CN105024279B - A kind of preparation method of stepped phase Bragg grating and its distributed feedback semiconductor laser - Google Patents
A kind of preparation method of stepped phase Bragg grating and its distributed feedback semiconductor laser Download PDFInfo
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Abstract
A kind of structure of stepped phase Bragg grating, wherein stepped phase Bragg grating be periodicity phase-shifted grating modulation format be withzAxle is in uniform stepped distribution, a width of P of each step of the ladder, the cycle of corresponding periodicity phase shift, a height of α of step corresponds to the phase shift size of periodicity phase shift, because the phase shift size is presented using 2 π as the cycle to grating performance impact, i.e., the grating of two 2 π phase shifts of difference has identical reflectance spectrum；Therefore only need byIt is taken as。
Description
Technical Field
The invention belongs to the technical field of photoelectrons, relates to optical fiber communication, photonic integration and other photoelectric information processing, and provides a fiber Grating, a planar waveguide Grating, a distributed feedback laser and a multi-wavelength laser array based on a step-phase Bragg Grating.
Background
The Bragg grating is specific periodic refractive index modulation, can produce special filter characteristic in certain wave band or some wave bands in the optical waveguide, common Bragg grating includes the fiber Bragg grating and the planar waveguide Bragg grating, its difference lies in the difference of the corresponding optical waveguide, the former waveguide is the optic fibre, the latter is the planar waveguide [1]. The fiber bragg grating is widely applied to fiber communication and fiber sensing systems, the manufacturing method mainly adopts a holographic exposure method utilizing the photosensitive characteristic of the fiber, and the applications of the fiber bragg grating include light filtering, dispersion compensation, wavelength division multiplexing and the like. The planar waveguide Bragg grating is mainly applied to information processing of distributed feedback lasers and photonic integrated planar waveguides, and the functional characteristics and the analysis method of the planar waveguide Bragg grating are similar to those of an optical fiber Bragg grating.
In terms of communication, due to the increasing data transmission requirements and the gradual maturity of technologies such as generation of optical signals, fabrication of optical fibers and the like, optical carrier communication systems gradually replace electrical carrier communication systems, and the fundamental reason is that light has a large bandwidth relative to electricity, which means a higher transmission rate. For optical communication, wavelength Division Multiplexing (WDM) is a very dramatic technology, and its basic meaning is to multiplex signals simultaneously in time and space using different wavelengths of light as carriers [2]. The first use of wavelength division multiplexing is to divide the light into 1310 and 1550 bands, which correspond to the 80nm and 120nm wavelength ranges of 1310nm and 1550nm, respectively, where the loss in the optical fiber is low. With the development of technology, coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM) have been developed, where the coarse wavelength division multiplexing represents that the band is divided into sub-channels with a distance of about 20nm, so as to greatly increase the information rate transmitted by one optical fiber, and the dense wavelength division multiplexing represents that the 1550 band is divided into a plurality of sub-channels with adjacent 0.8nm (100 GHz), at the same time 150 channels can be transmitted in the same optical fiber, so as to greatly increase the information transmission rate, and therefore, with the rapid development of the current information society, the dense wavelength division multiplexing method will be an extremely important choice for future information transmission main trunk and even many branch trunks.
The communication system mainly comprises three parts: the optical fiber sensor comprises an optical signal generator, an optical signal transmission medium, namely an optical fiber, and an optical signal detector. The importance of the optical signal generator is in the first place, which provides the power for the whole system, as compared to the engine for an airplane, while the most important component of the optical signal generator is the laser. In a communication system, intensity modulation is the most commonly used modulation method of a laser, and can be divided into direct modulation and external modulation, wherein direct modulation refers to directly loading a signal on a driving current of the laser, and the method has the advantages that an optical signal generator can be simply and cheaply realized, and has the defects that the modulation rate is limited by the response characteristic of the laser, so that a higher modulation rate, such as 40GHz, cannot be achieved, and meanwhile, a signal generated by the optical signal generator has larger frequency chirp, and the transmission distance of the optical signal generator is greatly influenced due to the existence of single-mode fiber dispersion; external modulation refers to that modulation is achieved by adding a modulator outside a laser and controlling the intensity of light emitted by the laser based on an acousto-optic effect, a magneto-optic effect, a Franz-Keldysh effect, a quantum stark effect and the like, and common external modulators include an electro-absorption modulator (EML) and a mach-zehnder Modulator (MZI), which have high cost but can achieve a higher modulation rate, such as 100GHz, but do not introduce frequency chirp, so that the laser is more suitable for modern communication systems [3].
