CN112147740A

CN112147740A - Multi-working-frequency-band programmable microwave photonic filter based on integrated silicon waveguide

Info

Publication number: CN112147740A
Application number: CN202011140313.9A
Authority: CN
Inventors: 廖莎莎; 包航; 张甜甜; 刘继伟
Original assignee: Chongqing University of Post and Telecommunications
Current assignee: Chongqing University of Post and Telecommunications
Priority date: 2020-10-22
Filing date: 2020-10-22
Publication date: 2020-12-29

Abstract

The invention relates to a multi-working-frequency-band programmable microwave photonic filter based on an integrated silicon waveguide, which belongs to the technical field of filtering and comprises a coupler, a fixed delay structure, a phase modulation structure, a power modulation structure, an adjustable delay structure and a reflection structure which are sequentially connected on a silicon-based chip, wherein the coupler divides an input signal into n uniform signals, and n is more than or equal to 4; the fixed delay structure is used for adding equal-difference delay quantity to each path of signal; the phase modulation structure is used for adjusting the phase of each path of optical signal; the power modulation structure is used for adjusting the power of each path of optical signal; the adjustable delay structure is used for adding different delay quantities to each path of signal; and finally, returning the signal in the original path through the reflection structure.

Description

Multi-working-frequency-band programmable microwave photonic filter based on integrated silicon waveguide

Technical Field

The invention belongs to the technical field of filtering, and relates to a multi-working-frequency-band programmable microwave photonic filter based on integrated silicon waveguide

Background

The microwave photon technology combines microwave communication and photon technology, fully utilizes the advantages of high sampling rate, low loss, large time bandwidth product, parallel transmission realization and the like of the photon technology, can effectively break through the bottleneck of generating and processing microwave signals by traditional electricity, and improves the communication capacity and the signal processing bandwidth of a microwave communication system. The microwave photon filter realized by adopting the microwave photon technology is far superior to the traditional electric filter in the aspects of signal processing frequency, bandwidth and flexibility, can process signals with larger bandwidth, higher frequency and larger dynamic range, and has important significance in the aspects of promoting the development of novel communication technologies such as 5G and the like, expanding the application scene of the filter and the like.

The current methods of microwave photon filters can be mainly divided into two categories: the first is to directly construct a Finite Impulse Response (FIR) structure, and the second is to control the optical signal sidebands containing microwave signal information. The first method is the most common method for realizing microwave photon filter, and the method modulates the microwave signal to be processed on the optical signal, realizes microwave signal branches with different amplitudes, phases and delays by modulating and delaying the optical signal, and finally realizes the effect of microwave filtering. Since 2008, research teams at home and abroad will directly construct FIR structure Microwave Photonic filters, and the FIR structure Microwave Photonic filters are pushed from simple positive coefficients (Mora, j., Chen, l.r., capmann, j.single-Bandpass Microwave Filter With Tuning and Reconfiguration capabilities, Journal of Lightwave Technology,2008) to enable the positive and negative coefficients to be re-expanded to Complex coefficients (Zhang, c., Yan, l.s., Pan, w., et. a Tunable Microwave Photonic Filter With a Complex Coefficient Based on Tuning. Photonic Journal,2013), and also to enable the center frequency and bandwidth functions to be Tunable. However, due to the addition of polarization modulators, optical parametric amplifiers, and other devices in the system, the filters become more complex and difficult to miniaturize and integrate.

The second approach is to implement microwave filtering by varying the relative powers between the sidebands and the optical carrier, the sidebands and the sidebands, and the shape of the sidebands or the optical carrier. The sideband of the control optical signal may be generated by Stimulated Brillouin Scattering (SBS) effect (Li, p., zuu, x., Pan, w., et al, Tunable Photonic Radio-Frequency Filter With a Record Out-of-Band emission. ieee Transactions on Microwave and technology, 2017), Fiber Bragg Grating (FBG) (gate, l., Zhang, j., chemical, x., Microwave Filter With waveguide Using a Modulator and an amplifier, and optical waveguide With a reflector, FBG) or by micro-wave Filter With a micro-oscillator, optical waveguide, and micro-oscillator, optical waveguide, micro-oscillator, micro-. The SBS effect has the advantages of low threshold, narrow natural line width, flexible control and the like. However, the SBS effect has high requirements for the pump device, and in order to control the sidebands of the optical signal, the pump light needs to be precisely controlled, which undoubtedly increases the difficulty of implementing the filter and also increases the cost of the filter system. In addition to this, the integration scheme of the SBS effect employs As₂S₃The waveguide is not only expensive, but also not compatible with the existing CMOS process, which is not beneficial to large-scale integration. Compared with the prior art, the scheme of the FBG and the micro-ring has much lower requirements on related instruments and low cost, and the reflection/resonance wavelengths of the FBG and the micro-ring can be effectively changed by methods of thermal tuning, electric tuning and the like, so that the center frequency of the filter is changed. However, because the shapes of the reflection spectrum and the resonance spectrum of the manufactured FBG and the micro-ring are difficult to change, the method for realizing the bandwidth adjustment range of the filter is narrow, and the filtering shape is difficult to change greatlyAnd (4) transforming.

