Bell believed the photophone's principles were his life's "greatest achievement", telling a reporter shortly before his death that the photophone was "the greatest invention [I have] ever made, greater than the telephone". The photophone was a precursor to the fiber-optic communication systems which achieved popular worldwide usage in the 1980s. Its master patent was issued in December 1880, many decades before the photophone's principles came into popular use.
Try to answer this question if possible (otherwise reply "unanswerable"): What modern technology takes the next step from the photophone?
fiber-optic communication