Patent Publication Number: US-2021194199-A1

Title: Fiber laser system with mechanism for inducing parasitic lights losses

Description:
RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 16/314,794 filed Jan. 2, 2019, which is a 371 of International No. PCT/US2017/40213, filed Jun. 30, 2017, which claims the benefit of U.S. Provisional Application No. 62/357,564, filed on Jul. 1, 2016, each of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to fiber laser systems configured with a wavelength discriminator (WD) receiving light at parasitic and signal wavelengths over free space and configured to maximize losses of light at the parasitic wavelength in the fiber laser system. 
     Prior Art Discussion 
     The lasing at unwanted, parasitic wavelengths in many optical fiber laser systems is undesirable and may be caused by several factors. One of these factors is the existence of nonlinear effects, such as stimulated Raman scattering (SRS). Another factor relates to gain mediums doped with different species of rare-earth ions and the existence of different lasing levels in the gain medium, such as ytterbium (Yb) radiation in erbium (Er) lasers based on Er/Yb doped medium. The list of the above disclosed is not exclusive and may be greatly expanded. Parasitic lasing severely limits the performance of high gain solid state lasers. It reduces the overall efficiency of the optical system, damages individual components and destabilizes the operation of the optical system. 
     The problem of radiation at parasitic wavelengths becomes even more severe in fiber laser systems characterized by uncontrollable backreflection. Particularly vulnerable are fiber laser systems used for processing various materials when the level of the back-reflection at parasitic wavelengths greatly changes depending on a laser treating process, material surface conditions and system&#39;s parameters. 
     Referring to  FIG. 1 , a typical industrial fiber laser system  10  for processing a work piece necessarily includes, among others, a light source, which may have one or more gain blocks  12  shown here as a laser, and a delivery fiber typically including feeding fiber  14  or feeding and process fibers which guide light towards surface  16  to be processed. The gain bock  12  is configured with an active fiber  22 , which is doped, for example, with Yb ions, and often input and output passive signal fibers  18 ,  20 , respectively. The passive fibers  18 ,  20  are coupled to respective opposite ends of active fiber  22  by splices  24 . The passive and active fibers together with passive feeding fiber  14  define a waveguide supporting propagation of light at the single wavelength between gain block  12  and surface  16 . Typically, the output end of the waveguide is coupled to an output optical coupler, such as beam expander  25  made of quartz and configured to reduce the power density of the output light. The output surface of beam expander  25  is typically provided with an antireflection (AR) layer or any other wavelength discriminator (WD) minimizing propagation of signal light, which is back reflected from the surface to be treated, into the waveguide. The AR layer is however transparent to light at other, parasitic wavelengths different from the signal wavelength of the signal light and originated in the fiber laser system and its delivery fiber due to relatively high optical intensities and fiber lengths. In other words, the signal light has a sufficient power to exceed a threshold for generating nonlinear effects resulting in light at the parasitic wavelength in the delivery passive fiber. 
     The light coupled into feeding fiber  14  has a power that can vary, based on the number of gain blocks  12 , anywhere between several hundred watts and hundred kilowatts; the length of feeding fiber  14  can be substantial depending on how far surface  16  is located from the laser light source. Both the power and fiber lengths are major contributors to the generation of nonlinear effects, such as SRS. As a result, the spectrum of light of  FIG. 3 , emitted from the output end of feeding fiber  14  typically includes signal and parasitic wavelengths 1070 and 1125±x nm, respectively, before it is incident on surface  16 . At least part of the incident light is back reflected into feeding fiber  14  which guides it towards the laser light source. 
     Since AR coating  27  is ineffective against back reflected parasitic light from surface  16 , a resonator, capable of lasing the parasitic light, is formed between surface  16  and any formation within system  10  of  FIG. 1  capable of retro-reflecting this light back to surface  16 . For example, high reflectivity fiber Bragg grating (HRFBG)  26  of the schematic of laser system  10  ( FIG. 1 ) retro-reflects the lion&#39;s share of back reflected parasitic light due to its reflectivity spectrum shown in  FIG. 2 . Besides HR FBG  26 , splices  24  of  FIG. 1  and other inner obstacles enabling the turnaround of the back reflected light at the parasitic wavelength, of course, are contributory factors to be reckoned with. 
