Pulse (physics)

A pulse is a small shake.

It travels through things like a rope. A pulse can move through empty space, too.

What happens at the end of the rope matters. If the end is held tight, it is a fixed end. If the end can move, it is a free end.

A pulse will bounce off a free end. It stays facing the same way.

But a pulse changes at a fixed end. An upward shake will flip over. It comes back as a downward shake.

It is fun to see how shakes move!

A pulse is a single disturbance. It moves through a medium. A medium is the stuff the pulse travels through. This could be matter, like a rope. It could also be a vacuum, which is empty space.

What happens when a pulse hits an end? It depends on how the end is held. A free end can move up or down. A fixed end is held very tight.

A pulse will reflect off a free end. It keeps its direction. An upward pulse will return as an upward pulse.

A pulse will reflect off a fixed end too. But it will invert. This means it flips over. An upward pulse will return as a downward pulse.

Pulses also change when they cross into new things. If a pulse moves into a lighter medium, it acts like it hit a free end. If it moves into a heavier medium, it acts like a fixed end. Scientists also study dark pulses. These are parts of a wave with less light. They can be made in special lasers.

A pulse is a single disturbance that moves through a medium. This medium is the stuff that carries the pulse. It might be matter, like a long rope. It could also be a vacuum, which is empty space.

Pulses are very important in physics. They help us understand how energy moves from one place to another. Scientists use math to describe how these pulses change as they travel.

Imagine sending a pulse through a rope or a slinky. When the pulse reaches the end, it will reflect. What happens next depends on the type of end. A free end can move up and down freely. A fixed end is held very tightly in one place.

If the end is free, the pulse stays the same. An upward pulse will bounce back as an upward pulse. This is called a reflection with no inversion.

A fixed end works in a different way. When a pulse hits a fixed end, it inverts. This means the pulse flips over completely. An upward pulse will return as a downward pulse.

You can also see this with different materials. If a pulse moves into a lighter medium, it acts like a free end. If it hits a heavier or denser medium, it acts like a fixed end.

Scientists also study a special kind of light called a dark pulse. Most light pulses are bright, but dark pulses are different. A dark pulse is a small drop in light intensity. It happens within a steady, bright background wave.

These pulses can be very stable in certain types of lasers. Researchers use math to show how these pulses form in a laser. They call this dark soliton shaping.

We have seen many dark pulses discovered in recent years. In 2008, scientists reported the first dark pulse laser. This happened in a quantum dot diode laser. In 2009, they achieved a dark pulse fiber laser. This used an erbium-doped fiber laser with a polarizer. Finally, in 2022, the first free space dark pulse laser was shown. It used a nonlinear crystal inside a solid state laser.

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.