Patent Publication Number: US-2011051755-A1

Title: Frequency Conversion Laser Head

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. application Ser. No. 12/152,047 filed with the US Patent and Trademark office on May 12, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a laser head with improved heat-diffusing capabilities. 
     2. Prior Art 
     Frequency converting laser heads are used in numerous fields including, but not limited to industrial and medical applications. Not all wavelength regions of interest are directly accessible with laser. Therefore, it is common to generate visible light by nonlinear conversion of light. Typically a beam of light of a narrow wavelength illuminates a nonlinear component which doubles, triples and etc. the output frequency of the light. 
     In a nonlinear frequency conversion process the efficiency of conversion of laser power at a fundamental frequency into power at combined frequencies, such as the second, third and other harmonic frequencies, is strongly dependent on the intensity of radiation interacting with the non-linear optical material. In practice, high intensities are needed to reach laser output powers up to tens of watts and higher so desirable by the market. The high intensities are accompanied by elevated temperatures which are detrimental to the desired functionality of a laser head explained in detail immediately hereinbelow. 
     Referring to  FIG. 1 , a laser unit  10  includes a laser module  12  and a frequency conversion laser head  14 . The laser module  12  may be configured as a solid state or all fiber laser outputting a pump light at a fundamental frequency. The pump is coupled to an output component  16  and delivered to laser head  14  where it impinges upon a frequency conversion component. The latter, in turn, converts the fundamental frequency of the pump radiation into the desired frequency. The output light at the desired frequency is finally delivered to the object to be treated. 
     Not all of the pumped radiation is converted into the desired frequency. In fact, only a small portion of the pumped light is usefully converted; the other, large portion of the pumped radiation remains unchanged and, therefore, useless. For example, the pump light, coupled to laser head  14 , has a power of 10 W. As a result of the frequency conversion, the output light delivered to the object to be treated at the desired frequency has a power of about 2 W. The other 8 watts of the pump light exiting the non-linear frequency component at the fundamental frequency create a thermally-hazardous situation within laser head  14 , which can possibly lead to a completely unsatisfactory operation of the laser. To somewhat minimize the thermal effect, the laser head has a greater volume that renders it space-ineffective. Yet, to the best of the applicant&#39;s knowledge, this configuration of the laser head is predominant in the laser industry. 
     It is, therefore, desirable to provide a frequency conversion laser head configured to minimize the thermal effect of the unconverted pump radiation. 
     It is also desirable to provide a laser assembly configured with the improved frequency conversion laser head, as disclosed immediately above. 
     SUMMARY OF THE INVENTION 
     These objectives are attained by a laser assembly configured in accordance with the present disclosure and capable of coupling a pump radiation at undesirable frequency out of the case of a frequency conversion laser head. In particular, the disclosed frequency conversion laser head is configured with a heat dump assembly operative to couple unconverted pump radiation out of the laser head&#39;s case. 
     Preferably, the disclosed dump assembly includes an optical filter capable of separating desired frequencies of output light which exits a non-linear conversion element from fundamental, unconverted frequencies. The unconverted output light is steered towards and coupled into a dump fiber that guides the output light outside the case of the laser head. 
     Evacuating the unconverted light from a laser head allows for a compact structure of the laser head and minimizes detrimental thermal effects on the proper functionality of components of the frequency conversion laser head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present disclosure will become more readily apparent from a further specific description accompanied by the following drawings, in which: 
         FIG. 1  is a schematic view of a typical laser assembly including the frequency conversion laser head of the known prior art. 
         FIG. 2  is a schematic view of a laser assembly configured in accordance with the present disclosure. 
         FIGS. 3A and 3B  are respective diagrammatic orthogonal views of different modification of the laser system of  FIG. 2 . 
         FIG. 4  is a diagrammatic optical schematic of the laser system configured in accordance with the present disclosure. 
         FIG. 5  is a view of the laser system of  FIG. 3   
     
    
    
     SPECIFIC DESCRIPTION 
     Reference will now be made in detail to the disclosed system. 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 far from precise scale. For purposes of convenience and clarity only, the terms “connect,” “couple,” and similar terms with their inflectional morphemes do not necessarily denote direct and immediate connections, but also include connections through mediate elements or devices. 
       FIG. 2  illustrates a diagrammatic view of the disclosed laser assembly  25  including a laser system or pump laser module  18  and a frequency conversion laser head  20 . An input component  22  couples a pump light beam Lpump at a fundamental frequency f f  to frequency conversion laser head  20 , which is operative to shift the fundamental frequency ff of pump light Lpump to a desired frequency fd. The light Lpump is processed inside the case of laser head  20  so that it splits into an output light beam Lout at the desired frequency fd and an output light Lout at the fundamental frequency ff, as explained hereinbelow. 
