CN113659440B - Ultrahigh-frequency optical frequency comb quantum dot mode-locked laser and preparation method thereof - Google Patents
Ultrahigh-frequency optical frequency comb quantum dot mode-locked laser and preparation method thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
- H01S5/0657—Mode locking, i.e. generation of pulses at a frequency corresponding to a roundtrip in the cavity
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Abstract
The invention particularly discloses an ultrahigh frequency optical frequency comb quantum dot mode-locked laser and a preparation method thereof. The mode-locked laser comprises a substrate, a negative electrode evaporated on the lower side of the substrate, and an N-type contact layer, an N-type cladding layer, an InAs quantum dot active region, a P-type cladding layer, a P-type contact layer and a positive electrode which are sequentially grown on the upper side of the substrate from bottom to top, wherein the N-type contact layer, the N-type cladding layer, the InAs quantum dot active region, the P-type cladding layer and the P-type contact layer are sequentially grown on the substrate through a molecular beam epitaxial growth cavity, and the P-type contact layer is divided into a saturable absorber and a gain region with the length ratio of 1:4-1:6 by utilizing electric isolation etching, so that the saturable absorber accounts for 14% -20% of the total cavity length of the mode-locked laser, a subpicosecond mode-locked state with stable voltage-free conditions can be achieved, the cost and the consumption are effectively reduced, and great convenience is provided on operation. Has the characteristics of low energy consumption, high efficiency, low cost, convenient operation and mass production.
Description
Technical Field
The invention relates to the technical field of mode-locked lasers, in particular to an ultrahigh frequency optical frequency comb quantum dot mode-locked laser and a preparation method thereof.
Background
The mode-locked laser is also called an ultrafast laser because of its narrow pulse width, high peak power, high repetition rate, wide spectrum, and other characteristics of the pulse wave generated. With the rapid development of laser technology, mode-locked lasers have been widely used in numerous front-end technologies. Such as: optical clock, nanomaterial processing, laser radar detection, millimeter wave, terahertz generation and the like. Mode locking of mode-locked lasers can be classified into active mode locking, passive mode locking and hybrid mode locking techniques. Wherein, the passive mode locking utilizes the nonlinear optical property of the saturable absorber: the bleaching effect is achieved on the input light pulse with high intensity, meanwhile, the bleaching effect is achieved on weak light, phase locking among longitudinal modes in the cavity is achieved, and then ultra-short pulses are generated easily.
The optical frequency comb breaks the mutually independent relation between the technical fields of radio frequency and optical frequency in the past twenty years, skillfully links the optical frequency with the microwave frequency, realizes the bidirectional conversion between the two, breaks through the bottleneck of signal processing, promotes the development of optoelectronics and brings unprecedented possibility for each field. Among the technologies for realizing the optical frequency comb, the semiconductor mode-locked laser has long been known as one of the most promising comb-shaped spectrum emitters by virtue of its advantages such as small size, low threshold value, simple structure, and convenient operation.
At present, optical frequency combs play a critical role in optical communication systems, in addition to applications in the technologies of gas component analysis, laser radar, global positioning system GPS, etc. In order to cope with the increasing information transmission demands, a Wavelength Division Multiplexing (WDM) concept has been proposed, that is, different wavelengths carry different signals to carry out communication transmission in the same optical fiber, so that the number of optical fibers can be reduced while the communication capacity is increased, and thus the cost and the energy consumption are reduced. The fine wavelength division multiplexing (DWDM) technology becomes the most effective scheme for expanding the bandwidth of the line due to the dense band interval (200 GHz/100GHz/50 GHz). In recent years, with the development of optical networks, the problem of high-speed short-distance transmission caused by chromatic dispersion limitation of 5G forwarding networks again gathers people's eyes on the unique advantage of low O-band chromatic dispersion cost. However, the current phase O-band DWDM technology is still in the search phase and lacks standardization, mainly because of its high technical requirements and lack of suitable light sources. The high repetition frequency (> 100 GHz) mode-locked lasers currently on the market have the following drawbacks:
1. basically located in the C band, lacking an O band light source;
2. the mechanism for generating the pulse wave is complex, and in order to obtain a stable ultra-high frequency optical frequency comb, optical path coupling among a plurality of active/passive optical devices is needed to be involved in actual operation, so that the requirements on the skills of operators are high, and the popularization and the use are not easy;
3. the material gain is insufficient to realize stable lasing of the laser device under the extremely short cavity length (hundreds of micrometers), so that the passive mode locking condition of the single-chip device cannot be met;
4. even the simplest two-section type passive mode locking device still needs to reach stable mode locking condition through the comprehensive action of voltage and current, a voltage source and a current source are inexhaustible, so that the defects of complex operation, high energy consumption, high cost and the like are unavoidable in the traditional two-section type passive mode locking device.
