CN117215132A - FP cavity electro-optic modulator array for multi-wavelength channel transmitter - Google Patents
FP cavity electro-optic modulator array for multi-wavelength channel transmitter Download PDFInfo
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Abstract
The invention discloses an FP cavity electro-optic modulator array which can be used for a multi-wavelength channel emitter. The modulator array is mainly formed by sequentially connecting N modulator units; the single modulator unit comprises a 2X 2 Fabry-Perot cavity structure, a substrate and a pair of modulation electrodes, wherein the 2X 2 Fabry-Perot cavity structure and the modulation electrodes are arranged on the upper surface of the substrate, the two modulation electrodes are symmetrically distributed on two sides of the 2X 2 Fabry-Perot cavity structure, the multiplexing end of the first mode demultiplexer is connected with the input end of one anti-symmetric multimode waveguide grating, the two anti-symmetric multimode waveguide gratings are connected through an intermediate waveguide, and the output end of the other anti-symmetric multimode waveguide grating is connected with the multiplexing end of the first mode demultiplexer. The invention has compact structure, excellent electro-optical modulation performance of the modulator unit, large modulation bandwidth and flexible cascading mode, can be used for a multi-wavelength channel transmitter, and is suitable for scenes such as large-capacity optical communication, optical calculation and the like.
Description
Technical Field
The invention belongs to the technical field of optical communication, and particularly relates to an FP (Fabry-Perot) cavity electro-optical modulator array for a multi-wavelength channel transmitter, which is low in energy consumption, large in bandwidth, compact in structure and capable of transmitting in multi-wavelength channels.
Background
In recent years, with rapid development in the fields of 5G communication, internet of things, data centers, and the like, demands of human society for communication capacity have been exponentially increased. Optical communication technology has become the mainstream technology of current communication by virtue of its anti-interference, low loss, high bandwidth, low crosstalk and the like. Among them, the photoelectric device is used as a core device, and gradually becomes a bottleneck for development of ultra-large capacity optical communication technology. Based on different materials and principles, solutions such as silicon-based modulators, indium phosphide modulators, electro-optic polymer modulators, thin film lithium niobate modulators, etc. have been widely studied and have emerged as a series of high performance devices. The thin film lithium niobate modulator has the characteristics of linear response, low optical loss, high stability and the like based on the principle of linear electro-optic effect, and becomes one of the schemes with the highest potential. However, lithium niobate has a relatively small electro-optic coefficient, and in X-cut thin film lithium niobate, light is transmitted along the ordinary axis of the crystal, and the external electric field is maximum along the extraordinary axis of the crystal, and the electro-optic coefficient is about 31pm/V at a wavelength of 1550nm. For mach-zehnder intensity modulators, a phase shifting arm on the order of millimeters to centimeters is typically required to accumulate sufficient phase change, reduce half-wave voltage, and must introduce traveling wave electrodes. For a micro-ring cavity modulator, the waveguide radius is typically large to avoid mode hybridization in the X-cut curved waveguide. Thus, both of the above structures have larger device dimensions.
In addition, lithium niobate modulators face a mutual constraint between modulation bandwidth and modulation efficiency. The method is characterized in that the modulation bandwidth of the Mach-Zehnder traveling wave electrode modulator is reduced along with the increase of the length of the phase-shifting arm due to the existence of microwave loss and refractive index mismatch; in a resonant modulator, the modulation efficiency of the microcavity of high quality factor is higher, however the intra-cavity photon lifetime is higher and the modulation bandwidth decreases with it.
There is an upper limit to the data transmission rate of a single modulator, and to meet the requirement of transmission capacity in a commercial optical communication system, using a multidimensional multiplexing technology, increasing the total transmission capacity by increasing the number of channels is a direct and reliable solution. Among them, wavelength division multiplexing technology has been widely studied and adopted in optical fiber communication systems. In such large-scale array scenarios, high performance lithium niobate modulators of small size and compact construction are particularly important. In the prior art, an electro-optic modulator array which can be used for a multi-wavelength channel high-speed transmitter and has a compact structure on a thin film lithium niobate platform is lacking.
Disclosure of Invention
In order to solve the problems in the background art, an object of the present invention is to propose an FP cavity (fabry-perot cavity) electro-optic modulator array that can be used in a multi-wavelength channel transmitter. The modulator array is formed by cascading N2X 2 Fabry-Perot cavity electro-optical modulator units with different working wavelengths, can realize larger communication capacity, more compact size and lower energy consumption, can compensate resonance peak wavelength errors caused by processing errors through subsequent upper-layer modification packages, has the advantages of simple design, simple process and the like, has expandability, and plays an important role in an on-chip integrated optical communication system.
