1. INTRODUCTION

First and foremost, we would like to thank the authors who contributed to this feature issue [1–15]. As stated in the call for papers, Applied Optics and the Laser Systems Technical Group welcomed submissions to a feature issue on “Applications of Lasers for Sensing and Free Space Communications” in conjunction with the Applications of Lasers for Sensing and Free Space Communications (LS&C) 2022 topical meeting held on 11–15 December 2022 in Barcelona, Spain [16]. Meeting participants were particularly encouraged to submit their work; however, this feature issue was open to all contributions in related areas.

This feature issue highlights the latest developments in laser-based sensing and free space communications. Sophisticated laser systems are increasingly being used in a wide variety of applications including remote imaging and object characterization, sensing for autonomous vehicles, probing of the atmosphere, and high-bandwidth free-space optical (FSO) communications. These applications spread across a variety of basing platforms, including ground, air, water, and space platforms, and encompass a range of laser-system implementations. Both direct detection measurements and coherent detection systems can find application in probing of the earth and atmosphere, as well as in FSO communications. This feature issue reports on multiple applications of lasers for sensing and FSO communications, the sophisticated systems that operate in these applications, and the enabling components and processing techniques used within.

It is worth highlighting that this feature issue was endorsed by the Laser Systems Technical Group [17], which encompasses novel laser system development for a broad range of scientific, industrial, medical, remote sensing and other directed-energy applications. The group addresses technical issues concerning sources that cover the full spectral range, including ultraviolet, visible, infrared, terahertz, and microwave, and has strong overlap with other technical groups (like the Fundamental Laser Sciences Technical Group [18]) that study and develop laser techniques and technologies. This feature issue is an extension of the group’s efforts to bring together researchers and engineers to produce novel laser systems with unique performance.

Moving forward, the articles published in this feature issue will be appropriately celebrated in webinars and in-person events, as well as in list-serve emails and social-media posts by the Laser System Technical Group.

In what follows, we review the topics of interest for this feature issue and provide the background story for the invited review paper published as part of this feature issue [15]. Put simply, the goal for this invited review paper was to celebrate the legacy of David L. Fried—an oft-cited friend and colleague to many researchers. Robert Q. Fugate served as the lead author for this invited review paper with co-authors: Brent L. Ellerbroek and Jeffrey D. Barchers. For additional context, we include their associated bios before the conclusion to this feature issue.

2. TOPICS OF INTEREST

As stated in the call, topics of interest included, but are not limited to, the following:

All publications needed to present original, previously unpublished work and were subject to the normal standards and peer review processes of Applied Optics.

3. BACKGROUND STORY FOR THE INVITED REVIEW PAPER

As part of the application for this feature issue [19], we asked Bob Fugate of the Air Force Research Laboratory to serve as the lead author for an invited review paper on David Fried. In practice, David’s publications are prolific and cited in almost every paper dealing with laser-based sensing and free space communications through the atmosphere. We knew that these publications had a rich history that Bob could expand upon. In Bob’s case, this rich history led to the very first demonstration of laser guide star adaptive optics. In addition to this rich history, we were hoping that Bob could find some co-authors to expand upon the technical details of what is written in David’s obituary [20]. The associated stories were very motivational to say the least.

Working with Bob, we nominally outlined a five-section tribute paper with an intro (in the same light as David’s obituary) that sets up three more technically minded sections thereafter. One section would be spearheaded by Bob discussing how he worked with David on the first-ever demonstration of laser guide star adaptive optics as the lead experimentalist. Another section would come from a co-author who worked with David as an employee at the Optical Sciences Company. That co-author would become Brent Ellerbroek of the Thirty Meter Telescope International Observatory. And one more section would come from a co-author who worked with David as an early career scientist/engineer. That co-author would become Jeff Barchers of nLight DEFENSE Systems. The conclusion would then sum up these three-distinct viewpoints.

4. ASSOCIATED BIOS

We greatly appreciate Bob, Brent, and Jeff’s efforts and highlight their own legacies with the following bios.

A. Robert Q. Fugate

Robert Q. Fugate received his B.S. in Physics from Case Institute of Technology in 1965, and a Ph.D. in Experimental Physics from Iowa State University in 1970. He worked as a civilian research scientist for the Air Force for over 35 years, first at Wright-Patterson Air Force Base (AFB) in Dayton, Ohio, then at Kirtland AFB near Albuquerque, New Mexico. His career focused on laser beam control and propagation in the atmosphere, remote detection of laser threats, and the development of laser guide star adaptive optics for military applications. He led a team at the Starfire Optical Range at Kirtland AFB that made the first measurements validating the concept that laser beacons could be used to measure the wavefront distortion induced by atmospheric turbulence. His team further developed the technology to demonstrate that laser guide star adaptive optics was possible on large aperture ground-based telescopes. He was the leading advocate in the Department of Defense for transitioning adaptive optics technology to the astronomy community.

