WO2013178847A1

WO2013178847A1 - Sampled delay line

Info

Publication number: WO2013178847A1
Application number: PCT/ES2013/070292
Authority: WO
Inventors: José CAPMAY FRANCOY; Ivana GASULLA MESTRE
Original assignee: Universitat Politècnica De València
Priority date: 2012-05-31
Filing date: 2013-05-08
Publication date: 2013-12-05
Also published as: ES2436873B1; ES2436873A1

Abstract

The invention relates to a sampled delay line comprising a homogeneous or heterogeneous multi-core optical fibre (2, 2a) used as a sampled delay line that can be reconfigured for implementation of microwave photonics functionalities, involving the processing of radio-frequency signals carried by one or more optical carriers. In particular, the invention relates to the use of heterogeneous multi-core fibres (2), the cores (3) of which have different chromatic dispersion properties, for implementing different incremental delays on a single signal supplied to each of the cores. The invention also involves supplying a single signal to all of the cores, which signal is wavelength multiplexed (WDM) or obtained from a single emitter laser having multiple channels or optical modes equally spaced in wavelength, using the modelocking technique, and supplying different independent WDM signals to each of the cores (3). The invention can be used as a tunable, fixed transverse filter, as an optical power supply for phased array antennas, and as an arbitrary generator of radio-frequency signals.

Description

SAMPLE DELAY LINE DESCRIPTION

OBJECT OF THE INVENTION

The present invention relates to a sampled delay line that aims to allow the use of multicore optical fibers (MCF) as sampled and reconfigurable delay lines for the implementation of various functionalities employed in Microwave Photonics, which involve the processing of radio frequency spectrum signals carried by one or more carriers located in the optical region of the spectrum. In particular, the use of heterogeneous multicore fibers, in which each core has a different composition and chromatic dispersion properties, allows the implementation of different incremental delays on the same signal fed to each of the nuclei.

Homogeneous multicore fibers can also be used to implement sampled delay lines, including at the exit of each core of the multicore fiber an additional fiber segment that provides the differential delay.

It is also the object of the invention to provide a great versatility of the proposed sampled delay line according to the multicore fiber optical feeding structure: feeding a single multichannel optical signal to all the cores, either generated by mode coupling or modeling. wavelength multiplexing (WDM, Wavelength Multiplexing Division), by using multiple optical sources (of different emission wavelengths), so that the multichannel optical signal feeds each core separately and, finally, by feeding Multichannel or WDM optical signals independent of each of the cores.

In either case the incremental delay can be constant or variable. Another object of the invention is the use of new connectors between multicore fibers and mononuclear fibers, both at their input and at their output, either for the injection of a single modulated optical signal at the input of the MCF, or for the joint detection of the N signal samples in the MCF output plane.

The present invention is within the framework of Microwave Photonics. Specifically, within the general scope of photonic processing of radiofrequency signals, microwaves and millimeter waves. It is also part of the field of multicore optical fiber applications, a carrier for recently developed broadband telecommunication systems to support spatial division multiplexing techniques. This new carrier, originally designed for high-speed digital baseband transmission applications, can also offer advantages in the transmission of analog optical signals, typical of radio-fiber systems and microwave photonics. Specifically and given its unique properties, it provides a unique platform for the development of new devices and photonic systems specific to this field of application, with distinctive and novel properties.

BACKGROUND OF THE INVENTION

In the state of the art, the use of multicore fiber optic fiber (MCF) is known, which has several cores in the same cover duly located in the sectional plane so that each core acts as a single-mode transmission guide (SMF, singlemode fibers) composed of a single independent core. Therefore, its incorporation into optical communications networks can offer transmission capacities far superior to those obtained with standard single core fibers (SCF). The first multicore fibers to be designed and manufactured were homogeneous multicore fibers, composed of N identical cores in terms of size and propagation characteristics. In this type of MCF, the N cores are arranged in the same diameter cover (b) and the density of cores is determined by the distance (Λ) between cores that guarantees a certain level of interference or crosstalk along a given propagation length (L). In the homogeneous fibers the N nuclei have the same diameter and the same propagation characteristics, that is to say the same refractive index (/ ¾), the same propagation constant β, and therefore the same group delay r, and the same chromatic dispersion (D).

Recently the design of heterogeneous multicore fibers has been proposed, where all or part of the nuclei that make it up have different propagation properties. This type of MCF can be designed to provide lower levels of interference between pairs of cores than the homogeneous MCF and, in addition, allows to obtain a higher density of cores for the same cover diameter, as detailed below.

Multicore optical fibers were initially conceived in response to the exponential growth in the demand for transmission capacity that is taking place in optical telecommunications networks. Fiber optic telecommunication systems have been using various multiplexing schemes to date, such as time division multiplexing (TDM, Time Division Multiplexing), optical wavelength division (WDM, Wavelength Division Multiplexing) and by polarization division (PDM, Polarized Division Multiplexing), thanks to which it is possible to transport transmission rates of up to several tens of Tb / s through standard single-mode fiber optic links. However, the increase in transmission rates to values around 100 Tb / s through a conventional single-mode fiber is perceived as difficult due, among others, to the bandwidth limitation of fiber amplifiers, to the requirements in the signal-to-noise ratio, to the limits in the injected optical power and to the inherent lines of the optical fiber. Consequently, and once the possible multiplexing dimensions were overloaded, various research groups began to consider new solutions, such as the spatial division multiplexing technique (SDM) using multicore optical fibers. Thus, as a particularization of multiple input multiple transmission systems output, the increase in transmission capacity is achieved by transmitting a different signal for each of the N cores that make up the MCF and acting as independent channels, requiring therefore the use of N transmitters at its input and N receivers at its output , that is, a transmitter connected to the input of each core and a receiver connected to the output of each core.

Through spatial multiplexing, links implemented by homogeneous MCF composed of 7 cores have been demonstrated, with constant separation between cores Λ, which have reached transmission capacities of up to 109 Tb / s in the optical bands C and L along 16.8 km and values of up to 56 Tb / s in the C band for a distance of 76.8 km. Improvements in the design of the MCF links have been achieved recently using separations between non-constant cores or using heterogeneous schemes. Through the first design and thanks to a circular structure, it has been possible to propose a substantial increase in the number of cores up to 19, being able to manufacture fibers composed of 10 cores in a cover diameter of 204.4 μΐΎΐ. The second proposal would also allow up to 19 cores, reducing the cover diameter to 125 μιη, thanks to the use of heterogeneous MCF, in whose structure the nuclei are distributed spatially in triangular (with the same un) or rectangular patterns (in a network with two values of Λ different).

