Super resolving systems: Toraldo Pupils

(PUTO Project)

 

 

 

 

A history coming from afar

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More than half century ago, in 1952, Giuliano Toraldo di Francia (photo on the right) presented a theoretical work* in which he demonstrated that with the use of special types of composed pupils (Toraldo Pupils) it was possible to increase the resolving power of an optical instrument. Thus was born the concept of super-resolution, which refers to various methods for improving the angular resolution of an optical system beyond the classical diffraction limit, which is related to the size of the pupil diameter D by the relation 1.22λ/D (𝜆 is the wavelength). The practical consequence of a limitation of this kind was that, with an optical instrument, it was not possible to see two points the distance of which is less than 1.22𝜆/D.

Over the years, much speculation has ensued in order to understand if (and how) it could be possible (or not) to go below this diffraction limit.

An advancement in knowledge was obtained with the development of microwave techniques in relation to research on highly directives antennas.  Probably, and paradoxically, this improvement was due to a certain ignorance on the part of the "microwaves technicians" in relation to wave optics principles. In fact, as pointed out by Toraldo, “microwave researchers were not very much concerned, or perhaps even acquainted, with the old and well-established theorems of wave optics, according to which no material improvement of the uniform pupil should have been possible. As a result, an entirely new theory has been set up, which contains many revolutionary implications”.

From 1952 to now, many efforts have been devoted to this topic, which is interesting also for its theoretical implications. With the advancement in technology it has been possible to perform experiments that have demonstrated the correctness of Toraldo predictions both in the field of microscopy and in the spectral region of microwaves, a region in which the first successful laboratory test of Toraldo Pupils was achieved in 2003: based on these results, a possible practical approach in the field of Radio Astronomy has been taken into account.

 

 

Something more about the theoretical model

 

 

 

 

 

* - G. Toraldo di Francia, Il Nuovo Cimento IX, Suppl. , 426 (1952).

     - G. Toraldo di Francia, Atti Fond. Giorgio Ronchi 7, 366 (1952).