Work of elements and systems of the solar power engineering is related with the redistribution in space and time and transformations of energy of sun radiations. The eventual result of such transformations is the effective receipt of electric and thermal energy suitable for the application by users. Energy effectiveness of sun equipment is largely determined by perfection of technologies of modulation, spatial control and spectral transformation of optical rays on the stages of collection and perception of sun radiations.  

Technologies of spatial control of optical radiations are based on the use of scanning devices - optical deflectors, intended for the change in accordance with the set law of spatial position of optical beam in time. The most known constructions of optical deflectors that are used in today’s solar power engineering are heliostats of surveillance after the Sun constructed on the basis of flat or profiled mirrors with the purpose of collection of maximal amount of sunbeams and direction of these rays onto active photo-voltaic (PV) elements (concentrator photovoltaic systems), or onto the active thermo-absorbing environments (thermal systems of tower type, though systems and dish systems). These simplified optical deflecting systems are not in a complete measure accomplished. When meeting with the optical radiations they distort wave front and, thus, can negatively affect the interference picture of focused sun radiations and diminish an effective light flux that interacts with the matter of photovoltaic receiver or thermo-absorbing body. Possibilities of optical radiations control within elements and systems of sun energy are broadening by modernization of this and adaptation of many other principles, which lay in a base of the work of optical deflectors. 

To the optical deflectors many publications both in our country and in foreign countries are devoted. Part of publications represents the examples of calculation of parameters. In other publications the value of parameters is brought only without the sufficient argued exposition of methods of their achievement. The purpose of the given article is to conduct of analysis of modern development state of optical deflectors, to acquaint of specialists with the features of construction and principles of action of separate types of optical deflectors, to give recommendations to the developers of optical-electronic apparatus in the choice of concrete type of optical deflector.

Classification and principles of action. Optical deflectors can be classified in accordance with different signs: in accordance with the mode of interaction of optical radiations with an active environment, physical principles and phenomena, that lie in a base of control of the parameters of optical environment, character of reflection of ray, time of switching, spatial motion of beam and possibility of focusing.

In accordance with the mode of interaction of optical radiations with an active environment optical deflectors can be divided onto the reflective, refractive, diffractive, birefringence (double refraction) and polarization devices; in obedience with the character of reflection of a ray – onto the continuous (gradient) and discrete devices; according to the spatial motion of the ray – onto single coordinate (1D) devices, two-coordinate (2D) devices and three-coordinate (3D) devices; according to the time of switching – onto inertias and low-inertial devices; in accordance with the possibility of focusing of optical rays – onto elements with focusing and without focusing. 

In accordance with physical principles, that lie in the basis of work, optical deflectors are divided into optical-mechanical (OM), optical-mechanical holographic (OMH), magnetic-electrical (ME) or galvanometers, electromagnetic (EM), piezoelectric (PE), magnetostrictive (MS), magnet-optical (MO), acousto-optic (AO) and electro-optical (EO) devices.

Parameters and characteristics. Optical deflector (fig.1) is characterized by concrete parameters which determine a possibility of his use in different devices and systems. The most essential parameters are: law of scanning, amplitude of angle of scanning, resolution, distortion of front of optical wave, frequency of scanning, range of frequencies of scanning, bandwidth, velocity, optical reduction, linear aperture of optical scanning beam, divergence of optical beam, spectral optical range of work, optical losses, electric voltage and current, sensitiveness to reflection, mass and overall sizes, firmness to vibrations, stability of characteristics with the change of condition of environment (temperature, pressure and etc.).

The law of scanning determines the character of motion of the ray. Scanning can be linear, sinusoidal, saw-like, circular, spiral or some other.

Amplitude of the angle of scanning Δα_max characterizes the maximal angular motion of optical beam. It is determined in radians or angular degrees.

Resolving power (Resolution) N is defined by the number of different directions of optical beam that are laid within the limits of the angle of scanning Δα_max. For estimation of the resolution a Rayleigh criterion is mainly used. If you look at an object, light entering your eye creates a diffraction pattern on your retina. When two objects are separated by a small angle, the diffraction patterns overlap. You are able to resolve the two objects as long as the central peaks in the two diffraction patterns don't overlap. The limit is when one central peak falls at the position of the first dark fringe for the second diffraction pattern. This is known as the Rayleigh criterion. When the central peaks overlap the two objects look like one.

In obedience with the Rayleigh criterion a divergence of optical beam is determined by the next relation γ = ξλ/(nD), where λ is a wavelength of radiations; D – aperture diameter of optical beam; n – index of refraction of optical environment; ξ – coefficient, that relies on the form of beam,  ξ = 1,27 for a beam with the Gaussian intensity distribution, ξ = 1,22 for the beam of circle form with the permanent distribution of intensity. Resolving power of one-axe scanning deflector at the absence of distortions that are brought in by a deflector into the aperture of optical beam, is determined by N = Δα_max/γ = Δα_maxnD/(ξλ).

The resolution of focusing scanning devices of the type of «deflector-lens» is estimated in the same way (fig.2). The diameter of optical spot d_spot in the focal plane of a lens with the operating aperture diameter D_lens and focal distance f¹ is determined by d_spot = ξλf¹/(nD_lens).

 The length of line of scanning is calculated as follows: l_scan = f¹Δα_max. In this case the resolving power N of optical deflector is determined by the relation of length of line of scanning l_scan to the diameter of optical spot d_spot or N = l/d_spot = Δα_maxnD_lens/(ξλ).