The technical scheme adopted by the invention is as follows:
the modulator array is mainly formed by sequentially connecting N modulator units; for an nth modulator cell of the N modulator cells, the nth modulator cell comprising a 2 x 2 fabry-perot cavity structure, a substrate, and a pair of modulation electrodes; the 2X 2 Fabry-Perot cavity structure and the modulation electrodes are arranged on the upper surface of the substrate, and the two modulation electrodes are symmetrically distributed on two sides of the 2X 2 Fabry-Perot cavity structure.
The 2X 2 Fabry-Perot cavity structure comprises a first mode demultiplexer, a second mode demultiplexer, a first antisymmetric multimode waveguide grating, a second antisymmetric multimode waveguide grating and an intermediate waveguide;
the multiplexing end of the first mode demultiplexer is connected with the input end of the first anti-symmetric multimode waveguide grating, the output end of the first anti-symmetric multimode waveguide grating is connected with the input end of the second anti-symmetric multimode waveguide grating through an intermediate waveguide, and the output end of the second anti-symmetric multimode waveguide grating is connected with the multiplexing end of the first mode demultiplexer; the mode port I and the mode port II of the first mode demultiplexer are respectively used as an input end and a reflection output end of the modulator unit, and the mode port I and the mode port II of the second mode demultiplexer are respectively used as a transmission output end and an uploading end of the modulator unit; the first anti-symmetric multimode waveguide grating, the second anti-symmetric multimode waveguide grating and the intermediate waveguide are all positioned between the two modulation electrodes.
The N modulator units are sequentially connected in a transverse cascade or longitudinal cascade mode;
when N modulator units are transversely cascaded, the N modulator units are transversely cascaded into a row, and the transmission output end of the N-1 level modulator unit is connected with the input end of the N level modulator unit through a connecting waveguide;
when N modulator units are longitudinally cascaded, the N modulator units are longitudinally cascaded into a column, and the reflection output end of the N-1 level modulator unit is connected with the input end of the N level modulator unit through a connecting waveguide.
In the nth modulator unit, the first anti-symmetric multimode waveguide grating and the second anti-symmetric multimode waveguide grating both perform TE of signals 0 Mode reverse coupling to TE 1 TE of modes or signals 1 Mode reverse coupling to TE 0 A pattern, both patterns satisfying the following phase matching conditions:
(n eff0 +n eff1 )Λ＝λ bragg
wherein n is eff0 、n eff1 TE respectively 0 、TE 1 The equivalent refractive index of the mode, lambda is the grating tooth period of the anti-symmetric multimode waveguide grating, lambda bragg Is the center wavelength of the reflection band of the anti-symmetric multimode waveguide grating.
Tooth-shaped structures are arranged on two sides of the first anti-symmetrical multimode waveguide grating and the second anti-symmetrical multimode waveguide grating along the waveguide transmission direction, and the tooth-shaped structures on two sides are arranged in a way that grating teeth and tooth grooves are oppositely and symmetrically arranged on two sides; the width of each grating tooth varies as a specific function along the waveguide propagation direction.
The specific function is a gaussian, sinusoidal or hamming function.
The first mode demultiplexer and the second mode demultiplexer are mainly composed of a straight-through waveguide and a coupling waveguide; in the coupling working area, the through waveguide and the coupling waveguide are tapered.
The substrate is mainly formed by sequentially laminating a substrate and a lower cladding layer from bottom to top, a 2X 2 Fabry-Perot cavity structure and a modulation electrode are both arranged on the upper surface of the lower cladding layer, and an upper cladding modification layer is arranged on the upper surface of an intermediate waveguide in the 2X 2 Fabry-Perot cavity structure.
The first mode demultiplexer, the second mode demultiplexer, the first antisymmetric multimode waveguide grating, the second antisymmetric multimode waveguide grating and the intermediate waveguide are all formed by etching an X-cut film lithium niobate material.
The invention combines the wavelength selection characteristics of the antisymmetric multimode waveguide grating and the Fabry-Perot cavity, and connects a plurality of 2X 2 Fabry-Perot cavity electro-optical modulator units in cascade in different modes to construct a multi-wavelength channel, thereby greatly improving the total communication capacity of the transmitter. The waveguide in the Fabry-Perot cavity is covered with the coating modification layer, so that resonance peak wavelength errors caused by machining errors can be compensated, and channel intervals are uniform. The invention has compact structure, excellent electro-optic modulation performance of the modulator unit, large modulation bandwidth, low energy consumption and small insertion loss, avoids the use of a circulator/isolator, and has the potential of large-scale array; the modulator array based on the cascade has simple framework and flexible cascade mode, can be used for a multi-wavelength channel transmitter, and is suitable for scenes such as high-capacity optical communication, optical calculation and the like.
The beneficial effects of the invention are as follows:
1. the invention adopts the 2 multiplied by 2 Fabry-Perot cavity electro-optical modulator as a basic structural unit, and the 2 multiplied by 2 Fabry-Perot cavity electro-optical modulator is cascaded to form an array, so that the invention has compact structure, flexible framework, simple design and convenient process.
2. The invention is based on a thin film lithium niobate platform, and the comprehensive performance of the invention in the aspects of modulation linearity, device loss and theoretical bandwidth limit is far superior to that of electro-optical modulators based on other platforms in the background introduction.
3. In the aspect of performance of the unit device, the unit device adopts the lumped electrode, has lower energy consumption compared with a travelling wave electrode modulator on the film lithium niobate, and the bandwidth is only limited by the quality factor of a cavity and the RC bandwidth of the electrode, so that higher modulation bandwidth can be realized; the invention can utilize the reflection output signal of the Fabry-Perot cavity, and has lower loss and higher modulation efficiency compared with the traditional transmission spectrum; the invention has 2X 2 port, which can realize cascade connection in various forms.
4. In the aspects of device array and cascade connection, the invention has more remarkable advantage of compact structure. In addition, the invention utilizes the wavelength selective characteristics of the anti-symmetric waveguide grating and the Fabry-Perot cavity to directly cascade, and does not need an additional filter; the framework is flexible, and the small channel wavelength interval or the large channel wavelength interval can be realized through different cascading modes; and the processing error can be compensated by the upper wrapping modification layer, so that the method has excellent expandability.
Drawings
FIG. 1 is a schematic diagram of a structure in which N2X 2 Fabry-Perot Luo Dianguang modulator cells are arranged laterally and cascaded with an input waveguide of a next stage to form a multi-wavelength channel emitter by a transmission output waveguide;
fig. 2 is a schematic structural diagram of a multi-wavelength channel transmitter formed by cascade connection of N2×2 fabry-perot Luo Dianguang modulator units with a reflective output waveguide and an input waveguide of a next stage;
FIG. 3 is a schematic diagram of a 2×2 Fabry-Perot Luo Dianguang modulator cell structure employed in the present invention;
FIG. 4 is a schematic diagram of an anti-symmetric multimode waveguide grating employed in the present invention;
FIG. 5 is a schematic diagram of a mode demultiplexer employed in the present invention;
FIG. 6 is a schematic cross-sectional view of a waveguide;
FIG. 7 is a graph showing the variation of transmission intensity with wavelength of a reflective output of a 2X 2 Fabry-Perot Luo Dianguang modulator cell at various voltages in accordance with an embodiment of the present invention;
FIG. 8 is a schematic diagram showing the variation of the transmission intensity of the output end of a multi-wavelength channel transmitter formed by cascading a transmission output waveguide with an input waveguide of the next stage according to the wavelength, wherein 2X 2 Fabry-Perot Luo Dianguang modulator units are arranged transversely in the embodiment of the invention;
FIG. 9 is a schematic diagram showing the variation of the transmission intensity of the output end of a multi-wavelength channel transmitter formed by cascading a reflective output waveguide with an input waveguide of the next stage along with the wavelength, wherein 42×2 Fabry-Perot Luo Dianguang modulator units are longitudinally arranged in the embodiment of the present invention;
in the figure: 1-modulator unit (101, 102, …, 10N), 2 a-first mode demultiplexer, 2 b-second mode demultiplexer, 3 a-first antisymmetric multimode waveguide grating, 3 b-second antisymmetric multimode waveguide grating, 4-intermediate waveguide, 5 a-first modulating electrode, 5 b-second modulating electrode, 201-through waveguide, 202-coupled waveguide, 601-substrate, 602-lower cladding, 603-X-cut thin film lithium niobate material, 604-upper cladding modification layer.
Detailed Description
The invention will be further described with reference to the drawings and examples.
The transmitter is mainly formed by sequentially connecting N modulator units 101, …,10N, …, 10N; for an nth modulator cell of the N modulator cells, the nth modulator cell comprises a 2 x 2 fabry-perot cavity structure, a substrate, and a pair of modulation electrodes 5a, 5b; the 2 x 2 fabry-perot cavity structure and the modulating electrodes 5a and 5b are arranged on the upper surface of the substrate, and the two modulating electrodes 5a and 5b are symmetrically distributed on two sides of the 2 x 2 fabry-perot cavity structure along the waveguide transmission direction.
The modulator unit is specifically a 2×2 fabry-perot cavity electro-optic modulator unit, and the 2×2 fabry-perot cavity electro-optic modulator unit includes a pair of mode demultiplexers 2a, 2b and a pair of antisymmetric multimode waveguide grating cavity mirrors 3a, 3b, so that input/output light is separated, and 2×2 routing is realized. The pair of modulating electrodes 5a, 5b comprises a first modulating electrode 5a and a second modulating electrode 5b.