     In accordance with one aspect of the disclosure, light Lout at fundamental frequency ff is coupled into a dump fiber  24  guiding light Lout outside frequency conversion laser head  20 . As a consequence and in contrast to the known prior art, the heat generated by the reflected light beam does not accumulate inside the laser head that, thus, does not experience overheating while featuring a compact structure. 
     Referring to  FIGS. 3A and 3B , frequency conversion laser head  20  may be configured as a separate component detachably coupled to laser system or pump laser module  18 , or may be manufactured and marketed in combination therewith. In either configuration, frequency conversion laser head  20  may be coupled to system  18 , so that these components are displaceably fixed to one another in a single case ( FIG. 3B ). In accordance with this modification, input and output dump fiber waiveguides  22  and  24 , respectively, are mounted inside laser system  18 . Alternatively, as shown in  FIG. 3A , frequency conversion laser  20  is configured as a separate component flexibly coupled to system  18  so that these two components are detachably coupled to and displaceable relative to one another. Typically, laser module  18  and head  20  are coupled by a flexible hollow component  26  ( FIG. 3A ) which houses input fiber  22  and dump fiber  26 . 
       FIG. 4  illustrates an optical schematic of disclosed assembly  25  in which frequency conversion laser head  20  is detachably coupled to input fiber waveguide  22  guiding pump light Lpump at fundamental frequency ff from pump  18 . The input fiber waveguide  22  outputs the Lpump light beam inside the case of laser head  20 . The pump light is guided along a light transmission path (LTP) by well known optical elements, such as a focusing lens  30  coupling the focused pump light to a nonlinear frequency conversion component  32  which is operative to shift the fundamental frequency ff to other desired frequencies fd. The light exiting nonlinear component  32  at fundamental and desired frequencies is incident upon a beam splitter  34  locating along the transmission light path. Configured to separate the desired frequency from the fundamental frequency, beam splitter  34  reflects light beam Lout at fundamental frequency ff along a light dump path (LDP) which extends in a plane transverse to the plane of the transmission light path. The light beam Lout at desired frequency fd propagates through beam splitter  34  without deviation from the light transmission path LTP and is coupled by an output focusing optics (not shown) to an output delivery fiber, which is preferably, but not necessarily, a single-mode fiber  40 . The beam splitter  34  preferably has at least one reflecting surface covered with dielectric material; as a consequence, beam splitter is transmissive to the entire light beam Lout at the desired frequency except for a small portion thereof which is tapped off for power monitoring purposes, as explained hereinbelow. 
     The nonlinear frequency conversion component  32  is based on a non-linear response of the polarization of the material caused by the electric field of the pump light beam. The degree of non-linear response is governed by the magnitude of the non-linear susceptibility of the material. The effect of this non-linear response is to shift the frequency of the pump light beam to other frequencies. Examples of non-linear frequency conversion component  32  includes: second harmonic generation, sum and difference frequency generator, third and higher harmonic generation, stimulated Raman shifting, optical parametric oscillation and others and any combination of these. 
     As the output light beam Lout at fundamental frequency ff propagates along the light dump path DLP, it initially impinges upon a dump mirror  36  which is configured similarly to beam splitter  34  and guides light Lout fundamental frequency ff towards an output dump focusing lens  38 . The latter couples the light beam Lout at fundamental frequency ff to multimode dump fiber  24  which guides it outside frequency conversion laser head  20 . In accordance with one modification of the present disclosure, multimode dump fiber  24  terminates inside the housing of system or pump module  18 . Alternatively, dump fiber may guide the unconverted light into free space, as illustrated in  FIGS. 2 and 4  by dash lines. 
     The desired wavelengths are subject only to a particular configuration of non-linear conversion component  32 . For example, frequency conversion laser head  20  is configured to operate in combination with CW or pulsed laser, which is pumped by pump light Lpump at 1064 and/or 1550 nm and has an average power approaching 20 W. The pump light radiation Lpump is launched in nonlinear frequency conversion component  32  configured from a 30 mm long lithium niobate crystal which is periodically poled. The crystal is operative to upconvert pump radiation Lpump to 775 nm output light beam Pout carried along the light transmission path with an output power of about 2 W. The unconverted pump light Lout at fundamental frequency ff is separated from the second radiation harmonic and further guided outside frequency conversion laser head  20  upon coupling into 100 micron-core MM fiber  24 . As readily apparent to one of ordinary skills in the art, the dielectric layer deposited on splitters  34  and  36  may be configured to fully reflect not only the light at fundamental frequency, but also a Raman component thereof which has a wavelength different from both, the fundamental and desired wavelengths. 