Various types of multicore fibers have been designed and manufactured to date, always seeking the lowest possible levels of propagation loss and interference between cores to obtain the highest possible density. The level of interference, that is, the maximum power transferred between cores, as well as the losses caused by micro and macrocurvatures, depend on the design of the refractive index profile of the cores (n _co ) of the cover refractive index {n _c ¡) and the geometry of the core network (distance between cores Λ). Among the companies that currently manufacture multicore fibers we can highlight OFS Labs (USA), NTT (Nippon Telegraph and Telephone Corporation, Japan), Sumitono Electric Industries (Japan) and Fujikura (Japan). The homogeneous 7-core MCFs manufactured by OFS Labs offer losses around 0.37dB / km and 0.23 dB / km respectively in the second and third transmission window for the central core, while passing values of 0.40 and 0.26 dB / km for the outer cores. Similar attenuation values, between 0.22 and 0.24 dB / km, were obtained for homogeneous MCFs with different separation between cores manufactured by Fujikura.

Another important issue to address regarding links implemented with MCF is the connectivity between MCFs and standard mononuclear fibers. The commercial use of MCFs as a means of parallel transmission in optical communications networks requires reliable low-cost devices to implement the coupling of signals at the input and output of each of the cores individually. To date, several devices have been designed for spatial multiplexing systems, such as the OFC labs TMC (Tapered Multicore fiber Connector) multicore fiber connector designed for 7-core MCF, through which 7 independent mononuclear fibers are fused together to match the transverse spatial arrangement of the nuclei in the MCF. In this way, insertion losses have been achieved between 0.38 and 1 .8 dB, with interference levels between cores below -38 dB. There are no known connectors that allow the connection of a single SCF fiber to an MCF fiber.

Multicore fibers have never been used as sampled delay lines, or with any other functionality, in the field of application of microwave photonics or radio-fiber systems. A variety of patents have been found that refer to the design and construction of various types of multicore fibers with controlled coupling or free of coupling between cores. As an example, patent documents can be cited: US201 10243517 (Uncoupled Multi-Core Fiber), US201 10182557 (Multi-Core Fiber), US201 10206330 (Multicore Optical Fiber) and US7418178 (Multicore Fiber.) It can also be cited patent document US201 1274435 (Multicore Fiber Transmission Systems and Methods) in which the application of optical fibers to systems is discussed. This patent relates to the transmission of data in digital CWDM / DWDM systems through the cores of multicore fibers using vertical cavity lasers and photodetector arrays.

In the state of the art, multicore optical fibers have been used as a transmission medium for N independent channels, and never as a delay line or in the fields of microwave photonics.

DESCRIPTION OF THE INVENTION

To achieve the objectives and solve the aforementioned drawbacks, the invention provides a new delay line characterized in that it comprises at least one optical source for generating an optical signal, at least one RF (Radio Frequency) modulator of the optical signal. , to modulate the optical carrier with the RF signal, and a multi-core optical fiber (MCF). Said MCF can be both a heterogeneous MCF and a homogeneous MCF.

In the case that a heterogeneous MCF is used, its N nuclei have the same length L and a different color dispersion D. The modulated signal is applied to each of the cores, to provide at the output of each core N that forms the delay line, the same signal but delayed according to different incremental delays depending on the chromatic dispersion of each core, providing then N samples spaced in time.

In the case of using a homogeneous MCF, its N nuclei have the same length L and the same chromatic dispersion D, and the modulated signal is applied to each of the nuclei. The output of each core N of the homogeneous MCF is connected to a single mode optical fiber, which can be single mode fibers of different lengths with the same chromatic dispersion D, or they can also be fibers with the same length and different chromatic dispersion D; to, in any of the cases, provide at the output of the delay line, corresponding to the output of each single mode fiber, a same signal but delayed according to different incremental delays, acting as a delay line sampled over time.

The incremental time delays of each nucleus of the heterogeneous MCF and of each single-mode fiber can be constant between each pair of adjacent or non-constant nuclei.

Each nucleus of the heterogeneous MCF has a linear group delay with a different slope wavelength. By changing the wavelength of the optical signal, the incremental delay value can be varied, acting as the sampled delay line in the reconfigurable time.

In a first embodiment of the invention it is envisioned that it comprises a multichannel M-channel optical source generated well by the modeling technique, which provides a pulse train with different wavelengths that are modulated with the same RF signal and applied to the N nuclei of the fiber, or generated by a plurality of optical sources M of different emission wavelengths, which are multiplexed in wavelength (WDM) and modulated with the same RF signal. Said pulse train equivalent to the optical multiplex is responsible for feeding each of the MCF cores, so that at the output of each core a delay line of the M modulated signals is obtained, each line experiencing a given incremental delay given by the particular dispersion characteristics of each core.

A second variant of the invention comprises an M-channel multichannel optical source, well generated by the modeling technique, which provides a pulse train with different wavelengths that are modulated with the same RF signal and applied to the N cores of the fiber or generated by a plurality of optical sources M of different emission wavelengths, which are multiplexed in wavelength (WDM) and modulated with the same RF signal. Said pulse train or optical multiplex is responsible for feeding each of the MCF cores, but in this case the set of outputs of the N cores is connected to a WDM demultiplexer. Through this configuration, at the output of the demultiplexer M delay lines are obtained, each corresponding to one of the M optical wavelengths, each consisting of N samples.

A third variant of the invention comprises various multichannel optical channels of M channels generated by the modeling technique or by pluralities of optical sources of different emission wavelengths, which are multiplexed in wavelength (WDM) by N different multiplexers, modulating each of the N multiplexed groupings with a different RF signal. Through this scheme, each core k that makes up the MCF is fed with a different WDM multiplex, consisting of M _k optical carriers. This configuration provides the output of each of the N cores, a different and independent delay line, where the number of samples is given by the number of wavelengths that make up a certain multiplex (Mk), while the incremental delay is defined according to the dispersion characteristics of each of the N cores

In any of the above cases, the separation between the wavelengths corresponding to adjacent optical sources may be constant or not constant, according to the required needs.

