Patent Document

CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present invention claims priority from U.S. Patent Application Ser. No. 60/896,630 filed on Mar. 23, 2007, the entire disclosure of which incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to methods, arrangement and apparatus for using certain electro-magnetic radiation source, and more particularly to methods, arrangements and apparatus for wavelength tuning and a wavelength-swept laser using exemplary optical wavelength filter systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been demonstrated in an extended-cavity semiconductor laser by using a diffraction grating filter design. Obtaining single-frequency laser operation and ensuring mode-hop-free tuning, however, may use a complicated mechanical apparatus and limit the maximum tuning speed. One of the fastest tuning speeds demonstrated so far has been limited less than 100 nm/s. In certain applications such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width may provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy. 
         [0004]    A line width on the order of 10 GHz can be achievable with the use of an intra-cavity tuning element (such as an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously demonstrated has been less than 1 kHz limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 25 kHz may be needed for video-rate (&gt;30 frames/s), high-resolution optical imaging in biomedical applications. 
         [0005]    A wavelength-swept laser that uses a diffraction grating and polygon scanner has provided high-speed wavelength tuning up to 20,000 nm/ms. While the high-speed polygon based wavelength-swept light source enabled high-speed imaging as fast as 200 frames/s, wavelength tuning rate as fast as 20,000 nm/ms keeping an instantaneous line width narrower than 0.15 nm has already reached to the limit of the polygon based wavelength-swept laser. In addition, a tuning rate of 7000-nm/ms is achieved with 65 mW of power over a wavelength range of 120-nm and with an instantaneous line-width ˜0.07 nm. 
         [0006]    Accordingly, for faster tuning and especially for wide wavelength tuning range (˜200 nm) and (or) narrow instantaneous line width (˜0.07 nm) at fast tuning rate, it may be beneficial to provide a further wavelength scanning filter and laser scheme/procedure and/or overcome at least some of the deficiencies described herein above. 
       OBJECTS AND SUMMARY OF EXEMPLARY EMBODIMENTS 
       [0007]    One of the objectives of the present invention is to overcome certain deficiencies and shortcomings of the prior art arrangements and methods (including those described herein above), and provide exemplary embodiments of arrangements and methods for wavelength tuning and a wavelength-swept laser using exemplary optical wavelength filter systems 
         [0008]    According to one exemplary embodiment of the present invention, an apparatus and a source arrangement can be provide for light wave filtering which may facilitate high-speed wavelength-swept light with broad spectral tuning range and narrow instantaneous linewidth. In one exemplary embodiment of the present invention, the optical filter can include a diffraction grating (or multiple diffraction gratings or prisms), and an angular scanning element(s) (including, but not limited to, a polygonal mirror, a (multi facets) planar mirror disposed on a rotating shaft, a (multi facets) mirror disposed on a galvonmeter, or an (multi) acousto-optic modulator(s)). An exemplary alignment of the diffraction grating (or multiple diffraction gratings) can facilitate a high-speed wavelength sweeping over a broad tuning range (˜120 nm) with narrow instantaneous linewidth (˜0.07 nm). 
         [0009]    In another exemplary embodiment, the wavelength-swept filter can be combined with a gain medium implementing a wavelength tunable light source. The filter and gain medium may further be incorporated into a laser cavity. For example, a laser can emit a narrow band spectrum with its center wavelength being rapidly swept over a broad wavelength range. The laser resonator may include a unidirectional fiber-optic ring, unidirectional combined fiber and free space optic ring, or a full free space linear cavity with a specially designed semiconductor optical gain medium to minimize the cavity length of the laser. 
         [0010]    Indeed, exemplary systems and methods for filtering an electromagnetic radiation can be provided. For example, at least one first arrangement can be provided which is capable of receiving at least one first electro-magnetic radiation and forwarding at least one second electro-magnetic radiation at different angles with respect to a direction of incidence of the first electro-magnetic radiation. At least one second wavelength dispersion arrangement can be provided which is configured to receive the second electro-magnetic radiation, forward at least one third electro-magnetic radiation to the first arrangement and further receive at least one fourth electro-magnetic radiation. The third electro-magnetic radiation can be based on the second electro-magnetic radiation, and the fourth electro-magnetic radiation can be based on the third electro-magnetic radiation. 
         [0011]    For example, the first arrangement can be a multi-faceted mirror arrangement. The first electro-magnetic radiation can impact a first facet of the multi-faceted mirror arrangement, and the third electro-magnetic radiation can impact a second facet of the multi-faceted mirror arrangement, with the first and second facets being different from one another. The first arrangement can also be a polygon beam scanning arrangement. T first electro-magnetic radiation can impacts a first facet of the polygon beam scanning arrangement, and the third electro-magnetic radiation can impact a second facet of the polygon beam scanning arrangement, with the first and second facets being different from one another. The polygon beam scanning arrangement is capable of continuously being rotated. 
         [0012]    According to another exemplary embodiment of the present invention, the second arrangement can be a defraction grating arrangement, a prism arrangement and/or a grism arrangement. The first and second arrangements may be positioned such that a particular electro-magnetic radiation that is based on the first electro-magnetic radiation may be received by the at least one first arrangement from the second arrangement more that twice. At least one third wavelength dispersion arrangement can be provided which is configured to physically separate one or more components of a particular electro-magnetic radiation based on a frequency of the particular electro-magnetic radiation. The first electro-magnetic radiation can be based on the particular electro-magnetic radiation. In addition, at least one fourth arrangement can be provided which is configured to receive at least some of the one or more components, and modify at least one characteristic of the received one or more components to provide the first electro-magnetic radiation which is associated with one or more further components of the particular electro-magnetic radiation. 
         [0013]    Yet another exemplary embodiment of the present invention can be provided. For example, a source arrangement can provide at least one particular electromagnetic radiation. Such exemplary source arrangement can include at least one emitter arrangement configured to provide the at least one electromagnetic radiation. At least one first arrangement may be provided which is capable of receiving the particular electro-magnetic radiation and forwarding at least one first electro-magnetic radiation at different angles with respect to a direction of incidence of the particular electro-magnetic radiation. Further, at least one second wavelength dispersion arrangement can be provided which is configured to receive the at least one first electro-magnetic radiation, forward at least one second electro-magnetic radiation to the first arrangement and further receive at least one third electro-magnetic radiation. The second electro-magnetic radiation can be based on the first electro-magnetic radiation, and the third electro-magnetic radiation can be based on the at least one second electro-magnetic radiation. 
         [0014]    The source arrangement can also include at least one laser cavity configured to receive the third electromagnetic radiation. The laser cavity can be a ring laser cavity. The emitter arrangement can be a semiconductor optical amplifier, a laser diode, a super-luminescent diode, a doped optical fiber, a doped laser crystal, a doped laser glass, and/or a laser dye. The particular electromagnetic radiation may have a frequency that is continuously swept in a positive wavelength direction. An optical circulator can also be included in the source arrangement. For example, wavelength ranges of the emitter arrangement can be distinct from one another. The first arrangement can be a multi-faceted mirror arrangement. The particular electro-magnetic radiation can impact a first facet of the multi-faceted mirror arrangement, and the second electro-magnetic radiation can impact a second facet of the multi-faceted mirror arrangement, with the first and second facets being different from one another. 
         [0015]    According to still another exemplary embodiment of the present invention, the first arrangement can be a polygon beam scanning arrangement. The particular electro-magnetic radiation can impact a first facet of the polygon beam scanning arrangement, and the second electro-magnetic radiation can impact a second facet of the polygon beam scanning arrangement, with the first and second facets being different from one another. The polygon beam scanning arrangement is capable of continuously being rotated. The second arrangement can be a defraction grating arrangement, a prism arrangement and/or a grism arrangement. The first and second arrangements can be positioned such that a further electro-magnetic radiation that is based on the first electro-magnetic radiation is received by the first arrangement from the second arrangement more that twice. 
         [0016]    At least one third wavelength dispersion arrangement can also be provided which is configured to physically separate one or more components of a further electro-magnetic radiation based on a frequency of the particular electro-magnetic radiation. The particular electro-magnetic radiation may be based on the further electro-magnetic radiation. At least one fourth arrangement can also be provided which is configured to receive at least some of the one or more components, and modify at least one characteristic of the received one or more components to provide at least one particular electro-magnetic radiation which is associated with one or more further components of the further electro-magnetic radiation. 
         [0017]    In yet another exemplary embodiment of the present invention, an apparatus and source arrangement for filtering an electromagnetic radiation can be provided which may include at least one spectral separating arrangement configured to physically separate one or more components of the electromagnetic radiation based on a frequency of the electromagnetic radiation. The apparatus and source arrangement may also have at least one continuously rotating optical arrangement, e.g., polygonal scanning mirror and spinning reflector disk scanner, which is configured to receive at least one signal that is associated with the one or more components. Further, the apparatus and source arrangement can include at least one beam selecting arrangement configured to receive the signal. 
         [0018]    Other features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Further objects, features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the present invention, in which: 
           [0020]      FIG. 1  is a block diagram of a first exemplary embodiment of an optical wavelength filter arrangement/apparatus according to the present invention; 
           [0021]      FIG. 2A  is an illustration of a second exemplary embodiment of the arrangement/apparatus which includes a diffraction grating and polygon scanner-based wavelength tuning filter according to the present invention; 
           [0022]      FIG. 2B  is an illustration of a third exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and polygon scanner based wavelength tuning filter according to the present invention; 
           [0023]      FIG. 2C  is an illustration of a fourth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and polygon scanner according to the present invention; 
           [0024]      FIG. 2D  is an illustration of a fifth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and polygon scanner according to the present invention; 
           [0025]      FIG. 3A  is an illustration of a sixth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating, a polygon scanner, and a reflector according to the present invention; 
           [0026]      FIG. 3B  is an illustration of a seventh exemplary embodiment of the arrangement/apparatus which includes two diffraction gratings and polygon scanner according to the present invention; 
           [0027]      FIG. 3C  is an illustration of an eight exemplary embodiment of the arrangement/apparatus which includes multiple diffraction gratings, the reflector and the polygon scanner according to the present invention; 
           [0028]      FIG. 4A  is an illustration of a ninth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating, the reflector, and the polygon scanner (or in combination with the other exemplary embodiments) according to the present invention; 
           [0029]      FIG. 4B  is an illustration of a tenth exemplary embodiment of the arrangement/apparatus which includes two diffraction gratings and the polygon scanner (or in combination with the other exemplary embodiments) according to the present invention; 
           [0030]      FIG. 5A  is a block diagram of an eleventh exemplary embodiment of an optical wavelength filter according to the present invention; 
           [0031]      FIG. 5B  is an illustration of a twelfth exemplary embodiment of the arrangement/apparatus which includes two diffraction gratings, a telescope, and the polygon scanner according to the present invention; 
           [0032]      FIG. 5C  is an illustration of a thirteenth exemplary embodiment of the arrangement/apparatus which includes two diffraction gratings, the telescope, the reflector and the polygon scanner according to the present invention; 
           [0033]      FIG. 5D  is an illustration of a fourteenth exemplary embodiment of the arrangement/apparatus which includes three diffraction gratings, the telescope and the polygon scanner according to the present invention; 
           [0034]      FIG. 6A  is an illustration of a fifteenth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and the polygon scanner based wavelength tuning filter using two (or N) gain mediums according to the present invention; 
           [0035]      FIG. 6B  is an illustration of a sixteenth exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and the polygon scanner based wavelength tuning filter using two (or N) gain mediums in series or parallel according to the present invention; 
           [0036]      FIG. 7  is an illustration of a seventeenth exemplary embodiment of the arrangement/apparatus which includes a short linear cavity laser using the diffraction grating and the polygon scanner based wavelength tuning filter according to the present invention; 
           [0037]      FIG. 8A  is an illustration of an eighteenth exemplary embodiment of the arrangement/apparatus which includes a fiber ring laser using the diffraction grating and the polygon scanner based wavelength tuning filter according to the present invention; 
           [0038]      FIG. 8B  is an illustration of a nineteenth exemplary embodiment of the arrangement/apparatus which includes a combined fiber and free space ring laser using the diffraction grating and polygon scanner based wavelength tuning filter according to the present invention; 
           [0039]      FIG. 8C  is an illustration of a twentieth exemplary embodiment of the arrangement/apparatus which includes a resonant cavity fiber ring laser using the diffraction grating and the polygon scanner based wavelength tuning filter according to the present invention; and 
           [0040]      FIG. 8D  is an illustration of a twenty-first exemplary embodiment of the arrangement/apparatus which includes the resonant cavity fiber Raman ring laser using the diffraction grating and the polygon scanner based wavelength tuning filter according to the present invention. 
       
