this invention is described in preferred embodiments in the following description with reference to the figures , in which like numbers represent the same or similar elements . reference throughout this specification to “ one embodiment ,” “ an embodiment ,” or similar language means that a particular feature , structure , or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention . thus , appearances of the phrases “ in one embodiment ,” “ in an embodiment ,” and similar language throughout this specification may , but do not necessarily , all refer to the same embodiment . the described features , structures , or characteristics of the invention may be combined in any suitable manner in one or more embodiments . in the following description , numerous specific details are recited to provide a thorough understanding of embodiments of the invention . one skilled in the relevant art will recognize , however , that the invention may be practiced without one or more of the specific details , or with other methods , components , materials , and so forth . in other instances , well - known structures , materials , or operations are not shown or described in detail to avoid obscuring aspects of the invention . high - power , pulsed fiber lasers are of great interest in applications such as laser micromachining , material processing , nonlinear optics , and laser sensing . high power fiber lasers are commonly achieved via the means of making a fiber - based master - oscillator - power - amplifier ( mopa ). fig1 illustrates the schematic of mopa configuration . the seed laser is amplified by a fiber amplifier . typically the seed laser is amplified by a rare - earth doped gain fiber , which is energized by pump laser . fig2 shows the cross section view of a rare - earth doped fiber . the pump laser is combined together with seed laser via the so - called signal and pump combiner . the amplified seed laser can be amplified again in order to achieve higher pulse energy and higher peak power . when more than one amplifier is used , the fiber amplifiers are called multi - stage amplifiers . in order to achieve high power , double - cladding rare - earth doped gain fiber is typically used . fig3 illustrate the typically cross section view of double cladding gain fiber . the core is used to guide the signal . here it is called seed laser . the inner cladding is used to confine the pump lasers . the core is typically rare - earth doped glass . the rare - earth ion produces gain . for example , ytterbium ion ( yb 3 + ) and neodymium ( nd 3 + ) offer gain near 1 micron wavelength , erbium ion ( er 3 + ) produces gain near 1 . 55 micron , thulium ion ( tm 3 + ) and holmium ion ( ho 3 + ) can produce gain near 2 micron wavelength . the inner cladding is typically undoped glass material with a lower refractive index in order to form waveguide in the core . the external cladding layer can be glass material or polymer material , which has a lower refractive index to confine the pump laser in the inner cladding . in order to generate polarization maintaining ( pm ) output , pm gain fiber is needed . fig4 illustrates the cross section view of typical pm fiber . high pulse energy and high peak power is needed for many applications . due to the strong transverse confinement and long interaction length , power scaling of fiber amplifier is limited by the onset of nonlinear effects . for single - frequency / narrow - band amplifiers , stimulated brillouin scattering ( sbs ) has the lowest threshold and possibly causes much of the signal light to be reflected back . for broader signal bandwidth , stimulated raman scattering ( srs ) can happen at higher power levels and transfer a lot of signal power into unwanted new wavelength components . the sbs threshold power for narrow band signal is determined by the following equation 1 : where b is a number between 1 and 2 which depends on polarization state . a c is the effective area . gb is the sbs gain coefficient . le is the effective transmission length of the fiber . the threshold power for srs can be described as the following equation ( 2 ) therefore , the threshold of optical nonlinearity in fiber increases with the effective area and decreases with the effective transmission length of the fiber . the effective area increase with the core diameter of the fiber and the mode filed diameter of the fiber . for single mode core , the mode field diameter is typically proportional to the physical core diameter of the fiber . in order to increase the pulse energy and peak power of the fiber laser one need to increase the threshold of the optical nonlinearity of gain fiber . in order to increase the threshold of the optical nonlinearity of gain fiber , the length of the gain fiber should be short and the core diameter of the gain fiber should be large . the length of the gain fiber is limited by pump absorption . cladding pumped fiber amplifiers often have a length of many meters for efficiently absorbing of pump light . a high doping concentration can improve the absorption and then shorten the length of the gain fiber . however , the doping concentration of typical silica fiber is limited . so typically a few meter long gain fiber is used . the core diameter is limited in order to ensure the fiber is single mode fiber . the beam quality will degrade and is no longer single mode when the v number of the fiber is more than 2 . 