Near-self-similarity amplification flat light frequency comb based on rare earth doped gain nonlinear optical waveguide

Abstract

The invention provides a near self-similarity amplification flat light frequency comb based on a rare earth doped gain nonlinear optical waveguide, and belongs to the technical field of light frequency combs. A pulse light source generates a light frequency comb seed light source; the pulse compression unit is used for compressing chirped pulses of the optical pulses; the pulse shaping unit is used for shaping the compressed pulse; according to the normal dispersion rare earth doped nonlinear optical waveguide amplifier, rare earth doped ions are injected / doped into a normal dispersion nonlinear optical waveguide, and the waveguide amplifier is formed under the pumping action of pumping laser with the corresponding proper wavelength. The near self-similar amplification process evolution of the light pulse is realized in the normal dispersion rare earth doped nonlinear optical waveguide amplifier based on the combined action of normal dispersion, gain, self-phase modulation and light wave splitting. The near self-similarity amplification evolution process of the light pulse in the normal dispersion rare earth doped nonlinear optical waveguide amplifier is utilized, and the performance of the output light frequency comb in the aspects of flatness and flat bandwidth can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of optical frequency combs, and in particular to a nearly self-similar amplified flat optical frequency comb based on a rare earth doped gain nonlinear optical waveguide. Background Art

[0002] The optical frequency comb is an important multi-wavelength light source, which is widely used in precision measurement, optical communication, laser radar and other fields. At present, there are four main schemes for generating optical frequency combs, namely: 1) Based on mode-locked lasers. The frequency interval of the optical frequency comb generated by this scheme is related to the cavity length of the laser, and it is difficult to achieve large-scale continuous tuning of the frequency interval. 2) Based on electro-optical modulation. This scheme can use cascaded electro-optical modulators to modulate the amplitude and phase of the input single-frequency light, and can flexibly control the center frequency and frequency interval of the optical frequency comb. In addition, the modulator can be placed in the resonant cavity for cyclic modulation, but the spectrum envelope of the generated optical frequency comb presents a triangular shape. 3) Based on nonlinear Kerr microring resonator. This scheme uses the third-order nonlinear Kerr effect of light in the microring resonator to realize the optical comb, but it also requires precise control of the dispersion, nonlinearity, gain, loss, frequency detuning, etc. of each order in the optical microcavity to obtain a stable optical frequency comb. 4) Based on nonlinear supercontinuum spectrum broadening. This scheme mainly obtains high-repetition-rate picosecond pulses through electro-optic modulators and pulse compression, and then utilizes the dispersion and nonlinear effects in highly nonlinear waveguides to achieve spectrum broadening and spectrum envelope flattening.

[0003] The scheme of generating optical frequency comb based on resonant cavity structure is limited by the inherent cavity length and is sensitive to external environmental interference such as temperature / vibration. The scheme of optical frequency comb based on cascaded electro-optic modulators and highly nonlinear optical fiber / waveguide to achieve cavity-free supercontinuum spectrum broadening shows better reliability in terms of environmental interference resistance. At present, the scheme of realizing cavity-free optical frequency comb based on highly nonlinear optical fiber requires hundreds of meters of optical fiber, and strict dispersion management of the entire length of optical fiber is also required to prevent normal / abnormal dispersion jump problems.

[0004] In contrast, the use of small-core multi-component glass fibers or integrated optical waveguides with large nonlinear refractive index coefficients can reduce the length of nonlinear fibers / waveguides and solve the dispersion jump problem. In 2022, the University of Southampton used a 17mm long integrated silicon core fiber as a parametric mixer to achieve a 30nm bandwidth optical frequency comb with 12dB flatness by self-phase modulation to broaden the spectrum (Sohanpal R, et al. All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre, Nat Commun, 2022), but it was affected by two-photon absorption, free carrier absorption, etc. In 2024, the Pengcheng Laboratory input picosecond pulses into a 5mm long AlGaAsOI waveguide, and used self-phase modulation and four-wave mixing effects under anomalous dispersion conditions to obtain a 22nm bandwidth optical frequency comb with a flatness of 20dB (Ma C., et al. Fast-reconfigurable frequency comb generation based on AlGaAsOI waveguide with electro-optic timelens, Commun Phys, 2024), but the flatness of the optical frequency comb achieved by anomalous dispersion was poor. In 2022, Charleston University of Technology in Sweden used electro-optically modulated picosecond Gaussian pulses as pumps, and used self-phase modulation effects to achieve spectrum broadening in a normal dispersion spiral silicon nitride waveguide with a total length of 20cm, and obtained a 50nm bandwidth optical frequency comb with a flatness of 10dB (Rebolledo Salgado, et al. Coherent supercontinuum generation in all-normaldispersion Si3N4 waveguides. Optics express, 2022). Although the above schemes have made significant progress in shortening the fiber / waveguide length and broadening the spectrum, there is still much room for improvement in the flat bandwidth and flatness performance of the optical frequency comb. The main reason is that the fiber / waveguide still has large losses. Summary of the invention