Also, in any of the above cases, the RF modulation can be an amplitude modulation or a phase modulation.

The above embodiments have the advantage that they can be adapted for use in various Microwave Photonics applications.

In a first type of adaptation, the sampled delay line is applied as a fixed and tunable transverse filter, where the outputs of the cores that make up the MCF are grouped and connected to a photodetector in order to detect the RF signal. Samples of the modulating signal can be independently weighed by variable optical attenuators. The simplest implementation of microwave transverse filtering considers a single modulated optical source, so that the MCF acts as a delay line Composed of N samples. Through this scheme, tuning the emission wavelength of the optical source allows the free spectral range (FSR) of the radio frequency response of the implemented filter to be varied.

A variant of the above scheme consists in using either a multichannel M channel optical source generated by the modeling technique or a plurality of M optical sources, of different emission wavelengths, which are multiplexed in wavelength (WDM) and They are modulated with the same RF signal. Thus, at the output of each core of the MCF, an independent transverse filter is established, characterized by a different FSR, tunable by varying the separation between emission wavelengths of the optical sources. To do this, the output of each MCF core is connected to a different RF photodetector.

Another variant is also provided in which the output of the delay line is connected to a WDM demultiplexer, so that in each of the M outputs of the demultiplexer, a different filter is established consisting of N samples, with different FSR. For this, each output of the demultiplexer is connected to a different photodetector.

In a second type of adaptation, the sampled delay line is applied to arrays of phase microwave antenna arrays (Phased array antennas, PAA). To do this, the output of each core of the MCF is photodetected independently and connected to a different antenna element, where each antenna element belongs to a set or array of antennas in phase. In this case, the pointing direction The resulting array factor is governing by varying the incremental group delay between cores through the tuning of the wavelength.

The previous case corresponds to an RF transmitter system. For reception systems of radiated RF signals, a variant of the above scheme includes the inclusion of a different optical source and electro-optic modulator at the input of each core. The outputs from the N Cores that make up the MCF are connected to a single photodetector for the reception of the radiated signals.

In the previous cases of application in array feeding systems, variable optical attenuators can be included at the exit of each core to implement various poisoning schemes.

In a third type of adaptation, the sampled delay line is applied to the arbitrary optical generation of RF signals. In a first variant, the device comprises an RF pulse generator connected to a first and a second electro-optic modulator, fed in regions with opposite slopes, by different voltages, so that the same electric pulse is injected into both modulators. Both electro-optical modulators are connected to a single optical emission source at their input. The first modulator is connected at its output to a 1 xN optical coupler followed by 2x1 optical switches that connect to each of the N cores of the MCF. The selection state of the 2x1 switch allows to provide a selection of Ν _Ϊ positive samples to be transmitted through a subset of Ν _Ϊ cores. On the other hand, the second electro-optical modulator is connected to a second 1xN optical coupler also followed by a series of 2x1 optical switches, in order to provide a selection of N ₂ negative samples to be transmitted by a second subset of N ₂ cores. The output of the N cores is connected to a single RF photodetector, obtaining the desired RF pulse pattern.

It is expected that it comprises variable optical attenuators at the output of the first and second 1 xN optical coupler, to control the amplitude of the samples individually.

In a second variant of arbitrary RF signal generation, the RF pulse generator is connected to a single electro-optical modulator, whose output is connected to the input of the N cores of the MCF. Each output of said cores is connected to a 1 x 2 optical switch, each of which is connected to a first and a second optical coupler. Nx1, which, in turn, are connected to a balanced photodetector, which allows the separation of a subset Ni of positive pulses and a subset N ₂ of negative pulses. Pulse selection is made by controlling the 1 x 2 switches at the output of each core that directs the signal to one of the balanced photodetectors. For example to the upper balanced photodetector, for positive coefficients, or to the lower balanced photodetector, for negative coefficients.

The inclusion of variable optical attenuators is provided in each of the inputs of the first and second optical coupler Nx1, to control the amplitude of the samples individually.

In order to make the connections of the input and / or output of the delay line, indicated above, the invention has developed a device responsible for connecting a single mononuclear fiber with the N cores of the MCF. Said element is arranged at the entrance of the MCF fiber in cases where it is required to perform the injection of a single modulated optical signal to the MCF input, while it is placed reciprocally at the MCF output in those cases. in which it is necessary to detect the N samples of the signal in the output plane of the MCF.

In accordance with all of the foregoing, the invention can be used for use as a fixed and tunable transverse filter, as optical feed of arrays of phase antennas or to implement arbitrary generators of radiofrequency signals

The advantages provided by the present embodiment over others corresponding to the state of the art reside mainly in the use of the parallelism that the MCFs offer in a reduced carrier size and capable of distributing the signals over long distances and with low losses. Providing such parallelism within a single physical structure also provides advantages in terms of uniformity in the length of the carriers and uniform behavior against mechanical and environmental variations. BRIEF STATEMENT OF THE FIGURES

Figure 1.- Shows a schematic view of the representation of a multicore fiber.

Figure 2.- Shows an embodiment of the invention in which a heterogeneous multicore fiber is used as a discrete optical delay line. In this example, a single optical carrier is applied at the input of the different N cores to obtain spatial diversity at its output.

Figures 3a and 3b.- Shows a graph of the group delay of each of the nuclei that make up a heterogeneous multicore fiber as a function of the optical wavelength. Each core has a different linear slope of a different slope, or what is the same, a different value of the chromatic dispersion parameter D. In figure 3a for a wavelength Λι an incremental delay Ti is obtained, and in the figure 3b for a Λ ₂ an incremental delay T ₂ is obtained.

Figure 4.- Shows an embodiment of the invention in which a sampled delay line based on a homogeneous multicore fiber and optical delay lines is obtained.

Figure 5.- Shows an embodiment of the invention in which a delay line based on a heterogeneous multicore fiber with multichannel optical input or WDM and with spatial diversity output of the WDM signal is obtained.

Figure 6.- Shows a graph of the group delay versus the wavelength to indicate the principle of operation of the delay line with multichannel or WDM optical input signals of Figure 5.

Figure 7.- Shows an embodiment of the invention in which a delay line based on a heterogeneous multicore fiber similar to that of Figure 5 is obtained, with multichannel optical input or WDM, but with the difference that the output is obtained in the wavelength domain through demultiplexing in WDM. Figure 8 shows an embodiment of the invention in which a delay line based on a heterogeneous multicore fiber with input from several different multichannel or multiplex WDM optical sources for each core and with output in spatial diversity is obtained.

