FIBER LASER BASICS AND DESIGN PRINCIPLES

FIBER LASER DEFINITION:

Fiber lasers are a sub-category of diode pumped solid state lasers which utilize a doped optical fiber core as the amplification medium. They contain a spool of fiber optic cable which has a core that has been doped with a variety of rare earth elements from the lanthanide family of the periodic table. Elements such as ytterbium and erbium are commonly used. The fiber doping element is selected and doped into the ultra-pure glass fiber core in order to achieve a desired lasing wavelength and/or power level. continue »

QUICK NAVIGATION:

• FIBER LASER DEFINITION
• FIBER LASER DESIGN PRINCIPLES
• BACKGROUND INFORMATION
  • Optics: Snell's Law
  • Optics: Refractive Index
  • Optics: Numercial Aperture
  • Lasers: Spontaneous Emission
  • Lasers: Stimulated Emission
  • Lasers: Population Inversion
  • Lasers: Amplification
• HOW A FIBER LASER WORKS
• KEY COMPONENTS USED TO BUILD A FIBER LASER
• DOPED OPTICAL FIBER
• DOUBLE CLAD FIBER
• YTERRBIUM (Yb) DOPANT
• PUMP LASER DIODES
• PUMP LASER COMBINERS
• FIBER BRAGG GRATINGS
• FIBER LASER WAVELENGTHS
• RESOURCES & FURTHER READING

Fiber Laser Diagram

DEFINITION (CONTINUED):

In order to make the doped optical fiber act as an amplifying medium, multiple semiconductor laser diodes in the wavelength range of 915nm to 980nm are coupled (spliced) into the spool of doped fiber. The laser light emitted from the pump laser diodes excites the rare earth ions embedded in the doped fiber core from their ground state to produce high excitation levels This excitation ultimately produces the laser light. This light resonates between Bragg gratings which are placed at each end of the fiber. The light is amplified as it resonates between these gratings. One of the gratings has a lower reflectivity than the other, which allows the laser light which has been created in the cavity (amplifying medium) to exit the fiber. The resultant beam propagates through a high power fiber and out of the fiber laser.

FIBER LASER DESIGN PRINCIPLES:

Background Information - Optics Fundamentals:

Before getting into the details of how fiber lasers create and deliver their light, it may be helpful to review several key principles of optics and a few of the primary design fundamentals of lasers.

• Snell’s Law: Because it is one of the most basic principles underlying how fiber optic cables and lasers interact, Snell’s Law is a fundamental concept to understand in order to learn how a fiber laser functions. It’s the formula which defines how light bends, or refracts, when it passes through a boundry between two different transparent materials. In the context of fiber lasers, Snell's Law plays a large role in modelling how a laser source will enter into an optical fiber and then travel down the fiber. More specifically, it describes how a laser source enters into and travels down several fibers that are wrapped around each other in an adjacent manner. As mentioned above, dopants are added to the ultra-pure glass which makes up the core of the optical fiber in a fiber laser. In a basic configuration, the core of the fiber will be doped and the adjacent layer(s) of glass will not be doped. The dopant, such as ytterbium, will change the refractive index. Depending on the refractive indices of the two different materials, such as the doped and non-doped glass, the light will bend at specific angles. These angles can be determined by applying Snell’s law. These angles are measured with respect to a normal line which is perpendicular to the boundry. This video provides a good overview of Snell's Law as it applies to fiber optic cable:

  Snell's Law as it Applies to Fiber Optic Cable

• Refractive Index: The refractive index of a fiber is a dimensionless unit which is used to represent how much the path of the laser beam bends when it enters a material. The core and the cladding of the fibers used in a fiber laser are made of ultra-pure glass. The core of the fiber is designed and manufactured to have a higher index of refraction than the cladding. The fiber laser manufacturer adds dopants into the ultra-pure glass core to change the refractive index. Deliberately changing the refractive index at intervals along the length of the fiber makes the fiber reflect the laser beam travelling through the fiber in a manner similar to a mirror. By changing the the refractive index at certain intervals, the amount of reflection for particular wavelength can be controlled.

