Defense

Adequate beam intensity distribution means sufficient energy transfer to the targeted material. This can mean successfully disabling a drone or interrupting satellite communication. Insufficient or excessive irradiance or fluence can introduce unwanted challenges such as undesirable or uncontrolled material damage. Regular beam profiling can help minimize these challenges and support a successful outcome.

The 2D beam profile to the left shows a Gaussian beam intensity profile where the intensity is greatest at the center, white, and decreases moving outward toward the outer circumference, blue. Profiles like this give relative information about the intensity distribution. For other beam shape profiles used in defense, see beam shape, below.

The power applied at the beam waist divided by the spot size also gives information about the power intensity (for continuous beams). Common laser power levels used in defense vary greatly considering the many laser processes. Some common values: ~1 kW - 300 kW+ and even experimental systems of up to 1 MW.

Note - For a continuous beam, the terms intensity, irradiance or power density are used: power divided by area, W/cm². For a pulsed beam, the term fluence or energy density are used: energy divided by area, J/cm². A pulse, repeating at the pulse frequency, will have peak irradiance and maximum pulse energy values reached during the pulse.

At the focus, the beam diameter reaches a minimum, often referred to as the spot size or beam waist diameter. Focusing a beam to a smaller spot size will increase the density in that spot and vice versa. It is important to apply the optimal amount of power or energy at the specified spot size. Too large or too small will affect the desired target location and may lead to some of the defects mentioned previously.

Common spot size values for the industry vary from ~5 µm up to even centimeters!

The focal plane of a non-collimated laser beam is generally where the beam is focused to its smallest spot size. The focal distance is the distance from the focusing lens along the axis of propagation to the focal plane and can vary depending on the presence of other optics: the laser source, focusing optics and possible beam shaping devices. The focal plane, in many cases, lines up with the target surface.

• Gaussian (intensity or fluence steadily decrease moving radially outward from beam center) — Directed energy over a long distance, the power increases steadily moving toward the beam center.
• Top hat or flat-top — Have uniform intensity or fluence across the beam, allowing a constant amount of energy to be applied over a larger area. Used in materials testing.
• Ring or donut is a beam shape used for beam control and adaptive optics systems.

Beam profilers provide a quick and effective means to quantify the relative intensity distribution of a beam to verify beam shape.

Beam propagation is the behavior of a laser beam propagating through free space and is described by M² (beam quality), divergence and pointing.

M² characterizes how close a power intensity profile is to a “Gaussian” beam, or standard bell shape, and can give a sense of how focused the beam is.

• M² = 1: Perfect Gaussian Beam
• M² near 1 (low M² values): Beams can be focused to small spot sizes and can also achieve better collimation
• M² > 1: Beams don’t focus as tightly, less Gaussian behavior

Divergence describes the angle the beam diverges outward from the beam waist into the far field, much beyond the Rayleigh length. In contrast, divergence near zero is a way to confirm a beam is collimated, for example before being focused. This helps to ensure that once the beam is focused, it will be at the correct spot size and location. For example: with directed energy or sensing, use a collimated beam.

Pointing is the angle of laser beam propagation with respect to the optical axis. A pointing value of zero means it is perfectly aligned with the optical axis. It characterizes how much a laser stays on center as it gets farther from the laser source, including accuracy and precision. Pointing measurements support better beam alignment, important for highly accurate and precise targeting applications. Misalignments can be caused by thermal fluctuations in the environment or in the laser system of high power lasers, as well as due to attenuation and time.