    
    
       [0041]    Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject invention will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described embodiments without departing from the true scope and spirit of the subject invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0042]      FIG. 1  shows a block diagram of a first exemplary embodiment of an arrangement/apparatus which includes an optical wavelength filter  1  in accordance the present invention. In this first exemplary embodiment, the optical wavelength filter  1  can be used in a variety of different applications, general and non-limiting examples of which are described below. In the example shown in  FIG. 1 , the filter  1  may be coupled to one or more applications  3  via a light source  2 . It should be understood that in certain exemplary applications, the filter  1  can be used with or connected to an application (e.g., one or more of the applications  3 ) via a device other than a light source (e.g. a passive or active optical element). 
         [0043]    In the first exemplary embodiment shown in  FIG. 1 , a broad spectrum light source and/or controller  2  (hereinafter referred to as “light controller”), may illuminate a beam deflecting device  4  and then be coupled to a wavelength dispersing element  5 . The wavelength dispersing element  5  may be coupled to several configurations which shall be described in further detail below in connection with  FIGS. 8A-8D , and then retrace the path back to the light controller  2 . The light controller  2  can be further coupled to one or more of the applications  3  that are adapted to perform one or more tasks with or for, including but not limited to, optical imaging processes and optical imaging systems, laser machining processes and systems, photolithography and photolithographic systems, laser topography systems, telecommunications processes and systems, etc. The light controller  2  can be one or more of various systems and/or arrangements that are configured to transmit a beam of light having a broad frequency (f) spectrum. 
         [0044]    In one exemplary embodiment of the present invention, the beam of light may be a collimated beam of light. The beam of light can include a plurality of wavelengths λ . . . λn, within the visible light spectrum (e.g., red, blue, green). Similarly, the beam of light provided by the light controller  2  can also include a plurality of wavelengths λ . . . λn that may be defined outside of the visible spectrum (e.g., ultraviolet, near infrared or infrared). According to another exemplary embodiment of the present invention, the light controller  2  can include a unidirectional light transmission ring, which shall be described in further detail below in connection with  FIGS. 8A-8D  which shows an exemplary embodiment of a wavelength tuning laser source. 
         [0045]    Further, in still another exemplary embodiment of the present invention, the light controller  2  can include a linear resonator system, which shall be described in further detail below in connection with  FIGS. 8A-D . In one exemplary embodiment of the present invention, the wavelength dispersing element  5  can include a light dispersion element, which may include but not limited to, a reflection grating, a diffraction grating, prism, or combinations of one or more of these elements. Furthermore, the dispersion element  5  is adapted to direct or steer and/or focus the wavelengths of light to a predetermined position(s) located on a beam deflecting device  4 . Moreover, the dispersion element  5  can be controlled to receive and selectively redirect one or more discrete wavelengths of light back to the beam deflecting device and back to the light controller  2 . Thereafter, the light controller  2  can selectively direct the received discrete wavelengths of light to any one or more of the applications. The device  4  can be provided in many different ways. For example, the beam deflecting device  4  can be provided from elements including, but not limited to, a polygonal mirror (or several polygon mirrors), a (multi facets) planar mirror disposed on a rotating shaft, a (multi facets) mirror disposed on a galvanometer, or an (multi) acousto-optic modulator(s). 
         [0046]      FIG. 2A  shows a diagram of an exemplary embodiment of the arrangement/apparatus which includes the diffraction grating and polygon scanner based wavelength tuning filter in accordance with the present invention. The exemplary optical wavelength filter  1 ′ can be configured as a reflection-type filter which may have substantially identical input and output ports. An input/output optical fiber  10  and a collimating lens  11  can provide an input from a light controller  2 ′ (which may be substantially similar to the light controller  2  described above with reference to  FIG. 1 ) to the optical wavelength filter  1 ′. The optical wavelength filter can include a collimated input/output beam  12 , a diffraction grating  13 , and a spinning polygon scanner  14 . Light input to the optical wavelength filter is provided as a collimated input beam  12 . Wavelength filtered output is retro-reflected as a collimated light output  12 . The mirror surface of the polygon arrangement  14  is placed such that the beam of light is reflected with a non-zero angle (rather than directly being reflected back to the light controller  2 ′ from the polygon arrangement&#39;s mirror facet  14 ). To have minimum beam clipping on the polygon facet, the following condition can be met, e.g.: 
         [0000]        D&lt;L  cos(ψ) 
         [0000]    where D, L, and ψ are 
         [0000]    
       
