Method and device for optical finite impulse response filtering and corresponding optical equipment

Abstract

The invention relates to an optical finite impulse response filtering device, comprising: - at least one 1 to N optical coupler (11) configured to separate an incident optical signal (10) into N optical signals transmitted at the input of N optical channels, respectively, - N optical channels each comprising an optical waveguide (12, 13) capable of propagating an optical signal transmitted over the channel, a module (122, 132) configured to attenuate or amplify the optical signal propagated by the optical waveguide and a delay module (131) capable of delaying propagation of the optical signal by the optical waveguide, - P photodiodes (14) arranged at the output of an N / P set of N optical channels, respectively, and configured so that optical signals outputted by the optical waveguides (12, 13) of the set of N / P optical channels are incident on a detection surface of the photodiode arranged at the output of the set of N / P optical channels.

Description

Method and device for optical filtering with finite impulse response and corresponding optical equipment

[0001] The field of the invention is that of fiber optic telecommunications, for example in the context of passive optical networks (PON for "Passive Optical Network" in English) allowing high transmission rates. More specifically, the invention relates to an optical filter with finite impulse response capable of being used in such a network, or more generally in the field of optical telecommunications. Prior art

[0002] The finite impulse response filter, known by the acronym FIR (Finite Impulse Response) filter, is commonly used in signal processing for signal equalization, i.e. to compensate for distortions introduced in the frequency domain by the different elements of the signal transmission chain. It is particularly increasingly used in optical access networks. An FIR filter is a filter whose impulse response is of finite duration.

[0003] The general principle of such a finite impulse response filter is illustrated in the.

[0004] In the case of an analog FIR filter, the input signal x[n] is separated into several channels, on each of which a delay z is applied -1 , as well as an attenuation or amplification whose amplitude is given by a coefficient b i, which corresponds to a coefficient of the filter transfer function. The different signals that encounter delays z -1 fixed and different amplifications / attenuations of coefficient b i , are then recombined at each stage of the filter. The signal y[n] obtained at the output of such an FIR filter is the result of the weighted sum of the different signals passing through the different channels of the filter, and is expressed in the form:

[0005] Either

[0006] Where N-1 denotes the filter order, and where the coefficients b i denote the coefficients of the filter transfer function. The above equation expresses a discrete convolution between the input signal x[n] and a function represented by the values ​​b i , these thus describing the impulse response of the filter.

[0007] The filter thus performs an equalization on the input signal x[n] by recombining with this same signal different delayed and amplified or attenuated versions of itself.

[0008] This type of filter is already widely produced and used in the electrical field to equalize electrical signals, both in the world of electronics and in the world of photonics, particularly for telecommunications.

[0009] Work on the development of such FIR filters, which would perform signal equalization not in the electrical domain, but in the optical domain, has been reported.

[0010] Indeed, for many years, there has been considerable interest in the possibility of producing all-optical FIR filters in order to utilize the wide bandwidth of optics. An all-optical filtering method could significantly improve signal processing performance when the signal to be processed is at high speed.

[0011] Thus, work has been carried out in free space, using the polarization properties of light, as discussed in the article by Y. Zhou, G. Zeng, F. Yu, and H. S. Kwok, “Study on optical finite impulse response filter,” Opt. Eng., vol. 42, no. 8, p. 2318, 2003. However, a disadvantage of the latter is that free-space optics is not well suited to the field of optical telecommunications, mainly due to its bulkiness, but also due to the difficulty of achieving and maintaining complicated optical alignments.

[0012] Other work has been done in integrated photonics on silicon on insulator, using properties of photonic crystals, presented for example in the article by M. Gayet al., “Silicon-on-Insulator RF Filter Based on Photonic Crystal Functions for Channel Equalization,” IEEE Photonics Technology Letters, vol. 28, no. 23, pp. 2756–2759, Dec. 2016.