Figure 9.- Shows an embodiment of the invention in which a delay line based on a heterogeneous multicore fiber is obtained whose nuclei have different group delay slopes D¡. Single optical carrier input with output in spatial diversity.

Figure 10.- Shows an embodiment of the invention in which a delay line based on a heterogeneous multicore fiber is obtained, with input from a multichannel optical source or WDM with non-constant wavelength separation ΔΛ, with output in spatial diversity .

Figure 11.- Shows an implementation of a microwave transverse photonic filter using a heterogeneous multicore fiber.

Figures 12a - 12d.- They show a series of graphs of the characteristic electrical transfer function of the microwave transverse photonic filter, highlighting the tunability and reconfigurability functionalities, according to the embodiment of the previous figure.

Figure 13.- Shows an implementation of a feed network in arrays of antennas in phase using a heterogeneous multicore fiber.

Figure 14.- Shows a scheme for the generation of arbitrary RF signals by phase inversion in a Mach-Zehnder modulator, using a heterogeneous multicore fiber.

Figure 15.- Shows a scheme for the generation of arbitrary RF signals by balanced detection, using a heterogeneous multicore fiber. Figure 16.- It shows the structure of the connectors that the invention provides to allow the connection of a mononuclear fiber with the N cores at the input of the MCF fiber and the connection of the N cores at the MCF output with a mononuclear fiber .

DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated in the background section, the multicore fibers comprise N cores that are arranged in the same cover of diameter b and the density of cores is determined by the distance Λ between cores that guarantees a certain level of interference or crosstalk at along a given propagation length L, as shown in Figure 1. In the homogeneous fibers the N nuclei have the same diameter and the same propagation characteristics, that is to say the same refractive index n _c ¡, the same propagation constant β, and therefore the same delay of group T, and the same dispersion chromatic D, unlike heterogeneous multicore fibers that have different propagation characteristics.

Next, the propagation characteristics of a multicore fiber are described, for this the most generic case of heterogeneous MCF is considered, where each of the different nuclei j of the N nuclei that make it up has the same diameter a and different index of refraction n _bearing Each core therefore acts as a waveguide that transmits a different propagation mode that has a propagation constant / ¾ ^■ , which can be expressed according to its second-order Taylor series development along the angular central frequency of emission of the optical source ω ₀ as: β _] (ω) (ω - ω ₀ )

one

= β ° + β \ {ω - ω ₀ ) + -β ^ ω - ω ₀ Υ.

The group delay η at a given optical wavelength λ can be derived from the above equation as

where L is the length of the MCF bond, c is the speed of light in a vacuum, _{v¡¡ =} \ i β represents the propagation group velocity of the jth core and D¡ is the scatter parameter associated to the nucleus ^' -th defined as

The particularization of the previous expressions to the case of a homogeneous MCF is obtained considering the same propagation constant β, and therefore the same group delay r, for each of the nuclei.

An example of the invention of an implementation of a sampled delay line using a heterogeneous multicore fiber optic 2 formed by heterogeneous dispersive cores 3 of different composition fed by the same input signal from an optical source is shown in Figure 2. 4, constituted by a laser, which provides a continuous carrier optical signal 5 (P1) that is applied to an electro-optic modulator 6 to modulate the signal 5 by a radio frequency (RF) signal 7. Radiofrequency modulation can well be performed in amplitude or phase.

The modulated signal 8 is divided between the N cores 3 that make up the heterogeneous multicore fiber 2. For which a connector 31 is provided, which is described later with the help of Figure 16. Although all the cores 3 have the same length L , each of them presents a linear group delay with the different slope wavelength, or what is the same, a different value of the chromatic dispersion parameter D, as shown by way of example in Figures 3a and 3b. Under linear approximation, and by the appropriate material design of each core, the characteristic of the group delay of each of them can be approximated by

T ₀ + jDL (A - Á ₀ ) where λ represents the wavelength of the optical carrier, j the core number, D the basic dispersion parameter of the least dispersive core and r ₀ a basic and equal group delay for all cores 3 that occurs at a wavelength reference λ ₀ , τ ₀ = Uv _g . The basic incremental delay between the output signals of two cores 3 that possess delay of adjacent groups is then given by:

This incremental group delay is fixed for a given value of the working wavelength. For example, Figure 3a shows the distribution of delays when the working wavelength is

. In this case, the delay line produces N replicas of the modulated signal at its input delayed respectively by values 0, Ti, 27Ί, ... (Λ / -1) 7Ί. To change the value of the basic delay, only the operating wavelength of the laser 4 must be changed. For example, the change of said basic delay of 7Ί → T ₂ is shown in Figure 3b, when the laser wavelength 4 feeder changes from Λι → λ ₂ . Thus, the proposed delay line provides N samples 10 time-equalized at the output 9 of the N cores 3, where the basic delay between samples 10 is tunable by modifying the wavelength. Therefore, samples 10 are obtained at spatially different outputs 9 and therefore this configuration is referred to as spatial diversity.

The heterogeneous multicore fiber 2 used in the scheme of Figure 2 can be replaced by a homogeneous multicore fiber 2a, followed by a set of optical delay lines 1 1 implemented by single-mode fiber segments (SMF, single-mode fibers) 1 1 mononuclear (SCF) , singlecore fiber) standard of various lengths, as illustrated in Figure 4. In this case each core 3a of the homogeneous multicore fiber has the same group delay

According to the above, a transverse filter with an incremental basic delay between samples T can be available, for which each core k must be coupled to a section k of SMF 1 1 of length L _S MF, k, such that the difference between delays associated with contiguous sections, it is precisely T _S MF = T. In the event that the SMF does not present chromatic dispersion, this would be given by

Lsmf

^ SMF ^~ (LsMF, k + l ^~ LsMF, k)

vgSMF ^v gSMF where L _S MF represents the basic difference between the lengths of contiguous sections and the group speed V ₉ SMF is the same for each fiber section 1 1. If, on the contrary, the SMF introduced has chromatic dispersion, the difference between contiguous delays would be

'SMF (λ) - (L _{SMF k + 1} - L _{SMF k} ) ¹ - + D (A - A ₀ ) ^~ LsMF ¹ - + D (A - A ₀ )

vgSMF ^v gSMF

In this case, the tuning of the emission wavelength of the laser 4 makes it possible to dynamically control the incremental basic delay.

