Welcome to the Age of Lasers

USS Harry S. Truman operations

LASERS ARE HERE TO STAY NOW, HOW DO YOU DETECT THEM?

By John DeGrassie and Christina Wright

The recent at-sea tests of the Navy’s Laser Weapon System have ushered in a new era—the age of “Laser Wars”—where there is an increasing role for directed-energy weapons in the battlespace. How will commanders detect, assess, and counter laser or laser-assisted threats in this new operational environment to protect warfighters and ensure mission success.

Lasers engage at the speed of light, effectively point-to-point along the line of sight, at wavelengths usually not visible to the human eye. This means that the laser systems entering this era have a low probability of being detected and intercepted.

A current laser warning receiver (LWR) that detects laser or laser-assisted threats relies on direct or near-direct illumination by the laser. This may be sufficient to protect individual Navy assets against a laser threat, but will be insufficient to meet the spectrum dominance and battlespace awareness that the new era of “Laser Wars” demands.

For example, to protect larger Navy assets (protecting a group of landing craft from laser-guided mortars or protecting Navy ships in foreign ports from laser threats) the current LWR capabilities will not scale practically.

Laser tracking or engagement attempts by hostile systems are point-to-point and therefore have a high degree of specificity when targeting. For small assets where LWRs can be located near critical targets, the LWR may effectively detect a laser threat. On the other hand, a large Navy asset like an aircraft carrier or groups of assets would require many LWRs to guard fully against potential threats. Perhaps more than 30 existing sensors, for instance, would be required to guard an aircraft carrier against potential laser threats. This could be cost prohibitive and would be extremely difficult to maintain.

In addition, to meet the battlespace awareness demands in an environment where lasers are present, lasers need to be detectable from reconnaissance platforms that are far away from the engagement.

To overcome the scalability problems with current LWRs and to enable greater spectrum dominance through increased battlespace awareness, a new laser detection capability that can detect indirect laser illumination is needed—an “off-axis” laser detection capability.

Space and Naval Warfare System Center Pacific’s (SSC Pacific) Atmospheric Propagation Branch is developing such a capability with the Laser Identification through Scattering and Beam Recognition (LITSABR) project. This effort supports counter directed-energy weapons (CDEW) efforts and intelligence, surveillance, and reconnaissance capabilities. LITSABR is currently being supported by the Office of Naval Research’s CDEW discovery and innovation program.

Detecting lasers at positions far from direct illumination is a difficult problem to solve; it requires a diverse set of expertise, including accurate atmospheric modeling, precise atmospheric characterization, sensor engineering, and careful laser physics modeling. Recognizing this, a variety of expert performers is collaborating to address the off-axis laser detection problem. They include the Naval Research Laboratory, with experience in high-energy laser physics and aerosol physics; the Naval Academy, with experience in laser propagation and energy absorption; the Georgia Tech Research Institute, with its one-of-a-kind lidar for precise atmospheric profile measurements; Nanohmics, Inc. and Sensing Strategies, Inc., with experience in designing sensitive laser detectors; and SSC Pacific and its LITSABR project.

How Does It Work?

If you have ever attended a music concert that employed fog machines and lasers to enhance the musical experience, then you know the basic fundamentals involved in off-axis laser detection.

The principles of how a laser beam can be detected off-axis are demonstrated when the colored laser beams pass through the fog-like substance at the concert and some of the laser light is scattered at an angle to its original line (or axis) of propagation toward off-axis observers, in this case the audience. The key process is the scattering of the laser light by a given medium: the fog in the laser light show. This redirects the laser signal, allowing it to be detected by the off-axis sensors, the eyes of the concert attendees in the audience. Without the presence of the fog medium, this would not be possible. The lasers would need to be pointed directly or nearly directly at the audience to be seen.

In the same way as the fog in the light show, particles in the atmosphere (molecules, aerosols, dust, soot, etc.) will scatter propagating laser energy of all wavelengths, visible to the human eye or not, into directions not collinear with the original line of propagation. Scattered laser energy is, in principle, detectable with a sensor or camera with adequate sensitivity at the wavelength of the laser light of interest–though the number of atmospheric scatterers is typically fewer than in the concert fog example and results in less scattered light. Nevertheless, the resulting scattered laser energy can enable laser warning receivers to detect and identify laser threats from positions that are far askew from the laser’s directed path of propagation. Scattering is always present to some degree in the atmosphere, even on clear days, and depends on the propagation geometry, location, time of day, time of year, and local weather.

With the Atmospheric Propagation Branch’s expertise in modeling the laser propagation environment, the LITSABR project is providing predictive models to aid in the design of future sensors and to help inform tactical decisions on the use of those sensors and related laser technologies. Knowledge gained from the program will be critical to answering where and when a laser is detectable by a given off-axis LWR.

