CN110212401B

CN110212401B - A kind of on piece distributed feed-back optical parametric oscillator

Info

Publication number: CN110212401B
Application number: CN201910705220.7A
Authority: CN
Inventors: 尹志军; 吴冰; 许志城
Original assignee: Nanjing Nanzhi Advanced Photoelectric Integrated Technology Research Institute Co Ltd
Current assignee: Nanzhixin Chain Technology Huzhou Co ltd
Priority date: 2019-08-01
Filing date: 2019-08-01
Publication date: 2019-11-19
Anticipated expiration: 2039-08-01
Also published as: DE112019007597T5; CN110212401A; GB202100323D0; GB2588065A; GB2588065B; DE112019007597B4; WO2021017385A1

Abstract

The application provides a kind of on piece distributed feed-back optical parametric oscillator, the on piece distributed feed-back optical parametric oscillator changes traditional optical parametric oscillator structure, remove the structures such as resonant cavity and hysteroscope from, by the top layer in the period polarized waveguide of ridge, refractive-index grating is set, so that pump light is filtered while propagation in the waveguide, to obtain the output light of narrow linewidth target wavelength, optical parametric oscillator provided by the present application reduces the volume of optical parametric oscillator significantly, can be applied to small-size chips.

Description

On-chip distributed feedback optical parametric oscillator

Technical Field

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

Background

An Optical Parametric Oscillator (OPO) is a Parametric Oscillator oscillating at an Optical frequency, which inputs an Optical signal having a frequency ofIs converted into two output lights with lower frequencies, signal lights, by second-order nonlinear optical interactionAnd an idlerThe sum of the two output optical frequencies is equal to the input optical frequency:+=。

an important feature of the optical parametric oscillator is that laser light which is coherent and has a wide spectral range (output light, range in which the center value of the frequency peak is movable) can be generated, that is, the output laser light has coherence, and the movable range of the center value of the frequency peak of the output laser light is large. When the pump intensity is significantly higher than the threshold, the two output optical waves are very close to coherent, and the linewidths of the signal light and the idler light are very narrow, usually only a few kHz. Narrow linewidth optical parametric oscillators are now widely used in spectroscopy.

The conventional optical parametric oscillator comprises a pumping light source, an OPO resonant cavity and the like, wherein the output linewidth of the laser can be controlled by the OPO resonant cavity (in a non-grating structure), the OPO resonant cavity comprises a waveguide and cavity mirrors arranged at two ends of the waveguide in the length direction, pumping light is transmitted in the waveguide, and light waves generated by the waveguide are filtered by the cavity mirrors to finally obtain output laser with a narrower linewidth.

Fig. 1a to 1d show schematic cavity shapes and optical path diagrams of several conventional OPO resonant cavities, in which fig. 1a shows a two-mirror linear straight cavity, fig. 1b shows a V-shaped folded cavity, fig. 1c shows an X-shaped straight cavity, and fig. 1d shows a four-mirror annular cavity. As can be seen from fig. 1a to 1d, a plurality of mirrors are required to be used in a conventional OPO cavity to continuously extend a light path, each mirror occupies 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, so that the cavity mirror and the nonlinear crystal must adopt a special fixing structure, and the mirror frame for fixing the lens is usually large in volume due to extremely high requirements of the cavity mirror on the adjustment freedom and the heat dissipation, and different OPO performances and different lengths of the light paths cause that the volume of the OPO cavity cannot be reduced. Therefore, the conventional OPO resonator is very long, for example, the length can reach 1.5m, and currently, an optical parametric oscillator using the conventional OPO resonator is known, the minimum length of the OPO resonator also reaches 10cm, and the more the reflection lens passed by the optical path, the larger the energy loss. Due to the large size and heat loss, the conventional optical parametric oscillator cannot be integrated on a chip.

The optical parametric oscillator has the characteristic that the output wavelength can be selected randomly, can be used for stepless regulation of the output wavelength in a small range, is particularly a miniaturized or even chip-type optical parametric oscillator, and is a core component for optical communication, quantum computation, sensing and the like.

Because of the need for OPOThe optical pumping, especially the pumping with laser with narrow spectrum and good beam quality, is required to ensure the conversion efficiency of the OPO. At present, the laser crystal is generally pumped by a semiconductor laser, for example, using Nd: YVO₃The pump laser crystal (neodymium-doped yttrium vanadate crystal), Nd: YAG (neodymium-doped yttrium aluminum garnet laser) and the like generate pump laser with the wavelength of 1064nm or frequency doubling of 532 nm. The OPO directly using semiconductor laser pumping is also reported at present, but the semiconductor for directly generating laser needs to be manufactured by a special process to narrow the line width of the output laser to below 1 nm. The existing OPO has the defects of complex structure, large volume, lower output power and poor reliability due to the factors.

