rus
Issue #5/2014
A.Belozyorov, N.Larionov , A.Lukin, A.Melnikov
On-axis computer-generated hologram optical elements: history of development and use. Part II
On-axis computer-generated hologram optical elements: history of development and use. Part II
Computer-generated hologram elements (CGHOEs) could be useful for solving experimental gas dynamics major problems, including development of optical systems for gas flow visualization in a vast observation field (up to 1000 mm).
Теги: a computer-generated hologram optical element alignment of a centered multicomponent optical system aspherical surface manufacturing technology асферическая поверхность синтезированный голограммный оптический элемент технология изготовления юстировка центрированной многокомпонентной оптической системы
When conducting research of gas dynamic flows, it is necessary to provide a possibility for a larger modeling scale, absence of any alterations of the flow during the experiment, high sensitivity of measurements (to a limit of a thousandth of a visible spectrum’s wavelength) and a series of other specific features [1].
On-axis CGHOEs have found their use in solving problems of experimental gas dynamics. In “NPO “State Institute of Applied Optics” (“NPO “GIPO”) under the supervision of A.F. Belozyorov D.Sc. a complex of research efforts has been conducted on using computer-generated holograms in interference instruments for gas flow visualization in wind tunnels and ballistic tracks and to produce collimator objectives for opto-physical measurement systems (OPM). These works were executed by L.T. Mustafina.
The reason for such research was the fact that technologies which became the basis of shadow, interference and shadow-interference instruments of the 1-st and the 2-nd generations did not fully solve the problems set by researchers. It was necessary to equip modern aerodynamic and ballistic installations with such unique systems which would have allowed to experimentally study the processes of flow along models of planes, helicopters, future aircrafts, automobiles, multi-purpose rockets in near-natural conditions [1]. That is why it was required to develop relatively cheap and compact systems for OPM with work field (visualization field) of 800 to 1000 mm and more.
Prior to that a unified series of unique hologram objects have been developed based on the theory of imaging properties of holograms and elements of hologram optics [2, 3] – complex optical systems of two “contrary” collimators of a large sizes. They perform functions of blocks for illuminating and receiving sections of shadow and interference instruments. Use of hologram objectives in modern OPM systems gave them new properties and considerably increased their technical parameters: diameter of the studied gas flow – 230 to 1000 mm and extremely high values of relative aperture obtained in collimator objectives (from 1:3,5 to 1:2 and even to 1:1). Similar systems are developed in gas dynamics laboratories of world’s leading aerospace companies.
In basic variations of principal schemes of technical formation of the object branch of OPM holographycal systems (Fig. 13) CGHOEs are used either as main objectives (a,b), or as compensators (c-e). When making instruments, combined use of such objectives is also possible because their aberrational characteristics differ substantially. From this point of view, in the illuminating section of the OPM system it is advisable to use high-aperture compact variations (Fig.13 c-e) and in the receiving section – those variations which provide the best quality of images of extensive objects (Fig.13 a,b).
Fig.14a shows frequency characteristics of CGHOEs in objectives and hologram lens objectives with diameters of 230 mm, where CGHOEs are used as spherical aberration compensators. Maximum compensating holograms’ frequency for objectives with relative aperture of 1:3 and 1:2 does not exceed 20 mm-1, whereas for objective holograms it is considerably greater. Estimation has been made for λ = 694 nm (pulse ruby laser). Similar curves for objectives with 400 and 800 mm diameters are set forth on Fig.14b and 14c. Two positions of the lens objective in relation to the compensator have been studied: in the first case – with its convex side towards the compensator, in the second case – with the flat side. Estimations of hologram lens objectives with diameters of 230, 400, 800 mm using 100 mm diameter compensator have been made.
Use of multi-level CGHOEs (meaning levels of etching depth of material in which grooves are “ruled”) which have high diffraction efficiency (up to 90% and more) and decreased noise level is possible in case of low frequency (no more than 30 mm-1). That is why, when making interference instruments of this class, it is more appropriate to use dual-component optical systems consisting of a lens objective and an accordingly estimated hologram compensator (Fig.15).