In the future, high density information transmission will lead to high density optical communication devices, and the demand for higher density optical communication will be a Photonic Integrated Chip (PIC), such as an electronic chip, and in order to achieve the processing capability for larger information rates, the electronic device gradually transits from the combination of multiple diodes to today's electronic integrated chip of billions of diodes. Therefore, the integration of the photonic device will be a processing scheme for optical communication in the near future, and meanwhile, the most central and most difficult integration of the photonic device is the integration of the semiconductor laser, on one hand, the indium-phosphorus-based material of the semiconductor laser is incompatible with the silicon-based material of the traditional optical transmission, which causes a coupling problem, and in addition, the yield of the semiconductor laser array is seriously affected due to the problem of the low yield caused by the process complexity of the semiconductor laser, and the more difficult integration is the multi-wavelength laser array used in the wavelength division multiplexing system. Since Distributed Feedback (DFB) semiconductor lasers can achieve both miniaturization of the laser and wavelength stability, DFB semiconductor lasers are the most commonly used light sources in communication systems, and their principle is to achieve single-mode lasing of the laser by introducing a pi phase shift in a uniform grating, whose lasing wavelength λ depends on the period Λ (λ =2 n) of the uniform grating eff Λ, where n eff Which is the effective refractive index of the waveguide, generally equal to about 3.2), and the wavelength interval in the dwdm system is 0.8nm, which means that the variation of the grating period of the adjacent wavelength laser is 0.125nm, i.e. the variation of the grating half period is 0.063nm, and the variation precision of the grating half period cannot be accurately realized for the electron beam exposure technology with the spot diameter of a few nanometers. Thing to doExperimental results show that]Reconstruction-equivalent chirp technique proposed by Chengxiang et al in 2007 [5]The multi-wavelength laser array with high wavelength interval precision can be realized by simple technologies such as conventional holographic exposure, micron-scale photoetching and the like, but due to the equivalent effect of the multi-wavelength laser array, the grating intensity is only about 1/3 of the actual uniform grating intensity, and the coupling efficiency of the laser is reduced, so that the narrow linewidth and the high directly-modulated rate of lasing are difficult to realize.
Reference documents:
[1] erdgan, fiber grating spectra, IEEE Journal of lightwave technology,1997,15 (8): 1277-1294
[2] Khare, fiber optics and optoelectronics, oxford University Press,2004, chapter 11
[3] N. binh, advanced digital optical communications, CRC Press,2015, chapter 3.1
[4] Y.Shi et al, high channel count and High precision channel spacing multi-wavelength laser array for future photonic integrated chips, scientific Reports,2014, DOI
[5] Y.Dai, X.Chen, DFB semiconductor laser based on reconstruction-equivalent chirp technology, optics Express,2007,15 (5): 2348-2353
Disclosure of Invention
The invention aims to provide a step phase Bragg grating, clarify the working principle and the application of the step phase Bragg grating and provide a preparation method of a distributed feedback semiconductor laser based on the grating structure. The method can be used in optical fiber and planar waveguide to provide the function of filter, and the phase shift form and equivalent phase shift form of the grating can be applied in distributed feedback semiconductor laser and its multi-wavelength array, especially in the multi-wavelength laser array of dense wavelength division multiplexing system, to overcome the problem that the wavelength interval precision of adjacent lasers can not be well controlled due to too large electron beam spot in electron beam exposure.
