WO2021017385A1

WO2021017385A1 - On-chip distributed feedback optical parametric oscillator

Info

Publication number: WO2021017385A1
Application number: PCT/CN2019/127790
Authority: WO
Inventors: 尹志军; 吴冰; 许志城
Original assignee: 南京南智先进光电集成技术研究院有限公司
Priority date: 2019-08-01
Filing date: 2019-12-24
Publication date: 2021-02-04
Also published as: CN110212401A; DE112019007597T5; DE112019007597B4; CN110212401B; GB2588065A; GB2588065B; GB202100323D0

Abstract

Disclosed is an on-chip distributed feedback optical parametric oscillator. The on-chip distributed feedback optical parametric oscillator changes the structure of a traditional optical parametric oscillator, and removes structures such as a resonant cavity and a cavity mirror. A refractive index grating (3) is arranged on the top of a ridge-shaped periodically poled waveguide (2), so that pump light is filtered while propagating in the waveguide (2) to obtain output light having a target wavelength and a narrow linewidth. The volume of the optical parametric oscillator is greatly reduced, so that same can be applied to small-size chips.

Description

An on-chip distributed feedback optical parametric oscillator

This application claims the priority of a Chinese patent application filed with the Chinese Patent Office on August 1, 2019, the application number is 201910705220.7, and the invention title is "an on-chip distributed feedback optical parametric oscillator", the entire content of which is incorporated herein by reference Applying.

Technical field

This application belongs to the field of laser technology, and particularly relates to an on-chip distributed feedback optical parametric oscillator.

Background technique

Optical Parametric Oscillator (OPO) is a parametric oscillator that oscillates at an optical frequency. It converts the input laser light (ie, pump light) with a frequency of ω _p through the second-order nonlinear optical interaction. There are two lower frequency output lights, signal light ω _s and idle frequency light ω _i . The sum of the two output light frequencies is equal to the input light frequency: ω _s +ω _i =ω _p .

An important feature of the optical parametric oscillator is that it can generate coherent lasers with a wide spectral range (output light, the range in which the center value of the frequency peak can be moved), that is, the output laser has coherence, and the output laser The center value of the frequency peak can move in a larger range. When the pump light intensity is significantly higher than the threshold, the two output light waves are very close to the coherent state, and the line width of the signal light and the idle frequency light is very narrow, usually only a few kHz. Now, the narrow linewidth optical parametric oscillator is widely used in spectroscopy.

The traditional optical parametric oscillator includes a pump light source, an OPO resonant cavity and other parts. Among them, the OPO resonant cavity (non-grating structure) can control the laser output line width, and the OPO resonant cavity includes a waveguide and is arranged in the length direction of the waveguide. The cavity mirrors at both ends, wherein the pump light is transmitted in the waveguide, and the light wave generated by the waveguide is filtered by the cavity mirror, and finally an output laser with a narrow line width is obtained.

Figures 1a to 1d show the cavity shapes and optical path diagrams of several conventional OPO resonators. Figure 1a shows a two-mirror linear straight cavity, Figure 1b shows a V-shaped cavity, and Figure 1c shows an X-shaped straight cavity. Figure 1d shows a four-mirror ring cavity. From Figure 1a to Figure 1d, it can be seen that the traditional OPO cavity needs to use multiple mirrors to continuously extend the light path, because each mirror needs to take up a certain volume, and the working efficiency of the OPO cavity depends on the construction of the cavity and the collimation of the light path. Therefore, cavity mirrors and nonlinear crystals must adopt a special fixed structure. Because cavity mirrors require extremely high adjustment freedom and heat dissipation, the frames used to fix the lenses are usually large in size. According to different OPO performances, optical paths The length is different, all of which cause the volume of the OPO cavity cannot be reduced. Therefore, the traditional OPO resonant cavity is very long, for example, the length can reach 1.5m. At present, it is known that the optical parametric oscillator using the traditional OPO resonant cavity must have a minimum length of 10cm. Moreover, the more reflective lenses the optical path passes through , The greater the energy loss. It is precisely because of the large volume and large heat loss that traditional optical parametric oscillators cannot be integrated on the chip.

