Over the last year we have been celebrating the 20th Anniversary of Optics Express, which published its first issue in July 1997. The all-electronic, open access model introduced by Optics Express proved to be highly successful. Despite competition from the stream of new open access publications that have since entered the optics and photonics space, Optics Express remains vigorous: we received more than 6,500 submissions in 2017, our most ever, and published more than 3,000 papers! We have an outstanding editorial board, which now includes more than 120 scientists and engineers, and maintain a very fast editorial and production process (2017 median time to publication: only 65 days). Optics Express enjoys a Google Scholar h-5 index of 104, which gives it a #3 ranking among publications in Optics & Photonics.

In a previous editorial I announced a number of special features in celebration of this anniversary year [1]. These include guest editorials from our previous Editors-in-Chief [2–4], Editors’ Picks Collections representative of important topic areas that have been covered by the journal, and a list of our 100 most cited articles. These and other materials may be found at our Anniversary web site.

To wrap up our celebration, we have recruited roughly a dozen distinguished authors to write invited review or perspective articles. I am excited now to announce the publication of the first three of these articles. Additional review papers will be published in groups within the next few months prior to end of our year-long anniversary celebration in July. All of the review papers will be compiled on our Anniversary web site; when complete, they will also constitute a virtual feature issue.

In the current group, an article by Dan Mittleman of Brown University is entitled “Twenty years of terahertz imaging” [5]. Mittleman points out that the field of terahertz (THz) imaging began around the same time as the launch of Optics Express and “has for the most part grown up alongside Optics Express.” Furthermore, Optics Express has been a highly attractive venue for papers in this field. As noted by Mittleman, searching for the phrase “terahertz imaging” on the Optics Express webpage brings up over 300 articles. This represents roughly a quarter of all the articles on “terahertz” published in Optics Express. After providing a historical background, the article gives a focused discussion on a selection of recent hot topics, including the push toward faster image acquisition, near-field THz imaging, and applications in nondestructive investigation of works of art and historical artifacts.

Frank Wise of Cornell University and colleagues have prepared a perspective entitled “Several new directions for ultrafast fiber lasers” [6], accompanied by an extensive bibliography numbering nearly 250 entries. Wise et al. point out that although nonlinearity is a key ingredient for mode-locking, mode-locked lasers have traditionally incorporated “just enough nonlinearity to obtain short pulses, and no more.” However, this philosophy limits attainable peak intensities, particularly for fiber laser systems. The article begins with a discussion of mode-locked fiber lasers that embrace new modes of highly nonlinear operation to enhance pulse energies by orders of magnitude, then moves on to provide a perspective on frontier topics such as nonlinear pulse propagation in multi-mode fibers with possible application to mode-locking in both space and time.

The final paper in this initial set is entitled “The harmonic state of quantum cascade lasers: origin, control, and prospective applications” [7]. Written by Federico Capasso of Harvard University and a group of collaborators, this paper takes a deep dive into a current hot topic involving multimode laser operation. Multimode oscillation of interband semiconductor lasers is well known and has been extensively studied. However, the unipolar, intraband character of quantum cascade lasers (QCLs) brings new physics, such as ultrafast gain recovery dynamics, which may lead to new multimode oscillation phenomena. The so-called “harmonic state” refers to an intriguing multimode regime characterized by oscillation on a series of widely spaced longitudinal modes separated by many free spectral ranges (FSRs), distinct from the more common dense multimode state, which features oscillation on adjacent FSRs. The authors report experimental and theoretical investigations into the physics of this state and discuss potential applications such as short pulse generation and broadband spectroscopy in the mid-infrared.

At this point I would like to express my appreciation to a number of individuals who helped with the organization of our Anniversary celebration, including selection of Editors’ Picks and recruitment and processing of invited reviews. Senior Deputy Editor Jim Leger led these selections, assisted by current and former Deputy Editors Miguel Alonso, Chris Dainty, John Dudley, Magnus Karlsson, Guifang Li, Takeshige Omatsu, Vitor Schneider, and Christian Seassal, as well as the Editor-in-Chief of Optical Materials Express, Sasha Boltasseva. I would also like to specifically thank members of the OSA Publications staff for their contributions to many aspects of the Anniversary celebrations: Kelly Cohen, John Long, Bob Sumner, Sika Dunyoh, Julie Rovesti, Daphne Greenwood, Rebecca Robinson, Sharon Jeffress, Jennifer Mayfield, and Marisol Velez.

Finally, I would like to thank you, our readers and authors. Please continue to keep Optics Express in mind for your ongoing innovations in all areas of optics and photonics!