     In accordance with a further aspect of the disclosure, frequency conversion laser head  20  is provided with a power monitor  28  operative to reliably measure the power of output light beam Lout at desired frequency fd which is carried by output fiber  40 . As mentioned above, beam splitters  34  and  36  may be configured so that a small portion of output light beam Lout at desired frequency fd is reflected along the light dump path LDP. The mirror  36  is transparent for the light at desired frequency fd which propagates through splitter/mirror  36  along a measuring light path (MLP) and is incident on power monitor  28  located downstream from mirror  36  along the measuring light path. The power monitor  28  is coupled to a processing unit (not shown) provided with software which is operative to alter the power of pump light beam Lpump, if a measured value deviates from the reference value. 
     The laser system/pump module laser  18 , as mentioned above, may be configured as a solid state laser or a fiber laser and operate in a CW or pulsed regime The solid state laser pump  18  may be configured based on, for example, the Nd:YAG laser rod, Al2O3Ti rod, alexandrite rod and many others, well known to one of ordinary skills in the laser art. The pulsed operation may include Q-switched operation. In conjunction with solid state laser, nonlinear conversion component  32  may include, for example, potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate (KD*P), ammonium dihydrogen phosphate (ADP), potassium titanium oxide phosphate (KTP), rubidium dihydrogen phosphate (RDP), rubidium dihydrogen arcenate (RDA), cesium dihydrogen arsenate (CDA), lithium iodate (LiIO3), lithium niobate (LiNbO3) and others. 
     Referring to  FIG. 5 , laser system  18  is based on a fiber waveguide doped with Er, Yb, Er/Yb, Th and other suitable rare earth elements. The fiber laser modules may include a single laser fiber block provided with a resonant cavity. In addition, it may be configured with a single or multiple amplification fiber blocks. 
       FIG. 5 , disclosed in conjunction with  FIG. 4 , illustrates a gain fiber block  50  of pump/laser system  18  based on a multimode (MM) fiber  52  which is pumped by one or more laser diodes (not sown). The MM active fiber  52  has a core  54  capable of supporting a fundamental mode at the desired frequency. To output light pump beam Lout, a single mode passive fiber  56  is directly spliced in the end-to-end configuration to MM fiber  52  without the use of a mode converter. 
     The core sizes of respective fibers  52  and  56  are chosen so that the fundamental mode spot size of MM fiber  52  substantially matches that of SM fiber  56  so that efficient coupling of the fundamental modes of the fibers is achieved between the fibers at their coupled ends. The fiber are so configured that the fundamental mode of MM fiber  52  propagates down its core  54  without significant coupling of power into higher order modes. 
     The laser of  FIG. 5  forces oscillation in the fundamental mode and actively prevents lasing in the higher order modes in the following way: oscillation of the fundamental mode is forced only by the monomode fiber waveguide and grating  60  which is written into SM fiber  56 . The grating partially reflects the fundamental mode signal back into MM fiber  52  such that the reflected fundamental mode signal does not couple to higher order modes. The grating  60 , in effect, ensures partial reflection of only the fundamental mode, which increases discrimination against higher order modes in MM fiber  52 . Therefore, due to highly predominant fundamental mode oscillation, emission is stimulated in the fundamental mode only. 
     As mentioned above, the fiber gratings can be replaced with mirrors. In fact, as long as some form of reflector arrangement with the required reflectivities is present in the system, whether they are internal reflectors written into the waveguides, external reflectors, or a combination of internal and external reflectors, the system should operate as described by outputting substantially SM pump light beam Lpump at the fundamental frequency ff which is coupled to input SM fiber  22  of  FIGS. 3 and 4 . 
     The fiber/gain block  50  is illustrated to have a resonant cavity defined between two reflective components, such as mirrors or, preferably, fiber gratings  58  and  60 . As well understood by one of ordinary skills in the laser art, a single or multiple amplification fiber blocks can be configured identically to fiber block  50  only without the reflective components. 
     The laser of  FIG. 5  operates by coupling a laser diode light beam to MM fiber  52 . The method of coupling may include an end pumping technique and a side pumping technique. The MM fiber  52  is, preferably, a large mode area, multi-cladding fiber. The laser diode light beam provides a signal at a first wavelength. In response, MM fiber  5  is capable of laser action with an emission at the fundamental frequency ff while exhibiting multimode behavior at the fundamental frequency. One of the opposite ends of SM fiber  56 , which exhibits substantially single mode behavior at the fundamental frequency, is fusion spliced to the opposing end of MM fiber  52 . Finally, the optical cavity is defined between reflective components  58  and  60 , respectively, and includes at portions of MM and SM fibers. Note that the amplification block would be provided with a MM fiber having its opposite ends spliced to respective SM fibers, wherein the upstream SM fiber would be spliced to SM fiber  56  of block  50 . 
     A variety of changes of the disclosed structure may be made without departing from the spirit and essential characteristics thereof. Thus, it is intended that all matter contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the disclosure being defined by the appended claims.