     The formation of the resonator is completed by a gain a medium between the surface and back reflecting formation. Thus the gain medium can be the entire waveguide including active  22 , passive  18  and  20  and feeding  14  fibers or any individual part thereof. The example of the latter is an Er laser amplifies light at parasitic, unwanted Er wavelength. Any additional output fiber and/or additional amplifying cascade coupled to the existing one only adds to the gain experienced by light at the parasitic wavelength. Thus, with the back reflection of light at the parasitic wavelength from external surface  16 , the quality (Q) factor of this parasitic resonator increases and the generation at the parasitic wavelength begins. 
     Accordingly, it is highly desirable that losses of unwanted light in the parasitic resonator be as high as possible so as to suppress external back reflection and decrease the Q factor. This general requirement is met by filtering the light at the parasitic wavelength. 
     The issue here is where, within the parasitic resonator, a loss-inducing mechanism should be installed. Several attempts directed at preventing back-reflection at parasitic wavelengths have been made in systems disclosed below. 
     For example, U.S. Pat. No. 7,912,099 discloses slanted fiber Bragg gratings (FBG) located between neighboring gain blocks which constitute a fiber laser light source. Furthermore, at least one FBG is written in the delivery fiber. The slanted FBGs, particularly those used in fiber laser systems where output light should propagate over free space after it is emitted from the laser source, may have certain disadvantages. For example, the FBG induces undesirable losses of signal light at the signal wavelength; its reflection bandwidth is rather narrow allowing a considerable part of back reflected parasitic light to propagate along the waveguide in opposite directions. The back reflected light at the parasitic wavelength is guided along a feeding fiber which functions as gain medium for both forward and back reflected light between the surface to be processed and the FBG. All in all, the disclosed FBG&#39;s configuration is not flexible: one cannot make it deeper and/or longer without detrimentally affecting the light at the signal wavelength. 
     Thus, it is desirable to provide a wavelength discrimination mechanism inducing losses of light at a parasitic wavelength and configured to be more flexible than the known wavelength discriminating elements. Also, it is desirable to provide a fiber laser system which is configured with the wavelength discriminating component inducing losses of back reflected parasitic light before the latter is coupled into the delivery fiber. 
     Still another example of fiber laser systems operative to minimize detrimental effects of back reflected radiation at a parasitic wavelength is disclosed in U.S. Pat. No. 8,081,376. The disclosed laser system includes an optical component located between gain stages and configured to attenuate light at the parasitic wavelength, which is generated in a delivery fiber. However, in this laser source, light at the parasitic wavelength uncontrollably propagates in the delivery fiber in opposite forward and backward directions which causes all the problems discussed above. 
     Typically, industrial fiber laser light sources, including the one disclosed in this patent, are provided with a delivery fiber guiding light to the surface to be laser treated. As a rule, the delivery fiber has a substantial length. One of ordinary skill in the art of the fiber laser systems is well aware that a greater fiber length lowers a threshold for generating SRS. 
     Many fiber laser systems include a passive process fiber delivering light to remote locations and thus often having a substantial length. Due to high optical intensities, which may reach hundreds of kilowatts, and great fiber lengths, the process fiber functions as gain medium or Raman-active medium. Typically, in high power fiber laser systems, the process fiber receives light from a feeding fiber, which is connected to the system&#39;s output fiber, by means of a bulk guide optical component, such as a beam coupler or beam switch. Left uncontrolled, parasitic light may be emitted from the delivery fiber at prohibitively high power levels. The result of this undesirable emission includes unwanted effects on the work piece to be laser treated. Many industrial applications however do not tolerate the presence of the parasitic light. Furthermore, not all parasitic light incident on the work piece is absorbed. Hence a part of the parasitic light back reflected from the work piece is coupled into the feeding fiber and propagates towards the gain blocks and pumps. This presents the problems discussed above in regard to fiber laser systems in which parasitic light propagates backwards through gain blocks to pumps. 
