ES2619506B2 - Multimode interference optical coupler device and method to tune the response of an optical signal to a multimode interference optical coupler - Google Patents

Abstract

Multimode interference optical coupler device and method for tuning the response of an optical signal into a multimode interference optical coupler. # The device comprises a multimode waveguide structure with a number N of single-mode input and output waveguides (10 , 20) to allow the propagation of an injected optical signal in said multimode interference optical coupler device, or MMI coupler, (1), where in a region of its interior a series of multiple images of said injected optical signal are formed and wherein the MMI coupler (1) comprises a unit (15) for generating a traveling surface acoustic wave (SAW) that affects said inner region of the MMI coupler (1) to modulate with an opposite phase the effective refractive index of the images multiple formed, the latter being separated by a distance determined by the expression: {la} {sub, SAW} / 2 + n {la} {sub, SAW}.

Description

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DESCRIPTION

Multimode interference optical coupling device and method for tuning the response of an optical signal into an optical multimode interference coupler

Technology Sector

The present invention concerns in general the field of optical devices. In particular, the present invention concerns an optical multimode interference coupler device and a method for tuning the response of an optical signal into an optical multimode interference coupler, wherein said optical coupler device (or MMI coupler) comprises a glider of Waves designed to allow the propagation of a large number of modes through the wave glutton.

State of the prior art

At present, most of the transmission of long-distance information is done by sending optical signals through optical fibers, a fact that has led to the laying of huge optical fiber transmission networks. As the complexity and volume of the data transferred has increased greatly over time, the need to have great flexibility in the networks has been demonstrated, in order to modify this way! traffic in each part of the network and respond in this way to changes in the demand for data transfer. However, although the signal is transmitted through optical fibers, the signal processing at the nodes thereof is currently performed almost completely electronically. If in the nodes of the networks it was possible to manipulate the optical signals without the need to use electron-photon conversion interfaces, transmission networks could be made that worked more efficiently and quickly, with all the electronic components of the nodes replaced by purely photonic elements.

Many investigations have been done to design active photonic devices that allow performing the above-mentioned functionalities. Some of these investigations consist of placing metal contacts in the region of the MMI coupler that needs to be modulated, to generate temperature changes (thermo-optical effect) or to apply electric fields (electro-optical effect). In the first case, the thermal inertia itself makes it very difficult to perform devices that are very fast (operation in the MHz range), in addition to being almost impossible to maintain the temperature gradient in the perfectly controlled material (it is difficult to control the diffusion of heat ). In the second case, it is possible to obtain

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very fast devices (operation in the GHz range), but since they are second-order effects, very long interaction lengths are generally required, resulting in very long devices making integration into photon circuits difficult. In addition, in both cases, the complexity at the time of manufacturing and contacting the electrodes increases markedly as the number of regions to be modulated increases.

Several modulators designed from tunable MMI couplers with different coupling length are known from US-B1-6571038. In this patent, it is proposed to modulate only the multiple images of a plane of interest by applying a current, a voltage or a light signal to the material formed by the wave glutton so as to modify its optical properties. Although in the device proposed in the aforementioned patent the wave glues in the active region are eliminated, thus contributing to the reduction of the additional phase errors that are introduced during the manufacture, in order to be able to modulate by applying a current The metallization of electrodes on wave glues manufactured using non-linear materials is necessary. By increasing the number of images to be modulated, the manufacture of the circuits becomes noticeably more complex, a task that can be really difficult for a high number of contacts. In the same way, to illuminate with a laser only a certain image in the plane of interest requires to have a very well focused point, of a width of a few microns. In any case, it is complicated to have precise control of the region in which the refraction index is being modified either with the placement of electrodes to apply after a current or voltage, or with the application of light, which hinders the operation of the device.

Also, in the document [1] the same authors of the aforementioned US-B1-6571038 patent explicitly expose the technical problems that exist to precisely control the flow of current in the different regions of the wave glutton, and therefore the problems generated in the control of the changes in the index of refraction of the material which finally generates problems in the correct functioning of the device.