By analyzing the existing microwave photon filter scheme, the adjustability of the filter is not strong, and meanwhile, the schemes of realizing the adjustability of the central wavelength and the bandwidth are fewer; the filter with the variable filter shape has few schemes, and the filter scheme with a special filter shape is hardly realized; the working frequency band of the filter can only meet the requirement of a single frequency band, and can not meet the filtering requirements of optical signals, microwave signals and millimeter wave signals at the same time.

Disclosure of Invention

In view of the above, the present invention provides an integrated silicon waveguide-based multi-operating-band programmable microwave photonic filter, which has a compact structure, is easy to manufacture and low in cost, integrates all functional modules on one silicon-based chip, can realize multi-operating-band programmable, and is more flexible, wider in application range and strong in usability.

In order to achieve the purpose, the invention provides the following technical scheme:

a multi-working-frequency-band programmable microwave photonic filter based on an integrated silicon waveguide comprises a coupler, a fixed delay structure, a phase modulation structure, a power modulation structure, an adjustable delay structure and a reflection structure which are sequentially connected on a silicon-based chip, wherein the coupler divides input signals into n uniform signals, and n is more than or equal to 4; the fixed delay structure is used for adding equal-difference delay quantity to each path of signal; the phase modulation structure is used for adjusting the phase of each path of optical signal; the power modulation structure is used for adjusting the power of each path of optical signal; the adjustable delay structure is used for adding different delay quantities to each path of signal; and finally, returning the signal in the original path through the reflection structure.

Further, the coupler is a silicon-based structure with a plurality of Y-branch power dividers connected in series, a plurality of 1 × 2MMI power dividers connected in series or 1 × n MMI power dividers.

Further, the fixed delay structure comprises n silicon-based waveguides with different lengths, and the n paths of delay amount are distributed in an arithmetic progression, namely T, 2T, 3T and … … nT.

Furthermore, the phase modulation structure is an electrically-tunable or thermally-tunable silicon-based straight waveguide structure, and different phases are introduced to each path of optical signal by changing voltage or temperature.

Further, the power modulation structure is an electrically-tunable or thermally-tunable MZI structure, and each optical signal is attenuated to different degrees by changing voltage or temperature.

Furthermore, the adjustable time delay structure is an electric or thermal silicon-based micro-ring array or an electric or thermal silicon-based chirped Bragg grating, different time delay amounts are introduced to each path of optical signal by changing voltage or temperature, and the silicon-based chirped Bragg grating is of a reflection type or a reverse coupling type.

Furthermore, the reflection structure is a silicon-based Sagnac reflector structure or a silicon-based Bragg grating structure and reflects signals.

The invention has the beneficial effects that: compared with a separation device, the silicon-based waveguide structure is compact in structure, easy to integrate, simple to manufacture and low in cost, can realize multi-working-frequency-band programmable filtering, and is more flexible, wider in application range and strong in usability.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.

Drawings

For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic structural diagram of a microwave photonic filter according to the present invention, FIG. 1(a) is a block diagram of a microwave photonic filter, and FIG. 1(b) is a schematic top view of a silicon waveguide system of the microwave photonic filter;

FIG. 2 is a schematic diagram of a microwave photonic filter;

fig. 3 is a diagram of programmable filtering results, in which fig. 3(a) is center frequency tunable filtering, fig. 3(b) is bandwidth tunable filtering, and fig. 3(c) -fig. 3(f) are filter shape tunable filtering (triangular, rectangular, gaussian, and super-gaussian filtering, respectively);

FIG. 4 is a graph of the result of the bit-multiplexed band filtering;

FIG. 5 is a schematic diagram of designing a 1 × 2MMI, wherein FIG. 5(a) is a 2-dimensional schematic diagram and FIG. 5(b) is a 3-dimensional schematic diagram;