Resolving power N is more important parameter, than the angle of scanning Δα. Because the angle of scanning can be increased  by the use of the proper optical system, and resolving power here remains unchanging or, in worst case, diminishes, so as expression ΔαD = ξλN remains unchanging. As it is visible from this expression, the value of resolving power can be attained in two ways: by deflecting of light beams of large linear aperture D onto the small angles and by deflecting of beams of small aperture on the large angles.

Distortion of front of optical beam can be estimated by the angle of distortion ψ_a.d and by a coefficient of linear distortion β_ld.

The angle of distortion ψ_a.d determines the measure of increase or reduction of angular divergence of optical beam after passing through optical deflector: ψ_a.d = γ_out – γ_in,, where γ_in and γ_out – angular divergence of optical beam on the entrance and output of optical deflector.

The coefficient of linear distortion β_ld determines the change of linear sizes of optical beam at his deflection. If an optical beam on the entrance into optical deflector has the rounded form of the transversal crossing, the coefficient of linear distortion estimates ellipticity: β_ld = [(D¹¹_out - D¹_out)/D¹_out]x100, where D¹¹_out and D¹_out – linear sizes of the transversal crossing of optical beam on the output of optical deflector in two mutually perpendicular directions. Angular and linear distortions of optical beam diminish a resolving power of optical deflectors.

Frequency of scanning (scanning frequency) f_s determines the number of periods of oscillations of optical beam at his spatial motion from one element to the other. Part of deflectors is working only on one fixed frequency, other part – within the range of frequencies f_s1 - f_s2. The range of frequencies of scanning is one of the most important parameters of optical deflector.

The bandwidth Δf characterizes the quality of optical deflector and is estimated by a product of scanning frequency onto the resolution. If the index of refraction of environment is equal of 1, Δf = Nf_s = Δα_maxf_sD/(ξλ).

The fast-acting of optical deflectors t_od determines a velocity of the spatial motion of optical ray at his transition from one resolution element onto a neighboring one. For continuous deflectors this value is t_od = 1/Δf = 1/Nf_s.

Optical reduction i_or estimates the measure of disparity of the angles of rotation of optical deflector α and of optical ray Δα: i_or = Δα/α.

A possible linear aperture D_in and possible angular divergence γ_in of optical beam determine the legitimate values of the named parameters, which provide a normal work of optical deflector. A possible linear aperture D_in of optical beam determines the maximal values of resolution, bandwidth and fast-acting of scanning devices.

A spectral optical range Δλ characterizes the range of wavelengths of optical radiations, within which optical deflector can work.

The optical losses are determined by the coefficient of transmission of flux of optical radiations: τ = Ф_e.out/Ф_e.in= I_e.out/I_e.in,, where Ф_e.in and Ф_e.out – fluxes of radiations upon entrance and exit of deflector, I_e.in and I_e.out – density of flux of radiations upon entrance and exit of optical deflector.

Some materials that can be used in the scanning devices are estimated by optical density D_dens. It is calculated as logarithm of relation of density of input flux to density of output flux: D_dens = lg(1/τ) = lg(I_e.in/I_e.out). Optical density of material that is attributed to the layer of material with thickness of 1 sm is called the coefficient of absorption K. The coefficient of transmission, optical density and coefficient of absorption rely on a wavelength λ of optical radiations.

Electric voltage u_el.max and current i_el.max determine the electric parameters of optical deflector, at which the maximal values of amplitude of deflection angle or of resolution are achieved.

Sensitiveness S_sens characterizes the value of deflection angle of optical beam or amount of elements of resolution at controlling action of certain value. If deflector is controlled by electric voltage, a sensitiveness S_sens is determined by correlation S_sens = Δα/u_el or S_sens = N/u_el. For more complete parameter specification and estimation of the possibility of the use of optical deflector in the certain systems it is necessary to define their frequency characteristic, amplitude characteristic and volt-ampere characteristic.

Frequency characteristic determines dependence of resolution N or of amplitude of deflection angle Δα from the frequency of controlling electric voltage: N = f(f) and Δα = f(f). On the basis of this description a working frequency of line scanning f_lin or a range of working frequencies f_s1 - f_s2 can be chosen.

Amplitude characteristic determines dependence of resolution N or amplitude of deflection angle Δα from electric power P_el (electric voltage u_el or strength of electric current i_el): N = f(P_el);  Δα = f(P_el); N = f(u_el);  Δα = f(u_el); N = f(i_el);  Δα = f(i_el). With the frequency control the amplitude characteristic determines dependence of amplitude of deflection angle or of resolution from a frequency of controlling signal.

In subsequent sections the analysis of parameters and characteristics of existent optical deflectors will be conducted.

By Vasil Sidorov on September 04, 2010 in queltanews.com

Technopark QUELTA,

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com

References

1.     Rebrin Y.K., Sidorov V.I. // Optical Deflectors. Kiev: Tékhnika, 1988. 136 pp.

2.     Rebrin Y.K., Sidorov V.I. Optical mechanical and holographic deflectors // Results in science and technology. Radio engineering. Vol. 45. - Moscow: VINITI, 1992. - 252 pp.

3.     Rebrin Y.K., Sidorov V.I. Holographic devices of control of optical ray. – Kiev: KHMAES, 1986. - 124 pp.

4.     Rebrin Y.K., Sidorov V.I. Piezoelectric multielement devices of control of optical ray. – Kiev:  KHMAES, 1987. - 104 pp.