In document [2] an acoustically modulated AWG (arrayed waveguide grating) device is disclosed, whose structure is constructed from MMI couplers of equal length with five input and output glues, respectively. Thus, in [2], both couplers are connected by a set of straight and curved glues (S-bends) whose relative length is calculated to produce wavelength dispersion at the output of the device. Two interdigitated transducers (or IDTs) generate two traveling acoustic waves that interfere with each other to generate a stationary acoustic wave that modulates the device

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AWG For this, the straight glutton sections placed between the transducers are properly spaced to allow acoustic modulation of the AWG response. By modulating the response using acoustic waves, each wavelength can quickly oscillate between all output channels with the frequency of the acoustic wave if sufficient power is applied to the RTDs. Once a complete reconfiguration of the AWG channels has been achieved, it is possible to obtain modulated light at higher harmonics of the acoustic frequency by further increasing the power that is applied to the IDTs. Unlike the present invention, the device of [2] consists of two MMI couplers and has a set of wave glues connected to both couplers that introduce wavelength dispersion. Likewise, the device of [2] requires two flat IDTs (with Z / ngers / straight electrodes) to generate two traveling acoustic waves that interfere with each other to generate a stationary acoustic wave that modulates the device correctly.

The document [3] discloses a Mach-Zehnder interferometer constructed from MMI couplers of different length joined by curved wave glues (S-bends). The first MMI coupler divides the light into two beams of equal intensity, which travel through curved glues connected to two straight gluttony sections. It is precisely in these straight glues that the acoustic phase difference that modulates the response of the device is introduced. These straight glues are connected to the next MMI coupler by two other curved glues. A single transducer with straight electrodes generates the acoustic traveling wave that modulates the device. In the absence of acoustic modulation, the device divides the signal into two parts of the same intensity. By modulating the response by acoustic waves, the light begins to oscillate rapidly between the two output channels with the frequency of the acoustic wave. Thus, it is possible to obtain modulated light at higher harmonics of the acoustic frequency by increasing the power that is applied to the IDT.

Likewise, the device proposed in [3] presents some imperfections introduced during manufacturing that can become problematic for the correct operation of the devices. Thus, the imperfections that are introduced in the arms that connect both MMI couplers are especially critical, since these additional phases degrade the interference in the second coupler, often preventing the correct functioning of the samples. This problem becomes more critical as the number of wave glides increases in the active region of the devices.

There is, therefore, the need to offer a new optical multimode interference coupler, and a method that uses said optical coupler, compact and reconfigurable,

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capable of being acoustically modulated by means of a unique interdigitating focusing transducer, which does not have wavelength dispersion, and which can be used in combination with wavelength multiplexers.

References:

[1] Multimode Interference Couplers with Tunable Power Splitting Ratios, Juerg Leuthold and Charles H. Joyner, MAY 2001 | VOL. 19, NO. 5 | JOURNAL OF LIGHTWAVE TECHNOLOGY.

[2] Acoustically driven arrayed waveguide grating, 10 Aug 2015 | Vol. 23, No. 16 | DOI: 10.1364 / OE.23.021213 | OPTICS EXPRESS 21213.

[3] Synchronized photonic modulators driven by surface acoustic waves, A. Crespo-Poveda et al, 9 September 2013 | Vol. 21, No. 18 | DOI: 10.1364 / OE.21.021669 | OPTICS EXPRESS 21669.

Explanation of the invention

To that end, examples of realization of the present invention provide according to a first aspect, a multimode interference optical coupling device, which comprises, like the optical devices known in the state of the art, a wave glutton structure multimode with a number N of single mode waveguides of input and output (of a width less than the width of the multimode wave glutton) to allow the propagation of an optical signal injected into said multimode interference optical coupling device, or coupler MMI

The said MMI coupler has a certain length and a certain width and in a region of its interior a series of multiple images of said injected optical signal are formed, the number of which depends on the position along the length of the MMI coupler and the position of the single mode access guide in which the optical signal is injected, and whose response can be tuned by introducing a set of additional phases in said multiple images.

Typically, and unlike the known proposals, the proposed device also comprises a unit configured and adapted to generate a traveling surface acoustic wave of a certain width and of a certain wavelength that affects perpendicularly to the direction of light propagation in said inner region of the MMI coupler to modulate with opposite phase the Index of effective refraction of the multiple images formed, wherein the multiple images formed are separated by a distance that is determined according to the expression: ASAW / 2 + nASAW.

For a preferred embodiment, the number N of single-mode input and output waveguides is an even number of waveguides, equal to or greater than 2, which are

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they find a Weff / N distance spaced from each other, Weff being the effective width of the MMI coupler.

Also, said unit comprises an interdigitated transducer that includes a series of curved geometry metal electrodes.

Preferably, the length of the MMI coupler is 3Ln (where Ln is the beating length) and the inner region is located at L / 2 (where L is the length of the MMI coupler).

In an example of realization, the number of multiple images formed in the inner region of the MMI coupler is two. In this case, the device, in a first operative mode, allows to adjust the separation of the two multiple images formed with respect to the wavelength of the acoustic wave by selecting the appropriate width of the MMI coupler and the appropriate positions of the single-mode waveguides of entry and exit. Alternatively, in a second operating mode, the device allows to adjust the wavelength of the acoustic wave with respect to the separation of the two multiple images formed by selecting the spacing of the metal electrodes of the interdigitated transducer.

