Nestled between Nuremberg and the Franconian Switzerland in southern Germany, Erlangen is a mid-sized college town. Its pleasant, but unspectacular, outward appearance belies the high regard with which Erlangen is held within the optics community. Its outsized international reputation relative to its size is due largely to Adolf Lohmann and the Lehrstuhl für Angewandte Optik (Chair in Applied Optics) that he founded in 1974 at the Friedrich Alexander Universität and led until his retirement in 1991. For imaging scientists and researchers in optical processing, visiting Erlangen was a rite of passage.

Prof. Lohmann passed away at age 87 in 2013, but his legacy is broad and deep. Most notably, the Max Planck Institute for the Science of Light sits across from the original location of the Lehrstuhl in a new facility that opened in 2016.

Each of us had a personal connection to Prof. Lohmann. Two of us (MT and STS) were students, who worked directly with him, and the third (JM) was a visiting professor on two occasions in Erlangen, once in 1986 and again in 1990.

As a teacher, Adolf Lohmann was revered by students and colleagues for his unconventional style invariably attempting to discover a new twist to seemingly well-known phenomena. Much of this style is captured in his lecture notes [1].

This feature issue of Applied Optics pays tribute to Adolf Lohmann’s research. It is a testimony to the relevance and influence of his work today, decades after its publication.

A trademark of Adolf Lohmann’s work is his care for detail and the interest in a comprehensible interpretation of optical phenomena. One of his earliest publications [2] uses what is now known as the angular spectrum method to reinterpret diffraction and image formation. The angular spectrum method is one of the corner stones for numerical simulation of free-space diffraction, and the contribution of Brenner and Mehrabkhani in the feature issue is proof of the continued interest in this approach.

By interpreting free-space propagation of periodic wavefronts in terms of the fractional Fourier transform, Schnebelin and Guillet de Chatellus, in fact, combine several themes championed by Adolf Lohmann, namely, self-imaging, also known as the Talbot effect, and the fractional Fourier transformation, to provide a new formulation of paraxial free-space propagation.

The work by Awwal et al. connects Adolf Lohmann’s work on Fourier phase information [3] to the design of optical matched filters necessary to automate the alignment of high-energy laser beams. The technique is used to initiate nuclear fusion reactions at the National Ignition Facility (Lawrence Livermore National Laboratory).

It was Adolf Lohmann’s lifelong passion to apply optical technology to signal and information processing. When he was selected as the first recipient of the Optical Society of America’s Emmett Leith medal, he expressed satisfaction in a private conversation with one of us (MT) that the award recognized in particular his contributions to optical information processing. Many contributions to this feature reflect this emphasis in more than one way.

Adolf Lohmann was a strong promoter of the Wigner distribution function and phase space methods as an alternative and beneficial formulation of optical system theory. Testorf’s contribution to the feature issue tries to capture Adolf Lohmann’s unique approach to phase-space optics.

Early on, Adolf Lohmann recognized the significance of the fractional Fourier transform as a means to develop novel applications in optics and as a tool for optical system analysis. The paper by Oktem and Ozaktas on formulating bounds for the signal recovery from incomplete data recorded in two fractional Fourier transform planes, and the contribution by Mico et al. on optical encryption by changing the transforming properties of lenses through tilting, both exemplify these themes.

In describing progress in intensity interferometry, Shoulga’s and Ribak’s papers pay tribute to Adolf Lohmann’s work on the bispectrum [4] and triple correlation [5]. Similarly, Mait et al. highlight the continued importance of Lohmann and Rhodes’ contribution to aperture synthesis [6].

Adolf Lohmann was also a pioneer of holography and, apart from Emmett Leith, one of the first to advance the field by reinterpreting holography in terms of a communication system. Both single-sideband holography [7] and his seminal contributions to computer-generated holograms [8] resulted from this innovative perspective.

The principles of computer-generated holography lie at the heart of diffractive optics design. In this feature issue, Hasegawa et al. demonstrate the use of computer-generated holograms to 3D display technology, and Jahns’ contribution highlights the design of diffractive optics with rotational symmetry. Combining computer-generated holography with the concept of the Lohmann–Alvarez lens [9], Grewe et al. describe the design of diffractive optical elements where the element function can be altered by mutually rotating sandwiched elements.

Adolf Lohmann’s interest in combining diffractive optics and optical information processing is well represented in this feature issue by Shwartz’s work on diffractive optics for optical fiber communication as well as the paper by Ledesma-Carrillo et al. on pupil design for imaging with extended depth of field. In this context, the work of Wilde et al. connects diffractive optics to superresolution imaging, yet another area of research pioneered by Adolf Lohmann [10]. Tradonsky et al. describe the use of Talbot array illuminators to phase-lock laser modes, a further tribute to Adolf Lohmann’s interest in the Talbot effect. Finally, Bichra et al. describe the use of diffractive optics and Talbot self-imaging for innovative wavefront sensing.

While all contributions to this special feature celebrate Adolf Lohmann, the scientist, we also pay tribute to an exceptional human being. Ever conscious of the burden of history borne by Germany, Adolf Lohmann was committed to building bridges between people, not walls. In the same manner that he questioned technical assumptions, he was skeptical of those who saw the world in black and white.

Adolf Lohmann (shown in Fig. 1) used science to overcome boundaries between people. His scientific reputation shaped the Lehrstuhl für Angewandte Optik into a place of exceptional diversity. All three of us fondly remember a research environment, where an international group of scientists shared a passion for the optical sciences, regardless of their social, cultural, racial, or religious upbringing.

 

Fig. 1. Adolf Lohmann and his wife Karla. The photograph was taken in 2006 on the occasion of his 80th birthday (courtesy of Prof. Dr. Gerd Häusler).

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