The optical parametric oscillator has the characteristic that the output wavelength can be arbitrarily selected, and can be used to steplessly adjust the output wavelength in a small range, especially the miniaturized or even chip-type optical parametric oscillator, which is the core component of optical communication, quantum computing, sensing, etc.

Because OPO needs to be pumped with a laser, especially a laser with a narrow spectrum and good beam quality is needed to pump to ensure the conversion efficiency of OPO. At present, laser crystals are generally pumped by semiconductor lasers, for example, Nd:YVO ₃ (neodymium-doped yttrium vanadate crystal), Nd:YAG (neodymium-doped yttrium aluminum garnet laser) and other pumped laser crystals are used to generate a wavelength of 1064nm or more Pump laser with 532nm frequency. OPO pumped directly with a semiconductor laser has also been reported, but the semiconductor used to directly generate laser light needs to be manufactured with a special process to narrow the line width of the output laser to less than 1 nm. The above-mentioned many factors cause the current OPO to have a complicated structure, large volume, low output power and poor reliability.

Summary of the invention

This application changes the structure of the traditional optical parametric oscillator, eliminating the resonant cavity and cavity mirror and other structures. By setting the refractive index grating on the top layer of the ridge-type periodic polarization waveguide, the pump light is filtered while propagating in the waveguide. In order to obtain output light with a narrow linewidth target wavelength, the optical parametric oscillator provided in the present application greatly reduces the volume of the optical parametric oscillator, so that it can be applied to small-sized chips. The on-chip distributed feedback optical parametric oscillator provided by the present application includes: a substrate 1, a periodically polarized waveguide 2 laminated on the top of the substrate 1, a refractive index grating 3 laminated on the top of the periodically polarized waveguide 2, and Wherein, the periodically polarized waveguide 2 is periodically polarized perpendicular to the substrate 1; the refractive index grating 3 includes a high refractive index layer 31 and a low refractive index layer 32, and the high refractive index layer 31 and the low refractive index layer 31 The rate layers 32 are alternately distributed perpendicular to the substrate 1.

In an achievable manner, the periodically polarized waveguide 2 is a ridge waveguide.

Further, the periodically polarized waveguide 2 is a doped lithium niobate waveguide, and the doped lithium niobate waveguide includes an iron-doped lithium niobate waveguide and/or a zinc-doped lithium niobate waveguide.

Furthermore, the thickness of the high refractive index layer 31 and the thickness of the low refractive index layer 32 are both 1/4 of the oscillation wavelength.

Furthermore, the refractive index of the low refractive index layer 32 is smaller than the refractive index of the periodically polarized waveguide 2.

In another achievable manner, the substrate 1 is an undoped lithium niobate substrate.

In another achievable manner, the on-chip distributed feedback optical parametric oscillator further includes metal electrodes 4, the metal electrodes 4 are two pieces, one of which is arranged at the bottom of the substrate 1, and the other is arranged at the bottom of the substrate 1. The top end of the refractive index grating 3.

Compared with the traditional optical parametric oscillator, the on-chip distributed feedback optical parametric oscillator provided by the present application does not have a resonant cavity, and accordingly, it is free of cavity mirrors. The refractive index grating is laminated on the top of the periodic polarization waveguide to make The pump light is filtered while propagating in the waveguide. Because the polarization period of the periodically polarized waveguide is very short, and cooperates with the refractive index grating, the pump light propagates in the shorter period of the polarization waveguide. Multi-oscillation filtering can be performed, and the filtering efficiency is much higher than that of traditional optical parametric oscillators. The optical parametric oscillator can reduce the output spectral linewidth, maintain the stability of the output spectrum, and align the center characteristic wavelength of the output spectrum with the target wavelength. The total length of the optical parametric oscillator is reduced to the millimeter level and can be used in micro devices such as chips.