Disclosure of Invention
The invention aims to overcome the defects and shortcomings in the prior art, and provides an ultrahigh frequency optical frequency comb quantum dot mode-locked laser and a preparation method thereof. Has the characteristics of low energy consumption, high efficiency, low cost, convenient operation and mass production.
In order to solve the technical problems, the invention provides an ultrahigh frequency optical frequency comb quantum dot mode-locked laser which comprises a substrate, a positive electrode, a negative electrode, a GaAs N-type contact layer, an N-type cladding layer, an InAs quantum dot active region, a P-type cladding layer and a GaAs P-type contact layer, wherein the GaAs N-type contact layer, the N-type cladding layer, the InAs quantum dot active region, the P-type cladding layer and the GaAs P-type contact layer are sequentially grown on the upper side surface of the substrate 1 from bottom to top, the negative electrode is evaporated on the lower side surface of the substrate, the GaAs P-type contact layer comprises a gain region and a saturable absorber which are etched in electrical isolation and are arranged in parallel, the length ratio of the saturable absorber to the gain region is 1:4-1:6, and the positive electrode is evaporated on the gain region.
Preferably, the N-type cladding layer comprises a first N-type Al which is arranged from bottom to top in sequence x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The As buffer layer and the P-type cladding layer comprise a first P-type Al layer which is sequentially arranged from bottom to top x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x An As buffer layer, wherein x has a value range of [0.2,0.99 ]]。
Preferably, the substrate is a semi-insulating GaAs substrate or a semi-insulating silicon substrate.
The invention also provides a preparation method of the ultra-high frequency optical frequency comb quantum dot mode-locked laser, which comprises the following steps:
s1, conveying a substrate into a molecular beam epitaxial growth cavity for high-temperature deoxidation treatment;
s2, sequentially growing a GaAs N-type contact layer, an N-type cladding layer, an InAs quantum dot active region, a P-type cladding layer and a GaAs P-type contact layer on the substrate subjected to high-temperature deoxidation to obtain a mode-locked laser sample;
s3, performing ridge waveguide etching on the mode-locked laser sample obtained in the step S2;
s4, plating an insulating layer on the mode-locked laser sample after ridge waveguide etching and carrying out planarization treatment;
s5, evaporating a positive electrode on the GaAs P-type contact layer;
s6, carrying out electric isolation etching between the saturable absorber and the gain region so that the saturable absorber accounts for 14% -20% of the total cavity length of the mode-locked laser sample;
s7, thinning and evaporating a negative electrode on the lower side surface of the substrate;
s8, post-processing.
Preferably, in the step S1, when the substrate is a semi-insulating silicon substrate, the temperature of the molecular beam epitaxial growth chamber is 900-1100 ℃; when the substrate is a semi-insulating GaAs substrate, the temperature of the molecular beam epitaxial growth cavity is 500-650 ℃.
