FAQ - Fiber Lasers

Solid state lasers suffer from thermally induced stress, reducing the quality of the laser output especially when power scaling is carried out. Typically in cylindrical type of geometries, the temperature profile of the rod is the highest temperature in the centre and slowly coming to lower levels at the periphery in a parabolic shape, causing thermal lensing and consequent generation of thermal stress producing low quality laser output. On the other hand, the fiber optic laser is compact and it has long term stability, without any thermal lensing problems. Long length of the optical fibers offer large surface area due to their long length as well as having large ratio of surface to volume, ensures fast heat dissipation producing diffraction limited output. Thermal load also is distributed over the entire length of the fiber cable, thus reducing the thermal lensing build up problems as seen in the case of solid state lasers.

Fiber lasers involve optical fibers as the gain media. In general, the gain medium is a fiber doped with rare earth ions such as erbium, (Er³⁺), neodymium (Nd³⁺), ytterbium (Yb³⁺), thulium (Tm³⁺), or praseodymium (Pr³⁺). These lasers fall under the category of solid-state lasers because the gain medium of fiber lasers is similar to those of solid-state lasers.

There are six transmission bands for fiber optic communication. These are O Band (1.26 to 1.31 micron), E band (1.36 to 1.46 micron), S band (1.46 to 1.53 micron), C Band (1.53 to 1.565 micron), L band (1.565 to 1.625 micron) and U Band (1.625 to 1.675 micron). Seventh band which is also being used by private networks is around 850 nm.

Single Mode and Multimode: In multimode fibers the core diameter is greater than the core diameter of single-mode fibers, making the light to have several propagation modes, i.e. the light goes through the fiber core using several paths and not using a single path, like in single-mode fibers. Multimode fibers have a core diameter 50 to 100 microns (typical commercial values are 50, 62.5 and 100 microns) and a cladding diameter of 125 microns. Single-mode fibers are used in long-distance cables, but they require connectors with better precision and as such are expensive devices. In this kind of fiber the light has only one way of traveling inside the fiber core, hence its name. The core diameter is between 7 and 10 microns and its cladding diameter is around 125 microns, so both multi-mode and mono-mode cables have the same diameter, what makes the difference is the diameter of the core.

Multimode fibers can be classified into graded-index and step-index, depending on the refractive index between the core and the cladding; in graded-index there is a gradual change between the core and the cladding, while in step-index this change is abrupt, hence the name.

Step index fibers result in limited distances because of modal dispersion. Since the modes extend through the fiber at different angles, their lengths are slightly different. The result is that light takes less time to travel down some modes (the shorter ones) than others dispersing or spreading down the light pulse. With short fiber spans, there is a little spreading out of the pulse but on longer distances typically more than a kilometer, the spread is so much that it is of not much use.

A graded index fiber on the other hand virtually eliminates modal dispersion by gradually decreasing the refractive index out towards the cladding, where the modes are longest. Then waves on longer modes travel faster than on the shorter modes, so the entire pulse reaches at the receiver at almost the same time.

There are three types of single-mode fibers: non dispersion-shifted fiber (NDSF), dispersion-shifted fiber (DSF) and non zero-dispersion-shifted fibers (NZ-DSF). Non Dispersion Shifted Fibers (NDSF) carries signal generally in the O band transmission window at 1.310 micron. Although the range is greatly increased but there is other problem namely chromatic dispersion. Any light pulse, howsoever the precise laser may be, contains a large number of frequencies. Since the refractive index is frequency dependent, the waves end up traveling at different velocities resulting in pulse dispersion. At the same time wave-guide dispersion also affects the wave velocity. As a result, part of electric field and magnetic field extend into cladding resulting in making the wave faster. At 1.310 micron, chromatic dispersion and wave-guide dispersion cancel each other. However outside this wavelength, dispersion increases thus restricting the length of fiber.

S band (1.46 to 1.53 micron) and C Band (1.53 to 1.565 micron) transmission windows are better suited for longer distances and they have less attenuation and works well with optical amplifiers such as Erbium doped fiber amplifiers. Dispersion shifted fibers (DSF) move optimal dispersion points to higher frequencies by altering the core cladding interface. Zero dispersion shifted fibers (ZDSF) moves the zero dispersion frequency from 1.310 micron by increasing the wave-guide dispersion until it cancels out the chromatic aberration at 1.55 micron. However, devices like Wavelength Division Multiplexing (WDM), Dense Wavelength Division Multiplexing (DWDM) and Erbium doped fiber amplifiers operate in this range. Signals traveling over ZDSF can combine to create additional signals that may be amplified by Erbium doped fiber amplifiers and superimposed on DWDM channels resulting in noise like four wave mixing.

Non zero dispersion fibers avoids four wave mixing by moving the zero dispersion point above the range of Erbium doped fiber amplifiers. The signal still transmits in S and C band with only a moderate amount of dispersion. The operation in this manner is rather useful as it provides a minimal level of interference needed to separate DWDM channels from one another. Therefore non-zero dispersion fibers are being widely used. The idea is to move λ₀ to either end of the 1550 nm band, thus ensuring that all of the wavelength channels have slightly different optical speeds in the fiber. Common wavelengths are around 1.530 micron, 1.497 micron, 1.452 micron and 1.560 micron. The advantage that these fibers have over DSF is a compromise solution of a slightly lower degree of integrated dispersion compensation for a higher tolerance to non-linear distortion effects. These are either with positive dispersion Non-zero dispersion fibers and negative non-zero dispersion fibers. Some long-haul fiber paths usually alternate between positive non zero optical regime and negative non zero optical regimes to provide self-dispersion compensation with uniformly low dispersion across the minimum-loss window at 1550 nm.

The Numerical Aperture (NA) of a fiber is defined as the sine of the largest angle an incident ray can have for total internal reflectance in the core. Rays launched outside the angle specified by a fiber's NA will excite radiation modes of the fiber. A higher core index, with respect to the cladding, means larger NA.

Qualitatively, NA is a measure of the light gathering ability of a fiber. It also indicates how easy it is to couple light into a fiber. where α is the half acceptance angle.

The cutoff wavelength, λ_c, is the minimum wavelength in which a particular fiber still acts as a single mode fiber. Below the cutoff wavelength, higher order modes are able to propagate which means that the fiber becomes a multimode fiber at this wavelength. It is given as where a is the fiber core radius, n₁ = n_core and n₂ = n_clad are the refractive indices of core and cladding, V_c is the cutoff number.

V number is a normalized frequency parameter of a fiber and can be used to express many fiber parameters such as number of modes at a given wavelength, mode cut off conditions, propagation constants etc. The V number is a dimensionless parameter, which is often used in the context of step index fiber. It is defined as The V number can be interpreted as a kind of normalized optical frequency. It is relevant for various essential properties of a fiber:

If V is less than 2.405 then the fiber is a single mode fiber but if V is greater than 2.405 then fiber is multimode.

V number is related with the number of modes is the fiber as:
       M≈ V²/2 for step index fiber and
       M≈ V²/4 is the number of modes for graded index fiber.

Fiber lasers based on single mode fiber can generate a high quality laser beam that is almost diffraction limited, but it requires that the pump sources should also have a diffraction limited beam quality thus restricting the lasers to be generally a low power lasers. Multimode fibers on the other hand usually lead to poor beam quality. In order to overcome this problem, recently double clad fibers (see adjoining figure) are being widely used. In these types of fibers, the pumping laser light partly travels in the single mode core but the major portion of pumping light is restricted to the inner cladding. The inner cladding has a relatively larger area as compared to that of the core and also has a much higher numerical aperture, so that it can support a large number of propagation modes thereby allowing the efficient launch of the output of high power laser diodes.

End-pumped coupling technique, V type groove pump coupling and coupling through the micro-prism.

Wall-plug efficiency or radiant efficiency is the energy conversion efficiency with which the system converts electrical power into optical power. It is also defined as the ratio of the radiant flux (i.e. the total optical output power) to the input electrical power. Overall fiber-laser efficiency is the result of a two-stage process. First is the efficiency of the pump diode. Semiconductor lasers are very efficient, with an electrical to optical efficiency in the range of 50 - 60 %. If this output can be matched carefully to the fiber laser's absorption line, the result is the pump efficiency, which is of the order of 40 - 50%. The second is the optical-to-optical conversion efficiency. Usually high excitation and extraction efficiency can be achieved, producing optical-to-optical conversion efficiency on the order of 60% to 70%. The result is wall-plug efficiency in the 25% to 35% range.

Industrial fiber lasers are utilized in materials processing applications in all of the major high-power and low-power markets, including automotive welding and cutting, sintering, marking, scribing, drilling and heat treating. The single-mode lasers, with the ability to attain high fluency levels and to be focused to micron-sized spots, have changed previous beliefs relating to process parameters. On the kilowatt level, the fiber laser has attained higher speeds of cutting and weld penetration than conventional technology operating at the same power level. Fiber lasers have some advantages over other lasers for materials processing. For example, the near-IR wavelengths of fiber lasers are absorbed well by metals. The beam can also be delivered by fiber, which allows a robot to easily move the beam focus around for cutting and drilling. The industrial market is now the largest market for fiber lasers; much of the action right now is at the kilowatt-class power level. In case of high-power cutting and welding—for example, replacing resistance welding for high-speed sheet steel, solving the problem of material distortion caused by resistance welding. Power and other feedback controls allow fiber lasers to cut a very precise curve, especially going around corners. Particularly interesting is their use in automotive work. The automotive industry is moving to high-strength steel to produce cars that meet durability requirements but are relatively light for better fuel economy; the problem is how to cut the high-strength steel. And that's where they turn to fiber lasers. It's very difficult, for example, for conventional machine tools to punch holes in this kind of steel; however, fiber lasers (and other types of lasers as well) can easily cut these holes. In addition, fiber lasers now offering up to 100-kW output power have greatly increased the power available at the 1-μm region, up from the previous 5-kW level.

There is a requirement to combine a number of separate laser beams into a single beam. Most commonly, the need is to provide a high power levels in industrial lasers and particularly in laser directed energy weapons has led to an interest in scalable systems in which an arbitrary number of otherwise identical laser beams can be added together to realise overall power levels in the 10s to 100s of kW. In the most elemental form of beam combining we might envisage using some form of semi-transparent mirror (beam splitter), which transmits one beam and reflects the other. Unfortunately, this simplistic technique doesn't work efficiently because we always lose half the total power that is available. For example, a 50:50 beam splitter would lose 50% from each beam and any other splitting ratio would lose power from the beams proportionally e.g. a 70:30 beam splitter would lose 70% of one beam and 30% of the other. Perhaps the most commonly encountered beam combining element is the well-known dichroic mirror, which transmits one wavelength or range of wavelengths whilst reflecting others. This has been used effectively for many years and enables beams of different wavelengths to be combined with high efficiency. If the beams are polarized, as with many laser systems, this property can be exploited to provide an efficient combination method using a polarizing beam splitter. Reflective polarizers reflect s-polarized light whilst transmitting the p-polarizations.

The simplest approach is incoherent beam combining, which directs many laser beams in the same direction, increasing total power but not increasing the beam brightness. Incoherent combination of the high-quality beams from fiber lasers can generate much more directional beams that are getting serious consideration for use in various applications. "Beam brightness at the source is of limited importance when considering realistic propagation scenarios in turbulent atmospheres".

One is combining beams coherently, so their amplitudes add constructively. An alternative is combining beams of different wavelengths, as in wavelength-division multiplexing, which avoids the complexities of phase matching but produces wider-band laser emission. Coherent Beam Combining (CBC) also helps to maintain the very unique properties of the laser emission with respect to its spectral and spatial properties. Such lasers are of major interest for many applications, including industrial, environmental, defense, and scientific applications. Recently, significant progress has been made in coherent beam combining lasers, with a total output power of 100 kW already achieved. The coherent beam combining techniques can be subdivided into side-by-side combining (tiled aperture) techniques, leading to a larger beam size but reduced divergence and filled-aperture techniques, where several beams are combined into a single beam with the same beam size and divergence.

• Commercially available High Power CW Fiber Lasers cover output power range from 1 kW to over 100 kW and feature a wide range of operating wavelengths, single-mode and multi-mode options, high stability and extremely long pump diode lifetime. These lasers are water-cooled and can be supplied with a built-in or standalone chiller. The lasers are available with a wide variety of fiber terminations, collimation optics and processing heads. IPG Photonics is one of the manufacturers.
• IPG manufactures high power CW Ytterbium lasers in 1 to >100 kW range and Erbium, Thulium and Raman fiber lasers in 1 to 5 kW range.
• After successfully testing Laser Weapon System (LaWS), which was deployed in 2014 on the amphibious transport dock USS Ponce, the US Navy plans to fire a 150-kw weapon system in 2018. And further plans to enhance the laser power to something like 250 kW or 500 kW by 2020.