     A need therefore exists for a method of increasing light losses at the parasitic wavelength in a “parasitic” resonator or gain medium for parasitic light. 
     Another need exists for a fiber laser system implementing the inventive method and provided with a wavelength discriminating mechanism which is configured to increase losses for radiation at the parasitic wavelength in the “parasitic” resonator. 
     SUMMARY OF THE DISCLOSURE 
     The above-identified needs are satisfied by a method providing placing of a wavelength discriminator (WD) in diverging, collimated or focused beam inside a delivery fiber system, which includes feeding and process fibers, or outside this delivery fiber system. The disclosed fiber laser system implements the above-described method with a wavelength discriminator (WD) configured to induce losses of parasitic light while transmitting signal light practically without any attenuation. The method and fiber laser system, as disclosed here, benefit single mode (SM) or multimode (MM) mid- and high-power fiber laser systems by minimizing losses at a signal wavelength and practically eliminating the influence of external uncontrollable parameters on the system&#39;s output stability. 
     In accordance with one aspect of the disclosure, the disclosed method includes providing the WD between an output end of the feeding fiber and the surface to be treated. As a result, the light at the parasitic wavelength circulating in the parasitic resonator experiences losses which lead to a high threshold for nonlinear effects which, in turn, translates into a higher, stable output power of signal light. 
     In another aspect of the disclosure, the above disclosed method of the above previous aspect also includes selecting the WD so as to minimize the back reflected light at the parasitic wavelength in the resonator. The WD is configured as a multilayer dichroic mirror coupled to a beam expander which is operatively connected to the output of the delivery fiber system. The light at signal and parasitic wavelengths propagates over free space within the expander before being filtered as it passes through the WD. This configuration is particularly, but not exclusively, advantageous for mid-power fiber laser systems outputting up to a few kW radiation. 
     Still in combination with any of the above aspects, the disclosed method further includes placing the WD in non-fiber components, such as couplers and beam switches, such that as light at signal or working and parasitic wavelengths propagates over the feeding fiber and free space before it is filtered by the WD. This aspect is particularly advantageous for high power fiber laser systems radiating ten to hundred kW outputs since the process fiber or fibers guide light practically free from parasitic frequencies to the work piece. 
     In still another aspect of the disclosure, the method of any of the above-discussed aspects includes preventing not only the back reflected light at the parasitic wavelength but also forward propagating parasitic light generated upstream from the fiber delivery system. The light at the parasitic wavelength is determined by a sum of two variables—one, internal reflection, which depends on parameters of the fiber laser system and thus is constant, and the other, external reflection, which is a function of variable back reflected light at the parasitic wavelength. Accordingly, with the back reflected parasitic light blocked from entering the resonator, whatever light at parasitic wavelength contained in the system&#39;s output, it is constant and thus easily determined. 
     In another aspect of the disclosure, the WD of any of the above aspects is positioned at an angle which is adjustable relative to an optical axis of the collimator so as to eliminate the back reflected light at the parasitic wavelength in the parasitic resonator without inducing significant losses to the light at the signal/working wavelength. 
     According to still another aspect, the method of the above-disclosed aspects further includes measuring a power and spectrum of the system output light and power of the back reflected light at the parasitic wavelength. 
     Another aspect of the disclosure is concerned with a method of maximizing losses of parasitic light in gain medium of the fiber laser system and method of any of the above disclosed aspects or independent therefrom. 