Preferably, the specific implementation manner of the step S2 includes:
s21, growing a layer of GaAs N-type contact layer with the thickness of 300nm on the substrate subjected to high-temperature deoxidation, wherein the carrier concentration of the GaAs N-type contact layer is 0.8x10 18 -1.2×10 18 cm -3 ；
S22, growing a first N-type Al on the GaAs N-type contact layer x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x An N-type cladding layer composed of an As buffer layer, wherein the first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The thickness of the As buffer layer is 20-100nm and 1-1 respectively5 μm and 10-100nm, a first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The carrier concentrations of the As buffer layers were all 1×10 17 -10×10 17 cm -3 ；
S23, growing a high-density InAs quantum dot active region with 8-10 cycles on the N-type cladding layer, wherein each cycle is performed according to the following growth steps: firstly growing a GaAs wetting layer with the thickness of 35.5nm, then depositing an InAs quantum dot material layer with the volume of 3ML, then depositing an InGaAs covering layer with the thickness of 3.7nm on the InAs quantum dot, and finally growing a GaAs spacer layer with the thickness of 35.5nm, so that the quantum dot density of an InAs quantum dot active region is not lower than 5.9x10 10 cm -2 ；
S24, growing a first P type Al on the InAs quantum dot active region x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al 0.2 Ga 0.8 A P-type cladding layer composed of an As buffer layer, wherein the first P-type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The As buffer layers have thicknesses of 20nm, 1.4 μm and 20nm, respectively, and are of the first P type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The carrier concentrations of the As buffer layers were 4.0X10 respectively 17 -4.4×10 17 cm -3 、6.5×10 17 -7.5×10 17 cm -3 And 1.2X10 19 -1.6×10 19 cm -3 ；
S25, growing a GaAs P-type contact layer with the thickness of 400nm on the P-type cladding layer, wherein the carrier concentration of the GaAs P-type contact layer is 1.8x10 19 -2.2×10 19 cm -3 。
Preferably, in the step S3, ridge waveguide etching is performed on the laser sample by using a photolithography or inductively coupled ion etching method, and the etching depth is 1.5-1.8 μm.
Preferably, in the step S4, an insulating layer is first plated on the laser sample by using a plasma enhanced chemical vapor deposition method, the thickness of the insulating layer is 150-200nm, then a patterning process and a top layer windowing process are performed on the insulating layer to expose the GaAs P-type contact layer of the ridge waveguide, and then a planarization process is performed.
Preferably, in step S6, the GaAs P-type contact layer is etched by using photolithography or wet etching techniques to achieve a length ratio between the saturable absorber and the gain region of 1:4-1:6, so as to ensure that the saturable absorber occupies 14% -20% of the total cavity length.
Preferably, in the step S7, the thickness of the substrate 1 is first thinned to 120-200 μm, then the negative electrode is evaporated on the lower side of the substrate by electron beam sputtering or magnetron sputtering, and finally rapid annealing is performed, wherein the annealing temperature is 700-750 ℃, and the annealing time is 15-60 seconds.
Compared with the prior art, the invention improves the material gain of the mode-locked laser by increasing the quantum dot density of the active region and optimizing the growth condition of the quantum dots, and adopts electric isolation etching to divide the length ratio of the saturable absorber and the gain region into 1:4-1:6, so that the saturable absorber accounts for 14-20% of the total cavity length, thereby being capable of achieving a subpicosecond mode-locked state with stable voltage-free condition, effectively reducing the cost and consumption, and providing great convenience in operation. Has the characteristics of low energy consumption, high efficiency, low cost, convenient operation and mass production.
Drawings
FIG. 1 is a schematic diagram of the structure of an ultra-high frequency optical frequency comb quantum dot mode-locked laser of the present invention;
FIG. 2 is a flow chart of a method for manufacturing an ultra-high frequency optical frequency comb quantum dot mode-locked laser according to the invention;
FIG. 3 is a spectral diagram of an ultra-high frequency optical frequency comb quantum dot mode-locked laser of the present invention;
fig. 4 is a pulse diagram of an ultrahigh frequency optical frequency comb quantum dot mode-locked laser in accordance with the present invention.
In the figure, a substrate, a 2-GaAs N-type contact layer, a 3-N-type cladding layer, a 4-InAs quantum dot active region, a 5.P-type cladding layer, a 6-GaAs P-type contact layer, a 61-gain region, a 62-saturable absorber, a 7-positive electrode and a 8-negative electrode.
Detailed Description
In order to better understand the technical solutions of the present disclosure, the following description will clearly and completely describe the technical solutions of the embodiments of the present disclosure with reference to the drawings in the embodiments of the present disclosure. It will be apparent that the described embodiments are merely embodiments of a portion, but not all, of the present disclosure. All other embodiments, which can be made by one of ordinary skill in the art without inventive effort, based on the embodiments in this disclosure, shall fall within the scope of the present disclosure.
