1. A reconfigurable optical all-pass filter, comprising: three optical couplers, N +2 optical phase shifters and N micro-rings, where N is an integer greater than or equal to 2; wherein the content of the first and second substances,

the upper arm of the output end of the first optical coupler (2) is connected with the upper arm of the input end of the second optical coupler (4) through a first waveguide, and the lower arm of the output end of the first optical coupler (2) is connected with the lower arm of the input end of the second optical coupler (4) through a second waveguide; a first optical phase shifter (3) is arranged on the first waveguide or the second waveguide;

the upper arm of the output end of the second optical coupler (4) is connected with the upper arm of the input end of the third optical coupler (8) through a third waveguide, and the lower arm of the output end of the second optical coupler (4) is connected with the lower arm of the input end of the third optical coupler (8) through a fourth waveguide; a second optical phase shifter (5) is arranged on the third waveguide, a third optical phase shifter (6) and a first micro-ring (7) coupled with the fourth waveguide are arranged on the fourth waveguide, or a second optical phase shifter (5) is arranged on the fourth waveguide, and a third optical phase shifter (6) and a first micro-ring (7) coupled with the third waveguide are arranged on the third waveguide;

the output end of the third optical coupler (8) is connected with a fifth waveguide, and N-1 optical phase shifters and N-1 micro-rings coupled with the fifth waveguide are arranged on the fifth waveguide;

tuning the coupling coefficients of the N micro-rings to make the zero point of the (i + 1) th micro-ring and the pole of the (i) th micro-ring mirror-symmetrical, wherein i is 1, … and N-1; and adjusting the first optical phase shifter to make the zero point of the first micro-ring and the pole of the Nth micro-ring mirror symmetric, thereby realizing all-pass filtering.

2. The optical allpass filter according to claim 1, wherein the coupling region of the j-th micro-ring and the waveguide provided with the j + 2-th optical phase shifter is designed as a tunable coupler, the tunable coupler is composed of two identical coupling regions and two equal-length waveguides, and the phase difference between the upper arm and the lower arm of the tunable coupler is changed by adjusting the j + 2-th optical phase shifter, so as to tune the coupling coefficient of the j-th micro-ring, where j is 1, …, N.

3. The optical all-pass filter according to claim 1 or 2, wherein the N micro-rings are provided with phase shifters for adjusting resonance wavelengths of the respective micro-rings.

4. The optical allpass filter of claim 3 wherein the phase frequency response center frequency of the allpass filter is tuned by changing the resonant wavelength of the microring using thermo-optic effects, carrier injection or optomechanical principles.

5. The optical allpass filter of claim 3 wherein said N +2 optical phase shifters and the phase shifters disposed in said N micro-rings are implemented using heater electrodes, PN junctions, or optical structures.

6. The optical allpass filter of claim 1 wherein the first optical coupler is a 1 x 2 multimode interference coupler or a Y-branch coupler or a directional coupler; the second optical coupler adopts a 2 x 2 multi-mode interference coupler or a directional coupler; the third optical coupler adopts a 2 x 1 multi-mode interference coupler or a Y-branch coupler or a directional coupler.

7. The optical all-pass filter according to claim 1 or 6, further comprising: a first coupling grating (1) and a second coupling grating (9);

the first coupling grating is connected with the input end of the first optical coupler and used as the input end of the optical all-pass filter;

and the second coupling grating is connected with the fifth waveguide and used as an output end of the optical all-pass filter.

8. The optical all-pass filter according to claim 1, wherein the filter is made of any one of the following materials: silicon, organic polymers, silicon nitride, silicon oxide; the filter adopts any one of the following waveguide structures: strip waveguides, ridge waveguides, slit waveguides, surface plasmon waveguides.

Background

An all-pass filter is a special filter that keeps the amplitude constant over a certain frequency range and only changes the phase information, which can also be called an all-pass network (phase filter). The traditional filter realizes signal filtering through the change of the amplitude-frequency response of the filter, and the all-pass filter changes the phase information of the signal through the phase-frequency response characteristic of the filter, so that the amplitude of the signal is not influenced except for fixed insertion loss. In electronics, this characteristic is often used to eliminate unwanted phase effects in the system and to introduce delay to a particular signal; in the optical field, the phase response of an all-pass filter can be designed correspondingly, so that an optical delay line, a dispersion compensator and the like can be constructed, and the all-pass filter is an important device in an all-optical signal processing system.