     The disclosed fiber laser system is configured with the WD and implements the method or methods disclosed in each of and any combination of the above aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects are discussed in detail in the following specific description which is accompanied with the drawings, in which: 
         FIG. 1  is a diagrammatic optical schematic illustrating of a fiber laser system in accordance with the state of the art; 
         FIG. 2  is a reflective spectrum of the high reflection fiber Bragg grating (HRFBG) at 1070 nm wavelength of  FIG. 1 ; 
         FIG. 3  is emission spectra of the laser of  FIG. 1  during generation of Raman signal; 
         FIG. 4  is an optical schematic of the disclosed fiber laser system with a WD operative to induce losses of light at parasitic wavelengths by retro-reflecting the back reflected light at parasitic wavelengths; 
         FIG. 5  is a reflective spectrum of the WD of  FIG. 4 ; 
         FIG. 6  is a schematic of experimental fiber laser system used for determining a threshold for nonlinear effects, such as SRS, with and without the WO of  FIG. 4 ; 
         FIGS. 7A-7C  illustrate watt-ampere characteristics of the output of the experimental system of  FIG. 6  at different lengths of the delivery fiber; 
         FIG. 8  illustrates a Raman generation threshold as a function of the length of the delivery fiber in the system of  FIG. 6 ; 
         FIGS. 9A-9B  illustrate emission spectra upstream from the HR FBG in the experimental laser system of  FIG. 6  with and without the WD of  FIG. 4  at output powers corresponding to respective thresholds for nonlinear effects, such as SRS; 
         FIG. 10  illustrates additional light losses at the parasitic Raman wavelength as function of external reflection of this light in the disclosed fiber laser system of  FIG. 4 ; 
         FIG. 11  illustrates the output power, representing the difference between outputs of system  4 , which correspond to respective thresholds of nonlinear effects, such as SRS, with and without the WD, from external reflection of the parasitic light, wherein two graphs represent the fiber laser systems with respective different lengths of the delivery fiber; 
         FIG. 12  is a general schematic of the disclosed fiber laser optical system provided with the WD which is configured to induce losses of parasitic light in Raman-active medium. 
         FIG. 13  is the detailed schematic of the fiber laser system of  FIG. 12 ; 
     
    
    
     SPECIFIC DESCRIPTION 
     Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are not to precise scale. The term “couple” and similar terms do not necessarily denote direct and immediate connections, but also include connections through intermediate elements or devices. 
       FIG. 4  illustrates one of the aspects of this disclosure. As shown, a fiber laser system  30  is configured to implement the inventive method of maximizing losses of light at a parasitic wavelength in a parasitic resonator. The latter is formed, during the operation of system  30 , by a gain medium, which is a fiber waveguide extending between a laser-treated surface  16  reflecting light and any formation in the fiber waveguide between the output end of a delivery fiber  38  capable of retro-reflecting the back reflected light. 
     The fiber laser system  30  is structured similarly to system  10  of  FIG. 1  and includes laser source with gain block  12  which is configured with light emitters-doped active signal fiber  22  input, passive signal fibers  18  and  20  and passive output fiber  21  which are spliced at  24  to one another. The gain block  12  may be configured as a laser or amplifier, and for the illustration purposes it is configured as the fiber laser defined between HR FBG  26  and low reflectivity (LF) FBG  28  written in respective passive signal fibers  18  and  20 . 
     The beam expander  25  is configured with a WD  32  realizing one of the main concepts of the disclosure: inducement of losses of light at a parasitic wavelength in fiber laser system  30 . In particular, WD  32  of  FIG. 4  increases losses of parasitic light in the parasitic resonator. As losses of the parasitic light increase in the resonator, a threshold for nonlinear effects, such as SRS, becomes higher which, in turn, leads to a higher and more stable output of system light and better processed surface  16  of the work piece. 
     The laser  12  is a single transverse mode (SM) Fabry-Perot oscillator operating in quasi-continuous regime (QCW) which emits signal light at a central signal λ 1sl  working wavelength of 1070 nm and optical intensities sufficient to exceed a threshold for generating parasitic light in feeding fiber  14 . In the context of this disclosure, parasitic light is a result of Raman scattering; yet, as one of ordinary skill readily realizes, other non-linear effects causing generation of light at parasitic wavelengths are also considered here. It may also be configured to output system light in multiple transverse modes and operate in CW or pulsed regimes at central wavelengths different from 1070 nm which depend on the type of light emitting ions in active fiber  22 . 