As shown in fig. 3, the 2×2 fabry-perot cavity structure includes a first mode demultiplexer 2a, a second mode demultiplexer 2b, a first antisymmetric multimode waveguide grating 3a, a second antisymmetric multimode waveguide grating 3b, and an intermediate waveguide 4;
the multiplexing end of the first mode demultiplexer 2a is connected with the input end of the first anti-symmetric multimode waveguide grating 3a, the output end of the first anti-symmetric multimode waveguide grating 3a is connected with the input end of the second anti-symmetric multimode waveguide grating 3b through a section of intermediate waveguide 4, and the output end of the second anti-symmetric multimode waveguide grating 3b is connected with the multiplexing end of the first mode demultiplexer 2 b; the mode port one and the mode port two of the first mode demultiplexer 2a serve as an input terminal and a reflection output terminal of the modulator unit 10n, respectively, and the mode port one and the mode port two of the second mode demultiplexer 2b serve as a transmission output terminal and an uploading terminal of the modulator unit 10n, respectively; the first anti-symmetric multimode waveguide grating 3a, the second anti-symmetric multimode waveguide grating 3b and the intermediate waveguide 4 are all located between the two modulating electrodes 5a, 5b.
The N modulator units are sequentially connected in a transverse cascade or longitudinal cascade mode;
when N modulator units are transversely cascaded, the N modulator units are transversely cascaded into a row, that is, the transmitter is mainly formed by uniformly and transversely arranging N modulator units in parallel at intervals along the transmission direction of the waveguide, as shown in fig. 1, the transmission output end of the N-1-th modulator unit 10 (N-1) and the input end of the N-th modulator unit 10N are connected through the connecting waveguide, the grating reflection bands of each modulator unit are different, and finally, the resonance wavelength of each modulator unit is different. Incident light with each wavelength is respectively incident from the uploading ends of the modulator units at each stage, and the modulated output light is combined and output at the transmission output waveguide of the modulator unit at the last stage;
when N modulator units are longitudinally cascaded, the N modulator units are longitudinally cascaded into a column, that is, the transmitter is mainly formed by uniformly and longitudinally arranging N modulator units in parallel at intervals along the direction perpendicular to the transmission direction of the waveguide, as shown in fig. 2, the reflection output end of the N-1-th modulator unit 10 (N-1) and the input end of the N-th modulator unit 10N are connected through the connecting waveguide, the reflection band of each stage of electro-optic modulator unit is the same, the equivalent cavity lengths are different, and finally, the resonance wavelength of each modulator unit is different. The incident light of each wavelength is incident from the input end of the 1 st stage unit device, and the modulated output light is combined and output in the transmission output waveguide of the last stage modulator unit.
In the nth modulator unit, the first and second anti-symmetric multimode waveguide gratings 3a and 3b perform signal processingTE of (2) 0 Mode reverse coupling to TE 1 TE of modes or signals 1 Mode reverse coupling to TE 0 A pattern, both patterns satisfying the following phase matching conditions:
(n eff0 +n eff1 )Λ＝λ bragg
wherein n is eff0 、n eff1 TE respectively 0 、TE 1 The equivalent refractive index of the mode, lambda, is the grating tooth period of the anti-symmetric multimode waveguide gratings 3a, 3b, lambda bragg Is the center wavelength of the reflection band of the antisymmetric multimode waveguide grating 3a, 3 b.
As shown in fig. 4, the first anti-symmetric multimode waveguide grating 3a and the second anti-symmetric multimode waveguide grating 3b are provided with tooth-shaped structures on both sides along the waveguide transmission direction, and the tooth-shaped structures on both sides are arranged in a way that grating teeth and tooth grooves are oppositely and symmetrically arranged on both sides; the maximum width of the grating teeth is delta 0 The width of each grating tooth varies along the waveguide propagation direction as a specific function, forming an intensity apodization.
The width of the grating teeth is specifically the length of the side of the grating teeth perpendicular to the direction of waveguide transmission.
The specific function is a gaussian, sinusoidal or hamming function.
As shown in fig. 5, the first mode demultiplexer 2a and the second mode demultiplexer 2b are of a dual-core waveguide structure, and each of the first mode demultiplexer 2a and the second mode demultiplexer 2b is mainly composed of a through waveguide 201 and a coupling waveguide 202; in the coupling operation region of the through waveguide 201 and the coupling waveguide 202, the through waveguide 201 and the coupling waveguide 202 are tapered, and the widths in the waveguide transmission direction are respectively widened and narrowed.
