Why Are Continuous Wave Lasers Important

Lasers are everywhere around us. Surgeons use them for eye surgery and cancer treatments. Manufacturers use them for material processing to cut, mark, weld, clean, and texture various types of materials. Some people need them for tattoo or hair removal, and everyone has seen laser light shows during music concerts. More recently, new applications like laser holography are emerging.

Different types of lasers are needed for these applications. Based on their gain medium, lasers are classified into five main types:

• Gas Lasers
• Solid-State Lasers
• Fiber Lasers
• Liquid Lasers (Dye Lasers)
• Semiconductor Lasers (Laser Diodes)

Additionally, these five types of lasers can be divided into subcategories based on their mode of operation: continuous-wave lasers and pulsed lasers. Furthermore, there are also multiple types of pulsed lasers.

Before differentiating the types of lasers, it's good to remember what a laser actually is.

What Is a Laser?

A laser is a device that generates light in the form of a laser beam. A laser beam is different from a light beam in that its rays are monochromatic (a single color), coherent (of the same frequency and waveform), and collimated (going in the same direction).

Lasers provide this "perfect information" which is ideal for applications that require high precision.

Lasers are comprised of three main components:

• The energy source pumps light into a gain medium. It varies according to the type of laser. It could be a laser diode, an electrical discharge, a chemical reaction, a flash lamp, or even another laser.
• The gain medium emits light of a specific wavelength when excited by light. It is said to be the source of optical gain. Lasers are typically named after their gain medium. In a CO2 laser for example, the gain medium is CO2 gas.
• The resonator amplifies the optical gain through mirrors that surround the gain medium. These include bulk mirrors in solid-state lasers, cleaved or coated facets in laser diodes, and Bragg reflectors in fiber lasers.

Gas Lasers

A gas laser is a laser in which an electric current is sent through a gas to generate light through a process known as population inversion. Examples of gas lasers include carbon dioxide (CO2) lasers, helium–neon lasers, argon lasers, krypton lasers, and excimer lasers.

Gas lasers are used in a wide variety of applications, including holography, spectroscopy, barcode scanning, air pollution measurements, material processing, and laser surgery.

CO2 lasers are probably the most widely known gas lasers and are mainly used for laser marking, laser cutting, and laser welding.

Solid-State Lasers

Solid-state lasers use a solid (crystals or glasses) mixed with a rare earth element as their source of optical gain. The mixed element is typically neodymium, chromium, erbium, thulium, or ytterbium.

The most known solid-state laser is the ruby laser, since it is the first laser ever constructed. The Nd:YAG laser (neodymium-doped yttrium aluminum garnet) is also common in material processing applications.

Solid-state lasers are also used for LIDAR technology as well as various medical applications, including tattoo and hair removal, tissue ablation, and kidney stone removal.

Fiber Lasers

A fiber laser is a special type of solid-state laser that is a category of its own. In fiber lasers, the gain medium is an optical fiber (silica glass) mixed with a rare-earth element.

The light guiding properties of the optical fiber are what makes this type of laser so different: the laser beam is straighter and smaller than with other types of lasers, making it more precise. Fiber lasers are also renowned for their small footprint, good electrical efficiency, low maintenance and low operating costs.

Fiber lasers are used in a range of applications, including material processing (laser cleaning, texturing, cutting, welding, marking), medicine, and directed energy weapons.

Examples of fiber lasers used for these applications include ytterbium and erbium-doped fiber lasers.

Liquid Lasers (Dye Lasers)

A Liquid lasers use an organic dye in liquid form as their gain medium. They are also known as dye lasers and are used in laser medicine, spectroscopy, birthmark removal, and isotope separation.

One of the advantages of dye lasers is that they can generate a much wider range of wavelengths, making them good candidates to be tunable lasers, meaning that the wavelength can be controlled while in operation.

Photo Courtesy of Wikimedia Commons

In laser isotope separation for example, lasers are tuned to specific atomic resonances. They are then tuned to a specific isotope to ionize the atoms, making them neutral as opposed to negatively or positively charged. They are then separated with an electric field, achieving what is called isotope separation.