[0005] The purpose of the present invention is to provide a nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide to solve at least one technical problem existing in the above-mentioned background technology. The present invention adopts a rare earth doped nonlinear optical waveguide amplifier with normal dispersion as a nonlinear transmission medium, and generates a flat optical frequency comb at 1550nm based on nonlinear supercontinuum spectrum broadening. High repetition frequency optical pulses undergo a nearly self-similar amplification evolution process under the combined effects of normal dispersion, gain, self-phase modulation and light wave splitting, thereby achieving the broadening of the optical frequency comb spectrum and the flattening of the spectrum envelope.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a flat optical frequency comb based on a rare earth doped nonlinear optical waveguide amplifier, which mainly includes: a pulse light source, a pulse compression unit, a pulse shaping unit, and a normal dispersion rare earth doped nonlinear optical waveguide amplifier;

[0008] The pulse light source is the seed light source for generating the optical frequency comb. Depending on the pulse light source, the rare earth doped nonlinear optical waveguide amplifier can be pumped by electro-optical modulation cascade pulses, mode-locked lasers, phase-locked dual-frequency lasers, and other methods to achieve a flat optical frequency comb;

[0009] The pulse compression unit is composed of a normal dispersion flat high nonlinear optical fiber / electro-optic phase modulator, a cascaded standard single-mode optical fiber / chirped fiber grating, which can realize chirped pulse compression of optical pulses, and can achieve pulse compression and peak power enhancement. The center of the compressed pulse is close to the Fourier transform limit;

[0010] The pulse shaping unit is implemented by a commercial Waveshaper or a nonlinear optical ring mirror (NOLM) and can shape the pulse into a third-order super-Gaussian pulse or a pulse of other shapes;

[0011] The normal dispersion rare earth doped nonlinear optical waveguide amplifier is to inject / dope rare earth doped ions into a normal dispersion nonlinear optical waveguide, and form a waveguide amplifier under the pumping action of a pump laser of a corresponding appropriate wavelength, so as to provide gain for the optical pulse during the transmission process. The optical pulse in the normal dispersion rare earth doped nonlinear optical waveguide amplifier realizes the evolution of a nearly self-similar amplification process of the optical pulse based on the joint action of normal dispersion, gain, self-phase modulation and light wave splitting, thereby realizing the broadening of the optical frequency comb spectrum and the flattening of the spectrum envelope.

[0012] Optionally, the substrate materials of the rare earth-doped nonlinear optical waveguide include materials that are easy to realize rare earth doping ions, such as silicon nitride, lithium niobate, lithium tantalate, potassium oxide, aluminum oxide, yttrium oxide, silicon oxide, tantalum oxide, niobium oxide, tellurium oxide, gallium oxide, bismuth oxide, zinc oxide, phosphorus oxide, etc., or their mixed materials. It can also be combined with low-loss optical waveguide materials such as aluminum nitride, sulfide, silicon nitride, tantalum oxide, etc. to form a ridge waveguide, and at the same time have a strong third-order nonlinear effect.

[0013] Optionally, the rare earth doped nonlinear optical waveguide may be doped with erbium, erbium / ytterbium co-doped, ytterbium, bismuth, praseodymium, neodymium, thulium, holmium, thulium / holmium co-doped ions, etc. It may also be co-doped with bismuth, gallium, aluminum and other ions. Different doped ions may be pumped with respective appropriate wavelengths of pump laser sources, which may provide gains in different bands such as 1.5 μm band, 1 μm band, 1.3 μm band, and 2 μm band.

[0014] Optionally, the rare earth doped nonlinear optical waveguide has a length of several centimeters to several meters, has a compact spiral shape structure, and saves optical chip area. Euler bending / quasi-Euler bending is used to realize S-bend in the spiral waveguide, and the arc waveguide and the straight waveguide are connected to reduce the waveguide connection loss, only excite the fundamental mode, and suppress the high-order mode.

[0015] Optionally, the width of the rare earth doped nonlinear optical waveguide is in the order of thousands of nanometers to several micrometers, and the height is in the order of hundreds of nanometers to several micrometers. The height and width of the waveguide can be adjusted to obtain a flat normal dispersion with a suitable dispersion value. In addition, the dispersion value can be adjusted to change along different transmission positions of the waveguide. The waveguide is surrounded by a protective layer of low-loss material such as silicon dioxide, aluminum oxide or silicon oxynitride.

[0016] Optionally, the rare earth doped nonlinear optical waveguide is a multimode optical waveguide, and the waveguide loss can be as low as 1-10 dB / m when there is no corresponding pump laser pumping (ie, no gain), and the waveguide gain can be as high as 1-100 dB / m when there is a corresponding appropriate pump laser pumping.

