CN107037532B - Preparation method of long period waveguide grating - Google Patents

Abstract

The invention discloses a preparation method of a long-period waveguide grating. The waveguide grating structure comprises a lithium niobate substrate (5), a cladding plate waveguide (4), a buffer layer (2) and a coplanar waveguide electrode structure; the preparation method includes : Step 1, sample preparation; Step 2, carry out proton exchange for the first time; Step 3, make waveguide mask; Step 4, carry out second proton exchange; Step 5, carry out annealing treatment; Step 6, carry out end face polishing; 7. Perform periodic polarization treatment. Through the above steps, a core layer waveguide and a cladding slab waveguide with a periodic domain structure are prepared, thereby obtaining a long-period waveguide grating structure. The long-period waveguide grating manufactured by the preparation method of the invention has the advantages of low modulation voltage, high modulation bandwidth, easy modulation and the like.

Description

Preparation method of long period waveguide grating

technical field

The invention relates to the field of integrated optics, in particular to a preparation method of a long-period waveguide grating based on periodic domain structure lithium niobate.

Background technique

In recent decades, with the rapid growth of multimedia service demands, traditional electrical transmission networks can no longer meet people's needs, and there is an urgent need for ultra-long-distance and high-capacity optical fiber communication systems. Because optical fiber has the advantages of large transmission capacity, small transmission loss, and light weight, optical fiber communication has become the mainstream method in wired communication networks. The rapid development of large-capacity optical fiber communication networks has strongly promoted the demand and research of broadband, high-speed and ultra-high-speed optical devices.

The key to restricting the development of optical networks lies in the development of advanced optical devices, and wavelength selection and switching devices play a very critical role in optical networks, especially the grating structure whose refractive index changes periodically, because its optical transmission spectrum has The nature of the change with the wavelength of light, so it has very important value in the fields of optical fiber communication and sensing.

In 1996, A.M.Vengsarkar and others from Bell Laboratories in the United States fabricated the first long-period fiber grating (LPFG) in a hydrogen-loaded silicon-germanium fiber by using ultraviolet light. In 1998, D.D. Davis and others used laser pulses to write long-period fiber gratings in ordinary optical fibers, marking the development of long-period gratings to a new stage. The birth and development of fiber gratings have brought milestone changes to the fields of fiber optic communication and fiber optic sensing. Making long period gratings with optical fibers is not only simple in process, but also has the advantages of low loss, low reflection, integration and convenient connection. Therefore, devices based on long-period fiber gratings have developed rapidly, but long-period fiber gratings are still greatly limited by fiber materials and structures, and their functions are relatively limited. It is difficult to tune the transmission characteristics of gratings in real time. This tuning can It is the tuning of the resonant wavelength of the stop band of the grating, and it can also be the tuning of the intensity of the grating. Obviously, realizing these two kinds of tuning can greatly expand the functions and application fields of gratings. Considering the flexibility of the current optical waveguide manufacturing process, the diversity and multi-functionality of waveguide materials, the use of optical waveguides instead of optical fibers to manufacture long-period gratings can not only break through the above limitations, but also help achieve tunability of gratings and expand long-period gratings. The function and application fields of the grating are of great significance.

Planar optical waveguide gratings are generally realized by fabricating a periodic structure on the waveguide. However, this fixed periodic structure is not conducive to the tuning of the grating intensity or resonance wavelength. For this reason, there is also a tunable long-period waveguide grating (LPWG) by placing periodic electrodes on the waveguide and periodically modulating the refractive index of the waveguide by using thermo-optic or electro-optic effects. In 2008, Zheng Jiancheng and others proposed for the first time to place interdigitated electrodes on the z-cut y-transmission lithium niobate waveguide to realize the long-period waveguide grating (LPWG). The schematic diagram of the structure is shown in Figure 1. , so that the electric field is periodically distributed inside the lithium niobate waveguide. Due to the electro-optic effect, the refractive index is periodically distributed in the direction of the waveguide, thus forming a long-period waveguide grating, which can be controlled flexibly by electrifying or not. However, the long-period waveguide grating based on the interdigitated electrode structure has problems such as low modulation bandwidth and difficult modulation, which limits the further application of the long-period waveguide grating in the fields of optical filters, sensors, and optical modulators.