[0013] The basic diagram of such an optical FIR filter is illustrated in, in the simple example of a two-stage (or taps) FIR filter. The input optical signal 10 passes through an optical coupler 11, which separates it: on a first channel 12, the optical signal is not delayed, but is attenuated or amplified by means of a variable optical attenuator 122 (or VOA for the English "Variable Optical Attenuator") or an optical amplifier; on a second channel 13, the optical signal is subjected to a delay τ 131 and an attenuation / amplification 132. These two signals are then recombined, before being detected by a photodiode 14, which delivers an electrical signal at its output. In principle, such an optical FIR filter therefore corresponds to a Mach-Zehnder interferometer.

[0014] However, the realization of an FIR filter in guided optics poses a problem at the level of the signal detection at the output. Indeed, when several optical waves not traveling the same path are combined, they interfere, and this interference phenomenon can go as far as producing the extinction of the output signal, in the event of destructive interference.

[0015] This phenomenon is illustrated by the schematic representation of the, which is not to scale. The optical intensity detected by the photodiode 14 can be expressed in the form:

[0016] Where I1 denotes the intensity of the signal traveling along the first channel 12, I2 denotes the intensity of the signal traveling along the second channel 13, and where the complementary term of the equation corresponds to the interference term of the two signals traveling along the two channels, which is graphically transcribed by the pulsations represented on the upper part of the graph of the, within the envelopes of the two optical signals.

[0017] For optimum operation of an optical FIR filter, it is necessary to succeed in detecting only the amplitude of the two signals, i.e.: , which corresponds to the lower part of the graph of the.

[0018] It is therefore necessary to implement a solution for detecting optical beams at the filter output that does not allow them to interfere with each other. Previous work has shown that the use of multimode interferometers can cancel this effect. However, such multimode interferometers are expensive.

[0019] To date, there is no all-optical FIR filter that allows direct detection of the output signal without using a component preventing interference between the recombined signals of the different channels.

[0020] There is therefore a need for an optical FIR filter architecture that does not have these various drawbacks of the prior art. In particular, there is a need for such an optical FIR filter architecture on optical fiber coupled with a detection method making it possible to detect the signal at the output of the filter on a photodiode, without using a component canceling the interference effects between the output waves.

[0021] The invention meets this need by proposing an optical filtering device with a finite impulse response, comprising:- at least one 1 to N optical coupler, where N is a natural integer greater than or equal to 2, configured to separate an incident optical signal into N optical signals transmitted respectively at the input of N optical channels,- N optical channels each comprising an optical guide capable of propagating an optical signal transmitted on said channel, a module configured to attenuate or amplify said optical signal propagated by said optical guide and a delay module capable of delaying a propagation of said optical signal by said optical guide,- P photodiodes, where P is a natural integer greater than or equal to 1,respectively arranged at the output of a set of N / P of said N optical channels and configured so that optical signals delivered by said optical guides of said set of N / P optical channels are incident on a detection surface of said photodiode arranged at the output of said set of N / P optical channels.,

[0022] Thus, the invention is based on a completely new and inventive approach to an all-optical FIR filter architecture. Indeed, unlike the solutions of the prior art according to which the optical signals from the different paths of the filter were recombined in the same optical guide, arranged at the output of the filter, before projection of the resulting optical beam onto a photodiode, the claimed architecture does not recombine the optical signals from the N optical paths before projection onto the photodiode(s) arranged at the output of the device. Thus, N spatially separated optical beams are projected, at the output of N optical guides, onto the detection surface of P photodiodes. This eliminates the interference phenomenon that could result from the recombination of the N optical signals upstream of their detection by the photodiode.

[0023] The module configured to attenuate or amplify the optical signal propagating on each of the channels is for example a variable optical attenuator (VOA) or an optical amplifier, or more generally any type of optical or mechanical device allowing the light to be attenuated or amplified.

[0024] According to one embodiment, P=1, and the device comprises a photodiode arranged at the output of the N optical channels and configured so that N optical signals delivered by the N optical guides are incident on a detection surface of the photodiode.

[0025] This embodiment corresponds to the simplest case, in which all the optical beams from the different filter paths are projected onto the same photodiode. This embodiment has the advantage of being the least expensive.

