Lidars

Alongside with adaptive optics and its methods to struggle with distortions of optical signals by classical means, experts of ASC "Femto" consider a very perspective direction of the coherent optics that works directly with phases of optical signals. As phases of light waves of natural sources do not fall under influence in the given section of optics, it is necessary to work with lasers since frequencies and phases of their radiation give in to greater level of control.

The mentioned direction of engineering yet does not have generally accepted name. We call it "a coherent location" though sometimes solved problems are not, broadly speaking, connected with location.

To illustrate opportunities of a coherent location we shall consider some examples of the systems developed in ASC "Femto" which are intended for the solution of classical optical problems. First of all, it is the problem of increasing the angular resolution of optical systems. The angular resolution is one of the major characteristics of technical vision systems. Theoretically, the ratio of wavelength to diameter of the entrance aperture defines achievable angular resolution.

The essential increase of diameter of the receiving aperture encounters serious technological difficulties. The graceful exit from this drawback was found in a radar-location with development of the active synthesis method of the receiving aperture. Application of this method has enabled the increase of angular resolution of radars (for example, radar of the lateral review) on qualitatively new level. We have developed the method of synthesis of large space-time apertures in optical range of wavelengths. In the future, this approach may probably become the base for observation of the remote moving objects.

The essence of the method is the illumination of the observable object with narrow-band radiation of the laser. Because of movement (or rotation) of object relative to the position of transmitter-receiver, the reflected field moves along the plane of observation with the velocity determined by the tangential velocity of the object and by its angular speed of rotation. If a linear array of coherent receivers is arranged in the plane of observation, i.e. receivers in which the input field is mixed up with the reference light wave, the field reflected by the object can be registered in the large area, which at processing of input signal may be considered as the entrance aperture of the system. Due to coherent (homodyne or heterodyne) method of signal receiving, the data pertaining to the amplitude and phase of a signal is acquired.

Such method of synthesis of the aperture is called "space-temporal". On the spatial coordinate determined by a linear array of receivers, the given method is similar to well known passive synthesis. In temporal domain the method is equivalent to the method of Doppler spectra.

An additional useful advantage of the given method is the circumstance, that at coherent reception of an optical signal it is very simply to execute its narrow-band filtration, which enables equally effective day and night work (the coherent receiver is practically insensitive to sunlight). For the achievement of high angular resolution it is necessary to solve the problem of correction of space-temporal phase distortions on the synthesized aperture, caused by the various reasons:

  turbulent atmosphere
  time instability of phase of laser radiation
  radial acceleration of moving object
  initial unphasing of coherent receivers

The processing of data is carried out on a computer and the diffraction-limited image displayed on the monitor screen. The efficiency of this approach was confirmed by the numerous experiments.

ASC "Femto" delivers custom-built experimental installation for demonstration of this method by observation of rotating test objects at distances of up to several kilometers. The size of the synthesized receiving aperture in demonstration experiments may differ on Customer requirements from tens to hundreds of meters.

The problem is inversed when it is necessary to generate extra narrow laser beam of high power (for example, for the location of the removed objects) that is, to attain the high angular resolution not at reception of a signal, but at its transfer. In the course of resolution of this problem the increase of power and the decrease of beam divergence of individual irradiator encounters significant engineering difficulties, as soon as certain level of power is overcome. Besides, delivery of high density radiation is connected with emergence of thermo-optic deformations in the optical path and in the environment of propagation. Ii addition, the presence of turbulent distortion of atmosperic refraction index makes the problem of use of laser radiation at appreciable distances inevitably demanding the necessity of creation of complex and expencive adaptive systems.

Alternative way of creation of laser system with narrow beam is coherent summarizing of beams with a direct control of frequencies and phases. The development of this trend will allow the building of systems with easily increased level of power density; it will provide an opportunity to use the methods of adaptive optics for compensation of turbulent distortions without resorting to complex cooled deformable mirrors. ASC "Femto" has wide experimental experience in phasing of independent continuous CO2-lasers.

Direct phase measurement of an optical signal allows facing classical location problems in a new fashion. Thus, with the use of not so complex an equipment it is possible to create systems of speed measurement with accuracy of the order of 1mm/s or range measuring instruments with accuracy better then 1mm. The latter system is presently at the stage of manufacturing of a pre-production model.


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