         
           
             1 
             
                
               2 
             
           
         
       
     
         [0000]    width of the collimated beam  12  of each wavelength components at the focusing lens  11 , facet size, and incident angle, respectively. The sweep angle of the reflected light from the polygon arrangement  14  is double the polygon arrangement&#39;s  14  rotation angle  16 . The diffraction grating  13  is placed close to the polygon scanner facet (≦2 cm) to decrease beam displacement on the diffraction grating  13 . When each partial rotation of the polygon through an angle of 
         [0000]    
       
         
           
             θ 
             = 
             
               
                 2 
                  
                 π 
               
               N 
             
           
         
       
     
         [0000]    (e.g., the facet-to-facet angle  15  of the polygon), where N is the number of mirror facets, the sweep angle  16  of the reflected light is 2θ for a rotation of the angle θ of the polygon arrangement  14 . The reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  at Littrow&#39;s angle before retracing the path back to the light controller  2 . 
         [0047]    As is illustrated in the exemplary embodiment shown in  FIG. 2A , a beam deflection device  4  (e.g., which may include a polygon mirror or arrangement  14 ) is adapted to preferably reflect back only the spectral component within a narrow passband as a function Littrow&#39;s angle. The orientation of the incident beam  12  with respect to the normal axis of the polygon facet  14  and a rotation direction  17  of the polygon arrangement  14  can be used to determine the direction of wavelength tuning, e.g., a wavelength up (positive) scan or a wavelength down (negative) scan. The exemplary arrangement shown in  FIG. 2A  can generate a negative wavelength sweep. It should be understood that although the polygon arrangement  14  is shown in  FIG. 2A  as having twelve facets, polygon arrangements which have fewer than twelve facets or greater than twelve facets can also be used. While generally not considering practical mechanical limits, based upon conventional manufacturing techniques, a particular number of facets of the polygon arrangement  14  to use in any application may depend on a desired scanning rate and a scanning range for a particular application. 
         [0048]    Furthermore, the size of the exemplary polygon arrangement  14  may be selected based on preferences of a particular application, and preferably taking into account certain factors including, but not limited to, manufacturability and weight of the polygon arrangement  14 . It should also be understood the diffraction gratings that have different pitch may be provided. In addition, the diffraction grating may provide adjustable parameters which control the tuning range and linewidth. The Gaussian beam  12  can be utilized with a broad optical spectrum incident to the grating from the fiber collimator  11 . The exemplary grating equation is expressed as λ=2p sin(α) where λ is the optical wavelength, p is the grating pitch, and α is Littrow angle (or the incident angle (the diffracted angle) of the beam with respect to the normal axis  18  of the grating). FWHM bandwidth of the filter is defined by 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   δλ 
                   ) 
                 
                 FWHM 
               
               = 
               
                 
                   2 
                    
                   
                     
                       2 
                        
                       
                           
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                   
                    
                   λ 
                    
                   
                       
                   
                    
                   pm 
                    
                   
                       
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   D 
                 
               
             
             , 
           
         
       
     
         [0000]    where m is the diffraction order and D is 
         [0000]    
       
         
           
             1 
             
                
               2 
             
           
         
       
     
         [0000]    width of the Gaussian beam at the fiber collimator for double pass. 
         [0049]    The filter tuning range can be expressed as Δλ=2pΔα cos(α 0 ), where α 0  is the Littrow&#39;s angle at λ 0  (center wavelength). One of exemplary design parameters of the filter, originated from the multiple facet nature of the polygon mirror, is the free spectral range, which is described in the following. The polygon arrangement  14  may have a facet-to-facet polar angle given by θ=2π/N≈L/R, where L is the facet width, R is the radius of the polygon and the sweep angle  16  of the reflected light is 2θ for a rotation of the angle θ of the polygon arrangement  14 . The exemplary polygon arrangement  14  preferably does not retro-reflect more than one spectral component at a given time because the range of Littrow angle is equal to the sweeping angle, i.e. Δα=2θ. The spacing of the multiple spectral components simultaneously reflected, or the free spectral range, can be defined as (Δλ) FSR =4pθ cos(α 0 ). 
         [0050]    In an exemplary intra-cavity scanning filter application, the free spectral range of the filter can exceed the spectral range of the gain medium in order to avoid multiple frequency bands (in the case of an inhomogeneously broadened gain medium) or limited tuning range (in the case of a homogeneously broadened gain medium). 
         [0051]    The duty cycle of laser tuning by the filter can be, for example, 100% with no excess loss caused by beam clipping if preferable condition ca be met as follows: 
         [0000]        D&lt;L  cos(ψ)  (1) 
         [0000]    This exemplary equation may be derived from a condition that the beam illuminating polygon facet should be smaller than the facet width at the maximum incident angle of the beam with respect to the normal axis  18  of the polygon facet. 
         [0052]      FIG. 2B  shows a diagram of a third exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  for decreasing FWHM bandwidth of the filter with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  at an angle α  19  (not equal to Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates another polygon facet  21  (which are not necessary the adjacent faces of the polygon facet  14 ) before retracing the path back to the light controller  2 . 
         [0053]    The diffraction grating according to the third exemplary embodiment of the present invention is operative to provide one or more features as described above, as well as to convert a diverging beam from the polygon facet  14  into converging angular dispersion after the diffraction grating  13  on the polygon facet  21 . Such result may be advantageous for a proper operation of the filter. In addition, the diffraction grating  13  and the incident angle of the optical beam  12  on the polygon facet  14  may provide adjustable parameters, which control the tuning range and linewidth. In this exemplary embodiment, The grating equation can be expressed as λ=p(sin(α)+sin(β)) where λ is the optical wavelength, p is the grating pitch, and α and β are the incident and diffracted angles of the beam with respect to the normal axis  18  of the diffraction grating  13 , respectively. From simple geometry, one can find that φ=β, where φ is the angle between the second polygon facet  21  and the diffraction grating. FWHM bandwidth of the filter is defined by 
         [0000]    
       
         
           
             
               
                 ( 
                 δλ 
                 ) 
               
               FWHM 
             
             = 
             
               
                 
                   2 
                    
                   
                     
                       2 
                        
                       
                           
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                   
                    
                   λ 
                    
                   
                       
                   
                    
                   pm 
                    
                   
                       
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   D 
                 
               
               . 
             
           
         
       
     
         [0054]    The filter tuning range can be expressed as Δλ=p(Δα cos(α 0 )+Δβ cos(β 0 )) where α 0  and β 0  are the incident and diffracted angles at λ 0  (center wavelength). If the sweeping angle is equal to the range of the incident angle, i.e. Δα=2θ and the range of diffracted spectrum is equal to the facet angle, i.e. Δβ=θ, the polygon arrangement can retro-reflect one spectral component at a given time. The spacing of the multiple spectral components simultaneously reflected, or the free spectral range, can be defined as Δλ=pθ(2 cos(α 0 )+cos(β 0 )). 
         [0055]    The duty cycle of laser tuning by the filter can be, for example, 100% with no excess loss caused by beam clipping if preferable conditions are met as follows: 
         [0000]    
       
         
           
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                         
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         ψ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                       
                         cos 
                          