It should be noted that the terms "first," "second," and the like in the description and claims of the present disclosure and in the foregoing figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments of the disclosure described herein may be capable of operation in sequences other than those illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
Referring to fig. 1, fig. 1 is a schematic structural diagram of an ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to the present invention.
The ultra-high frequency optical frequency comb quantum dot mode-locked laser comprises a substrate 1, a positive electrode 7, a negative electrode 8, a GaAs N-type contact layer 2, an N-type cladding layer 3, an InAs quantum dot active region 4, a P-type cladding layer 5 and a GaAs P-type contact layer 6 which are sequentially grown on the upper side surface of the substrate 1 from bottom to top, wherein the negative electrode 8 is evaporated on the lower side surface of the substrate 1, the GaAs P-type contact layer 6 comprises a gain region 61 and a saturable absorber 62 which are etched in an electric isolation mode and are arranged in parallel, the length ratio of the saturable absorber 62 to the gain region 61 is 1:4-1:6, and the positive electrode 7 is evaporated on the gain region 61.
In this embodiment, a GaAs N-type contact layer 2, an N-type cladding layer 3, an InAs quantum dot active region 4, a P-type cladding layer 5 and a GaAs P-type contact layer 6 are sequentially grown on a substrate 1 from bottom to top, then the GaAs P-type contact layer 6 is divided into a saturable absorber 62 and a gain region 61 with a length ratio of 1:4-1:6 by using electric isolation etching, and finally a negative electrode 8 and a positive electrode 7 are respectively evaporated on the lower side surface of the substrate 1 and the gain region 61, thereby obtaining the mode-locked laser, and the mode-locked laser can achieve a subpicosecond mode-locked state with stable voltage-free conditions because the saturable absorber 62 occupies 14% -20% of the total cavity length of the mode-locked laser, thereby effectively reducing cost and consumption, and providing great convenience in operation. Has the characteristics of low energy consumption, high efficiency, low cost, convenient operation and mass production.
As shown in FIG. 1, the N-type cladding layer 3 comprises a first N-type Al arranged from bottom to top x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The As buffer layer and the P-type cladding layer 5 comprise a first P-type Al which is arranged from bottom to top in sequence x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x An As buffer layer, wherein x has a value range of [0.2,0.99 ]]。
In this embodiment, the first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The thickness of the As buffer layer is 20nm, 1.4 μm and 20nm respectively; the first P type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The thickness of the As buffer layer is 20nm, 1.4 μm and 20nm respectively, wherein the value of x can be in [0.2,0.99 ] according to the actual requirement]And carrying out corresponding adjustment.
In this embodiment, the first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The As buffer layer isFirst N type Al 0.2 Ga 0.8 As buffer layer, N type Al 0.4 Ga 0.6 As light confinement layer and second N type Al 0.2 Ga 0.8 An As buffer layer of the first P type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The As buffer layer is a first P type Al 0.2 Ga 0.8 As buffer layer, P-type Al 0.4 Ga 0.6 As light confinement layer and second P-type Al 0.2 Ga 0.8 An As buffer layer.
Wherein the substrate 1 is a semi-insulating GaAs substrate or a semi-insulating silicon substrate. In GaAs material systems, the effective refractive index is about 3.7 in the 1310 band, expressed by the formula l=c/(2*n) r * f) (wherein, c represents the light propagation speed, f represents the repetition frequency corresponding to the cavity length of the mode-locked laser, and n r Representing the optical refractive index of the material, L representing the cavity length of the mode-locked laser) can calculate that when the cavity length of the mode-locked laser is equal to or less than 405 microns, the repetition frequency of 100GHz or more can be achieved, and at the same time, the passive mode-locked laser can achieve a stable sub-picosecond mode-locked state of the saturable absorber 62 in the no-reverse bias state only when the length of the saturable absorber 62 is 14% -20% of the total cavity length.