[0026] In other implementations, several photodiodes can be provided, for example two photodiodes each receiving on their surface half of the optical beams generated at the output of the N channels of the filter, or three photodiodes each receiving on their surface a third of the optical beams generated at the output of the N channels of the filter. It should be noted that increasing the number of photodiodes increases the cost of the device. However, when the number N of optical channels is large, it may be necessary to provide several photodiodes, in order to ensure spatial separation of the optical beams incident on the detection surface of the photodiode.

[0027] In an extreme case, we can choose to place a photodiode at the output of each of the channels, therefore to provide N photodiodes, and to proceed with the recombination of the signals in the electrical domain.

[0028] According to one feature, the optical guides are optical fibers. Thus, the claimed optical FIR filter can be produced on optical fiber with a photodiode having a detection surface sufficiently large compared to the constraints of minimum spacing between the two fiber cores.

[0029] According to another feature, the optical guides are made using integrated photonics.

[0030] An integrated photonics implementation (on silicon for example) makes it possible to improve the stability and precision of the device.

[0031] According to one aspect, the delay module of one of said N optical channels belongs to the group comprising:- an optical loop of a length L i imposing a delay t i to the optical signal propagated on said optical path;- an optical ring resonator configured to produce an adjustable optical delay line.

[0032] Such an optical ring resonator is for example described in the article by G. Rostami and A. Rostami, “Tunable optical delay line using two port ring resonator,” in2006 Asia-Pacific Microwave Conference, Dec. 2006, pp. 1308–1312.

[0033] It will be noted that, in one embodiment, the signal propagating on one of the N optical channels does not undergo any delay, so that the delay module imposes a delay t on it i =0.

[0034] More generally, delay modules are configured to introduce delays t i different on each of the N optical channels. In one embodiment, the taps are spaced temporally, by means of delay lines, by a multiple of T or T / 2, where T denotes the symbol time.

[0035] In another aspect, the optical guides and the photodiodes are configured such that beams of the optical signals incident on the detection surface of one of the photodiodes are spatially separated.

[0036] Indeed, if one can detect the different optical beams spatially separated on the surface of the photodiode while avoiding the use of an optical coupler which would have the effect of interfering the signals, then only the amplitudes of the different beams are detected by the photodiode, and interference is avoided. This allows the implementation of such a device in a fiber architecture while avoiding the use of original optical components. The optimal case is the one where the N optical beams are completely spatially separated. Such a spatial decorrelation of the optical signals makes it possible to detect only their amplitudes, or the envelopes of the signals, and not the beats, or oscillations, corresponding to the interference terms of the signals between them.

[0037] This configuration is obtained by achieving an optimal compromise between the size of the detection surface of the photodiode(s) and the spacing between the different optical guides of each of the channels, at the output of the filter. This compromise depends of course on the technology used, and the components chosen for the production of the claimed optical filtering device.

[0038] According to yet another aspect, the optical guides and the photodiodes are configured so that a spatial overlap of the beams of the optical signals incident on the detection surface of one of the photodiodes is less than a determined overlap threshold. This configuration corresponds to an intermediate case, which appears when the beams partially overlap on the detection surface of the photodiode. An overlap threshold is set which must not be exceeded, below which the claimed device can continue to operate efficiently. It will be noted that when the overlap of the beams is significant or total (i.e. greater than the determined overlap threshold), the interference between the beams becomes dominant and the FIR filter becomes ineffective.

[0039] The invention also relates to an optical line terminal (OLT) of an optical communication network comprising an optical filtering device as described above.

[0040] The invention further relates to an optical network unit (ONU) of an optical communication network comprising an optical filtering device as described previously.

[0041] The invention finally relates to an optical filtering method with finite impulse response, comprising: - a separation of an incident optical signal into N optical signals transmitted respectively at the input of N optical channels each comprising an optical guide, where N is a natural integer greater than or equal to 2, - on each of the N optical channels, an attenuation or amplification of an optical signal propagated by the optical guide and an application of a propagation delay to the optical signal, - for each set of N / P optical channels, where P is a natural integer greater than or equal to 1, a projection of the optical signals delivered by the optical guides of said set of N / P optical channels onto a detection surface of a photodiode arranged at the output of the set of N / P optical channels, so that a spatial overlap of the beams of the optical signals projected onto the detection surface is less than a determined overlap threshold.

