CN115061292A

CN115061292A - Tunable optical delay line based on thin-film lithium niobate

Info

Publication number: CN115061292A
Application number: CN202210858438.8A
Authority: CN
Inventors: 蔡鑫伦; 柯伟
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2022-07-20
Filing date: 2022-07-20
Publication date: 2022-09-16

Abstract

The invention relates to the technical field of optical devices, and provides a tunable optical delay line based on thin-film lithium niobate, which comprises a grating coupler arranged on a lithium niobate thin film, a light source and a light source, wherein the grating coupler is used for coupling an input optical signal and coupling and outputting the delayed optical signal; the n-level spiral delay optical waveguide is used for delaying the input optical signal; and the n + 1-stage optical switch is used for controlling the optical signal to enter the delay optical waveguide path or the reference waveguide arm path. The input light loaded with microwave signals enters a tunable optical delay line through the grating coupler, the optical switch controls the optical signals to enter a delay optical waveguide path, and the spiral delay optical waveguide generates corresponding delay on the optical signals and then enters the next-stage optical switch; or the optical switch controls the optical signal to enter the path of the reference waveguide arm and enter the next-stage optical switch; the optical signal is output through the grating coupler after selecting a path through the last stage optical switch.

Description

Tunable optical delay line based on thin-film lithium niobate

Technical Field

The invention relates to the technical field of optical devices, in particular to a tunable optical delay line based on thin-film lithium niobate.

Background

The physical properties of the photon itself determine that it cannot be stored directly in the medium as a charge, and therefore optical delay lines are typically used to briefly confine the optical signal in the optical waveguide. Optical delay lines have a wide range of applications, such as signal synchronization and buffering in optical communications, high-precision filters in optical signal processing, and optical phase shifters in phased array radar.

Most of commercial optical delay lines are realized based on space optics or optical fibers, and the problems of large device size, low tuning speed, low delay precision and the like generally exist. Research on optical delay lines has been reported on a number of integrated optical platforms such as silicon, silicon nitride, silicon dioxide, and polymers. However, these platforms have their own advantages and disadvantages, such as the silicon platform has the advantages of compatibility with CMOS (complementary metal oxide semiconductor) process and small size, but its waveguide propagation loss is large; silicon nitride and silicon dioxide can realize low-loss ultra-long waveguide, but the device size is difficult to further reduce due to the characteristic of low refractive index difference; the polymer material has the advantages of low dielectric constant, high electrooptical coefficient and the like, but the preparation process is difficult, and the waveguide loss is high. At present, a reverse coupler adopting a single spiral structure is also proposed to effectively reduce the on-chip occupied area of a long waveguide delay line, but the waveguide with a bent structure still has the problem of larger waveguide propagation loss. In addition, the loss of the on-chip waveguide based on the lithium niobate thin film has certain advantages compared with other materials, but the small-size bent waveguide is difficult to prepare by the on-chip waveguide due to the small relative refractive index difference, and the problem of high difficulty in the preparation process exists.

In addition, the problem of direct current bias point drift exists on the existing lithium niobate thin film, a direct current bias voltage needs to be loaded to enable the electric modulation optical switch or modulator with the Mach-Zehnder structure to work at an orthogonal point when the electric modulation optical switch or modulator works, the bias voltage can generate a drift phenomenon along with time, and the phenomenon limits the use scenes of the optical switch and the modulator on the lithium niobate thin film.

Disclosure of Invention

The invention provides a tunable optical delay line based on thin-film lithium niobate, aiming at overcoming the defects of waveguide propagation loss and difficulty in preparing small-size bent waveguides in the prior art.

In order to solve the technical problems, the technical scheme of the invention is as follows:

a thin film lithium niobate based tunable optical delay line, comprising:

the grating coupler is arranged on the lithium niobate film and is used for coupling the input optical signal and coupling the delayed optical signal and then outputting the coupled optical signal;

the n-level spiral delay optical waveguide is used for delaying the input optical signal;

the n + 1-level optical switch is used for controlling an optical signal to enter a delay optical waveguide path or a reference waveguide arm path; wherein n is more than or equal to 2.

In the using process, input light loaded with microwave signals enters a tunable optical delay line through the grating coupler, the optical switch controls optical signals to enter a delay optical waveguide path, and the spiral delay optical waveguide generates corresponding delay for the optical signals and then enters the next-stage optical switch; or the optical switch controls the optical signal to enter the path of the reference waveguide arm and enter the next-stage optical switch; the optical signal is output through the grating coupler after selecting a path through the last stage optical switch.

Preferably, the i +1 th stage spiral delay optical waveguide is 2 times of the length of the i stage spiral delay optical waveguide; where i is 1, 2.

Preferably, the spiral delay optical waveguide is a multi-mode wide waveguide, and the spiral delay optical waveguide includes a curved structure having a curvature varying in a sinusoidal curve, a bezier function, or an euler curve.

Preferably, the optical switch comprises an electro-optical switch of 2 × 2 mach-zehnder interference structure.

As a preferred scheme, metal electrodes are respectively arranged between the phase shift arms of the optical switch and at two sides of the phase shift arms, and the metal electrodes are connected with an external drive control system through metal leads.

Preferably, the tunable optical delay line further comprises a directional coupler and an on-chip photodetector; the directional coupler is arranged on a reference arm straight waveguide of any optical switch of the front n stages, and an optical signal split by the directional coupler is guided into the on-chip photoelectric detector after passing through the output grating to monitor the output state of the optical switch.

Preferably, the on-chip photodetector is heterogeneously integrated above the output grating in a flip-chip bonding mode, and the on-chip photodetector is connected with an external drive control system through a metal electrode and a metal lead.

Preferably, the directional coupler is a directional coupler with an adjustable splitting ratio.

Preferably, the lithium niobate thin film is disposed on a substrate.

Preferably, a conductive silica cladding layer doped with zinc ions is deposited on the upper surface of the tunable optical delay line.

Compared with the prior art, the technical scheme of the invention has the beneficial effects that: the invention overcomes the characteristic of bias point drift of the lithium niobate thin film, utilizes the electro-optic effect of the lithium niobate material for tuning, can greatly reduce the switching time between different delays, adopts the spiral delay optical waveguide, reduces the proportion of the bent waveguide in the delay waveguide, and has the advantages of small size, low loss and low power consumption.

Drawings

Fig. 1 is a schematic structural diagram of a tunable optical delay line based on thin-film lithium niobate according to an embodiment of the present invention.

Fig. 2 is a schematic structural diagram of a spiral delay optical waveguide according to an embodiment of the present invention.

Fig. 3 is a schematic structural diagram of an optical switch according to an embodiment of the present invention.

FIG. 4 is a graph showing the output state change of an optical switch after depositing a common silica cladding layer according to an embodiment of the present invention.

Fig. 5 is a graph illustrating the output state change of an optical switch after deposition of a zinc ion doped silica cladding layer in accordance with an embodiment of the present invention.

Fig. 6 is an architecture diagram of an optical system according to an embodiment of the present invention.

The optical fiber coupler comprises a 1-lithium niobate thin film, a 2-grating coupler, a 3-spiral delay optical waveguide, a 4-optical switch, a 401-delay optical waveguide path, a 402-reference waveguide arm path, a 5-metal electrode, a 6-directional coupler and a 7-on-chip photoelectric detector.

Detailed Description

The drawings are for illustrative purposes only and are not to be construed as limiting the patent;

for the purpose of better illustrating the embodiments, certain features 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 technical solution of the present invention is further described below with reference to the accompanying drawings and examples.

Example 1

The present embodiment provides a tunable optical delay line based on thin film lithium niobate, and as shown in fig. 1, the present embodiment is a schematic structural diagram of the tunable optical delay line based on thin film lithium niobate.

The tunable optical delay line based on thin-film lithium niobate provided in this embodiment includes:

and the grating coupler 2 is arranged on the lithium niobate film 1 and is used for coupling the input optical signal and coupling the delayed optical signal and then outputting the optical signal.

And the n-stage spiral delay optical waveguide 3 is used for delaying the input optical signal.

An n + 1-stage optical switch 4 for controlling an optical signal to enter the delay optical waveguide path 401 or the reference waveguide arm path 402; wherein n is more than or equal to 2.

In this embodiment, the input light loaded with the microwave signal enters the lithium niobate optical waveguide through the grating coupler 2. In passing through each stage of the optical switch 4, the optical switch 4 controls the optical signal to enter the delay optical waveguide path 401 or the reference waveguide arm path 402. When the optical switch 4 controls the optical signal to enter the delay optical waveguide path 401, the spiral delay optical waveguide 3 generates a corresponding delay for the input optical signal, and then enters the next stage optical switch 4. When the optical switch 4 controls the optical signal to enter the reference waveguide arm path 402, the reference waveguide arm path 402 transmits the input optical signal to the next-stage optical switch 4. When the optical signal is transmitted to the last stage (n +1 th stage) optical switch 4, the optical switch 4 controls the optical signal to enter a target path for output, and the optical signal after a specific delay difference is obtained.

In an alternative embodiment, the lithium niobate thin film 1 is disposed on a substrate.

In this embodiment, the lithium niobate thin film 1 is an ideal material platform for realizing an on-chip adjustable optical delay line, and has the characteristics of a high electro-optic coefficient, low waveguide loss, and the like.

In an alternative embodiment, the (i + 1) th stage spiral delay optical waveguide 3 is 2 times the length of the (i) th stage spiral delay optical waveguide 3. Where i is 1, 2.

In this embodiment, the length of the current-stage spiral delay optical waveguide 3 is 2 times that of the previous-stage spiral delay optical waveguide 3, the spiral delay optical waveguides 3 are arranged in a winding manner, and the waveguide at the innermost ring needs to use the minimum-bending bent waveguide, so that the occupation ratio of the bent waveguide in the delay waveguide is reduced.

Further, the spiral delay optical waveguide 3 in this embodiment is a multi-mode broad waveguide, and the curved structure in the spiral delay optical waveguide 3 adopts a curved structure with curvature variation, such as a sinusoidal curve, a bezier function, or an euler curve.

Fig. 2 is a schematic structural diagram of the spiral delay optical waveguide 3 of this embodiment.

Losses in the optical waveguide are mainly caused by roughness of the side walls of the waveguide, and a mode optical field in the waveguide overlaps the side walls and is scattered by non-smooth particles on the side walls to generate losses, which are more obvious in a curved waveguide part because an optical mode field in the curved waveguide part is shifted to the side walls, and the scattering effect is more obvious. Increasing the waveguide width can improve this phenomenon to some extent, reducing the bending loss, and thus enabling the use of a smaller bend radius curved waveguide. However, increasing the waveguide width causes another problem that the waveguide no longer satisfies the single mode condition, and the TE0 mode light may be converted into other high-order mode light after being bent, thereby causing mode crosstalk.

In this regard, the present embodiment employs a multi-mode-width waveguide as the spiral delay optical waveguide 3, reduces the loss of the curved waveguide by increasing the waveguide width, and suppresses excitation of a higher-order mode by designing the curved waveguide with a curve having a curvature that varies.

Since the curvature in the euler curve is linearly varied, the curvature variation rate thereof can be more conveniently designed, and the high-order mode excitation can be effectively suppressed, so that a curved waveguide with a smaller bending radius can be used, the present embodiment preferably uses the euler curve structure.

In a specific implementation process, the spiral delay optical waveguide 3 is specifically designed in such a way that the longest theoretical waveguide length can be obtained by determining the waveguide pitch and dividing the area by the width, and the length problem is directly converted into the area problem.

For example, if the sum of the waveguide width and the waveguide pitch is 12.5 μm in a write field area of 500 μm by 500 μm, the longest waveguide length that can be obtained by a single write field is theoretically 2 cm. Because the waveguides need to be connected by bending, the area of the whole writing field cannot be occupied, and according to the design calculation of the spiral delay optical waveguide 3 of the embodiment, the waveguide which can actually reach 1.56cm in the same writing field of 500 μm by 500 μm can be obtained, so that the problem of larger device size caused by larger radius of the bent waveguide is effectively solved.

The limitation of the area to 500 μm is to consider that in the electron beam Exposure (EBL), there is a splicing error between the write fields of 10nm order, which also increases the waveguide loss, so that the waveguide length is designed based on the area of a single write field in order to reduce the number of times of splicing the write fields. And when the writing field area is increased, the length-area ratio of the delay waveguide can be further increased.

It should be noted that the design of the same structure should be protected by this patent as the available write field area is further increased.

In one embodiment, a tunable optical delay line structure as shown in fig. 1 is used, and a spiral delay optical waveguide 3 with n + 4 stages and an optical switch 4 with n + 1-5 stages are provided.

The input light loaded with microwave signals enters the tunable optical delay line shown in fig. 1 through the grating coupler 2, and the input optical signals are selectively output from one of two output ports of the optical switch 4 after passing through the optical switch 4, and enter the spiral delay optical waveguide 3 or the reference arm straight waveguide. Then the optical signal enters the next stage optical switch 4, and the above process is repeated.