As shown in fig. 2, the invention further provides a preparation method of the ultra-high frequency optical frequency comb quantum dot mode-locked laser, which comprises the following steps:
s1, conveying a substrate 1 into a molecular beam epitaxial growth cavity for high-temperature deoxidation treatment;
s2, sequentially growing a GaAs N-type contact layer 2, an N-type cladding layer 3, an InAs quantum dot active region 4, a P-type cladding layer 5 and a GaAs P-type contact layer 6 on the substrate 1 subjected to high-temperature deoxidation to obtain a laser sample;
s3, performing ridge waveguide etching on the laser sample obtained in the step S2;
s4, plating an insulating layer on the laser sample after ridge waveguide etching and carrying out planarization treatment;
s5, evaporating a positive electrode 7 on the GaAs P-type contact layer 6;
s6, performing electric isolation etching between the saturable absorber and the gain region so that the saturable absorber accounts for 14% -20% of the total cavity length of the laser sample;
s7, thinning and evaporating a negative electrode 8 on the lower side surface of the substrate 1;
and S8, post-processing, such as cutting and packaging.
When the substrate 1 is a semi-insulating silicon substrate, the temperature of the molecular beam epitaxial growth cavity is 900-1100 ℃; when the substrate 1 is a semi-insulating GaAs substrate, the temperature of the molecular beam epitaxial growth chamber is 500-650 ℃.
The specific implementation manner of the step S2 includes:
s21, growing a layer of GaAs N-type contact layer 2 with the thickness of 300nm on the substrate 1 subjected to high-temperature deoxidation, wherein the carrier concentration of the GaAs N-type contact layer 2 is 0.8x10 18 -1.2×10 18 cm -3 In this embodiment, the carrier concentration of the GaAs N-type contact layer 2 is 1×10 18 cm -3 ；
S22, growing a first N-type Al on the GaAs N-type contact layer 2 x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x An N-type cladding layer (3) composed of an As buffer layer, wherein the first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The As buffer layer has a thickness of 20-100nm, 1-1.5 μm and 10-100nm, respectively, and a carrier concentration of 1×10 17 -10×10 17 cm -3 The method comprises the steps of carrying out a first treatment on the surface of the In this embodiment, the N-type cladding layer 3 is made of a first N-type Al 0.2 Ga 0.8 As buffer layer, N type Al 0.4 Ga 0.6 As light confinement layer and second N type Al 0.2 Ga 0.8 An As buffer layer, wherein, the first N type Al 0.2 Ga 0.8 As buffer layer, N type Al 0.4 Ga 0.6 As light confinement layer and second N type Al 0.2 Ga 0.8 The As buffer layers have thicknesses of 20nm, 1.4 μm and 20nm, respectively, and are of the first N type Al 0.2 Ga 0.8 As buffer layer, N type Al 0.4 Ga 0.6 As light confinement layer and second N type Al 0.2 Ga 0.8 The carrier concentrations of the As buffer layers were all 6×10 17 cm -3 ；
S23, growing a high-density InAs quantum dot active region 4 with 8-10 cycles on the N-type cladding layer 3, wherein each cycle is performed according to the following growth steps: firstly growing a GaAs wetting layer with the thickness of 35.5nm, then depositing an InAs quantum dot material layer with the volume of 3ML, then depositing an InGaAs covering layer with the thickness of 3.7nm on the InAs quantum dots, and finally growing a GaAs spacer layer with the thickness of 35.5nm, so that the quantum dot density of the InAs quantum dot active region 4 is not lower than 5.9x10 10 cm -2 ；
S24, growing a first P-type Al on the InAs quantum dot active region 4 x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x A P-type cladding layer (5) composed of an As buffer layer, wherein the first P-type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The As buffer layers had thicknesses of 20nm, 1.4 μm and 20nm, respectively, and carrier concentrations of 4.0X10, respectively 17 -4.4×10 17 cm -3 、6.5×10 17 -7.5×10 17 cm -3 And 1.2X10 19 -1.6×10 19 cm -3 The method comprises the steps of carrying out a first treatment on the surface of the In this embodiment, the P-type cladding layer 5 is made of a first P-type Al 0.2 Ga 0.8 As buffer layer, P-type Al 0.4 Ga 0.6 As light confinement layer and second P-type Al 0.2 Ga 0.8 An As buffer layer, wherein, the first P type Al 0.2 Ga 0.8 As buffer layer, P-type Al 0.4 Ga 0.6 As light confinement layer and second P-type Al 0.2 Ga 0.8 The As buffer layers have thicknesses of 20nm, 1.4 μm and 20nm, respectively, and are of the first P type Al 0.2 Ga 0.8 As buffer layer, P-type Al 0.4 Ga 0.6 As light confinement layer and second P-type Al 0.2 Ga 0.8 The carrier concentrations of the As buffer layers were 4.2X10, respectively 17 cm -3 、7×10 17 cm -3 And 1.4X10 19 cm -3 ；
