rus
Issue #5/2023
A.V.Fomin, E.M.Filonenko, E.A.Anashkin
FEATURES OF THE ION-BEAM DEPOSITION TECHNOLOGY OF MULTILAYER OPTICAL COATINGS FOR INJECTION LASERS CAVITIES
FEATURES OF THE ION-BEAM DEPOSITION TECHNOLOGY OF MULTILAYER OPTICAL COATINGS FOR INJECTION LASERS CAVITIES
https://doi.org/10.22184/1993-8578.2023.16.5.272.280
The work is devoted to developing the optical coatings technology for injection lasers cavities on an ion-beam deposition facility. The optical coatings design has been performed to meet the requirements for facets reflection coefficients, and dielectric layers deposition modes have been determined. Development of the deposition modes consisted of determining the optimal parameters of an ion beam, the flow ratio rates of the ion-source and reagent gases, and substrate temperature. Calculations of the spectral characteristics of high-reflection and anti-reflection coatings obtained based on the proposed designs and experimental values of dielectric layers' refractive indices indicate that required reflection coefficients for injection lasers cavities have been achieved.
The work is devoted to developing the optical coatings technology for injection lasers cavities on an ion-beam deposition facility. The optical coatings design has been performed to meet the requirements for facets reflection coefficients, and dielectric layers deposition modes have been determined. Development of the deposition modes consisted of determining the optimal parameters of an ion beam, the flow ratio rates of the ion-source and reagent gases, and substrate temperature. Calculations of the spectral characteristics of high-reflection and anti-reflection coatings obtained based on the proposed designs and experimental values of dielectric layers' refractive indices indicate that required reflection coefficients for injection lasers cavities have been achieved.
Теги: injection lasers ion-beam deposition optical coatings refractive indices инжекционные лазеры ионно-лучевое нанесение коэффициенты преломления оптические покрытия
INTRODUCTION
Nowadays, the ever-increasing demand for powerful and reliable sources of laser radiation in the 915–980 nm spectral range makes it necessary to develop a technology for producing injection lasers with improved characteristics of their resonators resistant to catastrophic and gradual degradation. In turn, resistance of resonator ends is determined by preparation quality of their surfaces, the design of optical coatings applied to them, and selected technology of their application. The field of development of optical coating designs has been developing since the 1970s, with the majority of works in this field being motivated by the need to create a finished product with increased power and reliability values, as well as obtaining an intellectual property object, and less often including fundamental studies of the properties of the resulting coatings. The approach results in a number of patents for inventions in this field, often with ambiguous results, which do not allow to compare directly the advantages of the developed technologies and/or designs with the solutions specified in other patents, and as a consequence, to choose a ready-made solution when creating their own technology of optical coatings for facets of injection lasers.
The most used methods of optical coatings production are electron beam deposition [1], magnetron sputtering [2] and ion beam deposition [3]. The latter method has a number of advantages, since it allows to obtain thin films of high density, low absorption coefficient in the visible and infrared ranges [3], as well as to control stoichiometric composition by directly adjusting the gas flow ratio, energies and ion current. In turn, the issue of application of the ion-beam deposition method for optical coatings manufacturing on the ends of injection lasers is characterized by a small number of works and requires a comprehensive study. Thus, the aim of this study was to determine the peculiarities of ion-beam deposition application with subsequent development of our own technology for developing the optical coatings on laser cavities. The work included the optical coatings design with the required reflection coefficients, the study of monolayers deposition modes of dielectric materials included in the structure of optical coatings, and production of multilayer coatings on the cavitu facets of laser diode bars (LDBs) of the 915–980 nm spectral range, as well as approbation of the obtained optical coatings.
DESIGN OF OPTICAL COATINGS STRUCTURE
In injection laser design, the plane-parallel Fabry-Perot resonator is formed by two chipped facets of a semiconductor crystal with an R-value of ~30 %. Dielectric coatings are applied to the ends of injection lasers to protect their chipped surfaces from external influences, and also to obtain certain reflection coefficients at the output and rear ends of the resonator – up to 10 % (semi-transparent mirror) and more than 95 % (blind mirror), respectively - so that practically all the emitted power goes out through one facet of the cavity.