[0042] The aforementioned finite impulse response optical filtering method, optical line terminal (OLT) and optical network unit (ONU) have at least the same advantages as those provided by the optical filtering device according to the present invention. Presentation of figures

[0043] Other aims, characteristics and advantages of the invention will appear more clearly on reading the following description, given as a simple illustrative, and non-limiting, example, in relation to the figures, among which:

[0044] presents the general principle of a prior art finite impulse response filter;

[0045] illustrates the principle of an optical filter with finite impulse response in integrated photonics according to the prior art;

[0046] graphically describes the interference phenomenon that occurs with an optical filter according to the;

[0047] presents the architecture of an optical finite impulse response filtering device according to an embodiment of the invention, in the simple example of a two-stage filter;

[0048]

[0049]

[0050] : these three figures illustrate in schematic form the different configurations of the projections of the optical signals on the detection surface of the photodiode of;

[0051] presents the general architecture of a finite impulse response optical filtering device in an embodiment with N optical channels and P photodiodes.

[0052] Detailed description of embodiments of the invention

[0053] The general principle of the invention is based on an optical filtering device architecture with finite impulse response coupled with a detection method making it possible to detect the optical signal at the output of the filter on a photodiode without using components canceling the effects of interference between the optical waves at the output.

[0054] This general principle is illustrated, in a simple embodiment in which the filter comprises N=2 optical channels and P=1 photodiode, by the architecture of. Those skilled in the art will easily extend this teaching to other values ​​of N and P.

[0055] Thus, the schematic diagram of an optical FIR filter of order 1 is presented, i.e. presenting only two stages (“taps”), or N=2 optical channels.

[0056] The incident optical signal x[n] 10 is split into two by an optical coupler 11. On the upper arm, the signal encounters an element 122 for attenuating or amplifying the signal, for example a variable optical attenuator or VOA or an optical amplifier (semiconductor for example), for applying a multiplicative coefficient b0 to the optical signal circulating in the optical guide 12. On the lower arm, the signal encounters a delay 131 relative to the optical path of the arm as well as an attenuation / amplification element 132. The delay element 131 is for example produced by means of a delay loop extending the path of the optical signal. The attenuation / amplification element 132 is for example a variable optical attenuator or VOA or an optical amplifier, for applying a multiplicative coefficient b1 to the optical signal circulating in the optical guide 13.The optical beams circulating in the optical guides 12 and 13 are then projected, at the output of the filter, onto the surface of a photodiode 14. The signals resulting from the two arms 12, 13 of the filter are thus detected on the surface of the photodiode 14, which delivers an electrical signal y[n] at the output. These signals are not recombined in the same optical guide before projection onto the surface of the photodiode 14.

[0057] Figures 5A to 5C illustrate the different illumination configurations of the detection surface 141 of the photodiode 14.

[0058] Illustrates the optimal operating case of the FIR optical filter of the. In this configuration, the incident optical beams from the optical guides referenced 12 and 13 are projected into two completely separate zones, referenced 1 and 2. In this configuration, the detection surface 141 is thus large enough so that there is no overlap between the projections of the two beams; similarly, the orientation and spacing of the two optical guides 12, 13 make it possible to guarantee this absence of overlap on the surface of the photodiode 14.

[0059] Illustrates an acceptable case of operation of the FIR optical filter of the. In this configuration, the incident optical beams from the optical guides referenced 12 and 13 are projected into two partially overlapping zones, referenced 1 and 2. This partial overlap 15 is nevertheless sufficiently weak to allow detection of the intensities I1 and I2 of the optical signals projected respectively by the optical guides 12 and 13.

[0060] On the other hand, illustrates a configuration in which the projection of the optical beams from the optical guides 12 and 13 occurs on the same zone 16: the overlap is total. When the overlap of the beams is significant or total, the interference between the beams becomes dominant and the FIR filter becomes ineffective.