Compactness of optical systems of such instruments allows for a significant reduction of work space necessary for their placement. In some cases it becomes possible to combine two functions in instruments’ objectives: of optical elements and of protective glass. At that, relief phase structure of CGHOEs can be applied directly to protective glass of gas dynamic installations’ illuminators, which allows to cut the number of necessary large-dimensional optical elements in half.
One of existing methods of producing large-dimensional objectives’ of OPM systems is based on a principle of “wavefront circulation”. The basis of the principle is the use of a hologram optical element (HOE) with a registered wavefront which characterizes the quality of the corrected optical system [1], and transmission of a light wave, restored using HOE, through the optical system of the collimator in “counter motion” of light rays. In this case, due to mutual compensation of aberrations of the actual instrument and the wave, restored using HOE, “aberration-free” wave field is achieved at the exit.
Fig.16 shows a scheme of a four-mirror (3, 4, 12, 14) interferometer based on two contrary main objectives with a narrow reference beam. Each objective consists of a flat-convex lens (optical diameter is 400 mm) and a computer-generated compensating CGHOE hologram (optical diameter is 200 mm). Figures 17 and 18 show other variations of this holographical interferometer which have 400 mm diameter visualization fields.
With the help of an interferometer based on 4-axis computer-generated hologram lenses (Fig.19) an interferogram of gas flow inside the nozzle of an impact tunnel has been obtained (Fig.20). This scheme was used in a shadow-interference instrument IAB-462: visualization field – 230 mm, residual aberrations of the optical system of the instrument do not exceed 5λ, resolution of the optical system – at least 25 mm-1 within the whole field. It is worth mentioning that, in late 1970s – early 1980s, models of interferometers for wind tunnels based on the use of hologram objectives and hologram compensators with visualization field of 400 mm were produced by specialists of “NPO GIPO” for the first time in the world practice.
Fig.21 shows an interferometer assembled using Twyman-Green scheme with a hologram lens objective with a visualization field of 800 mm in diameter, a flat-convex lens 7 in an object branch (optical diameter – 800 mm, focal distance – 1506.6 mm) and a hologram compensator CGHOE 6 (optical diameter 200 mm). Residual wave aberration of the interferometer makes up about ~2λ (Fig.22). It can be substantially compensated using simple holographical aberration correction [4].
Theoretically, diffraction efficiency of physical HOEs, registered in oncoming beams, can reach 100%. It is known [4], that a HOE obtained in oncoming beams with on-axis spherical and off-axis flat wavefronts has same properties as an off-axis parabolic mirror. A shadow system (Fig.23) made using such HOEs acting as mirrors has a high quality.
To study three-dimensional (spatial) gas flows a “sharp focusing” [1] method is used. It is known that the depth of a sharply depicted area decreases with the increase of the aperture of a beam which transmits through a spatial phase object. That is why, in the context of the “sharp focusing” method, transmission of the phase object by a series of elementary beams, which form an aperture angle, is performed. Obtaining of interference images of sections of a spatial gas flow also has a practical interest. To explain the work principle of this method we shall use Fig.24. A coherent light beam W1 illuminates a disperser 1. Between it and a photographic plate 4 phase objects 2 and 3 are located along Oy axis at the distance of Δy from each other. With such illumination method light beams with aperture angles θ2 и θ3 pass through objects 2 and 3 accordingly. A portion of rays within the beam with aperture θ3 passes through a transparent heterogeneity 3. The hologram is obtained using a method of two expositions. The technology of the “sharp focusing” method passed an experimental verification in a gas dynamics experiment: in the wind tunnel of the G.M. Krzhyzhanovsky Energy Institute and at the ballistic track of the A.F.Ioffe Physico-Technical Institute (Fig.25).
Differences which can be observed between shadow images of flows which appear near flying balloons, first three of which (a-c) were obtained from a hologram using the “sharp focusing” method and the last one (d) – using a conventional shadow method, are shown on Fig.26.
One of many practical uses of the “sharp focusing” method is research using wind tunnels and plasma units, when impact of protective glass on the end result must be eliminated. Thing is, when observing gas dynamics tests, protective glass undergoes substantial heating, due to that high demands are made concerning its quality. Use of the “sharp focusing” method allows to decrease these demands by 20 and more times which is especially important for reduction of costs of produced OPM systems with increase of visualization field.