Optical all-pass filters generally have two structures: Gires-Tournois lumen and microring structure. Compared to the Gires-Tournois cavity, the formation of the micro-ring resonance does not require a cavity surface to provide optical feedback, with inherent integration advantages. The micro-ring has small volume, low cost, easy tuning and good phase-shifting characteristic, so the micro-ring structure is widely applied to the structure of the all-pass filter. Ideally, i.e., the waveguide has no transmission loss, the amplitude response of the microring is constant at 1, and has a phase response of 2 π. A series of micro-rings are cascaded, so that the phase response range and the group delay of the system can be infinitely expanded on the basis of not changing the amplitude response, and the system can be regarded as an ideal all-pass filter. In practice, however, the loss of the waveguide is not negligible, so that the amplitude response of the microring resonator is no longer a straight line with a constant 1, but a concave curve. Meanwhile, in order to meet the application requirements of more systems, an adjustable and reconfigurable all-pass filter with a larger phase-frequency response range is urgently needed.

Disclosure of Invention

Aiming at the defects and improvement requirements of the prior art, the invention provides a reconfigurable optical all-pass filter, aiming at obtaining an all-pass filter with large phase response range and high time delay while obtaining a flat amplitude-frequency response, and realizing the reconfiguration of the filter order, thereby enhancing the flexibility of the all-pass filter.

To achieve the above object, the present invention provides a reconfigurable optical all-pass filter, comprising: three optical couplers, N +2 optical phase shifters and N micro-rings, where N is an integer greater than or equal to 2; wherein the content of the first and second substances,

the upper arm of the output end of the first optical coupler is connected with the upper arm of the input end of the second optical coupler through a first waveguide, and the lower arm of the output end of the first optical coupler is connected with the lower arm of the input end of the second optical coupler through a second waveguide; a first optical phase shifter is arranged on the first waveguide or the second waveguide;

the upper arm of the output end of the second optical coupler is connected with the upper arm of the input end of the third optical coupler through a third waveguide, and the lower arm of the output end of the second optical coupler is connected with the lower arm of the input end of the third optical coupler through a fourth waveguide; a second optical phase shifter is arranged on the third waveguide, a third optical phase shifter and a first micro-ring coupled with the fourth waveguide are arranged on the fourth waveguide, or a second optical phase shifter is arranged on the fourth waveguide, and a third optical phase shifter and a first micro-ring coupled with the third waveguide are arranged on the third waveguide;

the output end of the third optical coupler is connected with a fifth waveguide, and the fifth waveguide is provided with N-1 optical phase shifters and N-1 micro-rings coupled with the fifth waveguide;

tuning the coupling coefficients of the N micro-rings to make the zero point of the (i + 1) th micro-ring and the pole of the (i) th micro-ring mirror-symmetrical, wherein i is 1, … and N-1; and adjusting the first optical phase shifter to make the zero point of the first micro-ring and the pole of the Nth micro-ring mirror symmetric, thereby realizing all-pass filtering.

Further, a coupling region of the jth micro-ring and a waveguide provided with a jth +2 optical phase shifter is designed to be an adjustable coupler, the adjustable coupler is composed of two identical coupling regions and two equal-length waveguides, the phase difference of the upper arm and the lower arm of the adjustable coupler is changed by adjusting the jth +2 optical phase shifter, and therefore the coupling coefficient of the jth micro-ring is tuned, wherein j is 1, … and N.

Furthermore, phase shifters are arranged on the N micro rings and used for regulating and controlling the resonance wavelength of the corresponding micro rings.

Further, the resonance wavelength of the micro-ring is changed by adopting the thermo-optic effect, carrier injection or optical force principle, so that the phase-frequency response center frequency of the all-pass filter is adjusted.

Further, the N +2 optical phase shifters and the phase shifters arranged on the N micro-rings are implemented by using heating electrodes, PN junctions or optical structures.

Further, the first optical coupler adopts a 1 x 2 multi-mode interference coupler or a Y-branch coupler or a directional coupler; the second optical coupler adopts a 2 x 2 multi-mode interference coupler or a directional coupler; the third optical coupler adopts a 2 x 1 multi-mode interference coupler or a Y-branch coupler or a directional coupler.