[0061] The filter can be made on optical fiber with a photodiode having a detection surface large enough for operation in the optimal case of the. A realization in integrated photonics (on silicon for example) is also possible, to improve the stability and precision of the device.

[0062] Such an all-optical FIR filter allows for all-optical signal processing and equalization functions to be performed, without resorting to converting the optical signal to the electrical world. In particular, it allows for the implementation of all-optical equalization to compensate for distortions and various transmission problems in optical telecommunications systems. It can advantageously be integrated into any equipment in an optical network in which signal equalization is useful, such as an OLT-type line termination equipment or an ONU-type optical network unit.

[0063] The present invention presents the more general architecture of an optical filtering device with finite impulse response of order N-1 in an embodiment with N optical channels and P photodiodes.

[0064] The incident optical signal x[n] 10 is separated into N by an optical coupler. On each of the N arms, the signal encounters a VOA or optical amplifier type element allowing the signal to be attenuated or amplified, and a delay (possibly zero on the path i=0) relative to the optical path of the arm. The delay element of the optical path, or arm, of index i allows a delay t to be applied ito the optical signal. In one embodiment, the delays of the different channels follow one another. For example, by designating the symbol time (i.e. the inverse of the bit rate) by T, we can construct a device in which the taps are spaced by T: the delay applied to channel 1 is then T, the delay applied to channel 2 is 2T, the delay applied to channel 3 is 3T, and the delay applied to channel i is therefore i*T, etc. The attenuation / amplification element of the optical channel, or arm, of index i, makes it possible to apply a multiplicative coefficient b i to the optical signal circulating on channel i.

[0065] In this example, we choose for illustrative purposes to group the optical paths in threes: thus, the optical beams circulating in the optical guides of index 0, 1 and 2 are projected, at the output of the filter, onto the surface of a photodiode 14 1 ; the optical beams circulating in the optical guides of index 3, 4 and 5 (not shown) are projected, at the output of the filter, onto the surface of a photodiode 14 2, and so on, up to the optical beams circulating in the optical guides of index N-3, N-2 and N-1, which are projected, at the output of the filter, onto the surface of a photodiode 14 P . The output signals of photodiodes 141 to 14 P are added to reconstruct the resulting electrical signal y[n].

[0066] The choice of the number N of filter stages and the number P of photodiodes depends on different criteria, the precision sought for the FIR optical filter and the intended application.

[0067] Indeed, the speed of the charge carriers, and therefore the bandwidth of a photodiode, is directly related to its active surface. Typically, for a bandwidth of 20 GHz, the diameter D

[0068] of the active surface of a photodiode is of the order of D ≥ 10 µm. This order of magnitude is taken from the specifications of the component manufacturers. The larger the active surface of the photodiode, the slower the photodiode. It is therefore necessary to aim for photodiode surfaces that are small enough for high-speed applications. On the other hand, it is understood that the larger the surface of the photodiode, the greater the number of optical beams that can be projected onto this surface without overlap (), or with acceptable overlap ().

[0069] In the case of an implementation of the FIR optical filtering device in the form of a fiber component, the characteristics of single-mode optical cables and fibers must also be taken into account, as described for example in the ITU (International Telecommunications Union) standard G. 652 (11 / 16 approved on November 13, 2016). In particular, the standard provides that the core diameter of the SSMF 28 single-mode fiber (used in networks) is D = 9 µm; the minimum spacing between two fiber cores is e ≥ 30 µm. This minimum spacing conditions the proximity of the different optical output arms of the optical FIR filter, and therefore, consequently, the number of distinct optical beams that can be projected, from these different optical arms, onto the active surface of the same photodiode.