As shown in fig. 6, the substrate is mainly composed of a substrate 601 and a lower cladding layer 602 which are sequentially laminated from bottom to top, the fabry-perot cavity structure and modulation electrodes 5a and 5b are both arranged on the upper surface of the lower cladding layer 602, and an upper cladding modification layer 604 is arranged on the upper surface of an intermediate waveguide 4 in the fabry-perot cavity structure, that is, the intermediate waveguide 4 is located between the oxygen-buried layer 602 and the upper cladding modification layer 604.
The substrate 601, the oxygen buried layer 602, and the fabry-perot cavity structure, which are sequentially stacked from bottom to top, constitute a lithium niobate thin film LNOI on an insulator. The lower surface of the connecting waveguide is also provided with a substrate and an oxygen buried layer, and the substrate 601/oxygen buried layer 602 under the fabry-perot cavity structure is respectively connected with the substrate/oxygen buried layer under the connecting waveguide into an integrated structure.
The first mode demultiplexer 2a, the second mode demultiplexer 2b, the first antisymmetric multimode waveguide grating 3a, the second antisymmetric multimode waveguide grating 3b, and the intermediate waveguide 4 are each etched from an X-cut thin film lithium niobate material 603. The structure made of the X-cut thin film lithium niobate material 603 forms a monolithic thin film lithium niobate ridge waveguide.
The fabry-perot cavity structure of the waveguide region is a partially etched ridge waveguide, and different-sized upper cladding modification layers 604 can be respectively added to the fabry-perot cavities of the modulator units 10n at each stage according to actual processing conditions, so that additional phase change is introduced, and resonance wavelength errors caused by processing deviation are compensated. The upper cladding modification layer 604 may be formed directly by an electron beam exposure glue or a photoresist such as SU8, ZEP520, A, HSQ, or may be a cladding material such as silicon dioxide.
The working process and principle of the invention are as follows:
the teeth and tooth grooves of the anti-symmetric multimode waveguide gratings 3a and 3b in the Fabry-Perot cavity are symmetrically arranged on two sides, and the phase matching condition is satisfied:
(n eff0 +n eff1 )Λ＝λ bragg
wherein n is eff0 、n eff1 Respectively TE 0 、TE 1 The equivalent refractive index of the mode, lambda, is the period of the grating teeth of the anti-symmetric multimode waveguide grating 3a, 3b, lambda bragg Is the center wavelength of the reflection band of the anti-symmetric multimode waveguide grating teeth 3a, 3 b. Within the reflection band of the anti-symmetric multimode waveguide grating 3a, 3b, the grating can transfer TE 0 The incident light of the mode being partially reflected as TE 1 Mode, part still in TE 0 Mode transmission; or TE is to 1 The incident light of the mode being partially reflected as TE 0 Mode, part still in TE 1 Mode transmission.
The period and the reflection band center wavelength of the first anti-symmetric multimode waveguide grating 3a and the second anti-symmetric multimode waveguide grating 3b are the same.
When the resonance condition of the fabry-perot cavity is satisfied, the transmission output Cheng Xiezhen peak of the modulator cell and the reflection output form a resonant recess. Light is incident from the input end of the modulator unit, and the mode of reflected light is different from that of the incident light, so that the reflected light is output from the reflected output end under the demultiplexing of the first mode demultiplexer 2 a; the mode of the transmitted light is the same as the incident light, and the transmitted light is output from the transmission output end. Similarly, when light is incident from the uploading end, reflected light is output from the transmission output end through the second mode demultiplexer 2b, and a transmitted light signal is output from the reflection output end.
The extending direction of the fabry-perot cavity is along the ordinary optical axis direction of the X-cut thin film lithium niobate material 603, and the connecting line direction of the modulating electrodes 5a, 5b is along the extraordinary optical axis direction of the X-cut thin film lithium niobate material 603. Due to the electro-optical effect of lithium niobate, when a voltage U is applied to the modulating electrodes 5a, 5b, the extraordinary optical axial refractive index of the X-cut thin film lithium niobate material 603 is changed by an electric field, and the change amount Δn is:
wherein n is the original refractive index of lithium niobate in a very optical axis direction, r 33 Is the electro-optic coefficient corresponding to the transmission of light along the ordinary axis of the crystal and the external electric field along the extraordinary axis of the crystal in lithium niobate, and d is the distance between the two modulating electrodes 5a, 5b. TE in thin film lithium niobate ridge waveguide due to change of refractive index of X-cut thin film lithium niobate material 603 0 、TE 1 Mode-dependent equivalent refractive index delta n eff0，1 Also change:
Δn eff0，1 ＝Γ 0，1 Δn
wherein Γ is 0，1 Is TE (TE) 0 Or TE (TE) 1 The coefficient of equivalent refractive index corresponding to the mode along with the change of the refractive index of the electro-optic material is determined by the mode field distribution of the optical field and the electric field.