• Numerical Aperture: The definition of numerical aperture (NA) varies slightly between its use in fiber coupled laser applications and other areas of optics. Applications such as microscopy and photography have a slightly different definition of NA. In fiber lasers, NA is a unit-less number that is used to quantify the angles that the laser beam can have (incident to the fiber) as it travels through the fiber. It is the sine of the largest angle of an incident ray with respect to the fiber axis. The NA is typically calculated using the refractive indices of the core and cladding. It is associated with the fiber coupled laser diode pumping source being coupled into the fiber in the fiber laser. It is important because it defines the ability of the fiber to gather light. And it also offers a way to define how easy or how difficult it will be to couple the pump source laser into the fiber. A basic way to understand the NA value is to understand that a fiber with a high numerical aperture will guide the beam more effectively and support multiple modes. A detailed definition and mathematical modeling of numerical aperture can be found in the Handbook of Solid State Lasers ». This resource is well written and easier to understand than most text books.

Background Information - Laser Fundamentals:

Starting from a basic level, when energy is applied to an atom in the form of light, the electrons orbiting the nucleus of the atom absorb the light energy and become excited. In the case of a fiber laser, the most common atom used as the source of this excitation is an ytterbium (Yb) atom. Yb atoms are embedded (doped) into the fiber core in a fiber laser. This excitation of the ytterbium atom electrons forces them to change their energy level from their original ground state of equilibrium to a new, higher energy state. For Yb doped fiber lasers, the energy source is typically a 976 nanometer diode laser pump source. Yb atoms absorb the wavelength 976nm very efficently, which is why Yb is so commonly used as a doping agent. When the energy source is removed the electrons will return to their ground state. When they return to their ground state, the law of conservation of energy is enacted and the excited electrons must release the energy they have gained. This engergy is released in the form of a photon. This newly created photon will have a vibrational frequency and wavelength that is dependent on the energy level at which the photon was released. In the case of fiber lasers, manufacturers typically design their lasers to emit a wavelength of 1064nm or 1030nm:

Photon Emission in a Fiber Laser

Having established the basics of how a photon is created in the section above, there are five design principles briefly described below which will give the reader a basic understanding of how all lasers works:

• Sponteous Emission
• Stimulated Emission
• Sustained Stimulated Emission
• Population Inversion
• Amplification

First, a quick refresher on the relationships between frequency, energy and wavelength:

Energy and Frequency: E2 - E1 = h v

• E is Energy, measured in Joules
• h is Planck's constant, measured in Joules-second
• v is Frequency, measured in increments of 1/seconds

Wavelength and Frequency: λ = c/v

• λ is wavelength measured in meters
• c is the speed of light in the active laser medium, measured in meters/second

Spontaneous emission occurs when electrons are excited to a point that they change their energy state and release photons. This is explained in more detail in the paragraph above. Stimulated emission occurs when an excited electron is in a higher energy state. Another photon at an energy level equal to the difference between its present level and the lower energy level can cause the first electron to return to its ground state. This causes the release of a second photon of equal energy and frequency. In addition to fiber lasers, this is how all lasers work. All of the photons which are being released are effectively synchronized and are operating at the same wavelength, phase and direction. Sustained stimulated emission means that there are enough electrons releasing photons to maintain a state of ongoing stimulated emission.

Laser Stimulated Emission

In addition to the principles described above, it's important to understand the principles of population inversion and amplification. Population inversion refers to the population, or total number of electrons, and the change in their respective energy state. Electrons are by default in a ground state. In order to produce the fiber laser light, they must be inverted from their ground state. In order to sustain a state of stimulated emission as efficiently as possible, their needs to be more electrons in higher energy states than at the ground level. The higher the energy level of the electron, the higher the degree of population inversion and the greater the chance of amplified emission. The energy source which is supplied for excitation of the electrons and to create the population inversion in a fiber laser comes from a semicondutor laser diode. This process of transfer of energy is referred to as optical pumping.

