mathvariant="italic">jω</mtext><mtext>)= </mtext><apply><sum /><lowlimit><mtext mathvariant="italic">k</mtext><mtext>=0</mtext></lowlimit><uplimit><mtext mathvariant="italic">N</mtext></uplimit><mrow><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>·</mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext>-</mtext><mtext mathvariant="italic">jkωτ</mtext></mrow></msup></mrow></apply></mrow></math><img file="EP0740173A2_D0001.tif" /></maths> and<maths id="math0002" num="(2)"><math display="block"><mrow><mtext mathvariant="italic">H</mtext><mtext>(</mtext><mtext mathvariant="italic">t</mtext><mtext>)= </mtext><apply><sum /><lowlimit><mtext mathvariant="italic">k</mtext><mtext>=0</mtext></lowlimit><uplimit><mtext mathvariant="italic">N</mtext></uplimit><mrow><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>·</mtext><mtext mathvariant="italic">δ</mtext><mtext>(</mtext><mtext mathvariant="italic">t</mtext><mtext>-</mtext><mtext mathvariant="italic">kτ</mtext><mtext>),</mtext></mrow></apply></mrow></math><img file="EP0740173A2_D0002.tif" /></maths> where δ (t) is the delta function.
The impulse response of the filter results from the Fourier transformation of H (jω) into the time domain. If, as is the case with a transversal filter according to Eq. (2), the impulse response is time-discrete, a periodic frequency response of the filter with a periodicity (<u>F</u>ree <u>S</u>pectral <u>R</u>ange FSR) of <maths id="math0003" num=""><math display="inline"><mrow><mtext>FSR = 1 / τ</mtext></mrow></math><img file="EP0740173A2_D0003.tif" /></maths>.
The following then applies to the transmission of the optical power as a function of the angular frequency ω:<maths id="math0004" num="(3)"><math display="block"><mrow><mtext>Transmission(</mtext><mtext mathvariant="italic">ω</mtext><mtext>) = |</mtext><mtext mathvariant="italic">H (jω)</mtext><msup><mrow><mtext>|</mtext></mrow><mrow><mtext mathvariant="italic">2</mtext></mrow></msup><mtext>;</mtext></mrow></math><img file="EP0740173A2_D0004.tif" /></maths> is the course of the phase φ (ω) over ω<maths id="math0005" num="(4)"><math display="block"><mrow><mtext mathvariant="italic">φ (ω)</mtext><mtext> = </mtext><mtext mathvariant="italic">arg {H (jω)}</mtext><mtext>.</mtext></mrow></math><img file="EP0740173A2_D0005.tif" /></maths> The group term T<sub>G</sub>(ω) is<maths id="math0006" num="(5)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">T</mtext></mrow><mrow><mtext mathvariant="italic">G</mtext></mrow></msub><mtext>(</mtext><mtext mathvariant="italic">ω</mtext><mtext>)=-</mtext><mfrac><mrow><mtext mathvariant="italic">dφ</mtext></mrow><mrow><mtext mathvariant="italic">dω</mtext></mrow></mfrac></mrow></math><img file="EP0740173A2_D0006.tif" /></maths> and the dispersion D (ω)<maths id="math0007" num="(6)"><math display="block"><mrow><mtext mathvariant="italic">D</mtext><mtext>(</mtext><mtext mathvariant="italic">ω</mtext><mtext>)=</mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">dT</mtext></mrow><mrow><mtext mathvariant="italic">G</mtext></mrow></msub></mrow><mrow><mtext mathvariant="italic">dλ</mtext></mrow></mfrac><mtext> ,</mtext></mrow></math><img file="EP0740173A2_D0007.tif" /></maths> resulting from the relationship <maths id="math0008" num=""><math display="inline"><mrow><mtext mathvariant="italic">c</mtext><mtext> = </mtext><mtext mathvariant="italic">λf</mtext></mrow></math><img file="EP0740173A2_D0008.tif" /></maths><maths id="math0009" num="(7)"><math display="block"><mrow><mtext mathvariant="italic">D</mtext><mtext>(</mtext><mtext mathvariant="italic">ω</mtext><mtext>) = </mtext><mfrac><mrow><msup><mrow><mtext mathvariant="italic">d</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext mathvariant="italic">φ</mtext><mtext>(</mtext><mtext mathvariant="italic">ω</mtext><mtext>)</mtext></mrow><mrow><msup><mrow><mtext mathvariant="italic">dω</mtext></mrow><mrow><mtext>2</mtext></mrow></msup></mrow></mfrac><mtext></mtext><mfrac><mrow><mtext>2</mtext><mtext mathvariant="italic">πc</mtext></mrow><mrow><msup><mrow><mtext mathvariant="italic">λ</mtext></mrow><mrow><mtext>2</mtext></mrow></msup></mrow></mfrac></mrow></math><img file="EP0740173A2_D0009.tif" /></maths> results. When considering frequency ranges that are small compared to the optical frequency, Eq. (7) for<i>λ</i> the middle wavelength are used.