The technical scheme of the invention is as follows: a step phase Bragg grating and a DFB semiconductor laser based on the grating structure and a manufacturing technology of a multi-wavelength array thereof are disclosed, wherein the step phase Bragg grating is a periodic phase shift grating, and the refractive index modulation of the grating is as follows:
where z refers to the position along the axis of the grating, Δ n 1 For the refractive index modulation of a uniform grating, c.c. represents the complex conjugate where each step is flat, i.e. corresponds to a uniform grating, Λ is the period of the uniform grating, and φ (z) is the phase portion of the refractive index modulation of the grating:
phi (z) = k.alpha, k ∈ N when k.P < z < (k + 1) · P
The mode of the grating is uniformly distributed along the z axis in a step shape, the width of each step of the step is P, the width corresponds to the period of the periodic phase shift, the height is alpha, and the phase shift size corresponds to the periodic phase shift, and the influence of the phase shift size on the grating performance takes 2 pi as the period, namely, two gratings with the phase shift difference of 2 pi have the same reflection spectrum; therefore, only alpha is required to be (-pi, pi), N is a natural number, and the other expression form of the grating is a periodic phase shift grating, wherein the period is the width of the step, and the phase shift is the height of the step.
To solve for the reflection spectrum properties of the echelle phase grating, the phase portion is written as
k belongs to N when k.P<z< (k + 1). P (2)
DiscoveryIs a periodic function with P as the period, shaped like a saw-tooth, in which within the interval [0, P) of one period
Thus can obtain
Due to the fact thatIs a periodic function with P as the period, thereby obtainingAlso a periodic function with P as the period, so that Fourier expansion can be performed to obtain
Wherein F m As coefficients of respective orders after Fourier transform
The resulting refractive index modulation of the step phase can be expressed as
The grating is thus equivalent to a superposition of a number of sub-gratings, where the equivalent grating period Λ of the mth order sub-grating m Satisfy the requirements of
Namely that
The wavelength positions corresponding to the m-level sub-gratings areWherein n is eff The m-th order and m + 1-th order channels are spaced by the effective refractive index of the waveguide
In particular for 0 order sub-gratings
That is, the position of the 0-level sub-grating is determined by the uniform grating period Λ, the step phase step height α and the step phase step width P.
At the same time, the relatively uniform grating of the m-th order grating has a grating strength of
In particular, when α = π/2, the relative grating intensity of the 0 order is | F 0 | =0.9; when α = π, the relative grating intensity of the 0 order is | F 0 |＝0.64。
In addition, an echelle phase bragg grating has its true phase-shifted version and equivalent phase-shifted version:
(1) True phase shift form
An arbitrary phase shift gamma is added in the middle position of a section of ladder phase Bragg grating, so that each sub-grating of the ladder phase Bragg grating is a phase shift grating, the phase shift is also gamma, and the transmission spectrum and the reflection spectrum shape of each channel in the corresponding multi-channel reflection spectrum correspond to the transmission spectrum and the reflection spectrum of the phase shift grating.
In the middle of a segment of the grating (z = z) 0 At) z 0 Half the grating length, with a phase shift of gamma inserted, when its refractive index is modulated to
The phase shift introduced by this structure is all of the magnitude gamma for the mth order sub-grating.
(2) Equivalent phase shift form
The right part of the middle position of a section of step phase Bragg grating is translated to the right by the delta P distance, and the translated delta P distance is filled according to the form of the right half part of the grating, so that phase shift is generated in the m-level sub-grating, and the phase shift is equal toIn particular whenWith α = π, an equivalent π phase shift can be made for a 0 order sub-grating.