Contents of the invention

In order to overcome the defects existing in the prior art, the present invention proposes a preparation method of a long-period waveguide grating, which uses coplanar waveguide electrodes to control the long-period waveguide grating, so as to solve the modulation of the existing interdigitated electrode structure long-period waveguide grating The bandwidth is not high, the modulation voltage is large, and it is difficult to modulate, so as to improve the performance of the long-period waveguide grating.

In the preparation method of the long-period waveguide grating of the present invention, the waveguide grating structure includes a lithium niobate substrate 5, a clad slab waveguide 4, a buffer layer 2, and a coplanar waveguide electrode structure from bottom to top; wherein, the buffer layer 2 covers On the upper surface of the clad slab waveguide 4, the lithium niobate substrate 5 covers the lower surface of the clad slab waveguide 4; the coplanar waveguide electrode structure further includes a signal electrode 6, ground electrodes 7, 8 The signal electrode 6 covers the middle part of the upper surface of the buffer layer 2; the core layer waveguide 3 is arranged in the middle part of the upper surface of the clad slab waveguide 4 and directly below the signal electrode 6; the ground electrode 7 , 8 are located on both sides of the signal electrode 6, and are located on the same plane as the signal electrode 6; the preparation method includes the following steps:

Step 1. Sample preparation. This step specifically includes: selecting an optical-grade Z-cut 1mm thick lithium niobate wafer as the initial material, and using a precision cutting machine to cut the wafer into a waveguide sample;

Step 2. Carry out the first proton exchange. This step specifically includes: put the exchange source benzoic acid into the sealed quartz glass tube built in the isothermal zone in the constant temperature diffusion furnace, heat to the exchange temperature and keep the temperature constant for two hours, and then put the cleaned The sample is placed in the exchange source for proton exchange; the exchange temperature is between the melting point of benzoic acid of 122°C and the boiling point of 249°C;

Step 3. Make a waveguide mask. This step specifically includes: coating a layer of metal mask on the surface of lithium niobate; using photolithography and etching processes to make the waveguide part without a mask, and the other parts still have a mask, and clean the waveguide sample ;

Step 4, carry out the second proton exchange, the specific operation is the same as step 2;

Step 5. Perform annealing treatment. This step specifically includes: cleaning the waveguide sample after the second proton exchange in step 4, putting it into the quartz fixture and pushing it into the quartz tube. After the quartz fixture is pushed into the predetermined position, the oxygen Carry out high-temperature heating in the atmosphere, the heating temperature is 200°C to 400°C, last for 3-6 hours, and then slowly pull out the fixture;

Step 6. Perform end face polishing, which specifically includes: successively polishing with polishing powder with a particle size of 20 μm, 7 μm, and 1.5 μm, and then polishing the end face of the waveguide with a polishing solution;

Step 7. Perform periodic polarization treatment, which specifically includes: using a high-voltage electric meter and an oscilloscope to connect to the circuit during the polarization process; using photolithography technology to make a periodic pattern on the waveguide surface of the waveguide sample; The period of the electrode mask plate is Λ; put the waveguide sample into the liquid electrode fixture and seal it with a silicone gasket; after that, inject a saturated LiCl solution into the liquid electrode fixture, and avoid the appearance of air bubbles during the injection of the solution; Connect the waveguide sample to the high-voltage square wave pulse generation system; set the polarization time to 0.85s;

Through the above steps, the core layer waveguide 3 and the cladding slab waveguide 4 with periodic domain structure are prepared, so as to obtain a long-period waveguide grating structure.