The change in mode equivalent refractive index brings about a change in the round-trip optical path length in the fabry-perot cavity, with a consequent change in resonant wavelength. At a specific wavelength around the resonance wavelength, the optical power changes, and electro-optical intensity modulation is realized. At wavelengths far from resonance, the optical power does not change substantially.
When the fabry-perot Luo Dianguang modulator cells are cascaded, the nth operating wavelength λ n Corresponds to the resonance wavelength lambda of the Fabry-Perot Luo Dianguang modulator unit at the nth stage res,n Nearby, and can pass through other channels almost without loss. Applying a modulation voltage to an nth stage fabry-perot Luo Dianguang modulator cell, an nth operating wavelength lambda n The optical power of the (c) is changed to form intensity modulation, and the power is not affected due to the fact that other wavelengths are far away from the resonance wavelength of the n-th-stage Fabry-Perot cavity.
Due to process deviation in the processing process and temperature change in the practical application scene, the resonance wavelength of the fabry-perot cavity may come in and go out from the design value. The minor changes in resonant wavelength brought about by the latter can be dynamically adjusted by means of direct current bias, thermo-optic tuning, etc. And the deviation value caused by the former can be adjusted by introducing the upper cladding modification layer 604. Additional phase shift introduced through the upper cladding modification layer 604 is requiredThe size of (2) should satisfy:
wherein FSR is the free spectral range of a single Fabry-Perot cavity, and Deltalambda is the deviation of resonant wavelength to be compensated. Additional phase shift of light round trip in fabry-perot cavityThe method comprises the following steps:
wherein Deltan' eff0 ，Δn' eff1 Respectively upper coating modifying layers604 resulting in TE in waveguide 0 、TE 1 The change in the mode's equivalent refractive index, L, is the length of the overcladding modification layer 604, λ res Corresponding to the resonant wavelength. Thus, additional phase shifts of a particular magnitude can be introduced by appropriate selection of the material, thickness, width, length of the over-coating modification layer 604.
Specific embodiments of the invention are as follows:
the following example is based on a ridge waveguide structure of a thin film lithium niobate platform on an insulator, the cross section of the ridge waveguide structure is shown in fig. 6, the core layer material of the ridge waveguide structure is an X-cut thin film lithium niobate material 603, the total thickness of the ridge waveguide structure is 400nm, the height of the ridge waveguide structure is 200nm, the height of the slab is 200nm, and the etching of lithium niobate usually generates an inclination angle of 30 degrees relative to the vertical direction; the material of the lower cladding 602 is silicon dioxide, which has a thickness of 3 μm, and the upper cladding is not selected. Refractive index n of X-cut thin film lithium niobate material 603 near 1550nm wavelength o ＝2.211，n e = 2.138, silica refractive index is 1.444.
Example 1
An electro-optical modulator array for multi-wavelength channel emitter is composed of two 2X 2 Fabry-Perot Luo Dianguang modulator units, the transmission output end of 1 st stage modulator unit is connected to the input end of 2 nd stage modulator unit, and the two wavelengths are lambda 1 、λ 2 The signal light of the (2) is input from the uploading end of the 1 st and 2 nd modulator units respectively. Two microwave modulation signals are respectively applied to the two modulation units through modulation electrodes, and the modulated two paths of optical signals are combined and output from the transmission output end of the 2 nd-stage modulator unit.
The modulating electrodes 5a and 5b are titanium-gold electrodes with thicknesses of 10nm and 150nm, respectively.
The fabry-perot cavity embodiment in a 2 x 2 fabry-perot Luo Dianguang modulator cell is shown in fig. 3. The first/second anti-symmetric multimode waveguide gratings 3a, 3b are laterally amplitude apodized with gaussian lines and the grating teeth are arranged completely anti-symmetrically. The two antisymmetric multimode waveguide gratings 3a and 3b in the same stage are identical, and the grating periods of the 1 st stage and the 2 nd stage are different.
In this example, a set of parameters for a typical fabry-perot cavity in the present invention is given: the intermediate waveguide 4 has a length of 5 μm and a width of 2 μm; the spacing d=4.5 μm between the two modulating electrodes 5a, 5b; the base width of the anti-symmetric multimode waveguide gratings 3a and 3b is 2 mu m, the tooth depth adopts Gaussian line type with Gaussian coefficient of 8 to carry out transverse amplitude apodization, the maximum width of the grating teeth is 0.55 mu m, the duty ratio of the grating teeth is 0.5, the number of grating periods is 180, and the grating periods of the anti-symmetric multimode waveguide gratings 3a and 3b in the 1 st level and 2 nd level modulation units are 442nm and 435nm respectively. The actual length of the Fabry-Perot cavity is about 160 mu m, and the equivalent cavity length is not more than 85 mu m. Therefore, the modulating electrodes 5a, 5b only need 85 μm in length, and the corresponding capacitance is only about 10fF, which can greatly reduce the energy consumption.