FIG. 6 is a schematic diagram of a fixed delay structure, in which FIG. 6(a) is a 2-dimensional schematic diagram and FIG. 6(b) is a 3-dimensional schematic diagram;

FIG. 7 is a modulation structure of a hot modulation mode, wherein FIG. 7(a) is a 2-dimensional schematic diagram of a designed hot modulation structure, FIG. 7(b) is a 3-dimensional schematic diagram of a designed hot modulation structure, FIG. 7(c) is a 2-dimensional schematic diagram of a designed hot MZI power modulation structure, and FIG. 7(d) is a 3-dimensional schematic diagram of a designed hot MZI power modulation structure;

fig. 8 is a schematic diagram of an adjustable delay structure and a reflection structure (which can implement the functions of delaying and reflecting signals) of a thermal modulation mode, where fig. 8(a) is a 2-dimensional schematic diagram and fig. 8(b) is a 3-dimensional schematic diagram.

Reference numerals: 1-coupler, 11-1 × 2MMI, 2-fixed delay structure, 3-phase modulation structure, 31-hot electrode, 4-power modulation structure, 41-thermoelectric stage, 5-adjustable delay structure, 51-reflection type chirp Bragg grating and 6-reflection structure.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.

Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.

Fig. 1 shows a schematic structural diagram of a microwave photonic filter implementation of the present invention. The microwave photonic filter based on the integrated silicon waveguide comprises a coupler 1, a fixed delay structure 2, a phase modulation structure 3, a power modulation structure 4, an adjustable delay structure 5 and a reflection structure 6 which are connected in sequence.

An optical signal to be processed is input into the microwave photon filter, the input signal is divided into n paths of uniform signals by the coupler 1 (n can be any integer larger than 4), the fixed delay structure 2 carries out equal-difference delay on each path of signal, the phase modulation structure 3 carries out phase modulation on each path of signal, the phase modulation structure comprises a hot electrode 31, the power modulation structure 4 carries out power modulation on each path of signal, the amplitude modulation structure comprises a hot electrode 41, the adjustable delay structure 5 adds different delays on each path of signal, and finally the reflection structure 6 reflects the signal to return the original path of the signal, wherein the two functions are realized by the reflection type chirp Bragg grating 51. All the functional modules of the microwave photon filter are manufactured on one silicon-based chip, so that the microwave photon filter is compact in structure and low in power consumption. In addition, the microwave photonic filter is different from the aforementioned scheme in principle, and is to transfer the microwave or millimeter wave signal filtering to the optical filtering, determine the microwave filtering curve by the optical filtering curve, and realize the improvement of the performance of the microwave and millimeter wave filter by designing the optical filtering curve. Therefore, the filter is also suitable for optical signal filtering. Fig. 2 shows a schematic diagram of the filtering technique of the integrated silicon waveguide based multi-operating-band programmable microwave photonic filter, which has the following principles:

suppose the frequency spectrum of the microwave/millimeter wave signal to be processed is M (f) and the central frequency is f_m(as shown by the spectrum at node B in fig. 2). The optical carrier adopts single-wavelength laser, and the optical carrier frequency is set as f_c(as shown by the spectrum at node a in fig. 2). The modulation of the signal to be processed by the modulator onto the optical carrier forms a plurality of sidebands, as shown by the spectrum at node C in FIG. 2, whose center frequencies are f_m+f_cAnd f_c-f_m. Suppose that the optical filter filters a spectrum H (f) and has a center frequency f₀The spectrum of the signal after passing through the optical filter is M (f). H (f). For large-bandwidth microwave/millimeter wave signals, the bandwidth of M (f). H (f) is approximately equal to that of H (f), and the center frequency is f₀. The optical carrier signal is then re-injected into the system, where the signal spectrum is shown in fig. 2 as the spectrum at node E. Finally, the spectrum of the final output signal after passing through the photodetector is shown as the spectrum at node F in FIG. 2, with center frequency F₀-f_c. That is, the filter has the same filter bandwidth as the optical filter and a center frequency equal to the difference between the optical carrier frequency and the center frequency of the optical filter. The microwave/millimeter wave filtering is transferred to optical filtering, namely, high-quality microwave/millimeter wave filtering can be realized only by improving the performance of the optical filter.