The output voltage of the high voltage DC power supply involved in the high voltage electric meter and the high voltage square wave pulse is 4.0kV.

Compared with the existing long-period waveguide grating based on the interdigitated electrode structure, the long-period waveguide grating based on the periodic domain structure lithium niobate produced by the present invention has obvious advantages such as low modulation voltage, high modulation bandwidth and easy modulation.

Description of drawings

FIG. 1 is a schematic diagram of the structure of a lithium niobate long-period waveguide grating with interdigitated electrodes in the prior art;

Fig. 2 is a kind of long-period waveguide grating structure schematic diagram that the present invention proposes;

3 is a schematic structural diagram of the optical modulator proposed by the present invention;

Reference signs: 1. interdigitated electrode, 2. buffer layer, 3. core layer, 4. cladding slab waveguide, 5. lithium niobate substrate, 6. signal electrode, 7, 8. ground electrode, 9. input optical fiber , 10. Long-period waveguide grating based on periodic domain structure lithium niobate, 11. Output optical fiber.

Detailed ways

The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 2, it is a schematic structural diagram of a long-period waveguide grating proposed by the present invention, including a signal electrode 6, ground electrodes 7, 8, a buffer layer 2, a core layer 3, a cladding slab waveguide 4 and a lithium niobate substrate 5. The upper surface of the clad slab waveguide 4 is covered with a buffer layer 2, and the lower surface is covered with a lithium niobate substrate 5; the middle part of the upper surface of the buffer layer 2 covers the signal electrode 6; the core layer 3 is arranged on the middle part of the upper surface of the clad slab waveguide 4 and is located at the signal electrode 6; the ground electrodes 7 and 8 are located on both sides of the signal electrode 6, and are located on the same plane as the signal electrode 6; through the coplanar waveguide electrode structure, a voltage is applied on the periodic domain structure lithium niobate substrate to form different direction domains. influence, the electric field is periodically distributed inside the lithium niobate waveguide; by adjusting the voltage applied to the traveling wave electrode, the distribution of the refractive index of the crystal in the direction of the waveguide can be changed, and the core layer 3 is optically coupled into the cladding slab waveguide 4 in.

All electrodes can be metal materials with good conductivity such as gold; wherein, the width W of the signal electrode 6 is 5-20 μm, the length L is 10-30 mm, and the thickness H is 5-30 μm; the width of the ground electrodes 7 and 8 is 800-20 μm. 2500 μm, the length L is 10-30 mm, which is the same as the length of the signal electrode 6 , and the thickness H is 5-30 μm, which is the same as the thickness of the signal electrode 6 .

The distance S between the signal electrode and the ground electrode is 10-30 μm; the distance S between the signal electrode 6 and the ground electrodes 7 and 8 is 10-30 μm.

The buffer layer 2 is made of silicon dioxide material.

The core layer 3 adopts a proton exchange strip waveguide with a width T of 5-10 μm.

The cladding slab waveguide 4 adopts proton exchange periodically poled lithium niobate slab waveguide, and the period Λ of the periodic domain structure is 40-100 μm. A voltage is applied on the lithium niobate crystal with periodic domain structure through the coplanar waveguide electrode structure, so that there is an electric field inside the lithium niobate waveguide, and the electric field has different effects on domains in different directions, so that the refractive index in the direction of the waveguide presents a periodic distribution.

A long-period waveguide grating based on periodic domain structure lithium niobate of the present invention, the specific working principle is as follows:

Assume that the effective electric field intensity around the strip waveguide in the core layer is E _z , under the action of the electric field, the refractive index of the lithium niobate substrate along the Z direction changes. For the lithium niobate domain structure in which the domain direction is the same as the electric field direction in the cladding slab waveguide, the refractive index change is Δn _z+ , the formula is as follows:

For the lithium niobate domain structure in which the domain direction is opposite to the direction of the electric field in the cladding slab waveguide, the refractive index change is Δn _z- , and the formula is as follows:

From the formulas (a) and (b), the relative change in the refractive index of the forward and reverse domains of the lithium niobate crystal can be obtained

Δn _z =Δn _z+ -Δn _z- =n _e γ ₃₃ E _z (3)

Among them, n _e is the extraordinary light refractive index of lithium niobate crystal, γ ₃₃ is the electro-optic coefficient of lithium niobate crystal, E _z is the effective electric field strength around the strip waveguide in the core layer.