The variation of the reflected output spectrum with modulation voltage in the level 1 modulator cell is shown in fig. 7. The modulation efficiency of the resonant wavelength shift with static electricity is about 11pm/V. Can be according to Q=lambda res /Δλ FWHM The quality factor Q of the Fabry-Perot cavity is estimated to be about 2800, wherein lambda res Is the resonant wavelength, deltalambda FWHM Is the full width at half maximum of the resonance peak. Theoretical 3dB modulation bandwidth f limited by intracavity photon lifetime 3dB ：
f 3dB ＝f 0 /Q
The 3dB modulation bandwidth is about 69GHz, where f 0 Is the optical frequency in vacuum.
The spectrum of the two-stage modulator cell cascade is shown in fig. 8, where the two curves are the spectrum of the transmission output port of the modulator cell 102 when input from the upload ports of the modulator cell 101 and the cell device 102, respectively. The loss of the output spectrum is less than 0.5dB, and the resonance wavelengths of the two modulator units are about 1570nm and 1550nm respectively. As shown in fig. 8, when an optical signal having a wavelength around 1570nm is incident from the upstream end of the modulator unit 101, the modulated reflected optical signal may be re-input from the transmission output waveguide to the modulator unit 102, and the modulator unit 102 may have a reflectivity of less than-40 dB at 1570nm, and the optical signal may be transmitted output almost without loss.
The embodiment shows that the 2×2 fabry-perot Luo Dianguang modulator unit of the present invention has the advantages of compact structure, large bandwidth, low energy consumption, low loss, simple process, etc.; based on the modulator unit and the high-speed transmitter cascaded in the mode of the embodiment, the characteristics of wavelength selectivity, high side mode rejection ratio, large free spectrum range and the like of the antisymmetric multimode waveguide grating can be directly utilized, the modulator unit is not limited by the free spectrum range of the Fabry-Perot cavity any more, the expandability of the number of wavelength channels is strong, and the total communication capacity of the system is greatly improved.
Example 2
An electro-optical modulator array for multi-wavelength channel emitter is composed of eight 2×2 Fabry-Perot Luo Dianguang modulator units, the reflection output end of n-1 stage modulator unit is connected to the input end of n-1 stage modulator unit, and eight wavelengths are lambda 1 ，λ 2 ，…，λ 8 Together with the light from the input of the level 1 modulator unit 101. Microwave modulation signals 1 to 8 are applied to each modulator cell via modulation electrodes 5a and 5b, respectively, and eight modulated optical signals are output from the reflection output terminal of the 8 th-stage modulator cell 108.
The specific forms of the modulating electrodes 5a, 5b and the fabry-perot cavity are the same as those of embodiment 1, and will not be described again.
The anti-symmetric multimode waveguide gratings 3a, 3b used by the eight modulator units are identical, specifically: the grating base width is 2.6 mu m, the tooth depth adopts Gaussian line type with Gaussian coefficient of 6 to carry out transverse amplitude apodization, the maximum width of grating teeth is 1.6 mu m, the duty ratio of grating teeth is 0.5, the number of grating periods is 110, and the grating period is 430nm; the spacing d=5.4 μm between the two modulating electrodes 5a, 5b; the intermediate waveguide 4 has a width of 2.6 μm and the intermediate waveguides 4 of each stage of modulator cells have a length of 4.1 μm,4.147 μm,4.194 μm, …,4.382 μm,4.429 μm in sequence. When the length of the intermediate waveguide 4 is gradually increased by 47nm step length as above, the resonant wavelength interval of the adjacent two-stage modulator units is about 1.6nm, namely, the frequency interval is 100GHz, and the total output spectrum of the modulator array after cascading is shown in fig. 9. The loss of the total output spectrum is lower than-1.8 dB, the quality factor of each resonant cavity is about 3200, and the extinction ratio is greater than 18dB. Each wavelength is modulated by the microwave signals of the corresponding channels, and the channels can be mutually not interfered by reasonably designing the quality factors and the control line widths of the Fabry-Perot cavities.
Therefore, the wavelength division multiplexing with smaller channel interval can be realized based on the modulator array cascaded by the mode described in the embodiment based on the 2×2 fabry-perot Luo Dianguang modulator unit, and the system has the advantages of flexible and various modes for calibrating resonance wavelength, strong expansibility and greatly improved total communication capacity of the system.