Description of the drawings

Figure 1a shows a traditional two-mirror linear straight cavity;

Figure 1b shows a traditional V-shaped cavity;

Figure 1c shows a traditional X-shaped straight cavity;

Figure 1d shows a traditional four-mirror ring cavity;

FIG. 2 shows a schematic diagram of the front view structure of an on-chip distributed feedback optical parametric oscillator preferred in an embodiment of the present application;

Fig. 3 shows a left side view of the on-chip distributed feedback optical parametric oscillator shown in Fig. 2;

Figure 4 shows the optical path of light propagating in the ridge waveguide;

FIG. 5 shows a spectrum diagram of the output laser spectrum in this embodiment;

FIG. 6 shows a spectrum diagram of the output laser spectrum after applying a voltage of 100V to the optical parametric oscillator provided by this embodiment.

Detailed ways

Here, exemplary embodiments will be described in detail, and examples thereof are shown in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings indicate the same or similar elements. The implementation manners described in the following exemplary embodiments do not represent all implementation manners consistent with the present invention. Rather, they are merely examples of devices and methods consistent with some aspects of the present invention as detailed in the appended claims.

The structure and working principle of the on-chip distributed feedback optical parametric oscillator provided in the present application will be described in detail below through specific embodiments.

The invention adopts the method of manufacturing the refractive index grating on the lithium niobate ridge waveguide to realize the on-chip optical parametric oscillator with automatic alignment, narrow line width and stable output. It can be used for chip integration, photonic chip light source, detector light source, etc. It can be used to detect cancer cells clinically in medicine, and it can also be used to detect air pollution in the field of environmental science.

In the embodiments of this application, the term "on-chip" refers to a small or microchip, and the "distributed feedback" refers to feedback throughout the entire path of the optical path, that is, the pump light is self-injected into the chip The periodic polarized waveguide of the distributed feedback optical parametric oscillator starts to continuously perform oscillation feedback until the output is output from the periodic polarized waveguide.

FIG. 2 shows a schematic diagram of the front view of an on-chip distributed feedback optical parametric oscillator according to an embodiment of the present application, and FIG. 3 shows a left view of the on-chip distributed feedback optical parametric oscillator shown in FIG. 2.

2 and 3, the on-chip distributed feedback optical parametric oscillator provided by the present application includes: a substrate 1, a periodically polarized waveguide 2 laminated on the top of the substrate 1, and a periodically polarized waveguide 2 laminated on the periodically polarized waveguide 2 The top refractive index grating 3.

In the art, waveguides used for optical parametric oscillators can include at least two configurations, buried waveguides and ridge waveguides. Among them, the structure of the ridge waveguide can be known with reference to Figures 2 and 3. Figure 4 shows the optical path of light propagating in the ridge waveguide. For ease of presentation, in this embodiment, the x-axis direction in Figure 4 is called the ridge In the longitudinal direction of the ridge waveguide, the y-axis direction is called the width direction of the ridge waveguide, and the z-axis direction is called the height direction of the ridge waveguide.

In this embodiment, the periodically polarized waveguide 2 is a ridge waveguide. The applicant believes that the left and right sides of the ridge waveguide in the width direction can be regarded as air substrates, or air waveguides. Further, The refractive index of air is lower than that of the ridge waveguide, and light naturally forms total reflection at the interface between the ridge waveguide and the air. Therefore, the ridge waveguide is selected as the basis of the on-chip distributed feedback optical parametric oscillator in this embodiment.

In this embodiment, the ridge waveguide can be obtained by cutting a rectangular parallelepiped base. Specifically, the top surface of the ridge waveguide can be set first, and then cut from the top surface of the ridge waveguide to the bottom surface, cut off the left and right sides of the ridge waveguide in the width direction, and the cutting depth is the height of the ridge waveguide, so as to obtain The ridge waveguide with a substrate shown in Figs. 2 and 3.

In this embodiment, the periodically polarized waveguide 2 is a doped lithium niobate waveguide, and the doped lithium niobate waveguide includes an iron-doped lithium niobate waveguide and/or a zinc-doped lithium niobate waveguide. It includes other doped lithium niobate waveguides that can be used for the on-chip distributed feedback optical parametric oscillator.