Disclosure of Invention

This application changes traditional optical parametric oscillator structure, removes structures such as resonant cavity and chamber mirror from, sets up the refracting index grating through the top layer at ridge periodic polarization waveguide for the pump light is in carry out the filtering in propagating in the waveguide, thereby obtains the output light of narrow linewidth target wavelength, the volume of the optical parametric oscillator that this application provided reduces greatly makes it can be applied to small-size chip. The application provides an on-chip distributed feedback optical parametric oscillator includes: the device comprises a substrate 1, a periodically polarized waveguide 2 laminated on the top end of the substrate 1, and a refractive index grating 3 laminated on the top end 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 high refractive index layers 31 and low refractive index layers 32, and the high refractive index layers 31 and the low refractive index layers 32 are alternately arranged perpendicular to the substrate 1.

In one realizable approach, the periodically poled waveguide 2 is a ridge waveguide.

Further, the periodic polarization 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.

Further, 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.

Further, the low refractive index layer 32 has a refractive index smaller than that of the periodically polarized waveguide 2.

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

In another implementation manner, the on-chip distributed feedback optical parametric oscillator further includes two metal electrodes 4, one of the two metal electrodes 4 is disposed at the bottom of the substrate 1, and the other metal electrode is disposed at the top of the refractive index grating 3.

Compared with the traditional optical parametric oscillator, the on-chip distributed feedback optical parametric oscillator provided by the application has no resonant cavity, accordingly, a cavity mirror is prevented from being arranged, the refractive index grating is arranged on the top end of the periodic polarization waveguide in a laminating mode, and the pumping light is filtered while being transmitted in the waveguide.

Drawings

FIG. 1a shows a conventional two-mirror linear straight cavity;

FIG. 1b shows a conventional V-fold cavity;

FIG. 1c shows a conventional X-shaped straight lumen;

FIG. 1d shows a conventional four-mirror annular cavity;

fig. 2 is a schematic diagram illustrating a front view structure of an on-chip distributed feedback optical parametric oscillator according to an embodiment of the present disclosure;

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

FIG. 4 shows the optical path of a light ray propagating in a ridge waveguide;

FIG. 5 is a spectrum diagram showing an output laser spectrum of the present embodiment;

fig. 6 shows a spectrum of an output laser spectrum after a voltage of 100V was applied to the optical parametric oscillator provided in this example.

Description of the reference numerals

1-substrate, 2-periodic polarization waveguide, 3-refractive index grating, 31-high refractive index layer, 32-low refractive index layer and 4-metal electrode.

Detailed Description

Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the invention, as detailed in the appended claims.

The structure and the operation principle of the on-chip distributed feedback optical parametric oscillator provided by the present application are explained in detail by specific embodiments below.

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

In the embodiment of the present application, the term "on-chip" refers to a small or micro chip, and the "distributed feedback" refers to feedback through the whole path of the optical path, that is, the pump light starts to continuously oscillate and feed back from the periodically polarized waveguide of the on-chip distributed feedback optical parametric oscillator until output from the periodically polarized waveguide.

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

With reference to fig. 2 and 3, the present application provides an on-chip distributed feedback optical parametric oscillator including: the grating comprises a substrate 1, a periodic polarization waveguide 2 laminated on the top end of the substrate 1 and a refractive index grating 3 laminated on the top end of the periodic polarization waveguide 2.

In the art, waveguides for optical parametric oscillators may include at least both buried and ridge waveguide configurations. For convenience of description, in this embodiment, the x-axis direction in fig. 4 is referred to as a length direction of the ridge waveguide, the y-axis direction is referred to as a width direction of the ridge waveguide, and the z-axis direction is referred to as a height direction of the ridge waveguide.

In the present embodiment, the periodically poled waveguide 2 is a ridge waveguide, and the applicant believes that the left and right sides of the ridge waveguide in the width direction may be regarded as air substrates, or air waveguides, and 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 air, so the ridge waveguide is selected as the basis of the on-chip distributed feedback optical parametric oscillator in the present embodiment.

In this embodiment, the ridge waveguide may be obtained by cutting a rectangular parallelepiped substrate. Specifically, the top surface of the ridge waveguide may be set first, and then the ridge waveguide may be cut from the top surface to the bottom surface, and portions on both left and right sides in the width direction of the ridge waveguide may be cut to a depth corresponding to the height of the ridge waveguide, so as to obtain the ridge waveguide with the substrate as shown in fig. 2 and 3.

In this embodiment, the periodically poled 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, and may further include other doped lithium niobate waveguides that can be used for the on-chip distributed feedback optical parametric oscillator.