[0017] Optionally, the rare-earth-doped nonlinear optical waveguide may be a fully-etched trapezoidal / rectangular structure waveguide, or may be a ridge structure waveguide or an inverted ridge structure waveguide.

[0018] Optionally, the pulse width of the pulse light source is between 0.1 ps and 100 ps.

[0019] Optionally, the pulse compression unit mainly includes a quartz-based normal dispersion high nonlinear optical fiber / electro-optic phase modulator, and a cascaded standard single-mode optical fiber / chirped fiber grating, which can achieve pulse narrowing and peak power enhancement.

[0020] Optionally, the pulse shaping unit is implemented using commercial Waveshaper or nonlinear optical ring mirror NOLM and other methods, which can shape the pulse into Gaussian pulses and super Gaussian pulses or pulses of other shapes, thereby improving the performance of the final optical frequency comb, increasing the spectral bandwidth of the optical frequency comb and improving the flatness.

[0021] Beneficial effects of the present invention: In a normal dispersion rare earth doped nonlinear optical waveguide amplifier with gain, the optical pulse can continuously obtain gain during the transmission process, so that the power of the optical pulse continues to increase. This not only enhances the nonlinear effect inside the optical waveguide, but also enables the generation of an optical frequency comb with good performance within a relatively short optical waveguide length, thereby increasing the spectrum bandwidth and improving the flatness. Secondly, the rare earth doped nonlinear optical waveguide is designed as a compact spiral structure based on Euler bending / quasi-Euler bending, which effectively saves the area of ​​the optical chip. Finally, during the transmission of the optical pulse in the rare earth doped nonlinear optical waveguide amplifier, normal dispersion, gain, self-phase modulation, light wave splitting, etc. work together to achieve near self-similar amplification of the optical pulse, and the output pulse is a parabolic-shaped pulse with a near-linear chirp. In the frequency domain, the spectrum envelope corresponding to the parabolic-shaped pulse presents a parabolic-shaped shape, and the central area is relatively flat. Therefore, the optical frequency comb generated by the near self-similar amplification process has been significantly improved in terms of flatness and flat bandwidth, thereby improving the overall performance of the optical frequency comb.

[0022] Additional aspects and advantages of the present invention will be given in part in the following description, which will become obvious from the following description, or may be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0024] Figure 1 Schematic diagram of a system for generating a flat optical frequency comb in a rare-earth doped nonlinear optical waveguide amplifier based on normal dispersion according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the fully etched erbium-doped lithium niobate optical waveguide structure according to an embodiment of the present invention.

[0026] Figure 3 The time domain pulse and frequency domain spectrum evolution of the optical frequency comb during the transmission process of the initial chirp-free hyperbolic secant pulse in the erbium-doped lithium niobate gain nonlinear optical waveguide amplifier described in the embodiment of the present invention; wherein, Figure 3 (a) is the time domain, Figure 3 (b) is the frequency domain spectrum envelope.

[0027] Figure 4 When the gain coefficient of the erbium-doped lithium niobate gain nonlinear optical waveguide amplifier is changed to 5dB, 0dB and loss 3dB respectively, the time domain pulse and frequency domain spectrum envelope of the final output optical frequency comb are compared. The other conditions are the same as Figure 3 The same; among them, Figure 4 (a) is the time domain, Figure 4 (b) is the frequency domain spectrum envelope.

[0028] Figure 5 This is a comparison diagram of the time domain pulse and frequency domain spectrum of the optical frequency comb output after the pulse shaper is used to shape the pulse into a hyperbolic secant pulse, a Gaussian pulse, a third-order super-Gaussian pulse, and a fifth-order super-Gaussian pulse, and the pulse is passed through an erbium-doped lithium niobate gain nonlinear optical waveguide amplifier; wherein, Figure 5 (a) is the time domain, Figure 5 (b) is the frequency domain spectrum envelope.

[0029] Figure 6 The time domain pulse / pulse chirp of the phase-locked dual-frequency laser after pulse compression and the corresponding frequency domain spectrum diagram according to the embodiment of the present invention; wherein, Figure 6 (a) is the time domain, Figure 6 (b) is the frequency domain spectrum.

[0030] Figure 7 The time domain pulse and frequency domain spectrum diagram of the optical frequency comb outputted by the phase-locked dual-frequency laser according to the embodiment of the present invention after pulse compression without pulse shaping and direct pumping of the erbium-doped lithium niobate gain nonlinear optical waveguide amplifier; wherein, Figure 7 (a) is the time domain, Figure 7 (b) is the frequency domain spectrum.

[0031] Figure 8 The phase-locked dual-frequency laser described in the embodiment of the present invention is first subjected to pulse compression, and then the compressed pulse is shaped into a hyperbolic secant pulse, a Gaussian pulse, and a third-order super-Gaussian pulse, and then input into a normal dispersion erbium-doped lithium niobate gain nonlinear optical waveguide amplifier, and then a comparison diagram of the time domain pulse and frequency domain spectrum envelope of the output optical frequency comb; wherein, Figure 8 (a) is the time domain, Figure 8 (b) is the frequency domain spectrum. DETAILED DESCRIPTION

[0032] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below by the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention.