It can be known from formula (3) that Δn _z varies with the external effective modulation electric field based on the periodic domain structure lithium niobate long-period waveguide grating And for a given material and electrode structure, _Δnz is completely controlled by the effective external effective electric field. By adjusting the voltage applied to the coplanar waveguide electrode structure, the effective electric field around the core strip waveguide can be changed to realize Tuning of the long period waveguide grating. Similarly, the effective modulation electric field of the long-period waveguide grating with the existing interdigitated electrode structure If you want to obtain the same magnitude of refractive index modulation, the required effective modulation electric field should satisfy E _z =E _z ', the modulation electric field is generated by the external modulation voltage, the signal electrode in the periodic domain structure lithium niobate long period waveguide grating based on the present invention It is very close to the strip waveguide, the effective electric field generated by the modulation voltage is very large, and the distance between each group of interdigital electrodes in the long-period waveguide grating with the existing interdigital electrode structure is relatively large, if you want to obtain the same size effective To modulate the electric field, a larger modulation voltage needs to be applied. Therefore, the lithium niobate long-period waveguide grating based on the periodic domain structure of the present invention has the advantage of low modulation voltage, and the lower modulation voltage is conducive to saving power consumption and promoting the practical application of the long-period waveguide grating.

The coplanar waveguide electrode structure adopted by the lithium niobate long-period waveguide grating based on the periodic domain structure is a traveling wave electrode structure; while the existing long-period waveguide grating with an interdigitated electrode structure adopts a lumped parameter electrode. Electro-optic modulation devices fabricated with traveling-wave electrodes as transmission lines have much larger modulation bandwidths than modulators fabricated with lumped parameter electrodes. According to theoretical calculations, the product of the modulation bandwidth and the electrode length of the lumped parameter lithium niobate modulation device is about 2.2 GHz cm (refer to non-patent literature 1), and existing experiments have verified that the traveling wave electrode lithium niobate modulation device has a performance greater than The product of modulation bandwidth and modulation electrode length of 200 GHz·cm (see Non-Patent Document 2). Therefore, the long-period waveguide grating tuned by the traveling wave electrode will have a higher modulation bandwidth.

In the existing long-period waveguide grating with interdigitated electrode structure, the electric field generated by the interdigitated electrodes is very uneven (refer to non-patent document 3) and there is strong interference between the electrodes. If you want to achieve more precise regulation, A fairly complex calculation of the electric field distribution has to be performed. However, in the long-period waveguide grating based on the periodic domain structure, the coplanar waveguide electrode has a symmetrical structure, which can generate a relatively uniform electric field around the strip waveguide in the core layer, so that the waveguide grating has good periodicity, and it is easy to realize the long-period waveguide grating. more precise control.

A method for preparing a long-period waveguide grating of the present invention, the specific steps are as follows:

Step 1, sample preparation, specifically: What the present invention selects is that the optical grade Z-cut 1mm thick lithium niobate wafer is used as the initial material, and the wafer is cut into samples of L (length) × 10 (width) (mm) using a precision cutting machine ;

Step 2. Perform the first proton exchange. This step specifically includes: put the exchange source into the sealed quartz glass tube built in the isothermal zone of the constant temperature diffusion furnace, heat it to the exchange temperature and keep the temperature constant for about two hours, and then put the cleaned sample Put it into the exchange source for proton exchange. The exchange source is generally benzoic acid. In order to reduce surface corrosion during the exchange process, a certain amount of lithium benzoate or sodium benzoate can be added to the exchange source. Given the volatility of benzoic acid, exchange devices are generally sealed. Benzoic acid has a melting point of 122°C and a boiling point of 249°C, so the exchange temperature should be between these two temperatures;