In addition to the atmospheric modeling, the LITSABR project is developing an off-axis laser detection and characterization capability using multistatic imaging sensors. The LITSBR project builds on research carried out in the Bistatic Laser Detection at Large Standoffs (BLDLS) project, a prior ONR CDEW-funded program. The system works by combining images of the scattered laser light taken by multiple cameras. From these images, the laser position, direction, and other characteristics can be determined. The technique is similar to 3D scene reconstruction from imagery used in computer vision research to create representations of buildings and topography. For the LITSABR capability, the multistatic off-axis detector reconstructs the 3D representation of the detected propagating laser.

The multistatic capability requires multiple cameras to take a single image or images of the scattered laser beam from multiple vantage points. Multiple images are necessary for depth perception since, with all other variables held constant, one image or vantage point cannot distinguish points that are far away from those that are near, thus the distance from the camera to the laser cannot be determined. The position of any single camera and the line delineating the laser beam axis define a plane in space called an “ambiguity plane.” On this plane, many different beam axis distances (locations) and orientations (propagation directions) will provide the same image projection to the camera, leaving the laser position and orientation uncertain or ambiguous without any additional information.

Introducing a second image or vantage point can break this ambiguity just as binocular vision provides depth perception. The position of a second camera and the laser beam axis define a second ambiguity plane in space. The two nonparallel ambiguity planes intersect in a line. The line in which the two camera-laser axis ambiguity planes intersect gives the actual orientation of the laser beam in space. Once the 3D coordinate representation of the laser is determined, the laser origin and direction is readily determined. The feasibility of using two cameras to detect and characterize a laser from images of the atmospheric scattering was demonstrated in the BLDLS project.

The LITSABR project is currently pursuing a multistatic sensor approach, using three or more sensors. Introducing additional sensors improves the effectiveness of the system, allowing the laser source to be located even for special cases when two cameras leave some ambiguity. It may also improve the accuracy of the laser location for cases when the atmospheric scattering is weak or inconsistent.

The multistatic approach also reduces the error in a bistatic measurement. With more camera-laser axis ambiguity planes this uncertainty can be reduced, and the LITSABR project already has demonstrated reduced errors with this method.

Using three or more sensors in the multistatic off-axis laser detection approach not only overcomes degenerate cases but anticipates a distributed network of sensors in the battlespace. With the multistatic approach the LITSABR capability can use existing sensors, systems, and imagery to create an off-axis laser detection capability with little additional cost. This approach can meet the demands of the new operational environment and grow organically along with other sensing capabilities by using any and all available imagery of scattered laser light. With a few off-axis LWRs or using existing deployed sensors or both, the capability developed in the LITSABR project, in conjunction with other CDEW programs, can provide an off-axis laser detection capability cost-effectively.

Bringing Lasers to the Fleet

The Navy is currently developing and testing a demonstration shipboard high-energy laser weapon, signaling an ever-growing role of lasers in naval operations. To counter any similar laser threats to its own operations, it is critical that the Navy has capabilities for early detection and characterization of laser threats. The LITSABR program enables this capability, enhances national security, and helps ensure the Navy can operate in an increasingly complex electromagnetic and electro-optical environment. The difficult challenges to realizing off-axis laser detection are being addressed and will enable greater situational awareness in today’s maritime battlespace.

The LITSABR program has demonstrated multistatic off-axis detection and characterization, incorporating more than two cameras, at the proof-of-concept level. Atmospheric models will need to be integrated with the multistatic capability and validated through outdoor atmospheric experiments. Once baselined, the models developed with the LITSABR and collaborative programs then can be used to validate other off-axis LWRs, and inform countermeasures utility decisions for laser threats. This is one area where collaboration with other Navy programs is most critical. Pulling together the best expertise and resources is absolutely necessary for off-axis LWR modeling and sensors to be validated.

In addition, the LITSABR program is planning to apply the multistatic capability to single, moving imagers and “swarms” of imagers in both real time and after analysis to make platforms into sensors for laser threat detection and characterization.

As lasers grow in civilian and military operations around the world, it is critical that any lasers present can be detected and characterized in order to assure access to the battlespace. With capabilities like those developed under the LITSABR program, warfighters have a method for detecting and characterizing lasers from off-axis standoff ranges far from direct laser illumination to enable and inform CDEW technologies.

About the authors:

John DeGrassie is a scientist in Space and Naval Warfare Systems Center Pacific’s Atmospheric Propagation Branch.

Christina Wrightis a staff writer at Space and Naval Warfare Systems Center Pacific. She is a captain in the Army Reserve specializing in public affairs.