If one specifies the transfer function according to magnitude and phase (or dispersion), the impulse response can be determined therefrom by means of Fourier transformation, and the impulse response results from sampling with the sampling period <maths id="math0010" num=""><math display="inline"><mrow><mtext>τ = 1 / FSR</mtext></mrow></math><img file="EP0740173A2_D0010.tif" /></maths> the filter coefficients a<sub>0</sub> ... a<sub>N</sub>.
For example, consider a filter with a dispersion of -1000 ps / nm over a bandwidth of 10 GHz. It can be seen from Eq. (7) that a quadratic course of the phase over ω is necessary for a constant dispersion:<maths id="math0011" num="(8)"><math display="block"><mrow><mtext mathvariant="italic">φ</mtext><mtext>(</mtext><mtext mathvariant="italic">ω</mtext><mtext>)=</mtext><mfrac><mrow><msubsup><mrow><mtext mathvariant="italic">λ</mtext></mrow><mrow><mtext>0</mtext></mrow><mrow><mtext>2</mtext></mrow></msubsup></mrow><mrow><mtext>4</mtext><mtext mathvariant="italic">πc</mtext></mrow></mfrac><msup><mrow><mtext mathvariant="italic">Dω</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>;</mtext></mrow></math><img file="EP0740173A2_D0011.tif" /></maths> is in it <i>λ</i><sub><i>0</i></sub> the wavelength (e.g. λ<sub>0</sub> = 1.55µm) at which the desired dispersion <i>D</i> should be achieved exactly. 3 shows the course of the amount and phase of the transfer function, which are freely selected within certain limits. In addition to the quadratic course, care was also taken in the phase that the difference between the phases at the area boundaries is an integral multiple of 360 °, so that continuous transitions occur (the section of the transfer function shown is repeated periodically). The course of the amount was chosen so that the range of negative dispersion is in the range of higher transmission. Here too, care must be taken to ensure that the transitions are as continuous as possible in order to keep the length of the associated impulse response as short as possible.
Regarding a periodic filter course, it should be noted that it allows simultaneous dispersion compensation at several wavelengths. The dispersion of the in the distance<maths id="math0012" num=""><math display="inline"><mrow><mtext>FSR = 1 / τ</mtext></mrow></math><img file="EP0740173A2_D0012.tif" /></maths> successive usable filter areas is dependent on the wavelength. The amount<img file="EP0740173A2_D0013.tif" /> in Eq. (7) points at intervals of <maths id="math0013" num=""><math display="inline"><mrow><mtext>FSR = 1 / τ</mtext></mrow></math><img file="EP0740173A2_D0014.tif" /></maths> always the same size, which is denoted by K. The dispersion can thus be represented as follows:<maths id="math0014" num="(12)"><math display="block"><mrow><msub><mrow><mtext>D</mtext></mrow><mrow><mtext>periodically</mtext></mrow></msub><mtext> = </mtext><mtext mathvariant="italic">K</mtext><mfrac><mrow><mtext>2</mtext><mtext mathvariant="italic">πc</mtext></mrow><mrow><msup><mrow><mtext mathvariant="italic">λ</mtext></mrow><mrow><mtext>2</mtext></mrow></msup></mrow></mfrac></mrow></math><img file="EP0740173A2_D0015.tif" /></maths>
The derivative according to λ results in:<maths id="math0015" num="(13)"><math display="block"><mrow><mfrac><mrow><msub><mrow><mtext mathvariant="italic">dD</mtext></mrow><mrow><mtext mathvariant="italic">periodically</mtext></mrow></msub></mrow><mrow><mtext mathvariant="italic">dλ</mtext></mrow></mfrac><mtext>=-</mtext><mtext mathvariant="italic">K</mtext><mfrac><mrow><mtext>4</mtext><mtext mathvariant="italic">πc</mtext></mrow><mrow><msup><mrow><mtext mathvariant="italic">λ</mtext></mrow><mrow><mtext>3</mtext></mrow></msup></mrow></mfrac><mtext>=</mtext><mfrac><mrow><mtext>-2</mtext></mrow><mrow><mtext mathvariant="italic">λ</mtext></mrow></mfrac><msub><mrow><mtext mathvariant="italic">D</mtext></mrow><mrow><mtext mathvariant="italic">periodically</mtext></mrow></msub></mrow></math><img file="EP0740173A2_D0016.tif" /></maths>
To compensate for the dispersion of 1 km of standard fiber