Because lithium niobate is a nonlinear crystal, at the same time, lithium niobate is also an electro-optic crystal, an acousto-optic crystal and a photorefractive crystal. Photorefractive crystal refers to a crystal whose refractive index changes through the spatial distribution of photogenerated carriers under the action of light radiation. By directing light into the light-refracting material, the crystal in the material will generate charge carriers (electrons or holes). Due to the effects of diffusion, drift, photovoltaics, etc., alone or in combination, the carriers will migrate in the crystal lattice until Being trapped in a new position, the generated space charge causes an electric field intensity distribution in the crystal, and the electric field changes the refractive index of the crystal correspondingly through the electro-optical effect.

Because the refractive index of doped lithium niobate, especially iron-doped lithium niobate and zinc-doped lithium niobate is higher than that of undoped lithium niobate, the waveguide material of this embodiment adopts doped niobate Lithium and non-doped lithium niobate are used as the substrate material, so that the pump light is totally reflected at the interface between the waveguide and the substrate, so that the pump light propagates in the periodically polarized waveguide 2.

Further, the doped lithium niobate waveguide described in the present application can be prepared by a preparation process of doped lithium niobate waveguide, for example, ion implantation or titanium diffusion.

Specifically, the periodically polarized waveguide 2 of this embodiment can be prepared by the following method: doping from the top of the periodically polarized waveguide 2 to its interior, and target doping elements such as iron or zinc in niobium A photorefractive layer is formed in the matrix of the lithium niobate, that is, a doped lithium niobate layer, the thickness of the doped lithium niobate layer is uniform; and then the doped lithium niobate layer is cut to two left and right sides in the width direction. In part, the cutting depth is the doping depth, thereby forming a ridge-type periodic polarization waveguide with lithium niobate as the substrate material and doped lithium niobate as the waveguide material.

In this embodiment, the periodically polarized waveguide 2 is periodically polarized perpendicular to the substrate 1, that is, the periodically polarized waveguide is periodically polarized along its length direction. Since the pump light propagates in the periodically polarized waveguide 2 along its length, the periodically polarized waveguide 2 is periodically polarized along its length, thereby achieving quasi-phase matching.

In this embodiment, corresponding to the periodically polarized waveguide 2, the substrate 1 is a lithium niobate substrate.

In this embodiment, as shown in FIG. 3, the refractive index grating 3 includes a high refractive index layer 31 and a low refractive index layer 32, and the high refractive index layer 31 and the low refractive index layer 32 are perpendicular to the substrate. 1 Alternate distribution, that is, the high refractive index layer 31 and the low refractive index layer 32 are alternately distributed along the length direction of the periodically polarized waveguide 2, and ultraviolet light is performed by double-beam interference or a mask method to form a refractive index Raster.

In this embodiment, the refractive index grating 3 has a central refractive index, that is, the refractive indexes of the high refractive index layer 31 and the low refractive index layer 32 are symmetrically distributed on both sides of the central refractive index, for example, the central refractive index is set Is 2.2, the refractive index of the high refractive index layer 31 may be 2.201, and the refractive index of the low refractive index layer may be 2.199.

In this embodiment, the central refractive index can be set according to the wavelength of the target output laser, and the refractive index of the high refractive index layer 31 and the low refractive index layer 32 can also be set according to the wavelength of the target output laser.

Furthermore, the thickness of the high refractive index layer 31 and the thickness of the low refractive index layer 32 are both 1/4 of the oscillation wavelength, so that the pump light passes through refraction during the propagation process of the periodically polarized waveguide 2. The rate grating performs distributed filtering.

Furthermore, the refractive index of the low refractive index layer 32 is smaller than the refractive index of the periodically polarized waveguide 2, so that the light of the oscillation wavelength can be totally reflected in the periodically polarized waveguide, thereby reducing the loss of the target output laser.

In this embodiment, the distribution period of the high refractive index layer 31 and the low refractive index layer 32 in the refractive index grating 3 is hereinafter referred to as "refraction period" and there is no correspondence between the polarization period of the periodically polarized waveguide, that is, The refraction period can be greater than, equal to, or less than the polarization period.

