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Fiber Lasers: Everything You Need to Know

authorIcon By Jerome Landry on November 26, 2020 topicIcon Industrial Lasers

Fiber lasers are everywhere in the modern world. Due to the different wavelengths they can generate, they are widely used in industrial environments to perform cutting, marking, welding, cleaning, texturing, drilling and a lot more. They are also used in other fields such as telecommunication and medicine. 

Fiber lasers use an optical fiber cable made of silica glass to guide light. The resulting laser beam is more precise than with other types of lasers because it is straighter and smaller. They also have a small footprint, good electrical efficiency, low maintenance and low operating costs. 

If you want to learn everything you need to know about fiber lasers, keep reading. 

When Was the Fiber Laser Invented?

An ytterbium-doped pulsed fiber laserElias Snitzer invented the fiber laser in 1961 and demonstrated its use in 1963. Serious commercial applications only emerged in the 1990s, however.

Why did it take so long? The main reason is that fiber laser technology was still in its infancy. For example, fiber lasers could only emit a few tens of milliwatts whereas most applications require at least 20 watts. There was also no means of generating high-quality pump light, as laser diodes did not perform as well as today.

Here are some of the key moments in the history of fiber laser technology, going back as early as 1917 when Albert Einstein established its foundations. 

  • 1917 – Stimulated emissions are discovered (Albert Einstein). 
  • 1957 – The theoretical framework for the laser is developed (Gordon Gould). 
  • 1960 – The first laser—a ruby laser—is constructed (Ted Maiman). 
  • 1960 – Continuous-wave laser beams are generated for the first time.
  • 1960 – The term “fiber optics” is coined (Narinder Kapany).
  • 1961 – Optical modes in glass fibers are invented (Elias Snitzer). 
  • 1962 – Q-switching, a technique to generate pulsed laser beams, is demonstrated (Robert W. Hellwarth and R.J. McClung). 
  • 1963 – The first fiber laser is demonstrated (Elias Snitzer). 
  • 1964 – A method is discovered to remove impurities from glass fibers, and hence limit light loss (Charles Kao and George Hockham). 
  • 1988 – The first double-clad fiber laser is demonstrated (Elias Snitzer). 
  • 1990 – The watt barrier is broken with a 4W erbium-doped fiber laser. 
  • 2004 – The single-mode silica fiber laser and amplifier is invented (David N. Payne). 

Today, important advances are still being made in fiber laser technology, making it more efficient, powerful and accessible. Some of the most upcoming applications include laser cleaning and laser texturing, which can replace polluting technologies and help make the world greener. 

What Are the Types of Fiber Lasers?

Generally speaking, fiber lasers can be categorized using the following criteria: 

  • Laser source: Fiber lasers vary according to the material with which the laser source is mixed. Some examples include ytterbium-doped fiber lasers, thulium-doped fiber lasers, and erbium-doped fiber lasers. All these types of lasers are used for different applications because they produce different wavelengths. .
  • Mode of operation: Different types of lasers release laser beams differently. Laser beams can either be pulsed at a set repetition rate to reach high-peak powers (pulsed fiber lasers), as is the case with “q-switched”, “gain-switched” and “mode-locked” lasers. Or, they can be continuous, meaning that they continuously send the same amount of energy (continuous-wave fiber lasers). 
  • Laser power: Laser power is expressed in watts and represents the average power of the laser beam. For example, you can have a 20W fiber laser, a 50W fiber laser, and so on. High-power lasers generate more energy faster than low-power lasers. 
  • Mode: The mode refers to the size of the core (where light travels) in the optical fiber. There are two types of modes: single-mode fiber lasers and multi-mode fiber lasers. The core diameter for single-mode lasers is smaller, typically between 8 and 9 micrometers, whereas it is larger for multi-mode lasers, typically between 50 and 100 micrometers. As a general rule, single-mode lasers convey laser light more efficiently and have a better beam quality. 

Fiber lasers can be categorized in many other ways, but the categories mentioned here are the most common. Follow these links if you want to see examples of fiber lasers integrated into products: 

What's the Difference Between Fiber Lasers and CO2 Lasers?

A fiber laser (left) and a CO2 laser (right)

The main difference between fiber and CO2 lasers is the source where the laser beam is created. In fiber lasers, the laser source is silica glass mixed with a rare-earth element. In CO2 lasers, the laser source is a mixture of gases which includes carbon dioxide.  

Due to the state of their source, fiber lasers are considered solid-state lasers, and CO2 lasers are considered gas-state lasers. 

These laser sources also produce different wavelengths. Fiber lasers, for example, produce shorter wavelengths, with some examples ranging between 780 nm and 2200 nm. CO2 lasers, on the other hand, produce longer wavelengths that typically range between 9,600 nm and 10,600 nm.  

They are used for different applications due to their different wavelengths. For example, 1064 nm fiber lasers are usually preferred for metal processing applications. Laser cutting is a notable exception, where CO2 lasers are often preferred to cut metals. CO2 lasers also react well with organic materials. 

If you’re debating between the two, read our post on choosing between a CO2 and a fiber laser.

What is a Fiber Laser Machine?