Specifically, the method comprises the following steps: the second half section (z is more than or equal to z) of the step phase Bragg grating 0 Where) is shifted by a distance Δ P, the refractive index is modulated by
In particular when z ≧ z 0 When the temperature of the water is higher than the set temperature,
wherein
Therefore, the Fourier transform form of the equivalent phase shift refractive index modulation can be obtained by substituting the formula (12) into
So for the m-th order sub-grating, the phase shift introduced by the structure is of the magnitudeIn particular, the phase shift of the 0-order sub-grating is pi when α = pi and Δ P = P.
The key technology to be used when preparing the fiber bragg grating or the planar waveguide grating of the step phase bragg grating is an electron beam exposure technology, the main process is to use an electron beam to carry out scanning exposure of a specified pattern on an optical fiber or a substrate covered with corresponding electron beam exposure glue, carry out glue washing after the exposure is finished, carry out chemical reaction and washing on the exposed glue, and finally carry out dry etching or wet etching to obtain the required step phase bragg grating. The true phase shift form and the equivalent phase shift form can be obtained by the same method.
The grating part structure of the distributed feedback semiconductor laser is the real phase shift or equivalent phase shift form of the stepped phase Bragg grating, wherein the phase shift can be selected within the rangeMade by an electron beam exposure method, the other parts may have the same structure as a conventional distributed feedback laser. The laser has a lasing wavelength corresponding to the Bragg wavelength of the 0-level sub-grating, which is determined by the height alpha and width P of each step of the step phase of the grating,which has the formula ofWherein n is eff The effective refractive index of the waveguide is indicated, and Λ is the grating period of the uniform grating part.
Each laser in the multi-wavelength laser array is a distributed feedback semiconductor laser with real phase shift or equivalent phase shift of step phase, the lasing wavelength is determined by a 0-level sub-phase shift grating, wherein the real phase shift or equivalent phase shift of the 0-level sub-grating is equal to that of the 0-level sub-gratingThe width P or alpha of the step phase Bragg grating is changed by each laser of the multi-wavelength laser array, and the wavelength of the 0-level sub-grating is obtained through the relational expressionThat is, each laser of the multi-wavelength laser array has a different P or α to generate a different lasing wavelength.
Therefore, under the condition of physical realization, a single-wavelength single longitudinal mode laser and a multi-wavelength laser array can be respectively prepared based on the real phase shift form and the equivalent phase shift form of the step phase Bragg grating.
When the single longitudinal mode single wavelength laser in a real phase shift form is prepared, the adopted grating is in the real phase shift form of the step phase grating, the 0-level wavelength position of the grating is in the central position of the gain peak of the material, and the +/-1-level wavelength position of the grating is relatively far away from the central position of the gain peak.
When the single longitudinal mode single wavelength laser in the equivalent phase shift form is prepared, the adopted grating is in the equivalent phase shift form of the step phase grating, the 0-level wavelength position of the grating is in the central position of the gain peak of the material, and the +/-1-level wavelength position of the grating is relatively far away from the central position of the gain peak.
When the true phase shift multi-wavelength laser array is prepared, the adopted grating is the true phase shift form of the step phase grating, and the 0-level wavelength position of each laserIs positioned close to the center of the gain peak of the material, and the +/-1 order wavelength position is relatively far away from the center of the gain peak. The width P of the step of each laser is required to be changed along with the change of the lasing wavelength of different lasers
When the multi-wavelength laser array in an equivalent phase shift form is prepared, the adopted grating is in an equivalent phase shift form of a step phase grating, the 0-level wavelength position of each laser is close to the central position of a material gain peak, and the +/-1-level wavelength position is relatively far away from the central position of the gain peak. The width P of the step of each laser is required to be changed along with the change of the lasing wavelength of different lasersIn particular, when P =20 μm, α = π/2, Λ =250nm eff And 3.2, when the wavelength of different lasers is changed by 0.8nm, the required changed step width is delta ≈ 400 × 0.8nm =320nm. For the multi-wavelength array of the conventional pi phase shift grating, the variation of the half period of the uniform grating of different lasers with the wavelength variation of 0.8nm is only delta-0.0625 nm, and the beam spot of an electron beam with the radius of a few nanometers is difficult to process.