At a wavelength of 1.55 µm, a dispersion of approximately -17 ps / nm is required. Eq. (13) shows an increase in the dispersion of the at intervals of<maths id="math0016" num=""><math display="inline"><mrow><mtext>FSR = 1 / τ</mtext></mrow></math><img file="EP0740173A2_D0017.tif" /></maths> successive filter ranges of 0.02 ps / nm<sup>2</sup>. In contrast, the standard fiber has a slope in the dispersion of approximately 0.06 ps / nm per kilometer in length<sup>2</sup> on. This slope is therefore not compensated for; it remains a value of 0.08 ps / nm<sup>2</sup>km, so that exact compensation is only achieved for one wavelength. In the case of wavelengths lying close to one another, however, periodic filter courses are nevertheless also suitable for wavelength division multiplexing (WDM).
The application of the Fast Fourier transform to the transfer function shown in FIG. 3 results in the impulse response shown in FIG. The each around<maths id="math0017" num=""><math display="inline"><mrow><mtext>τ = 1 / FSR</mtext></mrow></math><img file="EP0740173A2_D0018.tif" /></maths> support points (sample values) lying apart are the filter coefficients sought a<sub>k</sub> (in FIG 1). Since each sample corresponds to a branch in the transversal filter (in FIGS. 1 and 2), the smallest possible number of samples should be aimed for. The exemplary embodiment considered here is based on only the 13 sample values which lie in the time range delimited by dashed lines in FIG.
As can be seen from FIG. 4, the phases of the samples are predominantly not equal to 0 ° or 180 °, that is to say complex; so it applies<maths id="math0018" num="(9)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>=|</mtext><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>|·</mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext mathvariant="italic">jφk</mtext></mrow></msup></mrow></math><img file="EP0740173A2_D0019.tif" /></maths>
However, the impulse response of a transfer function that can be implemented cannot have samples with any phases, but must be real; if, as here, the desired transmission behavior is only required within a bandwidth that is small compared to the absolute (optical) frequency position, the filter can be implemented. The phases of the sampled values are implemented by corresponding runtimes:<maths id="math0019" num=""><math display="block"><mrow><mtable><mtr><mtd><mrow><mtable><mtr><mtd><mrow><mtable><mlabeledtr><mtext>(10)</mtext><mtd><mrow><mtext mathvariant="italic">H (jω)</mtext><mtext>=</mtext><apply><sum /><lowlimit><mtext mathvariant="italic">k</mtext><mtext>=0</mtext></lowlimit><uplimit><mtext mathvariant="italic">N</mtext></uplimit><mrow><mtext>|</mtext><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>|·</mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext mathvariant="italic">jφk</mtext></mrow></msup><mtext>·</mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext>-</mtext><mtext mathvariant="italic">jkωτ</mtext></mrow></msup></mrow></apply></mrow></mtd></mlabeledtr></mtable></mrow></mtd></mtr><mtr><mtd><mrow><mtable><mlabeledtr><mtext>(11)</mtext><mtd><mrow><mtext mathvariant="italic">H (jω)</mtext><mtext>=</mtext><apply><sum /><lowlimit><mtext mathvariant="italic">k</mtext><mtext>=0</mtext></lowlimit><uplimit><mtext mathvariant="italic">N</mtext></uplimit><mrow><mtext>|</mtext><msub><mrow><mtext mathvariant="italic">a</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>|·</mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext>-</mtext><mtext mathvariant="italic">jkω</mtext><mtext>(</mtext><mtext mathvariant="italic">τ</mtext><mtext>-</mtext><mtext mathvariant="italic">τk</mtext><mtext>)</mtext></mrow></msup></mrow></apply><mtext></mtext></mrow></mtd></mlabeledtr></mtable></mrow></mtd></mtr></mtable></mrow></mtd></mtr></mtable></mrow></math><img file="EP0740173A2_D0020.tif" /></maths> With<maths id="math0020" num=""><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">τ</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>=</mtext><mfrac><mrow><mtext mathvariant="italic">φk</mtext></mrow><mrow><msub><mrow><mtext mathvariant="italic">kω</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP0740173A2_D0021.tif" /></maths> and <maths id="math0021" num=""><math display="inline"><mrow><msub><mrow><mtext>ω ≈ ω</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></math><img file="EP0740173A2_D0022.tif" /></maths>.