[0033] It should be understood by those skilled in the art that unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention belongs.

[0034] It should also be understood that terms, such as those defined in commonly used dictionaries, should be understood to have a meaning consistent with that in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless as defined herein.

[0035] Those skilled in the art will appreciate that, unless otherwise stated, the singular forms "a", "an", "said" and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements and / or groups thereof.

[0036] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

[0037] In the description of this specification, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0038] In the description of this specification, the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present technology and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present technology.

[0039] Unless otherwise clearly specified or limited, the terms "installed", "connected", "connected", and "set" should be understood in a broad sense, for example, they can be fixedly connected or set, or detachably connected or set, or integrally connected or set. For ordinary technicians in this field, the specific meanings of the above terms in this technology can be understood according to specific circumstances.

[0040] To facilitate understanding of the present invention, the present invention is further explained below with reference to specific embodiments in conjunction with the accompanying drawings, and the specific embodiments do not constitute a limitation on the embodiments of the present invention.

[0041] Those skilled in the art should understand that the drawings are merely schematic diagrams of embodiments, and the components in the drawings are not necessarily necessary for implementing the present invention.

[0042] A flat optical frequency comb is realized based on near self-similar amplification in normal dispersion rare earth doped nonlinear optical waveguide amplifier. In a normal dispersion rare earth doped nonlinear integrated optical waveguide amplifier with gain, based on the combined effects of normal dispersion, gain, self-phase modulation, and light wave splitting, high repetition rate pulses undergo a near self-similar amplification evolution process to produce a flat optical frequency comb near the 1550nm band. Thin-film lithium niobate is selected as the base material of the waveguide due to its good nonlinear and electro-optical properties; in order to realize the optical frequency comb in the 1550nm band, erbium ions Er are selected. 3+ (It can also be erbium-ytterbium co-doped ions, bismuth-gallium-erbium-aluminum co-doped ions) as rare earth doping elements, and pumping with a 1480nm pump laser source (it can also be a 980nm pump laser source) can form an erbium-doped lithium niobate nonlinear optical waveguide amplifier.

[0043] The process of pulse propagation in a nonlinear waveguide with gain can be modeled and analyzed using the nonlinear Schrödinger equation (NLSE) with constant gain g:

[0044]

[0045] The pulse slowly varying envelope amplitude A means A(z,T)=A p (z,T)exp[iφ p (z,T)], where A p(z,T) represents the amplitude of the pulse, φ p (z,T) represents the phase of the pulse, and the asymptotic solution is obtained using an analytical method.

[0046] Solving equation (1) in the normal dispersion range (β2>0), when | T | ≤T p (z), the asymptotic solution is expressed as:

[0047] A(z,T)=A p (z,T)exp[iφ p (z,T)](2)

[0048]

[0049]

[0050] From this asymptotic solution, we can get that when |T|≤T p (z), the pulse width T p (z) with amplitude A p (z) changes linearly, the pulse width and amplitude change exponentially, and the pulse can evolve into a parabolic shape, where U in is the input pulse energy. The instantaneous frequency chirp of the pulse envelope is Ω(z,T) = -dφ(z,T) / dT. From equation (4), we can get that the pulse phase changes as a quadratic change of time T, which indicates that the amplified pulse can evolve into a linear chirped pulse. This solution can be called a self-similar solution. Due to this similarity, that is, the pulse width and amplitude change exponentially, after a sufficiently long distance, the pulse can evolve into a parabolic pulse with a linear chirp. Then the parabolic pulse evolves in a self-similar amplification manner, and the shape and chirp of the pulse remain unchanged.

[0051] According to the asymptotic solution analysis obtained by self-similar amplification, the self-similar amplified pulse generated by the combined effects of gain, nonlinearity and normal dispersion will have a parabolic shape and a linear chirp, with the central part showing good flatness characteristics. The linear chirp indicates that the shape of the time domain pulse and the frequency domain spectrum envelope are similar. Therefore, a flat optical frequency comb frequency domain spectrum can be obtained based on the process of self-similar amplification.

[0052] Figure 1This is a schematic diagram of generating a flat optical frequency comb in the 1550nm band based on a normal dispersion rare earth doped nonlinear optical waveguide amplifier in this embodiment. The optical frequency comb generation system mainly includes a pulse light source, a pulse compression unit, a pulse shaping unit and a normal dispersion rare earth doped nonlinear waveguide amplifier. According to the different pulse light sources, the generation scheme of the optical frequency comb in this embodiment selects two types of pulses, electro-optical modulated cascade pulses and phase-locked dual-frequency lasers, and pumps the normal dispersion rare earth doped nonlinear optical waveguide amplifier to achieve a flat optical frequency comb.