                         
                           ( 
                           β 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    The first equation may be derived from a condition that the beam width illuminating the first polygon facet  14  should be smaller than the facet width. The second equation can be derived from that the diffracted beam width illuminating the second polygon facet  21  should be smaller than the facet width. 
         [0056]      FIG. 2C  shows a diagram of a forth exemplary embodiment of the wavelength tunable filter arrangement  1  for decreasing FWHM bandwidth of the filter with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  at an angle α 19  (not equal to Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates another polygon facet  21  (preferably the adjacent faces of the polygon facet  14 ), and can be returned to the grating at Littrow&#39;s angle γ 23  before retracing the path back to the light controller  2 . 
         [0057]    The diffraction grating  13  according to the forth exemplary embodiment of the present invention as shown in  FIG. 2B  is operative to provide one or more features as described above, as well as to convert a diverging beam from the polygon facets  14  and  21  into converging angular dispersion after the diffraction grating  13  on the polygon facet  21 . Such result may be advantageous for a proper operation of the filter. In addition, the diffraction grating  13  and the incident angle of the optical beam  12  on the polygon facet  14  may provide adjustable parameters, which control the tuning range and linewidth. In this embodiment, The grating equations can be expressed as λ=p(sin(α)+sin(β)) and λ=2p sin(γ), where λ is the optical wavelength, p is the grating pitch, and α, β, γ are the incident, diffracted, and Littrow angles of the beam with respect to the normal axis  18  of the diffraction grating  13 , respectively. From simple geometry, one can find that 2φ=γ±β, where φ is the angle between the second polygon facet  21  and the diffraction grating  13 . By eliminating β and γ, the above equations give an expression, quadratic in λ, that depends only on the incident angle  19  and the angle between the second polygon facet  21  and the diffraction grating  13 . The solution is 
         [0000]    
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     
                       p 
                        
                       
                         { 
                         
                           
                             
                               
                                 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       α 
                                       ) 
                                     
                                   
                                    
                                   
                                     ( 
                                     
                                       1 
                                       + 
                                       
                                         0.5 
                                          
                                         
                                             
                                         
                                          
                                         
                                           cos 
                                            
                                           
                                             ( 
                                             
                                               2 
                                                
                                               
                                                   
                                               
                                                
                                               φ 
                                             
                                             ) 
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                 
                                 + 
                               
                             
                           
                           
                             
                               
                                 
                                   
                                     
                                       
                                         
                                           
                                             
                                               sin 
                                               2 
                                             
                                              
                                             
                                               ( 
                                               α 
                                               ) 
                                             
                                           
                                            
                                           
                                             
                                               ( 
                                               
                                                 1 
                                                 + 
                                                 
                                                   0.5 
                                                    
                                                   
                                                       
                                                   
                                                    
                                                   
                                                     cos 
                                                      
                                                     
                                                       ( 
                                                       
                                                         2 
                                                          
                                                         
                                                             
                                                         
                                                          
                                                         φ 
                                                       
                                                       ) 
                                                     
                                                   
                                                 
                                               
                                               ) 
                                             
                                             2 
                                           
                                         
                                         - 
                                       
                                     
                                   
                                   
                                     
                                       
                                         
                                           ( 
                                           
                                             
                                               
                                                 sin 
                                                 2 
                                               
                                                
                                               
                                                 ( 
                                                 α 
                                                 ) 
                                               
                                             
                                             - 
                                             
                                               
                                                 sin 
                                                 2 
                                               
                                                
                                               
                                                 ( 
                                                 
                                                   2 
                                                    
                                                   φ 
                                                 
                                                 ) 
                                               
                                             
                                           
                                           ) 
                                         
                                          
                                         
                                           ( 
                                           
                                             1.25 
                                             + 
                                             
                                               cos 
                                                
                                               
                                                 ( 
                                                 
                                                   2 
                                                    
                                                   φ 
                                                 
                                                 ) 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                 
                               
                             
                           
                         
                         } 
                       
                     
                     
                       ( 
                       
                         1.25 
                         + 
                         
                           cos 
                            
                           
                             ( 
                             
                               2 
                                
                               φ 
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0058]    As shown in the equation, the tuning range of the filter is fundamentally limited by the grating pitch, the incident angle  19 , and the angle between the second polygon facet and the diffraction grating  13 . In addition, it can be shown that the FWHM bandwidth of the filter (instantaneous line-width) is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     δ 
                      
                     
                         
                     
                      
                     λ 
                   
                   = 
                   
                     
                       2 
                        
                       p 
                        
                       
                           
                       
                        
                       λ 
                        
                       
                         
                           ln 
                            
                           
                               
                           
                            
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                     
                     
                       π 
                        
                       
                           
                       
                        
                       
                         D 
                         ( 
                         
                           1 
                           ± 
                           
                             
                               0.5 
                                
                               
                                   
                               
                                
                               
                                 cos 
                                  
                                 
                                   ( 
                                   β 
                                   ) 
                                 
                               
                             
                             
                               cos 
                                
                               
                                 ( 
                                 γ 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Equation (5) shows that the linewidth of this embodiment can be improved by a factor of 
         [0000]    
       
         
           
             1 
             + 
             
               
                 0.5 
                  
                 
                     
                 
                  
                 
                   cos 
                    
                   
                     ( 
                     β 
                     ) 
                   
                 
               
               
                 cos 
                  
                 
                   ( 
                   γ 
                   ) 
                 
               
             
           
         
       
     
         [0000]    compared to the previous embodiment. 
         [0059]    The filter tuning range can be expressed as Δλ=p(Δα cos(α 0 )+Δβ cos(β 0 )), and Δλ=2pΔγ cos(γ 0 ) where α 0 , β 0 , and γ 0  are the incident, diffracted, and Littrow angles at λ 0  (center wavelength). If the sweeping angle is equal to the range of the incident angle, i.e. Δα=2θ and the range of diffracted spectrum follows the following equality, i.e. 2θ=Δγ±Δβ, the polygon arrangement can retro-reflect one spectral component at a given time. The spacing of the multiple spectral components simultaneously reflected, or the free spectral range, can be defined as 
         [0000]    
       
         
           
             Δλ 
             = 
             
               2 
                
               θ 
                
               
                   
               
                
               
                 p 
                 1 
               
                
               
                 
                   
                     ( 
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           α 
                           0 
                         
                       
                       + 
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           β 
                           0 
                         
                       
                     
                     ) 
                   
                   
                     1 
                     ± 
                     
                       
                         
                           p 
                           1 
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               β 
                               0 
                             
                             ) 
                           
                         
                       
                       
                         2 
                          
                         
                           p 
                           2 
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               γ 
                               0 
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 . 
               
             
           
         
       
     
         [0060]    The duty cycle of laser tuning by the filter can be, for example, 100% with no excess loss caused by beam clipping if preferable conditions are met as follows: 
         [0000]    
       
         
           
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                         
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         ψ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                       
                         cos 
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                       
                         cos 
                          
                         
                           ( 
                           β 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0061]    Equation (6) may be derived from a condition that the beam width illuminating the first polygon facet  14  should be smaller than the facet width. Equation (7) can be derived from that the diffracted beam width illuminating the second polygon facet  21  should be smaller than the facet width. 
         [0062]      FIG. 2D  shows a diagram of a fifth exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  for decreasing FWHM bandwidth of the filter with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  at an angle α 19  (e.g., may be different from Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates another polygon facet  21  (not necessary the adjacent faces of the polygon facet  14 ) and is returned to the grating at an angle γ 23  (e.g., may be different from Littrow&#39;s angle) and diffracted light at angle φ 24  illuminates the third polygon facet  25  (not necessary the adjacent faces of the polygon facet  14  and  21 ) before retracing the path back to the light controller  2 . 
         [0063]    The diffraction grating according to the fifth exemplary embodiment of the present invention can be operative to provide one or more features as described above, as well as to convert a diverging beam from the polygon facets  14  and  21  into converging angular dispersion after the diffraction grating  13  on the polygon facet  21 . Such result may be advantageous for a proper operation of the filter. In addition, the diffraction grating  13  and the incident angle of the optical beam  12  on the polygon facet  14  may provide adjustable parameters, which control the tuning range and linewidth. In this embodiment, The grating equations are expressed as λ=p(sin(α)+sin(β)) and λ=p(sin(γ)+sin(φ)), where λ is the optical wavelength, p is the grating pitch, and α, β, γφ are the incident and diffracted angles of the beam with respect to the normal axis  18  of the diffraction grating  13 , respectively. 
         [0064]    It is possible that 2φ 1 =γ+β, where φ 1  is the angle between the second polygon facet  24  and diffraction grating  13 . In addition, φ 2 =φ, where φ 2  is the angle between the third polygon facet  25  and diffraction grating  13 . By eliminating β, γ, and φ, the above equations give an expression, quadratic in λ, that depends only on the incident angle and the angle between the second polygon and third polygon facets  21  and  25  and diffraction grating  13 . The exemplary solution can be as follows: 
         [0000]    
       
