Precision optical diagnostics

One of the important directions in the activities of the ND&O Division is precision optical metrology and development of methods of precision optical mea-surements. In the recent years, IAP RAS researchers under the direction of I. E. Kozhevatov solved a number of problems in the field of high-precision re-gistration of optical fields.

Measured phase distribution
in a laser beam

Point diffraction interferometer based on a single-mode optical fiber. An interference method for measuring optical fields from remote objects and external sources, in which a reference beam is obtained from a studied beam by filtering its spatial spectrum by a single-mode fiber is developed. This method of formation of a reference beam minimizes the impact of air fluctuations on the path of probe radiation on measurement accuracy. A new algorithm of reconstructing the phase of the interference pattern reduces significantly the influence on measurement accuracy of vibrations of the elements of interfero-meter optical scheme. To eliminate the impact on the measurement results of the phase errors caused by non-ideal elements of the optical scheme, a procedure for calibrating the interferometer was developed that permits determining errors and their further subtraction from the measured distributions of the field phase. The above mentioned developments allowed attaining nanometer accuracy in measuring the shape of remote optical elements and the phase of the wave fields from external sources.

Optical raster method for subwavelength resolution imaging. A raster method for measuring a near-field structure and characteristics of surfaces with subwavelength spatial resolution was proposed. The method gives information about the structure of a sample simultaneously over the entire surface, providing much faster data acquisition than the traditional near-field scanning optical microscopy. The idea underlying the method is the transformation of the near light field into propagating waves by a two-dimensional raster with subwavelength structure located in the immediate neighborhood of the surface under study. The structure of the surface is determined from measurements in the far field by conventional interferometry or microscopy. The method will allow obtaining three-dimensional images of biological objects and elements of future quantum computers with resolutions inaccessible to conventional optics. The method was tested in numerical experiments, a device for its experimental verification has been assembled.

Simplified diagram
of raster method

Calibration of the spatial distribution of multielement photodetector sensitivity. Accurate recording of the spatial structure of optical radiation intensity is essential for precision optical metrology, particularly, in registering optical wave fields and in three-dimensional imaging. The main factor limiting measurement accuracy of spatial intensity distributions is spatial inhomogeneity of the sensitivity of light recording devices — CCD and CMOS photodetector matrices.
A method of calibrating spatial distribution of photodetector matrix sensitivity was developed and tested experimentally. The main idea of the method is measurement of light beam intensity distribution stable in time at several different combinations of relative positions of the measured two-dimensional intensity distribution and the photodetector matrix. Under certain conditions for the number and combination of experiments, this method permits obtaining a data array from which the distribution of photodetector matrix sensitivity and the light beam intensity distribution are reconstructed.
The scheme of the experimental setup, operating instructions and algorithm of reconstructing spatial distributions of photosensitivity from results of mea-surements were developed. Experiments on calibrating spatial distributions of CCD matrix photosensitivity and measurements of intensity distribution of optical radiation by means of the calibrated matrices were performed.

Results of the experiment on calibrating spatial distribution of CCD matrix sensitivity. Top — measured intensity distribution (x, y) (left); reconstructed intensity distribution I (x, y) (center) and photosensitivity (right) of CCD matrix M (x, y). Bottom — the corresponding one-dimensional distributions at fixed coordinate y. The x and y coordinates are elements of the matrix