According to the different selection paths, 16 different delay differences can be generated in the embodiment, and the delay step is the delay difference generated by the first-stage delay waveguide.

The optical signal after the specific time delay is output through the grating coupler 2 at the output end and then converted into a microwave signal carrying the specific time delay by the photoelectric detector.

Alternatively, the present embodiment can achieve more and longer delay ranges by cascading more stages of optical switches 4 and spiral delay waveguides.

In the embodiment, the electro-optic effect of the lithium niobate material is used for tuning, so that the switching time between different delays can be greatly reduced, and meanwhile, the helical delay optical waveguide 3 is adopted, so that the proportion of the curved waveguide in the delay waveguide is reduced, and the advantages of small size, low loss and low power consumption can be achieved.

Example 2

This example is a further improvement over the tunable optical delay line based on thin film lithium niobate presented in example 1.

Further, the optical switch 4 in the present embodiment includes an electro-optical switch 4 of a 2 × 2 mach-zehnder interference structure.

Compared with the thermo-optic switch 4, the electro-optic switch 4 has faster switching speed and lower power consumption, does not need to additionally consider the heat dissipation of a chip, and is more beneficial to realizing the integration of a plurality of single-chip devices.

In an alternative embodiment, metal electrodes 5 are respectively arranged between the phase shift arms of the optical switch 4 and on two sides of the phase shift arms, and the metal electrodes 5 are connected with an external drive control system through metal leads.

Fig. 3 is a schematic structural diagram of the optical switch 4 of the present embodiment.

In this embodiment, the external drive control system applies a corresponding voltage to the metal electrode 5 through the metal lead wire, so as to select the optical signal entering the delay optical waveguide path 401 or the reference waveguide arm path 402.

In the specific implementation process, parameters such as the extinction ratio and the driving voltage of the optical switch 4 need to be designed. In order to realize a high extinction ratio, most importantly, whether the optical switch 4 using the multimode interferometer (MMI) can achieve the 1:1 light splitting performance needs to be considered during the design, and manufacturing errors in the process manufacturing process need to be considered, so that the structure needs to have a large tolerance, two MMIs need to be used for one optical switch 4, and the loss of the MMI needs to be considered during the multi-stage adjustable delay.

The optical switch 4 of the present embodiment adopts the electro-optical switch 4 of 2 × 2 mach-zehnder interference structure, and the metal electrodes 5 are respectively disposed between the phase shift arms and on both sides of the phase shift arms, so that the push-pull driving design can reduce the length of the required phase shift arms by half, reduce the area ratio of the optical switch 4, and further reduce the loss.

In addition, the driving voltage of the optical switch 4 is an important parameter, and the lower the driving voltage, the better the power consumption of the device can be reduced. The driving voltage is optimized mainly by optimizing the length of the optical switch 4 and the electrode distance in the design process, and the size of the optical switch 4 needs to be reduced while the driving voltage is reduced.

In this embodiment, a conductive silica cladding doped with zinc ions is deposited on the upper surface of the tunable optical delay line to solve the dc drift phenomenon existing in the lithium niobate thin film 1, so that the optical switch 4 can be controlled by a low-frequency dc signal.

Fig. 4 and 5 respectively show the electrical tuning characteristics of the optical switch after depositing the ordinary silica cladding and the silica cladding doped with zinc ions, where fig. 4 shows that the output state of the optical switch changes with time after applying a dc voltage to the optical switch after depositing the ordinary silica cladding, and fig. 5 shows that the output state of the optical switch is stable and does not change with time after applying the dc voltage to the optical switch after depositing the silica cladding doped with zinc ions.

In an alternative embodiment, the tunable optical delay line further comprises a directional coupler 6 and an on-chip photodetector 7; the directional coupler 6 is arranged on a reference arm straight waveguide of any one of the optical switches 4 of the front n stages, and an optical signal split by the directional coupler 6 is guided into the on-chip photoelectric detector 7 through an output grating to monitor the output state of the optical switch 4.

The additional directional coupler 6 is used for splitting the optical signal transmitted by the reference arm straight waveguide, and monitoring the optical signal transmitted by the reference arm straight waveguide through an output grating connected with the directional coupler and the on-chip photoelectric detector 7, and the on-chip photoelectric detector 7 is used for monitoring and feeding back the output state of the optical switch 4, so that the optical signal is completely output from a certain path.

Further, the on-chip photoelectric detector 7 is heterogeneously integrated above the output grating in a flip chip bonding mode, and the on-chip photoelectric detector 7 is connected with an external drive control system through the metal electrode 5 and a metal lead.