Because the lithium niobate is a nonlinear crystal, the lithium niobate is also an electro-optic crystal, an acousto-optic crystal and a photorefractive crystal. A photorefractive crystal is a crystal whose refractive index changes under the action of light radiation through the spatial distribution of photogenerated carriers. By refracting the light through the material, the crystal in the material generates charge carriers (electrons or holes), the charge carriers migrate in the crystal lattice until being trapped at a new position by a trap due to the single or comprehensive action of the effects of diffusion, drift, photovoltaic generation and the like, and the refractive index of the crystal is correspondingly changed by the electric field due to the electric field intensity distribution caused by the generated space charges in the crystal.

Since the refractive index of the doped lithium niobate, especially the iron-doped lithium niobate and the zinc-doped lithium niobate, is higher than that of the undoped lithium niobate, the doped lithium niobate is used as the waveguide material in this embodiment, and the undoped lithium niobate is used as the substrate material, so that the pumping light is totally reflected at the interface between the waveguide and the substrate, and the pumping light is propagated in the periodically polarized waveguide 2.

Further, the doped lithium niobate waveguide described herein may be prepared by a doped lithium niobate waveguide preparation process, such as ion implantation or titanium diffusion.

Specifically, the periodically poled waveguide 2 described in the present embodiment can be prepared by the following method: doping the periodic polarization waveguide 2 from the top end thereof to the interior thereof, forming a photorefractive layer, namely a doped lithium niobate layer, in the lithium niobate medium matrix by using target doping elements such as iron element or zinc element, wherein the thickness of the doped lithium niobate layer is uniform; and cutting the left and right redundant parts of the doped lithium niobate in the width direction, wherein the cutting depth is the doping depth, so that the ridge type periodic polarization waveguide taking the lithium niobate as a substrate material and the doped lithium niobate as a waveguide material is formed.

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

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

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, the high refractive index layer 31 and the low refractive index layer 32 are alternately distributed perpendicular to the substrate 1, 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 irradiation is performed by a two-beam interference method or a mask method to form the refractive index grating.

In the present embodiment, the refractive index grating 3 has a central refractive index, that is, 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 to be 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 may be set according to the wavelength of the target output laser light, and the refractive indices of the high refractive index layer 31 and the low refractive index layer 32 may be set according to the wavelength of the target output laser light.

Further, 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 is distributed filtered by the refractive index grating during the propagation in the periodically polarized waveguide 2.

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

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, hereinafter referred to as "refraction period" does not have a corresponding relationship with the polarization period of the periodic polarization waveguide, that is, the refraction period may be greater than, equal to, or less than the polarization period.

The embodiment can naturally filter the pump light in the waveguide propagation process by arranging the specific refractive index grating, and can form pump light oscillation in the minimum waveguide length, thereby providing a foundation for reducing the volume of the optical parametric oscillator.

In this embodiment, the metal electrodes 4 are plate-type electrodes, and include two metal electrodes, which are respectively laminated on the top layer of the refractive index grating 3 and the bottom layer of the substrate 1, and the shapes and sizes of the two metal electrodes 4 are respectively matched with those of adjacent components.

The metal electrode 4 is used for applying voltage to the mu refractive index grating 3, and adjusting the refractive indexes of the high refractive index layer 31 and the low refractive index layer 32 by adjusting the voltage intensity, so that the wavelength of target output laser is controlled, and the alignment of the central characteristic wavelength of actual output laser and the central characteristic wavelength of preset output laser is realized.

The working principle and effect of the optical parametric oscillator provided by the present application are described as follows:

the thickness of the optical parametric oscillator substrate was 2 μm, the thickness of the periodically polarized waveguide was 6 μm, the width was 8 μm, the thickness of the refractive index grating was 2 μm, the central refractive index of the refractive index grating was 2.2, and the refractive index change amount was about 0.001, that is, the refractive index of the high refractive index layer was 2.201, and the refractive index of the low refractive index layer was 2.199, and the results are shown in fig. 2 and 3.

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

formula (1)

Wherein,

k represents the wavevector, and L represents the thickness of the dielectric layer;

further, the wave vector k may be calculated according to the following formula (2):

formula (2)

Wherein n represents a refractive index; λ represents the spectral wavelength.

Further, transfer matrix of multi-layer dielectric layerM _sCan be calculated according to the following formula (3):

formula (3)

Wherein N represents the number of dielectric layers.

Further, the reflectance r can be calculated according to the following formula (4):

formula (4)

Wherein M is₂₁Representing a second row and a first column in the Ms matrix;

M₂₂representing a second row and a second column in the Ms matrix;

i is an imaginary unit;

is the wavevector of the leftmost refractive index layer of the refractive index grating;

is the wavevector of the rightmost refractive index layer of the index grating.