The MMI coupler in an exemplary embodiment is a photonic router, which can be installed in at least one node of an optical transmission network to redirect an optical signal to the desired point, without using conversion interfaces to convert it into a electronic signal, making the transmission of signals more efficient and faster.

Examples of embodiment of the present invention provide according to a second aspect a method for tuning the response of an optical signal in a multimode interference optical coupler, wherein the method comprises propagating an injected optical signal in a multimode optical interference coupling device. , or MMI coupler, through an N number of single mode input and output waveguides of a width smaller than a multimode section of the MMI coupler; and forming in an inner region of the MMI coupler a series of multiple images of the injected optical signal, and whose response can be tuned by introducing a set of additional phases in said multiple images, wherein said MMI coupler comprises a certain length and a certain width

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Unlike the known proposals, the method also comprises modulating the multiple images formed in the inner region of the MMI coupler with an opposite phase, by generating a traveling surface acoustic wave of a certain width and a certain wavelength, which it affects the inner region of the MMI coupler where multiple images are formed, with multiple images formed separated by an ASAw / 2 + nASAw distance (where ASAW is the wavelength of the acoustic wave and n is an integer).

Therefore, the present invention makes it possible to use a single traveling surface acoustic wave, so that it is only necessary to place a single interdigitated transducer, which greatly simplifies the fabrication and subsequent handling of the device. In addition, the use of surface acoustic waves allows to design very efficient and fast devices, and of great simplicity.

On the other hand, the present invention, by eliminating the waveguides of the active part of the MMI coupler in which the acoustic phase differences are introduced, eliminates possible imperfections that could be introduced in the manufacture and that affect subsequent performance. At the same time, the absence of these wave glues allows very compact and robust multimode interference optical coupling devices that are easily integrated into photonic circuits of greater complexity.

Brief description of the drawings

The foregoing and other advantages and characteristics will be more fully understood from the following detailed description of some examples of realization with reference to the attached drawings, which should be taken by way of illustration and not limitation, in which:

Fig. 1 is a schematic diagram of a multimode interference optical coupling device. In this case, a single acoustic beam of width WU modulates multiple images of the input field (injected optical signal and that propagates through the MMI coupler structure), allowing the optical signal introduced into an arbitrary single mode input glutton oscillate between two well-defined single mode output glues.

Figs. 2A and 2B schematically illustrate the simplified operation of the proposed coupling device for a particular case in which the number of single-mode input and output waveguides is equal to 4. In this case, when there is no acoustic excitation (Fig. 2A), the MMI coupler acts as a 'cross-coupler, cross coupler.

Detailed description of the invention and some examples of realization

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MMI couplers are optical components widely used in the design of integrated photonic circuits, mainly due to their robustness and reduced size. The central structure of an MMI coupler is a wave glutton designed to allow the propagation of a large number of modes through it, typically greater than three. In order to be able to couple and extract the light from the multimode gluttony, a set of glues of smaller width, generally single mode (allows the propagation in a single way) is placed both at the beginning and at the end of this. The operation of this type of couplers is based on the self-image properties of multimode wave glues (self-imaging). This ability to generate multiple images of the initial field, that is, of the optical signal injected into the MMI coupler, at intervals of the direction of light propagation makes them very useful for dividing and recombining optical signals in photon integrated circuits.

An example of preferred realization of the proposed multimode interference optical coupling device is shown in Fig. 1. In this particular case, the MMI 1 coupler comprises a multimode waveguide structure with a number N, preferably even, of single mode waveguides of input 10 and output 20, whose response can be tuned by a traveling acoustic surface wave SAW, generated by an interdigitated unit or transducer 15, which modulates the effective refraction index of a narrow section of the MMI 1 coupler itself and, therefore, its response.

The aforementioned single mode 10 and output 20 waveguides are spaced apart by a Weff / N distance, Weff being the effective width of the MMI 1 coupler.

The MMI 1 coupler can be used as a reconfigurable photonic router, which can be located on the nodes of an optical transmission network to dynamically modify the data traffic. In addition, it is a very compact device, which lacks wave glues of its active region, thus greatly facilitating its integration with other photonic devices on chips of a very contained size.

The structure of the MMI coupler 1 according to this preferred embodiment comprises a length L of coupling 3Ln, which is the distance at which a single mirror image of the input field is formed (ie, the optical signal injected into the MMI coupler 1) , and a width W.

