How Lasers work

The laser is an exotic light source invented in 1960. Since then, many types of lasers have emerged. Lasers are now used in science, medicine, communication, and manufacturing. Let's explore how lasers work and how they differ from ordinary light sources like light bulbs.

A Pressure Cooker for Light

Imagine a pressure cooker. Water inside a closed chamber is boiled and converted to steam. As the temperature of the steam rises, so does the pressure inside the chamber. To prevent dangerous pressure levels, there's a small escape valve on top to release some steam. The energy from the stove burner is ultimately converted to a "beam of steam."

Now, imagine doing something similar with light. You find a material that emits light when "pumped up" with energy. Surround this material with mirrors to form an enclosed chamber. When you turn on the "pump," the light emitted by the energized material builds up and bounces around inside the chamber. A small opening allows a fraction of the light to escape, forming a useful beam.

This analogy is a good starting point, but it doesn't tell the whole story. The light beam from a laser typically has a specific wavelength, which we perceive as a particular color. This can be red (longer wavelength), green (medium wavelength), or blue (shorter wavelength), depending on the type of laser. The light is also highly collimated, meaning it travels in a specific direction. Lasers have features that discriminate against light that is not of the desired wavelength or direction. To understand these features, let's first look an ordinary light source.

Disorganized Light

When electrical current passes through a tungsten wire in a light bulb, the wire gets very hot. A small fraction of that energy converts to light. However, this light is of various wavelengths across the spectrum, which combine to produce what we perceive as white light.

The tungsten filament also emits light in all directions. So, this is highly disorganized light. How does a laser produce more organized light?

Selecting a Wavelength and Amplifying It

While being pumped with energy, the material in a laser selectively emits light in a particular range of wavelengths and amplifies this light. Below is an animation of a hypothetical laser material amplifying a green-wavelength light wave passing through it. As the light wave travels, it picks up energy stored in the material from pumping, causing the wave to strengthen. This amplification process is called stimulated emission. The word laser is an acronym that stands for "light amplification by stimulated emission of radiation."

Adding the Mirrors

The final step in building a laser is to place the amplifying material between parallel mirrors. This setup selectively amplifies light traveling perpendicular to the mirrors. It's similar to Darwinian survival of the fittest. Only light traveling in this direction can bounce back and forth many times through the material, getting amplified repeatedly. One of the mirrors is partially transparent, allowing a small fraction of the light to escape and form a useful beam. This is akin to the escape valve on the pressure cooker.

This structure also helps select a particular wavelength. The light bouncing between the mirrors forms a self-reinforcing standing wave if the wavelength is just right. This is similar to the standing wave you can create on a jump rope by wiggling one end at the correct frequency.

This animation shows the laser in steady operation, where there is a balance between the energy gained by the light wave on each round trip between the mirrors and the energy lost because some fraction escapes through the partially transparent mirror.

Applications of Lasers

Lasers have a wide range of applications across various fields. In medicine, they are used for surgeries, skin treatments, and vision correction. In manufacturing, lasers cut and engrave materials with precision. They also play a crucial role in telecommunications, enabling high-speed data transmission.

Medical Uses

Lasers are revolutionizing medical procedures. They allow for minimally invasive surgeries, reducing recovery times and improving patient outcomes. For instance, laser eye surgery corrects vision by reshaping the cornea. Similarly, lasers are used in dermatology to treat skin conditions like acne scars and wrinkles.

Industrial Applications

In manufacturing, lasers are used for cutting, welding, and engraving materials. Their precision allows for intricate designs and high-quality finishes. Industries ranging from automotive to aerospace rely on lasers for their efficiency and accuracy.

Communication Technologies

Lasers are essential in modern communication technologies. They enable fiber-optic networks, which transmit data over long distances at high speeds. This technology underpins the internet, making it possible for us to connect and communicate globally.

To Learn More

You can, of course, Google the topic of lasers and find many articles about details not covered here. This Laserax site discusses different types of lasers and the energy sources used for pumping. Our understanding of stimulated emission is a fruit of the most successful theory in the history of physics: quantum mechanics. That may be a topic for another post!