Directional photonic coupler with independent tuning of coupling factor and phase difference

Abstract

The present invention discloses a directional photonic coupler (1) with independent tuning of the coupling factor and phase difference. The coupler comprises: two waveguides (4, 5), with respective propagation constants “β.sub.1, β.sub.2”, on which phase shifters (6, 7) configured to modify the propagation coefficients are located. Both phase shifters are configured such that, by independent modification (differential or unique) of the propagation coefficients, the power coupling factor (K) between an input signal (2a or 2b) and the output signals (3b and 3a) is tuned, and by equal and simultaneous modification of the propagation coefficients, the common phase difference of the optical output signals (3 a, 3b) is tuned. A third phase shifter (15) can be used to retune the phase difference at the input/output of one of the waveguides. The coupler is of particular interest in PIC circuits, coupled resonators, Mach-Zehnder interferometers and mesh structures.

Claims

1. A directional photonic coupler independent tuning of a coupling factor and a phase difference, the directional photonic coupler comprising: a first waveguide with a propagation coefficient, denoted as β.sub.1, and a second waveguide with a propagation coefficient, denoted as β.sub.2; an input and an output of the first waveguide and an input and an output of the second waveguide; a first phase shifter, located at a predetermined distance from the first waveguide, configured to modify the propagation coefficient, denoted as β.sub.1, of the first waveguide; and a second phase shifter, located at a predetermined distance from the second waveguide, configured to modify the propagation coefficient, denoted as β.sub.2, of the second waveguide; wherein the first phase shifter and the second phase shifter are configured such that, by independent modification of the propagation coefficient, denoted as β.sub.1, of the first waveguide and of the propagation coefficient, denoted as β.sub.2, of the second waveguide, respectively, a coupling factor, denoted as “K,” between an optical input signal of one of the first waveguide or the second waveguide and optical output signals of the first waveguide and the second waveguide, is tuned, and wherein, by equal and simultaneous modification of the propagation coefficient, denoted as β.sub.1, of the first waveguide and of the propagation coefficient, denoted as β.sub.1, of the second waveguide, respectively, a common phase difference of the optical output signals of the first waveguide and the second waveguide is tuned.

2. The directional photonic coupler of claim 1, further comprising a substrate and a cladding, wherein the cladding is located on the substrate, which comprises therein at least the first waveguide and the second waveguide, with the first phase shifter and the second phase shifter located on the cladding.

3. The directional photonic coupler of claim 2 further comprising a third phase shifter located in an input of one of the first waveguide or the second waveguide, wherein the third phase shifter is configured to introduce a phase difference before the phase difference introduced by the first phase shifter and the second phase shifter.

4. The directional photonic coupler of claim 3, wherein the microprocessor is additionally connected to a plurality of optical power monitors at one or both outputs of the directional photonic coupler for reading and calculating the coupling factor, denoted as K.

5. The directional photonic coupler of claim 2 further comprising a third phase shifter located in an output of one of the first waveguide or the second waveguide, wherein the third phase shifter is configured to introduce a phase difference after the phase difference introduced by the first phase shifter and the second phase shifter.

6. The directional photonic coupler of claim 2 further comprising a microprocessor connected to the first phase shifter and to the second phase shifter for the activation thereof, wherein the microprocessor calculates the change in the propagation coefficient, denoted as β.sub.1, of the first waveguide to obtain the coupling factor, denoted as K, and wherein the microprocessor also calculates the simultaneous variation of the propagation coefficient, denoted as β.sub.1, of the first waveguide and the propagation coefficient, denoted as, β.sub.2, of the second waveguide to obtain the phase difference.

7. The directional photonic coupler of claim 1 further comprising a third phase shifter located in an input of one of the first waveguide or the second waveguide, wherein the third phase shifter is configured to introduce a phase difference before the phase difference introduced by the first phase shifter and the second phase shifter.

8. The directional photonic coupler of claim 7 further comprising a microprocessor connected to the first phase shifter and to the second phase shifter for the activation thereof, wherein the microprocessor calculates the change in the propagation coefficient, denoted as β.sub.1, of the first waveguide to obtain the coupling factor, denoted as K, and wherein the microprocessor also calculates the simultaneous variation of the propagation coefficient, denoted as β.sub.1, of the first waveguide and the propagation coefficient, denoted as, β.sub.2, of the second waveguide to obtain the phase difference.

9. The directional photonic coupler of claim 1 further comprising a third phase shifter located in an output of one of the first waveguide or the second waveguide, wherein the third phase shifter is configured to introduce a phase difference after the phase difference introduced by the first phase shifter and the second phase shifter.

10. The directional photonic coupler 9 further comprising a microprocessor connected to the first phase shifter and to the second phase shifter for the activation thereof, wherein the microprocessor calculates the change in the propagation coefficient, denoted as β.sub.1, of the first waveguide to obtain the coupling factor, denoted as K, and wherein the microprocessor also calculates the simultaneous variation of the propagation coefficient, denoted as β.sub.1, of the first waveguide the propagation coefficient, denoted as β.sub.2, of the second waveguide to obtain the phase difference.

11. The directional photonic coupler of claim 10, wherein the microprocessor is additionally connected to the third phase shifter the activation thereof.

12. The directional photonic coupler of claim 10, wherein the microprocessor is additionally connected to a plurality of optical power monitors at one or both outputs of the directional photonic coupler for reading and calculating the coupling factor, denoted as “K”.

13. A photonic integrated circuit “(PIC)” comprising the directional photonic coupler of claim 1.

14. A coupled resonator comprising the directional photonic coupler of claim 1.

15. The directional photonic coupler of claim 1 further comprising a microprocessor connected to the first phase shifter and to the second phase shifter for the activation thereof, wherein the microprocessor calculates the change in the propagation coefficient, denoted as β.sub.1, of the first waveguide to obtain the coupling factor, denoted as K, and wherein the microprocessor also calculates the simultaneous variation of the propagation coefficient, denoted as β.sub.1, of the first waveguide and the propagation coefficient, denoted as, β.sub.2, of the second waveguide to obtain the phase difference.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an exemplary embodiment of a directional photonic coupler according to the present disclosure in a section view (FIG. 1a), a plan view (FIG. 1b), and a 3D view (FIG. 1c).

(2) FIG. 2a shows the variation of the coupling factor as a function of the equal increase in the propagation coefficients of the waveguides with the directional photonic coupler of the present disclosure.

(3) FIG. 2b shows the variation of the phase difference as a function of the equal increase in the propagation coefficients of the waveguides with the directional photonic coupler of the present disclosure.

(4) FIG. 3a shows two resonators coupled by means of the directional photonic coupler of the present disclosure.

(5) FIG. 3b shows the application of the directional photonic coupler of the present disclosure in a Mach-Zehnder interferometer.

(6) FIGS. 4a to 4d show different structures in which the directional photonic coupler of the present disclosure can be applied. FIG. 4a: a triangular structure, FIG. 4b: a square structure; FIG. 4c: a hexagonal structure, and FIG. 4d: a mesh structure.

(7) FIG. 5 shows the directional photonic coupler of the present disclosure with three phase shifters, wherein one of them is at the input or at the output of the waveguides.

(8) FIG. 6 shows a laboratory embodiment for experimental measurements of the directional photonic coupler of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

(9) An embodiment of the disclosure is described in detail below according to the FIGS. that are shown.

(10) FIG. 1 shows an exemplary embodiment of a directional photonic coupler according to the present disclosure wherein a phase shifter can be seen for each waveguide. Specifically, FIG. 1c shows the directional photonic coupler 1 comprising two waveguides 4 and 5 within a cladding 9, which is located on the substrate 8. Respective phase shifters 6 and 7 are located on each of the waveguides. The waveguides 4 and 5 have their inputs 2a and 2b, respectively, and their outputs 3a and 3b, respectively. Any of the inputs 2a and 2b can be connected to a light source which will supply an input signal 10 with a specific optical power. For the specific case in which the input 2a is fed by the input signal 10, the direct signal 11a will be obtained at the output 3a and the coupled signal 11b will be obtained at the output 3b, the power and phase of which will depend on the coupling factor K with the input signal, as is known in the prior art (with the phase shifters being disconnected). FIG. 1b shows a plan view of the directional photonic coupler 1, but only the waveguides 4 and 5 and the phase shifters 6 and 7 are shown. FIG. 1b also shows how the coupling A.sub.i(z) between signals propagating through the waveguides between z=0 and z=Lc depends. Lastly, FIG. 1a shows a section view of the directional photonic coupler 1 in which there is shown the substrate 8, on which the cladding 9 including the two waveguides 5 and 6 arranged parallel to one another and separated by a distance “g”, is deposited, and finally, the phase shifters 6 and 7 having a width “w” arranged parallel to one another and space by a distance “d”, are located on the cladding 9. In the particular case of the directional photonic coupler shown in FIG. 1, the virtual joining of the waveguides 5 and 6 would form a plane parallel to the virtual plane formed by the phase shifters 6 and 7. As can be seen in FIG. 1a, each phase shifter has a radius of action 12a, 12b on the waveguide on which it is located.