[0053] The first solution is to use an electro-optical modulation cascade to achieve a high repetition rate pulse light source. The repetition rate of the pulse light source is determined by the modulation frequency of the electro-optical modulator. Since the pulse width of the selected light source is generally in the range of 0.1ps to 1ps, there is no need to use a pulse compression unit in the process of generating an optical frequency comb through pulse transmission. The implementation system mainly includes three units: a pulse light source, a pulse shaping unit, and a rare earth doped nonlinear optical waveguide amplifier.

[0054] The input peak power P0 of the pulse light source is 25W, and the 1 / e half width T0 of the pulse is 0.5ps. In this embodiment, an initial chirp-free (chirp coefficient C=0) hyperbolic secant pulse is used as the input light pulse, and the formula of the hyperbolic secant pulse is:

[0055]

[0056] In this embodiment, the doping ions in the normal dispersion rare earth doped nonlinear optical waveguide amplifier are erbium ions. 3+ , so that the waveguide gain of the TE0 fundamental mode is 5dB / m. The base material of the waveguide is thin-film lithium niobate (materials with high third-order nonlinear effects, such as silicon nitride, lithium tantalate, etc.). The erbium-doped thin-film lithium niobate optical waveguide is surrounded by silicon dioxide material. The erbium-doped thin-film lithium niobate optical waveguide is a fully etched trapezoidal structure waveguide. The upper base width of the waveguide is thousands of nanometers, the thickness is hundreds of nanometers, and the inclination angle is between 60° and 80°. The dispersion control optimization of the optical waveguide should be based on the transmission length formula of the light wave splitting, and the nonlinear coefficient γ, the peak power of the pump pulse, the pulse width, the waveguide gain and other parameters should be comprehensively considered for selection. Ensure that the normal dispersion and nonlinear coefficient of the optical waveguide are more appropriate, and a relatively flat optical frequency comb can be obtained. In order to obtain a flat normal dispersion of appropriate size, the width, height and angle of the optical waveguide can be adjusted. Finally, the height H = 650nm, the width W = 2800nm, and the angle θ = 75° of the erbium-doped lithium niobate optical waveguide are selected. Through finite element simulation, the second-order dispersion of the TE0 fundamental mode of the erbium-doped lithium niobate waveguide at a wavelength of 1550nm is obtained as β2=50.6159ps 2 / km, third-order dispersion is β3 = 0.0379146ps 3 / km, nonlinear coefficient γ=0.490276181W -1 ·m -1 .

[0057] In the process of realizing a flat optical frequency comb by using nearly self-similar amplification in rare-earth-doped gain nonlinear optical waveguide amplifiers, the combined effects of gain, normal dispersion, self-phase modulation, and light wave splitting play a key role in the broadening and flattening of the spectrum. The self-similar amplified pulse generated will have a parabolic shape and a linear chirp, with the central part showing good flatness characteristics. The linear chirp indicates that the shape of the time domain pulse and the frequency domain spectrum envelope are similar, so a flat optical frequency comb can be obtained according to the process of self-similar amplification. In a waveguide amplifier with a gain coefficient of g, the energy of the optical pulse increases with the increase of the transmission distance, so the self-phase modulation effect is significantly enhanced. At this time, the effective length of the waveguide is expressed as L eff =[exp(gL)-1] / g, the effective length of the waveguide is much longer than the waveguide length L, so compared with ordinary undoped and gainless optical waveguides, the use of rare earth doped nonlinear optical waveguide amplifiers can quickly broaden the spectrum within a relatively short waveguide length. The use of optical wave splitting can improve the frequency envelope flatness of the optical frequency comb. In the case of no gain, the bandwidth of the optical frequency comb is equal to the normal dispersion waveguide length Z described in the occurrence of optical wave splitting. OWB It is related to the nonlinear coefficient, the initial pulse peak power, the waveguide second-order dispersion and the initial pulse width. The empirical formula is:

[0058]

[0059] Among them, ω SPM represents the maximum frequency shift of spectrum broadening caused by self-phase modulation during light wave splitting, and ω0 represents the center frequency of the pulse. Therefore, in order to obtain an optical frequency comb with a large spectrum bandwidth and good flatness, it is necessary to reasonably set the values ​​of β2, γ, P0, and T0 parameters. In the case of gain in the optical waveguide, for formula (8), the optical frequency comb broadens the bandwidth even wider. For formula (9), the waveguide transmission distance required for light wave splitting is shorter.

[0060] In order to avoid the overlapping of adjacent pulses that may occur during the pulse evolution process, the length of the normal dispersion gain nonlinear optical waveguide is set to 1.5m. In order to save the area of ​​the optical chip, a compact helical optical waveguide structure is used. Euler bending / quasi-Euler bending is used to realize S-bend in the helical waveguide, and the arc waveguide and straight waveguide are connected to reduce the waveguide connection loss, only excite the fundamental mode, and suppress the high-order mode.

[0061] When the pulse is transmitted in a normal dispersion erbium-doped lithium niobate gain nonlinear optical waveguide amplifier, the time domain / frequency domain evolution of the generated optical frequency comb is shown in the following figure: Figure 3As shown in the figure, since the optical pulse can continuously obtain gain in the erbium-doped lithium niobate gain nonlinear optical waveguide, as the transmission distance increases, the peak power of the time domain pulse continues to increase (different from the case of a waveguide with no gain and loss), and the shape of the pulse also changes, gradually evolving into a parabolic shape. For the frequency domain spectrum, at the beginning of the transmission process, the spectrum begins to broaden under the action of normal dispersion, gain, and self-phase modulation. As the transmission distance increases, under the combined action of gain, normal dispersion, light wave splitting, and self-phase modulation, the spectrum is further broadened, and the flatness of the spectrum envelope is significantly improved. Finally, after a transmission distance of 1.5m, an optical frequency comb with a 3dB flat bandwidth of 50nm is generated in the 1550nm band.

[0062] Compare the cases where pulses generate optical frequency combs in nonlinear optical waveguides with different gain coefficients / loss characteristics. Other conditions remain unchanged, only the gain coefficient / loss characteristics of the optical waveguide are changed, and the time domain pulses and frequency domain spectra of the output optical frequency combs are compared, such as Figure 4 As shown. It can be observed that with the increase of the gain coefficient, the flatness and flat bandwidth of the output optical frequency comb spectrum are significantly improved. Specifically, when the loss of the lithium niobate waveguide is 3dB / m (i.e. g = -3dB / m), a 4dB flat bandwidth of 25nm is generated; when the gain provided by the erbium-doped lithium niobate gain nonlinear waveguide is balanced with the loss in the waveguide (i.e. g = 0dB / m), a 3dB flat bandwidth of 32nm is generated; when the gain provided by the erbium-doped lithium niobate gain nonlinear waveguide reaches 5dB / m, a 3dB flat bandwidth of 50nm is generated, and the flat bandwidth and flatness of the output optical frequency comb are significantly improved. Therefore, using a normal dispersion rare earth doped nonlinear optical waveguide amplifier as a nonlinear transmission medium for optical pulses to achieve supercontinuum spectrum broadening can significantly optimize and improve the performance of the output optical frequency comb.

[0063] The effects of pulses of different shapes on the optical frequency comb were compared and studied. The input high repetition frequency pulses were shaped into Gaussian pulses, third-order super-Gaussian pulses, and fifth-order super-Gaussian pulses using a pulse shaper. Subsequently, the shaped pulses were input into an erbium-doped lithium niobate nonlinear optical waveguide amplifier, and the time domain and frequency domain spectrum envelopes of the optical frequency comb generated by pulses of different shapes were simulated, such as Figure 5 As shown in the figure, it can be seen that when the input pulse is a third-order Gaussian pulse, the 2dB flat bandwidth of the output optical frequency comb reaches 85nm. Compared with the hyperbolic secant pulse, the flat bandwidth of the optical frequency comb is significantly increased. Therefore, pulse shaping can significantly improve the performance of the output optical frequency comb.

[0064] The second solution is to use a phase-locked dual-frequency laser as a pulse light source. Since the pulse width of the selected phase-locked dual-frequency laser light source is large, a pulse compression unit needs to be introduced during the pulse transmission process. Therefore, this solution mainly includes a phase-locked dual-frequency laser pulse light source, a pulse compression unit, a pulse shaping unit, and a rare-earth-doped nonlinear optical waveguide amplifier to realize the optical frequency comb.

[0065] The phase-locked dual-frequency laser is generated by a DFB laser. The total power P of the dual-frequency laser input is total is 1W, the ratio of high-frequency laser power to low-frequency laser power is x=1, and the laser frequency interval v is selected m =0.1THz (can be changed as needed), the wavelengths of the dual-frequency laser are 1549.6nm and 1550.4nm respectively, and the expression can be written as:

[0066]

[0067] The pulse compression unit uses normal dispersion flat high nonlinear fiber and standard single-mode fiber, which can perform chirped pulse compression on the phase-locked dual-frequency laser to achieve pulse width narrowing and peak power improvement. The second-order dispersion, nonlinear coefficient, loss, and fiber length of normal dispersion flat high nonlinear fiber are 2ps respectively. 2 / km, 10.7W -1 km -1 , 0.8dB / km, 150m, the second-order dispersion, nonlinear coefficient, loss, and fiber length of standard single-mode optical fiber are -20.5ps respectively 2 / km, 1.3W - 1 km -1 , 0.2dB / km, 85m. The length selection of the two optical fibers is related to the total power, power ratio, and frequency interval of the phase-locked dual-frequency laser. The phase-locked dual-frequency laser is affected by self-phase modulation in a high-nonlinear optical fiber with normal dispersion, and a quasi-linear chirp is generated in the center of the time-domain pulse, causing spectrum broadening. However, the standard single-mode optical fiber has anomalous dispersion in the 1550nm band, and the chirp it produces is opposite to the chirp introduced by the high-nonlinear optical fiber in the center of the pulse. Therefore, the pulse will be compressed in the single-mode optical fiber. The pulse after chirped pulse compression is as follows: Figure 6 As shown, the peak power of the pulse is about 5.6W.