When a fiber laser system is engineered into a solution that is ready to be used, that solution is called a fiber laser machine. Whereas the OEM laser system is the tool that performs the operation, the laser machine is the framework in which the tool is integrated.  

Laser machines can make sure that: 

  • Workers are 100% safe by providing laser safety and fume extraction 
  • Mechanical components are included to automate operations or facilitate the operator’s work 
  • The laser process is fine-tuned for a specific operation 

For example, the fiber laser machine shown here includes a rotary table, a rotary indexer, a Class-1 laser safety enclosure, a fume extractor, a vision camera and an HMI.

 

Follow these links if you want to see more examples of fiber laser machines:

How Long Does a Fiber Laser Last?

Most online sources claim that fiber lasers last 100,000 hours whereas CO2 lasers last 30,000 hours. This is not entirely true. These numbers refer to a value called “mean time between failures” (MTBF), which isn’t the same for all fiber lasers. In reality, you will see different numbers for different types of fiber lasers. 

The MTBF measures the reliability of a laser by indicating how many hours the laser is expected to function before a failure occurs. It is obtained by testing multiple laser units, and then dividing the total number of operational hours by the total number of failures.  

Although this value does not exactly tell you how long a fiber laser can last, it still provides a good idea of the laser’s reliability. 

If you really want to know the exact lifespan of a fiber laser, you'll be disappointed as there’s no real answer. In truth, fiber lasers have critical points in their lifetime when they can fail. 

Here’s what you need to know if your laser experiences failures at any of these moments:  

  • Early life: If a fiber laser has fabrication errors, it will likely have failures early on. You should ensure that you have a purchase guarantee that covers fabrication errors so that the laser can be replaced at no fees. 
  • Normal life: Once you’ve passed the first critical period of early life, the MTBF value provides you a good idea of your laser’s chances of failure. A high MTBF is a good assurance that everything will go smoothly, but not a guarantee. You can prepare for failures during the normal life in different ways: have a spare laser readily available, rent a laser while yours is being repaired, or have a prolonged purchase guarantee.  
  • End of life: When fiber lasers are near their end life, chances of failure increase drastically. Even then, a high-quality industrial laser can often operate way past its MTBF. 

How Does a Fiber Laser Work (And What Are Its Components)?

Fiber lasers use pump light from what is called laser diodes. These diodes emit light that is sent into the fiber-optic cable. Optical components located in the cable are then used to generate a specific wavelength and amplify it. Finally, the resulting laser beam is shaped and released. 

Here’s how each component is used to perform this operation. 

Step 1. Light is Created in the Laser Diodes 

A laser diode emitting light to be pumped into a fiber laserLaser diodes transform electricity into photons—or light— to be pumped into the fiber-optic cable. For this reason, they are also known as the “pump source”

To generate light, diodes use two semiconductors charged differently: 

  • The first one is charged positively, which means that it needs an extra electron. 
  • The second one is charged negatively, which means it has an extra electron, or a free electron. 

When the positive and negative charges meet, they try to combine. But to do so, the free electron must be released as a photon. As current flows through the semiconductors, the quantity of photons quickly increases. 

The resulting light is pumped into the fiber-optic cable and will be used to generate the laser beam. 

Step 2. Pump Light is Guided in the Fiber-Optic Cable 

In nature, light goes in all directions. To focus light into a single direction and obtain a laser beam, fiber-optic cables use two basic components: the fiber core and the cladding. 

  • The core is where light travels. It is made of silica glass and is the only part of the cable that includes a rare-earth element. 
  • The cladding is the material that surrounds the core. When light hits the cladding, it bounces back into the core. This occurs because the cladding provides total internal reflection. 

Total internal reflection occurs because the cladding has a lower refractive index than the core. You can see similar effects in nature. For example, if you look at submerged objects, they appear deformed. This is because when light travels from air to water, it hits a different refractive index and changes direction. The same applies when light travels from the core to the cladding, except that the change in direction produces a reflection. 

Without the cladding, light would go in all directions and exit the core. But thanks to the cladding’s refractive index, light remains in the core and continues its path. 

To visualize how light travels in fiber cables, you can watch this video: 

 

Step 3. Light is Amplified in the Laser Cavity  

As pump light travels through the fiber-optic cable, it eventually enters the laser cavity—a small region of the cable where only light of a specific wavelength is produced. Physical engineers say that the fiber is “doped” in this region because it has been mixed with a rare-earth element.  

As particles from the doped fiber interact with light, their electrons rise to a higher energy level. When they fall back to their basic state, they release energy in the form of photons or light. Physical engineers refer to these phenomena as “electron excitation” and “electron relaxation”.

A fiber-optic cable showing the Bragg gratings in the coreThe laser cavity also acts as a resonator where light bounces back and forth between what is called “fiber Bragg gratings”. This leads to “Light Amplification by the Stimulated Emission of Radiation”, or LASER. Put simply, this is where the laser beam is formed.

There are two types of Bragg gratings: 

  • The first acts as a mirror, reflecting light back into the cavity.  
  • The second acts as a selective mirror, allowing some of the light to exit the cavity, but reflecting the rest back into the cavity. 