405 , where λ is the vacuum wavelength , a is the radius of the fiber core , and na is the numerical aperture . as can be seen in the equation ( 3 ), a lower na value can compensate the increased core size and keep the v number as low as possible . however there is also a limit to reduce the na for conventional step index fiber . u . s . pat . no . 8 , 774 , 590 discloses a refractive index difference between the core and the cladding of 0 . 05 to 0 . 30 % of silica fiber . this patent teaches that a light storing effect of the optical fibers cannot be sufficiently obtained when the relative refractive index difference between the core and the cladding is lower than 0 . 05 %. the refractive index of silica glass is approximately 1 . 45 . the refractive index of the core glass is 1 . 4507 . so the na of the fiber should be near 0 . 04 by using the following equation 4 : na =√{ square root over ( n core 2 − n clad 2 )}, ( 4 ) when the na is 0 . 046 , the single mode core diameters are 16 . 65 micron for 1 micron wavelength laser , 25 . 8 micron for 1 . 55 micron wavelength laser , and 33 . 3 micron for 2 micron wavelength laser in according to equation ( 3 ). although u . s . pat . no . 8 , 774 , 590 claims a core diameter of 20 to 30 micron for ytterbium doped fiber laser ( ytterbium doped fiber laser wavelength is 1 micron ), the v number is already larger than 2 . 405 , which means it is not truly single mode fiber anymore . fiber bending is needed in order to filter out the higher order mode . so the true single mode core diameter near 1 micron is approximately 16 . 65 micron . further , silica fibers for u . s . pat . no . 8 , 774 , 590 are formed using mcvd ( modified chemical vapor deposition ) or vad ( vapor axial deposition ) method to deposit the core material . a problem , however , rises with these conventional optical fibers in that current optical fiber manufacturing methods are restricted in their ability to precisely control the indices of refraction of the core material ( n core ) and the cladding material ( n clad ). because of this restricted ability , in commercially practical fiber , the difference between n core and n clad is usually limited by design to no less than 0 . 1 %. this , in turn , restricts the designed size of the core diameter for a given wavelength , and / or restricts the wavelengths of single - mode operation of a fiber for a given core diameter . for example , one common optical fiber manufacturing method referred to as flame hydrolysis uses a burner to fire a combination of metal halide particles and sio 2 ( called a “ soot ”) onto a rotating graphite or ceramic mandrel to make the optical fiber perform . see keiser , optical fiber communications , 2nd ed ., mcgraw - hill ( 1991 ), which is incorporated by reference herein , at pp . 63 - 68 . the index of refraction is controlled by controlling the constituents of the metal halide vapor stream during the deposition process . the process is “ open loop ” without a feedback mechanism to precisely control the ultimate index of refraction of the optical material . moreover , the metal halide vapor stream is limited in its controllability and in its ability to control the ultimate index of refraction of the optical material . during the process a good portion of the material will be vaporized , therefore , it is extremely difficult to control the difference of the refractive index difference to close to 0 . 05 % ( equals to na of 0 . 046 ). so most gain fibers have na of 0 . 08 or larger . another approach is to use the so - called photonic crystal fiber ( pcf ) design to achieve a large core diameter . a photonic crystal fiber ( also called holey fiber , hole - assisted fiber , microstructure fiber , or microstructural fiber ) is an optical fiber which obtains its waveguide properties not from a spatially varying glass composition but from an arrangement of very tiny and closely spaced air holes which go through the whole length of fiber . such air holes can be obtained by using a preform with holes , made e . g . by stacking capillary and / or solid tubes and inserting them into a larger tube . these fibers are not step index fibers and their guiding mechanism is different from step index fibers . laser - active pcfs for fiber lasers and amplifiers can be fabricated , e . g ., by using a rare - earth - doped rod as the central element of the preform assembly . rare earth dopants ( e . g . ytterbium or erbium ) tend to increase the refractive index , the guiding properties are determined by the photonic microstructure only and not by a conventional - type refractive index difference . for high - power fiber lasers and amplifiers , double - clad pcfs can be used , where the pump cladding is surrounded by an air cladding region ( air - clad fiber ). due to the very large contrast of refractive index , the pump cladding can have a very high numerical aperture ( na ), which significantly lowers the requirements on the pump source with respect to beam quality and brightness . such pcf designs can also have very large mode areas of the fiber core while guiding only a single mode for diffraction - limited output , and are thus suitable for very high output powers with excellent beam quality . but pcf ( microstructured fiber ) has many disadvantages including difficulty for fabrication , difficulty for fusion splicing , poor thermal conductivity of the air - gap , and relatively low doping in the core of the fiber . therefore , it is strongly desired to have a step index fiber with a large core diameter , which is truly single mode fiber . we disclose a type of gain fiber , which has a numerical aperture of between 0 . 01 and 0 . 04 , resulting an extremely large single mode core diameter . here the host of the rare - earth ions , the gain elements , is the multicomponent glasses , which is different from the most commonly used silica glass . it is well known that silica fibers are made with vapor deposition method , which contains almost no alkali metal ions nor alkaline earth metal ions because these ions are not compatible with vapor deposition process . the total content should be less than 0 . 1 weight percent . multicomponent glasses always contain alkali metal ions or alkaline earth metal ions , which is at least more than 1 weight percent . the alkali metals include lithium ( li ), sodium ( na ), potassium ( k ), and the alkaline earth metals are beryllium ( be ), magnesium ( mg ), calcium ( ca ), strontium ( sr ), and barium ( ba ). these alkali metal ions or alkaline earth metal ions are called glass network modifier in multicomponent glasses . other metal ions such as zn and pb can act as glass network modifiers , which again is not compatible with vapor deposit process . multicomponent glasses include phosphate glasses , silicate glasses , tellurite glasses , germanate glasses , et al . u . s . pat . no . 6 , 816 , 514 , in the name of jiang disclose rare - earth doped phosphate - glass fiber for fiber laser application . u . s . pat . no . 6 , 859 , 606 in the name of jiang , disclose erbium doped boro - tellurite glasses for 1 . 5 micron fiber amplification . u . s . pat . no . 7 , 298 , 768 in the name of jiang , disclose germanate glasses for fiber lasers . u . s . pat . no . 8 , 121 , 154 to jiang discloses silicate glasses for fiber laser applications . multicomponent glass fibers are used for fiber laser application because of theirs capability of high doping concentrations . these patents limit their advantages of using a relatively shorter piece of gain fiber compared to silica glass fiber . but for high pulse energy fiber lasers , a large core diameter is critical . applicants have discovered that a large core diameter can be obtained from multicomponent glass gain fibers . the numerical aperture can be from 0 . 01 to 0 . 04 . therefore , the core diameter can be from 25 micron to 60 micron for 1 micron wavelength , 35 micron to 90 micron for 1 . 55 micron wavelength , and 45 micron to 120 micron for 2 micron wavelength . applicants dope high rare - earth ions into the fiber , so the total length of the gain fiber is not longer than 60 cm . therefore the gain fiber can be packaged straight . no bending is necessary . because of the extremely large core diameter and relatively short length of gain fiber , a peak power of greater than 50 kw can be achieved without optical nonlinearity . applicants have developed a new cladding pumped polarization maintaining yb doped fiber based on silicate materials . with large mode size , high yb doping level and low na , the fiber amplifier has achieved record high threshold for nonlinear effects while keep excellent diffraction limited beam quality . table 1 compares the parameters of applicants &# 39 ; yb - doped fiber with most popular commercial cladding pumped yb fibers . table 2 compares the sbs / srs thresholds for different input signals between applicants &# 39 ; fiber and commercial fibers . the nonlinear threshold of applicants &# 39 ; fiber is many times higher than commercial fibers . the threshold of applicants &# 39 ; fiber is always many times higher than the typical commercial fiber , which means high pulse energy can be achieved . while the preferred embodiments of the present invention have been illustrated in detail , it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention .