How a Fiber Laser Works

hi I am your new fern photon most people are familiar with electrons and electronics similarly we photons enable a whole field called photonics the science of light I'm going to talk to you a little about light optical fibers and the development of optical fiber lasers photons can be portrayed as either particles or waves mathematically but for our purposes we will use a hybrid also in my presentation some technical terms will be in italics this way you'll know they might have external references for instance wavelength represented by the lowercase Greek symbol lambda in the optical spectrum wavelength refers to the color of light blue is a shorter wavelength around 400 nanometers and red is a longer wavelength around 700 nanometers light of course extends through a far greater spectrum than the human eye can see from x-rays through 2 terahertz radiation most optical systems operate from the UV through to the infrared optical fibers became possible with the practical understanding of Snell's law light transitioning between transparent materials bends in proportion to the change in its velocity like a rod in a beaker appears bent at the air-water interface the ratio of the velocity difference is the refractive index in optics we add materials called dopants into glass to change the refractive index by putting two pieces of glass with different refractive indices adjacent to each other we can emulate the bent rod effect imagine that a glass rod with a high refractive index core surrounded by a low refractive index cladding would guide light continuously down its axis well that's exactly what happens and we can also change the effective angle at which light is accepted by adjusting the refractive index of the two glasses this is called the numerical aperture high numerical aperture means broad angles of acceptance if the core size is large so that multiple wavelengths of light or modes can enter by a variety of angles and can propagate down the core the core is considered to be multimode 'add if the core size is small enough to only allow the passage of one wavelength at a specific injection angle the core region is considered to be single mode depending on the size of the glass rod the refractive index and the wavelength the core can be both single or multi moded we strive for single mode so we generate certain wavelengths and use glass rods with diameters in the micrometers less than two human hairs in diameter small enough to be greatly flexible and resilient fibers fibers usually have one core and one cladding along with a protective coating most are made of quartz glass because of its purity we use standardized nomenclature to specify the relative sizes of the core to the cladding for example 10 125 represents a single-mode fiber with a core of 10 micrometers in diameter and a cladding of 125 micrometers a 105 micrometer core with a 125 micrometer cladding is represented by 105 125 even though we know we can send light down a fiber there are many variables that come into play fiber type core size numerical aperture refractive index and doping all contribute to expand the range and possibilities of light propagation however we strive for single mode remember launching light down a single mode fiber is difficult and the light source is expensive how can we strike a balance between power efficiency and economy through experimentation it was discovered that changing the refractive index periodically along the length of the fiber could make the fiber reflect light this creates a grading that takes advantage of Bragg's law to create a type of mirror by changing the intensity and period of the grading the amount of reflection for particular wavelengths can also be controlled this quickly led to the development of a fiber laser a fiber laser works by reflecting light through an optical cavity so that the stream of photons stimulates atoms that store and release light energy at useful wavelengths on the periodic chart these elements appear in the lanthanide series ytterbium is the most common lazing atom elie Schnitzer a pioneer in optical fibers learn to dope the core of an optical fiber with ytterbium to control the refractive index and photon absorption practically speaking the etre have an absorption curve that looks like this a broader spectrum of light can be absorbed at the shorter 915 nanometer wavelength and a more efficient absorption can occur at the slightly longer but narrower 976 nanometer wavelength the photon absorbed by the ytterbium dopant disappears and electrons whizzing around the atomic nucleus move to higher orbitals on account of the absorbed energy this process is called pumping to indicate that energy is being injected into and stored by the atoms in the fiber within about a millisecond the electrons otherwise unstimulated drop to their original or ground state and emit a photon at a wavelength of 1064 nanometers the energy efficiency of this absorption and re-emission is known as the quantum efficiency and it is simply the ratio of the pump over the emission wavelength it's impossible to get a higher optical efficiency than this number with its closed pump and lazing wavelengths ytterbium can be made to lase with astounding optical efficiency pump store emit over and over again is a realistic oversimplification of the process to maximize the coordination of the pumping and emission and ingenious fiber optical cavity sometimes called a resonator is constructed at the pump end of the cavity a high reflector made using a fiber bragg grating is spliced to a fiber that is doped with ytterbium atoms at the output end a similar fiber Bragg grating with a modest reflectivity of around 10% is installed thus a very simple monolithic single mode laser device is created monolithic and fiber terminology means that once the light is in the fiber it stays in the fiber end to end however the length of the doped fiber is important as it determines how much of the pump light is absorbed we use the term absorption length for the combination of length with the amount of doping typically a fiber laser has an absorption length of about 95% of the pump energy this is to avoid having a section of unpark a beti a none pumped section inevitably charges itself and self lazes resulting in a failed fiber to pump this device an optical laser diode can be installed on the pump end because such a laser is single mode its