[0070] In the case of implementing the FIR optical filtering device as an integrated photonic component, the design constraints of the optical guides must also be taken into account. The work of Y. Huang, Q. Zhao, L. Kamyab, A. Rostami, F. Capolino, and O. Boyraz in the article “Sub-micron silicon nitride waveguide fabrication using conventional optical lithography,” Opt. Express, vol. 23, no. 5, p. 6780, Mar. 2015, for example, reports an order of magnitude to the state of the art of the typical size of an optical guide d ≤ 1 µm. It should be noted that this value is indicative, as it is very highly dependent on the technologies, wavelengths, desired guidance, etc. Similarly, the minimum spacing between two guides is of the order of e ≥ 3-4 µm.

[0071] All these values ​​are indicative and vary greatly with the technologies, manufacturing methods, materials as well as the parameters of the final photonic device (guiding, wavelength, etc.). They must nevertheless be taken into account to satisfy the compromise necessary for the operation of the claimed optical FIR filtering device, namely the choice of the size of the active surface of the photodiode on the one hand, and the choice of the number of distinct optical beams projected by the output waveguides of the device onto this active surface on the other hand, in order to ensure that the optical beams are spatially separated at the surface of the photodiode, or overlap sufficiently little so that the operation of the optical FIR filter remains acceptable in terms of performance.

Claims

Optical filtering device with finite impulse response, comprising:- at least one 1 to N optical coupler (11), where N is a natural integer greater than or equal to 2, configured to separate an incident optical signal (10) into N optical signals transmitted respectively at the input of N optical channels,- N optical channels each comprising an optical guide (12, 13) capable of propagating an optical signal transmitted on said channel, a module (122, 132) configured to attenuate or amplify said optical signal propagated by said optical guide and a delay module (131) capable of delaying a propagation of said optical signal by said optical guide,- P photodiodes (14; 141-14 P), where P is a natural integer greater than or equal to 1, respectively arranged at the output of a set of N / P optical channels among said N optical channels and configured so that optical signals delivered by said optical guides of said set of N / P optical channels are incident on a detection surface of said photodiode arranged at the output of said set of N / P optical channels. Optical filtering device according to claim 1, characterized in that P = 1, and in that said device comprises a photodiode (14) arranged at the output of said N optical channels and configured so that N optical signals delivered by said N optical guides are incident on a detection surface (141) of said photodiode. Optical filtering device according to any one of claims 1 and 2, characterized in that said optical guides (12, 13) are optical fibers. Optical filtering device according to any one of claims 1 and 2, characterized in that said optical guides (12, 13) are produced using integrated photonics. Optical filtering device according to any one of claims 1 to 4, characterized in that said delay module (131) of one of said N optical channels belongs to the group comprising: - an optical loop of a length L i imposing a delay t i to the optical signal propagated on said optical path;-an optical ring resonator configured to produce an adjustable optical delay line. Optical filtering device according to claim 5, characterized in that said delay modules (131) are configured to introduce delays t i different on each of said N optical channels. Optical filtering device according to any one of claims 1 to 6, characterized in that said optical guides and said photodiodes are configured so that beams of said optical signals incident on said detection surface of one of said photodiodes are spatially separated. Optical filtering device according to any one of claims 1 to 6, characterized in that said optical guides and said photodiodes are configured so that a spatial overlap (15) of the beams of said optical signals incident on said detection surface (141) of one of said photodiodes is less than a determined overlap threshold. Optical line terminal (OLT) of an optical communication network comprising an optical filtering device according to any one of claims 1 to 8. Optical network unit (ONU) of an optical communication network comprising an optical filtering device according to any one of claims 1 to 8. Optical method of filtering with finite impulse response, comprising:- a separation of an incident optical signal (10) into N optical signals transmitted respectively at the input of N optical channels each comprising an optical guide, where N is a natural integer greater than or equal to 2,- on each of said N optical channels, an attenuation or amplification of an optical signal propagated by said optical guide and an application of a propagation delay to said optical signal,- for each set of N / P optical channels among said N optical channels, where P is a natural integer greater than or equal to 1, a projection of the optical signals delivered by said optical guides of said set of N / P optical channels onto a detection surface of a photodiode arranged at the output of said set of N / P optical channels, so that a spatial overlap (15) of the beams of said optical signals projected onto said detection surface (141) is less than a determined overlap threshold.

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