Further, the directional coupler 6 is a directional coupler 6 with adjustable splitting ratio. Optionally, a directional coupler 6 with a split ratio of 1:99 is employed.

In this embodiment, the on-chip photodetector 7 is connected to the external drive control system through the metal electrode 5 and the metal lead, and feeds back the working state of the optical switch 4 monitored by the on-chip photodetector to the external drive control system, and the external drive control system adaptively adjusts the voltage applied to the metal electrode 5 of the optical switch 4 according to the working state of the optical switch 4, thereby further ensuring that the optical signal is completely output from a certain path.

Example 3

The present embodiment provides an optical system, as shown in fig. 6, which is an architecture diagram of the optical system of the present embodiment.

The optical system proposed in this embodiment includes a laser, a modulator, a photodetector, and a tunable optical delay line as proposed in embodiment 1 or embodiment 2.

In this embodiment, the laser outputs an optical signal, the optical signal is loaded with a microwave signal through a modulator and then input into the tunable optical delay line for delay, the delayed optical signal is detected by the photodetector and output, and the microwave signal subjected to specific delay is output.

Microwave signals are loaded on optical waves through a modulator and enter the tunable optical delay line through the grating coupler 2, and optical signals which generate specific delay after being output by the tunable optical delay line are converted into microwave signals carrying specific delay through the photoelectric detector.

The same or similar reference numerals correspond to the same or similar parts;

the terms describing positional relationships in the drawings are for illustrative purposes only and are not to be construed as limiting the patent;

it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A tunable optical delay line based on thin film lithium niobate, comprising:

the grating coupler (2) is arranged on the lithium niobate film (1) and is used for coupling an input optical signal and coupling and outputting the delayed optical signal;

the n-level spiral delay optical waveguide (3) is used for delaying the input optical signal;

an n + 1-stage optical switch (4) for controlling an optical signal to enter the delay optical waveguide path (401) or the reference waveguide arm path (402); wherein n is more than or equal to 2;

the input light loaded with microwave signals enters a tunable optical delay line through the grating coupler (2), the optical switch (4) controls optical signals to enter a delay optical waveguide path (401), and the spiral delay optical waveguide (3) generates corresponding delay for the optical signals and then enters a next-stage optical switch (4); or the optical switch (4) controls the optical signal to enter the reference waveguide arm path (402) and enter the next stage optical switch (4); the optical signal is output through the grating coupler (2) after selecting a path through the last stage optical switch (4).

2. The tunable optical delay line of claim 1, wherein the spiral delay optical waveguide (3) is coiled, and the (i + 1) th order spiral delay optical waveguide (3) is 2 times the length of the (i) th order spiral delay optical waveguide (3); where i ═ 1, 2., n.

3. The tunable optical delay line of claim 2, wherein the spiral delay optical waveguide (3) is a multi-mode broad waveguide, and the spiral delay optical waveguide (3) comprises a curved structure with a curvature change of a sinusoidal curve, a Bessel function or an Euler curve.

4. The tunable optical delay line of claim 1, wherein the optical switch (4) comprises an electro-optical switch of 2 x 2 mach-zehnder interference structure.

5. The tunable optical delay line of claim 4, wherein metal electrodes (5) are respectively disposed between the phase shift arms and on both sides of the phase shift arms of the optical switch (4), and the metal electrodes (5) are connected to an external drive control system through metal leads.

6. The tunable optical delay line of claim 5, wherein a cladding layer of conductive silica doped with zinc ions is deposited on the upper surface of the tunable optical delay line.

7. The tunable optical delay line of claim 5, further comprising a directional coupler (6) and an on-chip photodetector (7); the directional coupler (6) is arranged on a reference arm straight waveguide of any one optical switch (4) of the front n stages, and an optical signal split by the directional coupler (6) is guided into the on-chip photoelectric detector (7) through an output grating to monitor the output state of the optical switch (4).

8. The tunable optical delay line of claim 7, wherein the on-chip photodetector (7) is heterogeneously integrated above the output grating by flip-chip bonding, and the on-chip photodetector (7) is connected with an external drive control system through a metal electrode (5) and a metal lead.

9. The tunable optical delay line of claim 7, wherein the directional coupler (6) is a directional coupler (6) with adjustable splitting ratio.

10. The tunable optical delay line of any one of claims 1 to 9, wherein the lithium niobate thin film (1) is disposed on a substrate.