Population Inversion in a Fiber Laser

The final principle, laser amplification, occurs when population inversion and stimulated emission are both present in a fiber laser. When population inversion is achieved, it means that the rate of stimulated emission exceeds that of absorption. This happens when there are more electrons at an excited energy state and more photons are being released from each incident photon. In a fiber laser, this is accomplished by encapsulating the movement of electrons in an optical cavity which is made up of rare earth doped fiber. The doped fiber is also known as the gain or amplification medium.

Laser Amplification Medium

HOW A FIBER LASER WORKS:

Having reviewed the optics and laser fundamentals in the sections above, we can now apply these fundamentals to the fiber laser category to understand how they function. Fiber lasers utilize a doped fiber core as the amplification medium. In order to make the doped fiber act as the amplifying medium, multiple semiconductor laser diodes are spliced into the spool of doped fiber. These laser diodes are referred to as pumps. The pumps are typically in the 915nm to 980nm range and can deliver from 500 milliwatts up to roughly 600 watts of optical power per laser diode. There are typically multiple pump laser diodes spliced into the fiber.

Fiber Laser Block Diagram

The light from the pump laser diodes passes through a pump combiner which is spliced into the “active” fiber to excite the active element in the fiber core. In this context, “active” refers to the ytterbium doped fiber core and "passive" refers to the non-doped fiber sections. The laser light emitted from the pump lasers excites the electrons of the ytterbium atoms embedded in the doped fiber core from their ground state to an increased energy state. This produces the excitation levels that lead to the spontaneous emission state described above. The electrons are pumped to reach an energy level corresponding to the wavelength of the pump light. Through design, they are then dropped to a lower metastable state. In the case of an ytterbium doped fiber, a photon at 976nm which is absorbed by the ytterbium atom forces the electrons around the atomic nucleus of the Yb atom to move to higher orbitals. This happens because the the electrons have absorbed the 976nm pumping energy.

After transitioning to a higher energy orbital, the excited electrons will drop to their original ground state and emit a photon at a wavelength of 1064nm. This drop occurs on a very short time scale of less than one millisecond. The relationship of the 976nm pump laser wavelength input to the 1064nm fiber laser output is inherint to the element ytterbium. Other wavelengths can also be generated. This National Institute of Health publication describes a 1018nm ytterbium-doped fiber laser ». If the intensity of the 976nm pump light is high enough, the number of electrons in the metastable state exceeds the number of electrons that remain in the ground state. This state represents both the stimulated emission and the population inversion principles. The transition of the electrons back to the ground state causes emission of light with a wavelength corresponding to the energy difference. In this case, a photon at 1064nm wavelenth.

To complete the process of light creation in a fiber laser, the spool of fiber is bound by fiber Bragg gratings at each end. Gratings are described in more detail below. They are essentially mirrors that create a high power resonator when they are placed at both ends of the doped fiber. The longer wavelength light (the 1064nm output) resonates between these gratings. The gratings reflect light at an angle determined by the geometry of the reflecting surface. As the light is bounced back and forth between the two gratings, it is being amplified with every pass. The grating on the output of the fiber laser is partially transparent allowing the longer wavelength laser light which has been created to exit the fiber.

The spool of doped fiber can be from a few meters up to kilometers in length. This allows them to have a very wide output power range and makes them capable of reaching very high output power levels.

We have offered a quick summary of how a fiber laser works and the underlying laser and optics principles. To get a more in-depth understanding, this video from Nufern does an excellent job explaining the basic components and processes:

How a Fiber Laser Works by Nufern

FIBER LASER COMPONENTS:

  Key Components Used in Fiber Lasers

• Doped Optical Fiber

  An optical fiber is a cylindrical strand of very pure silica-glass. Their function is to guide a laser beam by internally reflecting the laser light as is travels down the fiber. They are manufactured in lengths of several meters up to kilometers. The ability to manufacture fiber in very long lengths is one of the key factors that allowed the fiber laser to supersede most competing high power laser technologies. Because optical fibers offer a very high surface area relative to the volume of laser light travelling through them, they offer excellent heat removal. This makes cylindrical optical fiber a very good medium for supporting high optical power levels. In addition to cylyndrical fiber, new research on developing ribbon fiber to enable much higher power levels » is being done at Lawrence Livermore National Labs.