Here is <maths id="math0022" num=""><math display="inline"><mrow><msub><mrow><mtext>ω</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><msub><mrow><mtext> = 2πc / λ</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></math><img file="EP0740173A2_D0023.tif" /></maths> the angular frequency at which the phase should be reached exactly (e.g. at λ<sub>0</sub> = 1.55µm).
Using the 13 sample values limited in FIG. 4, the courses shown in FIG. 5 result for transmission and dispersion. The dispersion has a relatively large ripple, which is due to the fact that it was not the complete impulse response that was considered, but only 13 samples.
In order to achieve better properties even with such a limited number of samples, an optimization can be carried out with computer assistance in which the amplitudes (amounts) and phases (arguments) of the samples are varied. 6 shows the result of such an optimization, on the basis of which the filter receives a relatively constant dispersion of -1000 ps / nm over a bandwidth of 10 GHz, the transmission having an equally broad, pronounced maximum in this area.
The example considered here is based on a Free Spectral Range FSR = 31.6 GHz.
As the scaling on the transmission curve (in FIGS. 5 and 6) shows, the filter has an attenuation of approximately 5.5 dB. It is also possible to design a filter with frequency-independent transmission for the same dispersion, whereby the attenuation can be approximately 7.5 dB.
In order to understand the attenuation that is in principle present in transversal filters, the following should be noted here: The sum of the amounts of all samples of an ideal lossless transversal filter is one. However, the transmission will only be one if all waves are added in phase when the individual power components are combined. Due to the maturity differences of the individual shares, this will only be possible in a very narrow band. However, filters for dispersion compensation should be as broad-banded as possible, in which case all phases of the power components will not match at any frequency across the entire bandwidth.
As has already been said above, in the exemplary embodiment according to FIG. 2, the amplitudes of the individual filter coefficients are realized by the coupling factors of the individual branchers and combiners, the dimensioning also having to take into account the effect of the couplers located in front of the respective branch or behind the respective combiner and the filter structure is expediently symmetrical, ie the same coupling factors are provided for coupling and decoupling, so that additional losses when combining the power components are avoided. The following coupling factors result for the optical transversal filter outlined above:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="4" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">k<sub>0</sub>= -22.97 dB,</entry><entry namest="col2" nameend="col2" align="left">k<sub>1</sub>= -20.28 dB,</entry><entry namest="col3" nameend="col3" align="left">k<sub>2</sub>= -17.91 dB,</entry><entry namest="col4" nameend="col4" align="left">k<sub>3</sub>= -13.45 dB,</entry></row><row><entry namest="col1" nameend="col1" align="left">k<sub>4</sub>= - 9.77 dB,</entry><entry namest="col2" nameend="col2" align="left">k<sub>5</sub>= - 5.68 dB,</entry><entry namest="col3" nameend="col3" align="left">k<sub>6</sub>= - 4.63 dB,</entry><entry namest="col4" nameend="col4" align="left">k<sub>7</sub>= - 2.54 dB,</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">k<sub>8</sub>= - 2.46 dB,</entry><entry namest="col2" nameend="col2" align="left">k<sub>9</sub>= - 2.57 dB,</entry><entry namest="col3" nameend="col3" align="left">k<sub>10</sub>= -2.85 dB,</entry><entry namest="col4" nameend="col4" align="left">k<sub>11</sub>= -1.99 dB.</entry></row></tbody></tgroup></table></tables>
The path length corresponding to the travel time difference τ = 31.64 ps required at FSR = 31.6 GHz (with a refractive index of n = 1.5) is 6.3 mm.