S25, growing a layer with the thickness of 400nm on the P-type cladding layer 5The carrier concentration of the GaAs P-type contact layer 6 is 1.8X10 19 -2.2×10 19 cm -3 The method comprises the steps of carrying out a first treatment on the surface of the In this embodiment, the carrier concentration is 2×10 19 cm -3 。
In the step S3, a ridge waveguide etching is performed on the laser sample (i.e. the wafer that has been grown) by using a photolithography technique or an inductively coupled ion etching (Inductive coupledplasma, ICP) technique, and the etching depth is 1.5-1.8 μm.
In the step S4, an insulating layer is first plated on the laser sample by using a plasma enhanced chemical vapor deposition (Plasma enhanced chemical vapor deposition, PECVD) method, the thickness of the insulating layer is 150-200nm, then a patterning process and a top layer window are performed on the insulating layer to expose the GaAs P-type contact layer 6 of the ridge waveguide, and finally a planarization process is performed. The insulating layer material is silicon dioxide or nitrogen dioxide, and the planarization treatment specifically comprises spin coating and planarization etching of a planarization material so as to ensure that no material residue exists on the ridge waveguide.
In the step S6, the GaAs P-type contact layer 6 is electrically isolated etched by using photolithography or wet etching technology to achieve a length ratio between the saturable absorber 62 and the gain region 61 of 1:4-1:6, so as to ensure that the saturable absorber 62 occupies 14% -20% of the total cavity length.
In the step S7, the thickness of the substrate 1 is first thinned to 120-200 μm, then the negative electrode 8 is evaporated on the lower side of the substrate 1 by electron beam sputtering or magnetron sputtering, and finally rapid annealing treatment (Rapid thermal annealing process, RTP) is performed, wherein the annealing temperature is 700-750 ℃, and the annealing time is 15-60 seconds. Wherein the substrate 1 is subjected to thickness reduction treatment by a grinder to reduce the influence of thermal effects on the device, and at the same time, ohmic contact with low resistance can be formed by rapid annealing treatment.
In the embodiment, firstly, a layer of GaAs N-type contact layer 2, an N-type cladding layer 3, an InAs quantum dot active region 4, a P-type cladding layer 5 and a GaAs P-type contact layer 6 are sequentially grown on a semi-insulating substrate 1 by utilizing a molecular beam epitaxial growth cavity; then, ridge waveguide etching, insulating layer plating, planarization treatment and positive electrode evaporation 7 are carried out on the grown laser; then, the GaAs P-type contact layer 6 is subjected to electric isolation etching by utilizing a photoetching or wet etching technology, so that the length ratio between the saturable absorber 62 and the gain region 61 is 1:4-1:6, the saturable absorber 62 is ensured to occupy 14% -20% of the total cavity length, and further, the mode-locked laser can achieve a subpicosecond mode-locked state with stable voltage-free condition, the cost and the consumption are effectively reduced, and great convenience is provided for operation; finally, thinning and evaporating the negative electrode 8, cutting and packaging are carried out on the mode-locked laser, so that the ultra-high frequency optical frequency comb quantum dot mode-locked laser is obtained. The ultra-high frequency optical frequency comb quantum dot mode-locked laser has the characteristics of low energy consumption, high efficiency, low cost, convenience in operation and mass production.