One of the suitable choices for forming a AR coating is Al2O3 with bulk refractive index n = 1.76, satisfactory values of thermal conductivity (0.2–0.3 W/(cm·K)) and forbidden band width (6.5 eV).
Typical systems for obtaining HR coatings of injection laser resonators in the near-IR range are SiO2/TiO2, SiO2/Ta2O5, SiO2/Si, Si3N4/Si. As a rule, the optical thickness of each layer is a quarter wave (QWOT – quarter wave optical thickness) or a multiple of it [4].
The refractive indices of the layers forming AR and HR coatings will depend on the methods and modes of deposition of the selected materials. In case of oxide deposition, the main difficulty is obtaining films of stoichiometric composition due to the effect of preferential sputtering of oxygen [5]. In this case, according to [5–7], the selected application rates and modes of reaction gas supply allow to obtain coatings with refractive indices close to those of bulk materials.
Based on the review of materials and their reference optical constants, as well as the principles of optical coatings design and required reflection coefficients, the schemes of anti-reflecting (AR) and highly reflective (HR) coatings were developed to form translucent and blind mirrors, respectively (Table 1).
STUDY OF OPTICAL COATINGS LAYERS FORMATION MODES BY ION-BEAM DEPOSITION METHOD
The existing ion-beam deposition unit is equipped with two Kaufman-type ion sources, a system for optical monitoring of the applied coatings, a quartz system for measuring their thickness, and a high-performance cryo-system allowing to reach a vacuum depth of 2∙10–6 bar. During the ion-beam deposition process, argon and oxygen are supplied as a source of ions for atomization of the target material and reagent gas, respectively. The gases are fed into a Kaufman-type ion gun [8], which allows the materials to be deposited at rates of ~0.01–0.2 nm/s. The parameters of the applied dielectric layers are monitored using a quartz sensor and an optical monitoring system, allowing thickness and spectral characteristics of the applied layers to be determined as they grow. The vacuum chamber contains four water-cooled targets fixed on copper bases. The design of the chamber is schematically presented in Fig.1.
The study of optical coating deposition modes consisted of deposition of each material included in the designed coatings in the form of monolayers on a control glass (CG1) with the subsequent determination of their optical constants and densities under different deposition modes. The parameters to be varied were the ion source gas ratio and reagent gas flow rates, monolayer deposition rates, and substrate temperature.
In the process of deposition of each monolayer, the ion beam parameters were selected in such a way as to obtain the maximum possible deposition rate at given ratios of ion source gas and reagent gas (O2/Ar) flow rates. The choice of optimal gas flow ratios was based on the analysis of deviations of experimental density values for the obtained monolayers from reference values of film densities of stoichiometric composition Δρ = ρтеор. – ρэксп. [9–11].
The ratios at which these deviations were minimal (Fig.2) were chosen for the deposition of layers. Subsequently, optimal substrate temperatures for each monolayer were searched at the established speeds and ratios of gas flow rates (Table 3). Table 3 shows the refractive indices values of the obtained monolayers at different substrate temperatures in case of the selected modes of application rate, and gas flow rate ratio.
According to Table 2, the most pronounced dependence of the refractive index on temperature at other selected modes is characteristic of the Al2O3 monolayer. Taking into account other selected modes of deposition, the optimal substrate temperature for the maximum refractive index is achieved in case of deposition of dielectric layers of SiO2 and TiO2 was 150 °C. Temperature for the maximum refractive index is achieved in case of Al2O3 monolayer deposition was 300 °C.
Calculations of spectral characteristics for the proposed designs of optical coatings (Table 1) at experimental values of refractive indices of the studied monolayers allowed to obtain the transmission coefficients of coatings on GaAs substrate and CG1 (Fig.3a, b) and to provide direct control of optical properties during subsequent fabrication of anti-reflecting (AR) and highly reflective (HR) coatings on the ion-beam deposition unit.