In this embodiment, by setting a specific refractive index grating, the pump light can be naturally filtered during the waveguide propagation process, and the pump light oscillation can be formed in the minimum waveguide length, which provides a basis for reducing the volume of the optical parametric oscillator.

In this embodiment, the metal electrode 4 is a plate electrode, including two pieces, which are respectively laminated on the top layer of the refractive index grating 3 and the bottom layer of the substrate 1. The shape and size of the two metal electrodes 4 Respectively match the shape and size of its neighboring parts.

The metal electrode 4 is used to apply a voltage to the μ refractive index grating 3, and adjust the refractive index of the high refractive index layer 31 and the low refractive index layer 32 by adjusting the voltage intensity, thereby controlling the wavelength of the target output laser to achieve Alignment of the actual output laser center characteristic wavelength with the preset output laser center characteristic wavelength.

The working principle and effect of the optical parametric oscillator provided in the present application are described below with an embodiment:

The thickness of the optical parametric oscillator substrate is 2 μm, the thickness of the periodic polarization waveguide is 6 μm, the width is 8 μm, the thickness of the refractive index grating is 2 μm, the central refractive index of the refractive index grating is 2.2, and the refractive index change is about It is 0.001, that is, the refractive index of the high refractive index layer is 2.201 and the refractive index of the low refractive index layer is 2.199. The results are shown in FIGS. 2 and 3.

According to the transfer matrix theory of the refractive index grating, the transfer matrix M of the refractive index grating can be calculated according to the following formula (1):

among them,

k represents the wave vector, L represents the thickness of the dielectric layer;

Further, the wave vector k can be calculated according to the following formula (2):

Among them, n represents the refractive index; λ represents the spectral wavelength.

Further, the transfer matrix Ms of the multilayer dielectric layer can be calculated according to the following formula (3):

M _s ＝M _N ·...·M ₂ ·M ₁ formula (3)

Among them, N represents the number of dielectric layers.

Furthermore, the reflectivity r can be calculated according to the following formula (4):

Among them, M ₂₁ represents the second row and first column of the M _s matrix;

M ₂₂ represents the second row and second column of the M _s matrix;

i is an imaginary unit;

k _L is the wave vector of the leftmost refractive index layer of the refractive index grating;

k _R is the wave vector of the rightmost refractive index layer of the refractive index grating.

Among them, k _L can be calculated according to the following formula (5):

Correspondingly, k _R can be calculated according to the following formula (6):

According to the above formula and the 1/4 wavelength theory of high reflective multilayer film, the following parameters are set:

The oscillation wavelength (the wavelength of the target output laser) is 1600 nm (the spectrum is shown in Figure 5), the refractive index of the high refractive index layer in the refractive index grating is n ₁ = 2.199, and the refractive index of the low refractive index layer is n ₂ = 2.201. The central refractive index of the refractive index grating is 2.200, and the two layers are arranged alternately, a total of 5000 layers, then the total length of the refractive index grating layer and the single layer lateral length L=1600nm/2.200/4. In this embodiment, the The lateral length of the single layer of the refractive index grating layer refers to the length of the single layer of the high refractive index layer or the low refractive index layer along the x-axis direction, so that the total length of the optical parametric oscillator is calculated to be about 1600nm/2.200/4×5000, It is about 0.91mm. Compared with the traditional optical parametric oscillator, the total length of the optical parametric oscillator is shortened to about 1mm, which provides the basis for its inlay on the chip.

Moreover, the optical parametric oscillator provided by the above embodiment is an optical parametric oscillator for pumping light, and the minimum size of the traditional optical parametric oscillator is also above 10 cm.

Further, when the three-dimensional matrix is symmetrical, it is converted into a two-dimensional matrix, that is, the electro-optical coefficient (γ _ij ) of the lithium niobate crystal can be calculated according to the following formula (7):

Among them, i represents the number of rows in the two matrix after transformation

j represents the number of columns in the two matrices after transformation;

γ ₂₂ represents the electro-optical coefficients of elements with i=2 and j=2 in the two-dimensional matrix, and the meaning of the remaining electro-optical coefficients can be deduced by analogy.