In the present invention, the positive electrode 7 and the negative electrode 8 are deposited on the gain region 61 and the lower surface of the substrate 1 by electron beam sputtering or magnetron sputtering, respectively.
In order to better understand the working principle and technical effect of the invention, the spectrum diagram and the pulse diagram of the ultra-high frequency optical frequency comb quantum dot mode-locked laser are described below.
Referring to FIG. 3, FIG. 3 shows a spectral diagram of an ultra-high frequency optical frequency comb quantum dot mode-locked laser, measured by an AQ6370D oscilloscope from YOKOGAWA, where the abscissa is Wavelength (Wavelength nm) and the ordinate is Power (Power dBm). As can be seen from fig. 3, the center wavelength is 1295nm, and the interval between two adjacent wavelengths is 0.56nm, by the formula:
Δf＝(Δλ*c)/(λ 1 λ 2 ) (1)
in the formula (1), deltalambda represents the interval between two adjacent wavelengths, c represents the light propagation speed, and lambda 1 、λ 2 Representing two adjacent wavelengths, respectively.
From equation (1), it can be seen that the wavelength interval of 0.56nm corresponds to a repetition frequency of 100GHz, thus confirming that the quantum dot mode-locked laser of the present invention can produce an optical frequency comb with a heavy frequency of up to 100 GHz.
Referring to fig. 4, fig. 4 shows a pulse diagram of an ultra-high frequency optical frequency comb quantum dot mode-locked laser, which is measured by using a pulsecack autocorrelation of a.p.e. company, wherein the abscissa is time (delay ps) and the ordinate is light pulse Intensity (Intensity a.u. ]), and a single pulse can obtain a pulse width of 0.7ps through Gaussian function curve (Gaussian) fitting, that is, it is explained that the quantum dot mode-locked laser of the present invention can be used as a sub-picosecond pulse wave output light source.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (8)
1. The ultra-high frequency optical frequency comb quantum dot mode-locked laser is characterized by comprising a substrate (1), a positive electrode (7) and a negative electrode (8), and a GaAs N-type contact layer (2), an N-type cladding layer (3), an InAs quantum dot active region (4), a P-type cladding layer (5) and a GaAs P-type contact layer (6) which are sequentially grown on the upper side surface of the substrate (1) from bottom to top, wherein the negative electrode (8) is evaporated on the lower side surface of the substrate (1), the GaAs P-type contact layer (6) comprises a gain region (61) and a saturable absorber (62) which are etched in electrical isolation and are arranged in parallel, the length ratio of the saturable absorber (62) to the gain region (61) is 1:4-1:6, and the positive electrode (7) is evaporated on the gain region (61);
the N-type cladding layer (3) comprises a first N-type Al which is sequentially arranged from bottom to top x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The As buffer layer and the P-type cladding layer (5) comprise a first P-type Al which is arranged from bottom to top in sequence x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x An As buffer layer, wherein x has a value of [0.2,0.99 ]]。
2. The ultra-high frequency optical frequency comb quantum dot mode-locked laser according to claim 1, characterized in that the substrate (1) is a semi-insulating GaAs substrate or a semi-insulating silicon substrate.