APPROBATION OF OPTICAL COATING DESIGNS
High-reflective and anti-reflecting coatings were applied to the ends of laser diode linacs (LDBs) by ion-beam deposition. Beforehand, four LDBs with the cavity length of 4 mm were stacked in a special clamp for LDBs, after this they were installed in a tooling for fixing in the ion-beam deposition chamber. The parameters of ion-beam deposition of layers, including temperature, deposition rates, and the ion source gas ratio and reagent gas flow rates were set according to the refractive indices of TiO2 (n = 2,35), SiO2 (n = 1,46) and Al2O3 (n = 1,65).
As a result of two processes, optical coatings were obtained on the output and rear end faces of the LDB, and the spectral characteristics of the coatings were controlled using CG1 glass witness substrates on a spectrophotometer with an accuracy of at least 1 % of T, where T is the transmission coefficient.
The obtained transmittance coefficients on CG1 for the spectral range of 915–980 nm amounted to 87.9 % and 2.7 % for anti-reflecting and highly reflective coatings, respectively. Deviation of experimental values from calculated values is not more than 1.5 % (Fig.4a, b) and can be due to the difference between the specified and actual refractive indices of the monolayers due to insignificant deviation of temperature and rate modes from application modes previously established as optimal.
Comparisons of the electro-optical characteristics of LDBs before and after application of optical coatings on the end faces of their cavities were made in order to estimate the power gain and the values of the actual reflection coefficients on the output and rear mirrors of LDBs. The measurements were carried out on the LDB certification unit in pulse generation mode in current range from 0 to 3A. Comparison of measurement results showed an increase in power value by a factor of 1.9 after application of anti-reflective coatingand highly reflective coatings. Figure 5 shows typical watt-ampere characteristics (W-I) of single LDB emitters before and after application of optical coatings.
The actual reflection coefficients of the mirrors (R1 и R2) formed at the ends of the cavities of length L = 4 mm were found from formula (1) for the radiation output losses αext, and were determined earlier for original laser heterostructure
. (1)
The actual reflection coefficients at the output (R1) and rear (R2) mirrors of the single emitter LDBs are summarized in Table 3.
The results of comparison of the W-I of single LDB emitters before and after optical coatings and calculation of actually obtained values indicate that required reflection coefficients at the rear and output facets of the injection laser cavities have been achieved.
CONCLUSIONS
The task of technology development to obtain optical coatings for resonators of injection lasers was successfully solved by the ion-beam deposition method. The conducted studies of the specifics of the method application allowed working out the deposition modes of dielectric monolayers included in design of the designed optical coatings. As a result of technology development, the required values of reflection coefficients were achieved, as evidenced by the results of comparing the output radiation powers of single LDB emitters before and after application of optical coatings, as well as calculations of actual reflection coefficients at output and rear ends of injection lasers resonators.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Nowadays, the ever-increasing demand for powerful and reliable sources of laser radiation in the 915–980 nm spectral range makes it necessary to develop a technology for producing injection lasers with improved characteristics of their resonators resistant to catastrophic and gradual degradation. In turn, resistance of resonator ends is determined by preparation quality of their surfaces, the design of optical coatings applied to them, and selected technology of their application. The field of development of optical coating designs has been developing since the 1970s, with the majority of works in this field being motivated by the need to create a finished product with increased power and reliability values, as well as obtaining an intellectual property object, and less often including fundamental studies of the properties of the resulting coatings. The approach results in a number of patents for inventions in this field, often with ambiguous results, which do not allow to compare directly the advantages of the developed technologies and/or designs with the solutions specified in other patents, and as a consequence, to choose a ready-made solution when creating their own technology of optical coatings for facets of injection lasers.