Step 3, making a waveguide mask, this step specifically includes: coating a layer of metal film (such as an aluminum film) on the surface of lithium niobate; photolithography and etching processes make the waveguide part without a mask, and other parts still have a mask, and the Sample cleaning;

Step 4, carry out the second proton exchange, the specific operation is the same as step 2;

Step 5, annealing, this step specifically includes: cleaning the sample after the proton exchange in step 4, putting it into a quartz fixture and pushing it into a quartz tube, and heating at a high temperature (200-400° C.) in an oxygen atmosphere; If the temperature rises too fast and it breaks, the speed of pushing it into the furnace should not be too fast. After pushing it into the predetermined position, it will last for a certain period of time (3-6h) in an aerobic atmosphere, and then slowly pull out the fixture.

Step 6. Polishing the end face of the optical waveguide. This step specifically includes: successively polishing with polishing powders with a particle size of 20 μm, 7 μm, and 1.5 μm, and then polishing the end face of the optical waveguide with a polishing solution;

Step 7. Perform periodic polarization treatment. This step specifically includes: using a high-voltage electric meter and an oscilloscope to connect to the circuit during the polarization process; using photolithography technology to make a periodic pattern on the waveguide surface of the sample; the electrodes selected during the photolithography process The period of the mask plate is Λ; the waveguide sample is put into the liquid electrode fixture and sealed with a silica gel gasket; after that, inject a saturated LiCl solution into the liquid electrode fixture, and the appearance of air bubbles should be avoided as much as possible during the injection of the solution; Enter the high-voltage square wave pulse generation system; set the polarization time to 0.85s, and the output voltage of the high-voltage DC power supply to 4.0kV;

Through the above steps, the proton exchange cladding slab waveguide and core proton exchange strip waveguide with periodic domain structure are prepared.

In addition, the present invention also proposes an optical modulator based on a periodic domain structure lithium niobate long-period waveguide grating, which realizes the intensity modulation of an optical signal by a single waveguide, and has a simple structure and is easy to manufacture.

As shown in Figure 3, it is a schematic diagram of the structure of an optical modulator realized by using a long-period waveguide grating based on a periodic domain structure lithium niobate proposed by the present invention, including an input optical fiber 9 and a long-period waveguide grating based on a periodic domain structure lithium niobate. waveguide grating 10 and output fiber 11. Wherein: the input optical fiber 9 is used to transmit light to the long-period waveguide grating 10; the long-period waveguide grating 10 is used to modulate the input light according to an electrical signal; the output optical fiber 10 is used to output the modulated light.

When an optical signal of a certain power enters the core waveguide through the input fiber, if the wavelength satisfies the resonance condition, that is, λ=Λ(N _o -N _m ), where λ is the resonance wavelength, Λ is the period of the long-period waveguide grating, N _o is the mode effective refractive index of the core waveguide, and N _m is the mode effective refractive index of the cladding slab waveguide; the transmitted optical signal will be coupled into the surrounding medium, and the loss of the output optical signal is the largest, and the output intensity of the optical signal is the smallest at this time ; If the wavelength does not meet the resonance condition, the loss of the output optical signal is small, and the output intensity of the optical signal is large. Therefore, changing the resonance condition can change the intensity of the output optical signal. The resonance condition of the optical signal in the waveguide is related to the effective refractive index of the surrounding medium of the strip waveguide. Applying a voltage on the long-period waveguide grating has different effects on the domains in different directions of lithium niobate, which will also change its effective refractive index. By adjusting the voltage applied to the long-period waveguide grating, the resonance condition of the optical signal transmitted in the waveguide can be changed, thereby realizing the intensity modulation of the input light.