  The cylindrical silica-glass used in fiber lasers ranges in width from several micrometers (the width of a human hair) up to hundreds of micrometers. As mentioned in the introduction, manufacturers use rare earth ions as additive agents to alter the optical properties of the glass fiber. This is referred to as doping a fiber. Doping a fiber changes the refractive index of the glass. To dope a fiber is to introduce a trace of a rare earth element into the silica-glass fiber. The rare earth elements most commonly used are ytterbium, erbium or thulium. Of these three elements, the most frequently used is ytterbium. A good summary of erbium doped fiber amplification can be found in this Lehigh University department of physics article ».

  It takes a very small amount of rare earth element dopant to produce the desired lasing properties. Specific rare earth ions are chosen to produce the desired wavelength. Corning, Nufern and OFS Optics have historically been leaders in the manufacturing of doped optical fiber. Nufern Ytterbium Doped Fiber (image courtesty of Nufern)

• Double-Clad Fiber

  The type of fiber most commonly used in high power fiber lasers is double-clad fiber. Double-clad fiber has a core which is doped with the rare earth dopants described above. A standard single mode fiber offers the beam quality characteristics that fiber laser manufacturer’s desire. But a standard single mode fiber core requires the use of single mode pump laser diodes. The single mode core diameter is kept small enough to permit the single-mode laser oscillation which will yield the high quality beam output. However, the single mode 9XXnm pumps which would need to be used as pump sources don’t offer enough power to reach the desired output power levels. They are also very expensive. Double-clad fibers solve this problem by having a doped single mode core with two layers of cladding around the core. The core has the highest refractive index. The inner layer of cladding which surrounds the core is pumped by high power and relatively low cost multi-mode pump laser diodes. The inner cladding can accept large amounts of pump light from multiple pump source coupled into the inner cladding layer. The outer cladding is designed to have a lower index of refraction than the inner cladding or the core. The double-clad approach offers a good compromise between high power and high beam quality. Double-Clad Fiber, Image courtesy of Kuniharu Himeno, Fujikura Technical Review, 2015

• Yterrbium (Yb) Fiber

  Ytterbium is the most common rare earth element used to dope fiber in fiber lasers. The ytterbium atom offers very good light absorption in the 900nm to 1100nm pump laser diode range. This pump range also happens to offer the most economical and highest power commercially available pumping sources. Ytterbium also offers the fluorescence that causes laser oscillation and output in the 1000nm to 1100nm range. This wavelength range is typically absorbed well by most metals. Therefore, Yb is the primary type of doped fiber used for fiber lasers used in metal processing. Metal processing is the single largest application segment for fiber lasers.

• Pump Laser Diodes

  Laser diodes are compact efficient semiconductor devices that convert electrical energy to laser light. A technical overview of laser diodes can be found in this article: "A Technical Introduction to Laser Diodes" » These devices are used to “pump” the doped fiber because of their brightness and their spectral characteristics. The laser beams emitted from the laser diode pump sources excite the rare earth ions which are embedded in the doped fiber. This excitation produces the commensurate high gain levels. The doping agent, such as Yb, is chosen in part due to its ability to absorb the light from these pump lasers. Diode lasers make excellent excitation sources for a number of technical and commercial reasons. First, they are very compact. They are built on a single semiconductor chip that contains everything necessary for a laser. They are roughly 40mm x 40mm and can be easily inserted into the fiber laser system. Secondly, they offer a relatively high efficiency rate. Their conversion rate from electrical energy to optical output energy is approximately 50%. They offer direct excitation with low current levels so that conventional transistor based circuits can supply the diode laser. This enables them to be driven by lower electrical power than other laser technologies.