(11) The effect of each phase shifter on its corresponding waveguide is known as “tuning” and there are currently different tuning technologies. The purpose of tuning technologies is to modify the phase of the (optical) signal circulating through the waveguide. This effect is achieved by modifying the optical properties of the waveguide. Most tuning elements require an electronic power supply that must be guided to the integrated device. Depending on the physics underlying the effect, some examples of tuning are: “thermo-optic tuning”: the phase difference is caused by the local modification of temperature. This effect can be produced by passing a current through a metallic layer close to the core of the guide and thereby releasing heat; electro-optical tuning: The passage of electric current through the guide itself modifies its propagation properties, producing the desired phase difference; “capacitive effects, electromechanical effects, MEMs”: the geometrical properties of the guide or the pressure in some of its materials are modified to alter/produce a phase difference; “optical tuning”: an optical pump or tuning signal is used for interfering with the target signal.

(12) With respect to the “g” or “w” values indicated above, they will depend on several factors such as the tuning technologies described above and/or the fabrication technologies (“Silicon on Insulator”, “Silica”, “Silicon nitride”, “Indium Phospore”, “Lithium Niobate on Silicon”). Typical “w” and “g” values are between 0.6 μm and 1.6 μm.

(13) With the configuration shown in FIG. 1 and for any tuning technology described above, the directional photonic coupler 1 of the present disclosure successfully varies the propagation coefficients β.sub.1 and β.sub.2 of the waveguides by means of the action of the phase shifter 6 and 7 for independently tuning the coupling factor (K) and the phase difference between the signals propagating through the waveguides 4 and 5.

(14) To achieve the desired coupling factor value between the transmitted signal and the coupled signal, tuning (changing the propagation coefficient β.sub.1) of one of the waveguides is sufficient, such that a difference is generated between the propagation coefficients of the waveguides. In other words, the propagation coefficient is kept constant if the propagation coefficient difference is kept constant. Moreover, changing the propagation coefficient β.sub.1 entails a phase change (phase difference) of the signal circulating through the waveguide. If a specific phase difference other than that generated when obtaining the desired coupling factor is desired, modifying the propagation coefficients β.sub.1 and β.sub.2 in the same proportion would be sufficient.

(15) To carry out the foregoing, the phase shifters 6 and 7 can be connected to a microprocessor (not shown) which will be responsible for calculating the change in the propagation coefficient β.sub.1 of the waveguide 4 to obtain the desired coupling factor, and also for calculating the simultaneous variation of the propagation coefficient β.sub.1 of the waveguide 4 and the propagation coefficient β.sub.2 of the waveguide 5. Once having calculated both propagation coefficients with which the desired coupling factor and phase difference is obtained, the microprocessor will activate the phase shifters 6 and 7 that will act on the waveguides 4 and 5 until the propagation coefficients β.sub.1 and β.sub.2 correspond with those calculated by the microprocessor. Additionally, the microprocessor can be connected to an optical power monitor (not shown), which are connected at one or both outputs of the directional photonic coupler for reading and calculating the coupling factor “K” instantaneously.

(16) It can be seen in FIGS. 2a and 2b that, by means of the directional photonic coupler of the present disclosure, the coupling factor (K) is kept constant as Δn.sub.eff,common increases (FIG. 2a) and the phase difference grows as Δn.sub.eff,common increases (FIG. 2b). It must be borne in mind that n.sub.eff=(β.sub.a+β.sub.p)2π/λ, and therefore, Δn.sub.eff,common=[((β.sub.a1+β.sub.p1)2π/λ−((β.sub.a2+β.sub.p2)2π/λ)]. So, changes in β.sub.1 y β.sub.2 modify the β.sub.a value (active part).

(17) FIGS. 3a and 3b show applications of the directional photonic coupler of the present disclosure in typical PIC (Photonic Integrated Circuit) designs. Specifically, FIG. 3a shows the directional photonic coupler applied to two coupled resonators 13a, 13b, and FIG. 3b shows the directional photonic coupler applied to a Mach-Zehnder interferometer 14. In both cases, the coupling factor can be programmed by accepting and modifying the power supply of each phase shifter. The phase shifter typically found in one of the arms of the Mach-Zehnder can, for example, be substituted if the TDC design includes the third coupler.