     As seen in  FIG. 5 , WD  32  is transparent to signal light at 1070±2 nm signal wavelength λ 1 . However, WD  32  can be configured to prevent external back reflection light at parasitic wavelengths λ3ebr and retro-reflect it back to surface  16 , as indicated by λ3eprr. The WD  32  includes a dichroic mirror. As known to one of ordinary skill, the dichroic filter is configured with alternating layers of optical coatings having different refractive indices built up upon a glass substrate. The interfaces between the layers of different refractive index and high reflectivity index (up to 99%) produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. By controlling the thickness and number of the layers, the wavelength of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation which otherwise leads to prohibitively high temperatures and eventual destruction of the filters. 
     The total light at parasitic wavelength in system  30  provided with WD  32  is a sum of two variables: internally reflected light from the WD and externally reflected light from system internal or external to the system obstacle, such surface  16 . The share of the internal parasitic light λip is defined as R*D 2 /(2*L*A) 2 , where D—diameter of delivery fiber, R—coefficient of reflection of WD at the parasitic wavelength, A—NA of laser radiation and L—distance at which laser light propagates over free space within the beam expander. The external light at the parasitic wavelength coupled in to the resonator has no effect on the internal light. 
     The share of externally generated parasitic back reflected light at parasitic wavelength λepbr is determined as R1*(1−R) 2 , where R1 is back reflected light at parasitic wavelength coupled into the resonator. Accordingly, this share of light can be considerably reduced and even practically eliminated by adjusting the reflectivity R of WD  32 . For example, with R=0.95, system  30  experiences a 400 fold increase in losses of light at parasitic wavelength compared to system  10  of  FIG. 1  with the normal WD. 
     Returning to  FIG. 4 , WD  32  is coated on output surface  40  of the end connector, such as beam expander  25  and configured to process parasitic light that propagates in both forward and back reflected propagating directions. Alternatively, WD  32  may be a separate element with both of the opposite faces reflecting light at the parasitic wavelength in respective directions. Regardless of the particular configuration, WD  32  induces losses to the light at the parasitic wavelength in the parasitic resonator. 
     The WD  32  scatters internally generated light at parasitic wavelength λ3ip which propagates over free space in beam expander  25  in the forward propagating direction before it is incident on WD  32 . Only an insignificant portion of the back reflected internally generated parasitic light λ3ipbr is coupled back into the core of feeding fiber  14 , the lion&#39;s share of this light is scattered and does not enter the core of fiber  14 , as indicated by blue arrows. As a result, WD  32  greatly minimizes propagation of the internally generated parasitic light towards surface  16  which remains largely unaffected by this light depending, of course, on the coefficient of reflection of WD  32 . 
     The externally generated back reflected light at parasitic wavelength λ3epbr, i.e., a portion of internally generated parasitic light that managed to propagate through WD  32 , is back reflected from surface  16  towards the waveguide of system  30 . However, WD  32  retro-reflects back reflected light λ3eprr and thus thus minimizes the external parasitic light from coupling into the resonator. Without WD  32 , if coupled into the resonator and amplified, the externally generated parasitic light may damage optical components of laser  12  in the back reflected direction and detrimentally affect surface  16  in the forward direction when it is retro-back reflected from any given formation in the waveguide. 
     As known, the generation of light, in this case parasitic light, in the laser occurs when the gain experienced by parasitic light in the resonator equals to the losses of this light. Accordingly, the additional losses of SRS lead to higher output powers. The following table illustrates the results continuing the above. 
                                                             Power at   Power at   Power at                   beginning of    beginning of    beginning of            Backreflection   Backreflection   Raman   Raman   Raman           at 1070 nm   at 1150 nm   generation.   generation.   generation.       WD   into core of   into core of   Fiber length   Fiber length   Fiber length       configuration   fiber   fiber   36 m   16 m   6 m                  Normal   −15.5 dB   −15.5 dB   430 W    690 W   1030 W       Disclosed   −15.5 dB   −39.8 dB   625 W   1027 W   1500 W                    
As follows from the above-disclosed table, the threshold for Raman generation in the system with the disclosed WD occurs at powers approximately 1.4-2-times higher than in systems with the normal WD, provided the length of the delivery fiber is the same in both systems.