[0068] The compressed pulse is input into the normal dispersion rare earth doped nonlinear optical waveguide amplifier. In this embodiment, the same waveguide material and waveguide structure as the first solution are selected. The height H of the erbium-doped lithium niobate waveguide is 650nm, the width W is 2800nm, and the angle θ is 75°. The second-order dispersion, nonlinear coefficient, and gain of the waveguide are 50.6159ps respectively. 2 / km, 0.490276181W -1 ·m -1 , 5dB / m. In order to avoid pulse overlap, the length of the erbium-doped lithium niobate waveguide in the phase-locked dual-frequency laser solution is selected to be 3.2m. Since the required erbium-doped lithium niobate waveguide is relatively long, the optical waveguide also adopts a spiral structure design to save the area of ​​the integrated optical chip. In the erbium-doped lithium niobate gain waveguide, normal dispersion, gain, self-phase modulation and light wave splitting are used to realize a nearly self-similar amplification process, achieve the broadening and flattening of the optical frequency comb spectrum, and generate a 5dB flat bandwidth of 55nm in the 1550nm band.

[0069] The influence of the input pulse shape on the performance of the generated optical frequency comb is compared and studied, and the chirped compressed pulse is input into the pulse shaping unit. The pulse shaping unit generally uses a commercial Waveshaper or a nonlinear optical ring mirror NOLM, which can shape the pulse into a variety of shapes such as hyperbolic secant pulses, Gaussian pulses, and third-order super-Gaussian pulses. Subsequently, it undergoes a nearly self-similar amplification evolution process in a normal dispersion erbium-doped lithium niobate nonlinear optical waveguide amplifier to achieve broadening and flattening of the optical frequency comb spectrum. Figure 8 The output optical frequency comb of pulses of different shapes in a normal dispersion erbium-doped lithium niobate gain waveguide amplifier is shown. Preferably, when the input pulse is shaped into a Gaussian pulse, the oscillation of the leading and trailing edges of the pulse is significantly weakened, and the 3dB flat bandwidth of the generated optical frequency comb is about 60nm, and its 2dB flat bandwidth is 52nm, that is, the flatness of the output optical frequency comb can be improved from 5dB to within 2dB after pulse shaping.

[0070] Those skilled in the art should understand that Figure 1 The number of various elements shown for simplicity may be less than the number of an actual optical frequency comb generation system, but such omission is undoubtedly based on the premise that it will not affect the clear and complete disclosure of the embodiments of the invention.

[0071] In summary, the optical frequency comb generation system of the embodiment of the present invention mainly realizes a broadband flat optical frequency comb with continuously tunable frequency interval in the 1550nm band, which is beneficial to the fields of ultra-high-speed coherent optical communication, microwave photonics, etc. The present invention realizes a flat optical frequency comb with nearly self-similar amplification based on a rare earth doped nonlinear waveguide amplifier. In a normal dispersion rare earth doped nonlinear optical waveguide amplifier, the spectrum of the optical frequency comb is broadened and flattened by utilizing the combined effects of normal dispersion, gain, self-phase modulation and light wave splitting. The realization of the optical frequency comb mainly includes a high repetition frequency pulse light source, a pulse compression unit, a pulse shaping unit and a normal dispersion rare earth doped nonlinear optical waveguide amplifier. A high repetition frequency pulse or a phase-locked dual-frequency laser is used as an input pulse light source; the pulse compression unit is used to narrow the pulse and increase the pulse peak power; the pulse shaping unit shapes the pulse shape, thereby improving the performance of the output optical frequency comb; the normal dispersion rare earth doped nonlinear optical waveguide amplifier is used as a nonlinear medium for optical pulse transmission, and a flat optical frequency comb is realized based on nonlinear supercontinuum spectrum broadening. In order to save the area of ​​optical chip and optimize the device size, the rare earth doped nonlinear waveguide adopts a compact spiral structure based on Euler bending / quasi-Euler bending. The present invention utilizes the nearly self-similar amplification evolution process of optical pulses in the normal dispersion rare earth doped nonlinear optical waveguide amplifier to effectively improve the performance of the output optical frequency comb in terms of flatness and flat bandwidth.