         
           
             λ 
             = 
             
               
                 p 
                  
                 
                   { 
                   
                     
                       
                         
                           
                             
                               sin 
                                
                               
                                 ( 
                                 α 
                                 ) 
                               
                             
                              
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   0.5 
                                    
                                   
                                       
                                   
                                    
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         2 
                                          
                                         φ 
                                       
                                       ) 
                                     
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                         
                       
                     
                     
                       
                         
                           
                             
                               
                                 
                                   sin 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   α 
                                   ) 
                                 
                               
                                
                               
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       0.5 
                                        
                                       
                                           
                                       
                                        
                                       
                                         cos 
                                          
                                         
                                           ( 
                                           
                                             2 
                                              
                                             φ 
                                           
                                           ) 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                             - 
                           
                         
                       
                     
                     
                       
                         
                           
                             ( 
                             
                               
                                 
                                   sin 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   α 
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   sin 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   
                                     2 
                                      
                                     φ 
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                            
                           
                             ( 
                             
                               1.25 
                               + 
                               
                                 cos 
                                  
                                 
                                   ( 
                                   
                                     2 
                                      
                                     φ 
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   } 
                 
               
               
                 ( 
                 
                   1.25 
                   + 
                   
                     cos 
                      
                     
                       ( 
                       
                         2 
                          
                         φ 
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    As shown in this equation, the tuning range of the filter is fundamentally limited by the grating pitch, the incident angle  19 , and the angle between the second polygon facet and the diffraction grating  13 . In addition, it can be shown that the FWHM bandwidth of the filter (instantaneous line-width) is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     δ 
                      
                     
                         
                     
                      
                     λ 
                   
                   = 
                   
                     
                       2 
                        
                       p 
                        
                       
                           
                       
                        
                       λ 
                        
                       
                         
                           ln 
                            
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                     
                     
                       π 
                        
                       
                           
                       
                        
                       
                         D 
                         ( 
                         
                           1 
                           + 
                           
                             
                               cos 
                                
                               
                                 ( 
                                 β 
                                 ) 
                               
                             
                             
                               cos 
                                
                               
                                 ( 
                                 γ 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Equation (8) shows that the linewidth of this embodiment has been improved by a factor of 
         [0000]    
       
         
           
             1 
             + 
             
               
                 cos 
                  
                 
                   ( 
                   β 
                   ) 
                 
               
               
                 cos 
                  
                 
                   ( 
                   γ 
                   ) 
                 
               
             
           
         
       
     
         [0000]    compared to the previous embodiment. 
         [0065]    The filter tuning range can be expressed as Δλ=p(Δα cos(α 0 )+Δβ cos(β 0 )) and Δλ=p(Δγ cos(γ 0 )+Δφ cos(φ 0 ) where α 0 , β 0 , γ 0 , and φ 0  are the incident, and diffracted angles at λ 0  (center wavelength). If the sweeping angle is equal to the range of the incident angle, i.e. Δα=2θ and the range of the first diffracted spectrum follows the following equality, i.e. 2θ=Δγ+Δβ, and the range of the second diffracted spectrum is equal to the facet angle, i.e. Δφ=θ, the polygon arrangement can retro-reflect one spectral component at a given time. The spacing of the multiple spectral components simultaneously reflected, or the free spectral range, can be defined as 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     λ 
                   
                   = 
                   
                     
                       
                         p 
                          
                         
                             
                         
                          
                         
                           θ 
                           ( 
                           
                             
                               
                                 
                                   
                                     
                                       cos 
                                        
                                       
                                         ( 
                                         
                                           ϕ 
                                           0 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       cos 
                                        
                                       
                                         ( 
                                         
                                           β 
                                           0 
                                         
                                         ) 
                                       
                                     
                                   
                                   + 
                                   
                                     2 
                                      
                                     
                                       cos 
                                        
                                       
                                         ( 
                                         
                                           γ 
                                           0 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       cos 
                                        
                                       
                                         ( 
                                         
                                           β 
                                           0 
                                         
                                         ) 
                                       
                                     
                                   
                                   + 
                                 
                               
                             
                             
                               
                                 
                                   2 
                                    
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         γ 
                                         0 
                                       
                                       ) 
                                     
                                   
                                    
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         α 
                                         0 
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                       
                         ( 
                         
                           
                             cos 
                              
                             
                               ( 
                               
                                 γ 
                                 0 
                               
                               ) 
                             
                           
                           + 
                           
                             cos 
                              
                             
                               ( 
                               
                                 β 
                                 0 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0066]    The duty cycle of laser tuning by the filter can be, for example, 100% with no excess loss caused by beam clipping if preferable conditions can be met as follows: 
         [0000]    
       
         
           
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                         
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         ψ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                       
                         cos 
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                       
                         cos 
                          
                         
                           ( 
                           β 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   D 
                   &lt; 
                   
                     L 
                      
                     
                       
                         cos 
                          
                         
                           ( 
                           γ 
                           ) 
                         
                       
                       
                         cos 
                          
                         
                           ( 
                           ϕ 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Equation (10) may be derived from a condition that the beam width illuminating the first polygon facet  14  should be smaller than the facet width. Equations (12) and (12) can be derived from that the diffracted beam widths illuminating the second and third polygon facets  21  and  25  should be smaller than the facet width. The FWHM bandwidth of the filter can be further decreased by illuminating several diffraction gratings with the same or different pitches. 
         [0067]      FIG. 3A  shows a diagram of a sixth exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  (with grating pitch p 1 ) at an angle α  19  (not equal to Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates a reflector  26  before retracing the path back to the light controller  2 . It can be shown that the FWHM bandwidth of this filter (instantaneous line-widths) is given by 
         [0000]    
       
         
           
             δλ 
             = 
             
               
                 
                   2 
                    
                   p 
                    
                   
                       
                   
                    
                   λ 
                    
                   
                     
                       ln 
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   D 
                 
               
               . 
             
           
         
       
     
         [0068]      FIG. 3B  shows a diagram of a seventh exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  (with grating pitch p 1 ) at an angle α 19  (not equal to Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates other diffraction grating  27  (with grating pitch p 2 ) at Littrow&#39;s angle γ 28  before retracing the path back to the light controller  2 . It can be shown that the FWHM bandwidth of this filter (instantaneous line-widths) is given by 
         [0000]    
       
         
           
             δλ 
             = 
             
               
                 
                   2 
                    
                   
                     p 
                     1 
                   
                    
                   
                     p 
                     2 
                   
                    
                   λ 
                    
                   
                     
                       ln 
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   
                     D 
                     ( 
                     
                       
                         p 
                         2 
                       
                       + 
                       
                         
                           
                             cos 
                              
                             
                               ( 
                               β 
                               ) 
                             
                           
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 γ 
                                 ) 
                               
                             
                           
                         
                          
                         
                           p 
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
         [0069]      FIG. 3C  shows a diagram of an eight exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates the diffraction grating  13  (with grating pitch p 1 ) at an angle α 19  (not equal to Littrow&#39;s angle). The diffracted light  20  at angle β 22  from the grating illuminates other diffraction grating  27  (with grating pitch p 2 ) at an angle γ 28  (not equal to Littrow&#39;s angle). The diffracted light at angle φ 29  illuminate a reflector  30  before retracing the path back to the light. It can be shown that the FWHM bandwidth of this filter (instantaneous line-widths) is given by 
         [0000]    
       
         
           
             δλ 
             = 
             
               
                 
                   2 
                    
                   
                     p 
                     1 
                   
                    
                   
                     p 
                     2 
                   
                    
                   λ 
                    
                   
                     
                       ln 
                        
                       
                           
                       
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   
                     D 
                     ( 
                     
                       
                         p 
                         2 
                       
                       + 
                       
                         
                           
                             cos 
                              
                             
                               ( 
                               β 
                               ) 
                             
                           
                           
                             cos 
                              
                             
                               ( 
                               γ 
                               ) 
                             
                           
                         
                          
                         