 
     Assume that additional losses of 24.3 dB are introduced by the disclosed WD in system  30 . In order to reach a threshold for lasing parasitic Raman light, its gain should be 24.3/2=12.3 dB higher than that of system  10  of  FIG. 1 . The latter is explained by the fact that the signal in the resonator is twice amplified, while the losses occur only once upon back reflection of radiation into the resonator. 
     The data in the above-disclosed table has been obtained by a system  45  of  FIG. 6  which is similar to systems of respective  FIGS. 1 and 4 . In particular, the laser source includes passive fibers  18 ,  20  and active fiber  22  each with a 14 μm mode field diameter (MFD) for light at the parasitic wavelength. The source is configured with a Fabry-Perot resonator operating in a QCW regime at 1070 nm signal working wavelength and outputting a peak power of 1500 W. The output end of the system is provided with SM optical connector/beam expander  25 . The test was conducted by alternating between beam expander  25  with the normal coating and disclosed WD  32 , which is transparent to signal light at 1070 nm and reflects parasitic light in a spectral region between 1115 and 1150 nm. The occurrence of the lasing threshold for Raman generation is controlled by determining the spectrum of laser radiation and radiation power increase upstream from HR FBG  26 , and saturation of the power in the power meter downstream from plate  34 . 
       FIGS. 7A-7C  illustrate the output peak power of the system as a function of the input current at diode pumps  38  at each length of delivery fiber  14  disclosed in the table and obtained by experimental system  45  of  FIG. 6  during the above-disclosed test. The disclosed system with WD  32  is characterized by substantially higher peak powers. 
       FIG. 8  illustrates a Raman generation threshold as a function of the length of delivery fiber  14 . Confirming the conceptual basis of the present disclosure, the disclosed system with WD  32  has a substantially higher threshold than that of system  10  of  FIG. 1 . 
       FIGS. 9A and 9B  illustrate respective spectra of the laser emission upstream from HR FBG  26  of system  45  of  FIG. 6  at powers triggering the Raman generation,  FIG. 9A  represents system  45  with disclosed WD  32 , while  FIG. 9B  represents system  45  provided with the normal WD and characterized the threshold at lower powers than that of  FIG. 9A . 
     Based on the above disclosed test results, the loss difference in the resonator for Raman generation of system  45  with and without WD  32  clearly depends on the level of back reflected light at the parasitic wavelength and is equal to the difference between the external and internal variables,  FIG. 10  is the graph of additional losses of the parasitic light in system  45  of  FIG. 6  with beam expander  25  provided with WD  32  and with the normal WD. 
       FIG. 11  shows the difference in the maximal output power of the laser emission in the disclosed and prior art systems (Wods-Wopa), as a function of the level of back reflected parasitic light before the threshold for nonlinear effects is reached. The blue curve corresponds to the output the difference between these systems provided with a 4-meter long delivery fiber, whereas the red curve corresponds to the disclosed and prior art systems provided with a 10-meter long delivery fiber. Even at the miniscule back reflection of approximately 0.1% coupled into the core of the delivery fiber, the difference in max output powers is about 150-200 W, which is substantial compared to a nominal output power in a 1000-1500 W range. 
     In summary, based on the foregoing, the disclosed WD incurs substantial additional losses of the parasitic light in the resonator as a result effective suppression of the back reflected light at parasitic wavelength. The suppression of the back reflected light contributes to high thresholds for generation of parasitic light in the resonators. The difference in the output power of the disclosed and prior art systems before reaching the threshold for nonlinear effects growths as the back reflection of the parasitic light in the resonator increases (See  FIG. 11 ). For SM QCW lasers, depending on the level of back reflection and length of the delivery fiber, this difference may as high as 750 W. 