Further, the optical all-pass filter further includes: a first coupling grating and a second coupling grating;

the first coupling grating is connected with the input end of the first optical coupler and used as the input end of the optical all-pass filter;

and the second coupling grating is connected with the fifth waveguide and used as an output end of the optical all-pass filter.

Further, the filter adopts any one of the following materials: silicon, organic polymers, silicon nitride, silicon oxide; the filter adopts any one of the following waveguide structures: strip waveguides, ridge waveguides, slit waveguides, surface plasmon waveguides.

Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:

(1) the optical all-pass filter provided by the invention adopts the all-pass filter with constant amplitude-frequency response obtained after the adjustment of amplitude, phase and coupling coefficient, and the specific implementation method comprises the following steps: tuning the coupling coefficients of the N micro-rings to ensure that the zero point of the (i + 1) th micro-ring and the pole of the (i) th micro-ring are in mirror symmetry, wherein i is 1, … and N-1; meanwhile, the first optical phase shifter is adjusted, so that the zero point of the first micro-ring and the pole of the Nth micro-ring are in mirror symmetry, and all-pass filtering is realized. Compared with the prior art, the all-pass filter is realized without eliminating waveguide loss, and the problem that the cascaded micro-ring all-pass filter is sensitive to waveguide loss and is difficult to manufacture is solved. In addition, the phase-frequency response of the device is the superposition of the phase-frequency responses of the micro-ring resonators, so that the all-pass filter has an ultra-large phase-frequency response range.

(2) Compared with the low-order all-pass filter, the optical all-pass filter provided by the invention has the advantages that the rolling edge of the phase frequency response is steeper and the phase frequency change range is larger due to the adoption of the plurality of micro-rings. Therefore, the microwave photon delayer realized based on the structure can realize high delay, and the delay amount can be further improved by cascading more micro-rings. Meanwhile, due to the steep edge rolling, the microwave photon phase shifter based on the structure can work in a lower frequency region, and the working frequency range of the phase shifter is favorably improved. In addition, the structure has the characteristic of reconfigurable orders, and has potential application value in the fields of phased array radar, signal processing and the like.

(3) The optical all-pass filter provided by the invention realizes the tuning of the filter through the optical phase shifter on the micro-ring; the resonance wavelength of the micro-ring can be changed by using, but not limited to, thermo-optic effect, carrier injection principle or optical force principle, so that the center frequency of the phase filtering of the optical all-pass filter can be adjusted. Meanwhile, an adjustable coupler is introduced into the structure, and the coupling coefficient of the micro-ring and the waveguide is adjustable, so that the phase response and the order of the high-order all-pass filter are reconfigurable.

Drawings

FIG. 1 is a schematic structural diagram of an optical all-pass filter provided in the present invention;

FIG. 2 is a schematic view of a micro-ring unit structure;

FIG. 3 is a diagram illustrating magnitude-frequency response simulation results of an optical all-pass filter according to the present invention;

FIG. 4 is a diagram illustrating a simulation result of phase-frequency response of the optical all-pass filter according to the present invention;

FIG. 5 is a diagram illustrating a simulation result of delay response of an optical all-pass filter according to the present invention;

the same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein: 1-a first coupling grating, 2-a first optical coupler, 3-a first optical phase shifter, 4-a second optical coupler, 5-a second optical phase shifter, 6-a third optical phase shifter, 7-a first microring, 8-a third optical coupler, 9-a second coupling grating.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

Referring to fig. 1, the present invention provides a reconfigurable optical all-pass filter, comprising: the optical coupler comprises a first coupling grating 1, a second coupling grating 9, three optical couplers, N +2 optical phase shifters and N micro-rings, wherein N is an integer greater than or equal to 2; wherein the output end of the first optical coupler 2, the input end of the second optical coupler 4 and the first optical phase shifter 3 form MZI₁Structure (Mach-Zehnder Interferometer); the output of the second optical coupler 4, the input of the third optical coupler 8, the second optical phase shifter 5, the third optical phase shifter 6 and the first microring 7 together form a MZI₂And (5) structure.