Among them, the specific value of the electro-optic coefficient is:

When a 100v electric field is applied to the refractive index grating, the refractive index ellipsoid equation is shown in the following equation (8):

Among them, n _e represents the refractive index of e light;

E _z represents the electric field strength in the z direction;

n ₀ represents o-ray refractive index;

X, Y, and Z respectively represent coordinate axes in three directions in a coordinate system, and the coordinate system is shown in FIG. 4.

Further, after a voltage is applied to the refractive index grating layer, the refractive indexes of the high refractive index layer and the low refractive index layer in the refractive index grating layer will both change, and the high refractive index layer and the low refractive index layer The refractive index change values of the layers are the same. Specifically, the refractive index change value can be calculated according to the formula shown in the following formula (9):

According to the calculation of the above formula (9), Δn≈0.0017.

Then put the above-mentioned refractive index change value into the transfer matrix formula-formula (3), because the optical path in the refractive index grating layer changes, it can be calculated that the reflection center wavelength shifts about 0.9nm. The output spectrum is shown in Figure 6.

It can be seen from Fig. 5 and Fig. 6 that only the peak position of the target output laser is shifted, while the remaining parameters are almost unchanged. The optical parametric oscillator provided in this application can well ensure the stability of the output spectrum and the actual output laser The center characteristic wavelength of the spectrum is aligned with the center characteristic wavelength of the preset output laser spectrum.

From the above description of the optical parametric oscillator provided by the present application, it can be seen that the optical parametric oscillator provided by the present application can shorten the length of the optical parametric oscillator from at least 10 cm to the millimeter level, and the thickness is only micron level, so that the light The parametric oscillator can be applied to microchips, and the required electric field intensity can be reversed for different target output laser center wavelengths, and the output wavelength can be adjusted. Therefore, the optical parametric oscillator provided in the present application can achieve narrow When the line width and wavelength are adjustable, the actual output laser spectrum can be accurately aligned with the preset laser output spectrum.

The application has been described in detail above with reference to specific implementations and exemplary examples, but these descriptions should not be understood as limitations on the application. Those skilled in the art understand that without departing from the spirit and scope of the present application, various equivalent substitutions, modifications or improvements can be made to the technical solutions of the present application and the embodiments thereof, and these all fall within the scope of the present application. The protection scope of this application is subject to the appended claims. specific contents

Claims

An on-chip distributed feedback optical parametric oscillator, characterized in that the on-chip distributed feedback optical parametric oscillator comprises: a substrate (1), and a periodic polarization waveguide (2) laminated on the top of the substrate (1) , A refractive index grating (3) laminated on the top of the periodically polarized waveguide (2), wherein,

The periodically polarized waveguide (2) is periodically polarized perpendicular to the substrate (1);

The refractive index grating (3) includes a high refractive index layer (31) and a low refractive index layer (32), and the high refractive index layer (31) and the low refractive index layer (32) are perpendicular to the substrate (1). ) Alternate distribution;

The periodically polarized waveguide (2) is a ridge waveguide;

The on-chip distributed feedback optical parametric oscillator further includes a metal electrode (4), the metal electrode (4) is a plate electrode, the metal electrode (4) is two pieces, one of which is arranged on the substrate (1) At the bottom, another piece is arranged at the top of the refractive index grating (3).
The on-chip distributed feedback optical parametric oscillator according to claim 1, wherein the periodically polarized waveguide (2) is a doped lithium niobate waveguide, and the doped lithium niobate waveguide comprises iron-doped lithium niobate Waveguide and/or zinc doped lithium niobate waveguide.
The on-chip distributed feedback optical parametric oscillator according to claim 1, wherein the thickness of the high refractive index layer (31) and the thickness of the low refractive index layer (32) are both 1/4 of the oscillation wavelength.
The on-chip distributed feedback optical parametric oscillator according to claim 1, wherein the substrate (1) is an undoped lithium niobate substrate.
The on-chip distributed feedback optical parametric oscillator according to claim 1, wherein the refractive index of the low refractive index layer (32) is smaller than the refractive index of the periodically polarized waveguide (2).