The most used methods of optical coatings production are electron beam deposition [1], magnetron sputtering [2] and ion beam deposition [3]. The latter method has a number of advantages, since it allows to obtain thin films of high density, low absorption coefficient in the visible and infrared ranges [3], as well as to control stoichiometric composition by directly adjusting the gas flow ratio, energies and ion current. In turn, the issue of application of the ion-beam deposition method for optical coatings manufacturing on the ends of injection lasers is characterized by a small number of works and requires a comprehensive study. Thus, the aim of this study was to determine the peculiarities of ion-beam deposition application with subsequent development of our own technology for developing the optical coatings on laser cavities. The work included the optical coatings design with the required reflection coefficients, the study of monolayers deposition modes of dielectric materials included in the structure of optical coatings, and production of multilayer coatings on the cavitu facets of laser diode bars (LDBs) of the 915–980 nm spectral range, as well as approbation of the obtained optical coatings.
DESIGN OF OPTICAL COATINGS STRUCTURE
In injection laser design, the plane-parallel Fabry-Perot resonator is formed by two chipped facets of a semiconductor crystal with an R-value of ~30 %. Dielectric coatings are applied to the ends of injection lasers to protect their chipped surfaces from external influences, and also to obtain certain reflection coefficients at the output and rear ends of the resonator – up to 10 % (semi-transparent mirror) and more than 95 % (blind mirror), respectively - so that practically all the emitted power goes out through one facet of the cavity.
One of the suitable choices for forming a AR coating is Al2O3 with bulk refractive index n = 1.76, satisfactory values of thermal conductivity (0.2–0.3 W/(cm·K)) and forbidden band width (6.5 eV).
Typical systems for obtaining HR coatings of injection laser resonators in the near-IR range are SiO2/TiO2, SiO2/Ta2O5, SiO2/Si, Si3N4/Si. As a rule, the optical thickness of each layer is a quarter wave (QWOT – quarter wave optical thickness) or a multiple of it [4].
The refractive indices of the layers forming AR and HR coatings will depend on the methods and modes of deposition of the selected materials. In case of oxide deposition, the main difficulty is obtaining films of stoichiometric composition due to the effect of preferential sputtering of oxygen [5]. In this case, according to [5–7], the selected application rates and modes of reaction gas supply allow to obtain coatings with refractive indices close to those of bulk materials.
Based on the review of materials and their reference optical constants, as well as the principles of optical coatings design and required reflection coefficients, the schemes of anti-reflecting (AR) and highly reflective (HR) coatings were developed to form translucent and blind mirrors, respectively (Table 1).
STUDY OF OPTICAL COATINGS LAYERS FORMATION MODES BY ION-BEAM DEPOSITION METHOD
The existing ion-beam deposition unit is equipped with two Kaufman-type ion sources, a system for optical monitoring of the applied coatings, a quartz system for measuring their thickness, and a high-performance cryo-system allowing to reach a vacuum depth of 2∙10–6 bar. During the ion-beam deposition process, argon and oxygen are supplied as a source of ions for atomization of the target material and reagent gas, respectively. The gases are fed into a Kaufman-type ion gun [8], which allows the materials to be deposited at rates of ~0.01–0.2 nm/s. The parameters of the applied dielectric layers are monitored using a quartz sensor and an optical monitoring system, allowing thickness and spectral characteristics of the applied layers to be determined as they grow. The vacuum chamber contains four water-cooled targets fixed on copper bases. The design of the chamber is schematically presented in Fig.1.
The study of optical coating deposition modes consisted of deposition of each material included in the designed coatings in the form of monolayers on a control glass (CG1) with the subsequent determination of their optical constants and densities under different deposition modes. The parameters to be varied were the ion source gas ratio and reagent gas flow rates, monolayer deposition rates, and substrate temperature.
In the process of deposition of each monolayer, the ion beam parameters were selected in such a way as to obtain the maximum possible deposition rate at given ratios of ion source gas and reagent gas (O2/Ar) flow rates. The choice of optimal gas flow ratios was based on the analysis of deviations of experimental density values for the obtained monolayers from reference values of film densities of stoichiometric composition Δρ = ρтеор. – ρэксп. [9–11].