Dr. Fugate retired from the Air Force as Senior Scientist for Atmospheric Compensation in 2006 and joined the staff at New Mexico Tech, in Socorro, New Mexico. He also worked part time as a consultant to military, commercial, and academic organizations. As of 2022, he serves in an emeritus role for the Air Force Research Laboratory, principally for the Starfire Optical Range.

Dr. Fugate first met Dr. David L. Fried in Boston in the fall of 1981 when Julius Feinleib first proposed the idea of laser guide stars for adaptive optics to the government. When it became clear that DARPA was going to sponsor an experiment to test the concept, Fugate enlisted Fried to help with the design of the experiment and the data processing to test his theory of focus anisoplanatism. After a successful first experiment, Fugate and Fried continued a close collaboration on theory and experiment through the development of laser guide star adaptive optics and its military applications until 2020.

Dr. Fugate is a member of the National Academy of Engineering, received the Meritorious Presidential Rank Award, has an asteroid named in his honor for contributions to astronomy, and is the author of over 100 publications in professional journals. Bob is an avid photographer and specializes in nature, landscapes—especially under the night sky—and deep sky astrophotography. His work has been published on the covers of five magazines, three books, and two international journals, and has been featured at NASA’s website, Astronomy Picture (APOD) and Earth Science Picture of the Day (EPOD). He is an invited contributor to Healing Images and was awarded the prestigious Photographic Society of America’s Progress Medal.

B. Brent L. Ellerbroek

Brent L. Ellerbroek received his B.A. in Mathematics from UCLA in 1975 and his Ph.D. in Mathematics from Caltech in 1979. His work since then has concentrated on the design, analysis, and implementation of laser guide star multi-conjugate adaptive optics (MCAO) for ground-based optical and near-infrared telescopes. He worked for David Fried at the Optical Sciences Company (tOSC) from 1984 to 1987, and for Robert Fugate at the Starfire Optical Range from 1990 to 1999. His 1993 and 1994 publications in MCAO with multiple laser guide stars generalized the analytic approaches developed earlier in collaboration with Dr. Fried and others at tOSC. He contributed to adaptive optics demonstrations on the SOR 1.5- and 3.5-m telescopes and collaborated with Jeff Barchers on the development of wavefront sensing and control concepts for compensating both amplitude and phase. More recently he has worked at Gemini Observatory (1999–2003) and the Thirty Meter Telescope (TMT) International Observatory (2004–2017). Dr. Ellerbroek’s leadership at both Gemini and TMT have led to hardware realizations at an increasing number of observatories of the joint vision created several decades earlier through the efforts of Fried, Fugate, and Ellerbroek. He continues consulting for TMT on a part-time basis.

C. Jeffrey D. Barchers

Jeffrey D. Barchers has worked in the field of High Energy Laser (HEL) and Adaptive Optical (AO) systems for more than 25 years. After completing his B.S. at the U.S. Air Force Academy and his M.S. at MIT, Jeff started his career as a 1LT at the Starfire Optical Range in 1997, where he had opportunity to work for Dr. Robert Fugate and learn from and work with Dr. Brent Ellerbroek and Dr. David Fried, among many other mentors. After working at SAIC from 2002–2007, Jeff joined Nutronics, Inc. Jeff grew Nutronics until its sale to nLIGHT, Inc. in 2019. During the period from 2008–2013 Jeff served as the JOSA A Topical Editor for Atmospheric Optics. Currently the President for nLIGHT DEFENSE Systems (recently re-named from Nutronics) in Longmont, Colorado, Jeff has developed one of the nation’s leading capabilities in HEL, AO, and advanced high-speed tracking systems. Most recently, Jeff’s team developed the world’s first 300 kW coherently combined HEL in support of the High Energy Laser Scaling Initiative (HELSI) at the Office of the Under Secretary of Defense for Research and Engineering (OUSD(R&E)).

5. CONCLUSION

Once again, we would like to thank the authors who contributed to this feature issue [1–15]. Put simply, the 15 papers published in Applied Optics highlight the latest developments in laser-based sensing and free space communications. We would also like to thank Robert Q. Fugate, Brent L. Ellerbroek, and Jeffrey D. Barchers for their efforts in writing the invited review paper associated with this feature issue. David L. Fried’s legacy is appropriately celebrated as a result.