power is limited to the output of a single-mode pump diode times the quantum efficiency of the laser single-mode pump diodes are expensive and relatively low-powered to achieve higher power either more pumps need to get connected to a fiber or the fiber will need to get much fatter in fact over time both came to be it was discovered that light once guided by the core continues to propagate for a significant distance even when the core is tapered until it is essentially gone the light can then be recaptured by a new core that intersects the light path this path is called the evanescent field by extending this concept it was discovered that multiple fibers can feed into or split out of one evanescent field leading to the development of a fused by conic coupler there are limits to this phenomenon however the brightness defined by the core area multiplied by the numerical aperture must be conserved for example a coupler can be made with two input fibers of a 100 micrometer cord diameter and a numerical aperture of 0.12 and an output of 100 micrometers with a numerical aperture of 0.2 for if we now change our laser by adding the newfound coupler to the front end we can see that the laser power can be doubled unfortunately practical limits quickly set in when we try to add a third diode for at rippling effect since each input would require a pump diode with a very small numerical aperture in ultra high brightness these diodes have a matching ultra high price not practical at all more economical high power pump diodes have a larger numerical aperture and a larger diameter output fiber using these economical pumps requires the use of double clad fibers the invention of double clad fibers enabled very high power fiber lasers with good manufacturing economy great electrical efficiency and astoundingly high brightness in principle a double clad laser fiber works the same way as the conventional fiber we just described however the double clad fiber is larger in diameter and has a second pump cladding with a very high numerical aperture now even conserving brightness this will allow a coupler with six legs at point to two numerical aperture each pumps of up to 600 watts per leg are commercially and competitively available for this coupler this is encouraging once the laser is assembled as before and we apply pump power to the diodes we quickly see that much of the pump light simply passes down the cladding without intersecting successfully with a dopant ion in the core without this intersection and transfer of power the laser cannot work the solution to this problem is to create extra reflecting facets on the outside of the first cladding schnitzer remember him found that changing the shape causes enough redistribution of the light in the cladding to force a high level of interaction with the core now we have created another situation too much light as power and the laser goes up beyond a few hundred watts photons in the core start to behave differently much like surging traffic on a congested highway nonlinear effects start to appear such as brilliant scattering and Raman scattering the solution is the same as on a highway we add more lanes for fiber means increasing the diameter of the core this obviously reduces the light intensity quickly because the core area increases with the square of the radius so a doubling of the diameter quadruples the light carrying capacity and with this increase of core diameter comes the loss of the single mode beam quality that makes fiber lasers so effective what to do we've determined that the acceptance angle in the fiber is dependent on the refractive index difference between the core and the cladding daav kleiner and Jeff Kaplow found that bending the fiber also affects how the core accepts light they also recognized that the single mode or the fundamental mode propagates down the fiber with a slightly lower numerical aperture than any of the higher modes the combination of a low numerical aperture in the core and a bend in the fiber allow the higher modes to escape leaving the fundamental mode in the core the evanescent field of the single mode beam expands to fill the core diameter avoiding nonlinear effects best of all the amount of energy from the core that escapes as a higher mode is minimal serious extremely high beam quality fiber lasers were now possible laser technology has evolved over the past half-century the power to cut and weld and do amazing things with directed-energy has spread to all parts of the globe many refinements are still being developed advances have also been made in using laser fibers for specialized telecommunications scientific industrial and directed energy applications lower power diodes can create and manipulate seed signals often as pulses however the signal can attenuate over distance which requires amplification to do this we need to modify our six to one coupler into a six plus one-to-one coupler and send the seed signal directly into the core of the fiber with this configuration we can eliminate the Bragg reflectors and simply stimulate the laser emission with a seed signal of our choice an avalanche rather than a resonance effect however realize that a residual pump power totaling about 5% is quite significant with the fiber laser operating at one or more kilowatts this valueless power could cause all manner of safety and engineering problems in beam delivery systems so additional components such as residual pump strippers are added to the very output end of the fiber the stripper has a higher refractive index than the nominal cladding on the fiber this then catches the high numerical aperture light remaining from the pump and releases it out of the fiber as lost thermal energy further refinements such as sophisticated electronic feedback and control mechanisms are still in development so you see creating a viable high-powered laser is a result of intense research and finely tuned techniques we sincerely hope that this introduction was useful and informative for more information the parts to build your own laser an educational kit or one of our many complete industrial scientific or defense oriented lasers please give us a call or find us on the web Cheers you

Channel: nuferncorporation

Views: 524,267

Keywords: optical fibers, coil winding, optical lasers, fiber lasers, fiber amplifiers

Id: ofEqFlqkiS0

Length: 13min 21sec (801 seconds)

Published: Mon Apr 07 2014