Hence, we have shown that in “NPO GIPO” several types of holographical interferometers had been developed based on the use of CGHOEs, which underwent practical experimental verification and have no current analogues. Uniqueness of this equipment is in having a work field with 230 to 1000 mm, having the ability to multiply the volume of information obtained from a single gas dynamics experiment, to facilitate measurement sensitivity and the start of three-dimensional gas flows study.
On-axis CGHOEs have found their use in solving problems of experimental gas dynamics. In “NPO “State Institute of Applied Optics” (“NPO “GIPO”) under the supervision of A.F. Belozyorov D.Sc. a complex of research efforts has been conducted on using computer-generated holograms in interference instruments for gas flow visualization in wind tunnels and ballistic tracks and to produce collimator objectives for opto-physical measurement systems (OPM). These works were executed by L.T. Mustafina.
The reason for such research was the fact that technologies which became the basis of shadow, interference and shadow-interference instruments of the 1-st and the 2-nd generations did not fully solve the problems set by researchers. It was necessary to equip modern aerodynamic and ballistic installations with such unique systems which would have allowed to experimentally study the processes of flow along models of planes, helicopters, future aircrafts, automobiles, multi-purpose rockets in near-natural conditions [1]. That is why it was required to develop relatively cheap and compact systems for OPM with work field (visualization field) of 800 to 1000 mm and more.
Prior to that a unified series of unique hologram objects have been developed based on the theory of imaging properties of holograms and elements of hologram optics [2, 3] – complex optical systems of two “contrary” collimators of a large sizes. They perform functions of blocks for illuminating and receiving sections of shadow and interference instruments. Use of hologram objectives in modern OPM systems gave them new properties and considerably increased their technical parameters: diameter of the studied gas flow – 230 to 1000 mm and extremely high values of relative aperture obtained in collimator objectives (from 1:3,5 to 1:2 and even to 1:1). Similar systems are developed in gas dynamics laboratories of world’s leading aerospace companies.
In basic variations of principal schemes of technical formation of the object branch of OPM holographycal systems (Fig. 13) CGHOEs are used either as main objectives (a,b), or as compensators (c-e). When making instruments, combined use of such objectives is also possible because their aberrational characteristics differ substantially. From this point of view, in the illuminating section of the OPM system it is advisable to use high-aperture compact variations (Fig.13 c-e) and in the receiving section – those variations which provide the best quality of images of extensive objects (Fig.13 a,b).
Fig.14a shows frequency characteristics of CGHOEs in objectives and hologram lens objectives with diameters of 230 mm, where CGHOEs are used as spherical aberration compensators. Maximum compensating holograms’ frequency for objectives with relative aperture of 1:3 and 1:2 does not exceed 20 mm-1, whereas for objective holograms it is considerably greater. Estimation has been made for λ = 694 nm (pulse ruby laser). Similar curves for objectives with 400 and 800 mm diameters are set forth on Fig.14b and 14c. Two positions of the lens objective in relation to the compensator have been studied: in the first case – with its convex side towards the compensator, in the second case – with the flat side. Estimations of hologram lens objectives with diameters of 230, 400, 800 mm using 100 mm diameter compensator have been made.
Use of multi-level CGHOEs (meaning levels of etching depth of material in which grooves are “ruled”) which have high diffraction efficiency (up to 90% and more) and decreased noise level is possible in case of low frequency (no more than 30 mm-1). That is why, when making interference instruments of this class, it is more appropriate to use dual-component optical systems consisting of a lens objective and an accordingly estimated hologram compensator (Fig.15).
Compactness of optical systems of such instruments allows for a significant reduction of work space necessary for their placement. In some cases it becomes possible to combine two functions in instruments’ objectives: of optical elements and of protective glass. At that, relief phase structure of CGHOEs can be applied directly to protective glass of gas dynamic installations’ illuminators, which allows to cut the number of necessary large-dimensional optical elements in half.