Specifically, the upper arm of the output end of the first optical coupler 2 is connected with the upper arm of the input end of the second optical coupler 4 through a first waveguide, and the lower arm of the output end of the first optical coupler 2 is connected with the lower arm of the input end of the second optical coupler 4 through a second waveguide; the first waveguide or the second waveguide is provided with a first optical phase shifter 3 for changing the phase difference of the upper and lower arm signals. The upper arm of the output end of the second optical coupler 4 is connected with the upper arm of the input end of the third optical coupler 8 through a third waveguide, and the lower arm of the output end of the second optical coupler 4 is connected with the lower arm of the input end of the third optical coupler 8 through a fourth waveguide; the third waveguide is provided with a second optical phase shifter 5, the fourth waveguide is provided with a third optical phase shifter 6 and a first micro-ring 7 coupled with the fourth waveguide, or the fourth waveguide is provided with a second optical phase shifter 5, the third waveguide is provided with a third optical phase shifter 6 and a first micro-ring 7 coupled with the third waveguide, and the second optical phase shifter 5 is used for changing the phase difference of the upper and lower two-arm signals. The output end of the third optical coupler 8 is connected with the second coupling grating 9 through a fifth waveguide, and the fifth waveguide is provided with N-1 optical phase shifters and N-1 micro-rings coupled with the fifth waveguide.

Referring to fig. 2, the micro-ring has two ports, i.e., port 1 and port 2, the coupling region between the jth micro-ring and the waveguide provided with the jth +2 optical phase shifter is designed as an adjustable coupler, the adjustable coupler includes two identical coupling regions and two equal-length waveguides, and the phase difference between the upper arm and the lower arm of the adjustable coupler is changed by adjusting the jth +2 optical phase shifter, so as to tune the coupling coefficient of the jth micro-ring, where j is 1, …, and N.

The working principle of the optical all-pass filter is as follows: by adjusting MZI₁Inner first optical phase shifter 3 realizes MZI₂Regulating and controlling the light splitting ratio of the upper arm and the lower arm; MZI₂The upper arm signal light is coupled through a first micro-ring 7, and the lower arm signal is transmitted through a waveguide and then input into a third optical coupler 8; MZI by adjusting the second optical phase shifter 5₂The lower arm signal produces a phase difference of an integral multiple of pi compared to the upper arm signal; two paths of signals subjected to amplitude and phase regulation are input into the third optical coupler 8 for interference, signals obtained by two paths of light interference are output from the third optical coupler 8, and are output from the second coupling grating 9 after being coupled by a plurality of cascaded micro-rings (a second micro-ring to an Nth micro-ring). The present invention is a multi-zero pole system that requires mirror symmetry of the zeros and poles about the Z-domain unit circle in order to obtain an all-pass filter with constant amplitude response. And performing Z transformation on the transmission spectrum of the single all-pass micro-ring, so that a zero point and a pole exist on a Z domain. The pole of the first micro ring is symmetrical to the zero point of the second micro ring in a mirror image mode by tuning the coupling coefficient of each micro ring, the pole of the second micro ring is symmetrical to the zero point of the third micro ring in a mirror image mode, and the like is carried out until the pole of the (N-1) th micro ring is symmetrical to the zero point of the (N) th micro ring. Finally, in order to obtain a zero point which is in mirror symmetry with the pole position of the Nth micro-ring, MZI is utilized₁The position of the zero point of the first micro-ring is changed by the interference characteristic of the structure, specifically, the first optical phase shifter 3 is adjusted to make the zero point of the first micro-ring and the pole of the Nth micro-ring mirror symmetricThus realizing the all-pass filtering of the whole structure. Furthermore, by regulating MZI₂The self-coupling coefficient of the cascade micro-ring after the structure is 0 or 1, so that the order of the all-pass filter can be reconstructed.

Fig. 3, fig. 4, and fig. 5 are simulation results of amplitude-frequency response, phase-frequency response, and delay response of the optical all-pass filter, respectively, and respectively show simulation results of one micro-ring, two micro-rings, and three micro-rings in an all-pass state under the condition that the coupling coefficients of the first micro-ring 7 are the same. In fig. 3, the amplitude-frequency response of the three all-pass filter structures can be kept constant regardless of the number of micro-rings, but the loss of the whole structure increases with the increase of the number of micro-rings, so that the all-pass filter of the structure can cascade enough micro-rings under the condition that the insertion loss allows. Fig. 4 shows the phase frequency response of three all-pass filter structures, and it can be seen from the figure that as the number of micro-loops increases, the range of the phase frequency response increases by a multiple of 2 pi, and the edge rolling of the phase frequency response becomes steeper accordingly. Fig. 5 shows the delay response of three all-pass filter structures, and it can be seen that a higher order all-pass filter can achieve a higher delay than a lower order all-pass filter.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.