                           p 
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
         [0070]    The FWHM bandwidths of the filter configurations shown in  FIGS. 3A-3C  can be decreased further by increasing the number of diffraction grating. 
         [0071]      FIG. 4A  shows a diagram of a ninth exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement for doubling the FSR of the filter with the same polygon rotation speed according to the present invention. In this exemplary embodiment, the reflected light from the polygon scanner facet  14  illuminates a reflector (or a folded telescope) and illuminates the other polygon facet and can be coupled to all previous described filter configurations. The sweep angle of the reflected light from the polygon arrangement is quadraple of the polygon rotation angle. When the facet-to-facet angle  15  of the polygon, e.g., angle θ, the sweep angle of the reflected light is 4θ for a rotation of the angle θ of the polygon arrangement. The reflector can be placed near the polygon scanner facet  14  to decrease beam displacement on the diffraction grating and avoid beam clipping on the second polygon facet. 
         [0072]      FIG. 4B  shows a diagram of a tenth exemplary embodiment of the arrangement/apparatus which includes the wavelength tunable filter arrangement  1  for increasing the tuning speed of filter with the same polygon rotation speed and without increasing the number of polygon facets according to the present invention. By placing two (or N) diffraction gratings  100 ,  101  with the angle 2θ/N between each other, which preferably direct the reflected beam of light from the polygon arrangement back to the polygon arrangement, and to the light controller  2 , N wavelength scans from λ 1  to λ N  are achieved for the polygon rotation of the one facet-to-facet angle, θ. In this exemplary embodiment, the filter FSR decreases to 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   Δ 
                    
                   
                       
                   
                    
                   λ 
                 
                 ) 
               
               FSR 
             
             = 
             
               
                 
                   4 
                    
                   p 
                    
                   
                       
                   
                    
                   θ 
                    
                   
                       
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       
                         α 
                         0 
                       
                       ) 
                     
                   
                 
                 N 
               
               . 
             
           
         
       
     
         [0073]      FIG. 5A  shows a block diagram of an eleventh exemplary embodiment of the arrangement/apparatus which includes the optical wavelength filter  1 ′ in accordance the present invention. In this exemplary embodiment, the optical wavelength filter  1 ′ can be used in a variety of different applications, general examples of which are described below. In this example, the filter  1 ′ may be coupled to one or more applications  3 ′ via a light source  2 ′. It should be understood that in certain exemplary applications, the filter  1 ′ can be used with or connected to an application (e.g., one or more of the applications  3 ′) via a device other than a light source (e.g. a passive or active optical element). In this exemplary embodiment as shown in  FIG. 5A , a broad spectrum light source and/or controller  2 ′ (hereinafter referred to as “light controller”), may be coupled to a wavelength dispersing element  4 ′. The light controller  2 ′ can be further coupled to one or more of the applications  3 ′ that are adapted to perform one or more tasks with or for, including but not limited to, optical imaging processes and optical imaging systems, laser machining processes and systems, photolithography and photolithographic systems, laser topography systems, telecommunications processes and systems, etc. The wavelength dispersing element  4 ′ can be coupled to a lens system  6 ′, which is further coupled to a beam deflection device  5 ′ and wavelength dispersing element  7 ′. 
         [0074]    The light controller  2 ′ can be one or more of various systems and/or arrangements that are configured to transmit a beam of light having a broad frequency (f) spectrum. In one exemplary embodiment, the beam of light may be a collimated beam of light. The beam of light can include a plurality of wavelengths λ . . . λn, within the visible light spectrum (e.g., red, blue, green). Similarly, the beam of light provided by the light controller  2 ′ can also include a plurality of wavelengths λ . . . λn that may be defined outside of the visible spectrum (e.g., ultraviolet, near infrared or infrared). In one exemplary embodiment of the present invention, the light controller  2 ′ can include a unidirectional light transmission ring, which shall be described in further detail below in connection with  FIGS. 8A-D  which shows an exemplary embodiment of a wavelength tuning laser source. Further, in another exemplary embodiment of the present invention, the light controller  2 ′ can include a linear resonator system, which shall be described in further detail below in connection with  FIGS. 8A-D . 
         [0075]    The wavelength dispersing element  4 ′ of the optical wavelength filter  1 ′ can include one or more elements that are specifically adapted to receive the beam of light from the light controller  2 ′, and to conventionally separate the beam of light into a plurality of wavelengths of light having a number of directions. The wavelength dispersing element  4 ′ is further operative to direct portions of light having different wavelengths in equal angular directions or displacements with respect to an optical axis . . . . In one exemplary embodiment of the present invention, the wavelength dispersing element  4 ′ can include a light dispersion element, which may include but not limited to, a reflection grating, a transmission grating, a prism, a diffraction grating, an acousto-optic diffraction cell or combinations of one or more of these elements. 
         [0076]    The lens system  6 ′ of the optical wavelength filter  1 ′ can include one or more optical elements adapted to receive the separated wavelengths of light from the wavelength dispersing element. Light at each wavelength propagates along a path which is at an angle with respect to the optical axis  3  . . . . The angle is determined by the wavelength dispersing element  4 ′. Furthermore, the lens system  6 ′ is adapted to direct or steer and/or focus the wavelengths of light to a predetermined position located on a beam deflection device  5 ′. 
         [0077]    The beam deflection device  5 ′ can be controlled to receive and selectively redirect one or more discrete wavelengths of light to the wavelength dispersing element  7 ′. The wavelength dispersing element  7  redirect back one or more discrete wavelengths to the beam deflection device  5 ′ and then along the optical axis through the lens system  6 ′ to the wavelength dispersing element  4 ′ and back to the light controller  2 ′. Thereafter, the light controller  2 ′ can selectively direct the received discrete wavelengths of light to any one or more of the applications. The beam deflecting device  5 ′ can be provided in different ways. For example, the beam deflecting device  5 ′ can be provided from elements including, but not limited to, a polygonal mirror, a planar mirror disposed on a rotating shaft, a mirror disposed on a galvonmeter, or an acousto-optic modulator. 
         [0078]      FIG. 5B  shows a schematic diagram of a twelfth exemplary embodiment of the arrangement/apparatus which includes the optical wavelength filter  1 ′. The exemplary optical wavelength filter  1 ′ can be configured as a reflection-type filter which may have substantially identical input and output ports. An input/output optical fiber  53  and a collimating lens  56  can provide an input from a light controller  2 ′ (which may be substantially similar to the light controller  2  described above with reference to  FIG. 5A ) to the optical wavelength filter  1 ′. The optical wavelength filter  1 ′ includes a diffraction grating  50 , optical telescoping elements  5 ′ (hereinafter referred to as “telescope  6 ′” and may possibly be similar to the lens system  6  of  FIG. 1A ), and a polygon mirror scanner  54 . The telescope  6 ′ can include two lenses, e.g., first and second lenses  51 ,  52  with 4-f configuration. 
         [0079]    In this embodiment of the optical wavelength filter  1 ′ shown in  FIG. 5B , the mirror surface of the polygon arrangement  54  is placed substantially a distance F 2  from lens  22 , and the beam of light is reflected with a non-zero angle (rather than directly being reflected back to the telescope from the polygon arrangement&#39;s  54  mirror facet). The sweep angle of the reflected light from the polygon arrangement  54  is double the polygon arrangement&#39;s  54  rotation angle. When the incident angle difference  90  between λ 1  and λN with respect to the polygon arrangement  54  is approximately the same as the facet-to-facet angle  57  of the polygon, e.g., angle θ, the sweep angle  58  of the reflected light is 2θ for a rotation of the angle θ of the polygon arrangement  54 . By illuminating a diffraction grating  55  at Littrow angle  59  which preferably direct the reflected beam of light from the polygon arrangement  54  back to the polygon arrangement  54 , and to the telescope (e.g., similar to the telescope  6 ′ of  FIG. 5B ), with the angle θ between each other, twice wavelength scans from λ 1  to λN are achieved for the polygon rotation of the one facet-to-facet angle. In addition, the linewidth of this exemplary filter can be improved as compared to the previous polygon scanner filter using telescope and end reflector by illuminating other diffraction grating. 
         [0080]    The first lens  51  may be located at a first distance from the wavelength dispensing element  4 ′ (e.g., diffraction grating  50 ), which can approximately be equal to the focal length F 1  of the first lens  51 . The second lens  52  may be located at a second distance from the first lens  51 , which can be approximately equal to the sum of the focal length F 1  of the first lens  51  and the focal length F 2  of the second lens  52 . Using such exemplary arrangement, the first lens  51  can receive one or more collimated discrete wavelengths of light from the wavelength dispersing element  4 ′, and can effectively perform a Fourier Transform on each one of the collimated one or more discrete wavelengths of light to provide one or more approximately equal converging beams that are projected onto an image plane IP. 
         [0081]    The image plane IP can preferably be located between the first lens  51  and the second lens  52  and at a predetermined distance from the first lens  51 . According to one exemplary variation of the present invention, such predetermined distance may be defined by the focal length F 1  of the first lens  51 . After such one or more converging beams are propagated through the image plane IP, these one or more converging beams form equal or corresponding one or more diverging beams that are received by the second lens  52 . The second lens  52  is adapted to receive the diverging beams and provide approximately an equal number of collimated beams having predetermined angular displacements with respect to the optical axis. Thus, the second lens  52  can direct or steer the collimated beams to predefined portions of the beam deflection device  5 ′. 
         [0082]    The telescope  6 ′ according to this exemplary embodiment of the present invention can be operative to provide one or more features as described above, as well as to convert a diverging angular dispersion from the grating into converging angular dispersion after the second lens  52 . Such result may be advantageous for a proper operation of the filter. In addition, the telescope  6 ′ may provide adjustable parameters which control the tuning range and linewidth and reduce the beam size at the polygon mirror to avoid beam clipping. As is illustrated in the exemplary embodiment of  FIG. 5B , a beam deflection device  5 ′ (e.g., which may include a polygon mirror or arrangement  54 ) is adapted to preferably reflect only the spectral component within a narrow passband as a function of the angle of the front mirror facet of the polygon arrangement  54  with respect to the optic axis  38 . The reflected narrow band light illuminates the diffraction grating  55  and diffracted and received by the optical fiber  53 . 
         [0083]    In this exemplary embodiment, the equations can be expressed as λ=p 1 (sin(α)+sin(β)) and λ=2p 2  sin(γ), where λ is the optical wavelength, p 1  and p 2  are the grating pitches, and α, β, γ are the incident, diffracted, and Littrow angles of the beam with respect to the normal axes of the diffraction gratings  50 ,  55 , respectively. 
         [0084]    It can be shown that the FWHM bandwidth of the filter (instantaneous line-width) may be provided by 
         [0000]    
       
         
           
             
               
                 
                   δλ 
                   = 
                   
                     
                       2 
                        
                       
                         p 
                         1 
                       
                        
                       
                         p 
                         2 
                       
                        
                       λ 
                        
                       
                         
                           ln 
                            
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           α 
                           ) 
                         
                       
                     
                     
                       π 
                        
                       
                           
                       
                        
                       
                         D 
                         ( 
                         
                           
                             p 
                             2 
                           
                           + 
                           
                             
                               
                                 0.5 
                                  
                                 
                                     
                                 
                                  
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     β 
                                     ) 
                                   
                                 
                               
                               
                                 cos 
                                  
                                 
                                   ( 
                                   γ 
                                   ) 
                                 
                               
                             
                              
                             
                               p 
                               1 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Equation (13) shows that the linewidth of this embodiment has been improved by a factor of 
         [0000]    
       
         
           
             1 
             + 
             
               
                 
                   0.5 
                    
                   
                       
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       β 
                       ) 
                     
                   
                 
                 
                   cos 
                    
                   
                     ( 
                     γ 
                     ) 
                   
                 
               
                
               
                 
                   p 
                   1 
                 
                 
                   p 
                   2 
                 
               
             
           
         
       
     
         [0000]    compared to the previous embodiment. 
         [0085]    The tuning range of the filter may be limited by the finite numerical aperture of the first lens  51 . The acceptance angle of the first lens  51  without beam clipping may be defined by Δβ=(D 1 −W cos β 0 /cos α)/F 1  where D 1  and F 1  are the diameter and focal length of the first lens  51 . Such formulation relates to the filter tuning range via the filter tuning range can be expressed as 
         [0000]    
       
         
           
             
               
                 Δ 
                  
                 
                     
                 
                  
                 λ 
               
               = 
               
                 
                   
                     
                       F 
                       2 
                     
                      
                     
                       p 
                       1 
                     
                      
                     
                       Δβ 
                       ′ 
                     
                   
                   
                     F 
                     1 
                   
                 
                  
                 
                   cos 
                    
                   
                     ( 
                     
                       β 
                       0 
                     
                     ) 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    and Δλ=2p 2 Δγ cos(γ 0 )=Δγ√{square root over (4p 2   2 −λ 0   2 )} where α 0 , β 0 , and γ 0  are the incident, diffracted, and Littrow angles at λ 0  (center wavelength). One of exemplary design parameters of the filter, originated from the multiple facet nature of the polygon mirror, is the free spectral range, which is described in the following. A spectral component after propagating through the first lens  51  and the second lens  52  may have a beam propagation axis at an angle β′ with respect to the optic axis  38 , e.g., β′=−(β−β 0 )·(F 1 /F 2 ), where F 1  and F 2  are the focal lengths of the first lens  51  and the second lens  52 , respectively. 
         [0086]    The polygon arrangement  54  may have a facet-to-facet polar angle given by θ=2π/N≈L/R, where L is the facet width, R is the radius of the polygon and N is the number of facets. If the range of β′ of incident spectrum is greater than the facet angle, i.e. Δβ′=Δβ·(F 1 /F 2 )&gt;θ, the polygon arrangement  24  can retro-reflect more than one spectral component at a given time. If the sweeping angle is equal to the range of the incident angle, i.e. Δβ′=2θ and the range of diffracted spectrum follows the following equality, i.e. 2θ=Δγ, the polygon arrangement can retro-reflect one spectral component at a given time. The spacing of the multiple spectral components simultaneously reflected, or the free spectral range, can be defined as Δλ=2θ√{square root over (4p 2   2 −λ 0   2 )}, when 
         [0000]    
       
         
           
             
               
                 F 
                 2 
               
               
                 F 
                 1 
               
             
             = 
             
               
                 
                   
                     
                       4 
                        
                       
                         p 
                         2 
                         2 
                       
                     
                     - 
                     
                       λ 
                       0 
                       2 
                     
                   
                 
                 
                   
                     p 
                     1 
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       
                         β 
                         0 
                       
                       ) 
                     
                   
                 
               
               . 
             
           
         
       
     
         [0087]    The duty cycle of laser tuning by the filter can be, for example, 100% with no excess loss caused by beam clipping if two preferable conditions can be met as follows: 
         [0000]    
       
         
           
             
               
                 
                   W 
                   &lt; 
                   
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         α 
                          
                         
                             
                         
                          
                         
                           F 
                           1 
                         
                       
                       
                         cos 
                          
                         
                             
                         
                          
                         β 
                          
                         
                             
                         
                          
                         
                           F 
                           2 
                         
                       
                     
                      
                     L 
                      
                     
                         
                     
                      
                     and 
                      
                     
                         
                     
                      
                     W 
                   
                   &lt; 
                   
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         α 
                       
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           β 
                           0 
                         
                       
                     
                      
                     
                       
                         ( 
                         
                           
                             F 
                             2 
                           
                           - 
                           S 
                         
                         ) 
                       
                       · 
                       θ 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0088]    The first equation may be derived from a condition that the beam width after the second lens  52  should be smaller than the facet width. The second equation can be derived from that the two beams at the lowest  60  and highest wavelengths  61  of the tuning range, respectively, which should not overlap each other at the polygon arrangement  54 . S in equation (14) denotes the distance between the second lens  52  and the front mirror of the polygon arrangement  54 . 
         [0089]      FIG. 5C  shows a schematic diagram of a thirteenth exemplary embodiment of the arrangement/apparatus which includes the optical wavelength filter  1 ′. In this exemplary embodiment, a diffraction grating  55  is illuminated at an angle  61  (not equal to Littrow angle). The diffracted light  62  at angle β from the grating illuminates a reflector  56  before retracing the path back to the light controller  2 . It can be shown that the FWHM bandwidth of this filter (instantaneous line-widths) is given by 
         [0000]    
       
         
           
             δλ 
             = 
             
               
                 
