Fiber laser

Fiber laser

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A fiber laser or fibre laser is a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 in which the active gain medium is an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

 doped with rare-earth elements such as erbium
Erbium
Erbium is a chemical element in the lanthanide series, with the symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements on Earth...

, ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...

, neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...

, dysprosium
Dysprosium
Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...

, praseodymium
Praseodymium
Praseodymium is a chemical element that has the symbol Pr and atomic number 59. Praseodymium is a soft, silvery, malleable and ductile metal in the lanthanide group. It is too reactive to be found in native form, and when artificially prepared, it slowly develops a green oxide coating.The element...

, and thulium
Thulium
Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides . It is an easily workable metal with a bright silvery-gray luster...

. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...

, such as stimulated Raman scattering
Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....

 or four-wave mixing
Four-wave mixing
Four-wave mixing is an intermodulation phenomenon in optical systems, whereby interactions between 3 wavelengths produce a 4th wavelength in the signal. It is similar to the third-order intercept point in electrical systems...

 can also provide gain and thus serve as gain media for a fiber laser.

Advantages and applications



The advantages of fiber lasers over other types include:
  • Light is already coupled into a flexible fiber: The fact that the light is already in a fiber allows it to be easily delivered to a movable focusing element. This is important for laser cutting, welding, and folding of metals and polymers.

  • High output power: Fiber lasers can have active regions several kilometers long, and so can provide very high optical gain. They can support kilowatt levels of continuous output power because of the fiber's high surface area
    Surface area
    Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...

     to volume
    Volume
    Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

     ratio, which allows efficient cooling.

  • High optical quality: The fiber's waveguiding
    Waveguide
    A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

     properties reduce or eliminate thermal distortion of the optical path, typically producing a diffraction-limited
    Diffraction-limited
    The resolution of an optical imaging system — a microscope, telescope, or camera — can be limited by factors such as imperfections in the lenses or misalignment. However, there is a fundamental maximum to the resolution of any optical system which is due to diffraction...

    , high-quality optical beam.

  • Compact size: Fiber lasers are compact compared to rod
    Solid-state laser
    A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...

     or gas laser
    Gas laser
    A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate "on the principle of converting electrical energy to a laser light output...

    s of comparable power, because the fiber can be bent and coiled to save space.

  • Reliability: Fiber lasers exhibit high vibrational stability, extended lifetime, and maintenance-free turnkey
    Turnkey
    A turn-key or a turn-key project is a type of project that is constructed by a developer and sold or turned over to a buyer in a ready-to-use condition.-Common usage:...

     operation.


Fiber laser can also refer to the machine tool that includes the fiber resonator.

Applications of fiber lasers include material processing
Laser ablation
Laser ablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma...

, telecommunications, spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

, and medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

.

Design and manufacture



Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing
Fusion splicing
Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as...

 different types of fiber; fiber Bragg gratings
Fiber Bragg grating
A fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a...

 replace conventional dielectric mirror
Dielectric mirror
A dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...

s to provide optical feedback
Optical feedback
Optical feedback is the optical equivalent of acoustic feedback. A simple example is the feedback that occurs when a loop exists between an optical input, e.g., a video camera, and an optical output, e.g., a television screen or monitor...

. Another type is the single longitudinal mode operation of ultra narrow distributed feedback lasers (DFB)
Distributed feedback laser
A distributed feedback laser is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured as a diffraction grating...

 where a phase-shifted Bragg grating overlap the gain medium. Fiber lasers are pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 by semiconductor laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...

s or by other fiber lasers.

Double-clad fibers




Many high-power fiber lasers are based on double-clad fiber
Double-clad fiber
Double-clad fiber is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding...

. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...

 propagates in the core, while a multimode
Multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus...

 pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness
Radiance
Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...

 into a much higher-brightness signal. As a result, fiber lasers and amplifiers are occasionally referred to as "brightness converters."