The foregoing is merely illustrative of the present invention, and the present invention is not limited thereto, and any person skilled in the art will readily recognize that variations or substitutions are within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (9)
1. An FP cavity electro-optic modulator array for use in a multi-wavelength channel transmitter, comprising:
the modulator array is mainly formed by sequentially connecting N modulator units (101, …,10N, …, 10N); for an nth modulator unit of the N modulator units, the nth modulator unit comprising a 2 x 2 fabry-perot cavity structure, a substrate and a pair of modulation electrodes (5 a, 5 b); the 2X 2 Fabry-Perot cavity structure and the modulating electrodes (5 a, 5 b) are arranged on the upper surface of the substrate, and the two modulating electrodes (5 a, 5 b) are symmetrically distributed on two sides of the 2X 2 Fabry-Perot cavity structure.
2. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 1 wherein: the 2X 2 Fabry-Perot cavity structure comprises a first mode demultiplexer (2 a), a second mode demultiplexer (2 b), a first antisymmetric multimode waveguide grating (3 a), a second antisymmetric multimode waveguide grating (3 b) and an intermediate waveguide (4);
the multiplexing end of the first mode demultiplexer (2 a) is connected with the input end of the first anti-symmetric multimode waveguide grating (3 a), the output end of the first anti-symmetric multimode waveguide grating (3 a) is connected with the input end of the second anti-symmetric multimode waveguide grating (3 b) through an intermediate waveguide (4), and the output end of the second anti-symmetric multimode waveguide grating (3 b) is connected with the multiplexing end of the first mode demultiplexer (2 b); the mode port I and the mode port II of the first mode demultiplexer (2 a) are respectively used as an input end and a reflection output end of the modulator unit (10 n), and the mode port I and the mode port II of the second mode demultiplexer (2 b) are respectively used as a transmission output end and an uploading end of the modulator unit (10 n); the first anti-symmetric multimode waveguide grating (3 a), the second anti-symmetric multimode waveguide grating (3 b) and the intermediate waveguide (4) are all located between the two modulation electrodes (5 a, 5 b).
3. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: the N modulator units are sequentially connected in a transverse cascade or longitudinal cascade mode;
when N modulator units are transversely cascaded, the N modulator units are transversely cascaded into a row, and the transmission output end of the N-1 level modulator unit (10 (N-1)) and the input end of the N level modulator unit (10N) are connected through a connecting waveguide;
when N modulator cells are cascaded in a longitudinal direction, the N modulator cells are cascaded in a longitudinal direction, and the reflective output end of the N-1 th stage modulator cell (10 (N-1)) and the input end of the N-th stage modulator cell (10N) are connected by a connecting waveguide.
4. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: in the nth modulator unit, the first anti-symmetric multimode waveguide grating (3 a) and the second anti-symmetric multimode waveguide grating (3 b) perform TE of signals 0 Mode reverse coupling to TE 1 TE of modes or signals 1 Mode reverse coupling to TE 0 A pattern, both patterns satisfying the following phase matching conditions:
(n eff0 +n eff1 )Λ＝λ bragg
wherein n is eff0 、n eff1 TE respectively 0 、TE 1 Mode equivalent refractive index, Λ is antisymmetricGrating tooth period lambda of multimode waveguide grating bragg Is the center wavelength of the reflection band of the anti-symmetric multimode waveguide grating.
5. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: the first anti-symmetric multimode waveguide grating (3 a) and the second anti-symmetric multimode waveguide grating (3 b) are provided with tooth-shaped structures along two sides of the waveguide transmission direction, and the tooth-shaped structures on two sides are arranged symmetrically on two sides of the grating teeth and tooth grooves; the width of each grating tooth varies as a specific function along the waveguide propagation direction.
6. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 5 wherein: the specific function is a gaussian, sinusoidal or hamming function.
7. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: the first mode demultiplexer (2 a) and the second mode demultiplexer (2 b) are mainly composed of a through waveguide (201) and a coupling waveguide (202); in the coupling working area, the through waveguide (201) and the coupling waveguide (202) are tapered.
8. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: the substrate is mainly formed by sequentially laminating a substrate (601) and a lower cladding layer (602) from bottom to top, a 2X 2 Fabry-Perot cavity structure and modulation electrodes (5 a and 5 b) are arranged on the upper surface of the lower cladding layer (602), and an upper cladding modification layer (604) is arranged on the upper surface of an intermediate waveguide (4) in the 2X 2 Fabry-Perot cavity structure.
9. An FP cavity electro-optic modulator array for a multi-wavelength channel emitter as claimed in claim 2 wherein: the first mode demultiplexer (2 a), the second mode demultiplexer (2 b), the first antisymmetric multimode waveguide grating (3 a), the second antisymmetric multimode waveguide grating (3 b) and the intermediate waveguide (4) are all formed by etching an X-cut film lithium niobate material (603).
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