The invention can etch in the optical fiber or the planar waveguide by using an electron beam exposure method, so that the effective refractive index modulation of the optical fiber or the planar waveguide is the refractive index modulation of the step phase Bragg grating.
The invention has the beneficial effects that: the function of the invention is to form the response of the multi-wavelength channel with equal spacing, and the wavelength corresponding to 0 level isThe wavelength interval between adjacent channels is aboutWherein n is eff Is the effective refractive index of the waveguide and the relative of the channelsThe grating strength of the uniform grating isIn particular, by adding a certain phase shift on the basis of the structure, the actual phase shift of the multiple channels can be achieved; or by shifting the second half by a length, the equivalent phase shift for a particular channel can be achieved. Therefore, the preparation of Distributed Feedback (DFB) semiconductor lasers and arrays thereof can be realized by introducing phase shift, which has the advantages of being able to manufacture laser arrays with high wavelength interval precision and simultaneously ensuring relatively large grating strength of lasing channels.
The invention uses a novel Bragg grating to manufacture the fiber grating and the plane waveguide grating, and manufactures the distributed feedback semiconductor laser and the multi-wavelength array thereof based on the Bragg grating.
Drawings
FIG. 1 step phase(a) And equivalents thereof (b);
FIG. 2 is a schematic diagram of an echelle phase Bragg grating;
FIG. 3 shows a reflection spectrum of an echelle phase Bragg grating; relative intensity value | F of sub-raster of each channel m L, where n eff ＝3.2,Δn eff =0.003, p =15 μm, Λ =242nm, (a) - (e) correspond to different values of the phase shift magnitude α:
FIG. 4 step phases of the actual phase shift form (a) and the equivalent phase shift form (b)Phase of a bit Bragg gratingA schematic diagram;
FIG. 5 typical transmission spectra of gratings of the actual phase-shifted version (a) and the equivalent phase-shifted version (b);
FIG. 6 is a schematic diagram of a true phase shift pattern of a echelle phase Bragg grating in a multi-wavelength laser array; (a), (b), (c) and (d) correspond to four wavelengths in the multi-wavelength laser array;
FIG. 7 is a schematic diagram of a method for fabricating a step-phase Bragg grating by electron beam exposure;
FIG. 8 is a schematic diagram of a method for fabricating a step phase Bragg grating by a nanoimprint lithography; a. b and c respectively correspond to three steps of a nano-imprinting method: manufacturing a nano-imprinting template, a nano-imprinting process and ion reaction etching;
1. nano-imprinting the template; 2. nano-imprinting glue; 3. substrate materials (e.g., optical fibers, planar waveguides, etc.);
FIG. 9 is a schematic diagram of a four-wavelength array of stepped phase Bragg grating based semiconductor lasers;
101. a metal n-electrode; inp buffer layer (substrate); 103. a lower waveguide layer; 104. a multi-quantum well active layer MQWs;105. a grating layer; 106. an upper waveguide layer; 107. an upper cladding layer (protective layer); 108. a ridge waveguide; 109. an ohmic contact layer; 110. a metal p-electrode; 111. an insulating layer.