Here’s how amplification takes place: when photons hit other excited particles, these particles also release photons; since the Bragg gratings reflect photons back into the cavity, and more pump light is sent into the cavity, an exponential number photons are released. 

As a result of this stimulated emission of radiation, laser light is created. 

Step 4. Laser Light of a Specific Wavelength is Created 

The wavelength produced by the doped fiber varies according to the doping element of the laser cavity. This is very important, as different wavelengths are used for different applications. The doping element could be erbium, ytterbium, neodymium, thulium, and so on. Ytterbium-doped fiber lasers, for example, generate a wavelength of 1064 nm and are used for applications like laser marking and laser cleaning.

Different doping elements produce different wavelengths because specific particles release specific photons. As such, photons generated in the laser cavity all have the same wavelength. This explains why each type of fiber laser generates a specific wavelength—and only that wavelength. 

Step 5. The Laser Beam is Shaped and Released  

Photons that exit the resonant cavity form a laser beam that is extremely well collimated (or straight) due to the fiber’s light guiding properties. In fact, it is too collimated for most laser applications.  

To give the laser beam a desirable shape, different components can be used, such as lenses and beam expanders. For example, our fiber lasers are equipped with a 254 mm focal length lens for laser applications that dig into the material (i.e., laser engraving and laser texturing). This is because their short focal length allows us to focus more energy onto an area. 

Other types of lenses provide different advantages, which is why experts choose them carefully when optimizing a laser for a specific application. 

What Are the Laser Parameters?

Not all lasers and laser applications use the same parameters. For example, different ones need to be adjusted for laser cutting and laser marking. Some parameters, however, are used for all types of fiber lasers. Here are the ones you are most likely to encounter. 

Wavelength 

The electromagnetic spectrum showing the entire range of wavelengths that exist

Image Courtesy of the National Institute of Standards and Technology 

The wavelength produced by a fiber laser corresponds to the level of electromagnetic radiation of the laser light. Typically, fiber lasers produce wavelengths between 780 nm and 2200 nm, which is located in the infrared spectrum and is invisible to the human eye. This range of infrared light tends to react well with metals, rubber and plastics, making it useful for a wide range of materials processing applications. 

Some fiber lasers such as green lasers produce visible light which can react well with soft materials such as gold, copper, silicone and soft glass. Green fiber lasers are also used for holography, therapy and surgery, among other things.

These lasers require additional components to generate visible light. John Wallace from Laser Focus World explains how this is done: 

 

[…] there is actually no fiber laser on the market that produces visible laser light from within the lasing fiber itself. Visible light can, however, be obtained from a near-infrared (IR)-emitting fiber laser by external frequency conversion—for example, Raman-shifting, frequency-doubling, frequency sum-mixing, or combinations of these approaches. 

Excerpt from Photonics Products: Fiber Lasers: Visible fiber lasers do red, green, and now bluish by Laser Focus World

Mode of Operation 

The mode of operation is the way in which the laser beam is released. Fiber lasers typically operate in the continuous-wave or in the pulsed mode.  

In the continuous-wave operation mode, a continuous, uninterrupted laser beam is released, which is ideal for applications like laser welding and laser cutting. 

In the pulsed operation mode, short pulses are released at a set repetition rate. Pulsed laser beams reach higher peak powers and are ideal for laser engraving and laser cleaning. This mode includes the following parameters: 

  • Pulse Energy: The pulse energy is the number of millijoules contained in each pulse. Typically, each pulse contains 1 mJ of energy. 
  • Pulse Duration: The pulse duration, also known as pulse length and pulse width, is the duration of each pulse. Shorter pulses concentrate the same energy in a shorter time, and hence reach higher peak powers. The pulse duration can be expressed in microseconds, nanoseconds, picoseconds or femtoseconds. 
  • Repetition Rate: The pulse repetition rate is the number of pulses released per second. It is also known as the pulse frequency, which is expressed in kHz. 100 kHz is equal to 100,000 pulses per second. 

Power  

The laser power is the amount of energy that can be produced by the laser over one second. It is also known as “average power” and “output power”.  

Pulsed lasers may also indicate a peak power, which is a different parameter. The peak power is the maximum amount of energy reached by a single pulse. For example, a 100W pulsed fiber laser can easily reach 10,000W of peak power. This is because pulsed lasers do not distribute energy evenly over time as opposed to continuous-wave lasers. 

Beam Quality 

The beam quality indicates how close the beam is to what is called a Gaussian beam. In actual applications, this is relevant because it indicates how well focused the laser beam is.  

Mathematically speaking, a perfect beam quality is expressed as M2=1. Laser beams that are well-focused concentrate more energy in a smaller area. High-quality laser beams are required for applications like laser engraving and laser cleaning, whereas lower beam qualities may be more appropriate for applications where ablation is not desired, such as laser welding. 

Jerome Landry's picture

Jerome Landry

Trained in physics and physical engineering, Jerome has been working in the high-tech industry for more than 4 years. He is currently a technical sales specialist at Laserax. He has hands-on experience with laser processes and their interaction with materials, as well as with industrial traceability standards. This allows him to guide clients toward the best laser solution.