The ratios at which these deviations were minimal (Fig.2) were chosen for the deposition of layers. Subsequently, optimal substrate temperatures for each monolayer were searched at the established speeds and ratios of gas flow rates (Table 3). Table 3 shows the refractive indices values of the obtained monolayers at different substrate temperatures in case of the selected modes of application rate, and gas flow rate ratio.
According to Table 2, the most pronounced dependence of the refractive index on temperature at other selected modes is characteristic of the Al2O3 monolayer. Taking into account other selected modes of deposition, the optimal substrate temperature for the maximum refractive index is achieved in case of deposition of dielectric layers of SiO2 and TiO2 was 150 °C. Temperature for the maximum refractive index is achieved in case of Al2O3 monolayer deposition was 300 °C.
Calculations of spectral characteristics for the proposed designs of optical coatings (Table 1) at experimental values of refractive indices of the studied monolayers allowed to obtain the transmission coefficients of coatings on GaAs substrate and CG1 (Fig.3a, b) and to provide direct control of optical properties during subsequent fabrication of anti-reflecting (AR) and highly reflective (HR) coatings on the ion-beam deposition unit.
APPROBATION OF OPTICAL COATING DESIGNS
High-reflective and anti-reflecting coatings were applied to the ends of laser diode linacs (LDBs) by ion-beam deposition. Beforehand, four LDBs with the cavity length of 4 mm were stacked in a special clamp for LDBs, after this they were installed in a tooling for fixing in the ion-beam deposition chamber. The parameters of ion-beam deposition of layers, including temperature, deposition rates, and the ion source gas ratio and reagent gas flow rates were set according to the refractive indices of TiO2 (n = 2,35), SiO2 (n = 1,46) and Al2O3 (n = 1,65).
As a result of two processes, optical coatings were obtained on the output and rear end faces of the LDB, and the spectral characteristics of the coatings were controlled using CG1 glass witness substrates on a spectrophotometer with an accuracy of at least 1 % of T, where T is the transmission coefficient.
The obtained transmittance coefficients on CG1 for the spectral range of 915–980 nm amounted to 87.9 % and 2.7 % for anti-reflecting and highly reflective coatings, respectively. Deviation of experimental values from calculated values is not more than 1.5 % (Fig.4a, b) and can be due to the difference between the specified and actual refractive indices of the monolayers due to insignificant deviation of temperature and rate modes from application modes previously established as optimal.
Comparisons of the electro-optical characteristics of LDBs before and after application of optical coatings on the end faces of their cavities were made in order to estimate the power gain and the values of the actual reflection coefficients on the output and rear mirrors of LDBs. The measurements were carried out on the LDB certification unit in pulse generation mode in current range from 0 to 3A. Comparison of measurement results showed an increase in power value by a factor of 1.9 after application of anti-reflective coatingand highly reflective coatings. Figure 5 shows typical watt-ampere characteristics (W-I) of single LDB emitters before and after application of optical coatings.
The actual reflection coefficients of the mirrors (R1 и R2) formed at the ends of the cavities of length L = 4 mm were found from formula (1) for the radiation output losses αext, and were determined earlier for original laser heterostructure
. (1)
The actual reflection coefficients at the output (R1) and rear (R2) mirrors of the single emitter LDBs are summarized in Table 3.
The results of comparison of the W-I of single LDB emitters before and after optical coatings and calculation of actually obtained values indicate that required reflection coefficients at the rear and output facets of the injection laser cavities have been achieved.
CONCLUSIONS
The task of technology development to obtain optical coatings for resonators of injection lasers was successfully solved by the ion-beam deposition method. The conducted studies of the specifics of the method application allowed working out the deposition modes of dielectric monolayers included in design of the designed optical coatings. As a result of technology development, the required values of reflection coefficients were achieved, as evidenced by the results of comparing the output radiation powers of single LDB emitters before and after application of optical coatings, as well as calculations of actual reflection coefficients at output and rear ends of injection lasers resonators.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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