By changing the transit time τ, an exchange between bandwidth B and dispersion D can be achieved insofar as the product B<sup>2</sup>D remains constant for the filter in question. With a variation of the transit time τ, an expansion or compression of the frequency scale of the otherwise unchanged filter characteristic is achieved; a doubling of the bandwidth B manifests itself in a reduction in the dispersion D by a factor of 4. In principle, any values for B and D can be provided. With larger values of B<sup>2</sup>D, however, requires a larger number of samples, which places limits on the implementation. In practice it has been shown that samples that are smaller than 2.5% of the maximum sample value do not have to be realized. This boundary condition is also given in the exemplary embodiment explained here.
While at constant bandwidth the dispersion increases approximately linearly with the number of samples, the increase in the attenuation caused by the filter principle decreases with increasing number of samples, so that the ratio of dispersion to attenuation improves. For example, this ratio is 709 ps / nm / dB with a bandwidth of 10 GHz and 57 samples; at 250 ps / nm / dB, as is currently achieved with dispersion-compensating fibers, this transversal filter should bring about 26 dB additional attenuation, in order to achieve overall better results than with dispersion-compensating fibers. It should be remembered that these properties of the transversal filter are not restricted to a wide bandwidth compared to the fiber; at a bandwidth of 20 GHz, the dispersion based on the fundamental attenuation would have a value of 177 ps / nm / dB in the example.
To fine-tune the phases (arguments) of the individual (complex) samples, heating elements W can be fitted in the branches of the optical transversal filter, as is also indicated in FIG. 2. Tunable directional couplers can also be provided, which also allow the amounts of the sampled values to be set. Heating elements or also tunable couplers must also be provided for optical transversal filters per se (from J. Lightwave Technol., Vol.12 (1994), pp 664 ... 669) is known, so that no further explanation is required here.
The transmission (see FIG. 6) can be used to stabilize the filter to the wavelength of the transmission laser if the filter is appropriately dimensioned. The filter curve can be shifted by evenly changing the running times of all branches using the heating elements. Conversely, it is also possible to stabilize the transmitter wavelength on the filter curve, which will be considered in particular with large filters. In this case, a one-time adjustment of the filter would suffice, if necessary, and only the temperature of the filter would have to be kept constant during operation.
27 sheets
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Every citation, both ways
| Document | Relation | Office | Cited during |
|---|---|---|---|
| DE10147063A1 | Cited by | Germany | Search report |
| US6856724B2 | Cited by | United States of America | Applicant |
| US5602666A | Cites | United States of America | Search report |
11 members in 7 offices
Priority claims5
| Document | Office | Kind | Date |
|---|---|---|---|
| 19515158 | Germany | A | |
| 19515158 | Germany | A | |
| 19515158 | Germany | – | |
| 19515158 | – | – | – |
| DE1995115158 | – | – | – |
Members11
| Document | Office | Kind | |
|---|---|---|---|
| DE19515158C1 | Germany | C1 | |
| CA2174820A1 | Canada | A1 | |
| EP0740173A2This record | European Patent Office (EPO) | A2 | |
| AU5085796A | Australia | A | |
| RU2115145C1 | Russian Federation | C1 | |
| EP0740173A3 | European Patent Office (EPO) | A3 | |
| US5867293A | United States of America | A | |
| AU710268B2 | Australia | B2 | |
| EP0740173B1 | European Patent Office (EPO) | B1 | |
| AT321280T | Austria | T | |
| DE59611335D1 | Germany | D1 |
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Numbers
- Publication
- 0740173
- Publication, DOCDB
- 0740173
- Publication, EPODOC
- EP0740173
- Application
- 96105810
- Application, DOCDB
- 96105810
- Application, EPODOC
- EP19960105810
Titles3
- German
- Schaltungsanordnung zur Dispersionskompensation in optischen Übertragungssystemen mittels eines optischen Filters
- English
- Circuit structure for dispersion compensation in optical communication systems by means of an optical filter
- French
- Structure de circuit pour compenser la dispersion dans les systèmes de télécommunications optiques au moyen d'un filtre optique
Classification
- CPC, 6
- G02B6/12007
- G02B6/29346
- G02B6/29394
- G02B6/29395
- G02B6/29398
- H04B10/25133
- IPC, 3
- G02B6 28
- G02B6 34
- H04B10 2513
Designated states10
- Contracting states, 10
- Austria
- Belgium
- Switzerland
- Germany
- France
- United Kingdom
- Italy
- Liechtenstein
- Netherlands (Kingdom of the)
- Sweden