Another embodiment of the invention is shown in Figure 5 in which the sampled delay line is fed by a multiplex of optical sources. In this case the fed signal comes from a WDM wavelength multiplex of modulated lasers 4a-4c, instead of a single optical source. An equivalent configuration consists in using a single emitting laser of different channels or wavelength optical modes as a power source using the phase phase modeling or modeling technique, which provides a pulse train with different wavelengths that are modulated with the same RF signal and apply to the N fiber cores. Referring to said figure 5, the optical power signal is composed of M lasers 4A-4M whose wavelengths of the signals 5A

- 5M they produce follow the law Á¡ = λ ₀ + i, i = 1, 2, ... M. Such signals 5A

- 5M are applied to a multiplexer 13 whose output is applied to the radiofrequency modulator 6. In case of using a modeling source the multiplexer WDM is not necessary. In this way the multiplex is modulated 8A-8M by a radiofrequency signal 7 that feeds each of the nuclei N of a heterogeneous MCF 2. Accordingly, as shown in Figure 6, which illustrates the generation of the incremental delays, each multiplex suffers in each core j a different basic delay that can be expressed as: j = Tj (Á _k ) - τ ¡(Á _k _i) = jDLAÁ

That is, each output 9 of the cores 3 corresponds to a delay line of M samples 15A-15N, each delayed by a different incremental delay value.

The delay line provides a two-dimensional delay configuration where the delay experienced by a sample carried by the wavelength Á¡ = λ ₀ + / ΔΛ and leaving through the core j experiences a delay given by:

^T ¡, j ^{= T} or + ijDLÁÁ

This functionality can be exploited in two ways depending on whether the spatial domain or the wavelength domain is contemplated. In Figure 5 the first case was described, where the output 9 of each core 3 provides a sampled delay line whose incremental delay is obtained by setting the value of j in the previous expression and calculating the difference between the delays experienced by the length of wave i + and /, that is 7} = jDL. Thus N delay lines are enabled, each providing M samples. The second case is shown in Figure 7, where the N outputs 9 of the fiber 2 are grouped by applying to a demultiplexer 14, which demultiplexes them in wavelength to obtain M delay lines of N samples 16A-16M each. In this case the basic delay for each delay line of each wavelength is obtained by setting / and calculating the difference between the delays experienced by contiguous cores j + and j, T¡ = iDL. In order to allow the grouped connection of the output of the N cores, the invention has developed a connector 20 which will be described with the aid of Figure 16. The above schemes can be extended and / or modified, as shown in Figures 8, 9 and 10.

In the embodiment of Figure 8, the heterogeneous optical fiber 2 is fed, with a different WDM 17A-17N multiplex for each of the cores, (or by an equivalent configuration that would use a single emitting laser of different channels as a power source or wavelength-balanced optical modes using the phase-mode or modeling technique) each in turn modulated by a different 7A - 7N radio frequency signal, with RF modulators 6A - 6N, whose modulated signal 8A -8N is applied to each of the nuclei 3 of the heterogeneous multicore fiber 2 taking its outputs 9 separately, in which case the structure functions as a set of N different and independent delay lines. Each of the delay lines has a different basic delay 7} = DLAÁ _j where _j expresses the wavelength separation of two contiguous carriers of the WDM multiplex that is injected into the core j. In each output 9 samples 15A-15N are obtained, each delayed by a different incremental delay value.

In the embodiment of Figure 9, a single optical carrier signal 5 (P1) and a heterogeneous fiber 2 of cores 3 are used, as in the description made for Figure 2, with the difference that the cores 3 have slopes of different group delay D / (Di, ..., D _k ..., D _N ), unrelated through a constant increase, obtaining 10 'samples at its output.

With respect to Figure 10, WDM multiplex signals are used at the input of the heterogeneous fiber 2, in the same way as described for Figure 5, but with the difference that non-constant wavelength separation ΔΛ is expected, obtaining the samples 15Ά - 15'N.

Some adaptations of the previous embodiments are described below, for use in various Microwave Photonics applications: a) Firstly, an adaptation for its application as a fixed and tunable transverse filter is described. One of the most interesting applications in the field of microwave photonics is the use of photonic devices in order to implement tunable and reconfigurable filters to perform signal processing tasks in RF systems. In the most generic scheme of a microwave photonic filter of the state of the art, the optical carrier modulated in intensity (or in phase) by the RF signal crosses an optical processing stage that is responsible for sampling the signal in the domain of the weather. The resulting optical signal is detected by a photodetector 19. Depending on the nature of the optical delay lines that make up the optical processing stage, the microwave photonic filters of the prior art can be classified according to: a.1) Filters with lines of delay based on parallel fiber optic sections.

Using a single optical source 4, this type of filters generates N samples of the modulated optical signal that are independently weighed by variable optical attenuators (not shown) and delayed by delay lines determined according to the optical path along which They spread. Through this scheme, constant delays are obtained with the laser emission wavelength. a.2) Filters with dispersive delay lines.

In this case the use of multiple optical sources allows N optical carriers to be modulated with the same RF signal. The dispersive element will generate a different delay for each of the propagated optical wavelengths, that is, for each of the samples.

In the particular case of the invention, the implementation of delay lines sampled by heterogeneous multicore fibers 2, where each core 3 has different dispersive characteristics, allows the implementation of filters with dispersive delay lines using a single optical source. 4, as illustrated in Figure 1 1. The optical signal is applied to an intensity modulator 21 Mach-Zehnder MZM and to a phase modulator 22, which modulate the optical carrier by means of an RF signal 7 and are applied to the different cores 3, whose outputs are connected to a photodetector 19 By said scheme, the joint photodetection of each of the N delayed samples of the modulating RF signal results in a microwave FIR (finite impulse response) filtering phenomenon, characterized by the frequency response:

N-l

H (Q) = ∑a _n e- ^{j {nnT + you} where T is the basic delay unit of the filter, while a _n and φ _η represent, respectively, the amplitude and phase of the π-th coefficient. The above expression identifies an electrical transfer function with a periodic spectral characteristic, as can be seen in Figure 12a - 12d, whose frequency period or free spectral range (FSR) is inversely proportional to the basic delay between T samples. Thus, the control of T, which is achieved by tuning the emission wavelength of the optical source, allows reconfiguring the filter response, changing the FSR, Fig. 12a. The selectivity of the filter, given by the quality factor Q: Q = FSR / AQFWHM, where AQFWHM denotes the width of the resonance when the response falls 3 dB with respect to its maximum, is related to the number of samples N, as It can be seen in Fig. 12b. Therefore, the number of cores that will make up the MCF allows the filter to be designed with specific selectivity characteristics. By including variable optical attenuators at the input or output of the MCF (not shown), poisoning or apodization techniques can be implemented in the amplitude values of the coefficients at _n , which allows controlling the rejection level of the eliminated bands , primary to secondary lobe level (MSLR, mam to secondary lobe ratio) or secondary lobe level (SLL), Fig. 12c. Finally, the tunability of the filter, that is the change in the position of its center frequency, can be controlled by varying the phase φ _η of the samples, Fig. 12d. In the vertical axis of said figures 12a-12d the variation of the module of the frequency response normalized to the unit is represented and in the horizontal axis the frequency.

The extension of the filtering scheme proposed in Figure 1 to the assumption that the optical signal is composed of M optical carriers multiplexed in wavelength (or generated by a single emitting laser of different channels or optical modes equiespaced in wavelength by means of the technique of hitching in mode phase or modeling), it allows two extensions of the filtering functionality of special interest in the field of microwave photonics.

First, as described previously in Figure 5, by having at the output of each core j that composes the MCF an optical delay line of M samples with basic delay 7), (given by the difference between the delays experienced by adjacent wavelengths, 7} = jDL), a filter with different FSRj can be implemented at the output of each core 3, each of which is connected to a photodetector 19, for detecting the signal in the domain of the frequency This structure allows N filters to be compactly arranged with different FSR, tunable by varying the emission wavelength separation of the optical sources 4A-4M. The control of the emission power level of each of the 4A -4M lasers makes it possible to apply apodization techniques equally to each of the N cores.

The second extension is based on the grouping of the outputs of the N cores and their subsequent wavelength demultiplexing, as illustrated in Figure 7, so that it is possible to obtain M different filters composed of N samples each one. For this, each output of the demultiplexer 14 is connected to a photodetector 19. In this case, each filter is characterized by an FSR¡ inversely proportional to the basic delay given by the difference between the delays experienced between adjacent cores, T¡ = iDL. The implementation of poisoning techniques in this case would require the inclusion of N variable optical attenuators at the input or output of each of the cores.

The implementation of microwave transverse filters can also be obtained using as a base element the delay line composed of a homogeneous multicore fiber 2a and various sections of 1 1 single-mode fiber. b) Secondly, an adaptation is described for its application as optical feed of arrays of antennas 23 in phase.

Thus the second application is found in the microwave array antenna feed systems 23 in phase (Phased array antennas, PAA). These radiating structures are composed of a group of multiple antennas, arranged in one or two dimensions, which are fed individually and coherently by a phase control system or a time delay (true time delay), so that it is possible to produce a radiation pattern determined at a given angular direction. The direct application of this technology is found in radar systems that can be found both in the civil field (radio astronomy, climatology, space communications, terrestrial broadcasting) and in the military. One of the facets of the use of optical power networks in PAA, in addition to the advantages inherent in the use of microwave photonics technology, lies in the possibility of distributing the microwave signals generated from / to a central unit to / from a remote location.

The key element of a PAA is found in the beam forming network composed of several independent physical paths and responsible for connecting the radiating elements with an RF source (uplink: the RF signal is processed through the forming network and radiated to free space) or with an RF detector (downlink: the incident radiation is received by the antennas and transferred to the same RF receiver). The transmission characteristics of each path and, therefore, the aiming direction of the shaped beam can be modified if amplitude and phase / delay control elements are included. As far as optical forming networks are concerned, we find in the literature various techniques that can be classified into two categories: fiber-based networks and / or in integrated waveguides and networks based on free space optics. The solution proposed by the invention relates to fiber optic based networks and is based on the use of a heterogeneous multicore fiber 2 as a parallel feed network. As shown in Figure 13, the implementation scheme is very similar to that described in Figure 1 1 for microwave photonic filters, with the proviso that at the final end of MCF 2, the optical signal is detected from independently of the output of each of the N cores by means of a photodetector 19. It should be noted that Figure 1 1 corresponds to the case of RF signal emission, while for systems receiving radiated signals, the N battery Photodetectors are replaced by N transmitters (not shown) each composed of a laser 4 emitting in continuous wave and an electro-optic modulator 6. In that case, at the output of the MCF 2 the optical signal is detected together by a single photodetector 19.

For the description of the radiation pattern of a linear antenna group in one dimension, the Array Factor (AF) is used, by which the radiation pattern of the cluster is obtained, as the product of the array factor and the radiation pattern of the antenna element. The variation in the direction of aiming of the beam Θ is achieved by varying the complex weights or coefficients at _r that make up the array factor:

AF (e) = ∑a _r e where u = sin (6) and d _x the separation distance between the radiating elements on the x-axis. In the invention, said control is implemented by varying the incremental basic group delay T between adjacent cores 3 of the MCF 2, that is, thanks to the tuning of the emission wavelength of the laser A, as it has been previously described:

T = T _{j + l} (A) - Tj (A) = DL (A - A ₀ )

Through this delay or true time delay control system, the coefficients at _r can be expressed as .rVíd _x u ₀ with what the AF would result

where T = d _x Uo / c is fulfilled for u ₀ = sin (e ₀ ), where θ _{0 is} the aiming angle by which the array factor is maximum.

It should be noted that the inclusion of variable optical attenuators at the exit of each core would allow the implementation of various poisoning schemes.