Wherein,k _Lcan be calculated according to the following equation (5):

formula (5)

Accordingly, the number of the first and second electrodes,k _Rcan be calculated according to the following equation (6):

formula (6)

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

an oscillation wavelength (wavelength of the target output laser light) is 1600nm (a spectrum of the oscillation wavelength is as shown in fig. 5), a refractive index n1=2.199 of a high refractive index layer in the refractive index grating, a refractive index n2=2.201 of a low refractive index layer, a central refractive index of the refractive index grating is 2.200, and 5000 layers are alternately arranged in two layers, so that a total length of a single layer of the refractive index grating layer is L =1600nm/2.200/4, in this embodiment, a single layer of the refractive index grating layer is a length of the high refractive index layer or the low refractive index layer along an x-axis direction, and a total length of the optical parametric oscillator is calculated to be about 1600nm/2.200/4 × 5000 and about 0.91 mm.

Moreover, the above-mentioned embodiments provide an optical parametric oscillator for pumping, and the minimum size of the conventional optical parametric oscillator is also above 10 cm.

Further, the three-dimensional matrix is converted into a two-dimensional matrix under the condition of symmetry, namely, the electro-optic coefficient of the lithium niobate crystal () Can be calculated according to the following equation (7):

=formula (7)

Wherein i represents the number of rows of elements in the transformed two-matrix

j represents the number of columns where the elements in the transformed two matrixes are located;

the electro-optic coefficients representing the i =2, j =2 elements of the two-dimensional matrix, the meaning of the remaining electro-optic coefficients and so on.

The specific numerical values of the electro-optic coefficient are as follows:

when an electric field of 100v is applied to the refractive index grating, the refractive index ellipsoid equation is shown in the following formula (8):

formula (8)

Wherein,n _erepresents the refractive index of e light;

represents the z-direction electric field strength;

n ₀represents the refractive index of o light;

x, Y and Z respectively represent coordinate axes in three directions in a coordinate system as shown in fig. 4.

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

formula (9)

According to the calculation of the above formula (9),。

then the refractive index change value is substituted into a transfer matrix formula-formula (3), and the reflection center wavelength shift of about 0.9nm can be calculated because the optical path in the refractive index grating layer is changed. The output spectrum is shown in fig. 6.

As can be seen from fig. 5 and 6, only the peak position of the target output laser is shifted, and the remaining parameters are hardly changed, the optical parametric oscillator provided by the present application can well ensure the stability of the output spectrum, and the alignment between the central characteristic wavelength of the actual output laser spectrum and the central characteristic wavelength of the preset output laser spectrum.

By the above explanation to the optical parametric oscillator provided by the application, the optical parametric oscillator provided by the application can shorten the length of the optical parametric oscillator to the millimeter level by more than at least 10cm, and the thickness is only the micron level, thereby making the optical parametric oscillator can be applied to the microchip, can reverse the required electric field intensity to the central wavelength of different target output lasers, realizes that the output wavelength has the adjustability, thereby the optical parametric oscillator provided by the application can realize the accurate alignment of actual output laser spectrum and preset laser output spectrum when realizing narrow linewidth and wavelength adjustable technical index.

The present application has been described in detail with reference to specific embodiments and illustrative examples, but the description is not intended to limit the application. Those skilled in the art will appreciate that various equivalent substitutions, modifications or improvements may be made to the presently disclosed embodiments and implementations thereof without departing from the spirit and scope of the present disclosure, and these fall within the scope of the present disclosure. The protection scope of this application is subject to the appended claims.

Claims

1. An on-chip distributed feedback optical parametric oscillator, comprising: a substrate (1), a periodically poled waveguide (2) laminated on top of the substrate (1), a refractive index grating (3) laminated on top of the periodically poled waveguide (2), wherein,

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

the refractive index grating (3) comprises high refractive index layers (31) and low refractive index layers (32), the high refractive index layers (31) and the low refractive index layers (32) being alternately distributed perpendicular to the substrate (1);

the periodic polarization waveguide (2) is a ridge waveguide;

the on-chip distributed feedback optical parametric oscillator further comprises metal electrodes (4), wherein the metal electrodes (4) are plate-type electrodes, two metal electrodes (4) are arranged, one of the two metal electrodes is arranged at the bottom of the substrate (1), and the other metal electrode is arranged at the top end of the refractive index grating (3).

2. The on-chip distributed feedback optical parametric oscillator of claim 1, wherein the periodically poled waveguide (2) is a doped lithium niobate waveguide, the doped lithium niobate waveguide comprising an iron-doped lithium niobate waveguide and/or a zinc-doped lithium niobate waveguide.

3. The on-chip distributed feedback optical parametric oscillator of 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.

4. The on-chip distributed feedback optical parametric oscillator of claim 1, wherein the substrate (1) is an undoped lithium niobate substrate.

5. An on-chip distributed feedback optical parametric oscillator according to claim 1, wherein the low refractive index layer (32) has a refractive index less than the refractive index of the periodically poled waveguide (2).