       FIGS. 12 and 13  illustrate a high power fiber laser system  50 ,  55 , respectively, configured with a dichroic WD  56  such as a bulk filter that in principle can be used in laser system  30  of  FIGS. 4-11 . The WD  56  may be located in a fiber-to-fiber coupler  60  ( FIG. 12 ) which provides optical communication between feeding fiber  14  and passive process fiber  62  guiding light towards the final destination. Alternatively WD  56  may mounted in a beam switch  54  ( FIG. 13 ) typically distributing the radiation from the laser source to different light recipients through respective passive process fibers  62 . In either case, feeding fiber  14  is coupled to the passive output fiber of laser source  12  and functions as gain medium for the parasitic light while dichroic WD  56  operates to reflect forward propagating parasitic light before it is coupled into process fiber/gain medium  62 . The amount of parasitic light coupled into gain medium  62  depends on coefficient of reflection of WD  62 , which may prevent of up to 99% of this light from entering the gain medium without drastically attenuating signal light. 
     Turning specifically to  FIG. 12 , high power fiber laser system  50  may be limited to laser  12  or have one or more amplifying cascades or gain blocks  52  each including at least an active fiber, but typically having a combination of active and two passive input and output, fibers. The system output light is coupled into feeding fiber  14  guiding it to a fiber-to-fiber coupler  60  which provides optical communication over free space between feeding and process fibers  14  and  62  respectively. 
     The system  50  further includes a bulk light guiding and beam shaping unit  58 . The system output light is incident on WD  56  that, along with unit  58  and possibly other optical components, may be housed, for example, in a casing  60 . 
     The signal and parasitic light propagate through process fiber  62  which functions as additional gain medium for the unwanted light. Even if system  50  is configured with slanted fiber gratings of U.S. Pat. No. 7,912,099, which is fully incorporated herein by reference, process fiber  62  alone or in combination with feeding fiber  14  constitutes gain medium for parasitic light, such as Raman. As can be understood from  FIGS. 12 and 13 , system  50  may have a delivery (feeding and process) fiber covering a few hundred meter distance and, with the shown configuration, typically operates at high power levels of tens to hundreds kW. The fiber length and optical power density are the system characteristics which either individually or in combination with each other create favorable conditions for generation of parasitic light even without the parasitic resonator, let alone its presence. The damage that the forward propagating emitted parasitic light may inflict on surface  16  can be irreparable. In addition, if used in the surface treating process, the detrimental role of a parasitic resonator and back reflected parasitic light cannot be ignored. Accordingly, inducing losses of parasitic light in the gain medium substantially increases the system output at signal light wavelength and improves the quality of surface processing. 
     Referring specifically to  FIG. 13 , the advantages of using WD  56  capable of minimizing Raman generation in gain medium  62  were tested in system  55  that instead of the fiber-to-fiber coupler has a beam switch  54 . The system is operative to measure and control the spectrum of the output beam and its power realized by unit  66  spaced from a beam expander  68  which is coupled to the output end of process fiber  62 . The latter receives laser generated light from feeding fiber  14  via beam switch  54  where the light propagates over free space while being guided by mirrors  70  mounted along with a collimator  72  and WD  56  in the housing of beam switch  54 . 
     The laser system  55  is an experimental system for determining the reflected SRS suppression at the output of process fiber  62 . The WD  56  is configured as a plate with a 50 mm diameter and 2.5 mm thickness which transmits signal light at 1050-1080 nm working wavelength and reflects parasitic light at 1129-1200 nm wavelength and has transmission as shown in  FIG. 15 . The WD  56  is mounted in the housing at a 1°-relative to the optical axis of the collimator to prevent influence of reflected SRS radiation on laser  12 . Hence the reflected SRS beam is deflected at 3.5 mm from the output end of feeding fiber  14 . 
     The experimental system  55  is configured to output 31 kW using a Ø 100 μm, 112 m long feeding fiber. Two tests, one with WD  56  and the other without it, resulted in respective two spectra as shown in  FIG. 16 . The suppression of the SRS level, i.e., the difference between these spectra is 27 dB at 1135 nm. Additional tests were conducted with the same system operating at different full output powers with and without WD  56 . The results are presented in  FIG. 17  and demonstrate that the installation of dichroic WD  56  allows increasing the output power at more than 30%. 
     Although the present disclosure has been described in terms of the disclosed example, numerous modifications and/or additions to the above-disclosed embodiments would be readily apparent to one skilled in the laser arts without departing however from the scope and spirit of the following claims.