Pulse (physics)

In the field of physics, a pulse is defined as an individual disturbance. This disturbance propagates, or moves, through a transmission medium. The medium serves as the carrier for the pulse. It can be composed of matter, such as a rope or a slinky. It can also be a vacuum, which is empty space. This is how electromagnetic radiation travels. The medium may be finite or it may be indefinitely large.

Scientists use complex mathematics to describe how these pulses behave. Specifically, they use a partial differential equation (PDE). This is a mathematical way to model how the pulse moves and changes. The specific type of PDE used depends on the disturbance. For example, a pulse might be described by a hyperbolic PDE or a parabolic PDE. These equations help researchers predict the exact movement of the energy through the medium.

When a pulse travels through an elastic medium, it eventually reaches an end. What happens at that end depends on how the medium is held. There are two primary types of ends: fixed ends and free ends. A fixed end is held firmly in place. Imagine a person holding one end of a rope very tightly. This prevents the end from moving at all. A free end is different because it can move. For instance, a rope might be attached to a stick. The end can move up or down along that stick freely.

Reflection is the process of the pulse bouncing back from the end. The behavior of this reflection changes based on the end type. If a pulse reaches a free end, it reflects without inverting. This means the direction of displacement stays the same. An upward pulse will return as an upward pulse.

Conversely, a pulse reaching a fixed end will undergo inversion. In this case, the pulse flips. An upward pulse will reflect and return as a downward pulse.

This inversion is clearly shown in animations of the wave equation.

This same logic applies when a pulse crosses between two different media. The density or weight of the materials determines the reflection style. If a pulse moves from one medium into a less heavy or less dense medium, it behaves like it hit a free end. It will not invert. However, if the pulse travels into a heavier or denser medium, it behaves like it hit a fixed end. In this scenario, the pulse will invert. This shows how the physical properties of matter change the path of the disturbance.

Researchers also study a unique phenomenon called a dark pulse. Most pulses are bright, but a dark pulse is characterized by a localized reduction in intensity. This happens against a more intense, continuous wave background. In certain lasers, these are known as dark solitons. Scalar dark solitons, or linearly polarized dark solitons, can be quite stable. They are formed in all-normal dispersion fiber lasers through a method called nonlinear polarization rotation. Vector dark solitons are much less stable. This instability is caused by the cross-interaction between two polarization components.

The history of dark pulse discovery is quite recent. In 2008, the first dark pulse laser was reported. This was achieved using a quantum dot diode laser with a saturable absorber. The following year, in 2009, scientists successfully created a dark pulse fiber laser. This used an all-normal dispersion erbium-doped fiber laser with a polarizer in the cavity. Numerical simulations suggest these form due to dark soliton shaping. Most recently, in 2022, the first free space dark pulse laser was demonstrated. This setup used a nonlinear crystal inside of a solid state laser.