                   2 
                    
                   
                     p 
                     1 
                   
                    
                   
                     p 
                     2 
                   
                    
                   λ 
                    
                   
                     
                       ln 
                        
                       
                           
                       
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                    
                   
                     cos 
                      
                     
                       ( 
                       α 
                       ) 
                     
                   
                 
                 
                   π 
                    
                   
                       
                   
                    
                   
                     D 
                     ( 
                     
                       
                         p 
                         2 
                       
                       + 
                       
                         
                           
                             cos 
                              
                             
                               ( 
                               β 
                               ) 
                             
                           
                           
                             cos 
                              
                             
                               ( 
                               γ 
                               ) 
                             
                           
                         
                          
                         
                           p 
                           1 
                         
                       
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
         [0090]      FIG. 5D  shows a schematic diagram of a fourteenth exemplary embodiment of the arrangement/apparatus which includes the optical wavelength filter  1 ′. In this exemplary embodiment, reflector  56  is replaced in the previous embodiment with several gratings to narrow the instantaneous line-width of the filter. 
         [0091]      FIG. 6A  shows a fifteenth exemplary embodiment of the arrangement/apparatus of the present invention which provides a polygon tuning filter accommodating two light inputs and outputs. For example, in order to support two or more inputs and outputs of this filter, two or more sets of optical arrangements, each respective set including an input/output fiber  70 ,  70 ′, a collimating lens  71 ,  71 ′, a diffraction gratings (or previous described filter configurations)  72 ,  72 ′, may share the same polygon arrangement  73 . Because the scanning mirror of the polygon arrangement  73  is structurally isotropic about the rotation axis, certain optical arrangements that can deliver the beams of light to the polygon arrangement  73  can be accommodated from any directions. Since both sets of optical arrangement in the embodiment of  FIG. 6A , utilize the same polygon scanner, their respective scanning optical transmission spectra are synchronized. It should be understood that the embodiment of  FIG. 6A  can be extended to include multiple (e.g., greater than 2) optical arrangements each having its own input and output optical channel. 
         [0092]    One exemplary application of the above-described exemplary polygon tuning filter according to the tenth embodiment of the present invention may be a wide band wavelength scanning light source. In  FIG. 6A  which shows a fifteenth exemplary embodiment of the present invention, a first broadband light source  74  provides a light signal which may have a wavelength λ 1  to λi, and a second broadband light source  74 ′ provides another light signal having a wavelength λi-j to λN. When the two optical arrangements supporting the wavelengths λ 1  to λi and the wavelengths λi-j to λN, respectively, are synchronized to output approximately the same wavelength at the same instance, such exemplary arrangement may become a wide band wavelength scanning light source with linear scan rate from λ 1  to λN. Since the FSR of the polygon scanning filter can be adjusted to be 350 nm or wider without any optical performance degradation, two or more broadband light sources with different center wavelengths can be combined with this filter to provide linear scanning light source over 350 nm tuning bandwidth. It should be understood that the embodiment of  FIG. 6A  can be extended to include multiple (e.g., &gt;2) optical arrangements and multiple (e.g., &gt;2) broadband light sources. 
         [0093]    The exemplary embodiment of the arrangement/apparatus shown in  FIG. 6A  can also be configured so that the wavelength tuning bands of each optical arrangement and broadband light source are discontinuous. In such a configuration, the tuning bands can be swept in a continuous or discontinuous sequential manner or be swept simultaneously. 
         [0094]      FIG. 6B  shows a sixteenth exemplary embodiment of the present invention of the arrangement/apparatus for increasing the filter FSR by combination of two or more gain media  74  and  75  (parallel or serial) whose gain spectra are distinct. This exemplary arrangement/approach has advantage compared to the twelfth exemplary embodiment because there is less preference for multiple (e.g., ≧2) optical arrangements and synchronizing the wavelength sweep of the independent resonators. 
         [0095]      FIG. 7  shows an exemplary embodiment of the arrangement/apparatus which includes the wavelength-swept laser using the grating and polygon scanner filter. Collimated light output  80  from a semiconductor optical amplifier (SOA)  81  is directly coupled into the grating and polygon scanner filter. A small portion of the light from the reflection facet side of the SOA  82  can be coupled into the single mode fiber  83  providing output of the laser  84 . 
         [0096]    A frequency downshift in the optical spectrum of the intra-cavity laser light may arise as the light passes through the SOA gain medium, as a result of an intraband four-wave mixing phenomenon. In the presence of the frequency downshift, greater output power can be generated by operating the wavelength scanning filter in the positive wavelength sweep direction. Since the combined action of self-frequency shift and positive tuning allows higher output to be obtained and enables the laser to be operated at higher tuning speed, the positive wavelength scan may be the preferable operation. The output power can be decreased and the instantaneous linewidth can be broadened with an increasing tuning speed. A short cavity length may be desired to reduce the sensitivity of the output power and instantaneous linewidth to the tuning speed. 
         [0097]    With a short length wavelength scanning filter based on the grating and polygon scanner filter and direct free-space coupling between the gain medium and the optical wavelength filter, the total cavity round trip length can be shorter than 20 cm, which is advantageous for reducing the sensitivity of the output power and instantaneous linewidth to the tuning speed. 
         [0098]      FIG. 8A  shows another embodiment of the arrangement/apparatus which includes the wavelength-swept laser using the grating and polygon scanner filter. Fiber ring cavity  92  can be coupled to the grating and polygon scanner filter via collimating lens  95 . For the applications where the high speed tuning is not essential so that the relatively long cavity length can be allowed, fiber ring cavity with a conventional dual port SOA  93  can be an optional configuration. 
         [0099]      FIG. 8B  shows another embodiment of the arrangement/apparatus which includes the wavelength-swept laser using the grating and polygon scanner filter. Fiber and free space ring cavity can be coupled to the grating and polygon scanner filter via collimating lens  106 , beam cube splitter  108 , and optical isolator  109 . For the applications where the high speed tuning is essential so that the short cavity length can be important, combination of fiber and free space ring cavity with a conventional dual port SOA  103  can be an optional configuration. 
         [0100]      FIG. 8C  shows an exemplary embodiment of the arrangement/apparatus which includes a fiber ring wavelength swept-laser with long cavity length. Increasing the cavity length so that the laser light can become resonant after a round trip of the cavity is another way to reduce the sensitivity of the output power and instantaneous linewidth to the tuning speed. Additional length of fiber  91 , whose length depends on the tuning repetition rate, in the ring cavity  92  enables resonant tuning. Cavity length variation of the laser cavity with the grating and polygon scanner filter may be smaller than that of the polygon scanner based laser, therefore better resonant may be obtainable. 
         [0101]      FIG. 8D  shows an exemplary embodiment of the resonant cavity fiber Raman ring laser using the grating and polygon scanner filter. Since long length of optical fiber  90  is needed for resonant wavelength tuning, Raman gain can be induced in the long length of fiber  90  with proper pump light  111  supplied through a WDM coupler  110 . Special type of fiber can be used as a long length fiber  90  in the cavity to enhance the Raman gain efficiency. Since the Raman gain wavelength band is determined by the wavelength band of the pump light, wavelength swept-laser with arbitrary wavelength tuning band may be obtained as far as the pump light with proper wavelength band is available. 
         [0102]    Further, depending on the pump light power and the Raman gain efficiency in the fiber, high power wavelength-swept laser may be implemented. Pump light for the Raman gain can be also provided in backward direction to the laser light and both forward and backward pumps can be used simultaneously to obtain higher gain. The pump light is not limited to the light with a single wavelength component. To obtain a broad bandwidth Raman gain, a multiple wavelength pump light can be preferably utilized. This scheme can be further expanded to achieve a laser tuning range beyond the filter free spectral range by using multiple Raman pump light staggered in wavelength, whose gain bandwidth is broader than the free spectral range of the filter, that are progressively cycled on and off. 
         [0103]    The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Indeed, the arrangements, systems and methods according to the exemplary embodiments of the present invention can be used with any OCT system, OFDI system, SD-OCT system or other imaging systems, and for example with those described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004, the disclosures of which are incorporated by reference herein in their entireties. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present invention. In addition, to the extent that the prior art knowledge has not been explicitly incorporated by reference herein above, it is explicitly being incorporated herein in its entirety. All publications referenced herein above are incorporated herein by reference in their entireties.

Technology Category: g