Deeper Dive into Laser Target Designation
Laser Source Requirement for Modern Laser Designator Systems
Combat zones can be extremely chaotic. With a massive influx of sensory input, implementing clear communication, rugged tools, and reliable instrumentation is key to a successful mission. One of the most challenging tasks in any aerial combat situation is determining which assets on the ground to target and which ones to avoid. This determination is particularly challenging when enemy assets are camouflaged or hidden amongst civilian assets. Because of this challenge, a practice, commonly referred to as “painting the target,” was developed and has been successfully deployed for many years, allowing ground forces to identify and designate targets for successful engagement by aerial support for superior air control. Of course, on the modern battlefield, soldiers aren’t using cans of spray paint for pilots to attempt target location. Instead, they use portable laser designator systems, designed to illuminate the target with infrared radiation, which is then easily detected and tracked by their aerial counterparts. Global positioning system (GPS) guided munitions are also an option. However, GPS guided munitions typically have a circular error probability (CEP) of around 5 meters, while the CEP of laser guided munitions is typically 3 to 1 meters. In this application note, we will examine the critical laser requirements for laser designation systems and discuss what types of lasers are ideal for these systems.
Clearing Up Some Misconceptions
There is sometimes confusion about the difference between a designator vs laser. The laser is a critical component of a laser designator, but you still need the optics, opto-mechanical, and electronic components to be able to utilize the proper laser source for your laser designator module. There is also some confusion on the difference between a laser designator and a laser range finder. The simplest answer is that a range finder merely provides a distance between the unit and the target, while a designator actually “paints the target” with a coded signal. More on this in a moment.
How a Laser Designator Works
There are many different types of laser designation systems used by the military or air force today, as shown in the figure below, courtesy of Areté Associates (e.g., laser designator for rifle, laser guided munitions or laser guided bombs). Still, even with different laser designator specifications, they all share the same basic functionality and outcome (each type designates targets). In a laser designation system, an infrared laser is fixed onto a target of interest and pulsed with a predetermined pulse repetition frequency code (PRF code). These laser designator PRF codes allow the infrared receiver on the plane or missile to efficiently recognize targets and lock onto the pulsed signal which is scattered off the target (reflected light). This method enables the effective delivery of the projectile on target. At a glance, the laser requirements seem relatively straightforward. The laser designator wavelength needs to be invisible to the human eye, and it needs to have a programmable pulse rate. Still, when you look in more detail, many small factors add up to big problems if not appropriately addressed.
Laser Requirements: Divergence and Pointing Stability Considerations
According to the NATO standard STANAG 3733, the laser beam divergence must be small enough that 90% of its energy is incident upon the target for 95% of the time, assuming a target size of 2.3m2. Ideally, a soldier would be positioned at a safe distance (up to 5km away) from the designation target. Given this great distance, it is critical that the laser source has excellent beam divergence and pointing stability ratings so the laser designator range is still useful. For example, if a laser beam has a half-angle divergence of 1 mrad, its radius will expand at a rate of 1 mm per meter. While this slight increase in radius may not seem like much, at a distance of 2km, an initial laser beam diameter of 1 mm will have expanded to 4 meters in diameter at the target. The figure below gives a visual representation of how rapidly a laser with a half-angle divergence of 1 mrad can balloon in size. This rapid divergence is problematic when locking onto smaller targets without more than 10% of the light missing the mark. Furthermore, it dramatically reduces the overall intensity of the reflected light, making it harder to be detected by the missile’s targeting system. Read more about about divergence here: “Why a larger beam results in a smaller spot size in laser designation?” Similarly, problems can arise regarding the laser’s pointing stability, given that any minuscule variations in beam angle at the source can result in massive variances of the beam’s position at a distance. These significant variances in beam position can be devastating when trying to meet the 95% stability requirement also laid out in the NATO standards.
Laser Requirements: Pulse Jitter and Size, Weight, and Power (SWaP) Considerations
The accuracy of the pulse repetition rate is another critical property that one must consider. The infrared receiver on the plane or missile must confirm that the illuminated target matches the predetermined frequency code. Otherwise, the guidance system will not be able to lock on, regardless of the signal’s brightness. Therefore, if there is jitter in the pulse triggering, the frequency and timing can be altered, distorting the frequency code and hindering the receiver’s ability to confirm the signal. Take a deeper dive into the critical nature of pulse jitter in this article: “Why is a Low Jitter Feature Important in Actively Q-Switched DPSS Lasers?” Lastly, in addition to the optical considerations laid out in the last paragraph, ruggedization, miniaturization, and power consumption requirements for the laser are equally important, especially when considering handheld laser target designators. Having reliable, rugged tools, built to withstand vibration and shock, and designed with low SWaP in mind – to be easily integrated with various equipment, where space and resource constraints limit your options – is key to mission success.
Why a Larger Beam Results in a Smaller Spot Size for Laser Designation
We recently updated a blog post titled “Laser Source Requirements for Modern Laser Designator Systems,” in which we took a deep dive into the fundamentals of laser designation as well as the military’s requirements for such a laser. In that blog post we cited NATO standard STANAG 3733, which required that the laser beam have a divergence small enough so that 90% of its energy is on target 95% of the time assuming a 2.3 x 2.3 m target. We went on to explain that most laser designators are designed to be used at distances up to 5 km. Therefore, one of the most critical factors when choosing a laser source is the beam divergence.
In this post, we are discussing one method for decreasing the beam divergence by increasing the beam size. While this may seem counter-intuitive, it is, in fact, the case that as the collimated beam diameter increases, the beam divergence decreases. This happens for the same reason that a laser diode has a slow and fast axis. That is, whenever light is contained in a small area, it tends to diffract or diverge. The same process happens to the laser beam itself. If a laser is perfectly collimated it is considered to be diffraction limited. This means that the laser beams divergence is solely determined by the area it is confined in, hence its beam diameter. Therefore, when you expand the laser beam to a larger diameter and re-collimate it the divergence will decrease since the light isn’t being squeezed into as small of an area as it once was. As a general rule of thumb, the beam divergence is inversely proportional to the beam diameter. So, every time you double the beam diameter you half the beam divergence.
In laser designation, you can take advantage of this fact, to ensure a smaller spot size on the target by starting out with a larger beam exiting the designator. Two primary beam expander designs are used today, Keplerian and Galilean. From the image below you can see that the fundamental difference between these two designs is the existence of an intermediate focus. While the Keplerian design can be beneficial for lower power applications such as microscopy, for most high power and field-deployable applications the Galilean design is far superior. The Galilean is preferred in applications such as laser designation not only because it is more compact, but also because it doesn’t run the risk of thermal instabilities due to heating from the intermediate focus.
While using a beam expander is an excellent way to decrease the beam divergence and ensure a longer range (smaller spot at a distance) for laser designation it is still imperative that you start out with as low a beam divergence laser as possible. For example, the Airtrac-6M from Arete Associates has a beam parameter produced (BPP), of approximately 6 mm-mrad. This means that at a beam diameter of 6mm the laser will have a beam divergence of 1 mrad. Therefore, if this laser beam was expanded to a collimated diameter of 24 mm it would have a beam divergence of 0.25 mrad. By definition a mrad means that after 1 m the radius of the beam will increase by 1 mm, so this means that at a distance of 5 km our beam (with a 0.25 mrad divergence) will be approximately 2.5 m in diameter. Which is more than acceptable for the NATO guidelines.
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