  The primary diode laser types used fiber laser are 915nm ~ 980nm pump sources and 1060nm seed sources. The 1060nm seed sources are used as a trigger source in ultra-fast pulsing fiber lasers. While 10+ years ago many of the fiber pump sources were single emitter (single laser diode chip) devices, next generation multi-emitters have become the dominant source. Multi-emitters are based on the principle of combining multiple laser diode chips in series in a single package. They combine the multiple laser chip apertures and use micro-optics to focus the resultant beam into the optical fiber They provide pumping power levels in the 100W to 300W range from a 105µm diameter, 0.22NA fiber.

  . 976nm, 300W Laser Diode Pump Excitation Source

• Pump Laser Combiners

  Beam combiners in fiber lasers are designed to additively increase the output power of the pump sources that are being used to excite the doped ions. They are passive fiber based components. For example, multiple 976nm multimode single emitter laser diodes can be combined to pump a 120 watt Yb doped fiber laser. Four of the 976nm pumps are spliced into a 4 (input) x 1 (output) passive combiner. Five of these 4 x 1 combiners can be used to combine the power from 20 laser diodes. Output power from each of the combiners is approximately 36 watts each, yielding approximately 180 watts of pumping power. This level of pumping power will produce a > 120 watt fiber laser output power at 1064nm. Efficiency levels of pump power to fiber laser output power of > 65% are common. For more information, this article on high power beam combining Yb-doped fiber lasers » may be a useful reference. . Pump Laser Combiner, Image Courtesty of Laser Solution Technology, Gwangju, South Korea

• Fiber Bragg Gratings

  A fiber Bragg grating (FBG) is a type of distributed Bragg reflector which is constructed in a short segment of optical fiber. An FBG is used as an in-line wavelength filter to block certain wavelengths or is used as a wavelength-specific reflector. A Bragg grating is simply a section of glass in a fiber which has stripes etched into the glass. With respect to fiber lasers, FBG’s are used to create a cavity in the doped fiber which traps the ytterbium or erbium atoms and keeps them in the fiber core. Bragg gratings are used in fiber lasers as the wavelength reflector. At a very basic level, a fiber laser works by reflecting light through the optical cavity formed by the grating in a way that forces the photons to stimulate the ytterbium atoms doped into the fiber. On both sides of the Yb doped “active” fiber, fiber Bragg gratings are placed which act as mirrors reflecting the light of the desired wavelength back and forth. The mirrors constitute a laser resonator. The desired wavelength of light is reflected selectively by the grating. This produces the induced emission in the resonator. This induced light then propagates in the resonator and is reflected by the both FBGs to stimulate further induced emission. The repetition of induced emission results in laser oscillation and ultimately the laser light is emitted from the output port which has the lower reflectivity grating. Fiber Bragg Grating

FIBER LASER WAVELENGTHS:

Most fiber lasers employ ytterbium or erbium as the rare earth element doping agent in the fiber core. Ytterbium offers photon emission at wavelengths in the one micrometer range. Specifically, at 1030nm, 1064nm and 1080nm. Erbium is the element of choice to produce an output in the 1550nm range. Erbium offers a range of roughly 1528nm to 1620nm to 1620nm. Pulsed 1550nm fiber lasers are becoming quite common in sensing applications because they offer very high power in the 1.5 micron eye-safe wavelength range. To get output power levels over 5 watts at 1550nm, it is fairly common to use a combination of erbium and ytterbium as the doping agents. In this case, the ytterbium is excited and the energy is then transmitted to the erbium ions.

RESOURCES AND FURTHER READING:

K. Himeno: “Basics and Features of High-Power Fiber Laser”, Fujikura Technical Review no. 44, 2015.

Lawrence Livermore National Labs: "How Lasers Work"

Bill Shiner: Fiber Lasers for Material Processing: June 22, 2011, NEW ENGLAND FIBER OPTIC COUNCIL