In the aspect of optical filtering, the microwave photonic filter of the invention utilizes the FIR principle, and can realize programmable characteristics, namely three filtering functions of adjustable central frequency, adjustable bandwidth and variable filtering shape, by finely controlling the phase modulation structure and the power modulation structure in each branch of the FIR; by changing the delay amount of the adjustable delay structure in each branch of the FIR, the filter can adapt to the requirements of different working frequency bands of microwave, millimeter wave and optical filtering, and the related derivation is as follows:

assuming an input light field of f_in(t) Fourier transform thereof into F_in(ω). Assuming that the electric field intensities of n output paths after being reflected by the reflecting structure are respectively E_out1、E_out2、E_out3……E_outnThen the final output of the system is:

set its Fourier transform to F_out(ω) then

The system transfer function H (ω) is then:

wherein alpha is_kThe power modulation coefficient introduced by the power modulation structure on the kth light path is between 0 and 1;

the phase change introduced by the phase modulation structure on the kth light path is between 0 and 2 pi; t is the fixed delay difference between two adjacent paths; delta T_kIs the adjustable delay amount of the k-th path. From the final H (ω) form, the output of the optical filter is in the form of a fourier series.

According to the derivation, the optical signal to be processed is divided into multiple paths by the coupler 1, different delays are carried out by the fixed delay structure 2, and power and phase adjustment is carried out by the phase modulation structure 3 and the power modulation structure 4, so that the sum of any Fourier series can be realized, and the programmable filtering signal can be realized in the frequency domain. And the period of the filter can be changed by changing the delay amount in the adjustable delay structure 5, so that the adjustable delay structure is suitable for different working frequency bands.

An optical signal can introduce an equal-difference delay amount after passing through a fixed delay structure 2, the fixed delay structure 2 comprises n silicon-based waveguides with different lengths, and each path of delay amount is distributed in an equal-difference number sequence, namely a first path 1T, a second path 2T, and a third path 3T … … nth path nT. The phase modulation structure 3 and the power modulation structure 4 are both finely controlled in an electric or thermal regulation mode, so that the center frequency, the bandwidth and the filtering shape of the filter are controllable, and programmable filtering is realized. For the hot phase modulation bit/power modulation structure, parameters of the phase power modulation structure 3 and the power modulation structure 4 are set, programmable filtering can be realized only by changing the temperature, and the output result is shown in fig. 3, in this embodiment, n is 8, T is 50ps, and Δ T is shown in fig. 3_k＝0(k＝1,2,……8)。

The adjustable delay structure 5 is also finely controlled by means of electrical or thermal tuning. For the thermal tuning delay structure, the delay amount in the optical path can be changed by changing the temperature, the period of the filter can be changed correspondingly, the requirements of different working frequency bands are met, the output result is shown in fig. 4, in the embodiment, n is 8, T is 50ps, and Δ T is_k0 (dot-dash curve) and Δ T_k16.7 kps (solid curve), Δ T _k50 kps (dashed curve), where k is 1,2, … … 8.

In this particular embodiment, the silicon layer on the silicon waveguide is 220nm, and the strip waveguide is formed by etching 220nm and has a width of 500 nm. Taking an 8-path optical path as an example, the coupler 1 can be implemented by a series 3-stage 1 × 2MMI11 structure, each 1 × 2MMI11 implements a power division effect, and after 3 stages, the input optical signal is equally divided into 8 parts. A 2 and 3 dimensional representation of a 1 x 2MMI11 in the present invention is shown in fig. 5.

In this particular embodiment the fixed delay structure 2 is implemented by waveguides of different lengths, the 2 and 3 dimensional schematic of which is shown in fig. 6 (different delay amounts are achieved by cascading a plurality of the above structures), where the curved waveguide has a radius of curvature of 10 μm. The difference of the delay time between every two adjacent optical paths is 50 ps.

In this particular embodiment, the phase modulation structure 3 and the power modulation structure 4 are implemented by thermal modulation, the principle of which can be simply explained as that temperature variation causes the effective refractive index of the silicon waveguide to vary, resulting in phase variation, which can implement phase modulation. The phase modulation structure is manufactured on one arm of a Mach-Zehnder interferometer (MZI) structure, and power modulation can be achieved. Under the condition of 1550nm optical signal and room temperature, the refractive index n of the silicon waveguide can be expressed by the following relation of the change T with the temperature:

and the relation between the transmission phase change of the optical signal in the waveguide and the refractive index of the waveguide can be expressed as follows:

wherein L is an optical path through which the optical signal passes, L is an actual length of the waveguide, λ is an optical signal wavelength, and k is a wave vector of the optical signal.