One of existing methods of producing large-dimensional objectives’ of OPM systems is based on a principle of “wavefront circulation”. The basis of the principle is the use of a hologram optical element (HOE) with a registered wavefront which characterizes the quality of the corrected optical system [1], and transmission of a light wave, restored using HOE, through the optical system of the collimator in “counter motion” of light rays. In this case, due to mutual compensation of aberrations of the actual instrument and the wave, restored using HOE, “aberration-free” wave field is achieved at the exit.
Fig.16 shows a scheme of a four-mirror (3, 4, 12, 14) interferometer based on two contrary main objectives with a narrow reference beam. Each objective consists of a flat-convex lens (optical diameter is 400 mm) and a computer-generated compensating CGHOE hologram (optical diameter is 200 mm). Figures 17 and 18 show other variations of this holographical interferometer which have 400 mm diameter visualization fields.
With the help of an interferometer based on 4-axis computer-generated hologram lenses (Fig.19) an interferogram of gas flow inside the nozzle of an impact tunnel has been obtained (Fig.20). This scheme was used in a shadow-interference instrument IAB-462: visualization field – 230 mm, residual aberrations of the optical system of the instrument do not exceed 5λ, resolution of the optical system – at least 25 mm-1 within the whole field. It is worth mentioning that, in late 1970s – early 1980s, models of interferometers for wind tunnels based on the use of hologram objectives and hologram compensators with visualization field of 400 mm were produced by specialists of “NPO GIPO” for the first time in the world practice.
Fig.21 shows an interferometer assembled using Twyman-Green scheme with a hologram lens objective with a visualization field of 800 mm in diameter, a flat-convex lens 7 in an object branch (optical diameter – 800 mm, focal distance – 1506.6 mm) and a hologram compensator CGHOE 6 (optical diameter 200 mm). Residual wave aberration of the interferometer makes up about ~2λ (Fig.22). It can be substantially compensated using simple holographical aberration correction [4].
Theoretically, diffraction efficiency of physical HOEs, registered in oncoming beams, can reach 100%. It is known [4], that a HOE obtained in oncoming beams with on-axis spherical and off-axis flat wavefronts has same properties as an off-axis parabolic mirror. A shadow system (Fig.23) made using such HOEs acting as mirrors has a high quality.
To study three-dimensional (spatial) gas flows a “sharp focusing” [1] method is used. It is known that the depth of a sharply depicted area decreases with the increase of the aperture of a beam which transmits through a spatial phase object. That is why, in the context of the “sharp focusing” method, transmission of the phase object by a series of elementary beams, which form an aperture angle, is performed. Obtaining of interference images of sections of a spatial gas flow also has a practical interest. To explain the work principle of this method we shall use Fig.24. A coherent light beam W1 illuminates a disperser 1. Between it and a photographic plate 4 phase objects 2 and 3 are located along Oy axis at the distance of Δy from each other. With such illumination method light beams with aperture angles θ2 и θ3 pass through objects 2 and 3 accordingly. A portion of rays within the beam with aperture θ3 passes through a transparent heterogeneity 3. The hologram is obtained using a method of two expositions. The technology of the “sharp focusing” method passed an experimental verification in a gas dynamics experiment: in the wind tunnel of the G.M. Krzhyzhanovsky Energy Institute and at the ballistic track of the A.F.Ioffe Physico-Technical Institute (Fig.25).
Differences which can be observed between shadow images of flows which appear near flying balloons, first three of which (a-c) were obtained from a hologram using the “sharp focusing” method and the last one (d) – using a conventional shadow method, are shown on Fig.26.
One of many practical uses of the “sharp focusing” method is research using wind tunnels and plasma units, when impact of protective glass on the end result must be eliminated. Thing is, when observing gas dynamics tests, protective glass undergoes substantial heating, due to that high demands are made concerning its quality. Use of the “sharp focusing” method allows to decrease these demands by 20 and more times which is especially important for reduction of costs of produced OPM systems with increase of visualization field.
Hence, we have shown that in “NPO GIPO” several types of holographical interferometers had been developed based on the use of CGHOEs, which underwent practical experimental verification and have no current analogues. Uniqueness of this equipment is in having a work field with 230 to 1000 mm, having the ability to multiply the volume of information obtained from a single gas dynamics experiment, to facilitate measurement sensitivity and the start of three-dimensional gas flows study.
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