Detailed Description
The step phase Bragg grating is a key technology, and corresponding fiber gratings and planar waveguide gratings can be manufactured based on the technology. The phase portion of the grating of the step phase grating is uniformly stepped (fig. 1), wherein each step is uniform, the width of each step is P, and the height is α. Another intuitive expression of a echelle phase bragg grating is a periodic phase shifted grating (fig. 2), where the period is the width P of the step and the phase shift is the height α of the step. According to the formula (8) and the formula (10), the step phase brad can be obtainedThe wavelength of different channels of the grid grating and the relative grating strength of the different channels. Based on the most common transmission matrix method for studying Bragg gratings, FIG. 3 lists the time when n is eff ＝3.2,Δn eff =0.003, p =15 μm, Λ =242nm, grating reflection spectrum at different step height values and corresponding relative intensity value | F of each channel sub-grating m L wherein It can be found that the proportion of the 0-order grating intensity relative to other orders is increased significantly with the decrease of α, and for α with the same absolute value and different signs, the influence is that the reflection spectrum is symmetrically transformed with the 0-order reflection spectrum as the center, and at the same time, under the condition that the step width P is not changed, the 0-order position of the reflection spectrum is changed with the change of α, and under the condition that α tends to be 0, the 0-order wavelength position of the reflection spectrum is closer to the bragg wavelength of the corresponding uniform grating.
The step phase has a true phase shift form and an equivalent phase shift form, the grating phase distribution of the true phase shift form is shown in fig. 4 (a), the core of the step phase grating is that a true phase shift is added into the grating on the basis of the step phase bragg grating, a phase shift grating described by formula (11) can be generated, and the transmission spectrum of the phase shift grating is shown in fig. 5 (a); the phase profile of the equivalent phase shift form of the grating is shown in fig. 4 (b), and the core is to shift the second half of the echelle phase bragg grating by a small distance, which can produce a phase shift grating as described in equation (13), and the transmission spectrum is shown in fig. 5 (b).
According to the formula (8), the bragg wavelength of the 0-level sub-grating can be determined by the step width P and the step height α, and the larger the step height, the stronger the 0-level sub-grating, and in order to ensure the grating strength of the 0-level sub-grating and the adjustability of the 0-level wavelength, the step height α is set to beIntroduction of truesThe phase shift form or equivalent phase shift form of the bragg grating ensures that the phase shift value γ of the 0-level sub-grating is pi according to the formula (11) of the real phase shift form or the formula (13) of the equivalent phase shift form, and the 0-level wavelength can be changed by changing the step width P, so that the bragg grating can be used for array narrow-band filters and multi-wavelength laser arrays, the schematic diagram of the multi-wavelength laser based on the real phase shift form of the step phase bragg grating is represented by fig. 6, the corresponding array numerical value is as shown in table one, and an eight-wavelength laser array which meets the dense wavelength division multiplexing and meets the ITU standard and has the channel interval of 0.8nm can be manufactured. Wherein the step heightγ＝π，n eff Λ =242nm, and it can be observed from the table that with a channel spacing of 0.8nm, the step width difference that needs to be changed is about 200 to 400nm, whereas with the conventional pi phase shift grating technique the half period of the seed grating is changed by 0.065nm, the error tolerance is improved by thousands of times with this method.
Step width value of eight-wavelength laser with 0.8nm channel interval
The preparation method comprises the following steps:
1. specific preparation method of fiber grating and planar waveguide grating based on stepped phase Bragg grating
(1) Electron beam exposure method
Firstly, coating a layer of uniform electron beam exposure glue, which is usually PMMA (polymethyl methacrylate), on a corresponding part of an optical fiber or a planar waveguide, then using an electron beam exposure technology, as shown in fig. 7, scanning an electron beam on the exposure glue and forming a required pattern of a stepped phase Bragg grating by changing the exposure of the electron beam, as shown in fig. 2 and fig. 6, then using an organic solvent to dissolve the PMMA with less exposure, and then using ICP (inductively coupled plasma) dry etching or wet etching based on chemical reaction to etch the material, so as to obtain the required pattern, namely the optical fiber grating or the planar waveguide grating based on the stepped phase Bragg grating. Based on the same principle of the steps, the multi-wavelength passive optical grating filter array can be prepared.