For a fixed waveguide length l, the relationship between the temperature and the phase change of the optical signal transmitted in the waveguide can be expressed by the following equations (4) and (5):

wherein n is₀The refractive index of the silicon waveguide when unheated, and Δ T is the change in temperature.

In this particular embodiment, the 2-and 3-dimensional schematics of the thermo-phase modulating structure 3 are shown in fig. 7(a) and 7(b), respectively, with the waveguide structure unchanged, wherein the thermo-modulating region length is 200 μm. The 2-dimensional and 3-dimensional schematic diagrams of the thermally tuned MZI power modulation structure 4 are shown in fig. 7(c) and 7(d), respectively, and the curved waveguide has a radius of curvature of 10 μm and a length of 240 μm for two arms, and the length of the thermally tuned region of one arm is 200 μm. It should be noted that the longer the thermal modulation region, the larger the refractive index change at the same temperature change. If the size needs to be reduced, the same requirement can be met only by increasing the temperature of the heater.

In this particular embodiment, the tunable delay structure 5 and the reflective structure 6 are implemented by a thermally tuned reflective chirped bragg grating 51, whose 2-and 3-dimensional diagrams are shown in fig. 8(a) and 8(b), respectively. The structure consists of a waveguide with gradually changing width and square bulges which are periodically distributed, and the working principle of the structure is similar to that of a chirped fiber grating. The square bumps with the period of lambda enable the effective refractive index of the waveguide to change periodically, and the waveguide structure reflects optical signals with specific wavelengths. Unlike chirped fiber gratings, which achieve chirp by varying the refractive index perturbation period, the variation in the reflection wavelength achieved in a waveguide grating depends on the width variation of the waveguide. This is because the period Λ of the square protrusion in the waveguide grating is usually only several hundred nm, and fine control of the period Λ will greatly increase the requirement for process accuracy and also reduce the process tolerance of the chip. When the waveguide width is increased from w₁Slowly changing to w₂In this case, the effective refractive index of the waveguide is also reduced, and the wavelength of the reflected optical signal is also slowly red-shifted. There may be a delay difference between the optical signals of different frequencies, since the optical signals of different frequencies are reflected at different locations of the waveguide. When the temperature of the structure is changed, the change of the temperature can cause the refractive index of the waveguide to change, thereby causing the change of the reflection position of the optical signal and realizing the adjustment of the delay amount.

Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.

Claims

1. A multi-working-frequency-band programmable microwave photonic filter based on integrated silicon waveguides is characterized in that: the silicon-based chip phase-delay circuit comprises a coupler, a fixed delay structure, a phase modulation structure, a power modulation structure, an adjustable delay structure and a reflection structure which are sequentially connected on a silicon-based chip, wherein the coupler divides an input signal into n uniform signals, and n is more than or equal to 4; the fixed delay structure is used for adding equal-difference delay quantity to each path of signal; the phase modulation structure is used for adjusting the phase of each path of optical signal; the power modulation structure is used for adjusting the power of each path of optical signal; the adjustable delay structure is used for adding different delay quantities to each path of signal; and finally, returning the signal in the original path through the reflection structure.

2. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the coupler is a silicon-based structure with a plurality of Y-branch power dividers connected in series, a plurality of 1 multiplied by 2 multi-mode interference coupler MMI power dividers connected in series or 1 multiplied by n MMI power dividers.

3. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the fixed time delay structure comprises n silicon-based waveguides with different lengths, and the n paths of time delay are distributed in an arithmetic progression, namely T, 2T, 3T and … … nT.

4. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the phase modulation structure is an electrically-tuned or thermally-tuned silicon-based straight waveguide structure, and different phases are introduced to each path of optical signals by changing voltage or temperature.

5. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the power modulation structure is an electrically-tunable or thermally-tunable Mach-Zehnder interferometer MZI structure, and each path of optical signal is attenuated to different degrees by changing voltage or temperature.

6. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the adjustable time delay structure is an electric-regulation or thermal-regulation silicon-based micro-ring array or an electric-regulation or thermal-regulation silicon-based chirped Bragg grating, different time delay amounts are introduced into each path of optical signal by changing voltage or temperature, and the silicon-based chirped Bragg grating is of a reflection type or a reverse coupling type.

7. The integrated silicon waveguide-based multi-operating band, programmable microwave photonic filter of claim 1, wherein: the reflection structure is a silicon-based Sagnac reflector structure or a silicon-based Bragg grating structure and reflects signals.