CN113156735A - Inverse ridge type AlGaAs waveguide and method thereof - Google Patents

Abstract

The invention discloses an inverse ridge type AlGaAs waveguide for generating a high-coherence octave mid-infrared supercontinuum and a frequency comb and a method thereof, wherein the method comprises the following steps: a wave-guiding layer and a substrate; the material of the wave guide layer is Al_0.18Ga_0.82As, the substrate material is Al_0.8Ga_0.2As; the Al is_0.18Ga_0.82The width of the As wave guide layer is W; the Al is_0.18Ga_0.82As waveguide layer partially embedded with Al_0.8Ga_0.2As substrate, partially not embedded with Al_0.8Ga_0.2As substrate, embedded substrate portion having a height of H_dIs not embeddedThe height of the substrate-entering part is H_u(ii) a The length of the AlGaAs waveguide is 3mm, and the Al is_0.18Ga_0.82The width of the As wave guide layer is 6 μm, and the height of the non-embedded substrate part is H_u0.7 μm, and the embedded substrate portion has a height H_dAnd was 1.4 μm. The inverse ridge type AlGaAs waveguide has the advantages of simple structure, easy preparation and larger nonlinear coefficient of the waveguide.

Description

Inverse ridge type AlGaAs waveguide and method thereof

Technical Field

The invention relates to the field of nonlinear waveguides, in particular to an inverse ridge type AlGaAs waveguide capable of being used for generating an octave mid-infrared supercontinuum and a method thereof.

Background

The generation of the supercontinuum refers to a phenomenon that when a short pulse pump light beam propagates in a nonlinear medium, the spectrum is greatly widened due to the combined action of nonlinearity and dispersion effects. It has important application in the fields of spectroscopy, biomedicine, optical imaging, optical coherence tomography, environmental detection and the like. Generally, the quality of the supercontinuum is evaluated by two characteristics, coherence and spectral width. In the normal dispersion region, the pumping is mainly used for broadening the spectrum through the self-phase modulation effect, and the generated super-continuum has high coherence but limited spectral width broadening. In the anomalous dispersion region, pumping depends on high-order nonlinear effects such as soliton self-frequency shift, soliton splitting and the like, so that octave supercontinuum can be generated, but due to noise caused by modulation instability, the coherence of the supercontinuum can be influenced.

Currently, supercontinuum can be generated from the following media:

bulk media: alfano first discovered that injecting 5mJ picosecond pulses into bulk BK7 glass produced a 400-700 nm white spectrum covering the entire visible spectrum. Subsequently, some studies based on the generation of supercontinuum in materials such as crystals and glass have been reported in succession. The generation of supercontinuum in bulk media has the advantages of high efficiency, low cost, etc., but the generation of supercontinuum in bulk materials is a complex process involving complex coupling between spatial and temporal effects.

Optical fiber: the generation of supercontinuum in optical fibers involves a purely time-kinetic process, the cross-mode properties being determined only by the linear waveguide properties. Most conventional optical fibers have a small difference in refractive index between the core and the cladding. Photonic crystal fibers are considered to be superior nonlinear media for generating supercontinuum due to their characteristics of single mode transmission, better mode field confinement, and easily tunable dispersion and nonlinearity.

Waveguide: compared with an optical fiber, the waveguide can realize the generation of the supercontinuum in a shorter transmission distance, and the on-chip supercontinuum light source is lower in cost, higher in efficiency and more stable. Common waveguide structures include stripe, ridge and groove types, and a floating structure is usually added when the generation of the mid-infrared supercontinuum is studied. By adjusting the structural parameters of the waveguide, the waveguide can have different dispersion characteristics in different wave bands. At present, many studies have been reported on the generation of supercontinuum by waveguides based on materials such as silicon, chalcogenide, silicon nitride, lithium niobate, etc. However, due to the centrosymmetric crystal structure, materials like silicon and silicon nitride lack the inherent second-order nonlinear effect to produce efficient frequency conversion, while materials possessing high second-order and third-order nonlinear coefficients are more prone to produce octave broadband supercontinuum.

Disclosure of Invention

In view of the above technical problems, the present invention aims to provide an inverse ridge type algan waveguide for generating a high coherence octave mid-infrared supercontinuum and a frequency comb, and a method thereof, and is implemented by the following technical scheme:

the invention provides an inverse ridge type AlGaAs waveguide for generating a high-coherence octave mid-infrared supercontinuum and a frequency comb, which comprises the following components: a wave-guiding layer and a substrate; the material of the wave guide layer is Al_0.18Ga_0.82As, the substrate material is Al_0.8Ga_0.2As；

The Al is_0.18Ga_0.82The width of the As wave guide layer is W;

the Al is_0.18Ga_0.82As waveguide layer partially embedded with Al_0.8Ga_0.2As substrate, partially not embedded with Al_0.8Ga_0.2As substrate, embedded substrate portion having a height of H_dThe height of the portion of the non-embedded substrate is H_u；

The length of the AlGaAs waveguide is 3mm, and the Al is_0.18Ga_0.82The width of the As wave guide layer is 6 μm, and the height of the non-embedded substrate part is H_u0.7 μm, and the embedded substrate portion has a height H_dAnd was 1.4 μm.

Optionally, the algan waveguide is an inverse ridge waveguide having 3 zero dispersion points, where the 3 zero dispersion points are 3.74 μm, 6.56 μm, and 8.89 μm, respectively; the 3 zero dispersion points divide the dispersion region into four parts, namely a normal dispersion region from 0 to 3.74 μm, an anomalous dispersion region from 3.74 μm to 6.56 μm, a normal dispersion region from 6.56 μm to 8.89 μm, and an anomalous dispersion region from 8.89 μm onward.

Optionally, the nonlinear coefficient of the inverse ridge type AlGaAs waveguide at 4.9 μm can be 2.09W^-1m^-1。

The invention also provides a method for generating a high-coherence octave mid-infrared supercontinuum and a frequency comb by using the inverse ridge type AlGaAs waveguide, which comprises the following steps:

injecting 50 chirp-free hyperbolic secant pulse sequences with the repetition frequency of 100MHz into the reverse ridge type AlGaAs waveguide; the center wavelength of the non-chirped hyperbolic secant pulse is 4.9 microns, the full width at half maximum is 120fs, the peak power is 900W, and the random noise coefficient is 0.0001.

Producing a mid-infrared supercontinuum over 2.7 octaves at-40 dB level from 2.2 to 14.5 μm with a first order coherence of 1 over the whole spectral range;

and obtaining the frequency combs with stable amplitude and equal intervals.

The invention provides an inverse ridge type AlGaAs waveguide which can be used for generating a high-coherence octave intermediate infrared supercontinuum and an optical frequency comb, and has the following advantages:

the inverse ridge type AlGaAs waveguide has the advantages of simple structure, easy preparation and larger nonlinear coefficient of the waveguide.

The dispersion curve of the reverse ridge type AlGaAs waveguide has 3 zero dispersion points which are respectively positioned at the positions of 3.74 mu m, 6.56 mu m and 8.89 mu m and penetrate into a middle infrared region, and the absolute value of dispersion is smaller and the dispersion curve is flatter.

When the pumping wavelength is 4.9 mu m, the full width at half maximum is 120fs, the peak power is 900W and the noise coefficient is 0.0001, the inverse ridge type AlGaAs waveguide can generate a mid-infrared supercontinuum which is more than 2.7 octaves and is from 2.2 to 14.5 mu m and is in a level of minus 40dB only in a waveguide with the length of 3 mm.

The super-continuum spectrum generated by the reverse ridge type AlGaAs waveguide has good coherence, and the first-order coherence is close to 1 in the whole spectrum range.

The inverse ridge type AlGaAs waveguide can obtain a high-coherence octave mid-infrared supercontinuum, and on the basis, incident pulses are changed into 50 pulse sequences with the repetition frequency of 100MHz, so that frequency combs with stable amplitudes and equal intervals can be obtained.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1 is a cross-sectional view of an inverted ridge AlGaAs waveguide according to the present invention.

FIG. 2 shows the effective mode field area and nonlinear coefficient of the dispersion characteristic of the inverse ridge AlGaAs waveguide.

FIG. 3 is a supercontinuum diagram obtained from an inverted ridge AlGaAs waveguide.

FIG. 4 is a graph of coherence achieved by an inverse ridge AlGaAs waveguide of the present invention.

FIG. 5 is a time domain and frequency domain pulse evolution diagram in an inverse ridge AlGaAs waveguide.

FIG. 6 is an enlarged view of the frequency comb obtained from the inverted ridge AlGaAs waveguide and its details.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention.

AlGaAs (aluminum gallium arsenide) is an alloy of GaAs and AlAs, and a change in refractive index can be achieved by changing the content of Al, which facilitates adjustment of the dispersion characteristics of the waveguide. The AlGaAs has higher nonlinear Kerr coefficient, second-order nonlinear coefficient, higher third-order polarizability, lower two-photon absorption effect and wider transparent window (0.7-17 mu m), is a very promising nonlinear waveguide material, and is suitable for researching the generation of a supercontinuum of a middle infrared band which can cover a molecular fingerprint region. The inverse ridge type waveguide structure is beneficial to confining mode field energy in the waveguide layer and is convenient for flexibly adjusting dispersion characteristics.

A highly coherent and octave supercontinuum can be used to generate the optical frequency comb. An optical frequency comb refers to a coherent light source in the frequency domain consisting of equally spaced discrete lines. The optical frequency comb can be obtained by methods such as a mode-locked laser, supercontinuum generation and Kerr frequency comb, wherein the theoretical model for obtaining the optical frequency comb based on the supercontinuum generation is simplest. The optical frequency comb has the advantages of narrow line width, fine frequency interval, constant amplitude, high stability and the like, so that the optical frequency comb has rich applications in the fields of science, engineering and the like, such as chemical sensing, precision measurement, infrared imaging, molecular detection, timing, distance measurement and the like.

An embodiment of the present invention is an inverse ridge type AlGaAs waveguide for generating high coherence octave mid-infrared supercontinuum, and its three-dimensional structure diagram is shown in FIG. 1. In this embodiment, the AlGaAs waveguide structure includes a waveguide layer and a substrate, and the waveguide layer is made of Al_0.18Ga_0.82As, the substrate layer is made of Al_0.8Ga_0.2As, the width W of the waveguide layer is 6 μm, the height Hu of the portion of the waveguide layer not embedded in the substrate is 0.7 μm, and the height Hd of the portion embedded in the substrate is 1.4 μm. The length of the reverse ridge type AlGaAs waveguide is 3 mm.

Fig. 2(a) shows the dispersion characteristics of the inverted ridge type algan waveguide according to the present invention, and fig. 2(b) shows the effective mode field area (solid line) and the nonlinear coefficient (broken line) of the inverted ridge type algan waveguide according to the present invention. As can be seen from FIG. 2(a), the designed inverse ridge AlGaAs waveguide has 3 zero dispersion points, which are located at 3.74 μm, 6.56 μm and 8.89 μm, respectively. The dispersion region is divided into four parts by three zero dispersion points, namely a normal dispersion region from 0 to 3.74 μm, an anomalous dispersion region from 3.74 μm to 6.56 μm, a normal dispersion region from 6.56 μm to 8.89 μm, and an anomalous dispersion region from 8.89 μm onward. The existence of a plurality of zero dispersion points enables the designed inverse ridge type AlGaAs waveguide to reduce the condition of phase mismatch in the process of generating the supercontinuum, so that the obtained supercontinuum has better coherence. As can be seen from FIG. 2(b), the inverse ridge AlGaAs waveguide of the present invention has a large nonlinear coefficient, and the nonlinear coefficient at 4.9 μm can reach 2.09W^-1m^-1。

The invention also provides a method for generating a high-coherence octave mid-infrared supercontinuum and frequency comb by using any one of the inverse ridge type AlGaAs waveguides, which comprises the following steps:

step one, injecting 50 chirp-free hyperbolic secant pulse sequences with the repetition frequency of 100MHz into the reverse ridge type AlGaAs waveguide; the center wavelength of the non-chirped hyperbolic secant pulse is 4.9 microns, the full width at half maximum is 120fs, the peak power is 900W, and the random noise coefficient is 0.0001.

And step two, generating the intermediate infrared supercontinuum with more than 2.7 octaves from 2.2 to 14.5 mu m under the-40 dB level, wherein the first-order coherence degree in the whole frequency spectrum range is 1.

And step three, obtaining frequency combs with stable amplitude and equal intervals.

FIG. 3 is a middle infrared supercontinuum diagram generated by the inverse ridge type AlGaAs waveguide when the center wavelength of the incident hyperbolic secant pulse is 4.9 μm, the full width at half maximum is 100fs, the peak power is 900W, and the noise coefficient is 0.0001. As can be seen from FIG. 3, the supercontinuum generated by the inverse ridge AlGaAs waveguide covers from 2.2 μm to 14.5 μm at the-40 dB level, deep into the mid-infrared region, over 2.7 octaves. In this embodiment, the pump wavelength is located in the anomalous dispersion region between the first zero dispersion point and the second zero dispersion point of the inverted ridge type algan waveguide, so that high-order solitons are generated. Then, under the influence of high-order dispersion and Raman effect, the high-order solitons with instability are split, and the fundamental solitons with blue shift and red shift are generated. In the short wavelength direction, as the blue-shifted fundamental order solitons satisfy the phase matching condition, blue-shifted dispersion waves occur, so that the frequency spectrum is broadened in the short wavelength direction. In the long wavelength direction, a series of nonlinear effects such as self-phase modulation and cross-phase modulation occur, so that the frequency spectrum is further broadened, and finally the mid-infrared region is reached.

