Temporal coherence length and speckle: a simultaneous approach to those problems in holography; authors' reply to comment.
It appears that the comment of Yu on our Letter, which relates to one of his previous works, has arisen due to misunderstanding. We believe the reason is due to the lack of sufficient explicitness in the papers involved. The main assertion of our Letter (third paragraph) is that a forceful reduction of one hologram into smaller subholograms for speckle averaging does not, in general, improve the over-all quality of the image (Fig. 3, Ref. 2) unless the entire object spectrum can be contained effectively within each of the subholograms. Yu and Wang's claim that the "hologram image speckle can be made to disappear" has been contradicted by Kermisch's analysis, which establishes an upper limit in the reduction of speckle contrast. It is true that Yu and Wang briefly stated that the "speckle reduction is possible" at the cost of resolution. But their paper does not make the following points explicitly clear: (1) that, due to the reduction in area of any one of their sampling subapertures, the average physical size of the individual speckle increases, and (2) that their random-aperture-sampling introduces a severe reduction in the object contrast (explicitly analyzed by Kermisch and also reported by Hariharan and Hegedus). Besides, we assert that the over-all quality of the image of a general object reconstructed from the entire hologram at a time is better than the incoherently speckleaveraged image after dividing the same hologram into many subholograms.