There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design. The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.

Power scaling



Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited
Diffraction-limited
The resolution of an optical imaging system — a microscope, telescope, or camera — can be limited by factors such as imperfections in the lenses or misalignment. However, there is a fundamental maximum to the resolution of any optical system which is due to diffraction...

 beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area (LMA) fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...

 powers from Yb-doped fiber lasers have increased from 100 W in 2001 to >50 kW. Single mode lasers have reached 10 kW in CW power: "http://www.laserfocusworld.com/articles/364754"

Previously unattainable powers can now be achieved with commercially available off-the-shelf fibers and components. As a result, fiber laser technology is expected to have a profound effect on a broad variety of industrial applications. This white paper describes the technology in greater detail: "KW-power fiber lasers with single transverse mode output".
Nonlinear polarization rotation

When a linearly polarized light is incident to a piece of weakly birefringent fiber, the polarization of the light will generally become elliptically polarized in the fiber. The orientation and ellipticity of the final light polarization is fully determined by the fiber length and its birefringence. However, if the intensity of the light is strong, the nonlinear optical Kerr effect in the fiber must be considered, which introduces extra changes to the light polarization. As the polarization change introduced by the optical Kerr effect depends on the light intensity, if a polarizer is put behind the fiber, the light intensity transmission through the polarizer will become light intensity dependent. Through appropriately selecting the orientation of the polarizer or the length of the fiber, an artificial saturable absorber effect with ultra-fast response could then be achieved in such a system, where light of higher intensity experiences less absorption loss on the polarizer. The NPR technique makes use of this artificial saturable absorption to achieve the passive mode locking in a fiber laser. Once a mode-locked pulse is formed, the nonlinearity of the fiber further shapes the pulse into an optical soliton and consequently the ultrashort soliton operation is obtained in the laser. Soliton operation is almost a generic feature of the fiber lasers mode-locked by this technique and has been intensively investigated.
Semiconductor saturable absorber mirrors

Semiconductor saturable absorbers were used for laser mode-locking as early as 1974
when p-type germanium is used to mode lock a CO2 laser which generated pulses
~500 ps . Modern SESAMs are III-V semiconductor single quantum well (SQW)
or multiple quantum wells grown on semiconductor distributed Bragg reflectors
(DBRs) . They were initially used in a Resonant Pulse Modelocking (RPM)
scheme as starting mechanisms for Ti:Sapphire lasers which employed KLM as a fast
saturable absorber . RPM is another coupled-cavity mode-locking technique.
Different from APM lasers which employ non-resonant Kerr-type phase nonlinearity
for pulse shortening, RPM employs the amplitude nonlinearity provided by the
resonant band filling effects of semiconductors . SESAMs were soon developed
into intracavity saturable absorber devices because of more inherent simplicity with
this structure . Since then, the use of SESAMs has enabled the pulse durations,
average powers, pulse energies and repetition rates of ultrafast solid-state lasers to be
improved by several orders of magnitude. Average power of 60 W and repetition rate
up to 160 GHz were obtained . By using SESAM-assisted KLM, sub-6 fs
pulses directly from a Ti: Sapphire oscillator was achieved . A major advantage
SESAMs have over other saturable absorber techniques is that absorber parameters
can be easily controlled over a wide range of values . For example, saturation
fluence can be controlled by varying the reflectivity of the top reflector while
modulation depth and recovery time can be tailored by changing the low temperature
growing conditions for the absorber layers . This freedom of design has
further extended the application of SESAMs into modelocking of fiber lasers where a
relatively high modulation depth is needed to ensure self-starting and operation
stability. Fiber lasers working at ~ 1μm and 1.5μm were successfully demonstrated.
.
Graphene saturable absorbers

Graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...

 is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Optical absorption from graphene can become saturated when the input optical intensity is above a threshold value. This nonlinear optical behavior is termed saturable absorption
Saturable absorption
Saturable absorption is a property of materials where the absorption of light decreases with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities ....

 and the threshold value is called the saturation fluency. Graphene can be saturated readily under strong excitation over the visible to near-infrared region, due to the universal optical absorption and zero band gap. This has relevance for the mode locking of fiber lasers, where wideband tunability may be obtained using graphene as the saturable absorber. Due to this special property, graphene has wide application in ultrafast photonics. Furthermore, comparing with the SWCNTs, as graphene has a 2D structure it should have much smaller non-saturable loss and much higher damage threshold. Self-started mode locking and stable soliton pulse emission with high energy have been achieved with a graphene saturable absorber in an erbium-doped fiber laser.
Atomic layer graphene possesses wavelength-insensitive ultrafast saturable absorption, which can be exploited as a “full-band” mode locker. With an erbium-doped dissipative soliton fiber laser mode locked with few layer graphene, it has been experimentally shown that dissipative solitons with continuous wavelength tuning as large as 30 nm (1570–1600 nm) can be obtained.

Active mode locking


Active mode-locking is normally achieved by modulating the loss (or gain) of the
laser cavity at a repetition rate equivalent to the cavity frequency, or a harmonic
thereof. In practice, the modulator can be acousto-optic or electro-optic modulator,
Mach-Zehnder integrated-optic modulators, or a semiconductor electroabsorption
modulator (EAM). The principle of active mode-locking with a sinusoidal modulation. In this situation, optical pulses will form in such a way as to
minimize the loss from the modulator. The peak of the pulse would automatically
adjust in phase to be at the point of minimum loss from the modulator. Because of the
slow variation of sinusoidal modulation, it is not very straightforward for generating
ultrashort optical pulses (< 1ps) using this method.

For stable operation, the cavity length must precisely match the period of the
modulation signal or some integer multiple of it. The most powerful technique to
solve this is regenerative mode locking i.e. a part of the output signal of the
mode-locked laser is detected; the beatnote at the round-trip frequency is filtered out
from the detector, and sent to an amplifier, which drives the loss modulator in the
laser cavity. This procedure enforces synchronism if the cavity length undergoes
fluctuations due to acoustic vibrations or thermal expansion. By using this method,
highly stable mode-locked lasers have been achieved . The major advantage of
active mode-locking is that it allows synchronized operation of the mode-locked laser
to an external radio frequency (RF) source. This is very useful for optical fiber
communication where synchronization is normally required between optical signal
and electronic control signal. Also active mode-locked fiber can provide much higher
repetition rate than passive mode-locking. Currently, fiber lasers and semiconductor
diode lasers are the two most important types of lasers where active mode-locking are
applied.

Dark soliton fiber lasers


In the non-mode locking regime,the first dark soliton fiber laser has been successfully achieved in an all-normal dispersion erbium-doped fiber laser with a polarizer in cavity. Experimentally finding that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations we interpret the dark pulse formation in the laser as a result of dark soliton shaping.

Multiwavelength fiber lasers


Recently,multiwavelength dissipative soliton in an all normal dispersion fiber laser passively mode-locked with a SESAM has been generated. It is found that depending on the cavity birefringence, stable single-, dual- and triple-wavelength dissipative soliton can be formed in the laser. Its generation mechanism can be traced back to the nature of dissipative soliton.

Fiber disk lasers




Another type of fiber laser is the fiber disk laser
Fiber disk laser
A fiber disk laser is a fiber laser with transverse delivery of the pump light. They are characterized by the pump beam not being parallel to the active core of the optical fiber , but directed to the coil of the fiber at an angle...

. In such, the pump is not confined within the cladding of the fiber (as in the double-clad fiber
Double-clad fiber
Double-clad fiber is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding...

), but pump light is delivered across the core multiple times because the core is coiled on itself like a rope. This configuration is suitable for power scaling
Power scaling
Power scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design....

in which many pump sources are used around the periphery of the coil.