FIG. 4 is a coherence curve of the supercontinuum obtained in the above embodiment of the inverted ridge AlGaAs waveguide of the present invention. As can be seen from fig. 4, the mid-infrared supercontinuum generated by the inverse ridge type algaas waveguide is almost kept at about 1 in the first order coherence degree in the whole spectrum range. It can be seen that the obtained mid-infrared supercontinuum has good coherence.

FIG. 5 is a diagram of the evolution of the time domain pulse and the frequency domain pulse in the inverse ridge AlGaAs waveguide in the above embodiment. As can be seen from fig. 5(a), the time domain pulse is gradually compressed as the transmission distance increases, and at 2mm, the time domain pulse starts to split. As can be seen from fig. 5(b), when the waveguide length is less than 2mm, the spectrum is symmetrically broadened under the influence of the self-phase modulation effect. After 2mm, the dispersion wave and the cross-phase modulation equivalent should be such that the spectrum is further broadened. Finally, in the reverse ridge AlGaAs waveguide, a supercontinuum covering the range from 2.2 mu m to 14.5 mu m is obtained at the level of-40 dB, and the spectrum extends into the middle infrared region and exceeds 2.7 octaves.

Fig. 6(a) is a frequency comb diagram obtained by injecting 50 hyperbolic secant pulse trains having a repetition frequency of 100MHz into the inverted ridge type aluminum gallium arsenic waveguide, and the center wavelength, full width at half maximum, and peak power of the preferred hyperbolic secant pulses are the same as those in the above example. Fig. 6(b) and (c) are enlarged views of the detail at 35THz and 105THz, respectively, and the window size of sampling is 500 MHz. As can be seen from FIG. 6, the frequency comb generated by the inverse ridge AlGaAs waveguide has the interval of 100MHz between the comb teeth, and the spectral line shows a fine and dense comb tooth in the sampling bandwidth, and the amplitudes of the comb teeth are substantially equal. Therefore, the frequency comb with stable amplitude and equal intervals can be obtained through the inverse ridge type AlGaAs waveguide.

Although the invention has been described in detail above with reference to a general description and specific examples, it will be apparent to one skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (4)

1. An inverse ridge AlGaAs waveguide for generating a high coherence octave mid-infrared supercontinuum and frequency comb, comprising: a wave-guiding layer and a substrate; the material of the wave guide layer is Al_0.18Ga_0.82As, the substrate material is Al_0.8Ga_0.2As；

The Al is_0.18Ga_0.82The width of the As wave guide layer is W;

the Al is_0.18Ga_0.82As waveguide layer partially embedded with Al_0.8Ga_0.2As substrate, partially not embedded with Al_0.8Ga_0.2As substrate, embedded substrate portion having a height of H_dThe height of the portion of the non-embedded substrate is H_u；

The length of the AlGaAs waveguide is 3mm, and the Al is_0.18Ga_0.82The width of the As wave guide layer is 6 μm, and the height of the non-embedded substrate part is H_u0.7 μm, and the embedded substrate portion has a height H_dAnd was 1.4 μm.

2. The AlGaAs waveguide of claim 1, wherein the AlGaAs waveguide is an inverted ridge waveguide having 3 zero dispersion points, wherein the 3 zero dispersion points are 3.74 μm, 6.56 μm and 8.89 μm, respectively; the 3 zero dispersion points divide the dispersion region into four parts, namely a normal dispersion region from 0 to 3.74 μm, an anomalous dispersion region from 3.74 μm to 6.56 μm, a normal dispersion region from 6.56 μm to 8.89 μm, and an anomalous dispersion region from 8.89 μm onward.

3. The AlGaAs waveguide of claim 1, wherein the inverse ridge AlGaAs waveguide has a nonlinear coefficient energy of 2.09W at 4.9 μm^-1m^-1。

4. A method of generating a high coherence octave mid-infrared supercontinuum and frequency comb using the inverted ridge algan waveguide according to any one of claims 1 to 3, comprising:

injecting 50 chirp-free hyperbolic secant pulse sequences with the repetition frequency of 100MHz into the reverse ridge type AlGaAs waveguide; the center wavelength of the non-chirped hyperbolic secant pulse is 4.9 microns, the full width at half maximum is 120fs, the peak power is 900W, and the random noise coefficient is 0.0001.

Producing a mid-infrared supercontinuum over 2.7 octaves at-40 dB level from 2.2 to 14.5 μm with a first order coherence of 1 over the whole spectral range;

and obtaining the frequency combs with stable amplitude and equal intervals.

Images