By introducing phase differences in the appropriate locations of the MMI 1 coupler, it is possible to alter the way in which the light interferes with the MMI coupler 1 and modify the position of the mirror image that is formed at the output of the MMI 1 coupler. According to this preferred embodiment, the appropriate region of the interior of the MMI 1 coupler for

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Entering these additional phase differences is in the middle of the MMI 1 coupler, for a distance of 3Ln / 2. In this plane, two multiple images of the input field are formed whose position depends on the input single mode waveguide 10 of the MMI coupler 1 that is excited.

A simple way to introduce additional phase differences in the two multiple images is to divide the MMI 1 coupler into two parts along the plane of interest, which are connected by a set of single-mode wave glues into which these additional phases The position of the multiple images in the middle of the MMI coupler 1 makes it possible to use a single interdigitated transducer 15 to generate the said SAW traveling surface acoustic wave that modulates both multiple images with opposite phase. In the proposed device, the interdigitated transducer 15 generates a very narrow acoustic beam that has a perpendicular impact on the direction of light propagation on the MMI coupler, so that the use of wave glues in the modular region (active region) is not necessary. ), so that the result is a compact device formed by a single MMI 1 coupler (instead of two or more, which is usual).

The interdigitated transducer 15, with its curved geometry metal electrodes, allows the elimination in the MMI coupler 1 of the waveguides of the active region of the MMI coupler 1 by generating said SAW traveling surface acoustic wave, with a very narrow beam WU and of a certain Asaw wavelength, which strikes the body of the MMI coupler and modulates the two multiple images of the input field that are formed in the middle of the MMI coupler 1. Similarly, advantageously, it also allows an optical signal introduced into an arbitrary single-mode waveguide 10, arbitrary, oscillate between two output single-mode waveguides 20 perfectly set with the frequency of the acoustic wave that modulates the MMI coupler 1. In addition, the generation of harmonic modulated light is possible higher acoustic frequency through an increase in the acoustic power introduced.

The two images that are formed in the inner region of the MMI 1 coupler are separated by a distance that is determined according to the expression:

ASAW / 2 + n ASAW,

where Asaw is the wavelength of the acoustic wave and n is an integer, so that when applying a unique traveling surface acoustic wave SAW each image is modulated with an opposite phase.

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The Asaw wavelength of the SAW traveling acoustic surface wave generated by said interdigitated transducer 15 is determined by the separation between the metal electrodes (fingers) of the interdigitated transducer 15, whereby, according to an example of realization or first operating mode, It is possible to adjust the separation of the multiple images formed inside the MMI1 coupler with respect to Asaw by properly adjusting the width W of the MMI 1 coupler with respect to Asaw. Alternatively, according to an exemplary embodiment or second operating mode, the spacing of the metal electrodes of the interdigitated transducer 15 can be adjusted to adjust Asaw to the width of the MMI 1 coupler that one wishes to use. In this way, the surface acoustic traveling wave SAW generated by the interdigitated transducer 15 has a perpendicular impact on the inner region of the MMI coupler 1 in which the two multiple images are formed, modulating its refractive index with opposite phase and, therefore, maximizing acoustic-modulation modulation.

With reference now to Figs. 2A and 2B, the simplified operation of the proposed device is shown therein in the case where the number of single mode waveguides of input 10 and output 20 is equal to 4, ie N = 4. According to this embodiment, the light enters through the single-mode input waveguide 1. In Fig. 2A, the MMI 1 coupler operates in the absence of acoustic modulation (ie no SAW signal), so that the light introduced in the single mode waveguide input 10 i = 1 is sent to the single mode waveguide output 20 k = 4. In Fig. 2B, the operation of the proposed device is shown when the power required to completely reconfigure the channels is applied on the interdigitated transducer 15, so that the signal continuously oscillates between the output single mode wavelets 20 k = 1 and k = 4 Since the SAW traveling surface acoustic wave is a traveling wave, the multiple images that are formed in the inner region of the MMI 1 coupler will not undergo maximum acoustic modulation at all times. Thus, assuming that the nodes of the SAW traveling acoustic surface wave are initially on the images at t = 0, they will not undergo any modulation at t = 0 and t = TSAW / 2 (where TSAW is the period of the SAW acoustic wave), so the light will follow the path (i, k) = (1,4) as in the absence of modulation. On the other hand, the two images will undergo maximum modulation with opposite phases at t = TSAW / 4 and t = 3TSAW / 4, so that the light will follow the path (i, k) = (1,1).

A person skilled in the art could make changes and modifications in the described embodiments described without departing from the scope of the invention as defined in the appended claims.