(2) Nanoimprint method
As shown in fig. 8, a is the manufacturing of a nano-imprinting template, the shape of the template is complementary to the shape of the required step phase bragg grating, the template is generally manufactured by an electron beam exposure method or a nano-soft imprinting method, the electron beam exposure method is similar to (1), the nano-soft imprinting method is to repeatedly manufacture the nano-imprinting template used here by the manufactured concave-convex complementary nano-imprinting template, and the principle is similar to the method; b is the process of nanoimprint, which changes the properties and shape of the nanoimprint paste by pressure, thereby transferring the pattern of the imprint template onto the substrate material using an ICP or ion reactive etching method in the process c.
2. Phase shift Distributed Feedback (DFB) semiconductor laser based on stepped phase Bragg grating and multi-wavelength array thereof
The structure of the distributed feedback semiconductor laser is that an epitaxial n-type InP buffer layer, an undoped lattice-matched InGaAsP waveguide layer, a strained InGaAsP multiple quantum well, an InGaAsP grating material layer, an InGaAsP waveguide layer, an InP limit layer and an InGaAs ohmic contact layer are sequentially formed on an n-type substrate material; the grating of the InGaAsP grating material layer is a step phase Bragg grating, namely a grating used for laser lasing; the surface of the equivalent grating for laser lasing adopts SiO with the thickness of 200-400nm 2 An insulating layer.
The fabrication of a DFB laser based on a stepped phase bragg grating and an array with an operating wavelength in the 1550nm range is described below.
The epitaxial material of the device is mainly made by MOVPE technique, described as follows: firstly, an n-type InP buffer layer (with the thickness of 200nm and the doping concentration of about 1.1 multiplied by 10) is epitaxially coated on an n-type substrate material for one time 18 cm -2 ) A non-doped lattice matching InGaAsP waveguide layer (lower waveguide layer) with the thickness of 100nm, a strained InGaAsP multiple quantum well (the optical fluorescence wavelength is 1.52 microns,7 quantum wells: well width of 8nm,0.5% compressive strain, barrier width of 10nm, lattice matching material) and 100nm thick p-type lattice matching InGaAsP (doping concentration of about 1.1 × 10) 17 cm -2 ) An upper waveguide layer. And then forming the grating structure of the required laser by utilizing an electron beam exposure technology through the designed real phase shift or equivalent phase shift form of the step phase Bragg grating. After the grating part is manufactured, p-InP and p-type InGaAs (100 nm, doping concentration is more than 1 × 10) are grown by secondary epitaxy 19 cm -2 ) And etching to form a ridge waveguide and a contact layer, wherein the length of the ridge waveguide is 400 microns, the width of the ridge is 3 microns, the width of a groove on the ridge side is 20 microns, and the depth of the groove is 1.5 microns. Filling SiO into the periphery of the ridge by Plasma Enhanced Chemical Vapor Deposition (PECVD) 2 Or organic BCB forms the insulating layer. Finally, a Ti-Au metal P electrode is plated.
And both end surfaces of the device are coated with anti-reflection films (AR). The typical value of the threshold current of the laser is about 8mA, and the side mode suppression ratio reaches more than 40 dB.
The method is also applicable to a multi-wavelength laser array, wherein different grating parts are manufactured by an electron beam exposure technology, different step widths P selected by different lasers are different, the secondary epitaxy process is respectively repeated to form a plurality of ridge waveguides, and the effect image of the 4-wavelength laser array is as shown in figure 9.
Claims (7)
1. A structure of echelle phase bragg grating, wherein the echelle phase bragg grating is a periodic phase shift grating, and the refractive index of the grating is modulated as follows:
where z refers to the position along the axis of the grating, Δ n 1 For the refractive index modulation of a uniform grating, c.c. represents the complex conjugate where each step is flat, i.e. corresponds to a uniform grating, Λ is the period of the uniform grating, and φ (z) is the phase portion of the refractive index modulation of the grating:
phi (z) = k.alpha, k ∈ N when k.P < z < (k + 1) · P
The mode of the grating is uniformly distributed along the z axis in a step shape, the width of each step of the step is P, the width corresponds to the period of the periodic phase shift, the height of the step is alpha, the phase shift corresponds to the size of the periodic phase shift, and the influence of the size of the phase shift on the grating performance takes 2 pi as the period, namely, two gratings with the phase shift difference of 2 pi have the same reflection spectrum; therefore, only alpha needs to be (-pi, pi), and N is a natural number.