[0072] The above description is only a preferred specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make various changes and modifications without departing from the spirit and scope of the present invention. For example, other materials with reasonable length and normal dispersion and high third-order nonlinearity are selected as optical waveguides or optical fibers, and various rare earth ions are doped such as erbium, erbium / ytterbium co-doping, ytterbium, bismuth, praseodymium, neodymium, thulium, holmium, thulium / holmium co-doping, co-doping with bismuth, gallium, aluminum and other ions to form various types of rare earth-doped nonlinear optical waveguide amplifiers in different bands, such as rare earth-doped silicon nitride optical waveguide amplifiers, rare earth-doped TeO2 tellurite optical waveguide amplifiers, rare earth-doped Ta2O5 optical waveguide amplifiers, rare earth-doped LiNbO3 optical waveguide amplifiers, rare earth-doped LiTaO3 optical waveguide amplifiers, rare earth-doped GeSbS amplifiers, rare earth-doped GeSbSe chalcogenide optical waveguide amplifiers and other high nonlinear integrated optical waveguide amplifiers or optical fiber amplifiers. Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative work on the basis of the technical solution disclosed in the present invention should be included in the scope of protection of the present invention.

Claims

1. A nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide, characterized in that: include: Pulse light source, pulse compression unit, pulse shaping unit, normal dispersion rare earth doped nonlinear optical waveguide amplifier; The pulse light source is used to generate a seed light source for an optical frequency comb; The pulse compression unit is composed of a normal dispersion flat high nonlinear optical fiber / electro-optic phase modulator, a cascaded standard single-mode optical fiber / chirped fiber grating, and is used to compress the chirped pulse of the optical pulse to achieve pulse compression and peak power enhancement. The center of the compressed pulse is close to the Fourier transform limit. The pulse shaping unit is used to shape the compressed pulse; The normal dispersion rare earth doped nonlinear optical waveguide amplifier is to inject / dope rare earth doped ions into a normal dispersion nonlinear optical waveguide, and form a waveguide amplifier under the pumping action of a pump laser of a corresponding appropriate wavelength, so as to provide gain for the optical pulse during the transmission process. The optical pulse in the normal dispersion rare earth doped nonlinear optical waveguide amplifier realizes the evolution of a nearly self-similar amplification process of the optical pulse based on the combined action of normal dispersion, gain, self-phase modulation and light wave splitting, thereby realizing the broadening of the optical frequency comb spectrum and the flattening of the spectrum envelope.

2. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The substrate material of the rare earth doped optical waveguide is a mixed material of one or more of silicon nitride, lithium niobate, lithium tantalate, potassium oxide, aluminum oxide, yttrium oxide, silicon oxide, tantalum oxide, niobium oxide, tellurium oxide, gallium oxide, bismuth oxide, zinc oxide or phosphorus oxide.

3. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The rare earth doping ions in the waveguide are erbium, erbium / ytterbium co-doped, ytterbium, bismuth, praseodymium, neodymium, thulium, holmium or thulium / holmium co-doped ions, or co-doped bismuth, gallium or aluminum ions; different doping ions use pump laser sources with corresponding appropriate wavelengths to provide gains in different bands.

4. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The rare earth doped nonlinear optical waveguide has a helical structure, and uses Euler bending / quasi-Euler bending to realize S-bend in the helical waveguide, and connects the arc waveguide and the straight waveguide.

5. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: By adjusting the height and width of the rare-earth doped nonlinear optical waveguide, a flat normal dispersion with a suitable dispersion value can be obtained; Alternatively, the dispersion value is adjusted to change at different transmission positions along the waveguide; the waveguide is surrounded by a silicon dioxide / aluminum oxide / silicon oxynitride protective layer.

6. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The rare earth doped nonlinear optical waveguide is a multimode optical waveguide, and the waveguide loss can be as low as 1-10 dB / m when there is no corresponding pump laser pumping, and the waveguide gain can be as high as 1-100 dB / m when there is a corresponding appropriate pump laser pumping.

7. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The rare earth doped nonlinear optical waveguide is a fully etched trapezoidal / rectangular structure waveguide, a ridge structure waveguide or an inverted ridge structure waveguide.

8. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The pulse width of the pulse light source is between 0.1 ps and 100 ps.

9. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The pulse compression unit comprises a quartz-based normal dispersion flat high nonlinear optical fiber / electro-optic phase modulator and a cascaded standard single-mode optical fiber / chirped optical fiber grating to achieve pulse narrowing and peak power enhancement.

10. The nearly self-similar amplified flat optical frequency comb based on rare earth doped gain nonlinear optical waveguide according to claim 1, characterized in that: The pulse shaping unit is implemented by using a commercial Waveshaper or a nonlinear optical ring mirror (NOLM) method to shape the pulse into a Gaussian pulse or a super-Gaussian pulse, thereby improving the performance of the optical frequency comb generated in the end, increasing the spectrum bandwidth of the optical frequency comb and improving the flatness.

Images