Claims (15)

5 10 fifteen twenty 25 30 35 1. A multimode interference optical coupling device, comprising a multimode waveguide structure with a number N of single-mode input and output waveguides (10, 20) of a width less than a width of the multimode waveguide to allow propagation of an optical signal injected into said multimode interference optical coupler device, or MMI coupler, (1), wherein said MMI coupler (1) has a certain length (L) and a certain width (W) and in a region of its interior forms a series of multiple images of said injected optical signal whose number depends on the position along the length (L) of the MMI coupler (1) and the position of the single mode access guide in the that the optical signal is injected, and whose response can be tuned by introducing a set of additional phases in said multiple images, the device being characterized because: - the MMI coupler (1) also comprises a unit (15) configured and adapted to generate a traveling surface acoustic wave (SAW) of a certain width (WU) and of a certain Asaw wavelength that affects perpendicularly to the direction of propagation of the light in said inner region of the MMI coupler (1) to modulate with an opposite phase the effective refraction index of the multiple images formed; Y - the multiple images formed are separated by a distance that is determined according to the expression: ASAW / 2 + nASAW, where Asaw is the wavelength of the traveling surface acoustic wave (SAW) and n is an integer. 2. Device according to claim 1, characterized in that the number N of single-mode input and output waveguides (10, 20) comprises an even number of waveguides equal to or greater than 2, spaced apart by a Weff / N distance, Weff being an effective width of the MMI coupler (1). Device according to claim 1, characterized in that the unit (15) comprises an interdigitated transducer that includes a series of curved geometry metal electrodes. Device according to any one of the preceding claims, characterized in that said certain length (L) is 3Ln and the inner region of the MMI coupler (1) is located at L / 2. 5. Device according to claim 3, characterized in that the number of multiple images formed in the inner region of the MMI coupler (1) is two and because in a first operating mode the device adjusts the separation of the two multiple images formed with respect to the length wave of the acoustic wave (SAW) by selecting the width 5 10 fifteen twenty 25 30 35 (W) of the MMI coupler (1) and the positions of the single-mode input and output waveguides (10, 20). 6. Device according to claim 3, characterized in that the number of multiple images formed in the inner region of the MMI coupler (1) is two and because in a second operating mode the device adjusts the wavelength of the Asaw acoustic wave with respect to the separation of the two multiple images formed by selecting a spacing of the metal electrodes of the interdigitated transducer. 7. Device according to any one of the preceding claims, characterized in that the MMI coupler (1) is a photonic router. 8. Device according to claim 7, characterized in that the photonic router is installed in at least one node of an optical transmission network. 9. Method for tuning the response of an optical signal into a multimode interference optical coupler, comprising: - propagating an injected optical signal in a multimode interference optical coupling device, or MMI coupler (1), through a number N of single-mode input and output waveguides (10, 20) of a width smaller than a multimode section of the MMI coupler; Y - forming in an interior region of the MMI coupler (1) a series of multiple images of said injected optical signal, and whose response can be tuned by introducing a set of additional phases in said multiple images, wherein said MMI coupler (1) It comprises a certain length (L) and a certain width (W), the method being characterized in that it also comprises: - modulate the multiple images formed in said inner region of the MMI coupler (1) with an opposite phase by generating a traveling surface acoustic wave (SAW) of a certain width (WU) and of a certain Asaw wavelength that affects perpendicularly to the direction of light propagation in the inner region of the MMI coupler (1) where the multiple images are formed, the multiple images formed separated by an ASAW / 2 + nASAW distance, where Asaw is the wavelength of the wave superficial acoustic traveler (SAW) and n is an integer. 10. Method according to claim 9, characterized in that the number N of single-mode input and output wavelets (10, 20) comprises an even number of wavelets equal to or greater than 2, spaced apart by a Weff / N distance, Weff being an effective width of the MMI coupler (1). Method according to claim 9, characterized in that the MMI coupler (1) comprises an interdigitated transducer that includes a series of curved geometry metal electrodes. 12. Method according to claim 11, characterized in that the acoustic wave perpendicularly affects the direction of propagation of the light on said inner region of the MMI coupler (1) to modulate with multiple phase the multiple images formed, which are two, by means of a adjustment of the width (W) of the MMI coupler (1) with respect to the length of 5 Asaw wave of the acoustic wave (SAW) generated. 13. Method according to any of claims 9 to 12, characterized in that said certain length (L) of the MMI coupler (1) is 3Ln and the inner region of the MMI coupler (1) is located at L / 2. 14. Method according to any one of claims 9 to 13, characterized in that the MMI coupler (1) is a photonic router. 15. Method according to claim 14, characterized in that the photonic router is installed in at least one node of an optical transmission network.