Finally, the implementation of optical shaping networks for antenna arrays 23 can also be obtained using as a base element the delay line composed of a homogeneous multicore fiber 2a and various sections of single mode fiber 1 1. c) Arbitrary optical generation of radio frequency signals

The proposed optical delay line can also be applied to the arbitrary optical generation of radiofrequency signals and, as a particular case, to one of the formats that has been most interesting in recent years: the ultrawideband standard (UWB). This functionality is mainly applicable to radar systems and wireless communications networks, where benefits can be obtained, as discussed above for PAA, referring to the advantages offered by optical propagation in the distribution of RF signals from a central unit to a remote location

An optical RF pulse generator is based on a discrete microwave photonic filtering scheme where the possibility of synthesis of a specific impulse response is sought. In said filtering scheme the possibility of obtaining positive and negative coefficients together is required. The use of MCF allows two different schemes to be obtained to obtain negative coefficients. The first scheme, illustrated in Figure 14, is based on the use of two electro-optic modulators 6A, 6B that are fed in regions with opposite slopes through two supply voltages.

and V _dC 2, respectively. Both modulators 6A, 6B are modulated by the same electrical pulse from an RF 24 pulse generator. The modulated optical signal from modulator 6A will be released to a subset of Ni cores and will be responsible for providing the Ni positive samples to the line. delay set by the N-core MCF 3. On the other hand, the optical signal from 6B will be injected into a subset of N ₂ cores, thus providing the N ₂ negative samples that are required for the synthesis of the final pulse. The selection of the polarity of the pulses to be transmitted by the MCF 2 is carried out at the input of each core 3 by means of an optical coupler 26 of type 1 xN followed by a switch 27 or optical switch 2x1 that allows to select the signal from one or Another modulator dynamically. At the output 9 of the MCF 2, a photodetector 19 is arranged to perform the joint detection of all samples, duly delayed according to the dispersion characteristics of each of the cores, allowing to obtain the desired RF pulse 30. The control over the amplitude of the samples individually for positive and negative polarities can be implemented by introducing variable optical attenuators (not shown) to each of the outputs of the optical couplers 26 of type 1 xN.

By way of example, figure 14 (and subsequent figure 15) illustrates at the output of photodetector 19 a pulse 30 of the doublet type, corresponding to the case where N = 3 (Ni = 2 and Λ / ₂ = 1) for a sample amplitude selection of [0.5, - 1, 0.5].

In the second scheme for generating electrical pulses, each of the N total samples comes from the same electro-optic modulator, as shown in Figure 15. The separation between positive and negative coefficients is carried out by means of an architecture of balanced photodetection 19, whereby the desired subset of Ni positive samples and the subset of N ₂ negative samples are selected at the output of each core 3 by means of an optical switch 28 of type 1 x2 followed by an optical coupler 29 of type Nx1. Pulse selection is done by controlling switches 28 of type 1 x2 at the output of each core 3, which directs the signal to one of the balanced photodetectors 19. For example, to the upper balanced photodetector, for coefficients positive, or to the lower balanced photodetector, for negative coefficients. The amplitude of the pulses can be controlled individually by the introduction of variable optical attenuators in each of the inputs of the optical couplers 29 of type Nx1.

It is important to note that both schemes provide the generation of RF pulses with great flexibility since, in addition to the generation of the most common pulses of the unicycle type (N = 2) or doublet (N = 3), it allows the design of pulses of higher order.

The tuning of the optical emission wavelength of the laser 4 makes it possible to vary the incremental basic delay between T pulses, which, when using heterogeneous MCF 2, in which the difference between dispersion slopes of adjacent nuclei is constant (D / c + i - Dk = D), remains identical between pairs of adjacent pulses. If, on the contrary, the final synthesis of the RF pulse required samples with a non-constant incremental basic delay Tk, we would use heterogeneous MCFs whose nuclei had dispersion slopes not subject to said constant relationship.

Both schemes proposed for the generation of arbitrary RF pulses can also be implemented using as an optical delay line the one composed of a homogeneous multicore fiber 2a and various sections of 1 1 single-mode fiber.

It should be noted that the RF 24 generated optical pulses can also be used to perform pulse coding techniques by employing various modulation formats, such as, for example, pulse position modulation (PMM), modulation by pulse polarization or bi-phase modulation (BPM), amplitude modulation Pulse Rate (PAM), On-Off Keying Modulation (OOK) and Orthogonal Pulse Modulation (OPM).

Finally, it should be noted that to make the connections of the input and / or output, indicated in the previous examples, of the multicore fiber optic 2, 2a,

5 with a mononuclear fiber 18, the invention has developed the corresponding connectors that allow such connections to be made. Figure 16 shows the structure of a connector 12 for the connection between a mononuclear fiber 18 and the N cores of the MCF, SMC connector (Single to Multicore Connector), which allows the injection of a single optical signal modulated to the io MCF entry. It also shows the structure of a connector 20, for the connection between the MCF cores and a single-core fiber 18, MSC connector (Multi to Singlecore Connector), for the joint detection of the N signal samples in the output plane of the MCF.

Claims

one . SAMPLE DELAY LINE, characterized by comprising:

- At least one optical source for generating an optical signal,

- At least one RF modulator of the optical signal, to modulate the optical carrier with the RF signal,

- A multicore optical fiber (MCF), selected between a heterogeneous MCF and a homogeneous MCF,

- where in the case of a heterogeneous MCF, their nuclei N have the same length L and a different color dispersion D, and to each of which the modulated signal is applied to them, to provide the corresponding delay line output at the exit of each core N, the same signal but delayed according to different incremental delays depending on the chromatic dispersion of each core, providing N samples spaced in time,

- where in the case of a homogeneous MCF, their nuclei N have the same length L and the same chromatic dispersion D, and to each of which the modulated signal is applied; the output of each core N of the homogeneous MCF being connected to a single mode optical fiber, selected from single mode fibers of different length with the same chromatic dispersion D, and fibers with the same length and different chromatic dispersion D; to provide at the output of the delay line, corresponding to the output of each of the N single-mode fibers, the same signal but delayed according to different incremental delays, acting as the time-sampled delay line.

2. SAMPLED DELAY LINE, according to claim 1, characterized in that the incremental time delay of each of the N nuclei of the heterogeneous MCF, and of the structure formed by the Homogeneous MCF followed by stretches of single-mode fiber, is selected between a constant value and a non-constant value.

3. SAMPLED DELAY LINE, according to claim 2, characterized in that each of the N nuclei of the heterogeneous MCF, and of the structure formed by the homogeneous MCF followed by stretches of single-mode fiber, have a linear group delay with the length of a different slope wave, for changing the wavelength of the optical signal, changing the incremental delay value, acting as the sampled delay line in the reconfigurable time.