3. The preparation method of the ultra-high frequency optical frequency comb quantum dot mode-locked laser is characterized by comprising the following steps of:
s1, conveying a substrate (1) into a molecular beam epitaxial growth cavity for high-temperature deoxidation treatment;
s2, sequentially growing a GaAs N-type contact layer (2), an N-type cladding layer (3), an InAs quantum dot active region (4), a P-type cladding layer (5) and a GaAs P-type contact layer (6) on the substrate (1) subjected to high-temperature deoxidation to obtain a mode-locked laser sample;
s3, performing ridge waveguide etching on the mode-locked laser sample obtained in the step S2;
s4, plating an insulating layer on the mode-locked laser sample after ridge waveguide etching and carrying out planarization treatment;
s5, evaporating a positive electrode (7) on the GaAs P-type contact layer (6);
s6, performing electric isolation etching between the saturable absorber (62) and the gain region (61) so that the saturable absorber (62) accounts for 14% -20% of the total cavity length of the mode-locked laser sample;
s7, thinning the substrate (1) and evaporating a negative electrode (8) on the lower side surface of the substrate (1);
s8, post-processing;
the specific implementation manner of the step S2 includes:
s21, growing a layer of GaAs N-type contact layer (2) with the thickness of 300nm on the substrate (1) subjected to high-temperature deoxidation, wherein the carrier concentration of the GaAs N-type contact layer (2) is 0.8x10 18 -1.2×10 18 cm -3 ；
S22, growing a first N-type Al on the GaAs N-type contact layer (2) x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x An N-type cladding layer (3) composed of an As buffer layer, wherein the first N-type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The thickness of the As buffer layer is 20-100nm, 1-1.5 μm and 10-100nm respectively, the first N type Al x Ga 1-x As buffer layer, N type Al x Ga 1-x As light confinement layer and second N type Al x Ga 1-x The carrier concentrations of the As buffer layers were all 1×10 17 -10×10 17 cm -3 ；
S23, growing a high-density InAs quantum dot active region (4) with 8-10 periods on the N-type cladding layer (3), wherein each period is performed according to the following growth steps: firstly growing a GaAs wetting layer with the thickness of 35.5nm, then depositing an InAs quantum dot material layer with the volume of 3ML, then depositing an InGaAs covering layer with the thickness of 3.7nm on the InAs quantum dot material layer, and finally growing a GaAs spacer layer with the thickness of 35.5nm, so that the quantum dot density of the InAs quantum dot active region (4) is not lower than 5.9x10 10 cm -2 ；
S24, growing a first P-type Al on the InAs quantum dot active region (4) x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x A P-type cladding layer (5) composed of an As buffer layer, wherein the first P-type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The As buffer layers have thicknesses of 20nm, 1.4 μm and 20nm, respectively, and are of the first P type Al x Ga 1-x As buffer layer, P-type Al x Ga 1-x As light confinement layer and second P-type Al x Ga 1-x The carrier concentrations of the As buffer layers were 4.0X10 respectively 17 -4.4×10 17 cm -3 、6.5×10 17 -7.5×10 17 cm -3 And 1.2X10 19 -1.6×10 19 cm -3 ；
S25, growing a layer of GaAs P-type contact layer (6) with the thickness of 400nm on the P-type cladding layer (5), wherein the carrier concentration of the GaAs P-type contact layer (6) is 1.8x10 19 -2.2×10 19 cm -3 。
4. The method for preparing the ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to claim 3, wherein in the step S1, when the substrate (1) is a semi-insulating silicon substrate, the temperature of a molecular beam epitaxial growth cavity is 900-1100 ℃; when the substrate (1) is a semi-insulating GaAs substrate, the temperature of the molecular beam epitaxial growth cavity is 500-650 ℃.
5. The method for preparing the ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to claim 3, wherein in the step S3, ridge waveguide etching is performed on the laser sample by using a photoetching or inductively coupled ion etching method, and the etching depth is 1.5-1.8 μm.
6. The method for preparing the ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to claim 5, wherein in the step S4, an insulating layer is firstly plated on a laser sample by a plasma enhanced chemical vapor deposition method, the thickness of the insulating layer is 150-200nm, then pattern processing and top layer windowing are performed on the insulating layer to expose a GaAs P-type contact layer (6) of a ridge waveguide, and then planarization processing is performed.
7. The method for preparing the ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to claim 6, wherein in the step S6, the GaAs P-type contact layer (6) is electrically isolated etched by photolithography and wet etching techniques to achieve a length ratio between the saturable absorber (62) and the gain region (61) of 1:4-1:6, so as to ensure that the saturable absorber (62) occupies 14% -20% of the total cavity length.
8. The method for preparing the ultrahigh frequency optical frequency comb quantum dot mode-locked laser according to claim 7, wherein in the step S7, the thickness of the substrate (1) is firstly thinned to 120-200 μm, then the negative electrode (8) is evaporated on the lower side surface of the substrate (1) by utilizing electron beam sputtering or magnetron sputtering, and finally rapid fire fading treatment is carried out, wherein the fire fading temperature is 700-750 ℃, and the fire fading duration is 15-60 seconds.
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