Semiconductor Lasers (Laser Diodes)

Laser diodes, also called diode lasers and semiconductor lasers, are similar to regular diodes in that they have a positively-negatively (PN) charged junction. The difference is that laser diodes have an intrinsic layer at the PN junction made of materials that create spontaneous emission. The intrinsic layer is polished so that the generated photons are amplified, ultimately converting the electric current into laser light.

Photo Courtesy of Wikimedia Commons

Although most semiconductor lasers are diode lasers, a few of them are not. This is because there are semiconductor lasers that do not use the diode structure, such as quantum cascade lasers and optically pumped semiconductor lasers.

Like fiber lasers, laser diodes can be classified as solid-state lasers since their gain medium is solid. However, they are in a category of their own because of their PN junction.

Laser diodes are often used as energy sources to pump other lasers. These lasers are referred to as diode-pumped lasers. In these cases, laser diodes are typically arrayed to pump more energy, as shown in the following image.

Laser diodes are extremely common. They are used in barcode readers, laser pointers, laser printers, laser scanners, and several other applications.

Laser Types by Mode of Operation

All types of lasers can operate using one of two methods: their laser beams can either be pulsed or continuous. This is what we call their mode of operation.

• With continuous-wave lasers, there is a constant flow of energy, meaning that the laser continuously shoots a single, uninterrupted laser beam. The most common example of this is a laser pointer's uninterrupted beam. Continuous-wave lasers are commonly used for laser cutting and laser welding.
• With pulsed lasers, the laser beam is interrupted at regular intervals to allow the energy to build up and reach a higher peak power than continuous-wave lasers. The laser beam is released as pulses that have a specific duration called the pulsed duration. These high energy densities are required for many applications like spot welding and engraving.

Continuous-wave lasers may seem more powerful than pulsed lasers because the advertised laser power is typically much higher, but this can be misleading. This is because lasers are named according to their average laser power, and the average power of pulsed lasers is usually lower even if they reach higher peaks of power.

For example, a 6,000W continuous-wave laser continuously releases 6,000W of laser power. Conversely, a 100W pulsed laser can release pulses of 10,000W each.

Laser Types by Pulse Duration

Pulsed lasers are divided into several categories based on the duration of their pulses.

A modulator is used to control the number of pulses per second. As a result, each pulse has a precise duration, called pulse duration, pulse length, or pulse width. The pulse duration is the time between the beginning and the end of a pulse.

Several modulating methods are used to pulse laser beams: q-switching, gain-switching, and mode-locking are some examples. The shorter the pulse, the higher the energy peaks. Here are the most common units used to express pulse duration.

• Milliseconds (one thousandth of a second) are the longest time units used to express pulse duration and have hence the lowest energy peaks. For example, laser hair removal pulses may vary between 5 ms and 60 ms depending on the hair thickness.
• Microseconds (one millionth of a second) are probably the least common pulse durations They can be used for material processing applications, but the following pulse durations are more commonly used, as they offer more precision. Microsecond lasers can also be used for applications like spectroscopy and hair removal.
• Nanoseconds (one billionth of a second) are very common pulse durations used for applications like laser material processing, distance measurements, and remote sensing. Laserax, for example, uses nanosecond fiber lasers to perform laser marking, cleaning, texturing, and engraving.
• Picoseconds (one trillionth of a second) and femtoseconds (one quadrillionth of a second) are the shortest pulse durations, which is why the terms ultrashort pulses and ultrafast lasers are used. These lasers offer the most precise results and have the lowest heat affected zones. This prevents undesirable melting and allows for very precise engravings. They are used in material processing, medicine (such as eye surgery), microscopy, measurements, and telecommunications.

More Types of Lasers

As you can see, there are many ways to categorize lasers. Another way is by the laser wavelength, where you have infrared, near-infrared, visible, ultraviolet, and X-ray lasers.

Laser experts keep pushing the limits of laser technology, with new developments being made every year. As a result, the types of lasers are constantly evolving, and anyone looking to explore this world can expect a lifetime of discoveries.

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Source: https://www.laserax.com/blog/types-lasers

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