4. SAMPLE DELAY LINE, according to claim 1, characterized in that it comprises means for generating a pulse train of different wavelengths that are selected from:

- a plurality of M optical sources of different emission wavelengths, which are multiplexed in wavelength (WDM) and modulated with an RF signal, so that the same multiplex feeds each of the MCF cores ,

- a single laser emitter of different optical channels by means of the technique of phase-mode hitching (modeling), which provides a pulse train with different wavelengths that are modulated with the same RF signal and applied to the N cores of the fiber to obtain at the exit of each core a delay line in both cases composed of M samples delayed by different incremental delay values.

5. SAMPLE DELAY LINE, according to claim 1, characterized in that it comprises means for generating a pulse train of different wavelengths that are selected from: - a plurality of M optical sources of different emission wavelengths, which are multiplexed in wavelength (WDM) and modulated with an RF signal, so that the same multiplex feeds each of the MCF cores ,

- a single laser emitter of different optical channels by means of the technique of phase-mode hitching (modeling), which provides a pulse train with different wavelengths that are modulated with the same RF signal and applied to the N cores of the fiber; the output of the delay line being in both cases connected to a WDM demultiplexer, to obtain M delay lines of N samples each.

SAMPLED DELAY LINE, according to claim 1, characterized in that it comprises M groups of means for generating a pulse train of different wavelengths that are selected from:

- various pluralities of optical sources of different emission wavelengths, which are multiplexed in wavelength (WDM) and modulated with a different RF signal, thus feeding each of the N cores with a different WDM multiplex

- various emitting lasers of different optical channels by means of the phase-mode modeling technique, which are modulated with a different RF signal and applied to the N fiber cores;

Obtaining in both cases in the output plane, N different and independent delay lines.

SAMPLED DELAY LINE, according to claims 4, 5 or 6, characterized in that the wavelength separation between adjacent optical sources has a value selected between a constant value and a non-constant value.

8. SAMPLED DELAY LINE, according to claims 1, 4, 5 or 6, characterized in that the RF modulation is selected between an RF amplitude modulation and an RF phase modulation.

9. MICROWAVE TRANSVERSAL PHOTONIC FILTER, according to claim 1, characterized by an optical delay line where the output of each of the N cores that compose it are grouped and connected to a photodetector, to perform microwave photonic transverse filtering functions; where the tuning of the wavelength allows reconfiguring the filter response.

10. MICROWAVE TRANSVERSAL PHOTONIC FILTER, according to claim 4, characterized by an optical delay line where the output of each of the N cores of the MCF is connected to a different photodetector to establish N filters with different FSR (Free Spectral Range) at the output of each MCF core, tunable by varying the separation between emission wavelengths of the optical sources; .

eleven . MICROWAVE TRANSVERSAL PHOTONIC FILTER, according to claim 5, characterized by an optical delay line, where the output of each of the N cores of the MCF is connected to a wavelength grouping demultiplexer of the N outputs, and where each The demultiplexer output comprises a different photodetector, to establish M different filters composed of N samples each, with different FSR.

12. MICROWAVE TRANSVERSAL PHOTONIC FILTER, according to claim 1, characterized in that each input or output of the MCF N cores is connected to a variable attenuator to poison the amplitude of the samples.

13. OPTICAL FEEDING OF PHASE ANTENNA ARRAYS, according to claim 3, characterized by an optical delay line where the The output of each of the N cores of the MCF is connected to an antenna through a radiofrequency photodetector; where each of the N antennas corresponds to an element of the array of antennas in phase, to obtain a parallel feed network, and where the pointing direction of the array factor is governing by varying the incremental group delay between cores through the tuning of the wavelength.

14. OPTICAL FEEDING OF PHASE ANTENNA ARRAYS, according to claim 13, characterized by an optical delay line where the input of each of the N nuclei of the MCF is connected to an optical source through an electro-optic modulator; all the outputs of the N cores being grouped and connecting said grouping to a single photodetector for reception systems of radiated signals.

15. OPTICAL FEEDING OF PHASE ANTENNA ARRAYS, according to claims 13 or 14, characterized by an optical delay line incorporating variable optical attenuators at the exit of each core to implement various poisoning schemes.

16. ARBITRARY OPTICAL GENERATION OF RADIO FREQUENCY SIGNS, characterized by an optical delay line, according to claim 1, comprising an RF pulse generator connected to a first and a second electro-optic modulator fed in regions with opposite slopes, by different voltages, to modulate with the same electric pulse; the input of both electro-optical modulators being connected to a single optical emission source: where the output of the first modulator connected to a first 1 xN optical coupler followed by 2x1 optical switches that connect to each of the N cores, to provide a selection of positive samples to a first subset of Ni cores and where the output of the second electro-optic modulator is connected to a second optical coupler 1xN followed by 2x1 optical switches to provide a selection of negative samples to a second subset of N ₂ cores, comprising means for selecting the signal from the first or second dynamically electro-optic modulator; and comprising at the output of the N cores a photodetector, to generate a desired RF signal.

17. ARBITRARY OPTICAL GENERATION OF RADIO FREQUENCY SIGNS, characterized by an optical delay line, according to claim 16, comprising variable optical attenuators at the output of the first and second 1xN optical coupler, to control the amplitude of the samples individually for positive polarities and negative

18. ARBITRARY OPTICAL GENERATION OF RADIO FREQUENCY SIGNS, characterized by an optical delay line, according to claim 1, comprising an RF pulse generator connected to a single electro-optical modulator, which is connected to a single optical emission source and to the input of the N cores of the MCF, where the output of each of said N cores is connected to a 1 x 2 optical switch, each of which is connected to a first and a second Nx1 optical coupler, which, in turn, They are connected to a balanced photodetector, allowing differential detection between a subset of Ni positive pulses and a subset of N ₂ negative pulses by controlling the 1x2 switches at the output of each core that direct the signal to one of said photodetectors balanced.

19. ARBITRARY OPTICAL GENERATION OF RADIO FREQUENCY SIGNS, characterized by an optical delay line, according to claim 18, comprising variable optical attenuators at each of the inputs of the first and second 1xN optical coupler, to control the amplitude of the samples individually for positive and negative polarities.

20. CONNECTOR FOR SAMPLED DELAY LINE BASED ON MULTI-OPTIC FIBER, according to claim 1, characterized in that it comprises a single-core fiber connector with the N cores at the input of the MCF fiber for injection of a single optical signal modulated at the input of MCF; and a connection connector of the N cores at the output of the MCF with a mononuclear fiber, for the detection of the N samples of the signal in the output plane.