(18) Another particularly interesting and highly relevant application of the directional photonic coupler in PICs is the field of “waveguide meshes”. In a manner similar to the mode of operation of FPGAs (field-programmable gate array), programmable PICs implementing multiport beam splitters can be configured by means of conventional circuit discretization in a prefabricated waveguide mesh structure with pairs of coupled waveguides, known as Tunable Basic Units (TBU). By configuring each TBU, constructive, destructive, or partial interference can be achieved in each complementary output port, which leads to the routing of the signal and the definition of the circuit topology and design parameters. Although these circuits sacrifice footprint, power consumption, and optical gain, they provide an unprecedented versatility and flexibility, which allows applications that are not possible in a standard PIC of specific applications. FIGS. 4a to 4d illustrate different waveguide mesh combinations and topologies proposed in the literature for this purpose, wherein the directional photonic coupler of the present disclosure has been included as a TBU (“Tunable Basic Unit”). Specifically, FIG. 4a shows a photonic structure of a triangular structure 16, FIG. 4b shows a square structure 17, FIG. 4c shows a hexagonal structure 18, and FIG. 4d shows a mesh structure 19 with arrows indicating the input and the output.

(19) To program complex and extensive waveguide mesh structure-based systems, moderate TBU losses (0.25 dB/TBU) seriously decreases the overall circuit performance. To overcome this limitation, the incorporation of the directional photonic coupler of the present disclosure to replace the current TBU design based on 3-dB MZI devices reduces losses in programmed waveguide mesh circuits, which leads to FIGS. comparable to those of similar circuits designed using ASPICs (Application Specific Photonic Integrated Circuits). When compared with the balanced 3-dB MZI TBU approach, due to the miniaturization capacity that does not compromise losses, a triple improvement in time resolution is also obtained.

(20) Additionally, the directional photonic coupler of the present disclosure may have a third phase shifter 15 as shown in FIG. 5. With the third phase shifter 15, an additional phase difference independent of that introduced by the phase shifters 6 and 7 on any of the propagating signals can be included at the output or at the input of any of the waveguides 4, 5. The third phase shifter 15 can therefore be located at the input (FIGS. 5a and 5c) or at the output (FIGS. 5b and 5d) of the waveguides 4, 5.

(21) Lastly, FIGS. 6a and 6b show a laboratory fabrication for measuring experimental results of the directional photonic coupler of the present disclosure. It has been designed and fabricated under a Multi Project Wafer (MPW), running a directional photonic coupler like the one of the present disclosure in a silicon nitride platform, illustrated in FIG. 6A. A tunable laser sweeping from 1520 to 1620 nm has been used for measurements, followed by a polarization controller before accessing the chip by means of optical fibers. The data was acquired by an optical spectrum analyzer for each programmed electrical power value.

(22) In this case, a single-mode waveguide having a width of 1 μm and a height of 300 nm was used to propagate a TE (Transverse Electric) field. The gap between the waveguides (g) was set to 1.5 μm, leading to a theoretical total coupling length of 717 μm. However, the decision was made to increase the final coupler length L to 1235 μm to increase the safety of the thermal tuners (phase shifters) and to check the analytical model rather than to find a perfect passive cross state, and before proceeding to an optimization round. For the metal layer, a distance between phase shifters (d) of 2 μm was considered. The optical crosstalk was kept between 15 and 21 dB for the cross and bar operations, while a bandwidth >5 nm was obtained for a ±2% uniformity. The total excess loss was insignificant and estimated to be below 0.1 dB. FIG. 6b illustrates the change of the power coupling factor K versus the applied electric current in four different wavelengths. The model was validated and predicts fabrication errors in the width range of 15 nm and gap variation of 70 nm.

(23) For the directional photonic coupler shown in FIG. 6a, the power consumption needed for the coupling factor reconfigurability from 1 to 0 is greater than in a conventional MZI approach if a thermal adjustment mechanism is used (i.e., a power consumption of 270 mW is measured for the MZI approach and 460 mW is estimated for the TDC approach in the same integration platform). The reason behind this is the proximity of the two waveguides and the resulting un-optimized thermal interference that more seriously affects the common phase change rather than the differential phase change. However, if the structure is optimized, accordingly by changing “d” and “g”, the electrical power consumption can be considerably reduced. With the state of the art, TDCs with phase shifting capacities of less than 700 μm and 100 μm in silicon nitride and silicon on insulator platforms could be achieved, respectively, representing a more than three-fold length decrease with respect to the MZI-based TBU approaches. Furthermore, alternative adjustment mechanisms like the electromechanical effect seem to be a promising option to achieve low-power, low-loss, and shorter TDCs.