The core layer strip waveguide and the cladding slab waveguide in the present invention can also be made by the titanium diffusion method, using the technical solution described in the present invention, or those skilled in the art are inspired by the technical solution of the present invention to design Similar technical solutions that achieve the above-mentioned technical effects all fall within the protection scope of the present invention.

Non-Patent Document 1:

Alferness R C.Waveguide Electrooptic Modulators[J].IEEE Transactions on Microwave Theory&Techniques,1982,30(8):1121-1137.

Non-Patent Document 2:

Macario J, Yao P, Shi S, et al.Full spectrum millimeter-wave modulation.[J].Optics Express,2012,20(21):23623-23629.

Non-Patent Document 3:

Jiang Qiang, Wang Xuan, Li Zhiyuan. Electro-optic coefficient measurement of electro-optic polymer thin film devices with coplanar intercalated finger electrodes[J]. Acta Optics Sinica, 2013(9):290-294.

Claims (1)

1. A preparation method for long-period waveguide grating, characterized in that, the long-period waveguide grating structure comprises lithium niobate base (5), cladding slab waveguide (4), buffer layer (2) and common Surface waveguide electrode structure; wherein, the buffer layer (2) covers the upper surface of the clad slab waveguide (4), and the lithium niobate substrate (5) covers the clad slab waveguide (4) The lower surface; the coplanar waveguide electrode structure further includes a signal electrode (6), ground electrodes (7), (8); the signal electrode (6) covers the middle part of the upper surface of the buffer layer (2); in the A core layer waveguide (3) is arranged in the middle of the upper surface of the clad slab waveguide 4 and directly below the signal electrode (6); the ground electrodes (7), (8) are located on both sides of the signal electrode (6) side, and on the same plane as the signal electrode (6), the preparation method includes the following steps: Step 1. Sample preparation. This step specifically includes: selecting an optical-grade Z-cut 1mm thick lithium niobate wafer as the initial material, and using a precision cutting machine to cut the wafer into a waveguide sample; Step 2. Carry out the first proton exchange. This step specifically includes: put the exchange source benzoic acid into the sealed quartz glass tube built in the isothermal zone in the constant temperature diffusion furnace, heat to the exchange temperature and keep the temperature constant for two hours, and then put the cleaned The sample is placed in the exchange source for proton exchange; the exchange temperature is between the melting point of benzoic acid of 122°C and the boiling point of 249°C; Step 3. Make a waveguide mask. This step specifically includes: coating a layer of metal mask on the surface of lithium niobate; using photolithography and etching processes to make the waveguide part without a mask, and the other parts still have a mask, and clean the waveguide sample ; Step 4, carry out the second proton exchange, the specific operation is the same as step 2; Step 5. Perform annealing treatment. This step specifically includes: cleaning the waveguide sample after the second proton exchange in step 4, putting it into the quartz fixture and pushing it into the quartz tube. After the quartz fixture is pushed into the predetermined position, the oxygen Carry out high-temperature heating in the atmosphere, the heating temperature is 200°C to 400°C, last for 3-6 hours, and then slowly pull out the fixture; Step 6. Perform end face polishing, which specifically includes: successively polishing with polishing powder with a particle size of 20 μm, 7 μm, and 1.5 μm, and then polishing the end face of the waveguide with a polishing solution; Step 7. Perform periodic polarization treatment, which specifically includes: using a high-voltage electric meter and an oscilloscope to connect to the circuit during the polarization process; using photolithography technology to make a periodic pattern on the waveguide surface of the waveguide sample; The period of the electrode mask plate is Λ; put the waveguide sample into the liquid electrode fixture, and seal it with a silicone gasket; after that, inject a saturated _LiCl solution into the liquid electrode fixture, and avoid bubbles during the injection process. appears; connect the waveguide sample to the high-voltage square wave pulse generating system; set the polarization time to 0.85s; Through the above steps, a core layer waveguide and a cladding slab waveguide with a periodic domain structure are prepared, thereby obtaining a long-period waveguide grating structure.