2. The structure of echelle phase bragg grating as claimed in claim 1 wherein said echelle phase bragg grating has a plurality of uniform sub-gratings like a sampled grating, the reflection spectrum properties of which also exhibit a multi-channel property, each channel corresponding to a sub-grating of one of the stages, the phase portion being written as
When k.P<z< (k + 1). P (2)
Is a periodic function with period P, shaped like a saw-tooth, in which the interval [0, P) of one period
Thus obtaining
Due to the fact thatIs a periodic function with P as the period, thus obtainingIs also a periodic function with P as the period, then Fourier expansion is carried out, and the method can be obtained
Wherein F m As coefficients of respective orders after Fourier transform
The resulting refractive index modulation of the step phase can be expressed as
The grating is thus equivalent to a superposition of many sub-gratings, where the equivalent grating period Λ of the m-th order sub-grating m Satisfy the requirement of
Namely, it is
The wavelength positions corresponding to the m-level sub-gratings areWherein n is eff The m-th order and m + 1-th order channels are spaced by the effective refractive index of the waveguide
The relatively uniform grating of the mth order grating has a grating strength of
3. The method for manufacturing an optical fiber grating or a planar waveguide grating based on the echelle phase bragg grating as claimed in claim 1 or 2, wherein an arbitrary phase shift γ is added to the middle of a section of the echelle phase bragg grating in the true phase shift form of the echelle phase bragg grating, so that each sub-grating of the echelle phase bragg grating is a phase shift grating, the phase shift is γ, and the transmission spectrum and the reflection spectrum of each channel in the corresponding multi-channel reflection spectrum have the shape corresponding to the transmission spectrum and the reflection spectrum of the phase shift grating.
4. The method for preparing a fiber grating or a planar waveguide grating based on a echelle phase bragg grating as claimed in claim 1 or 2, characterized in that the equivalent phase shift form of the echelle phase bragg grating translates the right part of the middle position of a section of echelle phase bragg grating to the right by the Δ P distance and fills the translated Δ P distance in the form of the right half grating, which acts to generate a phase shift in the m-level sub-grating with a phase shift of the magnitude of
5. The method for preparing the fiber grating or the planar waveguide grating of the echelle phase Bragg grating as claimed in claim 4, wherein the step phase Bragg grating is prepared by a method comprising the steps ofWith α = π, an equivalent π phase shift is formed for the 0 order sub-grating.
6. A method for preparing distributed feedback semiconductor laser, which is characterized by using the planar waveguide grating based on step phase Bragg grating in true phase shift mode as claimed in claim 3 or the planar waveguide grating based on step phase Bragg grating in equivalent phase shift mode as claimed in claim 4, wherein the phase shift is selected in the rangeIs made by an electron beam exposure method, and the other parts have the same structure as a traditional distributed feedback laser; the laser has a lasing wavelength corresponding to the Bragg wavelength of the 0-level sub-grating, which is determined by the height alpha and width P of each step of the step phase of the grating, and the formula isWherein n is eff The effective refractive index of the waveguide is indicated, and Λ is the grating period of the uniform grating part.
7. A method according to claim 6, wherein the multi-wavelength laser array is formed by real phase-shifting or equivalent phase-shifting distributed feedback semiconductor lasers with step phase, the lasing wavelength is determined by 0-order sub-phase-shifting grating, and the real phase-shifting or equivalent phase-shifting of the 0-order sub-grating is equal toThe width P or alpha of the step phase Bragg grating is changed by each laser of the multi-wavelength laser array, and the wavelength of the 0-level sub-grating is obtained through the relational expressionI.e. each laser of the multi-wavelength laser array has a different P orA can produce different lasing wavelengths.
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